Publications by authors named "Gopinathan Sankar"

40 Publications

Elucidation of copper environment in a Cu-Cr-Fe oxide catalyst through high-resolution XANES investigation.

Phys Chem Chem Phys 2021 Mar;23(10):5888-5896

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.

Copper containing materials are widely used in a range of catalytic applications. Here, we report the use of Cu K-edge high resolution XANES to determine the local site symmetry of copper ions during the thermal treatment of a Cu-Cr-Fe oxide catalyst. We exploited the Cu K-edge XANES spectral features, in particular the correlation between area under the pre-edge peak and its position to determine the local environment of Cu2+ ions. The information gained from this investigation rules out the presence of Cu2+ ions in a tetrahedral or square planar geometry, a mixture of these sites, or in a reduced oxidation state. Evidence is presented that the Cu2+ ions in the Cu-Cr-Fe oxide system are present in a distorted octahedral environment.
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http://dx.doi.org/10.1039/d0cp06468hDOI Listing
March 2021

Following the Formation of Silver Nanoparticles Using X-ray Absorption Spectroscopy.

ACS Omega 2020 Jun 4;5(23):13664-13671. Epub 2020 Jun 4.

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.

The formation of silver and Au@Ag core@shell nanoparticles reduction of AgNO by trisodium citrate was followed using X-ray absorption near-edge structure (XANES) spectroscopy and time-resolved UV-visible (UV-vis) spectroscopy. The XANES data were analyzed through linear combination fitting, and the reaction kinetics were found to be consistent with first-order behavior with respect to silver cations. For the Au@Ag nanoparticles, the UV-vis data of a lab-scale reaction showed a gradual shift in dominance between the gold- and silver-localized surface plasmon absorbance bands. Notably, throughout much of the reaction, distinct gold and silver contributions to the UV-vis spectra were observed; however, in the final product, the contributions were not distinct.
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http://dx.doi.org/10.1021/acsomega.0c00697DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7301364PMC
June 2020

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study.

Phys Chem Chem Phys 2020 Sep 24;22(34):18882-18890. Epub 2020 Apr 24.

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.

In situ studies on the physical and chemical properties of Au in inverse ceria alumina supported catalysts have been conducted between 295 and 623 K using high energy resolved fluorescence detection X-ray absorption near edge spectroscopy and X-ray total scattering. Precise structural information is extracted on the metallic Au phase present in a 0.85 wt% Au containing inverse ceria alumina catalyst (ceria/Au/alumina). Herein evidence for the formation of an Au hydride species at elevated temperature is presented. Through modelling of total scattering data to extract the thermal properties of Au using Grüneisen theory of volumetric thermal expansion it proposed that the Au Hydride formation occurs synergistally with the formation of a cerium oxyhydride. The temperature reversible nature, whilst remaining in a reducing atmosphere, demonstrates the activation of hydrogen without consumption of oxygen from the supporting ceria lattice.
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http://dx.doi.org/10.1039/d0cp00455cDOI Listing
September 2020

Electronic and Geometric Structures of Rechargeable Lithium Manganese Sulfate LiMn(SO) Cathode.

ACS Omega 2019 Jul 1;4(7):11338-11345. Epub 2019 Jul 1.

School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore.

Here, we report the use of LiMn(SO) as a potential energy storage material and describe its route of synthesis and structural characterization over one electrochemical cycle. LiMn(SO) is synthesized by ball milling of MnSO·HO and LiSO·HO and characterized using a suite of techniques, in particular, ex situ X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy on the Mn and S K-edges to investigate the electronic and local geometry around the absorbing atoms. The prepared LiMn(SO) electrodes undergo electrochemical cycles to different potential points on the charge-discharge curve and are then extracted from the cells at these points for ex situ structural analysis. Analysis of X-ray absorption spectroscopy (both near and fine structure part of the data) data suggests that there are minimal changes to the oxidation state of Mn and S ions during charge-discharge cycles. However, X-ray photoelectron spectroscopy analysis suggests that there are changes in the oxidation state of Mn, which appears to be different from the conclusion drawn from X-ray absorption spectroscopy. This difference in results during cycling can thus be attributed to electrochemical reactions being dominant at the surface of the LiMn(SO) particles rather than in the bulk.
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http://dx.doi.org/10.1021/acsomega.9b00356DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6682148PMC
July 2019

High-Pressure Study of the Elpasolite Perovskite LaNiMnO.

