VII. Catalysis by Surfaces EEW508 VII. Catalysis by Surfaces

Chemical technology

80% of chemical processes use catalysts. Environmental and energy catalysts – approximately $13 billion The product made by catalysts – approximately $1 trillion EEW508 VII. Catalysis by Surfaces

Surface catalytic process

The reaction occurs repeatedly by a sequence of elementary steps that includes adsorption, surface diffusion, the chemical rearrangements (bond breaking, bond forming, molecular rearrangement) of the adsorbed reaction intermediates and the desorption of the products. EEW508 VII. Catalysis by Surfaces Brief history of surface catalysis

The oxidation of hydrogen by air over platinum was observed by H. Davy (1817) and E. Davy (1820). Dobereiner (1823) constructed a ‘tinderbox (부시깃통)’ to produce flame when a small dose of hydrogen generated by the reaction of zinc and hydrochloric acid reacts with air in the presence of platinum. Dobereiner also found that platinum aid the oxidation of CO and ethanol. Faraday found that ethylene adsorption deactivates the platinum surface temporarily while the adsorption of sulfur deactivates platinum permanently. He measured the rate of hydrogen oxidation, suggested a mechanism, and observed its deactivation and regeneration. In 1836, Berzelius defined this phenomena and called it catalysis. EEW508 VII. Catalysis by Surfaces

Catalytic action

Catalysts: aiding in rapidly achieving chemical equilibrium for certain chemical reactions.

Hydrogen oxidation ( H2 + ½ O2 → H2O)

Let’s assume that we have O2 and H2 gas mixture in a glass bulb → no chemical reaction. The thermal energy of gas is RT (~0.6 kcal/mol).

To form H2O, H2 or O2 should be dissociated first. But the dissociation energy is 103 kcal/mol for H2, and is 117 kcal/mol for O2. Also subsequent atom- molecule reactions (H+ O2 or H2 +O) still requires an activation energy of about 10 kcal/mol. Therefore the gas-phase reaction is very improbable.

If we just drop a high-surface-area platinum gauze, the reaction occurs instantaneously and explosively. On the Pt surface, both molecules

dissociate to atoms with near zero activation energy (H2 + Pt → 2H-Pt or O2 +Pt→ 2O+Pt). The atom-atom interaction takes place on the surface with very low or no activation energies in contrast to that in the gas phase. EEW508 VII. Catalysis by Surfaces

Ammonia synthesis ( N2 + 3 H2 → 2NH3)

For reaction, N2 should be dissociated. However, the large dissociation of N2 (280 kcal/mol) makes it virtually impossible for this reaction to occur in the gas phase.

On an iron surface, N2 dissociate with a small activation energy (3 kcal/mol). Iron also atomize the hydrogen molecules. The chemisorbed nitrogen

atoms then react with hydrogen atoms on the surface to produce NH, NH2, and finally NH3 molecules that desorb into the gas molecules.

Therefore, the surface catalytic reaction involves atomization of the diatomic molecules with large bonding energies by forming chemisorbed atomic intermediates on the surface and to lower the activation energy for the subsequent reactions on the surface. EEW508 VII. Catalysis by Surfaces

Kinetics

Turnover frequency (TOF), R

R = f/N, where f is the number of product molecules per second, and N is the number of active surface sites available on the catalyst surface. Therefore, R has the unit of molecules/site/sec.

In experiment, measurement of TOF requires the estimation of number of active sites.

For single crystal, N = nA, where n is the surface atomic concentration, and A is the surface area of the catalysts. This type of analysis assumes that every surface site is active. However, the number of catalytically active sites is smaller than the total number of available surface sites. Therefore this type of estimation gives rise to conservative lower limit of the catalytic turnover rate. For 3D nanoparticle systems or industrial catalysts, the number of active sites usually is measured with the chemisorption experiment with hydrogen or CO. EEW508 VII. Catalysis by Surfaces

Reaction probability

Reaction probability RP shows the overall efficiency of the catalytic process under the reaction conditions.

RP = [rate of formation of product molecules] / [rate of incidence of reactant molecules].

RP = TOF/ F(rate of molecular incidence), where F = P/(2  MRT)1/2

Catalytic reaction rate, R can be expressed R = k x f(P) where P is the partial pressure of the reactants.

