Nickel-based cerium zirconate inorganic complex structures for CO2 valorisation via dry reforming of methane

The increasing anthropogenic emissions of greenhouse gases (GHG) is encouraging extensive research in CO2 utilisation. Dry reforming of methane (DRM) depicts a viable strategy to convert both CO2 and CH4 into syngas, a worthwhile chemical intermediate. Among the different active phases for DRM, the use of nickel as catalyst is economically favourable, but typically deactivates due to sintering and carbon deposition. The stabilisation of Ni at different loadings in cerium zirconate inorganic complex structures is investigated in this work as strategy to develop robust Ni-based DRM catalysts. XRD and TPR-H2 analyses confirmed the existence of different phases according to the Ni loading in these materials. Besides, superficial Ni is observed as well as the existence of a CeNiO3 perovskite structure. The catalytic activity was tested, proving that 10 wt.% Ni loading is the optimum which maximises conversion. This catalyst was also tested in long-term stability experiments at 600 and 800°C in order to study the potential deactivation issues at two different temperatures. At 600°C, carbon formation is the main cause of catalytic deactivation, whereas a robust stability is shown at 800°C, observing no sintering of the active phase evidencing the success of this strategy rendering a new family of economically appealing CO2 and biogas mixtures upgrading catalysts.

Optimizing biogas methanation over nickel supported on ceria-alumina catalyst: Towards CO2-rich biomass utilization for a negative emissions society

Biogas methanation emerges as a prominent technology for converting biogas into biomethane in a single step. Furthermore, this technology can be implemented at biogas plant locations, supporting local economies and reducing dependence on large energy producers. However, there is a lack of comprehensive studies on biogas methanation, particularly regarding the technical optimization of operational parameters and the profitability analysis of the overall process. To address this gap, our study represents a seminal work on the technical optimization of biogas methanation obtaining an empirical model to predict the performance of biogas methanation. We investigate the influence of operational parameters, such as reaction temperature, H2/CO2 ratio, space velocity, and CO2 share in the biogas stream through an experimental design. Based on previous research we selected a nickel supported on ceria-alumina catalyst; being nickel a benchmark system for methanation process such selection permits a reliable data extrapolation to commercial units. We showcase the remarkable impact of studied key operation parameters, being the temperature, the most critical factor affecting the reaction performance (ca. 2 to 5 times higher than the second most influencing parameter). The impact of the H2/CO2 ratio is also noticeable. The response surfaces and contour maps suggest that a temperature between 350 and 450 °C and an H2/CO2 ratio between 2.5 and 3.2 optimize the reaction performance. Further experimental tests were performed for model validation and optimization leading to a reliable predictive model. Overall, this study provides validated equations for technology scaling-up and techno-economic analysis, thus representing a step ahead towards real-world applications for bio-methane production.

Boosting Low-Temperature CO2 Hydrogenation over Ni-based Catalysts by Tuning Strong Metal-Support Interactions

Rational design of low-cost and efficient transition-metal catalysts for low-temperature CO2 activation is significant and poses great challenges. Herein, a strategy via regulating the local electron density of active sites is developed to boost CO2 methanation that normally requires >350 °C for commercial Ni catalysts. An optimal Ni/ZrO2 catalyst affords an excellent low-temperature performance hitherto, with a CO2 conversion of 84.0 %, CH4 selectivity of 98.6 % even at 230 °C and GHSV of 12,000 mL g-1  h-1 for 106 h, reflecting one of the best CO2 methanation performance to date on Ni-based catalysts. Combined a series of in situ spectroscopic characterization studies reveal that re-constructing monoclinic-ZrO2 supported Ni species with abundant oxygen vacancies can facilitate CO2 activation, owing to the enhanced local electron density of Ni induced by the strong metal-support interactions. These findings might be of great aid for construction of robust catalysts with an enhanced performance for CO2 emission abatement and beyond.

