Catalysis by Single-crystal Surfaces

BY R.W. JOYNER

1 Introduction

Judged by two important criteria, the field of catalysis by single-crystal surfaces has prospered significantly since the earlier reviews in this series, the later of which covered the period to 1977. The number of studies has increased to an undeniable extent and these form almost the whole matter of this report. Little room has been left to detail other areas of 'Surface Science' that might be of interest to the catalytic practitioner. This Reporter also believes that the quality and sophistication of information in the area of single-crystal catalysis has improved markedly since 1977 and that a more detailed understanding of many important catalytic processes is being achieved. The remainder of this report attempts to put a gloss onto these statements, each section dealing with an individual reaction or class of reactions. Sections are designed, if the reader desires, to be read independently. Each opens with an account of advances in our knowledge of adsorption, structure, theory, or other topics of relevance. Investigations of the reaction in question are then outlined. No attempt is in general made to describe the experimental techniques used or to discuss detailed interpretation of results. A short glossary of acronyms is given in Table 1. Nearly all of the reactions considered are of industrial or technological significance and most of the catalysts described are metallic. This reflects the much greater ease of preparation of metal single crystals vis à vis large oxide or sulphide single crystals. Catalysis by zeolites, now of immense significance in catalytic cracking, has, however, been excluded by reason of space and lack of competence of the reviewer.

There are three reasons why catalysis by single crystals can usefully be reviewed as a sphere of activity apart from catalysis in general. The first is that crystal structure is itself an influence on catalytic rate. However, as Boudart has reminded us changing the surface structure usually changes the rate of reaction by less than a factor of twenty, while changing the metal may alter the rate by many orders of magnitude. A second point is that single crystals are somewhat easier to characterize than supported catalysts, where the role of surface and bulk phase transformation and the influence of impurities may be more insidious. The main justification for isolating single-crystal catalysis for study is that it is often performed in conjunction with some of the formidable array of surface science techniques listed in Table 1. The information obtained can also be related to that obtained in studies of adsorption and structure of single crystals. A more detailed and fundamental understanding of the reaction of interest may therefore be expected.

The reactions of hydrocarbons are considered in three groups, the first involving synthesis, and the second dealing with isomerization and hydro- genolysis. The third is involved largely with oxidation reactions and reactions of oxidized hydrocarbons. CO and NO oxidation are then examined, followed by NH3 synthesis and oxidation.

2 Hydrogen Reactions

Synthesis – Successive oil crises have focused attention on forthcoming shortages of liquid hydrocarbons. A consequence of this has been a revived interest in the Fischer–Tropsch (FT) synthesis [equation (l)], of which methanation, [equation (2)] is a special case. Fe, doubly promoted by K2O and Al2O3

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

CO + 3H2 [right arrow] CH4 + H2O (2)

The adsorption of both reactants on transition metals has been much studied. H2 adsorption on Pt, as evidenced by the rate of H2-D2 exchange, is structure sensitive. H2 adsorption has been shown to cause metal surface reconstruction, rather than simple 'on top' chemisorption, of the (001) face of W, a surface which itself has been shown to reconstruct at low temperature.

Of major significance for our understanding of the FT reaction has been the recognition that CO adsorption may be dissociative on many surfaces. First suggested by King et al. in 1972, the unequivocal demonstration of CO dissociation was an early success for photoelectron spectroscopy studies of surfaces." The involvement of dissociated CO in FT synthesis on supported CO catalysts has been firmly established by the ratio-tracer studies of Biloen et al. and on Fe by the work of Perrichon et al.

Those metals which dissociate CO have been identified by Rhodin et al. as shown in Figure 1, and the relation of dissociation to the existence of stable carbides has been noted." The existence of dissociative CO adsorption on Ru has not been established by XPS or UPS. On the close-packed (001) plane Menzel et al. suggest that electron bombardment is required. However Singh and Grenga have shown C build-up resulting from CO dissociation at edge sites on a spherical single crystal. McCarty and Wise have demonstrated that isotopic mixing occurs in CO above a supported Ru catalyst at 350K.

There is clear evidence for structural sensitivity in CO dissociation on some metals. On Ni dissociation was observed on evaporated films and ion-bombarded single crystals, but not on a well annealed (111) surface. Detailed confirmation of these early results comes from a study of CO adsorption on Ni(111) and the stepped plane [5(111) x (110)]. Temperature-programmed desorption from the stepped plane showed a CO peak at ~ 820 K in addition to that observed from the low-index surface, at 430 K. Erley and Wagner attribute this higher temperature peak to activated recombination of C and 0 adatoms formed by CO dissociation. Erley et al. give further proof of dissociation in an HREELS study. Figure 2 shows the loss spectrum resulting from the adsorption of 0.4 Langmuir of CO at 180 K. The peak at 1520 cm-1 appears before those at 1890 and 2010 cm-1, and is assigned to CO adsorbed at the steps. 1520 cm-1 represents a very low C=O stretching frequency and is in the range associated by Nguyen and Sheppard with CO triply bonded to the surface. Erley et al. suggest that this species is the precursor of the dissociatively adsorbed species indicated in curve (b) of Figure 2, observed after heating the surface to 430 K. This spectrum shows no...