Fast Flow Reactor Research Articles

ChemInform Abstract: Heterogeneous Reactions of Chlorine Nitrate and Hydrogen Chloride on Type I Polar Stratospheric Clouds.

A fast-flow reactor coupled with a quadrupole mass spectrometer was used to study the heterogeneous reactions ClONO2 + HCl yields Cl2 + HNO3 (1) and ClONO2 + H2O yields HOCl + HNO3 (2) on vapor-deposited HNO3-H2O ice substrates. It was found that the sticking coefficient of HCl on these substrates was a strong function of the substrate composition, ranging from about 2 x 10 exp -5 at nitric acid trihydrate composition to 6 x 10 exp -3 at 45 wt pct HNO3. The HNO3-H2O ice substrates were found to have large internal surface areas, and corrections for gas-phase diffusion within the porous ices were applied to observed loss rates.

H 2 production from reactions between water and small molybdenum suboxide cluster anions

Reactions between molybdenum suboxide cluster anions, Mo(x)O(y)(-) (x=1-4; y < or = 3x), and water (H(2)O and D(2)O) have been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. Product distributions vary with the number of metal atoms in the cluster. Within the MoO(y)(-) oxide series, product masses correspond to the addition of one water molecule, as well as a H/D exchange with MoO(4)H(-). Within the Mo(2)O(y)(-) oxide series, product evolution and distribution suggest sequential oxidation via Mo(2)O(y)(-)+H(2)O/D(2)O-->Mo(2)O(y+1)(-)+H(2)/D(2) reactions for y<5, while for Mo(2)O(5)(-), Mo(2)O(6)H(2)/D(2)(-) is produced. Mo(2)O(6)(-) does not appear to be reactive toward water. For the Mo(3)O(y)(-) oxide series, sequential oxidation similarly is suggested for y<5, while Mo(3)O(5)(-) reactions result in Mo(3)O(6)H(2)/D(2)(-) formation. Mo(3)O(6)(-) appears uniquely unreactive. Mo(3)O(7)(-) and Mo(3)O(8)(-) react to form Mo(3)O(8)H(2)/D(2)(-) and Mo(3)O(9)H(2)/D(2)(-), respectively. Lower mass resolution in the Mo(4)O(y)(-) mass range prevents unambiguous mass analysis, but intensity changes in the mass spectra do suggest that sequential oxidation with H(2)/D(2) evolution occurs for y<6, while Mo(4)O(y+1)H(2)/D(2)(-) addition products are formed in Mo(4)O(6)(-) and Mo(4)O(7)(-) reactions with water. The relative rate constants for sequential oxidation and H(2)O/D(2)O addition for the x=2 series were determined. There is no evidence of a kinetic isotope effect when comparing reaction rates of H(2)O with D(2)O, suggesting that the H(2) and D(2) losses from the lower-oxide/hydroxide intermediates are very fast relative to initial reaction complex formation with H(2)O or D(2)O. The rate constants determined here are two times higher than those determined in identical reactions between W(2)O(y)(-)+H(2)O/D(2)O.

CO 2 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.

Experimental and Theoretical Study of the Reactions between Vanadium−Silicon Heteronuclear Oxide Cluster Anions withn-Butane

Vanadium and silicon heteronuclear oxide cluster anions VxSiyOz− (x + y ≥ 2, z ≥ 4) are prepared by laser ablation and reacted with n-butane (C4H10) in a fast flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. The observation of hydrogen-containing products (V2O5)m(SiO2)nOH− (m = 1, n = 1−4; m = 2, n = 1) strongly suggests the following reactions: (V2O5)m(SiO2)nO− + C4H10 → (V2O5)m(SiO2)nOH− + C4H9. Although V2O6− is produced in the cluster source, no V2O6H− product is produced under the same experimental condition. It indicates that specific heteronuclear oxide clusters V2O5(SiO2)1−4O− and (V2O5)2SiO2O− are more reactive than the homonuclear oxide cluster V2O6− (or V2O5O−). Density functional theory (DFT) calculations are performed to study reaction mechanisms of V2O5SiO2O− (or V2SiO8−) + C4H10. The calculated results are in good agreement with the experimental observations. The structural and bonding properties of (V2O5)m(SiO2)nO− ...

