Michael Bowker | Cardiff University (original) (raw)
Papers by Michael Bowker
Topics in Catalysis, 2004
We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to c... more We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO-NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal ''light-off'' phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O 2 dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2CO a + O 2g + 2S fi 2CO 2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. ''Fast XPS'', carried out in situ during the course of the reaction, shows domination of the surface by CO a below 390 K and domination by O a above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO-NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. ''Light-off'' is dictated by the onset of the dissociation of NO a , which occurs at~400 K. Once N a and O a are formed, N 2 O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing O a as CO 2g . Again, the reaction self-accelerates and there is a rapid change of surface coverage from NO a to O a at~450 K. The overall self acceleration is due to the following overall reaction, 2NO a + CO g + S fi N 2 O g + CO 2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by O a and to the reduced coverages of the molecular species.
We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO 2 (1 1 0) using metal vapou... more We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO 2 (1 1 0) using metal vapour deposition (MVD) to form particles in the size range 1–50 nm, which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using scanning tunnelling microscopy (STM). Using these methods we are able to identify the atomic level mechanism responsible for certain phenomena in catalysis, for which molecular level models have previously been proposed from macroscopic measurements. In this paper we address two such phenomena, namely spillover and the so-called strong metal–support interaction (SMSI) effect. Oxygen spillover from Pd particles to the titania support occurs due to the fast adsorption of oxygen on Pd compared with titania, and is driven by reaction with Ti 3+ ions in the vicinity of the particles. The SMSI state is identified at atomic resolution as being due to the appearance of Ti at the surface of the Pd particles. These Ti layers are partially oxidised and form very well defined structures of two main types—a rectangular lattice and hexagonal unit cells of large dimension. These layers passivate the surface for the adsorption of CO.
Catalysis Today, Feb 14, 1992
ABSTRACT
Catalysis Lett, 1993
Page 1. Catalysis Letters 21 (1993) 139-148 139 Direct synthesis of formate from methanol oxidati... more Page 1. Catalysis Letters 21 (1993) 139-148 139 Direct synthesis of formate from methanol oxidation on the CuPd[85 9 15] { 110 } p(2 x 1) surface: induction of a new reaction pathway via a ligand effect Mark A. Newton and Michael ...
Abstract Au/TiO2 catalysts were prepared by incipient wetness impregnation and tested for the roo... more Abstract Au/TiO2 catalysts were prepared by incipient wetness impregnation and tested for the room temperature photocatalytic reforming of methanol in aqueous solution to produce hydrogen. These catalysts proved to be active for this reaction, with the dependence on ...
Journal of Physics Condensed Matter
ABSTRACT
Studies in Surface Science and Catalysis, 1996
The ‘Holy Grail’ of catalysis has been to identify what Taylor described as the ‘active site’ tha... more The ‘Holy Grail’ of catalysis has been to identify what Taylor described as the ‘active site’ that is, that ensemble of atoms which is responsible for the surface reaction involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on plnar surfaces. This gives a greater
Journal of Physics Condensed Matter
ABSTRACT
Journal of Catalysis
ABSTRACT
Current Opinion in Solid State and Materials Science
ABSTRACT
![Research paper thumbnail of Copper-palladium alloy surfaces: I. Cu/Pd[85:15]{110}, surface structure and reactivity](https://attachments.academia-assets.com/41951422/thumbnails/1.jpg)
](Both ordered and disordered phases of Cu:Pd[85 :15]{110} have been studied using low-energy elect... more Both ordered and disordered phases of Cu:Pd[85 :15]{110} have been studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). Low-energy ion scattering (LEIS) has also been employed to study the ordered surface. In the case of the ordered phase, these techniques have been used to ascertain the structural and electronic influence of the Pd on this surface in comparison to that observed for Cu{110}. In addition temperature-programmed desorption (TPD) has been employed to determine the chemical reactivity of this surface towards the decomposition of adsorbed formic acid (HCOOH) and C-deuterated formic acid (DCOOH).This is compared to the same reaction on Cu{110} and a kinetic isotope effect is noted in the decomposition of the two acids. Segregation of the Cu component of the alloy is observed upon the thermally induced transition from disordered to ordered surface phases, leaving the surface layer ess...
