Surface analysis and the complexity of real catalysts

Abstract

This paper is written for specialists of surface techniques not perfectly familiar with catalysis, as well as for scientists working in the area of heterogeneous catalysis. It is aimed at suggesting the problems of characterization that surface analysis is faced with, because of the complexity of real catalysts. We first present the various typical 'architectures' to which catalysts may correspond. We shall then illustrate typical prohlems, and their solutions, by selecting examples in the field of hydrodesulfurization (HDS) catalysts. We shall show how 10 out of the 12 theories proposed for explaining catalytic activity were eliminated thanks to surface techniques. HDS catalysts are remarkable in several respects, especially because they undergo many 'metamorphoses' (activation, modifications during catalytic work, ageing, regeneration) from the time of their preparation to that of their death by irreversible deactivation. They therefore provide a large body of illustrative examples concerning the characterization of several 'architectures'. In spite of the achievements of surface techniques in clarifying many features of HDS catalysts during their life, investigators are still faced with unsolved problems; many such problems are related to the very fine scale (of the order of 1 nm or less) of the details of importance for catalysis. This might suggest directions for new improvements or the development of new surface techniques. As a conclusion, we shall recall that, because complete characterization needs the joint use of several techniques, a methodology of study had to be developed, now exists, and still continues to improve. The main lines of this general methodology are not restricted to catalysts, but can be used for the study of all complex highly dispersed solids.

Reactions of Dibenzothiophene with Hydrogen in the Presence of Selected Molybdenum, Iron, and Cobalt Compounds

“…The nature of the active species and the mechanism of catalytic action remain a subject of debate. For example, Delmon cataloged a dozen theories explaining the behavior of these catalysts , These investigators subsequently showed that these Co atoms on the edges of the MoS 2 layers seem to be located in the same plane as the Mo atoms, with a Mo−Co distance of 0.28 nm, which is close to that observed in Co−Mo−S cluster compounds.…”

Novel bimetallic dispersed catalysts for temperature-programmed coal liquefaction. Technical progress report, October 1995--December 1995

“…For example, Delmon cataloged a dozen theories explaining behavior of these catalysts. 5 Topsge and his colleagues established that the basic structural units are small MoS2-like domains with Co atoms on the edges of the layers in the structure.6.7 These investigators subsequently showed that these Co atoms on the edges of the MoS2 layers seem to be located in the same plane as the Mo atoms, with a Mo-Co distance of 0.28 nm, which is close to that observed in Co-Mo-S cluster compounds.8…”

Surface Chemical Composition

“…Important properties such as activity, selectivity and also poisoning of a catalyst are determined by the chemical species on its surface [1][2][3][4][5][6]. with characteristic results.…”