Jia-Bi Ma

Characterization and reactivity of oxygen-centred radicals over transition metal oxide clusters

We introduce chemical structures and reactivity of oxygen-centred radicals (O(-)˙) over transition metal oxide (TMO) clusters based on mass spectrometric and density functional theory studies. Two main issues will be discussed: (1) the compositions of TMO clusters that have the bonding characteristics of (or contain) the O(-)˙ radicals; and (2) the dependences (cluster structures, sizes, charge states, metal types, etc.) of the chemical reactivity and selectivity for the O(-)˙ radicals over TMO clusters. One of the goals of cluster chemistry is to understand the elementary reactions involved with complex heterogeneous catalysis. The study of the O(-)˙ containing TMO clusters permits rather detailed descriptions for how mono-nuclear oxygen-centred radicals may exist and react with small molecules over TMO based catalysts.

Hydrogen-atom abstraction from methane by stoichiometric vanadium-silicon heteronuclear oxide cluster cations.

Vanadium-silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O(2) background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V(2)O(5))(n)(SiO(2))(m)](+) (n=1, m=1-4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V(4)O(10) (+) with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O(t)*) were found in all of the stable isomers. These O(t)* radicals are active sites of the clusters in reaction with CH(4). The O(t)* radicals in [V(2)O(5)(SiO(2))(1-4)](+) clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH(4) activation on silica-supported vanadium oxide catalysts.

Methane activation by V3PO10˙+ and V4O10˙+ clusters: A comparative study

A series of vanadium and phosphorus heteronuclear oxide cluster cations (V(x)P(y)O(z)(+)) are prepared by laser ablation and the reactions of V(3)PO(10)˙(+) and V(4)O(10)˙(+) with methane in a fast flow reactor under the same conditions are studied. A time of flight mass spectrometer is used to detect the cluster distribution before and after reactions. In addition to previously identified reaction of V(4)O(10)˙(+) + CH(4)→ V(4)O(10)H(+) + CH(3)˙, the observation of hydrogen atom pickup cluster V(3)PO(10)H(+) suggests the reaction: V(3)PO(10)˙(+) + CH(4)→ V(3)PO(10)H(+) + CH(3)˙. The rate of the reaction of V(4)O(10)˙(+) with CH(4) is approximately 2.5 times faster than that of V(3)PO(10)˙(+) with CH(4). Density functional theory (DFT) calculations predict that structure of V(3)PO(10)˙(+) is topologically similar to that of V(4)O(10)˙(+), as well as that of P(4)O(10)˙(+), which is very similar to V(4)O(10)˙(+) in terms of methane activation in previous studies. The facile methane activation by the homo- and hetero-nuclear oxide clusters can all be attributed to the presence of an oxygen-centered radical (O˙) in these clusters. Further theoretical study indicates that the O˙ radical (or spin density of the cluster) can transfer within the high symmetry V(4)O(10)˙(+) and P(4)O(10)˙(+) clusters quite easily, and CH(4) molecule further enhances the rate of intra-cluster spin density transfer. In contrast, the intra-cluster spin density transfer within low symmetry V(3)PO(10)˙(+) is thermodynamically forbidden. The experimentally observed reactivity difference is consistent with the theoretical consideration of the intra-cluster spin density transfer.

CH Activation on Aluminum–Vanadium Bimetallic Oxide Cluster Anions

Aluminum-vanadium bimetallic oxide cluster anions (BMOCAs) have been prepared by laser ablation and reacted with ethane and n-butane in a fast-flow reactor. A time-of-flight mass spectrometer was used to detect the cluster distribution before and after the reactions. The observation of hydrogen-containing products AlVO(5)H(-) and Al(x)V(4-x)O(11-x)H(-) (x=1-3) strongly suggests that AlVO(5)(-) and Al(x)V(4-x)O(11-x)(-) (x=1-3) can react with ethane and n-butane by means of an oxidative dehydrogenation process at room temperature. Density functional theory studies have been carried out to investigate the structural, bonding, electronic, and reactive properties of these BMOCAs. Terminal-oxygen-centered radicals (O(t)(.)) were found in all of the reactive clusters, and the O(t)(.) atoms, which prefer to be bonded with Al rather than V atoms, are the active sites of these clusters. All the hydrogen-abstraction reactions are favorable both thermodynamically and kinetically. To the best of our knowledge, this is the first example of hydrogen-atom abstraction by BMOCAs and may shed light on understanding the mechanisms of C−−H activation on the surface of alumina-supported vanadia catalysts.