Heterogeneous Reaction of NO2 on Fresh and Coated Soot Surfaces

The heterogeneous reaction of nitrogen dioxide (NO(2)) on fresh and coated soot surfaces has been investigated to assess its role in night-time formation of nitrous acid (HONO) in the atmosphere. Soot surfaces were prepared by incomplete combustion of propane and kerosene fuels under lean and rich flame conditions and then processed by heating to evaporate semivolatile species or by coating with pyrene, sulfuric acid, or glutaric acid. Uptake kinetics and HONO yield measurements were performed in a low-pressure fast-flow reactor coupled to a chemical ionization mass spectrometer (CIMS), using atmospheric-level NO(2) concentrations. The uptake coefficient and the HONO yield upon interaction of NO(2) with nascent soot depend on the type of fuel and combustion regime and are the highest for samples prepared using fuel rich flame. Heating the nascent soot samples before exposure to NO(2) removes the organic material from the soot backbone, leading to a significant increase in NO(2) uptake coefficient and HONO yield. Continuous exposure to NO(2) reduces the reactivity of soot because of irreversible deactivation of the surface sites. Our results support the oxidation-reduction mechanism involving adsorptive and reactive centers on soot surface where NO(2) is converted to HONO and other products. Coating of the soot surface by different materials to simulate atmospheric aging has a strong impact on its reactivity toward NO(2) and the resulting HONO production. Coating of pyrene has little effect on either reaction rate or HONO yield. Sulfuric acid coating does not alter the uptake coefficient, but significantly reduces the amount of HONO formed. Coating of glutaric acid significantly increases NO(2) uptake coefficient and HONO yield. The results of our study indicate that the reactivity and HONO generating capacity of internally mixed soot aerosol will depend on the chemical composition of the coating material and hence will vary considerably in different polluted environments.

Absolute Photoionization Cross Section of the Methyl Radical

The absolute photoionization cross section of the methyl radical was determined relative to that of NO at photon energies of 10.54 eV using the CH(3) + NO(2) --> CH(3)O + NO reaction. Kinetics of this reaction was studied in a fast flow reactor coupled with VUV laser photoionization. Simulation of the kinetics of the decrease of the methyl signal and the corresponding increase of the NO signal (in combination with the NO absolute photoionization cross section determined by Watanabe (Watanabe, K. J. Chem. Phys. 1954, 22, 1564; Watanabe, K.; Matsunaga, F. M.; Sakai, H. Appl. Opt. 1967, 6, 391)), yields the absolute photoionization cross section of the methyl radical: sigma(CH(3))(10.54 eV) = 5.1 (1.2) x 10(-18) cm(2) (95% confidence interval). This result is in good agreement with the recently published measurements by Taatjes et al. (Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Fan, H.; Pratt, S. T. J. Phys. Chem. A 2008, 112, 9336) and by Gans et al. (Gans, B.; Mendes, L. A. V.; Boyé-Péronne, S.; Douin, S.; Garcia, G.; Soldi-Lose, H.; Cunha de Miranda, B. K.; Alcaraz, C.; Carrasco, N.; Pernot, P.; Gauyacq, D. J. Phys. Chem. A 2009, 114, 3237).

Heterogeneous Chemistry of Alkylamines with Sulfuric Acid: Implications for Atmospheric Formation of Alkylaminium Sulfates

The heterogeneous interaction of alkylamines with sulfuric acid has been investigated to assess the role of amines in aerosol growth through the formation of alkylaminium sulfates. The kinetic experiments were conducted in a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The measurements of heterogeneous uptake of methylamine, dimethylamine, and trimethylamine were performed in the acidity range of 59-82 wt % H(2)SO(4) and between 243 and 283 K. Irreversible reactive uptakes were observed for all three alkylamines, with comparable uptake coefficients (gamma) in the range of 2.0 x 10(-2) to 4.4 x 10(-2). The measured gamma value was slightly higher in more concentrated sulfuric acid and at lower temperatures. The results imply that the heterogeneous reactions of alkylamines contribute effectively to the growth of atmospheric acidic particles and, hence, secondary organic aerosol formation.

