Elsevier

Applied Catalysis A: General

Volume 542, 25 July 2017, Pages 296-305
Applied Catalysis A: General

In-situ XANES and XPD studies of NiO/Ce0.9Zr0.1O2 IT-SOFCs anode nanomaterial as catalyst in the CPOM reaction

https://doi.org/10.1016/j.apcata.2017.05.040Get rights and content

Highlights

  • NiO(60 wt.%)/Ce0.9Zr0.1O2 is an excellent catalyst for CPOM reaction, with excellent levels of methane conversion to synthesis gas.

  • In situ XPD and XANES experiments confirmed that Ni° is the active centre to produce the partial oxidation of methane.

  • In-situ XANES at Ce-LIII confirmed that cerium cations are responsible for providing lattice oxygen to form COx mixtures.

  • The occurrence of total or partial oxidation of methane is dependent on the feed ratio CH4:O2 and related to Ce4+/Ce3+ ratio.

Abstract

The aim of this paper is to study the oxidation states of metal cations and the changes in the crystalline phases in NiO/Ce0.9Zr0.1O2 nanocatalysts under typical conditions of methane catalytic partial oxidation reaction by in-situ X ray absorption near edge spectroscopy (XANES) and X-ray powder diffraction (XPD) studies. The Ce0.9Zr0.1O2 mixed oxide was obtained by glycine-nitrate-combustion method, being the nickel incorporated by incipient wetness impregnation. The evolution of the crystalline structure with temperature and operating conditions was followed by in-situ XPD experiments and the oxidation states of Ce and Ni cations by in-situ XANES experiments at the Ce-LIII and Ni-K absorption edges. It was observed that NiO is completely reduced to Ni° at temperatures above 650 °C while Ce0.9Zr0.1O2 fluorite-like phase showed changes in lattice parameter due to cerium reduction and crystallite growth, but no phase transformations or segregations were observed. It was confirmed that Ni° is the active centre to activate methane molecule while Ce4+/Ce3+ ratio is strongly related with CO/CO2 concentration in the exhaust gas flow.

Introduction

The development of the global economy has led to intensive use of fossil fuels, with a consequent increase in environmental pollution and emissions of greenhouse gases. The concern about the level of air pollution and global warming has given great impulse to the development of more efficient and cleaner energy production technologies. The utilization of renewable sources of energy and the development of sustainable processes are key issues in the energy economy. In this context, the energy production from biomass [1], solar cells [2], and fuel cells [3] are some of the main subjects in current investigations seeking to contribute to the solution of environmental pollution. Among the different types of fuel cells under development, solid oxide fuel cells (SOFCs) show great advantages over the others, due to the possibility of feeding the cells with fossil fuels like methane. SOFCs present better performance when operating with synthesis gas instead of methane, due to the faster electro-oxidizing of CO and H2. In the high-temperature SOFCs operating at temperatures above 900 °C, synthesis gas is obtained by methane reforming in the anode or an external chamber. This method is not efficient enough when applied in intermediate-temperature SOFCs (IT-SOFCs) that operate in the 500 °C–800 °C temperature range. One of the possibilities to increase the performance of methane-fueled IT-SOFCs is to convert methane to synthesis gas by catalytic partial oxidation (CPOM). This reaction can be described by the following equation:CH4(g) + 0.5O2(g)  2 H2(g) + CO(g) ΔH0 = −36 kJ/mol

Noble metals like Rh, Ru, Pt and Pd are catalysts very active for this reaction, showing a direct conversion mechanism of methane into synthesis gas without the appearance of carbon deposits [4], [5], [6], [7], [8], [9]. However, the use of these high cost and low availability metals is totally discouraged in the development of cleaner and more efficient energy production systems. Cheaper transition metals, especially nickel, are preferred to catalyze this reaction with lower costs. In this regard, the use of CeO2-based oxides as nickel phase support prevents accumulation of carbon deposits on the surface of the catalyst helping to maintain the level of activity [10].

Many researchers have studied ceria-based supports for CPOM reaction with nickel catalysts [11], [12], [13], [14], [15]. In this work, CeO2-ZrO2 was selected as support based on the better reducibility and oxygen storage capacity observed in Zr-doped ceria. Among the different Zr contents that could be reached in the CexZr1-xOd system, those corresponding to x < 0,5 showed the better redox behavior. Nevertheless, the compositions in that region are metastable and some decomposition due to aging should be expected. Samples corresponding to the cerium rich region (x > 0.8) are thermodynamically stable with good reducibility an oxygen availability [16], [17], [18].

