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Alternative Methods for the Preparation of Gold Nanoparticles
Supported on TiO2
Rodolfo Zanella, Suzanne Giorgio, Claude R. Henry, and Catherine
Louis*,Laboratoire de ReactiVite de Surface, UMR 7609 CNRS,
UniVersite Pierre et Marie Curie,4 place Jussieu, 75252 Paris Cedex
05, France, and CRMC2 CNRS, Campus de Luminy, case 913,13288
Marseille Cedex, FranceReceiVed: December 10, 2001; In Final Form:
May 23, 2002
The best current way to prepare Au/TiO2 catalysts is the method
of deposition-precipitation with NaOH (DPNaOH) developed by Haruta
and co-workers. With this method, it is possible to obtain small
gold metalparticles (2-3 nm), but the corresponding gold loading
remains rather low (3 wt %). The main goal of thiswork is to
investigate other methods of preparation of Au/TiO2 catalysts to
obtain small gold metal particles(2-3 nm) and a higher Au loading.
It is shown that anion adsorption with AuCl4- (AA) does not produce
Auloading higher than 1.5 wt % and the average particle size is not
very small (4 nm). Cation adsorption withAu(en)23+ (CA) leads to
small particles (2 nm) when the solution/support contact time is
moderate (1 h), butthe Au loading does not exceed 2 wt %. The most
promising method of preparation appears to be
deposition-precipitation with urea (DP urea). Indeed, samples with
gold particles as small as those obtained with DPNaOH (2 nm) can be
prepared, and all gold in solution is deposited on TiO2 in contrast
to DP NaOH. TheDP urea samples reported in this paper can reach a
Au loading as high as 8 wt % using a TiO2 support witha surface
area of 45 m2 g-1. The possible mechanisms of deposition of gold on
the TiO2 support by thedifferent methods of preparation are
discussed.
I. Introduction
Gold metal becomes catalytically active in several
chemicalreactions when it is finely divided and supported on
metaloxides.1-3 The most remarkable catalytic properties of
supportedgold were observed by Haruta et al. in 19874-6 in CO
oxidationat subambient temperature. Since then, the most studied
catalysthas been gold supported on TiO2. Taken separately, Au
andthe TiO2 support are catalytically inactive for this reaction,
butAu/TiO2 shows a drastic synergetic effect for the reaction ofCO
oxidation. The optimum size of gold particles is smallerthan 5 nm
for catalytic applications and about 2-3 nm for COoxidation.2,7,8
Such particle sizes can be achieved by a carefulcontrol of the
conditions of preparation.
The catalytic activity of Au/TiO2 for CO oxidation dependson the
preparation method; for example, Au/TiO2 catalystsprepared by
coprecipitation are less active than catalystsprepared by
deposition-precipitation.9,10 The parameters usedin the
preparations are also important. For instance, for
thedeposition-precipitation method, Haruta et al.3 described howthe
catalytic activity is sensitive to gold concentration, pH
andtemperature of the solution, calcination temperature, and
additionof magnesium citrate.
Haruta developed a preparation method of Au/TiO2 catalystsby
deposition-precipitation with NaOH as precipitating agent(DP
NaOH).11,12 With a nominal amount of 13 wt % of Au insolution and
within a pH range between 7 and 10, this methodpermits the
deposition of up to 3 wt % of Au and the formationof small metal
particles with an average size of about 3 nm.Higher Au loading of 8
wt % could be achieved at pH 5.5, but
much larger particles were obtained (10 nm). It may be notedthat
the amount of Au deposited on TiO2 by DP NaOH is alwayslower than
the amount of Au contained in solution, that is, theyield of DP is
lower than 100%.
It may be also noted that this method of preparation doesnot
exactly correspond to the principle of the method of DP,largely
developed by Geus13,14 and then extensively studied byour group for
the preparation of Ni/SiO2 samples.15-18 In theDP method, the metal
precursor is added to an aqueoussuspension of the support and
subsequently precipitated as ahydroxide by raising the pH. The
surface of the support acts asa nucleating agent, and this method,
if it is properly performed,leads to the greater part of the active
precursor being attachedto the support. The key factor of this
preparation is theprevention of precipitation away from the support
surface. Themethod of deposition-precipitation developed by Geus et
al.13,14using urea (CO(NH2)2) as the precipitating base permits
thegradual and homogeneous addition of hydroxide ions throughoutthe
whole solution, CO(NH2)2 + 3H2O f 2NH4+ + CO2 +2OH-, and avoids
local increase in pH and the precipitation ofmetal hydroxide in
solution.
In the preparation of Au/TiO2 catalysts, one can also takeinto
account the fact that TiO2 is an amphoteric oxide
(isoelectricpoint, IEPTiO2 ) 6 19). Therefore, this oxide can be
used forpreparation (i) by cation adsorption when the solution pH
ishigher than IEPTiO2 (the main surface species is O-, so the
TiO2surface is negatively charged) and (ii) by anion adsorption
whenthe pH is lower than IEPTiO2 (the main surface species is
OH2+,so the TiO2 surface is positively charged).
