sdarticle (6)
Transcript of sdarticle (6)
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–158
such as, nitrobenzene [14], styrene, cyclohexene, cyclooctene,
acetophenone [15], furfural [16], buta-1,3-diene [17] and 1,3-
cyclopentadiene [18], using hydrogen gas supply. However,
correlation of the hydrogenation activity with the P content of the
Ni–P alloy has never been performed. A lot of experimental
results have been carried out to elucidate the promoting effect of
the alloying metalloid on the catalytic activity of the alloys but
conclusions concerning the electronic effect are very contro-
versial. Based on XPS spectra and EXAFS data, it is widely
accepted that the promoting effect of alloying is attributed to the
electronic effect of the non-metallic element on the active Ni sites
and in turn to the catalytic properties [19–21]. However, while the
higher hydrogenation activity of Ni–P amorphous alloy,
compared to Ni–B, is explained on basis that hydrogen
dissociative chemisorption is more favourable on electron-
deficient Ni centers (Ni–P) than on electron-enriched ones (Ni–
B) [17], the same argument does not account for the increase of
the catalytic activity of Ni–B amorphous alloy with the increase
of the B content (more electrons may transfer from the alloying B
to Ni with the increase of the B content) observed by Li et al. [22].
In this case, the authors advocate that the increase of the electron
density on the Ni active sites would weaken the C C bonding,
which would facilitate the hydrogenation of this group.
Concerning the structural effect there is also no agreement in
the literature regarding the P content at which the transition
from crystalline to an amorphous structure takes place. Besides,
divergences in the effect of the P content on the catalytic
activity towards the hydrogen evolution reaction (HER) are also
found. Some authors advocate that an increase of the P content
of the alloy is accomplished by an increase of its catalytic
activity [12,13] but others authors support an opposite effect
[23,24]. This incongruence seems to be related to the
amorphous/crystalline character of the alloy, i.e. whether the
catalytic activity comparison is performed between crystalline
and amorphous Ni–P alloys or just between amorphous Ni–P
alloys. On analysing the literature data, it may be concluded
that some experimental results bring out the electronic effects
of metal alloying on the catalytic activity towards HER (on
comparing amorphous alloys containing different P content),
while others make more striking the structural effects (on
comparing Ni–P alloys with different amorphous character).
This could explain the volcano type plot, which is obtained by
T. Burchardt when the rate of HER is plotted versus the P
content of the Ni alloy (maximum catalytic effect was observed
for 19 at.%) [12].
This prelude illustrates well the complexity of the overall
system and the need of considering multiple effects on
correlating the hydrogenation activity with the P content of the
Ni–P alloy.
The novel methodology will be attempted using 3-buten-1-ol
as the model molecule and Ni–P electroless alloys films
containing 7–14 at.% of P as the catalytic material. Within this
phosphorous content range, a significant effect of Ni–P alloy
composition on the HER activity has been reported [13]. Alike
the study previously performed on a Pd-black film [9], 3-buten-
1-ol was chosen as a model compound because of its high
solubility in aqueous solution and the small number of
hydrogenation products that could be formed (1-butanol is the
only product expected). Comparison of the proposed metho-
dology with the electrocatalytic hydrogenation (ECH) will be
performed, using the same organic molecule and the same
catalytic substrate.
2. Experimental
2.1. Ni–P alloy preparation and characterization
The Ni–P alloys were prepared by electroless deposition on a
10 mm � 50 mm Cu plain weave mesh with 0.38 mm nominal
aperture and 0.25 mm wire diameter (from Goodfellow). The
experimental procedure for the films deposition was adapted
from [13], on which the Ni–P alloy films deposition on carbon
steel substrate is described. The deposition of 9 and 14 at.% P
alloys was carried out from a solution of 0.1 M NiSO4 + 0.20 M
NaH2PO2 + 0.15 M CH3COONa, pH 5.5 (adjusted with
CH3COOH), at 80 8C, under solution stirring. By changing
the deposition time, variations in the P content were achieved (9
and 14 at.% P films for 20 and 40 min, respectively). A solution
with the same composition, but a lower H2PO2� concentration
(0.1 M) was used for plating the alloy containing 7 at.% P
(45 min deposition). Despite the important composition gradient
found on the deposits prepared from the 0.20 M NaH2PO2
solution, a negligible composition gradient was detected when
the films were obtained from a lower hypophosphite concentra-
tion solution (0.10 M). EDS analysis on several points of the as-
prepared films allowed to evaluate accurately the composition of
the uppermost film (surface) layer: 7.4� 0.5, 9.3� 0.5,
13.6 � 0.4. For simplicity, these Ni–P alloy films are identified,
respectively as 7, 9 and 14 at.% of P.
