Monoclinic ZrO 2 and its supported materials Co/Ni/ZrO 2 for N 2 O decomposition
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J. Lin Department of Physics, Faculty of Science, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511, Singapore
W. K. Teo and J. C. Wu Department of Chemical Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511, Singapore
K.L. Tan Department of Physics, Faculty of Science, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511, Singapore (Received 18 July 1994; accepted 14 November 1994)
Monoclinic ZrO2 and its supported materials Co/Ni/ZrO2 (Co:Ni = 1:1) for catalytic decomposition of N2O have been studied with GC, FTIR, EDAX, XPS, and the evaluation of catalytic activity of the materials. It is found that monoclinic ZrO2 alone has the catalytic effect for N2O decomposition, although higher activities are found for Co/Ni/ZrO2 systems. XPS study shows that only Co exists in the surface region of ZrO2, which is attributed to the formation of NiO-ZrO2 solid solution resulting from an interdiffusion between Ni2+ and ZrO2 matrix. The gas decomposition on Co/Ni/ZrO2 can be described as first order with respect to partial pressure of N2O. Surface reactions on ZrO2 and Co/Ni/ZrO2 will also be addressed.
I. INTRODUCTION
Nitrous oxide (N2O) in earth's atmosphere contributes to catalytic stratospheric ozone destruction and greenhouse warming. Though it is now generally considered an environmental pollutant, nitrous oxide still continues to increase in the atmosphere at an increasing rate of 0.2% per year. This increase appears to be caused mainly by anthropogenic activities.1 Man-made nitrous oxide may arise as a co-product from some chemical processes, such as the use of circulating fluidized beds for combustion, automotive exhaust emissions, and the production of large amounts of adipic acid, which is produced from the HNO3 oxidation of cyclohexanolcyclohexanone mixtures, for Nylon 6,6 and 6,12.1>2 Nitrous oxide produced from the above industrial applications ranges from several hundred ppm in a fluidized bed combustion of coals (2 and its supported materials Co/Ni/ZrC>2 for N2O decomposition
N 2 O molecule would seem to be a viable path for N 2 O adsorption. Taking the above experimental results into consideration, the decomposition mechanism of N 2 O on monoclinic ZrO 2 can be thus postulated as the following: N 2 O(g)
(4.1) N 2 (g)
(4.2)
O^(ad) — O-(flrf) + l/2O 2 (g)
(4.3)
It should be noted that, for a 99.5% or higher conversion, the monoclinic ZrO 2 catalyst 00Z has to be operated at a relatively high temperature [Fig. 2(a)], and at this temperature the oxide ZrO 2 would turn itself to a solid electrolyte for lattice anion O2~ transport, i.e., conducting the charge for the reaction. The loaded metal cobalt Co 2p3/2 XPS spectra measured from Co/Ni/ZrO 2 systems at room temperature are presented in Fig. 7. According to a very recent surface analysis for the cobalt oxide system,18 Co 3 O 4 is in thermodynamically stable form at room temperature, and a Co 3 O 4 to CoO transition occurs at around 620 K. For an air-f
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