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Applied Catalysis A: General
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old supported on ceria nanoparticles and nanotubes
renda Acostaa,b, Elena Smolentsevac, Sergey Beloshapkind, Ricardo Rangele,iguel Estradaa, Sergio Fuentesc, Andrey Simakovc,∗
Posgrado en Física de Materiales, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, B.C. 22860, MexicoPosgrado de la Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán, MexicoUniversidad Nacional Autónoma de México, Centro de Nanociencias y Nanotecnología, Km. 107 Carretera Tijuana a Ensenada, Ensenada, C.P. 22860, Bajaalifornia, MexicoMaterials & Surface Science Institute, University of Limerick, Limerick, IrelandFacultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán, Mexico
r t i c l e i n f o
rticle history:eceived 11 May 2012eceived in revised form0 September 2012ccepted 22 September 2012vailable online 8 October 2012
a b s t r a c t
The ceria nanotubes with different size have been prepared via a hydrothermal treatment of CeO2
nanoparticles at 120 ◦C with two different NaOH concentrations (5 or 10 M) for 36 h. The synthesizedceria samples characterized by SEM, TEM, XRD, and UV–visible spectroscopy were used as supports forAu/CeO2 catalysts preparation by DP technique using HAuCl4 as gold precursor and urea as precipitationagent. The formation of gold nanoparticles (Au NPs) has been studied by TEM, in situ UV–visible–mass
analysis at temperature programmed reduction and XPS spectroscopy. Three distinguishable steps inthe formation of Au NPs accompanied by profound ceria reduction particular for ceria nanotubes havebeen found. Au NPs stabilized on the ceria nanotubes have been characterized with higher activity in COoxidation than those supported on ceria nanoparticles. The structure and redox treatment of ceria nano-tubes affects size of Au NPs and their catalytic activity in CO oxidation. Pre-oxidized Au/CeO2-nanotubes
vity in
V–vis in situO oxidation
manifest the highest acti
. Introduction
The unique catalytic properties of supported gold species weretudied extensively during the past decades. The remarkable abilityf these materials to catalyze different reactions at low temper-tures is attributed both to the presence of Au atoms with lowoordination state on the surface of gold nanoparticles (Au NPs) andhe mutual interaction of Au NPs and support [1–7]. The key rolef gold-support interface in CO oxidation was clear shown exper-mentally for ceria in [8], where drastic improvement of catalyticctivity of gold films was achieved by their decoration with ceriaanoparticles. It was definitely shown that the active sites for theO oxidation on Au/CeO2 are located at the interface of Au/CeO2.imilar effect of gold–ceria interface was found for CO oxidationeaction by the design of a set of ceria nanotower samples with theame surface area of Au and CeO2 but different interfacial lengths9].
Recent studies revealed that nanocrystalline CeO2 used as sup-
ort increases the activity of gold species in CO oxidation bywo orders of magnitude in comparison with conventional CeO210–13]. The activity of Au NPs deposited on ceria nanorods in CO
oxidation was higher than those supported on ceria nanoparticles[14]. It was established that not the crystallite size, but rather sur-face structure, more specifically the exposed surface planes of thecrystalline CeO2 support, is important for achieving a high redoxand catalytic activity of Au NPs supported on ceria [14–17]. Thestrong effect of the crystal plane of ceria on the activity of goldspecies was confirmed for WGS reaction [18,19], low-temperatureCO oxidation [14] and preferential CO oxidation [20,21]. Au NPssupported on {1 1 0} ceria planes presented as nanorods aremore active in water gas shift (WGS) reaction than those sta-bilized on {1 0 0} ceria planes of nanoparticles or nanocubes[18].
At the present time, there are different techniques to preparenanostructured ceria with different shape and size such asmicrowave assisted heating, flame spray pyrolysis, spray diffusionprocess, reverse micelles process/surfactant assisted, hydro-thermal treatment, sol–gel, chemical vapor deposition, thermaldecomposition of cerium organo-metallic compounds, templatedirected synthesis, alcoholthermal treatment, nonisothermal pre-cipitation, etc. This wide set of techniques permits to fabricateceria nanospecies in a shape of cubes, spheres, crystals, rods, wires,
discs, plates, tubes, hollow spheres, pyramids, towers and flow-ers [9,18,19,22–24]. Among other techniques the hydrothermaltreatment possesses extraordinary advantages of single step, lowtemperature, controlled composition and morphology, and high
owder reactivity because of low aggregation and high crystallinityf as obtained ceria species [22,23].
The aim of the present work was to prepare ceria nanotubes withifferent size by hydrothermal treatment of CeO2 nanoparticles inlkaline solution at high pressure and to evaluate interaction ofhem with supported Au NPs by means of different in situ and exitu techniques.
