Adsorption growth and catalysis

see also Moriaty section 4. Note that the node of chapter 4 is removedIn the web but the links below should workTopics:PhysisorptionChemisorptionAdsorption kineticsAdsorbate diffusiondesorptionWhat can we understand now from catalysis

Absorption generally refers to two phenomena which are largely unrelated. In one case, it refers to when atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. For instance, a sponge absorbs water when it is dry.

(Absorption also refers to the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make transition between two electronic energy levels. The photon is destroyed in the process. The absorbed energy may be re-emitted as radiant energy or transformed into heat energy. The absorption of light during wave propagation is often called attenuation. The tools of spectroscopy in chemistry are based on the absorption of photons by atoms and molecules. )

Adsorption is similar, but refers to a surface rather than a volume: adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution

Sorption: either on the surface or in the material

What the difference between absorption and adsorption and sorption?(from wiki.answers.com)

Physisorption:

-van der Waals bonding-weak-relative large separation adsorbed atom and surface-electronic structure of surface and adsorbed species intact-happens at low temperature (of the order of condensation temperature)-fast, no threshold to overcome

Where does the force come from: consider a point charge a distance z from a metal surfaceHow does the electric field lines look like?Near the particle (charge e) it should look like e/r2 The surface is conducting, so the field lines should be at the surface perpendicular to the surface (else a current would flow, readjusting the charge and thus rearranging the field lines)

If we consider the electric field of two charges: e and -e with the charge -e at the mirror image position relative to the surface. Then the field of these 2 particles alone is perpendicular to the surface and behaves like e/r2 near the charge. There is a theorem that states a solution that meets all boundary conditions is a solution of the problem.

Thus the energy V of a charge e z away from a conducting surface is :

z z

z

See http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=318.0

Consider a hydrogen atom above a surface:its energy can thus be calculated by consideringits interaction with anti-hydrogen at the mirror imageposition:

surface

which for u << z simplifies to:

proton, anti-proton

elec., positron proton, positron

elec., anti-proton

Attractive partdue to van der Waals force

Repulsive partdue to overlapof electron clouds

Ek is the kinetic energy of the arriving atom. It has to be transferredto the solid in order to be bound.

In the case of atomic hydrogen (very reactive) almost certainly a chemical bond willform if it approaches the surface. For molecular hydrogen, noble gases etc. the overlap of the substrate and molecule wave functions will lead to a strong repulsive potential for separation distances smaller than the molecular dimension.

EbEb

no barrier barrier

Chemisorption:

As a molecule:energy as a functionof distance from surface r:

As separate atoms: now a bond (bond energy Eb) has to be broken.

(green curve is the energy of two separate atoms)

For noble gases the closed shell system is stable and physisorption is the only thing possible. Other gases will arrive at the surface generally as a molecule.Then two more things are possible:

r

Energy

physisorption

chemisorption

Here chemisorption willoccur straight away

Here physisorption willoccur first, chemisorption after activationbarrier has been overcome

The real world is always more complex. case H2 on tungsten surface

Adsorption: sticking coefficienton clean surface

Rads= flux*sticking coefficient= flux*exp-(Eads)/kT

The Flux Z is proportional to density of atoms (i.e. pressure) and v (average velocity),to be precise:

Hertz Knudsen formula

Thus:

Arrhenius-type behaviour

unit of exposure Langmuir One Langmuir is the exposure at 10-6 torr for 1 sec.(this corresponds roughly to 1 monolayer per second if the sticking coefficient is 1)

Note: final coverage is two H per surface Si atom (2 monolayers) . Should thissurprise you? (check number of broken bonds per Si atom for 001 surface, see spm section)

Example H on Si

Irving Langmuir 1881 –1957The Nobel Prize in Chemistry 1932

Worked for General Electric Company , and was the firstindustrial scientist to receive Nobel priceVery versatilegreatly improved incandescent light bulb.surface sciencechemistryplasma physics (arc welding)

link

recap: chemical bond, the case of H2

bonding

antibonding

empty

atomiclevel

atomiclevel

Bring two H atomsclose together:

bring an H moleculeclose to a surface:

Fermi Level

den

sity of s ta

tes

of surfa

ce

distance to surface

broadening chargetransfer

density ofstates metal

bonding

anti-bonding

*

hydrogen molecule becomes slightly positive.

Fermi Level

den

sity of s ta

tes

of surfa

ce

distance to surface

broadening

chargetransfer

density ofstates metal

bonding

anti-bonding

*

hydrogen molecule becomes slightly negative.

More complete description (cartoon) of adsorption ofa molecule

r

Energy

physisorption

molecular chemisorptionsome charge transfer

atomic chemisorption

Start with bonding and anti-bonding orbital from molecules. Now as separation decreases:1 some broadening2 each orbital can now be in phase and out of phase with orbital of substrate: splitting develops3 splitting get so large that there is charge transfer “chemisorption”4 half the bonding orbital is empty half the anti-bonding orbital is filled. No chemical bond left between the two atoms5 bonding and anti-bonding orbitals between atom and surface

Desorption

r

Energy

Edes

Desorption rate:Again an Arrhenius behaviour

With: n attempt frequency (usual taken as the vibrational frequency)N concentration of adsorbed atomsEdes energy desorption barrier

Eads

DHads

example: n as 1012 Hz (a typical value) what is the dwell time of an adsorbate whose desorption barrier is 0.22 eV?

The bond energy is 0.22 eV or 0.3475 x 10-19 J. At 300 K, the dwell time is ~ 4.4 ns whereas at 77 K the dwell time is 159 seconds.

