Base metal oxidation catalysts, comparison

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

A Comparison of Platinum and Base Metal Oxidation Catalysts... 

Nb- and Ti-containing silica-based mesoporous molecular sieves were used as catalysts for photocatalytic oxidation of methane. It is found that methanol was formed at 323 K and water pre-adsorbed samples exhibited higher catalytic activity than their pure metal oxides. By comparison with Nb-MCM-41, Ti-MCM-41 gave better methane conversion and methanol yield although traces of formaldehyde were observed over Nb-MCM-41 catalyst. Interestingly, OH radical was detected by the in-situ ESR spectroscopy using DMPO as trapper. 

It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, etc.) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity (A) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers (N), but also on the specific activity of one center (propagation rate constant, Kp) and on the effective catalyst surface (Sen) as well ... 

The tltanla-based thin film catalyst models were constructed by first oxidizing the titanium surface In 5 x 10 torr of O2 for approximately 30 minutes at 775 K. This produced an AES llneshape consistent with fully oxidized TIO2. The metal was then vapor deposited onto the oxide support with the latter held at 130 K. The thickness of the metal overlayer and Its cleanliness were verified by AES. After various annealing and adsorption procedures, these thin films were further characterized using SSIMS, AES and TDS. For comparison, some work was done with Pt on Al20s. In this case a Mo foil covered with AI2O3 replaced the Tl(OOOl) substrate. 

The results obtained with the Tishchenko reaction of furfural using alkaline earth metal oxides as base catalysts are presented in Table III. Because the strength of basic sites increases in the order MgOacid strengths is the reverse 73), it was concluded that CaO and SrO— which have moderate acid and base sites in comparison with MgO and BaO—are appropriate... 

Hydrocarbon oxidation on base metal catalysts is also susceptible to lead poisoning, especially if the catalysts are exposed to relatively high temperatures, for at least part of their service time. It was noted above that lead retention, especially on base metal catalysts, also increases with temperature up to a certain point. This behavior is shown by the results of Yao and Kummer (81) in Fig. 18. One should note that the hydrocarbon used for testing catalyst activity, namely propylene, was quite reactive. With a less reactive test hydrocarbon one could expect a still sharper effect. The comparison with a reference production noble metal catalyst, given in Fig. 18, is quite instructive. 

Wang, H-F, Kaden, WE, Dowler, R, Sterrer, M, and Freund, H-J. Model oxide-supported metal catalysts - comparison of ultrahigh vacuum and solution based preparation of Pd nanoparticles on a single-crystalline oxide substrate. Phys Chem Chem Phys. 2012 14 11525-11533. 

As previously said, water does not inhibit the formation of species I at high temperatures (where species I can be formed by decomposition of species III), and this agrees with the absence of any detectable effects of water in the formation of NO by ammonia oxidation. According to the above mechanism, it is also expected that the reaction NO + NH3 will not be affected by the presence of H2O. In previous work it was found that water inhibits the SCR reaction on metal oxide based catalysts [2,16], but to a lower extent in comparison with ammonia oxidation. However, the SCR reaction occurs at lower temperatures with respect to ammonia oxidation to NO, so that at these conditions species III and species I are still in competition. 

Most of the work cited above has dealt with treating the soot in some way before doing the combustion experiments. We wish to report experiments conducted on soot from a diesel vehicle which has been deposited onto catalytic monolithic substrates. This sooted substrate is then placed in a laboratory apparatus where a synthetic gas mixture flows over the sample, and the soot combustion is monitored as a function of temperature. The laboratory set up simulates regeneration conditions on a vehicle. Using this technique we have been able to obtain kinetic information about the oxidation of soot and gaseous products. Comparisons of base metal and noble metal catalysts were also conducted and are reported. It is intended that this work will help elucidate the mechanism involved in the catalytic combustion of soot which should help in developing improved catalytic materials. 

These experiments provide a direct comparison of the initial activities of platinum and base metal catalysts. Differences in performance— produced by such variables as catalyst bed mass, exhaust gas space velocity, and catalyst temperature—are explained by the effect of converter size on warm-up rates and by the kinetic differences for oxidation reactions over the two types of catalysts. 

Comparison of Noble Metal and Oxide Catalysts. - A few studies have directly compared the activity of noble metal and oxide based catalysts. The combustion of a range of C5-C9 hydrocarbon VOCs in humidified air by 0.1% Pt/3% Ni/Al203 and ceria promoted hopcalite commercial catalysts has been compared [96]. The Pt based catalyst showed no deactivation during 253 continuous operation, whilst over 297 days the temperature of hopcalite catalyst required an 85°C increase to maintain > 99% conversion. However, the final operating temperature of the hopcalite catalyst was 400°C, 30°C lower than the isothermal operating temperature of the Pt system. A first order concentration deactivation model was developed and predicted a 362 day lifetime for the... 

Enzymes are biocatalysts and involve in the speed up of slow biochemical processes. In general, enzymes are released again after a reaction ceases and can continue in another reaction. Practically, this process cannot go forever, since more catalysts have limited stabilities and, slowly, they become inactive. In the food industry, enzymes are often used once and then they are discarded. In comparison to inorganic catalysts, i.e., acids, bases, metals, and metal oxides, enzymes have very specific functions. Enzyme actions are limited to specific bonds in their reactions with various compounds. An enzyme molecule usually binds to the substrate(s) and a specialized part(s) of it to catalyze the substrate into a product. For each type of reaction in a cell, there is a different enzyme. The specific actions of enzymes in industrial processes usually obtain high production yields with a minimum level of by-products. 

Noble metal-metal oxide dumbbell-shaped NPs have been synthesized based on seed-mediated growth. Metal oxides are grown over the pre-synthesized noble metal seeds by the thermal decomposition of the metal carbonyl followed by oxidation in air. They show enhanced catalytic activity towards CO oxidation in comparison with their counterparts [94]. Heterostructured Cu2S-ln2S3 with various shapes and compositions can be obtained by a high-temperature precursor-injection method wherein Cuj is used as the catalyst for the nucleation and growth of In Sj NPs [95]. 

Mn02 was the most active among the tested metal oxides under the reaction conditions used. Comparison of the activities of the Mn02-based catalytic system for the MW-assisted and CH methods in the benzyl alcohol oxidation (with O2 or air) revealed that MW irradiation significantly increases the reaction rate (by circa two times) with similar selectivities ( hot spot effect) [23]. The tested Mn02 catalyst can be recovered and recycled (the activity remains unchanged at least for a few cycles) the catalyst should be washed with deionized water and acetone and then dried at 120 °C [23]. 

The presence of thallium(0) led to an increase in activity and selectivity of metallic palladium catalysts supported on silica in aldose oxidation reactions. However, silica-supported thallium(0) had no activity by itself (entry 3). ° Similarly, the bimetallic catalyst platinum-thallium/ZSM-5, prepared by impregnation of thallium sulfate and chloroplatinic acid on Zeolite Socony Mobil-5 (ZSM-5), showed greater selectivity in propane aromatisation and almost the same catalytic activity as monometallic thallium/ZSM-5 (entry 4). Similar comparison of vanadium-caesium-copper and vanadium-caesium-copper-thallium catalysts supported on TiOa.SiC demonstrated that addition of thallium improved the catalytic activity in partial oxidation of p-tert-butyltoluene to p-tert-butyl-benzaldehyde (entry The application of solid-supported thallium-based catalysts in different processes includes (a) iron-thallium catalysts in carbon monoxide hydrogenations to form hydrocarbons and alcohols, and catalytic reforming of... 

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