Diversity of Adsorbed Hydrogen on the TiC(001) Surface at High Coverages

Abstract

The catalyzed dissociation of molecular hydrogen on the surfaces of diverse materials is currently widely studied due to its importance in a broad range of hydrogenation reactions that convert noxious exhaust products and/or greenhouse gases into added-value greener products such as methanol. In the search for viable replacements for expensive late transition metal catalysts TiC has been increasingly investigated as a potential catalyst for H 2 dissociation. Here, we report on a combination of experiments and density functional theory calculations on the well-defined TiC(001) surface revealing that multiple H and H 2 species are available on this substrate, with different binding configurations and adsorption energies. Our calculations predict an initial occupancy of H atoms on surface C atom sites, which then enables the subsequent stabilization of H atoms on top of surface Ti atoms. Further H 2 can be also molecularly adsorbed over Ti sites. These theoretical predictions are in full accordance with information extracted from X-ray photoemission spectroscopy and temperature-programmed desorption experiments. The experimental results show that at high coverages of hydrogen there is a reconstruction of the TiC(001) surface which facilitates the binding of the adsorbate.

Critical Hydrogen Coverage Effect on the Hydrogenation of Ethylene Catalyzed by δ-MoC(001): An Ab Initio Thermodynamic and Kinetic Study

“…To investigate the effect of pre-adsorbed hydrogen on the overall process, several θ ML (14 H*), well within the limits observed for less H-affine TiC(001) surface. 31 Notice that when hydrogenating C 2 H 4 , the effective θ H decays 2 H* -0.13 ML, or 0.067 ML per hydrogenation step-although given the little impact on θ H on the reaction energetics, we will refer in the following to the initial considered θ H , this is, before C 2 H 4 first hydrogenation step. For θ H coverage values ranging from 0.13 to 0.50 ML H* occupies surface C sites, as predicted in previous works.…”

Critical effect of carbon vacancies on the reverse water gas shift reaction over vanadium carbide catalysts

“…The Perdew-Burke-Ernzerhof (PBE) exchange-correlation (xc) functional has been chosen [52], and dispersion -van der Waals-interactions have been included through the D3 method as proposed by Grimme and coworkers [53]. The PBE-D3 calculated energy differences are accurate enough to properly describe the interaction of reactants, intermediates, and products on TMC systems, as proven in the past on joint experimental and theoretical works [18][19][20]24,27]. The valence electron density has been expanded in a plane wave basis set with a kinetic energy cutoff of 415 eV.…”

“…VC has been selected because, compared to other TMCs, it displays a low affinity for oxygen [39][40][41], thus preventing the easy formation of oxycarbides. Furthermore, earlier DFT calculations showed that the hybridization between V d and C p orbitals yields hydrogen binding energies comparable to Mo 2 C and WC yet slightly smaller [19,22,42]. The small O affinity of VC anticipates a poor RWGS catalytic activity, destabilizing the adsorbed O adatom intermediate state formed during the CO 2 reduction to CO.…”

“…attach H 2 and split it [19][20][21], allows considering them as viable catalysts for CO 2 hydrogenation reactions [22].…”

Assessing the usefulness of transition metal carbides for hydrogenation reactions

“…"# of -0.60 eV. The Kubas mode for adsorbed H 2 , previously described in the literature for a number of the here contemplated TMCs, 14,[25][26][27] corresponds to an almost dissociated H 2 molecular state where both atomic H species are adsorbed on the same C atom, with H 2 bond lengths ranging 1.68 to 1.80 Å, see Table S2 of the ESI, and having a non-negligible interaction between them, estimated to be in between -0.55 to -0.92 eV, see Table S2 of the ESI. The adsorption energies reported on Table 1 show that the formation of the Kubas mode is actually thermodynamically favorable on group IV TMCs and -MoC, with adsorption energies stronger than those of molecular H 2 adsorption on top-C or top-M, yet Kubas H 2 is not thermodynamically favored on group V TMCs.…”