Methods of Catalyst Application

It must be appreciated, that surfaces of Nb, Ta, V and Zr will be immediately reoxidized or otherwise contaminated upon re-exposure to air, albeit with thitmer layers (or in the limit a monolayer) relative to thick native films often encountered before abrasion. As a minimum, monolayer adsorption of contaminants is almost certainly assured in all but the best ultra-high vacuum ( 1 x torr or 1 X 10 Pa) or in systems which simultaneously sputter substrate surfaces with argon or other inert ion during catalyst deposition. Some interfacial impurities between the substrate and the catalyst layer are tolerated in practice. However, the state of the substrate surface immediately prior to catalyst deposition is critical for wetting and adherence of the catalyst layers and for prevention of delamination. Theoretical flux maxima will not be achieved if thick impurity layers at the cata-lyst/substrate interface hinder hydrogen diffusion. 

Edlund et al. have used thin foils of palladium or platinum laminated onto Nb, Ta or V substrates. [46-52]. Edlund and Pledger used platinum-vanadium-palla-dium laminated composites at 973 K (700 °C) to decompose H2S at partial pressures up to 115 psia (7.9 bara) and total pressures up to 1000 psi (69 bar) [48]. Removal of hydrogen through the composite membranes aids the decomposition 

Hara et al., use a radio-frequency (RE) sputter deposition technique to deposit 100 nm thick films of palladium onto amorphous alloys of Zr3oNii54, Ti39Nioi and Hf3oNi64 [57, 58]. Eutectic alloy compositions are preferred for ease of melting and ease of forming amorphous alloys upon rapid quenching of molten alloys onto cold rotating plates. 

The group of Nishimura at Tsukuba has employed a wide variety of deposition techniques in their membrane work [59-68]. In their early studies, palladium was electroplated onto both sides of the substrates [61]. Electroless deposition using PdCl2 with SnCl2 sensitizers was also used [59, 60]. The authors recommend abrasion using 0.05 pm (50 nm) diameter alumina particles to remove thick native layers of contaminants after alloy fabrication and rolling of foils [60]. It should be noted that workers skilled in the art avoid abrasives such as diamond paste or silicon carbide (SiC) to nunimize possible formation of refractory carbides or silicides with the reactive substrates. 

The group of Nishimura has evaporated 100 nm thick films of palladium onto both sides of foils of vanadium alloyed with nickel, aluminum and molybdenum in an ultra-high vacuum system equipped for physical vapor deposition [62-66]. 

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