Carbon monoxide adsorptive mechanism

Gilman S. 1963. The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. I. Carbon monoxide adsorption and desorption and simultaneous oxidation of the platinum surface at constant potential. J Phys Chem 67 1989-1905. 

Numerous studies on carbon monoxide adsorption on Pt(100) hex structure have shown that this results in a reconstructed (1 x 1) overlayer of CO adsorbed on Pt(100). The deconstruction process follows a mechanism of migration, cluster formation, and surface conversion from hex (106 kJ mol ) to (1 x 1) layers (156 kJ mol-1) [43]. 

In general, the mechanism is well known and has been described by the classical Langmuir-Hinshelwood (LH) model [7-10], based on adsorption of oxygen in competition with adsorbed carbon monoxide. This mechanism was presented by Engel et al. [9] ... 

The gases that have been used most often are hydrogen, carbon monoxide, and oxygen. Hydrogen is by far the most useful, and it has the best established adsorption mechanism. It dissociates at room temperature on most clean metal surfaces of... 

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

The change in the nature of the adsorption with increasing coverage (dissociative followed by associative) has been explained by a statistical consideration of the reaction mechanism shown above120). Associative adsorption is expected to occur at vacant sites for which all adjacent olefin binding sites are occupied by earlier dissociation products (or carbon monoxide, as shown by Fig. 6b), because dissociative adsorption (formation of vinyl and hydride species, followed by hydride migration to another alkene) requires two adjacent vacant sites. 

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or C02. Various multi-step reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to-date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell [50] has proposed the following surface reaction mechanism ... 

Key (46) and Strickland-Constable (47) also support mechanism B for the carbon-carbon dioxide reaction. Strickland-Constable concludes from earlier measurements (46) that the rate of adsorption of carbon monoxide on carbon is too low to account for the retardation. 

To calculate the number of surface ions Cu+, the adsorption of krypton was determined and, assuming that the (001, Oil, 111) planes of the oxide are exposed in equal amounts, the average number of Cu+ sites can be estimated. The adsorption of oxygen exceeded the calculated number of these sites and indicated a penetration of the surface by oxygen, a suggestion which is compatible with the observation that carbon monoxide does not react with all the oxygen previously adsorbed producing carbon dioxide. The mechanism is illustrated pictorially in Fig. 18. 

The revival of interest in Fischer-Tropsch chemistry in the 1970s resulted in new observations that eventually led to the formulation of a modified carbide mechanism, the most widely accepted mechanism at present.202-204,206,214 Most experimental evidence indicates that carbon-carbon bonds are formed through the interaction of oxygen-free, hydrogen-deficient carbon species.206 Ample evidence shows that carbon monoxide undergoes dissociative adsorption on certain metals to form carbon and adsorbed oxygen ... 

From the calorimetric study of the first sequence of adsorptions, it is possible to propose a mechanism (I) for the catalytic oxidation of carbon monoxide on NiO(250). 

The same sequence of adsorptions has been studied on NiO(200) (8, 20). The interaction between oxygen preadsorbed on NiO(200) and carbon monoxide yields only adsorbed carbon dioxide. Therefore, on NiO(200), gaseous carbon dioxide is produced during the catalytic reaction through Mechanism I (8, 20), whereas on NiO(250) two reaction paths are probable (Mechanisms I and II). These results show clearly... 

Reaction Mechanism. The rate of production of heat as a function of time gives indications of the velocity of the process taking place on the catalyst surface (Figure 4B). For instance, it has been shown (20) that, on NiO(200), the adsorption of oxygen and the formation of CO.r(adfo ions are fast processes compared with the adsorption of carbon monoxide or the reaction between CO and CCV ds)- From calorimetric results and a kinetic study of the reaction, it has been concluded (8) that the decomposition of COa uds) ions by adsorbed carbon monoxide to yield carbon dioxide is the slowest step of the reaction mechanism on NiO(200) (Mechanism I). 

Although the basic principles behind this Intact ejection mechanism can be Illustrated with carbon monoxide, the extrapolation to large bloorganlc molecules Is not necessarily obvious. Calculations have been performed for a series of organic molecules adsorbed on a Ni(OOl) surface to understand the mechanisms of molecular ejection (8-12). The first molecules which have more than just a few atoms examined are benzene which it-bonds on a metal surface and pyridine which can either ir-bond or o-bond on a metal surface. Larger structures, whose sizes approach the diameter of bloorganlc molecules, are naphthalene, biphenyl and coronene whose adsorption structures are unknown. All the molecules except pyridine are assumed to ir-bond on the surface. 

The principal difficulty with metal only mechanisms is to find a plausible way of activating the oxygen molecule. The problem is discussed in Section 5.2.2 formation of the Oj ion, in which the 0-0 bond is considerably weakened (see Table 5.2), appears possible on small particles,130,131 and the suggestion that the adsorption of oxygen and of carbon monoxide is mutually supportive opens a possible route for a metal-only mechanism. 

Kinetic evidence for synergic adsorption of carbon monoxide and water on the low-temperature shift catalyst Cu/ZnO/Fe203 was obtained by van Herwijnen and deJong (113), and IR spectra of surface formate were detected on several oxide catalysts, including CuO/MgO, at temperatures as low as 20 JC and pressures of 20 Torr, as reported by Davydov et al. (104). Decomposition of the surface formate to C02 and H2 occurred at 100-150°C over the Cu/MgO catalyst and at 250 300°C over the MgO catalyst, and the promotion effect of copper was attributed to the formation and decomposition of a labile surface formate (HCOO)2Cu. Ueno et al. (117) have shown earlier that surface formates are formed on zinc oxide, from CO and H20 as well as from C02 and H2, and hence an associative mechanism of the shift and reverse-shift reaction, involving formate intermediate, is believed to operate on many oxide catalysts. 

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