Investigation of PD-CU alloying application in pd-based dense metallic membranes with h2 and sulfur interaction

Abstract

Energy consumption is increasing day by day all around the world as a result of rise in human population, civilization and quality of social life. Almost 77% of energy is produced from fossil fuels which negatively impacts our environment. To prevent the hazards of fossil fuels, the governments, industries and scientific world are trying to find better alternatives which are sustainable. Hydrogen is a strong candidate as an sustainable energy carrier due to its renewability, high energy content and high efficiency. However, hydrogen cannot be found in pure state naturally. It is generally found in mixture with different gases such as N2, CO, CH4, and CO2. Therefore, to utilize hydrogen in energy production such as in fuel cells, it is crucial to separate and purify it from these gas mixtures. There are different commercially available techniques such as pressure swing adsorption (PSA) and cryogenic distillation which require intense energy. Therefore, membrane separation technology has received more attention than other processes since it consumes less energy. Advanced membrane technology is also cost-effective for extracting pure hydrogen from a stream with low purity or low pressure, compared to PSA or cryogenic technologies. The properties of membranes affect the hydrogen yield at the end of the process, so they must possess excellent chemical, thermal and mechanical stability, high hydrogen permeability, and be cost-effective to prepare, energy efficient, and durable. Dense metallic Pd membranes have demonstrated great potential to achieve ultra-high pure hydrogen via separation from gas mixtures. However, susceptibility of Pd to sulfur poisoning and expensive production cost are common drawbacks for these membranes. Therefore, to decrease the cost and to create a surface which is sulfur-tolerant, Pd can be combined to form an alloy with other metals. In this study, palladium-copper alloy surfaces on support were prepared using sequential electroless plating (ELP) method due to low cost and high sulfur resistance of these alloys. α-Al2O3 was used as support material. Prior to plating, α-Al2O3 supports were cleaned, dried and then activated with Pd seeds. Following the seeding, activated supports were plated first with Pd and then, with Cu on Pd plated layer. The total plating time of Pd was 3 hours which was followed by 30 minutes, 1 hour and 3 hours of Cu plating. In some experiments, the plating procedures were repeated several times. Different drying temperatures were applied between these sequential plating procedures in oven. At the end of the deposition procedures, the supports contains both Pd and Cu were annealed under stagnant gas. Annealing is crucial to create a well-mixed Pd-Cu alloy. It was observed that as Cu layer thickness increased, the total annealing time and temperature had to be increased to provide enough driving force and time for Pd atoms to diffuse into Cu layer to create a well-mixed alloy. Our experiment results show the correlation between the film thickness and total annealing time which is directly proportional to achieve well-mixed Pd-Cu alloy. The total annealing time was found as 96 hours to create an FCC Pd-Cu alloy at 600 °C for 3 hours Pd and maximum 1 hour Cu deposition. However, these time and temperature conditions are not sufficient for the samples were undergone several 3 hours depositions such as 3 times 3 hours Pd deposition then 3 hour Cu deposition. Besides sulfur resistance of Cu, metal organic frameworks are also promising for gas separation processes with their good properties to prevent the sulfur poisoning of surface. They have variety of applications and this is one of them. Therefore, as a final step, UiO-66 (Zr) was plated on the Pd-Cu surfaces by growing the frameworks on the membrane surface. UiO-66 was preferred for the studies because of its high working capacity, good selectivity, cost-effective regenerability, and ease of modification. Through experimentation, a correlation was observed between the amount of Cu and the MOF deposits on the surface: when Cu amount is higher, the surface has a better characteristic to coat MOF. The Pd-Cu samples and MOF coated Pd-Cu samples were subjected to sulfur environment at 250-300 °C for 3 hours. SEM-EDS analysis was employed to characterize the surface morphology of the samples and determine the composition. According to SEM-EDS results, the chosen samples were also characterized by XRD to validate alloy formation and monitor changes in crystal structure upon sulfur exposure.