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Metal-support and preparation influence on the structural and electronic properties of gold catalysts
Applied Catalysis A: General 302 (2006) 309–316 www.elsevier.com/locate/apcata
Metal-support and preparation influence on the structural and electronic properties of gold catalysts Maria Pia Casaletto a,*, Alessandro Longo a, Anna Maria Venezia a, Antonino Martorana b, Antonio Prestianni b a
Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, Via Ugo La Malfa 153, Palermo 90146, Italy b Dipartimento di Chimica Inorganica e Analitica ‘‘S. Cannizzaro’’, Universita` di Palermo, Viale delle Scienze, Parco D’Orleans, Palermo 90128, Italy Received 15 November 2005; received in revised form 25 January 2006; accepted 2 February 2006 Available online 7 March 2006
Abstract Nanostructured gold catalysts supported on CeO2 and SiO2 were prepared by the deposition–precipitation (DP) and the solvated metal atom dispersion (SMAD) techniques. The structural and electronic properties of the catalysts were investigated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Gold was found as small metal nanoparticles (cluster size 2 nm) in the SMAD-prepared samples and in ionic state in the DP catalysts. The catalytic activity of the samples was tested in the reaction of low temperature CO oxidation. Gold nanosized particles in a pure metallic state exhibited a worse catalytic performance, both on ceria and silica. The presence of non-metallic Au species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. The higher activity of the Au/CeO2 (DP) sample with respect to the Au/SiO2 (DP) catalyst can be ascribed to a better stabilization of the Au+1 ions, probably as AuO- species, by the cerium oxide. # 2006 Elsevier B.V. All rights reserved. Keywords: CO oxidation; XPS; XANES; Oxidation state of gold; Particle size effect; Au/CeO2; Au/SiO2
1. Introduction A complete understanding of the structure-activity relationship is one of the most important goals of surface science studies related to heterogeneous catalysis. Gold nanostructured catalysts have attracted rapidly growing attention, due to their potential applicability to various reactions of industrial and environmental interest [1,2]. Supported gold nanoparticles have been widely investigated in the last few years and most of the studies have focused on the unusual low temperature CO oxidation activity [2,3]. Nevertheless, despite the extensive recent efforts addressing such extraordinary catalytic behaviour of gold nanoparticles, the origin of the structure sensitivity of the low-temperature oxidation of CO on supported Au clusters has been not yet completely unravelled. Different possible factors have been evoked, including the size of the Au clusters, the oxidation state of gold, the nature of the support material
* Corresponding author. Tel.: +39 091 6809378; fax: +39 091 6809399. E-mail address: [email protected] (M.P. Casaletto). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.02.005
(state and structure), the Au-support interface, the preparation method. The catalytic performances of gold nanoparticles have been correlated, in turn, with electronic (quantum size effect, oxidation state), structural and support (defects, perimeter interface) effects, but a general consensus has not been reached [4,5]. The sensational catalytic activity exhibited by gold upon sizing down its dimension [3] has captured the general attention on nanostructured catalysts, giving rise to a massive amount of articles in the literature. In the last few years the role of Au particle size has become less predominant in determining an high catalytic activity, since many other effects have been proven essential. Furthermore, recently Chen and Goodman [6] prepared well-ordered gold monolayers and bilayers that completely cover the titanium oxide, in order to eliminate particle shape and support effects. In this paper they reported that gold can be no longer in the form of nanoparticles, but as a supported bilayer film on TiO2 and still a catalytic activity for CO oxidation about 45 times higher than that of gold clusters supported on high surface area titanium oxide is found [6].
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In addition to the particle size of metallic Au, the role of the oxidation state of gold has received great attention, since oxidised gold species were proposed as the active sites for CO oxidation in differently supported Au catalysts [7–10]. Oxidised gold species were found responsible for the high catalytic activity in the low-temperature CO oxidation in a variety of supported gold systems: gold supported on TiO2, TiO2-ZrO2, Fe2O3 and Al2O3 by the DP method [9,10], on Fe2O3 by co-precipitation [8] and on Y-type zeolite by ion exchange [7,10]. High activity in the water-gas shift reaction is obtained by non-metallic Au species supported on Fe2O3 [10] and on CeO2 [11]. Even if essential for some oxidation reactions, in view of their totally different oxygen adsorption properties compared with those of bulk Au [4,12], metallic gold nanoparticles did not participate in the water-gas shift reaction, since the catalytic activity resulted unaffected by the removal of metallic Au species by cyanide leaching [11]. In our previous study on ceria-supported gold catalysts ionic gold species were found responsible for the achievement of the highest CO conversion in the low temperature catalytic CO oxidation [13]. A linear relationship between CeO2 surface oxygen reducibility, determined by H2-TPR, and CO conversion confirmed the strong interaction between ionic gold and ceria support that can play a crucial role in the catalytic performances of the samples. Gold ions in contact with the ceria support can modify the ceria properties by weakening the Ce–O bond and increasing the formation of CO2 [13]. Recently, on the basis of joint density functional theory (DFT) calculations and experimental evidence by a fast-flow reactor mass spectrometer, AuOn (n = 1–3) species were found to promote the oxidation of CO [14]. In particular, the AuO species dramatically decreased upon