© Copyright 1999 American Chemical Society
AUGUST 31, 1999 VOLUME 15, NUMBER 18
Foreword Third International Symposium: Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids Organized by
W. Rudzinski Maria-Curie University, Lublin, Poland
B. W. Wojciechowski Queen’s University, Kingston, Canada
M. Rozwadowski Nicholas Copernicus University, Torun, Poland Committee for Interface Chemistry of the Polish Chemical Society This issue of Langmuir is devoted to papers presented at the third of the symposia known as ISSHACn, n ) 1-3, and concerned with heterogeneous surfaces. All three of these meetings have been held in pleasant small towns (mainly) in Poland: the first in Kasimerz Dolny in July 1992; the second was split between Zakopane, Poland, and Levoca, Slovakia, in September 1995, and the third was held in Torun, Poland, in August 1998. In each case, the goal was to bring together workers in this field for discussion and exchange of ideas in a format similar to that of a Gordon Conference. Originally, their locations were chosen in a successful attempt to encourage attendance by scientists from Eastern European countries as well as from the worldwide community of researchers on this problem. Attendance has been limited to roughly 150 (plus accompanying persons) in order to facilitate personal interactions. In addition to accomplishing this goal, the size limitation has allowed the publication of the papers presented at each of the meetings in Langmuir: in Vol. 9, number 10 (1993), Vol. 13, number 5 (1997), and this issue. The presentations at the meetings were roughly evenly split between oral and poster. A glance at the Tables of Contents reveals that the subjects of the papers range widely over surface science and are united by their
emphasis upon the role of heterogeneity in experiment and theory. It has been known for many years that heterogeneity plays an extremely important role in determining the behavior of molecules and ions on or near surfaces. The specific definition of heterogeneity has changed somewhat with the passage of time and will no doubt continue to change. A simple but physically unrealistic definition of a homogeneous surface is one for which the adsorption energies are constant at all points over the area. The heterogeneous case is then everything else. However, to make this into something useful, some approximations must be introduced: the size of an element of the ideal homogeneous surface must be large so that edge effects upon its surface properties can be neglected and the variation in surface properties from point to point due to the effects of the atomic structure of the surface upon adsorption interactions and/or chemistry must also be negligible. Even so, this kind of definition is usually felt to be too restrictive, and one often sees homogeneous surfaces defined as those for which only a single crystal face is exposed with no edge effects. Many of the older theories that start from this kind of homogeneity give rise to models of localized adsorption on a lattice of sites which
10.1021/la9906469 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/24/1999
5682 Langmuir, Vol. 15, No. 18, 1999
are all identical. The alternative is to ignore the effects of atomic structure upon the properties of the surface. This leads to models of perfectly mobile adsorption. Such models can lead to specific predictions for the behavior of adsorbed films that are widely accepted as starting points for more realistic description of chemical and physical adsorption. Current work in this field has moved past such simple models both in their descriptions of adsorption interactions and in their treatments of the formation and properties of films on such surfaces. This is particularly evident from the emphasis upon heterogeneous surfaces that has inspired this series of meetings. There are two different but closely related aspects of studies of heterogeneous surfaces that must be considered: first, the characterization of the surface itself, and second, the properties of gases or liquid solutions adsorbed on the surface. By characterization, we mean the chemical and physical nature of the surface with special attention paid to imperfections such as impurities, crystallite edges, cracks, steps, or regions of differing chemical or crystalline nature. Modern surface science has produced a wide array of powerful tools to study such properties. For instance, atomic force or scanning tunneling microscopy and beam scattering, atomic or ionic, are sensitive probes of surface atomic structure for heterogeneous as well as homogeneous systems. A variety of electron spectroscopies can be used to determine the chemical nature of the groups on or near the surface. In some systems, infrared or Raman spectra yield valuable information about the bonding of surface species. Starting with the first of the ISSHAC meetings, the number and variety of the papers dealing with this aspect of heterogeneous surfaces has increased significantly. In part, this has been a consequence of the increasing emphasis on chemisorption and catalysis since these are highly sensitive to the details of surface composition and structure. However, efforts to tailor the structure and chemical composition of adsorbent surfaces are relevant both to chemical and physical adsorption. Oxides are the primary group of materials in this regard, especially since they include silicas and aluminosilicates. However, aluminum and titanium oxides are also among the materials whose surface properties are still the subject of detailed characterization and modification. An important aspect of these surfaces is their tendency to add hydroxyl groups, with the accompanying changes in surface electrostatic field and chemistry. One must of course add the various forms of carbon to this list, including
especially the various types of buckytubes that are currently being synthesized. As can be seen in these proceedings, papers discussing the synthesis and characterization of adsorbents are an increasingly important aspect of current research on heterogeneous surfaces. The second part of the research in this field is to better understand the relationship between surface properties and the thermodynamic and dynamic behavior of adsorbed films. The thermodynamic properties are obtained first from isotherms, which yield information concerning the chemical potential of the adsorbed film as a function of its density or concentration, and second from the heats of adsorption, which give the energy changes that occur when adsorbate is transferred from the bulk to the surface phase. Relevant dynamical properties are primarily rates of surface chemical reaction, surface diffusion, and adsorption-desorption rates as exemplified by temperatureprogrammed desorption (TPD). Of these three areas, the ISSHAC meetings show a greater emphasis on the last two than on the first, possibly because rates of chemical reaction on strongly heterogeneous surfaces are particularly resistant to theoretical analysis. It is notable that the specific nature of the gas-solid interactions for the adsorbents studied plays an important role in the analysis of the experiments. That is, emphasis has shifted away from attempts to extract surface site energy distributions solely from isotherm data. Currently, this “classic” approach is being augmented by including known information about the surface structure to obtain more physically realistic information about these systems. This is true for physical adsorption both from the liquid and from the gas. Heats of adsorption are being used more to characterize these energies of adsorbate-adsorbent interaction. Simulations and theory are being brought to bear on the relationships between diffusion and the adsorbateadsorbent interactions for heterogeneous surfaces, and an improved theoretical description of TPD is under development. The papers in this proceedings issue indicate that the study of heterogeneous surfaces is expanding rapidly both in the techniques employed and in the physical relevance of the information obtained. The fourth meeting of this kind is being organized for the summer of 2001. It promises to continue the tradition of presenting the best of current work in a delightful setting designed to produce extensive exchange of ideas.
William Steele Department of Chemistry, Penn State University, University Park, PA LA9906469
