Webster defines encyclopedia as “a book or set of books which gives instruction in all branches of knowledge.” The objective of this article is to instruct the educated and the neophyte in the “acid-base behavior of oxide and related surfaces.” In this context, the word “behavior” will be taken as the dictionary definition of “external appearance or action.” This discussion is not meant to be an exhaustive scientific document but rather one that provides an overview of a vast subject. There are several recent publications that provide comprehensive discussions and the interested reader can seek in these sources those details that are only outlined here.1, 2 Most of our understanding of the behavior of acid-base sites on solid surface has come from literature related to catalysis. This is understandable in that many industrially important reactions involve the conversion of hydrocarbons to useful products over acid catalysts. Therefore, the references that are cited will come predominantly from catalysis literature.

The concept of acidity and basicity is probably one of the most useful classification schemes in all of chemistry. It allows for a systematic placement of compounds into groups having similar structures and reactivities. An early student of this subject said, “There is nothing in the Universe but alkali and acid, from which Nature composes all things,” [Otto Tachenius (1671)]. Tachenius was an alchemist who recognized the versatility of these classes of materials in the “purging” (refining) of such elements as mercury and gold.

After a brief background, the sections to follow will discuss the acid-base behavior of oxide and related materials in relation to their structure (i.e., external appearance). The materials will be limited to simple amorphous oxides, mixed oxides, crystalline oxides, layered oxides, heteropoly acids, supported transition metal oxides, sulfated metal oxides, and activated carbons.