(6eh) Towards the Computational Design of Monolayer (Hydroxy)Oxide-Metal Bifunctional Catalysts

Ultrathin (hydroxy)oxide films grown on transition metal surfaces have widespread applications in processes related to electrocatalysis, photocatalysis, and heterogeneous catalysis, among others. These hybrid systems with novel properties are often prepared and characterized under very specific conditions, such as ultrahigh vacuum. When subsequently used in various applications, however, significant structural evolution, which depends strongly on the reaction conditions in-situ, is expected, and this evolution remains largely unexplored. In order to identify structure-property relationships and, ultimately, design/screen new materials with improved performance, the development of such understanding is essential.

The description of complex metal/hydroxyoxide interfaces has long been a challenge for modeling, because of the complications introduced by strong correlation effects in transition metal oxides and by the very rich interface structures. As a consequence, even many classic questions related to these systems, for example the origin of the strong metal support interaction (SMSI) that plays a central role in the catalysis of some oxide-supported metal nanoparticles, remain unanswered.

Recently, we have addressed the major challenge of developing self-consistent and highly accurate description of strongly-correlated transition metal oxides, hydroxides and oxyhydroxides, through synergistic use of a Hubbard U correction, a state-of-the-art dispersion correction, and a water-based bulk reference state for the calculations. The strong performance of our approach is illustrated on a series of bulk transition metal (Mn, Fe, Co and Ni) hydroxides, oxyhydroxides, binary, and ternary oxides, where the corresponding thermodynamics of redox and (de)hydration are described with standard errors of 0.04 eV per (reaction) formula unit.

We have applied these methodologies to investigate the growth of monolayer Ti- and Fe-(hydroxy)oxide films on VIIB group and IB group noble transitional metal surfaces, and have obtained mechanistic understanding of the origin of the strong metal support interaction. This understanding has been extended to predict the structure evolution of the other monolayer transitional metal (Mn, Co, and Ni) (hydroxy)oxide films on a variety of substrates in electrochemical environments. It has been demonstrated that the structure and oxidation state of the films can be systematically tuned by changing the applied electrode potential, the nature of substrates, and/or the size of the film cluster, and the predictions have been confirmed by synchrotron experiment on the monolayer Ni-(hydroxy)oxide/Pt, a hydrogen evolution (HER) electrocatalyst with improved performance in comparison with traditional Pt catalyst in alkaline conditions.

The above analyses have led us to conclude that the Ni (hydroxyl)oxide/Pt interface under hydrogen evolution HER conditions is composed of NiOH monolayers on Pt, and the predicted structure is subsequently used for a detailed structural and electrocatalytic analysis at three-phase boundaries of NiOH films, Pt substrates, and the surrounding water. The kinetic analysis shows that water can be dissociated to adsorbed OH* and H* groups in a bifunctional mechanism that significantly lowers the activation barriers for alkaline HER and provides a clearly measureable acceleration of the catalyst process as compared to mono-functional Pt catalysts. The mechanistic understanding and kinetic analysis also suggests a new catalyst, NiOH/Pt₃X, with improved performance in comparison with NiOH/Pt, which is further supported by electrochemical experiment.

The development of highly accurate description of strongly-correlated transition metal oxides, the mechanistic understanding of interface structure evolution and structure-properties relationships, and the successful prediction of new electrocatalyst has provided the foundation towards rational design of novel catalysts with increased activity, improved selectivity and controlled stability.

Selected Publications (20 total, 12 first (co-)author, h-index=10):

1. Z. H. Zeng, M. Chan, Z. Zhao, J. Kubal, D. Fan and J. Greeley, Synergistic effect of dispersion corrections and reference state for first principles-based prediction of electrochemical Pourbaix diagrams (submitted to Phys. Rev. Lett.)
2. Z. H. Zeng, J. Kubal, K.-C. Chang,  N. Markovic and J. Greeley, Towards Controlling the Structural Evolution of Monolayer (hydroxy)oxide-Metal on Noble Metal Substrate ( submitted to J. Am. Chem. Soc.)
3. Hemma Mistry, Rulle Reske, Z. H. Zeng, Zhi-Jian Zhao, J. Greeley, P. Strasser, B. R. Cuenya, J. Am. Chem. Soc. 2014, 136 (47), 16473-16476.
4. Z. H. Zeng, M. E. Björketun, S. Ebbesen, M. B. Mogensen and J. Rossmeisl, Phys. Chem. Chem. Phys. 15 (18), 6769 (2013)
5. M. E. Grass, D. R. Butcher, Z. H. Zeng, F. Aksoy, H. Bluhm, G. A. Somorjai, W. X. Li, B. S. Mun, Z. Liu, J. Am. Chem. Soc. 133 (50), 20319 (2011)