The thicknesses, evaluated from SEM analysis of the cross-
section of the deposits, were remarkably uniform and were
found to be 4–6 mm (Fig. 1).
The electroless deposition was induced by short-circuiting
the Cu mesh with an Al foil for 30 s in the plating bath. After
disconnecting the Al, the time deposition was monitored once
the open circuit potential reached approximately �650 mV,
which was the steady-state potential for the Ni–P/electroless
solution system at the present experimental conditions. A fresh
solution was used for each plating experiment.
The phosphorus content in the deposits and surface
morphology was determined by a Philips-FEI Quanta 400
SEM/EDS spectrometer. Structure of the alloys was assessed by
X-ray diffraction using a Panalytical, X’Pert PRO diffract-
ometer and CuKa X-ray source.
2.2. Cyclic voltammetry
A classical three-compartment cell was used for the cyclic
voltammetric experiments. The working electrode was a small
sample of the as-deposited alloy mesh (0.70 cm � 0.80 cm) with
a geometric area of 1.72 � 0.13 cm2. Calculation of the current
density relied on this geometric area. Each voltammogram was
registered on a new deposit as cycling changes the deposit
activity. All the voltammetric data was obtained by initiating the
Fig. 1. SEM images of as-prepared Ni–P films and of its cross section.
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–15 9
potential scan in the negative direction. The voltammetric scan
was 10 mV s�1. Solutions were deaerated with N2.
2.3. Catalytic transfer hydrogenation and electrocatalytic
hydrogenation
The CTH and electrocatalytic hydrogenation experiments
were carried out in one compartment cell having a Pt mesh
counter electrode. The electrolyte consisted of a 10 mM 3-
buten-1-ol + 0.1 M NaOH + 0.1 M NaH2PO2 aqueous solution,
except on the ECH experiments where a hypophosphite-free
solution was used. The solution was thermostatized at 25.0 8Cand stirred with a magnetic bar. The electrode potential was
controlled by an Autolab potentiostat model 100 and is referred
to a saturated calomel electrode (SCE).
The progress of the hydrogenation reaction was monitored
by gas chromotography (GC) analysis on a Dani model 1000
gas chromatograph equipped with a FID detector and a DB-
1701 capillary column (30 m long, 0.53 diameter). 0.5 mL of
the electrolyte solution was extracted with ethyl ether and was
immediately analysed by GC. An internal standard (1-pentanol)
was used on the quantification. The reaction product was
identified by comparing its retention time with that of an
authentic sample.
The catalytic activity of the catalyst towards the hypopho-
sphite ion dehydrogenation (and consequently, towards the
organic molecule hydrogenation) was followed by the open
circuit potential evolution. ECH was carried out by passing a
charge corresponding to two electrons per molecule of 3-buten-
1-ol (2 F mol�1). On the CTH experiments, dehydrogenation of
the hypophosphite ion was initially promoted by applying
�1.1 V potential for 30 s.
The material yield (or yield of 1-butanol), selectivity and
conversion were calculated as,
Yield of1-butanolð%Þ ¼�
n1-butanol
n03-buten-1-ol
�� 100 (1)
Selectivityð%Þ ¼�
yield1-butanol
conversion3-buten-1-ol
�� 100 (2)
Conversionð%Þ ¼�ðn0
3-buten-1-ol � n3-buten-1-olÞn0
3-buten-1-ol
�� 100 (3)
Fig. 2. XRD pattern of as-deposited Ni–P films with 7, 9 and 14 at.% P content.