reaction with CO, as evidenced by the intensity decrease of the AuO peak at 213 amu in the mass spectrum. Structural calculations and molecular dynamics simulations evidenced that a very fast and thermodynamically favoured oxidation reaction occurred. A proposed mechanism involves the oxidized Au species as effective active sites with the binding of CO and the subsequent CO2 release by the cleavage of either Au–O or Au–C bond [14]. A direct evidence of AuO , AuO2 and AuOH ion clusters on supported gold catalysts has recently been reported by a timeof-flight secondary ion mass spectroscopy (TOF-SIMS) study [15]. Even if oxidized gold was not detected by XPS, SIMS spectra of Au/g–Al2O3 and Au/TiO2 catalysts prepared by the DP process indicated the existence of oxidized Au clusters of AuO , AuO2 and AuOH , due to the higher detection sensitivity of this surface analytical technique. Oxidized gold species may be still present on the surface of the catalysts, even if not evidenced by XPS, as trace ions detected by TOF-SIMS [15]. A direct correlation between the Au+1 content and the catalytic activity for CO oxidation was found for Au/TiO2 and Au/TiO2–ZrO2 catalysts, containing both Au0, Au+1 and Au+3 species, as evidenced by Mo¨ssbauer spectroscopy [10]. On the basis of our previous findings of a better catalytic performance of ionic gold with respect to metallic species in Au/CeO2 catalysts [13] and considering the great emphasis still given to the role of metallic gold nanoparticles in literature, we
have planned a set of experiments in order to investigate the nature of gold species on different supports as a function of different preparation methods. To this aim Au/CeO2 samples were compared with Au/SiO2 catalysts, in which a negligible metal-support interaction is expected. In this article we will focus on the structural and electronic characterization of supported gold nanoparticles. The importance of catalyst preparation and the nature of support materials are addressed in view of the significance of gold particles size. Since metallic Au is reported to be catalytically important when the particles size is in the nanometer scale, the solvated metal atom dispersion (SMAD) technique was selected, besides the most common deposition–precipitation method, for obtaining the metal in the pure zero-valent state, for avoiding common residuals from the metal precursors and for the tailoring of the particle size distribution [16–18]. In order to look at intrinsic properties of gold small particles and possibly to relate the catalytic CO oxidation activity directly to Au particle size, a non-interacting oxide such as SiO2 was chosen as support. Since the interaction with the support is known to play a dramatic role, gold nanoparticles were supported on a different oxide like CeO2, by using the same preparation techniques and the same gold loading. The choice of cerium oxide as support is motivated by the interaction of gold with this ‘‘reducible’’ oxide, which has been proved to enhance the ceria surface oxygen reducibility and to be able to form a mixed oxide AuxCe1 xO2 d [11,13]. For a direct comparison, the same amount of gold nanoparticles (3.0 wt.% Au) was supported on ceria, both by the SMAD and the deposition–precipitation procedures. 2. Experimentals and methods 2.1. Preparation of catalyst Gold nanoparticles supported on CeO2 (Aldrich, specific ˚ ) and on SiO2 surface area = 79 m2/g; average pore size 24 A 2 (Aldrich, specific surface area = 545 m /g) powders were prepared by two different synthetic routes. The metal thermal evaporation from high purity Au foil (Aldrich, 99.99%) was performed according with the SMAD technique [16]. This procedure involves the following steps: (a) deposition of an organic solvent (acetone) on the walls of the metal vapor reactor cooled at 77 K (liquid nitrogen temperature); (b) vaporization of the metal under vacuum, followed by the rapid trapping of the atoms in the frozen solvent matrix on the walls; (c) warming up to room temperature (RT) of the condensate of solvated gold atoms and melting down to the bottom of the reactor with the formation of a colloid solution. The solvated metal atom solution was used to impregnate a weighted amount of powder support poured into the reactor. After impregnation, the samples were degassed and dried at room temperature under vacuum. Samples prepared by this technique will be referred to as Au/CeO2 (SMAD) and Au/SiO2 (SMAD), respectively. According to the method of deposition–precipitation (DP) reported by Haruta and co-workers [19], ceria and silica supports were dispersed in an aqueous solution of
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HAuCl43H2O (Aldrich, 99.9%) used as gold precursor. The pH value of the support dispersion was adjusted at pH 8 by addition of a 0.1 M NaOH solution. The aqueous dispersion was stirred at 353 K for 24 h. After filtration, the solid was washed with hot distilled water several times to remove residual Cl species, as tested by AgNO3. The powder was dried at 373 K in a dry oven and a portion was calcined at 573 K for 1 h. Samples obtained by this procedure will be denoted as Au/CeO2 (DP) 373 K and Au/SiO2 (DP) 373 K; Au/CeO2 (DP) 573 K and Au/SiO2 (DP) 573 K, respectively. The bulk amount of gold in the catalysts was quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a ICP Perkin-Elmer Optima 3000DV spectrometer. A portion of 0.02 g of catalyst was dissolved by Aqua Regia (HNO3:HCl, in 1:3 ratio) in a microwave oven. A 3.0 wt.% gold loading was obtained for all the Au/CeO2 and Au/SiO2 samples. 