Synthesis of fibrous silica supported noble metals catalysts for syngas production via partial oxidation of methane

Abstract

The ever-increasing worldwide energy consumption and released tons of energy-related CO2 gas into the atmosphere have driven the exploration of syngas in petrochemical industries and synfuels generation from Fischer-Tropsch synthesis (FTS). Among the technologies for syngas generation, catalytic partial oxidation of methane (POM) appears as a promising technique due to its short contact time at high space velocity and mildly exothermicity leading to excellent energy efficiency. However, the catalyst deactivation induced by carbon deposit is always a challenging issue for POM. Thermodynamic equilibrium assessment for POM was conducted in this research by using the Gibbs free energy minimization approach to study the tuning of syngas H2/CO ratio appropriate for downstream FTS. The results revealed that indirect combustion-reforming pathway was possibly the main contributory factor to the syngas yield during POM. In this research, silica materials with various morphology, namely commercial silica (SiO2), commercial Mobil Composition of Matter number 41 (MCM-41) and dendritic fibrous KAUST Catalysis Centre 1 (KCC-1) were prepared to study their properties and catalytic activity relationship for POM. The KCC-1 support was synthesized using microwave-assisted microemulsion. The addition of 0.5 wt.% M (M = Ru, Pd or Rh) on KCC-1 support were carried out using wetness impregnation methods to further enhance the POM performance. The catalysts were characterized using X-ray diffraction, N2 physisorption, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, electron spin resonance and Raman spectroscopy measurements. The effects of support morphology and active transition metals addition towards the catalytic performance, stability, resistibility to carbonaceous deposits and high-temperature oxidative regeneration of the KCC-1 supported catalysts were examined over a temperature range of 500–900 oC. Compared to SiO2 and MCM-41, the high concentration of oxygen vacancies in KCC-1 substantially contributed to the enhancement in POM performance which was highly associated with CH4 dissociation and adsorption of oxidizing agents (i.e., O2, CO2 and H2O). At 800 oC, turnover rate of CH4 dropped in the order of Rh/KCC-1 (30.1 min-1) > Pd/KCC-1 (18.1 min-1) > Ru/KCC-1 (15.1 min-1). The in-situ ESR and XPS studies corroborated that the oxygen vacancies were beneficial for the syngas formation by enhancing methane steam reforming and methane dry reforming reaction rates as well as carbon gasification process. Based on the achieved H2/CO ratio, Rh/KCC-1 appeared as a prospective candidate for use in POM application appropriate for downstream synfuel production. The mechanism-derived kinetic modelling determined that the POM over Rh/KCC-1 followed dual site dissociative adsorption of both CH4 and O2 with bimolecular surface reaction as rate-determining step. This study highlighted the new perspectives on the use of KCC-1 supported catalysts in alternative, renewable and sustainable energy technologies with respect to reaction engineering and catalysis, particularly from catalyst synthesis, characterization and application point of view.