The Catalyst Review September 2017 - 13

EXPERIMENTAL
Simultaneous NOx and Particulate Matter Removal from Diesel Exhaust by Hierarchical Fe-Doped
Ce−Zr Oxide...
Well-established catalytic protocols exist for removing noxious
emissions from gasoline-fueled cars. These processes rely on
precious group metals dispersed as nano-particles on suitable
oxide support materials to both oxidize CO and hydrocarbons and
reduce NOx to less harmful gasses. However, this approach cannot
be used to remove NOx from diesel engine exhausts because
they are too rich in oxygen. In addition, these exhausts are rich in
particulate matter (PM) which is a serious threat to human health.
Herein, the authors report the preparation (Figure 1) and use of a
Fe-doped, three-dimensionally ordered microporous (3DOM),
Ce−Zr oxide catalyst capable of simultaneously removing PM and
NOx from diesel exhausts. The Fe and Zr content of the 3DOM
mixed Fe−Ce−Zr oxide was optimized for low-temperature NOx
reduction and complete soot oxidation by mixing model soot
particles with an average size of 25 nm with the 3DOM mixed
oxide catalysts and exposing it to a simulated diesel exhaust
feed. Figure 2 shows the transient behavior of the catalyst
during temperature programmed reactions in which the effluent
CO2 concentration, produced by combustion of the model soot
particles, decreases at high temperature as combustion of the
model soot nears completion. The conversion of NO at high
temperature is limited because of the oxidation of NH3.
The authors also evaluated the
performance of the catalyst in the
presence of water. By adding 5% H2O to
the reactor feed, the catalytic performance
for PM oxidation was decreased, whereas
that for NOx reduction was only slightly
lower in comparison to the experiments
without water. The strong influence of
water on NO reduction is thought to be
due to competitive adsorption of NH3 and
H2O. This limits NH3 adsorption on acid
sites at low temperature, thus decreasing
low-temperature NOx reduction. On
the other hand, at high temperature,
the inhibiting effect of H2O slows NH3
oxidation, resulting in a higher NOx
reduction rate.

Figure 1. (a) Schematic representation of the synthesis
of the 3DOM mixed Ce−Fe−Zr oxide and its catalytic
function in diesel exhaust cleanup; (b) SEM and (c−e)
TEM images at different magnifications showing the
macroporous structure (c, d) and d-spacing of CeO2(111).

Figure 2. (left) CO2 concentration and (right) NO conversion as a function of
temperature upon exposure of 3DOM Ce0.9-xFexZr0.1O2 catalysts loosely mixed with
model soot particles in a gas feed containing 1000 ppm of NH3, 1000 ppm of NO, 3%
O2 and balance N2 at a gas hourly space velocity of 25,000 h−1.

The combined results of surface characterization and catalytic testing demonstrated the unique properties of Fe atoms doped into
ceria toward NO reduction with NH3 combined with soot oxidation leading the authors to suggest a mechanism for reduction of NO
with NH3 which involves adsorbed O2 as a catalytic surface intermediate and which may also be important in soot oxidation. Source:
Cheng Y, Song W, Liu J, et al. (2017). ACS Catal., 7: 3883−3892.

The Catalyst Review

September 2017 13

The Catalyst Review September 2017

Table of Contents for the Digital Edition of The Catalyst Review September 2017