2.2. Characterization of catalyst The physical and chemical characterization of the catalysts was performed by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The catalysts characterization was performed on the freshly prepared SMAD samples and after treating the DP samples at 373 and 573 K, in order to check the stability of the metallic particles towards sintering and the distribution of gold species in different oxidation states. XRD measurements were carried out by a Philips PW 1820 (vertical goniometer) using a Ni-filtered Cu Ka radiation ˚ ). A proportional counter and a 0.05 step size in (l = 1.5418 A 2u were used. The XRD patterns reported in this study resulted from the average of 20 measurements collected for each sample. The assignment of the crystalline phases was based on the JPDS powder diffraction file cards [20]. Gold particle sizes were calculated from the line broadening of the most intense Au reflections by using the Scherrer equation [21]. X-ray absorption spectra were recorded at RT on LIII Au absorption edge (11.920 KeV) at the BL 11.1 beamline of ELETTRA Sincrotrone Trieste S.C.p.A., operating at an energy of 2.4 GeV with a 130 mA storage ring current. The beamline was equipped with a Si (1 1 1) double crystal monochromator with energy resolution 2 10 4. In addition to catalysts samples, XANES spectra were also acquired for Au foil (internal standard reference for the energy calibration) and AuCl3 samples, as structural references for the Au clusters in the support matrix. XAFS measurements of reference samples and Au/SiO2 catalysts were performed in transmission mode by using two ionization chambers filled with a N2/Ar mixture at different composition for the incident I0 and transmitted I1 beam, respectively. Due to the different support nature, Au/ CeO2 samples were investigated in fluorescence mode (seven elements Ge EUROSYS detector). Au/CeO2 and Au/SiO2 samples for analysis were prepared by catalyst suspension in pure ethanol (Aldrich, 99.99%) and subsequent deposition on a suitable membrane. An edge jump of 0.2 in total absorption was obtained for the Au/SiO2 catalyst and, therefore, four
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measurements for each sample were needed to improve the signal-to-noise ratio. For Au/CeO2 catalysts five measurements for each sample were collected. All the spectra were recorded in the 11.700–13.000 KeV energy range. Samples were calibrated using the internal Au foil (LIII edge energy of 11.920 KeV) and normalized to an edge jump of unity. The surface chemical composition of the samples was studied by XPS in an ultrahigh vacuum chamber (base pressure 10 8 Torr). Photoemission spectra were collected by a VG Microtech ESCA 3000 Multilab spectrometer, equipped with a standard Al Ka excitation source (hn = 1486.6 eV), a ninechanneltrons detection system and a hemispherical analyzer operating at a constant pass energy of 20 eV. Gold catalysts were sampled as powders and pressed on a double-side adhesive tape. The binding energy (BE) scale was calibrated by measuring C 1s peak (BE = 285.1 eV) from the surface contamination and the accuracy of the measure was 0.1 eV. A non-linear least-square peak fitting routine was used for the analysis of XPS spectra, separating elemental species in different oxidation states. Relative concentrations of chemical elements were calculated by a standard quantification routine, including Wagner’s energy dependence of attenuation length [22] and a standard set of VG Escalab sensitivity factors. XPS quantification results are affected by a standard error lower than 10%. A Au 4f7/2 binding energy of 84.1 eV was obtained from a pure Au foil and used as the standard reference for metallic Au. 2.3. CO Catalytic oxidation tests The catalytic activities of the samples were compared by measuring CO conversion as a function of temperature (lightoff test). CO oxidation tests were performed in a quartz reactor loaded with 0.05 g powdered catalyst. A gas mixture containing 1% CO and 1% O2, in a balance of He as gas carrier, was fed continuously into the reactor at a total flow rate of 50 ml/min (STP), which corresponds to a weight hourly space velocity (WHSV) of 60.000 ml g 1 h 1. CO conversion was determined by analysing the effluent gases with a UV–IR Analyser (ABB Uras 14). 3. Results and discussion 3.1. Au/SiO2 catalyst XRD diffractograms of Au/SiO2 catalysts in the 348–708 range are shown in Fig. 1. The XRD pattern of the ‘‘as prepared’’ Au/SiO2 (SMAD) sample gives evidence of the presence of very broad diffraction peaks due to the f.c.c. structure of gold. A rough estimate of gold particle size by the Scherrer approximation results in Au clusters with an average ˚ . XRD analysis performed on the DP-prepared diameter of 20 A Au/SiO2 catalysts shows no evidence of metallic gold particles in the Au/SiO2 (DP) 373 K sample, whose pattern is that typical of the amorphous SiO2 support. Calcination at T = 573 K leads to gold particles agglomeration up to an average particle size of ˚ , as derived from the very intense and narrow Au about 150 A
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Fig. 1. XRD patterns of the Au/SiO2 catalysts prepared by the SMAD and DP procedures.
f.c.c. peaks detected in the XRD pattern of the Au/SiO2 (DP) 573 K catalyst. XANES data of the Au/SiO2 samples at RT are reported in Figs. 2 and 3, together with those of Au foil and AuCl3 taken as reference materials. The spectrum of Au0 from gold foil shows a shoulder at an energy 15 eV above the X-ray absorption edge (vertical dashed line in the figures) and intense peaks at 25 and 50 eV, labeled (a), (b) and (c), respectively. The spectrum of Au+3 shown in Fig. 3 is characterized by a very strong feature at 4 eV above the edge, which is absent in metallic gold, due to the complete occupancy of the d states, together with a shoulder at 15 eV and a very broad shoulder at 50 eV. An important evidence from Fig. 2 is represented by the shift and the broadening of the XANES peaks for the Au/SiO2 (SMAD) sample with respect to the pure Au foil. The XANES peaks labeled (a) and (b) in Fig. 2 are characteristic of a pure gold f.c.c. structure ordered up to the third shell. Balerna et al. [23] observed that a shift to higher energies and the broadening of these peculiar peaks occur as the cluster size decreases. In particular, the shift has been attributed to the shortening of the Au–Au first neighbour distance and the broadening to the
Fig. 2. XANES spectra of gold supported catalysts, containing Au metallic species. The energy scale is expressed relatively to the theoretical energy of the Au LIII edge at 11.920 KeV (vertical dashed line).
