PLATINUM METALS REVIEW · 2016. 1. 28. · UK ISSN 0032-1400 PLATINUM METALS REVIEW A quarterly...

UK ISSN 0032-1400

PLATINUM METALS REVIEW

A quarterly survey of research on the platinum metals and of developments in their application in industry

VOL. 41 OCTOBER 1997

Contents

Long Life Radioisotopic Power Sources Encapsulated in Platinum Metal Alloys By E. A. Franco-Ferreira, G. M. Goodwin, T. G. George and G . H. Rinehart

Platinum Metals Involvement in the Hydrogen Economy By F. A. Lewis

P a l l a d i d o r o u s Glass Catalysts for Heck Reactions

Inorganic and Co-ordination Chemistry By S. M. Godfiey

Highly Active and Enantioselective Rhodium Catalyst

Selective Ethanol Synthesis from Carbon Dioxide By Yasuo Izumi

Hexagonal Nanostructured Platinum

The Fifth Grove Fuel Cell Symposium By Donald S. Cameron

Harnessing the Unique Properties of Iridium By I.: I. Savchenko, I. A. Makaryan and I.: G. Dorokhov

Ruthenium Sensitisers in Solar Cells

Crystallographic Properties of Rhodium By J. W. Arblaster

Abstracts

New Patents

Index to Volume 41

NO. 4

154

163

163

164

165

166

170

171

176

183

184

190

195

199

Communications should be addressed to The Editor, Susan V. Ashton, Pluiinum M& Review

Johnson M a ~ h e y Public Limited Company, Hatton Garden, London ECl N 8JP

Long Life Radioisotopic Power Sources Encapsulated in Platinum Metal Alloys CASSINI MISSION TO STUDY SATURN AND ITS MOONS

By E. A. Franco-Ferreira and G. M. Goodwin

and T. G. George and G. H. Rinehart Oak Ridge National Laboratory, Tennessee, U.S.A.

Los Alamos National Laboratory, New Mexico, U.S.A.

The p la t inum metals alloys, DOP-26 ir idium and plat inum-30 per cent rhodium, have been successfully used to encapsulate plutonia fuel pellets for the Cassini Spacecraft. The iridium-encapsulated heat sources provide approximately 900 watts of electrical power for the spacecraft and its experiments, whereas the platinum alloy clad pellets will supply about 150 watts of heat to various parts of the spacecraft and its lunar probe, Huygens. The particular alloys used on this mission have been selected to fulf i l the critical function of maintaining fue l containment during normal service and f o r projected malfunction or accident scenarios. Their ability to perform satisfactorily has been demonstrated through extensive testing of their mechanical, physical and impact properties. The Cassini heat source manufacturing yields were significantly higher than those obtained for previous missions.

The last NASA grand-scale interplanetary voyage of this century, the Cassini/Huygens Mission, is scheduled to be launched during a window which occurs between October 6th and 30th, 1997 (1-3). The mission is a joint U.S.- European close-up study of Saturn and its moons.

The four-year study will begin in July 2004 and will include s i x t y orbits of Saturn and about forty fly-bys of Titan, its largest moon, by the Cassini orbiter. Huygens, the European-built lunar probe, will have a parachute descent last- ing for two and a half hours before landing on Titan. Among investigations will be Saturn’s atmosphere, its rings and its magnetosphere and additional insights will be sought into questions raised during the twin Voyager fly-bys of the 1980s (4). The atmosphere and surface of Titan and other icy moons will also be characterised. Titan is the only known body in the solar system, other than Earth, with a thick nitrogen atmosphere, and it is thought to be the only

other object in the solar system which has a liquid and solid surface with shorelines ( 5 ) .

Electrical power for the eighteen science insuu- ments and forty-four on-board processors (4) will be supplied by three Radioisotope Thermo- electric Generators (RTGs) which are powered by plutonia (238Pu02)-~elled General Purpose Heat Source (GPHS) modules. This source of power is necessary because of the great distance of Saturn Eom the sun and because of the dura- tion of the mission. Alternative, solar power, would have required a pair of arrays measur- ing at least 35 m x 9 m (2). Also, because ofthe distance from the sun, instruments and equipment on both the orbiter and probe will require external heaters to maintain their temperature within normal operating ranges. This heat is provided by plutonia-fuelled Light Weight Radioisotope Heater Units (LWRHUs) which are strategically located on their respective vehicles. The $144 million spent on the RTGs and LWRHUs represents approximately 4.5 per cent

Platinum Metals Rev., 1997,41, (4), 154163 154

Radioisotope Therrnoelectrlc Generator (RTG)

General purpose Heat source capsule heat source (GPHS)

Fig. 1 The Radioisotope Thermoelectric Generator (RTG), the General Purpose Heat Source (GPHS) Module and the Heat Source Capsule, showing the relationship between them. Each GPHS module contains four heat source capsulea

of the total cost of $3.2 billion for the 18 year long lifetime of the programme (4).

The plutonia fuel pellets for both the RTGs and LwRHus are completely contained in iridium alloy and platinum alloy claddings, respectively. This encapsulation is necessary both to maintain the integrity of the fuel forms during service and to prevent accidental release of fuel particles to the environment.

The two accident scenarios of most concern entail a launch accident or an inadvertent atmospheric re-entry as a result of an orbital abort or during the gravity-assisted Earth fly-by in 1999. Extensive impact testing of fuelled capsules has verified that the cladding materials can provide the requisite fuel containment under all credible accident conditions. The occasion of the launch presents an ideal opportunity to

review and update the use of platinum metals for the containment capsules, a topic that has been reported here previously (6).

Radioisotope Thermoelectric Generators

The electricity-producing RTGs are identical in design to those used on the Galileo Mission to Jupiter, launched in 1989, and the Solar-Polar Ulysses Mission, launched in 1990. The Cassini spacecraft will carry three RTGs, the largest number ever for a single launch. An RTG and its relationship to the fuelled capsules and the GPHS modules are shown in Figure 1.

TheRTGis 1.13mlong,426.7mmindiam- eter from fin to fin and weighs 55.8 kg. It contains eighteen GPHS modules powered by a total of seventy-two heat source capsules. The

Platinum Metals Rev., 1997, 41, (4) 155

electrical output is approximately 300 watts and results from thermoelectric conversion of the heat from the capsules which is delivered at an operating temperature of 1287°C.

Each heat source capsule, producing about 60 watts of heat, consists of a pressed and sintered pellet of plutonia weighing 15 1 g contained in a 0.685 mm thick shell of DOP-26 iridium alloy. The capsule, shown in Figure 2, is 29.97 mm long and 29.72 mm in diameter. Capsule closure is accomplished by an autogenous gas tungsten arc weld at the equator. Each capsule has a sintered iridium frit-vent at one pole to release the helium which is produced by a-decay within the fuel pellet. The frit-vent, covered by its 0.127 mm thick iridium decontamination cover, is visible in Figure 2.

As shown in Figure 1, each GPHS module contains four heat source capsules. The modules are assembled from a number of nesting graphite components which provide the required heat transfer to the thermopiles in the RTG. The graphite shells are also responsible, along with the RTG structure, for significant impact attenuation in accident conditions. To activate the vents, just prior to assembly of the GPHS module, the decontamination covers on the capsule frit-vents are removed.

Light Weight Radioisotope Heater Units

LWRHUs of the present design (7) were also used to heat components in the Galileo spacecraft (8), and were launched aboard the Mars Pathfinder Mission in December 1996. A maximum of a hundred and fifty-seven of these heat sources will be used on the Cassini spacecraft and the Huygens probe (9). Figure 3 shows various components of a LWRHU.

The fuelled capsule consists of a pressed and sintered plutonia pellet weighing 2.7 g encapsulated in a 0.875 mm thick shell of platinum- 30 per cent rhodium (Pt-30%Rh) alloy. This capsule, identified as the “clad” in Figure 3, is 12.85 mm long and 8.60 mm in diameter at its two ribs. Capsule closure is by means of an autogenous gas tungsten arc weld of a flat end cap at one end of the cylindrical capsule. Three

Fig. 2 A DOP-26 iridium Gknd Purpoee Heat So- capsule showing the equatorial tungsten arc weld and the iridium frit-vent at the pole to release the helium produced by the plutonia

capsules, with their final closure welds at the top, are shown in Figure 4. As in the GPHS modules, each LWRHU capsule is equipped with a frit-vent of sintered platinum in the centre of its lower end cap, which is activated prior to assembly. The remaining components of the L m , shown in Figure 3, are three pyrolytic graphite shells and the outer he-weave-pierced fabric graphite aeroshell, each with their respective end closures. These graphite components control the overall thermal balance of the heat source and enhance the performance of the fuel cladding in accident conditions.

Each LWRHU produces 1 watt of heat. The LWRHU units are mechanically attached to the spacecraft singly and in groups so as to maintain the temperature of critical instruments and mechanical devices. Some of the LWRHUs are mounted in thermostatically-controlled assem- blies which are able to vary the amount of heat provided either to the spacecraft components or radiated into space.

Alloy Selection and Properties The service environments experienced by space

power isotopic heat sources may involve exposure at elevated temperatures to low pressure oxygen as well as to carbonaceous insulating

Platinum Met& Rev., 1997, 41, (4) 156

- End cap (Wpn

-Shim (Pt-30Rh)

-Clad (Pt-30Rh)

-Frit (Sintered Pt)

Fig. 3 Components of the Light Weight Radioisotope Heater Unit; around the plutonia is a platinum-30% rhodium capsule with a welded flat end of the same material and sintered platinum frit-vent at the lower end. Thie is surrounded by three pyrolytic graphite (PG) shells with insulator plugs at each pole. The outer aeroshell is of fine-weave-pierced fabric (FWPF) graphite ,

materials. Refractory metals and alloys based on tungsten, tantalum, molybdenum and nio- bium do not perform well in environments of this type. In contrast, however, platinum group metals and alloys exhibit a high degree of resistance to oxidation and to embrittlement

by interstitial elements. They also have good high temperature strength and ductility, and the liquidus temperatures of their metal-carbon

Fig. 4 Three platinum-lO%o rhodium Light Weight Radio- isotope Heater Unit capsules


compounds are high enough to pose no danger during atmospheric re-entry or accidents, such as a fuel fire.

