A pyrene modified nickel cyclam catalyst (Ni(CycPy)2+ = Ni(1-(4-(pyren-1-yl)butyl)-1,4,8,11-tetraazacyclotetradecane) has been synthesised and electrochemically characterised under both CO2 and N2. ...The pyrene functional group forms a non-covalent interaction with carbon electrode supports and the immobilised Ni(CycPy)2+ complex remains electroactive. We report a Ni(CycPy)2+ modified gas diffusion electrode (GDE) that is tested in aqueous electrolyte and shown to be active towards CO production. This is the first successful demonstration of a nickel cyclam modified GDE in aqueous solvent and shows the potential of this class of catalysts for use in co-electrolysis devices.
The scaling-up of electrochemical CO2 reduction requires circumventing the CO2 loss as carbonates under alkaline conditions. Zero-gap cell configurations with a reverse-bias bipolar membrane (BPM) ...represent a possible solution, but the catalyst layer in direct contact with the acidic environment of a BPM usually leads to H2 evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO2 reduction can be achieved in a zero-gap BPM device using a pure water and CO2 feed. At a higher current density (100 mA cm–2), CO selectivity decreases, but was still >30%, due to reversible product inhibition. This study demonstrates the importance of developing acid-tolerant catalysts for use in large-scale CO2 reduction devices.
Pulsed electrolysis can significantly improve carbon dioxide reduction on metal electrodes, but the effect of short (millisecond to seconds) voltage steps on molecular electrocatalysts is largely ...unstudied. In this work, we investigate the effect pulse electrolysis has on the selectivity and stability of the homogeneous electrocatalyst Ni(cyclam)2+ at a carbon electrode. By tuning the potential and pulse duration, we achieve a significant improvement in CO Faradaic efficiencies (85%) after 3 h, double that of the system under potentiostatic conditions. The improved activity is due to in situ catalyst regeneration from an intermediate that occurs as part of the catalyst’s degradation pathway. This study demonstrates the wider opportunity to apply pulsed electrolysis to molecular electrocatalysts to control activity and improve selectivity.
Electrochemical CO2 reduction holds the potential to replace depleting petroleum reserves,harnessing renewable energy to produce fuels and chemical feed stocks. In this thesis, new approaches towards ...electrochemical CO2 reduction with molecular complexes have been investigated, addressing limitations through catalyst modification, electrolysis techniques as well as integrating CO2capture and conversion.Through the environmental drive to use aqueous electrolytes while overcoming the low solubility of CO2 in water, gas diffusion electrodes (GDE), often made of porous sp2 carboncloth or paper, have become prevalent in the CO2 reduction community.1,2 With this in mind apyrene modified Ni(cyclam) (cyclam = 1,4,8,11-tetraazacyclotetradecane) catalyst was synthesised and successfully immobilised to a range of carbon substrates, including GDE,where significant CO selectivity in both a flow (CO/H2 >1.5) and zero-gap cell (CO/H2> 1.0),outperforming previously reported N-alky lated Ni(cyclam) catalysts in aqueous electrolytes.While the possibility of this catalyst class in a co-electrolysis device was demonstrated, the stability of these electrodes was limited both by the water solubility and poisoning of the complex.Ni(cyclam), along with other molecular catalysts, suffer greatly from CO poisoning and overreduction, limiting both their long-term stability and performance at higher current densities.3,4 This has been previously mitigated with CO scavengers, 3 increased flow rates and long recovery periods,5,6 though this often only leads to temporary recovery. Recently, pulsedelectrolysis has shown to greatly improve the stability of CO2reduction with metal catalysts, 7 however this is largely unstudied on molecular catalysts. In Chapter 3, ms asymmetric anodicpulses were shown to significantly improve the selectivity and stability of Ni(cyclam) by in situ catalyst regeneration from an intermediate formed from the catalyst’s degradation pathway.Recently, pairing the capture of CO2 with its reduction has come into focus, with the possibility of streamlining the processes, over coming energetically demanding thermal regeneration of capture solutions. However, this is challenging, as during the capture process, CO2 undergoes conversion to carbamate and bicarbonate species, which conventional metal CO2 reduction catalysts remain largely inert towards.8,9 While molecular complexes have been shown, both spectroscopically and electrochemically to interact with these amine-CO2 species, this is generally done at low amine concentrations in organic solvents which is not representative of conventional capture solutions.10–12 In Chapter 4, a water-soluble Mn complex, is shown spectroscopically to interact with aqueous capture amines at industrially relevant concentrations, with initial electrolysis experiments showing CO/H2values of 2.1. While this preliminary selectivity is impressive, it was convoluted with issues with reproducibility which require resolution.