Fig. 3. XANES spectra of DP-prepared gold supported catalysts, containing oxidized and metallic Au species. The energy scale is expressed relatively to the theoretical energy of the Au LIII edge at 11.920 KeV (vertical dashed line).
cluster size distribution [23]. From XANES data, we can conclude that the Au/SiO2 (SMAD) sample contains typical f.c.c. gold clusters dispersed in the silica matrix. Due to the ˚) shortening of the lattice constant (from 4.079 to 4.030 A derived from XRD data in Fig. 1, the gold clusters dimension is probably defined in the nanometric range, in good agreement with Balerna et al. [23]. XANES of the Au/SiO2 (DP) 373 K sample shown in Fig. 3 clearly indicate the simultaneous presence of Au+3 and Au0 species. The oxidized species are identified by the diagnostic intense peak at 4 eVand the metallic Au species by the presence of type f.c.c. features that match peaks (b) and (c) in Fig. 3, characteristic of the reference Au foil XANES spectrum. The surface chemical composition of the catalysts has been investigated by XPS and a summary of the quantitative results is reported in Table 1. Samples of pure Au foil, SiO2 and CeO2 supports were also analysed and taken as reference. The Au/Si atomic ratio values are in good agreement with that expected for the bulk composition of samples with a 3.0 wt.% Au content (Au/Si = 0.02). The Au 4f spectra of all the investigated Au/SiO2 samples are shown in Fig. 4 and a shift of the spectrum of the DP-prepared sample towards higher BE values can be noticed. This evidence is confirmed by the curvefitting of the Au 4f core-level spectra by two spin-orbit splitted Au 4f7/2 and Au 4f5/2 components (DE = 3.7 eV), showing increasing BE values that correspond to a different chemical state of gold particles in the samples. In the Au/SiO2 (SMAD) sample the Au 4f7/2 photoelectronic peak is located at BE = 84.2 eV and this value is typical of pure metallic Au species [24]. The Au 4f spectrum of Au/SiO2 (DP) 373 K catalyst shown in Fig. 4b consists of two components located at BE = 84.2 and 86.3 eV, which can be assigned to metallic and oxidized gold species, namely Au0 and Au+3 ions [9,13,24]. In the Au/SiO2 (DP) 573 K sample the Au 4f7/2 peak detected at BE = 84.5 eV shown in Fig. 4c can be attributed to the presence of Au0 species on the surface [9,13,24]. The relative surface distribution of gold species in the catalysts is shown in Table 2, as determined by the curve-fitting of the Au 4f photoelectronic
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Table 1 Relative surface chemical composition of the samples Sample
Au 4f
Si 2p
O 1s
Ce 3d
Au/Si
O/Si
Au/Ce
O/Ce
SiO2 support Au/SiO2 (SMAD) Au/SiO2 (DP) 373 K Au/SiO2 (DP) 573 K CeO2 support Au/CeO2 (SMAD) Au/CeO2 (DP) 373 K Au/CeO2 (DP) 573 K
– 0.6 0.7 0.4 – 0.8 2.6 1.5
29.8 30.7 30.6 30.6 – – – –
70.2 68.7 68.7 69.0 60.9 62.7 56.8 59.4
– – – – 39.1 36.5 40.6 39.1
– 0.020 0.023 0.013 – – – –
2.4 2.2 2.3 2.3 – – – –
– – – – – 0.022 0.064 0.038
– – – – 1.6 1.7 1.4 1.5
Elemental concentrations are expressed as atomic percentage (atom.%). The Au/Si and Au/Ce atomic ratio values expected for the bulk composition of samples with a 3.0 wt.% Au loading are 0.02 and 0.05, respectively.
peak. Only metallic gold particles were detected by XPS in the SMAD-prepared Au/SiO2 sample, thus confirming XANES results (see Fig. 2) and the XRD pattern of small Au clusters (see Fig. 1). On the surface of the Au/SiO2 (DP) 373 K samples a major fraction of oxidized Au species were detected by XPS, as reported in Table 2. The presence of gold in an oxidized state in the Au/SiO2 (DP) 373 K catalyst is confirmed by XANES spectra in Fig. 3. The XRD pattern in Fig. 1 reveals that the size of metallic gold is below the XRD detection limit, since no diffraction peaks attributable to f.c.c. gold particles are evidenced. The change in gold species from oxidic to metallic phase in the bulk of the Au/SiO2 (DP) sample upon thermal
Table 2 Relative surface distribution of gold species in the samples, as derived from the curve-fitting of the Au 4f photoelectron peak Sample
Au 4f7/2 BE (eV)
Oxidation state
Peak area (%)
Au foil Au/SiO2 (SMAD) Au/SiO2 (DP) 373 K
84.1 84.2 84.2 86.3 84.5 84.4 84.5 85.8 84.2
Au 0 Au 0 Au 0 Au+3 Au 0 Au 0 Au 0 Au+1 Au 0
100 100 40 60 100 100 90 10 100
Au/SiO2 (DP) 573 K Au/CeO2 (SMAD) Au/CeO2 (DP) 373 K Au/CeO2 (DP) 573 K
(1.9) (2.0) (2.3) (2.2) (2.0) (2.0) (2.0) (2.0) (1.7)
The FWHM values are reported in parentheses. Concentrations are expressed as percentage of the total area of the Au 4f peak (taken as 100%).
