Harnessing Hydrogen Spillover for Sustainable Catalysis

Bert D. Chandler & Michael J. Janik

The Pennsylvania State University

Hydrogen is a key component of our energy infrastructure, and is widely used in petroleum refining, ammonia production (for fertilizers), and the production of clean oxidants such as hydrogen peroxide.  Green hydrogen generated from renewable energy sources and geologic formations will be a critical component of lowering CO₂ emissions making hydrogen an important part of our future energy infrastructure, as well.  Despite its broad technological importance, there remain significant fundamental questions around how hydrogen interacts with various materials, particularly materials that catalyze hydrogenation and dehydrogenation reactions, which are particularly important in hydrogen storage materials. 

We have been investigating a phenomenon called “hydrogen spillover”.  In this process, H₂ adsorption onto a metal particle splits the molecule into two H atoms absorbed on the metal surface.  The H atoms can then “spillover” onto a semiconductor support in the form of highly mobile proton-electron pairs, where the proton is associated with surface hydroxyl groups and the electron is injected into the surface conduction band of the titania.  This process is important for a number of industrial catalytic reactions in the energy supply chain.  While spillover was originally described several decades ago, it has been a poorly understood and, until now, largely unquantified adsorption phenomenon.  A better understanding of the spillover process has the potential to enable several hydrogen generation, utilization, and storage technologies and can play a key role in reducing the energy requirements for large scale industrial reactions such as methanol synthesis, CO₂ hydrogenation, and the water-gas shift reaction.    

This project builds on our recent discovery that hydrogen adsorption on Au/TiO₂ catalysts is primarily characterized as spillover hydrogen on the TiO₂ semiconductor support.  This offers the first opportunity to quantify spillover, which is a prerequisite to measuring adsorption energetics and understanding the nature of this important phenomenon.  Our early-stage data indicates H spillover is governed by a combination of surface entropy, metal oxide / hydroxide surface chemistry, and support electronics (i.e. band gap).  However, the interplay between these factors and their relative importance remains uncertain.  For example, while it is apparent high band gap materials exhibit little to no spillover, it is possible or even likely the oxide electronics become less important as the band gap decreases.  Similarly, it is clear surface entropy is critical for understanding spillover, but the role of configurational entropy and its relationship with surface hydroxyls is at best unclear. 

We will therefore refine our methods for quantifying and evaluating H spillover.   This will include extending studies to additional metals and oxides, particularly examining the surface chemistry of anatase and rutile titania, and the role of added bases in tuning adsorption enthalpy and configurational entropy.  As we improve our methods for studying spillover, we will examine how to intentionally utilize spillover in several proof-of-concept selective hydrogenation and dehydrogenation reactions.  Our goal will be to learn to control spillover thermodynamics so that we can intentionally transfer the surface H from one component of a catalyst to another.  Over time, we expect to build a comprehensive framework for understanding what factors control spillover so that we can intentionally incorporate H spillover into catalytic processes for energy conversion and environmental sustainability.