The Material- and Pressure-Gap Problem in Heterogeneous Catalysis

The aim of many surface science experiments is to provide the fundamental detail of a reaction over a well-characterized single crystal surface in order to establish structure-reactivity relationships. Supported catalytic particles, on the other hand, may have various exposed surface facets along with defect sites. A choice then has to be made as to which single crystal surface will provide the most accurate representation of the active surface facets of the support particles. In order to address the similarities or differences in the rate over ideal surfaces and those over supported particles, Kelly and Goodmanl l compared the rate of methane formation from CO and H2 catalyzed by an Ni(lOO) single crystal with that over a supported catalyst taken under the same conditions (see Fig. 2.12). 

For this particular reaction, the comparison of activation energies shows very similar results for the two systems. The agreement between the activation energies suggests that the practical catalysts and the single crystal catalytic surfaces have similar sites that 

In turning to the pressure-gap problem, we note that this same reaction carried out under UHV would show a marked decrease in the rate of reaction as measured by the catalytic turnover number. The turnover number is defined as the rate of a reaction normalized per reactive surface site. 

High reaction pressures are needed for many other systems as well in order to convert the surface into a uniquely reactive state such as has been found for ethylene epoxidation. The epoxidation reaction of ethylene catalyzed by silver shows a distinct pressure gap. Higher oxygen pressures are needed in order to convert the silver surface into a silver-oxide overlayer where weakly adsorbed oxygen atoms are formed, that selectively epoxidize ethylene ]. 

An example of such high-pressure effects are the studies of ethylene hydrogenation. Hydrogenation of ethylene by a Pt(lll) surface imder atmospheric conditions occurs by a weakly adsorbed ethylene species (tt-bonded), that predominantly forms only under reaction conditions, when the surface is nearly completely covered with strongly adsorbed ((T-bonded) ethylene. Similar results were also found computationally over Pd(lll) with the exception that both tt and weakly held di-a intermediates were found to be active. 

---