**1. Introduction**

The decrease of fossil fuel energy and the dilemma of environmental pollution urged a large number of researchers to maximize the conversions of methane and carbon dioxide into useful products such as hydrogen. Hydrogen is a benign source of energy. It is mainly obtained from biomass pyrolysis and thermal reforming. Methane, the main component of natural gas, can be obtained from various resources like shale gas and the fracking process, which has increased the availability of natural gas from infrequent deposits [1,2]. Moreover, the utilization of biogas is gaining momentum in recent years [3,4]. In the field of heterogeneous catalysis, particularly, in the latest decades, dry reforming of methane is regarded as one of the best prospective ways of conversion [5–7]. However, the dry reforming reaction as shown in Equation (1) is highly endothermic and thus requires high reaction temperatures. The process produces synthesis gas that has an appropriate ratio of H2 to CO suitable for Fischer–Tropsch synthesis [8]. Steam reforming of methane remains the best available industrial process for generating synthesis gas [9,10]. The requirement and the utilization of synthesis gas production are continuously increasing [11]. During methane dry reforming (DRM), CO2 is employed as an oxidant, which draws the interest

**Citation:** Abasaeed, A.; Kasim, S.; Khan, W.; Sofiu, M.; Ibrahim, A.; Fakeeha, A.; Al-Fatesh, A. Hydrogen Yield from CO2 Reforming of Methane: Impact of La2O3 Doping on Supported Ni Catalysts. *Energies* **2021**, *14*, 2412. https://doi.org/10.3390/ en14092412

Academic Editors: Vladislav A. Sadykov and Wasim Khan

Received: 2 March 2021 Accepted: 20 April 2021 Published: 23 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and the likelihood of seizing and recycling CO2 from the exhaust flue gases of industrial and power plants. DRM is presently not industrially applied because of the heavy coking and sintering of the catalysts in high-temperature reforming reactions [12]. Thus, it is essential to find an innovative catalyst that endures sintering and coking. The sintering of the metallic phase and carbon formation on the surface of the catalyst, which causes the deactivation, originate from operating conditions that facilitate the side reactions, which involve the cracking of the CH4 Equation (2) and the reverse Boudouard reaction Equation (3) resulting from the combinations of CO [13].

$$\text{CH}\_4 + \text{CO}\_2 \rightarrow 2\text{H}\_2 + 2\text{CO} \tag{1}$$

$$\text{CH}\_4 \rightarrow 2\text{H}\_2 + \text{C} \tag{2}$$

$$\text{CO}\_2 + \text{C} \to \text{CO} + \text{CO} \tag{3}$$

H2 yield through DRM is significantly affected by dissociation of CH4 over Nisupported surface and the gasification of carbon formed by CO2. The reverse water gas shift reaction and the action of the H2 spillover on the surface affect the H2 yield substantially. The hydrogen spill over enhances hydroxyl formation and catalytic activity toward CO oxidation at the metal/oxide interface. The hydroxyl groups at the metal/support interface react with CO to produce CO2. Similarly, the reverse water gas shift (CO2 + H2O → CO + H2) reduces the hydrogen; hence, the two phenomena involve the depletion of hydrogen and in turn influence the hydrogen yield. Noble metals like Pd, Ir, Pt, and Rh provide the extremely good performance of activity and stability but they are rare and expensive [14,15]. Transition metals like Ni are suitable alternatives for this reaction as they are stable and environmentally friendly [16,17]. Nonetheless, Ni-based catalysts are hampered by poor activity due to coke formations and sintering [18,19]. Hence, the major challenge is to come up with Ni catalysts capable of resisting carbon formation and sintering. Carbon generation may be opposed by controlling the reaction kinetics using suitable catalysts with proper constituents and supports. It has been established that metal–support interactions can alter both catalyst activity and activity maintenance. Bradford and Vannice had shown that support decoration of metal particle surfaces shattered big ensembles of metal atoms that served as active sites for carbon deposition and the sites in the metal–support interfacial region enhanced catalyst activity [20]. The choice of support type plays a vital role in DRM. Supports that possess O2 species at the surface of the catalyst help the carbon oxidation on the metal [21,22]. The balance between the rate of methane decomposition and the rate of carbon gasification regulates the catalyst stability [23]. The preparation of mesoporous oxides, like Al2O3, possessing high surface area and precise pores have revealed motivating results to disperse the metal over the support structure [24–26]. Bian et al., in their investigation of Ni supported home-made mesoporous alumina in methane dry reforming, indicated that the formation of NiAl2O4 spinel is advantageous to activity and stability towards DRM reaction [27].

