CO Oxidation on Ag

Figure 8.35. Effect of catalyst potential and work function on the rate enhancement ratio during CO oxidation on Ag. p02=3 kPa, pCo=5 kPa, , T=363°C, ro=2.7xl0"9 mol O/s A, T=390°C, r0 3.4xlO 9mol O/s , T=410°C, r0=5.5xlO 9mol O/s, A, T=390°C, r0=3.4xlO 9 mol O/s.38 Reprinted with permission from Trans Tech Publications.

The observed inverted volcano behaviour, as well as the kinetic behaviour are qualitatively similar to the case of CO oxidation on Ag/YSZ (Figs. 8.34 and 8.35)2 38 and thus present a nice example of promotional rule G4. Similar is the electrochemical promotion behaviour of CO oxidation on Au which has also been studied by Sobyanin and coworkers.40 41... 

Figure 52. Effect of Pco on regular (open-circuit) and NEMC A-induced rate of CO oxidation on Ag. T = 415°C P02 = 3 kPa , r = 0 , Vwr = +475 mV A, Vwr = -1300 mV. (Reprinted with permission from Elsevier Science Publishers, B.V., Amsterdam, from Ref. 4.)...

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. 

The CO oxidation on Pt was the second reaction, after C2H4 oxidation on Ag, for which a Non-Faradaic rate enhancement was observed.33 Typical measured A values were of the order 102-103 while p was typically below... 

Like CO oxidation on Ru, the understanding for ethylene epoxidation on Ag has continued to evolve. Many questions remain open, including the reaction mechanism on the Ag structures, and the role of intercalated oxygen atoms. Another dimension that is little explored so far is the surface states in a combined oxygen-ethylene atmosphere. Greeley et al. have reported recently that an ethylenedioxy intermediate may be present at appreciable coverage under industrial reaction conditions, the effect of which on the structure of the surface is unknown. More importantly, the implication of a dynamic co-existence of various surface oxides under reaction conditions for the reaction mechanism needs to be explored and understood at greater depth. 

Surface science studies of CO oxidation on Au(llO) single crystals have been made previously in which a Pt filament was used to adsorb oxygen adatoms ( o 0.25) on the Au(llO) surface and a CO titration was performed subsequently (7). CO did react to form CO2 with Eapp = 2 1 kcal/mole. Since CO was not observed to adsorb on Au(llO) at 125 K, it is only physisorbed (as on Au(lll)) and we can estimate that Elh = 7 kcal/mol on Au(llO). The difference from Au(lll) is probably due to a weaker Au-O bond on Au(lll) which leads to a lower barrier for reaction. No surface carbonate was formed from CO2 + Oa on Au(llO) either (7). This is in contrast to the behavior on Ag. Exposing oxygen covered Ag(llO) (16) or Ag(lll) (17) to CO2 produces carbonate species which are stable to 485 K on the surface. 

In catalytic reactions on metals a decrease in activation energy under UV illumi-tion is also observed 97 "). In experiments on CO oxidation on evaporated Ag, Au, Pd and Pt films 98 a decrease is observed in the activation energy due to... 

It was established that ethene oxide, CO and CO2 were formed on Ag-Sup at temperature range of 370-410 K. Such the products were formed at the presence of Ag-Im2, although the reaction took place at more high temperatures (450-500 K). The major product of partial oxidation on Ag-Ph was butene-2,3 oxide. It has been formed begining from 340 K with 100 % selectivity and ethene conversion being of 30-35 % at 408-415 K. The ethene oxide was detected in trace amounts. 

Gold deserves a special attention. It should be noted that the impetus given by the publications of Haruta in 1980s on low-temperature CO oxidation on Au nanoparticles [129] resulted in a flourishing field of research of the properties of Au NPs (and later Ag NPs) in diverse reactions of organic synthesis. Therefore, we will focus in the short review below on the most outstanding results (in our opinion) obtained in studying Au and Ag NPs on zeolite-like materials. 

Because Pd(II) salts, like Hgtll) salts, can effect electrophilic metallation of the indole ring at C3, it is also possible to carry out vinylation on indoles without 3-substituents. These reactions usually require the use of an equiv. of the Pd(ll) salt and also a Cu(If) or Ag(I) salt to effect reoxidation of the Pd. As in the standard Heck conditions, an EW substitution on the indole nitrogen is usually necessary. Entry 8 of Table 11.3 is an interesting example. The oxidative vinylation was achieved in 87% yield by using one equiv. of PdfOAcfj and one equiv. of chloranil as a co-oxidant. This example is also noteworthy in that the 4-broino substituent was unreactive under these conditions. Part B of Table 11.3 lists some other representative procedures. 

Figure 5.5. (a) Dependence of the NEMCA relaxation time constant x on 2FNc/I for C2H4 epoxidation on Agu and (b) for CO, C2H4 and CH3OH oxidation on Pt and Ag.12 Adapted from ref. 11 and reprinted from ref. 12 with permission from the American Chemical Society and from Elsevier Science respectively. 

Figure 6.4. Examples for the four types of global classical promotion behaviour. Work function increases with the x-axis. (a) Steady-state (low conversion) rates of ethylene oxide (EtO) and C02 production from a mixture of 20 torr of ethylene and 150 torr of 02 for various Cs predosed coverages on Ag(lll) at 563 K19 (b) Rate of water-gas shift reaction over Cu(l 11) as a function of sulphur coverage at 612 K, 26 Torr CO and 10 Torr H202° (c) Effect of sodium loading on NO reduction to N2 by C3H6 on Pd supported on YSZ21 at T=380°C (d) Effect of sodium loading on the rate of NO reduction by CO on Na-promoted 0.5 wt% Rh supported on Ti02(4% W03).22...

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