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We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite Sr0.8Ca0.2FeO3−δ) in chemical looping applications.

The oxidative dehydrogenation of propane (ODP) proceeds catalytically on a gas-solid interface (heterogeneous reaction) and/or in the gas phase (homogeneous reaction) via a radical chain process. ODP may therefore combine interrelated contributions from the heterogeneous dehydrogenation and gas-phase reactions, which can be initiated by a catalyst. This study demonstrates that relatively high propene and ethene selectivities (ca. 80 % and 10 %) and propane conversions (viz., 10 % at 500 degrees C) can be achieved with an empty quartz reactor, which is comparable to the performances of state-of-the-art ODP catalysts (boron-based or supported VOx). Optimization of the post-catalytic volume of a h-BN catalyst bed tested at 490 degrees C allows to increase the conversion of propane from 9 % to 15 % at a propene selectivity of 77 %, highlighting this parameter as an important variable for improving catalytic ODP performances., ChemCatChem, 15 (9), ISSN:1867-3880, ISSN:1867-3899

A sustainable hybrid aerogel based on β-lactoglobulin amyloid fibril/UiO-66-NH2 is developed for environmental remediation.

Alkali metal salt promoters, such as K2CO3 investigated in this work, are known to affect the cyclic performance of CaO-based sorbents – sometimes positively, sometimes negatively. Thus far, the influence of K2CO3 (and other salt-based promoters) is poorly understood, and detailed investigations on the interaction of K2CO3 with CaO are missing. We find that low amounts of K2CO3 in the sorbents improve the cyclic performance of CaO significantly (up to a factor of three), both under mild and harsh CO2 release conditions, while large amounts of K2CO3 (>1 mol.%) reduce the positive effect on the CO2 uptake performance. Aided by in-situ X-ray diffraction measurements, we observe that the gas environment has an important influence on the stability of K-containing phases in the sorbent. At typical carbonation conditions (650 °C, 15 vol% CO2/N2), CaO and K2CO3 readily form K-Ca double carbonates (K2Ca(CO3)2, which reacts further to form K2Ca2(CO3)3). When the calcination reaction is carried out in pure N2, the K-Ca double carbonates decompose above ∼730 °C, forming K2CO3 that becomes partially volatile near its melting point of ∼900 °C, and leads to a reduction of the K content in the samples with cycling. When the calcination reaction is carried out in a CO2-rich environment, the cyclic loss of K2CO3 is reduced substantially, but the sintering of the sorbent is increased. There are indications that very low quantities of K are incorporated into the CaO structure, and the formation of K-Ca double carbonates appears to influence the structural evolution of the CaCO3 phase formed during carbonation. With very low amounts of K2CO3 present in the sorbent, we observe an increase in pore volume for pore diameters between 20 and 80 nm, which ultimately may explain the improved CO2 uptake performance compared to the CaO sorbent without K2CO3. ISSN:0300-9467 ISSN:1385-8947 ISSN:1873-3212 ISSN:0923-0467

Electrocatalytic transformations of oxygen, i.e., the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), are key processes in renewable energy conversion, defining to a large extent, the efficiency of numerous energy conversion technologies, such as fuel cells, metal-air batteries or water electrolyzers. However, the development of highly effective, stable and inexpensive materials for such conversion processes is a bottleneck. Hence, establishing generic catalyst design principles by identifying structural features of catalysts that influence their performance would constitute a major step towards the rational engineering of advanced electrocatalysts. In this study, by investigating a series of metal-substituted manganese oxide (spinel), Mn3O4:M (M = Sr, Ca, Mg, Zn, Cu), nanoparticles as a model system, we demonstrate experimentally and rationalized the dependence of the activity of Mn3O4:M for ORR and the oxygen binding strength in Mn3O4:M oxides on the properties of the M substituent, viz. the enthalpy of formation of the binary MO oxide and the Lewis acidity of the M2+ substituent. Incorporation of elements M that have a low enthalpy of formation of MO (i.e., highly exothermic oxides featuring relatively strong M‒O bonding) enhances the oxygen binding strength in Mn3O4:M, which increases its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species on the catalyst surface. Our work provides a new perspective on the design of new compositions for oxygen electrocatalysis relying on the substitution by redox-inactive elements affecting the binding energy of oxygen to the surface of the complex metal oxide catalysts (ORR/OER activity descriptor). We speculate that this concept is general and transferrable to a broad selection of materials and processes involving oxygen adsorption and redox, beyond electrocatalysis.

