Lee Jennifer D, Qi Zhen, Foucher Alexandre C, Ngan Hio Tong, Dennis Kevin, Cui Jun, Sadykov Ilia I, Crumlin Ethan J, Sautet Philippe, Stach Eric A, Friend Cynthia M, Madix Robert J, Biener Juergen
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
J Am Chem Soc. 2022 Sep 21;144(37):16778-16791. doi: 10.1021/jacs.2c00830. Epub 2022 Sep 2.
The dissociation of H 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-D 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-D exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300-573 K. Pretreatment with flowing H is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H-D exchange rate is 5-7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H 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-D 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.
氢的解离是许多工业化学转化过程中必不可少的基本步骤，通常需要使用贵金属。在此，我们报道了一种掺杂少量钛的分级纳米多孔铜催化剂（npTiCu），与未掺杂的纳米多孔铜（npCu）催化剂相比，它能使氢-氘交换速率提高约一个数量级。通过在300-573K温度范围内的大气压流动条件下进行稳态氢-氘交换反应实验，测量了钛的促进作用。为了获得稳定的催化性能，需要用流动氢气进行预处理，并研究了523K和673K这两个温度。在优化的预处理和反应温度下，实验测定的npTiCu的氢-氘交换速率比未掺杂的铜材料高5-7倍。如原位常压X射线光电子能谱和X射线吸收光谱所示，氢气预处理导致制备的催化剂中氧化铜完全还原，表面钛氧化物物种部分还原。对于掺杂钛和未掺杂的npCu催化剂，氢-氘交换的表观活化能和指前因子有很大差异。密度泛函理论计算表明，铜主体表面孤立的金属钛原子可作为氢重组的活性表面位点。交换速率高于纯铜的增加主要是由于速率决定步骤从铜上的解离吸附转变为掺杂钛的铜上的氢/氘原子重组，同时产生相应的活化熵降低。