Experimental and theoretical study of neutral AlmCn and AlmCnHx clusters

Neutral Al(m)C(n) and Al(m)C(n)H(x) clusters are investigated both experimentally and theoretically for the first time. Single photon ionization through 193, 118, and 46.9 nm lasers is used to detect neutral cluster distributions through time of flight mass spectrometry (TOFMS). Al(m)C(n) clusters are generated through laser ablation of a mixture of Al and C powders pressed into a disk. An oscillation of the vertical ionization energies (VIEs) of Al(m)C(n) clusters is observed in the experiments. The VIEs of Al(m)C(n) clusters change as a function of the numbers of Al and C atoms in the clusters. Al(m)C(n)H(x) clusters are generated through an Al ablation plasma-hydrocarbon reaction, an Al-C ablation plasma reacting with H(2) gas, or through cold Al(m)C(n) clusters reacting with H(2) gas in a fast flow reactor. The VIEs of Al(m)C(n)H(x) clusters are observed to vary as a function of the number of H atoms in the clusters. Density functional theory and ab initio calculations are carried out to explore the structures, ionization energies, and electronic structures of the Al(m)C(n) and Al(m)C(n)H(x) clusters. C=C bonds are favored for the lowest energy structures for Al(m)C(n) clusters. H atoms can be bonded to either Al or C atoms in forming Al(m)C(n)H(x) clusters, with little difference in energy. Electron density plots of the highest occupied molecular orbitals (HOMOs) for closed shell species and the singly occupied molecular orbitals (SOMOs) for open shell species of Al(m)C(n) and Al(m)C(n)H(x) clusters are presented and described to help understand the physical and chemical properties of the observed species. VIEs do not simply depend on open or closed shell valence electron configurations, but also depend on the electronic structure details of the clusters. The calculational results provide a good and consistent explanation for the experimental observations, and are in general agreement with them. All calculated clusters are found to have a number of low lying isomeric structures.

Oxidation reactions on neutral cobalt oxideclusters: 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.

Active sites of stoichiometric cerium oxide cations (CemO2m+) probed by reactions with carbon monoxide and small hydrocarbon molecules

Cerium oxide cluster cations (Ce(m)O(n)(+), m = 2-16; n = 2m, 2m +/- 1 and 2m +/- 2) are prepared by laser ablation and reacted with carbon monoxide (CO) and small hydrocarbon molecules (CH(4), C(2)H(4), and C(2)H(6)) in a fast flow reactor. A time of flight mass spectrometer is used to detect the cluster distribution before and after the reactions. The observation of oxygen reduction and hydrogen pickup of Ce(m)O(2m)(+) clusters strongly suggests the following reactions: (1) Ce(m)O(2m)(+) + C(2)H(4) --> Ce(m)O(2m-1)(+) + C(2)H(4)O (m = 2-6); (2) Ce(m)O(2m)(+) + CO --> Ce(m)O(2m-1)(+) + CO(2) (m = 4-6); and (3) Ce(m)O(2m)(+) + CH(4)/C(2)H(6) --> Ce(m)O(2m)H(+) + CH(3)/C(2)H(5) (m = 2-4). Density functional theory (DFT) calculations are performed to study reaction mechanisms of Ce(2)O(4)(+) + X (X = CO, CH(4), C(2)H(4), and C(2)H(6)). The calculated results are in good agreement with the experimental observations. The structural and bonding properties of Ce(m)O(2m)(+) (m = 2-5) clusters are also investigated by the DFT calculations. The unpaired electron in each of the clusters is mainly distributed over one Ce atom (4f and 5p orbitals) and two O atoms (2p orbital) in a CeO(2) moiety, which can be considered as the active site in the cluster. To further understand the nature of the active sites in Ce(m)O(2m)(+) clusters, the fast flow reaction experiments are also carried out on zirconium oxide clusters Zr(m)O(n)(+), because both Zr ([Kr]4d(2)5s(2)) and Ce ([Xe]4f(1)5d(1)6s(2)) have the same number of valence electrons while the latter has one more f and one less d electrons. In addition to the oxygen transfer reactions such as Zr(m)O(2m)(+) + C(2)H(4) --> Zr(m)O(2m-1)(+) + C(2)H(4)O (m = 1-4) reported in the literature, hydrogen abstraction reactions Zr(m)O(2m)(+) + CH(4)/C(2)H(6) --> Zr(m)O(2m)H(+) + CH(3)/C(2)H(5) are also identified. The rate constants of CO oxidation as well as hydrogen abstraction by Ce(m)O(2m)(+) and Zr(m)O(2m)(+) are very different. The reactivity and selectivity of Ce(m)O(2m)(+) versus Zr(m)O(2m)(+) can be well rationalized based on the DFT calculations. The oxygen transfer and hydrogen abstraction reactions studied in this work are of widespread importance. The nature of the active sites of Ce(m)O(2m)(+) clusters is unique and may be considered in the use and design of cerium oxide based 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.

