Role of Oxygen Species toward the C–C Bond Cleavage in Steam Reforming of C₂₊ Alkanes: DFT Studies of Ethane on Ir(100)

Abstract

Understanding C−C bond cleavages has become important in developing a cost-effective technology for the steam reforming of natural gas, the primary form of hydrogen production. Previous studies of C−C bond cleavages in C 2+ steam reforming have focused solely on the C 2+ H x species. Here, we report the density functional theory (DFT) studies of 10 C−C bond cleavages of ethane decomposition (C 2 H x , x = 0−6) on Ir(100). We also investigated the effects of O and OH species that are present in steam ethane reforming (SER) on these cleavages. The DFT results demonstrate that coupling O with the C 2 H x species decreases activation energy, from >1 to 0.3 eV. Therefore, the C−C bond rupture in SER, along the minimum energy pathway, is likely to take place in C 2 H x O. The findings suggest that a good steam reforming catalyst also needs to facilitate the C 2 H x and O coupling, which would not only improve C−C bond cleavage but also prevent C deposition, a major cause of catalyst deactivation. Furthermore, the activation energy and reaction energy surfaces were constructed for 34 C−C bond cleavage reactions, which allow for direct performance comparisons among catalysts and the selection of a catalyst of interest beyond the minimum energy path.

Insights and Activation Energy Surface of the Dehydrogenation of C2HxO Species in Ethanol Oxidation Reaction on Ir(100)

“…The above model catalyst and the computational techniques are the same as those used in our studies of the other 18 dehydrogenation reactions of EOR. [40,70,76,82] We summarize below the numerical details of the DFT calculations.…”

“…Efforts have been devoted to the characterization of the complex chemical reaction networks in the chemical reaction space, [78] such as systematic enumeration of elementary reaction steps in surface catalysis, [79] utilization of machine learning, [80,81] and developments of variables to describe activation and reaction energy surfaces. [76,82] To describe the reaction network involving CÀ C bond cleavages in C 2 H x O y (x = 1-6; y = 0, 1), we used DoDH and DoBC to confer the activation energies of all 34 elementary reactions in an activation energy surface. [82] This not only allows a direct comparison among different reactions, but also provides a convenient vehicle to determine the relevant elementary reactions to be included in the kinetics studies under different reaction conditions.…”

“…[76,82] To describe the reaction network involving CÀ C bond cleavages in C 2 H x O y (x = 1-6; y = 0, 1), we used DoDH and DoBC to confer the activation energies of all 34 elementary reactions in an activation energy surface. [82] This not only allows a direct comparison among different reactions, but also provides a convenient vehicle to determine the relevant elementary reactions to be included in the kinetics studies under different reaction conditions. The energy graphs constructed including all 46 dehydrogenations using three variables, DoDH, DoBC, and p, are hypersurfaces.…”

“…If the CÀ O cleavage takes place, the CÀ C scissions of the generated C 2 H x intermediates become very challenging, leading to a CH 3 C/CHC poisoning on Ir(100). [82] When reaction temperature increases to 500 K (Figure 7(b) or higher like the ethanol steam reforming conditions, 45 dehydrogenations are likely to occur and the CÀ C bond cleavages may also take place in C 2 H 3 O and C 2 H 4 O species. Figure 7(b) shows clearly that all 23 dehydrogenation reactions are below the yellow plane and can take place readily.…”

“…In the DFT calculations, the top two layers of the model catalysts were allowed to fully relaxed without any constraints in single‐point, optimization, and transition state calculations and only the top layer was allowed to fully relaxed in frequency calculations. The above model catalyst and the computational techniques are the same as those used in our studies of the other 18 dehydrogenation reactions of EOR [40,70,76,82] . We summarize below the numerical details of the DFT calculations.…”