Methane dry reforming with CO 2 on CeZr-oxide supported Ni, NiRh and NiCo catalysts prepared by sol–gel technique: Relationship between activity and coke formation

Abstract

Ni, NiRh and NiCo catalysts supported on Ce–Zr-oxide synthesised by pseudo sol–gel method have been investigated. Monometallic samples were performed with two different Ce/Zr ratios and by conventional impregnation. BET, XPS, TPR, TPO and TEM were applied for sample characterization and dry reforming of methane was carried out with a feed mixture consisted of CH 4/CO 2/Ar = 10/10/80 or CH 4/CO 2 = 70/30 ratio. Co and Rh containing samples were proved to be stable catalysts, the impregnated Ni catalyst only slightly, while sol–gel Ni samples slowly deactivated during the long term overnight run. The amount of carbon on the sample's surface after catalytic runs varied between 1 and 12 mg C/100 mg of catalyst. The Ni samples prepared by sol–gel and impregnation method had a peak maximum in TPO at 400 °C and 600 °C, respectively. In the case of bimetallic sol–gel samples the carbon that gave TPO peak at 360 °C was slowly transformed to that at 600 °C, showing that after several carbon deposition (dry reforming) and gasification (TPO) cycles structural changes in the catalyst and in the carbon morphology take place and graphitic carbon and nanotube formation become prevailing as was detected by TEM. Upon high temperature pre-treatment and methane dry reforming reaction, alloyed NiRh, NiCo particles and sintered Ni were observed, with the simultaneous presence of carbidic carbon, carbon nanotubes and shell-type graphitic carbon deposition. Support degradation and segregation happened to some extent, but still there was a certain amount of Ni in strong interaction with the support probably still included in the oxide matrix. Broad metal particle size distribution seems to play a role in long term stability. When pure methane was decomposed on Ni catalyst prepared by sol–gel method, the carbonaceous deposit could completely be removed by the subsequent CO 2 treatment, emphasizing the active role of Ce–Zr mixed oxide support in gasification of surface coke.