Effects of Dopants The development of iridium alloys doped with

tungsten, thorium and aluminum (for use as containment materials and claddings) began at Oak Ridge National Laboratory more than twenty years ago (1 0). The addition of tungsten was found to improve alloy fabrication and increase yield strength, while additions of thorium and aluminum greatly improved ductility at very high strain rates by increasing grain boundary cohesion (1 1).

The optimised iridium alloy used for RTG fuel encapsulation, known as DOP-26, contains by weight 0.3 per cent tungsten, 60 ppm thorium and 50 ppm aluminum. The thorium dopant serves two important functions. At low levels, it appears that thorium segregates to the grain boundaries and improves grain boundary cohesion, while at higher dopant levels, the thorium combines with iridium in the form of IrsTh. These intermetallic precipitates pin the grain boundaries and inhibit grain coarsening while increasing alloy strength at high temperatures, with the optimum dopant level being 200 ppm thorium by weight ( 12).

Unfortunately, thorium can contribute to hot cracking in autogenous welds made in DOP-26. The iridium-rich end of the iridium-thorium binary phase diagram shows a eutectic reaction:

Liquid - (Ir) + Ir5Th at 2O8O0C, and congruent melting of IrsTh at 2260”C, both of which occur significantly below 2447”C, the melting point of iridium (1 3). This behaviour would be expected to lead to hot cracking (1 4) and, in fact, significant problems of this nature were experienced during GalileoKJlysses production ( 15, 16).

Liquation cracking was encountered in the GalileoNlysses capsule girth welds at the over- lap region (tie-in), but was essentially eliminated for Cassini by careful control of alloy composition, melting practice (17) and welding proce- dures (1 8). The dependence of weldability on

THORIUM CONTENT, ppm

Fig. 5 The effect of thorium content on hot cracking of DOP-26 iridium welds; threshold stress for eracking deereases by a factor of two between thorium contents of 34 and 94 pprn

composition is graphically shown in Figure 5, which plots hot cracking sensitivity (cracking stress) versus the thorium level. Note that the threshold stress for cracking, as measured by the Sigmajig test (a weld cracking test) (1 9), decreases by a factor of two over the thorium content range of 37 to 94 ppm. Each heat of production material was subjected to multiple chemical analyses and Sigmajig tested for weldability prior to fabrication of the cladding shells. (A “heat” of material is a quantity of material melted as a single lot during production). Thorium concentrations were maintained in the 60 to 70 ppm range, which still resulted in acceptable mechanical properties. Throughout production of the Cassini GPHS heat source capsules, not a single instance of weld hot cracking was encountered (1 8).

Design of Radioisotope Heater Units Radioisotope heater units used prior to the

Galileo mission employed relatively heavy, multilayer tantalum alloy containment systems, loaded with 80 per cent enriched plutonia fuel in the form of shards or pellets. The primary drawbacks to these systems were the weight of the multilayer containment and the inherently poor oxidation resistance of tantalum alloys to external oxidising environments and to oxygen released by the fuel.

A design effort at Los Alamos National Laboratory on a new light-weight radioisotope heater unit focused on methods of increasing the power density of the unit and its high

Platinum Metals Rev., 1997,41, (4) 158

temperature oxidation resistance. Because the weight and bulk of a multilayer Lontainment system significantly reduced the power density, a decision was made early-on to utilise a vented capsule design. Although the proposed use of a frit-vent permitted the helium produced by a- decay of the 238Pu02 to escape, and precluded the need for heavy pressure-vessel type containment, it increased the importance of the required oxidation resistance of the clad. As a result, the primary requirements for the LWRHU cladding were defined as:

a melting or eutectic point at least 200°C above the maximum predicted temperature during atmospheric re-entry,

sufficient strength and ductility to survive impact with the Earth with no loss of containment, and

chemical compatibility with both carbon (present in the graphite aeroshell surrounding the capsule) and oxygen over the range of operating and re-entry temperatures.

On the basis of thermodynamic studies previously conducted at Los Alamos National Laboratory, three platinum-based alloys were selected as potential encapsulation materials:

Subsequent testing of the three candidate alloys revealed that both Pt-8%W and Pt-30%Rh had sufficient ductility to warrant further investig- ation. In addition, these alloys were commercially available as sheet and tube. High strain-rate testing of Pt-30%Rh and Pt-8%W samples previously exposed to graphite at high temperatures (to simulate atmospheric reentry) revealed that after a 1 minute exposure at 1700°C the ductility of the Pt-30%Rh was approximately 75 per cent greater than that of the Pt-8%W alloy (at a strain rate of 45 m s”). Consequently, the Pt-30%Rh alloy was selected as the encapsulation material for the LWRHU.

Pt-30%Rhy PM%W and Pt-30%Rh-8%W (20).

GPHS Encapsulation Hardware The DOP-26 iridium alloy used to fabricate

the shells for GPHS fuel encapsulation was produced at Oak Ridge National Laboratory. The production procedure, implemented in 1989, is described below (1 7). Iridium powder was first

blended with tungsten powder to produce the Ir-0.3%W composition. The blended powder was compacted, hydrogen annealed and vacuum sintered. The sintered compacts were multiple electron beam melted into 500 g buttons. Final alloying with thorium and aluminum was performed by button arc melting under an argon partial pressure to minimise evaporative losses. The alloy buttons were drop-cast into 27 mm diameter ingots which were then joined end- to-end and vacuum arc remelted into 63 mm diameter ingots weighing about 10 kg. The ingots were hot extruded into rectangular bars at 1430°C and cold rolled to sheet. Circular blanks were cut ftom sheet that was surface ground to the final thickness, by electrical discharge machin- ing. The iridium alloy blanks were then con- verted into shell halves as described below (21).

Iridium Alloy Blanks The blanks were acid cleaned and each was

sandwiched between two tantalum barrier discs. The sandwiches were encapsulated between two 304L stainless steel sheets with electron beam circumferential closure welds. This “blank assembly” was deep drawn at 925°C in a two- draw operation. After drawing, the stainless steel and tantalum were chemically stripped from the cups with warm acid mixtures. After stripping and acid cleaning, the cups were vacuum annealed at 1375°C for one hour to achieve full recrystallisation. One cup ftom each heat treatment run was destructively tested for metallur- gical evaluation and chemistry verification.

The cups were sized to the required contour dimensions and a number of mechanical fea- ture details were then added. Each cup was 100 per cent dye penetrant inspected. The outer surfaces of the cups, excluding the weld zones, were grit blasted to enhance their emissivity. A final acid clean removed any residual metallic cont- aminants prior to frit-vent installation and shipping to Los Alamos National Laboratory.

LWRHU Encapsulation Hardware Capsule parts for the LWRHUs were fabri-

cated from Pt-30%Rh (22). Sheet and tube of Pt-30%Rh alloy were first subjected to hardness


testing and confirmatory chemical analyses, and subjected to visual and ultrasonic flaw inspec- tions. After verification of the material properties and chemical composition, capsule bodies were machined from the Pt-30%Rh tube-stock, and shims and end caps were machined from the sheet. The vent end-cap of the capsule was fabricated by cold-pressing and sintering 0.061 g of platinum powder into a porous frit, and then electron beam welding sheets of Pt- 3o%Rh around and over the platinum frit. A second electron beam weld was then used to attach the vent-cap assembly to the capsule body. The empty LWRHU shell was visually and dimensionally inspected and leak checked using pressurised helium.

Fuel Pellet Fabrication Plutonia powder was sampled for chemical

and isotopic analyses and introduced into the glovebox processing line at Los Alamos National Laboratory. The raw plutonia powder was heated to 775°C for several hours in an argon atmosphere saturated with l6O water vapour. This step was required to reduce the neutron emission rate from the plutonia (from the I7O and ''0 a-n reactions) by replacing the susceptible oxygen isotopes in the fuel with I6O. After the oxygen exchange treatment, the fuel powder was ball milled, slugged and screened into granules, and then seasoned at either 1100 or 1600°C. After seasoning, the high- and low-fired

Fig. 6 The General Purpose Heat Source automatic welding fixture, in which the iridium alloy half-.+hell.+ containing the plutonia fuel were rotated vertically in h n t ufthe welding torch

granules were mixed in a ratio of 40:60, and vacuum hot pressed at approximately 1500°C into GPHS or LWRHU pellets. After hot-pressing, the pellets were dimensionally inspected and then sintered at 1527°C.

Fuel Encapsulation The GPHS capsules were assembled and

welded in a helium-fUed glovebox at Los Alamos National Laboratory. Assembly entailed plac- ing the two DOP-26 iridium half-shells in the welding fixture, loading the fuel pellet into the lower shell, and bringing the upper shell into contact with the lower one at the weld joint. The lower shells contained a 0.127 mm thick iridium foil weld shield which was positioned behind the joint and prevented the weld root from con- tacting the fuel pellet. The welding fixture, shown in Figure 6, rotated the capsule around a vertical axis in front of a horizontally-positioned welding torch. The entire welding operation was automatic, under computer control. The autogenous, full-penetration weld was made at an up-tapering current of 1 15 to 1 16 A and a travel speed of 12.5 mm s-'. The entire welding cycle required 10.8 s of arc time, and the total cycle t ime for capsule assembly and welding averaged 15 minutes per unit (1 8).