Here, k may change to reflect the changing reaction mechanisms.

k = A exp[ - E*/RT] where A is the temperature-independent preexponential factor, R is the gas constant and E* is the apparent activation energy measured under the catalytic reaction conditions. EEW508 VII. Catalysis by Surfaces

Hydrocarbon conversion over Pt catalysts

Isomerization, cyclization, dehydrocyclization, and hydrogenolysis reaction have the activation energy of 35-45 kcal/mol. High temperature is required to carry out these reactions. Hydrogenation reactions – 6-12 kcal/mol, and can be performed at high rates at 300 K or below. EEW508 VII. Catalysis by Surfaces

Combustion reactions highly exothermic, and can be carried out at very high temperature. The mass transport of the reactants becomes the self-limiting step. The diffusion of the reactants in the gas phase to reach the surface is slower than the surface reaction process.

Hydrocarbon conversion The main reason for these low reaction probability is the formation of various hydrocarbon intermediates on the surface. For example, the dissociation of methane on the Pt(111) surface produce intermediates

such as –CH, -CH3, -CCH, and –CCH3 at various temperature as revealed by SFG vibration spectroscopy. These surface intermediates may block the active sites for the dissociation of methane and cause the low probability (~ 10 –8) of the dissociation process. EEW508 VII. Catalysis by Surfaces

Reaction models

Langmuir-Hinshelwood mechanism

Rideal-Eley mechanism EEW508 VII. Catalysis by Surfaces

CO oxidation – (10-6 – 760 Torr) Below ignition Above ignition

The reaction rate is governed by surface the surface is oxygen covered, reaction kinetics and the reaction is mass transport Langmuir- Hinshelwood kinetics limited by either CO approaching or CO2 leaving the CO(a) + O(a)→ CO2(g) surface.

This reaction is positive first order in O2 CO(g) + O(a) → CO2(g) and negative first order in CO pressure 14 kcal/mol on Pt(111) at a pressure of B Activation pA 40 Torr of CO and 100 Torr of O2 p AB energy: B A fast RDS  fast  k For both CO and O2, a positive half-  het 30 - 42  order dependence kcal/mol. in partial pressure Pt(111)

25kcal/mol for Rh(111) EEW508 VII. Catalysis by Surfaces

Catalyst Preparation

Support Porous materials with high surface area (100 – 400 m2/g) made of alumina, silica, or carbon, and other oxides (Mg, Zr, Ti, V oxides), phosphates, sulfides, or carbonates are prepared first.

Metal nanoparticles Transition metals are then deposited in the micropores, and then heated and reduced to produce small metal nanoparticles 1- 100 nm in size.

Additives Electron donors (alkali metals) or electron acceptor (halogens) adsorbed on the metal or on the oxide to act as bonding modifiers for the coadsorbed reactants. EEW508 VII. Catalysis by Surfaces

Incipient wetness impregnation

TEM image of 3 wt% Pt/Al2O3 catalysts prepared by incipient wetness from tetrammine platinum(II)-nitrate (calcination temperature 260 oC)

Solution of the catalyst precursor (for example, the platinum nitrate solution) is added to the porous support under continuous stirring, until the incipient wetness point is reached. At this point all the pores are filled with the impregnating solution. Then the catalysts is dried to remove the solvent, and is calcined, reduced or sulfide depending on the application. The size of metal particles are roughly controlled by amount of metal loaded into the catalysts. EEW508 VII. Catalysis by Surfaces

Turnover rate measurement

(a) Experimental setup of catalytic batch reactor for measuring the turnover rate of small area 2 Dimensional catalysts

(b) Flow reactor for 3 Dimensional high surface area catalyst systems. EEW508 VII. Catalysis by Surfaces

Case study: ethylene hydrogenation on Pt nanowires (A. M. Contreras et al. Catalysis Letter 100, 115, (2005). )

Nanoimprint lithography fabrication scheme Pt nanowires fabricated on silicon wafer EEW508 VII. Catalysis by Surfaces

Case study: ethylene hydrogenation on Pt nanowires (A. M. Contreras et al. Catalysis Letter 100, 115, (2005). )

All of the nanocatalyst arrays are cleaned by dosing with 1 x10-6 Torr of NO2 at 573 K for 20 min, followed by dosing the sample with 10 L of CO and flashing the temperature to 573 K to remove the remaining CO from the surface. This procedure has been established to be effective for cleaning supported Pt nanostructures and Pt(111) single crystals of their major surface impurities such as carbon and oxygen

Catalytic studies are carried out on all catalyst samples using 10 Torr C2H4, 100 Torr H2, and 650 Torr Ne gas.

