ORIGINAL PAPER

Promising Stability of Gold-Based Catalysts Prepared by DirectAnionic Exchange for DeNOx Applications in Lean BurnConditions

D.-L. Nguyen • S. Umbarkar • M. K. Dongare •

C. Lancelot • J.-S. Girardon • C. Dujardin •

P. Granger

Published online: 28 February 2013

� Springer Science+Business Media New York 2013

Abstract Supported gold catalysts on c-Al2O3 have been

investigated in the catalytic reduction of NOx in simulated

Diesel exhaust gas conditions. Different parameters have

been examined essentially the mode of gold incorporation

via classical deposition–precipitation and anionic exchange

methods and the nature of the pre-activation thermal

treatment. The resistance to thermal ageing under reactive

conditions at 500 �C was found completely different with a

significant rate enhancement on anionic-exchange samples.

Further comparisons also show that the nature of the

pre-activation thermal treatment influences the extent of

surface reconstructions during thermal ageing with a det-

rimental effect of reductive pre-treatment on the catalytic

performances.

Keywords Au/Al2O3 catalyst � Anionic-exchange �Deposition–precipitation � NOx abatement � Selective

catalytic reduction

1 Introduction

Up to now, most of research investigations on supported

gold based catalysts essentially concern oxidation reactions

at low temperature from stationary and mobile sources

[1–3]. The usual high catalytic performances of gold

nanosized particles in the CO/O2 reaction have been pre-

viously explained by the deposition of hemispherical

ultrafine gold particles on selected support materials [4].

On the other hand, only few investigations were devoted to

the development of gold in DeNOx catalysis and most of

them were performed far from real exhaust gas conditions

[5, 6]. Support effects earlier envisioned for oxidation

reactions still persist for the selective reduction of NOx to

nitrogen. By way of illustration, Ueda et al. [7] observed a

significant rate enhancement in NOx reduction to N2 by

C3H6 up to 5 vol.% O2 when Mn2O3 is added to Au/Al2O3.

These authors ascribed this beneficial effect to a sharp

enhancement in NO2 production which would react more

readily than NO with propene adsorbed on the surface of

gold particles. A prominent observation was also related to

the beneficial effect of water mainly associated to the

occurrence of the water–gas-shift (WGS) reaction.

Several obstacles may limit the development of gold

based catalysts in DeNOx. Under operating conditions,

particle sintering in running conditions at 500 �C might

considerably affect the catalytic performances of gold with

subsequent alterations of the selectivity behavior [8]. The

aggregation of nano-sized gold particles at high tempera-

ture usually leads to irreversible deactivation [9]. Previous

investigations showed that the thermal stability of gold

nano particles depends on the nature of the pre-activation

thermal treatment [10]. By way of illustration, Au/CeO2

pre-treated in 10 vol.% O2/N2 at 400 �C exhibits a higher

catalytic activity in the WGS reaction than that obtained on

a pre-reduced sample in 10 vol.% H2/N2 at the same

temperature [10]. On the contrary, a pre-reductive thermal

treatment induces a greater stability enhancement.

This study reports the behavior of supported gold cata-

lysts in the selective reduction of NOx by hydrocarbons in

D.-L. Nguyen � C. Lancelot � J.-S. Girardon � C. Dujardin �P. Granger (&)

Universite Lille Nord de France, CNRS UMR 8181, Unite de

Catalyse et de Chimie du Solide–UCCS, USTL, Batiment C3,

59650 Villeneuve d’Ascq, France

e-mail: pascal.granger@univ-lille1.fr

S. Umbarkar � M. K. Dongare

National Chemical Laboratory, Dr. Homi Bhabha Road, Pune

411008, India

123

Top Catal (2013) 56:157–164

DOI 10.1007/s11244-013-9946-z

lean conditions especially after ageing overnight at 500 �C

in the reaction conditions. Different parameters have been

examined such as the mode of gold incorporation via

deposition–precipitation or anionic exchange. The thermal

pre-treatment in reductive and/or oxidative conditions and

its relative impact on the extent of surface reconstruction

have been also examined.

