Alkali Metals and Cerium-Modified La–Co-Based Perovskite Catalysts: Facile Synthesis, Excellent Catalytic Performance, and Reaction Mechanisms for Soot Combustion

Abstract

A series of alkali metals and cerium-modified La− Co-based perovskite catalysts were successfully prepared by a simple method using glucose as a complexing agent. The physicochemical properties of catalysts were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N 2 adsorption, H 2 -temperature-programmed reduction (TPR), O 2 -temperature-programmed desorption (TPD), soot-TPR, NO-temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS), etc. Among the catalysts, La 0.9 Ce 0.05 K 0.05 CoO 3 possesses the highest catalytic activity for soot combustion, with T 10 , T 50 , and T 90 values of 269, 309, and 342 °C, respectively. In the presence of 10% H 2 O, T 90 is significantly reduced to 327 °C. As far as we know, the catalytic performance of the La 0.9 Ce 0.05 K 0.05 CoO 3 perovskite oxide catalyst is one of the best results in current reports for soot combustion, especially for T 50 and T 90 . The substitution of A sites by K and Ce ions produces numerous active sites of Co 2+ −O v on the surface of the La 0.9 Ce 0.05 K 0.05 CoO 3 catalyst and enhances the oxygen storage capacity by redox recycling between Ce 4+ and Ce 3+ . The La 0.9 Ce 0.05 K 0.05 CoO 3 catalyst also possesses a stronger ability of NO adsorption, storage, and NO-to-NO 2 oxidation compared to other prepared catalysts. Based on the results of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations, Langmuir− Hinshelwood (L−H) and Mars−van-Krevele (MVK) mechanisms were proposed as the main reaction mechanisms for soot combustion. More importantly, the La 0.9 Ce 0.05 K 0.05 CoO 3 catalyst exhibits good resistance ability for sulfur and water. These results provide a promising strategy for designing and preparing highly efficient and low-cost catalysts for the practical application of soot particle removal.

Catalytic Soot Combustion─General Concepts and Alkali Promotion

“…The disadvantage of the mobile alkali promoters is their loss due to volatilization at higher temperatures and leaching, caused by the dissolution of alkali in water vapor, which is a ubiquitous component of exhaust gases. 38 Typical stability studies in soot combustion consist of consecutive activity tests, which have been applied for both powdered 53,150,207,226,252 and monolithic catalysts. 182,229,250 Another method for stability testing is catalyst aging, 228 usually performed by overheating.…”

“…150 A promising method of alkali metal stabilization is their immobilization within an oxide matrix, by forming nanostructured phases 218 or glasses, 226,228,229 which can be further optimized by adding other dopants. 145 The stability of the soot combustion catalysts containing alkali promoters has been also investigated in the presence of contaminants such as SO 2 74,81,252,254 or water vapor. 252,254 Alkali-modified perovskites exhibit good resistance to both sulfur and water, 252 while potassium-promoted ceria shows low resistance to sulfur and water at high temperatures.…”

“…145 The stability of the soot combustion catalysts containing alkali promoters has been also investigated in the presence of contaminants such as SO 2 74,81,252,254 or water vapor. 252,254 Alkali-modified perovskites exhibit good resistance to both sulfur and water, 252 while potassium-promoted ceria shows low resistance to sulfur and water at high temperatures. 254 4.4.…”

Ultralight and spongy La–Mn-based perovskite catalysts modified by alkali metals and Ce: facile synthesis and excellent catalytic performance for soot combustion

“…The O-II and O-III peaks at 530.7–531.2 eV and 532.6–533.1 eV are related to chemically adsorbed oxygen on oxygen vacancies (O − ) and oxygen adspecies on the catalyst surface (O 2 − ). 56,57 The ratio of O − and O 2 − varies when La ions are replaced with the alkali and Ce cations. The observed tendency is consistent with the result of O 2 -TPD.…”

“…). 56,57 The ratio of O − and O 2 − varies when La ions are replaced with the alkali and Ce cations. The observed tendency is consistent with the result of O 2 -TPD.…”