Mixed-Anion Control of C–H Bond Activation of Methane on the IrO₂ Surface

Abstract

In this paper, an orbital correlation diagram is proposed for the purpose of understanding and predicting a surface reaction. To deal with the electronic structure of a surface, one may need to rely on a cluster model or a surface slab model. Band (crystal) orbitals calculated at the Γ point in the reciprocal space for the unit cell of the slab model with periodicity are found helpful for the construction of the correlation diagram. By using the diagram thus established, the C–H bond activation reaction of methane on an IrO2 surface is investigated. The energy level of the d z 2 orbital of a coordinatively unsaturated Ir atom in the surface is found to be important for determining the activation barrier of the reaction. The activation energy can be reduced by lowering the energy level of the d z 2 orbital. Conversely, a rise in the energy of the d z 2 orbital leads to an increase in the activation barrier. To make the d z 2 orbital energy change, the concept of mixed-anion compounds is adopted. The replacement of an oxide with a different anion allows one to tune the crystal field splitting of metal oxides. IrO2 doped with F as an axial ligand yields a lower-lying d z 2 orbital level, while the orbital goes up in energy when IrO2 is doped with N. This trend is consistent with what is expected from the electronegativity of each dopant. A perfect inverse linear correlation is found between the activation energy of the reaction and the electronegativity. By changing the dopant, one may have control over the reactivity of IrO2.

Electronic Origin of Catalytic Activity of TiH2for Ammonia Synthesis

“…A series of solid-state materials that contain multiple anionic species in a single phase are known as mixed-anion compounds. , Compared with a single-anion compound such as hydrides, mixed anion compounds such as the nitridated hydride would show a rich catalytic chemistry due to their unique coordination and electronic structures. In the literature, one can find a systematic investigation of the effects of the replacement of a sub-surface anion with another on catalysis …”

“…In the literature, one can find a systematic investigation of the effects of the replacement of a sub-surface anion with another on catalysis. 69 Whether the N atom is located in the subsurface site or on the surface, the activation barrier for the reaction of NH 2 * + H* → NH 3 * becomes lower than 0.9 eV. As discussed in sections S15 and S16 of the Supporting Information, a high activation energy step with an energy barrier of about 0.9 eV has been identified in the early stage of the reaction.…”

Competition between Hydrogen Bonding and Dispersion Force in Water Adsorption and Epoxy Adhesion to Boron Nitride: From the Flat to the Curved

“…This Walsh diagram is derived from eH calculations for the N­(BH 2 ) 3 structures optimized with DFT. One could draw the diagram with higher-level calculations, but for this purpose, we used the eH theory, the qualitative results of which do carry over to other levels. , The eH parameters used for this calculation are tabulated in the Supporting Information (SI).…”

Alkane Activation and Oxidation on Late-Transition-Metal Oxides: Challenges and Opportunities

“…IrO 2 (110) both shows a higher density of states above the Fermi level and is better aligned with the occupied orbitals of CH 4 , resulting in larger bonding contributions for cus -Ir versus cus -Ru (Table ). This understanding of CH 4 binding on rutile oxides along with an exploration of metal and anion doping of the rutile structure suggests that there are opportunities to optimize materials for C–H activation; ,− however, future work attempting to synthesize these model surfaces still is required.…”

“…Indeed, IrO 2 (110) is the only surface known to activate CH 4 and other light alkanes well below room temperature (∼100 K) and produces high coverages of C x H y groups. − Of significance is that these hydrocarbon intermediates remain stable over a wide temperature range (∼100–400 K) and are thus available, in principle, for selective transformations to value-added products. These recent discoveries have inspired efforts to understand the underlying factors responsible for the high activity of IrO 2 (110) toward alkane C–H cleavage − and develop insights needed to rationally design IrO 2 -based materials to selectively oxidize light alkanes to more valuable compounds, including olefins and organic oxygenates. − A key challenge for realizing this goal is to mitigate the extensive oxidation of light alkanes and instead promote their partial oxidation while maintaining high initial C–H activity.…”