1.3 1932: Adsorption of Gases by Solids; Faraday Discussion, Oxford

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes – physical adsorption and activated adsorption – and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten.

1.4 1940:: Seventeenth Faraday Lecture Langmuir

It was usually assumed that the (1 – θ) factor in the Langmuir equation, bP = θ/(1 – θ), took account of the fraction of the surface that was bare and that therefore the fraction of atoms (e.g. caesium on tungsten) that condense at the surface is proportional to (1 – θ). Langmuir in his lecture (given in 1938) drew attention to the physical assumptions underlying this factor (1 – θ) being very improbable, referring specifically to his experiments concerning the adsorption of caesium vapour on tungsten: when the coverage was close to unity, all the caesium atoms impinging on the surface were adsorbed, indicating that they sought vacant sites on the surface and were mobile.

Langmuir made the point in his lecture that the 'lifetime τ of an atom at the surface is not independent of the presence of other atoms, being given by τ = τ0(1 – θ) The shortening of the lifetime τ as θ approaches unity is the result of strong repulsive forces between pairs of atoms which occupy single sites'. We will see that this view is central to what STM revealed some 60 years later.

1.5 1950: Heterogeneous Catalysis; Faraday Discussion, Liverpool

Although there was the realisation that "clean" metal surfaces were essential to progress the understanding of adsorption and catalysis, it was J. K. Roberts and Otto Beeck who, as experimentalists, moved the subject forward, with Roberts' studies of hydrogen and oxygen adsorption at tungsten wires, cleaned by flashing to 2000 °C, and Beeck using large surface area metal films. Roberts had introduced earlier the distinction between immobile and mobile adsorption on fixed or localised sites with Miller discussing how statistical mechanics could be used to examine the equilibrium distribution in the mobile state and how it is related to the experimentally observed variation in the heat of adsorption with surface coverage.

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and "gas-induced" (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling's equation is applicable to the process 2M + H2 [right arrow] 2M – H.

Lennard-Jones in the Introduction to his paper stated "The literature pertaining to the sorption of gases by solids is now so vast that it is impossible for any, except those who are specialists in the experimental details, to appreciate the work which has been done or to understand the main theoretical problems which require elucidation". He goes on to describe what is still one of the cornerstones of adsorption behaviour, the Lennard-Jones potential energy diagram, its explanation of 'activated adsorption' and its relevance as an important concept in the understanding of surface catalysis. The paper by Volmer considers experimental evidence for the migration of molecules at surfaces from the viewpoint of crystal growth. He emphasises the need to search for experimental evidence for 'two-dimensional mobility' and discusses Estermann's data for silver on quartz and benzophenone on mica surfaces.

What was evident in 1950 was that very few surface-sensitive experimental methods had been brought to bear on the question of chemisorption and catalysis at metal surfaces. However, at this meeting, Mignolet reported data for changes in work function, also referred to as surface potential, during gas adsorption with a distinction made between Van der Waals (physical) adsorption and chemisorption. In the former the work function decreased (a positive surface potential) whereas in the latter it increased (a negative surface potential), thus providing direct evidence for the electric double layer associated with the adsorbate.

The work of Beeck and Roberts had a strong influence on the need to characterise the chemical state of metal surfaces under different preparative conditions, i.e. whether it was a metal filament, a high area metal film or a catalyst formed by the reduction of a metal oxide. Wheeler in 1952 highlighted the potential conflict between the "clean" surface and 'bulk catalyst' approaches to catalyst research. There was emerging a driving force to develop experimental methods which relied on ultra-high vacuum techniques, where the background pressure was 10-9 mbar or less, as prerequisites for studies of chemisorption and chemical reactivity of metal surfaces. In 1953, one of us (M.W.R.) attended a Summer School on 'The Solid State and Heterogeneous Catalysis' at the University of Bristol, 'intended for those engaged in research in University, Government and Industrial Laboratories'. This consolidated the messages that had emerged from the Faraday meeting of 1950, with Stone and Gray emphasising the defect solid state and Eley drawing attention to the problems associated with metal surfaces prepared by various methods. Mobility of surface atoms was anticipated to occur when the temperature of the solid was above 0.3Tm, whereas mobility of the bulk atoms occurred above 0.5Tm (the Tammann temperature), where Tm is the melting point in kelvin of the solid. In contrast to what we shall discuss later, surface mobility was considered to be a phenomenon to be associated with 'high temperatures' and therefore in accord with Langmuir's concept of the checkerboard model of a surface being homogeneous and consisting of fixed surface sites!

1.6 1954: Properties of Surfaces

This conference, organised by the New York Academy of Sciences, emphasised the contribution that fundamental studies carried out in industry were making, papers emanating from Bell Telephone Laboratories, Westinghouse Research Laboratories, du Pont Nemours, Kodak, Sylvania Electric Products and General Electric (the "home" of Irving Langmuir). Although there was much emphasis given to the physics of surfaces, we draw attention to two papers, the first by Becker and the second by H.A. Taylor. It is clear that Becker was greatly influenced by the development of the field emission microscope and what it revealed about "foreign atoms' adsorbed on metal surfaces and how the work function varies from one crystal face to another, and that "in adsorption the arrangement of the surface metal atoms plays an important part". Becker refers to the possibility of measuring sticking probabilities using the "flash filament" (later called temperature-programmed desorption TPD) technique with the emergence of ultra-high vacuum techniques and the ion gauge for pressure measurement. His paper emphasises how these developments led him to reappraise his article "The life history of adsorbed atoms and ions" published in 1929.