2 Experimental

2.1 Catalyst Synthesis and Characterization

Gold was deposited on alumina (450 m2 g-1) via a con-

ventional deposition–precipitation (DP) method [11] or

anionic exchange (AE) [12] by using HAuCl4�3H2O

(99,999 %, Alfa Aesa) as precursor salt. The pH of the

HAuCl4 solution was adjusted to 7 during the deposition–

precipitation by adding NaOH. Regarding anionic

exchange, alumina was added to a solution of HAuCl4 with

concentration of 1.0 9 10-4 mol/L heated to 70 �C for

1 h. The pH was maintained constant at * 4 to stabilize

the complex [Au(Cl)2(OH)2]- [12]. After exchange, the

solids were successively dried in air at 100 �C overnight

and calcined in air at 300 or 500 �C. Gold and chlorine

were analyzed by inductively coupled plasma emission

spectroscopy at the center chemical analysis of the CNRS.

Au and Cl contents are reported in Table 1. Nitrogen

physisorption measurements were carried out at -196 �C

on a Micromeritics TriStar II apparatus. X-ray diffraction

(XRD) patterns were recorded on a Bruker AXS D8

Advance diffractometer equipped with a Cu Ka(k = 0.154 nm) radiation. XPS spectra were obtained with

a Kratos AXIS Ultra DLT spectrometer using Al source.

2.2 Catalytic Measurements

Temperature-programmed reaction experiments (TPR)

were performed in a fixed bed flow reactor in the temper-

ature range 75–500 �C with a heating rate dT/dt =

2 �C min-1. 360 mg of catalyst were systematically

exposed to a synthetic reaction mixture close to the real

exhaust gas composition from lean burn engines containing

300 ppm NO, 300 ppm CO, 0.2 vol.% H2, 5 vol.% H2O,

10 vol.% CO2, 10 vol.% O2 with representative hydrocar-

bons such as propene (300 ppm) and decane (100 ppm)

diluted in He as reported elsewhere [13]. The inlet flow rate

was adjusted to 18 L h-1 to obtain a gas hourly space

velocity of 50,000 h-1. CO, H2, Nitrogen, propene and

nitrous oxides were analyzed by a CP4900 Varian mi-

croGC. The NO and NO2 concentrations were measured

from specific multigas MIR9000 analyzers supplied by SA

Environment. Before TPR experiments, the catalyst sam-

ples were systematically pre-heated in pure H2 at 250 �C or

500 �C. After a first TPR-1 experiment according to the

above-mentioned conditions, the catalysts were aged at the

final temperature of 500 �C overnight in the reaction con-

ditions and then cooled down at 75 �C to carry out TPR-2

experiments.

3 Results and Discussion

3.1 Influence of Preparation Method on the Bulk

and Surface Properties of Aged Au/Al2O3 Catalysts

Bulk properties of freshly-prepared and aged Au/Al2O3 were

characterized by X-ray diffraction in order to evaluate the

extent of structural modifications taking place during the

Table 1 Evolution of textural and surface properties of DP and AE Au/Al2O3 catalysts calcined in air at 500 �C or 300 �C and aged after

reaction

Catalyst T calcination Au content Cl content SSA (m2g-1) Cristallite sizea XPS analysis

