Impact of Sr Addition on Zirconia–Alumina-Supported Ni Catalyst for CO<i>_x</i>-Free CH₄ Production via CO₂ Methanation

Abstract

Zirconia-alumina-supported Ni (5Ni/10ZrO₂+Al₂O₃) and Sr-promoted 5Ni/10ZrO₂+Al₂O₃ are prepared, tested for carbon dioxide (CO₂) methanation at 400 °C, and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, surface area and porosity, infrared spectroscopy, and temperature-programmed reduction/desorption techniques. The CO₂ methanation is found to depend on the dispersion of Nickel (Ni) sites as well as the extent of stabilization of CO₂-interacted species. The Ni active sites are mainly derived from the reduction of 'moderately interacted NiO species'. The dispersion of Ni over 1 wt % Sr-promoted 5Ni/10ZrO₂+Al₂O₃ is 1.38 times that of the unpromoted catalyst, and it attains 72.5% CO₂ conversion (against 65% over the unpromoted catalyst). However, increasing strontium (Sr) loading to 2 wt % does not affect the Ni dispersion much, but the concentration of strong basic sites is increased, which achieves 80.6% CO₂ conversion. The 5Ni4Sr/10ZrO₂+Al₂O₃ catalyst has the highest density of strong basic sites and the highest concentration of active sites with maximum Ni dispersion. This catalyst displays exceptional performance and achieves approximately 80% CO₂ conversion and 70% methane (CH₄) yield for up to 25 h on steam. The unique acidic-basic profiles composed of strong basic and moderate acid sites facilitate the sequential hydrogenation of formate species in the CO_<i>x</i>-free CH₄ route.