H.A. Taylor considers kinetic aspects of surface reactions and starts from the proposition that although in discussions of reaction kinetics it is customary to divide the subject into two classes, homogeneous and heterogeneous, the inference that what may be true for one class cannot be true for the other. Taylor took the view that a single basis must underlie both classes of reactions, that each must be governed by the same basic principles and that no chasm exists between them. A paper with Thon questions the checkerboard model and that the surface plays an active rather than a passive role as implied in the Langmuir model. He questions the use of orders of reaction as providing unambiguous models of surface reactions. Taylor was particularly attracted to the views of the Russian scientist Semenov, who regarded the solid surface in a catalytic reaction as a source for generating and terminating radical reactions. The Taylor–Thon view led to the rejection of mechanisms based on the reaction between two chemisorbed species and favoured the reaction between a chemisorbed species and a gaseous reactant (essentially an Eley–Rideal mechanism). The Semenov view was that even in a heterogeneously catalysed reaction the product was formed by the reaction of a free radical and an "inert molecule", just as in a homogeneous chain reaction.

Can STM throw light on whether homogeneous gas-phase and heterogeneous surface reactions encompass a common theme – the participants of surface radicals in a "two-dimensional gas"?

1.7 1957: Advances in Catalysis; International Congress on Catalysis, Philadelphia

The first International Congress on Catalysis to be held in America was in Philadelphia in 1956 and according to Farkas it was "in view of the tremendous growth in the industrial applications of catalysis and the ever increasing scientific activity in the field". There was at this meeting an obvious step-change in the science, with new experimental methods being introduced to investigate solid surfaces and their chemical reactivity. In particular, there was the emergence of low-energy electron diffraction (LEED) (Schlier and Farnsworth), infrared studies (Eischens and Pliskin), magnetic studies (Selwood), isotopic exchange studies (Bond and Kemball), electron spin resonance (Turkevich), conductivity studies (Suhrmann) and flash-filament, later renamed temperature-programmed desorption (Ehrlich). Surface cleanliness of metal surfaces had become a fundamental issue and a pointer to the development of what became referred to as the surface science approach to catalysis.

1.8 1963: Conference on Clean Surfaces with Supplement: Surface Phenomena in Semiconductors, New York

This was an outstanding meeting held in New York, which to at least to one of us (M.W.R.) marked a turning point in surface science. In the Panel Discussion, R.S. Hansen, a chemist, made the telling comment: "The semiconductor physicist is encountering a number of chemical problems that he is not trained to solve; the chemist on the other hand is unaware of these potentially very interesting problems. I can say from a chemist's viewpoint I am sure that part of this difficulty is the language barrier between the physicist and the chemist and that certain of the concepts of the physicist are stated in language, where he is very much at home, that are purely phenomenological and have no strictly scientific context". Hansen goes on to give as an example "slow" or "fast" surface states in explaining conductivity changes in semiconductors. There were some outstanding papers which set the scene for the development of surface science: field emission (Müller); slow electron diffraction (Germer, also Farnsworth); work function and photoelectric measurements (Gobeli and Allen); adsorption at clean surfaces (Ehrlich); reactions of hydrocarbons with clean rhodium surfaces (R.W. Roberts); nucleation of adsorbed oxygen on clean surfaces (Rhodin); dynamic measurements of adsorption of gases on clean tungsten surfaces (Ricca); and oxygen complexes on semiconductor surfaces (Mino Green).

1.9 1966: Faraday Discussion Meeting, Liverpool

The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed.

1.10 1967: The Emergence of Photoelectron Spectroscopy

Siegbahn's publication of his group's development in Uppsala of what was described as ESCA (electron spectroscopy for chemical analysis) opened up the field of photoelectron spectroscopy, which through an understanding and interpretation of shifts in binding energy provided much more than the acronym suggested – chemical analysis. It is interesting to recall his comment regarding core-level binding energies: "We had discovered what we call the chemical shift. In the beginning we didn't like this: we were physicists and wanted to study systematically the behaviour of elements. There was now a problem as we had to be careful that the substance was not oxidized or changed in some other way". Kai Siegbahn was awarded the Nobel Prize for Physics in 1981. The surface chemist took advantage of the chemical shift in being able to distinguish different bonding states of the same element, for example N(a), NH(a) and NH3(a) and differentiating between molecular and dissociated states which had previously relied on whether first-order (molecular state) or second-order (dissociated state) desorption kinetics were observed.

1.11 1968: Berkeley Meeting: Structure and Chemistry of Solid Surfaces

This meeting was organised by Gabor Somorjai, driven by the rapid development of experimental methods in what was now developing as a sub-set of heterogeneous catalysis – surface science. It was evident that over the 2 years since the Faraday Discussion Meeting in 1966 the subject had moved on apace, with low-energy electron diffraction (LEED) following Germer's work being a dominant theme. Auger electron spectroscopy had just come into prominence with Weber and Peria, following Harris at General Electric's laboratories at Schenectady, realising that the LEED equipment could be easily adapted to enable Auger spectra to be obtained, which provided chemical analysis of the surface. There was an overwhelming emphasis on studies of single crystals.