Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti–Cu Catalysts

Abstract

The dissociation of H 2 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 H 2 −D 2 exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H 2 −D 2 exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300−573 K. Pretreatment with flowing H 2 is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H 2 −D 2 exchange rate is 5−7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H 2 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. The apparent activation energies and pre-exponential factors measured for H 2 −D 2 exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory 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 on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Boosting the H2–D2 Exchange Activity of Dilute Nanoporous Ti–Cu Catalysts through Oxidation–Reduction Cycle–Induced Restructuring

“…[ 36,37 ] Under these conditions, Ti oxide is thermodynamically favorable, leading to the segregation and oxidation of Ti on the surface. Oxidized Ti surface species are then reduced into metallic Ti as demonstrated in our recent work [ 38 ] and can be expected to remain on the surface at the end of the redox cycle.…”

“…To obtain the steady‐state HD conversions, we performed quantitative MS analysis following a previously reported method. [ 38,43 ]…”

“…To obtain the steady-state HD conversions, we performed quantitative MS analysis following a previously reported method. [38,43] Transmission Electron Microscopy: Scanning TEM (STEM) was performed with an aberration-corrected JEOL NEOARM, operating at 200 kV. A condenser lens aperture of 40 μm was used.…”

“…Additionally, it has been shown that the activity of a catalyst is highly influenced by its surface geometry. [38][39][40] In summary, the H 2 -D 2 exchange reactions tests shown in Figure 7 are the result of the modification of the surface structure upon redox cycling. They show the possibility to regenerate and improve the catalytic activity in nanoporous metal catalyst structures through a simple reduction-oxidation process.…”

“…[1,2] Dilute alloying with more reactive elements provides an opportunity to further improve activity and/or selectivity. [3] For example, in a recent study, we have shown that dilute Ti-alloying dramatically increases the activity of npCu for hydrogen dissociation, [4] which is a key reaction step for hydrogenation reactions. [5,6] In fact, the combination of dilute Ti with Cu modifies the adsorption energy of H 2 on the catalyst, which leads to improved activity and selectivity.…”