“Bridging the structure gap: Surface Chemistry at well defined defects”

One of the main goals of surface science is the understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase, in order to identify promoters and rate limiting factors at the atomic scale with the ultimate scope of designing more efficient catalysts. To reduce the complexity of the problem and focus attention on individual steps of the reaction of interest, most experiments have been performed so far under controlled, ultra-high vacuum conditions and on single crystal surfaces cut along low Miller index planes. On the other hand, catalytic reactions occurring industrially for the massive production of everyday life chemicals are far away from these well-defined conditions. Reactors work at high temperature and pressure, while the catalysts consist usually of supported powders exhibiting different atomic terminations and a high concentration of low coordinated sites (steps, kinks, vacancies etc.). The structural difference between the ordered samples used in surface science and the real catalysts is known as structure gap and, in the understanding of catalytic processes, it can be as relevant as the more widely invoked pressure gap, related to the difference in pressure between chemical reactors and ultra-high vacuum apparatuses. Although the importance of defect sites and the relevance of the structure gap have been evident for decades, the systematic study of defected surfaces began only recently, after a reasonable understanding of the simpler systems was reached. The most promising approach to this topic is the use of single crystal surfaces cut along high Miller index planes, i.e. stepped surfaces showing a high density of one majority low coordination site which mimics a defect. This approach allows a shortcut between the need for ordered substrates and controlled conditions and the availability of particular atomic configurations, a condition only partially mitigated with the advent of scanning probe microscopy with nanoscale resolution. Of course, only one defect-type at a time can be studied in this way. The present report summarizes the knowledge achieved so far for the gas–surface interaction in presence of well-defined defects and for simple reactions at such sites.