Reactivity of Atomic Oxygen Radical Anions Bound to Titania and Zirconia Nanoparticles in the Gas Phase: Low-Temperature Oxidation of Carbon Monoxide

Titanium and zirconium oxide cluster anions with dimensions up to nanosize are prepared by laser ablation and reacted with carbon monoxide in a fast low reactor. The cluster reactions are characterized by time-of-flight mass spectrometry and density functional theory calculations. The oxygen atom transfers from (TiO(2))(n)O(-) (n = 3-25) to CO and formations of (TiO(2))(n)(-) are observed, whereas the reactions of (ZrO(2))(n)O(-) (n = 3-25) with CO generate the CO addition products (ZrO(2))(n)OCO(-), which lose CO(2) upon the collisions (studied for n = 3-9) with a crossed helium beam. The computational study indicates that the (MO(2))(n)O(-) (M = Ti, Zr; n = 3-8) clusters are atomic radical anion (O(-)) bonded systems, and the energetics for CO oxidation by the O(-) radicals to form CO(2) is strongly dependent on the metals as well as the cluster size for the titanium system. Atomic oxygen radical anions are important reactive intermediates, while it is difficult to capture and characterize them for condensed phase systems. The reactivity pattern of the O(-)-bonded (TiO(2))(n)O(-) and (ZrO(2))(n)O(-) correlates very well with different behaviors of titania and zirconia supports in the low-temperature catalytic CO oxidation.

Collision-Induced Dissociation and Density Functional Theory Studies of CO Adsorption over Zirconium Oxide Cluster Ions: Oxidative and Nonoxidative Adsorption

Zirconium oxide cluster cations and anions are produced by laser ablation and reacted with CO in a fast flow reactor. The CO adsorption products Zr(x)O(y)CO(+) are observed for most of the generated cationic clusters (Zr(x)O(y)(+) = Zr(2)O(5,6)(+), Zr(3)O(7,8)(+), Zr(4)O(9,10)(+)...) while only specific anionic systems (Zr(x)O(y)(-) = Zr(3)O(7)(-), Zr(4)O(9)(-)...) absorb CO to produce Zr(x)O(y)CO(-). To study how the CO molecule is adsorbed on the clusters, the Zr(x)O(y)CO(±) products are mass-selected by a time-of-flight mass spectrometer (TOF-MS) and collided with a crossed helium beam. The fragment ions from collision-induced dissociation (CID) are detected by a secondary TOF-MS. Loss of CO and CO(2) is observed upon the collision of the helium beam with Zr(x)O(y)CO(+) and Zr(x)O(2x+1)CO(-), respectively. Density functional theory calculations indicate that oxidative and nonoxidative adsorption of CO takes place over Zr(3)O(7)(-) and Zr(3)O(7)(+), respectively. This is consistent with the CID experiments.

On the origin of the surprisingly sluggish redox reaction of the N2O/CO couple mediated by [Y2O2]+˙ and [YAlO2]+˙ cluster ions in the gas phase.

