CH4 Oxidation on Pt

Figure 4.28. Electrophobic behaviour Effect of catalyst work function on the activation energy E and catalytic rate enhancement ratio r/r0 for C2H4 oxidation on Pt p02 4.8 kPa, Pc2H4 0.4 kPa (a) and CH4 oxidation on Pt p02 =2.0 kPa, Pch4 =2.0 kPa (b)."4 Reprinted with permission from Elsevier Science.

It has been known since the early days of electrochemical promotion that upon varying Uwr and thus , not only the catalytic rates, r, are changing in a frequently dramatic manner, but also the activation energy of the catalytic reaction is also significantly affected. An example was already presented in Fig. 4.28 which shows that both C2H4 and CH4 oxidation on Pt/YSZ conform to equation (4.50) with an values of -1 and -3, respectively. 

Figure 8.22. Effect of step changes in applied positive and negative currents on Uwr and r during CH4 oxidation on Pt/YSZ29 at two different volumetric flowrate Fv showing that x is influenced by I but not by Fv.29 Reprinted with permission from Academic Press.

D.A. Hickman and L.D. Schmidt. Steps in CH4 Oxidation on Pt and Rh Surfaces High-Temperature Reactor Simulation. AIChE7., 39 1164-1177,1993. 

Hickman, D.A. and Schmidt, L.D. Reactors, kinetics and catalysis—Steps in CH4 oxidation on Pt and Rh surfaces High-temperature reactor simulations. AIChE Journal, 1993, 39 (7), 1164. 

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).

The CH4 oxidation on Pd31 exhibits a very pronounced NEMCA behavior at much lower temperatures (380-440°C) compared with those on Pt catalysts (650-750°C). In this temperature range the reaction exhibits inverted volcano behavior.31 For positive overpotentials the p values are as high as 89, with A values up to 105.31 Negative overpotentials also enhance the rate31 with p values up to 8. 

Corro, G., Eierro, J.LG. and Vazquez, C. (2005) Strong improvement on CH4 oxidation over Pt/Y-Al20j catalysts. Catal. Commun., 6, 287-295. 

Song X, Williams WR, Schmidt LD, Aris R Ignition and extinction of homogeneous-heterogeneous combustion CH4 and C3H8 oxidation on Pt, Proc Combust Inst 23 1129-1137, 1990. 

On a pure Pf/C catalyst, Rousseau et showed that the electro-oxidation of ethanol at the anode of a DEFC working at 80°C mainly led to the formation of acetaldehyde, acetic acid and carbon dioxide, with chemical yields of 47.5%, 32.5% and 20.0%, respectively. By comparing the mass yield and the faradic yields, they concluded that no other products were formed in a significant amount. This result confirms that Pt is able to break the C-C bond to some extent In situ infrared measurements on ethanol adsorption and electro-oxidation at platinum electrodes have clearly shown that the adsorbed CO species are formed from 0.3 V vs RHE at the platinum surface moreover Iwasita and Pastor found some traces of CH4 at potentials lower than 0.4 V vs RHE. Previous studies showed that the initial steps of ethanol adsorption and oxidation on Pt can follow two different modes ... 

Figure 8.23. Effect of catalyst potential and work function on the rate of CH4 oxidation to C02 on Pt for a low (1 1) CH4 to 02 feed ratio. Maximum methane conversion is 4%. pCH4= Po2as2 kPa, T, °C, r0, mol O/s.29 Reprinted with permission from Academic Press.

Figure 2. Effect of flow velocity on conversion and selectivity for CH4 oxidation over 10 layers of 40 mesh Pt-10% Rh gauze. The feed contained 16% CH4 in air and the front layer of gauze was maintained at 1227 5°C.

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

Thus, the deactivation observed on the used catalyst may be explained as follows. First, Pt is sintered and Rh is segregated on the surface of noble metal particles by the thermal effect [8, 9], and so, on only thermally deaetivated catalysts, the reactions due to Rh sueh as NO reduction are not slowed, although methane oxidation due to Pt is considerably slowed [3, 4]. However, when Pb is adsorbed on Rh, the catalytic activity of Rh is suppressed and the reaction rate of NO reduction is decreased to the same order as the rate of CH4 oxidation. Further, the surfaee of the wash eoat layer is covered with compounds consisting of Ca, P, Zn and Fe, and the effeetive surface area of the catalyst which the exhaust gas ean reach decreases, causing a considerable decrease in NO conversion and the disappearance of the window, accompanied wdth decreases in CH4 conversion and CO conversion on the rich side. 

Figure 7. Temperature-programmed oxidation of carbon deposited on Pt/Ce-ZrO2 after reaction at SOO C J CO2/ CH4 =1.2 feed ration, 14 atm...

In partial oxidation of CH4 >90% syngas is routinely obtained on Rh while for higher alkanes up to 70% olefins are produced on Pt. Olefins are a nonequilibrium product, and carbon is predicted under all conditions while none is observed. 

---