(560gc) Surface Lattice Oxygen Activation Via Zr4+ Cations Substituting on a2+ Sites of MnCr2O4 Forming ZrxMn1-xCr2O4 Catalysts for Enhanced NH3-SCR Performance

The
emissions of NO_x have caused great environmental issues, and the V₂O₅-WO₃/TiO₂
is the most widely used commercial catalyst for NH₃-selective
catalytic reduction (NH₃-SCR). However, there are many
disadvantages with this catalyst, such as the toxicity of vanadium, the high price of WO₃, and the high optimal
operating temperature range of 350-420 ^oC. Therefore, it is desirable to develop novel
catalysts with high efficiency, a broader and lower operating temperature
window, and environmental friendliness. Some spinel catalysts have been proven
to be effective in NH₃-SCR, and the catalytic performance
usually can be further enhanced via doping a new element. In this paper, Zr⁴⁺ cations were incorporated
into the lattice of MnCr₂O₄ spinel,
forminga
series of novel ternary Zr_xMn_1-xCr₂O₄
(x=0, 0.05, 0.1,
0.2)catalysts for the first time. The as-prepared
Zr-doped catalysts all exhibited higher NO_x conversion than undoped
MnCr₂O₄ catalyst. The Zr_0.05Mn_0.95Cr₂O₄
exhibited the best catalytic performance, with good stability in the presence
of H₂O, the lowest T₅₀ and T₉₀ (149 ^oC and 201 ^oC, respectively),
and the broadest T₈₀ operation
window (183-354 ^oC) under a gas hourly
space velocity of 112,000 h^-1. The structural properties,
morphologies, surface species, acidity, and redox ability of Zr_xMn_1-xCr₂O₄
catalysts were characterized by a series of characterizations. The
results revealed that Zr⁴⁺ cations are successfully incorporated,
and the resultant lattice deformation changed the physical and chemical
properties of the catalysts remarkably.
The introduction of zirconium increased the
concentration and intensity of the acid sites, produced higher levels of
reducible species, and activated surface lattice oxygen following a
Mars-van Krevelen mechanism in NH₃-SCR. Synergetic electron transfer effects might be
established
between Mn, Cr, and Zr cations, giving rise to higher levels of beneficial Mn³⁺,
Mn⁴⁺ and Cr⁵⁺ species (Mn²⁺
+ Cr³⁺ ↔ Cr²⁺ + Mn³⁺,
Mn³⁺ + Cr³⁺ ↔ Cr²⁺
+ Mn⁴⁺, Mn²⁺ + 2Zr⁴⁺ ↔ 2Zr³⁺ + Mn⁴⁺, Mn³⁺ + Zr⁴⁺
↔ Mn⁴⁺ + Zr³⁺,
Cr³⁺ + 2Zr⁴⁺↔ Cr⁵⁺
+ 2Zr³⁺). The promoted electrons
transfer effects and surface lattice oxygen activation are further
substantiated by density functional theory (DFT) results.

Key words: Zr_xMn_1-xCr₂O₄; NH₃-SCR;
surface lattice oxygen; electrons
transfer effects; DFT.

Fig. 1. SCR activity of Zr_xMn_1-xCr₂O₄
(x=0, 0.05, 0.1, 0.2) catalysts: (a) MnCr₂O₄; (b) Zr_0.05Mn_0.95Cr₂O₄;
(c) Zr_0.1Mn_0.9Cr₂O₄; (d) Zr_0.2Mn_0.8Cr₂O₄
and the schematic diagram showing the surface active species changes by Zr
doping and proposed reaction mechanism