posted on 2021-12-16, 16:41authored byYing Xin, Lu Cheng, Yanan Lv, Junxiu Jia, Dongxu Han, Nana Zhang, Jin Wang, Zhaoliang Zhang, Xiao-Ming Cao
Crystalline
structure of metal oxides is based on a close-packed
array of oxygen anions with metal cations occupying interstitial sites,
where the exposed facet determines the surface arrangement and coordination
of oxygen and metal ions. Owing to the different physicochemical properties
of various ions, the exposed crystal facets have a critical influence
on properties of metal oxides, including catalytic behavior. Understanding
the facet-dependent mechanisms of catalytic reactions is important
for improving the performance of metal oxide catalysts; however, this
understanding is currently lacking because research performed to date
has been limited by the unilateral experimental or theoretical evidence.
Herein, we aim to elucidate the effect of different exposed crystal
facets in Mn2O3 by constructing rod- and particle-like
Mn2O3 nanocrystals and investigating the effects
on the catalytic performance of these structures for NO oxidation.
Distinct catalytic behaviors of these different nanocrystals are investigated
by a combination of systematic structural and property characterizations
and density functional theory (DFT) calculations. Our results suggest
that NO oxidation on the surface of rod-like Mn2O3 with exposed (220) and (400) facets follows the Langmuir–Hinshelwood
mechanism, whereas the Mars–van-Krevelen mechanism is favored
for Mn2O3 nanoparticles with exposed (222) facets.
These different mechanisms lead to diverse catalytic performances
of Mn2O3, especially the poisoning resistance.
Specifically, rod-like Mn2O3 is shown to be
deactivated by H2O because hydroxylated oxygen inhibits
the adsorption of NO and adsorbed OH* hinders the chemisorption of
O2 on the (220) and (400) facets, while H2O
only slightly influences the activity of Mn2O3 nanoparticles with (222) facets. This work shows that regulating
which crystal facets of Mn2O3 are exposed allows
tuning the catalytic performance of this material and the reaction
pathways for NO oxidation. Furthermore, this work provides clear insight
that may increase the understanding of facet-dependent reaction mechanisms
of other metal oxide catalysts and establishes valuable design principles
for future studies to improve heterogeneous catalysts.