Hydrogen spillover
In heterogeneous catalysis, hydrogen molecules can be adsorbed and dissociated by the metal catalyst. The migration of hydrogen atoms from the metal catalyst onto the nonmetal support or adsorbate comprise the spillover phenomenon. Spillover, generally, is the transport of a species adsorbed or formed on a surface onto another surface. Hydrogen spillover can be characterized by three major steps, the first being where molecular hydrogen is split via dissociative chemisorption into its constitutive atoms on a transition metal catalyst surface, followed by migration from the catalyst to the substrate, culminating in their diffusion throughout the substrate surfaces and/or in the bulk materials.
The mechanism behind hydrogen spillover has been long disputed. Khoobiar’s work in 1964 marks the nascency of the spillover concept. In his findings, yellow WO3 can be reduced by H2 to a blue compound with the use of a platinum catalyst. Since the phenomenon was not found when using Al2O3 as the catalyst, he claimed that the dissociative chemisorption of H2 molecules on the Pt particles created hydrogen atoms. The hydrogen atoms migrated from the Pt surface to the WO3 particles and reduced them to blue WO3−x particles.
Essentially, hydrogen atoms would migrate from a hydrogen-rich to a hydrogen-poor surface. However, these atoms are usually not generated on the surface of a support metal. Hence, the two conditions for hydrogen spillover include the creation of hydrogen atoms (requires catalysts capable of dissociating and absorbing hydrogen) and the ability of hydrogen atoms to be transported.
Attempts to characterize the mechanism of hydrogen spillover have seen the use of radiation photoelectron spectroscopy to analyze the shift between different oxidation states of the support (commonly metal oxides) via their respective emission spectra. In general, the mechanism is thought to proceed via the transfer of neutral hydrogen atoms to the support upon overcoming an activation energy barrier. This has even been observed at temperatures as low as 180K in metal-organic framework (MOF) catalysts laced with Palladium nanoparticles (PdnP’s). Upon transfer to the support, they assume the role of Lewis bases where they donate electrons and reversibly reduce the sorbent. Additionally, the hydrodesulfurization of dibenzothiophene show that hydroxyl groups seem to favor the migration of spillover hydrogen, whereas sodium cations may trap the spillover hydrogen and are detrimental to hydrogenation pathway.
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