The primary non-destructive examination used for inspection of the GPHS girth welds was an immersed ultrasonic test. Automated scanning equipment produced a graphical (C-scan)


Fig. 7 The Light Weight Radioisotope Heater Unit automatic welding fixture, in which the platinum-30 per cent rhodium capsules containing the plutonia fuel were rotated vertically in front of the welding torch

presentation of the ultrasonic signal data from each side of the weld during a two-minute test, the principal objective being to locate centre- line hot cracks at the weld tie-in. This type of defect was a significant problem during the GalileoNlysses production campaign. Occasional instances of weld root fusion to the weld shield and internal joint mismatch, both considered innocuous, produced ultrasonic indications which were resolved by auxiliary techniques, such as tangential radiography.

Assembly and welding of the LWRHU capsules were carried out in a helium-filled glovebox at Los Alamos National Laboratory. The Pt-30%Rh capsules were supplied with their bottom end caps welded in place by electron beam welding. The empty capsule was loaded into the welding fixture, see Figure 7, and the fuel pellet was placed into the capsule, followed by a 0.1 mm thick Pt-30%Rh weld shield disc and the 1 .O mm thick top end cap. The welding fixture rotated the capsule around a vertical axis in front of a welding torch held at an angle of 45" to the axis of rotation. The automatic welding operation was controlled by a computer system identical to that used for the GPHS capsules. The autogenous full penetration weld was made at a down-tapering current of 80 to 74 A and a travel speed of 16.9 mm s-'. The entire welding cycle required 1.9 s of arc time and the total cycle time for capsule

assembly and welding averaged 7 minutes per unit (9).

The completed closure welds were examined visually at a magnification of 3OX, radiographed and tested for helium leaks. The vertical por- tion of the step had to be completely fused and the weld had to be free of linear defects exceed- ing 0.1 mm in length, as shown by radiography. Any pore or pores with an aggregate diameter in excess of 0.25 mm was a cause for rejection. The helium leak rate had to be less than 1 x 10" standard cm3 s-'. Because of product heat transfer requirements, the weld reinforcement on each flat end of the capsule had to be at least 0.076 mm at the lowest point and at a minimum of two out of three points at 90" intervals away from that location.

Production Results A total of three hundred and nineteen GPHS

capsules were built during the Cassini production campaign. The three RTGs required two hundred and sixteen capsules, the remainder being used for safety and performance testing, and as mission spares. There were thirty-six rejections for various reasons for an overall process yield of 88.7 per cent. Seven weld-related rejects were identified by non-destructive examination giving a welding process yield of 97.8 per cent. By comparison, the GalileoKJlysses program showed a net yield of 72.7 per cent


over a total production run of six hundred and fifteen capsules.

For LWRHU capsule production, one hundred and eighty-one heat sources were fabricated for the Cassini program, with at least twenty-three units being designated as mission spares or for safety testing. Only one capsule was rejected by non-destructive examination, for a radiographic indication interpreted as a weld crack, giving a net yield of 99.5 per cent. For Galileo the yield was 89.3 per cent on a production run of one hundred and fifty capsules.

Conclusions Because of their unique properties, two plat-

inum metals alloys, DOP-26 iridium and Pt- 30%Rh, have been used to encapsulate 238Pu02 fuel pellets for the Cassini Mission heat sources. Extensive physical, mechanical and impact prop- erty testing have shown that these alloys are capable of providing the requisite fuel containment during all credible accidendmalfunction conditions. Significant refinements in the production protocols for the DOP-26 alloy has led to improved manufacturing and enhanced weldability for those components. Overall process

yields were higher than ever achieved previously, and programme costs were accordingly lower.

Acknowledgements The authors wish to acknowledge the members of

the heat source community for their many valuable contributions to the programme.

Multi-Agency Effort The fabrication and supply of the RTGs and

LwRHus for use on the Cassini Mission was a multi- agency effort under the overall management of the U.S. Department of Energy’s Office of Engineering and Technology Development, Space and National Security Programs. Los Alamos National Laboratory (LANL) was responsible for GPHS and LWRHU design, fuel pelletising, encapsulation welding, and final assembly of the LWRHU. Westinghouse Savannah River Company supplied the plutonia fuel powder to LANL. Lockheed Martin Energy Systems operates Oak Ridge National Laboratory and fabricated the DOP-26 indium shells for the GPHS fuel encapsulation. EG&G-Mound Advanced Technol- ogies was responsible for the graphite components of the GPHS and LWRHU, procurement and fabrication of the PtdO%Rh hardware for the LWRHU capsules, final assembly of the GPHS modules, and loading, testing and shipping the three RTGs. Lockheed Martin Missiles and Space provided the housings and thermoelectrics for the RTGs, and was responsible for RTG spacecraft integration. Personnel from the Jet Propulsion Laboratory and Kennedy Space Center handled LWRHU spacecraft integration.

References 1 M. A. Dornheim, Aviat. Week & Space Technol.,

1996, 145, (24), 71 2 M. A, Tavern, A d Wmk &Space Techml., 1997,

146, (14), 44 3 B. A. Smith, Aviat. Week &Space Technol., 1997,



146, (19), 95 6 Platinum Metals Rev., 1985, 29, (l), 11; op. cit.,

H. Inouye, 1979, 23, (3), 100; op. cit., 1979,23,

7 R E. Tate, “The Lightweight Radioisotope Heater Unit ( L m : A Technical Description of the Reference Design”, LA-9078-MS, Los Alamos National Laboratory, January 1982

8 G. H. Rinehart, “Lightweight Radioisotopic Heater Unit (I!! Production for the Galileo Mission”, LA-1 1166-MS, Los Alamos National Laboratory, April 1988

9 E. A. Franco-Ferreira and G. H. Rinehart, “Welding One-Watt Heater Units for the Cassini Spacecraft”, LA-UR-97-1003, Los Alamos National Laboratory, March 1997

(I), 16

10 C. T. Liu and H. Inouye, “Development and Characterization of an Improved Ir-0.3% W Alloy for Space Radioisotopic Heat Sources”, ORNL- 5290, Oak Ridge National Laboratory, 1977

11 T. G. George and M. F. Stevens, “The High- Temperature Impact Properties of DOP-26 Iridium”,J. Met., 1988,40, (lo), 32

12 C. T. Liu, H. Inouye and A. C. Schaffhauser, Metall. Trans., 1981, 12A, (6), 993

13 “Binary Alloy Phase Diagrams”, Vol. 3, ed. T. B. Massalski, ASM International, 1990

14 S. A. David and C. T. Liu, Weld. 3., 1982,61, (9, 157-s

5 W. R. Kanne, Jr., Weld. 3., 1983,62, (8), 17 6 W. R. Kanne, Jr., “Weldability of General Purpose

Heat Source Iridium Capsules”, Proc. Fifth Symp. on Space Nuclear Power Systems, eds. M. S. El- Genk and M. D. Hoover, Am. Inst. Phys., 1988, p. 279

7 E. K. Ohriner, “Improvement in Manufacture of Iridium Alloy Materials”, Proc. Tenth Symp. on Space Nuclear Power Systems, eds. M. S. El- Genk and M. D. Hoover, Am. Inst. Phys., 1993, p.1093

18 E. A. Franco-Ferreira and T. G. George, Weld. 3., 1996, 75, (4), 69


19 G. M. Goodwin, Weld. J., 1987,66, (2), 33-s 2o T, G. George and T. A. cull,

21 G. B. Ulrich, “Metallurgicd Evaluation of Iridium Lightweight Clad Vent Set Cups”, Proc. First Int. Conf. on Radioisotope Heater Unit (LwRHu): Heat-Resistant Materials, eds. K. Natescum and

D. J. Tilack, 1991, p. 187 Development and Application”, eds. M. S. El- Genk and M. D. Hoover, Space Nuclear Power 22 D. McNeil, Private communication, 1997, EG&G Systems, Am. Inst. Phys., 1988, Vol. VII, p. 578 Mound Applied Technologies, Miamisburg, Ohio

Platinum Metals Involvement in the Hydrogen Economy International interest towards developments

in various academic and technological aspects of the hydrogen economy is actively continuing and was demonstrated recently at the second conference in the Hydrogen Power Series, HYPOTHESIS-11, held from 18th to 22nd August 1997 at Agder College, Grimstad, Norway. This conference fills the intermediate years between the World Hydrogen Energy Conferences and has the same objectives. Almost 200 participants enjoyed excellent facilities and a combined lecture and poster display programme approaching 100 contributions.

Contributed papers were programmed under the subdivisions: Production, Utilisation, Distribution, Transportation and Safety, with the latter topic including some analysis of liquid hydrogen technology. Platinum metals involvement in areas of electrolytic hydrogen methodology and fuel cell technology were included in a substantial paper presented by M. M. Jaksic and N. V. Krstajic of the University of Belgrade, which dealt comprehensively with alternative catalyst compositions including both alloys and intermetallic compositions of platinum and palladium.

The advantageous inclusion of platinum in a catalytic packing composition involved in studies of the H2-02 recombination reaction at low temperature was reported by G. Ioneta and I.

Stefanescu of the Institute of Cryogenics and Isotope Separation, Rm. Valcea, Romania.

Incorporation of platinum into the anodes of fuel cells developed for improved resistance to carbon monoxide inhibition were reported by F. Lufrano, E. Passalacqua, G. Squadrito and A. Pam, C.N.R. Institute for Storage of Energy, S. Lucia-Messina, Italy, and the inclusion of platinum in fuel cells for vehicular transport was discussed by V. Kazarinov, F. Pekhota, V. Rusanov and V. Fateev of the Hydrogen Energy and Technology Council on Fuel Cells, Moscow. Needs for conjoint considerations of hydrogen concentration and lattice expansion strain gra- dients on processes of hydrogen permeation and on estimations of hydrogen diffusion coefficients in palladium and palladium alloys were sum- marised by F. A. Lewis, Queen’s University, Belfast, R V. Bucur, University of Uppsala, X. Q. Tong, University of Southampton, Y. Sakamoto, University of Nagasaki, and K. Kandasamy, University of Jafha.

