Effects of Noble Metal Promoters on In Situ Reduced Low Loading Ni Catalysts for Methane Activation

Abstract

The commercial potential for a given catalytic process may be influenced by requirements on metal loading, in particular where noble metals are used. In an effort to substantially decrease the amount of catalyst material used for methane activation and catalytic partial oxidation (CPO), the effect of 0.005 wt% noble metal (Rh, Ru, Pd or Pt) on 0.5 wt% Ni/γ-Al₂O ₃ catalysts have been studied at temperatures below 1,173 K and 1 atm. The successful catalysts were activated directly by in situ reduction, without a calcination step, to promote formation of a highly dispersed (supported) metal phase from nitrate precursors. The obtained metal particles were not observable by XRD (size <  2–3 nm). This activation procedure had a decisive effect on catalyst activity, as compared to a catalyst which was calcined ex situ before in situ reduction. Adding a noble metal caused a significant drop in the ignition temperature during temperature programmed catalytic partial oxidation (TPCPO). The ignition temperature for partial oxidation coincides well with the temperature for methane dissociation, and is likely correlated to the reducibility of the noble metal oxide. Methane partial oxidation over 0.5 wt% Ni catalysts, both with and without promoter, yielded high selectivity to synthesis gas (>93%) and stable performance for continued operation, but synthesis gas production at temperatures below 1,073 K required a promoter when the catalyst was ignited by TPCPO. Ignition of the CPO reactions by introducing the feed at a high furnace temperature (1,073 K) also enabled formation of synthesis gas, but the reaction was then less stable than obtained with the TPCPO procedure. A dual bed concept attempted to beneficially use the activation and combustion properties of the noble metal followed by the reforming properties of Ni. However, it was concluded that co-impregnated catalysts yielded as high, or even higher conversion of methane and selectivity to synthesis gas.