Dry reforming of methane (DRM) is considered one of the most promising technologies for efficient greenhouse gas management thanks to the fact that through this reaction, it is possible to reduce CO2 and CH4 to obtain syngas, a mixture of H2 and CO, with a suitable ratio for the Fischer-Tropsch production of long-chain hydrocarbons. Two other main processes can yield H2 from CH4, i.e., Steam Reforming of Methane (SRM) and Partial Oxidation of Methane (POM), even though, not having CO2 as a reagent, they are considered less green. Recently, scientists\' challenge is to overcome the many drawbacks of DRM reactions, i.e., the use of precious metal-based catalysts, the high temperatures of the process, metal particle sintering and carbon deposition on the catalysts\' surfaces. To overcome these issues, one proposed solution is to implement photo-thermal dry reforming of methane in which irradiation with light is used in combination with heating to improve the efficiency of the process. In this paper, we review the work of several groups aiming to investigate the pivotal promoting role of light radiation in DRM. Focus is also placed on the catalysts\' design and the progress needed for bringing DRM to an industrial scale.

Direct conversion of biogas via the integrative process of dry reforming of methane (DRM) and catalytic methane decomposition (CDM) has received a great attention as a promising green catalytic process for simultaneous production of syngas and carbon nanotubes (CNTs). In this work, the effects of reaction temperature of 700-1100 °C and CH4/CO2 ratio of biogas were investigated over NiMo/MgO catalyst in a fixed bed reactor under industrial feed condition of pure biogas. The reaction at 700 °C showed a rapid catalyst deactivation within 3 h due to the formation of amorphous carbon on catalyst surface. At higher temperature of 800-900 °C, the catalyst can perform the excellent performance for producing syngas and carbon nanotubes. Interestingly, the smallest diameter and the highest graphitization of CNTs was obtained at high temperature of 1000 °C, while elevating temperature to 1100 °C leads to agglomeration of Ni particles, resulting in a larger size of CNTs. The reaction temperature exhibits optimum at 800 °C, providing the highest CNTs yield with high graphitization, high syngas purity up to 90.04% with H2/CO ratio of 1.1, and high biogas conversion (XCH4 = 86.44%, XCO2 = 95.62%) with stable performance over 3 h. The typical composition biogas (CH4/CO2 = 1.5) is favorable for the integration process, while the CO2 rich biogas caused a larger grain size of catalyst and a formation of molybdenum oxide nanorods (MoO3). The long-term stability of NiMo/MgO catalyst at 800 °C showed a stable trend (> 20 h). The experimental findings confirm that NiMo/MgO can perform the excellent activity and high stability at the optimum condition, allowing the process to be more promising for practical applications.

Plasma gasification melting (PGM) provides reliable disposal of toxic medical waste with a low heating value, which is capable of converting waste into energy. This study investigates the performance of experiments on plasma gasification for the treatment of chemical-pharmaceutical medical waste (CPMW) with an air medium. A comparative analysis is performed for gasification characteristics at three reactor temperatures (1000, 1400, and 1800 °C). Moreover, a thermodynamic equilibrium model is developed to assess performance features such as syngas yield, high heating value, and cold gas efficiency in the gasification temperature range of 1000-1800 °C. A comparison of the experiment and computational outcomes shows a good agreement. The results show that the quality of syngas and heating value is improved by increasing the temperature of the plasma gasifier so that at 1800 °C, H2, CO, and higher heating value (HHV) are obtained as 41 %, 37 %, and 10 MJ/Nm3, respectively. The obtained syngas is a clean fuel with low sulfur-containing and nitrogen-containing. The experimental results provide an extensive comprehension of CPMW gasification in a plasma reactor and consider a possibility for hydrogen and energy production.