The continuous and scalable synthesis of a porous organic cage (
CC3
), obtained through a 10-component imine polycondensation between triformylbenzene and a vicinal diamine, was achieved using twin ...screw extrusion (TSE). Compared to both batch and flow syntheses, the use of TSE enabled the large scale synthesis of
CC3
using minimal solvent and in short reaction times, with liquid-assisted grinding (LAG) also promoting window-to-window crystal packing to form a 3-D diamondoid pore network in the solid state. A new kinetically trapped 3+5 product was also observed alongside the formation of the targeted 4+6 cage species. Post-synthetic purification by Soxhlet extraction of the as-extruded 'technical grade' mixture of
CC3
and 3+5 species rendered the material porous.
Twin screw extrusion (TSE) is demonstrated as an alternative synthetic method for preparing a prolific porous organic cage.
Twin screw extrusion (TSE) is demonstrated as an alternative synthetic method for preparing a prolific porous organic cage.
The continuous and scalable synthesis of a porous organic cage (
CC3
), ...obtained through a 10-component imine polycondensation between triformylbenzene and a vicinal diamine, was achieved using twin screw extrusion (TSE). Compared to both batch and flow syntheses, the use of TSE enabled the large scale synthesis of
CC3
using minimal solvent and in short reaction times, with liquid-assisted grinding (LAG) also promoting window-to-window crystal packing to form a 3-D diamondoid pore network in the solid state. A new kinetically trapped 3+5 product was also observed alongside the formation of the targeted 4+6 cage species. Post-synthetic purification by Soxhlet extraction of the as-extruded ‘technical grade’ mixture of
CC3
and 3+5 species rendered the material porous.
We present a catalog of hard X-ray serendipitous sources detected in the first 80 months of observations by the Nuclear Spectroscopic Telescope Array (NuSTAR). The NuSTAR serendipitous survey 80 ...month (NSS80) catalog has an unprecedented ∼62 Ms of effective exposure time over 894 unique fields (a factor of 3 increase over the 40 month catalog, NSS40), with an areal coverage of ∼36 deg2, larger than all NuSTAR extragalactic surveys. NSS80 provides 1274 hard X-ray sources in the 3−24 keV band (822 new detections compared to the previous NSS40). Approximately 76% of the NuSTAR sources have lower-energy (<10 keV) X-ray counterparts from Chandra, XMM-Newton, and Swift-XRT. We have undertaken an extensive campaign of ground-based spectroscopic follow-up to obtain new source redshifts and classifications for 427 sources. Combining these with existing archival spectroscopy provides redshifts for 550 NSS80 sources, of which 547 are classified. The sample is primarily composed of active galactic nuclei (AGNs), detected over a large range in redshift (z = 0.012–3.43), but also includes 58 spectroscopically confirmed Galactic sources. In addition, five AGN–galaxy pairs, one dual AGN system, one BL Lac candidate, and a hotspot of 4C 74.26 (radio quasar) have been identified. The median rest-frame 10−40 keV luminosity and redshift of NSS80 are 〈L10−40 keV〉 = 1.2 × 1044 erg s−1 and 〈z〉 = 0.56. We investigate the optical properties and construct composite optical spectra to search for subtle signatures not present in the individual spectra, finding an excess of redder BL AGNs compared to optical quasar surveys, predominantly due to the presence of the host galaxy and, at least in part, due to dust obscuration.
We present a catalog of hard X-ray serendipitous sources detected in the first 80 months of observations by the Nuclear Spectroscopic Telescope Array (NuSTAR). The NuSTAR serendipitous survey ...80-month (NSS80) catalog has an unprecedented \(\sim\) 62 Ms of effective exposure time over 894 unique fields (a factor of three increase over the 40-month catalog), with an areal coverage of \(\sim \)36 deg\(^2\), larger than all NuSTAR extragalactic surveys. NSS80 provides 1274 hard X-ray sources in the \(3-24\) keV band (822 new detections compared to the previous 40-month catalog). Approximately 76% of the NuSTAR sources have lower-energy (\(<10\) keV) X-ray counterparts from Chandra, XMM-Newton, and Swift-XRT. We have undertaken an extensive campaign of ground-based spectroscopic follow-up to obtain new source redshifts and classifications for 427 sources. Combining these with existing archival spectroscopy provides redshifts for 550 NSS80 sources, of which 547 are classified. The sample is primarily composed of active galactic nuclei (AGN), detected over a large range in redshift (\(z\) = 0.012-3.43), but also includes 58 spectroscopically confirmed Galactic sources. In addition, five AGN/galaxy pairs, one dual AGN system, one BL Lac candidate, and a hotspot of 4C 74.26 (radio quasar) have been identified. The median rest-frame \(10-40\) keV luminosity and redshift of the NSS80 are \(\langle{L_\mathrm{10-40 keV}}\rangle\) = 1.2 \(\times\) 10\(^{44}\) erg s\(^{-1}\) and \(\langle z \rangle = 0.56\). We investigate the optical properties and construct composite optical spectra to search for subtle signatures not present in the individual spectra, finding an excess of redder BL AGN compared to optical quasar surveys predominantly due to the presence of the host-galaxy and, at least in part, due to dust obscuration.
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