Pt CO oxidation

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Nettesheim S, von Oertzen A, Rotermund FI FI and ErtI G 1993 Reaction diffusion patterns in the catalytic CO-oxidation on Pt(110) front propagation and spiral waves J. Chem. Rhys. 98 9977-85... 

The CO oxidation occurring in automobile exhaust converters is one of the best understood catalytic reactions, taking place on Pt surfaces by dissociative chemisoriDtion of to give O atoms and chemisoriDtion of CO, which reacts with chemisorbed O to give CO, which is immediately released into the gas phase. Details are evident from STM observations focused on the reaction between adsorbed O and adsorbed CO [12]. 

Various methods can be used to analy2e succinic acid and succinic anhydride, depending on the characteristics of the material. Methods generally used to control specifications of pure products include acidimetric titration for total acidity or purity comparison with Pt—Co standard calibrated solutions for color oxidation with potassium permanganate for unsaturated compounds subtracting from the total acidity the anhydride content measured by titration with morpholine for content of free acid in the anhydride atomic absorption or plasma spectroscopy for metals titration with AgNO or BaCl2 for chlorides and sulfates, respectively and comparison of the color of the sulfide solution of the metals with that of a solution with a known Pb content for heavy metals. 

V. N. Kusovkov, O. Kortluke, W. von Niessen. Kinetic oscillations in the catalytic CO oxidation on Pt single crystal surfaces Theory and simulation. J Chem Phys 705 5571-5580, 1998. 

The concentration dependence of CO oxidation over Pt at (02) (CO) l differs from the concentration dependence of CO oxidation over copper chromite at (02)°-2(C0). This can be explained by the fact that after the departure of a C02 molecule, the reoxidation of platinum surfaces is slow but the reoxidation of base metal oxide surfaces is fast. On the other hand,... 

J. Poppe, S. Voelkening, A. Schaak, E. Schuetz, J. Janek, and R. Imbihl, Electrochemical promotion of catalytic CO oxidation on Pt/YSZ catalysts under low pressure conditions, Phys. Chem. Chem. Phys. 1,5241-5249 (1999). 

In the following we will concentrate on three important cases, i.e. CO oxidation on alkali doped Pt, ethylene epoxidation on promoted Ag and synthesis gas conversion on transition metals. We will attempt to rationalize the observed kinetic behaviour on the basis of the above simple rules. 

Figure 2.39. (a) Effect of pcc/Pc>2 on the rate °f CO oxidation (measured as PC02) on Pt(l 11) covered with various Li coverages 0Lj 12 (b) Effect of Li coverage on the rate of CO oxidation at various fixed Pcc/po2 values.112 Reprinted with permission from Elsevier Science. 

Figure 4.16, Effect of catalyst potential, dimensionless catalyst potential n(=FUWR/RT), corresponding linearized51 Na coverage 0ns and pCo on the rate of CO oxidation on Pt/(T-A1203. T=350°C, po2=6 kPa.51 Reprinted with permission from Academic Press.

Typical examples of electrophilic reactions are the reduction of NO by ethylene on Pt32 and the CO oxidation on Pt under fuel-rich conditions.51,62... 

Figure 4.31. Transition from volcano-type behaviour at low Po2 to electrophobic behaviour at high po2 during CO oxidation on Pt/j3"-A]203.51 Effect of UWr and linearized51 Na coverage 0Na on the rate of CO oxidation on Pt/p"-Al203 at varying po2- Other conditions pco=2 kPa, T=350°C. The top part of the figure shows the corresponding variation of the actual51 Na coverage, 0Na, with UWr- Reprinted with permission from Academic Press.

The transition from volcano-type behaviour to electrophobic behaviour upon changing p02 is shown in Fig. 4.31 for the case of CO oxidation on Pt/p -Al203. Another example of volcano-type behaviour is shown in Fig. 4.32 for the case of NO reduction by C3H6 on Pt/ 3"-Al203.98,99... 