Inorg Chem 2019 Jul 26;58(14):9016-9027. Epub 2019 Jun 26.

ISIS Neutron and Muon Source , Rutherford Appleton Laboratory , Chilton, Didcot OX11 0QX , U.K.

Here we report a high-pressure investigation into the structural and magnetic properties of the double perovskite LaNiMnO using neutron scattering over a temperature range of 4.2-300 K at ambient pressure and over a temperature range of 120-1177 K up to a maximum pressure of 6.6 GPa. X-ray diffraction was also used up to a maximum pressure of 64 GPa, over a temperature range of 300-720 K. The sample was found to exist in a mixed rhombohedral/monoclinic symmetry at ambient conditions, the balance of which was found to be strongly temperature- and pressure-dependent. Alternating current magnetometry and X-ray absorption near-edge structure measurements were made at ambient pressure to characterize the sample, suggesting that the transition-metal sites exist in a mixed Ni/Mn and Ni/Mn state at ambient temperature and pressure. Analysis of the magnetic properties of the sample shows that the Curie temperature can be enhanced by ∼12 K with 2 GPa applied pressure, but it is highly stable at pressures beyond this. We report a pressure-volume-temperature equation of state for this material over this combined temperature and pressure range, with an ambient temperature bulk modulus of ∼179(8) GPa. The previously reported transition from monoclinic to rhombohedral symmetry upon heating to 700 K is seen to be encouraged with applied pressure, transforming fully by ∼1.5 GPa. Raman spectroscopy data were collected up to ∼8 GPa and show no clear changes or discontinuities over the reported phase transition to rhombohedral symmetry or any indication of further changes over the range considered. The ambient-pressure Grüneisen parameter was determined to be = 2.6 with a Debye temperature of 677 K. The individual modal parameters at ambient temperature were also determined from the high-pressure Raman data.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00404DOI Listing
July 2019

Insight into the Nature of Iron Sulfide Surfaces During the Electrochemical Hydrogen Evolution and CO Reduction Reactions.

ACS Appl Mater Interfaces 2018 Sep 11;10(38):32078-32085. Epub 2018 Sep 11.

Department of Chemistry , University College London , 20 Gordon Street , London WC1H 0AJ , U.K.

Greigite and other iron sulfides are potential, cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (FeS)-modified electrode were determined at -0.6 V versus standard hydrogen electrode (SHE) at pH 7, under conditions of the HER. In situ X-ray absorption spectroscopy was employed at the Fe K-edge to show that iron-sulfur linkages were replaced by iron-oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core-shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH as a product of the HER. Under these conditions, iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ infrared spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when -0.6 V versus SHE is applied. Saturation of the solution with CO resulted in the inhibition of the hydroxide formation, potentially due to surface adsorption of HCO. This study shows that the true nature of the greigite electrode under conditions of the HER is a core-shell greigite-hydroxide material and emphasizes the importance of in situ investigation of the catalyst under operation to develop true and accurate mechanistic models.
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http://dx.doi.org/10.1021/acsami.8b08612DOI Listing
September 2018

Optimized Atmospheric-Pressure Chemical Vapor Deposition Thermochromic VO Thin Films for Window Applications.

ACS Omega 2017 Mar 20;2(3):1040-1046. Epub 2017 Mar 20.

Department of Chemistry, Materials Chemistry Centre, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.

