Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

Abstract

Molecular hydrogen is the simplest and most abundant compound in the universe and is involved in numerous industrial chemical processes. In conventional chemistry, dihydrogen typically plays the role of a reductant and a reagent for homogeneous and heterogeneous hydrogenation processes such as the industrial and enzymatic ammonia formation, reduction of metallic ores and hydrogenation of unsaturated fats and oils. However, there are also processes in which molecular hydrogen participates as promoter, and even as catalyst. The catalytic role of the dihydrogen in free-valence migration in irradiated polymers and the interstellar isomerization of the formyl cation (protonated carbon monoxide) are well-documented examples of such processes. Recently, this issue has received new attention. Dihydrogen has been shown to play the role of a dehydrogenation catalyst (involving particularly metallocomplexes and inorganic materials), a relay (pass-on) transfer molecular agent and a transporter of protons.This review article, combined with original results, is focused on the mechanisms of the chemical processes where dihydrogen demonstrates catalytic behavior. We will call these processes (with somewhat broader meaning of the term) "dihydrogen catalysis" (DHC) which also includes the reactions mediated by transition metal dihydrides. Dihydrides are tentatively considered as pre-activated dihydrogen, coordinated to a metal center or implanted into a solid surface/support. DHC reactions are classified into five major reaction types: (i) dihydrogen-assisted relay transport of H-atoms (H2-RT); (ii) dihydrogen-assisted stepwise relay transport of H-atoms or of a free valence (sH2-RT); (iii) dihydrogen-assisted proton transport (H2-PT); (iv) dihydrogen-assisted dehydrogenation (H2-DeH); and (v) pre-activated dehydrogenation (PA-DeH). The classification of these mechanisms is Color versions of one or more of the figures in the article can be found online at www. tandfonline.com/lctr. 403 Downloaded by [George Mason University] at 02:43 28 September 2014 R. Asatryan and E. Ruckensteinbased on a detailed analysis of numerous potential energy surfaces studied by DFT and ab initio methods in conjunction with available experimental data. The H2-RT, H2-DeH, and PA-DeH processes occur via cyclic transition states. The relay H2-RT transport involves the H-H-H triad linked to both H-donor and H-acceptor centers, whereas the transition state ring in the H2-DeH dehydrogenation processes involves a H-H-H-H tetrad with the dihydrogen catalyst located in the middle. The H2-PT mechanism provides the transport of a proton mediated by dihydrogen combined in a triangular (H 3 + )-carrier unit. There are also practically important processes stimulated by dihydrogen such as the hydrogen spillover and hydrogen build-up in electronics, in which the catalytic role of dihydrogen is ambiguous, either because of the uncertainties in mechanisms, or prevailing traditional views. Some examples are briefly discussed in the framework of the concept of dihydrogen ...

Roaming-like Mechanism for Dehydration of Diol Radicals

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H2Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H2Activation at the Metal–Support Interface

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Atmospheric Chemistry of CH2OO: The Hydrolysis of CH2OO in Small Clusters of Sulfuric Acid

“…As a major precursor of atmospheric aerosols, H 2 SO 4 plays an important role in climate by affecting the characteristics of clouds. 23,24 It has been demonstrated 25−28 that H 2 SO 4 can exert a positive catalytic influence on atmospheric hydrolysis reaction routes 21,22,27−29 and hydrogen-transfer pro-cesses 25,26,28,30,31 by reducing the magnitude of the activation barrier 25−32 and promoting the effect of hydrogen transfer. 21,22 Compared with the catalysts H 2 O, NH 3 , and HCOOH, H 2 SO 4 may have better catalytic effects for some reaction systems.…”

“…As a major precursor of atmospheric aerosols, H 2 SO 4 plays an important role in climate by affecting the characteristics of clouds. , It has been demonstrated − that H 2 SO 4 can exert a positive catalytic influence on atmospheric hydrolysis reaction routes ,,− and hydrogen-transfer processes ,,,, by reducing the magnitude of the activation barrier − and promoting the effect of hydrogen transfer. , Compared with the catalysts H 2 O, NH 3 , and HCOOH, H 2 SO 4 may have better catalytic effects for some reaction systems. ,,,,,, Consequently, it is worth inquiring about whether H 2 SO 4 has a catalytic effect in promoting the hydrolysis of CH 2 OO. Besides, previous investigations − showed that the formation of H 2 SO 4 ···H 2 O or (H 2 SO 4 ) 2 is probably the first step in the process of nucleation when H 2 SO 4 is responsible for nucleation.…”