Ammonium formate decomposition over Au/TiO2: A unique case of preferential selectivity against NH3 oxidation

Article abstract of DOI:10.1039/c4cc00196f

The unique selectivity of Au/TiO₂ for converting ammonium formate to CO₂ in the presence of excess O₂ and H ₂O without oxidising NH₃ up to 300 °C is reported. The catalyst is highly stable and selective even after severe hydrothermal aging. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00196f

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Ammonium formate decomposition over Au/TiO2:
a unique case of preferential selectivity against
Cite this: Chem. Commun., 2014,
0, 6998
NH oxidation†
5
3
Received 9th January 2014,
Accepted 12th May 2014
a
a
ab
Manasa Sridhar,‡ Daniel Peitz,‡ Jeroen Anton van Bokhoven and
Oliver Krocher*
¨
ac
DOI: 10.1039/c4cc00196f
www.rsc.org/chemcomm
The unique selectivity of Au/TiO
2
for converting ammonium formate
2 2 2 3
to CO in the presence of excess O and H O without oxidising NH
up to 300 8C is reported. The catalyst is highly stable and selective
even after severe hydrothermal aging.
The advent of stringent measures curbing NO
has increased the demand for improved technologies for the
selective catalytic reduction (SCR) of NO in both stationary and
x
emissions
x
1
,2
mobile applications. Currently, urea is the most widely used
NH source implemented in mobile SCR. Immense interests in
3
replacing this material with alternative NH precursors, such as
3
concentrated guanidinium formate, ammonium formate
(
AmFo) and methanamide solutions, that are more thermally
stable, freeze at lower temperatures, have higher NH storage
capacities, and decompose more selectively have grown
3
Fig. 1 Product yields during AmFo decomposition over 0.5% Au/TiO
2
3
À1
À1
recently. One challenge is identifying highly active and robust (100 g L washcoat loading, GHSV = 19 490 h ; feed gas: 5% H
2
O, 10%
À1
O
2
, 85% N
2
and 0.05 mL min liquid spray of 40 wt% AmFo) showing
dedicated hydrolysis catalysts that selectively hydrolyse the
4
–6
nearly 100% NH
3
and CO
2
yields between 200 1C and 300 1C.
precursors and release NH without oxidation. Of the afore-
3
mentioned precursors, AmFo is experimentally the simplest
choice to study the activity and behaviour of the hydrolysis
catalysts towards selective NH
pression under realistic conditions.
3
release and side product sup- reactor setup to study the decomposition of liquid NH
3
precursor
7
compounds under spray conditions with precise temperature
5
,14
In this work, analogous to the extraordinary selectivity of control.
finely dispersed Au for preferential CO oxidation (PROX) in H -rich methanamide and NO
we highlight another phenomenological observation, complete decomposition of 40% AmFo over 0.5 wt% Au/TiO2.
wherein, Au supported on TiO catalysts are found to show absolute The carbon and nitrogen balances were closed using the
selectivity for AmFo decomposition in the presence of excess O2, molar feed and reactor outlet concentrations as quantified by
2 3
Fig. 1 illustrates the CO , CO, NH , formic acid,
2
x
(NO + NO ) yields released upon the
2
8–11
stream,
2
1
5
yielding 100% NH
as high as 300 1C. Anatase Au/TiO
modified incipient wetness impregnation method were coated 100 g L washcoat loading, which is approximately two-thirds
3
without any NH
3
oxidation up to temperatures FT-IR spectroscopy. Washcoat loading refers to the amount of
1
6
2
catalysts synthesised via a facile the catalyst deposited on the monolith per unit volume. At a
12
À1
1
3
onto 400 cpsi cordierite monoliths and tested in a laboratory of the typical catalyst loadings in automobiles, Au/TiO
2
reliably
converted AmFo into NH
3
, and CO
2
with 10% O
2
and 5% H O
2
a
b
c
Paul Scherrer Institut, 5232 Villigen, Switzerland. E-mail: oliver.kroecher@psi.ch
present in the feed. NH did not react under such conditions
3
ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland
E´ cole Polytechnique F ´e d ´e rale de Lausanne (EPFL), Institute of Chemical Sciences
rendering negligible NO and methanamide yields, the latter
x
being a side-product arising from the reaction between formic
and Engineering, 1015 Lausanne, Switzerland
7
acid and NH
3
. Methanamide may dehydrate further to form
†
Electronic supplementary information (ESI) available: Experimental details and
HCN; however, under these reaction conditions, no HCN was
calculation of pseudo-first order rate constants. See DOI: 10.1039/c4cc00196f
‡
These authors contributed equally to this work.
formed over Au/TiO
2
. To determine the stability of the catalysts
6998 | Chem. Commun., 2014, 50, 6998--7000
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Table 1 Mass based rate constants at various reaction temperatures
calculated using pseudo first order kinetics (refer to ESI)
À1 À1
Catalyst sample
Fresh
Temperature (K)
300
Rate constant kmass (L g
s )
12.5 Æ 0.03
6.1 Æ 0.01
4.4 Æ 0.02
4.4 Æ 0.02
2.7 Æ 0.01
2.0 Æ 0.02
3.7 Æ 0.03
2.4 Æ 0.02
2.0 Æ 0.01
3.2 Æ 0.04
0.8 Æ 0.04
0.3 Æ 0.03
2
1
50
90
5
1
h aged
300
2
1
50
90
0 h aged
300
2
1
50
90
Bare TiO
2
300
2
1
50
90
of generic Arrhenius behaviour. Hydrothermal aging at 800 1C for
h decreased the rate constants by over 60%, while, 5 h further
5
treatment resulted in a marginal drop of B15% when comparing
