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).