Nanoscale, 14 (45), ISSN:2040-3364, ISSN:2040-3372

Water scarcity is a worldwide concern with various implications, such as widespread pollution with heavy metals and organic pollutants, resulting in global threats to the environment and human health, and calling for innovative remediation strategies. We show here that the capability of protein-based carbon aerogels for removing contaminants from water can be greatly improved by transforming proteins into amyloids before their hydrothermal carbonization. Amyloid fibrils were prepared from whey protein isolate, a by-product of the dairy industry, and upon addition to different carbohydrates, the ensued mixtures were used for carbon aerogels production. Furthermore, the generality of the approach for producing carbon aerogels was demonstrated using vegetable amyloids from soy protein. The carbon aerogels are able to adsorb Au(III), Pt(II), Fe(III), and Ag(I) with excellent removal efficiencies. Moreover, the amyloid whey lactose carbon aerogel has an adsorption capacity of 650.08 mg/g for Au(III), significantly higher than that of a carbon aerogel derived from monomer whey lactose. The reusability of the carbon aerogel is also proven over three continuous adsorption/regeneration cycles, with a stable performance in terms of removal efficiency indicating good long-term use. Taken together, the results demonstrate amyloid carbon aerogels’ capability as an efficient, sustainable, and inexpensive solution for water purification., Chemical Engineering Journal, 449, ISSN:0300-9467, ISSN:1385-8947, ISSN:1873-3212, ISSN:0923-0467

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.

MgO-based CO

MgO-based materials possess favorable thermodynamic properties that enable them to be used as CO2 sorbents in the temperature range of 300–550 °C — at least theoretically. MgO-based sorbents have seen a boost in activity since the discovery of molten alkali metal salt promoters, and yet, they practically remove CO2 from a gas stream efficiently only at very high pressures (p > 10 bar), rendering them suitable for precombustion, but not for postcombustion CO2 capture schemes. Most of the current performance assessments rely on measurements of the transition between MgO and MgCO3 (or Mg-double carbonates) rather than the ability of the sorbents to actually remove CO2 from a gas stream, which may be misleading regarding their practical use and application. Further, reliable thermodynamic data for sorbents involving Mg-double carbonates is missing, although these materials have large potential for industrial CO2 capture applications.

This paper focuses on the experimental demonstration of a three-stage GST (gas switching technology) process (fuel, steam/CO2, and air stages) for syngas production from methane in the fuel stage and H2/CO production in the steam/CO2 stage using a lanthanum-based oxygen carrier (La0.85Sr0.15Fe0.95Al0.05O3). Experiments were performed at temperatures between 750–950 °C and pressures up to 5 bar. The results show that the oxygen carrier exhibits high selectivity to oxidizing methane to syngas at the fuel stage with improved process performance with increasing temperature although carbon deposition could not be avoided. Co-feeding CO2 with CH4 at the fuel stage reduced carbon deposition significantly, thus reducing the syngas H2/CO molar ratio from 3.75 to 1 (at CO2/CH4 ratio of 1 at 950 °C and 1 bar). The reduced carbon deposition has maximized the purity of the H2 produced in the consecutive steam stage thus increasing the process attractiveness for the combined production of syngas and pure hydrogen. Interestingly, the cofeeding of CO2 with CH4 at the fuel stage showed a stable syngas production over 12 hours continuously and maintained the H2/CO ratio at almost unity, suggesting that the oxygen carrier was exposed to simultaneous partial oxidation of CH4 with the lattice oxygen which was restored instantly by the incoming CO2. Furthermore, the addition of steam to the fuel stage could tune up the H2/CO ratio beyond 3 without carbon deposition at H2O/CH4 ratio of 1 at 950 °C and 1 bar; making the syngas from gas switching partial oxidation suitable for different downstream processes, for example, gas-to-liquid processes. The process was also demonstrated at higher pressures with over 70% fuel conversion achieved at 5 bar and 950 °C., Industrial & Engineering Chemistry Research, 60 (9), ISSN:1520-5045, ISSN:0888-5885

Thermochemical CO2 capture cycles such as chemical looping combustion (CLC) are an economically promising CO2 capture technology that rely on the combustion of a hydrocarbon fuel with lattice oxygen that is derived from a solid oxygen carrier. The oxygen carrier is typically regenerated with air. To increase the agglomeration resistance and redox stability of the oxygen carriers, the active phase is often stabilized with high Tammann temperature ceramics, resulting in the formation of so-called cermet structures. It has been hypothesized that the redox performance of the cermets depends critically on the conduction pathways for solid-state ionic diffusion and the activation energy for charge transport. Here, we investigate the influence of the formation of a percolation network on the electrical conductivity and the rate of oxidation for CeO2-stabilized Cu. We found that for oxygen carriers that contained 60 wt. % CuO, the charge transport occurred predominately via Cu/CuO conduction pathways. Below the percolation threshold of CuO, the conduction of charge carriers took place via CeO2 grains, which formed a continuous network. The measurements of charge transport and redox characteristics confirmed that the activation energy for charge transport through the cermet increased with decreasing Cu content. This indicates that the solid-state diffusion of charge carriers plays an important role during re-oxidation.

