Chemisorption of carbon monoxide

Figure 2.14. The molecular orbitals of gas phase carbon monoxide, (a) Energy diagram indicating how the molecular orbitals arise from the combination of atomic orbitals of carbon (C) and oxygen (O). Conventional arrows are used to indicate the spin orientations of electrons in the occupied orbitals. Asterisks denote antibonding molecular orbitals, (b) Spatial distributions of key orbitals involved in the chemisorption of carbon monoxide. Barring indicates empty orbitals.5 (c) Electronic configurations of CO and NO in vacuum as compared to the density of states of a Pt(lll) cluster.11 Reprinted from ref. 11 with permission from Elsevier Science.

Weinberg and co-workers study in 198220 of the chemisorption of carbon monoxide on Ru(0001) by LEED exemplifies how lateral interactions determine... 

Since the early work of Langmuir (1), the chemisorption of carbon monoxide on platinum surfaces has been the subject of numerous investigations. Besides its scientific interest, an understanding of CO chemisorption on Pt is of considerable practical importance for example, the catalytic reaction of CO over noble metals (such as Pt) is an essential part of automobile emission control. 

Surface groups consisting of atoms foreign to the structure can be formed on a great variety of substances. It is not intended to discuss all possibilities this would surpass the scope of an article limited in volume. Furthermore, research in this field has but begun surface compounds have been studied only on a selected group of substances. Most of the investigated substances, however, are very important from an industrial viewpoint. Therefore, in this article the chemistry of surface compounds will be described for a few characteristic and well-known examples. Borderline cases, such as the chemisorption of carbon monoxide on metals, will not be considered. 

B.E.T. method using nitrogen, since nitrogen is chemisorbed at — 196°C. The hydrogen adsorption at this temperature measures the surface more accurately and is in close agreement with the chemisorption of carbon monoxide at both liquid nitrogen and room temperature and with the van der Waal s adsorption of krypton. 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

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. 

Chemisorption of Carbon Monoxide. Chemisorption of carbon monoxide on NiO(250) does not change the electrical conductivity of the sample. The same result was obtained for NiO(200) (23). The curve of differential heats of adsorption of carbon monoxide on NiO(250) presents many similarities with the curve recorded in the case of NiO(200) (20). However, a few differences are noted. The heat of adsorption of the first dose (0.08 cc. per gram) of carbon monoxide on NiO(200) is high (42 kcal. per mole) (Table II). The adsorption of the next dose on the same oxide releases only 29 kcal. per mole. The initial high value of the heat adsorption was explained by the interaction of CO with excess surface oxygen (Table I), giving C02(ainitial heat of adsorption amounts to 29 kcal. per mole (Table II). It seems, therefore, that the surface excess oxygen... 

Kummer and Emmett 311) studied the chemisorption of carbon monoxide on an iron ammonia catalyst by the same method and found a partial mixing of the desorbed gas, as if the surface consisted of a heterogeneous complex of homogeneous parts. Similar results were obtained by Eischens 312). 

Chemisorption of carbon monoxide on massive gold surfaces123... 

Recognition of the easier chemisorption of carbon monoxide on atoms of low CN as indicated by its preference for rough films formed at low temperature led to a study of the stepped Au(332) surface,63 although even here desorption was complete when temperature was raised to ambient. The use of isotopically labelled molecules showed that the IR band at 2120 cm-1 contained four components, although TPD revealed only two. The band at 2120 cm-1 red shifted, and the surface potential increased linearly, with increasing coverage. 

We may conclude that the chemisorption of carbon monoxide on gold surfaces is generally somewhat weak it cannot for example lift the reconstruction of the Au(110)(l x 2) surface to restore the normal (1 x 1) phase.61 It prefers atoms of low CN and it desorbs below room temperature. Nevertheless, under the appropriate conditions considerable information can be obtained about its adsorbed state. 

Figure 5.6 Modes of chemisorption of carbon monoxide. (A) Combination of donation from the 5a orbital to the metal s vacant d-orbital + back-donation from a filled d-orbital to the 2ty antibonding orbital.

Table 5.3 Chemisorption of carbon monoxide on supported gold catalysts IR bands assigned to linear CO on Au° particles (1) and associated bands (2).

There is no specific provision for the chemisorption of carbon monoxide on the gold, although this could occur before it reacts with an adjacent hydroxyl ion. The way in which hydrogen ions recombine to form molecules also remains unclear. 

Extensive information concerning distribution of the promoters, penetration below the promoters of adsorbed atoms, and chemical behavior of the promoters was obtained by Brunauer and Emmett (25,26). They used chemisorption of carbon monoxide, carbon dioxide, nitrogen, hydrogen, and oxygen, individually and successively measuring the influence of one type of chemisorption upon another type. It was concluded that CO and C02 were chemisorbed as molecules, H2 and N2 as atoms, and 02 probably as ions. C02 is chemisorbed on the alkali molecules located at the surface, whereas H2, N 2, CO, and 02 are chemisorbed on the iron atoms. From the effect of presorbed CO upon the chemisorption of C02 and vice versa it was concluded that the promoters are concentrated on the surface and are distributed so effectively that most surface iron atoms are near to a promoter atom. Strong indication... 

It must be emphasized that nondissociative chemisorption of carbon monoxide, which appears to be a necessary condition for its hydrogenation to methanol, is not a sufficient prerequisite. For example, nickel (29) or gold... 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

Several observations have already indicated that the amorphous copper solute induces irreversible chemisorption of carbon monoxide, and it therefore behooves us to correlate the methanol yields with the surface concentration of irreversibly chemisorbed CO. The best understanding of the synthesis pattern is provided with the help of the representation in Fig. 17. The turnover rate, defined as the number of CO molecules converted to methanol in 1 sec on one site titratable by irreversibly bound carbon monoxide, depends smoothly on the concentration of these sites for each zinc... 

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