Reaction between sulfur dioxide and iron oxide cationic clusters

The reactivity of sulfur dioxide (SO2) molecules toward iron oxide cationic clusters (FemOn+) is studied by a homemade time-of-flight mass spectrometer coupled with a laser ablation/supersonic expansion cluster source and a fast flow reactor. The association products FemOnSO2+ can be observed for most of the clusters. The interesting result is that the cooperation effect of SO2 and water is in favor of the adsorption of gas phase water on specific scale iron oxide clusters (Fe2O2/+ and Fe3O3/+). The reactivity information obtained may be useful to investigate atmospheric heterogeneous chemistry of related systems.

Kinetics and mechanisms of the reaction of CH with H 2O

The rate constant for the CH + H 2O reaction has been measured in a fast-flow reactor to be (0.9 ± 0.4) × 10 −11 cm 3 molecule −1 s −1 at 330 K. The product branching ratio of H atoms probed by resonance fluorescence is determined to be 100%. Quantum chemistry and statistical calculations show that the rate limiting process is the isomerisation of the HCOH 2 adduct to H 2COH and allow us to propose k = (2.8 ± 1.4) × 10 −11( T/300) −1.22 ± 0.07exp(−(12 ± 12)/ T) cm 3 molecule −1 s −1 in the range 100–700 K for the rate constant of CH + H 2O → H + H 2CO and H 2 + H + CO.

Ground State Structures of Fe2O4−6+ Clusters Probed by Reactions with N2

Reactions of small cationic iron oxide clusters (Fe(2)O(4-6)(+)) with N(2) are investigated by experiments and first principle calculations. The cationic iron oxide clusters are generated by reaction of laser ablated iron plasma with O(2) in a supersonic expansion, and are reacted with N(2) in a fast flow reactor at near room temperature conditions. Cluster cations are detected by a time-of-flight mass spectrometer. The substitution reaction Fe(2)O(n)(+) + N(2) --> Fe(2)O(n-2)N(2)(+) + O(2) is observed for n = 5 but not for n = 4 and 6. Density functional theory calculations predict that the low-lying energy structures of Fe(2)O(4-6)(+) are with side-on (eta(1)-O(2)) or end-on (eta(2)-O(2)) bonded molecular oxygen unit(s). The calculations further predict that the substitution of eta(1)-O(2) and eta(2)-O(2) in Fe(2)O(4,6)(+) clusters by N(2) is exothermic and subject to negative and positive overall reaction barriers, respectively, at room temperature. We thus propose that the ground state structures of Fe(2)O(4)(+) and Fe(2)O(6)(+) contain eta(2)-O(2). In contrast, both the experiment and theory favor a eta(1)-O(2) in the ground state structure of Fe(2)O(5)(+).

Unusual products observed in gas-phase WxOy−+H2O and D2O reactions

Addition of H(2)O and D(2)O to small tungsten suboxide cluster anions W(x)O(y)(-) (x = 1-4; y < or = 3x) was studied using mass spectrometric measurements from a high-pressure fast flow reactor. Within the WO(y)(-) mass manifold, which also includes WO(4)H(-), product masses correspond to the addition of one to three H(2)O or D(2)O molecules. Within the W(2)O(y)(-) cluster series, product distributions suggest that sequential oxidation W(2)O(y)(-) + H(2)O/D(2)O --> W(2)O(y+1)(-) + H(2)/D(2) occurs for y < 5, while for W(2)O(5)(-), W(2)O(6)H(2)(-)/W(2)O(6)D(2)(-) is primarily produced. W(2)O(6)(-) does not appear reactive. For the W(3)O(y)(-) cluster series, sequential oxidation with H(2) and D(2) production occurs for y < 6, while W(3)O(6)(-) and W(3)O(7)(-) produce W(3)O(7)H(2)(-)/W(3)O(7)D(2)(-) and W(3)O(8)H(2)(-)/W(3)O(8)D(2)(-), respectively. Lower mass resolution in the W(4)O(y)(-) mass range prevents definitive product assignments, but intensity patterns suggest that sequential oxidation with H(2)/D(2) evolution occurs for y < 6, while W(4)O(y+1)H(2)(-)/W(4)O(y+1)D(2)(-) products result from addition to W(4)O(6)(-) and W(4)O(7)(-). Based on bond energy arguments, the H(2)/D(2) loss reaction is energetically favored if the new O-W(x)O(y)(-) bond energy is greater than 5.1 eV. The relative magnitude of the rate constants for sequential oxidation and H(2)O/D(2)O addition for the x = 2 series was determined. There are no discernable differences in rate constants for reactions with H(2)O or D(2)O, suggesting that the H(2) and D(2) loss from the lower-oxide/hydroxide intermediates is very fast relative to the addition of H(2)O or D(2)O.

