Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts

The dissociation of H2 is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H2-D2 exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The catalysts were prepared by dealloying of bulk alloys, Al80Cu20 and Al80Cu19Ti1, to produce the npCu and npTiCu catalysts, respectively. The promotional effect of Ti was measured via steady-state H2-D2 exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300─523 K. Pretreatment with flowing H2 is required for stable catalytic performance. Two pretreatment temperatures, 573 K and 673 K, were investigated for both the undoped and Ti-doped npCu catalysts. The experimentally-determined H2-D2 exchange rate is 5-7 times greater for npTiCu vs. the undoped material under optimized pretreatment and reaction temperatures. The H2 pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in-situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Both catalysts also contained similar, small amounts of residual aluminum from the dealloying which is present in the form of alumina nanoparticles that did not contribute to the differences in catalytic activity. The apparent activation energy and pre-exponential factors measured for H2-D2 exchange are substantially different for Ti-doped vs. undoped npCu catalysts. DFT calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption starting from gas phase H2 or D2 on Cu to H/D atom recombination towards chemisorbed H2/HD/D2 on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.