CO adsorption/desorption

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

The effect of oxidizing atmospheres on the reduction of NO over rhodium surfaces has been investigated by kinetic and IR characterization studies with NO + CO + 02 mixtures on Rh(lll) [63], Similar kinetics was observed in the absence of oxygen in the gas phase, and the same adsorbed species were detected on the surface as well. This result contrasts with that from the molecular beam work [44], where 02 inhibits the reaction, perhaps because of the different relative adsorption probabilities of the three gas-phase species in the two types of experiments. On the other hand, it was also determined that the consumption of 02 is rate limited by the NO + CO adsorption-desorption... 

Results were modelled on the basis of CO adsorption/desorption equilibrium with oxygen adsorption as the rate determining step and an irreversible Langmuir-Hinshelwood surface reaction. It was assumed that adsorption and... 

Carbon Monoxide-Hydrogen. - The reactions between CO and H2have been reviewed in the present series and previously by Vannice, who noted the paucity of studies with well defined alloys, although in both reviews it was possible to include CO adsorption/desorption and i.r. measurements involving alloys and bimetallic catalysts. The intrinsic importance of the catalyzed reaction makes it likely that much more work with bimetallic catalysts will be reported as indicated by the latest literature. The supported metals differ considerably in terms of product formation (which is also dependent on reaction conditions), methane (Ni), olefins (Ru, Co, and Ir), > C4 products (Co, Fe), and paraffin waxes (Ru), and it is tempting to suppose that bimetallics would combine desirable properties. 

An investigation of the mechanism of the charcoal catalysed reaction (10.17) reveals that the reaction proceeds via a strongly absorbed intermediate, identified as chloromethyl chloroformate, CHjC10C(0)Cl [1765]. This compound is found to form rapidly above 100 C in co-adsorption/desorption experiments, and decomposes rapidly above 170 C without significant desorption [1765], to give the final products, CH Clj and COj. The... 

The cyclotrimerization of acetylene to benzene has been studied by Rucker et al. (8J) over Pd(lll), (100), and (110) at pressure near 1 atm. The (110) surface was four-fold less active than Pd(lll) or (100), which contrasts with their relative selectivities during TDS under UHV conditions. The activity at high pressures was correlated with the fraction of the various surfaces that exposed clean Pd atoms, as probed by postreaction CO adsorption-desorption. In all cases, most of the surface was covered with a carbonaceous residue. The authors stated that the reaction rate is first-order in acetylene pressure for all three surfaces. The extensive data on the Pd(lll) surface clearly indicate first-order kinetics in that case. However, the limited data presented for Pd(110) seem (to the present author) to be better fitted by an order of —2.5, which is closer to the value of three suggested by the overall stoichiometry. 

As one extends this analysis to the chain growth probabilities and methane selectivities, an immediate conclusion reached is that, one has to include the effect of the alkali promoters on the CO adsorption kinetics and on olefin reincorporation steps as well. Since, no direct data on CO adsorption/desorption kinetics is available at the moment, we will leave this as a postulate. 

Ao Z, Li S, Jiang Q (2010) Correlation of the applied electrical field and CO adsorption/desorption behavior on Al-doped graphene. Solid State Commun 150 680-683... 

Previously PIj x = 1,2,3) on Si02 had shown different CO adsorption/desorption properties in TPD measurements already [14], however no CO oxidation had been studied at that point. 

Figure 15.5 Schematic of CHNS analyser. 1. Flushing gas (He), 2. Combustion gas (O ), 3. Gas flow meter, 4. Moisture trap (P2O5), 5. Carousel, 6. Ash finger, 7. Combustion tube, 8. Reduction tube, 9. Furnace, 10. SO adsorption / desorption tube, 11. H O adsorption / desorption tube, 12. CO adsorption / desorption tube, 13. Reference gas (He), 14 Thermal conductivity detector.

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

Besides the effect of the presence of alkali on CO adsorption, there is also a stabilizing effect of adsorbed CO on the adsorption state of alkali. Within the high alkali coverage range the number of CO molecules adsorbed on promoted surface sites becomes practically equal to the number of alkali metal species and their properties are not dependent on the CO coverage. In this region CO adsorption causes also stabilization of the adsorbed alkali, as indicated by the observed high temperature shift of the onset of alkali desorption. 

At low temperatures the reaction is negatively affected by the lack of oxygen on the surface, while at higher temperatures the adsorption/desorption equilibrium of CO shifts towards the gas phase side, resulting in low coverages of CO. As discussed in Chapter 2, this type of non-Arrhenius-like behavior with temperature is generally the case for catalytic reactions. 

Figure 12.5 CO stripping voltammogram with a CO- tee 0.1 M H2SO4 electrolyte. Compare the data in Fig. 12.4 the CO oxidation region begins at V = 0.43 V. After CO stripping, hydrogen adsorption/desorption peaks and the beginning of the Pt oxidation range are shown.

McEwen JS, Eichler A. 2007. Phase diagram and adsorption-desorption kinetics of CO on Ru(OOOl) from first principles. J Chem Phys 126 094701. 

Considering the above discussion, the currents in Fig. 15.5 are normalized to the surface areas estimated from CO stripping. Similar to massive electrodes, CVs for Pt nanoparticles show three characteristic potential regions (all potentials vs. RHE) (i) an Hupd region 0.05 Y < E < 0.40 V, followed by (ii) the so-called doublelayer region 0.40V < E< 0.60V and (iii) for E > 0.7 V, the oxygen adsorption/ desorption region. 

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

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