Influence of Stoichiometry and Charge State on the Structure and Reactivity of Cobalt Oxide Clusters with CO

Neutral noble-metal-free VCoO2 and CrCoO2 cluster catalysts for CO oxidation by O2

Neutral noble-metal-free metal oxide cluster catalysts (VCoO₂ and CrCoO₂) were developed for multiple CO oxidation reactions by O₂.

Tuning the Magnetic Moment of Small Late 3d-Transition-Metal Oxide Clusters by Selectively Mixing the Transition-Metal Constituents

Transition-metal oxide nanoparticles are relevant for many applications in different areas where their superparamagnetic behavior and low blocking temperature are required. However, they have low magnetic moments, which does not favor their being turned into active actuators. Here, we report a systematical study, within the framework of the density functional theory, of the possibility of promoting a high-spin state in small late-transition-metal oxide nanoparticles through alloying. We investigated all possible nanoalloys An-xBxOm (A, B = Fe, Co, Ni; n = 2, 3, 4; 0≤x≤n) with different oxidation rates, m, up to saturation. We found that the higher the concentration of Fe, the higher the absolute stability of the oxidized nanoalloy, while the higher the Ni content, the less prone to oxidation. We demonstrate that combining the stronger tendency of Co and Ni toward parallel couplings with the larger spin polarization of Fe is particularly beneficial for certain nanoalloys in order to achieve a high total magnetic moment, and its robustness against oxidation. In particular, at high oxidation rates we found that certain FeCo oxidized nanoalloys outperform both their pure counterparts, and that alloying even promotes the reentrance of magnetism in certain cases at a critical oxygen rate, close to saturation, at which the pure oxidized counterparts exhibit quenched magnetic moments.

Iodization Threshold in Size-Dependent Reactions of Lead Clusters Pbn+ with Iodomethane

Utilizing a magnetron-sputtering (MagS) source in tandem with a multiple-ion laminar flow tube (MIFT) reactor and a customized triple quadrupole mass spectrometer (TQMS), we have prepared clean Pb_n⁺ (n = 1-13) clusters and measured their reactivity with iodomethane under high carrier gas pressures. Strong size dependences are found for the reactivity of these cationic Pb_n⁺ clusters with CH₃I. For the Pb_n⁺ with n ≤ 4, iodinated clusters Pb_nI⁺ were found to be the dominant products, in strong contrast to n > 4 where no such products were seen. Quantum chemical studies show that with an increasing number of Pb atoms, the Pb-Pb interatomic interactions become stronger compared with the Pb-I bonding in Pb_nI⁺ clusters. Furthermore, the reactions of Pb_1-4⁺ with CH₃I have fairly small transition state energy barriers, in contrast to those for Pb_n>4⁺ clusters which have magnitudes that will prevent reactions under the ambient conditions.

Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2 (H2 O)n - , n=1-10, in the C-O Stretch Region

We investigate anionic [Co,CO2 ,nH2 O]- clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250-2234 cm-1 using an FT-ICR mass spectrometer. We show that both CO2 and H2 O are activated in a significant fraction of the [Co,CO2 ,H2 O]- clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C-O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C-O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 - . However, calculations find Co(HCOO)(OH)- as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590-1730 cm-1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH)- . Upon additional hydration, all species [Co,CO2 ,nH2 O]- , n≥2, undergo IRMPD through loss of H2 O molecules as a relatively weakly bound messenger. The main spectral features are the C-O stretching mode of the CO ligand around 1900 cm-1 , the water bending mode mixed with the antisymmetric C-O stretching mode of the HCOO- ligand around 1580-1730 cm-1 , and the symmetric C-O stretching mode of the HCOO- ligand around 1300 cm-1 . A weak feature above 2000 cm-1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

Ligand-Mediated Reactivity in CO Oxidation of Niobium-Nickel Monoxide Carbonyl Complexes: The Crucial Roles of the Multiple Adsorption of CO Molecules

The heteronuclear metal oxide complexes are of great significance in heterogeneous catalytic oxidation of CO. However, previous studies are mainly focused on the composition of metal oxide, charge state, the support and the active oxygen species, with little attention paid to adsorbed CO ligands. Herein, the ligand-mediated reactivity in CO oxidation of niobium-nickel monoxide carbonyl complexes has been successfully identified. The NbNiO(CO) _n^- ( n = 5-6) anions are determined to be O-bridged complexes. In contrast, the NbNiO(CO) _n^- ( n = 7-8) anions are characterized to be η²-CO₂-tagged complexes. The crucial roles of the multiply adsorbed CO molecules that can facilitate not only the competitive binding with bridging oxygen atom to the transition metal centers but also the electron accumulation of transition metal atoms have been discovered. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over heteronuclear metal oxide under CO-rich feed condition.

