Xuebin Liu

Effect of Metal–Support Interaction on Coking Resistance of Rh-Based Catalysts in CH4/CO2 Reforming

Abstract The effects of metal–support interaction on coking resistance of Rh/Al2O3, Rh/SiO2, and Rh/CeO2 catalysts during CH4/CO2 reforming were investigated. Temperature-programmed reduction and temperature-programmed oxidation (TPO) over fresh and used catalysts suggested that for Rh/Al2O3 and Rh/SiO2, the strong interaction between the metal and support led to a high dispersion of Rh after reduction. The CHx species formed on well-dispersed Rh with small Rh crystals could act as active intermediates to react with CO2 to form CO and H2. On the other hand, the carbon species formed on isolated Rh with large Rh crystals had a relatively low reactivity with CO2, which was attributed to the deactivation of catalysts. For the Rh/CeO2 catalyst, TPO and CO2 pulse experiment results indicated that the CHx species formed were more reactive than those formed on Rh/Al2O3 and Rh/SiO2. The unique Rh–CeO2 interaction resulted in the creation of CeO2–x and oxygen vacancies after partial reduction of CeO2, which facilitated the CO2 dissociation. As a result, the dissociated surface oxygen could readily react with CHx species, thereby inhibiting the carbon deposition over Rh/CeO2.

Light alkenes preparation by the gas phase oxidative cracking or catalytic oxidative cracking of high hydrocarbons

The preparation of light alkenes by the gas phase oxidative cracking (GOC) or catalytic oxidative cracking (COC) of model high hydrocarbons (hexane, cyclohexane, isooctane and decane in the GOC process and hexane in the COC process) was investigated in this paper. The selection for the feed in the GOC process was flexible. Excellent conversion of hydrocarbons (over 85%) and high yield of light alkenes (about 50%) were obtained in the GOC of various hydrocarbons including cyclohexane at 750°C. In the GOC process, the utilization ratio of the carbon resources was high; CO dominated the produced COX (the selectivity to CO2 was always below 1%); and the total selectivity to light alkenes and CO was near or over 70%. In the COC of hexane over three typical catalysts (HZSM-5, 10% La2O3/HZSM-5 and 0.25% Li/MgO), the selectivity to COX was hard to decrease and the conversion of hexane and yield of light alkenes could not compete with those in the GOC process. With the addition of H2 in the feed, the selectivity to COX was reduced below 5% over 0.1% Pt/HZSM-5 or 0.1% Pt/MgAl2O4 catalyst. The latter catalyst was superior to the former catalyst due to its perfect performance at high temperature, and with the latter, excellent selectivity to light alkenes (70%) and the conversion of hexane (92%) were achieved at 850°C (a yield of light alkenes of 64%, correspondingly).

Production of light alkenes with low CO2 emission from gas phase oxidative cracking (GOC) of hexane

Compared with non-oxidative pyrolysis of hexane, the rupture of C-C bonds of hexane in the GOC process becomes rather easy due to the change of thermodynamics and kinetics of the reaction with introduction of oxygen. CO predominates in the COx produced, and the ratio CO/CO2 can be as high as 16. The GOC process could be operated in an autothermic way, which would minimize energy consumption of the whole process and greatly decrease the CO2 emission. GOC seems to be an environmentally benign and promising alternative route for light alkenes production using heavy feedstock.

Studies of Carbon Deposition over Rh-CeO2/Al2O3 during CH4/CO2 Reforming

1. Abstract Three types of carbon species (a, β, γ-carbon) exist on highly dispersed Rh, Rh crystals, and Lewis acid sites of Al 2 O 3 support over Rh/Al 2 O 3 during CH 4 /CO 2 reforming, respectively. When CeO 2 was added, the more α-carbon has been observed, which was suggested to be located on the newly formed Rh-CeO 2 interface, and as an active intermediate is related to the reforming activity. The p-carbon was remarkably decreased and most likely responsible for the deactivation of catalyst. The γ-carbon formation was greatly suppressed, which is usually considered not to participate in the reforming reaction.