The structure and reactivity of surfaces revealed by scanning tunneling microscopy
- PDF / 475,074 Bytes
- 5 Pages / 585 x 783 pts Page_size
- 80 Downloads / 207 Views
Introduction The discovery of scanning tunneling microscope (STM) in the beginning of the 1980s was, to a large extent, an engineering feat,1 but STM has revolutionized the field of surface science with enormous impact in the basic and applied science of semiconductors and catalysis. STM is conceptually very simple and relies on the measurement of a small tunneling current (on the order of nA) flowing between an atomically sharp metal tip, which is raster-scanned at a subnanometer distance to the surface to be imaged. Atomic-scale control of the tip position is achieved through the use of piezoelectric materials such as lead zirconate titanate, which change their physical dimensions when a voltage is applied across the material. Also, great care must be taken to decouple the scanning tip from acoustic and ground vibrations that would otherwise smear the image.2 In this article, we focus on examples of unique STM applications to surface chemistry studies. As we shall see, together with information obtained from other microscopy and spectroscopy tools, as well as theory, STM is closer and better at providing insight to fundamental phenomena. This review will focus on examples of unique STM capabilities, including imaging the structure of active sites (e.g., steps, kinks, vacancies, and special atomic geometries in compounds); imaging dynamic processes such as diffusion of adsorbates and reactions; and surfaces with a high density of adsorbates under high pressures of gases. Such insight can be used to answer, and investigations are indeed very often motivated by, problems in catalysis, the
industrial application of which generates enormous economic value. Surprising perhaps to the outsider, atomic-scale insight into phenomena such as those listed previously can often be applied directly to understand the performance of catalysts employed in real-life applications. Examples are plenty, but in the present energy context, one might mention the three-way car catalyst,3 desulphurization of crude oil (see later),4 and fuel cells.5
Imaging the structure of active sites In the following examples, we will show how we are approaching a point where fundamental insights into surface structure and reactivity obtained by STM, in combination with other techniques, are enabling the design of new superior catalysts operating under technically relevant conditions, which can help meet today’s global energy challenges. The first example deals with steam-reforming catalysts, which are typically composed of Ni nanoclusters supported on a porous oxide. Steam reforming is a widely used catalytic process to yield carbon monoxide and hydrogen from water and a carbon source (methane, higher alkanes, etc.). Hydrogen, when produced, forms the feedstock in ammonia synthesis. One of the challenges of existing Ni-based catalysts is their rapid passivation by graphite, formed at high pressures and temperatures. While gold and nickel are immiscible in the bulk, high resolution STM images revealed6 that they form a surface alloy where Au atoms substitu
Data Loading...