. Experimental
.1. Ceria support preparation
Sample composed of ceria nanoparticles for following hydro-hermal treatment were prepared by the sol–gel technique usingerium acetate (Alfa-Aesar) and citric acid (J.T. Baker) via the citrateomplex method described in [25] and pre-calcined at 700 ◦C for 4 hnd ramp rate of 5 ◦C/min. The preparation of ceria nanotubes byydrothermal treatment of ceria nanoparticles was carried out at20 ◦C for 36 h in the presence of NaOH (5 or 10 M) according to [26].he final solids were washed with deionized water until reachingH 7 and then calcined at 700 ◦C for 2 h.
.2. Gold catalysts preparation
Gold (3 wt.%) was supported by deposition–precipitation (DP)echnique using urea as a precipitation agent as in [27]. The sup-ort was added to an aqueous solution of HAuCl4 (1.6 × 10−3 M)Alfa-Aesar) and urea (0.42 M) at the initial pH ∼ 2. The suspensionnder vigorous stirring was thermostated at 80 ◦C for 4 h, filterednd washed with ammonium hydroxide (25 M) for 30 min as in28]. Then samples were washed with water until pH 7, filtered andried at room temperature for 24 h. Selected portions of the driedamples were reduced in hydrogen or oxidized in oxygen flow with
temperature increase up to 350 ◦C and ramp rate of 20 ◦C/min, inrder to obtain supported metallic Au NPs. The catalysts obtainedere encoded as Au/CeO2-N, where N manifests the code of ceriaanoparticles (NP) or ceria nanotubes obtained after hydrothermalreatment in 5 or 10 M NaOH solutions (NT1 or NT2), respectively.
.3. Sample characterization
Scanning electron microscopy (SEM) measurements wereerformed by “JEOL 5300” microscope (Hitachi, Japan) usingn acceleration voltage 10 kV. X-ray diffraction (XRD) analysisas done with a Philips X’pert diffractometer applying CuK�
� = 0.154 nm) radiation. Transmission electron microscopy (TEM)as performed with a JEOL 2010 microscope. Before TEM measure-ents the samples were dispersed in isopropanol and dropped on
copper grid coated with carbon film. To estimate the value ofean diameter of Au NPs more than 200 particles were chosen.
he mean diameter (dm) of particle was calculated using formula:m =
∑i(xidi)/
∑ixi, where xi is the number of particles with diam-
ter di.The electronic state of gold species was studied by X-
ay photoelectron spectroscopy (XPS) with Kratos AXIS 165hotoelectron spectrometer using monochromatic AlK� radiationhv = 1486.58 eV) and fixed analyzer pass energy of 20 eV. All mea-ured binding energies (BE) were referred to the C1s line ofdventitious carbon at 284.8 eV. The spectra fitting were done usinghirley background estimation over the energy range of the fit.
Ex situ UV–visible spectra in diffuse reflectance mode foreria samples were recorded using UV–visible spectrometer Cary
00 Scan (Varian) equipped with a standard diffuse reflectancenit. Teflon PTF Halon (Varian) was used as a reference. In situV–visible–mass analysis of Au NPs formation under temperature-rogrammed reduction (TPR) of dried samples was performed in
: General 449 (2012) 96– 104 97
the flow reactor with simultaneous analysis of gas phase compo-nents and UV–visible spectra of the sample similar to that describedin [29]. UV–visible spectra were collected using an Avaspec-2048UV–visible spectrometer (Avantes) equipped with AvaLight-DHSlight source and high temperature optic fiber reflection probelocated close to the external surface of quartz reactor (∼5 mm).UV–visible spectra recorded each 15 s were obtained by subtrac-tion of initial spectrum recorded at room temperature. Reactorpacked with MgO was used as a reference. Analysis of gas phasecomponents was performed in-line with an HPR 20 mass spectrom-eter (Hiden). The mixture of 5 vol.% H2 and 5 vol.% Ar in heliumwas passed through U-shape quarts reactor with internal diam-eter of 4 mm packed with catalyst samples with fraction size of0.25–0.5 mm. The gases of ultrahigh purity (UHP) grade were used.The sample (0.08 g) was heated with ramp rate of 20 ◦C/min up to350 ◦C. The relative content of desorbed products was estimatedtaking into account all possible mass-fragments according to theHiden mass-library, while hydrogen uptake was calculated usingreference gas mixture.
