Design, fabrication and characterisation of H2-selective Pd-alloy membranes for application in pre-combustion CO2 capture cycles
Climit financing100 % from the Research Council
Project partnersStiftelsen SINTEF (SINTEF), Norges teknisk-naturvitenskapelige universitet (NTNU), United Technologies Corporation, United Technologies Research Centre (UTRC), Delft University of Technology (TU Delft)
Project period2012 – 2017
Goal of project:
The pre-memCO2 project aims to research and develop improved H2-selective membrane materials for integration in pre-combustion decarbonisation process schemes, thereby targeting the priority areas in CLIMIT. The project is a collaboration between two SINTEF departments, NTNU, the Technical University in Delft (the Netherlands), and United Technology Research Centre (USA) combining membrane fabrication and performance testing with ab-initio modelling and advanced characterisation. The objective is to obtain a more fundamental understanding of permeation membrane materials, which will enable improved membrane design and fabrication, and finally more efficient and robust membranes in power generation processes.
The objectives of the pre-memCO2 project will be achieved through a multidisciplinary approach combining theoretical modelling by first principle calculations, extensive materials characterization by high resolution microscopy and surface analytical techniques, ternary Pd-alloy manufacturing, and H2 permeation verification efforts to drive improved membrane design and process integration.
The technology that is being investigated is palladium (Pd) membrane technology for H2 separation from syngas, a key enabling concept for pre-combustion CO2 capture. Pd-alloy membranes have been studied in membrane reactors for water-gas shift (WGS-MR) and steam reforming (SR-MR) reactions to simultaneously achieve a high CO or methane conversion and production of pure H2. A key feature of this process intensification, achieving pre-combustion decarbonisation (PCDC), is that such a membrane reactor would produce both a high pressure CO2 stream and high-purity H2 for power generation. This can greatly facilitate the economics of power generation with carbon sequestration. The technology is therefore an alternative to post-combustion decarbonisation, where more diluted CO2 is removed, by various technologies, from the flue gas after combustion. In the PCDC membrane process the CO2 stream is kept at high pressure reducing the final compression work for storage.
At the temperatures needed for direct integration in the reforming of natural gas, or other fuels like bio-ethanol or hydrocarbons in general (typically 550 – 600 C), current membrane technology is insufficient. This is because the state-of-the-art thin Pd-based membranes suffer from pinhole formation at such high temperatures, rapidly decreasing membrane life-time. As a solution, the Pd membrane thickness is increased in order to reduce the effect of pinhole formation, leading unavoidably to a decrease in H2 permeance. Improved Pd-alloys and composite membrane structures are hence needed for the next generation of H2 separation membranes and membrane reactors. To solve this problem fundamental understanding of degradation mechanisms is required.
Results to date:
The project started officially on the 1st of June 2012, and has thus run for almost one and a half year. In June 2012 kick-off meetings were held with the NTNU and TU Delft where the focus and division of work has been decided upon. The Post-doc position at the NTNU was announced with a deadline of 1st of September, and the position was offered to Dr. Yanjie Gan. She started her post-doc position the 1st of April 2013. Since beginning to work in the project, Dr Gan has mastered the specific skills necessary to prepare samples for Transmission Electron Microscope examination of membranes. Examination of samples from the project has been started. Untested membranes are being characterised in order to provide a comparison with membranes that will be characterised after testing. Microstructure, grain-size, defects and variations in composition are being documented. In addition, orientation imaging microscopy is being evaluated on the membranes. This is a TEM-based technique that can map crystallographic orientation on the nanometre-scale where the SEM-based technique of electron backscattered diffraction has insufficient spatial resolution due to the fine grain-size of the membranes.
In the first year, preparation and characterisation work of Pd-alloy membranes at SINTEF has been focused on the ternary Pd-Cu-Ag alloy. This alloy is relevant for the development of highly permeable sulphur-tolerant Pd-Cu alloy membranes, and has therefore been used as model system for this study, aiming at obtaining relationships between micro/nano-structure and properties of the Pd-alloy materials. At SINTEF, nine Pd-Cu-Ag alloy samples with varying composition have been manufacturing by magnetron sputtering followed by appropriate characterization, and extensive H2 solubility and permeation evaluation. As-prepared Pd-alloy films have been characterised employing different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy Dispersive Spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Samples have also been sent to the NTNU.
Alloying of the Pd-Cu alloy with Ag results in solid solution Pd-alloys with expanded lattices as shown by the linearly increasing unit cell dimension obtained by XRD. For these materials, an increasing trend in H2 permeability with the fcc unit cell dimension has initially been observed. However, after that a maximum value for the H2 permeability is obtained at a Ag content of 1 at.%, a H2 permeability decrease with Ag content is observed up to 7 at.%. In contrast, the H2 solubility has been found to follow a continuing linear increase with Ag content and thus unit cell dimension. This would thus indicate that the H2 diffusivity shows a distinct maximum. This detrimental effect of an increased content of TM atoms in Pd-based ternary alloys can, for example, be due to pinning of hydrogen near the dissolved TM atoms, hindering efficient diffusion of hydrogen through the lattice. This behaviour is now under further investigation both experimentally and theoretically. The theoretical considerations are supported by atomistic calculations based on DFT. Atomistic models of grain boundaries have been created to test the hypothesis that the minority element segregates towards such boundaries, thus creating percolating regions with increased lattice constants and potentially enhanced hydrogen diffusivity.