For CO poisoning studies, 300 mTorr of CO is added to the manifold with the reaction gases. Gases are premixed in the gas manifold approximately 20 min before introduction to the catalyst and the reaction line.

The gases are circulated through the reaction line with a Metal Bellows re-circulation pump. A HP Series II gas chromatograph equipped with a FID detector and a 50-m alumina capillary column (J & W Scientific) was used to separate and analyze products.

UHV reactor system The GC was part of the reaction loop and sampled the circulating reaction gases every 2.5 min using an automatic sampling valve. EEW508 VII. Catalysis by Surfaces

The area of the EBL arrays both on alumina and silica are 36 mm2 equivalent to 3.6 x 109 Pt particles. Surface area of Pt nanowires was estimated, and then the number of active sites was calculated with the assumption that surface structure of Pt nanowires is the same with Pt(111) surface.

Typical ethane accumulation curve used to calculate turnover rates. This data was Arrhenius plots for ethylene hydrogenation obtained at 353 K using a Pt nanoparticle reactions on Pt nanoparticle arrays catalyst on a silica support. supported on (a) silica and (b) alumina EEW508 VII. Catalysis by Surfaces

these results suggest that oxide-metal interface sites can remain active under poisoning conditions EEW508 VII. Catalysis by Surfaces

Case study II: Catalytic activity of CO oxidation on nanoparticle arrays – bare nanoparticle versus coreshell nanoparticle EEW508 VII. Catalysis by Surfaces

Activation energy of CO oxidation on various catalyst systems

Catalytic systems Activation energy (kcal/mol) Pt (111) 14 (above ignition) 42 (below ignition) 15 nm Pt film on Si (100) 13.7 (above ignition), 26.9 (below ignition)

Pt/TiO2 nanodiodes 21-22 (below ignition)

Pt nanoparticles 27(below ignition)

Pt-SiO2 coreshell 10 (above ignition), nanoparticles 28 (below ignition) EEW508 VII. Catalysis by Surfaces

Catalyst Deactivation and Regeneration

Reasons for deactivation

Reactant hydrocarbons may decompose and deposit a thick layer of inactive carbon on the catalyst surface (coke) Some species can diffuse from the bulk to the surface, poisoning the catalytic reaction and reducing the catalytic activity. Surface oxidation can form the inactive thick oxide during the reaction

At the molecular scale, the deactivation takes place by blocking the active surface sites. For example, CO during hydrogenation reactions. In automobile catalytic converter, tetraethyl-lead from gasoline can poison Pt/Rh catalysts by depositing lead sulfate on the noble metal surface. EEW508 VII. Catalysis by Surfaces

Ethylene hydrogenation CO oxidation (40 oC, Pt/TiOx diode) (240 oC, Pt/TiOx diode) 0.03 0.16 35 30 0.14 0.025 30 25 0.12 0.02 25 0.1 20 20 0.015 0.08 15

0.06 15 TOF (/Pt/s) TOF TOF (a.u.) TOF 0.01 10 10

0.04 CO2 of conversion 0.005

0.02 5 5 conversion of ethane (%) ethane of conversion 0 0 0 0 0 5 10 0 5000 10000 15000 20000 number of run time (s) Self-cleaning under CO oxidation Site blocking by carbon adsorbate (10% change over 25% conversion) - poisoned CO2

C or CO C or CO Pt Pt EEW508 VII. Catalysis by Surfaces Catalyst Deactivation and Regeneration – formation of bulk oxide Turnover rate of CO oxidation (Ru thin film)

(100 Torr of O2 and 40 Torr of CO) 400000

350000 240oC 300000 250000 240oC 280oC 200000

150000

100000 CO2 turnover (/Pt site) (/Pt turnover CO2

50000

0 0 5000 10000 15000 20000 time (s)

CO 100Torr + He - reducing at 200 oC for 30min EEW508 VII. Catalysis by Surfaces

Compensation effect

The preexponential factor and the activation energy for the reaction vary greatly from catalyst to catalyst. However, they vary in such a way as to compensate each other, so that the rate constant remained almost constant.

ln A =  + E* /R

Where  is a constant and  is called the isokinetic temperature

One catalyst may have a large concentration of active sites where the reaction requires a high activation energy, while the other catalyst that has smaller concentration of active sites has low activation energy EEW508 VII. Catalysis by Surfaces

Compensation effect for the methanation reaction (CO hydrogenation) EEW508 VII. Catalysis by Surfaces