Carbon monoxide oxidation was performed in the same quartzflow reactor described above. Prior to the catalytic run, a sample(0.02 g) was pretreated in oxygen or hydrogen flow with a tem-perature increase up to 350 ◦C and ramp rate of 20 ◦C/min andwas finally purged with helium with a temperature decrease to25 ◦C. Experiments were carried out using gas mixture: 1 vol.%CO and 0.5 vol.% O2 in helium and the total flow rate 80 mL/min.Catalytic runs were performed with temperature increase within25–200 ◦C interval and ramp rate of 1 ◦C/min for catalysts and ramprate of 5 ◦C/min within 25–500 ◦C interval for supports. The reac-tants and products were analyzed by a gas chromatograph SRI8610 C equipped with thermal conductivity detector (TCD) and twocolumns packed with molecular sieves and silica gel for separationof O2, CO and CO2, respectively. The temperature of the catalyst wasmeasured by thin thermocouple placed inside of the reactor beingin contact with the catalyst bed. The catalyst sample was mixedwith 0.1 g of quartz granules (fraction 0.25–0.5 mm) in order toprevent sample overheating due to the exothermal effect of CO oxi-dation. Catalytic activity of the samples was characterized by valuesof CO conversion vs temperature and TOF values calculated at COconversion below 15%. Under TOF calculations only gold atomslocated on the surface of gold particles were taken into account.
3. Results and discussion
3.1. Supports characterization
3.1.1. SEM and TEM analysisThe ceria samples with quite different structure were obtained
by hydrothermal treatment of initial ceria sample. The SEM micro-graphs of obtained ceria samples are presented in Fig. 1. CeO2-NPsample was characterized with condensed structure while CeO2-NT1 sample was formed by differently packed fiber like species.Chaotically oriented long and narrow rods forming highly friablestructure were observed for CeO2-NT2 sample.
Fig. 2 displays the TEM images of Au/CeO2-NP, Au/CeO2-NT1and Au/CeO2-NT2 samples. The structure of CeO2-NP sample waspresented by chaotically and tightly packed ceria nanoparticleswhile samples after hydrothermal treatment (CeO2-NT1 and CeO2-NT2) consisted of the separate nanotubes different in width andlength according to TEM contrast as in [30]. CeO2-NT2 sample wasformed by long ceria nanotubes with diameter ∼30 nm while CeO2-
NT1 sample was characterized with the mixture of short and longceria nanotubes with diameter ∼10 nm. The light points in the TEMimages of the ceria nanotubes (see also insertion in Fig. 2 (mid-dle)) could be related with the presence of some structural defects
98 B. Acosta et al. / Applied Catalysis A: General 449 (2012) 96– 104
Fig. 1. The SEM micrographs of CeO2-NP (left), CeO2-NT1 (middle) and CeO2-NT2 (right) samples.
dle), A
onnliwb
3
r
FIr
Fig. 2. TEM images for Au/CeO2-NP (left), Au/CeO2-NT1 (mid
n the surface of these tubes. The package of wide and long ceriaanotubes seem to be resulted in the formation of ceria straighteedles while mixture of narrow short and long ceria nanotubes
ed to the formation of ceria curved nanofibres observed in the SEMmages. Well-detected dark points mostly of semispherical shape
ere assigned to gold metal NPs that properties will be discussedelow.
.1.2. X-ray diffractionThe analysis of XRD patterns of obtained samples (Fig. 3)
evealed typical peaks of cubic ceria structure (PDFWIN, 898436).
ig. 3. XRD patterns of Au/CeO2-NP, CeO2-NT2, CeO2-NT1 and CeO2-NP samples.nsertion presents the part of XRD pattern (multiplied by 50) of Au/CeO2-NP sampleeduced in hydrogen.
u/CeO2-NT2 (right) samples reduced in hydrogen at 350 ◦C.
It was concluded that hydrothermal treatment practically did notaffect ceria crystallographic structure. However, reconstruction ofceria nanoparticles into ceria nanotubes under hydrothermal treat-ment led to the decrease of ceria particle size because increaseof peak width in XRD patterns for CeO2-NT1 and CeO2-NT2 sam-ples in comparison with that for CeO2-NP sample. The lattercorrelated well with the changes of the specific surface areaof samples. The surface area of ceria nanotubes was about 17times higher in comparison with that for ceria nanoparticles (seeTable 1).
3.1.3. UV–visible spectroscopyIn UV–visible spectra of ceria samples recorded ex situ there
was an absorption edge in the near-UV region characteristic forceria (Fig. 4). The band-gap energy of these materials may beestimated from the adsorption edge wavelength of the interbandtransition. The most accepted method for determining the band-gap energy values of semiconductor is by plotting the square rootof the Kubelka–Munk function multiplied by the photon energyversus the photon energy and extrapolating the linear part of therising curve to zero [31,32]. The estimated values of band-gapenergy are presented in Table 1. The CeO2-NP sample was char-acterized with value of band-gap energy equal to 3.17 eV, whichis in a good agreement with value reported in [22,33] for ceriananoparticles with size 3–10 nm. The values obtained for ceriananotubes (3.07 and 3.04 eV) were similar for both samples andlower than that for CeO2-NP sample. Therefore, transformation ofceria nanoparticles into ceria nanotubes was accompanied withthe detectable changes in the electronic properties of the material.Appearance of structural defects in ceria (light points in the TEM
images) provoked appearance of electronic defects changing theband-gap energy. Therefore, one may expect structure irregularityboth on the external surface of ceria nanotubes and in thebulk.
