Synthesis of a Mo/Nb mixed carbide
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Synthesis of a MoyyNb mixed carbide V. L. S. Teixeira da Silvaa) NUCAT/PEQ/COPPE/Universidade Federal do Rio de Janeiro, Caixa Postal 68502, Rio de Janeiro, RJ-21945-970, Brazil, and Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061
M. Schmalb) NUCAT/PEQ/COPPE/Universidade Federal do Rio de Janeiro, Caixa Postal 68502, Rio de Janeiro, RJ-21945-970, Brazil
V. Schwartz and S. T. Oyamab) Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061 (Received 1 October 1996; accepted 10 November 1997)
Molybdenum and niobium carbides (b –Mo2 C, NbC), as well as mixed carbides of molybdenum and niobium, were synthesized by the temperature-programmed carburization method (TPC) using a 20 vol % CH4yH2 gas mixture. The starting materials were MoO3 , B-Nb2 O5 , and physical mixtures of B-Nb2 O5yMoO3 with Nby(Nb 1 Mo) atomic ratios varying from 0.2 to 0.8, respectively. Results from catalytic and temperatureprogrammed oxidation (TPO) measurements indicate that during the carburization of the Nb2 O5yMoO3 physical mixture with Nby(Nb 1 Mo) 0.8 there is, besides b –Mo2 C and NbC formation, the appearance of a carbidic phase not detectable by x-ray diffraction (XRD). This phase appears to be highly active and selective for the dibenzothiophene hydrodesulfurization (HDS) reaction.
I. INTRODUCTION
Although metallic carbides have been known to mankind since the 18th century when Berthollet et al.1 observed the changes produced in iron due to the addition of small amounts of carbon, only in the past few years has the study of transition metal carbides and nitrides become a substantial field of research.2 For many applications (ceramics, catalysts) small particle size or high surface areas are needed. Before 1973, when Levy and Boudart3 first observed the unusual selectivity of WC for the catalytic neopentane isomerization reaction, the number of papers published in the catalytic literature dealing with carbides and nitrides was relatively small. After that observation, there was an increase in interest in the surface properties of the materials with the number of publications growing substantially (Fig. 1). The samples used for catalytic studies in the 70’s and mid 80’s were prepared using metallurgical methods which, because of the high synthesis temperatures employed, led to materials of low specific surface areas (Sg ), and hence were not suitable for use as catalysts. The fact that such promising materials could not be synthesized in a high specific area led to a decrease in
a)
Present address: Departamento de Engenharia Qu´ımica/Instituto Militar de Engenharia, Pra¸ca General Tib´urcio 80, Praia Vermelha, Rio de Janeiro, RJ-22290-270, Brazil. b) Authors to whom correspondence should be addressed. J. Mater. Res., Vol. 13, No. 7, Jul 1998
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