High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides

Abstract

The reactions between methane and niobium oxide cluster cations were studied and compared to those employing vanadium oxides. Hydrogen atom abstraction (HAA) reactions were identified over stoichiometric (Nb2O5)N(+) clusters for N as large as 14 with a time-of-flight mass spectrometer. The reactivity of (Nb2O5)N(+) clusters decreases as the N increases, and it is higher than that of (V 2O5)N(+) for N ≥ 4. Theoretical studies were conducted on (Nb2O5)N(+) (N = 2-6) by density functional calculations. HAA reactions on these clusters are all favorable thermodynamically and kinetically. The difference of the reactivity with respect to the cluster size and metal type (Nb vs V) was attributed to thermodynamics, kinetics, the electron capture ability, and the distribution of the unpaired spin density. Nanosized Nb oxide clusters show higher HAA reactivity than V oxides, indicating that niobia may serve as promising catalysts for practical methane conversion.

Thermal Dehydrogenation of Methane Enhanced by μ2-Oxo Ligands in Tantalum Cluster Cations [TaxO]+, x = 4, 5

“…As observed in the comparison of [MCH 2 ] + and [MO 2 ] + (M = V, Nb, Ta), , the increased reactivity of tantalum-containing compounds is generally attributed to relativistic effects that enable the formation of very strong Ta–C bonds. Instead of dehydrogenation and concomitant H 2 evolution, a direct hydrogen-atom transfer (HAT) mechanism prevails for many metal oxide systems that contain terminal , (and less for bridging) oxygen atoms with localized spin density, for example, stoichiometric vanadium and niobium oxide clusters . In these systems, the local charge influences reaction barriers .…”

Desorption of Oxygen from Cationic Niobium Oxide Clusters Revealed by Gas Phase Thermal Desorption Spectrometry and Density Functional Theory Calculations

“…The Stuttgart/Dresden effective core potential (SDD) was used for Nb and V atoms, where 10 electrons of V and 28 electrons of Nb were treated as the core, whereas for O, the 6-311G(d) basis set was used. For even n clusters, previously reported structures 10,11,15,16,24 + and V 3 O 7 + from their ion mobility measurements and those calculated by Asmis et al for infrared multiple photon dissociation spectroscopy were referenced. 13,16,25 The most stable geometries had singlet spin multiplicity, except for V 3 O 6 + in the triplet state.…”

“…15 He et al found that niobium oxide and vanadium oxide clusters could dehydrogenate methane and determined the most stable structures of (Nb 2 O 5 ) n + clusters. 10 They showed that niobium oxide clusters had higher reactivity than vanadium oxide clusters and that larger niobium oxide clusters are less reactive. Misaizu et al determined the structure of vanadium oxide clusters by ion mobility mass spectrometry.…”

“…The gas-phase cluster is a good model to understand catalytic activity on an atomic and molecular level, and niobium oxide clusters have been investigated both theoretically and experimentally. − Castleman et al studied the reactivity of cationic niobium and tantalum oxide clusters with n -butane . They found that certain cationic niobium and tantalum oxide clusters could activate the C–C bond of n -butane, whereas vanadium oxide clusters transferred an O atom to n -butane .…”

Oxygen Release from Cationic Niobium–Vanadium Oxide Clusters, NbnVmOk+, Revealed by Gas Phase Thermal Desorption Spectrometry and Density Functional Theory Calculations

“…DFT calculations were carried out to determine the stable structures of Nb n V m O k + clusters ( n + m = 2, 3, and 4) using the Gaussian09 program, and to investigate the binding energies of the clusters . Becke’s three-parameter hybrid density functional with the Lee–Yang–Parr correlation functional (B3LYP) was used for all the calculations. , The Stuttgart/Dresden effective core potential (SDD) was used for the Nb and V atoms, where 10 electrons of V and 28 electrons of Nb were treated as the core, whereas for O the 6-311+G­(d) basis set was used. , For n + m = 2 clusters, previously reported structures of Nb 2 O 5 + were referenced. ,,− The initial geometries of the clusters of interest were set by adding one O atom to and removing one O atom from possible sites. The most stable geometries had doublet spin multiplicity.…”