NaBr Poisoning of Au/TiO\u3csub\u3e2\u3c/sub\u3e Catalysts: Effects on Kinetics, Poisoning Mechanism, and Estimation of the Number of Catalytic Active Sites

Sodium bromide was used to intentionally poison a commercial Au/TiO2 catalyst with the goals of understanding the nature of halide poisoning and evaluating the number and nature of the catalytic active sites. A series of eight poisoned catalysts were prepared by impregnating the parent catalyst with methanolic solutions of NaBr. Each catalyst was tested with CO oxidation catalysis under differential reactor conditions; O2 reaction orders and Arrhenius activation energies were determined for each material. All of the kinetic data, including a Michaelis−Menten analysis, indicated that the primary effect of adding NaBr was to reduce the number of catalytically active sites. Density functional theory calculations, employed to evaluate likely binding sites for NaBr, showed that NaBr binds more strongly to Au corner and edge atoms than it does to the titania support or to exposed Au face atoms. Infrared spectroscopy of adsorbed CO, along with a Temkin analysis of the data, was also used to evaluate changes to the catalyst upon NaBr deposition. These studies suggested that NaBr addition induces some subtle changes in the coverage dependent properties of CO adsorption, but that these did not substantially impact the CO coverage of the CO binding sites. The experimental and computational results are discussed in terms of possible poisoning mechanisms (siteblocking vs off-site binding and modification); the nature and number of active sites are also discussed in the context of the results

Selective Oxidation and Kinetic Modelling of Isobutane Over Phosphomolybdate Substrates

This thesis is concerned with the oxidation reaction processes associated with phosphomolybdic acid (H₃[PMo₁₂O₄ₒ]) and derivatives. The derivatives are synthesized via cation exchange of the protons of phosphomolybdic acid with various metal cations. These metals are sourced from selected regions of the periodic table, with a view to understanding and correlating the influence of elemental properties on oxidation activity and selectivity. All phosphomolybdates have been analysed by X-ray diffraction. The structure of these compounds varies significantly according to cation selection; triclinic, body centred cubic and primitive cubic phases were identified. Relationships between cation volume contributions and phase compositions of the phosphomolybdate structure have been established. Caesium phosphomolybdates were investigated using X-ray fluorescence in order to determine compositional information. It was found that this technique was unreliable and hence inappropriate for phosphomolybdates. UV-Visible spectroscopy was applied to solutions of phosphomolybdates in order to ascertain electronic transitional properties and a surface analysis was conducted to elucidate pore and surface area features

Comprehensive Study of Isobutane Selective Oxidation Over Group I and II Phosphomolybdates: Structural and Kinetic Factors

Various phosphomolybdates were synthesized using cations from Groups 1 and 2 of the periodic table. These compounds were of the form M x H3-xn [PMo12O40], with n being the cationic charge (+1 or +2). XRD analysis shows pure phosphomolybdic acid has a triclinic structure. A body centered cubic (BCC) structure gradually develops with addition of Group 1 cations, and the triclinic phase is completely replaced by the BCC phase once metal cations occupy a volume greater than 9-11 ų per phosphomolybdate anion. The Group 2 compounds do not form a cubic phase, however the triclinic phase distorts once cationic volume is greater about 5 or 6 ų and appears to become somewhat amorphous. Isobutane selective oxidation over the compounds yielded methacrolein (primary product), 3-methyl-2-oxetanone (lactone), acetic acid, propene, methacrylic acid, carbon dioxide and water as products. Propene was formed over the Group 1 compounds exclusively and methacrylic acid formation was observed with BaH[PMo12O40] only. Products form via two distinct processes: Category 1 product has an exponential profile and coverage is consistent with a Langmuir model, Category 2 formations are consistent with desorptions from within the bulk of the substrates. Methacrolein forms via both Category 1 and 2 processes, whilst all other products are formed by Category 2 exclusively. A rigorous kinetic analysis yielded accurate activation parameters. Category 1 methacrolein formation apparent activation energies ranged from 34.7 ± 1.3 to 119 ± 4 kJ mol⁻¹. Category 2 formations ranged from 34.3 ± 0.4 to 726 ± 172 kJ mol⁻¹. No relationship between activity and composition or structure could be ascertained, despite investigation into correlations using several different models