A review on high-pressure heterogeneous catalytic processes for gas-phase CO2 valorization

This review discusses the importance of mitigating CO2 emissions by valorizing CO2 through high-pressure catalytic processes. It focuses on various key processes, including CO2 methanation, reverse water-gas shift, methane dry reforming, methanol, and dimethyl ether synthesis, emphasizing pros and cons of high-pressure operation. CO2 methanation, methanol synthesis, and dimethyl ether synthesis reactions are thermodynamically favored under high-pressure conditions. However, in the case of methane dry reforming and reverse water-gas shift, applying high pressure, results in decreased selectivity toward desired products and an increase in coke production, which can be detrimental to both the catalyst and the reaction system. Nevertheless, high-pressure utilization proves industrially advantageous for cost reduction when these processes are integrated with Fischer-Tropsch or methanol synthesis units. This review also compiles recent advances in heterogeneous catalysts design for high-pressure applications. By examining the impact of pressure on CO2 valorization and the state of the art, this work contributes to improving scientific understanding and optimizing these processes for sustainable CO2 management, as well as addressing challenges in high-pressure CO2 valorization that are crucial for industrial scaling-up. This includes the development of cost-effective and robust reactor materials and the development of low-cost catalysts that yield improved selectivity and long-term stability under realistic working environments.

Synthesis of a Graphene-Encapsulated Fe3C/Fe Catalyst Supported on Sporopollenin Exine Capsules and Its Use for the Reverse Water-Gas Shift Reaction

Bioderived materials have emerged as sustainable catalyst supports for several heterogeneous reactions owing to their naturally occurring hierarchal pore size distribution, high surface area, and thermal and chemical stability. We utilize sporopollenin exine capsules (SpECs), a carbon-rich byproduct of pollen grains, composed primarily of polymerized and cross-linked lipids, to synthesize carbon-encapsulated iron nanoparticles via evaporative precipitation and pyrolytic treatments. The composition and morphology of the macroparticles were influenced by the precursor iron acetate concentration. Most significantly, the formation of crystalline phases (Fe3C, α-Fe, and graphite) detected via X-ray diffraction spectroscopy showed a critical dependence on iron loading. Significantly, the characteristic morphology and structure of the SpECs were largely preserved after high-temperature pyrolysis. Analysis of Brunauer-Emmett-Teller surface area, the D and G bands from Raman spectroscopy, and the relative ratio of the C=C to C-C bonding from high-resolution X-ray photoelectron spectroscopy suggests that porosity, surface area, and degree of graphitization were easily tuned by varying the Fe loading. A mechanism for the formation of crystalline phases and meso-porosity during the pyrolysis process is also proposed. SpEC-Fe10% proved to be highly active and selective for the reverse water-gas shift reaction at high temperatures (>600 °C).

The Need for Flexible Chemical Synthesis and How Dual-Function Materials Can Pave the Way

Since climate change keeps escalating, it is imperative that the increasing CO₂ emissions be combated. Over recent years, research efforts have been aiming for the design and optimization of materials for CO₂ capture and conversion to enable a circular economy. The uncertainties in the energy sector and the variations in supply and demand place an additional burden on the commercialization and implementation of these carbon capture and utilization technologies. Therefore, the scientific community needs to think out of the box if it is to find solutions to mitigate the effects of climate change. Flexible chemical synthesis can pave the way for tackling market uncertainties. The materials for flexible chemical synthesis function under a dynamic operation, and thus, they need to be studied as such. Dual-function materials are an emerging group of dynamic catalytic materials that integrate the CO₂ capture and conversion steps. Hence, they can be used to allow some flexibility in the production of chemicals as a response to the changing energy sector. This Perspective highlights the necessity of flexible chemical synthesis by focusing on understanding the catalytic characteristics under a dynamic operation and by discussing the requirements for the optimization of materials at the nanoscale.

