Carbon monoxide, chemisorbed reaction

A reaction mechanism is suggested which involves dissociative chemisorption of hydrogen and water in competition on one type of active sites and chemisorption of carbon dioxide on the other type. Chemisorption of carbon dioxide is so strong that it prevents chemisorption of carbon monoxide. Chemisorbed carbon dioxide and hydrogen are in equilibrium on the surface. Reverse shift takes place by dissociation of the reaction product into carbon monoxide and a chemisorbed hydroxyl-species. The shift reaction is taking place by reaction between carbon monoxide from the gas phase and hydroxyl-species on the surface. Methanol is formed by step-wise hydrogenation of chemisorbed carbon dioxide. 

That carbon monoxide could be oxidised in a facile reaction at cryogenic temperature (100 K) was first established in 1987 by XPS at an aluminium surface.21 The participation of reactive oxygen transients O 1 (s) was central to the mechanism proposed, whereas the chemisorbed oxide O2 state present at 295 K was unreactive. This provided a further impetus for the transient concept that was suggested for the mechanism of the oxidation of ammonia at a magnesium surface (see Chapter 2). Of particular relevance, and of crucial significance, was Ertl s observation by STM in 1992 that oxygen chemisorption at Al(lll) resulted in kinetically hot adatoms (Figures 4.1 and 4.7). 

Mechanisms A and B both state that carbon monoxide retards the gasification of carbon by carbon dioxide by decreasing the fraction of the surface which is covered by oxygen atoms under steady state conditions. In mechanism A, fli is decreased by the chemisorption of carbon monoxide by a fraction of the active sites. In mechanism B, 61 is decreased by the reaction of a portion of the chemisorbed oxygen with gaseous carbon monoxide to produce gaseous carbon dioxide. Reif (57) shows that only one of these reactions can control retardation at one time. 

In contrast to carbon dioxide, carbon monoxide is always chemisorbed with an electron transfer from CO to the solid. Following this concept, we get the surface reactions... 

At first sight, scheme (371) does not agree with the results of our adsorption experiments these experiments showed that activated charcoal does not chemisorb CO at 100°C. It should, however, be taken into consideration that the surface of charcoal subjected to activation or even simply after storage in contact with air is covered with chemisorbed oxygen. The studies of the reactions of carbon with C02 and steam (see Section XX) have demonstrated that oxygen chemisorbed on carbon is indistinguishable from chemisorbed carbon monoxide. So it may be reckoned that activated charcoal is already covered with carbon monoxide before the contact with this gas. 

THE RELATIONSHIP BETWEEN infrared spectra of chemisorbed carbon monoxide and the catalytic activity of metais for the methanization reaction is discussed in conjunction with experiments dealing with the effect of dissolved hydrogen on the catalytic activity of nickel. The purpose of this discussion is to illustrate the type of reasoning involved in seeking a relationship between spectra of chemisorbed molecules and catalytic activity. The underlying concepts of this relationship are extended to include recent advances made in studies of the effect of the semiconductor properties of the carrier on the activity of sup -ported metal catalysts. 

The pairing of copper with platinum and nickel with palladium was reminiscent of the work of Taylor and McKinney (3) who pointed out that copper and platinum were relatively poor catalysts for the hydrogenation of carbon monoxide to methane, while nickel and palladium were active catalysts for this reaction. Although this pattern could be coincidental, it is more reasonable and productive to assume a relationship between the spectroscopic results and the catalytic activities and to conclude that metals which chemisorb carbon monoxide in... 

The application of IR spectroscopy to catalysis and surface chemistry was later developed in the fifties by Eischens and coworkers at Texaco laboratories (Beacon, New York) in the USA [7] and, almost simultaneously, by Sheppard and Yates at Cambridge University in the UK [8]. Mapes and Eischens published the spectra of ammonia chemisorbed on a silica-alumina cracking catalyst in 1954 [6], showing the presence of Lewis acid sites and also the likely presence of Br0nsted acid sites. Eischens, Francis and Pliskin published the IR spectra of carbon monoxide adsorbed on nickel and its oxide in 1956 [9]. Later they presented the results of an IR study of the catalyzed oxidation of CO on nickel at the First International Congress on Catalysis, held in Philadelphia in 1956 [10]. Eischens and Pliskin also published a quite extensive review on the subject of Infrared spectra of adsorbed molecules in Advances in Catalysis in 1958, where data on hydrocarbons, CO, ammonia and water adsorbed on metals, oxides and minerals were reviewed [11]. These papers evidence clearly the two tendencies observed in subsequent spectroscopic research in the field of catalysis. They are the use of probes to test the surface chemistry of solids and the use of spectroscopy to reveal the mechanism of the surface reactions. They used an in situ cell where the catalyst sample was... 

This chapter focuses on metallic NMR behavior, i.e., on properties that are governed by electrons and holes in extended states with small energy differences, such as typically found in metals. Such NMR properties obviously allow us to determine whether the supported metal particles are indeed metallic and not simply small molecules built from atoms that would form a metal in the bulk. In addition, from NMR of adsorbed molecules, some adsorbates become a piece of the metal, (which tells us something about the nature of the chemisorption bond), as frequently happens with chemisorbed carbon monoxide and sometimes with hydrogen. This aspect of the NMR of these adsorbates is discussed later, but work related to their dynamics and reactions is only partially covered other adsorbates are not treated at all. 

Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NO ) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation slate between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. 

Following earlier work in which the intermediate in the formation of methane from carbon monoxide and hydrogen was found to be carbon, McCarty and Wise carried out a thorough study of the system. Four types of carbon were found to be formed from carbon monoxide on nickel at 550 50K. Chemisorbed carbon atoms reacted readily with hydrogen as did the initial layers of nickel carbide. Further deposits of the carbide, amorphous carbon, and crystalline elemental carbon were much less reactive and the kinetics of the reaction should be described by the established rate laws. Conversion of the more active to the less active forms of carbon occurred above approximately 600 K. 

Carbon monoxide becomes a versatile reagent when it is activated by transition metals, giving a large number of both homogeneous and heterogeneous reactions. In the former, reactions occur at metal-coordinated CO in the latter, chemisorbed CO is usually responsible for the observed catalytic reaction. 

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