Characterization of model catalysts

Freund H-J, Baumer M, Libuda J, Risse T, Rupprechter G, Shaikhutdinov S (2003) Preparation and characterization of model catalysts From ultrahigh vacuum to in-situ conditions at the atomic dimension. J Catal 216 223... 

Characterization of catalytic phenomena at oxide surfaces includes (1) characterization of established catalyst surfaces to improve the catalytic performance, (2) characterization of new catalysts in comparison with conventional catalysts, (3) characterization of specific model surfaces such as single crystals and epitaxial flat surfaces to transfer the knowledge so obtained to catalytic systems or even to create a new type of catalyst, and (4) characterization of catalysis... 

Before we discuss the template-controlled growth of model catalysts in more detail, we will have to consider a few aspects of STM imaging of these systems. This will be crucial for the characterization of the model catalyst surfaces. 

In this chapter, we introduce some of the most common spectroscopies and methods available for the characterization of heterogeneous catalysts [3-13], These techniques can be broadly grouped according to the nature of the probes employed for excitation, including photons, electrons, ions, and neutrons, or, alternatively, according to the type of information they provide. Here we have chosen to group the main catalyst characterization techniques by using a combination of both criteria into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characterization of real catalysts, and toward the end make brief reference to studies with model systems. Only the basics of each technique and a few examples of applications to catalyst characterization are provided, but more specialized references are included for those interested in a more in-depth discussion. 

Preparation and characterization of model and practical metallic catalysts... 

To understand heterogeneous catalysis it is necessary to characterize the surface of the catalyst, where reactants bond and chemical transformations subsequently take place. The activity of a solid catalyst scales directly with the number of exposed active sites on the surface, and the activity is optimized by dispersing the active material as nanometer-sized particles onto highly porous supports with surface areas often in excess of 500m /g. When the dimensions of the catalytic material become sufficiently small, the properties become size-dependent, and it is often insufficient to model a catalytically active material from its macroscopic properties. The structural complexity of the materials, combined with the high temperatures and pressures of catalysis, may limit the possibilities for detailed structural characterization of real catalysts. 

The cobalt and rhodium catalysts have one important difference between their respective mechanisms. Unlike in the rhodium-catalyzed process, there is no oxidative addition or reductive elimination step in the cobalt-catalyzed hy-droformylation reaction. This is reminiscent of the mechanistic difference between rhodium- and cobalt-based carbonylation reactions (see Section 4.2.3). The basic mechanism is well established on the basis of in situ IR spectroscopy, kinetic and theoretical analysis of individual reaction steps, and structural characterization of model complexes. 

The present contribution is a description of the technique of INS spectroscopy of catalysts and a summary of some recent experimental results that illustrate the usefulness of neutron spectroscopy. These include the characterization of model systems, commercial catalysts, mechanisms of coke deposition and catalyst deactivation, and the identification of atomic hydrogen in the topmost atomic layers of... 

The reproducible creation of model catalysts in the SMSI state has enabled the characterization of the chemisorption properties of model SMSI surface using a molecular beam reactor [8, 24]. The sticking probability of carbon monoxide is shown in Fig. 8.5 for model catalysts annealed to 573,633, and 693 K as a function... 

The electronic structure, morphology, and chemical reactivity of metal nanoclusters have attracted considerable attention due to their extensive technological importance. Chemical reactions and their catalytic relevance have been investigated on a variety of well-characterized, supported model catalysts prepared by vapor deposition of catalytically relevant metals onto ultrathin oxide films in ultrahigh vacuum conditions. Such ultrathin film supports are usually prepared by vaporizing a parent metal onto a refractory metal substrate in an oxygen atmosphere at a high temperature. These unique model systems are particularly well suited for surface-... 

The objective of this chapter is to show that particles in the mesoscopic regime have very different properties to the bulk phase and, specifically, to demonstrate how in-situ STM and FTIR spectroscopy have been successfully employed to determine information on the structure of model catalysts based on modification of substrate electrodes with metal particles of mesoscopic dimensions, and the effect of this structure on reactivity. It will be shown that studying these model electrodes helps provide a link between single-crystal electrodes, which have provided a wealth of useful information, and electrodes for real application. FTIR has long been invaluable as a probe for localized particle reaction on surfaces in electrochemical processes, and the present work will show how it can complement STM in providing excellent characterization of mesoscopic properties. 

To characterize the model catalysts in detail it is, however, necessary to identify the adsorption sites of the clusters. One possibility is to study the adsorption of probe molecules on the deposited clusters by means of thermal desorption (TDS)... 

The characterization of HDS catalysts has been the sub ject of a large number of papers, and virtually all the surface techniques and analytical tools available today, as well as powerful theoretical methods, have been extensively employed in order to tackle this exceedingly complicated problem [see e.g. ref. 15]. Tlie mass of information thus obtained has been interpreted in terms of several different models that have been evolving over the years into a rather sophisticated and well founded picture however, in spite of all the data available and of over seven decades of industrial practice, the exact nature and the structure of the catalytically active HDS sites of standard catalyst formulations continue to be the subject of controversy and frequent speculation. A great deal of the published work in this area has been devoted to the study of unpromoted catalysts in both calcined and sulfided forms, and this has resulted in the clarification of several important aspects nevertheless, for the sake of brevity, our description will concentrate essentially on the promoted Co-Mo catalysts in their sulfided forms, which are the ones most frequently used for practical purposes. Many excellent reviews widely cover the various theories and models which have been put forward for HDS active sites (see e.g. refs. 14, 15, and references therein) and thus there is no need to repeat that information at length here. 

Studies of Model Catalysts with Well-Defined Surfaces Combining Ultrahigh Vacuum Surface Characterization with Medium- and High-Pressure Kinetics... 

Transmission electron microscopy (TEM). Conclusions drawn from the above characterization studies were further corroborated by direct observation of model catalysts. These model catalysts were prepared by depositing the active components directly on gold microscope grids coated with a planar silica substrate. After deposition of the precursor salts, the grids were placed in a quartz flow reactor where in order to mimic the preparation of the real catalysts, they were subjected to the same pretreatments, as described in the catalyst preparation section. 

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