Flexible NiRu Systems for CO2 Methanation: From Efficient Catalysts to Advanced Dual-Function Materials

CO₂ emissions in the atmosphere have been increasing rapidly in recent years, causing global warming. CO₂ methanation reaction is deemed to be a way to combat these emissions by converting CO₂ into synthetic natural gas, i.e., CH₄. NiRu/CeAl and NiRu/CeZr both demonstrated favourable activity for CO₂ methanation, with NiRu/CeAl approaching equilibrium conversion at 350 °C with 100% CH₄ selectivity. Its stability under high space velocity (400 L·g^-1·h^-1) was also commendable. By adding an adsorbent, potassium, the CO₂ adsorption capability of NiRu/CeAl was boosted, allowing it to function as a dual-function material (DFM) for integrated CO₂ capture and utilisation, producing 0.264 mol of CH₄/kg of sample from captured CO₂. Furthermore, time-resolved operando DRIFTS-MS measurements were performed to gain insights into the process mechanism. The obtained results demonstrate that CO₂ was captured on basic sites and was also dissociated on metallic sites in such a way that during the reduction step, methane was produced by two different pathways. This study reveals that by adding an adsorbent to the formulation of an effective NiRu methanation catalyst, advanced dual-function materials can be designed.

Is the RWGS a viable route for CO2 conversion to added value products? A techno-economic study to understand the optimal RWGS conditions

Understanding the viability of the RWGS from a thermodynamic and techno-economic angle opens new horizons within CO₂ conversion technologies. Unfortunately, profitability studies of this technology are scarce in literature and mainly focused on overall conversion and selectivity trends with tangential remarks on energy demands and process costs. To address this research gap, herein we present a comprehensive techno-economic study of the RWGS reaction when coupling with Fischer-Tropsch synthesis is envisaged to produced fuels and chemicals using CO₂ as building block. We showcase a remarkable impact of operating conditions in the final syngas product and both CAPEX and OPEX. From a capital investment perspective, optimal situations involve RWGS unit running at low temperatures and high pressures as evidenced by our results. However, from the running cost angle, operating at 4 bar is the most favorable alternative within the studied scenarios. Our findings showcase that, no matter the selected temperature the RWGS unit should be preferentially run at intermediate pressures. Ultimately, our work maps out multiple operating scenarios in terms of energy demand and process cost serving as guideline to set optimal reaction conditions to unlock the potential of the RWGS for chemical CO₂ recycling.

Sustainable routes for acetic acid production: Traditional processes vs a low-carbon, biogas-based strategy

The conversion of biogas, mainly formed of CO₂ and CH₄, into high-value platform chemicals is increasing attention in a context of low-carbon societies. In this new paradigm, acetic acid (AA) is deemed as an interesting product for the chemical industry. Herein we present a fresh overview of the current manufacturing approaches, compared to potential low-carbon alternatives. The use of biogas as primary feedstock to produce acetic acid is an auspicious alternative, representing a step-ahead on carbon-neutral industrial processes. Within the spirit of a circular economy, we propose and analyse a new BIO-strategy with two noteworthy pathways to potentially lower the environmental impact. The generation of syngas via dry reforming (DRM) combined with CO₂ utilisation offers a way to produce acetic acid in a two-step approach (BIO-Indirect route), replacing the conventional, petroleum-derived steam reforming process. The most recent advances on catalyst design and technology are discussed. On the other hand, the BIO-Direct route offers a ground-breaking, atom-efficient way to directly generate acetic acid from biogas. Nevertheless, due to thermodynamic restrictions, the use of plasma technology is needed to directly produce acetic acid. This very promising approach is still in an early stage. Particularly, progress in catalyst design is mandatory to enable low-carbon routes for acetic acid production.

Feasibility of switchable dual function materials as a flexible technology for CO2 capture and utilisation and evidence of passive direct air capture

The feasibility of a Dual Function Material (DFM) with a versatile catalyst offering switchable chemical synthesis from carbon dioxide (CO₂) was demonstrated for the first time, showing evidence of the ability of these DFMs to passively capture CO₂ directly from the air as well. These DFMs open up possibilities in flexible chemical production from dilute sources of CO₂, through a combination of CO₂ adsorption and subsequent chemical transformation (methanation, reverse water gas shift or dry reforming of methane). Combinations of Ni Ru bimetallic catalyst with Na₂O, K₂O or CaO adsorbent were supported on CeO₂-Al₂O₃ to develop flexible DFMs. The designed multicomponent materials were shown to reversibly adsorb CO₂ between the 350 and 650 °C temperature range and were easily regenerated by an inert gas purge stream. The components of the flexible DFMs showed a high degree of interaction with each other, which evidently enhanced their CO₂ capture performance ranging from 0.14 to 0.49 mol kg^-1. It was shown that captured CO₂ could be converted into useful products through either CO₂ methanation, reverse water-gas shift (RWGS) or dry reforming of methane (DRM), which provides flexibility in terms of co-reactant (hydrogen vs. methane) and end product (synthetic natural gas, syngas or CO) by adjusting reaction conditions. The best DFM was the one containing CaO, producing 104 μmol of CH₄ per kg_DFM in CO₂ methanation, 58 μmol of CO per kg_DFM in RWGS and 338 μmol of CO per kg_DFM in DRM.

