Metal phosphate and precious metal catalysts for selective oxidation

The main objective researched in this thesis involves the selective oxidation of methanol to formaldehyde, using metal phosphate based catalysts. Molybdenum and vanadium phosphate based catalysts have been prepared, thoroughly characterised and tested as active catalysts for the selective oxidation of methanol to formaldehyde. Initial investigations highlighted the relatively low activity of both metal phosphate catalysts, however, significant enhancements in the catalytic activity and formaldehyde selectivity of both materials have been achieved in this research, primarily by supporting molybdenum phosphate catalysts using a range of supports, and also promoting both molybdenum and vanadium pyrophosphates with transition metals. It was discovered that a catalyst of 10 wt% (MoO2)2P2O7 supported on SiO2 containing 1 mol% vanadium (as a promoter) achieved significantly higher formaldehyde per pass yields (>20 %) than MoO3 supported on SiO2 (reported in the literature) and comparable activity to that of the commercial iron molybdate catalyst. Due to the promotional effect of vanadium, and the known activity of V2O5 catalysts for the oxidation of methanol to formaldehyde, molybdenum promoted (VO2)2P2O7 catalysts were tested for this reaction and reported for the first time. Catalytic studies revealed that there is a direct correlation between molybdenum content and catalytic activity, indicating a synergistic effect of the two transition metals. The penultimate chapter of this thesis involves the use of supported mono- and bi-metallic gold(palladium) catalysts and their use in both CO oxidation and selective methanol oxidation. A novel method of maintaining considerably small Au(Pd) nanoparticle size (unlike the standard thermal treatment method) has been discovered by the Hutchings group at Cardiff Catalysis Institute, involving the removal of stabilising IV ligands with a solvent extraction method. Using high resolution microscopy and a range of characterization techniques, the nanoparticle size was attributed to the surprisingly high activity achieved for both CO oxidation and methanol oxidation to methyl formate, at low temperatures which, particularly in the case of methanol oxidation, is a remarkable discovery.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Metal phosphate and precious metal catalysts for selective oxidation

The main objective researched in this thesis involves the selective oxidation of methanol to formaldehyde, using metal phosphate based catalysts. Molybdenum and vanadium phosphate based catalysts have been prepared, thoroughly characterised and tested as active catalysts for the selective oxidation of methanol to formaldehyde. Initial investigations highlighted the relatively low activity of both metal phosphate catalysts, however, significant enhancements in the catalytic activity and formaldehyde selectivity of both materials have been achieved in this research, primarily by supporting molybdenum phosphate catalysts using a range of supports, and also promoting both molybdenum and vanadium pyrophosphates with transition metals. It was discovered that a catalyst of 10 wt% (MoO2)2P2O7 supported on SiO2 containing 1 mol% vanadium (as a promoter) achieved significantly higher formaldehyde per pass yields (>20 %) than MoO3 supported on SiO2 (reported in the literature) and comparable activity to that of the commercial iron molybdate catalyst. Due to the promotional effect of vanadium, and the known activity of V2O5 catalysts for the oxidation of methanol to formaldehyde, molybdenum promoted (VO2)2P2O7 catalysts were tested for this reaction and reported for the first time. Catalytic studies revealed that there is a direct correlation between molybdenum content and catalytic activity, indicating a synergistic effect of the two transition metals. The penultimate chapter of this thesis involves the use of supported mono- and bi-metallic gold(palladium) catalysts and their use in both CO oxidation and selective methanol oxidation. A novel method of maintaining considerably small Au(Pd) nanoparticle size (unlike the standard thermal treatment method) has been discovered by the Hutchings group at Cardiff Catalysis Institute, involving the removal of stabilising IV ligands with a solvent extraction method. Using high resolution microscopy and a range of characterization techniques, the nanoparticle size was attributed to the surprisingly high activity achieved for both CO oxidation and methanol oxidation to methyl formate, at low temperatures which, particularly in the case of methanol oxidation, is a remarkable discovery

