New Trends in Catalysis for Sustainable CO₂ Conversion

Over the past few decades, there have been many advances in the world, leading to improvements in quality of life. Due to demographic and industrial growth, consumption has increased, as well as the amount of waste and pollution. Today, global warming and climate change are mainly attributed to the emission of anthropogenic greenhouse gases, with carbon dioxide (CO₂) being the most relevant one, due to the huge emissions of this gas into the atmosphere (mainly derived from the consumption of fossil fuels).

Carbon capture and storage (CCS) is a physical process consisting of separating the CO₂ (emitted by industry and the combustion processes for energy generation) and transporting it to geological storage to isolate it from the atmosphere in the long term. However, the most promising routes for CO₂ mitigation are those pursuing its catalytic valorization. By applying specific catalysts and suitable operating conditions, CO₂ molecules react with other components to form longer chains (i.e., hydrocarbons). Accordingly, effort should be made to catalytically valorize CO₂ (alone or co-fed with syngas) as an alternative way for reducing greenhouse gas emissions and obtaining high-value fuels and chemicals.

Carbon capture and utilization (CCU) is a developing field with significant demand for research in the following aspects:

• The development of new catalysts, catalytic routes, and technologies for CO₂ conversion;
• The study of new processes for obtaining fuels and chemicals from CO₂;
• Optimization of the catalysts and the reaction conditions for these processes;
• Further steps in advanced processes using CO₂-rich feeds (H₂ + CO₂ or CO₂ mixed with syngas), increasing product yields.

This Special Issue on “New Trends in Catalysis for Sustainable CO₂ Conversion” shows new research on the development of catalysts and catalytic routes for CO₂ valorization, and the optimization of the reaction conditions for the process.