Modelling approaches for biomass gasifiers: A comprehensive overview

Biomass resources have the potential to become a viable renewable technology and play a key role within the future renewable energy paradigm. Since CO₂ generated in bio-energy production is equal to the CO₂ absorbed during the growth of the biomass, this renewable energy is a net zero emissions resource. Biomass gasification is a versatile method for transforming waste into energy in which biomass material is thermochemically converted within a reactor. Gasification's superior flexibility, including both in terms of biomass type and heat generation or energy production alternatives, is what stimulates biomass gasification scientific and industrial potential. Downdraft gasifiers seem to be well-suited for small-scale generation of heat along with energy, whereas fluidised bed and entrained flow gasifiers currently attain significant economies of scale for fuel production. The operation of gasifiers is influenced by several factors, including operational parameters, feedstock types, and reactor design. Modelling is a valuable tool for building a unit based on the results of model predictions with different operational parameters and feedstock in such scenarios. Once verified, a suitable model may be used to assess the sensitivity of a gasifier's performance to changes in various operational and design factors. Effective models may help designers to theorise and predict the impacts of a variety of characteristics without the need for further empirical observations, which can help in the design and implementation of this technology. This work provides an overview of gasification technologies and a succinct guidance to the modelling decisions and modelling strategies for biomass gasification to enable a successful biomass to fuel conversion. A technical description and critical analysis of thermodynamic, kinetic, computational fluid dynamic and data-driven approaches is provided, including crucial modelling considerations that have not been explored in earlier studies. The review aims to aid researchers in the field to select the appropriate approach and guide future work.

Emerging natural and tailored perovskite-type mixed oxides–based catalysts for CO2 conversions

The rapid economic and societal development have led to unprecedented energy demand and consumption resulting in the harmful emission of pollutants. Hence, the conversion of greenhouse gases into valuable chemicals and fuels has become an urgent challenge for the scientific community. In recent decades, perovskite-type mixed oxide-based catalysts have attracted significant attention as efficient CO2 conversion catalysts due to the characteristics of both reversible oxygen storage capacity and stable structure compared to traditional oxide-supported catalysts. In this review, we hand over a comprehensive overview of the research for CO2 conversion by these emerging perovskite-type mixed oxide-based catalysts. Three main CO2 conversions, namely reverse water gas shift reaction, CO2 methanation, and CO2 reforming of methane have been introduced over perovskite-type mixed oxide-based catalysts and their reaction mechanisms. Different approaches for promoting activity and resisting carbon deposition have also been discussed, involving increased oxygen vacancies, enhanced dispersion of active metal, and fine-tuning strong metal-support interactions. Finally, the current challenges are mooted, and we have proposed future research prospects in this field to inspire more sensational breakthroughs in the material and environment fields.

Design of Full-Temperature-Range RWGS Catalysts: Impact of Alkali Promoters on Ni/CeO2

Reverse water gas shift (RWGS) competes with methanation as a direct pathway in the CO₂ recycling route, with methanation being a dominant process in the low-temperature window and RWGS at higher temperatures. This work showcases the design of multi-component catalysts for a full-temperature-range RWGS behavior by suppressing the methanation reaction at low temperatures. The addition of alkali promoters (Na, K, and Cs) to the reference Ni/CeO₂ catalyst allows identifying a clear trend in RWGS activation promotion in both low- and high-temperature ranges. Our characterization data evidence changes in the electronic, structural, and textural properties of the reference catalyst when promoted with selected dopants. Such modifications are crucial to displaying an advanced RWGS performance. Among the studied promoters, Cs leads to a more substantial impact on the catalytic activity. Beyond the improved CO selectivity, our best performing catalyst maintains high conversion levels for long-term runs in cyclable temperature ranges, showcasing the versatility of this catalyst for different operating conditions. All in all, this work provides an illustrative example of the impact of promoters on fine-tuning the selectivity of a CO₂ conversion process, opening new opportunities for CO₂ utilization strategies enabled by multi-component catalysts.