(�C) (%) (ppm) (nm) IAu/IAl Au 4f 7/2b Au/Alc

Au/Al2O3 Fresh 300 1.66 9400 417 16.9 5.7 9 10-2 84.1 1.8 9 10-3

(AE 300) Agedd 227 23.4 18.6 9 10-2 83.6 5.6 9 10-3

Au/Al2O3 Fresh 500 1.66 9400 367 20.5 8.1 9 10-2 83.7 2.7 9 10-3

(AE 500) Agedd 206 23.7 9.7 9 10-2 83.7 2.9 9 10-3

Au/Al2O3 Fresh 300 0.8 1200 335 – 6.1 9 10-2 84.2 1.8 9 10-3

(DP 300) Agedd 237 – 2.2 9 10-2 83.6 0.6 9 10-3

a Calculated from XRD measurementsb Binding Energy expressed in eV (accuracy ± 0.1 eV)c Surface atomic composition (relative accuracy ± 20 %)d Exposed overnight at 500 �C to 300 ppm NO, 300 ppm CO, 300 ppm propene, 100 ppm decane, 0.2 vol.% H2, 5 vol.% H2O, 10 vol.% CO2,

10 vol.% O2

158 Top Catal (2013) 56:157–164

123

ageing process. XRD patterns are collected in Fig. 1a, b for

calcined samples prepared by anionic exchange or deposi-

tion–precipitation and initially. Both exhibit the characteristic

X-ray lines ascribed to the boehmite structure after calcination

at 300 �C. Thermal ageing at 500 �C leads ultimately to the

stabilization of the c-Al2O3 phase. It is worthwhile to note that

the stabilization of c-Al2O3 is also observed after calcination

in air at 500 �C and subsequent thermal ageing does not

induce additional bulky detectable modifications. As a matter

of fact, the most significant observation on DP samples ini-

tially calcined at 300 �C resides in the lack of observation of

the characteristic X-ray lines of metallic gold currently

observed at 2h = 38.19, 44.4, 64.5, 77.58�, whereas they

clearly appear on Au/Al2O3(AE). The Scherrer equation was

used to calculate the crystallite sizes of gold. Data reported in

Table 1 show a slight increase of gold crystallite size on aged

AE samples previously calcined at 300 �C. On the other, hand

no significant crystallite growth seems to occur when the

calcination is performed at 500 �C. These observations are in

correct agreement with TEM measurements collected in

Figs. 2 and 3. As illustrated Au/Al2O3(AE) exhibits a much

broader gold particle size distribution than Au/Al2O3(DP) and

larger gold particles up to * 49 nm (see Fig. 3c). Further

ageing does not induce changes related to particle sintering.

TEM images recorded on aged Au/Al2O3(DP) also reveal a

relatively good thermal stability after ageing at 500 �C with

only a slight shift on the mean particle size from 17 to 19 nm.

All these observations, characterize the poor dispersion of

gold on Au/Al2O3(AE) which can be partly related to a much

higher residual chloride concentration which facilitates the

agglomeration of Au particles [14] as evidenced after cal-

cining Au/Al2O3(AE) at 500 �C with particle size up to 77 nm

(results not shown).

Nitrogen physisorption performed at -196 �C can be

correlated to XRD observations with a significant loss of

specific surface area after ageing partly attributed to the

boehmite transformation into c-Al2O3 phase. Thermal ageing

performed in the presence of 5 vol.% H2O leads to more open

porous structures associated with a significant increase of the

average pore size diameter from 3.6 to 6.0 nm. Surface

changes during thermal ageing have been investigated by

X-ray photoelectron spectroscopy. Binding energy values,

collected in Table 1, have been referenced to the characteristic

Al 2p core level currently observed at 74.6 eV in Al2O3. The

B.E. values Au 4f7/2 core levels reported in Table 1 charac-

terize the presence of metallic gold species currently reported

at 84 eV [15, 16]. No contribution corresponding to oxidic

Aud? and Au3? gold species earlier observed at 84.9 and

86.5 eV respectively is distinguishable. This underlines the

usual effect of X-ray irradiation which induces an extensive

reduction of Au3? to Au0 species [17]. As a general trend, a

significant shift of the B.E. to lower values for the character-

istic Au 4f7/2 core level occurs on aged DP and AE samples

becoming even lower than those currently obtained for a pure

Au foil [17]. As observed in Table 1, such a tendency also

characterized Au/Al2O3(AE) calcined at higher temperature

(500 �C) for which thermal sintering takes place significantly.

