Palladium surface area

Fig. 5 Plot of H2 STY at 200 °C versus palladium surface area for some catalysts reported in the literature.

Aben PC. Palladium areas in supported catalysts determination of palladium surface areas in supported catalysts by means of hydrogen chemisorption. J Catal. 1968 10 224. 

Scholten J J and van Montfoort A 1962 The determination of the free-metal surface area of palladium catalysts J. Catal. 1 85-92... 

To add surface area, the supports are uniformly coated with a slurry of gamma-alumina and recalcined under moderate conditions. The wash coat acts to accept the active metals, typically low levels of platinum and palladium, in a conventional impregnation process. In the United States in passenger car apphcations the spherical catalyst is used almost exclusively, and methods have been developed to replace the catalyst without removing the converter shell when vehicle inspection reveals that emission standards are not met. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

Platinum and palladium are the most common catalysts for alkene hydrogenations. Palladium is normally used as a very fine powder supported" on an inert material such as charcoal (Pd/C) to maximize surface area. Platinum is normally used as PtC, a reagent called Adams catalyst after its discoverer, Roger Adams. 

Two approaches were taken to provide Insight to the uniformity of these samples 1) perform extensive visual Inspection of the materials to ensure that no large palladium crystallites were present, which was the case for both samples, and 2) perform surface area measurements using hydrogen TPD to establish the relationship between the observed data by STEM and the estimated surface area from theoretical considerations. 

Table 1 gives the textural properties of the support and catalyst samples. As expected the pore volumes and the surface areas of the catalysts are lower than those of the support. This indicates that the palladium blocks some part of the... 

The smaller surface area values of the catalysts indicate that the palladium blocks some part of the surface of this mesoporous carbon. The decreased amount of the CO-yielding complexes on the catalyst surface compared to that of the carbon support indicates that the palladium is attached to CO generating groups. The increased concentration of C02 containing complexes on the catalysts surfaces can be due to the steps of the preparation. To clarify this it needs further experiments. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

The surface areas of the iridium and palladium catalysts were determined by chemisorption of hydrogen and carbon monoxide, respectively, the monolayer volume being determined from an adsorption isotherm taken at 20°C. 

Alternatively, it may be possible to demonstrate for the pure metals that the catalytic activity is independent of film weight in a certain weight range. For example, rates of ethylene oxidation were constant over pure palladium films, deposited and annealed at 400°C and weighing between 4 and 40 mg (73). Then, if electron micrographs show that the crystallite size is relatively independent of composition, a satisfactory comparison of catalytic activity can be made at the various alloy compositions. Finally, surface area measurements are less urgently needed when activity varies by orders of magnitude, or where the main interest lies outside the determination of absolute reaction rates. 

The investigated catalysts [9] were Pt-Pd/USY zeolite (SiO2/Al203 ratio 33.5, total- and mesopore surface areas 650 m2/g and 51 m2/g, total metal content 0,9%, Pd/Pt mass ratio 6 1 to 1 3, dispersion 0.41-0.55, acidity 0.20 mmol/g). Metal contents and Pd/Pt ratios are summarized in Table 1. As reference catalysts bimetallic Pt-Pd/Si02-Al203 (platinum-content 0.3%, palladium-content 0.6%, A1203 content 15%, surface area 292 m2/g, dispersion 0.41, acidity 0.18 mmol/g) and Pt-Pd/y-Al203 (platinum-content 0.3%, palladium-content 0.6%, surface area 182 m2/g, dispersion 0.35, acidity 0.12 mmol/g) were used. 

The N2-physisorption characterisation results show that, no significant variations (less than 5%) are observed on the BET surface area, the total pore volume and the micropore volume of the different Pd-ZSM-5 catalysts, when the preparation method, the pretreatment gas, the charge-balancing cations and the palladium loading are modified. This result suggests that the ZSM-5 texture is stable with respect to the preparative parameter variations and that the observed activity differences are not related to any... 

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