Methacrolein formation over cesium containing oxidation catalysts

Methyl methacrylate is produced on a large scale for use in the polymer industry. Synthesis of this acrylic monomer occurs almost exclusively via the acetone cyanohydrin process, which has many environmentally hazardous aspects. In particular the reactant HCN and the intermediate acetone cyanohydrin are very toxic and large amounts of ammonium sulfate, contaminated with organic compounds, are produced. Several companies now utilize alternative processes for methyl methacrylate production via methacrolein using molybdenum based heteropoly acids and isobutene oxidation. Improvements in catalytic selectivity and activity for these "green" processes can be achieved by a careful investigation of the kinetics of targeted catalysts. Furthermore, economic improvements can be made using isobutane as the gas-phase reactant. In this paper Keggin-type heteropoly compounds, which possess unique catalytic properties due to their favourable redox and acid properties, are examined

Selective isobutane oxidation over polyoxomolybdate Keggin-type structures

Keggin-type heteropoly compounds, such as H3[PMo12O40], show unique catalytic activity due to their acidity and redox properties. Such catalysts are used in industry for the oxidation of low-cost alkanes to more valuable alkenes, alcohols, aldehydes and carboxylic acids [1]. The activity and selectivity of polyoxomolybdates can be adjusted by varying the counter-cation [2]. Reported products from isobutane oxidation over Keggin polyoxometalates are isobutene, methacrolein, methacrylic acid and carbon oxides [1]. In the case of polyoxomolybdates, high yields of the aldehyde and carboxylic acid indicate that Mo(VI) can abstract hydrogen and add oxygen to the alkane. Recently we have developed a novel low-pressure technique to determine the kinetics of heterogeneous catalytic processes [3]. Molecular flow conditions and temperature-programming are used to accurately calculate activation energies and determine primary products for the rate-determining reaction. A quadrupole mass spectrometer monitors all gaseous species that effuse from the low-pressure reactor. Observed major products following exposure of isobutane to both H3[PMo12O40] and Cs2.5H0.5[PMo12O40] located in the low-pressure reactor were acetic acid, methacrolein, carbon dioxide and 3-methyl-2-oxetanone. Neither isobutene nor methacrylic acid was observed. Acetic acid formed and desorbed from the catalyst surface at low temperatures

Kinetic Simulation of Methacrolein and Lactone Production from the Catalytic Oxidation of Isobutane over Lanthanide Phosphomolybdates

Eight lanthanum- (La₀.₂₅H₂.₂₅[PMo₁₂O₄₀], La₀.₅H₁.₅[PMo₁₂O₄₀], La₀.₇₅H₀.₇₅[PMo₁₂O₄₀], La[PMo₁₂O₄₀]) and cerium- (Ce₀.₂₅H₂.₂₅[PMo₁₂O₄₀], Ce₀.₅H₁.₅[PMo₁₂O₄₀], Ce₀.₇₅H₀.₇₅[PMo ₁₂O₄₀], Ce[PMo₁₂O₄₀]) containing phosphomolybdate catalysts have been synthesized and analysed using a low pressure steady state technique. The products from isobutane oxidation using the catalysts were water, methacrolein, carbon dioxide and lactone. Methacrylic acid is not detected under the low-pressure molecular-flow conditions; lactone is postulated to be intermediary in methacrylic acid production. The methacrolein and lactone data were simulated using two different theoretical models in order to determine kinetic parameters. The activation barriers for methacrolein formation vary substantially throughout each lanthanum and cerium series, however the most active catalyst is determined to be Ce[PMo₁₂O₄₀]. Only three of the eight catalysts produced significant quantities of lactone (La₀.₇₅H₀.₇₅ [PMo₁₂O₄₀], La[PMo₁₂O₄₀] and Ce[PMo₁₂O₄₀]) and this activity is correlated with the most active catalysts for methacrolein formation. The trends in selective oxidation activity must be due to the acidity and redox properties, as well as the structural phases, but it is difficult to quantify in all cases

