Gregory P. Knowles

CO2 capture by adsorption: Materials and process development

Abstract Vacuum swing adsorptive (VSA) capture of CO 2 from flue gas and related process streams is a promising technology for greenhouse gas mitigation. Although early reports suggested that VSA was problematic and expensive, through the application of more logical process configurations that are appropriately coupled to the composition of the feed and product gas streams, we can now refute this early assertion. Improved cycle designs coupled with tighter temperature control are also helping to optimise performance for CO 2 separation. Simultaneously, new adsorbent materials are being developed. These separate CO 2 by selective (acid-base) reaction with surface bound amine groups (chemisorption), rather than on the basis of non-bonding interactions (physisorption). This report describes some of these recent developments from our own laboratories and points to synergies that are anticipated as a result of combining these improvements in adsorbent properties and VSA process cycles.

Hierarchical Mesoporous SnO2 Nanosheets on Carbon Cloth: A Robust and Flexible Electrocatalyst for CO2 Reduction with High Efficiency and Selectivity

Electrochemical reduction of CO2 into liquid fuels is a promising approach to achieve a carbon-neutral energy cycle. However, conventional electrocatalysts usually suffer from low energy efficiency and poor selectivity and stability. A 3D hierarchical structure composed of mesoporous SnO2 nanosheets on carbon cloth is proposed to efficiently and selectively electroreduce CO2 to formate in aqueous media. The electrode is fabricated by a facile combination of hydrothermal reaction and calcination. It exhibits an unprecedented partial current density of about 45 mA cm-2 at a moderate overpotential (0.88 V) with high faradaic efficiency (87±2 %), which is even larger than most gas diffusion electrodes. Additionally, the electrode also demonstrates flexibility and long-term stability. The superior performance is attributed to the robust and highly porous hierarchical structure, which provides a large surface area and facilitates charge and mass transfer.

Mesoporous Silica SBA-15 Supported Co3O4 Nanorods as Efficient Liquid Phase Oxidative Catalysts

Mesoporous silica SBA-15 supported cobalt oxide composites prepared via the “double-solvent”, impregnation and adsorption techniques were characterized by diffuse reflectance UV–Vis (DR UV–Vis), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The potential of these cobalt modified composites to oxidize norbornene, benzyl alcohol and 1-phenylethanol was determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). SEM images indicated that, irrespective of cobalt content (Co-content), the integrity of the mesoporous channels remained intact for all methods. DR UV–Vis exhibited a red-shift which increased with Co-content for catalysts prepared by the double-solvent technique and is attributed to more extensive crystal growth. The morphology of the cobalt oxide species had a direct impact on the pore volume (Pv) and surface area (SA) of the composites and this, in turn affected the catalytic activity. For catalysts prepared by the double-solvent technique, the activity was in reverse proportion to the Co-content. This is attributed to reduced Pv which limits the active SA available.

Amine-functionalised mesoporous silicas as CO2 adsorbents

Abstract Mesoporous silica substrates were functionalized with 3-aminopropyltrimethoxysilane (apts) and aminoethylaminopropyltrimethoxysilane (aeapts) to form hybrid products suitable for CO2 adsorption. Combined thermogravimetric/differential thermal analysis (TGA/DTA) was used to determine CO2 adsorption capacities and heats of adsorption. The extent of surface functionalisation varied with substrate morphology. At 20°C reasonable CO2 adsorption capacities were observed under both dry and wet conditions. However, the heats of adsorption differed significantly; it is thought, due to simultaneous formation of bicarbonate species under wet conditions.

