Surface studies of palladium based membranes and model systems

Abstract

The work of this thesis can be divided in to parts, one is based on the Pd/Ag alloy membranes and the other is about the model systems. It is well known that, Pd is a potential candidate membrane material, due to both its high solubility and permeability to hydrogen. Alloying of Pd with 23wt% Ag has been found to be an optimum composition for high hydrogen permeability.

Thin Pd/Ag 23wt% free standing films of thicknesses in the range 1.3 -10 μm were prepared by magnetron sputtering. The influence of thermal treatments in different gases on the hydrogen permeation behaviour of the selected membranes and the surface properties were investigated through a combination of hydrogen flow measurements and surface characterization. After thermal treatment in air at 300°C, we observed enhanced hydrogen permeation for ~1.3 to 5 μm membranes. Topography studies by atomic force microscopy showed that the increase in permeance correlated with increase in surface roughness and surface area. A similar behaviour in improvement of hydrogen flux was observed for an alternative treatment namely heat treatment in N2/Ar at 400°C for 1.5 μm membranes. The topography investigations showed increase in surface roughness and area also after this treatment.

To shed more light on the effect of the N2/Ar thermal treatments, we performed thermal treatments in this presumed inert atmosphere at three different temperatures (300°C, 400°C and 450°C) for three different film thicknesses, 2 μm, 5μm and 10 μm. For comparison we performed thermal treatment in air at 300°C for all selected films. After all treatments the hydrogen flux was found to be increase compared to before treatment for the 2 μm membranes, more strongly for the higher temperatures. In contrast for the thicker films the flux was decreasing after the treatments except for the 5 μm membranes thermally treated in N2/Ar at 450°C. On the other hand improvement in flux was obtained for all samples after air treatment at 300°C. Increase in surface roughness and surface area were accompanying the increased flux for the 2 μm membranes. No significant changes in surface roughness, at the initially rougher membrane surface, were observed for the membranes with decreasing flux. Summarizing all results from our group indicates bulk limited hydrogen permeation for the membranes of thickness down to 1.5-2 μm thermally treated in air at 300°C. Chemical composition analysis revealed Ag segregation to the surface for all N2/Ar thermal treated membranes and reverse segregation behaviour for air-treated on 1.5 μm and 5 μm membranes.

Long term stability of a 2.6 μm Pd-23%Ag/stainless steel composite membrane was examined in H2/N2 mixture as a function of both temperature and feed pressure. During continuous operation, the membrane showed good stability at 400°C while the N2 leakage increased very slowly at a temperature of 450°C (Pfeed = 10bar). After 100 days of operation (Pfeed = 5-20bar, T=350-450°C), the N2 permeance was 7.0 x 10-9 mol m-2s- 1Pa-1, which indicates that the H2/N2 permselectivity still lies around 500.Surface topography observed by atomic force microscopy (AFM) showed small changes in surface area after the long term stability measurements. Possible surface defects were also detected. In addition, segregation of Ag to the membrane surfaces was found from X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).

To address specific issues related to Pd based membrane systems, two selected Pd based model systems were studies as part of this thesis work. Molecular adsorption of methanol and its decomposition pathway on clean Pd(110) and an alloy Ag/Pd(110) surface were investigated. Multilayer methanol molecules adsorption on the surface was found at 100K. The methanol behaviour of these two surfaces was quite similar. Upon heating to 170K, two methanol species were identified. CO was formed on the surfaces at 250K and was desorbed at 500K. These studies show that silver at the amount deposited in the present work does not influence the methanol decomposition as compared to clean Pd(110).

As a model system for addressing interactions of adsorbed organonitrogen compounds on Pd(110) we have studied the methylamine molecular adsorption behaviour on this surface. Decomposition of adsorbed CH3NH2 to CN occurs mainly in the temperature range from 300 K to 350 K. The intermediate species CN remained at the surface even after annealing to 800K. Density function theory shows that methylamine is found to bind to Pd(110) surface via a lone pair state on the N atom.