SrCl2-Promoted REOx (RE = Ce, Pr, Tb) catalysts for the selective oxidation of ethane: A study on performance and defect structures for ethene formation

The performance and characterization of the SrCl2-promoted REO, (RE = Ce, P r, Tb) catalysts have been investigated for the oxidative dehydrogenation o f ethane (ODE) reaction. The doping of SrCl2 to REOx significantly reduced C2H4 deep oxidation and enhanced C2H4 selectivity and C2H6 conversion. It h as been shown that the catalytic performance increases in the order of 30 m ol% SrCl2/CeO2 < 30 mol% SrCl2/PrO1.83 < 40 mol% SrCl2/TbO1.75. We observed that Cl leaching was modest in the latter two catalysts but gradual Cl los s was observed over the first catalyst. Within a reaction period of 60 h, t he first catalyst degraded, whereas the latter two catalysts were stable. T he C2H6 conversion, C2H4 selectivity, and C2H4 yield measured 1 h after the start of the ODE reaction were, respectively, 72.6, 68.8, and 49.94 for 30 mol% SrCl2/CeO2, 79.1, 71.4, and 56.58 for 30 mol% SrCl2/PrO1.83, and 82.6 , 75.8, and 62.6% for 40 mol% SrCl2/TbO1.75 at 660 degreesC and 1.67 x 10(- 4) h g mL(-1) contact time. The results of X-ray photoelectron spectroscopy (XPS) and chemical analyses of chloride indicated that Cl- ions were unifo rmly distributed in 30 mol% SrCl2/PrO1.83 and 40 mol% SrCl2/TbO1.75, but we re not so in 30 mol% SrCl2/CeO2. The Ce 3d, Pr 3d, and Tb 4d spectra obtain ed in XPS studies demonstrated that there are RE3+ and RE4+ ions present in the SrCl2-doped catalysts and SrCl2 doping facilitates the redox cycle of the RE3+/RE4+ couple via RE3+ generation. The results of O-2 temperature-pr ogrammed desorption (TPD) studies showed that the addition of SrCl2 to REO, could obviously lower the desorption temperature of lattice oxygen. Temper ature-programmed reduction (TPR) results revealed that SrC2 doping causes t he reduction temperatures of lattice O2- in REO, to decrease; in other word s, the activity of lattice O2- was promoted. We consider that such behavior s are closely associated with the defect structures formed in ionic exchang es between the SrC2 and the REO, phases. X-ray diffraction (XRD) results in dicated that, among the three SrCl2-doped catalysts, 40 mol% SrCl2/TbO1.75 showed a cubic TbO1.75 lattice most significantly enlarged and a SrCl2 latt ice most pronouncedly shrunk. In situ laser Raman results indicated that th ere were dioxygen adspecies such as O-2(2-) and O-2(-) on the 30 mol% SrCl2 /CeO2 catalyst. XPS results indicated that there were O-, O-2(2-), and/or O -2(-) species on REOx, 30 mol% SrCl2/CeO2, 30 mol% SrCl2/PrO1.83, and 40 mo l% SrCl2/PTO1.75 On the basis of the results of in situ Raman, O-2-TPD, TPR , O-18(2-) and C2H6-pulsing, and XPS studies, we suggest that O-2(2-) and O -2(-) as well as surface lattice O2- species participate in the selective o xidation of ethane to ethene, whereas in excessive amount, the O- species t end to induce the deep oxidation of ethane. (C) 2001 Academic Press.