Selected papers will be published as a Proceedings Volume of HYPOTHESIS-I1 by Kluwer Press, Dordrecht. The next conference, HYPOTHESIS-111, will be held from July 5th to 8th, 1999, in Saint Petersburg State University, Russia; Fax: +7(812)428-7189, E-mail: egorov@efa. apmath. spb. su .

F. A. LEWIS

Palladium catalysts are often used for the acti- vation of C-H bonds, and in particular, there has been much work on palladium-catalysed Heck reactions. However, commercial development has so far been hampered by the low turnover numbers and turnover frequencies obtained. Palladium catalysts are also susceptible to poisoning, and therefore, relatively large amounts (1-5 mol%) of metal are required.

In order to overcome these problems, researchers from the CSIRO Division of Molecular Sciences, Australia, have used - 0.18 per cent palladium metal on porous glass tubing as a catalyst for liquid phase organic coupling reactions a. Li, A. W.-H. Mau and C. R. Strauss, Chem. Commun., 1997, (14), 1275-1276).

Palladium/Porous Glass Catalysts for Heck Reactions This catalyst, which could be used in contin-

uous or batchwise reactions, is resistant to oxida- tive deterioration and can be reused several times; in most cases, the reactions can be performed in air.

The regioselectivity observed is mostly con- sistent with previously reported Heck reactions with terminal alkenes yielding about 80:20 mixtures of 1- and 2-arylated alkenes. This catalyst also gave good yields for the coupling of terminal acetylenes with aryl iodides and bromides to give internal alkynes without the need for solubilising or activating ligands.

It is suggested that this system could find uses in other palladium catalysed reactions, such as hydrogenations and dehydrogenations.

Platinum Met& Rev., 1997, 41, (4) 163

Inorganic and Co-ordination Chemistry Chemistry of Precious Metals BY s . A. C O n O N , Blackie Academic & Professional, London, 1997, 374 pages, ISBN 0-7514-0413-6, E99.00

This book covers the fundamental chemistry of the six platinum metals as well as silver and gold. The binary chemistry of these elements and their fundamental co-ordination chemistry is thoroughly reviewed, and any trends in reac- tivity are highlighted. However, the organometallic chemistry receives less attention and is largely restricted to a-bonded complexes.

I was particularly impressed by the large quantity of physical data supplied by the author concerning bond lengths and vibrational spectroscopy. Such data is of great value to lecturers and can be surprisingly hard to access, particularly in a concise, compiled form. The explanation of trends in such data, provided by the author, is also very welcome.

The book is aimed at post-graduate students seeking a thorough grounding in the chemistry of the platinum group metals, as well as lecturers who are required to teach either an intro- duction to the chemistry of these elements or more advanced courses, particularly any involv- ing the co-ordination chemistry of the platinum group metals. I would again highlight the value of the tabulated physical data by the author for this purpose. Although the book is proba- bly of less utility to researchers involved in spe- cialised topics concerned with these elements, they may still find it of use as a reference text.

A consideration of the chemistries of ruthenium and osmium constitutes Chapter 1 and logically moves &om discussing the metals them- selves, through the halides (which are very well covered, including many structural illustrations) to the oxides, ending in an examination of the aqueous chemistry of these elements. The chapter then proceeds with a step-by-step docu- mentation of the chemistry of the co-ordination chemistry of ruthenium, broken down into sub- sections by oxidation state. Osmium then receives the same coverage. The breadth of the material is certainly impressive, with good use

of schematic reaction schemes to clarify the vast amount of study these elements have received. The frequent inclusion of structural illustrations throughout the chapter is very helpful, especially since important bond lengths and angles are included on the diagrams. The co-ordination chemistry of ruthenium and osmium in a variety of oxidation states is reviewed for complexes containing amine, tertiary phosphine, carboxylate, sulfide, sulfoxide, nitrosyl and EDTA donor ligands. Other topics investigated include complexes which contain polydentate ligands and porphyrin complexes. The simple aryl and alkyl species formed by the metals are also described.

Chapter 2 describes the chemistries of rhodium and iridium, both of which receive the same thorough coverage as ruthenium and osmium. The book continues with the same logical approach (beginning with binary chemistry and subsequently proceeding to co-ordination chemistry), maintaining an informative but very readable style. Good use is made of schematic reaction diagrams and I was particularly interested in the illustrated NMR diagrams which are very useful. Again the text is well supported by a variety of structural illustrations, all clearly labelled and discussed. Several molecular orbital (MO) schemes are included in the chapter and provide interesting theoretical support to some of the experimental observations concerning the chemistry of these elements. As in Chapter 1 , the co-ordination chemistry of rhodium and iridium with all the common donor ligands is included. Vaska’s compound, [IrcI(CO)(PPh,)z], receives special attention and this chapter contains an impressive review of addition complexes, both 5- and 6-co-ordinate. Tabulated infrared data and important bond lengths are also described and discussed, as is the mechanism of addition to Vaska’s compound. Additionally, dioxygen complexes of iridium are reviewed and

Platinum Metals Rev., 1997, 41, (4), 164-165 164

all such relevant compounds are tabulated, with the oxygen-oxygen bond lengths being quoted. A very informative and detailed section on complexes of these metals containing the ligand dimethylphenyl-phosphine is included in this chapter. Aspects of isomerism and variations in geometry and structure are critically discussed, with particular reference to NMR data. All reported crystal structures of such complexes are included. The last section of this chapter examines the simple a-bonded alkyl and aryl compounds of rhodium and iridium.

Chapter 3 continues in the same concise style, reviewing the chemistries of palladium and platinum. Considering the huge amount of research material available on these metals, Dr Cotton is to be congratulated on producing a thorough, concise and readable review which discusses all the known complexes with common donor ligands. Again, excellent use of physical data provides the reader with all the information required for a detailed understanding of the chemistry involved. A very good section describes the trans- effect, related to rates of reaction of various metal complexes. Theoretical explanation of the trans- effect (and trans-influence) is also included and critically reviewed. A series of Tables show spec- troscopic evidence for the trans-influence and helps to illustrate the importance of this effect to the reader. The concluding section in this

chapter concerns the anti-tumour activity of certain platinum complexes. This section is both interesting and informative, describing the complexes which exhibit anti-tumour properties and discussing their utilisation and toxicity. A section on how Cisplatin works (that is how it forms an adduct with DNA) is also of great value.

The concluding chapter concerns the chemistry of silver and gold. While this chapter (which constitutes just under a quarter of the book) was just as interesting and valuable as the preced- ing chapters, it is not reviewed here.

In conclusion, although I am well acquainted with review articles written by this author (which are always of a high standard), Dr Cotton is to be particularly congratulated on writing an excellent readable text, which contains a large quantity of valuable physical data. I would highly recommend this book to those requiring a sound knowledge of the binary and co-ordination chemistry of these elements. Anyone involved in teaching this chemistry should also seriously consider purchasing the book. This book will additionally serve as a very useful reference text for researchers concerned with any aspect of the co-ordination chemistry of the precious metals and I would recommend that research super- visors involved with the precious metals make this book available to their research students.

S. M. GODFREY

Highly Active and Enantioselective Rhodium Catalyst The development of platinum group metals

complexes with chiral ligands, and their potential uses have been discussed here recently, with enantioselective catalysis being singled out (1). Adding chiral bisphosphine ligands - BINAP being one of the best known - to catalysts has provided a range of enantioselective catalysts capable of a variety of asymmetric transformations.

Now, scientists at the Merck Research Labor- atories in New Jersey have used the planar chiral bisphosphine ligand, [2.2]PHANEPHOS, (4,12-bis(diphenylphosphino)- [2.2] -paracyclo- phane) with the rhodium complex, bis(l,5- cyclooctadiene)rhodium(I) d a t e to produce an active and highly enantioselective catalyst, "2.21- PHANEPHOS RhI'OTf which can hydrogenate dehydroamino acid methyl esters under very mild conditions (2). Formation of the catalyst before

substrate addition enabled complete conversions to be achieved in under 60 minutes, by bubbling hydrogen through the reaction mixture, at temperatures as low as 45°C.

Further activity of this catalyst was demonstrated by its reduction of tetrahydropyrazine to produce the HIV protease inhibitor Crixivan intermediate precursor, at -40°C and 1.5 bar in 6 hours with 100 per cent conversion and 86 per cent ee. Until now this reduction had incom- plete conversions, with only moderate enantio- selectivity, and thus required forcing conditions.

References 1 A. von Zelewsky, Platinum Metals Rev., 1996,40,

(31, 102 2 P. J. Pye, K. Rossen, R. A. Reamer, N. N. Tsou,

R. P. Volante and P. J. Reider, J. Am. Chem. Soc., 1997, 119, (26), 6207


Selective Ethanol Synthesis from Carbon Dioxide ROLES OF RHODIUM CATALYTIC SITES

By Yasuo Izumi Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Yokohama, Japan

Work on the synthesis of ethanol from carbon dioxide over a rhodium- selenium catalyst i s reported, and related reactions and characterisation studies are briefly reviewed. I n order to inhibit the formation of methane (complete reduction of carbon dioxide) and simultaneously activate carbon-carbon bond formation by the reaction of CH, with carbonyl derivatives, it is necessary to control the active rhodium sites. Based on a study of single crystal rhodium surfaces it is proposed that acetyl and acetate intermediates are formed. Recently it has been discovered that supported RhlTi02, promoted by selenium from inside the rhodium metal sites, is apotential catalyst for ethanol synthesis from carbon dioxide. The action of this catalyst is compared to related studies.

Emitted carbon dioxide has been implicated as one of the major sources of global warming. Therefore the conversion of carbon dioxide into alcohols is an environmentally desirable process, making use of an abundant, cheap resource.

