**1. Introduction**

The availability of natural gas (or shale gas) in large reserves makes CH4 serve as a suitable feedstock used in C1 chemistry to produce desired fuels and chemicals [1]. Unfortunately, the chemical inertness of CH4 results in direct conversion, which constitutes a grea<sup>t</sup> challenge for highly efficient utilization [2]. Ideally, the best use of CH4 occurs when it is converted into syngas, which can facilitate further downstream conversion [3] by means of the methanol route [4] and Fischer–Tropsch synthesis (FTS) [5–12] due to good reactivity, unlike the CH4 which has a high dissociation energy C–H bond [1]. Among the most widely investigated technologies, there are comparable advantages associated with the dry reforming of CH4 (DRM) with CO2 for producing syngas [13]. On the one hand, compared to the other reforming processes, there is a 20% lower operating cost for DRM [14]; on the other hand, the reforming of CH4 using CO2 not only produces high purity syngas [15,16] but also reduces the emissions of two abundantly available greenhouse gases to alleviate global climate change [17–20].

In spite of the above-mentioned merits, DRM suffers from serious carbon deposits on the surface of Ni nanoparticles, which leads to a remarkable loss of active sites [21–25]. Recently, DRM research efforts have resulted in strategies to improve the stability of the catalyst [26]. Based on the fact that smaller Ni nanoparticles efficiently improve catalytic performance by avoiding carbon accumulation [27–32], the general concept is to develop the catalyst preparation protocol to obtain small Ni nanoparticles encapsulated in the support or confined by the stable porous oxide layer to prevent sintering [33,34]. For example, Tomishige et al. reported that the solid solution catalyst of nickel–magnesia, which was prepared by the co-precipitation method, showed high and stable activity without carbon deposits for 100 days [35,36]. Kawi et al. synthesized a Ni-yolk@Ni@SiO2 nanocomposite with a yolk-satellite shell structure to efficiently inhibit the sintering of Ni, which resulted in negligible carbon deposition, and the CH4 conversion was 10% after the first 2 hours of reaction under the conditions of 800 ◦C, a gas hourly space velocity (GHSV) of 1440 <sup>L</sup>·g<sup>−</sup>1cat·h−1, a Wcat of 0.01 g, and a CO2:CH4:N2 ratio of 1:1:1 [37]. Similarly, Wang et al. pointed out that the Ni nanoparticle cores encapsulated by the mesoporous Al2O3 shells show superior coke resistance because of the confinement effects which prevent the Ni nanoparticles from agglomeration at high temperatures, and the CH4 and CO2 conversions under the reaction conditions of 800 ◦C, CO2/CH4 of 1/1, and a weight hourly space velocity (WHSV) of 36 <sup>L</sup>·h−1·gcat−<sup>1</sup> were about 88% and 92%, respectively [38].

Herein, different from the above-mentioned encapsulated Ni catalysts with relatively complicated preparation procedures, we propose a facile one-step strategy to prepare the SiO2 supported Ni catalysts toward the controlled formation of nanoparticle size and Ni-support interaction, which could lead to high activity and stability. Following the conventional impregnation method, glycine (C2H5NO2) and ammonium nitrate (NH4NO3) were introduced into the impregnated solution of nickel precursor (Ni(NO3)2·6H2O), as shown in Scheme 1. It was expected that the mixed materials with C2H5NO2 as fuel and NH4NO3 as combustion improver reacted exothermically after ignition which finished within a short time-frame with a very high temperature and release of a large quantity of gases, such as CO2, water, and N2. We thought this process might facilitate the formation of smaller crystalline materials and regulate the metal-support interaction, resulting in improved catalytic performance in the DRM reaction. To demonstrate the effects of the above combustion process on the catalytic performance, several characterizations, such as Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR) and thermogravimetric (TG), were employed to characterize the catalyst.

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

#### *2.1. Characterization of the Catalyst Sample*

As shown in Figure S1, all the fresh Ni/SiO2 catalysts exhibit apparent diffraction peaks at 2θ values of 37.3◦, 43.2◦, 63.0◦, 75.4◦, and 79.4◦ assigned to the NiO (JCPDS 22-1189). For the reduced catalysts (Figure 1a), Ni/SiO2-0/0 prepared by the conventional wetness impregnation method displayed the most intensive diffraction peaks at 2θ values of 44.5◦, 52.2◦, and 77.0◦, which are the characteristic peaks of metallic Ni (JCPDS 1-1206). According to Figure S1, the peak at 37.3◦ should be assigned to NiO. As the NH4NO3 was introduced into the impregnated solution with nickel nitrate, the resulting catalyst (Ni/SiO2-0/1) exhibited almost the same diffraction peak intensity at 44.5◦. However, for the case of C2H5NO2, Ni/SiO2-2/0 displays a much weaker diffraction peak. Interestingly, the addition of both C2H5NO2 and NH4NO3 results in almost no detectable diffraction peaks for Ni nanoparticles (Ni/SiO2-2/1), suggesting that smaller Ni nanoparticles can be obtained by synergistic effects of fuel and combustion improver in the combustion process, as presented in Scheme 1.

**Scheme 1.** One-step facile synthesis of Ni catalysts supported on silica (SiO2) prepared by the combustion of Ni(NO3)2–C2H5NO2–NH4NO3 impregnated in the porous SiO2.

TEM images of the reduced catalysts are depicted in Figure 1b,c. The Ni/SiO2-2/1 displays an average Ni nanoparticle size of only 6.1 ± 2.7 nm which is significantly smaller than that for Ni/SiO2-0/0 (31.3 ± 13.5 nm). The significant difference in the Ni nanoparticle size further confirms the synergistic effects of C2H5NO2 and NH4NO3 in reducing the Ni nanoparticle size. The combustion process between N2O and NH3 is highly exothermic. The decomposition of nickel nitrate produces N2O gas at 250 ◦C, while the decomposition of C2H5NO2 gives NH3 along with CO2 and H2O. The combustion process is triggered by the reaction between N2O and NH3 to form N2 and H2O [39]. When NH4NO3 is further added, NH3 and N2O can be formed via its decomposition at a low temperature of about 200 ◦C, thereby promoting combustion. The high-temperature stage in a short-duration favors the formation of ultra-small nanoparticles in a short time which may be in the order of seconds [40].

**Figure 1.** (**a**) XRD patterns of reduced Ni/SiO2 catalysts prepared with the combustion method by using different ratios of C2H5NO2 to NH4NO3. (**b**,**<sup>c</sup>**) TEM images and Ni size distribution of the reduced Ni/SiO2-0/0 and Ni/SiO2-2/1 catalysts, respectively. (**d**) H2-TPR profiles of the fresh Ni/SiO2 catalysts prepared with the combustion method.

Figure 1d exhibits the reduction behavior of fresh Ni/SiO2 catalysts with different molar ratios of C2H5NO2 to NH4NO3. As expected, NH4NO3 does not obviously change the H2-TPR profile compared to the case of Ni/SiO2-0/0, as both catalysts show a strong reduction peak at 300–450 ◦C with a small right shoulder peak at 450–510 ◦C. However, C2H5NO2 only (Ni/SiO2-2/0) notably weakens the peak at lower temperatures, accompanied by a shift in the right shoulder peak to the higher reduction temperature with enhanced intensity. For Ni/SiO2-2/1, the high temperature reduction peak is further intensified and shifts to a higher reduction temperature range. This result suggests that the smaller Ni nanoparticle size results in a more difficult reduction owing to a stronger metal-support interaction [41]. The reduction profiles correspond to the XRD and TEM results.