treatment is clearly indicated both by the XRD and XANES data in Figs. 1–3. Calcination of the Au/SiO2 (DP) sample at T = 573 K results in a thermal reduction and sintering of gold nanoclusters. The catalytic performances of silica-supported gold nanoparticles in CO oxidation are shown in Fig. 5, where the CO conversion is reported as a function of temperature (light-off test). All the investigated samples supported on an ‘‘inert’’ oxide like silica (i.e. Au/SiO2 catalysts) exhibit a poor catalytic activity, regardless the preparation procedure. Despite the small size of metallic gold particles (few nm) supported on silica, CO oxidation catalysis does not occur at low temperature as desired, since a 50% value of conversion is only attained well
Fig. 4. XPS curve-fitting of the Au 4f photoelectron peak in the Au/SiO2 (SMAD); Au/SiO2 (DP) 373 K and Au/SiO2 (DP) 573 K catalysts, (a), (b) and (c), respectively. The scatter points refer to the raw data, while the solid line to the curve-fitting results.
Fig. 5. CO conversion (%) as a function of temperature for all the investigated gold catalysts.
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beyond T = 500 K. The particle size does not seem to be a sufficient factor to obtain an efficient gold catalyst in the low temperature CO oxidation and this result argues against an electronic effect in small Au particles as a major factor contributing to the activity of gold catalysts. A little or no relationship between catalytic activity and particle size has already been claimed in the literature [25,26] and some other effects such as the metal-support interaction and the oxidation state of gold have in turn been considered as determining factors. The simultaneous presence of metallic and oxidized gold in the form of predominant Au+3 ions and minor Au0 species on the surface of the Au/SiO2 (DP) 373 K sample, as derived by XPS, does not improve the catalytic activity, as shown in Fig. 5. In order to further investigate the gold particle size effect, a ˚ Au/SiO2 sample with gold nanoclusters even smaller than 20 A was prepared by SMAD, after a convenient optimisation of the technique for the tailoring of the size of metallic nanoparticles [18]. In Fig. 6 the XRD pattern of this Au/SiO2 sample and that of the pure SiO2 support are reported. For a best detection of the f.c.c. Au clusters, the difference spectrum, shown at the bottom of the figure, was obtained by using a smoothed curve of the SiO2 diffractogram. The determination of the Au clusters dimensions was performed by using the Scherrer equation, including two distinct contributions for the Au (1 1 1) and Au (2 0 0) peaks, respectively, in a fitting procedure. Upon subtraction of a smooth background to the XRD Au/SiO2 ˚ was found. It is worth raw data, a clusters dimension of 15–20 A noting that this is the first experimental evidence of such small supported Au nanoparticles prepared by the SMAD technique. As already found for the Au/SiO2 (SMAD) shown in Fig. 1, a contraction of the Au lattice constant is detected, which is now ˚ ). Nevertheless, even more pronounced (from 4.079 to 4.000 A even if gold particles are tailored in one nanometer size, the catalytic performance of this Au/SiO2 sample in the low temperature CO oxidation is totally negligible (not shown in Fig. 5).
Fig. 6. XRD patterns of an Au/SiO2 sample prepared by the SMAD technique and pure SiO2 support. The difference spectrum at the bottom was obtained by using a smoothed curve of the SiO2 pattern.
Fig. 7. XRD patterns of the Au/CeO2 catalysts prepared by the SMAD and DP procedures. The inset refers to the presence of the Au (1 1 1) peak among the diffraction peaks of the ceria support, marked with a star.
3.2. Au/CeO2 catalyst In the XRD pattern of the Au/CeO2 (SMAD) sample shown in Fig. 7 only reflections of the fluorite structured CeO2 are evidenced and no diffraction peaks of gold are found. The presence of metallic gold in the sample can be inferred by the XANES spectrum in Fig. 2. On the surface of the Au/CeO2 (SMAD) catalyst gold is in the zerovalent state, as detected by the Au 4f7/2 photoelectron peak at BE = 84.4 eV, shown in Fig. 8a. As expected, the SMAD synthesis allows the formation of very small metallic gold clusters, not detectable by XRD, in good agreement with literature data [13,18]. Also in the Au/ CeO2 (DP) 373 K diffractogram reported in Fig. 7 no traces of gold f.c.c. particles are detected along the peaks of the pure ceria support. Still the XANES spectrum in Fig. 3 indicates that metallic gold species are present in the sample, evidently as very small particles undetectable by XRD, due to the higher scattering factor of ceria with respect to silica. The XPS spectrum in the Au 4f core-level region shown in Fig. 8b can be deconvoluted by two components at BE = 84.5 and 85.8 eV, which can be assigned to Au0 and Au+1species, respectively [9,13,24]. On the surface of the Au/CeO2 (DP) 373 K catalyst a small fraction of cationic gold species are found, together with the most abundant neutral gold particles, as indicated by the relative distribution of gold species in Table 2. The classical deposition–precipitation method is known to provide both metallic and cationic gold species [11,27]. Nevertheless, Au/ CeO2 samples prepared by the DP method may result in substantially different Au oxidation state, if small differences in the experimental procedure are introduced. In our previous work, we prepared a 3 wt.% Au/CeO2 (DP) 393 K catalyst, which exhibited only ionic gold species on the surface, and we found a high catalytic activity in CO oxidation below T = 273 K [13]. In the present case, the use of NaOH instead of Na2CO3, as precipitation agent, and a prolonged aging period of the solution at higher temperature (24 h at 353 K) resulted in reduced gold nanoparticles in the sample, predominantly as Au0 and partially as Au+1 species.
