(544do) High Throughput Alloy Catalysis across Composition Space

The purification of H₂ gas using Pd membranes has received significant attention as a cost effective replacement for traditional purification methods like cryogenic distillation and pressure swing adsorption (PSA). However, these membranes are susceptible to hydrogen embrittlement and catalyst poisoning unless they are alloyed with other metals, like Cu, Au, and Ag. The mechanism and energetics for hydrogen adsorption and desorption from Pd-alloys remain largely uncharacterized at real operation conditions. Furthermore, data regarding ternary alloys is especially sparse due to the difficultly of preparing such samples through traditional techniques. This project aims to use a high throughput approach to rapidly screen all ternary alloys of a Cu_xAg_yPd_1-x-y composition spread alloy film (CSAF), and quantify the kinetics of H₂ interactions. Component metals, Cu, Ag, and Pd, were vaporized and deposited onto a single substrate in order to form a CSAF that spans all of ternary composition space. The CSAF also includes binary combinations of the three metals, as well as regions of pure Cu, Ag, and Pd. Activity data for the H₂-D₂ exchange reaction was collected on the Cu_xAg_yPd_1-x-y CSAF using a homemade multichannel microreactor that allows simultaneous measurement of activity at 100 discrete alloy compositions. The microreactor array allows rapid screening of 100 alloy catalysts in a matter of hours, as opposed to the lengthy and costly process of individual catalyst testing. Experiments were performed across a temperature range of 333-593 K, at varying partial pressures of H₂, and at several total inlet flow rates. Fitting the activity data collected at 100 different catalyst compositions into a microkinetic model, the correlation between the energy barrier of the reaction and alloy composition can be obtained. Previous testing using a Cu_xAu_yPd_1-x-y CSAF confirms high exchange activity on the Pd-rich side of the CSAF. However, results also show that Pd can be alloyed with as much as 50% Cu before a significant decrease in conversion is observed. Energy barriers for dissociative H₂ adsorption range from approximately 0.1 eV on the Pd-rich side of the CSAF to 0.5 eV when the Pd content falls below 25%. The recombinative desorption barriers are significantly higher, going from 0.6 eV to 0.9 eV, indicating that the H₂-D₂ exchange reaction is desorption-limited. Further experimentation using the Cu_xAg_yPd_1-x-y CSAF allows exploration of a new realm of composition space. Quantification of the kinetic parameters for H₂ interaction with Cu_xAg_yPd_1-x-y alloys greatly accelerates the discovery and optimization of multicomponent catalyst materials.