Generally adsorption and desorption will occur at the same time

The resulting equilibrium coverage can be calculated easily forthe following simple model:

-[1]there are Nsites adsorption sites on a surface.-[2]each site can accommodate only one molecule.-[3]activation energies Eabs and E des are independent of coverage.

Let q be the fraction of sites occupied.Then:

In equilibrium both rates should be the same. Solve for q

This term because of condition [2]

The plots of q against P at a given temperature are called Langmuir isotherms

Langmuir isotherms for decreasing values of b (i.e. increasing temperature)

By changing the temperature the same equilibrium coverage is obtained at a different pressure

See also: http://www.chemistrylearning.com/langmuir-adsorption-isotherm/

What we can understand now about how a surface can be used in a catalytic reaction

Example: 2CO+O2 2CO2

This reaction requires 3 molecules to come together. In the gas phase this isextremely rare.

The reactionCO+O2 CO2 + O costs significant energy. So will not occur in a CO-O2 collision.

The surface solves this problem. If oxygen is adsorbed as an atom on a surfacethen the reaction CO+O CO2 can occur.

The resulting CO2 has to desorb, else the reaction will stop (catalyst poisoned )

So the following steps are required to occur on a catalytic active surface.

O2 adsorbed as two separate atoms on a surface

CO adsorbs as a molecule and diffuses until it finds an O atom

CO2 is formed and desorbs subsequently. New CO and O2 adsorbs and the process repeats

from:http://energy.nobelprize.org/presentations/ertl.pdf

Schematic energy diagram for the oxidation of CO and a Pt catalyst. (From data presented by G. Ertl in Catalysis: Science and Technology, J. R. Anderson and M. Boudart, Eds., vol. 4, Springer-Verlag, Berlin, 1983, p. 245.) All energies are given in kJ/mol. For comparison, the heavy dashed lines show a noncatalytic route.

1 eV (per atom) corresponds to 96.485 kJ/mol

The Haber–Bosch process for the synthesis of ammonia fromN2 and H2 that was developed in 1913 has been described as themost important invention of the twentieth century.

This removed a major limitation on the productivity of agriculture and was amajor factor in the tripling of the human population of the earth between 1900 and 2000.

The Haber–Bosch process is one of the earliest applicationsof surface science and the process was optimised by trail anderror and with very little detailed understanding of the basicsurface processes involved. The development of ultra-high vacuum(UHV) techniques in the 1960s and the subsequent explosionof surface science techniques for the study of surfaces in UHVhave provided some understanding of the mechanisms of theHaber–Bosch process

From link

1: N2 (g) → N2 (adsorbed)2: N2 (adsorbed) → 2 N (adsorbed) rate limiting step3: H2(g) → H2 (adsorbed)4: H2 (adsorbed) → 2 H (adsorbed)5: N (adsorbed) + 3 H(adsorbed)→ NH3 (adsorbed)6: NH3 (adsorbed) → NH3 (g)

production of H2 from CH4

iron-based catalystN2 and H2 adsorb on Fe

from wikipidea

!

from: G. Ertl: Primary steps in catalytic synthesis of ammoniaJ. Vac. Sci. Technol. A 1, 1247 (1983);

from: G. Ertl: Primary steps in catalytic synthesis of ammoniaJ. Vac. Sci. Technol. A 1, 1247 (1983);

In Kjoule/mol: 1 eV (per atom) corresponds to 96.485 kJ/mol

Withcatalyst

Withoutcatalyst

This was Weightman's view on Haber. But one can get also this story from the web:

Clara was the first wife of one of the great chemists, Fritz Haber. He invented the Haber-Boschprocess for synthesizing ammonia, which in turn is the key for making nitrates for fertilizers andexplosives. She herself was the first woman to receive a doctorate in chemsitry from the Universityof Breslau (now Wroclaw in Poland) in 1900. She is said to have contributed much to his work, buthated his efforts for Germany in the First World War. Science should be used for constructive ends,not for blind patriotism.

He, though, was a converted Jew and loyal above all else to his country. He pushed the military intothe use of chemical weapons. Their first major trial, of chlorine gas against French troops at Ypreson April 22, 1915, was a stunning success. Five thousand were killed and another few thousandincapacitated.

The German papers were full of praise, and Haber was promoted to captain. Finally a way had beenfound to break the stalemate of trench warfare. Haber threw a dinner party to celebrate. Clara andhe got into a furious argument. That night she took his army pistol and killed herself in theirgarden with a shot to the chest.

Haber left the next day for the Eastern front, not even staying for the funeral arrangements. Hecontinued to promote the use of poison gases, even when the true answer to the trenches turned outto be tanks. He was branded a war criminal after the Allied victory, but still won the Nobel Prizein 1918 for ammonia synthesis.

He remarried, and spent the years after war doing basic chemistry, but also searching desperatelyfor a way to extract gold from seawater in order to pay the Weimar's Republic's debts. He rosesteadily in honors, but nothing availed him when the Nazis came to power. Converting to Christianityand aiding one's country mightily in its worst struggle didn't count compared to his ancestry. Hemoved to a post at Cambridge, but died of a heart attack while traveling through Switzerland.

From link

How does nature fix N2 at room temperature and 1 atmosphere?Even much more complicated, based on Molybdenum containing moleculesFrom link One can only admire nature

From:

htp://spectrum

.ieee.org/energy/renewables/prospects-for-an-artfcial-leaf-are-grow

ing

Outstanding Challenges!!