In this context, the Ni° particle size and the interaction with the support are critical aspects that affect the catalyst performance [13]. Pre-reduction of nickel phase produces better catalyst performance, which was attributed to re-dispersion of metallic particles or better interaction with the support [11].

In this sense, Pal et al. [15] have recently reported the development of Ni/CeO2 catalyst with an optimized Ni content of 7.5 wt%, yielding a methane conversion of 98% at 800 °C with no observable deactivation after 50 h of time-on-stream. They believe that formation of the Cesingle bondOsingle bondNisingle bondOsingle bond layer with under-coordinated oxygen atoms on the surface may be responsible of the catalytic activity levels and good performance reached. Pantaleo et al. [19] have reported the analysis of 6 wt% Ni/CeO2 catalyst. They affirm that the crystallite sizes and the interaction between NiO and CeO2, which is controlled by varying the catalyst preparation procedure, are determinant factors for the activity and stability during the partial oxidation of methane. They did not observe deactivation of the catalyst and reported methane conversion of 90% at 650 °C over pre-reduced Ni/CeO2. We have previously reported the catalytic performance of NiO/Ce0.9Zr0.1O2 catalysts in total and partial oxidation of methane [20], [21]. In this catalyst the Ni phase was incorporated by incipient wetness impregnation of the support. The positive influence of the high oxygen mobility of the Ce0.9Zr0.1O2 support in the catalyst stability was observed [20]. Besides, we found that the NiO(60 wt.%)/Ce0.9Zr0.1O2 cermet was an excellent anode material for IT-SOFCs [21], [22]. The Ni° content should be greater than ∼30 vol.%, in order to provide a good electronic conductivity. A Ni° content of 50 vol.% is generally preferred, which is equivalent to a 60 wt.% of NiO [23].

Based on the previous information, it is worth to get a deep insight of the evolution of crystallite size, the stability of crystalline structure and the modification of the oxidation states of cerium and nickel cations of this anode material when it is exposed to atmospheres containing methane and oxygen in the temperature range used in IT-SOFCs. Besides, it seeks to establish some relationship between structural and metal cation oxidation states with product distribution in the reactor effluent.

Therefore, in this paper we study the crystallite size and the structure by in-situ X-ray powder diffraction (XPD), and the oxidation states of Ce and Ni cations by in-situ XANES experiments at the Ce-LIII and Ni-K absorption edges. The experiments were performed with synchrotron radiation in an extended temperature range and with different feed gas-flowcompositions. Mass spectrometry data were collected during in-situ tests in order to observe the species present at the exit of the reactor. The results of these experiments are analyzed and compared with those obtained in conventional catalytic tests conducted in a fixed-bed laboratory reactor.

Section snippets

Synthesis

Ce0.9Zr0.1O2 mixed oxide support was synthesized by the stoichiometric glycine-nitrate combustion route, starting from ZrO(NO3)2·6H2O (Fluka, Zr 27 wt.%), Ce(NO3)3·6H2O (Alfa Aesar, 99,95%) and glycine (Merck, >98.5%). The synthesis process was described elsewhere [24]. The as-obtained solids were calcined at 600 °C in air in order to eliminate any vestige of carbonaceous residues. This support will be referred to hereinafter as ZDC. Ni was incorporated to the ZDC support by incipient wetness

Conventional characterization studies

In Fig. 1, conventional XPD patterns of ZDC and fresh and spent samples of NiO/ZDC are plotted. ZDC support (Fig. 1(a)) shows the Bragg peaks corresponding to the fluorite-type crystal structure, while sample NiO/ZDC before the catalytic tests (Fig. 1(b)) shows the characteristic NiO peaks superimposed with those corresponding to ZDC support, indicating that NiO is in a separated phase. The XPD patterns of NiO/ZDC spent catalyst tested with a feed ratio CH4:O2 = 2 (Fig. 1(c)) exhibit the presence

Conclusions

By a multi-technique approach, we analyzed the crystalline structure, catalytic activity, and the oxidation state of metal cations in NiO(60 wt.%)/Ce0.9Zr0.1O2 catalyst under the conditions used in Partial Oxidation of Methane. The ZDC crystallites in the fresh support that was calcined at 600 °C are nanometric. When calcined at 1000 °C the ZDC crystallites grow but keep the nanometric size, confirming that the incorporation of zirconium in the ceria structure disables the growth of crystallites

Acknowledgements

This work was supported by the Brazilian Synchrotron Light Laboratory (LNLS) under proposals D06A-DXAS-9949 and XRD1-14413, Agencia Nacional de Promoción Científíca y Tecnológica (Argentina, PICT 2013 N°1493, 1032 and 1587), and CONICET (Argentina, PIP 112 2013 0100151 CO).

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