**1. Introduction**

Global warming has become an alarming issue. Emissions of greenhouse gases including carbon dioxide (CO2) and methane (CH4) actively contribute to global warming. Methods of transforming these CO2(g) and CH4(g) into useful products are an important area of study to generate industrially important fuels and chemicals [1–3]. In this context, numerous reforming reactions of CH4 have been employed using several oxidants (e.g., H2O, CO2, O2, etc.) to produce H2(g) or synthesis gas (syngas, a mixture of H2(g) and CO(g)) with an equimolar ratio of H2(g)/CO(g). Methane reforming processes include steam reforming, auto thermal reforming, tri-reforming, etc. [4–11]. Methane reforming using CO2, known as dry reforming (DRM), is attractive because it mitigates the emission of CH4 and CO2, produces syngas, the starting material in the Fischer-Tropsch process to generate hydrocarbons and oxygenates, and generates clean energy through the combustion of hydrogen [12]. CH4 is a

cost-effective feedstock for syngas production. The primary reaction that governs the process is as follows:

$$\text{CH}\_4 + \text{CO}\_2 \rightarrow 2\text{H}\_2 + 2\text{CO} \\ \Delta \text{H}\_{298} = +247 \text{ kJ mol}^{-1} \tag{1}$$

The reaction is energetically unfavorable, thus requiring high temperatures to achieve acceptable conversion. Both noble metals (i.e., Ru, Rh, or Pt) and first-row transition metals (i.e., Ni, Fe, Co) are common active elements in that catalyze CO2 reforming of CH4. Although noble metals display high activity and stability, their limited availability and high price have rendered them inappropriate for industrial use [13,14]. On the other hand, the first-row transition metals are cheaper and possess similar activity, but their stability is hampered by carbon deposition and particle sintering [15–18]. Therefore, development of Ni-based catalysts with high activity and resistance to deactivation due to carbon formation and metal sintering is essential for DRM. Catalytic performance can be influenced by many factors such as the active metal, support type, and texture. The support can enhance the catalyst selectivity, activity, and stability by increasing the surface area and dispersion of the active metal [19]. For example, Ni deposited on alumina supports result in high catalytic activity, but rapidly deactivates due to sintering, coke deposition, and formation of surface nickel aluminate phase. To increase the catalytic performance of Ni/ γ-Al2O3, various parameters can be incorporated in the catalyst.

Titania (TiO2) is characterized by low specific surface area and poor mechanical strength, and undergoes a phase transformation from anatase to rutile at high temperatures, making it unsuitable for high temperature reactions [20]. Previous studies have shown enhanced thermal stabilization of TiO2 by introducing a thermally stable second metal oxide (i.e., SiO2, Al2O3, etc.) [21,22]. Incorporation of TiO2 in Al2O3 supports can improve metal dispersion, reduce particle sintering, increase thermal stability, and enhance oxygen storage capacity to assists in gasifying carbon produced in reforming reaction [23]. Tauster et al. investigated the effects of support modification on the oxidation state of Ru and the catalytic performance of Ru/TiO2 catalysts under conditions of partial oxidation of methane. It was found that doping of TiO2 with small amounts of WO3 favored oxygen adsorption on Ru under reaction conditions, resulting in a stabilization of a fraction of the catalyst in its oxidized form [24]. Addition of metal oxide promoters has been used to improve Ni metal catalysts. For instance, Shamskar et al. investigated the addition of CeO2, La2O3, and ZrO2 to Ni/Al2O3 catalyst used for DRM and found that ceria-promoted catalyst reduced the carbon formation [25]. Ni-MgO-Al2O3 catalysts were used for steam reforming of methane by Jang et al. [26]. Al-Fatesh et al. studied the promotional effect of ceria in the catalytic DRM and found that the Ni doping with ceria resulted in an excellent activity and lowered coke formation [27]. MgO promoters enhance CH4 conversion and mitigated the effect of the potassium poisoning of the Ni-based catalyst. The MgO promoter is beneficial in suppressing carbon formation.