We have measured the neutral particle energy/nucleus distributions corresponding to the scatterin... more We have measured the neutral particle energy/nucleus distributions corresponding to the scattering of 1-6 keV H 2 and N~ beams incident at 1-5 ° to a Cu(111) surface. The atomic part of the distributions exhibit a series of regularly spaced peaks whose relative intensities, but not positions, change with incidence angle, azimuthal angle, and scattering angle. The magnitude of the final velocity of the atoms therefore determines the peak positions. We suggest that the oscillations are quantum mechanical in origin. We suggest an explanation in which the phase difference, accumulated by trajectories on the diabatic potential energy surfaces corresponding to the configurations H2-Cu and 2 Cu-H following traversal of the crossing seam on the exit half of the scattering trajectory, leads to interference and to the observed oscillatory structure. Assuming perpendicular energy scaling we appear to observe an entrance channel barrier for access of the 2 Cu-H configuration of between 0.3 and 0.6 eV. This is consistent with that inferred from experiments using hyperthermal beams.
Surface Science, 1995
... ELSEVIER Surface Science 343 (1995) L1175-L1181 surface science Surface Science Letters Molec... more ... ELSEVIER Surface Science 343 (1995) L1175-L1181 surface science Surface Science Letters Molecularly induced step faceting on Cu(110) surfaces FM Leibsle a.,, S. Haq ", BG Frederick ~, M. Bowker ab, NV Richardson aa Interdisciplinary Research Centre ... Stensgaard, Phys ...
Journal of Physics Condensed Matter
In a recent paper by Poulston et al STM images of Cu(110) surfaces exposed to methanol - oxygen m... more In a recent paper by Poulston et al STM images of Cu(110) surfaces exposed to methanol - oxygen mixtures were presented. Regions with 0953-8984/9/41/021/img1 periodicities were interpreted to consist of formate molecules. This comment questions the suitability of this interpretation and subsequent conclusions in light of previously published results which show that adsorbed methoxy can also form areas with 0953-8984/9/41/021/img1 periodicities.
Journal of the Chemical Society, Faraday Transactions, 1990
ABSTRACT
Journal of the Chemical Society, Faraday Transactions, 1994
Page 1. J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 1015-1022 1015 Adsorption and Decomposition of... more Page 1. J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 1015-1022 1015 Adsorption and Decomposition of Methanol on TiO, SrTiO, and SrO Nina Aas, Thomas J. Pringle and Michael Bowkert Leverhulme Centre for Innovative ...
Topics in Catalysis, 2004
We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to c... more We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO-NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal ''light-off'' phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O 2 dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2CO a + O 2g + 2S fi 2CO 2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. ''Fast XPS'', carried out in situ during the course of the reaction, shows domination of the surface by CO a below 390 K and domination by O a above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO-NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. ''Light-off'' is dictated by the onset of the dissociation of NO a , which occurs at~400 K. Once N a and O a are formed, N 2 O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing O a as CO 2g . Again, the reaction self-accelerates and there is a rapid change of surface coverage from NO a to O a at~450 K. The overall self acceleration is due to the following overall reaction, 2NO a + CO g + S fi N 2 O g + CO 2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by O a and to the reduced coverages of the molecular species.
We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO 2 (1 1 0) using metal vapou... more We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO 2 (1 1 0) using metal vapour deposition (MVD) to form particles in the size range 1–50 nm, which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using scanning tunnelling microscopy (STM). Using these methods we are able to identify the atomic level mechanism responsible for certain phenomena in catalysis, for which molecular level models have previously been proposed from macroscopic measurements. In this paper we address two such phenomena, namely spillover and the so-called strong metal–support interaction (SMSI) effect. Oxygen spillover from Pd particles to the titania support occurs due to the fast adsorption of oxygen on Pd compared with titania, and is driven by reaction with Ti 3+ ions in the vicinity of the particles. The SMSI state is identified at atomic resolution as being due to the appearance of Ti at the surface of the Pd particles. These Ti layers are partially oxidised and form very well defined structures of two main types—a rectangular lattice and hexagonal unit cells of large dimension. These layers passivate the surface for the adsorption of CO.
Catalysis Today, Feb 14, 1992
ABSTRACT
Catalysis Lett, 1993
Page 1. Catalysis Letters 21 (1993) 139-148 139 Direct synthesis of formate from methanol oxidati... more Page 1. Catalysis Letters 21 (1993) 139-148 139 Direct synthesis of formate from methanol oxidation on the CuPd[85 9 15] { 110 } p(2 x 1) surface: induction of a new reaction pathway via a ligand effect Mark A. Newton and Michael ...