Following these principles, we decided to prepare
Au/TiO2catalysts by three methods: (1) deposition-precipitation
withurea (DP urea) (Bond and Thompson,1 had suggested thismethod,
and Dekkers et al.20 had prepared Au/TiO2 samplesby this method.
However, they obtained rather large gold
* To whom correspondence should be addressed. E-mail:
[email protected].
Universite Pierre et Marie Curie. CRMC2 CNRS. Associated with
the Universities of Aix-Marseille II and III.
7634 J. Phys. Chem. B 2002, 106, 7634-7642
10.1021/jp0144810 CCC: $22.00 2002 American Chemical
SocietyPublished on Web 07/13/2002
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particles with an average size of 7.5 nm for a gold loading
of4.5 wt %); (2) anion adsorption (AA) with AuCl4- complex;(3)
cation adsorption (CA) with Au(en)23+ complex (en )ethanediamine)
(this method was first successfully developedby Guillemot et
al.21,22 for the introduction of gold into Yzeolites).
The goal of the study is to investigate whether it is possibleby
these three methods to prepare Au/TiO2 catalysts with smallmetal
particles, in the same range of size as samples preparedby DP NaOH
(2-3 nm),3,11,23 but with a better control of theAu loading,
especially with higher Au loading. In this paper,we show for the
first time that deposition-precipitation withurea (DP urea) can be
successfully applied to the preparationof Au/TiO2 catalysts with
high metal loading and small Auparticle sizes.
For comparison, other Au/TiO2 samples were also preparedby (i)
deposition-precipitation with NaOH (DP NaOH) in thepresence or in
the absence of magnesium citrate and (ii) incipientwetness
impregnation (Imp) with HAuCl4 (this is the very firstmethod
reported in the literature for the preparation of supportedgold
catalysts.28,29 However, the gold particle sizes are largeeven at
low metal loading. In addition, the samples contain largeamounts of
chlorides, which are known to poison catalysis formany
reactions).
II. Experimental Section
1. Au/TiO2 Preparations. Titania Degussa P25 was used asthe
support (BET surface area ) 45 m2 g-1, nonporous, 70%anatase and
30% rutile, purity > 99.5%) and solid HAuCl43H2O (Acros) as the
gold precursor. Before preparation, TiO2was previously dried in air
at 100 C for at least 24 h. All ofthe preparations were performed
in the absence of light, whichis known to decompose the gold
precursors. For most of thepreparations, 1 g of TiO2 was added to
100 mL of an aqueoussolution of gold precursor (4.2 10-3 M). The
amount of goldin solution corresponds to a maximum gold loading of
8 wt %on TiO2.
After deposition of gold onto TiO2 according to the
variousmethods described below, all the solids were submitted to
thesame procedure: (i) separation from the precursor solution
bycentrifugation (12 000 rpm for 10 min); (ii) washing (the
solidswere suspended in water (100 mL g-1), stirred for 10 min
atRT, and centrifuged again. This washing procedure was
repeatedfour times to remove residual Cl- and Na+ ions as well as
Auspecies not interacting with the support); (iii) drying
undervacuum at 100 C for 2 h; (iv) calcination at 300 C (300 mgof
sample was heated in a flow (30 mL min-1) of industrial air(Air
Liquide) from room temperature to 300 C with a rate of2 C min-1
then maintained at 300 C for 4 h. Calcinationtreatment leads to the
decomposition of the Au(III) complexesinto gold metal particles);
(v) storage of the samples away fromlight and under vacuum in a
desiccator at RT. Indeed, a strongincrease in the average particle
size of the calcined sampleswas observed when samples are stored in
air even for a shortperiod (for instance, from 1.8 to 3 nm for a
sample left in airfor about 10 days). After several months of
storage away fromlight in a desiccator, the average particle size
also slightlyincreases (for instance, from 1.7 to 2.1 nm after 10
months ofstorage). To avoid this effect, the samples have been
stored afterdrying, and calcination is performed when needed.
a. Deposition-Precipitation with NaOH. In the
standardpreparation conditions (similar to Harutas preparations24),
100mL of an aqueous solution of HAuCl4 (4.2 10-3 M) washeated to 80
C. The gold concentration in solution corresponds
to a theoretical Au loading of 8 wt % in the case of a
completedeposition-precipitation (DP yield ) 100%). This gold
loadingwas chosen because Haruta23 reported that the most
activecatalyst for CO oxidation contained 8 wt % of Au. The pH
wasadjusted to 8 by dropwise addition of NaOH (1 M), and then 1g of
TiO2 was dispersed in the solution, and the pH wasreadjusted to 8
with NaOH. The suspension thermostated at 80C was vigorously
stirred for 1 h and then centrifuged, and thesolid was washed,
dried, and calcined following the previouslyreported procedure. The
main parameters studied were the DPtime (1, 2, 4, or 16 h), the
addition of magnesium citrate (Mg3-(C6H5O7)2, 6.9 10-3 M) in the
suspension after the firstadjustment of pH, and the HAuCl4
concentration.
b. Incipient Wetness Impregnation. Titania was impregnatedwith
aqueous solutions of HAuCl4 (1.3 mL per g of TiO2) ofvarious
concentrations (0.04, 0.08, and 0.16 M) to obtainsamples with 1, 2,
and 4 wt % of Au, respectively. In all cases,the solution pH was
less than 1. The samples were aged at roomtemperature (RT) for 1 h.