In the present work, supported combinations of MgO, CeO2 and NiO catalysts were developed to retain high activity and stability while reducing the formation of coke during DRM. The effect of using MgO and CeO2 as separate and combined promoters, for 5.0 wt. % NiO supported over γ-Al2O3 doped with 3.0 wt. % TiO2 was studied. We determine the impact of each modifier on observed catalytic performance.

#### **2. Results and Discussion**

#### *2.1. X-ray Powder Diffraction (XRD)*

The XRD patterns of all the fresh catalysts are displayed in Figure S1 in the electronic supplementary information (ESI). All the patterns consisted of various metal oxides, where the presence metal oxide phases depended on the added components used to prepare the catalysts. Three metal oxides existed in all catalysts, where these metal oxides were the component of the support: cubic gamma-aluminum oxide, γ-(Al2O3)1.333 (PDF 01-075-0921), cubic synthesized honguiite titanium oxide, (TiO0.8)0.913 (PDF 01-085-1380), and aluminum silicate, Al0.5Si0.75 O2.25 (PDF 00-037-1460). Rhombohedral nickel oxide, NiO (PDF 00-044-1159) was found in Ti-CAT-I, Ti-CAT-II, Ti-CAT-III, and

Ti-CAT-V (these notations are defined in Section 3.2). When magnesium was added, cubic magnesium nickel oxide, MgNiO2 (PDF 00-024-0712) formed. Cubic synthesized cerianite (Ce) (ceria), CeO2 (PDF 00-034-0394), was detected in Ti-CAT-I, Ti-CAT-II, Ti-CAT-IV, and Ti-CAT-VI. Addition of magnesium strongly influenced the interaction of cerium with the other components of the catalyst. Monoclinic magnesium cerium oxide, MgCeO3 (PDF 00-004-0641), and cubic magnesium cerium titanium oxide, Mg2CeTiO6 (PDF 00-058-0550) were present in Ti-CAT-I and Ti-CAT-IV. Cubic periclase magnesium oxide, MgO (PDF 01-071-1176) was detected in Ti-CAT-I, Ti-CAT-III, and Ti-CAT-IV.

#### *2.2. Inductively Coupled Plasma Mass Spectroscopy (ICP-MS)*

ICP-MS analysis was carried out to quantify the metallic components as metal oxides for the best two catalysts. The results are shown in Table 1a,b.

Table 1 summarizes the results of ICP analysis of the metallic components in the prepared catalysts and compares it with the theoretical values. The experimental results were found to be in excellent agreemen<sup>t</sup> with the nominal values.


**Table 1.** ICP metal oxide microanalysis of Ti-CAT.

#### *2.3. Temperature Programmed Desorption (CO2-TPD)*

The CO2-TPD experiment was performed to study the basicity of the catalysts. The results obtained are shown in Figure 1. The basicity of the catalyst has a paramount influence on the catalytic performance in DRM due to the acidic nature of CO2. Thus, strong basic sites can enhance catalytic activity and increase the chemisorption and reaction of reacting gases [28]. Distribution of basic sites on the catalyst (i.e., weak, intermediate, strong, and very strong) correspond to the different desorption peaks in the temperature ranges of 20–150, 150–300, 300–450, and >450 ◦C, respectively, in the CO2-TPD profile [29,30].

All catalysts, except Ti-CAT-V and Ti-CAT-VI, showed the same basic site classification, because the CO2 desorption peaks appeared at almost the same different temperature ranges (Figure 1). Both Ti-CAT-V and Ti-CAT-VI have basic sites correspoding to site of high and strong basicity centered at a temperature around 310 ◦C.

For the peaks appearing at different temperature ranges, peaks in the temperature range of 50–125 ◦C correspond to weak basic sites, peaks at 160–185 ◦C fall under the category of intermediate strength basic sites, while the peaks at 260 ◦C correspond to strong basicity sites. An elbow peak was observed for all of the samples, except for Ti-CAT-V and Ti-CAT-VI, at temperature centered around 500 ◦C. This peak had no significant CO2 uptake.

**Figure 1.** CO2-TPD profiles of the synthesized catalysts.
