Atomic Layer Metal Deposition from Ethanol for Catalytic Applications

Core-shell architectures have been proven beneficial in enhancing nanocatalysts’ activity, selectivity and durability while increasing utilization of precious metals such as Pt, an expensive but highly active catalyst for various electrochemical and chemical reactions, for example the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells. In order to fully utilize benefits of this approach, a reliable synthesis method is essential to produce core-shell nanoparticles with uniform shell thickness tunable in 1 – 3 monolayer range. Usually the Pt monolayer shell is formed via electrochemical routes, such as galvanic displacement of an underpotentially deposited Cu layer.^[1]Here we report a surfactant-free, ethanol-based wet chemical approach to coating metal nanoparticles with uniform Pt atomic layers with high reproducibility and scalability. The principles applied in the coating method will be illustrated with two practical examples.

     The first example is ordered Ru-Pt core-shell nanocatalysts with enhanced carbon monoxide (CO) tolerance and dissolution resistance for the anodic HOR in PEM fuel cells.^[2] While the hydrogen produced via steam reforming followed by water gas shift reaction contains about 1% of CO which can severely deactivate the current fuel-cell catalysts, recent advances have been made in removing CO to ~ 10 ppm via preferential oxidation of CO in hydrogen feeds (PROX)^[3], renewing the interest in developing CO-tolerant catalysts for using inexpensive reformates (hydrogen with CO impurity) as the fuel other than pure hydrogen. The partial alloying between Ru and Pt is avoided, and thus single crystalline particles are formed though Ru and Pt have distinctly different crystal structures, hexagonal close-packed (hcp) for Ru, and face-centered cubic (fcc) for Pt. Ordered lattice structural transition from Ru(hcp) to Pt(fcc)at the core-shell interface is verified by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM), coupled with density functional theory (DFT) calculations. Furthermore, the Ru-Pt electrocatalysts with optimized structures and improved Pt utilizations exhibit significantly enhanced CO tolerance and excellent dissolution resistance at ultra-low catalyst loadings for the HOR in fuel-cell tests. This new level of atomic control also solves the dilemma in using a dissolution-prone metal (Ru) for alleviating the deactivating effect of CO.

     The second example is Pt monolayer coated on Pd nanoparticles with high activities and stabilities for the ORR, the cathodic reaction in PEM fuel cells. The uniformity of Pt shells is verified by various characterization techniques, and DFT calculations also show that two-dimensional growth of Pt on Pd is energetically favorable. The as-prepared Pt monolayer electrocatalysts exhibit high electrocatalytic performance toward the ORR.

     In conclusion, we have demonstrated a surfactant-free and high-yield chemical route for coating of Pt atomic layers on other metal nanoparticles, which ensures high reproducibility and scalability. The strategy illustrated here could be applicable to the fabrication of other bimetallic or multimetallic core-shell nanoparticles for various applications.

Acknowledgements

This research was performed at Brookhaven National Laboratory (BNL) under contract DE-AC02-98CH10886 with the U.S. Department of Energy (DOE).

References

[1]          R. R. Adzic, Electrocatalysis 2012, 3, 163-169.

[2]          Y.-C. Hsieh, Y. Zhang, D. Su, V. Volkov, R. Si, L. Wu, Y. Zhu, W. An, P. Liu, P. He, S. Ye, R. R. Adzic, J. X. Wang, Nat Commun 2013, 4, 2466.

[3]          K. Liu, A. Wang, T. Zhang, ACS Catalysis 2012, 2, 1165-1178.