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Critical Role of Mullite-type Oxides’ Surface Chemistry on Catalytic NO Oxidation Performance
Thampy, Sampreetha,Ashburn, Nickolas,Dillon, Sean,Liu, Chengfa,Xiong, Ka,Mattson, Eric C.,Zheng, Yongping,Chabal, Yves J.,Cho, Kyeongjae,Hsu, Julia W. P. American Chemical Society 2019 The Journal of Physical Chemistry Part C Vol.123 No.9
<P>By combining low energy ion scattering spectroscopy and density functional theory calculation, we study the surface composition and surface formation energy of AMn<SUB>2</SUB>O<SUB>5</SUB> (A = Sm, Bi) mullite-type oxides synthesized by different methods and their effects on NO catalytic performance. It is well-known that hydrothermal (HT) synthesis at low temperatures produces materials with higher specific surface areas (SSAs) compared with those synthesized by coprecipitation (CP) and high-temperature calcination; however, it is less clear how synthesis methods affect surface chemistry. We find that the NO oxidation performance of SmMn<SUB>2</SUB>O<SUB>5</SUB>-HT does not match the SSA increase when compared to the lower SSA SmMn<SUB>2</SUB>O<SUB>5</SUB>-CP. Combined experimental and theoretical investigation reveals that SmMn<SUB>2</SUB>O<SUB>5</SUB>-HT includes a higher fraction of inactive Sm-terminated surfaces, which explains its lower than expected activity. However, the surface chemistry change depends strongly on the A-site element. The exposed surfaces of BiMn<SUB>2</SUB>O<SUB>5</SUB>-CP are predominantly terminated by Bi and exhibit a very low activity, while BiMn<SUB>2</SUB>O<SUB>5</SUB>-HT contains active Mn-terminated surfaces. This study shows that catalytic performance is determined predominantly by surface chemistry, which depends critically on the A-site element and synthesis method and less by physical surface area.</P> [FIG OMISSION]</BR>
Zheng, Yongping,Thampy, Sampreetha,Ashburn, Nickolas,Dillon, Sean,Wang, Luhua,Jangjou, Yasser,Tan, Kui,Kong, Fantai,Nie, Yifan,Kim, Moon J.,Epling, William S.,Chabal, Yves J.,Hsu, Julia W. P.,Cho, Kye American Chemical Society 2019 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.141 No.27
<P>The correlation between lattice oxygen (O) binding energy and O oxidation activity imposes a fundamental limit in developing oxide catalysts, simultaneously meeting the stringent thermal stability and catalytic activity standards for complete oxidation reactions under harsh conditions. Typically, strong O binding indicates a stable surface structure, but low O oxidation activity, and <I>vice</I><I>versa</I>. Using nitric oxide (NO) catalytic oxidation as a model reaction, we demonstrate that this conflicting correlation can be avoided by cooperative lattice oxygen redox on SmMn<SUB>2</SUB>O<SUB>5</SUB> mullite oxides, leading to stable and active oxide surface structures. The strongly bound neighboring lattice oxygen pair cooperates in NO oxidation to form bridging nitrate (NO<SUB>3</SUB><SUP>-</SUP>) intermediates, which can facilely transform into monodentate NO<SUB>3</SUB><SUP>-</SUP> by a concerted rotation with simultaneous O<SUB>2</SUB> adsorption onto the resulting oxygen vacancy. Subsequently, monodentate NO<SUB>3</SUB><SUP>-</SUP> species decompose to NO<SUB>2</SUB> to restore one of the lattice oxygen atoms that act as a reversible redox center, and the vacancy can easily activate O<SUB>2</SUB> to replenish the consumed one. This discovery not only provides insights into the cooperative reaction mechanism but also aids the design of oxidation catalysts with the strong O binding region, offering strong activation of O<SUB>2</SUB>, high O activity, and high thermal stability in harsh conditions.</P> [FIG OMISSION]</BR>