Cluster reactivity experiments: Employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions

Abstract

Mass spectrometry is the most widely used tool in the study of the properties and reactivity of clusters in the gas phase. In this article, we demonstrate its use in investigating the molecular-level details of oxidation reactions occurring on the surfaces of heterogeneous catalysts via cluster reactivity experiments. Guided ion beam mass spectrometry (GIB-MS) employing a quadrupole-octopolequadrupole (Q-O-Q) configuration enables mass-selected cluster ions to be reacted with various chemicals, providing insight into the effect of size, stoichiometry, and ionic charge state on the reactivity of catalyst materials. For positively charged tungsten oxide clusters, it is shown that species having the same stoichiometry as the bulk, WO 3 ؉ , W2O 6 ؉ , and W3O 9 ؉ , exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene (C 3H6), suggesting the formation of propylene oxide (C 3H6O), an important monomer used, for example, in the industrial production of plastics. Furthermore, the same stoichiometric clusters are demonstrated to be active for the oxidation of CO to CO 2, a reaction of significance to environmental pollution abatement. The findings reported herein suggest that the enhanced oxidation reactivity of these stoichiometric clusters may be due to the presence of radical oxygen centers (W-O•) with elongated metal-oxygen bonds. The unique insights gained into bulk-phase oxidation catalysis through the application of mass spectrometry to cluster reactivity experiments are discussed.catalysis ͉ tungsten oxide ͉ oxygen radical ͉ carbon monoxide ͉ propylene M ass spectrometry has been crucial to the field of cluster research since its inception (1-9). A variety of cluster formation methods, primarily including laser vaporization (LaVa) (10-12), pulsed-arc discharge (PACIS) (13-15), electrospray ionization (ESI) (16), gas aggregation (17-18), and inert gas sputtering (CORDIS) (18) have enabled the creation of both positively and negatively charged as well as neutral gas-phase clusters across a large size range and with diverse elemental composition. The thermodynamic properties of clusters, including bond-dissociation energies (19-21), endothermic-reaction barriers (22), heat capacities (23), and the enthalpy, entropy, and free-energy changes associated with clustering reactions (24, 25), including those composed of hydrogen-bonded and van der Waals systems, have been widely studied, employing both GIB-MS and flow-tube experiments. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) (26) along with other linear ion trapping techniques (27) have been used to investigate the time-dependent kinetics of cluster molecule reactions, providing insight into reaction mechanisms, activation energies, and reaction intermediates. The structural properties of clusters, including bonding motifs and collision cross-sections have been examined through collision-induced dissociation (CID) (28) and high-pressure drift cell experiments (29). Time-of-flight (TOF) mass spectrometry...

Cluster Structure and Reactions: Gaining Insights into Catalytic Processes

“…One valuable approach employs the use of clusters whose chemistry can be used to follow the course of reactions influenced by various reactive centers/defect sites, and the present invited perspective focuses on past developments and future prospects in this area [2,5,[7][8][9][10]. While acquiring reproducible defect sites for the purpose of elucidating their influence on the course of catalytic reactions is an arduous task, it is a relatively straight forward procedure to recreate and conduct repetitive studies on model clusters which mimic specific reaction centers.…”

Closed-Cage Tungsten Oxide Clusters in the Gas Phase

“…The Al 13 cluster does not show any reactivity with oxygen, while other neighboring clusters did. Structure and reactivity of specific Al clusters such as Al 12 , Al 16 , Al 17 , and Al 18 have been extensively explored by Castleman and Khanna. 6 Magic-numbered silver clusters formed by matrix-assisted laser desorption mass spectrometry were reported by Kéki et al 7 Apart from monometallic clusters, they also reported bimetallic Ag-Au clusters.…”

“…15 Bridgeman and Cavigliasso investigated the structure and bonding in [W 10 O 32 ] nisopolyanions theoretically. 16 Various kinds of metal-carbon cage clusters were explored in great detail by Guo et al 17 Molecular level details of small WO clusters on oxidative catalysis have been reported by Johnson et al 18 recently. Rothgeb et al 19 reported the unusual reactivity of small anionic WO clusters with H 2 O and D 2 O.…”

“…24 Mass selected WO showed characteristic reactivity in the cracking of hydrocarbons. 18 Magicnumbered clusters such as W 13 -are especially stable and exhibit much larger relative abundances compared to the neighboring clusters. We have performed relativistic quantum chemical calculations to understand the structure and stabilities of these clusters as a function of their sizes and also to interpret the observed LDI mass spectra.…”

Mass spectrometry across the sciences

“…Some metal ions in H 2 O clusters suffer charge reduction, whereas others show different types of electron and ion solvation. Bulk W n O 3n catalyzes the oxidation of propene to propene oxide and CO to CO 2 (9). In this reaction, gaseous W n O 3n ϩ species of selected n values exhibit enhanced activity and selectivity, indicating a radical oxygen center (W-O•) mechanism.…”