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Fig. 8. XPS curve-fitting of the Au 4f photoelectron peak in the Au/CeO2 (SMAD); Au/CeO2 (DP) 373 K and Au/CeO2 (DP) 573 K catalysts, (a), (b) and (c), respectively. The scatter points refer to the raw data, while the solid line to the curve-fitting results.
The agglomeration of these very small metallic gold particles occurs upon calcination at T = 573 K, as shown by the presence of the Au (1 1 1) reflection at 2u = 38.38 in the XRD pattern of Au/CeO2 (DP) 573 K catalyst (see the inset in Fig. 7). A rough estimate of gold particle size by the Scherrer approximation indicates gold clusters with an average diameter ˚ . The presence of metallic gold in the Au/CeO2 (DP) of 150 A 573 K catalyst is confirmed by the XANES data reported in Fig. 2 and by the Au 4f7/2 photoelectron peak detected by XPS at BE = 84.2 eV, as shown in Fig. 8c and in Table 2. Catalytic results of the Au/CeO2 samples in the reaction of CO oxidation are reported in Fig. 5. Better performances in the low temperature range are displayed by the ceria-supported gold catalysts with respect to the silica ones, thus confirming the importance of the nature of support for gold nanoparticles, well assessed in the literature [1]. The presence of very small pure metallic gold particles in the Au/CeO2 (SMAD) sample is not a key-factor in the low temperature CO catalysis, as already ˚ gold particles size. A found for the Au/SiO2 samples with 20 A significant improvement in the light-off temperature is obtained by using the Au/CeO2 (DP) 373 K catalyst, which reaches a 50% CO conversion at T = 250 K. This evidence may be ascribed to Au+1 species in the sample that were detected by the XPS along with the most abundant neutral gold particles (see
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Table 2). The higher catalytic performances of the Au/CeO2 samples with respect to the Au/SiO2 catalysts can be attributed to a better stabilization of the Au+1 species by cerium cations. The influence of the addition of cerium oxide on the surface electronic states of supported gold has been studied in the literature [28]. The addition of CeO2 was found to stabilize the ionic states of supported gold and to increase the effective charge of gold ions [28]. The stabilization of gold ionic states in Au–Ce samples was attributed to the direct influence of CeO2 on the one-charged Au+1 cations, which had proven to be the active sites in the partial oxidation of alcohols, as M+1 cations of the Cu subgroup metals [29]. The co-existence of gold metallic and cationic species with hydroxyl ligands has been suggested as a chemical model of the active site in CO oxidation catalysis. Nevertheless, controversial reports are found in the literature, claiming the catalytic effectiveness of Au+1 rather then Au+3 cationic species [30,31]. In order to corroborate the role of Au+1–OH species as active sites, the presence of Au+1 cations in an active catalyst was determined by direct spectroscopic evidence, while the participation of hydroxyl groups to the reaction was inferred by the effect of water vapour [31]. In a different article, gold cations supposed as Au+3 were proposed as a ‘‘chemical glue’’ for binding gold particles to the support and preventing their sintering, while the presence of gold atoms was considered necessary as a site for CO chemisorption [30]. With respect to the various and controversial literature reports concerning the nature of the active site in CO catalytic oxidation, our experimental evidences seem to corroborate the hypothesis of the promoting effect of Au+1 ions, probably in the form of AuO species, as derived by theoretical calculations [14]. The comparison between Au/SiO2 and Au/CeO2 catalysts highlights the important role of Au+1 species, which are better stabilized by suitable cations from a rare earth oxide support like CeO2 [28,29]. The presence of pure metallic gold nanoparticles (size dimension in the range: 1–6 nm) has proven once again not to be the major requisite for the low temperature catalytic oxidation of CO to CO2. Moreover, also the morphology of the support in terms of porosity should be taken into account to explain the better catalytic performances of the Au/CeO2 (DP) 573 K sample with respect to the Au/ CeO2 (SMAD) catalyst. The small sized Au nanoparticles obtained by the SMAD technique may be entrapped inside the porous structure of the support, thus decreasing the surface Au/ Ce ratio (see Table 1). It is worth noticing that in the Au/CeO2 (DP) catalysts the Au/Ce atomic ratio values reported in Table 1 are in close agreement with that expected for the bulk composition of samples with a 3.0 wt.% Au loading (Au/ Ce = 0.05). 4. Conclusions The structural and surface analyses evidenced the presence of small gold metal crystallites (cluster size 2 nm) in the case of the SMAD-prepared samples and oxidized gold species in the case of the DP catalysts. By comparing gold nanoparticles supported on ceria, the presence of Au+1 species seems to play
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an important role in catalysis. By comparing the activities of different catalysts, the presence of non-metallic Au species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. Pure nanosized metallic gold particles exhibit a worse catalytic performance, both on a ‘‘reducible’’ support like ceria and on an ‘‘inert’’ support like silica. The particle size is not an essential pre-requisite to obtain efficient gold catalysts in the low temperature CO oxidation. The metal-support interaction may account for the higher activity of Au/CeO2 (DP) samples with respect to Au/SiO2 (DP) catalysts that can be ascribed to a better stabilization of Au+1 ions by the cerium oxide, possibly as AuO species. The lower activity of the Au/CeO2 (SMAD) sample with respect to Au/CeO2 (DP) 573 K catalyst, both containing pure metallic gold nanoparticles, can be attributed to the embedding of the small gold particles within the porous structure of the support. Acknowledgments Financial support from the European Community (Grant COST, Project D 15), from CNR (Consiglio Nazionale delle Ricerche) and from the University of Palermo (‘‘Fondi Ricerca Scientifica’’) are gratefully acknowledged. References [1] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319. [2] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [3] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [4] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [5] L. Guczi, G. Peto¨, A. Beck, K. Frey, O. Geszti, G. Molnar, C. Daro´czi, J. Am. Chem. Soc. 125 (2003) 4332. [6] M.S. Chen, D.W. Goodman, Science 306 (2004) 252.
