Bulk platinum catalyst

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

The exchange of this compound has been examined quite thoroughly. Kauder and Taylor 11) used platinized platinum as a catalyst and Thompson et al. 13) used a cobalt-thoria Fischer-Tropsch catalyst. The reaction has also been studied over a series of metal films by Kemball 18) and more recently over a number of bulk metal catalysts by Addy and Bond 44)-... 

Although platinum catalysts once were used in the manufactme of sulfuric acid, the only catalysts presently in use employ supported vanadia. ° For om problem we shall use a catalyst studied by Eklimd, whose work was echoed extensively by Donovan" in his description of the kinetics of SO, oxidation. The catalyst studied by Eklimd was a Reymersholm V2O5 catalyst deposited on a pumice carrier. The cylindrical pellets had a diameter of 8 mm and a length of 8 mm, with a bulk density of 33.8 Ib/ft. Between 818 and 1029°F, the rate law for SO, oxidation over this particular catalyst was... 

Platinum and platinum alloy electrocatalysts are by far the most studied in the PEFC and in model, bulk, metal catalyst/ionomer interfacial systems devised to study the electrocatalytic process in a PEFC environment. The reason is that, to date, no other electrocatalyst has been demonstrated to approach the specific activity exhibited by platinum (or platinum alloys) in PEFCs, particularly in the more demanding... 

Osmium, iridium, and platinum catalysts with dispersions (ratio of surface metal atoms to total metal atoms) in the range 0.7 to 1 have been studied by Via, Sinfelt, and Lytle.As expected for small particles, the average co-ordination numbers were between 7 and 10, significantly lower than the value of 12 for the bulk metals. This result is in agreement with gas chemisorption data. Also, the disorder of the metal atoms, represented by the r.m.s. deviation of interatomic distance about its equilibrium value, was found to be greater by factors of 1.4—2.0 than for atoms in the bulk metals. Information on such disorder has not been available previously. 

A hypothetical reaction has been used to develop the previous results. It is of interest to know the magnitude of the global rate for real situations. Fortunately, considerable experimental data are available. Measurements for the oxidation of SO2 with air on a platinum catalyst, deposited on 1/8 x 1 /8-in. cylindrical pellets," gave a global rate of 0.0956 g mole/(hr)(g catalyst). The bulk temperature of the gas was 465°C and the gas velocity in the catalyst bed was 350 lb/(hr) (ft ). The bulk composition corresponded to a 10% con- R. W. Olson, R. W. Schuler, and J. M. Smith, Chem. Eng. Progr., 42, 614 (1950). 

Olson and Smith measured the rate of oxidation of sulfur dioxide with air in a differential fixed-bed reactor. The platinum catalyst was deposited on the outer surface of the cylindrical pellets. The composition and the rates of the bulk gas were known. The objective was to determine the significance of external diffusion resistance by calculating the magnitude of — C. If this difference is significant, then the values must be used in developing a rate equation for the chemical step. 

X-Ray Diffraction Studies. The dependence of the lattice parameter of bulk platinum-iridium alloys on composition is shown in Figure 4.22 (4,45). Lattice parameters are commonly obtained from X-ray diffraction measurements. For platinum-iridium catalysts, X-ray diffraction measurements provide a way of demonstrating the presence of bimetallic clusters of platinum and iridium, if the metal dispersion is not too high. 

In relation to platinum highly dispersed as particles with sizes of the order of 10 A, Sinfeld makes the points (i) that X-ray diffraction is of little use for such cases and (ii) that semi-empirical analysis of the observed platinum EXAFS indicates the average coordination number of the platinum atoms in such catalysts to be lower than for bulk platinum metal [96a]. Furthermore, Sinfeld et al. have prepared bimetallic Ru/Cu clusters of very small size dispersed on oxide supports of very large surface area. The resultant high proportion of surface relative to bulk metal atoms represented favourable conditions in which surface metal atoms and their environment would dominate EXAFS. Semi-empirical analysis of the EXAFS led Sinfeld et al. to conclude that Cu atoms in surface layer(s) of the small bimetallic Cu/Ru clusters had (on average and in contrast to microscopic non-miscibility) several near-neighbour Ru atoms [96b]. 

Catalyst Properties. Of the four catalysts evaluated, two were prepared at the General Motors Research Laboratories (GMR) and two were commercially formulated, with one base metal and one platinum catalyst in each group (Table IV). All the catalysts were bulk, pelleted types... 

Surface Area Compared to bulk catalysts the supported catalysts have the advantage that they utilize the catalytically active metal much more efficiently. In bulk catalysts most of the metal is used to actually support the active metal and thus is lost for the catalytic process. Therefore, the support offers a high surface area for maximum dispersion of the metal crystallites. This is very important for precious metal catalysts, where the value of the precious metal (Pd, Pt, Rh, Ru) is very high. For example 100 kg of a commercial 5 wt% platinum catalyst contains 5 kg of platinum metal with a very high value. Current state-of-the-art precious metal powder catalysts have crystallite sizes in the range 2 to 20 nm. The crystallite size can be adjusted by the preparation method (e.g., it is possible to achieve 2.5-nm crystallites with a very tight size distribution on a commercial... 

In order to improve the electrocatalytic properties of methanol electrodes, and to reduce the poisoning phenomenon usually observed with bulk platinum, different platinum based alloys were considered such as Pt-Ru, and Pt-Sn, etc. [153]. Therefore such alloys were also dispersed into electron conducting polymers. Hable et al. [53] were apparently the first authors to disperse Pt-Sn catalyst particles in a polyaniline matrix, in order to activate the oxidation of methanol. They evaluated the Pt/Sn ratio by X-ray Photoelectron Spectroscopy and found that small amounts of Sn (e.g. Pt/Sn ratios of 10/1) were sufficient to enhance the electrocatalytic oxidation of methanol. Pt was found to be in the Pt(0) state whereas Sn was in an oxidized form. Similar observations concerning the enhanced electrocatalytic activity of Pt-Sn particles incorporated in PAni films were made by Laborde et al. [154]. Such Pt-Sn alloys are also very active for the electrocatalytic oxidation of ethanol [68,154]. 

Pt-NPs are used as catalysts for electrochemical hydrogen peroxide (H2O2) detection, where they act as modifiers of the electrode surface and electrocatalyze the oxidation of H2O2 observed by a lower oxidation peak potential when compared with the bulk platinum electrode [30]. As the H2O2 is a product of many enzymatic reactions, this electrode has a vast potential application as an electrochemical biosensor for many substances [15]. 

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