Carbon monoxide coverage

B. D. Chandler and L. H. Rgnolet, DRIFTS studies of carbon monoxide coverage on highly dispersed bimetalhc R-Cu and R—Au catalysts, Catal. Today 65,39—50 (2001). 

To understand the behaviour of preferential oxidation catalysts, the operating principle requires explanation. The common feature of these catalysts is the preferential adsorption of carbon monoxide at low temperature. When the reaction temperature increases, the carbon monoxide coverage decreases and reaction with oxygen (when it is present in the gas phase) takes place. At even higher temperatures and lower coverage of active sites with carbon monoxide, hydrogen oxidation occurs in parallel. Thus, an operating window exists for preferential oxidation catalysts. 

Fig. 28. Vibrational spectra of the saturation carbon monoxide coverage chemisorbed on Pt(lll) at 300 K as a function of preadsorbed potassium coverage...

For example, by examining the initial and differential heats of adsorption measured on Pt/Al203 powders calcined at different temperatures, Uner and Uner [50] concluded that CO adsorption processes is not structure-sensitive. CO heats of adsorption values obtained by the authors are plotted against carbon monoxide coverage in Fig. 12.4. The heat of adsorption data for all catalysts fell on the same curve. 

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.

This chapter is concerned entirely with the insertion of carbon monoxide into transition metal-carbon cr-bonds. Sulfur dioxide insertion 154, 239), also common among transition metal-carbon complexes, will be treated in a complementary review, which is to appear later. Subject to the restrictions given at the beginning of Section VI, an attempt has been made at a complete literature coverage of the insertion of CO. Particular emphasis focuses on recent results, especially those of a kinetic and stereochemical nature. 

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... 

Carbon Monoxide Oxidation on Platinum Coverage Dependence of the Product Internal Energy... 

Figure 7. Total internal reflection sum frequency generation (TIR-SFG) vibrational spectroscopy of high-pressure room temperature adsorption of carbon monoxide on PVP-protected Pt cube monolayers and calcined (373 K, 3h) monolayers [27], The infrared spectra demonstrate CO is adsorbed at atop sites, but is considerably red-shifted on the PVP-protected Pt cubes. After calcination, the atop frequency blueshifts to 2085 cm in good agreement with CO adsorption on Pt(l 0 0) at high coverages [28], (Reprinted from Ref [27], 2006, with permission from American Chemical Society.)...

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 9. Plot of the CO% production rate (divided by the coverages of oxygen and carbon monoxide) as a function of time for the titration of O by CO on Ir (111).

Carbon Monoxide. Carbon monoxide, a fuel in high-temperature cells (MCFC and SOFC), is preferentially absorbed on noble metal catalysts that are used in low-temperature cells (PAFC and PEFC) in proportion to the hydrogen-to-CO partial pressure ratio. A particular level of carbon monoxide yields a stable performance loss. The coverage percentage is a function of temperature, and that is the sole difference between PEFC and PAFC. PEFC cell limits are < 50 ppm into the anode major U.S. PAFC manufacturers set tolerant limits as < 1.0% into the anode MCFC cell limits for CO and H20 shift to H2 and C02 in the cell as the H2 is consumed by the cell reaction due to a favorable temperature level and catalyst. 

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).

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