Monoclinic vanadium(IV) oxide (VO) has been widely studied for energy-efficient glazing applications because of its thermochromic properties, displaying a large change in transmission of near-IR wavelengths between the hot and cold states. The optimization of the reaction between VCl and ethyl acetate via atmospheric-pressure chemical vapor deposition (APCVD) was shown to produce thin films of monoclinic VO with excellent thermochromic properties (Δ = 12%). The tailoring of the thermochromic and visible light transmission was shown to be possible by altering the density and morphology of the deposited films. The films were characterized by X-ray diffraction, atomic-force microscopy, scanning electron microscopy, ellipsometry, and UV-vis spectrometry. This article provides useful design rules for the synthesis of high-quality VO thin films by APCVD.
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http://dx.doi.org/10.1021/acsomega.7b00042DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6640985PMC
March 2017

Controlling the Cross-Sensitivity of Carbon Nanotube-Based Gas Sensors to Water Using Zeolites.

ACS Appl Mater Interfaces 2016 Oct 5;8(41):28096-28104. Epub 2016 Oct 5.

Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, United Kingdom.

Carbon nanotube-based gas sensors can be used to detect harmful environmental pollutants such as NO at room temperature. Although they show promise as low-powered, sensitive, and affordable monitoring devices, cross-sensitivity of functionalized carbon nanotubes to water vapor often obscures the detection of target molecules. This is a barrier to adoption for monitoring of airborne pollutants because of the varying humidity levels found in real world environments. Zeolites, also known as molecular sieves because of their selective adsorption properties, are used in this work to control the cross-sensitivity of single-walled carbon nanotube (SWCNT)-based sensors to water vapor. Zeolites incorporated into the sensing layer are found to reduce interference effects that would otherwise obscure the identification of NO gas, permitting repeatable detection over a range of relative humidities. This significant improvement is found to depend on the arrangement of the SWCNT-zeolite layers in the sensing device, as well as the hydrophilicity of the chosen zeolite.
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http://dx.doi.org/10.1021/acsami.6b10042DOI Listing
October 2016

Structure of Nano-sized CeO Materials: Combined Scattering and Spectroscopic Investigations.

Chemphyschem 2016 Nov 20;17(21):3494-3503. Epub 2016 Sep 20.

Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK.

The structure of several nano-sized ceria, CeO , systems was investigated using neutron and X-ray diffraction and X-ray absorption spectroscopy. Whilst both diffraction and total pair distribution functions (PDFs) revealed that in all of the samples the occupancy of both Ce and O are very close to the ideal stoichiometry, the analysis using Reverse Monte Carlo technique revealed significant disorder around oxygen atoms in the nano-sized ceria samples in comparison to the highly crystalline NIST standard. In addition, the analysis revealed that the main differences observed in the pair correlations from various X-ray and neutron diffraction techniques were attributable to the particle size of the CeO prepared by the reported three methods. Furthermore, detailed analysis of the Ce L - and K-edge EXAFS data support this finding; in particular the decrease in higher shell coordination numbers with respect to the NIST standard, is attributed to differences in particle size.
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http://dx.doi.org/10.1002/cphc.201600697DOI Listing
November 2016

Rutile to anatase phase transition induced by N doping in highly oriented TiO2 films.

Phys Chem Chem Phys 2016 Sep 22;18(35):24722-8. Epub 2016 Aug 22.

Department of Chemistry, University College London, 20 Gordon Street, London, WC1E 0AJ, UK.

Highly oriented TiO2 thin films were deposited onto Al2O3(0001), SrTiO3(001), and LaAlO3(001) substrates by spin coating a titanium alkoxide precursor solution followed by annealing. The films were nitrogen doped by two different routes: either by adding tetramethyethylenediamine (TMEDA) to the precursor solution or alternatively by high temperature ammonolysis. Undoped TiO2 films were highly oriented and the phase was dependent on the substrate. N doping by ammonolysis led to transformation of rutile films to anatase, confirmed by XRD and by XPS valence band spectroscopy. Significant differences were observed in the spatial distribution of the nitrogen dopant depending upon which synthesis method was used. These two factors may shed light on the increased photocatalytic efficiencies reported in N doped TiO2.
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http://dx.doi.org/10.1039/c6cp04003aDOI Listing
September 2016

Phase control during the synthesis of nickel sulfide nanoparticles from dithiocarbamate precursors.