Samples (2 and 4 wt %) were thendivided into two parts. One part
was directly dried (Impsamples), whereas the other one was washed
before drying(ImpW samples) to determine whether some Au species
interactwith the TiO2 support. All of the samples were calcined at
300C.
c. Anion Adsorption. A total of 1 g of TiO2 was added to100 mL
of an aqueous solution of HAuCl4 (4.2 10-3 M).Under such
conditions, the solution pH (2) was lower thanthe IEPTiO2, that is,
in adequate conditions of pH for anionadsorption. The suspension
thermostated at 25 or 80 C wasvigorously stirred for 15 min, 1 h,
or 15 h and finally centri-fuged. The solids were washed, dried,
and calcined at 300 C.
d. Cation Adsorption. Gold was also deposited on TiO2 bycation
adsorption of the Au(en)23+ complex (en ) ethanedi-amine) the
synthesis of which was described by Block andBailar.25 Au(en)2Cl3
was dissolved in 100 mL of water (4.2 10-3 M). By dropwise addition
of an ethanediamine solution(1 M), the pH was adjusted to a value
of 9.4 or 10.3, that is, ata pH higher than the IEPTiO2. Hence, the
adsorption of the Au-(en)23+ complex is theoretically possible. The
suspension wasvigorously stirred for 1 or 16 h at 80 C in a
thermostated vessel.After the adsorption, the samples were
centrifuged, washed,dried under vacuum, and calcined at 300 C.
e. Deposition-Precipitation with Urea. In the so-calledstandard
preparation conditions, 1 g of TiO2 was added to 100mL of an
aqueous solution of HAuCl4 (4.2 10-3 M) and ofurea (0.42 M). The
initial pH was 2. The suspensionthermostated at 80 C was vigorously
stirred for 4 h (pHincreases) and then centrifuged, washed, dried,
and calcined at300 C. The following parameters were studied: the DP
time(1, 2, 4, 16, and 90 h), the temperature of DP (80 and 90
C),the gold concentration (1.1 10-3, 1.6 10-3, and 4.2 10-3M), the
urea concentration (0.42 and 0.84 M), and the additionof magnesium
citrate (6.9 10-3 M).
2. Techniques of Characterization. Chemical analysis ofAu, Cl,
Mg, Na, C, and N in the samples was performed byinductively coupled
plasma atom emission spectroscopy at theCNRS Center of Chemical
Analysis (Vernaison, France). Thedetection limit is 300 ppm for Cl
and 1000 ppm for C and N.Chemical analysis was performed after
sample calcination. TheAu weight loading of the samples is
expressed in grams of Auper grams of sample calcined at 1000 C: wt
% Au ) [mAu/(mAu + mTiO2)] 100.
Calcined Au/TiO2 samples were examined by transmissionelectron
microscopy (TEM) with a JEOL 2000FX electron
Small Metal Particles in Au/TiO2 J. Phys. Chem. B, Vol. 106, No.
31, 2002 7635
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microscope. Except when especially mentioned, the histogramsof
the metal particle sizes were established from the measure-ment of
300 to 1000 particles. The size limit for the detectionof gold
particles on TiO2 is about 1 nm. The average particlediameter, dh,
was calculated from the following formula: dh )nidi/ni, where ni is
the number of particles of diameter di.The standard deviation was
calculated from the formula )[((di - dh)2)/ni]1/2. In some samples,
the presence of Au wasalso determined by energy-dispersive X-ray
spectroscopy coupledto TEM observations.
III. Results
1. Deposition-Precipitation with NaOH. Table 1 shows thatwhen
the pH of the DP NaOH solution is 8 (DPN1 sample),the average Au
particle size is 1.8 nm and the Au loading is1.8 wt % (Figure 1).
When the pH is lowered to 7 (DPN6sample), the average particle size
is smaller (1.4 nm) and theAu loading is higher (3.3 wt %). Another
parameter studied isthe DP time, from 1 to 16 h (samples DPN1 to
DPN4). Thegold loading slightly increases from 1.8 to 2.4 wt %,
while theaverage particle size seems to slightly decrease from 1.8
to 1.5nm (Table 1). The particle size distribution becomes
narrower.
