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

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.

Keywords: CO2 methanation; CO2 valorization; CeO2; ceria based oxides; gas to fuel technologies; methane dry reforming to syngas.

Figure 1

Proposed mechanism for CO₂ activation on ceria surface.

Figure 2

Two CO₂ adsorption configurations on reduced ceria (110) with in-plane (A) vacancy [Rv(InP)] and (B) split vacancy (Rp5). The surface oxygen nearest the vacancy is labeled as O_v. Both structural parameters and energies of adsorption are depicted (from Cheng et al., , reproduced with permission of AIP Publishing).

Figure 3

Schematic of solid FLPs on CeO₂(110) and CeO₂(100) constructed by surface oxygen vacancy regulation. White and red balls represent Ce and O atoms, respectively. Atoms labeled by blue circles represent the Lewis acid (Ce) or Lewis base (O) of solid FLPs. The position of oxygen vacancy is labeled by V_O in blue (Huang et al., , Reprinted with permission from American Chemical Society).

Figure 4

Reaction energy profile for the CH₄ → CH₃ + H reaction on isolated Ni atoms and Ni₄ clusters on the CeO₂(111) (A) and Ce₂O₃(0001) surfaces (B), in comparison to Ni(111). The structures shown to the left and right of the reaction pathways correspond to the side views of the optimized molecularly adsorbed and dissociated states used in the search of the transition state structure. All energies are relative to CH₄ in the gas phase. Reproduced from Lustemberg et al. (2016), with permission of ACS publications.

Figure 5

In presence of ceria the kinetics follows a Mars-van Krevelen mechanism, where active carbon is oxidized by CeO₂, while oxygen vacancies are replenished by O from CO₂ dissociation; the carbon fibers growth is hindered by CeO₂, which provides extra oxygen for their gasification. Adapted from Liang et al. (2018) with permission of ACS publications.

Figure 6

Strategies to stabilize and activate Ni/CeO₂ catalysts.

Figure 7

Methane dry reforming activity of Ni catalysts prepared by wet impregnation of supports of CeO₂, CeO₂ doped with Nd or Zr and of CeO₂ co-doped with Zr and Nd. (GHSW = 8,000 h⁻¹, CH₄/CO = 1.5). It was possible to obtain an improvement of ceria MDR activity only by co-doping, which affects both redox and acic-basic properties of support. Adapted from Pappacena et al. (2018), “Open Access” and licensed by the respective authors in accordance with the Creative Commons Attribution (CC-BY) license.

Scheme 1

Summary of concepts reviewed in this article.