Coprecipitation versus chemical vapour deposition to prepare Rh/Ni bimetallic catalysts

Abstract

The Rh/Ni interaction in bimetallic Rh/Ni/Mg/Al hydrotalcite-type (HT) derived catalysts for the catalytic partial oxidation (CPO) of CH4 was studied modifying the preparation method. The conventional incorporation of both Rh and Ni by coprecipitation (CP) of HT precursors was compared with the addition of Rh by chemical vapour deposition (CVD) to reduced Ni/Mg/Al HT-derived catalysts. Two Ni loadings were selected (about 8 and 2wt.%), whereas the Rh content was kept constant (0.18wt.%). Ni- and Rh-monometallic catalysts were prepared as reference materials.The addition of Rh resulted in an increase in Ni reducibility and dispersion for both CP and CVD catalysts. The Rh/Ni interaction was stronger in CP catalysts, while in CVD ones Rh overspread either on the support or Ni particles, with a preferential deposition on the latter. The Ni loading, regardless of the Rh incorporation method, was the main factor to determine the formation and growth of metallic particles. CO-DRIFTS spectra revealed that the dispersing and reducibility effect of Rh was more evident in the samples with a lower Ni-loading.The small amount of Rh in the catalysts yielded to higher CH4 conversions, especially in low Ni-containing catalysts wherein the Ni0 oxidation was avoided. All the catalysts were activated during catalytic tests at high temperature by reduction of hardly reducible species. The strongest Rh/Ni interaction in CP catalysts yielded to better performances with a lower carbon formation and high resistance towards metallic particle sintering.