Ni catalyst on mixed support of CeO₂–ZrO₂ and Al₂O₃: Effect of composition of CeO₂–ZrO₂ solid solution on the methane steam reforming reaction

Abstract

In this study, catalysts containing 5 wt.% Ni deposited on a support composed of a CeO₂–ZrO₂ solid solution deposited on alumina were tested in the steam reforming of methane. The supports, with various ratios of Ce to Zr, were prepared by co-precipitation of the oxide precursors, followed by calcination in synthetic air. The catalysts were then prepared by Ni impregnation of the supports. The prepared solids were characterized by temperature-programmed reduction with H₂ (TPR‐H₂), in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) spectroscopy. The XRD analysis confirmed the formation of a solid solution between ZrO₂ and CeO₂. In the catalytic tests, it was found that catalysts with higher Ce content did not exhibit deactivation during 6 h of reaction. The catalyst with highest Ce content, Ni(_0.8Ce_0.2Zr)Al, provided the best result, with the highest rate of conversion of methane and the lowest carbon deposition, which may be partly due to the smaller Ni⁰ crystallites in this sample and also the segregated CeO₂ particles may have favored H₂O adsorption which could lead to higher C gasification.

Introduction

Hydrogen is an important strategic material used in several industrial processes and also as a clean fuel, in fuel cells [1]. The industrial process generally used to generate H₂ is the catalytic steam reforming (SR) of natural gas, whose main component is methane. The steam reforming of methane (SRM) reaction is represented by Eq. (1), but this process is usually accompanied by the water gas shift reaction (WGSR), Eq. (2).

$$\begin{array}{ll}C{H}_4+H_{2}O\leftrightarrows CO+3H_2\hfill & \Delta H{°}_{298\text{K}}=206\text{kJ}{\text{mol}}^{-1}\hfill \\ \hfill & \Delta G{°}_{298\text{K}}=142\text{kJ}{\text{mol}}^{-1}\hfill \end{array}$$

$$\begin{array}{ll}CO+H_{2}O\leftrightarrows C{O}_2+H_2\hfill & \Delta H{°}_{298\text{K}}=-41.2\text{kJ}{\text{mol}}^{-1}\hfill \\ \hfill & DG{°}_{298\text{K}}=-28.5\text{kJ}{\text{mol}}^{-1}\hfill \end{array}$$

Supported nickel catalysts are used in steam reforming processes, but, compared to noble metal-based catalysts, they are less active and can suffer deactivation by carbon deposition and by oxidation of the active phase, Ni⁰ [2]. Moreover, the high temperatures and pressures used in the SRM may lead to catalyst deactivation by sintering [3].

To improve the catalytic performance and minimize coking of the catalyst, several conditions can be modified, such as characteristics of the support, metal content, preparation method and the introduction of promoters into the catalyst formulation [4]. According to the literature, the first step of the SRM reaction is the decomposition of methane, after which the carbon species formed on the active surface react with steam or surface oxygen species to form CO and/or CO₂ [5]. The catalyst support plays an important role in H₂O adsorption and some supports enhance the adsorption of steam [6].

Several studies in the literature report that reducible oxides, such as cerium oxide and/or zirconium oxide, improve the performance of some metal catalysts such as Ni, Pt and Pd and promote greater stability during methane reforming reactions. Moreover, when reduced by the reactants, these materials have a high reducing power and ability to store oxygen. Roh et al. [7] studied nickel catalysts supported on CeO₂, ZrO₂ and Ce–ZrO₂ and found that the sample supported on the mixed oxides of cerium and zirconium showed better activity, selectivity and stability in the methane steam and autothermal reforming reactions. Kambolis et al. [8], also studying Ni/Ce–ZrO₂ catalysts, showed that these mixed oxides promoted stability and good activity in the dry reforming of methane. The good performance of these catalysts was associated with the presence of redox sites responsible for the decomposition of CO₂ and H₂O.

According to the literature, [9], the incorporation of zirconium in the CeO₂ lattice creates a high concentration of defects in the structure, which improve the O²⁻ mobility that is responsible for the ability to store and liberate oxygen. This high oxygen mobility increases the oxygen vacancies of the support, which increase its reducibility and favor a continuous removal of carbon deposits from the active sites [10].

In view of the above mentioned considerations and also considering the properties of the CeO₂–ZrO₂ solid solution, the goal of this work was to study the effect of the CeO₂–ZrO₂ solid solution composition on the performance of Ni catalysts supported on mixed oxides of Al₂O₃ and CeO₂–ZrO₂ in the reaction of methane steam reforming.