Biogas upgrading to biomethane as a local source of renewable energy to power light marine transport: Profitability analysis for the county of Cornwall

In this work, the use of biomethane produced from local biogas plants is proposed as renewable fuel for light marine transport. A profitability analysis is performed for three real biogas production plants located in Cornwall (United Kingdom), considering a total of 66 different scenarios where critical parameters such as distance from production point to gas grid, subsidies, etcetera, were evaluated. Even though the idea is promising to decarbonize the marine transport sector, under the current conditions, the approach is not profitable. The results show that profitability depends on the size of the biogas plant. The largest biogas plant studied can be profitable if feed-in tariffs subsidies between 36.6 and 45.7 €/MWh are reached, while for the smallest plant, subsidies should range between 65 and 82.7 €/MWh. The tax to be paid per ton of CO₂ emitted by the shipping owner, was also examined given its impact in this green route profitability. Values seven times greater than current taxes are needed to reach profitability, revealing the lack of competitiveness of renewable fuels vs traditional fuels in this application. Subsidies to make up a percentage of the investment are also proposed, revealing that even at 100% of investment subsidized, this green approach is still not profitable. The results highlight the need for further ambitious political actions in the pursuit of sustainable societies.

K-Promoted Ni-Based Catalysts for Gas-Phase CO2 Conversion: Catalysts Design and Process Modelling Validation

The exponential growth of greenhouse gas emissions and their associated climate change problems have motivated the development of strategies to reduce CO₂ levels via CO₂ capture and conversion. Reverse water gas shift (RWGS) reaction has been targeted as a promising pathway to convert CO₂ into syngas which is the primary reactive in several reactions to obtain high-value chemicals. Among the different catalysts reported for RWGS, the nickel-based catalyst has been proposed as an alternative to the expensive noble metal catalyst. However, Ni-based catalysts tend to be less active in RWGS reaction conditions due to preference to CO₂ methanation reaction and to the sintering and coke formation. Due to this, the aim of this work is to study the effect of the potassium (K) in Ni/CeO₂ catalyst seeking the optimal catalyst for low-temperature RWGS reaction. We synthesised Ni-based catalyst with different amounts of K:Ni ratio (0.5:10, 1:10, and 2:10) and fully characterised using different physicochemical techniques where was observed the modification on the surface characteristics as a function of the amount of K. Furthermore, it was observed an improvement in the CO selectivity at a lower temperature as a result of the K-Ni-support interactions but also a decrease on the CO₂ conversion. The 1K catalyst presented the best compromise between CO₂ conversion, suppression of CO₂ methanation and enhancing CO selectivity. Finally, the experimental results were contrasted with the trends obtained from the thermodynamics process modelling observing that the result follows in good agreement with the modelling trends giving evidence of the promising behaviour of the designed catalysts in CO₂ high-scale units.

Characterization of emissions of condensable particulate matter under real operation conditions in cement clinker kilns using complementary experimental techniques