B. Acosta et al. / Applied Catalysis A: General 449 (2012) 96– 104 99
Table 1Conditions of hydrothermal treatment, BET surface area and values of band-gap energy for different ceria samples.
Sample Conditions of CeO2 hydrothermal treatment Structural and electronic properties
a The values were obtained from the width of diffraction peak at 28.6◦ using Scherrer’s
200 30 0 40 0 50 0 60 0 70 0 80 00.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Ab
so
rba
nce
, a
.u.
Wavelength , nm
CeO2-NP
CeO2-NT1
CeO2-NT2
2.9 3.0 3.1 3.2 3.30
50
100
150
200
250
300
350
400
[F(R
)*E
]
Photon energy, eVPhoton energy, eV
FK
3
3
wwoanrs
AAootShameo
TAa
ig. 4. UV–visible spectra of ceria samples. Inset presents plots of square root of theubelka–Munk function multiplied by the photon energy vs the photon energy.
.2. Gold catalysts characterization
.2.1. TEM analysisAnalysis of TEM images revealed that Au NPs were characterized
ith almost semispherical shape (Fig. 2). It is easy to see that Au NPsere distributed uniformly on the ceria surface. Estimated values
f average diameter of Au NPs for reduced and oxidized samplesre shown in Table 2. Usually the size of Au NPs supported on ceriaanoparticles was bigger than that on ceria nanotubes. In addition,eduction or oxidation of samples affected significantly the Au NPsize (see Table 2).
There are different factors controlling the final size of Au NPs.mong them are the value of surface area of support and process ofu NPs redistribution via their interaction with oxygen vacanciesf the support. The last process proceeds under partial reductionf support with some reducing agent as it was shown experimen-ally for gold–ceria samples exposed to the electron beam in [34].imilar effect could be expected for other reducing agent such asydrogen. Therefore, in these terms the data obtained for samples
ctivated in oxygen, when process of ceria reduction did not occur,ay be explained only by the comparison of surface area for differ-
nt supports. The size of Au NPs supported on ceria nanotubes afterxidation treatment was practically the same (around 3.7 nm) for
able 2verage diameter of gold nanoparticles according to TEM data for oxidized (TPO)nd reduced (TPR) gold–ceria samples.
Sample Average diameter of gold nanoparticles (nm)
both samples, while Au NPs with diameter 5.6 nm were observedfor oxidized Au/CeO2-NP sample. It was possible to propose thatAu(OH)3 particles formed by gold precursor hydrolysis with urea[35] and then deposited on the ceria nanotubes with higher sur-face area than that for ceria nanoparticles were characterized withrelatively high distance between each other and, therefore, lowprobability to be agglomerated.
The size of Au NPs obtained in this work was smaller thanthat for Au NPs supported on mesoporous ceria (∼6 nm) and ceriananorods (11 nm) [36], in contrary with similar values of surfacearea for ceria samples mentioned above and samples prepared inthe current work. This effect could be explained by the difference intechniques applied for the Au NPs preparation. Na2CO3 as precipi-tation agent was used in [36] instead of urea in the present work.According to [27] urea is quite effective for gold precursor hydrol-ysis with formation of uniform and small Au(OH)3 particles [37],which, in addition, were stabilized strongly with following washingof prepared samples with concentrated ammonia water solutionaccording to procedure developed in [28]. Strong stabilization ofdeposited Au(OH)3 particles led to the formation of small Au NPsdue to low probability of gold species migration and agglomerationduring thermal decomposition of Au NPs precursor.
In case, when partial reduction of ceria by hydrogen occursunder thermal treatment of gold–ceria samples, it is possible toexpect redistribution of Au NPs due to their interaction with oxygenvacancies on the ceria surface. Indeed, the heating of as prepareddried samples in hydrogen resulted in the formation of Au NPs withsmaller size than that for oxidized samples (see Table 2). In addi-tion, this effect depends on the nature of ceria used. Among thereduced samples, the Au NPs formed on CeO2-NT1 and CeO2-NT2were characterized with smaller size (2.7 and 1.8 nm, respectively)than that for Au/CeO2-NP sample (4.5 nm). According to mecha-nism proposed in [34], it was obvious to expect that efficiencyof small Au NPs stabilization depends on easiness of ceria vacan-cies formation, by other words on the reducibility of ceria. Highreducibility of ceria nanotubes compared to that for ceria nanopar-ticles (data will be presented below) resulted in the formation ofsmall Au NPs on ceria nanotubes. The smaller size of Au NPs formedon big ceria nanotubes (CeO2-NT2) compared with that for smallceria nanotubes (CeO2-NT1) indicated the highest amount of activeoxygen species on the surface of big ceria nanotubes.
