The dissociation of H2 is an essential elementary step in catalytic hydrogenation reactions, typically requiring the use of precious metals. One approach to mitigating the use of precious metals is to employ dilute alloys of copper as selective hydrogenation catalysts, based on the principle that a more reactive metal can initiate the catalytic cycle. This approach has been demonstrated using late transitional metal dopants, e.g., Ni and Pd, in Cu catalysts. By contrast, early transition metal dopants have received limited attention despite their high reactivity. In this work, we demonstrate that doping of Ti in hierarchical nanoporous (np) Cu increases the rate of H2-D2 exchange by a factor of seven, suggesting a dilute alloy bimetallic catalyst of reduced cost may be effective for selective hydrogenation. A combination of experiment and theory indicates that isolated, metallic Ti atoms in the surface layer of the Cu host facilitate H2 dissociation and are the active sites for H2 or D2 dissociation; ensuing migration between Ti centers leads to HD production. The DFT calculations further reveal that the higher activity of npTiCu compared to pure npCu is associated with entropic effects due to a change in the rate-limiting step. Furthermore, the promotional effect of Ti observed for H2 dissociation translates well to facilitating hydrogenation reactions, including the hydrogenation of 1-hexyne and crotonaldehyde. In both reactions, higher activities were observed over npTiCu compared to pure npCu. The promotional effect of Ti observed in this work opens up opportunities to expand dilute alloy catalyst design to include early transition metals for hydrogenation reactions.

Dilute nanoporous Ti-Cu catalysts for hydrogen-deuterium exchange and selective hydrogenation reactions

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