Fig. 3. Cyclic voltammograms in 0.1 M NaOH solution for different P content
on Ni–P alloy films: - - - 7 at.%, — 9 at.%, – – – 14 at.%.
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–1510
where n1-butanol, n3-buten-1-ol and n03-buten-1-ol represent, respec-
tively, the amount (in mol) of 1-butanol formed, 3-buten-1-ol
not hydrogenated, and the initial amount of 3-buten-1-ol. The H-
donor efficiency of the catalytic hydrogenation was defined as,
H-donor efficiencyð%Þ ¼�
n1-butanol
nhypophosphite consumed
�� 100 (4)
Quantitative determination of the hypophosphite ion concen-
tration at the end of the hydrogenation reaction was performed
by the iodometric method [25]. Current efficiency was calcu-
lated on the basis of the amount of 1-butanol formed and the
charge passed on the cell.
3. Results
3.1. Ni–P alloy characterization
Observed by the naked eye, the composite coatings are all
smooth and semi-bright as deposited. However, on analyzing
them by SEM (Fig. 1) differences on the surface morphology
were found. Apart from the lowest P content film, spherical
particles with a diameter ranging from 3 to 10 mm, distributed
preferentially in the cross wire zone of the mesh, are detected.
The composition of these particles, determined by EDS, was
found to be approximately the same as the layer beneath them.
Apparently, the amount and average size of these particles
increases as the P content also increases. In order to confirm
whether this effect was exclusively related to the film
composition, a 14 at.% P film was prepared from a solution
presenting the same composition but a different pH (4.8 instead
of 5.5, time deposition 60 min). An identical morphology to the
one observed on the Ni–P alloy containing 7 at.% P was found.
This result suggests that the presence of spherical particles is
dependent on very particular experimental conditions and
should not be directly correlated to the P content of the film.
Underneath these microparticles the Ni–P films present a
smooth layer with nodular nature, typical of the electroless Ni–
P deposits. Apparently, this layer presents the same morphol-
ogy in the three different composition alloys, which contrasts
with the AFM data obtained by Fundo and Abrantes [13] on Ni–
P films deposited on carbon steel, containing the same
phosphorus content. These authors observed significant
morphological differences upon the P content of the film and
concluded that the surface roughness was approximately four
times higher on the 7 at.% P than on the 14 at.% P film. Further
work will be needed to characterize in more detail the
morphology of these films and to confirm if the apparent
different morphological results are related to the different metal
substrate used on the deposition process.
The XRD patterns of Ni–P coatings with different P-contents
are shown in Fig. 2. It can be seen that the full width at half
maximum of the Ni peaks increases as the phosphorus content of
the Ni–P alloy increases as well, which is indicative of a
progressive increase of the alloy amorphicity. However, it is
important to remark that contrasting to the EDS data, the XRD
analysis concerns the whole thickness of the film, which means
that a direct correlation between the XRD data and the alloy
composition may be misleading on films presenting a
concentration gradient of P on the alloy. Probably, if glancing
incidence X-ray diffraction has been used, the crystallinity
displayed by 7 and 9% at. P films would have been lower. Despite
the amorphicity exhibited by the 14 at.% P film, the possibility
that the deposit may be amorphous but incorporating micro-
crystallites, not detectable by XRD, cannot be excluded [26].
Characterization of the Ni–P deposits after the hydrogena-
tion reaction revealed that the films exhibit the same properties,
in terms of their structure, morphology, composition and
adherence to the metal substrate.
3.2. Cyclic voltammetry
Fig. 3 shows the voltammograms obtained in a 0.1 M NaOH
solution at the different prepared Ni–P electroless alloys. The
results reveal the enlargement of the anodic peak at ffi�0.78 V,
and concomitant shift of the peak potential towards more
positive potentials, with the increase of % of P on the Ni–P
alloy. Despite this peak resembles the anodic peak observed at a
Ni electrode (which is ascribed to the a-Ni(OH)2 formation),
several authors propose its assignment to the P oxidation to
hypophosphite [27].
