Biomethane reforming over Ni catalysts supported on PrO2-ZrO2 solid-solutions
Introduction
Human activities lead to the dumping of a huge amount of organic matter that accelerates the emissions of greenhouse gases resulting in the alteration of the balance of the environmental system. The exploration of organic matter for energy production has been enlarged significantly nowadays aiming to reach sustainable development [1]. The widely known anaerobic digestion process applied to organic matter, at the present is applied to any wastes of vegetable and animal origin (ex. agricultural, and municipal solid wastes) to obtain biogas for energy uses and to encourage waste reduction and sustainability [2].
Biogas can be considered a first-generation biofuel, and its production is very attractive owing to its low production cost and is ecofriendly when compared to the other biofuels. The two major components of biogas are CH4 and CO2 (CH4:CO2 ratio depends on the substrate), and other products as minor components (H2S, NH3, and H2) [3], [4]. The methane obtained by the purification process of biogas (to clean principally NH3 and H2S) is denominated biomethane. One different way to explore the potential of biogas or biomethane is the production of Syngas (H2/O2), as demonstrated previously in references [3], [5], [6], [7], [8].
The direct reforming of biogas occurs by Dry Reforming of Methane (DRM, Reaction 1) because biogas is rich in CH4 and CO2 (CH4 is in excess in this case). Partial Oxidation of Biomethane occurs by the addition of oxygen in a sufficient amount to reach a molar ratio of 2CH4:1 O2 (Partial Oxidation of Methane, POM, Reaction 2). To realize the Oxidative Reform of Biogas, the addition of O2 to the biogas is needed to partially oxidize the excess of methane as shown in Reaction 3 (Oxidative Reform of Methane, ORM).DRM: 1.5 CH4 + CO2 → 2 CO + 2H2 + 0.5 CH4 ∆Hº= 260.5 kJ mol−1POM: 2 CH4 + O2 → 2 CO + 4H2 ∆Hº= −22.6 kJ mol−1ORM: 1.5CH4 + CO2 + 0.25O2 → 2.5 CO + 3 H2 ΔHº = 249.2 kJ mol−1
Syngas (H2/CO = 1,2,3.) is a very important starting material for a wide number of products such as hydrogen, methanol, liquid fuels (through Fischer-Tropsch process), Di-methyl ether (DME), ammonia, among others. The Syngas produced from reactions 1, 3 (DRM and ORM) is environmentally attractive since the two main greenhouse gases (in this case CH4 and CO2) are converted into a highly valuated raw material. The Syngas production from biomethane or biogas (Reactions 1–3) is an attractive eco-friendly process and turns it sustainable.
To enable the biogas or biomethane reforming process to produce Syngas, a good catalyst is needed. Transition metals of Group VIIIB are good catalysts for methane reforming (Ex. Fe, Ni, and Co) showing high efficiency in the conversion of methane; special Ni-based catalysts have been extensively used in Steam Reforming Methane (SRM), Dry Reforming of Methane (DRM), and Partial Oxidation of Methane (POM). The industrial process of SRM to produce Syngas (often followed by Water Gas Shift Reaction to produce a hydrogen-rich mixture) uses Ni-catalyst (supported on Al2O3); however, this catalyst causes some troubles related to carbon deposits (which leads to catalyst deactivation and a dangerous increase of the reactor pressure). Among different ways to minimize the carbon deposition rates in these cases we can list: the use of zirconia-based solid solutions as catalyst supports [7], [9], [10], [11], decreasing the crystallite size of the active metal catalyst [12], [13], [14], the use of Noble-metal promoters in non-noble catalysts [8], [12], among others. Solid solutions based on praseodymium have excellent redox properties and oxygen storage capacity which are very good for oxidation reactions and may be interesting in the reforming of biomethane or biogas [15], [16], [17]. In this connection, it possible to affirm that the Nickel catalysts containing solid-solution based on praseodymium are quite interesting.
Given the appointed in the paragraphs above the goal of this research work was to investigate the effect of different proportions of PrO2-ZrO2 solid-solution in nickel catalysts and, evaluate their catalytic performances in Dry Reforming of Methane, Oxidative Reforming of Biogas and Partial Oxidation of Biomethane. It is hoped that the results of this research be useful to enable the sustainable production of Syngas from biomethane (or biogas).
Section snippets
Synthesis of Ni catalysts
The Ni catalysts were synthesized by the One-Step Polymerization method (OSP method), the salt precursors of Ni, Zr, and Pr were Ni(NO3)2.6 H2O, Zr(CO3)2.1.5 H2O, Pr(NO3)3.6 H2O, and the polymer precursors were citric-acid and ethylene-glycol. All reactants were of analytic degree (99.9%<). The salt precursors of Ni, Zr, and Pr were dissolved (nitric acid used for Zr carbonate, and water for nitrates of Ni and Pr) separately, the detailed procedure of the One-Step Polymerization method was
Characterization of the catalysts
Fig. 1a shows the XRD patterns of the NiZr, Ni1PrZr, Ni4PrZr, and Ni8PrZr catalysts at room conditions. In these patterns, the principal peaks are related to the crystal lattice structure of fcc (face-centered cubic) of NiO (JCPDS 78–0643) and tetragonal ZrO2 (JCPDS 81-1549). The XRD pattern of the NiPr catalyst is shown in Fig. 1d, this pattern shows the main phase of perovskite PrNiO3 (JCPDS 79-2453) and a minor phase of fcc crystal lattice structure of NiO (denoted by weak intensity peaks).
Conclusions
The catalyst composed of NiO/PrO2/ZrO2 mixtures can be used in the Dry Reforming of Clean Biogas, Oxidative Reforming of Biomethane, as well as in the Partial Oxidation of Biomethane. The composition of the Ni8PrZr catalyst was the most promising.
The addition of PrO2 in the NiO/ZrO2 system improved the performance of the catalysts in all the methane reactions studied in this work. Catalysts with 4% and 8% of PrO2 showed the highest reactant conversion rates, the catalyst with 8% (Ni8PrZr)
CRediT authorship contribution statement
Yvan J.O. Asencios: Conceptualization, Methodology, Writing – original draft, Investigation, Writing – review & editing, Supervision. Cristiane B. Rodella: Methodology. Elisabete M. Assaf: Supervision. Methodology.
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
The authors thank the Brazilian National Council for Scientific Development (CNPq, Brazil, Grant No. 407097/20), the São Paulo Research Foundation (FAPESP, Brazil) for the financial support (Grant No.: 2014/24940-5), and the Brazilian Synchrotron Light Laboratory (LNLS) for the XPS and XANES analysis.
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2023, Journal of CO2 UtilizationCitation Excerpt :For this reason, these catalysts are being actively researched to improve the catalyst deactivation problem [1,26–29]. ZrO2-supported Ni catalysts have been broadly studied for the dry reforming reaction because ZrO2 has good thermal stability, acidic/basic properties, and good oxygen mobility [12,28,30–34]. In our previous study, we demonstrated that the number of Ni active sites on the catalyst surface was a major factor influencing the catalyst performance [35].