Strong metal-support interaction Metal-Semiconductor Mixed Catalysts

methane oxidation on ZnO/Ag, (CH4 + 2O2 → CO2 + 2H2O)

30 50 ZnO/Ag ZnO/Ag ZnO (Torr) 25 ZnO/Ag 40 ZnO ZnO/Ag

2 Ag O (Torr)O

2 Ag 20 ZnO 30 15 20 Ag 10 Ag

5 10 ZnO

0 0 Partialpressure H of Partialpressure COof 100 200 300 400 100 200 300 400 T ( oC) T ( oC)

“The most striking fact is that the mixed catalyst gives an extremely high yield in water vapor and carbon dioxide“ “the catalytic promotor effect in a mixed catalyst is traced back to an electron exchange between support and catalyst.”

“Combined Action of Metal and Semiconductor Catalysts” G.-M. Schwab and K. Koller, JACS 90, 3078 (1968). EEW508 VII. Catalysis by Surfaces

Strong metal-support interaction (SMSI) effect

Changes in catalytic activity when the group VIII metals Fe, Ni, Rh, Pt, Pd, and Ir, are supported on certain

oxides (TiO2, TaO5, CeO2, NbO, etc.)

For example, methane formation

from CO or CO2 and H2 is enhanced by 3 orders of magnitude [4]. AFM images of 50 nm Pt nanoparticles

[1] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [2] S.J. Tauster and S.C. Fung, J. Catal. 55 (1978) 29. [3]K. Foger, in: Catalysis, Science and Technology eds. J.R. Anderson and M. Boudart, Vol. 6 (Springer-Verlag, Berlin, 1984) ch. 4. [4] G.L. Haller and D.E. Resasco, Adv. Catal. 36 (1989) 173. [5] J.P. Hindermann, G.J. Hutchings and A. Kiennemann, Catal. Rev. Sci. Eng. 35 (1993) 1. EEW508 VII. Catalysis by Surfaces

Structure sensitivity and insensitivity

A catalytic reaction is structure sensitive if the rate changes markedly as the particle size of the catalyst changes.

A catalytic reaction is structure insensitive if the rate is not influenced by changing of the particle size. EEW508 VII. Catalysis by Surfaces

Ammonia synthesis

There are two possible reasons for the high activity of the (111) and (211) faces compared to other (210), (100), and (110) orientation: Their high surface roughness and/or the presence of unique active sites. EEW508 VII. Catalysis by Surfaces

Most frequently used catalysts materials

Reforming reactions to produce cyclic and aromatic molecules from alkanes to improve the octane number are carried out over Pt or Pt-based alloy.

Steam forming of natural gas to produce CO and hydrogen is an important large- volume catalytic process. This reaction can take place on Ni catalysts. EEW508 VII. Catalysis by Surfaces

Cost and catalytic activity – smart design of catalytic materials

J. K. Nørskov et al. “towards the computational design of solid catalysts” Nature Chemistry, 1 (2009).

(a) A price versus catalytic- performance plot for methanation over a range of elemental metals and alloys

(b) Experimental confirmation that NiFe alloys are more active than pure nickel. EEW508 VII. Catalysis by Surfaces

Selective catalysis Ni CO + H2 → methane (CH4)

Copper and zinc oxide

CO + H2 → methanol (CH3OH)

The reaction of n- hexane in the presence of hydrogen over Pt catalysts produce benzene, cyclic molecules, branched isomers, or shorter chain species. EEW508 VII. Catalysis by Surfaces

Selectivity

Catalytic reaction involves (a) Successive kinetic steps leading to the final product

R1 R2 A → B → C (b) Simultaneous reaction paths yielding two or more products

B R1

R2 A C

R3 D

For a reaction with n pathways, we define the fractional catalytic selectivity, Si as the fraction of reacting molecules which are converted along the ith pathway

Ri Si = n  R j j=1 EEW508 VII. Catalysis by Surfaces

Catalytic Selectivity - Key concept to achieve the Green Chemistry VII. Catalysis by Surfaces

High pressure STM and SFG

|> G. A. Somorjai and VIS SFG J. Y. Park

“Molecular Factors in |1> IR |0 Catalytic Selectivity” >

Angewandte Chemie intensity SFG (Reviews, 47, 9212- 9228, 2008) Frequency Surface mobility Reaction intermediates G