Reactivity of 4Fe+(CO)n=0-2 + O2: oxidation of CO by O2 at an isolated metal atom

The kinetics of ⁴Fe⁺(CO)_n=0-2 + O₂ are measured under thermal conditions from 300-600 K using a selected-ion flow tube apparatus. Both the bare metal and n = 2 cations are inert to reaction over this temperature range, but ⁴Fe⁺(CO) reacts rapidly (k = 3.2 ± 0.8 × 10^-10 cm³ s^-1 at 300 K, 52% of the collisional rate coefficient) to form FeO⁺ + CO₂. This is an example of the oxidation of CO by O₂ occurring entirely on a single non-noble metal atom. The reaction of the bare metal reaction is known to be endothermic, such that this result is expected; however, the n = 2 reaction has highly exothermic product channels available, such that the lack of reaction is surprising in light of the n = 1 reactivity. Stationary points along all three reaction coordinates are calculated using the TPSSh hybrid functional. These surfaces show that the n = 1 reaction is an example of two-state reactivity; the reaction proceeds initially on the sextet surface over a submerged barrier to a structure with an O-O bond distance longer than that in O₂, but must cross to the quartet surface in order to proceed over a second submerged barrier to rearrange to form CO₂. The n = 2 reaction does not proceed because, on all spin surfaces, the transition state corresponding to O-O separation is at higher energy than the separated reactants. The difference between the n = 1 and n = 2 reactions is not a result of steric effects, but rather because the O₂ is more strongly bound to Fe in the entrance well of the n = 1 case, and that energy is available to overcome the rate-limiting barrier to O-O cleavage. Experimental verification of some of these details are provided by guided ion beam tandem mass spectrometry results. The kinetic energy dependence of the n = 1 reaction shows evidence for a curve crossing and yields relevant thermochemistry for competing reaction channels.

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

Progress on the activation and transformation of important inorganic and organic molecules by negatively charged bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others.

The reactivity of stoichiometric tungsten oxide clusters towards carbon monoxide: the effects of cluster sizes and charge states

Density functional theory (DFT) calculations are employed to investigate the reactivity of tungsten oxide clusters towards carbon monoxide. Extensive structural searches show that all the ground-state structures of (WO3)n(+) (n = 1-4) contain an oxygen radical center with a lengthened W-O bond which is highly active in the oxidation of carbon monoxide. Energy profiles are calculated to determine the reaction mechanisms and evaluate the effect of cluster sizes. The monomer WO3(+) has the highest reactivity among the stoichiometric clusters of different sizes (WO3)n(+) (n = 1-4). The reaction mechanisms for CO with mono-nuclear stoichiometric tungsten oxide clusters with different charges (WO3(-/0/+)) are also studied to clarify the influence of charge states. Our calculated results show that the ability to oxidize CO gets weaker from WO3(+) to WO3(-) as the negative charge accumulates progressively.

Adsorption of carbon monoxide on small aluminum oxide clusters: Role of the local atomic environment and charge state on the oxidation of the CO molecule

We present extensive density functional theory (DFT) calculations dedicated to analyze the adsorption behavior of CO molecules on small AlxOy (±) clusters. Following the experimental results of Johnson et al. [J. Phys. Chem. A 112, 4732 (2008)], we consider structures having the bulk composition Al2O3, as well as smaller Al2O2 and Al2O units. Our electron affinity and total energy calculations are consistent with aluminum oxide clusters having two-dimensional rhombus-like structures. In addition, interconversion energy barriers between two- and one-dimensional atomic arrays are of the order of 1 eV, thus clearly defining the preferred isomers. Single CO adsorption on our charged AlxOy (±) clusters exhibits, in general, spontaneous oxygen transfer events leading to the production of CO2 in line with the experimental data. However, CO can also bind to both Al and O atoms of the clusters forming aluminum oxide complexes with a CO2 subunit. The vibrational spectra of AlxOy + CO2 provides well defined finger prints that may allow the identification of specific isomers. The AlxOy (+) clusters are more reactive than the anionic species and the final Al2O(+) + CO reaction can result in the production of atomic Al and carbon dioxide as observed from experiments. We underline the crucial role played by the local atomic environment, charge density distribution, and spin-multiplicity on the oxidation behavior of CO molecules. Finally, we analyze the importance of coadsorption and finite temperature effects by performing DFT Born-Oppenheimer molecular dynamics. Our calculations show that CO oxidation on AlxOy (+) clusters can be also promoted by the binding of additional CO species at 300 K, revealing the existence of fragmentation processes in line with the ones experimentally inferred.

Structures of [CoO(CO2)n]- and [NiO(CO2)n]- clusters studied by infrared spectroscopy

We present infrared spectra of [CoO(CO2)n](-) and [NiO(CO2)n](-) clusters and interpret them in the framework of computational results employing density functional theory. We find that both [CoO(CO2)n](-) and [NiO(CO2)n](-) clusters are generally composed of the same core isomers. The dominant isomers consist of an η(2) CO2 ligand and a CO3 moiety that can be bound to the metal atom with monodentate (η(1)) or bidentate (η(2)) connectivity. Minor structural isomers observed are composed of a C2O4 moiety with a lone oxygen atom or a CO3 unit.

