Chemisorbed Superoxide Species Enhanced the High Catalytic Performance of Ag/Co₃O₄ Nanocubes for Soot Oxidation

Abstract

The respective action mode between surface-adsorbed oxygen and bulk lattice oxygen during catalytic soot oxidation is still not fully recognized. Herein, a series of Ag-loaded Co3O4 catalysts with different Ag loading amounts were prepared by the impregnation method, and 5% Ag/Co3O4 presented competitive catalytic activity toward soot combustion with a T 50 below 290 °C in 10% O2/N2. This remarkable improvement in catalytic performance could be primarily attributed to the enhanced Ag–Co3O4 metal–support interaction induced by the formation of uniform, dispersive, and suitable size metallic Ag nanoparticles. The activation, activity, consumption–regeneration, identification, and reaction of surface-adsorbed oxygen along with the activity of bulk lattice oxygen were characterized by various designed and in situ techniques. The results demonstrated that the chemisorbed superoxide species (O2 –) play the potentially responsible role for boosting soot combustion, while the bulk lattice oxygen is much less active within the tested temperatures, inducing a negligible activity contribution. Moreover, soot-temperature programmed reduction, isothermal kinetic study, and density functional theory calculation provided supplementary support for the enhancement effect of Ag–Co3O4 combination in the activation and utilization of surface-adsorbed oxygen. The overall objective of this work is to identify the role of surface-adsorbed oxygen and bulk lattice oxygen for soot oxidation over Ag/Co3O4 catalysts.

Rapid Discrimination of Adsorbed Oxygen and Lattice Oxygen in Catalysts by the Cataluminescence Method

“…In the classic Langmuir–Hinshelwood theory, it is believed that reactants and oxygen are first co-adsorbed on the surface of a catalyst, and then oxygen gets electrons and reacts with reactants . Therefore, earlier attention was concentrated on tuning the adsorbed oxygen to improve the catalytic efficiency. − In comparison, lattice oxygen in the bulk phase is considered to be too stable but it can selectively participate in some oxidation reactions. − In addition, lattice oxygen is able to reduce competitive adsorption, has high safety, and is easy to replenish, and thus, it has the potential to replace gas-phase oxygen. , Nowadays, there are three main relationships between adsorbed oxygen and lattice oxygen in catalytic reactions, i.e., synergy, ,− primary/secondary effects, , and opposition/alone. − More importantly, both adsorbed oxygen and lattice oxygen can realize mutual transformation under certain circumstances. − Therefore, it appears as a crucial need to develop an operation-simple method to identify lattice oxygen and adsorbed oxygen in catalysts.…”

“…5−9 In addition, lattice oxygen is able to reduce competitive adsorption, has high safety, and is easy to replenish, and thus, it has the potential to replace gas-phase oxygen. 10,11 Nowadays, there are three main relationships between adsorbed oxygen and lattice oxygen in catalytic reactions, i.e., synergy, 3,12−14 primary/secondary effects, 15,16 and opposition/alone. 17−19 More importantly, both adsorbed oxygen and lattice oxygen can realize mutual transformation under certain circumstances.…”

Metal–Support Interactions on Ag/Co3O4 Nanowire Monolithic Catalysts Promoting Catalytic Soot Combustion

“…Numerous catalysts for soot combustion have been reported using noble metals [7][8][9], perovskites [10,11], spinel-type oxides [12,13], hydrotalcites [14,15], alkaline metal oxides [16,17], transition metal oxides [18], and rare earth metal oxides [19]. Ag-based catalysts are promising candidates for catalytic soot oxidation reactions because of their low price among noble metals, and particularly the Ag 0 species has a high ability to activate oxygen [20][21][22]. Interestingly, the Ag 0 species can be automatically obtained by directly supporting Ag salt on reducible metal oxides (Co 3 O 4 , CeO 2 , MnO 2 , etc.)…”

“…Some reducible, noble metal-free metal oxides, such as CeO 2 and MnO 2 and particularly Co 3 O 4 , are active in soot oxidation [27]. Moreover, Co 3 O 4 exhibits a high ability for NO oxidation to NO 2 [22,28], which is a more effective oxidant than O 2 for soot oxidation. Thus, Co 3 O 4 becomes a good candidate for supporting noble metal catalysts in this reaction.…”

Soot Combustion over Cu–Co Spinel Catalysts: The Intrinsic Effects of Precursors on Catalytic Activity

“…The slight decrease in soot catalytic activity may be due to the minor attenuation of the relative content of O ads /(O ads + O latt ). Chen et al also reported the catalytic soot performance of Ag/Co 3 O 4 and found that the decrease in adsorbed oxygen species was an important reason for the decrease of catalytic soot combustion [ 50 ].…”