Hydrogen dissociation catalysts

In addition to innovative coatings for the protection of the hydrogen dissociation catalyst, mesoporous metal oxide layers for enhanced performance of the sensor elements were investigated. The results demonstrated the suitability of an electrodeposited tungsten oxide (WO3) as a chemochromic layer for hydrogen detection, providing a valuable alternative to vacuum deposition. 

Fig. 8.8 Schematic of a composite membrane coated with hydrogen dissociation catalysts on both sides. Catalyst poisoning by sulfur and competitive adsorption by molecules such as CO must be considered...

Membrane Hydrogen Dissociation Catalysts and Protective Layers... 

Role of Hydrogen Dissociation Catalysts on Hydrogen Transport Membranes... 

In one embodiment patented by Makrides et al., tantalum foils were electrolyti-cally etched in hydrofluoric acid, washed with acetone and, while still wet with acetone, placed in vacuum and dried by evacuation. Using an argon plasma at a pressure of about 1 mm Hg (133.3 Pa), palladium was deposited onto both sides of the membranes to recommended thicknesses between 10 and 100 nm. Membranes of niobium and vanadium were prepared in the same manner, except that, in addition, vanadium was degassed in vacuum at 1273 K (1000 °C) to remove oxygen. Unalloyed palladium as well as Pd-Ag, Pd-Au and Pd-B, were also patented as hydrogen dissociation catalysts and as protective layers for the highly reactive niobium, tantalum and vanadium. 

That is, hydrogen dissociates in the presence of the catalyst, forming hydrogen ions and giving up electrons to the anode. The hydrogen ions are transported across the membrane to the cathode. At the cathode, hydrogen ions react with oxygen to form H2O. 

Apparently, in the reaction of olefins with hydrogen on catalysts such as palladium and platinum, both the dissociative and the associative mechanisms operate for isomerization and exchange. However, the dissociative mechanism accompanies those factors which tend to slow the addition or accelerate the removal of hydrogen from either substrate or intermediate. These factors may be any of the independent variables, such as the pressure of hydrogen, the structure of the substrate, or the catalyst (5). 

From the H/M values for the catalysts NiSn-BM (Sn/Ni = 0.29) and PtSn-BM (Sn/Pt = 0.71), and the H/M values for the corresponding monometallic ones, it can be inferred that Sn blocks about 70% of the originally accessible M atoms. For these systems, based on the dispersion measured for Pt and Ni, the atomic ratios Sn/M correspond to values higher than 1. Notably, even in these cases, an important portion of the metallic surface has sites accessible to hydrogen dissociative adsorption, which is essential for the phase to be active in hydrogenation reactions. 

In any of the schemes presented, hydrogen dissociative adsorption on Pt is possible after Sn addition, as was checked by hydrogen chemisorption. From these results, it is possible to think of a scheme to represent the main reaction pathway during the hydrogenation of a,(3-unsaturated aldehydes. Depending on the catalyst used, such a scheme is shown in Figure 6.12, which summarizes the results from the characterizations and catalytic tests performed in this work [47]. 

New catalysts such as a Raney Cu-based system containing Zr49 and ultrafine CuB with Cr, Zr, and Th50 exhibit good characteristics. The improved catalyst performance observed for a Cu-ZnO catalyst with added Pd is explained by the relative ease of hydrogen dissociation by the incorporated Pd particles and then spillover to the Cu-ZnO.51... 

Estimate the dispersion of the nickel. Hydrogen dissociatively adsorbs on nickel whereas it does not interact with the catalyst support and is not significantly adsorbed within the nickel crystal lattice. Therefore, the amount of uptake of hydrogen by the catalyst is a measure of how well the nickel has been dispersed when deposited on the support. 

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