3.2.2. X-ray photoelectron spectroscopyThe XPS spectra of Ce 3d for Au/CeO2 samples heated in hydro-
gen or oxygen at 350 ◦C are shown in Fig. 5. All spectra of Ce 3drevealed two principal signals at binding energy about 882.2 and900.6 eV for components of Ce 3d5/2 and Ce 3d3/2 separated by18.4 eV. All presented Ce 3d spectra were similar to the standardCeO2 spectrum, showing well resolved Ce4+ lines [38]. Anotherintensive peak in the spectra at around 916.4 eV and the band at907.2 eV was attributed to the satellite arising from Ce 3d3/2 ion-
ization, while the bands at 888.7 and 898.0 eV were due to Ce 3d5/2ionization. All XPS spectra were similar to each other independentlyon the ceria structure. The detected Ce 3d level did not reveal anyappreciable shift in the binding energy from one sample to another.
100 B. Acosta et al. / Applied Catalysis A: General 449 (2012) 96– 104
920 910 900 890 880
In
ten
sity
3d3/2 3d
5/2
NT2
NT1
NP
920 910 900 890 880
3d3/2 3d
5/2
NT2
NT1
NP
oxyge
T(
gwablf4fehsss[ssp
3r
pidhg(f
TX
I
Binding energy, eV
Fig. 5. Ce 3d XPS spectra for Au/CeO2 samples: calcined in
he values for Ce3d peak position and full width at half-maximumFWHM) are reported in Table 3.
The XPS spectra of Au 4f for Au/CeO2 samples heated in hydro-en or oxygen at 350 ◦C are presented in Fig. 6. The Au 4f spectraere characterized with the two spin–orbit components Au 4f7/2
nd Au 4f5/2 separated by 3.67 eV. The spectra fitting inicated theands at binding energy 83.7 and 87.4 eV that are typical for metal-
ic gold species [38,39]. Only metallic gold species were found bothor reduced and oxidized samples. Intensity of XPS spectra of Auf for Au NPs supported on ceria nanotubes was lower than thator those on ceria nanoparticles that could be explained by differ-nce in surface area for these samples. As usual, gold catalysts withigh surface area are characterized with low intensity of XPS Au 4fpectra due to predominant location of gold species in the pores ofupport out of XPS sampling zone [37]. As a rule, well crystallizedpecies are characterized with low FWHM values in XPS spectrum38]. Therefore, it could be expected that decrease of Au NPs sizehould be reflected by the increase of FWHM value. Indeed, theize of Au NPs were in a good aggrement with the FWHM valuesresented in Table 3.
.2.3. In situ UV–vis–mass analysis of temperature programmededuction of gold catalysts
Fig. 7 displays the TPR profiles and in situ UV–vis spectra for asrepared dried Au/CeO2-NT1 sample heated in hydrogen flow. The
n-line analysis of the gas phase products with mass-spectrometeruring the TPR of Au/CeO2-NT1 sample allowed detecting not only
ydrogen uptake, but desorption of water, CO2, ammonia and nitro-en (see Fig. 7, left). Some portion of desorbed water and CO250–175 ◦C) could be caused by the removal of those compoundsrom the sample being pre-exposed to air, because similar peaks
able 3PS data of Au 4f7/2 and Ce 3d5/2 values of binding energy (eV) referred to as C 1s (284.8 e
n parentheses shown the peak position for gold and ceria species and the full width at ha
Binding energy, eV
n at 350 ◦C (left) and reduced in hydrogen at 350 ◦C (right).
were found for support as well. On the other hand, simultaneousdesorption of water, CO2 and particular nitrogen detected withinthe temperature interval of 175–350 ◦C (not observed in case ofceria only) may correspond to the thermal decomposition of ureaor its reaction with gold hydroxide. The presence of urea or prod-ucts of its partial decomposition on the sample surface may beexplained by urea intercalation into gold deposit formed duringsample preparation as it was described in [35,40] and observedalso for nickel hydroxide particles [41].
The hydrogen consumption accompanied with desorptionof ammonia was observed within the temperature interval of180–280 ◦C. Presence of ammonia on the sample surface seemedto be caused by washing of freshly prepared sample with concen-trated ammonia solution (see Section 2). Some portion of ammoniacould also be formed due to partial urea decomposition as well.It should be emphasized that ammonia desorption was observedexactly at the same temperature interval as hydrogen consumption.Simultaneous desorption of nitrogen and ammonia evidenced thatammonia oxidation may occur even in a hydrogen atmosphere dueto the reaction of ammonia with such oxidant as gold hydroxide.
