Abstract
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.
Highlights
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 two-three decades (US, EPA, 2016; IPCC report, 2017)
Ambientpressure X-ray Photoelectron Spectroscopy (XPS) analysis and Scanning Tunneling Microscopy, (STM) characterization of NiO supported on CeO2(111) substrate revealed that when Ni atoms or small clusters are in tight contact with ceria the methane can be activated at room temperature and the dry reforming reaction become possible at a much lower temperature than that typically reported for conventional Ni based catalysts (427◦ C)
A strong bonding between Ni and CeO2 that inhibited Ni particle sintering was obtained via a pre-reduction of Ni/CeO2 in H2 in the temperature range of 500–700◦C, while higher temperatures of reduction (≥600◦C) induced decoration/encapsulation of Ni nanoparticles by a thin layer of reduced ceria support with partial coverage of Ni surface
Summary
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 two-three decades (US, EPA, 2016; IPCC report, 2017). Shape and size of the ceria nanocrystals can be designed and controlled by different strategies of preparation and treatments to boost redox properties and enhance catalytic activity (Wu et al, 2016; Trovarelli and Llorca, 2017; Ma et al, 2018) With this basis the review analyzes the role of ceria and ceria doped oxides in the reforming processes for the valorization of CO2, highlighting advantages and disadvantages of their use. It was demonstrated that following a high temperature (500◦C) reduction treatment the transient activity of a Rh/CeO2 catalyst was much higher compared to rhodium deposited on other oxides This behavior was attributed to the presence of oxygen vacancies on the surface of reduced ceria, that were filled by the water formed during reaction explaining the transient nature of the high activity recorded for Rh/CeO2. It is clear that different preparation routes and/or different metal loadings can change the way ceria interacts with the metal itself, affecting the reaction mechanism
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