It may be noted that our preparations can provide smallergold
particles than those obtained by Haruta: at pH 8, 1.8 nmfor 1.8 wt
% of Au (DPN1, Table 1) to be compared to 2.9 nmfor 2.2 wt %,12 and
at pH 7, 1.4 nm for 3.4 wt % (DPN6) to becompared to 3.3 or 3.6 nm
for 3.6 wt %.12 As for Harutassamples, the Au loading in our
samples is lower than thenominal amount of gold in solution,
indicating that the yield ofDP is less than 100% and that part of
the gold is not depositedon TiO2. When the concentration of gold in
solution is higher(8.4 10-3 M), the gold loading on TiO2 is higher
(3.6 wt %in DPN7) but the average particle size is much larger (3.3
nm)and the size distribution much broader.
A sample was also prepared in the presence of magnesiumcitrate.
According to Haruta et al.,3,11,23 magnesium citrate leadsto higher
Au loadings and small metal particles because it stickson the TiO2
surface and prevents gold particles from sinteringduring
calcination. Magnesium citrate is also known to reducethe gold(III)
precursors into metallic gold.11,26 Table 1 showsthat when
magnesium citrate is added (DPN5), the Au loadingis indeed higher
(3.1 compared to 1.8 wt % for DPN1), and theaverage particle size
is smaller (1.4 instead of 1.8 nm) (Figure2).
The gold particles in DPN5 are also smaller than thoseobtained
by Haruta et al. under close experimental conditions:1.4 nm to be
compared to 2.8-3.8 nm.27 However, DPN5 doesnot reach gold loadings
as high as 6 or 8 wt % as reported byHaruta in two papers.23,27
However, in the first one,27 it is notclear whether the Au loadings
reported are those in solution or
on TiO2, and in the second one,23 the amount of gold in
solutionis not reported.
2. Incipient Wetness Impregnation. Table 2 summarizesthe results
obtained for the Au/TiO2 samples prepared byimpregnation and by
impregnation followed by washing. Allof the Imp samples contain
only a few large Au particles (>10nm) after calcination (Table
2). These results are consistent withpublished results. Indeed,
Haruta et al. 28 obtained gold particlesbetween 10 and 30 nm for 1
wt % Au/TiO2 catalysts, andVannice et al. 29 obtained gold
particles between 25 and 35 nmfor 2 wt % Au/TiO2 sample. Although
X-ray fluorescence andchemical analyses indicate the presence of
Au, only a smallnumber of Au particles could be observed by TEM in
the Impsamples, so the values for the particle size distribution
are notreliable. This problem had already been reported by Haruta
etal.7
Chemical analysis shows that about 0.9 wt % of gold remainson
TiO2 after washing whatever the initial Au loading, and theaverage
particle size is between 3 and 5 nm after calcination(Table 2).
Hence, part of the gold species deposited duringimpregnation is in
strong interaction with TiO2 because itremains on the support after
washing. Because of this stronginteraction, sintering of gold
particles is prevented duringcalcination, so the metal particles
are smaller than those obtainedafter impregnation.
It may be noted that although the Au loading of Imp3 is
twicethat of Imp2, both the Au loading and the average particle
sizeof ImpW3 are smaller than those of ImpW2. We did not
attempt
TABLE 1: Au/TiO2 Samples Prepared byDeposition-Precipitation
with NaOH at 80 C
preparation results
sample
DPtime(h) pH
theor Auloading(wt %)
Auloading(wt %)
Clloading(wt %)
averageparticle
size (nm)standarddeviation
(nm)particle sizedistribution
(nm)DPN1 1 8 8 1.8 0.026 1.8 0.62 0.7-4.3DPN2 2 8 8 2.4 0.025
1.5 0.40 0.7-3.5DPN3 4 8 8 2.1 0.035 1.6 0.36 1.0-3.1DPN4 16 8 8
2.4 0.077 1.5 0.30 1.0-2.4DPN5a 1 8 8 3.1 0.09 1.4 0.36 0.7-2.7DPN6
1 7 8 3.3 0.031 1.4 0.34 0.7-2.4DPN7 1 8 16 3.6 0.030 3.3 1.92
0.7-16.0
a Addition of magnesium citrate (6.9 10-3 M).
Figure 1. (a) TEM image of calcined sample DPN1 prepared
bydeposition precipitation with NaOH (DP time ) 1 h, pH ) 8, 1.8
wt% Au); (b) size histogram of gold particles.
7636 J. Phys. Chem. B, Vol. 106, No. 31, 2002 Zanella et al.
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to repeat these experiments to check these unexpected
resultsbecause the main goal of these experiments was to
determinewhether some gold remained in interaction with the support
afterwashing.