Thermochemical redox cycles such as chemical looping combustion (CLC) are an economically promising COThe online version contains supplementary material available at 10.1007/s43938-022-00013-2.

Chemical looping is a promising combustion technology utilizing the cyclic oxidation and reduction of a metal oxide to transport oxygen for a variety of applications related to carbon capture and s...

The integration of a CaO-based CO2 sorbent into catalytic schemes to remove CO2 from the product stream provides an effective means to reduce greenhouse gas emissions of chemical processes and to improve the yield and purity of the desired product. A key requirement for such so-called sorbent-enhanced processes is the availability of cyclically stable CO2 sorbent. To this end, we have developed CaO-based CO2 sorbents that combine favourable structural features and a high thermal stability by introducing a thin, conformal layer of Al2O3 (forming Ca3Al2O6 with CaO upon calcination) by atomic layer deposition. The structure and pore volume of the sorbent were found to play a key role in its CO2 capture. Functionalizing such CO2 sorbents with Ni nanoparticles yielded a highly effective bi-functional material for the sorption-enhanced water-gas shift (SE-WGS) reaction. The material showed a high yield of hydrogen of high purity and minimal CO slip over several cycles of repeated SE-WGS/regeneration operation.

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 Sr1−xCaxFeO3−δ and SrFe1−xCoxO3−δ (0 ≤ x ≤ 1) to guide the design of more effective oxygen carriers for chemical looping applications at low temperatures (i.e. 400–600 °C). Ca A- or Co B-site substituted SrFeO3−δ show an increased reducibility, resulting in a higher oxygen capacity at T ≤ 600 °C 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 °C). Among the examined samples, Sr0.8Ca0.2FeO3−δ exhibited the highest oxygen storage capacity (2.15 wt%) at 500 °C, 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 °C, while the perovskite structure remains stable above 770 °C. ISSN:1463-9084 ISSN:1463-9076

As a promising approach for carbon dioxide capture, chemical looping combustion has been extensively investigated for more than two decades. However, the chemical looping strategy can be and has been extended well beyond carbon capture. In fact, significant impacts on emission reduction, energy conservation, and value-creation can be anticipated from chemical looping beyond combustion (CLBC). This article aims to demonstrate the versatility and transformational benefits of CLBC. Specifically, we focus on the use of oxygen carriers or redox catalysts for chemical production – a $4 trillion industry that consumes 40.9 quadrillion BTU of energy. Compared to state-of-the-art chemical production technologies, we illustrate that chemical looping offers significant opportunities for process intensification and exergy loss minimization. In many cases, an order of magnitude reduction in energy consumption and CO2 emission can be realized without the needs for carbon dioxide capture. In addition to providing various CLBC examples, this article elaborates on generalized design principles for CLBC, potential benefits and pitfalls, as well as redox catalyst selection, design, optimization, and redox reaction mechanism. ISSN:1754-5692 ISSN:1754-5706

The oxidative dehydrogenation of ethane (ODH) to produce ethylene offers advantages compared to the industry standard steam cracking, but its industrial application is hindered by costly air separation units needed to supply oxygen. A chemical-looping-based oxidative dehydrogenation (CL-ODH) scheme is presented, in which oxygen carriers supply gaseous oxygen in situ, which then reacts with ethane in the presence of a catalyst at a comparatively low temperature (500 °C). A common challenge of chemical looping processes beyond combustion is to suppress the overoxidation of hydrocarbons to COx to enable high product yields. It is demonstrated that the overoxidation of ethane can be eliminated completely through structural engineering of the perovskite oxygen carrier involving alkali-metal-based carbonate coatings, while maintaining the materials’ ability to generate oxygen. Through CL-ODH, higher ethylene selectivity (≈91%) and yields (≈39%) are achieved compared to the conventional ODH scheme without oxygen carrier and cofeeding air/ethane. 18O-labeling experiments demonstrate that the carbonate layer functions like a diffusion barrier for ethane while being permeable for oxygen. Both the CL-ODH scheme and the material design strategy can be extended to other catalytic oxidation or dehydrogenation reactions requiring oxygen at different temperatures, offering enormous potential to intensify such processes., Advanced Energy Materials, 12 (13)

