Platinum-based metal catalysts

With regard to the electro-catalyst the main research issue is to identify a platinum-based catalyst, i.e. a binary, ternary or quaternary catalyst composed of platinum and one or more transition metals that will be more active (and thereby further reducing the applied potential), exhibit an improved lifespan, and have reduced platinum loadings to reduce the cost. The NWU, located in the North-West province of South Africa where the majority of the world s platinum is mined and produced, is currently setting itself up for the synthesis, characterisation and testing of platinum-based electro-catalysts specifically for normal water electrolysis as well as for S02 electrolysis. 

Commercial SCR catalyst used in connection with coal-based power stations are generally composed of base metals, since platinum-group metal catalysts are too readily poisoned and have too narrow an operating temperature window for this application. Favored compositions are titania-based together with active components, normally oxides of vanadium, tungsten, or molybdenum. For these systems the optimum reaction temperature is usually in the range 3(XM00°C. 

All licensors agree on the necessity of hydrotreating the feed to lower the level of poisons for the platinum-based reforming catalyst. Temporary poisons are sulfur and nitrogen, while As, Pb, and other metals are permanent poisons. Proper conditions of hydrogen, pressure, temperature, and space velocities are able to reduce these poisons to the acceptably low levels of modern catalysts. Numerous process design modifications and catalyst improvements have been made in recent years. 

In practical terms this has constrained the development of only three basic control strategies (refs 9,10) in the context of stringent legislation. These are all based upon application of supported platinum group metal catalysts. The strategies are ... 

The activities of fresh, supported platinum and base metal oxidation catalysts are evaluated in vehicle tests. Two catalysts of each type were tested by the 1975 FTP in four 600-4300 cm3 catalytic converters installed on a vehicle equipped with exhaust manifold air injection. As converter size decreased, base metal conversions of HC and CO decreased monotonically. In contrast, the platinum catalysts maintained very high 1975 FTP CO conversions (> 90% ) at all converter sizes HC conversions remained constant 70% ) at volumes down to 1300 cm3. Performance of the base metal catalysts with the 4300-cm3 converter nearly equalled that of the platinum catalysts. However, platinum catalysts have a reserve activity with very high conversions attained at the smallest converter volumes, which makes them more tolerant of thermal and contaminant degradation. 

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. 

FTP Emissions. The overall system performance during the 1975 FTP tests for HC and CO emissions as a function of catalytic converter volume is plotted in Figures 1 and 2 respectively. Emissions are expressed in g/mile of vehicle operation with the cold start, hot stabilized, and hot cycle emissions weighted as prescribed (3). There was a distinct difference in the performance of the base metal and platinum catalysts with decreasing converter volume. Both HC and CO emissions with base metal catalysts increased monotonically as converter volume was decreased. In contrast, when platinum catalysts were used, both HC and CO emissions decreased to a minimum at 1300 cm3 and then increased at the smallest volume. 

For both catalysts, the rate of increase in bed temperature was greater for smaller converter volumes although there was little difference between the 600- and 1300-cm3 converters. Catalyst temperature increased more rapidly with the platinum catalyst. Greater conversion efficiencies (Table V) and hence greater energy release rates were attained with this catalyst during warm-up cycles 1-5 of the FTP. The fraction converted by the platinum catalysts was maximum at the 1300-cm3 converter volume. Conversion efficiencies with the base metal catalysts increased continuously as volume increased. 

Conversion efficiencies relative to the base line vehicle emissions are tabulated in Table VI. In general, the response of both types of catalyst to decreasing converter volume was similar to that for the 1975 FTP emissions. Both HC and CO emissions continually increased as base metal catalyst volumes were decreased. HC emissions were relatively constant for platinum catalyst volumes greater than 1300 cm3. CO conversions of 97-98% were produced by all combinations of platinum catalyst and converter volume. 