The synthesis of methanol from synthesis gas (CO + H2) is a well known process (1-3). The dissociation energy for carbon monoxide is large (1 07 1.8 kJ mol-I). It chemisorbs on metal surfaces via the carbon atom through the 5 0 orbital, which is the HOMO (highest occupied molecular orbital). When adsorbed CO, CO(ads), is completely hydrogenated, the product will either be methanol or methane, depending on whether the C - 0 bond is retained or cleaved, respectively.

The C-0 dissociation energy for carbon dioxide (526.1 kJ mol-') is smaller than for carbon monoxide. The HOMO, In,, is on the two oxygen atoms, which suggests that the reactions of carbon dioxide occur in a direction perpen- dicular to the 0-C-0 bond. Due to these bonding differences, carbon dioxide can undergo a much wider range of chemical transformations on a heterogeneous catalyst surface. For example, protonation forms formate species,

HCOz(ads), while reaction with a surface hydroxyl group or oxygen should produce carbonate species, COs(ads). Cleavage of one of the C-0 bonds often occurs on a transition metal surface, making the reaction more complex, due to the formation of CO as an intermediate species. (In this case the selectivity reflects the reaction path.)

Under reducing conditions, methane and lower hydrocarbons (2 I C 5 = 10) are predominantly produced from C 0 2 + H2 on most platinum group metal catalysts, and here we examine some studies for controlling the conversion of carbon dioxide to ethanol. In relation to our new RhloSe catalyst, a proposed reaction intermediate, which forms during ethanol synthesis, is described, and the controlled reaction path over our new ethanol synthesis catalyst is reported. The structure and electronic state of the rhodium catalyst sites are studied.

Proposed Mechanism Based on an Acetate Intermediate

Model studies of ethanol decomposition and oxidation on Rh( 1 10) have been reviewed (4). Stable acetate species are formed on the surface

Platinum Metals Rev., 1997,41, (4), 166-170 166

(a 1

CHI 0 iH’ \cH zx / c \ o

/”\ I \ /

(b )

Scheme The stabilisatione of:

(a) acyl species by adsorbed oxygen and (b) propanoyl speeies by promoter selenium

of pure rhodium. Acyl species, formed from the decomposition of adsorbed ethanol, interact with surface oxygen atoms to form the more stable acetate, see Scheme (a). Indeed, it has been proposed that the role of promoters, which preferably exist as metal oxides, is to effect this stabilisation of acetyl species (evident in temperature programmed desorption) by forming surface acetates.

Findings similar to this study of a model rhodium surface have been reported for selenium-doped Rh/ZrOz (5) and FWSiOz catalysts (6). During ethene hydroformylation (CZH~ + CO + Hz) propanoyl species (C2H5CO) were detected predominantly on rhodium metal sites, by FT-IR (Fourier Transform Infrared) spectroscopy, at 1770 and 1740 for Se-Rh/Zr02 and Se-Rh/SiOz, respectively. Propanoyl, cor- responding to the acetyl species formed from ethanol decomposition on the Rh( 1 10) surface, was formed by the migration of C2H5(ads) to CO(ads). The doped selenium had an electrical charge of -2 and - -1 for Se-Rh/ZrOz and Se-Rh/SiOz, respectively, and is suggested to function as a base to stabilise the propanoyl intermediate by “locking” the carbonyl carbon, see Scheme (b) (based on temperature-programmed desorption and in-situ FT-IR studies

(5)). Propionate species formed from the reaction of propanoyl with O(ads) were also detected on these catalysts; however, they were mainly stabilised on the surface of the support, and were in equilibrium with propanoyl species on the rhodium surface.

Lithium or Iron-Promoted Rh/SiOz Catalysts ’Extensive surveys of carbon dioxide hydro-

genation have been reported on promoted Rh/Si02 catalysts (7, 8). On unpromoted Rh/SiOz methane was the major product. Among promoter atoms that have been examined, it was found that co-impregnation with lithium, stron- tium, iron and silver promoted ethanol formation (7). With lithium, ethanol selectivity was 15.5 per cent at 5 MPa and 513 K, and with iron (at a molar ratio iron:rhodium = 2) the ethanol selectivity was 16.0 per cent at 5 MPa and 533 K (8).

Poisoning and Promotion Effects of Selenium &om Inside Rh,, Cluster Sites

As described above, in general, supported rhodium metal sites easily dissociate the C-0 bonds of COz leading to the formation of methane and lower hydrocarbons. For example, when the iron:rhodium ratio was varied in iron-promoted WSiOZ, the selectivity and conversion into ethanol reached a maximum around iron:rhodium = 2 (8). (The dependence of selectivity and conversion is often observed having a maximum as a function of the amount of promoter.) However, the reason for the dependence is not completely clear because an excessive amount of doping by promoter can affect the result by steric (geometric) poisoning andor by electronic induction, and it is hard to distin- guish between the two.

The synthesis of ethanol from carbon dioxide and hydrogen over [RhloSe]/TiOz is examined in the Table (9). At the lower temperature (523 K), the initial rate of formation of ethanol was 1.9 x mol h-’gc$ and the selectivity to ethanol was 83 per cent. The synthesis rate increased with reaction temperature, to reach 6.0 x mol h-’g,;’ at 723 K. However, the


Ethanol Synthesis on Supported [RhloSe] and Conventional Rh/Ti02*Catalysts

Catalyst

[R hloSe]/Ti02

[R hroSe]/A1203 [Rh&e]/MgO [R hloSe]/Si02 [R heC]ITi02 [R h&Ti02 RhITiOn

(A) Using carbon dioxide and hydrogen

T,,,,,, Initial rate, lo” mol h - ’ g i l Ethanol

Ethanol Methane + CO Ethane rnol % K selectivity,

523 1.9 0.4 0 83 623 3.7 1.4 0 71 723 6.0 5.8 0 51 623 0 0.15 0 0 623 0 0.04 0 0 623 0 0 0 0 623 0.4 1.2 2.4 10 623 0 6.2 0 0 623 0 1.8 0 0

Catalyst

[Rh,oSeITTiOz

Treact. Initial rate, mol h ‘gG1 Ethanol

Ethanol Ethane rnol % K selectivity,

623 0 0.03 0

Total pressure 47 kPa: PCO? (or PCO) ’ P H ~ = 1 : 2: Rh 1.3 for Ti02 and A1203. 2.8 for Si02 and 1.6 w.% for MgO

RhTTiO, was in vacuum at 523 K, then in OdH, at 523 K As pretreatment, incipient catalyst was in vacuum at 593473 K. then in hydrogen at 623 K for 1 h.

selectivity gradually decreased with increasing reaction temperature due to the larger activa- tion energy for methane + CO.

The model catalyst was prepared from the organometallic cluster [RhloSe(CO)22]2- and the starting supported catalyst was totally decar- bonylated by heating in vacuum (9-1 2). The structure of the active sites was studied by EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy, which is tunable to the absorption edge of each element. Both selenium K- and rhodium K-edge EXAFS spectra were measured for [RhloSe]/Ti02 catalyst (12). By curve fitting analysis of selenium K-edge EXAFS, based on model EXAFS parameters of [Rh3Se2(CO)6]- (Se-Rh bond) and Se02 (Se- 0 bond), the Se-Rh bond distance was found to be 2.41 A. The fit was dramatically improved by two-wave fitting, suggesting the existence of a surface oxygen atom at a distance of 1.97 A from selenium. The model parameters for rhodium K-edge curve fitting were extracted from rhodium foil (Rh-Rh bond), Rh3Ses (Rh-

Se bond) and Rh203 (Rh-0 bond). The best fit for rhodium K-edge EXAFS showed the bonding distances for Rh-Rh, Rh-Se and Rh-0 to be 2.72,2.41 and 2.09 A, with co-ordination numbers 4.0, 1 .O and 1.5, respectively. In comparison with [RhloSe(CO)22]2 (4.8), the co-ordination number, Nm.,, (4.0) became smaller, due to partial decomposition of the [RhloSe] core by the formation of Rh-O(surf) bonds.

Based on all the bond distances and co-ordination numbers obtained by EXAFS, a surface cluster model in which four rhodium atoms are interacting with O(surf) of Ti02 was proposed (12).

It is interesting to compare this model cluster consisting of ten rhodium atoms and one selenium atom (atomic ratio Se:Rh = 0.1) to other reports. When dimethyl selenide (gas) was reacted with [Rh,]/MgO, prepared from Rh6(CO) 16, the ethene hydroformylation reaction rate to propanal + propanol reached a maximum at around Se:[Rhh] = 0.6 (10). The atomic Se:Rh ratio was 0.1, equivalent to that for


[RhloSe]/TiOz. However, this may be coinci- dental, due to the complex effects of the selenium promoter. When the amount of selenium increased to the ratio Se: [a6] = 2, the selenium-doped [Rhb]/MgO lost all its catalytic activity. It appears that geometric hindrance of the reactants was significantly affected by poisoning of [Rhs]/MgO (doped onto the rhodium surface). Selenium in the [Rhlo] framework should affect the poisoning only electronically.

Different supported rhodium clusters have been prepared and compared with [RhloSe] for the COz + Hz reaction, see the Table (9). The [Rh6]/TiOz was the most active, but only produced methane and/or CO. Rh/TiOz, prepared by conventional impregnation from the Rh salt, also only formed methane + COY but at a lower rate resulting from the larger rhodium particle size, as observed by EXAFS. [RhsC]/Ti0~ gave ethanol, but significant amounts of methane (+ CO) and ethane were also produced, suggesting that inhibition of the total carbon dioxide reduction was not as successful on [Rh6C]/TiOz (ethanol selectivity was 10 per cent) compared to [RhloSe]/TiOz (71 per cent). Special features of using [RhloSe] for ethanol synthesis should be the retention of the [RhloSe] cluster under the reaction conditions (stabilisation by selenium) and an effective electronic modification by selenium (anionic selenium contacts all the ten rhodium atoms in the cluster).

