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

Abstract

Late-transition-metal oxides have emerged as promising materials for enabling direct catalytic conversions of light alkanes to value-added products due to the ability of certain facets of these oxides to promote alkane C–H activation at low temperature. This review discusses the current understanding of alkane activation and oxidation on crystalline surfaces of late-transition-metal oxides, determined mainly from ultrahigh-vacuum (UHV) surface science experiments and density functional theory (DFT) calculations of alkane chemistry on the PdO(101), RuO2(110), and IrO2(110) surfaces. These studies show that chemically adsorbed alkane σ-complexes serve as precursors for initial C–H activation and that the formation of these adsorbed intermediates is critical to the high C–H activity of late-transition-metal oxides. The article discusses relationships between the surface structure and alkane C–H activity of these oxides. The binding and initial activation of alkane σ-complexes is also discussed, with an emphasis placed on establishing the underlying factors that produce differences in alkane activity among different oxides and alkanes. Thereafter, the current understanding of the mechanisms for C1–C3 alkane oxidation on the IrO2(110) surface is presented. This work reveals key reaction steps that govern the oxidation activity and selectivity of IrO2(110) and thereby provides insights into properties that may be manipulated to promote partial oxidation pathways, resulting in enhanced selectivity toward the conversion of alkanes to value-added products. On the basis of the current knowledge developed from fundamental studies, general challenges and opportunities for utilizing late-transition-metal oxides in catalysts for selective alkane oxidation are identified.

Metathesis by Partner Interchange in σ‐Bond Ligands: Expanding Applications of the σ‐CAM Mechanism

“…[26,31,33] s-Alkane complexes have also been directly characterized on metal oxide surfaces,s uch as RuO 2 or PdO,a tl ow temperatures (e.g.90K)using acombination of temperatureprogrammed desorption, surface IR spectroscopy and DFT techniques. [51] Reassuringly,t hese M•••H-C interactions are broadly similar to those observed and calculated for molecular species,albeit now with the possibility of interaction with multiple surface metal sites for alkanes larger than methane (Figure 2). Theinteraction of cyclic alkanes with small (Ru 13 ) nanoparticles has been studied using DFT computational methods to understand empirically observed H/D exchange processes.T hese calculations indicate the formation of salkane complexes on the nanoparticle surface prior to C À H bond cleavage.…”

“…90 K) using a combination of temperature‐programmed desorption, surface IR spectroscopy and DFT techniques. [51] Reassuringly, these M⋅⋅⋅H‐C interactions are broadly similar to those observed and calculated for molecular species, albeit now with the possibility of interaction with multiple surface metal sites for alkanes larger than methane (Figure 2 ). The interaction of cyclic alkanes with small (Ru 13 ) nanoparticles has been studied using DFT computational methods to understand empirically observed H/D exchange processes.…”

“…σ‐Alkane complexes have also been directly characterized on metal oxide surfaces, such as RuO 2 or PdO, at low temperatures (e.g. 90 K) using a combination of temperature‐programmed desorption, surface IR spectroscopy and DFT techniques [51] . Reassuringly, these M⋅⋅⋅H‐C interactions are broadly similar to those observed and calculated for molecular species, albeit now with the possibility of interaction with multiple surface metal sites for alkanes larger than methane (Figure 2).…”

Activity of Ir(100) and IrO2(110) for the Catalytic Oxidation of Methane

“…The IrO 2 (110) surface has a rectangular unit cell (3.16 × 6.36 Å 2 ) and is composed of separate rows of coordinatively unsaturated Ir (Ir cus ) and bridging O atoms (O br ) (Figure S1). Both the Ir cus and O br atoms have single coordination vacancies, and the presence of these metal/oxygen pairs is responsible for the high activity of IrO 2 (110) toward alkane oxidation . The stoichiometric termination of IrO 2 (110) features equal concentrations of Ir cus and O br atoms.…”

Graphene Materials for Miniaturized Energy Harvest and Storage Devices

“…To address these problems, extensive electrode materials of different dimensions including 0D quantum dots, [ 12 ] 1D nanotubes and graphene nanoribbons, [ 13–15 ] nanowires and nanorods, [ 16,17 ] 2D nanosheets, [ 18 ] etc., have sprung up. Among these materials, 2D materials such as graphene and graphene oxide (GO), [ 19 ] MXene, [ 20 ] transition metal dichalcogenides, [ 21 ] transition metal oxides (TMOs), [ 22 ] and black phosphorus (BP) [ 23 ] have been widely used in flexible and miniaturized wearable electronic devices due to their accessible active sites, shortened ion transfer distance, excellent mechanical strength and flexibility caused by in‐plane covalent bonds, and inter‐layer van der Waals interactions. In particular, unlike the MXene that are easily oxidized in water/oxygen environment, poor conductive TMOs, and intrinsically unstable BP, graphene as a conjugated single layered carbon structure possesses high electrical conductivity, robust mechanical flexibility and stability, [ 24 ] exhibiting great potential in the application of miniature energy harvesting and storage devices (MEHSDs).…”