Tetragonal Rutile SnO₂ Solid Solutions for NO_x-SCR by NH₃: Tailoring the Surface Mobile Oxygen and Acidic Sites by Lattice Doping

Abstract

To understand the active sites requirements for NO x -SCR by NH 3 and for designing better catalysts, four SnO 2 -based solid solution samples with the SnO 2 lattice doped by metal cations possessing redox ability (Ce 4+ and Cu 2+ ) or acidity (In 3+ and W 6+ ) have been purposely designed. It is revealed that the metal cations have entered into the SnO 2 matrix to form a tetragonal rutile solid solution phase. Compared with individual SnO 2 , all the modified catalysts possess higher surface areas, lower crystallinity, more abundant surface defects, facile oxygen, and acidic sites. Therefore, they display improved reaction performance. Both surface facile oxygen and acidic sites play vital roles, and the balance between them controls the reaction. For the catalysts modified by Ce 4+ and Cu 2+ , a better balance can be generated. Therefore, they display the best performance among all the samples. NO x -SCR by NH 3 on the SnO 2 -based solid solutions follows a Langmuir−Hinshelwood mechanism.

Investigation of lattice capacity effect on Cu2+-doped SnO2 solid solution catalysts to promote reaction performance toward NO -SCR with NH3

“…As seen in Fig. 3(a), pure SnO2 exhibited significant NOx reduction activity due to the coexistence of both surface active oxygen and acidic sites [11,24,35], with the highest NOx conversion of 40% achieved at 400 °C. In contrast, pure CuO showed obviously better activity at a much lower temperature, and offered the highest NOx conversion of ~40% at 250 °C.…”

“…According to the rules of solid solution formation [25,26], it is highly likely for Cu 2+ cations to dissolve in the SnO2 lattice matrix to form a substitution solid solution with a certain lattice capacity. Indeed, our previous studies have proven that Cu 2+ cations can dissolve in the lattice matrix of SnO2 with different morphologies when its content is low [20,24].…”

“…Over the past 8 years, using SnO2 as a model solvent metal oxide, we have systematically investigated the dissolution behavior of several metal cations in the lattice matrix of tetragonal rutile SnO2 to form solid solution catalysts for various reactions [20][21][22][23][24]. In addition to evident changes in the texture and physical property, doping the secondary cations into SnO2 lattice to form non-continuous solid solutions also increased the number of surface deficient oxygen sites on SnO2, which could also be stabilized even at temperatures higher than 400 °C [20,22,23].…”

“…In addition to evident changes in the texture and physical property, doping the secondary cations into SnO2 lattice to form non-continuous solid solutions also increased the number of surface deficient oxygen sites on SnO2, which could also be stabilized even at temperatures higher than 400 °C [20,22,23]. Moreover, due to the expanded surface area, Sn 4+ cations enrich the catalysts' surfaces, thus increasing their Lewis acidity [24]. As a consequence, the activity of the prepared solid solution catalysts increased for different reactions [21].…”

“…We have recently demonstrated that SnO2-based solid solutions modified by metal cations possessing redox ability (Ce 4+ and Cu 2+ ) or acidity (In 3+ and W 6+ ) exhibited improved reaction performance in NOx-SCR with NH3 compared to pure SnO2 alone, due to the increased number of both surface active oxygen sites and enhanced acidity [24]. Although the Cu 2+ -SnO2 solid solution catalyst displays a slightly lower maximum NOx conversion than the Ce 4+ -SnO2 catalyst, it shows a much higher low-temperature activity (below 250 °C).…”