Carbon monoxide structure sensitivity

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

It is evident from these data that the transition frequency fluctuation correlation function of samples that have the same probe molecule (azide) embedded into two different proteins (hemoglobin and carbonic anhydrase), or of the sample with different probe molecules (azide, carbon monoxide) embedded to one protein (hemoglobin), all differ considerably. This, we believe, is a consequence of sensitivity of this spectroscopic technique to the local structure, which is different in each case. This result must be contrasted with electronic dephasing, where it was found that the energy gap fluctuation correlation function reflects the response of the bulk solvent and is essentially independent of the chromophore used as a probe (81). 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

Carbon monoxide. Carbon monoxide is one of the most commonly used probe molecules in the study of the chemical properties of metal surfaces. CO represents a step in the direction of complexity compared to atomic adsorbates and diatomic molecules. On one hand, the bonding involves molecular orbitals and it is sensitive to the detailed electronic structure of the metal surface. This allows one to use the CO bonding properties as a probe of changes in surface electronic structure. Yet at the same time, in many cases CO retains aspects of the simplicity that atomic adsorbates have. 

It is very sensitive to the action of heat on warming to about 200° C. under ordinary pressure it decomposes completely to iron and carbon monoxide. This decomposition, however, takes place at a much lower temperature in the presence of substances having a porous structure. Thus in the presence of iron, about 6o° C. is sufficient and in the presence of activated carbon, magnesium oxide, etc., it takes place even at ordinary temperatures. ... 

Structural sensitivity of the catalytic reactions is one of the most important problems in heterogeneous catalysis [1,2]. It has been rather thoroughly studied for metals, while for oxides, especially for dispersed ones, situation is far less clear due to inherent complexity of studies of their bulk and surface atomic structure. In last years, successful development of such methods as HREM and STM along with the infrared spectroscopy of test molecules has formed a sound bases for elucidating this problem in the case of oxides. In the work presented, the results of the systematic studies of the bulk/surface defect structure of the oxides of copper, iron, cobalt, chromium, manganese as related to structural sensitivity of the reactions of carbon monoxide and hydrocarbons oxidation are considered. 

Ladas et al. (321) have studied the oxidation of carbon monoxide over model Pd/Al203 catalysts formed by vacuum deposition of Pd on an a-AI2O3 single-crystal support. Little structure sensitivity is found (curves 4 and 5 of Fig. 20), and the rates for low FE agree with those reported for Pd... 

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