3. Anion Adsorption. Because some gold species (e0.9 wt%, Table
2) remain adsorbed on the TiO2 surface after washingof the
impregnated samples, preparations by anion adsorptionwith AuCl4-
have been attempted. Table 3 shows that underour experimental
conditions the Au loading does not exceed1.5 wt % and the average
particle size is about 4-6 nm whateverthe samples. As in the case
of Imp samples, only a few particlesare observed by TEM, so the
average size measurements arenot very accurate. However, it can be
mentioned that (i) thegold loading is higher when anion adsorption
is performed at80 C (AA4) rather than at RT (AA1) and (ii) the
equilibriumadsorption seems to be reached fast (10Imp3 4
>10ImpW2a 2 0.9 0.09 4.9 1.36 1.9-8.5 (165 particles)ImpW3a 4
0.6 0.08 2.7 1.52 1.4-5.8 (78 particles)
a The ImpW samples are washed after impregnation.
TABLE 3: Au/TiO2 Samples Prepared by Anion Adsorptionwith
AuCl4-
resultspreparation
sampletime ofcontact
T(C) pH
Auloading(wt %)
Clloading(wt %)
averageparticle
size (nm)standarddeviation
(nm)particle sizedistribution
(nm)AA1 15 h 25 2.5 1.0 0.16 5.7 1.38 5.0-7.0
(4 particles)AA2 15 min 80 2 1.3 0.16 3.7 1.30 1.7-6.8
(67 particles)AA3 1 h 80 2 1.0 0.07 5.6 1.50 2.0-8.3
(23 particles)AA4 15 h 80 2 1.5 0.15 4.4 1.29 2.0-7.5
(60 particles)
TABLE 4: Au/TiO2 Samples Prepared by CationAdsorption with
Au(en)23+ at 80 C
preparation results
sample
time ofcontact
(h) pHAu
loading(wt %)
Clloading(wt %)
averageparticle
size (nm)standarddeviation
(nm)particle sizedistribution
(nm)CA1 1 9.4 1.1 - 2.1 0.66 1.0-3.5
(134 particles)CA2 16 9.4 6.1 0.03 4.1 1.01 1.7-6.4
(140 particles)CA3 1 10.3 1.7 0.04 1.8 0.33 1.0-2.5
(88 particles)CA4 16 10.3 6.4 0.08 4.6a 2.74 1.7-15.8
(157 particles)a Plus some very large particles (80-150 nm) not
taken into account
in the calculation of the average size.
Small Metal Particles in Au/TiO2 J. Phys. Chem. B, Vol. 106, No.
31, 2002 7637
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citrate is added to the suspension (DPU8). When the
goldconcentration is lower (DPU10 and DPU9), 100% of the goldis
still deposited on TiO2 and the average particle size issmaller: 2
nm for 2 wt % Au (DPU9) and 2.3 nm for 3 wt %Au (DPU10) instead of
2.7 nm for 8 wt % Au (DPU3).
It may be noted that our preparations by DP urea providemuch
better results than the very first preparations performedby Dekkers
et al.20 They report particles with an average sizeof 7.5 nm for a
gold loading of 4.5 wt % in a Au/TiO2 sampleprepared by DP urea at
80 C (pH ) 8.5) and then washed,
dried at 80 C, and calcined at 400 C. The reason for
thedifferences in the particle size is not straightforward
becausesome preparation parameters are missing and it is
well-knownthat every step of preparation may have an influence on
thefinal state of the catalysts. One relevant point is that the
pHreached by Dekker et al. is higher than ours.
If one compares the DP urea samples (Table 5) to our ownDP NaOH
samples (Table 1), one can note that the averageparticle sizes are
slightly larger and the size distributions slightlybroader, except
for the preparation performed at 90 instead of80 C (DPU6 in Table
5). Samples of each type of preparation,
Figure 3. (a) TEM image of calcined sample CA1 prepared by
cationadsorption with Au(en)23+ complex (CA time ) 1 h, pH ) 9.4,
1.1 wt% Au); (b) size histogram of gold particles.
TABLE 5: Au/TiO2 Samples Prepared by Deposition-Precipitation
with Ureapreparation results
sample
DPtime(h)
T(C) pHa
theor Auloading(wt %)
Auloading(wt %)
Clloading(wt %)
averageparticle
size (nm)
standarddeviation
(nm)
particle sizedistribution
(nm)DPU1 1 80 2.99 8b 7.8 0.041 5.6 1.66 2.3-10.2DPU2 2 80 6.26
8 6.5 0.122 5.2 1.10 2.0-7.5 (135 particles)DPU3 4 80 7.04 8
7.7
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DP NaOH (DPN1) and DP urea (DPU6), with the same averagesmall
particle size were recently tested in the reaction of COoxidation
(25 mg of catalyst, T ) 5 C, 1% CO, and 4% O2 inN2, 99.3 cm3
min-1).30 The results show that the activityexpressed in
molCOconverted gcat-1 s-1 is much higher for DPU6than for DPN1.
This result is fully consistent with the higherAu loading in the DP
urea samples. More interesting is the factthat DPN1 and DPU6 show
the same activity when it isexpressed in molCOconverted molAu-1
s-1. This result confirms thatthe DP urea method leads to the same
dispersion of gold ontotitania as DP NaOH and that it is an
outstanding method ofpreparation of Au/TiO2.