Catalytic conversion of harmful gases produced in fossil-fuel combustion or in large-scale chemical transformations, such as CO or the oxides of nitrogen into nitrogen and carbon dioxide, is of utmost importance both environmentally and economically. For example, N2O is a potent greenhouse gas with a warming potential exceeding that of CO2 by a factor of 300,1 and its role in the depletion of stratospheric ozone is well known.2 While these redox reactions are exothermic, for example ΔrH=−357 kJ mol−1 for the process N2O + CO→N2 + CO2, they do not occur directly to any measurable extent at either room or elevated temperatures because of high energy barriers that exceed the 193 kJ mol−1 for the N2O/CO couple. Catalysts are required to open-up new, energetically more favorable pathways,3 and the first example of a homogeneous catalysis in the gas phase, whereby atomic transition-metal cations bring about the efficient reduction of N2O by CO, was reported in a landmark study by Kappes and Staley,4 which was followed in the ensuing decades by numerous investigations.5 Recently, these studies addressed more specific questions, for example, “catalyst poisoning”, and these experiments revealed remarkable effects of both the cluster size and the charge state of the catalysts.6 For example, the active species of the Pt7+ cluster are Pt7+, [Pt7O]+, [Pt7O2]+, and [Pt7(CO)]+ and it has a turnover number >500 at room temperature. The adsorption of more than one CO molecule onto the Pt7+ cluster, however, completely quenches the catalytic activity. Thus, coverage effects for any cluster sizes can be studied at a strictly molecular level. Similarly, the concept of “single-site catalysts”,7 the proper characterization and identification of which constitutes one of the challenges and intellectual cornerstones in contemporary catalysis, can be probed directly in gas-phase experiments with mass-selected heteronuclear metal-oxide clusters. For example, catalytic room-temperature oxidation of CO by N2O can be mediated by the bimetallic oxide cluster couple [AlVO4]+./[AlVO3]+..8 In the presence of CO, the cluster ion [AlVO4]+. is efficiently reduced to [AlVO3]+., and if N2O is added, the reverse reaction occurs. Both processes are clean and proceed with efficiencies (ϕ) of 59 % and 65 %, respectively, relative to the collision rates. Most interestingly, the two redox reactions occur at the Al-Ot. unit of the cluster (Ot: terminal oxygen atom); bond activation involving the V—O moiety cannot compete kinetically and thermochemically. Thus, the existence and operation of an “active site” of a catalyst can already be demonstrated in a rather small heteronuclear cluster.9

Experimental and theoretical study of the reactions between vanadium oxide cluster cations and water.

Vanadium oxide cluster cations V(x)O(y)(+) (x = 2-6) are prepared by laser ablation and are reacted with D(2)O in a fast flow reactor under room temperature conditions. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Observation of the products (V(2)O(5))(1-3)D(+) indicates the deuterium atom abstraction reaction (V(2)O(5))(1-3)(+) + D(2)O → (V(2)O(5))(1-3)D(+) + OD. In addition, significant association products (V(2)O(5))(1-3)D(2)O(+) are also observed in the experiments. Density functional theory calculations are performed to study the reaction mechanisms of V(4)O(10)(+) with H(2)O. The calculated results are in agreement with the experimental observations and indicate that H(2)O is dissociatively rather than molecularly adsorbed in V(4)O(10)H(2)O(+) complex.

Characterization of Mononuclear Oxygen-Centered Radical (O-˙) in Zr2O8- Cluster

Zirconium oxide cluster anions Zr(x)O(y)(-) are prepared by laser ablation and are reacted with n-butane in a fast flow reactor under near room temperature conditions. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Observation of a hydrogen atom pickup product Zr(2)O(8)H(-) indicates a C-H activation reaction: Zr(2)O(8)(-) + n-C(4)H(10) → Zr(2)O(8)H(-) + C(4)H(9). Density functional theory calculations predict that the oxygen-very-rich cluster Zr(2)O(8)(-) contains one mononuclear oxygen-centered radical (O(-)(•)), which leads to a high C-H activation reactivity, in agreement with the experiments. This study provides one example for how the highly oxidative O(-)(•) radical may be generated by adsorption of O(2) onto unreactive metal oxide clusters.

Activation of Methane and Ethane as Mediated by the Triatomic Anion HNbN−: Electronic Structure Similarity with a Pt Atom

Investigations of the intrinsic properties of gas-phase transition metal nitride (TMN) ions represent one approach to gain a fundamental understanding of the active sites of TMN catalysts, the activities and electronic structures of which are known to be comparable to those of noble metal catalysts. Herein, we investigate the structures and reactivities of the triatomic anions HNbN(-) by means of mass spectrometry and photoelectron imaging spectroscopy, in conjunction with density functional theory calculations. The HNbN(-) anions are capable of activating CH4 and C2H6 through oxidative addition, exhibiting similar reactivities to free Pt atoms. The similar electronic structures of HNbN(-) and Pt, especially the active orbitals, are responsible for this resemblance. Compared to the inert NbN(-), the coordination of the H atom in HNbN(-) is indispensable. New insights into how to replace noble metals with TMNs may be derived from this combined experimental/computational study.