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Metal-support and preparation influence on the structural and electronic properties of gold catalysts Maria Pia Casaletto a,*, Alessandro Longo a, Anna Maria Venezia a, Antonino Martorana b, Antonio Prestianni b a
Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, Via Ugo La Malfa 153, Palermo 90146, Italy b Dipartimento di Chimica Inorganica e Analitica ‘‘S. Cannizzaro’’, Universita` di Palermo, Viale delle Scienze, Parco D’Orleans, Palermo 90128, Italy Received 15 November 2005; received in revised form 25 January 2006; accepted 2 February 2006 Available online 7 March 2006
Abstract Nanostructured gold catalysts supported on CeO2 and SiO2 were prepared by the deposition–precipitation (DP) and the solvated metal atom dispersion (SMAD) techniques. The structural and electronic properties of the catalysts were investigated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Gold was found as small metal nanoparticles (cluster size 2 nm) in the SMAD-prepared samples and in ionic state in the DP catalysts. The catalytic activity of the samples was tested in the reaction of low temperature CO oxidation. Gold nanosized particles in a pure metallic state exhibited a worse catalytic performance, both on ceria and silica. The presence of non-metallic Au species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. The higher activity of the Au/CeO2 (DP) sample with respect to the Au/SiO2 (DP) catalyst can be ascribed to a better stabilization of the Au+1 ions, probably as AuO- species, by the cerium oxide. # 2006 Elsevier B.V. All rights reserved. Keywords: CO oxidation; XPS; XANES; Oxidation state of gold; Particle size effect; Au/CeO2; Au/SiO2
1. Introduction A complete understanding of the structure-activity relationship is one of the most important goals of surface science studies related to heterogeneous catalysis. Gold nanostructured catalysts have attracted rapidly growing attention, due to their potential applicability to various reactions of industrial and environmental interest [1,2]. Supported gold nanoparticles have been widely investigated in the last few years and most of the studies have focused on the unusual low temperature CO oxidation activity [2,3]. Nevertheless, despite the extensive recent efforts addressing such extraordinary catalytic behaviour of gold nanoparticles, the origin of the structure sensitivity of the low-temperature oxidation of CO on supported Au clusters has been not yet completely unravelled. Different possible factors have been evoked, including the size of the Au clusters, the oxidation state of gold, the nature of the support material
* Corresponding author. Tel.: +39 091 6809378; fax: +39 091 6809399. E-mail address: [email protected] (M.P. Casaletto). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.02.005
(state and structure), the Au-support interface, the preparation method. The catalytic performances of gold nanoparticles have been correlated, in turn, with electronic (quantum size effect, oxidation state), structural and support (defects, perimeter interface) effects, but a general consensus has not been reached [4,5]. The sensational catalytic activity exhibited by gold upon sizing down its dimension [3] has captured the general attention on nanostructured catalysts, giving rise to a massive amount of articles in the literature. In the last few years the role of Au particle size has become less predominant in determining an high catalytic activity, since many other effects have been proven essential. Furthermore, recently Chen and Goodman [6] prepared well-ordered gold monolayers and bilayers that completely cover the titanium oxide, in order to eliminate particle shape and support effects. In this paper they reported that gold can be no longer in the form of nanoparticles, but as a supported bilayer film on TiO2 and still a catalytic activity for CO oxidation about 45 times higher than that of gold clusters supported on high surface area titanium oxide is found [6].
