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P.R. Ettireddy et al. / Journal of Catalysis 292 (2012) 53–63
deformation of NH3 coordination to Lewis acid sites, appears to be
important for high activity and selectivity. It was also proposed that
the reaction mechanism proceeds through the formation of nitrosa-
mide and azoxy species [20]. In order to improve our understanding
on the mechanism, additional studies involving isotopic labeled
gases were performed. Our main focus is to study the involvement
of surface oxygen species and its role played toward the mechanism.
In the present work, we investigated the mechanism of the
selective catalytic reduction of nitric oxide with ammonia using
transient labeling studies with 15NO, 15NH3, and 18O2 over the
16.7% Mn/TiO2 (Hombikat) catalyst. This catalyst demonstrated
high activity and selectivity in our previous studies [9]. The reac-
tion pathways have been examined by switching selected combi-
nation of gases, both labeled and unlabeled (NO, NH3, and O2).
The underlying thought was to investigate the role of surface labile
oxygen and thus propose the mechanism for the low-temperature
reduction of NO with NH3 reaction. By using the transient studies
of labeled gases, we have found that the surface oxygen species
play an important role in the low-temperature SCR reaction. How-
ever, the results acquired with the time resolution illustrated that
the reaction of ammonia with lattice oxygen was practically
instantaneous. The formation of N2O, NO, and H2O species is evi-
dent when ammonia came into contact with catalyst lattice oxy-
gen. It is remarkable to note that the formation of NO2 was not
observed over the Mn/TiO2 catalyst. The formation of cross-labeled
species during the transient studies revealed the occurrence of
ammonia oxidation. In connection with the transient isotopic
labeling and in situ FT-IR studies, plausible mechanism for the
low-temperature SCR of NOx over titania-supported manganese
catalysts is proposed and the reaction is more likely a Mars–
van-Krevelen type mechanism.
300 W. The spectra were recorded in the fixed analyzer transmis-
sion mode with pass energies of 89.45 and 35.75 eV for recording
survey and high-resolution spectra, respectively. The powdered
catalysts were mounted onto the sample holder and degassed
overnight at room temperature at a pressure on the order of
10ꢀ7 torrs. Sample charging effects were eliminated by correcting
the observed spectra with the C 1s binding energy (BE) value of
284.6 eV. An estimated error of 0.1 eV can be considered for all
the measurements.
Oxygen pulse chemisorption measurements were performed to
determine the dispersion of manganese on the support surface. He-
lium was used as the carrier gas (30 mL/min). Before analysis,
approximately 50 mg of catalyst samples was reduced in flowing
10% H2 in helium (50 mL/min) at 250 °C for 2 h and then flushed
for 30 min in prepurified helium at the same temperature. Then,
oxygen pulses (1 mL loop volume) were injected onto the carrier
gas until saturation of the sample was attained. The oxygen uptake
was quantified by a TCD connected to a Micromeretics Autochem
2910 instrument.
In situ FT-IR spectra were recorded using a Bio-Rad (FTS-40)
Fourier transform instrument and a heatable IR cell connected to
a conventional volumetric operator. The scans were collected at a
scan speed of 5 kHz, resolution of 2.0, and an aperture opening of
2.0 cmꢀ1. Sixteen scans were averaged for each normalized spec-
trum. Circular self-supporting thin wafers (8 mm diameter) con-
sisting of 12 mg of material were used for the study. The wafers
were placed in a high-temperature cell with CaF2 windows and
purged in situ in the IR cell with prepurified grade helium
(30 mL minꢀ1, Wright Brothers) at 473 K for 2 h to remove any ad-
sorbed impurities. Then, the samples were cooled to 323 K, and the
selected gasses were introduced into the cell with a flow of
30 mL minꢀ1 for 1 h at 323 K to ensure complete saturation of
the sample. Physisorbed gasses were removed by flushing the wa-
fer with prepurified helium for 3 h at 373 K. Subsequently, the
in situ FT-IR spectra were recorded by evacuation at different
temperatures.
2. Experimental
2.1. Catalysts preparation
The TiO2 used in this study was Hombikat UV 100 from Sachtle-
ben Chemie. As determined by N2 adsorption, it had a specific sur-
face area of 309 m2/g, a pore volume of 0.37 cm3/g, and a pore
diameter of 4.5 nm. In a typical synthesis, 50 mL of deionized
water was added to a 100 mL beaker containing 1.0 g of support.
The mixture was heated to 70 °C under continuous stirring condi-
tions. A predetermined quantity of nitrate precursor was then
added, and the mixture was evaporated to dryness. The resulting
material obtained was further dried overnight at 110 °C, ground,
and sieved (80–120 mesh). Finally, the catalyst was calcined at
240 °C for 4 h in continuous air flow.
2.3. Reaction studies
The premixed gases namely oxygen (4% in He, Wright Brothers),
ammonia (3.99% in He, Wright Brothers), and nitric oxide (2.0% in
He, Matheson) were used as received. The labeled gases were iso-
topic oxygen (4% 18O2 (99 atom%) in balance helium, Isotech),
ammonia (4.06% 15NH3 (98 atom%) in balance helium, Isotech),
and nitric oxide (2.54% 15NO (99 atom%) in balance helium, Icon).
The labeled SCR reaction experiments were performed using a feed
mixture of 2500 ppm nitric oxide, 2500 ppm ammonia, and 2 vol.%
oxygen with helium as the balance gas (reaction 1).
4NO þ 4NH3 þ O2 ! 4N2 þ 6H2O
2.2. Characterization of catalyst
The specific surface areas were measured by nitrogen adsorp-
tion at ꢀ196 °C by the BET method using a Micromeritics Gemini
2360 surface area instrument. Prior to the analysis, 0.05–0.10 g
of catalysts was degassed at 150 °C for 2 h under helium
atmosphere.
A 50–70 mg of catalyst sample was used in this experiment. The
reactions were performed at 175 °C and total flow rate was 50 mL/
min. The catalyst was first pretreated at reaction temperature for
two hours with oxygen. After the pretreatment, the reactions were
carried out at the steady state for two hours with unlabeled com-
ponents before switched to labeled gas. The experiments for the
investigation of the interaction of the catalyst oxygen with the
gas phase oxygen and the nitric oxide were done by using
150 mg of catalyst.
A MKS PPT Quadrupole Residual Gas Analyzer was used to de-
tect the species at their different atomic mass units (amu). The sig-
nals of ammonia needed to be corrected from the interference with
oxygen by using the relative intensities of the m/e = 32 and
m/e = 16 peak. The data for the correction are obtained from cali-
bration experiments. The water peak at m/e = 18 was corrected
XRD studies were performed on a Siemens D500 Diffractometer
with a monochromated Cu K
a radiation source (wavelength
1.5406 Å). The scans were performed for 2h values ranging from
5° to 70° with step size 0.1° and time step 1.0 s to assess the crys-
tallinity of the manganese loading. XRD phases present in the cat-
alyst samples were identified using JCPDS powder data files.
X-ray photoelectron spectroscopy (XPS) was used to analyze the
atomic surface concentration on each catalyst. The spectra were re-
corded on a Perkin–Elmer Model 5300 X-ray photoelectron spec-
trometer using Mg K
a (1253.6 eV) as a radiation source at