Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La_1–xSr_xCoO₃Perovskite Catalysts

Abstract

NO oxidation rates over La 1−x Sr x CoO 3 (x = 0−0.3) perovskite catalysts are reported as a function of Sr doping in the absence and presence of NO 2 in the feed. Sr substitution is found to increase the rate of oxidation and to diminish the inhibitory influence of NO 2 . Temperature programmed desorption and isotopic exchange (TPIE) experiments were used to identify surface species and oxygen exchange processes expected to correlate with NO oxidation activity. Oxygen exchange in the LaCoO 3 perovskites occurred primarily through a heteroexchange process that was enhanced by doping with Sr. Density functional theory (DFT) calculations were used to further investigate the oxygen exchange processes on (100) facets of undoped and doped LaCoO 3 . Vacancy formation is predicted to be more facile on CoO 2 -terminated than LaO-terminated surfaces. The Sr dopant segregates to the LaO-terminated surface and diminishes oxygen bonding consistent with the TPIE results. The results suggest a model in which multiple oxygen species contribute to low-and high-temperature oxygen exchange.

DFT Analysis of NO Oxidation Intermediates on Undoped and Doped LaCoO3 Perovskite

“…17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

“…17 First principle periodic supercell plane-wave DFT calculations were performed using the Vienna ab initio Software Package (VASP). 23 The core electronic states were treated with projector augmented wave (PAW) method 24 exchange and correlation treating within the PW91 generalized gradient approximation (GGA).…”

“…A variety of mixed metal materials have been shown to have NO oxidation activity. 15 In particular, Kim et al 16 found that supported perovskite catalysts LaMO 3 (M = Co, Mn) exhibited NO to NO 2 conversions comparable to to greater than that of a supported Pt catalyst at temperature ranges between 200-400 • C. Sr-, 16,17 Ag-, 18 and Cu-doping 19 are all found to have promoting effects on oxidation activity. Choi et al 17 showed that the doped material retained the perovskite structure and related its promoting effect to observed decreases in NO, NO 2 , and O 2 desorption temperatures in doped samples.…”

Density Functional Theory Study of the Oxygen Chemistry and NO Oxidation Mechanism on Low-Index Surfaces of SmMn2O5 Mullite

“…18,25 In order to discuss the thermodynamic stability of surfaces, we have calculated the surface grand potential (Ω), which has been previously used in studies of low-index surfaces of binary and ternary compounds. 43 , 44 The detailed computational process can be found in SI.…”

“…23,24 The oxygen chemistries and NO oxidation mechanisms on selected perovskites, such as LaCoO 3 and LaMnO 3 , have been reported. [25][26][27][28][29] In contrast, the high activities of SmMn 2 O 5 for NO oxidation have not been well understood yet. It has been proposed that Mn dimers on stepped (110) surfaces are active sites towards NO oxidation via ER mechanism based on density functional theory (DFT) calculations.…”