(426c) Understanding the Adsorption and Hydrogenation of Model Bio-Oil Compounds in Aqueous Phase

Aqueous phase catalytic reactions are particularly important for sustainable processes such as waste remediation,¹ and fuel or chemical production.² The solvent can play a crucial role in the reaction kinetics,³ either by changing reactant adsorption strength on the catalyst surface⁴ or by unlocking new reaction mechanistic pathways. However, the role of solvent is not well-understood, making it difficult to apply intuition from vapor-phase reactions to the liquid phase, and often causing difficulty in interpreting kinetic data obtained in condensed phase. Additionally, the solvent can make it challenging, relative to the gas phase, to probe the catalyst surface, necessitating modifications in the experimental spectroscopy systems used. If the adsorption and reaction of molecules in aqueous phase catalysis can be better understood, it would greatly benefit catalyst design, particularly from ab initio simulations.

In this talk we will discuss the observed role of solvent (water) on kinetic behavior for hydrogenation and reduction reactions of model bio-oil compounds (e.g., phenol, furfural and benzaldehyde). First, we will show the enhancement of phenol hydrogenation reaction rates by controlling the solution phase pH.⁵ Next, we will discuss how these observed hydrogenation rates can be understood in part by considering the role of the solvent on adsorption of reactants and intermediate species. To do this, we discuss some techniques to determine the adsorption energy on metal catalyst surfaces (e.g., by generating an adsorption isotherm to derive adsorption energies). We demonstrate that by ensuring these adsorption measurements are reversible and path independent, we can derive adsorption energies in the aqueous phase. These aqueous-phase adsorption energies of phenol and other bio-oil model compounds are compared to liquid phase calorimetry to show the agreement and validity of the method, and also contrasted to calculations and literature gas phase calorimetry measurements for Pt⁶ and Rh to discuss the reasons for differences. We will also discuss spectroscopic techniques in the aqueous phase that we use to better understand the adsorption structure of model bio-oil compounds and their intermediates, including in situ X-ray Absorption Spectroscopy (near edge and extended fine structure) and surface enhanced Raman spectroscopy.

References

(1) Duca, M.; Koper, M. T. M. Powering Denitrification: The Perspectives of Electrocatalytic Nitrate Reduction. Energy Environ. Sci. 2012, 5, 9726–9742.

(2) Mukherjee, S.; Vannice, M. A. Solvent Effects in Liquid-Phase Reactions. I. Activity and Selectivity during Citral Hydrogenation on Pt/SiO2 and Evaluation of Mass Transfer Effects. J. Catal. 2006, 243, 108–130.

(3) Zhao, Z.; Bababrik, R.; Xue, W.; Li, Y.; Briggs, N. M.; Nguyen, D.-T.; Nguyen, U.; Crossley, S. P.; Wang, S.; Wang, B.; Resasco, D. E. Solvent-Mediated Charge Separation Drives Alternative Hydrogenation Path of Furanics in Liquid Water. Nat. Catal. 2019, In press. DOI: 10.1038/s41929-019-0257-z.

(4) Singh, U. K.; Vannice, M. A. Kinetics of Liquid-Phase Hydrogenation Reactions over Supported Metal Catalysts - A Review. Appl. Catal. A Gen. 2001, 213, 1–24.

(5) Singh, N.; Lee, M.-S.; Akhade, S. A.; Cheng, G.; Gutiérrez, O. Y.; Camaioni, D. M.; Glezakou, V.-A.; Rousseau, R.; Lercher, J. A.; Campbell, C. T. Impact of pH on Aqueous-Phase Phenol Hydrogenation Catalyzed by Carbon-Supported Pt and Rh. ACS Catal. 2019, 9, 1120–1128.

(6) Carey, S.; Zhao, W.; Mao, Z.; Campbell, C. T. Energetics of Adsorbed Phenol on Ni(111) and Pt(111) by Calorimetry. J. Phys. Chem. C 2019, 123, 7627–7632.