Nanoscale 2016 Jun 13;8(21):11067-75. Epub 2016 May 13.

Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK.

Square-planar nickel bis(dithiocarbamate) complexes, [Ni(S2CNR2)2], have been prepared and utilised as single source precursors to nanoparticulate nickel sulfides. While they are stable in the solid-state to around 300 °C, heating in oleylamine at 230 °C, 5 mM solutions afford pure α-NiS, where the outcome is independent of the substituents. DFT calculations show an electronic effect rather than steric hindrance influences the resulting particle size. Decomposition of the iso-butyl derivative, [Ni(S2CN(i)Bu2)2], has been studied in detail. There is a temperature-dependence of the phase of the nickel sulfide formed. At low temperatures (150 °C), pure α-NiS is formed. Upon raising the temperature, increasing amounts of β-NiS are produced and at 280 °C this is formed in pure form. A range of concentrations (from 5-50 mM) was also investigated at 180 °C and while in all cases pure α-NiS was formed, particle sizes varied significantly. Thus at low concentrations average particle sizes were ca. 100 nm, but at higher concentrations they increased to ca. 150 nm. The addition of two equivalents of tetra-iso-butyl thiuram disulfide, ((i)Bu2NCS2)2, to the decomposition mixture was found to influence the material formed. At 230 °C and above, α-NiS was generated, in contrast to the results found without added thiuram disulfide, suggesting that addition of ((i)Bu2NCS2)2 stabilises the metastable α-NiS phase. At low temperatures (150-180 °C) and concentrations (5 mM), mixtures of α-NiS and Ni3S4, result. A growing proportion of Ni3S4 is noted upon increasing precursor concentration to 10 mM. At 20 mM a metastable phase of nickel sulfide, NiS2 is formed and as the concentration is increased, α-NiS appears alongside NiS2. Reasons for these variations are discussed.
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http://dx.doi.org/10.1039/c6nr00053cDOI Listing
June 2016

Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts.

Nat Commun 2015 Mar 5;6:6451. Epub 2015 Mar 5.

Catalysis Engineering, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.

Depletion of crude oil resources and environmental concerns have driven a worldwide research on alternative processes for the production of commodity chemicals. Fischer-Tropsch synthesis is a process for flexible production of key chemicals from synthesis gas originating from non-petroleum-based sources. Although the use of iron-based catalysts would be preferred over the widely used cobalt, manufacturing methods that prevent their fast deactivation because of sintering, carbon deposition and phase changes have proven challenging. Here we present a strategy to produce highly dispersed iron carbides embedded in a matrix of porous carbon. Very high iron loadings (>40 wt %) are achieved while maintaining an optimal dispersion of the active iron carbide phase when a metal organic framework is used as catalyst precursor. The unique iron spatial confinement and the absence of large iron particles in the obtained solids minimize catalyst deactivation, resulting in high active and stable operation.
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http://dx.doi.org/10.1038/ncomms7451DOI Listing
March 2015

Time resolved in situ X-ray diffraction study of crystallisation processes of large pore nanoporous aluminophosphate materials.

Faraday Discuss 2015 ;177:237-47

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom.

Time resolved high-resolution X-ray powder diffraction was utilized to obtain detailed changes in the crystal structure parameters during the hydrothermal crystallization process of the nanoporous aluminophosphate AlPO-5 (AFI) structure. This in situ study offered not only the influence of metal ions on the onset of crystallization and estimation of the activation energy of the process, but also allowed us to determine in detail the changes in lattice parameters during this process. More importantly the time-resolved study clearly showed the lattice expansion in the divalent metal ions substituted system right from the on-set of crystallization process, compared to the one without any dopant ions, which suggest that an amorphous or poorly crystalline network is formed prior to crystallization that contains the large divalent ions (compared to Al(iii), the substituting element), which is in agreement with the combined XAS/XRD study reported earlier. A mechanism based on this and the earlier study is suggested.
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http://dx.doi.org/10.1039/c4fd00179fDOI Listing
October 2015

Single-step co-deposition of nanostructured tungsten oxide supported gold nanoparticles using a gold-phosphine cluster complex as the gold precursor.