IV. Discussion
Although this is not the main goal of the present paper,
adiscussion on the mechanism of deposition of gold on TiO2during
the various preparations described above is attempted,on the basis
of literature data only, because no characterizationstudies of the
Au/TiO2 samples before calcination have beenperformed up to
now.
1. Impregnation and Impregnation-Washing. This prepa-ration was
performed at RT with gold solutions at pH < 1 andconcentrations
between 0.04 and 0.16 M. Under these condi-tions, the main gold
species in solution is AuCl4-.31-36 Becauseat this low pH the TiO2
surface is positively charged, someAuCl4- can electrostatically
interact with the TiO2 surface. Thiswould explain that part of the
Au remains on the support afterwashing (1 wt %, Table 2).
2. Anion Adsorption. Under our conditions of preparation(RT or
80 C, 4.2 10-3 M of Au, and pH 2), the mainspecies in solution is
AuCl3(OH)- at RT34 and remains the sameup to 150 C.37 Because the
solution pH is lower than IEPTiO2(6), AuCl3(OH)- could
electrostatically interact with the TiO2surface. However, according
to the literature data related to theadsorption of gold hydroxy
chlorides on various oxides,38-43the mechanism of adsorption of
gold hydroxy chlorides is notan electrostatic interaction but the
formation of a surfacecomplex by reaction with surface OH, that is,
the formation ofan inner-sphere complex such as
For instance, Nechayev et al.41 reported that the maximum ofAu
adsorption on alumina occurs at pH close to the IEPAl2O3) 8, that
is, at a pH at which the number of neutral OH ismaximum. Machesky
et al.38 reported that the adsorption of goldhydroxy chloride on
goethite (IEPFeOOH ) 8.1) increases as pHincreases from 4 to 7,
which is opposite to typical behavior foranion adsorption on
positively charged oxide surfaces. Thisretrograde adsorption is
attributed to (i) a shift in Au speciationwhen pH increases from
AuCl4- at pH 4 to AuCl(OH)3- at pH7 and AuCl3OH- and AuCl2(OH)2- at
intermediate pH and (ii)the fact that gold hydroxy chloride
complexes preferentially reactwith the OH of the support and all
the more easily because theamount of substituted OH in the gold
coordination sphereincreases.
Because the amount of neutral OH surface species on TiO2is low
at low pH, this could explain that the amount of goldadsorbed is
low in the AA samples (e1.5 wt %, Table 3). Inaddition, the fact
that the Au loading does not depend on thesolution/support contact
time and on the temperature (Table 3),that is, that gold adsorption
is fast, is in agreement with amechanism of gold adsorption via a
surface complex formation.If this interpretation is correct, this
also means that (i) the natureof the interaction between Au species
and TiO2 is different fromthat occurring during impregnation and
(ii) the term anionadsorption used to designate this preparation
procedure isimproper because it does not reflect the phenomenon of
surfacecomplex formation.
3. Cation Adsorption. In the case of cation adsorption
withAu(en)23+, the solution pH (9.4 or 10.3) is higher than
IEPTiO2.If one assumes that all of the surface OHs are
deprotonatedand that each Au(en)23+ interacts with 3 O- of the TiO2
surfaceto compensate the surface charge, a maximum gold loading of3
wt % is expected (TiO2 Degussa support contains 6 OH/nm2 44).
Because a higher Au loading is reached (Table 4), thisindicates
that another chemical phenomenon rather than cationadsorption is
probably involved, but more characterization isneeded to elucidate
this point.
4. Deposition-Precipitation with NaOH. Haruta
etal.4-7,9-12,23,24,27,28 reported only a few pieces of
informationconcerning the possible mechanism of
deposition-precipitationof Au on TiO2 with NaOH. In ref 12, they
showed that theamount of gold deposited reaches a maximum at pH 6,
at thepH of IEPTiO2, that is, when the surface charge is neutral.
Inthat paper, they reported that the number of Au particles
isalmost constant within the pH range of 6-8. They concludedthat
gold is deposited on specific sites of the TiO2 surface. FromXANES
measurements, they concluded that Au is not metallicbut bound to
oxygen, most probably as gold hydroxide. Theyproposed that this
gold hydroxide is deposited from Au(OH)3Cl-on specific sites of
TiO2, which may act as nucleation sites forAu(OH)3. The increasing
amount of Au with increasing pH up
Figure 5. (a) TEM image of calcined sample DPU6 (DP time ) 4 h,T
) 90 C, 7.1 wt % Au); (b) size histogram of gold particles.
TiOH + AuCl3(OH)- T TiOAuCl2 + H2O + Cl- (I)
Small Metal Particles in Au/TiO2 J. Phys. Chem. B, Vol. 106, No.
