Title : Stability behaviour of Ce promoted Ni catalysts supported on modified ZrO2 with La2 O3, PO4 & WO3, for dry reforming of methane
Abstract:
The dry reforming of methane reaction into a valuable synthesis gas (H2 and CO), is a potential candidate dealing with the dwindling energy resources and global warming challenges simultaneously. Syngas is known as the building block for petrochemical industry for the production of hydrogen and liquid fuels like methanol, paraffin, olefins, oxygenates and aromatic through Fischer-Tropsch synthesis process. An incipient wet impregnation method was used to prepare Ni supported on high surface area ZrO2 and modified ZrO2 catalysts.The support modifiers are La2 O3 , PO4 and WO3 . Dry reforming of CH4 experiments over Ni/ZrO2 catalysts, subjected to calcination at 550 °C for 3 hr, were performed at 700°C for 400 min and at atmospheric pressure in a vertical stainless steel fixed-bed tubular (9.1 mm i.d. and 0.3 m long). Prior to each test, the catalysts were activated under a continuous flow of H2 (20 ml/min) for 1 hr at the 600°C temperature. Experiments were done using a feed gas mixture (CH4 , CO2 , and N2 ) at ratio 6/6/1 and the overall gas flow rate of 65ml/min (space velocity: 39000 ml (h.gcat.)-1(. It is well known that dry reforming reactions is negatively affected by some side reactions, for instance, Boudouard reaction, water gas shift reaction, and methane decomposition, which are mainly responsible for the deactivation of catalysts due to coke deposition. The physicochemical properties of various obtained catalysts were assessed using XRD, TPD, TPR, BET, TGA and TEM techniques. The support modification resulted in a substantial increase of the Lewis basicity of modified catalysts. La2 O3 incorporation produced the stable t-ZrO2 phase, consequently, Ni/La-ZrO2 catalyst, displayed the best performance among all the catalysts in terms of both CH4 and CO2 conversions. When La2 O3 was incorporated into ZrO2 , microscopic study displayed the formation of carbon nanotubes (MWCNTs) with an average external diameter of 28 nm, while the metallic species were located both inside as well as on the external surface of MWCNTs ( the majority of metal particles were positioned on the external side). The size of metal particles was as large as 6nm. Ni/Zr had a very low CH4 conversion and the activity declined rapidly, while La doping had overwhelmingly improved the catalytic activity. Compared with Ni/Zr, the percent rise in CH4 conversion was 91% and CO2 conversion was 48%. TGA analysis of spent catalysts was performed to evaluate the anti-coking ability of pure and La-modified catalysts. Lamodified catalyst had a weight loss of about 90% and combusting at relatively higher temperature (800°C) depicting the formation of stable carbon in the form of encapsulated or filamentous carbon. When the catalysts were promoted using 1% CeO2 , monoclinic ZrO2 phase transformed to tetragonal ZrO2 phase. After the CeO2 promotion, the amount of carbonaceous materials accumulated on the surface of catalysts significantly reduced and weight loss amounted about 50%. Furthermore, the corresponding peaks shifted to the lower temperature, which implies that the less stable carbon material had formed which burnt at relatively lower temperature. The 1% CeO2 promoter enhanced the stability of the Ni/La-ZrO2 catalyst.
