Proceeding Paper Cobalt Nanocomposites as Catalysts for Carbon Dioxide Conversion to Methanol † Javier Moral-Vico * , Anna Carrasco García and Antoni Sánchez GICOM, Departament of Chemical, Biological and Environmental Engineering, Escola d’Enginyeria, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain; anna.carrasco@uab.cat (A.C.G.); antoni.sanchez@uab.cat (A.S.) * Correspondence: antoniojavier.moral@uab.cat † Presented at the 1st International Electronic Conference on Processes: Processes System Innovation, 17–31 May 2022; Available online: https://sciforum.net/event/ECP2022. Abstract: Carbon capture and utilisation (CCU), has arisen as an alternative to the reduction of CO2 concentration in the atmosphere by converting it into value-added products. CO2 conversion to methanol presents certain drawbacks, such as high pressure and temperature conditions and, to solve these issues, new materials are being investigated. Among them, cobalt stands out due to its abundance and low price compared to noble metals. Cobalt and its oxides exhibit interesting electronic and magnetic properties and are being used as catalysts in a wide range of reactions. In this work, we present a systematic comparison of different cobalt and cobalt oxide nanocomposites in terms of their efficiency as catalysts for CO2 hydrogenation to methanol, and how porous and non-porous supports can enhance their catalytic capacity. For this purpose, a fixed bed reactor operating with continuous flow is used, under mild temperature (160–260 ◦ C) and pressure (10–15 bar) conditions. Several parameters are measured to evaluate the efficiency of the catalysis: CO2 conversion; space–time yield (STY), which indicates the methanol production yield per mass unit of catalyst and reaction time, and methanol selectivity, which evaluates the production of reaction side products such as carbon monoxide. How the adsorption capacity provided by the porous supports can enhance the catalytic capacity of cobalt and cobalt oxide is confirmed, as well as how porous supports such as zeolite and graphene clearly improve this capacity compared with a non-porous support such as silicon dioxide. Citation: Moral-Vico, J.; García, A.C.; Sánchez, A. Cobalt Nanocomposites Keywords: methanol; nanocomposites; heterogeneous catalysis; cobalt as Catalysts for Carbon Dioxide Conversion to Methanol. Eng. Proc. 2022, 19, 32. https://doi.org/ 10.3390/ECP2022-12614 1. Introduction Academic Editor: Antoni Sánchez In 2018, 33 gigatons of CO2 were released into the atmosphere. This was enough Published: 17 May 2022 to provoke an increase in the concentration of this gas in the atmosphere, from 280 ppm to 410 ppm [1]. CO2 is one of the main causes of global warming, which stands as one Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in of the main concerns that humanity is facing at present. For this reason, research on published maps and institutional affil- reducing the presence of CO2 is important. Part of this research focuses on capturing iations. gas to store it and then inject it underground. These are the so-called carbon capture and storage (CCS) methods. These processes, while not entirely environmentally sustainable because they require the use of fossil fuels as an energy source, are such efficient methods that they could result in an almost 20% reduction in carbon dioxide concentration in the Copyright: © 2022 by the authors. atmosphere. However, CCS methods have a very high cost when building facilities at the Licensee MDPI, Basel, Switzerland. industrial level, and 75% of the total costs are dedicated to the CO2 capture method [2]. This article is an open access article Due to the high costs of the CCS methods, carbon capture and utilization (CCU) methods distributed under the terms and are arising as more economically viable processes to address the excess of atmospheric conditions of the Creative Commons CO2 concentrations. These methods are based on the use of CO2 to obtain other valuable Attribution (CC BY) license (https:// products, such as chemical precursors and renewable fuels. Methanol stands out due to creativecommons.org/licenses/by/ its utility as a feedstock for obtaining other chemical products and its use as a renewable 4.0/). Eng. Proc. 2022, 19, 32. https://doi.org/10.3390/ECP2022-12614 https://www.mdpi.com/journal/engproc Eng. Proc. 2022, 19, 32 2 of 4 utility as a feedstock for obtaining other chemical products and its use as a renewable Eng. Proc. 2022, 19, 32 energy source [3]. In this framework, the conversion from CO2 to methanol 2has of 3 been demonstrated to be efficiently achieved in heterogeneous catalytic systems. Many different catalysts exist for the completion of the CO2 hydrogenation-to-meth- energy anol reaction. source [3]. In this However, framework, many of themthe conversion have fromasCO high costs, to methanol in2the has been and case of palladium demonstrated to be efficiently achieved in heterogeneous catalytic systems. gold [4]. Other metals and metallic oxides that entail lower costs are under study; copper Many different catalysts exist for the completion of the CO2 hydrogenation-to-methanol stands out due to its abundance on earth. Many copper catalysts have been synthesized reaction. However, many of them have high costs, as in the case of palladium and gold [4]. to optimize the interfaces between copper and metal oxide supports, Other metals and metallic oxides that entail lower costs are under study; copper due tostands copper’s out low effectiveness in activating due to its abundance on earth.CO 2 when Many copperused alone. catalysts The synthesized have been clearest example to optimize is the Cu/ZnO/Al the interfaces between copper and metal oxide supports, due to copper’s low effectiveness CO2 2O 3 catalyst, which is used industrially for the hydrogenation of CO and [5].inHowever, activating copper CO2 whencompounds used