(144d) Enhanced DeNOx Activity of Ag/Al2O3 Catalyst by a Bifunctional Reductant: Autocatalytic Synergy Between HC- and NH3-SCR Technologies

              The
selective catalytic reduction of NO_x by hydrocarbons remains
one of promising technologies for removing NO_x under lean
conditions, even though its overall deNO_x performance is not
yet sufficient for viable commercial implementation, especially due to its poor
low-temperature performance. ^1,2  Recently, the use of
oxygenated hydrocarbon as a reductant (OHC-SCR) has been reported as an
alternative way to improve the low-temperature deNO_x activity
of the HC-SCR.^3,4  However, its deNO_x
activity in the low temperature region below 300 ^oC is still too low
to be commercially applicable to the after-treatment of diesel engine exhausts. 
Another way to enhance the low-temperature deNO_x performance
may be the addition of H₂ to the feed stream over the Ag/Al₂O₃
catalyst.^5,6

              In
the present study, we report a strong autocatalytic synergism between the
conventional HC-SCR and the NH₃-SCR observed during the reduction of
NO by a bifunctional reductant over a Ag/Al₂O₃ catalyst,
which has resulted in a dramatic increase of the deNO_x
performance compared to that by a corresponding unifunctional reductant.  To
demonstrate the synergistic interaction between the HC-SCR and the NH₃-SCR,
we have chosen monoethanolamine (MEA) as a representative bifunctional
reductant, because it contains both partially oxidized hydrocarbon with a
hydroxyl group (–OH) for the OHC-SCR and an amino group (–NH₂) as
the precursor of NH₃ for the NH₃-SCR.  MEA is one of
the bulk commodity chemicals widely used in industry as a gas-scrubbing agent
for absorption and removal of CO₂ and H₂S from refinery
and natural gas streams.⁷  Plausible reaction pathways have
been proposed to elucidate the enhanced deNO_x activity of
Ag/Al₂O₃ by MEA, based on comparative catalytic activity
tests and the catalyst surface analysis by an in situ FTIR.

              Ag/Al₂O₃
catalysts were prepared by the incipient wetness method with AgNO₃ aqueous
solution.  All catalysts prepared in the present study were oven-dried at
110 ^oC overnight and calcined in a muffle furnace at 550 ^oC
for 5 h.  A conventional packed-bed flow reactor system was used for the
catalytic activity test.⁸  The standard feed stream contained
400 ppm NO, 400 ppm NH₂(CH₂)₂OH, or 400 ppm C₂H₅OH
+ 400 ppm NH₃, or 400 ppm C₂H₅OH, or 400 ppm
NH₃, 6% O₂, 2.5% H₂O and He balance (GHSV:
60,000 h^-1).

              Fig.
1 compares the deNO_x performance of the Ag(3.8)/Al₂O₃
catalyst in terms of the NO_x conversion by various reductants
at the C₁/NO_x feed ratio of 2 with a constant NO
feed concentration of 400 ppm for all C-containing reductants such as ethanol
and MEA.  The NH_x/NO_x molar feed ratio
was kept constant at unity for all N-containing reductants including MEA and ammonia. 
Note in Fig. 1 that NH₃ did not directly react with NO_x
over the Ag(3.8)/Al₂O₃ catalyst, but produced more NO_x
above 400 ^oC through its oxidation, resulting in a negative
conversion of NO_x.  However, when ammonia was in the
feed stream together with ethanol, NO_x conversion by (ethanol
+ ammonia) was higher than the sum of the individual NO_x
conversion by ethanol and ammonia, suggesting that the reactive intermediates
for NO_x conversion can be formed more efficiently in the co-presence
of ethanol and ammonia in the feed, a clear indication of a synergistic
interaction of ethanol and ammonia enhancing the deNO_x
performance of the Ag(3.8)/Al₂O₃ catalyst over the entire
temperature range tested.