Upper limit map of a background of gravitational waves

We searched for an anisotropic background of gravitational waves using data from the LIGO S4 science run and a method that is optimized for point sources. This is appropriate if, for example, the gravitational wave background is dominated by a small number of distinct astrophysical sources. No signal was seen. Upper limit maps were produced assuming two different power laws for the source strain power spectrum. For an f^-3 power law and using the 50 Hz to 1.8 kHz band the upper limits on the source strain power spectrum vary between 1.2e-48 Hz^-1 (100 Hz/f)^3 and 1.2e-47 Hz^-1 (100 Hz /f)^3, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between 8.5e-49 Hz^-1 and 6.1e-48 Hz^-1. As a side product a limit on an isotropic background of gravitational waves was also obtained. All limits are at the 90% confidence level. Finally, as an application, we focused on the direction of Sco-X1, the closest low-mass X-ray binary. We compare the upper limit on strain amplitude obtained by this method to expectations based on the X-ray luminosity of Sco-X1.Comment: 11 pages, 9 figures, 2 table

Upper limit map of a background of gravitational waves

We searched for an anisotropic background of gravitational waves using data from the LIGO S4 science run and a method that is optimized for point sources. This is appropriate if, for example, the gravitational wave background is dominated by a small number of distinct astrophysical sources. No signal was seen. Upper limit maps were produced assuming two different power laws for the source strain power spectrum. For an f^-3 power law and using the 50 Hz to 1.8 kHz band the upper limits on the source strain power spectrum vary between 1.2e-48 Hz^-1 (100 Hz/f)^3 and 1.2e-47 Hz^-1 (100 Hz /f)^3, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between 8.5e-49 Hz^-1 and 6.1e-48 Hz^-1. As a side product a limit on an isotropic background of gravitational waves was also obtained. All limits are at the 90% confidence level. Finally, as an application, we focused on the direction of Sco-X1, the closest low-mass X-ray binary. We compare the upper limit on strain amplitude obtained by this method to expectations based on the X-ray luminosity of Sco-X1.Comment: 11 pages, 9 figures, 2 table

Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Operando monitoring of temperature and active species at the single catalyst particle level

The development of improved catalysts requires insights into the relationship between catalytic activity and catalyst structure, including the underlying reaction mechanism. Here, we demonstrate a unique set of catalyst extrudate sensors that allow for the simultaneous detection of local temperature by luminescence thermometry, and of surface species by shell-isolated nanoparticle-enhanced Raman spectroscopy. This sensing approach was applied to the characterization of direct conversion of syngas into hydrocarbons and C2+ oxygenates over supported Rh and RhFe catalysts. Luminescence thermometry demonstrated a mismatch between the set temperature and the local catalyst temperature, with variations up to 40 °C. Furthermore, by investigating the surface species on varying extrudate and catalyst compositions, we identified tilted carbonyl species on the Rh/SiO2 interface that are probable precursors for the hydrogen-assisted CO dissociation. The implementation of extrudate catalyst sensors as a characterization tool provides a unique approach towards the further understanding of the relevant parameters in catalysis

Operando monitoring of temperature and active species at the single catalyst particle level

The development of improved catalysts requires insights into the relationship between catalytic activity and catalyst structure, including the underlying reaction mechanism. Here, we demonstrate a unique set of catalyst extrudate sensors that allow for the simultaneous detection of local temperature by luminescence thermometry, and of surface species by shell-isolated nanoparticle-enhanced Raman spectroscopy. This sensing approach was applied to the characterization of direct conversion of syngas into hydrocarbons and C2+ oxygenates over supported Rh and RhFe catalysts. Luminescence thermometry demonstrated a mismatch between the set temperature and the local catalyst temperature, with variations up to 40 °C. Furthermore, by investigating the surface species on varying extrudate and catalyst compositions, we identified tilted carbonyl species on the Rh/SiO2 interface that are probable precursors for the hydrogen-assisted CO dissociation. The implementation of extrudate catalyst sensors as a characterization tool provides a unique approach towards the further understanding of the relevant parameters in catalysis