Abstract
A single precursor RuxNiyMg1-x-y(OH)(OCH3) derived from solvothermal synthesis was used to generate Ru-Ni-MgO catalysts for methane reforming with CO2. Calcination-reduction pretreatment of precursors could easily cause segregation of Ru and formation of both large and small metallic particles as RuO2 has limited solubility in NiO-MgO solid solution. Uniform small Ru-Ni alloy particles could only be produced within their limited alloying composition range through direct reduction pretreatment of the precursor. Catalysts derived from calcination-reduction exhibited low initial activity that increased with time-on-stream, whereas catalysts derived from direct reduction demonstrated high and steady activity. Over spent catalysts, Ru was found to have changed the type of deposited carbon from a recalcitrant graphitic one that could only be gasified by O2 to a soft type that can be facilely gasified by CO2. Kinetic studies showed that Ru increased the activation barrier for the rate determining CH4 dissociation step and thereby slows down the carbon deposition rate. A first order reaction dependence for CH4 pressure variation and zeroth for CO2 pressure change for pristine Ni- and Ru-catalysts was identified, while a first order and a deviation from zeroth order for CH4 and CO2 pressure variation were observed on bimetallic Ru-Ni catalyst. Such a deviation is associated with the oxyphilic nature of Ru that is enriched in the alloy surface under reforming conditions. The effects of Ru on carbon gasification over spent catalysts were investigated using a modified CO2-TPO measurement based on an extrapolated Wigner–Polanyi equation for carbon gasification. Ru was found to accelerate carbon gasification by increasing the pre-exponential factor for CO2 oxidation of carbon, albeit a disfavored elevated activation barrier was obtained, thus showing a strong compensation effect. Carbon gasification is favored in high concentration of CO2 and at high temperatures for Ru-Ni catalyst.
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