Historically, the emission of particles from clinker kiln stacks has been one of the main environmental concerns in cement manufacturing processes. Up to now, environmental regulations have only focused on determining and controlling filterable particulate matter (FPM) in industrial emission sources. However, in recent years a growing interest in determining and analysing condensable particulate matter (CPM) has been evidenced due to the significant and established contribution of CPM to total emissions of particulate matter (PM). In this work, total PM (FPM + CPM) emissions from a clinker kiln in a cement manufacturing process have been characterized. A series of tests were performed to simultaneously collect FPM and CPM using a sampling train patented by University of Seville. The results showed very low level of emissions compared to regulatory limits. The average FPM and CPM concentrations obtained in the kiln were in the same order of magnitude, at 3.4 mg/Nm³ and 2.8 mg/Nm³, respectively. The CPM analysed was predominantly inorganic and represented 46% of total PM emissions. In addition, a microscopic morphological analysis was carried out on the samples and confirmed the presence of CPM with a size of less than 2 μm, as well as establishing the principal constituent elements of the same. The main element components were Al, Ca, Fe, Si, C and O. Compounds such as CaCO₃, alite, ferrite and dolomite were detected with analytical characterization techniques, such as infrared spectroscopy (FTIR) analysis and X-ray diffraction (XRD), providing a better understanding of the sources of contamination within CPM.

Preparation of sorbents derived from bamboo and bromine flame retardant for elemental mercury removal

This work showcases cost-effective elemental mercury capture strategy enabled by bamboo saw dust and bromine flame retardant (BFR) derived sorbent prepared by a novel hydrothermal-pyrolysis method. The hydrothermal treatment of bamboo and BFR blend was conducted in subcritical water resulting in a hydrothermal char. Subsequently, the hydrothermal char was pyrolyzed in nitrogen atmosphere leading to an improved pore architecture. The resulting biomaterials were proven highly effective for Hg removal. A thorough analysis of the physicochemical properties of the samples was conducted by means of BET, SEM, XRD, XPS and FT-IR. Key parameters such as bamboo/BFR ratio, hydrothermal temperatures and pyrolysis temperatures influence Hg⁰ removal capacity of our bio-sorbents. Overall, the optimal bamboo/BFR ratio, hydrothermal temperature and pyrolysis temperature are 2:1, 320 °C and 800 °C, respectively. Under these optimized conditions, a very promising elemental mercury removal efficiency of 99% is attained. The kinetics and mechanism of Hg⁰ removal are also proposed. The experimental data fit well with a pseudo-second-order model, indicating that Hg⁰ adsorption over sorbents was dominated by chemisorption. Our results indicate that the C-Br groups in sorbents provide active sites for oxidizing Hg⁰ into HgBr₂.

Biogas Conversion to Syngas Using Advanced Ni-Promoted Pyrochlore Catalysts: Effect of the CH4/CO2 Ratio

Biogas is defined as the mixture of CH₄ and CO₂ produced by the anaerobic digestion of biomass. This particular mixture can be transformed in high valuable intermediates such as syngas through a process known as dry reforming (DRM). The reaction involved is highly endothermic, and catalysts capable to endure carbon deposition and metal particle sintering are required. Ni-pyrochlore catalysts have shown outstanding results in the DRM. However, most reported data deals with CH₄/CO₂ stoichiometric ratios resulting is a very narrow picture of the overall biogas upgrading via DRM. Therefore, this study explores the performance of an optimized Ni-doped pyrochlore, and Ni-impregnated pyrochlore catalysts in the dry reforming of methane, under different CH₄/CO₂ ratios, in order to simulate various representatives waste biomass feedstocks. Long-term stability tests showed that the ratio CH₄/CO₂ in the feed gas stream has an important influence in the catalysts' deactivation. Ni doped pyrochlore catalyst, presents less deactivation than the Ni-impregnated pyrochlore. However, biogas mixtures with a CH₄ content higher than 60%, lead to a stronger deactivation in both Ni-catalysts. These results were in agreement with the thermogravimetric analysis (TGA) of the post reacted samples that showed a very limited carbon formation when using biogas mixtures with CH₄ content <60%, but CH₄/CO₂ ratios higher than 1.25 lead to an evident carbon deposition. TGA analysis of the post reacted Ni impregnated pyrochlore, showed the highest amount of carbon deposited, even with lower stoichiometric CH₄/CO₂ ratios. The later result indicates that stabilization of Ni in the pyrochlore structure is vital, in order to enhance the coke resistance of this type of catalysts.