The first visible changes in the in situ UV–vis spectra forAu/CeO2-NT1 sample were observed at 65 ◦C with appearance ofan adsorption band at around 540 nm (marked with arrow in Fig. 7,right). This band is characteristic for the optical absorption oflight excited oscillating conductivity electrons of metallic Au NPs:the so called “plasmon resonance” [42–44]. The rise of Au NPsformation indicated by rapid changes of plasmon peak intensity
in the UV–visible spectra within 165–220 ◦C temperature inter-val (marked with arrows in Fig. 7, right) coincided with the startof hydrogen uptake and intensive ammonia desorption accord-ing to TPR profiles (Fig. 7, left). Further temperature increase in
B. Acosta et al. / Applied Catalysis A: General 449 (2012) 96– 104 101
90 88 86 84 82
In
ten
sity
Au0
NT2
NT1
NP
90 88 86 84 82
Au0
NT2
NT1
NP
oxyge
2m
muFtm∼
iuttduppawt
Binding energy, eV
Fig. 6. Au 4f XPS spectra for Au/CeO2 samples: calcined in
20–350 ◦C interval resulted in the negligible changes in the plas-on peak intensity (Fig. 7, right).The changes of the maximum position for plasmon peak, nor-
alized plasmon intensity in UV–visible spectra and hydrogenptake vs temperature for all studied samples are presented inig. 8. The profile of the plasmon intensity changes implied thathere are three distinguishable steps in the process of Au NPs for-
ation: the first slow step at 50 to ∼200 ◦C, the second fast step at200 to ∼230 ◦C and the third slow step at ∼230–350 ◦C.
The first step proceeded with intense water and CO2 evolv-ng without any hydrogen consumption. The absence of hydrogenptake at this temperature (see TPR profiles in Fig. 7, left) permit-ed to propose the appearance of metallic Au NPs (about 20% ofotal Au NPs according to plasmon intensity in Fig. 8) at this stepue to thermal decomposition of Au(OH)3 or its reduction withrea and/or ammonia. Indeed, desorption of nitrogen as probableroduct of ammonia or urea oxidation was observed at this tem-
erature interval. The formation of Au NPs was accompanied with
slight shift of the plasmon resonance (Fig. 8) that could be relatedith the changes in the particle size. The red shift of peak posi-
ion may be explained by the suggestion that growing Au NPs were
Fig. 7. TPR profiles (left) and in situ UV–visible spectra (right) for Au/CeO2-NT1
Binding energy, eV
n at 350 ◦C (left) and reduced in hydrogen at 350 ◦C (right).
covered with a layer of compounds characterized with a high valueof dielectric function. This model is based on the theoretical estima-tions described in [44]. Thus, it can be proposed that Au NPs wereformed in this step mainly inside the particles of gold hydroxidesupported on ceria surface during the samples preparation.
Fast formation of Au NPs during second step (200–230 ◦C) wasaccompanied with significant red shift of the plasmon peak posi-tion, particular for ceria nanotubes (see Fig. 8). The direction ofplasmon shift permitted to propose that forming Au NPs duringthis step were still covered by Au(OH)3. On the other hand, the for-mation of Au NPs at this step was accompanied by the hydrogenuptake (Fig. 8) and the desorption of ammonia (see Fig. 7, left). Itwas proposed that at the end of this step the Au(OH)3 film, whichcovered the Au NPs, was finally reduced to Au0 causing the desorp-tion of ammonia because of its lower affinity toward the metallicgold compared to gold hydroxide. Note, that for all samples thehydrogen uptake exceeded the value required for complete reduc-
tion of Au(OH)3 in these catalysts (Fig. 9). Therefore, it could beconcluded that simultaneously with Au(OH)3 reduction the reduc-tion of ceria occurred being more profound for ceria nanotubes. Thiscould be confirmed by comparison of dynamics of Au NPs formation
recorded in time of as prepared dried sample reduction with hydrogen.
102 B. Acosta et al. / Applied Catalysis A: General 449 (2012) 96– 104
ig. 8. Position of plasmon maximum (black curve), normalized plasmon intensityV–visible–mass analysis of Au NPs formation under temperature-programmed re
nd curves of hydrogen uptake presented in Fig. 8. It was clear thatfter point (marked with vertical dot line for all samples), whenost of Au NPs was formed the hydrogen consumption still pro-
eeded. Note that the deepness of ceria reduction was correlatedell with values of band gap energy estimated for ceria samples
see Section 3.1.3). There is a relation between values of band gapnergy and content of structural defects in ceria samples. The loweralue of band gap energy of ceria corresponded to the higher con-ent of structural defects and, as a rule, to the higher mobility of bulkxygen. Similar effect of simultaneous reduction of supports andold precursor is also observed for Au/CeO2 and Au/Fe2O3 catalystsn [45,46] and for Au/Ce–Al catalysts in [35].
Reduction of ceria, in turn, can affect also the position of plas-on peak by changing media around freshly formed Au NPs. The
ppearance of oxygen vacancies in CeO2 caused the changes of theielectric function of ceria according to the theoretical estimationsescribed in [47]. Indeed, the shift of plasmon peak was correlatedell with deepness of ceria reduction (see Figs. 8 and 9). The mostrofound peak shift (about 100 nm) during TPR was observed foru/CeO2-NT1 catalyst based on small ceria nanotubes character-
zed with the highest value of hydrogen uptake (see Fig. 9).