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–15 11
Comparison of the results obtained by cyclic voltammetry in a
H2PO2� solution and in a solution containing a H2PO2
� + 3-
buten-1-ol, allows to predict the effect of the organic compound
on the catalytic activity towards the hypophosphite dehydro-
genation and oxidation. The voltammograms were recorded by
initiating the potential scan from �0.70 V in the negative
direction. Despite at this potential, the surface is covered by a
nickel hydroxide film (which has been spontaneously formed as
soon as the electrode is immersed in the solution), this potential
was chosen because it is neither too anodic to oxidise irreversibly
the surface electrode (into b-Ni(OH)2), neither too cathodic to
promote the H–P cleavage of H2PO2�. The results (Fig. 4) reveal
Fig. 4. Cyclic voltammograms for different P content on Ni–P alloy films in a
solution containing 10 mM 3-buten-1-ol + 0.1 M H2PO2� + 0.1 M NaOH (- - -)
and in a 3-buten-1-ol free-solution (—). n = 10 mV s�1.
that despite the unsaturated alcohol gives rise to an important
decrease on the catalytic activity of the Ni–P films, the
voltammetric features characteristic of the hypophosphite
oxidation reaction are still depicted (an anodic peak at
ffi�1.0 V) [5,6]. It is also shown that the extension of the
inhibition effect of 3-buten-1-ol on the hypophosphite oxidation
reaction depends clearly on the phosphorous content of the Ni–P
alloys films: it is much more prominent on the Ni–P film
containing 14 at.% of P than on the 7 or 9 at.% P film. This
behaviour is indicative that 3-buten-1-ol and the H2PO2� ion
compete with each other to adsorb on the Ni–P surface and that
the adsorption ability of the unsaturated alcohol is stronger on the
highest content P film. In addition, the notable decrease of the
anodic peak atffi�0.72 Vin the 3-buten-1-ol containing solution
is indicative that this organic compound affects the Ni–P ability
to oxidize as well. This decrease is also larger on the 14% P
sample than on the less phosphorus content films.
Fig. 5 is representative of the differences in the first and
second voltammetric scan of a Ni–P electrode (9 at.% of P) in a
0.1 M H2PO2� + 0.1 M NaOH solution. It reveals that the
catalytic activity of the electrode towards the hypophosphite
dehydrogenation and oxidation is strongly reduced upon
cycling. On observing the effect of the anodic limit potential
on the second scan voltammogram, it was concluded that the
pronounced decrease on the catalytic activity only occurs when
the anodic limit potential lies below �0.80 V, i.e. within a
potential range suitable for the Ni–P oxidation. This behaviour
reminds the voltammetric data previously observed on nickel
[6,28,29] and it suggests that a pronounced modification of the
surface state occurs in a potential range below �0.80 V. This
surface modification seems to inhibit further hypophosphite
dehydrogenation and oxidation.
By comparing the anodic peak at ffi�0.72 V in the
hypophosphite solution to that obtained in a hypophosphite-
free solution (Fig. 3) it can be inferred that this peak enlarges,
its current increases and the potential peak shifts towards more
positive potentials in the presence of the hypophosphite ion.
This result is strongly indicative that the oxidation of the Ni–P
alloy is itself promoted by the presence of hypophosphite ions;
Fig. 5. Cyclic voltammograms for a Ni–P alloy film (9 at.% of P) in 0.1 M
H2PO2� + 0.1 M NaOH solution. The solid and dashed curves refer to the first
and second consecutive cycles, respectively.
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–1512
however, whether this oxidation gives rise to a partial leaching
of phosphorous or to a partial leaching of nickel (or both) from
the surface is not clear for the moment. Some authors support
that Ni–P anodic oxidation causes partial dissolution of
phosphorus in an alkaline medium [30,31]. However, evidence
has also been given for formation of soluble Ni(II) compounds
(Ni aquocomplexes) in a hypophosphite solution in alkaline
medium [32]. The latter phenomenon has also been reported for
a nickel electrode in the acid medium [7] and has been
explained on basis of the hypophosphite ability to complex Ni2+
ions, giving rise to a bidentate monohypophosphite complex,
Ni(H2PO2)+ [33–35]. Clearly, a better understanding of the
surface phenomena occurring on Ni–P electroless alloys will
require, in the future, the use of very sensitive surface
techniques (such as XPS or AES).