This Special Issue includes ten articles and three reviews. In the paper by Amin et al., waste biomass is converted into activated carbon, and then a carbon-supported iron-based catalyst is prepared (Fe–C) [1]. The catalyst is used to assess the influence of temperature on the subsequent transformation of syngas to liquid fuels. Potassium is used as a structural promoter in the iron carbon-supported catalyst to boost catalyst activity and structural stability. Potassium promoter increases gasoline conversion from 36.4% (Fe–C) to 72.5% (Fe–C–K), and diesel conversion from 60.8% (Fe–C) to 80.0% (Fe–C–K). The influence of copper content and particle morphology on the performance of Cu/FeO_x catalysts in the hydrogenation of CO₂ is analyzed by Simkovičová et al. [2]. All the catalysts, with a copper content between 0 and 5 wt%, are found to be highly efficient, with CO₂ conversion reaching 36.8%. Their selectivity towards C₁ (versus C₂–C₄, C₂–C₄₌, and C₅₊ products) is dependent on the catalyst’s composition, and morphology, and on the temperature. The results indicate new potential methods of altering the morphology and composition of iron-oxide-based particles. The electrolysis of CO₂ in molten carbonate as an alternative mechanism to synthesize carbon nanomaterials is introduced in the paper by Liu et al. [3]. This study focuses on controlled electrochemical conditions in molten lithium carbonate to split CO₂ absorbed from the atmosphere into carbon nanotubes (CNT), and into various macroscopic assemblies of CNT, which may be useful for nano-filtration. Different CNT morphologies are prepared electrochemically by varying the anode’s and cathode’s composition and architecture, varying the electrolyte’s composition pre-electrolysis processing, and varying the application and density of currents. The paper by Liu et al. shows that CO₂ can be transformed into distinct nano-bamboo, nano-pearl, nano-dragon, solid and hollow nano-onion, nano-tree, nano-rod, nano-belt, and nano-flower morphologies [4]. The capability to produce these unusual nanocarbon allotropes at high purity by a straightforward electrolysis process opens an array of inexpensive unique materials with high strength, electrical and thermal conductivity, flexibility, charge storage, lubricant, and robustness properties. A new photocatalyst for CO₂ reduction is presented by Morawski et al. [5]. The photocatalyst is prepared from a combination of a commercial TiO₂ with a mesopore structure and carbon spheres with a microporous structure with high CO₂ adsorption capacity. The combined TiO₂–carbon spheres/silica cloth photocatalysts show higher efficiency in the two-electron CO₂ reduction towards CO than in the eight-electron reaction to methane. The 0.5 g graphitic carbon spheres combined with 1 g of TiO₂ results in almost 100% selectivity to CO. In the article by Li et al., a series of Ni-xSi/ZrO₂ catalysts (x = 0, 0.1, 0.5, 1 wt% of Si) are prepared by co-impregnation with ZrO₂ as support and Si as a promoter [6]. The effect of different amounts of Si on the catalytic performance is investigated for CO₂ methanation. It is found that adding the appropriate amount of Si improves the catalytic performance of the Ni/ZrO₂ catalyst at a low reaction temperature (250 °C). It is observed that Si enhances the interaction between Ni and ZrO₂ and increases Ni dispersion, the number of active sites (including surface Ni⁰), the number of oxygen vacancies, and the number of strong basic sites on the catalyst surface. Sim et al. report efficient CO₂ decomposition results using a nonperovskite metal oxide, SrFeCo_0.5O_x, in a continuous-flow system [7]. In this study, the authors obtain enhanced efficiency, reliability under isothermal conditions, and catalytic reproducibility through cyclic tests using SrFeO_3-δ. Activated oxygen-deficient SrFeO_3−δ decomposes CO₂ into carbon monoxide (CO) and carbon (C). In the work by Ju et al., a new type of electrolyte solution constituted by ionic liquids and propylene carbonate is used as the cathodic solution to study the conversion of CO₂ on an Ag electrode [8]. Linear sweep voltammetry and Tafel characterization indicate that the chain length of 1-alkyl-3-methyl imidazolium cation has strong influences on the catalytic performance for CO₂ conversion. Electrochemical impedance spectroscopy shows that the imidazolium cation absorbed on the Ag electrode surface stabilizes the anion radical (CO₂^•−), leading to the enhanced efficiency of CO₂ conversion. The synthesis and characterization of both binary (Cu₂O, Fe₂O₃, and In₂O₃) and ternary (Cu₂O–Fe₂O₃ and Cu₂O–In₂O₃) transition metal mixed-oxides as photocatalysts for solar-driven CO₂ conversion into energy-rich species is reported by Marcolongo et al. [9]. Two different preparation techniques (high-energy milling and coprecipitation) are compared. The composition and synthetic methodologies of mixed-oxides, the reactor geometry, and the method of dispersing the photocatalyst sample play key roles in the light-driven reaction of CO₂–H₂O. Willauer et al. study the behavior on CO₂ hydrogenation of an iron-based catalyst in a commercial-scale fixed-bed reactor under different feed rates and product recycling conditions [10]. CO₂ conversion increases from 26% to as high as 69% by recycling a portion of the product stream, and CO selectivity is greatly reduced from 45% to 9% in favor of hydrocarbon production. In addition, the catalyst is successfully regenerated for optimum performance. The review by Weidlich and Kamenická summarizes new developments in the ring opening of epoxides with a subsequent CO₂-based formation of cyclic carbonates [11]. The review highlights recent and major developments for sustainable CO₂ conversion from 2000 to the end of 2021. The syntheses of epoxides, especially from bio-based raw materials, are described, such as the types of raw materials used (vegetable oils or their esters) and the reaction conditions. The aim of this review is also to compare the types of homogeneous non-metallic catalysts. Weber et al. review recent advances in the mitigation of catalyst deactivation of CO₂ hydrogenation to light olefins [12]. The authors provide a brief summary of the two dominant reaction pathways (CO₂–Fischer–Tropsch and methanol-mediated pathways), mechanistic insights, and catalytic materials for CO₂ hydrogenation to light olefins. Then, they describe the main deactivation mechanisms caused by carbon deposition, water formation, phase transformation, and metal sintering/agglomeration. The main focus of this review is to provide a useful resource for researchers to correlate catalyst deactivation and the recent research effort on catalyst development for enhanced olefin yields and catalyst stability. CO₂ oxidative dehydrogenation is a greener alternative to the classical dehydrogenation method. The availability, cost, safety, and soft oxidizing properties of CO₂, with the assistance of appropriate catalysts at an industrial scale, can lead to breakthroughs in the pharmaceutical, polymer, and fuel industries. The review by Rahman et al. focuses on several applications of CO₂ in oxidation and oxidative dehydrogenation systems [13]. These processes and catalytic technologies reduce the cost of utilizing CO₂ in chemical and fuel production, which may lead to commercial applications in the imminent future.

In summary, these manuscripts clearly show the relevance of CO₂ conversion for the production of fuels and raw materials, avoiding CO₂ emission into the atmosphere and reducing global warming. Nowadays, efforts are being made on the co-feeding of CO₂ with syngas and on the use of new catalytic processes for CO₂ conversion under mild reaction conditions.

We would like to thank all the authors for their works and the reviewers for improving the quality of the papers with their comments. We are honored to be the Guest Editors of this Special Issue. We are also grateful to all the staff of the Catalysts Editorial Office.