Sci Technol Adv Mater 2014 Dec 9;15(6):065004. Epub 2014 Dec 9.

Department of Chemistry, University College London, 20 Gordon Street, WC1H 0AJ London, UK.

The use of a molecular gold organometallic cluster in chemical vapour deposition is reported, and it is utilized, together with a tungsten oxide precursor, for the single-step co-deposition of (nanostructured) tungsten oxide supported gold nanoparticles (NPs). The deposited gold-NP and tungsten oxide supported gold-NP are highly active catalysts for benzyl alcohol oxidation; both show higher activity than SiO supported gold-NP synthesized via a solution-phase method, and tungsten oxide supported gold-NP show excellent selectivity for conversion to benzaldehyde.
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http://dx.doi.org/10.1088/1468-6996/15/6/065004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5090393PMC
December 2014

Energy-resolved electron-yield XAS studies of nanoporous CoAlPO-18 and CoAlPO-34 catalysts.

J Synchrotron Radiat 2014 Jul 16;21(Pt 4):744-50. Epub 2014 May 16.

Netherlands Organization for Scientific Research (NWO), DUBBLE@ESRF, BP 220, 6 Rue Jules Horowitz, 38043 Grenoble, France.

Energy-resolved electron-yield X-ray absorption spectroscopy is a promising technique for probing the near-surface structure of nanomaterials because of its ability to discriminate between the near-surface and bulk of materials. So far, the technique has only been used in model systems. Here, the local structural characterization of nanoporous cobalt-substituted aluminophosphates is reported and it is shown that the technique can be employed for the study of open-framework catalytically active systems. Evidence that the cobalt ions on the surface of the crystals react differently to those in the bulk is found.
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http://dx.doi.org/10.1107/S1600577514008108DOI Listing
July 2014

Local structure and speciation of platinum in fresh and road-aged North American sourced vehicle emissions catalysts: an X-ray absorption spectroscopic study.

Environ Sci Technol 2014 Apr 12;48(7):3658-65. Epub 2014 Mar 12.

Johnson Matthey Technology Centre , Blount's Court, Sonning Common, Reading RG4 9NH, United Kingdom.

Given emerging concerns about the bioavailability and toxicity of anthropogenic platinum compounds emitted into the environment from sources including vehicle emission catalysts (VEC), the platinum species present in selected North American sourced fresh and road-aged VEC were determined by Pt and Cl X-ray absorption spectroscopy. Detailed analysis of the Extended X-ray Absorption Fine Structure at the Pt L3 and L2 edges of the solid phase catalysts revealed mainly oxidic species in the fresh catalysts and metallic components dominant in the road-aged catalysts. In addition, some bimetallic components (Pt-Ni, Pt-Pd, Pt-Rh) were observed in the road-aged catalysts from supporting Ni-, Pd-, and Rh-K edge XAS studies. These detailed analyses allow for the significant conclusion that this study did not find any evidence for the presence of chloroplatinate species in the investigated solid phase of a Three Way Catalyst or Diesel Oxidation Catalysts.
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http://dx.doi.org/10.1021/es404974eDOI Listing
April 2014

Tracking the structural changes in pure and heteroatom substituted aluminophosphate, AIPO-18, using synchrotron based X-ray diffraction techniques.

Phys Chem Chem Phys 2013 Jul;15(28):11766-74

University College London, Department of Chemistry, 20 Gordon Street, London, WC1H 0AJ, UK.

