| AVS 58th Annual International Symposium and Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS+TF-ThM |
| Session: | Plasma Deposition and Plasma Enhanced ALD |
| Presenter: | Colin Wolden, Colorado School of Mines |
| Correspondent: | Click to Email |
We introduce a new class of composite membranes based on transition metal carbide as economical alternatives to palladium for high temperature purification of H2. In this talk we describe two membrane concepts that were synthesized using plasma-enhanced chemical vapor deposition (PECVD) and magnetron sputtering. The first is a surface diffusion membrane comprised of nanostructured Mo2C deposited on porous ceramic supports. Stoichiometric Mo2C was fabricated using a two step synthesis process. Dense molybdenum oxide films were first deposited by plasma-enhanced chemical vapor deposition (PECVD) using mixtures of MoF6, H2, and O2. Oxide films 100 – 500 nm in thickness were then converted into molybdenum carbide using temperature programmed reaction using mixtures of H2 and CH4. Permeation testing of these membranes showed very high flux, but limited selectivity. To address this issue we describe a counterflow PECVD approach that we are developing which is used to both modify the pore size of the original supports as well as to repair pinholes that develop during the carburization.
The second strategy is to produce dense composite membranes comprised of Mo2C layers sputtered onto BCC metal foils. BCC metals (V, Ta, Nb) and their alloys have extremely high permeability for atomic hydrogen, but negligible catalytic activity for hydrogen dissociation. Platinum group metals have been used as catalysts, particularly palladium, but at elevated temperature they alloy with the underlying metal and rapidly lose their activity. In contrast, the Mo2C/V membranes described in this work displayed no change in permeability when operated at high temperature for >160 hours, and transmission electron microscopy confirmed that negligible interdiffusion occurs between these materials during testing. Hydrogen dissociation is the primary factor limiting hydrogen transport, as evidenced by the sensitivity of performance to carbide morphology. Sputter parameters were systematically varied to optimize the crystal structure and morphology. These composite membranes are perfectly selective to H2, with permeability values approach and in fact exceed that of pure palladium. These findings demonstrate the potential of low cost group V metals for H2 separations with simultaneous carbon capture at temperatures compatible with the processes used for H2 generation.