Newnham et al. synthesized nanostructured Ni-incorporated mesoporous alumina with various Ni loadings by hydrothermal method and tested them as catalysts for CO2 reforming of methane [28]. The result displayed excellent stability when 10%Ni was used due to the fact that the Ni nanoparticles in these catalysts being highly stable towards migration/sintering under the reaction conditions. The presence of strong Ni–support interaction and/or active metal particles being confined to the mesoporous channels of the support. Al-Fatesh et al. studied dry reforming of methane using a series of nickel-based catalysts supported on γ-alumina promoted by B, Si, Ti, Zr, Mo, and W [5]. They concluded that the promoters enhanced the interaction between NiO and γ-alumina support and, hence, Ni dispersion and stability. On the other hand, ZrO2 is prominent support with high thermal stability able to go through alteration in their acid–basic sites [29]. Numerous studies have displayed that the presence of ZrO2 improves the thermal resistance, redox properties, oxygen storage capacity and gasification of deposited carbon [30,31]. Hu et al. examined the dry reforming of methane over Ni/ZrO2 catalysts prepared via

decomposition of nickel precursor under the influence of dielectric barrier discharge (DBD) plasma at ~150 ◦C [32]. It was found to improve activities due to the exposition of Ni (111) facets, smaller metal particles, and more tetragonal zirconia with increased oxygen vacancies. In the course of dry reforming of methane, oxygen species over the catalyst surface affected the catalytic performance and carbon deposition. Zhang et al. studied the effects of the surface adsorbed oxygen species tuned by doping with metals like La, Ce, Sm, and Y on the catalytic behavior [33]. Their results confirmed that the surface adsorbed oxygen species promoted both CO2 activation and CH4 dissociation. Doping La2O3 in supported Ni catalysts favor the CO2 adsorption on the surface of the catalyst [34], alters the chemical and electronic state of Ni at the interface with the support and decreases the chemical interaction between Ni and the support causing the intensification of reducibility and higher dispersion of nickel [35]. Tran et al. studied the enhancement of La2O3 in the physicochemical features of cobalt supported over alumina for DRM using different temperatures and feed compositions [36]. Their results displayed that the La2O3 improved the H2 activation; enriched oxygen vacancy and lowered the apparent activation energy of CH4 consumption. The work of Lui et al. elaborated the promotional effects of La, Al, and Mn on Fe-modified clay supported by Ni catalysts used for dry reforming of methane [37]. The result of adding La, Al, and Mn altered the surface area, the basicity of the catalysts, and produced a smaller metallic Ni size. Moreover, Yabe et al. performed dry reforming of methane using several transition metals supported on ZrO2 catalysts [38]. Their results exhibited high activity and low carbon deposition upon using 1 wt%Ni/10 mol%La-ZrO2 catalysts.

In the present work, we assess the effect of the lanthanum oxide as a textual promoter of alumina and zirconia supports over Ni catalysts in the catalytic reforming of CH4 with CO2. The impact of different loadings of lanthanum oxide will be examined and their influence on the hydrogen yield. The output data will be further associated with the characterization results of BET, XRD, TPR, TEM, TPD, and TGA before and after the reaction. The difference in the basic supports originating from alumina and zirconia in terms of sintering and coking will be explored.

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