Figure 5.54. Effect of sodium coverage on the change AUWR of polycrystalline Pt catalyst potential UWr and on the catalytic rates of CO oxidation (solid lines37) and C2H4 oxidation (dashed lines36). Comparison with the theoretical Na coverage required to form the Pt(l 11)-(12xl2)-Na adlayer 0 is based on the number of surface Pt atoms 09a is based on the number of surface O atoms corresponding to the Pt(l 1 l)-(2x2)-0 adlattice. Reprinted from ref. 78 with permission from Elsevier Science,...

K. Asakura, J. Lanterbach, H.H. Rothermund, and G. Ertl, Spatio-temporal pattern formation during catalytic CO oxidation on a Pt(100) surface modified with submonolayers of Au, Surf. Sci. 374, 125-141 (1997). 

S. Kelling, S. Cerasari, H.H. Rotermund, G. Ertl, and D.A. King, A photoemission electron microscopy (PEEM) study of the effect of surface acoustic waves on catalytic CO oxidation over Pt(110), Chem. Phys. Lett. 293, 325-330 (1998). 

Figure 6.1. Effective double layer during CO oxidation on Pt/YSZ (top) and Pt/(3"-Al203 (bottom).

Figure 6.3. Examples for the four types of global electrochemical promotion behaviour (a) electrophobic, (b) electrophilic, (c) volcano-type, (d) inverted volcano-type, (a) Effect of catalyst potential and work function change (vs I = 0) for high (20 1) and (40 1) CH4 to 02 feed ratios, Pt/YSZH (b) Effect of catalyst potential on the rate enhancement ratio for the rate of NO reduction by C2H4 consumption on Pt/YSZ15 (c) NEMCA generated volcano plots during CO oxidation on Pt/YSZ16 (d) Effect of dimensionless catalyst potential on the rate constant of H2CO formation, Pt/YSZ.17 n=FUWR/RT (=A(D/kbT).

Figure 6.11 comes from the classical promotion literature and refers to CO oxidation on Pt(lll) promoted with Li.83 As with every alkali promoter,... 

Figure 6.11. Effect of Pco Po2 ratio at fixed Li coverage (left) and of Li coverage at fixed PC0/P02 ratio (right) on the rate of CO oxidation on Pt(l 1l).83 Reprinted with permission from Elsevier Science.

Figure 6.12. Example of rules Gl, G2 and G3 Effect of pCo (=Pd) and of Na coverage and corresponding UWr and AO values on the rate of CO oxidation on Pt films deposited on P"-A1203 at fixed Po2=6 kPa71 Note that dr/dO(= dr/edUWR) always traces dr/dpco for negative, positive and zero (volcano peak) values. In the right figure the raw data (left) have been fitted to a polynomial expression.71 Reprinted with permission from Academic Press.

The behaviour is qualitatively similar for CO oxidation on Pt deposited on (3"-Al203, a Na+ conductor and promoter donor,71 as shown in Fig. 6.12 which comes from the electrochemical promotion literature.71 Here when the rate is negative order in CO (=D) electrophilic behaviour (dr/d<0) is obtained (Fig. 6.12, rule G2). When the rate is positive order in CO a weak electrophobic behaviour is observed (Fig. 6.12, rule Gl). Note that the rate exhibits volcano-type behaviour both with respect to 0 and to pCo (Rule G3). The present rules enable one to model mathematically the data of Fig. 6.12 in a semiquantitive manner as shown in the next section. 

Figure 6.25, Experimental71 (left) and modelled simulated" (right) dependence of the rate of CO oxidation on Pt deposited on J3"-A1203 as a function of pco, catalyst potential UWR and dimensionless catalyst work function Il(=A

Finally the success of the model can be judged from Figures 6.25a and b which show the experimental and model-predicted rate dependence on pCo and work function during CO oxidation on Pt/pM-Al203.71 Note the transition from a classical Langmuir-Hinshelwood to a positive order rate dependence on pco with decreasing work function. Also notice that on every point of the experimental or model predicted rate dependence, the basic promotional mle ... 

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