ABSTRACT
The textbook definition of a catalyst is that it is a material which will change the rate of a chemical reaction by its presence but which is unchanged chemically in the process (Glasstone 1955). The process by which the rate is changed is termed catalysis. In more modem textbooks, the terms "catalyst" and "catalysis" has come to mean material and a process which causes the rate to be increased by the catalysts presence, respectively (Adamson, 1986). The term "negative catalyst" is sometimes found referring to material which would slow down the rate of chemical reaction. However, the term "inhibitor" has come to be used as an alternative for "negative catalyst," but it is often used to cover materials which may be consumed during 'the reaction. Catalyst can be divided up into two classes: homogeneous and heterogeneous catalysis. Homogeneous catalysts refers to the class of catalysts where the process occurs in one phase only. Copper compounds are found acting as homogeneous catalysts, but in this review, the use of copper compounds in heterogeneous catalysis is primarily addressed. In heterogeneous catalysis, the catalyst forms a separate phase distinct from that of the reactants. Generally in heterogeneous catalysis, the catalyst is a solid phase and the reactants are liquids or gases. In heterogeneous catalysis, there may be small changes in the stoichiometry of the catalyst which renders the classic definition of the term "catalyst" inade-
quate. It might be more suitable to simply say that catalysts alter the ·rate of chemical reactions, but without certain restrictions, this is probably too broad a definition.