Infrared spectra and structures of anionic complexes of cobalt with carbon dioxide ligands

We present infrared photodissociation spectra of [Co(CO2)n](-) ions (n = 3-11) in the wavenumber region 1000-2400 cm(-1), interpreted with the aid of density functional theory calculations. The spectra are dominated by the signatures of a core ion showing bidentate interaction of two CO2 ligands with the Co atom, each forming C-Co and O-Co bonds. This structural motif is very robust and is only weakly affected by solvation with additional CO2 solvent molecules. The Co atom is in oxidation state +1, and both CO2 ligands carry a negative charge.

CO Oxidation Promoted by Gold Atoms Loosely Attached in AuFeO3(-) Cluster Anions

Time-of-flight mass spectrometry experiment shows that upon the interactions with carbon monoxide, the mass-selected AuFeO3(-) oxide cluster anions can evaporate neutral gold atoms in a hexapole collision cell and oxidize CO into CO2 in an ion trap reactor. The computational studies identify that the gold atom is loosely attached in the AuFeO3(-) cluster, and the different reaction channels can be attributed to different cluster velocities. The structure of the AuFeO3(-) cluster is very flexible, and the approach of CO induces significant geometrical and electronic structure changes of AuFeO3(-), which facilitates the exposure of the positively charged gold atom to trap and oxidize CO. The CO oxidation by the AuFeO3(-) cluster follows the Au-assisted Mars-van Krevelen mechanism, in which the direct participation of the surface lattice oxygen (O(2-)) is proposed.

CO2 reduction by group 6 transition metal suboxide cluster anions

Reactions between small group 6 transition metal suboxide clusters, M(x)O(y)(-) (M = (98)Mo or (186)W; x = 1-4; y < or = 3x) and both CO(2) and CO were studied in gas phase using mass spectrometric analysis of high-pressure, fast flow reaction products. Both Mo(2)O(y)(-) and W(2)O(y)(-) show evidence of sequential oxidation by CO(2) of the form, M(2)O(y)(-)+CO(2)-->M(2)O(y+1)(-)+CO for the more reduced species. Similar evidence is observed for the trimetallic clusters, although Mo(3)O(6)(-) appears uniquely unreactive. Lower mass resolution in the M(4)O(y)(-) range precludes definitive product mass assignments, but intensity patterns suggest the continued trend of sequential oxidation of the more reduced end of the M(4)O(y)(-) oxide series. Based on thermodynamic arguments, cluster oxidation by CO(2) is possible if D(0)(O-Mo(x)O(y)(-)) > 5.45 eV. Although simple bond energy analysis suggests that tungsten oxides may be more reactive toward CO(2) compared to molybdenum oxides, this is not born out experimentally, suggesting that the activation barrier for the reduction of CO(2) by tungsten suboxide clusters is very high compared to analogous molybdenum suboxide clusters. In reactions with CO, suboxides of both metal-based oxides show CO addition, with the product distribution being more diverse for Mo(x)O(y)(-) than for W(x)O(y)(-). No evidence of cluster reduction by CO is observed.

Oxidation reactions on neutral cobalt oxide clusters: experimental and theoretical studies

Reactions of neutral cobalt oxide clusters (Co(m)O(n), m = 3-9, n = 3-13) with CO, NO, C(2)H(2), and C(2)H(4) in a fast flow reactor are investigated by time of flight mass spectrometry employing 118 nm (10.5 eV) single photon ionization. Strong cluster size dependent behavior is observed for all the oxidation reactions; the Co(3)O(4) cluster has the highest reactivity for reactions with CO and NO. Cluster reactivity is also highly correlated with either one or more following factors: cluster size, Co(iii) concentration, the number of the cobalt atoms with high oxidation states, and the presence of an oxygen molecular moiety (an O-O bond) in the Co(m)O(n) clusters. The experimental cluster observations are in good agreement with condensed phase Co(3)O(4) behavior. Density functional theory calculations at the BPW91/TZVP level are carried out to explore the geometric and electronic structures of the Co(3)O(4) cluster, reaction intermediates, transition states, as well as reaction mechanisms. CO, NO, C(2)H(2), and C(2)H(4) are predicted to be adsorbed on the Co(ii) site, and react with one of the parallel bridge oxygen atoms between two Co(iii) atoms in the Co(3)O(4) cluster. Oxidation reactions with CO, NO, and C(2)H(2) on the Co(3)O(4) cluster are estimated as thermodynamically favorable and overall barrierless processes at room temperature. The oxidation reaction with C(2)H(4) is predicted to have a very small overall barrier (<0.23 eV). The oxygen bridge between two Co(iii) sites in the Co(3)O(4) cluster is responsible for the oxidation reactions with CO, NO, C(2)H(2), and C(2)H(4). Based on the gas phase experimental and theoretical cluster studies, a catalytic cycle for these oxidation reactions on a condensed phase cobalt oxide catalyst is proposed.