Precious-metal catalysts, evaluation

Some of the earliest work on precious metal catalysts for CPO of methane was carried out by Ashcroft et al.41 In their work, Pr2Ru207 was evaluated for CPO performance. A methane conversion of >90% and selectivity of 94%-99% were achieved. Pure Ru metal was detected on the catalyst surface and believed to be the active component during the reaction. 

The engineering target is to eventually create a catalyst that is both highly selective and highly active over the entire operating temperature range of the diesel engine. This paper concentrates on evaluation and characterization of base-metal oxide catalysts and precious metal catalysts for total oxidation of ethanol. 

Research supporting the development of a compact water gas shift (WGS) reactor subsystem included catalyst screening studies, kinetic model development, and test reactor design and performance evaluation. Water gas shift catalysts obtained from commercial and other developers were converted into an engineered form and tested versus temperature, space velocity, and steam-to-gas ratio in single-channel reactors. Both base metal and precious metal catalyst formulations were included in the studies. 

Industry is seeking high yield, zero waste processes. Companies are evaluating the replacement of some base metal catalysts by more selective precious metal catalysts to eliminate troublesome by-product formation and/or contaminated waste waters. In the case of Chlorofluorocarbons (CFC s) industry is trying to develop cost effective processes to manufacture non-toxic Hydrofluorocarbons (HFC s) of zero ozone depleting potential to replace existing CFC s. 

The effect of supported precious metal catalysts on PAH emissions from diesel and gasoline engines has been evaluated with the following conclusions ... 

The performance of abase metal catalyst in pellet form was evaluated with acrylonitrile and results indicated that the manganese-based catalyst was more active than the chromium-based catalyst. Both manganese and manganese doped with copper showed complete conversion of acrylonitrile at around 260°C, while the chromium catalyst showed only about 50% conversion at 500 C, as shown in Fig. 7.17. When the performance of the precious metal catalysts was compared to that of the base metal catalysts it was found that ... 

Details about preparation and characterization of dispersed microcrystals can be found in review chapters [322] and will not be dealt with here. All investigations indicate that the properties of microcrystals differ considerably from those of bulk metals (and from those of adatoms and thin films as well) [328], and that they can also be influenced by the nature and texture of the support. In particular, micro-deposits of precious metals on various inert supports (Ti, Ta, Zr, Nb, glassy carbon etc.) exhibit enhanced electrocatalytic effects as evaluated per metal atom, while the mechanism of H2 evolution remains the same [329], and the enhancement increases as the crystallite size decreases [326, 331] (Fig. 17). However, while this is the case with Rh, Pt, Os and Ir, Pd shows only an insignificant increase, whereas for Ru even a drastic decrease is observed [315, 332]. Thus, the effect of crystal size on the catalytic activity appears to depend on the nature of the catalyst (without any relation with the crystal structure group) [330]. 

Evaluate new cathode catalyst and materials for higher operating efficiencies at lower precious metal loadings. 

Most often, in the heterogeneous catalysis the catalysts are soUds — usually metals, very often precious metals — while the reactants are liquid or gaseous. The catalysts are usually composed of a number of (metal) components in various combinations. The tests are performed in parallel under the otherwise equal temperature and pressure conditicms in a large number of microreactors. fii each microreactor the combination of the tested catalyst components is different and this results in a different yield or selectivity of the test reactirm. This is usually evaluated by composition analysis of the reaction products. 

Pressure differential scanning calorimetry (PDSC) provides a faster method for the evaluation of precious metal reduction catalysts. The efficient... 

U.S. Department of Energy s 2014 Atmual Merit Review and Peer Evaluation Meeting Presentation (2014) Non-precious metal fuel cell cathodes catalyst development and electrode strucmre design, http //www.hydrogen.energy.gov/pdfs/review 14/fc 107 zelenay 2014 o.pdf... 

Although the shaped nanocrystals have shown a high potential for the enhancement of catalytic activity, selectivity, and long-term stability, there are several issues that need to be resolved to expand their use to practical applications. The shaped nanocrystals are typically too large to effectively utilize expensive precious metals. The stability of the shaped nanocrystals should be carefully evaluated. The shape is often deformed too easily, especially at high temperatures where many practical gas-phase reactions occur. Shaped nanoparticles should also operate stably for long periods of time. Mass production of shaped nanocrystals is another important issue to be addressed properly. Indeed, much work on shaped nanoparticles is still needed to fundamentally understand their catalytic reactions and develop practical catalysts with better catalytic properties. 

Nickel boride has been used [43] as electrocatalyst for the H2 oxidation in recent years. An evaluation of this process on the basis [44] of different catalyst loadings demonstrated that the performance improved considerably with temperature between 60° and 115°C at 4 mg nickel boride/cm. A linear relation between catalyst loading and performance was not found. Anodes with low loadings performed less well than those with high loadings. None of the nickel boride electrodes could sustain a current of 5 mA at room temperature. The performance of anodes with precious metal loadings of 1.25 mg/cm was markedly superior to that of any of the nickel boride electrodes at a given temperature. 

An intermediate approach to limit the use of Pt and instead use Pd-based metal alloys has shown reasonable success in activity improvement. Although fairly precious, Pd is currently four times less expensive than Pt. Pd alloyed with Mo, Ta, W, Re, and Cu have all been evaluated. A Pd-Cu (1 1) catalyst composed of 20 nm nanoparticles prepared by co-impregnation showed activity within range of Pt and is being further pursued [63, 83]. 

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