3.3. Catalytic transfer hydrogenation
The electrochemical promotion of the CTH is only required
in the first stage of the experiment (during a few seconds) in
order to induce the hypophosphite dehydrogenation. Once the
dehydrogenation reaction starts, it is self-sustained [5,7,10].
According to previous studies, the electrochemical promotion
of the hypophosphite dehydrogenation by the negative
polarization of the catalyst (�1.1 V), relies on the formation
of a free-oxide/hydroxide surface that endorses a H2PO2� ion
adsorption configuration that favours the P–H breakdown [32].
Since the OCP reflects the catalyst ability to generate atomic
hydrogen [8], it allows to follow the catalyst transfer hydro-
genation in the 10 mM 3-buten-1-ol + 0.1 M NaOH + 0.1 M
H2PO2� solution after the induction step, Fig. 6. Typically, the
OCP profile is characterized by the development of a rather
constant potential which lies between �1.3 and �1.1 V and an
abrupt potential increase (in two or three stages) to�0.3 V. Such
behaviour is similar to that observed on Pd-black [9]. However,
while in that study the potential raise observed was ascribed to a
steep pH decrease, in this system the steep potential increase
relies on the catalyst deactivation. In effect, from this point,
Fig. 6. Typical OCP profile of the Ni–P alloy films in 10 mM 3-buten-1-ol
+ 0.1 M H2PO2� + 0.1 M NaOH solution after the first electrode activation by
applying �1.1 V for 30 s.
further conversion of 3-buten-1-ol was no longer detected. The
pH of the solution was measured at the beginning and at the end
of the reaction, and for all cases, it remained nearly constant.
It can also be inferred from Fig. 6 that the time lapse for the
catalyst deactivation increases with the increase of the P content
on the Ni–P alloy. The reason for the deactivation phenomena
will be analysed in more detail in Section 3.5. Regardless the
catalyst deactivation, it was found that it could be reactivated in
the same solution by applying a �1.1 V potential for 30–90 s.
Deactivation/reactivation of the catalyst proceeded several
times till the catalyst was irreversibly deactivated. Data on the
reaction yield, selectivity and H-donor efficiency, obtained
after the irreversible deactivation of the catalyst, is summarized
in Table 1.
It is shown that the Ni alloy activity, in terms of 1-butanol
yield, increases with the P content, which seems to indicate that
the catalyst activity is related to the amorphous character of the
Ni alloy. The same trend was found by some authors for the
catalytic activity towards the HER, which was explained on the
basis that the amorphous phase presents a highest ability to
absorb hydrogen [12,13,36,37]. The hypothesis that the catalytic
activity of the Ni–P electroless alloys for the CTH may be also
correlated to the surface roughness of the films cannot also be
excluded. In effect, the highest activity is displayed by the films
presenting apparently, a higher surface area.
Despite 1-butanol was the only product detected, a low
selectivity was found. This result may be indicative that a by-
product, not detectable by GC, should have been formed. As
previously discussed [9], this behaviour may be accounted by
the formation of aqueous soluble species (sodium monoalk-
enylphosphite). Another contribution for the low selectivity
may rely on losses of 1-butanol due to evaporation during the
experiment. Even though, its evaluation has revealed that the
mass lost by evaporation does not exceed 15%. Concerning the
H-donor efficiency of the hydrogenation reaction, it is revealed
that this is three times lower on the 14 at.% P than on 9 at.% P
alloy. This result points out that a significant part of the atomic
hydrogen formed on the 14 at.% P alloy is not used on the
hydrogenation reaction, which is in agreement with the higher
intensity of gas evolved that was observed on this film,
compared to the other Ni electroless films.
