An introduction to geocatalysis

Heterogeneous catalysis at mineral surfaces, or geocatalysis, may be important in natural systems. This contribution reviews how catalyst may influence reactions and several examples, mostly taken from the geochemical literature, are presented to illustrate the most important concepts. Two key characteristics that define a catalyst are activity and selectivity. Activity is the extent to which the catalyst can facilitate the rate of conversion of reactant to product. The selectivity of the catalyst refers to the ability of a catalyst to facilitate a specific reaction pathway where a reaction may proceed via multiple pathways. Photocatalysis is presented as a separate class of catalysis because reactions are only catalyzed when the system is illuminated with light of suitable wavelength. Three mechanisms of photocatalysis (photolysis, photoelectrochemical reaction, and charge injection) are discussed. Geocatalysis is particularly important for reactions with high activation energies, such as isotopic exchange reactions involving SO4-H2S, H2O-SO4, H2O-PO4, CO2-CH4 and transformations of humic acids and kerogen. While photocatalysis is limited to surficial aquatic environments, it may be of great importance in regulating the concentrations of redox-sensitive elements in the photic zones of lakes, streams, hot spring, and oceans. Photoelectrochemical reactions, such as the reduction of dinitrogen and carbon dioxide are thought to have been important in the origin of life on Earth. Geocatalysis may also prove to be useful in designing new environmental remediation technologies. For example, it is shown that sphalerite and ilmenite are capable of degrading chlorinated carbon compounds via a photocatalytic mechanism. (C) 1998 Elsevier Science B.V. All rights reserved.