alone.deactivate The clearestunder CO2is’sthe example conversion Cu/ZnO/Al to 2methanol O3 catalyst,condi- which tions [6], isleading used industrially for the to increased hydrogenation interest in studyingof CO andearth-abundant other CO2 [5]. However, copperthat metals com- are al- pounds deactivate under CO ’s conversion to methanol conditions [6], leading ready used for another type of catalysis [7]. Cobalt meets these conditions, which makes it 2 to increased an interest interestingin studying other earth-abundant metals that are already used for another type material for study in its elemental state and as an oxide, and both materials of catalysis [7]. Cobalt meets these conditions, which makes it an interesting material for have been studied with different supports, such as graphene, zeolite, and silicon dioxide. study in its elemental state and as an oxide, and both materials have been studied with different supports, such as graphene, zeolite, and silicon dioxide. 2. Results and Discussion 2. The Results and Discussion catalytic activity of the different cobalt compounds was investigated for the cat- The catalytic alytic conversion of activity of the different CO2 to methanol. cobalttests Catalytic compounds was investigated were performed at a mildforoperating the catalytic conversion pressure (10 bar) and a flow of CO 2 to methanol. Catalytic tests were performed at a mild operating rate of 10 mL/min, with temperature varying between 180 pressure (10 bar) and a flow rate of 10 mL/min, with temperature varying between 180 and 260 °C.◦ Five different cobalt compounds were tested: elemental cobalt, cobalt oxide and 260 C. Five different cobalt compounds were tested: elemental cobalt, cobalt oxide (Co(Co 3O4), cobalt oxide supported on zeolite (Co3O4—zeolite), cobalt oxide supported on 3 O4 ), cobalt oxide supported on zeolite (Co3 O4 —zeolite), cobalt oxide supported on graphene graphene (Co(Co3O4—graphene), and cobalt oxide supported on silicon dioxide (Co3O4— 3 O4 —graphene), and cobalt oxide supported on silicon dioxide (Co3 O4 — SiOSiO 2). Their catalytic results were expressed in terms of space–time yield (STY), which 2 ). Their catalytic results were expressed in terms of space–time yield (STY), which expresses expresses thethemethanol methanolproduction productionperper gram gram of catalyst andhour catalyst and hourofofexperiment, experiment,withwith re- sults shown results shown in Figure in Figure1. 1.AsAsexpected, expected, the results resultsofofelemental elemental cobalt cobalt showshow the lowest the lowest amount amount ofofmethanol methanol that that was wasobtained, as as obtained, single metals single are normally metals less active are normally lessthan oxides active than ox- or bimetallic systems ides or bimetallic systems [8].[8]. 2.50 2.00 STY (mg metOH/gcat·h) 1.50 1.00 0.50 0.00 200 210 220 230 240 250 260 T(°C) Cobalt Oxide Elemental Co Cobalt Oxide - Zeolite Cobalt Oxide - Graphene Cobalt Oxide - SiO2 Figure 1. STY results of different cobalt compounds. Tests were performed at 10 bar. Figure 1. STY results of different cobalt compounds. Tests were performed at 10 bar. Eng. Proc. 2022, 19, 32 3 of 3 The use of a support enhances methanol production, since STY is increased for all materials with a support. However, the use of porous supports is beneficial for the methanol synthesis. This is in agreement with the literature, where it has been demonstrated that the presence of porous materials improves CO2 adsorption and the distribution of the active sites [9]. In terms of conversion and selectivity, the results for cobalt compounds are not much better than those observed in the literature for zinc or copper oxides, and no significant differences were found between the different studied cobalt materials. 3. Conclusions Cobalt was shown to be a good catalyst for the conversion from CO2 to methanol, and cobalt oxide has a better performance than the elemental cobalt material. The use of a support was studied, confirming the enhancement of cobalt oxide performance in terms of catalysis when supports are used. This effect is improved when the supports are porous materials, such as zeolite and graphene. Author Contributions: Conceptualisation, A.S. and J.M.-V.; methodology, A.C.G.; validation, J.M.-V. and A.C.G.; formal analysis, A.C.G. and J.M.-V.; investigation, A.C.G. and J.M.-V.; resources, A.S. and J.M.-V.; data curation, A.C.G.; writing—original draft preparation, J.M.-V.; writing—review and editing, A.C.G., J.M.-V. and A.S.; visualisation, A.C.G.; supervision, J.M.-V. and A.S.; project administration, A.S.; funding acquisition, A.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Fundación Ramón Areces in the hallmark of the META2NOL project granted in the XIX National Contest of Life and Matter Sciences Projects, grant number CIVP19A5952. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: Anna Carrasco García thanks UAB for the PIF scholarship granted for the completion of her PhD thesis. Conflicts of Interest: The authors declare no conflict of interest. References 1. International Energy Agency. Key World Energy Statistics 2019. Available online: https://www.iea.org/publications/ freepublications/publication/KeyWorld2019.pdf (accessed on 10 April 2022). 2. Yadav, S.; Mondal, S.S. A review on the progress and prospects of oxy-fuel carbon capture and sequestration (CCS) technology. Fuel 2022, 308, 122057. [CrossRef] 3. Ott, J.; Gronemann, V.; Pontzen, F.; Fiedler, E.; Grossmann, G.; Kersebohm, D.B.; Weiss, G.; Witte, C. Methanol-An Industrial Review. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH GmbH: Weinheim, Germany, 2012. 4. Fan, L.; Fujimoto, K. Development of active and stable supported noble metal catalysts for hydrogenation of carbon dioxide to methanol. Energy Convers. Manag. 1995, 36, 633–636. [CrossRef] 5. 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