Palladium cobalt binary doping of molecular sieving silica membranes

This work investigates the preparation and performance of palladium cobalt binary doped silica (PdCoSi) membranes. In view that palladium nitrates precipitate under typical silica sol-gel synthesis conditions, membranes were prepared via the acid catalysis of tetraethylorthosilane (TEOS) with ethanol, palladium and cobalt chlorides. The largest H2 fluxes reached 0.6×10−6 mol m−2 s−1 Pa−1 at 500 °C for H2 permeation for the reduced membranes, whilst the best He/N2 permselectivies of 70 were observed for the oxidised membranes. The redox effect resulted in membranes with different performance as the reduction increased the total pore volumes of xerogels. However, XPS analysis revealed that palladium monoxide preferentially reduced to metal palladium, whilst reduction was not significant for cobalt oxide. Hence, the reduction to metallic palladium tailored molecular sieving domains through the loss of oxygen. As a result, the permeance of the larger molecular gases CO2 and N2 increased, whilst only a small increase was observed for the smaller molecular gas He. Interestingly, the membranes essentially remained as molecular sieves as evidenced by temperature dependent activated transport and high permselectivities. The molecular sieving domains created preferential pathways for CO2 and N2 permeance, though the silica matrix integrity remained generally intact. Hence, binary and multi doping of metal oxides in silica matrices can be used as a strategy to attain further functionalities not previously attained by pure silica or single metal oxide doped silica membranes using TEOS as a precursor.