Carbon monoxide alloy effects

Measurements of the infrared spectra of carbon monoxide on supported palladium and Pd-Ag atoms (75a) shed light on the relative importance of the ensemble and ligand effects. Three CO absorption bands were observed on palladium and its alloys at 2060,1960, and 1920 cm-1. 

Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255) discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table III, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256). 

The idea is not new that dissolved hydrogen can modify the activity of metal catalysts. Hall and Emmett (9) list a large number of workers who have supplied supporting evidence. Emmett, Kokes, and Hall showed that the behavior of dissolved hydrogen can be quantitatively related to the behavior Of copper-nickel alloys. This important contribution bears directly on the present discussion because the effect of adding copper to nickel is also amenable to study by means of the spectra of chemisorbed carbon monoxide. 

Early studies of carbon monoxide chemisorbed on copper-nickel alloys were complicated by the failure to realize the importance of dissolved hydrogen when the experiments were conducted (10). However recent infrared studies have shown that addition of 1 to 2% copper to nickel causes the band to shift to lower frequencies due to linear chemisorbed carbon monoxide (11). This shift supports the idea that copper and dissolved hydrogen have similar modifying effects on the electronic properties of the nickel. An argument developed below, based on the spectral changes, shows that these modifying effects are consistent with the view that electrons are transferred to the nickel. 

H.A. Gasteiger, N.M. Markovic, P.N. Ross, Structural effects in electrocatalysis Electrooxidation of carbon monoxide on Pt3Sn single-crystal alloy surfaces. Catal. Letters 1996, 36(1-2), 1-8. 

Evidence for ensemble effects in VIIIC/IB alloys has been obtained by examination of carbon monoxide adsorption by infrared spectroscopy. This technique has been applied to the systems Pd-Ag, Ni-Cu, and Pd-Au. It is generally accepted that carbon monoxide may chemisorb in bridged or linear forms, the former providing an absorption band in the region 1900-1950 cm and the other in the region 2000-2050 cm . There may be a distinguishable contribution to the former from CO bonded... 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

The catalytic activity of CaNis hydrogen storage alloy is studied for decomposition of diethyl ether in the temperature range between 570 and 720 K by an atmospheric flow method. The alloy showed a catalytic activity at temperatures higher than 570 K. Following mechanism is proposed on the basis of the effect of W/F on the composition of products. Ethane and acetaldehyde are the initial products, while methane and carbon monoxide are the secondary products, formed by the decomposition of the acetaldehyde. 

F. Sakamoto, Effect of carbon monoxide on hydrogen permeation in some palladium-based alloy membranes, Int. J. Hydrogen Energy 1996, 21(11/12), 1017-1024. 

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