the activities at 300 1C. At lower temperatures, the rate constants
tended to nearly identical values for 5 h and 10 h aged catalysts. To
elucidate the influence of Au, a control experiment using similar
Fig. 2 Product yields and AmFo conversion during AmFo decomposition
washcoat loading of bare TiO
conditions. The resultant rate constants were found to be a
magnitude of at least 4 lower than that of the fresh Au/TiO
catalysts. Importantly, over bare TiO , the conversion of formic
acid selectively produced CO; there was no CO formation, under
these conditions.
Because, formic acid undergoes secondary reactions to form
9.5% and tested under identical conditions of GHSV and feed CO and CO , the evolution of their selectivities as a function of
2
was performed under identical
À1
over (a) fresh (b) 5 h aged, and (c) 10 h aged 0.5% Au/TiO
2
catalysts (0.5 g L
O, 10% O
À1
washcoat loading, GHSV = 19490 h ; feed gas: 5% H
2
2
, 85% N
2
À1
2
and 0.05 mL min liquid spray of 40 wt% AmFo), showing nearly 100% NH
3
yields between 200 1C and 300 1C.
2
2
at partial conversion, the washcoat loading was reduced by
9
2
composition. Additionally, the fresh catalyst was subjected to temperature over the fresh and aged catalysts is interesting.
two incremental hydrothermal aging steps each lasting for 5 h Fig. 3(a) and (b) show that the temperature dependence of the
at 800 1C in air containing 10% H
repeated.
2
O, and the activity tests were selectivities for CO and CO
remained essentially unchanged even after 10 h aging. A decrease
selectivity was accompanied by an increase in the CO
2
formation and formic acid conversion
Fig. 2(a–c) depict the yield of all products formed from the in CO
2
decomposition of 40 wt% AmFo over fresh, 5 h and 10 h selectivity with increasing temperature for all catalysts. Formic
hydrothermally aged 0.5 wt% Au/TiO , respectively. 100% conversion acid conversion was significantly affected by the hydrothermal
2
for AmFo decomposition was achieved with all catalysts. NH , formic aging translating to a drop in conversion from B57% to B17%
3
acid and CO
formed in low yields. At 300 1C, the CO yields decreased from B10% of fresh and 10 h hydrothermally aged 0.5 wt% Au/TiO
2
were the major products, while CO and methanamide after 10 h treatment in 10% H
2
O. The HAADF-STEM images
are
2
over fresh catalyst to B3% over aged catalyst. Methanamide, which presented in Fig. S1 (ESI†). It appears that while the support
À1
was not observed in the case of 100 g L catalyst, started to appear (TiO
2
) morphology exhibited signs of sintering upon 10 h hydro-
À1
when using 0.5 g L catalyst and accounted for B3% yield or lesser thermal aging at 800 1C, the particle sizes of the visible Au
over both the fresh and aged catalysts across all temperatures. CO particles were only subtly affected. TiO support particles of the
yields were decreased due to aging from B43% over the fresh aged catalyst are in an aggregated state upon sintering which is in
2
2
2
À1
catalyst to B20% and B17% over 5 h and 10 h aged catalysts, consensus with the lowering of BET surface area from B80 m g
2
À1
respectively, at 300 1C. The product yield patterns over the 5 h and to B50 m g (Table S1, ESI†). An increase in average crystallite
0 h aged catalysts suggest a kind of stabilisation of the catalytic size from B25 nm to B32 nm was obtained from XRD measure-
activity after the first aging step. Moreover, NH yield always ments (Fig. S2, ESI†). However, no phase transformation of TiO
remained close to 100%, which further reiterates the discriminative from anatase to rutile was evident from the XRD patterns. Further
oxidation behaviour of our catalyst in contrast to the typical NH mechanistic study is underway to determine the role of Au and
TiO in our reaction.
The time on stream activities of the fresh and 10 h hydro-
the relative activities of catalysts tested under identical operating thermally aged catalysts were evaluated over 48 h (Fig. 3(c)).
1
3
2
3
17,18
oxidation behaviour of precious metal containing catalysts.
Pseudo-first-order kinetic constants were calculated to assess
2
1
9–21
conditions.
calculated for the fresh, 5 h aged, 10 h aged 0.5 wt% Au/TiO
and fresh bare TiO catalysts. The rate constants progressively stable activities, selectivities and nearly 100% NH
decreased for all catalysts with decreasing temperature, indicative no signs of deactivation with time-on-stream.
Table 1 lists the mass based rate constants A low temperature (190 1C) was chosen to examine the low
conversion stability. Both the fresh and aged catalysts exhibited
yields with
2
2
3
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Martin Elsener for his technical support and guidance. We
extend our gratitude to Dr Tarik Mohammed and Dr Mario El
Kazzi for aiding us with ICP-OES and XPS analysis, respectively.
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Au/TiO demonstrating unique selectivity against NH oxidation
in a highly oxidising environment during AmFo decomposition is
reported here for the first time. The catalysts display excellent
2
3
1
1
stability giving close to 100% NH
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7000 | Chem. Commun., 2014, 50, 6998--7000
This journal is ©The Royal Society of Chemistry 2014