Gas switching technology (GST) was introduced to facilitate operation and scale-up of pressurized chemical looping-based technologies thus bringing the expected benefits of reducing costs and energy penalty of CO2 capture. GST has so far been applied to generate heat/power, hydrogen, syngas, and oxygen using fossil fuel gas (but also from biomass for negative CO2 emissions) with integrated CO2 capture at minimal energy penalty generating over 50 publication studies demonstrating the technical feasibility of the technology and quantifying the potential energy and cost savings. In contrast to conventional chemical looping, GST inherently avoids solids circulation by alternating oxidizing and reducing conditions into a single fluidized bed reactor with an oxygen carrier, thus removing many of the technical challenges that hinder the scale-up of the technology. GST has successfully been applied and demonstrated for combustion, steam/dry methane reforming, and water splitting, using different oxygen carriers, showing the ease of operation under both atmospheric and pressurized conditions and achieving high products separation efficiency. This paper summarises the different studies completed on the Gas Switching Technology covering experimental demonstration (including the experience from a 50 kWth cluster), process modelling and techno-economics, highlighting the advantages and disadvantages of the technology and discussing the way forward., International Journal of Greenhouse Gas Control, 114, ISSN:1750-5836, ISSN:1878-0148

Industrial & Engineering Chemistry Research, 59 (15), ISSN:1520-5045, ISSN:0888-5885

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 due to the higher affinity of Cu for the partially reduced MXene surface (in preference to the SiO2 surface) and the mobility of Cu under H2 at 500 °C. With increasing 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 increased rate of CO2 hydrogenation to methanol. The critical role of the interface between Cu and Mo2CTx is further highlighted by density functional theory calculations that identify formate and methoxy species as stable reaction intermediates. [Figure not available: see fulltext.], Nature Catalysis, 4 (10), ISSN:2520-1158

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO

Hydrogen (H2) is a clean energy carrier and a major industrial feedstock, e.g., to produce ammonia and methanol. High-purity H2 can be produced efficiently from methane (CH4) using chemical looping-based approaches. In this work, we report on the development of a calcium-iron-based oxygen carrier (Ca2Fe2O5) doped with Ni or Cu and investigate its redox performance for H2 production when CH4 is used as the fuel. The experimental results suggest that the rapid formation of metallic Ni or Cu through exsolution promotes the reducibility of Ca2Fe2O5 with CH4. It was found that the reversible exsolution of Ni or Cu nanoparticles and their reincorporation in the Ca2Fe2O5 structure is key to avoid particle sintering and deactivation. Having the potential of converting a larger fraction of steam to H2 than pure iron oxide in addition to its higher reactivity with CH4, the doped calcium-iron-based oxygen carrier is a promising material for chemical looping H2 production.

The preparation of bifunctional CaO/CuO composites with high performance is essential for the development of the combined Ca–Cu looping process, in which the exothermic reduction of CuO with methane is used in situ to provide the heat required to calcine CaCO3. However, the rapid decline in CO2 uptake of CaO/CuO composites remains an important problem to be solved, despite their excellent redox characteristic. Herein we report a facile one-pot template-free synthesis approach to yield CaO/CuO hollow microspheres, aimed at enhancing the CO2 capture performance of CaO/CuO composites. CaO/CuO hollow microspheres feature highly porous shells and a homogeneous elemental distribution, and demonstrate significantly enhanced CO2 capture performance. After ten repeated cycles in a fixed-bed reactor, the CO2 uptake capacity of the best-performing CaO/CuO hollow microspheres exceeded that of the reference materials, i.e., CaO/CuO composites synthesized via wet mixing or a co-precipitation method, by 222% and 114%, respectively. Moreover, from cycle number eight onwards, the CO2 uptake was very stable over the tested 20 cycles, suggesting good cyclic stability of CaO/CuO hollow microspheres. Oxidation was always fast with O2 uptake capacities greater than 0.13 gO2 gmaterial−1. On the basis of N2 adsorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations, the significantly enhanced CO2 capture performance of the CaO/CuO hollow microspheres resulted from the unique hollow microsphere structure with highly porous shells, which were retained throughout the cyclic operations.