Electronic Activity Relationship for Methacrolein Formation Over 4th Period Transition Metal Phosphomolybdates

Phosphomolybdate compounds have been investigated for their structural characteristics and oxidation activity toward isobutane. The phosphomolybdates were synthesized from phosphomolybdic acid and the fourth period transition metal cations Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+ Cu2+, Cu+ and Zn2+. Two compounds were synthesized per transition metal: where (i) all the protons had been replaced by the particular transition metal, and (ii) only partial proton replacement leaving a single proton per phosphomolybdate. X-ray diffraction analysis has shown that a primitive cubic phase is apparent with some of the transition metal phosphomolybdates. Each solid was exposed to isobutane using the anaerobic low-pressure steady-state technique. Category 1 exponential-like distributions of methacrolein were observed with all the transition metal phosphomolybdates, except the lower oxidation state iron and copper salts, Fe1.5[PMo12O40] and Cu3[PMo12O40] respectively. Activation energies ranged from 51.31 ± 0.27 kJ mol−1 (Cr[PMo12O40]) to over 200 kJ mol−1 (Zn1.5[PMo12O40]). Phosphomolybdates with counter cations which are one or two electrons deficient from either a 3d5 or 3d10 configuration (in this case 3d3, 3d8 or 3d9) had the lowest activation barriers for methacrolein formation. A computational investigation into HOMO-LUMO band gap energies agrees with the association. The presence of protons also enhances Category 1 product formation and may be attributed to migration of H+ through the bulk of the solid

Kinetic Analysis of Methacrolein and Lactone Formation over Lanthanide Phosphomolybdate Catalysts

Eight lanthanum (La{0.25}H{2.25}[PM{o12}O{40}], La{0.5}H{1.5}[Pmo{12}O{40}], La{0.75}H{0.75}[Pmo{12}O{40}], La[Pmo{12}O{40}]) and cerium (Ce{0.25}H{2.25}[Pmo{12}O{40}], Ce{0.5}H{1.5}[Pmo{12}O{40}], Ce{0.75}H{0.75}[Pmo{12}O{40}], Ce[Pmo{12}O{40}]) containing phosphomolybdate catalysts have been synthesized and analysed using a low-pressure steady-state technique. The products from isobutane oxidation using the catalysts were water, methacrolein, carbon dioxide and lactone. The methacrolein and lactone data were simulated using two different theoretical models in order to determine kinetic parameters. The activation barriers for methacrolein formation vary substantially throughout each lanthanum and cerium series, however the most active catalyst is determined to be Ce[Pmo{12}O{40}]. Only three of the eight catalysts produced significant quantities of lactone (La{0.75}H{0.75}[Pmo{12}O{40}], La[Pmo{12}O{40}] and Ce[Pmo{12}O{40}]) and its formation is correlated with methacrolein. The correlation between lactone and methacrolein formation is likely due to the presence of a distinct structural phase

Accurate low-pressure kinetics for isobutane oxidation over phosphomolybdic acid and copper(II) phosphomolybdates

A low-pressure steady-state technique has been used to investigate the rates and mechanisms of the oxidation of isobutane over H₃[PMo₁₂O₄₀], CuH₄[PMo₁₂O₄₀]₂, Cu₂H₂[PMo₁₂O₄₀]₂, Cu₂.₅H[PMo₁₂O₄₀]₂, and Cu₃[PMo₁₂O₄₀]₂. Observed oxidation products over all catalysts are methacrolein, 3-methyl-2-oxetanone, acetic acid, carbon dioxide and water. The most selective catalyst for methacrolein formation at low temperatures (<496°C) is Cu₂.₅H[PMo₁₂O₄₀]₂, where both Cu(II) reduction and acid sites play a role. The least active catalyst at low temperatures is phosphomolybdic acid followed by Cu₃[PMo₁₂O₄₀]₂. This activity is reversed at higher temperatures. The 3-methyl-2-oxetanone is a unique product and is likely to be the precursor to methacrylic acid. Acetic acid is also probably a precursor to complete oxidation. Catalyst deactivation or restructuring is significant only over H₃[PMo₁₂O₄₀]