Surface structure Adsorbate-induced restructuring reactant

Product 1 Gibbs free energy free Gibbs Product 2 Reaction Selectivity Composition Charge transport Oxidation Metal-oxide 2CO 2CO2 states O interfaces 2 chemi- A current

e- Pt Ambient

XPS signal XPS Pressure n-type TiO2 -XPS Catalytic nanodiode Binding Energy (eV) + H2 Shape selectivity in benzene hydrogenation + Cyclohexane Benzene Cyclohexene Pt Single crystal Pt nanoparticles

Pt (111) Pt (100) Cuboctahedra (12.6 nm) Cube (13.4 nm)

CHA: cyclohexane 0.30 CHE: cyclohexene 0.10 Bratlie et . CHA al., Nano 0.25 CHA Pt(111) 0.08 Letters 7, 3097 cubes 0.20 (2007). CHA 0.06 0.15 Pt(100) 0.04 0.10 CHA 0.02 cuboctahedra

0.05 (molecule/site/s) TOF TOF (molecule/site/s) TOF CHE CHE 0.00 Pt(111) 0.00 Cuboctahedra (x10) 300 320 340 360 380 400 300 320 340 360 380 400 Temperature, K Temperature, K Effect of Particle Size on Selectivity in Hydrocarbon Conversion Reactions: Cyclohexene hydrogenation/dehydrogenation

10 Torr C6H10, 200 Torr H2, 480 K

25 1 1 + H2 0.9 0.9 20 0.8 Cyclohexene Cyclohexane 0.8 Benzene

0.7 0.7 15 ) selectivity )

12 0.6 0.6

H 10

6 0.5 0.5 (C 0.4 5 0.4 Apparent E (kcal/mol) Cyclohexane

0.3 0.3 Benzene (C6H6) Benzene selectivity

- H2 0 0.2 0.2 0 2 4 6 8 Particles size (nm) 0.1 Cyclohexane 0.1 Cyclohexene Benzene 0 0 0 2 4 6 8 Apparent Ea for cyclohexene Particle size (nm) dehydrogenation increases with increasing particle size Benzene selectivity decreases with increasing particle size Catalytic Conversion of Crotonaldehyde: Effect of Nanoparticle Size

Platinum Nanoparticles

0.8

Butyraldehyde 0.6 Butyraldehyde H

H3C O

H 0.4 H3C OH H3C O 1-Butanol Selectivity Selectivity Crontonaldehyde Crotyl alcohol

H3C OH 0.2 Crotyl alcohol 1-Butanol H3C CH2 + CO Propene Propene 0 1 3 5 7 Particle size (nm)

16 Torr C4H6O, 315 Torr H2, 393 K Dendrimer Encapsulated ~ 1 nm Nanoparticles

Metal ion NaBH4

Complexation Reduction

Loading

62 Tertiary Amines

64 Terminal SBA-15 Groups 20 nm 4th Generation PAMAM Dendrimer G4-OH Pt40/SBA-15 Pyrrole Hydrogenation over Pt NPs: Structure Sensitivity H H

+ +H N +2H 2 H2 2 N H2N +NH3 n-butylamine Butane and Pyrrole Pyrrolidine ammonia

4 Torr pyrrole, 400 Torr H2, 413 K The Effect of Oxidation State Changes With the Size of Pt Nanoparticles

Size (nm): 0.8 1.1 1.5 2.0 2.9 5.0

x+ x+ Pt /Pt(0) = 13 Ptx+ Pt (0) Pt /Pt(0) = 0.16 Ptx+ Pt (0)

4f 4f 4f5/2 5/2 7/2 4f7/2

0.8 nm Pt Nanoparticles 1.5 nm Pt Nanoparticles

For clarity, the deconvoluted peaks for Pt 4f5/2 are not shown in the XPS spectra. VII. Catalysis by Surfaces Size, shape, composition of nanoparticle versus catalytic selectivity

Tuning of catalytic activity and selectivity via tailoring the size, composition and shapes of nanoparticles

Catalytic Selectivity EEW508 VII. Catalysis by Surfaces

Oxide Deposits Enhance the Rate of Methane Formation [Boffa et al. J. of Catalysis (1994)]

High bandgap – insulating – no state in the semiconductor, thus no hot electron flow

Low bandgap – reversible transport – no flow Bandgap NbOx : 3.5 eV

TiOx :3.2 ~ 3.3 eV Methane Methane formation (a. u.) TaOx : 4.4 eV NiO : 3.6-3.7 eV

ZrOx : 5.5 eV VOx : 2.3 – 2.4 eV WOx : 2.2-3.0 eV FeOx :2.0-2.2 eV