**1. Introduction**

Dry, or carbon dioxide reforming of methane (DRM) has gained attention in recent decades, mainly due to the fact that DRM consumes prevalent greenhouse gases i.e., methane and carbon dioxide to produce synthetic gas, which serves as an important raw material for liquid hydrocarbon formation [1–8]. Hence, DRM offers two benefits: (a) conversion of major greenhouse gases into a value-added product, and (b) the DRM product, i.e., syngas, offers equimolar H2 and CO, which results in hydrocarbon production via Fischer–Tropsch (FT) synthesis [9–13]. The catalytic activity and stability are mainly dependent on the choice of a suitable catalyst [14]. Non-noble-metal-based catalysts, particularly transition-metal-based catalysts including catalysts of Ni, and Co, are mostly studied for DRM since these catalysts offer advantages such as their abundance, quick turnover rates, and low cost [15–18]. The bottlenecks associated with Ni-based catalysts include the loss of active metal surface area due to sintering and carbon formation during DRM which results in catalyst deactivation and also influences the selectivity of the syngas produced [19].

Generally, the basic supports or promoters, such as CeO2, La2O3, and Sr2+, have demonstrated better catalytic activity and enhanced chemisorption of CO2 than acidic supports. Many researchers have reported that ceria and its modified supported catalysts

**Citation:** Ahmad, N.; Alharthi, F.; Alam, M.; Wahab, R.; Manoharadas, S.; Alrayes, B. Syngas Production via CO2 Reforming of Methane over SrNiO3 and CeNiO3 Perovskites. *Energies* **2021**, *14*, 2928. https:// doi.org/10.3390/en14102928

Academic Editors: Wasim Khan and Giorgio Vilardi

Received: 2 April 2021 Accepted: 12 May 2021 Published: 18 May 2021

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provide a promising platform for endothermic DRM processes due to their basicity, to promote CO2 adsorption, and their high oxygen storage capacity/oxygen vacancy for CO2 activation or the gasification of different kinds of carbon precursors [20–23]. Perovskites have shown excellent performance in catalytic and photovoltaic industries and Ni-based perovskites are favored for DRM as perovskites offer high metal dispersion and thermal stability [24,25]. The perovskites in which the B-site cation is replaced with transition metals such as Ni need to be researched in depth [26,27]. Generally, several factors contribute to the catalytic performance of a perovskite [28], (a) the choice of element(s) for B-site cation, (b) controlling vacancy and valency through the proper selection of A-site element(s) and/or partially substituting companion metal(s), (c) high dispersion obtained due to the formation of fine particles, which leads to higher specific surface area, (d) the synergy between A-site and B-site elements.

Ren et al. [29] investigated the role of an Mo2C-Ni/ZrO2 catalyst in the steam–CO2 dual reformation of methane and found the catalyst exhibited high catalytic activity (~75% CH4 conversion) and unexpected coke-resistant stability, as evidenced by TGA, even after 30 h time-on-stream. In other research, LaBO3 (B = Ni, Fe, Co, and Mn) perovskites were studied for the reduction in pollution from vehicles fueled with natural gas [30]. Moreover, the effect of adding Pd to LaBO3 perovskites on oxidation activity performance showed that a smaller amount of Pd contributes to improving not only lattice oxygen mobility, but also enhances the reducibility of the B-site in LaBPd0.05O3 perovskites. Hence, Pd addition significantly enhanced catalytic activity of the perovskites.

Messaoudi et al. [31] studied the role of bulk LaxNiOy and supported LaxNiOy/ MgAl2O4 catalysts in DRM and found that the supported catalysts exhibited higher nickel dispersions and specific surface areas. These factors contributed to enhanced activity and stability, with minimal carbon formation during a 65 h time-on-stream. They also discovered that the supported catalysts had intact metallic nickel active sites after a long-term stability test, as verified by XRD results. The study of the impact of catalyst preparation methods, gas hourly space velocity, and reaction temperatures on catalytic performance of ternary perovskites AZrRuO3 (A = Ca, Ba, and Sr) revealed that the SrZrRuO3 catalyst exhibited the highest conversion and best stability among the tested perovskites [32]. Wang et al. [33] utilized perovskite (La2O3-LaFeO3) as the support to load Ni and Co to synthesize bimetallic catalysts, to explore their performance in DRM. The loading of a suitable amount of Co increased the catalytic activity and suppressed carbon deposition, which is attributed to the crystalline structure of the perovskite.

In this work, SrNiO3 and CeNiO3 perovskites were synthesized and investigated for CO2 reformation of methane. The activity performance in terms of CH4 and CO2 conversions and the catalyst durability, i.e., activity as function of time, are specifically elucidated herein. Overall, the aim of the study is to provide insights into the replacement of cerium with strontium and their respective responses to perovskite activity and stability under reforming conditions. The catalysts were characterized before and after activity and stability tests to understand and discuss the catalytic findings in relation to analysis results.

### **2. Materials and Methods**

#### *2.1. Preparation of SrNiO3 and CeNiO3 Nanocrystals*

Nanocrystals of SrNiO3 and CeNiO3 were synthesized by the self-combustion method using metallic nitrates and glycine as a precursor. Firstly, 1 mmol Ce(NO3)3·nH2O (Sigma Aldrich, St. Louis, MO, USA—99.9%), 1 mmol Ni(NO3)2·6H2O (Sigma Aldrich, St. Louis, MO, USA—99.9%), and 1 mmol Sr(NO3)2 were separately dissolved in 100 mL deionized water. The solutions of Ce/Sr and Ni were mixed in a 1:1 ratio to obtain a clear and homogeneous solution. Then, glycine (purity 99.5%), used as an ignition promoter, was added to the metal nitrate solutions (glycine:metal ions ≈ 1). The mixtures were thoroughly stirred by a magnetic mixer to eliminate the water at 60–70 ◦C until a homogeneous sol-like solution was formed. The gel was heated up to around 250 ◦C, at which temperature the ignition reaction occurred, producing a powdered precursor which still contained some

carbon residue. Finally, the powders were calcined in air at 700 ◦C for 6 h to eliminate the remained carbon, resulting in the formation of the perovskite structure.
