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Byun, Dong Wook (Michael)

University of British Columbia

Selective separation of hydrogen has many potential benefits, but no existing hydrogen membranes can be operated at temperatures higher than 550 ºC without deactivation. Molten gallium and indium membranes have recently been reported to allow high temperature operation but the overall flux falls short of state-of-the-art palladium-based membranes due to slow dissociative adsorption of hydrogen. In order to increase the rate of dissociative adsorption, this thesis investigates molten alloy of transition metals with low melting metals for the first time. Five criteria were used to identify the candidate alloys for testing: vapour pressure, oxide and hydride stability, toxicity, state of matter, and material cost. Hydrogen dissociation on surfaces of molten metals and metal alloys was quantified using H₂-D₂ isotopic exchange reaction experiments in the temperature range of 450 – 650 ºC. Dissociative adsorption rates were higher for alloys: activation energies of hydrogen dissociation dropped from 187 kJ/mol for pure molten bismuth to 91 kJ/mol for molten Cu₀.₀₃Bi₀.₉₇. Hydrogen sorption into molten bismuth and bismuth alloys was quantified based using Sievert’s apparatus. Alloying copper and nickel to bismuth accelerated the sorption rate. Hydrogen sorption rates were determined to be between 7.10 × 10⁻⁵ mol/m²·s and 1.22 × 10⁻⁴ mol/m²·s, almost two orders of magnitudes slower than the rate of dissociative adsorption at 550 ºC. This indicated that diffusion was rate-limiting and the diffusion coefficients of 9.4 × 10⁻⁹ m²/s to 1.9 × 10⁻⁸ m²/s were calculated. Alloying transition metals with low melting metals increased the rate of dissociative adsorption without penalty to the rate of diffusion. Hydrogen permeation experiments in a small lab-scale membrane apparatus were challenged due to poor wetting of the top candidates identified on quartz. In order to investigate this, wetting of copper-bismuth alloys to porous substrates was evaluated. Non-reactive wetting of Cu₀.₀₃Bi₀.₉₇ and Cu₀.₀₃Sn₀.₉₇ to porous quartz substrate exhibited contact angles of 110º and 120º respectively. Reactive wetting of copper-bismuth alloys was also investigated by adding chromium which forms a chromium carbide layer on thin graphite, resulting in a decrease of contact angle from 90º to 45º.

Text

2025-04-04

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/90576

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/