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'New Insights on the Selective Oxidation of Methanol to Formaldehyde on Femo Based Catalysts'

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'New Insights on the Selective Oxidation of Methanol to Formaldehyde on Femo Based Catalysts' ‘New Insights on the Selective Oxidation of Methanol to Formaldehyde on FeMo Based Catalysts' Catherine Brookes School of Chemistry Cardiff University Thesis for the degree of Doctor of Philosophy i This thesis was submitted for examination in August 2015. ii Abstract The selective oxidation of methanol has been studied in detail, with particular focus on gaining insights into the surface active sights responsible for directing the selectivity to formaldehyde. Various Fe and Mo containing oxides have been investigated for their reactivity with methanol, to gain an understanding of the different roles of these components in the industrial catalyst employed, which is a mixed phase comprised of MoO 3 and Fe 2(MoO 4)3. Catalysts have primarily been tested through using TPD (temperature programmed desorption) and TPPFR (temperature programmed pulsed flow reaction). The reactivity of Fe 2O3 is dominated by combustion products, with CO 2 and H 2 produced via a formate intermediate adsorbing at the catalyst surface. For MoO 3 however, the surface is populated by methoxy intermediates, so that the selectivity is almost 100 % directed to formaldehyde. When a mixture of isolated Fe and Mo sites co-exist, the surface methoxy becomes stabilised, resulting in a dehydrogenation reaction to CO and H 2. CO and CO 2 can also be observed on Mo rich surfaces, however here a consequence of the further oxidation of formaldehyde, through a linear pathway. TPD and DRIFTS identify these intermediates and products forming. Since the structure of the industrial catalyst is relatively complex, in that it contains both MoO 3 and Fe 2(MoO 4)3, it is difficult to identify the active site for the reaction with methanol. A novel approach to understanding this further, has involved the synthesis of a series of MoO x modified Fe 2O3 catalysts in an attempt to make core-shell oxidic materials of the type MoO x/Fe 2O3. Various monolayer loadings are investigated. It is conclusively shown that for all coverages the Mo stays in the surface regio n, even after annealing to high temperatures, only reacting with the iron oxide surface when the material is annealed above 400 ̊ C. From drying at 120 ̊ C to calcining at 500 ̊ C, the Mo converts from a MoO 3-like octahedral layer to Fe 2(MoO 4)3, with Mo in a tetrahedral structure. Although changes in the Mo phase are clearly evident, it is shown that for all catalysts a one monolayer equivalent of amorphous octahedral MoO x also remains at the surface, regardless of the calcination temperature employed. It is this layer which is deemed as the surface active layer, since all catalysts at varying monolayer overages and anneal temperatures show a similar reaction with methanol. This overlayer is unique, and is suggested to be comparable to the surface terminating layer in bulk catalysts such as Fe 2(MoO 4)3. Successive work involved studying the reactivity of this upper layer, with suggestions of a two site Mo-Mo surface species forming on adsorption of methanol. Concluding work involves an investigation into the redox properties of Fe 2(MoO 4)3, to address the significance of this mixed oxide in commercial materials. Fe 2(MoO 4)3 forms the iii majority of the industrial catalyst, and although it shows a superior performance in terms of its activity, it cannot compete with the near 100 % selectivity of MoO 3 to formaldehyde. Other supports have been trialled for their performance under reaction with methanol. It is shown that Fe 2(MoO 4)3 has increased bulk lattice oxygen mobility. Under normal reaction conditions, the reaction is carried out aerobically. However if oxygen supplies are restricted, Fe 2(MoO 4)3 is able to demonst rate a satisfactory performance above 300 ̊ C, as lattice oxygen is able to replace lost surface oxygen. This can continue for some time, until reduced phases containing Mo(IV) form. At this point formaldehyde selectivity drops, matched by a rise in CO production. High oxidation states are crucial to catalyst performance, with the reaction continuously cycling between Mo(VI) and Mo(VI), with a very short lifetime for the Mo(IV) species. iv Acknowledgements Firstly, I would like to express my sincere gratitude to my supervisor, Professor Mike Bowker, for his continual support and guidance throughout the course of my PhD. Whether it was a chat about ongoing work, a reassurance that I was on track, or a general catch up at the onsite Costa, I thank him for his patience, immense knowledge and his ability to calculate any sum without a calculator! I could not have imagined having a better supervisor. In addition to Mike I would like to thank the following people who have provided me