Multicomponent alloy materials are used in a variety of applications in which surface properties are critical. For Pd-based alloys used as dense metal membranes in hydrogen purification processes, critical surface properties include the ability of the alloy surface to dissociate H2 and its ability to resist contamination. A fundamental alloy characteristic that can influence both of these properties is surface segregation, the propensity of the alloy's surface composition to differ from its bulk composition. The proposed effort will develop and apply a novel, high-throughput methodology for fundamental study of these surface properties in Pd100-x-yAgxRuy and Pd100-x-yAgxCuy alloys. This approach will enable measurement of segregation, H2 dissociation kinetics, and sulfur poisoning across continuous regions of alloy composition space, providing an unprecedented and comprehensive understanding of how these surface properties depend on composition.
One of the challenges inherent in the study of multicomponent materials is that of understanding their properties over a wide range of composition space without the need to prepare and characterize a prohibitively large set of discrete, fixed-composition samples. For efficient study of alloy surfaces, the PIs have recently developed two unique tools that will serve as the basis for the proposed investigation. The first is a deposition source for preparation of Composition Spread Alloy Films (CSAFs), such as Pd100-x-yAgxRuy, with lateral gradients in composition across their surfaces, thus exposing all possible alloy compositions on a single compact (~1 cm2) substrate. The second tool is a 10x10 multichannel microreactor for spatially resolved measurement of reaction kinetics on CSAF surfaces. In the proposed research, these tools will be combined with spatially resolved surface analysis techniques to deliver a fundamental understanding of the composition dependence of a number of alloy surface properties that are critical to the performance of dense metal hydrogen separation membranes.
The tools and methods for preparation and characterization of CSAFs that are developed and refined during the proposed research program will be applied broadly to the study of alloy properties far beyond those relevant to hydrogen purification membranes. Highthroughput study of alloy hardness, corrosion resistance, catalytic activity for fuels conversion reactions, etc., will generate comprehensive data sets across all possible compositions for a nearlimitless number of binary, ternary and even higher order alloys.
The proposed work will train students in the application of high-throughput approaches to a wide variety of problems in alloy materials science. Collaborations with Universidad Nacional del Litoral in Santa Fe, Argentina and the Department of Energy's National Energy Technology Laboratory will give students opportunities to work with world-class scientists outside Carnegie Mellon; we plan to pursue separate funding that would support student travel to work in Argentina. Students from Argentina will also benefit from their exposure to this research and their interactions with Carnegie Mellon researchers.
The results of the proposed work will be disseminated to the scientific community through publications and presentations at national meetings. Beyond the research impact of the proposed work, the PIs are very active in the research community; they serve several professional organizations in various capacities, including organization of research symposia on topics related to high-throughput catalysis and surface science. They are also active in service to professional student organizations.
Multi component materials often have unique properties that are superior to those of their individual components. Examples include materials used as catalysts for chemical conversions, membranes for gas separations, and corrosion-resistant steels. Identifying the best components--and the optimal amount of each--for a specific application can be time-consuming. To accelerate discovery and undersanding of complex materials, we have developed a high-throughput approach based on composition spread materials libraries. Composition spreads are thin-film samples prepared so that their compositions vary continuously across their surfaces. A single composition spread library can contain all possible compositions of a three-component system on a compact (~1 cm2) substrate. Measurement of film properties at different locations (i.e., different compositions) on the composition spread surface allow rapid characterization of relationships between composition and material properties across composition space. In this project, we developed high-throughput tools and applied them to understanding and optimization of Pd alloys (PdCu and PdCuAu) for use as membranes for recovery of hydrogen (H2) from mixed gas streams, such as those generated in advanced coal conversion processes. We measured the reactivity of the alloy surfaces for H2 dissociation--the first step in the separation sequence--as a function of alloy composition and mapped reactivity onto the details of the alloys' valence electronic structures. Our approach complements efforts of first principles computational modelers to provide a comprehensive understanding of alloy catalysts and their design for a variety of specific applications. We also measured the extent of sulfur corrosion that occurs across the libraries' surfaces upon exposure to H2S, a common contaminant in fossil-derived gas streams, and identified specific composition regions that have potential for corrosion resistance. Collaborators at Universidad Nacional del Litoral in Santa Fe, Argentina, prepared prototype membranes of the preferred compositons and demonstrated their corrosion resistance in the separation application. Our results contribute to the design basis for Pd-alloy membrane systems for H2 separation (and "rejection" of CO2 for sequestration) in advanced coal conversion applications, which enable use of the US' vast coal reserves for electricity generation with minimal environmental impact. The high-throughput tools and methodologies that were developed and refined as part of this project will enable rapid discovery, optimization and fundamental understanding of multicomponent materials for applications in a variety of important applications--including, but not limited to, catalysis, separations, chemical sensing and corrosion resistance.