Chemical looping reforming of methane (CLRM) with Fe-based oxygen carriers is widely acknowledged as an environmentally friendly and cost-effective approach for syngas production, however, sintering-caused deactivate of oxygen carriers at elevated temperatures of above 900 °C is a longstanding issue restricting the development of CLRM. Here, in order to reduce the reaction temperature without compromising the chemical-looping CH4 conversion efficiency, we proposed a novel operation scheme of CLRM by manipulating the reaction pressure to shift the equilibrium of CH4 partial oxidation towards the forward direction based on the Le Chatelier\'s principle. The results from thermodynamic simulations showed that, at a fixed reaction temperature, the reduction in pressure led to the increase in CH4 conversion, H2 and CO selectivity, as well as carbon deposition rate of all investigated oxygen carriers. The pressure-negative CLRM with Fe3O4, Fe2O3 and MgFe2O4 could reduce the reaction temperature to below 700 °C on the premise of a satisfactory CLRM performance. In a comprehensive consideration of the CLRM performance, energy consumption, and CH4 requirement, NiFe2O4 was the Fe-based OCs best available for pressure-negative CLRM, especially for an excellent syngas yield of 23.08 mmol/gOC. This study offered a new strategy to address sintering-caused deactivation of materials in chemical looping from the reaction thermodynamics point of view.

Biosolids is a by-product of wastewater treatment that needs to be further processed. Traditional biosolids treatment and disposal technologies are inefficient under the current demanding standards. Thermochemical conversion technologies have been employed for biosolids management, with gasification being the most promising due to the production of syngas, a gaseous product that may be used for the production of energy or high-added-value substances through reforming reactions. Gasification is a complex thermochemical process; its performance and yield are strongly affected by the type of feedstock, but also by the system configuration and process conditions. Gasification usually takes place at temperatures between 700 and 1,200 °C, but it may also occur at lower temperatures (above 375 °C: supercritical water gasification) or at higher temperatures (above 3,000 °C: plasma gasification). The present review briefly presents the biosolids management practices, focusing on the gasification process and syngas treatment, while the state of the art in biosolids gasification is critically presented and discussed. A number of types of gasifiers (more frequently fluidized bed, but also fixed bed, rotary kiln, downdraft, etc.), gasifying agents, and operational conditions have been used for biosolids gasification. The key results of the study regarding biosolids gasification are: (i) the increase of temperature and equivalence ratio enhances the gasification performance, resulting in high syngas yield and quality, high cold gas efficiency, and low tar and char production; (ii) the calorific value of the obtained syngas tends to decrease with the increase of equivalence ratio; and (iii) the use of catalysts has been proven to substantially improve the gasification performance, compared to non-catalytic gasification. The proper selection of technical parameters determines the effectiveness of biosolids gasification, which is considered as a promising technology for the energy recovery from biosolids, so to upgrade wastewater treatment and improve environmental quality.

Exploring low-cost visible light photocatalysts for CO2 reduction to produce proportionally adjustable syngas is of great significance for meeting the needs of green chemical industry. A S-Scheme CeO2/g-C3N4 (CeO2/CN) heterojunction was constructed by using a simple two-step calcination method. During the photocatalytic CO2 reduction process, the CeO2/CN heterojunction can present a superior photocatalytic performance, and the obtained CO/H2 ratios in syngas can be regulated from 1 : 0.16 to 1 : 3.02. In addition, the CO and H2 production rate of the optimal CeO2/CN composite can reach 1169.56 and 429.12 μmol g-1 h-1, respectively. This superior photocatalytic performance is attributed to the unique S-Scheme photogenerated charge transfer mechanism between CeO2 and CN, which facilitates rapid charge separation and migration, while retaining the excellent redox capacity of both semiconductors. Particularly, the variable valence Ce3+/Ce4+ can act as electron mediator between CeO2 and CN, which can promote electron transfer and improve the catalytic performance. This work is expected to provide a new useful reference for the rational construction of high efficiency S-Scheme heterojunction photocatalyst, and improve the efficiency of photocatalytic reduction of CO2, promoting the photocatalytic reduction of CO2 into useful fuels.

Electrochemical carbon dioxide reduction reaction (eCO2RR) to generate syngas is an appealing strategy for CO2 net reduction. However, it suffers from the inferior faradaic efficiency (FE), selectivity, and difficult modulation of hydrogen/carbon monoxide (H2/CO) ratio. To address these issues, a series of magnesium-nickel (Mg-Ni) dual atomic catalysts with different Ni contents are fabricated on the nitrogen-doped carbon matrix (MgNiX-NC DACs) by one-step pyrolysis. MgNi5-NC electrocatalyst generates 0.51-0.79 H2/CO ratios in a potential range of -0.6 to -1.0 V vs. reversible hydrogen electrode (RHE) and the total FE reaches 100 % with good stability. While a wider range of H2/CO (0.95-4.34) is achieved for MgNi3-NC electrocatalyst in the same overpotential range, which is suitable for typical downstream thermochemical reactions. Introduction of Ni species accelerates the generation of CO, however, there is much less influence on the H2 production as compared with Mg-based single atomic electrocatalyst. According to the experimental results and density functional theory (DFT) calculations, the synergistic effect between Mg and Ni achieves the satisfied results rather than each one fulfill its own duty for selective producing H2 and CO, respectively. This work introduces a feasible approach to develop atomic catalysts on main group metal for more controllable CO2RR.

