High-performance catalysts, structural studies

In summary, the compilation of relevant case studies shows that XRD of working catalysts is a widely applicable technique. It gives rich and useful information about synthesis and activation of catalysts as well as deactivation by structural transformations. The pertinent question about the structure of the active sites is not accessible directly by this method despite such claims in the literature. It must be pointed out that this shortcoming of a technique involving characterization of samples in reactive atmospheres is common to all methods when one is concerned with high-performance catalysts in which the active sites are a small fraction of the active surface. Model systems do a better job in this respect, provided that they are active for the reaction of interest and not only in proxy reactions. 

Microwave-induced, catalytic gas-phase reactions have primary been pursued by Wan [63, 64], Wan et al. [65] have used pulsed-microwave radiation (millisecond high-energy pulses) to study the reaction of methane in the absence of oxygen. The reaction was performed by use of a series of nickel catalysts. The structure of the products seemed to be function of both the catalyst and the power and frequency of microwave pulses. A Ni/Si02 catalyst has been reported to produce 93% ethyne, whereas under the same irradiation conditions a Ni powder catalyst produced 83% ethene and 8.5 % ethane, but no ethyne. 

Three kinds of PAV films was prepared using methoxy pendant precursors. The chemical structures and synthetic route of the PAV films used in this study are shown in Fig. 19. The details of synthesis of the methoxy pendant precursors have been described in refs. 29 and 30. The precursors were soluble in conventional organic solvents, for example, chloroform, dichloromethane, benzene and so on. The precursor polymer thin films were spin-coated on fused quartz substrates from the chloroform solutions. The precursor films were converted to PAV films by the heat-treatment at 250 0 under a nitrogen flow with a slight amount of HC1 as a catalyst. This method provided high performance PAV films with excellent optical quality. 

Development of powerful spectroscopy and microscopy techniques, which allow us to study underlying chemical transformations that govern the performance of catalysts, including reaction mechanisms and the evolution of catalyst structure, with high spatial and temporal resolutions and at relevant conditions [2-6]. Development of density functional theory (DFT) methodology, which is utilized to study chemical transformations at the elementary step level with reasonable accuracy and efficiency [7]. DFT is particularly well suited for the treatment of extended metallic structures, which are often ideal model systems for heterogeneous catalytic processes [8-11]. 

Unfortunately, while catalyst components and structures in low-temperature MEAs have attracted considerable attention, optimization of high-temperature catalyst layer structures and components seems little studied. Lobato et al. [83] investigated the effect of the catalytic ink preparation method on the performance of HT-PEMFCs. They employed two methods for catalyst layer preparation the solution method and the colloid method. In the solution method, catalyst ink was prepared by mixing the catalyst (20% Pt/C) and PBl solution (5% PBl in dimethylacetamide). In the colloid method, acetone was added to the mixture of catalyst and PBI solution, which made the PBI form a colloid suspended in the solvent. They found that electrodes prepared by the solution method showed better performances at 150 °C and 175 °C, and that the electrodes prepared by die colloid method gave a better performance at 125 °C. This is probably due to differences in catalyst layer structure (see Section 18.2.7). 

In the literature, few studies have focused on performance improvement and mitigation of high-temperature catalyst layers. For LT-PEMFCs, materials used in the catalyst layer preparation are commercially available. For HT-PEMFCs, materials are not only different from those in LT-PEMFCs but also differ from study to study. For example, in PBI membrane-based MEAs, Pt/C catalyst and PBI ionomer were used in the catalyst layer [80-83]. However, in a CSH2PO4 membrane-based catalyst layer, no ionomer was used [22]. It is expected that improvement and mitigation of a high-temperature catalyst layer should depend on the materials used, and the catalyst layer structures should be optimized according to the materials employed. 

The same type of fused iron catalyst may exhibit different structures and activities after reduction under different conditions (e.g., temperature, pressure, space velocity and gas composition etc.). Reduction condition is the external factor which affects the physical-chemical properties of catalysts. Thus, different reduction conditions are required for catalysts with different t3rpes, particle sizes or different types and content of promoters. The selection of the optimized reduction condition is very important to obtain a high performance for ammonia synthesis catalysts. It is the main reason to study the reductive performance and related kinetics of catalysts. 

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