PNNL

The Science

Biomass represents an attractive domestic source of fuels and other key chemicals but requires catalytic processing to generate useful molecules. The persistent presence of water adds an additional level of complexity, as it affects catalyst surfaces and their mechanisms. Researchers used differently faceted titanium dioxide—TiO₂(001) and TiO₂(101)—nanoparticles, and isopropanol (IPA) dehydration as a model system to probe the surface interactions between the catalyst, water, and alcohol. They found that water does not simply block active sites during the reaction: it reorganizes the adsorbed alcohol into facet-specific surface complexes that alter the reaction barrier. On TiO₂(001), isopropanol adsorbs dissociatively and forms a strongly hydrogen-bonded complex with water, causing a large increase in the apparent activation barrier and a more severe activity loss. On TiO₂(101), isopropanol remains molecularly adsorbed and interacts more weakly with water, resulting in less inhibition.

The Impact

Water is commonly present during biomass-derived alcohol conversion, but its inhibitory effect has commonly been treated as simple competition for catalytic sites. This work addresses the fundamental problem of why and how water suppresses activity differently on distinct surfaces of the same oxide catalyst. The study provides a new molecular-level conclusion: water inhibition is controlled not only by how strongly water adsorbs, but also by how the catalyst surface organizes water and alcohol into specific interfacial complexes. This finding introduces interfacial molecular organization as a design principle for improving water tolerance in oxide catalysts.

Summary

Water is ubiquitous in biomass-derived feeds, yet its molecular-level impact on oxygen elimination reactions remains poorly understood. Researchers used TiO₂ nanocrystals with well-defined dominant (101) and (001) facets to probe the system to determine the mechanism of the facet-dependent effects of water. They observed that inhibition for dehydration of IPA on the TiO₂(001) surface is about four times more severe than TiO₂(101). Through a combination of in situ solid state nuclear magnetic resonance spectroscopy, in situ infrared spectroscopy, kinetic studies, and theoretical calculations, the team demonstrated that the disparity arises from the formation of distinct alkanol-water complex intermediates. On TiO₂(001), IPA undergoes dissociative adsorption to form an isopropoxide-H₂O complex that readily drives the surface into a complex-dominated regime. This pathway increases the activation barrier for C–H cleavage by 40 kJ mol⁻¹ by inducing a disordered transition state. In contrast, TiO₂(101) favors molecular IPA adsorption with weak hydrogen bonding to water, resulting in a smaller complex formation constant and a much smaller activation barrier increase (25 kJ mol⁻¹). By quantitatively linking facet-dependent complex coverage to transition-state destabilization, this work moves beyond simple site-blocking models and provides a conceptual framework for designing catalysts that remain active in environments that contain water.

Contact

Jianzi Hu, Pacific Northwest National Laboratory, jianzhi.hu@pnnl.gov 

Zdenek Dohnalek, Pacific Northwest National Laboratory, Zdenek.dohnalek@pnnl.gov 

Funding

This work was supported by the U.S. Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences within the Catalysis Science program (DE-AC05-76RL01830, FWP-47319). We thank Pacific Northwest National Laboratory (PNNL), a multiprogram national laboratory operated for the DOE by Battelle for all the characterizations. Part of the work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory, an Office of Science user facility at PNNL.