(9 Copyright 1993 American Chemical Society
OCTOBER 1993 VOLUME 9,NUMBER 10
Introduction Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids Studies of the effects of surface heterogeneity upon adsorption processes have a long history. Hundreds of papers, several extensive review articles, and two monographs devoted to this problem have been published. Papers dealing with this problem are more and more often presented at general scientificconferences. The scientists working on this phenomenon expressed the need for a symposium devoted only to this problem as a forum for intensive exchange of ideas, opinions, sorting out our knowledge, and speeding up further research in this field. There was also a desire to meet personally the people possessing the same scientific passion. Consequently, several dozen participants and accompanying pqsons assembled at the symposium whose proceedingg are published in this issue of Langmuir. The meeting was held in the small city of Kazimierz, located in a beautiful region on the Vistula River. This is one of the most picturesque little towns in Poland, situated on the banks of the river at the foot of rolling hills and featuring architectural monuments dating from the 14th century. Only some of the problems concerning surface heterogeneity effects in adsorption systems were discussed during the symposium. In particular,there was insufficient presentation in the field of catalysis. However, the common opinion was that the symposiumwas an important event in the study of surface heterogeneity. A brief historical review of many of the topics discussed follows: It was Langmuir himself who first suggested that his equation should be generalized to describe adsorption on energetically heterogeneous surfaces. He pointed out that his original equation could be applied to each type of site by the appropriate choice of parameters and that the total amount adsorbed could be obtained by averaging using appropriate weights. This aspect of Langmuir’s work was overshadowedby the extensive use of his simple equation. It was not until the end of the 1930sthat serious attempts were made to take into account heterogeneity effects in single gas adsorption on solid surfaces. The work was initiated in the former USSR by Zhuchovitski, Roginsky, Todes, and Bondareva but was disrupted by World War
11. Meanwhile, in the 1940s the problem of surface heterogeneity was taken up by workers in U.S.A., mostly in the papers published by Taylor, Halsey, Sips, and Zettlemoyer. In the 1970s Rudzinski, Jaroniec, and co-workers in Poland started an extensive study of surface heterogeneity effects in mixed-gas adsorption on solid surfaces. They applied the Integral Equation Approach to generalize the isotherm expressions for mixed-gas adsorption on a homogeneous solid surface. Further progress along these lines was made in the 1980s by Moon and Tien, Sircar, and Yang and co-workers in U.S.A. and Schliinder and co-workers in Germany. In the 1970s Myers generalized the IAS (Ideal Solution Approximation) approach to mixed-gas adsorption on heterogeneous surfaces. Nowadays we are aware that geometric and energetic heterogeneity is a fundamental feature of solid surfaces. The formation of such surfaces is predicted by statistical theories and computer simulations of crystal growth and by the theories of formation of amorphous structures. In most cases energetic surface heterogeneity is the consequence of geometric surface heterogeneity. It is surprising, therefore, that the theories of adsorption on energetically heterogeneous and porous surfaces have developed for a long time along two separate routes. It was an important moment when Hobson and Armstrong (Canada) showed that the Dubinin-Radushkevich isotherm used to describe adsorption by porous activated carbons also represented adsorptionon flat but energetically heterogeneous surfaces at low adsorbate pressures. Next, it was found that the Langmuir-Freundlich isotherm used to describe adsorption on flat but energetically heterogeneous surfaces also described adsorption in porous carbons at high surface loading. The empirical Dubinin-Astakhov equation which is now applied for that region is simply a hybrid between the Dubinin-Radushkevich and Freundlich isotherm. Dubinin, Stoeckli, and McEnaney have found relations between the geometric and energetic characteristics of porous heterogeneous surfaces. Their ideas have been extended further by Pfeifer, Jaroniec, and Avnir who seek
0743-746319312409-2483$04.O0/0 0 1993 American Chemical Society
Introduction
2404 Langmuir, Vol. 9, No. 10,1993
