Carbon Dioxide Reforming Of Methane Over Ni-Co Bimetallic Catalyst

Carbon dioxide reforming of methane (or CO2reforming of methane, CRM)provides an attractive way not only to utilize these two abundant greenhouse gases,but also to transform these gases into syngas with a low or adjustable H2/CO ratio,which is more preferable for the Fischer-Tropsch process and methanol synthesis.Moreover, thermochemical energy storage property of CH4-CO2reformingprocess in the implementation of solar energy collection-storage-recyclingprocess has also attracted the wide attention from the international community.Therefore, carbon dioxide reforming of methane has broad application prospectsand potential development value.The key technique of CRM process is to develop a catalyst with high activityand carbon resisitence. In this paper, the catalysts were designed on the basis ofCRM reaction mechanism. We chose MgO as the support and Ni-Co as the activemetal components. Ni-Co/MgO catalysts were prepared by using a multiple-impregnation method. Catalyst structure and properties were analysized by XRD, N2-adsorption, H2-TPR, XPS, SEM and TEM. The effects of Ni-Co bimetallicinteraction for catalyst structure and performance were investigated. Furthermore,in order to optimize the preparation and reduction condition for Ni-Co/MgObimetallic catalyst, the effects of impregnation methods for active metals,calcination temperature and reduction time of Ni-Co/MgO bimetallic catalystwere investigated. The main research contents and conclusions in this paper aredescribed as follows:(1) Appropriate amount of Co can enhance the Ni dispersion, and the activemetal particle size is about10nm after reduction of Ni-Co bimetallic catalyst. H2-TPR and XPS analysis confirmed the interaction between Ni and Co, and theirintrinsic electronic and geometric structures improve the catalytic activity andcarbon resistence.(2)2Co-8Ni bimetallic catalyst with Ni/Co of4presented the best catalyticperformance duo to its metal particles better dispersed and stablelized. Thecatalytic performance were investigated in a fixed bed verticle stainless steelreactor (13mm i.d.) at800oC,0.1MPa, gas hourly space velocity (GHSV) is36000mL·g-1·h-1. The CH4and CO2conversion rate were90%and95%respectively, the H2and Co yield were90%and92%, respectively, the amount ofcarbon deposition was less than1.5wt%after8h CRM reaction.(3) Active metal impregnation way is one of the important factorsinfluencing the bimetallic catalyst structure and performance. First impregnationCo, then Ni is the best way to promote the active metal dispersion and obtain small active metal particles, which were essential to the mitigation of carbon formation.(4) Calcination temperature is the important factor influencing the activemetal reduction degree and dispersion. The results show that with the increase ofcalcination temperature, a significantly decrease of active metal reduction degreewas found, which resulted in the decrease of catalyst activity.500oC is suitablefor the Ni-Co/MgO.(5) Reducing catalysts with hydrogen is an important process for carbondioxide reforming methane since metallic active sites are exposed and dispersedduring the reduction process. Activity evaluation showed that catalysts reducedfor1h exhibited superior catalytic activity with methane and carbon dioxideconversion to be92%and97%, respectively. The high activity is ascribed to thebetter metal dispersion (10.5%) and smaller active metal particle size (10nm).(6) Finally, based on CRM unit of Coal based integrated system，the stabilityof Ni-Co/MgO catalyst for CRM was investigated in a mixture gas atmosphere(H2/CO/CO2/CH4). Results show that Ni-Co/MgO bimetallic catalyst exihibitedexcellent activity and stability during the40h tests, the conversion rate of CH4and CO2were77%and96%, respectively, which close to the equilibriumconversion (78.8%and97%for CH4and CO2, respectively).