Reaction Mechanism of Ethanol Synthesis on [RhloSe]/TiOt Catalyst

The reaction mechanism for the formation of ethanol from COz + Hz on [RhloSe]/TiOz was studied by in-situ FT-IR ( 1 1). The intensity of two peaks (1 6 14 and 1243 mi') increased during the course of the reaction. They were assigned to bidentate carbonate species, based on the frequencies for [Co(NH3)4(CO3)]' (1635-1593 and 1292-1265 cm-') (13). Two peaks were observed in the same region (1570 and 1230 cm-') for unimpregnated TiOz in COZ + H2. Two unresolved peaks between the two carbonate peaks were also measured. On the basis of several references in (4), these two peaks may be assigned to acetate species. The

carbonate species on the [RhloSe]/TiOz should be affected by the [RhloSe] cluster, as they are either near the [RhloSe] or on the Rh of [RhloSe]. A peak at 3356 c d was also observed for unimpregnated TiO, which shifted to 2460 cm-' in deuterium gas, showing that the peak was due to hydroxyl species on Ti02.

The increases in intensity of the peaks at 2965, 2883, 2919 and 2853 cm-' were proportional to the ethanol synthesis rates, and were assigned to CH3 vas, vs, CH, vs., and vE, respectively. The dissociation of the C-0 bonds of carbon dioxide occurred on rhodium and the four peaks rapidly disappeared when the supply of reaction gas (COz + Hz) was discontinued. This suggests that the methyl and methylene species are formed mainly on rhodium. The intensities of the CO stretchingpeaks (2174 and 2108 cm-') were weak compared to those for [RhloSe] supported on Alz03, MgO and SOz, which exhib- ited lower total activities and no activities to ethanol.

The reason why CO(ads) was not stabilised on [RhloSe]/TiOz is unclear, but the different co-ordination number around rhodium, compared with that around conventional rhodium or supported [RhLoSe]/Al~03, [RhloSe]/SiO~ or [RhloSe]/MgO catalysts, may be important (12). These two wavenumbers, 2 174 and 2 108 ern-', were relatively high, suggesting that CO was bound to relatively positive rhodium sites.

Based on these assignments and on the peak intensity changes, a mechanism for ethanol synthesis on [RhloSe]/TiOz is proposed, see Figure 1. The CH3(ads) or CHz(ads) on rhodium may first react with C03(ads) or COZY followed by hydrogenation to ethanol on the rhodium sites. The reaction of CO + H2 was very slow and totally different from the reaction of COZ + HZ on [RhloSe]/TiOz, suggesting that ethanol synthesis does not proceed via CO for this catalyst.

On many rhodium catalysts, the order of reaction with respect to CO is negative, thus high CO concentrations would poison the reaction. Hence, it is possible in this case that COZ provides a small, steady state amount of CO for this reaction (14). The observed acetate species may support the reaction mechanism, not via CO as

Platinum Metak Rev., 1997, 41, (4) 169

Fig. 1 The proposed reaction mechanism of ethanol synthesis from COZ + H1 on [RhlaSe]fl’i02 catalyst. The reaction between CO(ads) and CH,(ads) to form ethanol is also possible

, o 1243cm-’ H 3 3 5 6 c m ’ 0

/ / ///TiO//////// / /

-Rhodium selenide’ - like dectronic structure (rRh-SC = 2 . 4 1 A )

shown in Figure 1. The stability of the Cl%(ads) on rhodium should be a key point, the rhodium site being affected by the interior negatively charged selenium (-1 - -2 by XPS) (12). The “rhodium se1enide”-like electronic state inhib- ited methane formation, but tuned the reaction path to ethanol by promoting the C-C bond formation of CH, with the carbonyl derivative.

Summary Controlling the reaction path of COz + Hz to

give ethanol was found to be possible due to the presence of carbon, oxygen, selenium, lithium and iron around rhodium sites. Electronic control by selenium for ethanol formation was investigated by several techniques, and electronic modification of the platinum metal cluster from within should be applicable to other systems.

References 1 E. L Muetterties and M. J. Krause, Angm. Chem.,

Int. Ed. Engl., 1983,22, 135 2 E. L. Muetterties and J. Stein, Chm. Rev., 1979,

79,479 3 K. Klier, Adu. Catal., 1982, 31, 243 4 M. Bowker, Catal. Today, 1992, 15, 77 5 Y. Izumi, K. Asakura and Y. Iwasawa, J. Catal.,

1991,127,631 6 Y. Izumi, K. Asakura and Y. Iwasawa, J. Catal.,

1991,132,566 7 H. Kusama, K. Sakama, K. Okabe and H.

Arakawa, Nippon Kagaku Kaishi, 1995,875 8 H. Kusama, K. Okabe, K. Sakama and H.

Arakawa, Energy, 1997,22,343 9 H. Kurakata, Y. Izumi and K. Aika, Chem.

Commun., 1996,389 10 Y. Izumi and Y. Iwasawa, J. Phys. Chem., 1992,

96,10942

1 1 Y. Izumi, H. Kurakata and K. Aika, Hyomen- Kagaku (Su$ace Science), 1996, 17, 242

12 Y. Izumi, H. Kurakata and K. Aika, submitted toJ. Catal.

13 K. Nakamoto, “Infrared and Raman Spectra of Inorganic and Coordination Compounds”, Third Ed., John Wiley & Sons, New York, 1970

14 For example, (a) J. S. Lee, K. H. Lee, S. Y. Lee and Y. G. Kim, J: Catal., 1993,144,414; (b) R. A. Koeppel, A. Baiker and A. Wokaum, Appl. Catal. A , 1992, 84, 77

Hexagonal Nanostructured Platinum Platinum particles can be prepared as nano-

structures or monolayers, in polymers and in templates of ceramic materials by the reduction of platinum acids or salts. Researchers fiom the U.K. and Germany now report the use of a liquid- crystalline phase template to form nanostructured platinum (G. S Attard, C. G. Goltner, J. M. Corker, S. Henke and R. H. Templer, Angew. Chem. Znt. Ed End., 1997,36, (12), 13151317).

Hexachloroplatinic acid and ammonium tetra- chloroplatinate were added to the surfactant, octaethyleneglycol monohexadecyl ether, which was used to prepare the lyotropic liquid-crystalline phases because it forms a wide hexagonal mesophase. The platinum salts were reduced, forming platinum powder of particle size 90 to 500 nm of hexa onal nanostructure with cylin-

thick platinum walls. The lyotropic phase may act as a structure-directing medium. Platinum is stabilised at the hydrophobidhydrophilic inter- face as small colloidal particles which agglom- erate and coalesce to a stable wall thickness. Only fast reductions allow this structure to form before the liquid-crystal phase rearranges.

This nanostructured platinum is mesoporous and of large surface area - fea,tures useful for catalytic, fuel cell and sensor applications.

drical pores 30 x in diameter, separated by 30 A


The Fifth Grove Fuel Cell Symposium By Donald S. Cameron The Interact Consultancy, Reading, England

The Fifth Grove Fuel Cell Symposium was held at the Commonwealth Institute in London from the 22nd to 25th September, attended by almost 400 delegates from 28 countries. The theme “Fuel Cells - Investing in a Clean Future” was intended to encompass the views of fuel cell users and producers in the light of demonstration programmes worldwide, and to promote commercial exploitation of this highly efficient, clean method of electric power generation. The symposium was sponsored by the Energy Technology Support Unit (ETSU) on behalf of the U.K. Department of Trade and Industry, the International Energy Agency Advanced Fuel Cells Programme, the European Fuel Cell Group, and Elsevier Advanced Technology.

Mrs Eryl McNally, Cochair of the Research and Energy Committee of the European Parliament opened the meeting. Later, she presented the platinum Grove Medal to Firoz Rasul, President and Chief Executive Officer of Ballard Power Systems, Canada, in recognition of their role in developing proton exchange membrane fuel cells to the point where these are being demonstrated in a host of applications, including passenger and public service vehicles.

Present and Future Capabilities The conference was structured to show the

present and future capabilities of fuel cells, and to gauge their likely progress against the challenges still to be overcome. To encourage questions and observations from all participants, two discussions were held.

Developing a thriving fuel cell industry will require major investments on a number of levels in order to:

provide exploitable technology demonstrate viability and commercial

generate interest and commitment among

All the speakers emphasised that fuel cells will

prospects

potential purchasers.

only become a commercial reality if they offer clearly definable financial benefits to the pur- chaser, and provide a profit to the manufacturer. Other characteristics, such as high efficiency and freedom from pollution are additional bonuses, but alone will not be sufEcient to result in widespread exploitation.

Currently, fuel cells are more expensive than conventional power plants such as gas turbines and diesel generators, and several speakers addressed the efforts being made by their organ- isations to reduce costs. These include developing less expensive materials of construction, increasing the power obtained from each cell, and scaling up the number of power plants sold in order to benefit from mass production. For some fuel cells, niche markets have emerged even at their present prices, which justify continued development of the technology while the numbers of sales increase.

Fuel Cell Technology Phosphoric Acid Fuel Cells

Platinum catalysed phosphoric acid fuel cells, operating at about 200°C are the most highly developed. Rick Whitaker, of ONSI, reported that 144 of their 200 kW PC25 generators have been sold, and 91 are operating in North America, Japan and Europe. Early models have been in service for over 37,000 hours, with over 95 per cent availability for service. One unit has established a record for any power plant by running for 9500 hours (1 3 months) continuously at full rated load. Their reliability is such that they are being marketed as un-interruptible power sources (UPSs) for installations such as computer facilities and hospitals.