Covalent organic framework (COF) has attracted increasing interest in photocatalytic CO2 reduction, but it remains a challenge to achieve high conversion efficiency owing to the insufficient active site and fast charge recombination. Rationally optimizing the electronic structures of COF to regulate the local charge of active sites precisely is the key point to improving catalytic performance. Herein, intercalated single Co sites coordinated by imine-N motifs have been designed by using trinuclear copper-based imine-COFs with distinct electronic moieties via a molecular engineering strategy. It is confirmed that the charge delivery property and local charge distribution of these heterometallic frameworks can be profoundly influenced by electronic structures. Among these featured structures with mixed-state copper clusters, Co/Cu3-TPA-COF stands out for an exceptional photocatalytic CO2 reduction activity and tunable syngas (CO/H2) ratio by changing various bipyridines. Experimental and theoretical results indicate that interlayer Co-imine N motifs on the donor1-acceptor-donor2 structures facilitate the formation of a highly separated electron-hole state, which effectively induces the oriented electron transfer from dual electron donors to Co centers, achieving an enhanced CO2 activation and reduction. This work opens up an avenue for the design of high-performance COF-based catalysts for photocatalytic CO2 reduction.

The present paper compared, through life cycle assessment (LCA), the production of aviation biofuel from two hydrothermal routes of microalgae cultivated in wastewater. Hydrothermal liquefaction (HTL) and gasification followed by Fischer-Tropsch synthesis (G + FT) were compared. Both routes included biomass production, hydrotreatment for biofuel upgrading, and product fractionation. Secondary data obtained from the literature were used for the cradle-to-gate LCA. G + FT had a higher impact than HTL in the 18 impact categories assessed, with human carcinogenic toxicity exerting the most harmful pressure on the environment. The catalysts were the inputs that caused the most adverse emissions. The solvent used for bio-oil separation also stood out in terms of impacts. In HTL, emissions for global warming were -51.6 g CO2 eq/MJ, while in G + FT, they were 250 g CO2 eq/MJ. At the Endpoint level, HTL resulted in benefits to human health and ecosystems, while G + FT caused environmental damage in these two categories, as well as in the resources category. In the improvement scenarios, besides considering solid, aqueous, and gaseous products as co-products rather than just as waste/emissions, a 20% reduction in catalyst consumption and 90% recovery were applied. Thus, in HTL, 39.47 kg CO2 eq was avoided, compared to 35.44 kg CO2 eq in the base scenario. In G + FT, emissions decreased from 147.55 kg CO2 eq to the capture of 8.60 kg CO2 eq.

While the mild production of syngas (a mixture of H2 and CO) from CO2 and H2O is a promising alternative to the coal-based chemical engineering technologies, the inert nature of CO2 molecules, unfavorable splitting pathways of H2O and unsatisfactory catalysts lead to the challenge in the difficult integration of high CO2 conversion efficiency with produced syngas with controllable H2/CO ratios in a wide range. Herein, we report an efficient plasma-driven catalytic system for mild production of pure syngas over porous metal-organic framework (MOF) catalysts with rich confined H2O molecules, where their syngas production capacity is regulated by the in situ evolved ligand defects and the plasma-activated intermediate species of CO2 molecules. Specially, the Cu-based catalyst system achieves 61.9 % of CO2 conversion and the production of pure syngas with wide H2/CO ratios of 0.05 : 1-4.3 : 1. As revealed by the experimental and theoretical calculation results, the in situ dynamic structure evolution of Cu-containing MOF catalysts favors the generation of coordinatively unsaturated metal active sites with optimized geometric and electronic characteristics, the adsorption of reactants, and the reduced energy barriers of syngas-production potential-determining steps of the hydrogenation of CO2 to *COOH and the protonation of H2O to *H.