Hence, such a shift on the B.E. of Au 4f core level reflects a

strengthening of the metallic character which seems to be

consistent with a particle growth particularly on AE sample

calcined at 500 �C. Semi-quantitative analysis provides

divergent observations relative to the evolution of the atomic

Au/Al ratio. Obviously, a significant increase in surface Au

concentration is noticeable on aged AE sample. On the other

hand, the reverse tendency characterizes DP samples with

lower gold concentration at the surface after ageing.

3.2 Consequence of Thermal Ageing on the Catalytic

Properties

Typically, the conversion profiles of NO to N2 and N2O

versus temperature (see Figs. 4, 5) correspond to a

20 30 40 50 60 70 80

2θ(degree)

2θ(degree)

a

d

****

(400

)

(311

)

(440

)

(511

)

Ο Ο

20 30 40 50 60 70 80

a

b

****

(400

)

(311

)

(511

)

(440

)

Ο Ο

a

b

Fig. 1 XRD diffraction patterns recorded on a Au/Al2O3(AE)

calcined in air at 500 �C (a) and then aged overnight at 500 �C in

the reacting mixture (b), calcined in air at 300 �C (c) and then

aged overnight at 500 �C in the reacting mixture (d) and on bAu/Al2O3(DP) calcined in air at 300 �C (a) and then aged overnight at

500 �C in the reacting mixture (b)—asterisk stands for the charac-

teristic X-ray lines of bulky metallic gold species and circle for the

boehmite phase

Top Catal (2013) 56:157–164 159

123

volcano-type curve usually observed in lean conditions

with the presence of a large excess of oxygen [18]. Such

catalytic features are currently related to competitive and

successive reactions involving the intermediate formation

of NO2 and its reaction with hydrocarbons in the selective

production of N2 [18]. In these specific conditions, several

limitations have to be taken into account such as the slow

oxidation of NO to NO2 which limits the SCR reaction at

low temperature and, at high temperature, the formation of

NO2 which becomes thermodynamically limited inducing a

loss in NO conversion.

3.2.1 Effect of the Mode of Gold Incorporation Via

Deposition–Precipitation or Anionic Exchange

Figure 4 compared the TPR curves recorded on pre-

reduced (TPR-1) and aged Au/Al2O3 (TPR-2). Catalysts

were initially calcined at 300 �C irrespective of the method

implemented for Au incorporation. On DP samples, the

maximum in NO conversion recorded on TPR-1 is

approximately 87 %. An attenuation of the maximum NO

conversion is discernible by examining TPR-2 as well as a

shift of the conversion profile to lower temperatures par-

allel to a loss of NO conversion at high temperature.

Ageing has no significant effect on propene conversion

while an enhancement in hydrogen conversion is clearly

distinguishable. In all cases, NO is selectively reduced to

nitrogen. This differs from previous observations on DP

samples showing a significant amount of N2O formed

during the cold start engine [18]. Now, regarding the cat-

alytic performance of Au/Al2O3(AE), poorer NO conver-

sions are recorded from TPR-1 than on DP sample. The

maximum NO conversion also strongly attenuates at 45 %.

As earlier discussed the conversion profile for NO corre-

sponds to that of propene conversion which seems more

selective than H2 with conversion shifted to much lower

temperatures. Accordingly, the NO/H2 reaction should not

be promoted at least at low temperature. Thermal ageing

induces similar tendencies than those previously depicted

but more accentuated on AE samples with a strong

enhancement in NOx conversion starting above 280 �C and

reaching a maximum of 80 % NO conversion at 380 �C.