31, 2002 7639
-
to 6 can be accounted for by a decrease in the ion
exchangecapacity or a decrease in ion interaction between TiO2 and
theAu species due to the decrease in the positive charge or
both.The decreasing trend when pH increases above 6 is explainedby
the increasing solubility of Au(OH)3 with pH.
In our samples, the Au loading barely changes with the DPtime
nor does particle size (Table 1). This indicates that, as inthe
case of anion adsorption, gold adsorption is fast. However,the
solution pH is much higher (7 or 8 instead of 2), so goldspeciation
in solution is different, probably AuCl2(OH)2- orAuCl(OH)3- or both
at 80 C, according to Murphy et al.37The TiO2 surface charge is
different as well; it is negativelycharged. It may be noted that
electrostatic adsorption of anionicgold complexes cannot occur. Our
interpretation is that a goldsurface complex can form (Ti-O-AuCl2)
in the same way asin eq I:
This is consistent with Harutas results:12 (i) the amount of
golddeposited reaches a maximum pH 6, that of the IEPTiO2, that
is,when the surface charge is neutral; (ii) Au is bound to
oxygenatoms (XANES data), provided that these oxygens are those
ofthe TiO2 surface (Au-O-Ti) and not those of gold hydroxide.It may
be noted that in the literature data on the interaction ofgold
chloride with oxide surfaces at various pHs,38-43,45 theauthors
never mention the possibility of the formation of goldhydroxide. In
addition, when one considers the chemistry ofHAuCl4 in aqueous
solution,46,47 AuCl4- is hydrolyzed, andhydroxochloro complexes
form by replacement of Cl- ligandsby OH- ligands when pH increases
without formation of goldhydroxide.
5. Deposition-Precipitation with Urea. It is proposed thatthe
first chemical step occurring during DP urea preparation ofAu/TiO2
is surface complex formation, as for anion adsorptionor DP NaOH.
When the DP time increases and before thesolution pH reaches
IEPTiO2 (Figure 6), the positive surfacecharge decreases, there are
more and more neutral surface OHs,and so more and more sites for
the formation of gold surfacecomplex (retrograde adsorption, see
section IV.2.).
However, this reasoning does not explain the fact that all ofthe
gold precipitates during DP urea and not during DP NaOH.A second
chemical phenomenon must be involved. If one refersto literature
data related to adsorption of gold hydroxy chlorideson reducible
oxides, this second chemical phenomenon couldbe the formation of
gold colloids. For instance, Greffie et al.45
identified two forms of gold species on iron oxides
(ferrihydriteand goethite) after coprecipitation: metallic gold
colloids andAu(III) species, which corresponded to 1-5% of all
thedeposited gold). The formation of gold colloids was attributedto
oxidation-reduction reactions between Au(III) and Fe(II)present as
traces: adsorption of gold on oxidizable mineralsurfaces induces
the reduction of Au(III) to elemental gold,which is subsequently
sorbed on oxides. Indeed, colloidal goldparticles are negatively
surface-charged,48,49 so they can reactwith positively charged iron
oxides by electrostatic interaction(IEP ferrihydrite and goethite )
8-9).
It is highly probable that TiO2 can also reduce gold becauseit
usually contains defects of Ti3+ ions. Let us then form
thehypothesis that gold colloids can form. The difference in
goldloading between DP NaOH samples and DP urea samples canbe
explained by the differences in solution pHs. Because goldcolloids
are negatively charged on the surface48,49 and becausethe TiO2
surface is also negatively charged during DP NaOHpreparations
because the solution pH (8 or 7) is higher thanIEPTiO2, gold
colloids could not interact with the TiO2 surfacebut would be
released into the solution. Thus, only the Au(III)surface complex
would be present on TiO2. This interpretationis consistent with the
low Au loading of the samples and withHarutas XANES results,12
which show that the gold is notmetallic.
For DP urea, all of the gold is deposited on the TiO2 duringthe
first hour (Table 5). It can be inferred that gold colloidscan
interact with the TiO2 surface, which is positively chargedbefore
the pH reaches IEPTiO2. In consequence, gold species inthe DP urea
samples would be a mixture of Au(III) surfacecomplexes and Au(0)
colloids. UV-visible spectroscopy of thesamples gathered after
centrifugation and washing (before dryingto avoid possible
decomposition of Au(III) into Au(0)) shouldallow us to check these
hypotheses. Detection of gold colloidsshould also be attempted in
the solution of DP NaOH gatheredafter centrifugation.