Operating on natural gas, they are 40 per cent efficient for electrical output, providing a further 45 per cent of low grade heat. Their twenty year development has cost $200 million, funded by United Technologies Corporation, Toshiba (Japan), Ansaldo (Italy), the U.S. Government,


and the Electric Power Research Institute (U.S.A.). Costs are being reduced partly by design simplifications, and also by increasing production. The early units were sold for about half their construction cost of almost $1 million each. However, ONSI predict that at present cost levels there is a market for un-interruptible power sources, and other applications will become feasible as prices are reduced.

Proton Exchange Membrane Fuel Cells. These fuel cells languished after their suc-

cess in the early U.S. Gemini space programme until intensively developed by Ballard Power Systems. Progress was hindered by the need for expensive construction materials and high platinum content. The use of less expensive polymers, graphite separator plates and dramatically reduced platinum requirements have trans- formed their prospects. Their solid polymer electrolyte provides freedom from possible leakage, and their operating temperature of less than 80°C means that they can be quickly started from cold. Hence they are ideally suited to transport and vehicle applications. Platinum metal loadings have been reduced from around 15g/kW to less than 1 g/kW with present cells and this figure is still falling.

Proton exchange membrane fuel cells are susceptible to poisoning by impurities such as carbon monoxide in the hydrogen supply. Efforts were reported first to render the fuel cells more tolerant of contaminated hydrogen, by using alloy catalysts such as platinum-ruthenium at the anode, and secondly to develop reformers supplying high purity hydrogen.

Reinhart Rippel described the Siemens AG efforts on he1 cells, which include development of a 100 kW alkaline he1 cell air independent propulsion system for the German Class 205 (Ul) fuel cell submarine trials. In 1989 Siemens began work on proton exchange membrane cells, culminating in development of a 34 kW module of 72 cells, producing 650 amps at 52.3 volts, at 70 to 80°C, and operating at 72 per cent efficiency at 25 per cent of rated output.

At present Siemens are collaborating with MAN and Linde to produce a low-floor city bus,

which will have 120 kW of proton exchange membrane fuel cell power, with the hydrogen fuel stored in pressurised cylinders.

Molten Carbonate Fuel Cells The 650°C operating temperature of molten

carbonate fuel cells offers the option of reforming natural gas or other hydrocarbon fuels inside the fuel cells (internal-reforming) or outside (external reforming), and also the benefit of high temperature recovered heat. However, the high operating temperature and corrosive alkaline electrolyte continue to provide many challenges in finding materials of construction.

Michael Bode of MTU, Germany, described their programme to develop and manufacture these fuel cells as 300 kW to 10 MW stationary generators. MTU claim to have made break- throughs in the main problems of high degradation rates and migration of electrolyte, and, with their 280 kW unpressurised "Hot Module", to have operated the largest molten carbonate fuel cell stack module to date.

Hiroo Yasue, of the MCFC Research Association, Japan, described the Japanese col- laborative effort on molten carbonate fuel cells, supported by the Government Moonlight and New Sunshine programmes. With three companies developing stacks and four companies building the balance of plant, a 1000 kW system is being assembled to evaluate the technology. This will have an external reformer running on natural gas, providing an overall electrical efficiency of 45 per cent, and a target life of 5000 hours, when it comes into operation in November 1998.

Solid Oxide Fuel Cells Operating at around 1 OOO"C, solid oxide elec-

trolyte fuel cells are capable of reforming hydrocarbon fuels internally. A 25 kW generator built by Westinghouse Electric Corporation in the U.S.A. has been operated in Japan for over 13,000 hours with degradation rates of less than 0.1 per cend1000 hours and 11 start-up cycles.

It is acknowledged that production costs need to be reduced by orders of magnitude to make the system commercially successful, and Allan


Casanova of Westinghouse Electric Corporation described their programme to achieve these reductions. This is being addressed by first increasing the size of individual cells (up to 150 cm long ceramic tubes), and by manufacturing them on a larger scale. A 100 kW unit is being constructed by Westinghouse for a demonstration in The Netherlands, with start-up due in November 1997, and a 250 kW unit is planned for Fort Meade, Maryland, U.S.A. Ultimately, static generators of 250 kW to 7 MW axe planned.

Demonstration Programmes Phosphoric Acid Fuel Cells

Most fuel cell demonstration programmes to date have involved phosphoric acid fuel cells. Lars Sjunnesson, of Sydkraft, the largest power utility in Sweden, explained the reasons for their trials with two units, built by ONSI and Fuji Electric, and operating on natural gas. They are attractive for many reasons, including their ease of siting at, or close to, the consumer, with the option of using them in a combined heat and power mode. However, plants of less than 5 MW face competition fiom diesel generators costing less than U.S. $1500 per kilowatt, while larger generators must compete with gas turbine power plants priced at less than $1000 per kilowatt.

One issue which must be addressed at an early stage is the regulatory system controlling their installation. Fuel cells represent new, unfamil- iar technology which is regarded with suspicion by local authorities and safety regulators. Hence the need for demonstration and familiarisa- tion programmes in parallel with technology development.

Tom Damberger related the experience of using fuel cells at several hospitals run by Kaiser Permanente, a major health care provider in the United States. On several occasions, cuts in electricity supplies to hospitals have been followed by failure of their emergency diesel generators. For this reason, fuel cell generators have been evaluated at Anaheim, Sacramento and Riverside, California since 1993. Two ONSI PC25 generators at Riverside have provided 99.5 per cent and 98.2 per cent availability for power generation, and in 1995 Kaiser signed

orders for six further units. As an indication of the potential market, a 300 bed hospital typically requires 1 to 4 MW of power with high security of supply. Kaiser Permanente have 70 MW of connected power demand in California, with an estimated market for 160 fuel cells in that state, and 500 across North America. Ideally, the system would provide high grade steam for heating and sterilising, although all the hot water provided by the PC25 units has been utilised.

Kiyokazu Matsumoto of Osaka Gas Company gave the reasons why Japanese gas and electricity industries have supported development of phosphoric acid fuel cells - they anticipate the need for an additional 172 gigawatts of electricity gen- erating capacity in Japan, by the year 20 10. The need for efficient utilisation of gas is emphasised by the annual import of 44 million tonnes of liquid natural gas - 95 per cent of Japanese requirements. To date, 87 fuel cells, mostly phosphoric acid types from various suppliers in sizes ranging up to 11 MW, have been evaluated.

Proton Exchange Membrane Fuel Cells Exciting developments include two prototype

passenger vehicles powered by proton exchange membrane fuel cells, shown recently at the Frankfurt Motor Show by Daimler Benz and Toyota, and a fuel cell powered submarine under construction for both the German and Italian Navies.

In a deal worth $340 million, Daimler Benz have formed a joint venture, DBB Fuel Cell Engines, with Ballard Power Systems of Vancouver, Canada, to exploit fuel cell technology. Dr Ferdinand Panik described how Daimler Benz have constructed a series of vehicles, culminating in the NECAR 111. This is based on a Mercedes Class A subcompact, which incorporates a reformer, enabling it to operate on liquid methanol fuel, which together with the 50 kW proton exchange membrane fuel cell fits under the floor and bonnet of the vehicle. Daimler Benz have also built the NEBUS fuel cell powered passenger bus for demonstration and testing.

Ballard Power Systems have built the power


plants for a first 3-bus demonstration programme for Chicago Transportation Authority which was started on 18th September, with a second 3-bus trial in Vancouver due to start shortly. These buses have 250 kW of fuel cell power, operating at between 45 to 55 per cent efficiency depending on load conditions, running on hydrogen stored under pressure. The bus field trials are scheduled to last through 1997/98, with commercial sales due to begin in the year 2000, and full production starting in 2004.

A host of automotive manufacturers, such as Chrysler, Ford, General Motors, Honda, Nissan, Volkswagen AG, and AB Volvo are involved in fuel cell vehicle programmes, many using fuel cells supplied by Ballard. Shigeyuki Kawatsu of Toyota Motor Corporation described their activities, which include building a RAV4 sport utility vehicle incorporating a methanol reformer. A 25 kW fuel cell and auxiliary battery together provide a total of 50 kW, giving a maximum speed of 125 km h-’ and a 500 km range. The entire power train is sufficiently compact to fit under the bonnet and floor of the vehicle.

Ballard Generation Systems, a joint venture between Ballard Power Systems and U.S. energy company, GPU International, are constructing two 250 kW stationary power generators which will be installed and tested by the latter.

Gunter Sattler of Ingenieurkontor Lubeck, a privately owned naval construction company in Germany, detailed the development of airless power systems for submarines by the German Navy. Initial trials were carried out with a Class 205 submarine using alkaline fuel cells, fed with hydrogen from a hydride store, and oxygen carried as cryogenic liquid. A second, Class 212 vessel is under construction and will be com- missioned in 2003, with 300 kW proton exchange membrane fuel cells, supplied with hydrogen generated by a methanol reformer, which will provide for cruising at 8 knots. Liquid oxygen stores supply the fuel cell, reformer and crew breathing requirements. Efficiency of power generation varies fi-om 70 per cent at 5 per cent load to 50 per cent at 100 per cent of rated load. Fuel cell stacks have been supplied by Ballard,

30 to 50 kW with suitable characteristics for operating on pure oxygen. An agreement has been signed enabling the Italian Navy to build the Type 2 12 submarine in Italy.

Molten Carbonate Fuel Cells A 2 MW fuel cell installation has been built

at Santa Clara, California, based on internal- reforming, molten carbonate electrolyte fuel cells built by Energy Research Corporation. Paul Eichenberger described the construction and commissioning of the power station, and extensive testing of the balance of plant, before starting up the sixteen fuel cells. These ran for 720 hours connected to the grid, supplying 1710 gigawatt hours of power, before the plant was shut down due to voltage anomalies. Following various difficulties, the plant was shut down in March this year, and new fuel cell stacks are currently being built. The programme was intended mainly to provide experience of constructing and operating the plant, and it is hoped to re-start trials in the first quarter of 1999.