Comparing this data with that obtained for the CTH
performed on the Pd-black film using the same H-donor and
unsaturated organic compound [9], it can be concluded that the
CTH on the Ni–P alloys (containing 14% at. of P) gives rise to a
lower reaction yield (51% on Ni–P and 89% on Pd-black) and
selectivity (77% on Ni–P and 97% on Pd-black), but a higher
Table 1
Material yield, selectivity and H-donor efficiency in hydrogenation experiments
performed in 0.1 M NaOH + 0.05 M H2PO2� + 10 mM 3-buten-1-ol solution
with the different P content films
% at. P on the
Ni–P alloys
Yield
%
Selectivity
(%)
H-donor
efficiency (%)
7 4 59 ffi100
9 27 72 38
14 51 77 14
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–15 13
H-donor efficiency (14% on Ni–P and 8% on Pd-black) and a
remarkable film stability (without peeling from the copper
substrate) is achieved.
3.4. Electrocatalytic hydrogenation
In order to better evaluate the performance of the
electrochemically promoted catalytic transfer hydrogenation
method, its comparison with another hydrogenation methodol-
ogy is required. Regarding this purpose, the ECH of the same
organic compound, on the same catalysts (which are used as
electrodes) and in the same composition of the electrolyte
(H2PO2-free) was performed. The results, Table 2, clearly
reveal that ECH is much less efficient to hydrogenate 3-buten-
1-ol than the electrochemically promoted CTH. It can also be
concluded that the relation between the films catalytic activity
(in term of the material yield) and the P content of the alloy
follow a different trend from the one observed on the CTH. The
highest catalytic activity is displayed by the 9 at.% of P alloy.
Supplementary experiments performed on a 12 at.% P alloy
(the effective composition is 11.6 � 0.2) confirmed such
unpredicted tendency.
All these results seems to point out that a poisoning effect by
the strong adsorption of 3-buten-1-ol, which would prevent the
water adsorption and consequently the generation of chemi-
sorbed hydrogen, is responsible for the low activity of the
electrodes to the ECH. This hypothesis is corroborated by the
reaction yield increase that is observed on decreasing the organic
compound concentration (performed on the 12 at.% P alloy) and
is also supported by the dependence of the films P content with
the apparent catalyst ability to adsorb 3-buten-1-ol (previously
described in Section 3.2). Hence, differences on CTH and ECH
activity could be explained qualitatively on the basis of
adsorption competition between the organic compound and
Table 2
The effect of the electrode potential and organic substrate concentration on the
electrocatalytic hydrogenation of 10 mM 3-buten-1-ol + 0.1 M NaOH solution
at different P content films
% at. P E (V) F mol�1 Yield
(%)
Selectivity
(%)
Current
efficiency (%)
Sol. 10 mM 3-buten-1-ol + 0.1 M NaOH
7 �1.2 2.0 5 69 4
�1.1 1.9 a – –
9 �1.2 2.0 30 85 22
�1.3 2.5 22 72 14
12 �1.1 2.7 a – –
�1.2 2.7 4 38 2
�1.3 1.9 8 72 8
14 �1.1 3.8 a – –
�1.2 2.0 4 25 4
�1.3 2.0 5 41 5
Sol. 3 mM 3-buten-1-ol + 0.1 M NaOH
14 �1.1 2.4 4 26 3
�1.2 2.6 10 51 7
�1.3 2.5 18 73 14
a On these experiments the reaction yield lies below data precision (�3%).
the supplier source of hydrogen. When the hydrogen source is the
hypophosphite ion, its high ability to adsorb on the catalyst would
prevent a rather high surface coverage of 3-buten-1-ol and
consequently the surface blockage by the strong adsorption of
this compound. On the ECH, in a hypophosphite-free solution,
the strong adsorption of the unsaturated alcohol compared to the
hydrogen supplier (water), would make more difficult the
approach of the water molecules to the surface of the catalyst,
leading to its partial poisoning. Nevertheless, it was found that by
regulating the Ni/P ratio, a compromise between the electrode
ability to generate chemisorbed hydrogen and to adsorb the
organic molecule can be found, making the Ni–P alloy a rather
effective catalyst for the electrocatalytic hydrogenation.
