不同温度予处理Pt/Al_2O_3催化剂对其表面结构和反应性能的影响

THE INFLUENCE OF PRETREATMENT AT DIFFERENT TEMPERATURES ON THE SURFACE STRUCTURE AND PERFORMANCE OF Pt/AI_2O_3

由于链烷烃在Pt/Al_2O_3催化剂上反应的复杂性,许多学者为了深入研究包括催化剂制备、反应机理等的内在规律,进行了大量的研究,取得了许多重要成果。近几年来,较多的工作着重从分散态金属的几何和电子因素,及金属同担体的相互作用进行考察,在催化剂制备方面,更多的注意添加第2、第3组元、杂质或毒物等对金属催化性能的调变作用,以期开发出高选选择性催化剂。而对不同气氛。

The effects of different pretreatments on the activity and selectivity of the Pt/Al_2O_3 catalysts for the reactions of C_6 alkane with hydrogen were studied. The different pretreatments used were treatments at different temperatures in hydrogen or nitrogen and poisoning of the catalysts with thiophene. The catalysts were prepared by impregnating γ-Al_2O_3 with H_2PtCl_6. Prior to use, the catalysts were calcined in air at 480℃ and reduced in hydrogen at 450℃. Catalyst compositions were determined by atomic emission spectroscopy. Pt particle sizes were measured with a high resolution transmission electron microscopy. Catalyst surface compositions were determined by XPS. A pulse microreactor with an attached gas chromatograph was used to study catalytic activity and selectivity. The reactor was a stainless steel tube (length 300 mm, i.d. 4mm) with a catalyst charge of 50mg and a hydrogen carrier gas flow of 30ml/min. The alkanes used were at least 99 mol% pure.The reactions of n-C_6,2MP and cyclohexane with hydrogen on Pt/Al_2O_3 were studied. When the catalysts were treated at high temperatures in hydrogen,it was found: (1) Pt particle sizes were unchanged, (2) The surface atom ratios, Pt/Al, O/Al and C/Al were only slightly changed, and (3) Hydrogenolysis of n-C_6 was drastically decreased. This activity decrease occured in samples treated at high temperature in both gases, hydrogen or nitrogen. It is suggested that the activity change is due to differences in metal-support interaction caused by the high temperature treatment, rather than to a covering of the Pt particles by the support or to Pt-Al alloy formation. When the catalysts were prepoisoned with thiophene, it was found that their activity for the reactions of n-C_6 was rapidly reduced while the activity for dehydrogenation of eyclohexane remained unchanged. The above results suggest that there are two kinds of Pt sites:a strongly adsorbing site,which is easily poisoned and affected byhigh temperature treatments, and is active for hydrogenolysis; and a weakly adsorbing site, which is not easily poisoned or affected by high temperature treatments, and is active for dehydrogenation.