Important Features of Solid Catalysts

Three features determine how a solid catalyst behaves in practice. (a) Catalytic activity: a catalyst's activity depends on the strength of chemisorption between it and the reactant molecules -- if large numbers of reactant molecules (gas or liquid) are strongly adsorbed on the catalyst's surface, the catalyst is considered active. But this bonding must not be so strong that the adsorbed molecules become immobilised on the surface (which would block the reaction from proceeding, since the reactants need enough mobility to actually react and desorb as products). Certain d-block metals -- Fe, V and Cr -- are found to be strongly active toward O2, C2H2, C2H4, CO, H2, CO2 and N2, while Mn and Cu are unable to adsorb N2 or CO2 at all; Mg and Li instead adsorb O2 selectively. (b) Catalytic selectivity: some solid catalysts are selective, meaning the same reactants give different products depending on which catalyst is used. For example, gaseous ethylene and O2 give acetaldehyde with a Pd/Al2O3 catalyst (C2H4 + O2 --[Pd/Al2O3]--> 2 CH3CHO) but ethylene oxide with an Ag/Al2O3 catalyst (2 C2H4 + O2 --[Ag/Al2O3]--> 2 C2H4O); similarly, CO and H2 give methane with a nickel catalyst (CO + 3 H2 --[Ni]--> CH4 + H2O) but methanol with a Cu/ZnO-Cr2O3 catalyst (CO + 2 H2 --[Cu/ZnO-Cr2O3]--> CH3OH). (c) Shape-selective catalysis by zeolites: zeolites are aluminosilicates with a three-dimensional network of silicate units in which some silicon atoms are replaced by aluminium, giving an Al-O-Si framework with a microporous structure. Because the reactions that occur inside a zeolite depend on the size and shape of the reacta …