Carbon monoxide chemisorption, room

Later studies by Garner and his co-workers showed that the fraction of carbon monoxide or hydrogen reversibly chemisorbed at room temperature varied from oxide to oxide. Zinc oxide was shown to be a case where the adsorption of carbon monoxide at room temperature was completely reversible. The heat of adsorption, determined both calorimetrically (5) and isosterically 6), was in the range 12-20 kcal./mole. For several other oxides, however, notably chromia, Mn20s and Mn20s Cr20s, the heat of adsorption of carbon monoxide was higher and the chemisorption was... 

Palladium, etc. Carbon monoxide chemisorption has often been used to avoid the problem of hydrogen aZ>sorption into the metal but suitable conditions can be found to minimize absorption while still forming the monolayer. The subject is still being studied, e.g., the variation of solubility with metal dispersion. Small Pd particles of 7 nm mean size can also be completely oxidized to PdO at 538-580 K and therefore the oxygen uptake (on reduced and evacuated Pd at 580 K) measures total Pd atoms, Pdt. As the oxygen adsorption at room temperature measures surface atoms, Pdg, then the dispersion Pdg/Pdt is readily obtained from two gas uptake measurements. 

Carbon Monoxide Chemisorption. As for hydrogen adsorption, the number of active sites for CO adsorption is obtained by extrapolation at zero pressure of the isotherms recorded at room temperature (Table 5). The pretreatment of the catalysts was exactly identical. 

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. 

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. 

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... 

The chemisorption studies of Parris and Klier (43) using the Cu/ZnO catalyst have been mentioned earlier. Carbon monoxide was irreversibly bonded at room temperature to the surface of the binary catalysts that were also active in methanol synthesis however, this irreversible adsorbate could be desorbed as CO, which indicates that it was not a surface carbonate but rather a strongly bonded carbonyl-type CO. Infrared studies of this chemi-sorbate are lacking and it would be very desirable to determine the structure of this surface species. 

It had been found that removal of hydrogen from nickel at 350°C, instead of at room temperature, produced a profound difference in the properties of the nickel with respect to ethylene chemisorption (4). Therefore the chemisorption of carbon monoxide was repeated using nickel which had been degassed at 350°C. This produced a startling difference compared to the results shown in Fig. I now most of the carbon monoxide was chemisorbed in the linear structure (5). Similar experiments have not been made with palladium but it is reasonable to predict that the ratio of linear to bridged carbon monoxide would be increased by a more thorough removal of hydrogen from this metal. 

The catalytic oxidation of carbon monoxide on nickel oxides prepared at 200 and 250° has been studied at room temperature. First, chemisorption of the reactants (CO, O2) and of the product of the reaction... 

Removal of lattice oxygen from the surface of nickel oxide in vcumo at 250° or incorporation of gallium ions at the same temperature [Eq. (14)] causes the reduction of surface nickel ions into metal atoms. Nucleation of nickel crystallites leaves cationic vacancies in the surface layer of the oxide lattice. The existence of these metal crystallites was demonstrated by magnetic susceptibility measurements (33). Cationic vacancies should thus exist on the surface of all samples prepared in vacuo at 250°. However, since incorporation of lithium ions at 250° creates anionic vacancies, the probability of formation of vacancy pairs (anion and cation) increases and consequently, the number of free cationic vacancies should be low on the surface of lithiated nickel oxides. Carbon monoxide is liable to be adsorbed at room temperature on cationic vacancies and the differences in the chemisorption of this gas are related to the different number of isolated cationic vacancies on the surface of the different samples. 

In considering the nature of platinum-rhenium catalysts, we begin with a comparison of the chemisorption properties of alumina-supported rhenium, platinum, and platinum-rhenium catalysts (40). Data on the chemisorption of carbon monoxide and hydrogen at room temperature are given in Table 4.4 for catalysts with platinum and/or rhenium contents in the range of interest for reforming applications. 

CO chemisorption was carried out in the same pulse injection system used for oxygen chemisorption(9). The samples were reduced at 300 C for 30 minutes and cooled to room temperature before injecting pulses of carbon monoxide. A stoichiometry of 1 1 was taken for evaluating the dispersity from the amount of CO adsorbed. 

In conclusion, by means of the foregoing analysis the heterogeneous character of chemisorption of nitrogen by a clean tungsten surface at room temperature is established. While all crystal surfaces accessible for examination appear to chemisorb nitrogen, some surfaces react relatively slowly. Surfaces of the latter type include the 100 planes that are expected to be atomically smooth. Thus, low reactivity may be correlated with surface smoothness. Apparently this factor is significant for the chemisorption of oxygen and carbon monoxide as well. 

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