Molten Salt-Promoted MgO Adsorbents for CO2 Capture: Transient Kinetic Studies

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO₂ associated with Mg²⁺-O^2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO₂ molecular path in MgO-based CO₂ adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO₂ on the surface of MgO exhibited the highest CO₂ uptake (15.7 mmol g^-1) at 350 °C compared to less than 0.1 mmol g^-1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO₂ at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO₃ formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO₂ partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO₂-capturing performance gained and its considerable significance for future practical designs of precombustion CO₂ capture.

Molybdenum Oxide Supported on Ti3AlC2 is an Active Reverse Water-Gas Shift Catalyst

MAX phases are layered ternary carbides or nitrides that are attractive for catalysis applications due to their unusual set of properties. They show high thermal stability like ceramics, but they are also tough, ductile, and good conductors of heat and electricity like metals. Here, we study the potential of the Ti₃AlC₂ MAX phase as a support for molybdenum oxide for the reverse water-gas shift (RWGS) reaction, comparing this new catalyst to more traditional materials. The catalyst showed higher turnover frequency values than MoO₃/TiO₂ and MoO₃/Al₂O₃ catalysts, due to the outstanding electronic properties of the Ti₃AlC₂ support. We observed a charge transfer effect from the electronically rich Ti₃AlC₂ MAX phase to the catalyst surface, which in turn enhances the reducibility of MoO₃ species during reaction. The redox properties of the MoO₃/Ti₃AlC₂ catalyst improve its RWGS intrinsic activity compared to TiO₂- and Al₂O₃-based catalysts.

Synergizing carbon capture and utilization in a biogas upgrading plant based on calcium chloride: Scaling-up and profitability analysis

Herein we analyze the profitability of a novel regenerative process to synergize biogas upgrading and carbon dioxide utilization. Our proposal is a promising alternative which allows to obtain calcium carbonate as added value product while going beyond traditional biogas upgrading methods with high thermal energy consumption. Recently we have demonstrated the experimental viability of this route. In this work, both the scale-up and the profitability of the process are presented. Furthermore, we analyze three representative scenarios to undertake a techno-economic study of the proposed circular economy process. The scale-up results demonstrate the technical viability of our proposal. The precipitation efficiency and the product quality are still remarkable with the increase of the reactor size. The techno-economic analysis reveals that the implementation of this circular economy strategy is unprofitable without subsidies. Nonetheless, the results are somehow encouraging as the subsides needed to reach profitability are lower than in other biogas upgrading and carbon dioxide utilization proposals. Indeed, for the best-case scenario, a feed-in tariff incentive of 4.3 €/MWh makes the approach profitable. A sensitivity study through tornado analysis is also presented, revealing the importance of reducing bipolar membrane electrodialysis energy consumption. Overall our study envisages the big challenge that the EU faces during the forthcoming years. The evolution towards bio-based and circular economies requires the availability of economic resources and progress on engineering technologies.

In-situ formation of carboxylate species on TiO2 nanosheets for enhanced visible-light photocatalytic performance

It still remains challenge for expanding the photo-response range of TiO₂ with dominant {0 0 1} facets due to the hardly achieving modification of the electronic structure without destroying the formation of TiO₂ high energy facets. Herein, we report the construction of carboxylate species modified TiO₂ nanosheets with dominant {0 0 1} facets by employing ethanol as a carbon source through a low-temperature (300 °C) carbonization method. The as-obtained samples were investigated in detail by using various characterization techniques. The results indicate that the carboxylate species derived from the oxidation and carbonization of ethanol are coordinated to the {0 0 1} facets in a bidentate bridging mode. The electron-withdrawing carboxylate species induce TiO₂ to form a lower valence band edge and a narrower bandgap, which enhances the oxidation ability of photogenerated holes and expands the photo-response range. The partially carbonized carboxylate species can also act as a photosensitizer to induce visible-light photocatalytic activity of TiO₂ nanosheets. In addition, the carboxylate species can further promote the separation of photogenerated charge carriers. The findings of this work may provide a new perspective for tuning the band structure of TiO₂ with dominant {0 0 1} facets and improving its photocatalytic performance.

