Transition metal phosphides (TMPs) have emerged as promising alternatives to traditional noble metal-based catalysts because of their excellent catalytic properties for hydrogen evolution reaction, hydrotreating, and hydrogenation applications.^[1-3] While noble metal-based hydrogenation catalysts can be effectively tuned by organic ligands,^[4-5] there are only a few such organic surface modification strategies known for TMPs so far.^[6-7] The limited range of available surface functionalities for TMPs makes it very challenging to understand how the properties of the surface groups influence TMP-based catalysis. This hampers accessing the full potential of ligand-directed catalysis for the more earth-abundant TMP catalysts. Methods that allow the introduction of surface organic groups with systematically varied properties are needed for rational improvement of TMP-based catalysts.

Herein, we present a novel approach to covalently modify cobalt phosphide with organic functional groups at the surface using a sonication-assisted approach. This enabled a systematic variation of electronic and steric properties of the surface groups, as well as changing their surface coverage. Combined X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and elemental analysis showed the successful surface modification with a range of different aryl and alkyl groups. We probed the effect of different surface groups with systematically varied properties by testing the modified materials as catalysts for the hydrogenation of α, β-unsaturated aldehydes. This analysis provided clear insight into the factors that govern catalytic properties and identified optimal electronic and steric properties and density of the surface groups for a selective C=O hydrogenation. Overall, our research significantly expands the current scope for the surface covalent functionalization of transition metal phosphides and shows the potential of these strategies for the tuning of the catalysis.

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[3] Y. Liu, A. J. McCue, D. Li, ACS Catalysis 2021, 11, 9102-9127.

[4] L. Zhang, M. Zhou, A. Wang, T. Zhang, Chem Rev 2020, 120, 683-733.

[5] C. A. Schoenbaum, D. K. Schwartz, J. W. Medlin, Acc. Chem. Res. 2014, 47, 1438-1445.

[6] I. A. Murphy, P. S. Rice, M. Monahan, L. B. Zasada, E. M. Miller, S. Raugei, B. M. Cossairt, Chem. Mater. 2021, 33, 9652-9665.

[7] R. Gao, L. Pan, H. Wang, Y. Yao, X. Zhang, L. Wang, J. J. Zou, Adv Sci (Weinh) 2019, 6, 1900054.