Calcium looping is a high-temperature CO(2) capture technology applicable to the postcombustion capture of CO(2) from power station flue gas, or integrated with fuel conversion in precombustion CO(2) capture schemes. The capture technology uses solid CaO sorbent derived from natural limestone and takes advantage of the reversible reaction between CaO and CO(2) to form CaCO(3); that is, to achieve the separation of CO(2) from flue or fuel gas, and produce a pure stream of CO(2) suitable for geological storage. An important characteristic of the sorbent, affecting the cost-efficiency of this technology, is the decay in reactivity of the sorbent over multiple CO(2) capture-and-release cycles. This work reports on the influence of high-temperature steam, which will be present in flue (about 5-10%) and fuel (∼20%) gases, on the reactivity of CaO sorbent derived from four natural limestones. A significant increase in the reactivity of these sorbents was found for 30 cycles in the presence of steam (from 1-20%). Steam influences the sorbent reactivity in two ways. Steam present during calcination promotes sintering that produces a sorbent morphology with most of the pore volume associated with larger pores of ∼50 nm in diameter, and which appears to be relatively more stable than the pore structure that evolves when no steam is present. The presence of steam during carbonation reduces the diffusion resistance during carbonation. We observed a synergistic effect, i.e., the highest reactivity was observed when steam was present for both calcination and carbonation.

Facilitated by redox catalysts capable of catalytic reactions and reactive separation, chemical looping offers exciting opportunities for intensified chemical production.

Development of efficient catalysts for the direct hydrogenation of CO2 to methanol is essential for the valorization of this abundant feedstock. Here we show that a silica-supported Cu/Mo2CTx (MXene) catalyst achieves a higher intrinsic methanol formation rate per mass Cu than the reference Cu/SiO2 catalyst with a similar Cu loading. The Cu/Mo2CTx interface can be engineered owing to the higher affinity of metallic Cu for the partially reduced MXene surface (in preference to the SiO2 surface) and the mobility of Cu under H2 at 500 C.Increasing the reduction time, the Cu/Mo2CTx interface becomes more Lewis acidic due to the higher amount of Cu + sites dispersed onto the reduced Mo2CTx and this correlates with an 2 increased rate of CO2 hydrogenation to methanol. The critical role of the interface between Cu and Mo2CTx is further highlighted by DFT calculations that identify formate and methoxy species as stable reaction intermediates.

Calcium looping, a CO2 capture technique, may offer a mid-term if not near-term solution to mitigate climate change, triggered by the yet increasing anthropogenic CO2 emissions. A key requirement for the economic operation of calcium looping is the availability of highly effective CaO-based CO2 sorbents. Here we report a facile synthesis route that yields hollow, MgO-stabilized, CaO microspheres featuring highly porous multishelled morphologies. As a thermal stabilizer, MgO minimized the sintering-induced decay of the sorbents’ CO2 capacity and ensured a stable CO2 uptake over multiple operation cycles. Detailed electron microscopy-based analyses confirm a compositional homogeneity which is identified, together with the characteristics of its porous structure, as an essential feature to yield a high-performance sorbent. After 30 cycles of repeated CO2 capture and sorbent regeneration, the best performing material requires as little as 11 wt.% MgO for structural stabilization and exceeds the CO2 uptake of the limestone-derived reference material by ~500%.

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO2 emissions. Solid materials capable of reversibly absorbing CO2 have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO2 absorption both at surfaces and within solid materials.

Perovskite-structured materials, owing to their chemical-physical properties and tuneable composition, have extended their range of applications to chemical looping processes, in which lattice oxygen provides the oxygen needed for chemical reactions omitting the use of co-fed gaseous oxidants.To optimise their oxygen donating behaviour to the specific application a fundamental understanding of the reduction/oxidation characteristics of perovskite structured oxides and their manipulation through the introduction of dopants is key. In this study, we investigate the structural and oxygen desorption/ sorption properties of Sr 1Àx Ca x FeO 3Àd and SrFe 1Àx Co x O 3Àd (0 r x r 1) to guide the design of more effective oxygen carriers for chemical looping applications at low temperatures (i.e. 400-600 1C).Ca A-or Co B-site substituted SrFeO 3Àd show an increased reducibility, resulting in a higher oxygen capacity at T r 600 1C when compared to the unsubstituted sample. The quantitative assessment of the thermodynamic properties (partial molar enthalpy and entropy of vacancy formation) confirms a reduced enthalpy of vacancy formation upon substitution in this temperature range (i.e. 400-600 1C). Among the examined samples, Sr 0.8 Ca 0.2 FeO 3Àd exhibited the highest oxygen storage capacity (2.15 wt%) at 500 1C, complemented by excellent redox and structural stability over 100 cycles. The thermodynamic assessment, supported by in situ XRD measurements, revealed that the oxygen release occurs with a phase transition perovskite-brownmillerite below 770 1C, while the perovskite structure remains stable above 770 1C.

No abstract

A novel calcium looping (CaL) process integrated with a spent bleaching clay (SBC) treatment is proposed whereby fuels and/or heat from regeneration of SBC provide supplemental energy for the calcination process; in addition, the regenerated SBC could be used to synthesize enhanced CaO-based sorbents. Composite samples were prepared with various doping ratios together with the regenerated SBC via a pelletization process. All pellets were subjected to thermogravimetic analysis (TGA) tests employing severe reaction conditions to determine the optimal doping ratios and regeneration method for the SBC-based sorbents. These results demonstrate that pellets containing combustible components showed higher CO 2 uptake, due to the improved pore structure, which was verified by N 2 adsorption measurements. The as-prepared sorbent "L-10PC" (90% CaO/10% pyrolytic SBC) achieved a final CO 2 uptake of 0.164 g(CO 2) g(calcined sorbent)-1 after 20 cycles, which was 67.3% higher than that of natural limestone particles. A new larnite (Ca 2 SiO 4) phase was detected by XRD analysis; however, the weak XRD peak associated with it indicated a low content of larnite in the pellets, which produced a smaller effect on performance compared to cement. A synergistic effect was achieved for a sample designated as "L-5PC-10CA" (85% CaO/5% pyrolytic SBC/10% cement), which resulted in the highest final uptake of 0.208 g(CO 2) g(calcined sorbent)-1. Considering the simplicity of the pyrolysis regeneration process and the excellent capture capability of pellets doped with pyrolytic SBC, the proposed system integrating CaL with SBC pyrolysis treatment appears to be promising for further development.