In-Situ-Activated N-Doped Mesoporous Carbon from a Protic Salt and Its Performance in Supercapacitors

Protic salts have been recently recognized to be an excellent carbon source to obtain highly ordered N-doped carbon without the need of tedious and time-consuming preparation steps that are usually involved in traditional polymer-based precursors. Herein, we report a direct co-pyrolysis of an easily synthesized protic salt (benzimidazolium triflate) with calcium and sodium citrate at 850 °C to obtain N-doped mesoporous carbons from a single calcination procedure. It was found that sodium citrate plays a role in the final carbon porosity and acts as an in situ activator. This results in a large surface area as high as 1738 m2/g with a homogeneous pore size distribution and a moderate nitrogen doping level of 3.1%. X-ray photoelectron spectroscopy (XPS) measurements revealed that graphitic and pyridinic groups are the main nitrogen species present in the material, and their content depends on the amount of sodium citrate used during pyrolysis. Transmission electron microscopy (TEM) investigation showed that sodium citrate assists the formation of graphitic domains and many carbon nanosheets were observed. When applied as supercapacitor electrodes, a specific capacitance of 111 F/g in organic electrolyte was obtained and an excellent capacitance retention of 85.9% was observed at a current density of 10 A/g. At an operating voltage of 3.0 V, the device provided a maximum energy density of 35 W h/kg and a maximum power density of 12 kW/kg.

Cadmium oxide/alkali metal halide mixtures – a potential high capacity sorbent for pre-combustion CO2 capture

A series of cadmium oxide based materials were prepared by mixing cadmium carbonate with alkali metal halides. Subsequent heat treatment then transformed the cadmium carbonate into oxide to yield the active carbon dioxide sorbent. It was observed from thermogravimetric analysis that neat cadmium oxide does not sorb significant amounts of carbon dioxide, whereas doping the material with alkali halides facilitates conversion to cadmium carbonate. The cadmium oxide/sodium iodide mixture, in particular, was found to reversibly bind up to 24 wt% carbon dioxide in the temperature range of 250 to 300 °C, which is consistent with an almost stoichiometric conversion of the cadmium oxide to cadmium carbonate. The carbon dioxide could subsequently be released, in the same temperature range, when the gas supply was switched from carbon dioxide to an inert gas flow. The formation of the carbonate was separately verified by both infrared spectrometry and powder X-ray diffraction (XRD). In addition, XRD provided simultaneous detection of both the oxide and carbonate phases thus demonstrating their inter-dependency and is consistent with the absence of other cadmium phases. Le Bail refinement of the unit cell parameters did not reveal a significant change in the unit cell size of the cadmium oxide or carbonate due to mixing with alkali metal halides. Transmission electron microscopy on a 17.5% NaI sample indicated that the material consists of spherical particles of ∼250 nm diameter. Nitrogen physisorption experiments showed that the sodium iodide-enhanced material is non-porous and of a low surface area.

Investigation of the capacity decay of a CdO–NaI mixed sorbent for pre-combustion CO2 capture

The mechanisms for the loss of both CO2 working capacity and mass from a CdO–NaI composite were investigated to better assess the potential use of the material to facilitate the pre-combustion capture of CO2 from syngas. Fresh activated material was used to analyse sorption and desorption using a CO2–N2 mixture. Mass spectrometric analysis of the exit gas revealed the loss of elemental iodine from the system over the period, attributed to the oxidation of iodide. Thermogravimetric analysis using iodine vapour suggested the iodide loss reaction to be a partially reversible equilibrium. X-ray photoelectron spectroscopy revealed the formation of a highly oxidised iodine species on the surface of the sorbent during initial calcination in both air and N2, but this compound vanished after the use of the sorbent in 25 CO2 sorption cycles. Elemental mapping showed that iodine was dislocated from the sodium, which it was considered to be originally associated to, supporting the theory of oxidation and evaporation (and possible re-deposition). Transmission electron microscopy revealed that the sorbent consisted of regular, spherical nanoparticles of approx. 250 nm diameter, which became more irregularly-shaped after CO2 sorption cycles, considered to be due to void/crack formation caused by density changes upon calcination and carbonation. In situ powder X-ray diffraction revealed an increase in crystallinity of both CdO and NaI upon heating to CO2 sorption temperature of 325 °C in N2 atmosphere, compared to room temperature. If the oxidation of the iodide promoter can be inhibited, this is likely to improve the multicyclic CO2 sorption stability of this material for future use.