3.5. Study of the catalyst deactivation phenomena
In order to assess whether the steep increase of the OCP
observed in the 10 mM 3-buten-1-ol + 0.1 M H2PO2- + 0.1 M
NaOH solution was related to the surface poisoning by a
hydrogenation product, the OCP was recorded in a solution not
containing the unsaturated organic molecule, Fig. 7. The
obtained data allows to conclude that the abrupt OCP increase
is an intrinsic characteristic of the electrode / H2PO2� + NaOH
system. In effect, the results reveal that the potential raise
recorded in this system occurs even earlier in the presence of 3-
buten-1-ol on the 9 and 14 at.% P films. On analyzing the
curves profile, it is recognized that, excepting the region that
just precede the abrupt potential increase (a potential oscillation
is depicted), the shape of the curve resembles those obtained in
a 3-buten-1-ol containing solution.
To evaluate the relationship between the electrode potential
attained in the OCP and the modifications that may occur on the
alloy surface, cyclic voltammograms were run in 0.1 M NaOH
solution after the electrode has rested at OCP in the
hypophosphite solution until �0.30 V (Fig. 8) has been
achieved. Comparing this voltammogram with those obtained
in Fig. 3, it is clearly depicted that a new cathodic peak (A)
emerges within �0.7 to �0.8 V. A similar result was obtained
when the electrode rested at OCP in the hypophosphite solution
Fig. 7. Typical OCP profile of the Ni–P alloy films (- - - 7 at.%, — 9 at.%, – – –
14 at.%) in 0.1 M H2PO2� + 0.1 M NaOH solution after the first electrode
activation by applying �1.1 V for 30 s.
Fig. 8. Cyclic voltammograms in 0.1 M NaOH solution for the different P
content on Ni–P alloy films (- - - 7 at.%, — 9 at.%, – – – 14 at.%) after its
removal from a +0.1 M H2PO2� +0.1 M NaOH solution upon achieved�0.30 V
at OCP.
Fig. 9. The effect of the anodic limit potential on the first (—) and second (– – –)
scan of the cyclic voltammograms of a 7 at.% P alloy in 0.1 M NaOH solution
after its removal from a 0.05 M H2PO2� +0.1 M NaOH solution upon achieved
�0.30 V at OCP. Inset first voltammograms in 0.1 M NaOH solution for
different P content on Ni–P alloy films which have not been submitted to the
hypophosphite solution: - - - 7 at.%, — 9 at.%, – – – 14 at.%.
M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–1514
until �0.50 V was achieved. The new cathodic peak is more
discernible on the lowest P content films. The dependence of
peak A on the scan rate confirmed that this peak is related to
surface reactant species. On comparing the first and second
voltammetric scan at different anodic limit potentials (Fig. 9), it
was found that this peak was related to an anodic peak (A0) at
�0.45 V. Cyclic voltammograms recorded with electrodes that
have not been submitted to the hypophosphite solution have
also revealed the presence of this anodic peak, but with a much
less intensity (Fig. 9 inset). These data are indicative that the
OCP raise to �0.50 or �0.30 V must be allied to an oxidation
phenomenon that is allied to the appearance of peak A0.According to several authors [27,30,38] the Ni–P oxidation
leads to the hypophosphite ion or to the nickel hypophosphite
formation, but experimental evidence for the A (or A0) peak
assignment has never been given. On account of the present
experimental results, it is much more plausible than peak A is
ascribed to a surface species, such as nickel hypophosphite,
however the accurate assignment of these peaks will require the
use of surface characterization techniques which are not under
the scope of this work.
Another important remark on the cyclic voltammograms of
the modified electrodes (Figs. 8 and 9) concerns the drastic
current decrease that is observed on the anodic peak assigned to
a-Ni(OH)2. Two proposals are suggested to explain this result:
(a) during the electrode rest at OCP in the hypophosphite
solution, part of the a-Ni(OH)2 formed is irreversible
transformed into b-Ni(OH)2; (b) the surface of the Ni–P alloy
film becomes partly blocked by some surface species that
inhibit further surface oxidation to a-Ni(OH)2.
