Kinetic Consequences of Subsurface Hydrogen in Alloy Catalysis

The design of catalysts for various industrial processes requires a rapid yet thorough search through multicomponent alloy composition space. High-throughput techniques have been developed to characterize the physical and electronic properties of alloy catalysts, as well as their reactivity and selectivity. Of particular interest in the field of heterogeneous catalysis are catalysts for reactions involving hydrogen due to their ubiquity in many chemical processes. This thesis applies high-throughput approaches to study the interaction between hydrogen and alloy catalysts to better understand the mechanism and energetics relevant to different reactions. The core of our methodology is a catalyst library known as a Composition Spread Alloy Film (CSAF) which is used in conjunction with a multichannel microreactor array to spatially isolate the alloys contained on the film. Combining the activity measurements using the microreactor with measurements of alloy composition and electronic structure using energy dispersive X-ray spectroscopy (EDX) and X-ray photoemission spectroscopy (XPS) allows determination of structure-property relationships, which can greatly accelerate catalyst discovery.

This work focuses on the kinetics of H₂-D₂ exchange, ethylene (C₂H₄) hydrogenation, and ethylene deuteration across AgPd binary alloys. These reactions were chosen because they are some of the simplest hydrogen-containing reactions but can still provide insight into the reaction of hydrogen with more complex molecules. In our study of H₂-D₂ exchange on a Pd catalyst, reaction data collected using the microreactor was used for kinetic parameter estimation and quantification of uncertainty limits when applying different rate laws. We have derived rate laws for the H₂-D₂ exchange reaction using three different microkinetic models: one that follows the traditional Langmuir-Hinshelwood (LH) framework and two that incorporate the effect of subsurface hydrogen (H’) on the reaction rate. The reaction mechanism that considers a second-order effect of subsurface H’, known as the Dual Subsurface Hydrogen (2H’) mechanism, has been shown to be more consistent with experimental data, and results in the prediction of kinetic parameters that agree with literature values for the adsorption of H₂ on Pd. These results support the growing evidence that non-Langmuirian reaction models may be more appropriate when hydrogen is present in the system.

In this thesis, we reformulate the microkinetic models developed for H₂-D₂ exchange so that they can be applied to ethylene hydrogenation and ethylene deuteration. Ethylene is the simplest molecule containing a C=C bond and its hydrogenation, with and without isotopic labelling, has been well-studied in the field of catalysis. Our proposed reaction mechanisms are the first to incorporate H’ into the rate laws for the hydrogenation and deuteration of ethylene. For comparison with the reaction models, we have collected high-throughput kinetic data for ethylene hydrogenation and deuteration using a Ag_xPd_1-x CSAF. Our results indicate that the mechanisms including the effect of H’ are more consistent with experimental data, as for H₂-D₂ exchange. In the process of collecting these datasets, we have observed that the catalytic activity of Ag_xPd_1-x alloys is enhanced in the presence of O₂, which supports surface segregation phenomena reported in the literature.

To study these reactions rigorously, we have made significant improvements to the microreactor protocol and data analysis techniques. The methodology detailed herein is generalizable to any reaction of interest, allowing this thesis to serve as a reference text for future high-throughput studies.