Trevor Brown - Academia.edu (original) (raw)

Papers by Trevor Brown

The technique of Very Low-Pressure Pyrolysis (VLPP) has been used in or der to obtain data on bot... more The technique of Very Low-Pressure Pyrolysis (VLPP) has been used in or der to obtain data on both collisional energy transfer in the gas phase, and for RRKM parameters for a number of unimolecular reaction systems .. Results are reported for the temperature and pressure dependence of the rate coefficients for the thermal unimolecular decomposition of ethyl acetate, over the range 800-1150 K, both under conditions where only reactant gas/wall collisions occur and also (at 837K) dilute in a number of bath gases (He, A:r, Ne, Kr, N 2 and c 2 H 4). Solution of the• appropriate r(?action-diffusion master equation yields from these data both the ectrapolated high-pressure rate coefficient (10 12 "7 exp(-201. 5kJ mol-1 /RT)s-1) a.ud the average downward collisional energy transfer (~Ed) • own These { ~ Ed) values are compared with those obtained for the same own cc,llision partners at ca. 345K using mul tiphoton decomposition (mpd), riJported by Eberhardt et C,tl. ,(1982). It is found that for monand d atomic bath gases<~ Ed) decreases significantly with increasi ng own timperature, consistent with simple statistical theories for this process. This combination of thermal and mpd techniques is generally applicable to obtaining energy transfer data over a very wide range of temperatures, r(Jactants and collision partners. Tl,e cyclobutanone reaction system has been investigated. Quantitative k .netic analysis has not been carried out, due to the appearance of a h~.t herto unreported and still unidentified minor isomerizat:on product , wbich further decomposes at higher temperatures. The expected major ,, 11 8 ..

Zeitschrift für Physikalische Chemie, 2000

The temperature dependence of the gas-phase collisional relaxation of highly vibrationally excite... more The temperature dependence of the gas-phase collisional relaxation of highly vibrationally excited aromatic molecules has been studied using large scale classical trajectory calculations. The investigations have focused on azulene collisions with different colliders (He, Ar and N

Catalysts, 2016

The selective oxidation energetics of isobutane to methacrolein over phosphomolybdic acid and cop... more The selective oxidation energetics of isobutane to methacrolein over phosphomolybdic acid and copper(II) phosphomolybdates have been investigated using low-pressure, pseudo-steady-state and temperature-programming techniques. Time-varying flexible least squares methods were used to determine variations in oxidation activation energies as the temperature increases at 5 • C•min −1. Catalyst activity stabilizes by the fourth consecutive temperature-programmed run. Rate parameters increase linearly with temperature in two sinusoidal, oscillating wave packets. For H 3 PMo 12 O 40 , three distinct reaction pathways are apparent in the fourth run with activation energies 76 ± 3, 93 ± 7 and 130 ± 3 kJ•mol −1 , and under these experimental conditions are observed at the optimum temperatures 704 ± 7 K, 667 ± 25 K and 745 ± 7 K, respectively. Over the copper-containing catalysts, two pathways are apparent: 76 ± 3 kJ•mol −1 at 665 ± 9 K and 130 ± 3 kJ•mol −1 at 706 ± 9 K. The three activation energies indicate either different reaction pathways leading to methacrolein or distinct active sites on the catalyst surface. The intermediate activation energy, 93 kJ•mol −1 , only observed over phosphomolybdic acid, may be linked to hydrogen bonding. Differences in optimum temperatures for the same activation energies for H 3 PMO 12 O 40 and for the copper catalysts indicate that compensating entropy changes are smaller over H 3 PMo 12 O 40. The inclusion of copper enhances catalyst stability and activity.

Catalysts, 2013

Deconvolution of the evolved isobutene data obtained from temperature-programmed, low-pressure st... more Deconvolution of the evolved isobutene data obtained from temperature-programmed, low-pressure steady-state conversion of isobutane over HZSM-5 has yielded apparent activation energies for isobutane dehydrogenation, isobutene dimerization and desorption. Intrinsic activation energies and associated isobutane collision frequencies are also estimated. A combination of wavelet shrinkage denoising, followed by time-varying flexible least squares of the evolved mass-spectral abundance data over the temperature range 150 to 450 °C, provides accurate, temperature-dependent, apparent rate parameters. Intrinsic activation energies for isobutane dehydrogenation range from 86 to 235.2 kJ mol −1 (average = 150 ± 42 kJ mol −1) for isobutene dimerization from 48.3 to 267 kJ mol −1 (average = 112 ± 74 kJ mol −1) and for isobutene desorption from 64.4 to 97.8 kJ mol −1 (average = 77 ± 12 kJ mol −1). These wide ranges reflect the heterogeneity and acidity of the zeolite surface and structure. Seven distinct locations and sites, including Lewis and Brønsted acid sites can be identified in the profiles. Isobutane collision frequencies range from 10 −0.4 to 10 22.2 s −1 and are proportional to the accessibility of active sites, within the HZSM-5 micropores or on the external surface.