Influence of Reaction Parameters on the Catalytic Upgrading of an Acetone, Butanol, and Ethanol (ABE) Mixture: Exploring New Routes for Modern Biorefineries

Here we present a comprehensive study on the effect of reaction parameters on the upgrade of an acetone, butanol and ethanol mixture-key molecules and platform products of great interest within the chemical sector. Using a selected high performing catalyst, Fe/MgO-Al2O3, the variation of temperature, reaction time, catalytic loading, and reactant molar ratio have been examined in this reaction. This work is aiming to not only optimize the reaction conditions previously used, but to step toward using less energy, time, and material by testing those conditions and analyzing the sufficiency of the results. Herein, we demonstrate that this reaction is favored at higher temperatures and longer reaction time. Also, we observe that increasing the catalyst loading had a positive effect on the product yields, while reactant ratios have shown to produce varied results due to the role of each reactant in the complex reaction network. In line with the aim of reducing energy and costs, this work showcases that the products from the upgrading route have significantly higher market value than the reactants; highlighting that this process represents an appealing route to be implemented in modern biorefineries.

Understanding the effect of Ca and Mg ions from wastes in the solvent regeneration stage of a biogas upgrading unit

This paper reveals the effect of calcium and magnesium ions in carbonation experiments carried out to regenerate sodium hydroxide from a biogas upgrading unit. This novel study arises as an alternative to standard physical process whose elevated energy consumption imposes economic restrictions. Previous works employed alkaline waste to turn them into value added product. Nevertheless, no attractive economical results were obtained due to the low regeneration efficiencies. Our hypothesis is that both calcium and magnesium waste composition percentages have an impact in the result, hence this work propose an isolated study aiming to determine the of each one in the global performance. To this end, the operational parameters (reaction time, reaction temperature and molar ratio) were tuned as well as physicochemical properties of the final solid samples were analyzed by several techniques. The results indicate that calcium is much more prone than magnesium to reach high efficiencies in aqueous carbonation experiments. Additionally, higher quality products were achieved with calcium. The results of this study suppose an important step for understanding the aqueous carbonation through waste in the path to achieve a more sustainable city and society.

Au/CeO2-ZnO/Al2O3 as Versatile Catalysts for Oxidation Reactions: Application in Gas/Liquid Environmental Processes

The present work showcases the versatility of nanogold systems supported on Zn-doped ceria when applied in two important environmental processes, the total CO oxidation, and the liquid phase oxidation of glucose to gluconic acid. In the CO oxidation the suitability of these materials is clearly demonstrated achieving full conversions even at sub-ambient conditions. Regarding the glucose oxidation our materials display high conversion values (always over 50%) and very importantly full or almost full selectivity toward gluconic acid-an added value platform chemical in the context of biomass upgrading routes. The key factors controlling the successful performance on both reactions are carefully discussed and compared to previous studies in literature. To our knowledge this is one of the very few works in catalysis by gold combining liquid and gas phase reactions and represents a step forward in the flexible behavior of nano gold catalysts.

Synergizing carbon capture storage and utilization in a biogas upgrading lab-scale plant based on calcium chloride: Influence of precipitation parameters

Herein a strategy for biogas upgrading in a continuous flow absorption unit using CaCl₂ as capturing agent is reported. This process is presented as an alternative to the standard physical regeneration processes to capture carbon dioxide (CO₂) from biogas effluents with inherent high energy penalties. This work showcases a systematic study of the main parameters (reaction time, reaction temperature, and molar ratio reactant/precipitator) affecting calcium carbonate (CaCO₃) precipitation efficiency in a reaction between sodium carbonate (Na₂CO₃) and CaCl₂. In addition, the purity and main characteristics of the obtained product were carefully analysed via in a combined characterization study using Raman, XRD, and SEM. Our results indicate that acceptable precipitation efficiencies between 62 and 93% can be reached by fine tuning the studied parameters. The characterization techniques evidence pure CaCO₃ in a calcite structure. These results confirmed the technical feasibility of this alternative biogas upgrading process through CaCO₃ production.

Robust mesoporous bimetallic yolk–shell catalysts for chemical CO2 upgrading via dry reforming of methane

A new generation highly efficient and stable mesoporous ZnO/Ni@silica yolk–shell catalyst is designed for chemical CO₂ recycling, to solve the coking and sintering issues of traditional catalysts.