0

5

10

15

20

25

a b

c

1 5 9 13 17 21 25 29 33

Au/Al2O3(DP) calcined

Au/Al2O3(DP) aged

Fre

quen

cy (

%)

Particle size (nm)

Fig. 2 TEM observation on

Au/Al2O3(DP) calcined in air at

300 �C (a) aged overnight at

500 �C in the reacting mixture

(b) particle size distribution (c)

160 Top Catal (2013) 56:157–164

123

Obviously, convergent catalytic performances are obser-

vable on aged DP and AE samples while they initially

strongly diverge. Such modifications in catalytic properties

cannot be strictly related to modifications in the particle

size distribution on aged samples especially for explain-

ing the gain observed in activity at low temperature on

Au/Al2O3(AE). On the other hand, XPS might provide

more significant information since thermal ageing induces

a sharp increase in Au concentration on Au/Al2O3(AE) (see

Table 1). In addition, the examination of B.E. values shows

a strengthening of the metallic character of gold after

ageing which could explain the shift of the conversion

profile to lower temperatures because more metallic gold

sites would be available for enhancing the rate of NO

conversion to NO2. Such tendency has already been

observed and discussed on silver based catalysts [19]. As

explained, the increasing amount of metallic Ag at the

surface would be responsible of the DeNOx performances

in the temperature range below 300 �C. Hence, the relative

increase in Au metallic sites would enhance the NO oxi-

dation to NO2 and subsequently the HC-SCR of NOx to N2.

It is worthwhile to note that an extra production of CO

takes place on DP and AE samples. Previous catalytic

measurements showed that the presence of decane origi-

nates the formation of CO via reforming and/or partial

oxidation of decane producing CO and H2 [18]. This

production persists after ageing and even intensifies shift-

ing to lower temperature on both catalysts. There is a clear

correlation between the gain in NO conversion on aged

samples and the shift to lower temperature of CO pro-

duction. These observations are consistent with previous

findings on Au/Al2O3 prepared via the same method. This

exhibits a comparable behavior related to a significant

enhancement in NO conversion to N2 when decane con-

version to CO and H2 coupled to successive WGS reaction

are promoted. Hence, these results seem in qualitative

agreement with previous workers who pointed out the

beneficial effect of water but ruled out the occurrence of

the NO/H2 reaction taking place at relatively low temper-

ature [6]. In fact, hydrogen in the inlet gas phase and/or

in situ produced by reforming reaction might induce a

beneficial effect probably related to a lowering of the

inhibiting effect of decane consumed by reforming reaction

[18] and preserving an optimal Aud?/Au0 ratio. Regarding,

this latter parameter there are strong similarities with the

catalytic behavior of silver based catalysts exhibiting the

same rate enhancement when H2 is added to the reaction

mixture [18]. As previously discussed, the promotion of the

simultaneous oxidation of NO to NO2 would occur on Ag0

catalysts while SCR would take place on Agd? [20, 21].

Returning to our observations in Fig. 4, the loss of con-

version at high temperature, whereas the conversion of NO

0

5

10

15

a b

c

1 5 9 13 17 21 25 29 33 37 41 45 49

Au/Al2O3(AE) calcined

Au/Al2O3(AE) aged

Particle size (nm)

Fre

quen

cy (

%)

Fig. 3 TEM observation on

Au/Al2O3(AE) calcined in air at

300 �C (a) aged overnight at

500 �C in the reacting mixture

(b) and c particle size

distribution

Top Catal (2013) 56:157–164 161

123

starts more readily on aged samples, might reflect change

in the Aud?/Au0 ratio with partial agglomeration of

small Aud? clusters during thermal ageing especially on

Au/Al2O3(DP) then partly suppressing the SCR activity at

high temperature.

