Catalyst, general surface, geometry

Because of the presence of electron-donor or electron-acceptor centers with special surface geometries and the fact that redox reactions are favored, general statements about the poisoning of semiconductor catalysts can hardly be made. Any molecule that is strongly adsorbed on the surface is a potential poison. 

Equation 8.5-11 applies to a first-order surface reaction for a particle of flat-plate geometry with one face permeable. In the next two sections, the effects of shape and reaction order on p are described. A general form independent of kinetics and of shape is given in Section 8.5.4.5. The units of are such that is dimensionless. For catalytic reactions, the rate constant may be expressed per unit mass of catalyst (k )m. To convert to kA for use in equation 8.5-11 or other equations for d>, kA)m is multiplied by pp, the particle density. 

The use of N2O to determine Cu surface areas requires great care to avoid subsurface oxidation. The frontal chromatography method, in which a dilute mixture of N2O and He (typically, 2% N2O) is passed over a large bed of catalyst until no further N2O is reacted, appears to be the most reliable method. In the pulse method the extent of subsurface oxidation depends on the temperature (a very serious problem above about 100 °C), the size of the N2O pulse, the size of the catalyst sample, the metal loading of the sample, and the geometry of the catalyst bed. In general, small pulses of N2O should be used, at temperatures below about 60 °C, with a deep catalyst bed (>1 cm). 

A procedure of general use to obtain Pt/C catalysts with different particle sizes is based on the variation of the time and temperature of activation. Figures 12 and 13 show the mass activity (mA g Pt) and specific activity (pA cm Pt), respectively, for O2 reduction at 900 mV, in 98% phosphoric acid at 170 °C. The mass activity for oxygen reduction appears to reach a maximum at a Pt particle size of 3nm, corresponding closely to the particle size at which the maximum in the fraction of (111) and (100) surface atoms on Pt particles of cubo-octahedral geometry occurs [32]. The specific activity increases gradually with an increase in Pt particle size and closely follows the trend observed between the surface fraction of (111) and (100) Pt... 

It has long been known that catalytic reaction rates and selectivity can depend sensitively on the size of catalyst particles (37-39). Such structural sensitivity has generally been explained by models whereby the activity or selectivity of the reaction was assumed to vary markedly with the local geometry of the surface sites. Using the methods outlined above, it has now been unequivocally proved that, indeed, many steady-state high-pressure catalytic reactions depend sensitively on the crystal-surface orientation of the model catalysts, and that others do not. This subject has been recently reviewed by Boudart (40), who points out the potential utility of single-crystal surfaces as standards against which to compare industrial catalysts. 

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