PD MONOLAYER - TRANSFORMATIVE CONCEPT FOR EFFICIENT ELECTROLYTIC H - ISOTOPE SEPARATION

Platinum and Palladium are considered as very attractive catalysts for electrolytic hydrogen isotope separation. The high cost of Pt and Pd makes it necessary to minimize the amount of the catalyst, if any practical application is considered. The solution to reduce the cost of the Pd and Pt catalyst and increase their activity is two-dimensional (2D) monolayer (ML) morphology where every catalyst atom is on the surface. The issue is that deposition of noble metals as 2D ML at room temperature is quite challenging. To overcome this challenge, an underpotential deposition (UPD) phenomenon was studied as critical step for successful 2D catalyst monolayer synthesis and design. The Pd monolayer was deposited on Au (111) and Ru (0001) single crystal substrates with the help of conventional electrochemical techniques and investigated by in-situ scanning tunneling microscopy (STM). The epitaxial strain as a result of lattice mismatch with substrate was induced within the catalyst monolayer. Analysis of the strain effects in monolayer catalysts (PdML/Au (111) and PdML/Ru (0001)) on the energy of hydrogen and deuterium adsorption bond was investigated by subtractively normalized interfacial FTIR spectroscopy (SNIFTIRS). The results suggest that tensile strain in bimetallic monolayer catalyst (PdML/Au (111)) increases the bond strength of atop adsorbed hydrogen and deuterium, compared to bimetallic monolayer catalyst with compressive strain (PdML/Ru (0001)) and bulk Pd catalyst. Results from SNIFTIR studies used as input for theoretical calculations to predict the isotope separation factor between PdML/Au (111) ,PdMLRu (0001) and bulk palladium. Data showed that tensile strain in Pd monolayer deposited on Au increases the value of hydrogen isotope separation factor by 20%, whereas for Pd monolayer on Ru this value is less. Mass spectrometry measurements with pre-determined ratio of H2O/D2O solution mixture was also performed on HER products for monolayer catalysts and bulk Pd. The experimental data confirms that when the Pd surface is stretched, activity of the surface is increased, resulting in higher H isotope separation efficiency. Kinetic measurements for hydrogen evolution reaction (HER) and deuterium evolution reaction (DER) was performed on PdML/Au (111) and PdML/Ru (0001), and used for hydrogen separation efficiency analysis. Results suggest that surface strain affects the recombination of atoms on the surface, at small overpotential regions where the recombination of H/D atom is the rate determining step. Agreement between theoretical calculations and experimental data confirms that 2D monolayer catalyst under tensile strain (Pd monolayer deposited on Au substrate) can improve hydrogen isotope separation efficiency.