We report the structural changes that occur during the thermal removal of organic template molecules that occlude the pores of small pore nanoporous zeolitic solids, AlPO-18, SAPO-18, CoAlPO-18, ZnAlPO-18 and CoSAPO-18. The calcination process is a necessary step in the formation of active catalysts. The studies performed using time-resolved High Resolution Powder Diffraction (HRPD) and High Energy X-ray Diffraction (HEXRD) techniques at various temperatures reveal that changes that take place are dependent on the type of heteroatom present in the nanoporous solids. While time-resolved HRPD shows clear changes in lattice parameters during the removal of physisorbed water molecules and subsequent removal of the organic template, HEXRD data show changes in various near neighbour distances in AlPO-18, SAPO-18, CoAlPO-18, CoSAPO-18 and ZnAlPO-18 during the calcination process. In particular HEXRD reveals the presence of water molecules coordinated to Al(III) ions in the as-synthesised materials. Upon removal of the template and water, these solids show contraction in the cell volume at elevated temperatures while first and second neighbour distances remained almost unchanged.
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http://dx.doi.org/10.1039/c3cp00015jDOI Listing
July 2013

Elucidation of structure and nature of the PdO-Pd transformation using in situ PDF and XAS techniques.

Phys Chem Chem Phys 2013 Jun 8;15(22):8555-65. Epub 2013 May 8.

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.

The PdO-Pd phase transformation in a 4 wt% Pd/Al2O3 catalyst has been investigated using in situ X-ray absorption spectroscopy (XAS) and in situ X-ray total scattering (also known as high-energy X-ray diffraction) techniques. Both the partial and total pair distribution functions (PDF) from these respective techniques have been analysed in depth. New information from PDF analysis of total scattering data has been garnered using the differential PDF (d-PDF) approach where only correlations orginating from PdO and metallic Pd are extracted. This method circumvents problems encountered in characerising the catalytically active components due to the diffuse scattering from the disordered γ-Al2O3 support phase. Quantitative analysis of the palladium components within the catalyst allowed for the phase composition to be established at various temperatures. Above 850 °C it was found that PdO had converted to metallic Pd, however, the extent of reduction was of the order ca. 70% Pd metal and 30% PdO. Complementary in situ XANES and EXAFS were performed, with heating to high temperature and subsequent cooling in air, and the results of the analyses support the observations, that residual PdO is detected at elevated temperatures. Hysteresis in the transformation upon cooling is confirmed from XAS studies where reoxidation occurs below 680 °C.
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http://dx.doi.org/10.1039/c3cp50600bDOI Listing
June 2013

What are the active species in the photoinduced H2 production with terpyridyl Pt(II) complexes? An investigation by in situ XAFS.

Chemphyschem 2013 Apr 6;14(6):1122-5. Epub 2013 Mar 6.

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.

Origin of the species: In situ X-ray absorption measurements show that monomeric Pt(II) species are the active components for photocatalytic H2 evolution under visible-light illumination for both [PtCl(tpy)]Cl on its own and a three-component system comprising Ru(bpy)3Cl2, MV(2+) and [PtCl(tpy)]Cl in the presence of a sacrificial electron donor (EDTA).
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http://dx.doi.org/10.1002/cphc.201201093DOI Listing
April 2013

Structure and speciation of chromium ions in chromium doped Fe2O3 catalysts.

Phys Chem Chem Phys 2013 Jan 13;15(1):168-75. Epub 2012 Nov 13.

University College London, Department of Chemistry, 20 Gordon Street, London, WC1H 0AJ, UK.

In this paper, we report a detailed characterisation of chromium doped iron oxide catalysts using a range of techniques to establish the nature of chromium species in the near surface and bulk of iron oxide, high-temperature shift (HTS) catalysts. In particular we have employed X-ray absorption spectroscopy Cr K-edge near edge and extended fine structure data for comparison with chemical and X-ray photoelectron spectroscopy. There was excellent agreement between the techniques in terms of identification and quantification of Cr(6+) and Cr(3+) species as a function of calcination temperatures between 100 and 500 °C.
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http://dx.doi.org/10.1039/c2cp43307aDOI Listing
January 2013

Catalytic reaction mechanism of Mn-doped nanoporous aluminophosphates for the aerobic oxidation of hydrocarbons.

Chemistry 2010 Dec;16(46):13638-45

Department of Chemistry, 20 Gordon Street, University College London, WC1H 0AJ London, UK.