The catalytic hydrogenation of CO2 to methanol depends significantly on the structures of metal-oxide interfaces. We show that doping a high-valency metal, viz. tungsten, to CeO2 could render improved catalytic activity for the hydrogenation of CO2 on a Cu/CeW0.25Ox catalyst, whilst making it more selective towards methanol than the undoped Cu/CeO2. We experimentally investigated and elucidated the structural-functional relationship of the Cu/CeO2 interface for CO2 hydrogenation. The promotional effects are attributed to the irreversible reduction of Ce4+ to Ce3+ by W-doping, the suppression of the formation of redox-active oxygen vacancies on CeO2, and the activation of the formate pathway for CO2 hydrogenation. This catalyst design strategy differs fundamentally from those commonly used for CeO2-supported catalysts, in which oxygen vacancies with high redox activity are considered desirable., Applied Catalysis B: Environmental, 306, ISSN:1873-3883, ISSN:0926-3373

CO2 capture using alkali metal salt (AMS)-promoted MgO-based sorbents at intermediate temperatures (300 – 500 °C) has gained increased interest recently. The prospects of such materials for CO2 capture were assessed in this work. We investigated the most reactive MgO-based sorbents that have been reported in the literature, i.e., MgO promoted with a combination of various AMS (incl. NaNO3, LiNO3, K2CO3 and Na2CO3), and examined how particle size (from powder to pelletized 500 μm particles) and reaction conditions (calcination/carbonation temperature, and partial pressure of CO2) affect the cyclic CO2 uptake using a thermogravimetric analyzer (TGA) at ambient pressure. The TGA results showed that the CO2 uptake of the sorbents decreased significantly after pelletization, losing 74 % of its initial capacity. However, the CO2 uptake capacity of the pelletized sorbents continued to increase over 100 cycles and reached a value (~ 0.46 gCO2/gsorbent) close to that of the powdery sample (~ 0.53 gCO2/gsorbent). Analysis via X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscope (SEM) and N2 physisorption suggests that the increase in CO2 uptake was related to a change of the nature of the alkali species within the molten phase that is reflected by their re-crystallization behavior when cooling them down to room temperature, and appeared to be affected by the CO2 partial pressure present during carbonation. Finally, the CO2 capture performance of the best-performing sorbents was evaluated in a packed bed reactor, in order to assess whether the most reactive sorbents are capable of removing a significant amount of CO2 from a gas stream at ambient pressure. The CO2 uptake of the sorbents in the packed bed experiments was very close to that in the TGA experiments; however, the CO2 capture efficiency was less than 10 %, which currently appears too low for an industrial post-combustion CO2 capture process to be viable. New material developments should not only focus on improving the rate of formation of MgCO3 from MgO, but also assess whether CO2 removal with such sorbents is actually feasible.

Carbon dioxide capture and mitigation forms 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, 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 mid- 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.

ACS Sustainable Chemistry & Engineering, 9 (17), ISSN:2168-0485

An integrated Ca/Cu looping process has been proposed recently for CO2 capture. It uses the exothermic in-situ reduction of CuO with methane or natural gas to supply the heat required for the endothermic calcination of CaCO3 to regenerate CaO for the following CO2 sorption cycle via bifunctional CaO/CuO composites. CaO/CuO composites possess excellent redox characteristics, but the rapid decline in CO2 capture performance remains an unresolved problem. Two different types of stabilizers, i.e., Al2O3 that can form a mixed phase with CaO, and MgO that does not form a mixed phase with CaO under reaction conditions, were incorporated into CaO/CuO composites via a Pechini method, and investigated by thermogravimetry. The results showed that the incorporation of Al2O3 or MgO enhanced cyclic stability significantly, and the formation of calcium-aluminum mixed oxides improved the cyclic CO2 capture stability more than did MgO. The best performing Al2O3- and MgO-stabilized CaO/CuO composites exhibited CO2 uptake of 0.14 and 0.09 gCO2/gmaterial after ten repeated cycles, exceeding the unstabilized CaO/CuO composites by 100% and 29%, respectively. We observed that 15 mol. % Al2O3 is the optimal quantity for Al2O3-stabilized CaO/CuO composites, ensuring a high and cyclically stable CO2 uptake capacity (0.11 gCO2/gmaterial after 45 cycles). Moreover, the cyclic stability was enhanced remarkably with increasing carbonation/oxidation temperature (from 550 to 750 °C). In-situ X-ray powder diffraction results suggest that the intermediate phase Cu2O played an important role in the enhanced performance of the composites at high carbonation/oxidation temperatures., Chemical Engineering Journal, 403, ISSN:0300-9467, ISSN:1385-8947, ISSN:1873-3212, ISSN:0923-0467

