A quick and versatile one step metal-organic chemical deposition method for supported Pt and Pt-alloy catalysts

Abstract

A simple, modified Metal-Organic Chemical Deposition (MOCD) method for Pt, PtRu and PtCo nanoparticle deposition onto a variety of support materials, including C, SiC, B₄C, LaB₆, TiB₂, TiN and a ceramic/carbon nanofiber, is described. Pt deposition using Pt(acac)₂ as a precursor is shown to occur via a mixed solid/liquid/vapour precursor phase which results in a high Pt yield of 90-92% on the support material. Pt and Pt alloy nanoparticles range 1.5-6.2 nm, and are well dispersed on all support materials, in a one-step method, with a total catalyst preparation time of ∼10 hours (2.4-4× quicker than conventional methods). The MOCD preparation method includes moderate temperatures of 350 °C in a tubular furnace with an inert gas supply at 2 bar, a high pressure (2-4 bar) compared to typical MOCVD methods (∼0.02-10 mbar). Pt/C catalysts with Pt loadings of 20, 40 and 60 wt% were synthesised, physically characterised, electrochemically characterised and compared to commercial Pt/C catalysts. TEM, XRD and ex situ EXAFS show similar Pt particle sizes and Pt particle shape identifiers, namely the ratio of the third to first Pt coordination numbers modelled from ex situ EXAFS, between the MOCD prepared catalysts and commercial catalysts. Moreover, electrochemical characterisation of the Pt/C MOCD catalysts obtained ORR mass activities with a maximum of 428 A g_Pt ^-1 at 0.9 V, which has similar mass activities to the commercial catalysts (80-160% compared to the commercial Pt/C catalysts).

Fig. 1. Image of reactor and furnace setup, under argon atmosphere with a feed of 2 bar at room temperature, when heated to 350 °C. The manual control valves (CV1 and CV2) are open during purging (∼1 h) and closed after initial purge. The quartz wool in the reactor outlet is to contain the precursors in the reactor and a beaker filled with water on the exit flow is placed while flowing Ar to monitor the gas flow and prevent air from entering the reactor.

Fig. 2. Thermogravimetric analysis (top) and differential thermal analysis (bottom) on Pt(acac)₂ under air or N₂. Under air: a temperature ramp of 5 °C min⁻¹ from 20–800 °C; under N₂: a ramp of 4 °C min⁻¹ from 20–350 °C.

Fig. 3. Proposed Pt(acac)₂ phase diagram, plotted with the decomposition range and temperature excursion of the reactor.

Fig. 4. Proposed Pt(acac)₂ deposition mechanism onto a support material, starting with decomposition of Pt(acac)₂, adsorption of Pt, nucleation of Pt and finally, desorption of the acetyl acetone ligand.

Fig. 5. Transmission Electron Microscope images of different Pt and Pt alloy nanoparticles deposited onto different support materials (as indicated on each image) using the MOCD deposition mechanism.

Fig. 6. Transmission Electron Microscope images of MOCD prepared catalysts (A) 20 wt% Pt/C, (B) 40 wt% Pt/C, (C) 60 wt% Pt/C, and commercial catalysts (D) 20 wt% Pt/C, (E) 40 wt% Pt/C, (F) 60 wt% Pt/C, (G) MOCD Pt/C catalyst particle size distribution and (H) commercial Pt/C catalyst particle size distribution.

Fig. 7. X-ray diffraction patterns of the (A) 20, 40 and 60 wt% Pt/C MOCD catalysts and (B) 20, 40 and 60 wt% Pt/C commercial catalysts, showing the carbon (002) and Pt(111), (200), (220), (311) and (222) peaks with an inset of Pt(111) for each set of catalysts.

Fig. 8. Tafel plots of ORR specific activity for (A) 20 wt%, (B) 40 wt% and (C) 60 wt% MOCD and commercial Pt/C catalysts (HiSPEC 3000 for 20 wt% Pt/C) measured in O₂ saturated 0.1 M HClO₄ electrolyte at room temperature with a scan rate of 20 mV s⁻¹.