Investigation of the reactions of small neutral iron oxide clusters with methanol

Reactions of neutral iron oxide clusters (Fe(m)O(n), m=1-2, n=0-5) with methanol (CH(3)OH) in a fast flow reactor are investigated by time of flight mass spectrometry. Detection of the neutral iron oxide cluster distribution and reaction intermediates and products is accomplished through single photon ionization by a 118 nm (10.5 eV) VUV laser. Partially deuterated methanol (CD(3)OH) is employed to distinguish reaction products and reaction mechanisms. Three major reactions are identified experimentally: CH(3)OH association with FeO; methanol dehydrogenation on FeO(1,2) and Fe(2)O(2-5); and (CH(2)O)Fe formation. Density functional theory calculations are carried out to identify reaction products, and to explore the geometric and electronic structures of the iron oxide clusters, reaction intermediates, and transition states, and to evaluate reaction pathways. Neutral formaldehyde is calculated to be formed on FeO(1,2) and Fe(2)O(2-5) clusters. Hydrogen transfer from methanol to iron oxide clusters occurs first from the O-H moiety of methanol, and is followed by a hydrogen transfer from the C-H moiety of methanol. Computational results are in good agreement with experimental observations and reveal reaction mechanisms for neutral iron oxide clusters taking methanol to formaldehyde through various reaction intermediates. Based on the experimental results and the calculated reaction mechanisms and pathways, complete catalytic cycles are suggested for the heterogeneous reaction of CH(3)OH to CH(2)O facilitated by an iron oxide catalyst.

Reactions of Neutral Vanadium Oxide Clusters with Methanol

Reactions of neutral vanadium oxide clusters with methanol and ethanol in a fast-flow reactor are investigated by time-of-flight mass spectrometry. Single-photon ionization through soft X-ray (46.9 nm, 26.5 eV) and vacuum ultraviolet (VUV, 118 nm, 10.5 eV) lasers is employed to detect both neutral cluster distributions and reaction products. In order to distinguish isomeric products generated in the reactions V(m)O(n) + CH(3)OH, partially deuterated methanol (CD(3)OH) is also used as a reactant in the experiments. Association products are observed for most vanadium oxide clusters in reaction with methanol. Products VOD, V(2)O(3)D, V(3)O(6)D, and V(4)O(9)D are observed for oxygen-deficient vanadium oxide clusters reacting with methanol, while oxygen-rich and the most stable clusters can extract more than one hydrogen atom (H/D) from CD(3)OH to form products VO(2)DH(0,1), V(2)O(4)DH(0,1), V(2)O(5)DH(0,1), V(3)O(7)DH(0,1), and V(4)O(10)DH(0,1). Species VO(2)(CH(3))(2), VO(3)(CH(3))(2), V(2)O(5)(CH(3))(2), V(3)O(7)(CH(3))(2), and V(3)O(8)(CH(3))(2) are identified as some of the main products generated from a dehydration reaction for V(m)O(n) + CH(3)OH. A minor reaction channel that generates VOCH(2)O (VOCD(2)O) and VO(2)CH(2)O (VO(2)CD(2)O) can also be identified. An obviously different behavior appears in the reaction V(m)O(n) + C(2)H(5)OH. The main observed products for this reaction are association products of the form V(m)O(n)C(2)H(5)OH. In order to explore the mechanism of V(m)O(n) + CH(3)OH reactions, DFT calculations are performed to study the reaction pathways of VO(2) + CH(3)OH and VO + CH(3)OH reaction systems. The calculation results are in good agreement with the experimental observations.

