A temperature-programmed-reduction study on alkali-promoted, carbon-supported molybdenum catalysts

The reducibility of a series of carbon-supported, molybdenum-based catalysts with or without a potassium promoter was investigated using temperature-programmed reduction (TPR). The Mo loading and the K-doping level influence the reducibility of the K-Mo/C catalysts. The TPR spectra of both Mo/C and K-promoted Mo/C catalysts generally consist of four peaks. Two of these (located around 350 and 440 degrees C) are attributed to the reduction of octahedrally coordinated Mo [Mo(O)], and a third peak (located around 740 degrees C) is attributed to the reduction of tetrahedrally coordinated Mo [Mo(T)]. The fourth peak (located around 850 degrees C) is assigned to CO desorption from the decomposition of the oxidized carbon support. The distribution of the different Mo species is greatly affected by the Mo loading and the K-doping level. In the absence of the K promoter, Mo(O) species predominate at relatively low metal loadings while Mo(T) species predominate at higher loadings. The reducibility of both species, quantified by the hydrogen consumed, or the average Mo valence, decreases with increasing Mo loadings. The K/C runs indicate that K itself is not reduced, but it may modify the reducibility of other metal compounds. A small amount of K, when added to the Mo/C, increases the area of the low-temperature Mo(O) peaks at the expense of the high-temperature Mo(T) peak. Under these conditions, the Mo(T) is significantly more reduced than is the Mo(O). Larger amounts of K decrease the Mo(T) peak area and cause less reduction in both the Mo(T) and Mo(O) species. K and Mo interact in K-promoted Mo/C catalysts, with the interaction being most pronounced for molar ratios of K/Mo between 0.2 and 1. (C) 2000 Academic Press.