Dihydrogen catalysis: A degradation mechanism for N2-fixation intermediates

Document Type

Article

Publication Date

11-29-2012

Abstract

Molecular hydrogen plays multiple roles in activation of nitrogen. Among others, it inhibits the overall process of N2-reduction catalyzed by nitrogenase enzyme. The H2-assisted dehydrogenation and the H-atom transfer reactions (called dihydrogen catalysis, DHC) are suggested as possible mechanisms for the degradation and removal of potential intermediates formed during the reduction of nitrogen. Several iron-organic model reactions associated with the core stereospecific reaction (cis-N2H2 + H2 → N2 + H2 + H2) are examined using a comprehensive density functional theory and ab initio analysis of the corresponding potential energy surfaces. A variety of energetically feasible decomposition pathways are identified for the DHC-oxidation of iron-bound [NxHy]-species. A liberated diazene intermediate (HN=NH) is suggested to interact in situ with two proximal hydridic H-atoms of an activated (hydrided) Fe-catalyst to produce N2 and H2 with a low or even no activation barrier. The majority of identified pathways are shown to be highly sensitive to the electronic environment and spin configuration of metallocomplexes. The H 2-assisted transport of a single H-atom from a bound [N xHy] moiety to either the proximal or distal (Fe, S or N) active centers of a catalyst provides an alternative degradation (interconversion) mechanism for the relevant intermediates. The two types of molecular hydrogen-assisted reactions highlighted above, namely, the H 2-assisted dehydrogenation and the transport of H-atoms, suggest theoretical interpretations for the observed H2-inhibition of N 2 activation and HD formation (in the presence of D2). The DHC reactions of various [NxHy] moieties are expected to play significant roles in the industrial high-pressure hydrodenitrification and other catalytic processes involving the metabolism of molecular hydrogen. © 2012 American Chemical Society.

Identifier

84870412155 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp303692v

e-ISSN

15205215

ISSN

10895639

First Page

11618

Last Page

11642

Issue

47

Volume

116

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