CO2 capture using alkali metal salt (AMS)-promoted MgO-based sorbents at intermediate temperatures (300–500 °C) has gained increased interest recently. The prospects of such materials for CO2 capture were assessed in this work. We investigated the most reactive MgO-based sorbents that have been reported in the literature (i.e., MgO promoted with a combination of various AMS (including NaNO3, LiNO3, K2CO3, and Na2CO3)), and examined how particle size (from powder to pelletized 500 μm particles) and reaction conditions (calcination/carbonation temperature and partial pressure of CO2) affect the cyclic CO2 uptake using a thermogravimetric analyzer (TGA) at ambient pressure. The TGA results showed that the CO2 uptake performance of the sorbents decreased significantly after pelletization, losing 74% of its initial capacity. However, the CO2 uptake capacity of the pelletized sorbents continued to increase over 100 cycles and reached a value (∼0.46 gCO2/gsorbent) close to that of the powdery sample (∼0.53 gCO2/gsorbent). Analysis via X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscope (SEM), N2 physisorption, and in situ XRD suggests that the increase in CO2 uptake was related to a change of the nature of the alkali species within the molten phase that is reflected by their recrystallization behavior when cooling them down to room temperature, and appeared to be affected by the CO2 partial pressure present during carbonation. Finally, the CO2 capture performance of the best-performing sorbents was evaluated in a packed bed reactor, in order to assess whether the most reactive sorbents are capable of removing a significant amount of CO2 from a gas stream at ambient pressure. The CO2 uptake of the sorbents in the packed bed experiments was very close to that in the TGA experiments; however, the CO2 capture efficiency was less than 10%, which currently appears too low for an industrial postcombustion CO2 capture process to be viable. New material developments should not only focus on improving the rate of formation of MgCO3 from MgO but also assess whether CO2 removal with such sorbents is actually feasible. ISSN:2168-0485

The effect of NaNO3 and its physical state on the thermal decomposition pathways of hydrated magnesium hydroxycarbonate (hydromagnesite, HM) towards MgO was examined by in situ total scattering. Pair distribution function (PDF) analysis of these data allowed us to probe the structural evolution of pristine and NaNO3-promoted HM. A multivariate curve resolution alternating least squares (MCR-ALS) analysis identified the intermediate phases and their evolution upon the decomposition of both precursors to MgO. The total scattering results are discussed in relation with thermogravimetric measurements coupled with off-gas analysis. MgO is obtained from pristine HM (N2, 10 °C min−1) through an amorphous magnesium carbonate intermediate (AMC), formed after the partial removal of water of crystallization from HM. The decomposition continues via a gradual release of water (due to dehydration and dehydroxylation) and, in the last step, via decarbonation, leading to crystalline MgO. The presence of molten NaNO3 alters the decomposition pathways of HM, proceeding now through AMC and crystalline MgCO3. These results demonstrate that molten NaNO3 facilitates the release of water (from both water of crystallization and through dehydroxylation) and decarbonation, and promotes the crystallization of MgCO3 and MgO in comparison to pristine HM. MgO formed from the pristine HM precursor shows a smaller average crystallite size than NaNO3-promoted HM and preserves the initial nano-plate-like morphology of HM. NaNO3-promoted HM was decomposed to MgO that is characterized by a larger average crystallite size and irregular morphology. Additionally, in situ SEM allowed visualization of the morphological evolution of HM promoted with NaNO3 at a micrometre scale., Nanoscale, 12 (31), ISSN:2040-3364, ISSN:2040-3372

Author(s): Bell, Alexis; Deretzis, Ioannis; Donat, Felix; Gallant, Betar; Lee, Jung-Ho; Morante, Joan Ramon; Ogale, Satishchandra; Risch, Marcel; Simonov, Alexandr; Warren, Emily; Neil, Simon; Darby, Claire

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 Sr1-xCaxFeO3-δ and SrFe1-xCoxO3-δ (0 ≤ x ≤ 1) to guide the design of more effective oxygen carriers for chemical looping applications at low temperatures (i.e. 400-600 °C). Ca A- or Co B-site substituted SrFeO3-δ show an increased reducibility, resulting in a higher oxygen capacity at T ≤ 600 °C 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 °C). Among the examined samples, Sr0.8Ca0.2FeO3-δ exhibited the highest oxygen storage capacity (2.15 wt%) at 500 °C, 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 °C, while the perovskite structure remains stable above 770 °C.

The combustion of fuel in oxygen rather than in air is one route to allow for the large-scale capture and storage of CO2. An alternative to the conventional air separation to produce oxygen, which imposes a significant energy penalty, is chemical looping air separation (CLAS). CLAS exploits the cyclic oxidation and reduction of solid oxygen materials. Here, the equilibrium partial pressure curves and the redox behavior for two potential materials (the perovskites SrFeO3-δ and SrMn0.1Fe0.9O3-δ) are derived from experiments. For the redox tests, a low dead volume micro reactor, operated as a differential packed bed, was used. This system enabled measuring the process of reduction at the 10 ms scale. Experiments were carried out between 798 and 898 K, with pO2 varied between 0 and 0.21 atm. All perovskites showed good performance during experiments lasting 1000 cycles. Despite similar chemical composition, the measured oxygen chemical potential and reduction kinetics differed between the tested materials significantly.