During the third step (230–350 ◦C), the formation of Au NPs
as finished. A further temperature increase up to 350 ◦C affectedlightly the intensity and position of plasmon indicating minor
0.0
2.0x10-4
4.0x10-4
6.0x10-4
8.0x10-4
Au/CeO2-NT2Au/CeO
2-NT1
No
rma
lize
d H
2 u
pta
ke
, m
ol H
2/g
Au/CeO2-NP
Samples
ig. 9. Normalized hydrogen uptake for Au/CeO2 samples. Dot line indicates thealue of H2 uptake (2.28 × 10−4 mol H2/g) required for complete reduction of goldrecursor.
black curve) and hydrogen uptake (grey curve) vs temperature according to in situn for Au/CeO2-NP (left), Au/CeO2-NT1 (middle) and Au/CeO2-NT2 (right) samples.
changes of Au NPs size. The low rate of this step was due to thedecomposition of the residual part of the gold precursor.
3.3. CO oxidation
Fig. 10 presents the curves of CO conversion vs temperature forgold catalysts and their supports. Ceria itself manifested catalyticactivity in CO oxidation starting from 200 ◦C for ceria nanotubes and300 ◦C for ceria nanoparticles. The presence of structural defectson the surface of ceria nanotubes was indicated by high catalyticactivity of ceria nanotubes in CO oxidation than that for ceriananoparticles. Similar effect is observed in [48]. One can see thatceria nanotubes with different size were characterized with prac-tically the same catalytic activity (see Fig. 10, left).
Of course, deposition of Au NPs on the ceria nanotubes and ceriananoparticles affected drastically catalytic activity in CO oxidation(see Fig. 10 and Table 4). The catalytic activity of gold–ceria catalystsdepended on both the nature of support and catalyst pretreatment.Au NPs supported on ceria nanotubes were more active than thosesupported on ceria nanoparticles. Note that pre-oxidized catalystswere more active and characterized with less difference in activitythan pre-reduced ones (see Fig. 10 and Table 4).
To the best of our knowledge, among all gold–ceria cata-lysts described in the literature the best activity in CO oxidationwas observed for Au NPs supported on ceria prepared in supercritical conditions [13]. However, this system was not stable
in the course of the reaction due to agglomeration of ceriaparticles. The obtained values of catalytic activity in the cur-rent work (0.23–0.58 mol CO mol Au−1 s−1 and 0.078–0.15 molCO mol Au−1 s−1 for oxidized and reduced samples, respectively)
Table 4Catalytic activity of pre-oxidized and pre-reduced Au/ceria samples in CO oxidationat room temperature presented in the current work and in the literature.
Au/CeO2-NP 0.23 0.078 This workAu/CeO2-NT1 0.52 0.12 This workAu/CeO2-NT2 0.58 0.15 This workAu/ceria nanorods 0.22 [14]Au/CeO2-HSb 0.056 [48]Au/CeO2 0.45 0.17 [35]Au/CeO2 0.045 [49]Au/CeO2-HSb 0.65 [13]Au/CeO2-SCc 1.08 [13]
a TOF values were estimated using steady-state CO conversion at 24 ◦C (notexceeding 15%) and content of Au atoms on the surface of gold particles.
b Catalyst based on ceria with high surface area.c Catalyst based on ceria prepared in super critical conditions.
B. Acosta et al. / Applied Catalysis A: General 449 (2012) 96– 104 103
25 50 75 100 125 150 175 200
mpera
Au/CeO2-SG
Au/CeO2-HT1
Au/CeO2-HT2
25 50 75 100 125 150 175 200
Au/CeO2-SG
Au/CeO2-HT1
Au/CeO2-HT2
100 200 300 400 5000.0
0.2
0.4
0.6
0.8
1.0
CO
co
nve
rsio
n, p
art
s
CeO2-NP
CeO2-NT1
CeO2-NT2
t) and
wArC
oefapttsbtbcicttc
Aibomtoocvmoacsmamtsafptdt
TeTemperature, oC
Fig. 10. CO conversion vs temperature for ceria samples (lef
ere comparable or higher than those reported in [49,50] foru/CeO2 (0.056 mol CO mol Au−1 s−1), (0.045 mol CO mol Au−1 s−1),espectively, and gold catalysts supported on porous flowerlikeeO2 microspheres (0.032 mol CO mol Au−1 s−1) [51].
