Template-Free Synthesis of Mesoporous Transition Metal Nitride Materials from Ternary Cadmium Transition Metal Oxides Minghui Yang and Francis J. DiSalvo* Department of Chemistry, Cornell University, Ithaca, New York 14853-1301, United States ABSTRACT: A simple process for preparing mesoporous transition metal nitrides by the ammonolysis of bulk ternary oxides that contain cadmium is reported. Mesoporous NbN, VN, Ta 3 N 5 , and TiN have been obtained. The products were characterized by Rietveld refinement of powder X-ray diffraction patterns, scanning electron microscopy (SEM), and nitrogen adsorption/ desorption analysis. Pore sizes ranging from 10 to 40 nm are easily accessible. KEYWORDS: mesoporous, transition metal nitride, synthesis ■ INTRODUCTION Transition metal nitrides (TMNs) have long been studied and their properties as metallic ceramic materials have been used in a wide variety of applications, such as fuel cells, optical coatings, electrical contacts, and catalysts. 1,2 The diversity of TMN applications stems largely from their unique and varied properties. Like their parent metals, TMNs are electrically conductive. However, their extremely high melting point, hardness and corrosion resistance resemble that of ceramic materials. TMNs are also more catalytically active and selective for some reactions such as alkylation, hydroprocessing and hydrotreating than pure metals 3-6 and show great electro- chemical stability in harsh conditions, including high temper- ature and acidic conditions. 7 TMNs have been synthesized in a variety of ways including reacting metals with gas-phase reagents (N 2 , NH 3 , hydrazine, urea, etc.), liquid phase methods, high pressure, etc. 8-10 Most traditional high-temperature synthesis methods produce products with low surface areas because significant sintering occurs at high temperatures. Low-temperature syntheses are therefore favored for applications where obtaining higher surface areas is a consideration. 11 This report extends our recent work on the nitriding of Zn containing transition metal oxides in order to produce smaller pores and lower processing temperatures. In the Zn case, ammonolysis at temperatures near 700 °C and above produces mesoporous nitrides resulting from the condensation of atomic scale voids created by the loss of Zn by evaporation, the replacement of 3 oxygen anions by 2 nitrogen anions, and in most cases the loss of oxygen to form water on the reduction of the transition metal. Since Cd is more easily reduced than Zn, and since Cd has a higher vapor pressure than Zn at a given temperature, we hypothesized that mesoporous nitrides with smaller pores could be obtained from Cd oxides at lower processing temperatures than from Zn oxides. Indeed, this report confirms that mesoporous NbN, VN, TiN, and Ta 3 N 5 with much smaller pores (below 10 nm) can be obtained by the ammonolysis of cadmium containing oxide precursors at temperatures as low as 450 °C. The TMNs were analyzed by X-ray diffraction (PXRD), scanning electron microscopy (SEM) and nitrogen absorption/desorption analysis. ■ EXPERIMENTAL PROCEDURES Cd metal oxides were prepared by solid-state reactions between CdO and Nb 2 O 5 ,V 2 O 5 , TiO 2 , or Ta 2 O 5 in a stoichiometric ratio. All chemicals used are commercially purchased and with highest possible purities (≥99.99%). Cd 4 V 2 O 9 and CdTiO 3 were prepared at 600 °C for 20 h. Cd 2 Nb 2 O 7 and Cd 2 Ta 2 O 7 were prepared at 1000 °C for 10 h in a platinum crucible. These oxides were placed in an alumina boat. The boat was then placed in a silica tube with airtight stainless steel end-caps that had welded valves and connections to input and output gas lines. All gases were purified to remove trace amounts of oxygen or water using pellet copper, nickel, palladium and platinum with zeolites as support. The silica tube was then placed in a split tube furnace and the appropriate connections to gas sources made. Argon gas was passed over the sample for 15 min to expel air before establishing a flow of ammonia gas (Anhydrous, Air Gas). The sample was heated to the above reaction temperatures at 150 °C/h. After treatment for the specified period, the furnace power was turned off and the product cooled to room temperature in ∼4 h under an ammonia flow. Before the silica tube was taken out of the split tube furnace, argon gas was flowed through the silica tube to expel the ammonia gas. The silica tube was left in lab for 24 h with one valve open in order to expose the ammonolysis product to air slowly. This latter procedure resulted in the formation of only a very thin oxide on the nitride surface. Finely ground powders were examined with a Rigaku Ultima VI powder X-ray diffractometer (PXRD) with CuK α radiation (K α1 , λ = 1.5406 Å and K α2 , λ = 1.5444 Å). Crystal structures of the oxides and resultant nitrides were confirmed by PXRD profiles using the GSAS package. 12 Scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX) were performed with a LEO-1550 field emission SEM (FSEM). Nitrogen adsorption/desorption isotherms were measured at -196 °C using a Micromeritics ASAP 2020 system. The samples were degassed at 200 °C for 24 h on a vacuum line. The Received: August 22, 2012 Revised: October 26, 2012 Published: November 1, 2012 Article pubs.acs.org/cm © 2012 American Chemical Society 4406 dx.doi.org/10.1021/cm302700w | Chem. Mater. 2012, 24, 4406-4409