Carbon monoxide dissociative chemisorption

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

Park S, Tong YT, Wieckowski A, Weaver MJ. 2002a. Infrared spectral comparison of electrochemical carbon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir 18 3233-3240. 

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... 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42), a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NjO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrogen adatoms [with their characteristic N(ls) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. 

Figure 2.16. Calculated dissociative nitrogen ( ), carbon monoxide ( ), and oxygen ( ) chemisorption energies over different 3d transition metals plotted as a function of the center of the transition metal rf-bands. A more negative adsorption energy indicates a stronger adsorbate-metal bond. Reproduced from [32].

To exhibit such an active and selective catalytic effect, the catalyst must be a fairly good hydrogenation catalyst that is able to activate molecular hydrogen. It must also activate carbon monoxide without dissociating it. A nondissociative chemisorption permits the hydrogenation of carbon monoxide to occur on both oxygen and carbon. Considering the formation of surface methoxide in the second mechanism [Eq. (3.43)], a further requirement is that the catalyst not form a too stable metal methoxide. 

Non-dissociative, dissociative. If a molecule is adsorbed without fragmentation, the adsorption process is non-dissociative. Adsorption of carbon monoxide is frequently of this type. If a molecule is adsorbed with dissociation into two or more fragments both or all of which are bound to the surface of the adsorbent, the process is dissociative. Chemisorption of hydrogen is commonly of this type. 

Oxidations of the two molecules appear to proceed almost but perhaps not quite wholly independently of each other the dissociative chemisorption of hydrogen is not much affected by the carbon monoxide, which, in view of what was said in Chapter 5, is somewhat surprising, as there it seemed that both interacted with lowly coordinated gold atoms. The dependence of kinetic parameters on the hydrogen concentration has not been... 

The facile dissociative adsorption of CO on transition metals at low temperatures has been demonstrated by XPS or pulse techniques for Ti, V, Cr and Mn (96] and at elevated temperatures for Ni, Co and Ku with Fc as the borderline case [96, 97J. A more detailed study by Somorjai for Pt (111) surfaces showed that dissociation occurs at the step sites only, and once these are filled, carbon monoxide is absorbed moiccularly [98]. All of the XPS studies on chemisorption on iron, except at very low temperatures, are indicative of dissociative surpikm being the first step in Fischer-Tropsdi reactions (99 101). However, photoelectron spectroscopy has so far not delineated a logical sequence of precursors and intermediates 1102. ... 

The electrocatalytic oxidation of ethanol has been investigated for many years on different platinum-based electrodes, including Pt/X alloys (with X = Ru, Sn, Mo, etc ), and dispersed nanocatalysts. Pme platinum smooth electrodes are rapidly poisoned by some strongly adsorbed intermediates, such as carbon monoxide, resulting from the dissociative chemisorption of the molecule, as shown by the first experiments in infrared reflectance spectroscopy (EMIRS). Both kinds of adsorbed CO, either linearly-bonded or bridge-bonded to the platinum surface, are observed. Besides, oth-... 

Adsorption of carbon monoxide (Eq.(5a)) is assumed to occur in parallel with the dissociative chemisorption of hydrogen (Eq. (5b)) (M is a catalyst site), hydrogen oxidation current is assumed to be generated by process (5c), and adsorbed carbon monoxide would oxidize electrochemically to CO2 according to Eq. (5d), reacting with a water-derived adsorbed oxygen species. 

Carbon monoxide is a key molecule in the electro-oxidation of Cl compounds and of many alcohols, since it is always produced by the dissociative chemisorption of the molecule, and since it may block the active catalytic sites. Therefore, its electrooxidation on platinum-based metals dispersed in an electron-conducting polymer, such as PAni, was investigated for a long time in our laboratory [8,28,34]. 

There is little doubt that both the carbon monoxide and the hydrogen molecule undergo chemisorption on the catalyst surface resulting in bond dissociation to give carbide, oxo, and (monoatomic) hydrido species (Scheme 1). 

An alternative route to surface-methylene groups involves stepwise reduction of carbon monoxide rather than dissociative chemisorption. Scheme 4 describes a number of plausible intermediates, all of which have been seen as isolable, well-characterized molecular compounds [11] carbonyl (a), formyl (b), hydroxycarbene (c), hydroxymethyl (d), and methylene (e). They are summarized in Ref. [8-12]. 

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