K-Mo/γ-Al_2O_3催化剂的表面物种和吸附性质

Surface Species and Adsorption Properties of K-Mo/γ-A1_2O_3 Catalysts

本文应用LRS、TEM和脉冲吸附等技术分析了KCl助剂及其含量变化对MoO_3/γ-Al_2O_3催化剂表面钼物种的存在形式、聚集状态及其低温O_2、CO和300℃H_2吸附性能的影响.助剂钾和表面钼物种之间产生相互作用,导致其存在形式发生改变,促使氧化态样品中钼物种聚结及其相应硫化态样品中MoS_2微晶的长大.这种作用在K/Mo比0.65～0.8附近最大;当K/Mo比超过0.8时,γ-Al_2O_3表面出现KCl微晶,并有少量KCl和钼物种作用,影响了钾钼作用物种的形成,部分抑制了钼Mo(VI)的硫化或还原,使得硫化态样品中MoS_2的微晶晶型变差.由于MoS_2微晶聚集及少量和钼作用的KCl部分覆盖表面配位不饱和银位而致相应的O_2、CO和H_2吸附能力随钾含量增加而下降.

In our previous work, the structures of Mo/γ-AI2O3catalysts with the modification of different KC1 promoter concentrations have been characterized by XPS, XRD, ESR and TPR. Here, the LRS, TEM and pulse adsorption techniques are employed to explore further the effect of KC1 promoter upon the formation of surface species and adsorption properties of the catalysts. It was found that as a result of the interaction between promoter and molybdenum components in the oxidic catalysts the potassium promotes the polymerization of the surface molybdenum species to form the K-Mo interacting species as K6Mo7O24 compound. The sulphurization of this interacting species gives rise to the redispersion of the molybdenum species and the formation of MoS2 microcrystallites which can be verified by LRS, but are not large enough to be detected by XRD. At a higher loading of potassium promoter, the large KC1 crystallites seen by XRD are evidently formed on the catalyst surface. Furthermore, the reduced LRS peak areas of the interacting species inoxidic and MoS2 in sulphided samples suggest that the over-loaded KC1 partly retard the crystallization of the molybdenum components. The TEM characterization demonstrates further the above mentioned facts. TEM images and electron diffraction patterns of the sulphided catalysts show that surface morphology of MoS2 is improved with the increase of potassium content, while at high KC1 loading with the K/Mo ratio over 0. 8 a poor crystallization of MoS2 evolved. Comparing the TEM determinations with XRD results of the catalysts with K/Mo ratio over 0. 8, it could be deduced that in addition to the KC1 crystallites which are formed by most of the potassium there is still a small amount of KC1 interacted with, or even dissolved into the bulk of molybdenum components, which result in the retardation of crystallization of the molybdenum components. The pulse adsorption measurements of H2 at 300℃ and O2 and CO at - 83. 6℃ over the sulphided catalysts show a monotonic drop of the adsorption uptakes with the increase of K/Mo ratios. The adsorption capacity of the probe molecules on the molybdenum catalysts is mainly related to the amount of the coordinately unsaturated molybdenum sites, so-called Mo(CUS). In the light of the interaction between potassium and molybdenum components which result in the molybdenum aggregation, the drop in uptakes could be ascribed to the decrease of Mo(CUS) site numbers. At high K/Mo ratio of over 0. 8, the continuous falling of the uptakes should be due to the existence of small amount of the surplus KC1 which interacts with the molybdenum species and possibly covers part of the Mo(CUS) sites.