Surface and electrochemical properties of molybdenum nitrides.
Lee, Hyuek Joon
1995
Abstract
The purpose of this research was to elucidate the surface and electrochemical properties of molybdenum nitrides. Molybdenum nitride powders were synthesized using temperature programmed reaction method, and tested for electrochemical stability. These materials ($\gamma$-Mo$\sb2$N) were stable up to 1.122 V (vs. NHE) in acid media. Hydrogen adsorption/desorption and hydrogen evolution were observed within this window. The electrocatalytic activities for the hydrogen evolution and oxidation reactions of Mo nitrides were evaluated for a series of high surface area thin film electrodes. The hydrogen evolution reaction (HER) rates were greater than those of Pt on a mass basis. However, the intrinsic activities of the Mo nitrides based on their BET surface areas were two orders of magnitude less than those of Pt. The hydrogen evolution and oxidation activities of the Mo nitrides depended on their surface areas and phase constituents, indicating structure-sensitivity. In order to evaluate the influence of crystalline phase and texture on the electrocatalytic properties of Mo nitrides, a series of phase-pure $\beta$-MO$\rm\sb{16}N\sb7,\ \gamma$-$\rm Mo\sb2N,\ \delta$-MoN films were prepared using ion-beam assisted deposition (IBAD). Hydrogen adsorption/desorption were observed during cyclic voltammetry, and the characteristics depended strongly on the phase and structure of the film. The double layer capacitances were in the range of 38 to 410 $\mu$F/cm$\sp2.$ The hydrogen evolution activities increased in the following order: $\delta$-MoN(200) $< \gamma$-Mo$\rm\sb2$N(200) $<\ \beta$-Mo$\rm\sb{16}N\sb7(400) < \beta$-$\rm Mo\sb{16}N\sb7(203).$ The hydrogen evolution and oxidation activities were one to two orders of magnitude lower than those of Pt, and comparable to that of tungsten carbides. The results demonstrated that hydrogen evolution and oxidation were structure-sensitive over the Mo nitrides. Hydrogen and ammonia temperature programmed desorption (TPD) was used to further investigate effect of surface structure on the properties of $\beta$-$\rm Mo\sb{16}N\sb7(400),\ \gamma$- $\rm Mo\sb2N(200),\ \delta$-MoN(101) films. Two types of H$\sb2$ desorption sites were detected; a low energy site producing a peak at $\sim$370 K, and a high energy site at $\sim$800 K (heating rae of 6 K/s). The low temperature peak was due to a surface site and the high temperature peak was due to a subsurface site. It is possible that hydrogen was stored in the subsurface layers during polarization, and this may explain the high capacitances for the Mo nitrides. The temperature programmed desorption of NH$\sb3$ was used to examine the acidity of sites at the Mo nitride surfaces. The desorption of Hz produced as a consequence of NH$\sb3$ decomposition depended on the structure of the films. The maximum desorption rates following saturation of the surface with NH$\sb3$ increased in the following order: $\delta$-MoN(101) $< \beta$-$\rm Mo\sb{16}N\sb7(400) < \gamma$-Mo$\sb2$N(200). No unambiguous relationship between the acidity of the Mo nitrides and electrocatalytic activities for hydrogen evolution/oxidation were observed.Other Identifiers
(UMI)AAI9610180
Subjects
Engineering, Chemical
Types
Thesis
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