Elsevier

Journal of Energy Chemistry

Volume 71, August 2022, Pages 547-561
Journal of Energy Chemistry

Optimizing the oxide support composition in Pr-doped CeO2 towards highly active and selective Ni-based CO2 methanation catalysts

https://doi.org/10.1016/j.jechem.2022.04.003Get rights and content

Abstract

In this study, Ni catalysts supported on Pr-doped CeO2 are studied for the CO2 methanation reaction and the effect of Pr doping on the physicochemical properties and the catalytic performance is thoroughly evaluated. It is shown, that Pr3+ ions can substitute Ce4+ ones in the support lattice, thereby introducing a high population of oxygen vacancies, which act as active sites for CO2 chemisorption. Pr doping can also act to reduce the crystallite size of metallic Ni, thus promoting the active metal dispersion. Catalytic performance evaluation evidences the promoting effect of low Pr loadings (5 at% and 10 at%) towards a higher catalytic activity and lower CO2 activation energy. On the other hand, higher Pr contents negate the positive effects on the catalytic activity by decreasing the oxygen vacancy population, thereby creating a volcano-type trend towards an optimum amount of aliovalent substitution.

Graphical abstract

Pr doping of CeO2 up to 10at% (Ce0.9Pr0.1O2-δ) in Ni/CeO2 increases the oxygen vacancy population and reduces the Ni particle size, improving CO2 methanation activity and lowering the activation energy.

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Introduction

The dreadful effects that anthropogenic climate change will have on ecosystems and human societies alike has led to a spike of research interest in carbon capture and utilization (CCU) technologies [1]. Furthermore, safety concerns, as well as the high costs and material requirements associated with H2 storage and transportation motivates the search for alternative energy-rich molecules (energy buffers) to apply in the so-called “Power-to-Gas” processes [2], [3]. The CO2 methanation reaction (Eq. (1)) attempts to tackle both problems, by utilizing CO2 captured from flue gases of conventional fossil-fuel powered units and green H2 produced via electrolysis using electricity harnessed from renewable sources, into CH4 or synthetic natural gas [4], [5], [6].CO2 + 4H2 → CH4 + 2H2O

CH4 is a gas with a much higher volumetric energy density than H2 (by a factor of 3.2 at any given pressure [7]), can be easily stored and transported and thus constitutes a perfect candidate as an energy buffer for storing intermittent renewable energy [3], [4]. CO2 emitted from the combustion of synthetic natural gas is compensated by the initial CO2 capture step, thus creating a closed carbon process with net zero CO2 emissions [4], [6].

The most common metals that can catalyze the transformation of CO2 and H2 into CH4 are Rh, Ru and Ni [8], [9], [10]. Rh and Ru are precious (or noble) metals and thus Rh- and Ru-based catalysts are associated with much higher costs compared to Ni-based ones, which makes them unsuitable for industrial implementation [10]. Although the metallic Ni surface alone is generally fairly active towards catalyzing the CO2 methanation reaction [11], the choice of the metal oxide support in Ni catalysts can drastically impact the catalytic activity [12]. Ni/CeO2-type catalysts are considered to be by far the most active and selective catalysts for CO2 methanation, compared to Ni catalysts supported on other types of supports (e.g., Al2O3 or SiO2), a property attributed to the great oxygen mobility of the CeO2 support, which originates from its rich defect chemistry (Ce4+ ↔ Ce3+ cycle) [8], [9], [13]. Cárdenas-Arenas et al. [14] have indicated that the CO2 methanation mechanism over Ni/CeO2 differs from Ni/Al2O3, with Ni/CeO2 exhibiting different sites for H2 (metallic Ni surface) and CO2 (NiO-CeO2 interface) dissociation, while also being able to transport oxygen species throughout the CeO2 lattice and thereby not blocking the catalytically active sites. Therefore, the use of a CeO2-based support is often considered essential in order to guarantee a sufficient catalytic activity for Ni-based catalysts [8], [9], [10], [13], [14].

The beneficial contribution of defect sites and especially oxygen vacancies that are located at the support surface during the CO2 methanation reaction has been addressed by numerous works in the literature [15], [16], [17], [18], [19]. Therefore, a rational strategy to improve the CO2 methanation performance of Ni/CeO2 catalysts would be to try to increase the oxygen vacancy population in the CeO2 support [16]. This can most notably be achieved via aliovalent substitution of Ce4+ cations in the CeO2 lattice with other metal cations of lower valence, e.g., Y3+, La3+, Pr3+ or Sm3+ [16], [18], [19]. Pr can readily dissolve into the crystalline lattice of CeO2 and, like Ce, can adopt both +4 and +3 oxidation states, the difference being that Pr4+ reduces far easier to Pr3+ compared to the Ce4+ → Ce3+ transition [20], [21]. Therefore, it has been reported that Pr doping of CeO2 up to around 10 at%–20 at% (x = 0.1–0.2 in Ce1−xPrxO2−δ solid solutions) can greatly enhance the population of oxygen vacancies and oxygen uptake of the mixed oxide [20]. The substitution of Ce4+ cations in the CeO2 lattice with Pr3+ ones can be described by the following Kröger-Vink equation (Eq. (2)).Pr2O3(CeO2)2PrCe+VO··+3OOx

