Chemical processes at oxide surfaces

Metal oxides are omnipresent and thus attract considerable attention with regard to various technologically important aspects. Presently, the most important applications are related to heterogeneous catalysis. A necessary—but not necessarily sufficient—condition for unravelling the fundamental principles in this complex field is to analyse in detail chemical reactions taking place at oxide surfaces. Unfortunately, the experimental investigation of this class of materials represents a formidable problem since their surfaces often exhibit a large density of defects and are not easy to prepare. In case of ZnO, for example, the number of papers providing results from vibrational spectroscopy on single crystal surfaces is extremely small. In fact, this issue of PCCP will contain some of the first vibrational spectra for hydroxyl (OH)-species on ZnO substrates (papers B515489H and B515418A). This lack of information on single crystalline substrates is contrasted by many results available for ZnO powders for which, of course, the assignment of vibrational bands is extremely difficult since the powder particles exhibit many structurally different surfaces.

From a theoretical point of view the investigation of these systems is a demanding problem, too, since very often the theoretical modeling of oxidic substrates is complicated by electrostatic instabilities in the calculations arising from the charged nature of the ionic species.

In the second part of the last decade understanding microscopic phenomena at surfaces of metal oxides has made a significant leap. Mostly triggered by advances in the theoretical description and by the availability of high-resolution microscopic data, the understanding of processes at oxidic surfaces now reaches the level established for surfaces of clean metals. This issue of Physical Chemistry Chemical Physics contains a selection of articles from a number of different groups demonstrating that in the last few years the understanding of oxide surface properties and the interaction of small molecules with these substrates has improved significantly. The fact that many of the papers in this issue describe theoretical results manifests the major advances in the theoretical methodology developed for oxide surfaces. Also, the application of novel experimental techniques to study ionic surfaces has recently contributed significant new information. For example, a systematic investigation of hydrogen atoms on single crystal oxide surfaces, a species very crucial to determining the oxide surface chemical properties, has only recently become possible by applying the scattering of thermal energy He atoms (see paper B515553C). But probably the most significant advance on the experimental side has been accomplished by scanning tunneling microscopy, or STM. The availability of high-resolution STM data has not only helped to unravel the geometric structure of oxide surfaces (see papers B515179A and B515464B) but also makes it possible to identify defect sites, which are the active sites for many chemical reactions occurring at oxide surfaces.

With regard to the interaction of simple molecules on oxide surfaces recently a general consensus appears to emerge from the theoretical and experimental results for the geometric and electronic structure of oxide surfaces. Particularly striking is the advance which has been achieved with the materials ZnO and TiO₂. We expect that in the next few years we will see a consistent description of the adsorption and reaction of surface species leading to a full understanding of chemical processes at these surfaces based on first principles. Thus, eventually the bridging of the materials and pressure gap for oxidic catalysts used at high pressures will be feasible, including a consistent microkinetic modelling of catalytic reactions (see paper B515651C).

The papers presented in this issue describe a number of different approaches to unravel chemical processes at oxide surfaces:

• Theoretical description of the adsorption of molecules and metal particles on different metal oxide surfaces. This research focuses on perfectly ordered surfaces (single crystals) as well as on “model” defects.

• Experimental investigations on the geometric and electronic structure of oxide surfaces and adsorbed particles.

• Kinetic studies with regard to the reaction dynamics on oxide surfaces. Again, the surfaces of single crystals are of interest as well as surfaces of powders (microkinetic analysis). Particularly important will be the matching of reaction dynamics determined from research on single crystal surfaces with kinetic results obtained from reactor studies with the corresponding oxide powders.

Some of the material contained in the papers of this Special issue has been presented at the 89th International Bunsen Discussion Meeting “Chemical processes at oxide surfaces: from experiment to theory”, which was organized by Katharina Al-Shamery (Oldenburg) and Christof Wöll (Bochum). The meeting attracted about 100 attendees and took place in Meschede (near Dortmund, Germany) on June 15–17, 2005.

The papers contained in this special issue were grouped according to the different oxidic substrates by aiming to mix experimental studies with theoretical contributions. Articles which focus on related aspects were grouped together.

Professor Christof Wöll

Ruhr University at Bochum, Germany

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