3.2.2 Influence of Pre-Oxidative and Pre-Reductive

Thermal Treatment Prior to Reaction: Impact

on Thermal Ageing

We have examined the impact of the nature of the pre-

treatment, under oxidative or reductive atmospheres, on the

extent of surface reconstructions and related catalytic

performances in term of stability. A similar procedure was

implemented with the extent of activation/deactivation

processes qualitatively discussed based on the comparison

of TPR-1 and TPR-2. As exemplified in Fig. 5a, a pre-

oxidative thermal treatment in air at 500 �C has a strong

beneficial effect on the conversion of NO. On the contrary,

subsequent pre-reductive thermal treatment induces a sig-

nificant loss of activity especially for high H2-reduction

temperature at 500 �C. As previously discussed calcined

sample at 300 �C exhibits a poor activity in comparison

with that obtained after calcination at higher temperature.

The highest activity recorded after calcination at 500 �C is

consistent with low B.E. values suggesting a strengthening

of the metallic character of gold. As illustrated in Fig. 5b

ageing improves the conversion of NO with broader oper-

ating windows and significant increase in maximum NO

conversion. It is also worthwhile to note that those ten-

dencies affect more significantly reduced samples at mod-

erate temperature with conversion curves superimposed to

Fig. 4 Temperature-programmed experiments for the selective NO

reduction to nitrogen on calcined Au/Al2O3 catalysts at 300 �C then

pre-reduced in H2 at 250 �C and further exposed to 300 ppm NO,

300 ppm CO, 300 ppm propene, 100 ppm decane, 0.2 vol.% H2,

5 vol.% H2O, 10 vol.% CO2, 10 vol.% O2 diluted in He wt %.

Au/Al2O3 prepared by deposition–precipitation using NaOH (DP)

(a) and anionic exchange (AE) (b). Open symbols for conversion/

concentration recorded on fresh catalysts full symbols on aged

samples. N2 selectivity on fresh times and dash aged samples

162 Top Catal (2013) 56:157–164

123

that recorded on aged sample initially calcined at 500 �C.

Some differences are observable at high temperature since

calcined sample retains a much higher conversion. Never-

theless, it seems obvious that surface modifications taking

place under real exhaust gas conditions attenuate the dif-

ferences initially observed on pre-oxidized and pre-reduced

samples. Hence the convergence to comparable catalytic

performances could reflect the adjustment of the Aud?/Au0

ratio towards an optimal value.

4 Conclusion

This investigation deals with the catalytic behavior of

aged Au/Al2O3 in the selective reduction of NO by

hydrocarbons under real exhaust gas conditions. Particular

attention was paid to deactivation processes after exposure

overnight in reaction conditions at 500 �C. It was found

that the mode of gold incorporation significantly influences

the nature of interactions between gold and alumina. Gold

redispersion on aged anionic exchange samples can be

correlated to the rate enhancement in NO conversion to

nitrogen. The influence of thermal pre-treatment on the

catalytic performance was also observed with stronger

interactions and better stability on samples initially cal-

cined at 500 �C. Pre-reduction in H2 has a strong detri-

mental effect especially at 500 �C. On the other hand,

moderate reduction temperature allows significant surface

reconstructions with catalytic performance on aged sample

converging to that observed on calcined samples. This fact

emphasizes the existence of an optimal Au0/Aud? ratio to

get a high conversion of NOx. Finally, Au/Al2O3 clearly

exhibits much higher activity in NOx conversion in the

temperature range 300-450 �C than noble metals, espe-

cially Pt [20] and offers new alternative to silver based

catalysts. Probably, the most important finding compare to

Ag resides in the activation of Au/Al2O3 under reactive

conditions while previous investigations reported deacti-

vation of supported gold catalysts in DeNOx catalysis [8].

The fact that larger particles seem more active than smaller

ones is an additional argument in favor of further practical

developments since under usual operating temperature

conditions the agglomeration of unstable small Au particles

would originate a gain in the selective conversion of NOx.

Acknowledgments We would like to thank the CNRS which

stimulates this research project through the International Associate

Laboratory between UCCS and NCL with a Ph’D fellowship (D.L.

Nguyen). We thank Arnaud Beaurain and A.S. Mamede who con-

ducted XPS measurements.

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1a

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130 180 230 280 330 380 430 480 530

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Temperature (°C)

NO

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