Rate constants and the H atom branching ratio of the reactions of the methylidyne CH(X2Π) radical with C2H2, C2H4, C3H4(methylacetylene and allene), C3H6(propene) and C4H8(trans-butene)

The reactions of the CH radical with several unsaturated hydrocarbons C2H2 (acetylene), C2H4 (ethylene), C3H4 (methyl-acetylene and allene), C3H6 (propene) and C4H8 (trans-butene) were studied at room temperature, in a low-pressure fast-flow reactor. CH(X2pi, v = 0) radicals were obtained from the reaction of CHBr3 with potassium atoms. The overall rate constants at 300 K are CH + C2H2: (3.6 +/- 0.6) x 10(-10), CH + C2H4: (3.1 +/- 0.6) x 10(-10), CH + C3H4 (methyl-acetylene): (3.4 +/- 0.6) x 10(-10), CH + C3H4 (allene): (3.6 +/- 0.6) x 10(-10), CH + C3H6 (propene): (4.2 +/- 0.8) x 10(-10) and CH + C4H8 (trans-butene): (4.0 +/- 0.80) x 10(-10) cm3 molecule(-1) s(-1) (errors are cited at the level of +/- 1 sigma). Absolute atomic hydrogen production was determined by vacuum ultra-violet (VUV) resonance fluorescence, H production from the CH + CH4 reaction being used as a reference. Observed H branching ratios for these CH reactions were: C2H2: 0.90 +/- 0.08, C2H4: 0.94 +/- 0.08, C3H4 (methyl-acetylene): 0.98 +/- 0.08, C3H4 (allene): 0.97 +/- 0.08, C3H6 (propene): 0.78 +/- 0.10, C4H8 (trans-butene): 0.69 +/- 0.12 (errors are cited at the level of +/- 1 sigma). A compilation of the available kinetic data on these reactions has been made in order to propose rate coefficients for each possible channel of the different reactions for astrochemical models.

Experimental and theoretical studies of the reaction between cationic vanadium oxide clusters and acetylene

The time of flight mass spectrometer coupled with a laser ablation/supersonic expansion cluster source and a fast flow reactor was adopted to study the reactivity of cationic vanadium oxide clusters (VmOn+) toward acetylene (C2H2) molecules under gas phase (P, ∼ 1.14 kPa), under near room temperature (T, ∼ 350 K) conditions. Association products, VmOnC2H2+ (m,n = 2,4; 2,6; 3,7–8; 4,9–11; 5,12–13; 6,13–16, and 7,17), are observed. The oxidation of C2H2 by (V2O5)n+ (n = 1–3) is experimentally identified. The reactivity of (V2O5)n+ decreases as n increases. Density functional theory (DFT) calculations were carried out to interpret the reaction mechanisms. The DFT results indicate that a terminal oxygen atom from V2O5+ can transfer overall barrierlessly to C2H2 at room temperature, which is in agreement with the experimental observation. Other experimental results such as the observation of V2O6C2H2+ and nonobservation of V2O7,8C2H2+ in the experiments are also well interpreted based on the DFT calculations. The reactivity of vanadium oxide clusters toward acetylene and other hydrocarbons may be considered in identifying molecular level mechanisms for related heterogeneous catalysis.

Experimental and Theoretical Study of the Reactions between Small Neutral Iron Oxide Clusters and Carbon Monoxide

Reactions of small neutral iron oxide clusters (FeO(1-3) and Fe(2)O(4,5)) with carbon monoxide (CO) are investigated by experiments and first-principle calculations. The iron oxide clusters are generated by reaction of laser-ablation-generated iron plasma with O(2) in a supersonic expansion and are reacted with carbon monoxide in a fast flow reactor. Detection of the neutral clusters is through ionization with vacuum UV laser (118 nm) radiation and time-of-flight mass spectrometry. The FeO(2) and FeO(3) neutral clusters are reactive toward CO, whereas Fe(2)O(4), Fe(2)O(5), and possibly FeO are not reactive. A higher reactivity for FeO(2) [sigma(FeO(2) + CO) > 3 x 10(-17) cm(2)] than for FeO(3) [sigma(FeO(3) + CO) approximately 1 x 10(-17) cm(2)] is observed. Density functional theory (DFT) calculations are carried out to interpret the experimental observations and to generate the reaction mechanisms. The reaction pathways with negative or very small overall barriers are identified for CO oxidation by FeO(2) and FeO(3). The lower reactivity of FeO(3) with respect to FeO(2) may be related to a spin inversion process present in the reaction of FeO(3) with CO. Significant reaction barriers are calculated for the reactions of FeO and Fe(2)O(4-5) with CO. The DFT results are in good agreement with experimental observations. Molecular-level reaction mechanisms for CO oxidation by O(2), facilitated by condensed phase iron oxides as catalysts, are suggested.