Pr-doped CeO2 oxides have been applied in many catalytic applications, either as stand-alone oxidation catalysts, or as oxide supports for other metal-based catalysts (e.g., Ni). Zhang et al. [22] employed Pr-doped CeO2 in the Prins condensation-hydrolysis of isobutene with formalin and noted that the increased population of surface oxygen vacancies, which peaked for a Pr/(Pr + Ce) atomic ratio of 0.2, promoted the catalytic performance via enhancing the adsorption of HCHO. Regarding Ni/Pr-CeO2 catalysts, Makri et al. [23] found that an enhanced transfer of lattice oxygen species in Pr-doped CeO2 could promote carbon gasification during the dry reforming of methane, whereas Xiao et al. [24] reported that Pr doping of CeO2 up to 20 at% could increase the oxygen vacancy population, decrease the crystallite size of Ni and thereby improve the catalytic activity, stability and coking resistance during the steam reforming of ethanol. Concerning the CO2 methanation reaction, Siakavelas et al. [18], [19] prepared Ni catalysts supported on CeO2 and La2O3-CeO2, modified with Mg, Sm and Pr (10 at%) and showed that the increase in the population of surface oxygen vacancies (especially upon Pr modification) was the property that led to the rise in catalytic activity. Finally, Rodriguez et al. [25] investigated the effect of Pr content on Ru/CeO2 catalysts and found that a low Pr loading (3 wt%) had a positive influence during CO2 methanation by enhancing the oxygen mobility of the CeO2 support, whereas high Pr loadings (25 wt%) negatively impacted the initial chemisorption and dissociation of CO2, which takes place at the Ru-CeO2 interface.

This work investigates the effect of the Pr content in the CeO2 support of Ni-based catalysts, which to the best of our knowledge has not yet been studied for the CO2 methanation reaction. To examine the physicochemical properties of the mixed metal oxide supports and reduced catalysts, a multitude of characterization methods is employed, namely X-ray diffraction (XRD), N2 physisorption, H2-temperature-programmed reduction (H2-TPR), CO2-temperature programmed desorption (CO2-TPD), H2-TPD, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS). Catalytic performance is evaluated at the temperature region of 200–500 °C and the CO2 activation energy of the catalysts is calculated. The catalytic stability is evaluated under 24 h time-on-stream experiments and the spent catalysts are investigated to examine potential degradation effects (e.g., carbon deposition or nanoparticle sintering). Moreover, the reaction pathway is investigated via in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments.

Section snippets

Catalyst preparation

Pr-doped CeO2 oxide supports (Ce1-xPrxO2-δ, x = 0, 0.05, 0.1, 0.2, 0.5) were prepared via the citrate sol–gel method. Calculated amounts of Ce(NO3)3·6H2O (Sigma-Aldrich, 99%) and Pr(NO3)3·6H2O (Sigma-Aldrich, 99.9%) were initially dissolved in 100 mL of d-H2O under stirring. Citric acid (Fluka, 99.5%) was then introduced to the metal nitrate aqueous solution at a molar ratio of citric acid to total metal cations equal to 1.5. The solution was then heated up at 80 °C until the evaporation of

Catalytic performance as a function of reaction temperature

The CO2 methanation performance of the prepared catalysts was first evaluated under Experimental Protocol #1, i.e., with a WHSV of 25000 mL gcat−1h−1. The results regarding CO2 conversion and CH4 selectivity are presented in Fig. 1, whereas CH4 yield can be found in Fig. S1. The dotted lines present in the graphs represent thermodynamic equilibrium calculations using Aspen Plus for ratio H2:CO2 = 4:1 and total pressure 1 atm. A comparison of the catalytic activity of the five prepared catalysts

Conclusions

In this work, the effect of aliovalent doping of the CeO2 support in Ni/CeO2 catalysts with Pr was investigated. It was found that substituting a small part of Ce4+ lattice cations with Pr3+ ones (ideally 10 at%) can boost the oxygen vacancy population and decrease the Ni nanoparticle size, thus promoting the availability of active CO2 and H2 chemisorption sites. A volcano-type trend was unveiled, where a small degree of Pr doping can increase the catalytic activity and lower the CO2 activation

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

AIΤ, NDC and MAG acknowledge support of this work by the project “Development of new innovative low carbon energy technologies to improve excellence in the Region of Western Macedonia” (MIS 5047197) which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Program “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

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