Table 5 also shows that as the pH increases, the averageparticle
size becomes smaller; this is observed when (i) the DPtime
increases (DPU1 to DPU5), (ii) the DP temperatureincreases from 80
to 90 C (DPU2 and DPU6), and (iii) theurea concentration is twice
higher (DPU2 and DPU7). If wesuppose that the particle size
measured after calcination reflectsthe size of gold colloids before
calcination, it can be inferredthat the increase in pH as the DP
time increases and the factthat urea dissociates faster at 90 than
at 80 C and that moreurea dissociates when the urea concentration
is higher have aninfluence on the kinetics of redispersion of gold
colloids(because all of the gold is deposited within the first hour
ofDP). Such a phenomenon of redispersion of gold colloids
wasreported in a study on the formation of gold colloids in
solutionby reduction of gold chloride by sodium citrate.50 It
showedthat the first particles formed were large and had a
fluffymorphology, then they shrink over the course of the
reaction.The authors proposed that particles may nucleate in a
short burst,agglomerate weakly, and continue to grow within the
agglomer-ates. As the reaction proceeds, the large agglomerates
fall apart,giving rise to a continual increase in the number of
smallparticles. To the light of this interpretation, the mechanism
ofDP urea can be revisited. It is proposed that the formation
ofgold surface complexes and colloids takes place simultaneously.At
the beginning of the DP, the pH is low (Figure 6) and thereare few
OH sites for the formation of gold surface complexand therefore few
nucleation sites for colloid growth. As DPproceeds and pH
increases, more and more surface OH sites
Figure 6. pH versus time of deposition-precipitation with urea
at 80C.
TiOH + AuCl2(OH)2- T TiOAuCl2 + H2O + OH- (II)
7640 J. Phys. Chem. B, Vol. 106, No. 31, 2002 Zanella et al.
-
are available, so more and more gold surface complexes form,and
there are more and more sites for colloid redispersion.
In conclusion, only on the basis of literature data, we caninfer
that the chemical phenomena occurring during DP NaOHand DP urea are
different from each other and also from themechanism described by
Geus et al.13,14 and Burattin et al.15(see Introduction) for the
preparation of Ni/SiO2 catalysts byDP urea. In contrast to nickel,
no gold hydroxide can precipitateon oxide supports according to the
literature data, and the goldloading does not depend on the DP time
(Tables 1 and 5). Inaddition, upon solution basification during DP
urea, the TiO2surface charge switches from positive to negative
when the pHpasses the IEPTiO2 at 6 (Figure 6), whereas in the case
of silica,the pH was always higher than its IEPSiO2 (2). In
consequence,the gradual basification of the gold solution upon urea
decom-position is not performed to avoid hydroxide precipitation
insolution as in the case of the preparation of Ni/SiO2 by DP
ureabut to favor the deposition of all of the gold as Au(III)
andgold colloids and then the redispersion of gold colloids
beforethe pH reaches IEPTiO2. It may be noted that the
termdeposition-precipitation used to designate this
preparationprocedure is improper because it does not reflect the
chemicalphenomena occurring during preparation.
V. Conclusion
This paper shows that the method of preparation by
deposi-tion-precipitation with NaOH developed by Haruta et al.10
givesslightly smaller metal particles than those obtained in
theirprevious studies (e2 nm with Au loading of 3 wt %).However,
the yield of DP was always lower than 100%.
In the present work, deposition-precipitation with urea
issuccessfully used for the preparation of Au/TiO2. Comparisonof
the results obtained by DP NaOH and DP urea shows thatthe DP urea
is a promising method of preparation. Indeed, (i)the gold particles
are as small as those obtained by Haruta, 2nm average particle size
and (ii) the yield of DP is about 100%,so the Au loading can reach
much higher values, as high as 8wt % for a TiO2 support with a
surface area of 45 m2 g-1. It iseven likely that higher Au loading
could be reached with moreconcentrated gold solutions.
On the other hand, preparation of Au/TiO2 samples byimpregnation
with HAuCl4 followed by washings revealed thatpart of the gold is
in strong interaction with TiO2. This led usto attempt to prepare
Au/TiO2 samples by anion adsorption withAuCl4-. In fact, the gold
complex is AuCl3OH- under theconditions of preparation. The Au
loading in these samples wasalways lower than 1.5 wt %, and the
average particle size wasnot very small (4 nm). The interaction
between gold and TiO2is different between the samples prepared by
impregnation-washing and anion adsorption because of the different
pHs andgold concentrations of the solutions, that is, because of
thedifferent gold speciation. AuCl4- can electrostatically
interactwith the TiO2 surface in the samples prepared by
impregnation-washing, while AuClxOHy- can form a surface complex
withTiO2 in the samples prepared by anion adsorption.
In an effort to exploit to the amphoteric properties of theTiO2
support (IEP 6), cation adsorption with the Au(en)23+complex was
also attempted. Small particles were obtained (2nm) when the
solution/support contact time was moderate (1h). However, the Au
loading did not exceed 2 wt %.
Acknowledgment. Rodolfo Zanella is indebted to CONA-CYT (Mexico)
and SFERE (France) for his Ph.D. grant, and toFESC, UNAM. We all
thank the Groupement De Recherche
CNRS Interfaces et Surfaces Sensibles a` la Structure
forsupporting Zanellas travel expenses between Paris and
Marseille.
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