to relate experiment to the fractal dimensionsof real solid surfaces. The computer simulations by Bakaev, McElroy, and Steele and co-workershave made impressive progress in understanding the relations between geometric and energetic heterogeneity of solid surfaces. Investigating the effects of surface energetic heterogeneity on critical phenomena in adsorption systems is an active part of present adsorption studies. Limited dimensions produce effects which may simulate to some extent effectsof surface heterogeneity. Here the computer simulations by Gubbins and co-workers have brought to light many interesting features of such systems. Until recently, most works treating gas adsorption on heterogeneous surfaces were devoted to adsorption equilibria. This was probably due to the fact that experimental studies of the time dependence of various adsorption processes were complicated in the past for technical reasons. However the Elovich equation, which was long applied to describe adsorption kinetics, was always associated with surface energetic heterogeneity. Aharoni’s theoretical works have emphasized the important role of surface heterogeneity in adsorption kinetics. The strong driving force for these studies is the desire to interpret TPD spectra from real surfaces in a quantitative way. This is one of the most commonly applied techniques to study the features of solid surfaces in catalysis, in particular. Catalysis is an area where the importance of the geometric and energetic surface heterogeneity can hardly be overestimated. One may hear sometimes even the extreme opinion that only various surface defects are the active catalytic sites. Hundreds of papers have been published showingthe importance of surface heterogeneity in catalysis,so it would be far too difficult to mention here even the most prominent names. In many important catalytic processes,surfacediffusion of reactants is the key step in reactions. Surface diffusion seems also to determine the industrial separation of gases by adsorption in many cases. So, it is no surprise that the f i t theoretical studies on the role of surfaceheterogeneity in surface diffusion started as long ago as the 1960s (Higashi,Ito, and Oishi). The last decade has brought an explosive development of the research in that area, mostly due to the theoretical works by Zgrablich (Argentina),Do (Australia), and Yang and co-workers in U.S.A. Most theoretical works on surface heterogeneity effects in adsorption have concernedthe gadsolid interface. This is because the mechanism of adsorption in these systems is generally less complicated than in the case of solid/ solution interfaces. However, we are aware now of the great role of surface heterogeneity in adsorption phenomena occurring at solid/solution interfaces. Perhaps
the most impressive proof came from the experimental and theoretical work of the Cases school of adsorption in France. These were studies of surfactant adsorption from aqueous solutions onto various solid surfaces. His theory of surfactant adsorption based on a model of a heterogeneous solid surface was a milestone. The most recent theoretical works by Rudzinski and co-workers and the experimental works by Partyka and co-workers provide further proof that it is difficultto explain the basic features of surfactant adsorption without accepting the concept of surface heterogeneity. Rudzinski, Jaroniec, and Dabrowski have published also numerous works on surface heterogeneity effects in adsorption from nonelectrolyte mixtures onto real solid surfaces. However, the solid/solution interface that is probably the most important in science, life, and technology is the oxide/electrolyte interface. The fact that the surfaces of the real oxidesare geometricallydistorted and energetically heterogeneous has been known for a long time. It is surprising that until very recently, the theories of adsorption within the electrical double layer formed at the oxide/electrolyte interface were based on models of the homogeneous oxide surface. Works reporting low-concentration adsorption of heavy ions on oxides were the exception. At the end of the 1970s Garcia-Miragaya and Page and Street et al. reported a successful correlation by the Freundlich equation of the data for trace adsorption by both clay minerals and soils. Benjamin and Leckie found the same for the trace adsorption of Cu2+,Zn2+, Cd2+,and Pb2+onto amorphous iron oxyhydride. At the beginning of the 1980s Sposito drew attention to the fact that, as in gas adsorption, applicability of the Freundlich isotherm in these systemsshould be associatedwith surface energetic heterogeneity. Advanced studies of surface heterogeneity effects in ion adsorption at the oxide/ electrolyteinterfacewere initiated by Benjamin and Leckie and Kinninburgh (U.S.A.) at the beginning of the 1980s. Rudzinski, Partyka, and co-workers subjected such data to a theoretical analysis based on a model of the heterogeneous oxide surface. T e successof the first meeting and the encouragement on t e part of many participants made us plan another symposiumsomewherein Central Europe. Such a location will be convenient for many participants whose funds for scientific trips are limited. For those who do not have financial problems, exploring Central Europe can be a fascinating experience. More detailed information about the place and time of the next symposium will be published in Langmuir in the near future. W.Rudzinski and B. W.Wojciechowski
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