Mechanisms of the hydrogen reduction of molybdenum oxides

The two stages of the hydrogen reduction Of MoO3 to Mo were investigated in a thermal balance under well defined reaction conditions. Starting with different grain and agglomerate sizes for both stages, the influence of a set of parameters (temperature, local partial pressure of H2O, gas flow, etc.) on the reaction progress and the final result were studied in detail. Depending on the set of parameters used, different reaction mechanisms like pseudomorphic transformation or chemical vapour transport (CVT) were observed. Taking into account that grains and agglomerates deviate from a spherical shape and a definite grain size, the extent of reaction is well described by standard theoretical gas-solid reaction models such as the shrinking core model or the crackling core model (CCM). Thermo-gravimetric analysis, X-ray diffraction, scanning electron microscopy, surface area measurements (BET method) and laser diffraction were used for these studies. Under all conditions, the first stage shows a reaction path MoO3 --> Mo4O11 --> MoO2 via CVT. The reaction extent follows the CCM. Depending on the local partial pressure of H2O during reduction, the formed Mo4O11 and MoO2 exhibit different size distributions and shapes of the grains. The extent of reaction of the second stage develops according to the shrinking core model. Depending on the local dew point, two different reaction paths can occur: Pseudomorphic transformation at low dew points and transformation via CVT at high dew points. This paper is an extract from the Ph.D. thesis of W.V. Schulmeyer Mechanismen der Wasserstoffreduktion von Molybdanoxiden, 1998, Darmstadt University of Technology, Institute of Material Science, Department of Chemical Analytics, FRG. It focuses on a phenomenological description of the most important results. (C) 2002 Elsevier Science Ltd. All rights reserved.