74 Oxidation catalysts: new trends Gabriele Centi* and Siglinda Perathoner Some highlights of the advances in the field of heterogeneous selective oxidation catalysts include, firstly, the design of new materials based on an understanding of the working state of solid catalysts, secondly, the search for new classes of catalytic materials and thirdly, the development of advanced catalysts that take effective advantage of new reactor options. active catalytic species. However, the catalysts used in these reactions are of the 'nonredox' type and thus do not demonstrate that carbonaceous species may form over clas- sical 'redox' type catalysts, that is, catalysts where lattice oxygen oxidizes hydrocarbons and is then replenished by gaseous oxygen. Addresses Dipartimento di Chimica Industriale, University of Messina, Salita Sperone 31,98166 Messina, Italy *e-mail: centi@scirocco.unime.it Current Opinion in Solid State & Materials Science 1999, 4:?4-?9 Electronic identifier: 1359-0286-004-00074 © Elsevier Science ttd ISSN 1359-0286 Introduction The development of catalysts and selective oxidation reac- tions is a main area of interest in catalysis, but the discov- ery of new innovative catalysts and reactions has progressed quite slowly in recent years. Much of the research activity has focused on obtaining a further under- standing of known catalytic materials rather than on inno- vative directions such as: 1. The design of new materials based on advances in understanding the working state of solid catalysts. 2. The search for new classes of catalytic materials which can open a field of new applications rather than being just a solution for a single, specific problem. 3. The development of advanced catalysts which take effective advantage of new reactor options. These advanced catalysts are based on a combination of a solid state and reaction engineering approach. This review covers articles published in the past year, selected according to the above three directions of research. Other examples of new concepts in selective oxi- dation have also been recently discussed elsewhere [1°°]. Discussion Catalyst design spurred by new aspects of surface chemistry The formation of carbonaceous deposits on catalysts is a known aspect of solid catalysts, but usually it is not con- sidered in the case of selective oxidation catalysts. Only in a few cases, such as catalysts for firstly, ethylbenzene oxidative dehydrogenation to styrene [2]; and secondly, cyclohexanone ammoximation to the corresponding oxime [3], has evidence been presented on either the formation of carbonaceous deposits or their direct involvement as Delmon and co-workers [4 °°] showed that carbonaceous species form over MoO3-containing catalysts in the selec- tive oxidation of isobutene to methacrolein. In particular, their work provided evidence for the formation of carbon balls 10 nm in diameter, organized in rows parallel to the [101] direction of the MoO 3 crystals. Evidence of the for- mation of an ordered array of carbonaceous species in situ during the catalytic reaction of a typical oxidation catalyst and the reaction of selective oxidation indicated that this phenomenon of the formation of carbonaceous type species during catalytic reactions is possibly a common aspect of the surface reactivity of oxides in selective oxi- dation reactions. In a study of the surface properties of (VO)zPzO 7 for n-butane oxidation [5] it was also observed that carbona- ceous species form during the reaction and two key roles of these species were suggested; firstly, control of the surface diffusion of adspecies; and secondly, selective inhibition of unselective sites towards maleic anhydride synthesis. Both these aspects are key factors to enhancing selectivity. The observation of Delmon and co-workers [4 °°] of the ordered deposition of carbonaceous species over molybdenum oxide during isobutene oxidation demonstrates the above suggestions and thus opens an interesting field on how to use this concept to tailor the surface properties of oxidation catalysts, for example, by a pretreatment which makes it possible to selectively create ordered patches of carbona- ceous species. The question of surface restructuring during a catalytic reaction is also a central aspect of understanding and designing oxidation catalysts, but usually only indirect evi- dence has been reported on this topic. Millar et al. [6°°,7"] have reported direct in situ imaging of surface phenomena occurring on a polycrystalline silver catalyst for methanol oxidation to formaldehyde in conditions close to those of industrial operation. Using environmental scanning electron microscopy they observed that pinholes were created in the vicinity of defects. These holes gradually increased in size and progressively reconstructed the entire surface. The methanol reacted with subsurface oxygen to produce sub- surface hydroxyl species which ultimately formed water in the subsurface layer of the silver. The resulting hydrostatic pressure forced the silver surface to adopt a 'hill and valley' conformation. The production of formaldehyde was corre- lated with the pinhole concentration. This result shows how