Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization

REVIEW published: 14 February 2019 doi: 10.3389/fchem.2019.00028 Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization Marta Boaro*, Sara Colussi and Alessandro Trovarelli Dipartimento Politecnico, Università di Udine, Udine, Italy Edited by: Cristina Artini, Università di Genova, Italy Reviewed by: Miguel Ángel Cauqui, University of Cádiz, Spain Radoslaw Debek, UMR6506 Laboratoire Catalyse et Spectrochimie (LCS), France Patrizia Frontera, Mediterranea University of Reggio Calabria, Italy *Correspondence: Marta Boaro Specialty section: This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry Received: 21 September 2018 Accepted: 11 January 2019 Published: 14 February 2019 Citation: Boaro M, Colussi S and Trovarelli A (2019) Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization. Front. Chem. 7:28. doi: 10.3389/fchem.2019.00028 Frontiers in Chemistry | www.frontiersin.org Reducing greenhouse emissions is of vital importance to tackle the climate changes and to decrease the carbon footprint of modern societies. Today there are several technologies that can be applied for this goal and especially there is a growing interest in all the processes dedicated to manage CO2 emissions. CO2 can be captured, stored or reused as carbon source to produce chemicals and fuels through catalytic technologies. This study reviews the use of ceria based catalysts in some important CO2 valorization processes such as the methanation reaction and methane dry-reforming. We analyzed the state of the art with the aim of highlighting the distinctive role of ceria in these reactions. The presence of cerium based oxides generally allows to obtain a strong metal-support interaction with beneficial effects on the dispersion of active metal phases, on the selectivity and durability of the catalysts. Moreover, it introduces different functionalities such as redox and acid-base centers offering versatility of approaches in designing and engineering more powerful formulations for the catalytic valorization of CO2 to fuels. Keywords: ceria based oxides, CeO2 , CO2 methanation, CO2 valorization, methane dry reforming to syngas, gas to fuel technologies INTRODUCTION For the first time since the Industrial Revolution the global CO2 atmospheric concentration have reached the threshold of 400 parts per million, increasing the average world temperature by 1.5◦ C within the next two-three decades (US, EPA, 2016; IPCC report, 2017). This poses a threat upon the environment and a great challenge for today’s society, that must combine the drive toward a continued economic growth and ever-increasing demand of energy and chemicals with the need to preserve the environment for future generations. Among the main strategies which have been considered to reduce or minimize CO2 atmospheric emissions, one is its capture and storage (Araújo and de Medeiros, 2017). Carbon capture and storage (CCS) comprises separation of CO2 from industrial sources, compression and transportation to a geologic site for storage, or to enhanced oil recovery. Taking into account that the potential sequestration capability and the industrial use of CO2 are more than 150 times lower than its production, the CCS approach should be adopted only if we are able to develop technologies to convert efficiently CO2 into chemicals or fuels, in an alternative strategy, identified as CCSU, CO2 capture-storageutilization (Cormos et al., 2018). Therefore, in an envision of a free-carbon footprint circular economy, CO2 should substitute fossil carbon as feedstock to produce fuels and chemicals, while solar, wind, and geothermal sources should be employed for the production of electricity and H2 (Martens et al., 2017). Nowadays, there are many emerging technologies based on chemical catalysis, electrocatalysis, photocatalysis, 1 February 2019 | Volume 7 | Article 28 Boaro et al. Ceria-Based Materials for CO2 Valorization and biocatalysis, but they are still not mature to make realistic the transition toward a mixed carbon-hydrogen economy (Kondratenko et al., 2013). Since the fuel market is many times larger than the chemicals market, several efforts are directed to develop technologies to recycle back CO2 emissions to a synthetic transportable fuel. In this field, the highest readiness have been achieved by the technologies based on catalytic processes such as the reverse water gas shift (CO2 +H2 ↔CO+H2 O, RWGS), the methane dry reforming (CH4 + CO2 ↔2CO + 2H2 , MDR), the methanation of CO2 (CO2 + 4H2 ↔CH4 + 2H2 O, CM), and CO2 conversion to oxygenates (Götz et al., 2016). The methane dry reforming reaction is an endothermic process that occurs at high temperature (>800◦ C). This implies that the catalysts employed (mainly Ni, Co and related alloys) can sinter and coke due to CH4 cracking, moreover they can be deactivated by the presence of sulfurous compounds in the stream (Lavoie, 2014). The direct hydrogenation of CO2 to methane is instead an exothermic process, thus thermodynamically favored at low temperature. A low operating temperature poses kinetic constrains which require efficient catalysts to be overcome. Various metals of the group VIIIB in the periodic table have been tested as catalysts for this reaction. Ru resulted one of the most active and selective as well as highly resistant to oxidizing atmosphere, however its high price limits its industrial application (Su et al., 2016). Nickel has been proved to be a valid alternative, especially for its low cost. The main disadvantage in using Ni is its high tendency to oxidize in the operating atmosphere, to poison in presence of sulfur gases and to volatilize forming nickel carbonyls, which are very toxic (Rönsch et al., 2016). The reverse water gas shift reaction (RWGS) is a slightly endothermic process promoted mainly by copper based and supported ceria catalysts. Its main advantage is the formation of CO, which can be used as building block in other processes such the Fisher-Tropsch and the methanol syntheses. The production of methanol through the RWGS resulted competitive against that obtained by a direct hydrogenation of CO2 , which is another possible reaction recently investigated to valorize CO2. Also in this case, the main issues are due to the need of a highly selective catalyst resistant to sintering and to poisoning due to coke and sulfur deposition (Daza and Kuhn, 2016). A common feature of the above mentioned reactions is the simultaneous occurrence of several equilibrium reactions that may limit the selectivity of the process considered. Moreover, they require a highly active catalyst since CO2 is a very stable molecule. An important aspect in developing suitable catalysts to overcome the issues of these processes is the choice of an appropriate support. The support has a pivotal function in controlling the morphology and the oxidation state of the meta (...truncated)