International Journal of Chemical Kinetics, 2013

Time-varying linear regression via flexible least squares is used to determine temperature-depend... more Time-varying linear regression via flexible least squares is used to determine temperature-dependent kinetic parameters during low-pressure, steady-state, temperatureprogrammed desorption from catalytic surfaces. The flexible least squares approach optimizes time-varying parameters by minimizing dynamic and measurement discrepancies between a linear theoretical model and experimental data using linear regression. The effectiveness of this approach is demonstrated by calculation of accurate temperature-dependent activation energies, preexponential factors, and differential conversion functions for the evolution of 3methyl-2-oxetanone (β-lactone) during the selective oxidation of isobutane over aluminum phosphomolybdates.

Catalysis Today, 2008

This article was published in an Elsevier journal. The attached copy is furnished to the author f... more This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Catalysis Letters, 2012

Carbon-carbon bond cleavage and dehydrogenation are observed following low-pressure activation of... more Carbon-carbon bond cleavage and dehydrogenation are observed following low-pressure activation of isobutane over HZSM-5. Temperature-dependent rate parameters were determined by flexible least squares modelling of the propene and isobutene formation data. Apparent activation energies are 73.9 ± 2.4 kJ mol-1 (C-C bond cleavage) and 62.8 ± 1.0 kJ mol-1 (dehydrogenation). Such low energy barriers under molecular flow conditions suggest activation steps that are obscured by higher-pressure catalysis.

Dalton Transactions, 2004

High-resolution X-ray diffraction data have been collected on the cubic polymorph of antimony(... more High-resolution X-ray diffraction data have been collected on the cubic polymorph of antimony() oxide (senarmontite) to determine the charge distribution in the crystal. The results are in quantitative agreement with crystal Hartree-Fock calculations for this polymorph, and have been compared with theoretical calculations on the orthorhombic polymorph (valentinite). Information about the nature of bonding and relative bond strengths in the two polymorphs has been extracted in a straightforward manner via topological analysis of the electron density. All the close contacts in both polymorphs are found to be similar in nature based on the value of the Laplacian, the magnitude of the electron density and the local energy density at the bond critical points, and these characterise the observed interactions as substantially polar covalent, similar to molecular calculation results on Si-O and Ge-O. Electrostatic potential isosurfaces reveal the octopolar nature of this function for senarmontite, and shed light on the observed packing arrangement of Sb 4 O 6 molecules in the crystal.

Journal of Catalysis, 1998

The reaction kinetics of evolved carbon monoxide and carbon dioxide during the temperature progra... more The reaction kinetics of evolved carbon monoxide and carbon dioxide during the temperature programmed oxidation (TPO) of an industrially spent fluid cracking catalyst have been investigated. Two pathways to CO and two to CO 2 evolution, all involving either undissociated or dissociated surface oxide complexes, were assumed. Rate coefficient parameters and O 2 reaction orders were then optimized to simulate the TPO profiles recorded over a range of conditions: heating rates of 2, 5 and 10 • C and O 2 partial pressures of 0.939, 1.11, and 5.0% in an atmosphere of N 2. Evolution rates of CO during TPO are independent of the O 2 partial pressure, whereas an order of 0.75 is indicated for CO 2 formation. Because of differences in assumed mechanisms and carbon substrates, calculated preexponential factors and activation energies can not be readily compared with literature values, although some comparisons are made. If the heating rate is high (≥5 • C min −1) and the oxygen partial pressure is low (≤1%), the shape of TPO profiles for highly saturated hydrocarbon coke deposited on cracking catalyst are affected by changes in the rate-determining step with increasing temperature.

Energy & Fuels, 2016

Isothermal adsorption data for n-butane and isobutane on BAX 1500 activated carbon reported by Wh... more Isothermal adsorption data for n-butane and isobutane on BAX 1500 activated carbon reported by Whittaker et al. [Whittaker, P. B.; Wang, X.; Zimmermann, W.; Regenauer-Lieb, K.; Chua, H. T. Predicting the Integral Heat of Adsorption for Gas Physisorption on Microporous and Mesoporous Adsorbents. J. Phys. Chem. C 2014, 118 (16), 8350−8358, DOI: 10.1021/jp410873v] were modeled with pressure-varying Langmuir adsorption parameters using flexible least squares for pressure-varying linear regression. Coverage varies with pressure and at distinct transitions; when the ratio of uptake to capacity is 0.69 ± 0.04, monolayer coverage is achieved or micropore volume is filled. Monolayer transitions are observed for the 298, 323, and 348 K isotherms, while micropore volume transitions are only apparent for the 298 K isotherms. The resultant adsorbent surface area is 1335 ± 25 cm2 g–1, and the micropore volume is 0.48 ± 0.03 cm3 g–1. Molecular areas, corresponding to excluded adsorbate areas, are dependent upon the temp...