Control of Pd dispersion in sol-gel-derived amorphous silica membranes for hydrogen separation at high temperatures

The sol-gel method was used to fabricate Pd-SiO2 membranes in which Pd particles 2-30 nm in size were dispersed in a SiO2 layer, and the thickness of the Pd-SiO2 layer was approximately 300 nm. The H2 permeation properties and the thermal and hydrothermal stabilities of the Pd-SiO2 membranes were evaluated. In the present study, the Pd-SiO2 layer was fabricated by (1) a 1-step method, whereby calcination occurred only at 550 °C under a H2 atmosphere for 1 h, and/or (2) via a 2-step method, whereby a first calcination was administered at 400 °C under a H2 atmosphere for 1 h prior to a second calcination at 550 °C under a H2 atmosphere for 1 h. The Pd-SiO2 membranes were quite stable under a N2 atmosphere at 500 °C, irrespective of the membrane fabrication method (1-step, 2-step calcination). However, under a H2 atmosphere, the N2 permeance of a membrane fabricated using the 1-step method increased approximately 10 times after exposure to H2 for the initial 3 h, and increased with time due to the formation of grain boundaries caused by the aggregation of Pd particles. A Pd-SiO2 membrane fabricated using the 2-step method was relatively stable under a H2 and steam atmosphere (500 °C, steam: 70 kPa), and showed H2 permeance of 5.0×10−7 mol m−2 s−1 Pa−1 with H2/N2 and H2/He permeance ratios of 260 and 2.2, respectively. The experimentally obtained H2/He permeance ratio for Pd-SiO2 membranes (Si/Pd=3/1, 3/7, 2/8, 1/9) showed reasonable agreement with a theoretical calculation based on a mixed-matrix structure (continuous phase: SiO2, dispersed phase: Pd).