Structural chemistry and selective CO2 uptake of a piperazine-derived porous coordination polymer

A new piperazine-derived ligand has been prepared and used to synthesise a porous coordination polymer which displays selective carbon dioxide uptake after solvent exchange and thermal activation. The ligand N,N′-bis(4-carboxyphenylmethylene)piperazine H2L1 was prepared from piperazine in three steps and good yield. A structure containing the deprotonated form K2L1·2H2O was determined and consists of a three-dimensional coordination polymer containing inorganic K2(COO)2(OH2) layers separated by the long organic bridging linker. The free compound H2L1 displays a one-dimensional hydrogen-bonded polymeric structure in the solid state with hydrogen bonding interactions between carboxylic acids and piperazine groups tightly linking molecules together. The two-dimensional polymeric complex [Zn3(L1)2(OH)2]·2DMF·0.5H2O 1 was prepared and analysed in the solid state to reveal tubular one-dimensional channels which, when activated by solvent exchange and evacuation, displayed selective affinity for CO2 over N2 and H2.

Multiple sorption cycles evaluation of cadmium oxide-alkali metal halide mixtures for pre-combustion CO2 capture

Cadmium oxide–alkali metal halide mixtures for pre-combustion CO2 capture were made using a wet mixing approach. Some of the samples were pelletised in as-synthesised state as well as with SBA-15 silica addition. In a multiple CO2 sorption cycle test via thermogravimetric analysis, the best performing powder material by capacity and kinetics (a cadmium oxide made from carbonate doped with 17.5 wt% sodium iodide) exhibited a sorption capacity loss from 17 to 2 wt% after 25 cycles of partial pressure swing sorption at atmospheric pressure and temperatures of 285 and 305 °C. When the initial decomposition of the carbonate took place in inert gas (Ar or N2 instead of air), the cyclic stability was improved. Water addition (1 vol%) to the sorption gas further improved the cyclic CO2 sorption stability and capacity. Elemental analysis of the samples after cyclic exposure to CO2 revealed that the capacity loss is associated with loss of iodine, whereas the sodium remains. Water addition, however, had no significant effect on this iodine loss. Pellets made from carbonate performed with a working capacity of 10 wt%, but lost their mechanical integrity during multicyclic sorption. If made in the oxide state, pellets remained sturdy, but showed almost no working capacity. The addition of 13.7 wt% SBA-15 improved the working capacity of the oxide pellet to a stable value of 5.2 wt% over 25 cycles. In situ powder X-ray diffraction showed the reversible isothermal phase transformation of CdO to CdCO3 during three cycles of sorption and also revealed the presence of a crystalline sodium iodide phase, which appeared to be lost with increasing number of sorption cycles.

Aminopropyl-functionalized silica CO2 adsorbents via sonochemical methods

Aminopropyl-functionalized hexagonal mesoporous silica (HMS) products, as are of interest for CO2 capture applications, were separately prepared by mixing aminopropyltrimethoxysilane (APTS) and HMS in toluene via a conventional stirred reactor and via sonication assisted methods, to investigate the potential of sonication to facilitate the preparation of products with higher tether loadings and correspondingly higher CO2 sorption capacities. Sonication was expected to improve both the dispersion of the substrate in the solvent and the diffusion of the silane throughout the mesoporous substrate. Structural properties of the products were determined by X-ray diffraction, N2 adsorption/desorption (77 K), helium pycnometry, and elemental analysis, and CO2 adsorption/desorption properties were determined via thermogravimetric and differential thermal analysis. The tether loadings of the sonication products (up to 1.8 tethersnm−2) were found to increase with sonication time and in each case were greater than the corresponding product prepared by the conventional approach. It was also found that the concentration of the reagent mixture influenced the extent of functionalization, that the crude products cured effectively under N2 flow as under vacuum, and that rinsing the crude products prior to curing was not essential. Sonication products with higher tether loadings were found to exhibit higher CO2 sorption capacities as expected.