Carbon monoxide spillover

Except in formal terms, the valence-state requirements needed for the spillover of molecules like carbon monoxide and ethene are more difficult to see. When associatively chemisorbed it appears that the carbon atom of carbon monoxide is always directed towards the surface leptons of most solids, an arrangement which could require the molecule to turn over during spillover, e.g. 

The addition of iridium to platinum on ij-AlaOa causes a marked increase in the H/M ratio of hydrogen uptake assuming that H/Ms = 1.69.70 value of two was not taken to be conclusive proof of spillover so the authors turned to the chemisorption of carbon monoxide and compared the ratios H/CO for uptake at saturation on the supported and unsupported bimetallic catalysts. The following results were obtained for the quotient of the two H/CO ratios at various catalyst compositions —... 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

Quantitative information about the activity of various catalysts is obt ned by rate me lsurements in a recirculation system. The results indicate that the observed syner sm between the noble metal and tin(IV)oxide is due to spillover of oxygen. The comparison of reaction rates, measured with catalysts mainly differing in the sorption capacity relative to oxygen, shows that the rate determining step of the carbon monoxide oxidation should be the migration of adsorbed oxygen. 

As stated above, the dependence of the reaxrtion rate on the carbon monoxide content shows the two kinetic regimes well known in the case of platinum catalysts. The synergism between platinum and tin(IV)oxide is operative in the domain where the rate is zero order with respect to carbon monoxide. It is often attributed to the spillover of reactive species, either carbon monoxide [14,15] or oxygen (16). We have discussed elsewhere [8,9], why we consider that oxygen is moving from the oxide to the platinum surface or to the three phase boundary, where it immediately reacts with adsorbed carbon monoxide as it is known from pure platinum. 

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