Chemical and Biological Engineering ETDs

Author

Aaron Roy

Publication Date

2-14-2014

Abstract

A method to synthesize high surface area α-MoC(1-x) without the temperature programmed reduction (TPR) of MoO3 is described. This method was then used, in conjunction with a novel in situ method to obtain highly dispersed PtMoz supported on α- MoC(1-x), to explore the suitability of such materials as catalysts for oxygen reduction reaction (ORR) in a proton exchange membrane fuel cell (PEMFC). Mg2(Mo3O8) and Zn2(Mo3O8), alternative tetravalent Mo intermediates to MoO2, were obtained through the thermal decomposition of Mg(MoO4) and Zn(MoO4) in mixtures of CO and H2. A novel sacrificial support method was developed where MgO and ZnO were investigated sacrificial support materials formed in situ as decomposition products of the Mo precursor. Reaction conditions were explored: including feed composition, synthesis temperature, and synthesis duration. X-Ray powder diffraction (XRD) was used to determine the time dependence on conversion, where 30 to 140 minutes at a temperature of ~700 °C was required to reach full conversion of the precursors. Scanning electron microscopy (SEM) revealed an equiaxed microstructure with primary particle sizes between approximately 15-30 nm, depending on the duration of high temperature exposure. Rapid heating rates along with limited exposure to high temperature required by this method resulted in gravimetric surface areas as high 45 m2/g, measured using four point Brunauer-Emmett-Teller (BET) analysis. A similar, ammonia-free, synthesis of β-MoN(1-x) in mixtures of N2 and H2 is also described. The ability of these precursors to be decomposed rapidly, in comparison to the time requirements for TPR, was utilized in the development of an in situ method for the preparation of PtMoz composite phases supported on α-MoC(1-x). Ammonium magnesium molybdate, (NH4)2Mg(MoO4)2, was developed as a precursor to α-MoC(1-x) which was found to form favorable interactions with ammonium derived (PtCl6)+2 species when co-precipitated in NH4(OH). When this co-precipitated, and amorphous, precursor was treated under the carbiding conditions developed for the synthesis of α-MoC(1-x), highly dispersed PtMoz on α-MoC(1-x) was obtained. Crystalline platinum phases were not observed by XRD and Transmission Electron Microscopy (TEM) revealed platinum phases with crystalline domains on the order of approximately 5 nm, homogeneously dispersed throughout the observed samples. The materials, as prepared, were found to be catalytically inert for ORR, leading to the development of an additional treatment designed to redistribute platinum phases in a manor conducive for yielding more labile surface species. In particular, it was found that a thermal treatment in a pure CO atmosphere directly following the initial material synthesis resulted in significant development of electrochemical surface area (ECSA). Materials were characterized using cyclic voltammetry (CV) in a rotating disc electrode setup. CV measurements were performed in both O2 and N2 saturated solutions of 0.1 M HClO4. The ECSA of this material was measured at ~36 m2/g and the specific mass activity for ORR was measured to be ~ 137 A/gpt. Catalyst durability measurements were also performed and the results are reported. The gravimetric surface area of the α-MoC(1-x) support was measured using BET N2 adsorption, measuring ~32 m2/g. The catalysts microstructure was also characterized using SEM. Transmission electron microscopy (TEM) was employed for the identification of various possible composite phases ranging from : α-Pt, α-PtMoz, β-Pt3Mo, and, α-(Pt2Mo3C)0.66. TEM results indicate a more broad distribution of these platinum phases, with the addition of what is likely β-MoC(1-x), in the materials following the CO surface treatment. X-ray photoelectron spectroscopy (XPS) was also used for surface analysis and the results are discussed.

Keywords

High Surface Area, Ceramics, Molybdenum Carbide, Molybdenum Nitride, Oxygen Reduction Reaction, Platinum

Sponsors

Los Alamos National Laboratory

Document Type

Thesis

Language

English

Degree Name

Chemical Engineering

Level of Degree

Masters

Department Name

Chemical and Biological Engineering

First Advisor

Ward, Tim

First Committee Member (Chair)

Atanassov, Plamen

Second Committee Member

Ward, Tim

Third Committee Member

Serov, Alexey

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