Low Temperature Steam Reforming Of Methane Over Ni/CeO₂-ZrO₂Catalysts

Steam reforming of methane (SRM) is the most mature technology and themost widely used method for industrial hydrogen production, accounting formore than2/3of industrial hydrogen production. Steam reforming of methane isa strongly endothermic reaction, and a large amount of energy is required inchemical industry so as to achieve a high CH4conversion. The elevatedtemperature consumes more energy and thus needs the reactor with bettermaterial. From the perspective of energy and economy, the method is notsuitable. Therefore, the main challenge in efficient SRM is to develop a newcatalyst which is suitable for low temperature and atmospheric pressure reaction.On the other hand, low temperature steam reforming of methane can use thewaste heat from hot power plant as heat supply system. In this case, the energy issaved, meanwhile the catalyst should not be sintered at low temperature, and canmaintain the good activity for a long time.In this investigation, the catalysts were prepared by coprecipitation method,taking Ni as an active metal, CeO2-ZrO2for double oxide composite carrier. Thestudy mainly focused on the influence of reaction process conditions (includingcatalysts particle size and space velocity) and catalyst preparation conditions(including Ni content, Ce/Zr, calcination temperature) on the catalyticperformance for low temperature steam reforming of methane. The XRD, BET,TPR and CO2-TPD were employed to analyze the catalyst crystal structure, specific surface area and pore structure, redox property and basic property. Theresults show that15%Ni-Ce0.8Zr0.2O2catalyst (calcination temperature is650oC)prepared using4mol/L KOH as the precipitation agent has an outstandingactivity and stability, and this result is associated with the structure of thecatalyst. The research content and the conclusions are described as follows:(1). The15%Ni-Ce0.8Zr0.2O2catalyst was applied for the preliminary study oflow temperature steam reforming of methane. Firstly, we discussed the mainfactors which affected the internal and external diffusion process of the lowtemperature steam reforming of methane. Results showed that when the size ofcatalyst particles was smaller than0.250-0.420mm, the influence of internaldiffusion of reaction process could be eliminated, when the space velocity wasgreater than26500mL/gcat h, the external of reaction process could beeliminated. These results provided the theoretical basis for the selection ofcatalysts particle size and space velocity in the later experiment process.(2). Explore the effects of Ni loading and the Ce/Zr ratio on low temperaturesteam reforming of methane reaction activity. With the Ni loading amount ofcatalysts increased, the activity of the catalysts has a trend of first increasing andthen decreasing,15%is the best Ni loading amount. The different Ce/Zr ratiosmainly affected the redox property of catalysts, and further affected thereduction behavior of NiO. When Ce/Zr is4, the catalyst has best catalystactivity.(3). Then we discussed the effects of the type and concentration of theprecipitating agent on low temperature methane steam reforming of15%Ni-Ce0.8Zr0.2O2catalysts. The results showed that when the4mol/L KOH wasused as the precipitating agent, the catalyst had the best activity, this wasbecause the active metal had the suitable force with the carrier, and it had abetter balance between the amount of active metal Ni and Ni particle size afterreduced. (4). Based on catalytic activity evaluation and TG, XRD, TPR and BETcharacterization, the15%Ni-Ce0.8Zr0.2O2catalyst calcined at650oC exhibitedsuperior catalytic activity with47%CH4conversion and63%for H2yield. Theremarkable catalytic performance should belong to the formation of CeO2-ZrO2solid solution structure with regular crystalline phase, the smaller size of Niparticles of catalysts which are reduced for1h and relatively modest surfacearea at this temperature.(5). The effect of different space velocity (90000、60000、30000mL/gcat h)on the activity and stability of the catalyst was studied over15%Ni-Ce0.8Zr0.2O2catalyst, which was the catalyst prepared in the above optimal conditions,namely15%Ni-Ce0.8Zr0.2O2catalyst calcined at650oC with4mol/L KOH asprecipitant agent. The results showed that the activity of catalyst was very stableat space velocity of30000and60000mL/gcat h, and the conversion of CH4is44%and33%respectively in100h running. However, when the space velocity roseto90000mL/gcat h, the activity of the catalyst decreased continuously untildeactivation. This might ascribe to the loss and oxidation of the active metal Niin the reaction.