This study demonstrates a novel“Gas Switching Water Splitting (GSWS)”technology for production of pure H2with integrated CO2capture. The reactor concept is based on the chemical looping technology where an oxygencarrier (metal oxide) is used to transport O2from air to the fuel for different redox reactions. Unlike the conven-tional chemical looping, Gas Switching Technology inherently avoids external circulation of the oxygen carrier byalternating the oxidizing and reducing gases in a single bubblingfluidized bed reactor. This greatly simplifies re-actor design leading to easier scale-up of the technology in comparison with the conventional chemical looping.Thefirst experimental demonstration of the GSWS concept was completed at atmospheric pressure and temper-atures ranging between 700 °C and 900 °C with iron-based oxygen carrier supported on alumina (~35 wt% Fe2O3on Al2O3). Approximately 99% H2purity was achieved at ~80% oxygen utilization. Significant fuel slippage wasobserved especially beyond 33% degree of reduction with some carbon deposition. The deposited carbon wasable to combust/gasify completely in the subsequent air stage which makes the concept robust in sustaining ox-ygen carrier life. However, the gas mixing between the GSWS stages reduced the H2purity, CO2purity, and CO2capture efficiency. Tominimizethe negative impact ofgas mixing, Cu dopedMg(Fe0.9Al0.1)2O4spinelwith 74 wt%active content was developed specifically for the second experimental demonstration. Despite the high stabilityand reactivity under redox conditions with TGA, this oxygen carrier did not perform optimally in 5 cm IDfluid-ized bed reactor because of excessive agglomeration at degree of reduction beyond 34%. In general, a range of theactive content between 35 and 70 wt% of the oxygen carrier was desired for optimal performance of the GSWSconcept C 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

The development of bifunctional CaO/CuO matrix composites with both high and stable reactivity is a research priority and key for the development of calcium looping integrated with chemical looping combustion (CaL-CLC), a new CO2 capture technology that eliminates the requirement for pure O2 for the regeneration of CaO-based sorbents. In this work, a simple but effective approach was first used, i.e., solution combustion synthesis (SCS), to produce various nanostructured CaO/CuO matrix composites with homogenous elemental distributions. All CaO/CuO matrix composites possessed increased CO2 uptake in the form of self-activation and excellent cyclically stable O2 carrying capacity over as many as 40 reaction cycles. For instance, the final carbonation conversion of CaO-CuO-1-800-30 was 51.3%, approximately 52.7% higher than that of the original material (33.6%). Here, the self-activation phenomenon have been observed for the first time in contrast to the rapid decay in CO2 uptake capacity previously reported, due mainly to the increase of both specific surface area and pore volume. In-situ X-ray diffraction (in-situ XRD) analysis revealed that no side reactions occurred between CaO/CaCO3 and CuO/Cu during the overall process. All of these results make CaO/CuO matrix composites an attractive candidate for CaL-CLC.

Calcium (or carbonate) looping (CaL) is a high-temperature process to capture CO2 from industrial installation using CaO-based materials as the sorbent. A large number of natural and synthetic CaO-based sorbents for CO2 capture have been developed and investigated in the laboratory by cycling the material between a low-temperature carbonation stage and a high-temperature calcination stage. We demonstrate the importance of the exact experimental protocol chosen to transition from the carbonation to the calcination step by varying the CO2 concentration and the heating rate while keeping the conditions during the actual carbonation (650 °C, 15 vol.% CO2) and calcination (950 °C, 80 vol.% CO2) stages constant. The experiments were performed in a thermogravimetric analyser, the equipment most frequently used in investigations of the cyclic CO2 uptake, using a natural limestone as the sorbent. Our results show that the reaction conditions under which the CaCO3 is decomposed to CaO determines the cyclic performance of the sorbent, whereas the effect of the CO2 concentration during the set calcination stage (which usually begins after most of the CaCO3 has been converted to CaO) appears to be insignificant. Higher heating rates facilitate a higher cyclic CO2 uptake because the carbonate phase, known to be prone to sintering, is exposed to higher temperatures for a shorter time. The reaction conditions during the transition period crucially affect the CO2 uptake determined in laboratory tests and hence need to be reported in full detail, yet this is currently hardly the case. Otherwise it is unclear whether differences in the CO2 uptake of different CO2 sorbents are due to modifications of the material or variations in the testing protocol.

