Wednesday, November 10, 2010: 10:10 AM
Alta Room (Marriott Downtown)
With heightened concerns for the clean air and clean environment, hydrogen energy derived from renewable sources in conjunction with fuel cell has recently been under investigation actively. Producing hydrogen from steam reforming of ethanol is considered as a preferred method. Ni-based catalyst is highly recommended in this process while the oxide supports play an important role on its selectivity and stability. Researches in the nanomaterials recently have revealed that catalytic nanoarchitecture with comparably sized metal and oxide domains can retain high activity because of an increase the interfacial contact area and hindering agglomeration of the active metal. It opens a new route in the development of highly active and stable catalysts for the steam reforming process. Here we present primary study on the impacts of zirconia particle size on the catalytic behavior of nanocomposite Ni-ZrO2 catalysts in the steam reforming of ethanol. Three nanocomposite Ni-ZrO2 catalysts with particle sizes of zirconia concentrated at 50 nm, 25 nm and 15 nm were carefully prepared by precipitation followed by impregnation method and nominated here as Ni-ZrO2-50, Ni-ZrO2-25 and Ni-ZrO2-15 respectively. The average nickel particle sizes in the first two catalysts were about 62 nm and 23 nm after reduction. To better understand the size effect of zirconia support, the Ni-ZrO2-15 catalyst underwent specially treated to enlarge the nickel particles to 80 nm in the reduced samples. The impact of weight flow rate on the catalytic activity was investigated at 400 ºC, ethanol/H2O = 1:3 for all the three catalysts. The catalysts showed complete conversion of ethanol at W/F higher than 2.0×10-4 (gcat h)/ml but dropped in an order of Ni-ZrO2-50, Ni-ZrO2-15 and Ni-ZrO2-25 catalysts in the reverse direction, indicating different activities of the catalysts. Analysis of the selectivity towards different carbonous compounds in the reactions operating at condition range that incomplete conversion of ethanol was achieved reveals clue in the reaction pathway of different catalysts, in which a main route of ethanol undergoing dehydrogenation to produce acetaldehyde followed by decomposition of acetaldehyde into CO and CH4 was found for all the three catalysts. Besides, trace of acetone and ethylene were also detected in the product stream of the Ni-ZrO2-50 and Ni-ZrO2-15 catalysts while little was found for the Ni-ZrO2-25 catalyst under the reaction condition range studied here. In ethanol steam reforming, acetaldehyde could also be oxidized into acetic acid and further transform into acetone by ketonization reaction, which could lead to unfavorable coke deposition on the surface of the support. Zirconia is reported to have high surface oxygen mobility especially in tetragonal phase when comparing with monoclinic phase. In the Ni-ZrO2-15 catalyst with a reduced zirconia particle size under the size limitation, predominant tetragonal phase along with trace of monoclinic phase of zirconia were detected in the XRD analysis while only a single monoclinic phase of zirconia was detected in the reduced Ni-ZrO2-50 and Ni-ZrO2-25 catalysts. The increase of oxidation activity due to the reduction of zirconia particle size has led to a higher selectivity towards acetone formation in the ethanol steam reforming for Ni-ZrO2-15 catalyst. In spite of the improvement in oxidation activity of the catalyst, the reduction in particle size of zirconia would not have significant positive impact on the catalytic activity unless the particle size of the nickel metal is also reduced into a limited range. Although the zirconia and the nickel particles are in a comparable range, 50 nm seems to be too large for the Ni-ZrO2-50 catalyst to perform a good activity in the steam reforming of ethanol. Reducing the particle of zirconia while keeping the nickel metal particle size in a range of larger than 50 nm, as in the situation of the Ni-ZrO2-15 catalyst, the improvement in the catalytic activity could only be in a limited range since insufficient C-C bond cleavage activity is supplied by the nickel metal. When both of the particle sizes of the zirconia and the nickel metal are reduced into a suitable range as in the situation of the Ni-ZrO2-25 catalyst, the increase of metal support interaction as revealed in the H2-TPR profiles lead to a significant improvement in both the activity and selectivity of the catalyst.