Model surfaces, infrared studies

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

Although much progress has been made toward understanding the nature and probable catalytic behavior of active sites on CoMo/alumlna catalysts, much obviously remains to be accomplished. Detailed explanation of the acidic character of the reduced metal sites evidently most important In HDS, and presumably In related reactions, must await the Increased understanding which should come from studies of simplified model catalysts using advanced surface science techniques. Further progress of an Immediately useful nature seems possible from additional Infrared study of the variations produced In the exposed metal sites as a result of variations In preparation, pretreatment, and reaction conditions. 

Tolbert, M. A., and A. M. Middlebrook, Fourier Transform Infrared Studies of Model Polar Stratospheric Cloud Surfaces Growth and... 

Tolbert, M.A., and Middlebrook, A.M. (1990) Fourier Transform infrared studies of model polar stratospheric cold surfaces growth and evaporation of ice and nitric acid/ice, J. Geophys. Res. 95, 22423-22431. 

There is a number of vibrational spectroscopic techniques not directly applicable to the study of real catalysts but which are used with model surfaces, such as single crystals. These include reflection-absorption infrared spectroscopy (RAIRS or IRAS) high-resolution electron energy loss spectroscopy (HREELS, EELS) infrared ellipsometric spectroscopy. 

Fourier transform infrared step-scan photoacoustic spectroscopy was developed to study the composition of thermoplastic olefin films, as a function of depth below the surface. Infrared bands associated with talc, polypropylene (PP), and ethylene-propylene mbber (EPR) were used as depth-profiling probes to identify photoacoustic signals. Experiments were done at various modulation frequencies, enabling a stratification model to be developed. The uppermost layer (0-3 micrometre) showed large changes in talc and PP concentration, whilst the layer below showed a significant decrease in both the phases. In the third layer (6-9 micrometre), all three phases showed the maximum values. In the fourth layer (9-12 micrometre), the talc concentration reduced, whilst concentrations of EPR and PP were observed, decreasing with depth. 32 refs. 

It is of course extremely difficult to model the detailed surface chemistry which might occur on grains since the nature of the grain material is very poorly characterised. However there is some indication that surface processes might contribute to molecule formation in addition to the accretion effects discussed in 2.1. Infrared studies of CH4 and SiH4 absorption lines show that the molecules exist in a region in which the terminal velocity of the outflow has been reached but the molecules have no absorption components at velocities less than the terminal velocity as, for example, does CO. Indeed detailed models of the 8184 lines show that this species cannot exist in IRC+10216 closer to the star than 30 R ... 

Surface bound ethylidene groups have been modelled in an infrared study by inter aUa,... 

Samples prepared as described above can be good models for working catalysts of either the oxide or metal types and many infrared studies of surface phenomena are undertaken in conjunction with catalytic investigations. Loose powders can alternatively be studied by diffuse reflection, with the advantage for kinetic studies that surface reactions, rather than diffusion processes, are more likely to be rate determining than is the case with the fine-pored pressed discs. 

The experimental data and arguments by Trassatti [25] show that at the PZC, the water dipole contribution to the potential drop across the interface is relatively small, varying from about 0 V for An to about 0.2 V for In and Cd. For transition metals, values as high as 0.4 V are suggested. The basic idea of water clusters on the electrode surface dissociating as the electric field is increased has also been supported by in situ Fourier transfomr infrared (FTIR) studies [26], and this model also underlies more recent statistical mechanical studies [27]. 

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