Abstract
Tin dioxide is an n-type semiconducting oxide with a broad forbidden energy gap. This is often used as an active element for gas detection [1±4]. This material is suitable for gas-sensing applications particularly for the detection of reducing gases because reaction of reducing gases with adsorbed oxygen species induces a conductivity change by surface adsorption leading to a modi®cation of the free electron density on the surface. By dispersing small amounts of noble metals on the surface of the SnO2, the sensitivity, selectivity and the rate of response can be markedly improved. Molybdenum nitride is known to have unique physical and chemical properties, including electronic, magnetic and catalytic properties [5], that are similar to noble metals in many aspects. During the last decade, a signi®cant number of works have been reported on its synthesis [6±8], catalytic properties [9±11] and characterization [12]. Molybdenum nitride has attracted much attention because of its excellent catalytic activity in a number of hydrogenation reactions [13] such as ammonia synthesis, hydrotreating, hydrogenolysis, etc. Volpe and Boudart [14] reported that high-surface-area a-Mo2N can be prepared by reducing MoO3 with NH3 in a temperature-programmed manner. Speci®cally supported molybdenum nitrides with high surface areas have been prepared and used in hydrotreating reactions [15±17], and these show high potential especially in industrial applications. The predominant phase in molybdenum nitride catalysts is aMo2N. The Mo atoms in a-Mo2N are arranged in a face-centered cubic array with nitrogen atoms randomly distributed in half of the octahedral interstices. These interstices may also be ®lled with other atoms such as oxygen [18]. As reviewed by Oyama [13], molybdenum nitride is produced by dissolving nitrogen atoms into the lattice of the Mo metal. This results in a Mo±Mo bond expansion from 272 to 417 pm to induce a d-band contraction and an increase of the density of states at the fermi level of the molybdenum atoms, leading to the resemblance of the nitride to the group VIII metals in electronic, magnetic and catalytic properties. As part of the ongoing work on the development of materials for detecting several hazardous gases, we have undertaken this problem to ®nd suitable alternatives to replace noble metal additives like Pt, Pd, Rh, etc. which are known to be expensive, by other less-expensive materials. These substitute additives must have properties similar to or comparable to those of the noble metals. Keeping this in mind, we have tried to prepare molybdenum nitride from molybdenum trioxide starting from ammonium molybdenate. In this letter, we report the preparation of tin dioxide by the sol±gel route and incorporation with molybdenum oxide and molybdenum nitride. The gas-sensing characteristics of pure SnO2, MoO3 and Mo2N-incorporated SnO2 have been studied and the results compared. A mixture of equal molar solution of tin tetrachloride, SnCl4 (Spectrochem India Ltd.) in deionized water (18 MU) and in ethyl alcohol were mixed. These two were mixed together and the mixture was kept stirring on a magnetic stirrer for 48 h, maintaining the temperature of the mixture at about 50 8C. The top water±alcohol mixture solution was decanted and replenished with fresh water± alcohol. This was repeated 4 to 6 times. The repeated solvent extraction and replenishment was found necessary to reduce both chlorine ion concentration [Cly] and hydrogen ion concentration [H]. The sol emulsion was allowed to harden into a xerogel [19]. Subsequent heat treatment of the airdried xerogel was found to improve its mechanical strength. The dried material was crushed and sintered at 500 8C for 2 h. X-ray diffraction (XRD) studies con®rmed the composition to be SnO2. This material was ground to a ®ne powder and fabricated into sensor elements. XRD data also show a broadening of the peaks, which indicates a decrease in grain size. The average crystallite size was calculated using the Scherrer formulae [20] and found to be about 70±80 AE . This is signi®cantly lower than that of SnO2 prepared by the conventional hydrolysis route and sintered at 500 8C. In the latter case, the grain size as calculated from the XRD data was about 120 AE . The Mo2N was synthesized by following a particular sequence of temperature programs. First, MoO3 was obtained by calcination of ammonium metamolybdatete tetrahydrate, H24Mo7N6O24 4H2O (Fluka) at 500 8C for 2 h under normal conditions. The MoO3 was con®rmed by XRD for impurities and complete conversion. This material was then used to prepare Mo2N in the following manner. The MoO3 obtained was uniformly heated in owing ammonia. After a quick increase (5 8C miny1) of the reaction temperature from room temperature to
Published Version
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