Al Figueroa, of San Diego Gas and Electric Utility, described a molten carbonate electrolyte fuel cell installation at a naval air station a t Miramar, San Diego. This uses a flat plate heat exchange reformer of advanced design, with an MC Power fuel cell, the system being commis- sioned in February 1997. Maximum output was lower than anticipated at 2 10 kW, which was attributed to voltage losses at the cathodes, although the plant was operated for 300 hours, producing 160,000 kwh of power and 346,900 pounds of steam. The trial has provided valuable experience of plant construction and operation, including automated control.

Solid Oxide Fuel Cells A significant development proposed for the

solid oxide fuel cell is to integrate the system with a gas turbine, with the fuel cell power section replacing the combustion stage of the turbine. The overall efficiency is potentially as high as 60 to 70 per cent, some of the electrical power being derived directly from a turbine-driven alternator, and some from the fuel cells. The

although Siemens are developing modules of solid oxide fuel cell efficiency is improved by


running at elevated pressure using air from the turbine compressor. The system poses problems of integration and control during operation, but the potential benefits are substantial.

Advances in catalysts for both molten carbonate and solid oxide fuel cells were described by Andrew Dicks of BG plc. Internal reformer catalysts can be adversely affected by alkali metals migrating from the electrolyte, while carbon formation can provide complications in solid oxide fuel cells. The latter can be largely overcome by subjecting the fuel to a pre-reforming process before entering the cell, which has the added benefit of minimising temperature differences across the electrodes due to the endothermic reactions taking place. In several cases, addition of small amounts of platinum group metal catalysts such as platinum or ruthenium supported on inert substrates, as well as potassium and molybdenum, have been beneficial in minimising carbon formation.

Conclusions The overall consensus was one of confidence

that technical development is almost complete, and that fuel cells have entered a demonstration and exploitation phase. The number of atten- dees representing the power utilities and oil supply companies reflected the increasing degree of interest from these groups. After numerous trials, the views of future users are being taken into consideration in designing new generators. All the fuel cell developers emphasised their efforts to make products which will directly compete on capital cost with conventional power generation equipment, without relying on other factors, such as higher efficiency or lower pollution, to influence the market.

One of the most important points raised is the need for fuel cells to be available for pur- chase and demonstration. Until they are widely demonstrated it will be difficult to increase the level of public, utility and regulator awareness to the point where they are accepted as com- monplace. Would-be purchasers are unlikely to be attracted by just one attribute, such as high efficiency, therefore the fuel cell must be shown to have a combination of desirable features,

for example simple installation and maintenance, reliability and durability, as well as competitive capital cost.

Proton exchange membranes fuel cells have made giant strides, both in technology and in credibility as power sources for transport and stationary applications. The disclosure at the Symposium that they are being installed in a submarine illustrates their degree of safety and reliability. Most of the major automotive manufacturers plan to evaluate them in passenger or public transport vehicles. Daimler Benz and Toyota, notably, have built small vehicles, with on-board reformers to enable the use of liquid fuel, while several bus mals are in progress or planned.

According to Firoz Rasul, the Ballard Power Systems strategy is to produce fuel cells which have generic applications, for engineering into a variety of systems and applications. By collaborating with numerous system developers, and providing almost a commodity “off the shelf” product, with the maximum number of uses, it is hoped to promote sac i en t sales to gain the maximum benefits from mass production.

The technical viability of phosphoric acid fuel cells is well established, and they continue to gain acceptance as highly reliable generators for combined heat and power applications. Increased competition among power companies will lead to greater demand for dispersed generators incorporating combined heat and power, particularly in applications such as hospitals, prisons and hotels, where security of supply is paramount.

Molten carbonate and solid oxide fuel cells lag several years behind the platinum catalysed, low temperature types, mainly due to challenges posed by conditions of operation. Faith in both technologies will be considerably enhanced when long term demonstrations at the 250 kW scale can be accomplished. In particular, the integration of solid oxide fuel cells with gas turbines, offering upwards of 60 per cent efficiency, is an exciting prospect for the future.

Proceedings of selected papers of the conference are expected to be published in a forth- coming issue of the Journal of Power Sources.


Harnessing the Unique Properties of Iridium ATTRACTIVE CATALYST FOR HYDROGENATION

By Y I. Savchenko, I. A. Makaryan and V. G. Dorokhov Institute of Chemical Physics of the Russian Academy of Sciences, Chernogolovka, Russia

With the exception of osmium, iridium is the least abundant of the six platinum group metals. It is of crucial importance i n a number of high technology applications and, at least in the past, has been regarded as a strategic material and stockpiled by majorgovernments. In recentyears it has become more readily available, prompting renewed interest by researchers, including those seeking to develop improved catalysts. In this paper we discuss the results of our investigations on the use of iridium as an hydrogenation catalyst. Data are presented on the characteristics of an iridium-based catalyst developed here for the effective and selective synthesis of substituted N-arylhydroxylamines and chloro-substituted anilines, symmetric and asymmetric azoxybenzenes, and unsaturated alcohols. These demonstrate that iridium is unique among the platinum metals.

Historically, the interest of researchers in the use of iridium as a catalyst for the hydrogenation of organic compounds has been intermit- tent. One recent period of activity occurred in the late 1960s to early 1970s when the characteristics and properties of various iridium-based catalysts were investigated. Russian scientists were involved in these researches; in particular, a new procedure to prepare a boron-promoted iridium-based catalyst, containing 0.5 to 20 wt.% iridium and 0.002 to 0.45 wt.% boron based on the weight of carrier, and a carrier which is inert towards various chemicals was developed by E. N. Bakhanova, M. L. Khidekel and colleagues at the Institute of Chemical Physics in Chernogolovka of the Russian Academy of Sciences (ICPC RAS) (1). This enabled the iridium-based catalyst to be produced on an industrial scale and to be used for the selective synthesis of 3,4-dichloroaniline by the hydrogenation of 3,4-dichloronitroben- zene, with more than 700 tonnes of high qual- ity amino product being manufactured from this reaction.

However, at that time the availability of iridium for use by the chemicals industry could not be assured. Supplies were limited and used

mainly for military purposes. More recently, with the easing of tension between the major powers and the reduction of their armed forces, the strategic importance of iridium has been downgraded, resulting in major changes in its availability and cost.

Thus, through the late 1980s and early 1990s the reduced strategic demand for iridium was matched by a decrease in its price, see Figure 1 (2). The lower prices offered opportunities for further economic use to be made of iridium and some increased demand has occurred in the 1990s. Even now, with supply and demand governed by normal commercial considerations price variations are always possible.

Iridium Catalyst Development Over the years, research into hydrogenation

has continued at ICPC RAS. It has been established that the iridium-based catalyst developed here has specific characteristics which are especially advantageous during the hydrogenation of various substances. In particular, in our research an iridium catalyst of the following composition was used: 4.7 to 4.9 weight per cent iridium and 0.08 to 0.1 weight per cent boron based on the weight of carrier, with an


activated carbon used as an inert carrier. Some of the reactions that may be facilitated by th is unique catalyst are presented here.

Selective Synthesis of Substituted N- Arylhydroxylamines

The useful properties of our iridium-based catalyst were determined during liquid-phase hydrogenations of substituted nitrobenzenes. This reaction typically proceeds in the presence of platinum group metals, in a neutral medium, according to the following scheme:

H Z H, HZ x-A~NO, - x-ArNO + x-ArNHOH

H2 x-ArNH2 - side products (9

where x-ArNOz, x-ArNO, x-ArNHOH and x- ArNH2 are substituted nitrobenzene, nitro- zobenzene, N-arylhydroxylamine and aniline, respectively, and x is a substituent, such as -H, -C1, -Br, -I, -F, -COOH, -CHO, -OH, -NH2, -OCH,, -COOCH,, -OC2H5, -NHCOCHs, etc.

The side products of the reaction are formed during transformation of the final x-ArNHz via hydrogenation of the benzene ring or elimination of one or several functional groups at a high volume of conversion (more than 95 to 98 per cent) of the initial x-&NOz. The nitrozo compound does not get into the solution, due to its extremely high adsorption on the catalytic surface, where it reacts immediately to give x- ArNHOH.

We investigated the hydrogenation of a number of nitro compounds in the presence of iridium-, platinum- and palladium-based catalysts, see Table I. A comparison of the hydrogenation rates indicates the relative similarities of the specific activities of the catalysts under consideration. It must be noted, however, that the iridium catalyst is less suitable for the hydrogenation of nitro compounds with electron-donating sub- stituents, especially by comparison with the palladium catalyst. At the same time, in the presence of the iridium-based catalyst, the hydrogenation of halogen-substituted nitro compounds proceeds at a high rate. Due to the very high halogen ion elimination rate, the palladium

Platinum Met& Rev., 1997,41, (4)

Fig. 1 The price of iridium fell significantly from the mid-1980s until 1992-93 when it stabilised before a recent recovery

catalyst was found to be totally unsuitable for this reaction.

Comparing the hydrogenation rates over iridium- and platinum-containing catalysts shows their similarities. However, a significant differ- ence in the distribution of products, compared with the platinum catalysts, is clearly observed during the iridium-catalysed hydrogenation. In particular, much higher concentrations of the intermediate, N-arylhydroxylamine, have been noted and it has a lower rate of transformation into the amino product.

Our researches show that the regularities of the product distribution during hydrogenation of substituted nitrobenzenes over various catalysts are similar. Data on the product distribution in the course of hydrogenations of p- chloronitrobenzenes in t

PLATINUM METALS REVIEW · 2016. 1. 28. · UK ISSN 0032-1400 PLATINUM METALS REVIEW A quarterly...

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