The fact that the Ni–P alloy film can be re-activated by
applying �1.1 V before being irreversible deactivated allows
supporting the second hypothesis, some surface species would
be responsible for the electrode blockage inhibiting in some
extent the formation of nickel hydroxide. On account that such
specie would be reversible reduced and that would be formed
while the hypophosphite oxidation occurs, it is proposed that
such specie is a Ni(II) compound. Such compound, probably
responsible for the peak A and A0, would remain adsorbed on
the surface, acting as a protective layer which would hinder the
approach of the hypophosphite ions or the water molecules
(or hydroxyl ions) to the surface. On applying a rather width
potential step (�1.1 V/30–90 s) such specie would be reduced
which means that new vacant surface sites would be formed and
so the surface would be re-activated. This explanation is
corroborated either by the promotion effect of the hypopho-
sphite ion on the nickel oxidation, either by the inhibition effect
of the oxidized surface on the hypophosphite dehydrogenation
and oxidation, previously observed. Under the light of this
explanation, the different time lapse for the steep raise of the
OCP in a 3-buten-1-ol solution and in an alcohol-free solution
can be accounted by the inhibition effect of the adsorbed
3-buten-1-ol on the Ni oxidation. In the presence of this
unsaturated alcohol, the adsorption of the hypophosphite ion
would be less extended and consequently its effect on the Ni
dissolution would be slowed down. The same interpretation
allows to understand the dependence of the time lapse for the
steep increase of the OCP on the film P content observed in
Fig. 6.
It cannot also be disclosed the hypothesis that during the Ni–
P alloy immersion in OCP the a-Ni(OH)2 is also formed
(although in a low amount) and that it is slowly converted in the
passive form, b-Ni(OH)2 which, at a rather high coverage,
would be responsible for the non-reactivation of the electrode.
Under this scope, the potential oscillation phenomena
observed just before the potential raise can be attributed to the
concomitant H2PO2� dehydrogenation – Ni oxidation process
that occur on the electrode surface. When Ni is oxidized to
Ni(II), the potential increases in the positive direction, until the
chemisorbed hydrogen that is simultaneously formed on other
free surfaces sites reduce the Ni(II) species compelling the
potential to drop off. As new vacant surface sites are formed,
the hypophosphite ions and the water molecules are able to
interact again with the surface electrode, promoting its
oxidation and so the potential rises once again. This cycling
process will proceed until the formation of Ni(II) species is high
enough to hinder the approach of the hypophosphite ion to the
surface, then the potential increases rapidly for about 600–
800 mV in the positive direction. Certainly, a crucial point not
addressed here is the step that triggers Ni oxidation and that is
responsible for the dynamical instability. Further studies will
have to be carried out to get a better insight on this subject.
4. Conclusions
The results showed that Ni electroless alloys are effective
catalysts for the CTH. The performance of the hydrogenation,
in terms of the reaction yield, selectivity, H-donor efficiency
and catalyst deactivation hindrance were found to depend on
the P content of the Ni alloy. It was concluded that the CTH
activity, in terms of 1-butanol yield, follows the same trend
than the amorphous character of the alloy. However, a different
dependence was observed on the electrocatalytic hydrogena-
tion using the same catalysts and organic compound. This
phenomenon was attributed to the strong adsorption of the 3-
buten-1-ol on the Ni alloy surfaces which would prevent the H
generation by water electroreduction. The cyclic voltammetric
results allowed to evaluate the catalyst ability to co-adsorb the
H-donor and the organic compound and revealed that the
adsorption ability of 3-buten-1-ol is stronger on the highest P
content film.
Regardless the deactivation of the catalyst in the course of
the hydrogenation run was observed, its reactivation by
electrochemical polarization, at �1.1 V for 30–90 s allowed
a high material yield and selectivity (51% and 77%,