In this work we apply state-of-the-art electronic-structure-based computational methods based on hybrid-exchange density functional theory to study the mechanism of the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates (Mn-AlPOs). We compare our results with available experimental data. We show that the catalytic efficiency of Mn-AlPOs in oxidation reactions is intrinsically linked to 1) the Mn redox activity, in particular between 2+ and 3+ oxidation states, and 2) the coordinative insaturation of tetrahedral Mn embedded in AlPO frameworks, which facilitates the reaction by stabilising oxo-type radicals through the formation of Mn complexes. Our mechanism demonstrates the crucial role of both Mn(III) and Mn(II) in the reaction mechanism: Mn(III) sites undergo an initial reaction cycle that leads to the production of the alkyl hydroperoxide intermediate, which can only be transformed into the oxidative products (alcohol, aldehyde and acid) by Mn(II). A preactivation step is required to yield the reduced Mn(II) sites able to decompose the hydroperoxide intermediates; this step takes place through a transformation of the hydrocarbon into the corresponding peroxo-derivative, stabilised by forming a complex with Mn(III) and yielding at the same time reduced Mn(II) sites. Both species enter a subsequent propagation cycle in which Mn(II) catalyses the dissociation of the hydroperoxide that proceeds until the formation of the oxidative products by two parallel pathways, through alkoxy- or hydroxy-radical-like intermediates, whilst the Mn(III)-peroxo complex enables further production of the hydroperoxide intermediate.
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http://dx.doi.org/10.1002/chem.201001876DOI Listing
December 2010

Tracking the formation of cobalt substituted ALPO-5 using simultaneous in situ X-ray diffraction and X-ray absorption spectroscopy techniques.

Phys Chem Chem Phys 2010 Jan 13;12(3):559-62. Epub 2009 Nov 13.

Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ.

We followed the formation of metal ion substituted aluminophosphate, a redox molecular sieve, using a newly developed in situ simultaneous X-ray diffraction and X-ray absorption spectroscopic technique. The study showed that a cobalt-phosphorous network forms prior to the crystallisation.
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http://dx.doi.org/10.1039/b920245eDOI Listing
January 2010

EXAFS and XRD characterization of palladium sorbents for high temperature mercury capture from fuel gas.

Phys Chem Chem Phys 2010 Jan 13;12(2):484-91. Epub 2009 Nov 13.

Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, UK RG4 9NH.

Removal of pollutants such as mercury at elevated temperatures provides improvements in the overall thermal efficiency during the process of coal gasification. The two high temperature sorbents studied were 5 wt% Pd/Al(2)O(3) and 5 wt% Pd/SiO(2): materials shown to have significantly different Hg adsorption capacities. A combination of XRD and EXAFS has been used to characterize the Pd-Hg alloy formed when these Pd-based sorbents were exposed to fuel gas (CO, CO(2), H(2)) containing Hg vapour at 204 degrees C. Significant differences were found in the nature of the alloy formed on the two sorbents following Hg exposure. The Pd/Al(2)O(3) sorbent produced a single homogeneous solid solution of Pd-Hg whilst the silica-supported Pd produced an alloy of varying composition.
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http://dx.doi.org/10.1039/b911941hDOI Listing
January 2010

A novel route for the inclusion of metal dopants in silicon.

Nanotechnology 2010 Jan 3;21(2):025304. Epub 2009 Dec 3.

Department of Physics, University College London, London, UK.

We report a new method for introducing metal atoms into silicon wafers, using negligible thermal budget. Molecular thin films are irradiated with ultra-violet light releasing metal species into the semiconductor substrate. Secondary ion mass spectrometry and x-ray absorption spectroscopy show that Mn is incorporated into Si as an interstitial dopant. We propose that our method can form the basis of a generic low-cost, low-temperature technology that could lead to the creation of ordered dopant arrays.
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http://dx.doi.org/10.1088/0957-4484/21/2/025304DOI Listing
January 2010

Elucidating the genesis of Bi2MoO6 catalyst by combination of synchrotron radiation experiments and Raman scattering.