with significant help and support which I have greatly appreciated. My sincere thanks must go to Dr Peter Wells. My journey to obtaining this thesis has been one of a unique kind, entering as the first student in to the UK Ca talysis Hub based at Harwell. It was a rocky start…with just three of us based permanently in the office, and a number of empty labs! Although there were initial doubts, Peter was there to ensure me that things would work out for the best. They did, of course, and I am now proud to be working at the fully developed catalysis institution here today. Thank you Peter for putting up with me, and for the continual encouragement. I finally got ‘that’ paper finished, even if it did come after a lacrosse match or two! In all seriousness, thank you for all the support, all the time you have put aside, and for your positive attitude towards the work of others. I should also here mention Dr Nikolaos Dimitratos, who was one of the three who began on the journey with us at Harwell. I still miss our daily coffees as the original ‘dreamteam’ which must include the infamous Ricardo! I acknowledge the contributions of David Morgan, David Mora Fonz, Emma Gibson, and Diego Gianolio, and also thank the support staff at both institutions. I thank Diamond Light Source and the EPSRC for their funding of this PhD. I thank my fellow labmates for the stimulating discussions and office banter, and for the fun we have had over the last few years. This includes both those at Harwell and Cardiff University. In particular I’d like to thank Wilm Jones and Scott Rogers for their valuable input. v Of course I would like to thank Macey for his support during the stressful process of writing up. I’m so glad I could look forward to w eekends away cycling with you, visiting friends. I’m not sure I would have been able to get through the PhD if it hadn’t been for some our amazing trips abroad, and your ability to always put a smile on my face! Last but not least I would like to thank my parents. My mum and best friend Eve Rees- thank you for always being there for me, no matter what, even when I acted like the ‘PhD zilla’. Your support extends to every aspect of my life, and I really would not have been able to get where I am today without you. And finally, my father John Brookes. From you I have inherited my academic drive and discipline. I hope you would be proud. vi Glossary MeOH Methanol AHM Ammonium Heptamolybdate ATR Attenuated Total Reflectance B.E Binding Energy B.N Boron Nitride C.N Co-ordination Number CH 3OH Methanol CH 4 Methane CO Carbon Monoxide CO 2 Carbon Dioxide DME Dimethyl Ether DRIFTS Diffuse Reflectance Infrared Fourier Transform Spectroscopy EDX Energy Dispersive X-Ray EM Electron Microscopy EXAFS Extended X-Ray Absorption Fine Structure Fe 2(MoO 4)3 Iron Molybdate Fe 2O3 Iron Oxide FT Fourier Transform FTIR Fourier Transform Infrared Spectroscopy FWHM Full Width Half Maximum GC Gas Chromatography GPE Gibb’s Potential Energy H2 Hydrogen H2CO Formaldehyde HC Hydrocarbon HNO 3 Nitric Acid HTF Heat Transfer Fluid HTF High Temperature Furnace IPFR Isothermal Pulse Flow Reaction IR Infrared K.E Kinetic Energy LCF Linear Combination Fitting LEIS Low Energy Ion Scattering vii MFC Mass Flow Controller MFP Mean Free Path ML Monolayer MoO 3 Molybdena MPW Multi Pole Wiggler MS Mass Spectrometry MVK Mars-Van Krevelen O.D Outer Diameter Oh Octahedral QMS Quadrupole Mass Spectrometer RDS Rate Determining Step RF Radio Frequency RPM Revs Per Minute SEM Scanning Electron Microscopy SR Synchrotron Radiation STEM Scanning Transmission Electron Spectroscopy TCD Thermal Conductivity Detector Td Tetrahedral TEM Transmission Electron Microscopy TOF Turn Over Frequency TPD Temperature Programmed Desorption TPO Temperature Programmed Oxidation TPPFR Temperature Programmed Pulsed Flow Reaction TPR Temperature Programmed Reduction VOC’s Volatile Organic Compounds w/t Weight XAFS X-ray Absorption Fine Spectroscopy XANES X-ray Absorption Near Edge Spectroscopy XPS X-ray Photoelectron Spectroscopy XRD X-ray Diffraction viii Contents Chapter 1: Introduction ..........................................................................................................1 Chapter 2: Experimental .......................................................................................................50 Chapter 3: ..Bulk Metal Oxides for Methanol Oxidation.....................................................117 Chapter 4: Core-Shell Structures of the Type MoO x/Fe 2O3, Probing the Nature of the Structural Motifs Responsible for Methanol Oxidation Catalysis................................................................................................................................189 Chapter 5: In situ studies for Understanding Surface Structural Changes.................................................................................................................................241
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