Stability of Ni/SiO2 in Partial Oxidation of Methane: Effects of W Modification

With the discovery and large-scale exploitation of natural gas resources such as shale gas and combustible ice, which are mainly composed of methane, their effective utilization has become a national strategic interest. Partial oxidation of methane (POM) to synthesis gas is one of the important methods for the utilization of natural gas and shale gas resources. The commonly used Ni/SiO2 catalyst for POM easily deactivates due to carbon deposition on the surface. To solve this problem, a urea precipitation method was employed in this work to prepare Ni-based catalysts modified with different amounts of tungsten (at W/Ni molar ratios of 0, 0.01, 0.03, 0.05, 0.07, and 0.10), and the catalyst stability in POM as well as the role of W were investigated. From characterizations such as X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (H-2-TPR), and X-ray photoelectron spectroscopy (XPS), we obtained the following results. The amount of W added to the Ni-based catalysts has a significant influence on their catalytic performances in POM and their physicochemical properties. The particle size of Ni in the catalysts decreases with W addition, and the Ni particle distribution on the support surfaces becomes more uniformed; however, the catalytic activity for POM is not significantly influenced. However, W-modified Ni-based catalysts show an increasing improvement in their stability in POM with increasing W/Ni molar ratio, with an optimum at the W/Ni molar ratio of 0.07; at the W/Ni molar ratio of 0.10, they exhibit a rapid deactivation in POM in a short time. Although interactions between Ni and SiO2 in the as-prepared catalysts are weak, the presence of adequate tungsten (W/Ni molar ratio of 0.05 and above) in the Ni-based catalysts can reduce the Ni particle size to some extent, and lead to the formation of strong interactions between Ni and W, which leads to an improvement in the dispersion of Ni on the support surface and imparts resistance for Ni particle growth in the POM reaction. The increased interaction between Ni and W changes the chemical state or oxygen affinity of Ni particles on the catalyst surfaces, and some of the partially oxidized Ni species (Ni delta+) on the catalyst surfaces coexist with reduced Ni species (Ni-0) during POM. Using an adequate amount of W-modified Ni catalysts results in almost no carbon deposition on the surfaces during POM, but using only a moderate amount results in good catalytic behavior and stability in POM. This finding suggests that the presence of W can not only enhance the anti-coking ability of the Ni-based catalysts and sustain their good stability in POM if the W content is low (i.e., W/Ni molar ratio of 0.07 and below), but also lead to the deactivation of W-modified catalysts in POM if the W content is high (i.e., W/Ni molar ratio of 0.10 and above), due to high oxygen affinity or the presence of more Ni species in oxidized form. In addition, alpha-WC (tungsten carbide) was identified using XRD to be formed on the surface of the moderate-amount W-modified Ni catalysts after POM, and it could inhibit or eliminate carbon deposition on the Ni-based catalyst surfaces. The catalytic performance evaluation of the catalysts in POM under a long time period confirmed that alpha-WC is stable.