Chemical looping-based approaches allow for the production of high purity hydrogen from methane with an inherent separation of the coproduced carbon dioxide. In such a process, first methane is oxidized using lattice oxygen from a solid oxygen carrier. In the second half-cycle, the reduced oxygen carrier is reoxidized with steam to yield hydrogen. In this work, we report on the development of an iron-based oxygen carrier with an excellent redox stability and probe the synergistic effect of adding Mn2O3 and ZrO2, leading to an enhancement of the reactivity of iron oxide with methane and increasing the hydrogen yield over multiple redox cycles. The promoting effects of Mn2O3 and ZrO2 in the oxygen carrier were elucidated by combining TPR, STEM-EDX, reactivity tests, and conductivity measurements complemented by SIMS analysis. The addition of ZrO2 promoted the oxidation reactivity of the material, whereas the addition of Mn2O3 accelerated the reduction of iron oxide. Conductivity measurements revealed that t...

Ethylene is an essential building block in the petrochemical industry and it is almost exclusively produced via ethane steam cracking, a well-established albeit highly energy and carbon dioxide intensive process. The oxidative dehydrogenation of ethane is a promising alternative to steam cracking reactions due to its exothermic nature, which decreases the overall energy requirements and carbon footprint. The need of a capital intensive air separation unit for producing oxygen limits its potential for industrial application. The current study investigates an alternative route, i.e. the production of oxygen via chemical looping, where oxygen is released in-situ by suitable oxygen carriers. The chemical looping oxidative dehydrogenation, supported by original experimental data, and the steam cracking processes are simulated with ASPEN Plus®. A comprehensive analysis of the energy requirements and an economic assessment are carried out for both processes. Compared with state-of-the-art ethane steam cracking, the proposed process provides ~28% energy savings per tonnes of ethylene produced and ~21% reduction in the resulting ethylene price. Sensitivity analysis show that the economy of the chemical looping oxidative dehydrogenation process is strongly sensitive to the feedstock price., Renewable and Sustainable Energy Reviews, 149, ISSN:1364-0321

Germanium telluride (GeTe), a phase-change material, is known to exhibit four different structural phases: three at room temperature (one amorphous and two crystalline, $\alpha$ and $\gamma$) and one at high temperature (crystalline $\beta$). Because transitions between the amorphous and crystalline phases lead to significant changes in material properties (e.g., refractive index and resistivity), GeTe has been investigated as a phase-change material for photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the temperature-dependent phase transitions in GeTe have been studied for bulk and thin-film GeTe, both fabricated by sputtering. Colloidal synthesis of nanoparticles offers a more flexible fabrication approach for amorphous and crystalline GeTe. These nanoparticles are known to exhibit size-dependent properties, such as an increased crystallization temperature for the amorphous-to-$\alpha$ transition in sub-10\,nm GeTe particles. The $\alpha$-to-$\beta$ phase transition is also expected to vary with size, but this effect has not yet been investigated for GeTe. Here, we report time-resolved X-ray diffraction of GeTe nanoparticles with different diameters and from different synthetic protocols. We observe a non-volatile amorphous-to-$\alpha$ transition between 210$^{\circ}$C and 240$^{\circ}$C and a volatile $\alpha$-to-$\beta$ transition between 370$^{\circ}$C and 420$^{\circ}$C. The latter transition was reversible and repeatable. While the transition temperatures are shifted relative to the values known for bulk GeTe, the nanoparticle-based samples still exhibit the same structural phases reported for sputtered GeTe. Thus, colloidal GeTe maintains the same general phase behavior as bulk GeTe while allowing for more flexible and accessible fabrication. Therefore, nanoparticle-based GeTe films show great potential for applications, such as in active photonics.

Chemical looping can provide attractive alternative process routes in which solid oxygen carriers function as lattice oxygen transfer agents, for example for the partial oxidation of methane. We report on the development of a perovskite-based oxygen carrier (La0.85Sr0.15Fe0.95Al0.05O3-δ) that enables the complete conversion of CH4 to a synthesis gas (a mixture of H2 and CO, selectivity > 99 %) through donation of its lattice oxygen (up to 9 wt%) at temperatures > 900 °C. The thermodynamic properties of the oxygen carrier permit its lattice oxygen to be replenished with CO2 or H2O, of which > 94 % is converted into CO or H2, respectively. The potential of this compositionally and structurally flexible oxygen carrier is demonstrated in a continuous experiment lasting more than 45 days (∼ 4050 redox cycles), in which all CH4 (reductant) and all CO2 (oxidant) is converted into a synthesis gas without CO2 contamination., Applied Catalysis B: Environmental, 278, ISSN:1873-3883, ISSN:0926-3373