Abstract Au/TiO2 catalysts were prepared by incipient wetness impregnation and tested for the roo... more Abstract Au/TiO2 catalysts were prepared by incipient wetness impregnation and tested for the room temperature photocatalytic reforming of methanol in aqueous solution to produce hydrogen. These catalysts proved to be active for this reaction, with the dependence on ...
Journal of Physics Condensed Matter
ABSTRACT
Studies in Surface Science and Catalysis, 1996
The ‘Holy Grail’ of catalysis has been to identify what Taylor described as the ‘active site’ tha... more The ‘Holy Grail’ of catalysis has been to identify what Taylor described as the ‘active site’ that is, that ensemble of atoms which is responsible for the surface reaction involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on plnar surfaces. This gives a greater
Journal of Physics Condensed Matter
ABSTRACT
Journal of Catalysis
ABSTRACT
Current Opinion in Solid State and Materials Science
ABSTRACT
Both ordered and disordered phases of Cu:Pd[85 :15]{110} have been studied using low-energy elect... more Both ordered and disordered phases of Cu:Pd[85 :15]{110} have been studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). Low-energy ion scattering (LEIS) has also been employed to study the ordered surface. In the case of the ordered phase, these techniques have been used to ascertain the structural and electronic influence of the Pd on this surface in comparison to that observed for Cu{110}. In addition temperature-programmed desorption (TPD) has been employed to determine the chemical reactivity of this surface towards the decomposition of adsorbed formic acid (HCOOH) and C-deuterated formic acid (DCOOH).This is compared to the same reaction on Cu{110} and a kinetic isotope effect is noted in the decomposition of the two acids. Segregation of the Cu component of the alloy is observed upon the thermally induced transition from disordered to ordered surface phases, leaving the surface layer ess...
We have measured the neutral particle energy/nucleus distributions corresponding to the scatterin... more We have measured the neutral particle energy/nucleus distributions corresponding to the scattering of 1-6 keV H 2 and N~ beams incident at 1-5 ° to a Cu(111) surface. The atomic part of the distributions exhibit a series of regularly spaced peaks whose relative intensities, but not positions, change with incidence angle, azimuthal angle, and scattering angle. The magnitude of the final velocity of the atoms therefore determines the peak positions. We suggest that the oscillations are quantum mechanical in origin. We suggest an explanation in which the phase difference, accumulated by trajectories on the diabatic potential energy surfaces corresponding to the configurations H2-Cu and 2 Cu-H following traversal of the crossing seam on the exit half of the scattering trajectory, leads to interference and to the observed oscillatory structure. Assuming perpendicular energy scaling we appear to observe an entrance channel barrier for access of the 2 Cu-H configuration of between 0.3 and 0.6 eV. This is consistent with that inferred from experiments using hyperthermal beams.
Surface Science, 1995
... ELSEVIER Surface Science 343 (1995) L1175-L1181 surface science Surface Science Letters Molec... more ... ELSEVIER Surface Science 343 (1995) L1175-L1181 surface science Surface Science Letters Molecularly induced step faceting on Cu(110) surfaces FM Leibsle a.,, S. Haq ", BG Frederick ~, M. Bowker ab, NV Richardson aa Interdisciplinary Research Centre ... Stensgaard, Phys ...
Journal of Physics Condensed Matter
In a recent paper by Poulston et al STM images of Cu(110) surfaces exposed to methanol - oxygen m... more In a recent paper by Poulston et al STM images of Cu(110) surfaces exposed to methanol - oxygen mixtures were presented. Regions with 0953-8984/9/41/021/img1 periodicities were interpreted to consist of formate molecules. This comment questions the suitability of this interpretation and subsequent conclusions in light of previously published results which show that adsorbed methoxy can also form areas with 0953-8984/9/41/021/img1 periodicities.
Journal of the Chemical Society, Faraday Transactions, 1990
ABSTRACT
Journal of the Chemical Society, Faraday Transactions, 1994
Page 1. J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 1015-1022 1015 Adsorption and Decomposition of... more Page 1. J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 1015-1022 1015 Adsorption and Decomposition of Methanol on TiO, SrTiO, and SrO Nina Aas, Thomas J. Pringle and Michael Bowkert Leverhulme Centre for Innovative ...