              Surprisingly,
the deNO_x efficiency of MEA is even higher than that of
(ethanol + ammonia) over the entire temperature range of our interest,
suggesting a greater synergistic interaction of –OH and –NH₂ groups
in MEA than that of ethanol and ammonia.  The superb deNO_x
efficiency of MEA compared to that of (ethanol + ammonia) is a clear indication
of an efficient synergistic interaction between the –OH and –NH₂
groups, which is attributable to the strong intra-molecular H-bonding
between the –OH and –NH₂ groups in MEA.^9,10

              A
series of separate experiments has confirmed that NH₃ can be
produced during MEA oxidation, which can be subsequently oxidized to N₂
and NO over the Ag/Al₂O₃ above 300 ^oC and 350
^oC, respectively.  In addition, hydrogen is produced from MEA
decomposition or steam reforming in the absence or presence of oxygen and water,
respectively, from 200 to 500 ^oC.  These findings indicate that
the deNO_x activity of the Ag/Al₂O₃ catalyst
is initiated with the production of hydrogen and NH₃ from MEA,
triggering the autocatalytic NH₃-SCR reaction in addition to the
conventional HC-SCR reaction over the Ag/Al₂O₃ catalyst.

              Possible
reaction intermediates during the MEA-SCR reaction were investigated by in situ
FTIR as shown in Fig. 2.  A band at 1670 cm^-1 when MEA was employed
with or without NO can be attributed to the
formation of oxime on the surface of the Ag/Al₂O₃
catalyst.¹¹  Oxime has been reported to be a precursor to
active reaction intermediates for the reduction of NO to N₂.^12,13 
Both intra- and inter-molecular formation of oxime from MEA may be possible due
to the strong intra- and inter-molecular H-bonding between the –OH and –NH₂
groups in MEA.^9,10  Another peak assigned to the isocyanate
species at 2235 cm^-1, widely recognized as a key reaction
intermediate to convert NO to N₂ by the HC-SCR reaction, was
observed at 200 ^oC when MEA was used as the reductant with or
without NO in the feed.  It reveals that the formation of both oxime and
isocyanate species is mainly due to MEA in the present catalytic deNO_x
system.  When (ethanol + ammonia) was used as the reductant, a weak peak
for the formation of the isocyanate species also appeared at 200 ^oC
with or without NO, consistent with its weak synergy compared to that of MEA,
as shown in Fig. 1.  However, with ethanol as the reductant, the
isocyanate species was hardly formed at 200 ^oC, even when NO was
present in the feed.  These findings indicate that the deNO_x
activity is enhanced by the reaction of NO with the reaction intermediates such
as oxime and isocyanate species at low temperatures over the Ag(3.8)/Al₂O₃
catalyst.

              Based
on all the observations, possible reaction pathways for the MEA-SCR reaction
over Ag/Al₂O₃ are proposed as shown in scheme 1. 
Starting from the nominal reductant MEA for simplicity, scheme 1 shows that MEA
can produce highly reactive reductants such as H₂, NH₃
and –NCO species in two parallel reaction pathways.  Note that both NH₃-SCR
and HC-SCR can proceed simultaneously on the surface of Ag/Al₂O₃
catalyst due to the production of highly reactive reductants by the reaction of
MEA with other reactants.

              In
summary, a bifunctional reductant (MEA) is capable of transforming the normally
single-functional Ag/Al₂O₃ catalyst active only for
HC-SCR into a dual-functional catalyst active for both HC-SCR and NH₃-SCR,
resulting in a dramatic enhancement of its deNO_x activity
through an autocatalytic synergism.

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Fig. 1. Comparison of NO_x conversion by various reductants
over Ag(3.8)/Al₂O₃ catalyst.

Fig. 2. In situ FTIR spectra over Ag(3.8)/Al₂O₃
catalyst in a flow of (NO) + MEA + O₂, (NO) + EtOH + NH₃
+ O₂ or (NO) + EtOH + O₂ at 200 ^oC.

Scheme 1. Reaction pathways and collective autocatalytic
reaction of MEA-SCR over Ag/Al₂O₃.