It is well known that the reaction of CO oxidation proceedsn the Au–CeO2 interface [8,9,12,52]. Therefore, it was obvious toxpect that high concentration of active oxygen species on the sur-ace of ceria nanotubes found in the TPR experiments describedbove will cause the high efficiency in CO oxidation particular forre-oxidized samples (Fig. 10, middle). In case of samples pre-reated in oxygen TOF value for Au/CeO2-NT2 was slightly higherhan that for Au/CeO2-NT1, which could be related with higherurface concentration of active oxygen species on the surface ofig ceria nanotubes compared to small ceria nanotubes (see Sec-ion 3.2.1). Further temperature increase caused slight differenceetween them, keeping more active Au NPs supported on smalleria nanotubes. This tendency could be explained by higher mobil-ty of oxygen species for small ceria nanotubes than that for bigeria nanotubes according to TPR data (see Section 3.2.3). Adequateransport of oxygen from the bulk to the surface of small ceria nano-ubes provided the highest activity of pre-oxidized Au/CeO2-NT1atalyst in CO oxidation.
Another order of activity was found for pre-reduced samples:u/CeO2-NT2 > Au/CeO2-NT1. Note that pre-treatment of samples
n hydrogen resulted in the formation of smaller Au NPs on theig ceria nanotubes than that on small ceria nanotubes. Analysisf TOF values vs diameter of Au NPs taking into account differentodels of active sites located on the perimeter of particle [53], on
he steps [54] and on the corner sites [55] confirmed the key rolef gold–ceria interface in the catalytic activity of Au NPs supportedn the ceria nanotubes. The contribution of these factors in thease of Au/ceria catalysts could be analyzed by comparison of TOFalues for Au/CeO2-NP sample after reductive and oxidative treat-ents (see Table 4). Reduction treatment resulted in the formation
f Au NPs with slightly low size than those on oxidized ceria (4.5nd 5.6 nm, respectively) (Table 2). Usually, TOF values rise drasti-ally within this range of Au NPs size particular for non-reducibleupports [56]. However, TOF values found for these two samplesanifested opposite tendency. Similar behavior was observed for
ll studied samples. It was proposed, that Au–Ce interface playsore important role in such specific case, than Au NPs size. Reduc-
ive treatment of sample decreases the content of ceria oxygenpecies diminishing ability of ceria to activate gas-phase oxygennd to the end the level of catalytic activity. Difference in activityor pre-reduced samples became more profound at elevated tem-
eratures (see Table 4 and Fig. 10, right). It was obvious to expecthat preliminary reduction of samples before catalytic test shouldecrease amount of active oxygen species on the ceria surface par-icular for small ceria nanotubes according to TPR data described
ture, oC Temperature,
oC
Au/CeO2 samples after TPO (middle), and after TPR (right).
above. Therefore, the deepest reduction of small ceria nanotubesled to noticeable decrease of activity of Au/CeO2-NT1 catalyst.
4. Conclusions
The hydrothermal treatment of ceria nanoparticles at differ-ent NaOH concentrations results in the formation of two sets ofceria samples: (i) narrow and short nanotubes; and (ii) wide andlong nanotubes. Prepared ceria nanotubes are characterized withdifferent electronic properties and reducibility.
There are three distinguishable steps in the Au NPs formationon ceria: (i) slow thermal decomposition of gold hydroxide; (ii)fast formation of Au NPs due to thermal decomposition of Au(OH)3and its reduction with hydrogen, and, (iii) slow transformationof residual part of gold precursor. These steps are accompa-nied by profound ceria reduction particular for ceria nanotubesthat was shown in detail experimentally by in situ UV–vis–massanalysis.
Formation of Au NPs on the surface of ceria depends strongly onthe gas atmosphere (hydrogen or oxygen) and support surface. Thesize of Au NPs formed under the sample heating in oxygen is mainlydetermined by thermal stability of deposited gold hydroxide andits agglomeration on the support. Thus, Au NPs with similar sizewere formed in oxygen on different ceria nanotubes with compa-rable surface area. The gold interaction with freshly formed oxygenvacancies on the ceria surface at sample treatment in hydrogenresults in the formation of different Au NPs on ceria nanotubes. Thesmaller size of Au NPs formed on big ceria nanotubes (CeO2-NT2)compared with that for small ceria nanotubes (CeO2-NT1) indicatedthe highest amount of active oxygen species on the surface of thesample composed of big ceria nanotubes.
Au NPs stabilized on the ceria nanotubes are characterizedwith higher activity in CO oxidation than those supported on ceriananoparticles. The structure and redox treatment of ceria nano-tubes affects size of Au NPs and their catalytic activity in COoxidation. Pre-oxidized Au/CeO2-nanotubes manifest the highestactivity in CO oxidation.
Acknowledgement
Brenda Acosta thanks the scholarship provided by CONACyTand ECOES program. This research project was supported partly byDGAPA–PAPIIT (UNAM, Mexico) through Grants Nos. IN 224510,IN 110208 and CONACyT research grant 50547-Q. The authors
thank E. Flores, I. Gradilla, V. Garcia, P. Casillas, J. Palomares, F.Ruiz, G. Vilchis, E. Aparicio, J. Peralta and M. Sainz for technicalsupport. M.I. Perez Montfort corrected the English version of themanuscript.
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