CO Control. Over base metal catalysts, the catalytic oxidation of CO is of the order of 0.7-1.0 in CO with little dependence on 02 concentration (8, 9, 11, 12). CO oxidation over platinum catalysts is of negative first order dependency on CO concentration (13) with some tendency toward a positive dependency at very high conversion (9). However, this last observation may have been the result of mass transfer limited kinetics (9). 

Supported platinum and base metal catalysts were evaluated in vehicle tests with converter volumes of 600-4300 cm3. The initial oxidation activity of the catalysts was determined as the vehicle was operated over the 1975 FTP. The ability of the base metal catalysts to control exhaust HC and CO emissions was strongly dependent on the catalyst volume. HC and CO conversion decreased quite rapidly as the converter size was decreased. 

Base metal catalysts have been used but not with the success achieved with platinum. Further work is now being done at various places on base metal catalysts and excellent progress is being made in this direction. Attempts are also being made to carry out the reaction with oxygen rather than air In order to facilitate absorption of the nitrogen oxides formed or to enable the product to be obtained in the form of liquid nitrogen tetroxide. This liquid... 

The development of these catalysts occurred in an atmosphere of tight secrecy, and little has been published on those efforts. All three were of the particulate type one contained noble metal(s) while the other two were noble-metal promoted base-metal catalysts (16). The UOP noble metal catalyst was provided several years later to GM and to Ford for evaluation with unleaded fuel and for other studies, and it can be deduced that it was supported on low-density (about 0.32 g/mL apparent bulk density) 1/8 inch spheres (17) and may have contained about 0.47 troy ounces of platinum per cubic foot (ca. 0.16 weight percent), with the platinum concentrated in a subsurface shell some distance below the exterior surfaces of the spheres for improved poison resistance (18) The Cyanamid catalyst was later studied by Ford (16, 19) it apparently was an extrudate (1/8" diameter x 1/8" long) of about 0.67 g/mL ABD, with about 125 ppm (weight) of palladium and 5 weight % each of CuO and 2 5 Si02 95% AI2O3 support of about... 

In extension to the dehydrogenation studies of amine boranes and the transfer hydrogenations of alkenes catalyzed by the Re(I) complexes 16 and 17, highly efficient hydrogenations of alkenes were established based on the co-catalytic systems of 16/Me2NH-BH3 or 17/Lewis acid, which exhibited catalytic activities comparable to those of Wilkinson- or Schrock-Osbom-type hydrogenations accomplished with platinum group metal catalysts [24]. 

As described before, a route to improve the electrocatal34ic properties of platinum, and to decrease the poisoning of its surface by adsorbed CO or CO containing species, is to prepare alloys with a second metal (or third metal). Many different binary and ternary platinum-based anode catalysts, such as PtRu, PtSn, PtMo, PtRuMo, PtRh/Sn02, or PtRuW, have been examined... 

Fuel Cells, Non-Precious Metal Catalysts for Oxygen Reduction Reaction Platinum-Based Cathode Catalysts for Polymer Electrolyte Fuel Cells... 

The catalysts based on ruthenium undoubtedly have priority in the TH reactions of ketones due to their excellent, sometimes approaching enzyme-like, efficiencies, selectivities, and their rich chemistry which allows the introduction of diverse types of chiral ligands. However, the possibility of using other platinum-group metal catalysts have been demonstrated as a valid alternative to ruthenium systems. Since the first reports of Ir-based ATH of ketones, for example by Graziani and coworkers in 1982 [87], interest in iridium catalysts, which have often been successfully used in TH of olefins, has been growing [88]. 

The study includes the use of both honeycomb and pelleted catalysts. The precious metal catalysts (PM catalysts) are platinum (Pt) and platinum/palladium (Pt/Pd), and the base metal catalysts are manganese, manganese copper and chromium. The platinum catalysts consist of one with a high surface area (150 m /g) and one with a low surface area (25 m /g). The platinum high surface area catalyst will be referred as the Pt (HS) catalyst in the text and the one with low surface area will be the Pt (LS) catalyst. 

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