Carbon monoxide surface coverage

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Figure 2.2. Thermal desorption spectra of carbon monoxide, measured mass spectrometically at mass 28 (atomic units, a.u.), on a platinum (100) surface upon which potassium has been pre-adsorbed to a surface coverage of 0K.7 Reprinted with permission from Elsevier Science.

Carbon monoxide chemisorbs in atop sites on the clean Nl(lOO) surface for coverages up to 0=0.50 where a well-ordered c(2x2) lattice is observed. A further Increase In CO coverage to the... 

Before analyzing the results of these, or similar, thermochemical cycles, the assumptions which have been made must be critically examined. Since the cycles are tested for different surface coverages, it is assumed first that the Q-0 curves represent correctly, in all cases, the distribution of reactive sites—the energy spectrum—on the surface of the adsorbent. This point has been discussed in the preceding section (Section VII.A). It is assumed moreover that, for instance, the first doses of carbon monoxide (8 = 0) interact with oxygen species adsorbed on the most reactive surface sites (0 = 0). This assumption, which is certainly not acceptable in all cases, ought to be verified directly. This may be achieved in separate experiments by adsorbing limited amounts of the different reactants in the same se-... 

Fig. 27. Differential heats versus coverage for the successive adsorptions, at 30°C, of carbon monoxide (A), oxygen(B), and, again, carbon monoxide (C) on the surface of lithium-doped nickel oxide. Reprinted from (54) with permission J. Chim. Phys.

Winslow, P., and Bell, A. T. 1985. Studies of the surface coverage of unsupported ruthenium by carbon- and hydrogen-containing adspecies during carbon monoxide hydrogenation. J. Catal. 91 142-54. 

Figure 2.9 Thermal desorption of carbon monoxide from two rhodium surfaces in ultrahigh vacuum, as measured with the experimental set-up of Fig. 2,10. Each curve corresponds to a different surface coverage of CO. At low coverages CO desorbs in a single peak indicating that all CO molecules bind in a similar configuration to the surface. At higher coverages, an additional desorption peak appears, indicative of a different adsorption geometry (courtesy of M.J.P. Hopstaken and W.E. van Gennip [141).

Carbon Monoxide The presence of CO in a H2-rich fuel has a significant effect on anode performance because CO affects Pt electrodes catalysts. The poisoning is reported to arise from the dual site replacement of one H2 molecule by two CO molecules on the R surface (40, 41). According to this model, the anodic oxidation current at a fixed overpotential, with (ico) and without (in2) CO present, is given as a function of CO coverage (0co) by Equation (5-11) ... 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

However, the analysis of the data was carried out in such a way as to cast doubt on the validity of these conclusions (Vayenas54). Okamoto, Kawamura and Kudo48 went on to use the e.m.f. interpretation from the above work49 to further investigate the mechanism of CO oxidation over platinum by using the cell as a probe of the surface coverage of carbon monoxide. 

Potentiometric techniques have been used to study autonomous reaction rate oscillations over catalysts and carbon monoxide oxidation on platinum has received a considerable amount of attention43,48,58 Possible explanations for reaction rate oscillations over platinum for carbon monoxide oxidation include, (i) strong dependence of activation energy or heat of adsorption on coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to adsorption or desorption of inert species, (iv) periodic oxidation or reduction of the surface. The work of Sales, Turner and Maple has indicated that the most... 

It can be concluded from the calorimetric data that the interaction between adsorbed oxygen and carbon monoxide is a multiple process. For low surface coverages—i.e., on the most active sites—CO " ions... 

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