Carbon adsorbents hydrogen spillover

For metal-on-carbon systems two aspects should be noted (i) metals and sulfided metals can act as sources of spiltover hydrogen species, and (ii) as active carbon adsorbs hydrogen by itself at high temperatures [T > 300°C (75)], the effect of spillover is to increase the rate of uptake but not the extent. With the inorganic oxides, spillover most often results in an increase in the amount of adsorption. 

Wang, L. and Yang, R. T. Hydrogen storage on carbon-based adsorbents and storage at ambient temperature by hydrogen spillover. Catalysis Rev. Sci. Eng. 2010, 52(4), 411 61. 

In the previous example the supported metal oxide onto which the metal precursor was adsorbed did not reduce which will be the case for many promoted systems. In many systems, however, the supported metal oxide will reduce, especially through hydrogen spillover, and a bimetallic catalyst can be synthesized. The idea is illustrated in Figure 3.11a for the Pd/Co/C electrocatalyst system. The idea will be to adsorb Pd complexes onto a well-dispersed, carbon-supported C03O4 phase, and reduce to get bimetallic Pd/Co particles that are perhaps core-shell in morphology. 

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. 

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