Chem Commun (Camb) 2009 Aug 3(32):4850-2. Epub 2009 Jul 3.

Unité de Catalyse et de Chimie du Solide, UMR CNRS 8181, Université des Sciences et Technologies de Lille, ENSCL, BP 90108, 59652 Villeneuve d'Ascq Cedex, France.

Combination of in situ Raman scattering with high-resolution XRD and XAS techniques has proven to be a powerful tool to elucidate the crystal growth of gamma-Bi2MoO6 under hydrothermal conditions.
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http://dx.doi.org/10.1039/b907935aDOI Listing
August 2009

New insights into the formation of microporous materials by in situ scattering techniques.

Faraday Discuss 2007 ;136:157-66; discussion 213-29

Daresbury Laboratory, Warrington, Cheshire, UK WA4 4AD.

The formation of zeolite A (LTA) in the presence of tetramethylammonium cations and CoA1PO-5 (AFI structure) in the presence of tetraethyl ammonium hydroxide was studied using time-resolved, in situ energy dispersive X-ray diffraction (EDXRD) and small angle and wide angle X-ray scattering (SAXS/WAXS) techniques. The in situ SAXS measurements show the formation of homogeneous precursors, ca. 10 nm in size, prior to the crystallization of LTA, and consumed during the crystallization. The crystal size is estimated by fitting the SAXS patterns with an equation for a cubic particle, and it is revealed that the final crystal size of the LTA depends on the synthesis temperature. The crystallisation of CoA1PO-5 occurs through the formation of poly-dispersed particles with an average size of the precursor particle of ca. 50 nm.
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http://dx.doi.org/10.1039/b700090cDOI Listing
November 2007

Selective oxidation of benzene to phenol over FeAlPO catalysts using nitrous oxide as oxidant.

Chem Commun (Camb) 2006 Dec 9(47):4955-7. Epub 2006 Oct 9.

Davy Faraday Research Laboratory, The Royal Institution of Great Britain, London, W1S 4BS, United Kingdom.

A tetrahedrally coordinated iron in framework substituted microporous AlPO-5 catalysts are shown to be active and selective for the hydroxylation of benzene to phenol, using nitrous oxide as the oxidant.
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http://dx.doi.org/10.1039/b608982hDOI Listing
December 2006

In situ observation of homogeneous nucleation of nanosized zeolite A.

Phys Chem Chem Phys 2006 Mar 10;8(11):1335-9. Epub 2006 Feb 10.

Department of Chemical System Engineering, The University of Tokyo, Tokyo, 113-8656, Japan.

The formation of zeolite A (LTA) in the presence of tetramethylammonium cations is studied using in situ small angle and wide angle X-ray scattering (SAXS/WAXS) techniques. The SAXS measurements show the formation of homogeneous precursors 10 nm in size prior to the crystallization of LTA which were consumed during the crystallization. The crystal size is estimated by fitting the SAXS patterns with an equation for a cubic particle, and it is revealed that the final crystal size of the LTA depends on the synthesis temperature. However, although such temperature dependence is noted for the final crystal size, the initial precursor particles size appears to be closely similar (ca. 10 nm) irrespective of the synthesis temperature.
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http://dx.doi.org/10.1039/b510963aDOI Listing
March 2006

A new approach to the determination of atomic-architecture of amorphous zeolite precursors by high-energy X-ray diffraction technique.

Phys Chem Chem Phys 2006 Jan 10;8(2):224-7. Epub 2005 Nov 10.

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

The structure of amorphous precursor species formed under hydrothermal conditions, prior to the onset of crystallization of microporous aluminosilicate zeolites, is determined employing high-energy X-ray diffraction (HEXRD). The investigation, combined with the use of reverse Monte Carlo modelling suggests that even numbered rings, especially 4R (R: ring) and 6R, which are the dominant aluminosilicate rings in zeolite A, have already been produced in the precursor. The model implies that the formation of double 4Rs occurs at the final step of the crystallization of zeolite A.
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http://dx.doi.org/10.1039/b512840dDOI Listing
January 2006