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In addition to the particle size of metallic Au, the role of the oxidation state of gold has received great attention, since oxidised gold species were proposed as the active sites for CO oxidation in differently supported Au catalysts [7–10]. Oxidised gold species were found responsible for the high catalytic activity in the low-temperature CO oxidation in a variety of supported gold systems: gold supported on TiO2, TiO2-ZrO2, Fe2O3 and Al2O3 by the DP method [9,10], on Fe2O3 by co-precipitation [8] and on Y-type zeolite by ion exchange [7,10]. High activity in the water-gas shift reaction is obtained by non-metallic Au species supported on Fe2O3 [10] and on CeO2 [11]. Even if essential for some oxidation reactions, in view of their totally different oxygen adsorption properties compared with those of bulk Au [4,12], metallic gold nanoparticles did not participate in the water-gas shift reaction, since the catalytic activity resulted unaffected by the removal of metallic Au species by cyanide leaching [11]. In our previous study on ceria-supported gold catalysts ionic gold species were found responsible for the achievement of the highest CO conversion in the low temperature catalytic CO oxidation [13]. A linear relationship between CeO2 surface oxygen reducibility, determined by H2-TPR, and CO conversion confirmed the strong interaction between ionic gold and ceria support that can play a crucial role in the catalytic performances of the samples. Gold ions in contact with the ceria support can modify the ceria properties by weakening the Ce–O bond and increasing the formation of CO2 [13]. Recently, on the basis of joint density functional theory (DFT) calculations and experimental evidence by a fast-flow reactor mass spectrometer, AuOn (n = 1–3) species were found to promote the oxidation of CO [14]. In particular, the AuO species dramatically decreased upon reaction with CO, as evidenced by the intensity decrease of the AuO peak at 213 amu in the mass spectrum. Structural calculations and molecular dynamics simulations evidenced that a very fast and thermodynamically favoured oxidation reaction occurred. A proposed mechanism involves the oxidized Au species as effective active sites with the binding of CO and the subsequent CO2 release by the cleavage of either Au–O or Au–C bond [14]. A direct evidence of AuO , AuO2 and AuOH ion clusters on supported gold catalysts has recently been reported by a timeof-flight secondary ion mass spectroscopy (TOF-SIMS) study [15]. Even if oxidized gold was not detected by XPS, SIMS spectra of Au/g–Al2O3 and Au/TiO2 catalysts prepared by the DP process indicated the existence of oxidized Au clusters of AuO , AuO2 and AuOH , due to the higher detection sensitivity of this surface analytical technique. Oxidized gold species may be still present on the surface of the catalysts, even if not evidenced by XPS, as trace ions detected by TOF-SIMS [15]. A direct correlation between the Au+1 content and the catalytic activity for CO oxidation was found for Au/TiO2 and Au/TiO2–ZrO2 catalysts, containing both Au0, Au+1 and Au+3 species, as evidenced by Mo¨ssbauer spectroscopy [10]. On the basis of our previous findings of a better catalytic performance of ionic gold with respect to metallic species in Au/CeO2 catalysts [13] and considering the great emphasis still given to the role of metallic gold nanoparticles in literature, we
have planned a set of experiments in order to investigate the nature of gold species on different supports as a function of different preparation methods. To this aim Au/CeO2 samples were compared with Au/SiO2 catalysts, in which a negligible metal-support interaction is expected. In this article we will focus on the structural and electronic characterization of supported gold nanoparticles. The importance of catalyst preparation and the nature of support materials are addressed in view of the significance of gold particles size. Since metallic Au is reported to be catalytically important when the particles size is in the nanometer scale, the solvated metal atom dispersion (SMAD) technique was selected, besides the most common deposition–precipitation method, for obtaining the metal in the pure zero-valent state, for avoiding common residuals from the metal precursors and for the tailoring of the particle size distribution [16–18]. In order to look at intrinsic properties of gold small particles and possibly to relate the catalytic CO oxidation activity directly to Au particle size, a non-interacting oxide such as SiO2 was chosen as support. Since the interaction with the support is known to play a dramatic role, gold nanoparticles were supported on a different oxide like CeO2, by using the same preparation techniques and the same gold loading. The choice of cerium oxide as support is motivated by the interaction of gold with this ‘‘reducible’’ oxide, which has been proved to enhance the ceria surface oxygen reducibility and to be able to form a mixed oxide AuxCe1 xO2 d [11,13]. For a direct comparison, the same amount of gold nanoparticles (3.0 wt.% Au) was supported on ceria, both by the SMAD and the deposition–precipitation procedures. 2. Experimentals and methods 2.1. Preparation of catalyst Gold nanoparticles supported on CeO2 (Aldrich, specific ˚ ) and on SiO2 surface area = 79 m2/g; average pore size 24 A 2 (Aldrich, specific surface area = 545 m /g) powders were prepared by two different synthetic routes. The metal thermal evaporation from high purity Au foil (Aldrich, 99.99%) was performed according with the SMAD technique [16]. This procedure involves the following steps: (a) deposition of an organic solvent (acetone) on the walls of the metal vapor reactor cooled at 77 K (liquid nitrogen temperature); (b) vaporization of the metal under vacuum, followed by the rapid trapping of the atoms in the frozen solvent matrix on the walls; (c) warming up to room temperature (RT) of the condensate of solvated gold atoms and melting down to the bottom of the reactor with the formation of a colloid solution. The solvated metal atom solution was used to impregnate a weighted amount of powder support poured into the reactor. After impregnation, the samples were degassed and dried at room temperature under vacuum. Samples prepared by this technique will be referred to as Au/CeO2 (SMAD) and Au/SiO2 (SMAD), respectively. According to the method of deposition–precipitation (DP) reported by Haruta and co-workers [19], ceria and silica supports were dispersed in an aqueous solution of
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HAuCl43H2O (Aldrich, 99.9%) used as gold precursor. The pH value of the support dispersion was adjusted at pH 8 by addition of a 0.1 M NaOH solution. The aqueous dispersion was stirred at 353 K for 24 h. After filtration, the solid was washed with hot distilled water several times to remove residual Cl species, as tested by AgNO3. The powder was dried at 373 K in a dry oven and a portion was calcined at 573 K for 1 h. Samples obtained by this procedure will be denoted as Au/CeO2 (DP) 373 K and Au/SiO2 (DP) 373 K; Au/CeO2 (DP) 573 K and Au/SiO2 (DP) 573 K, respectively. The bulk amount of gold in the catalysts was quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a ICP Perkin-Elmer Optima 3000DV spectrometer. A portion of 0.02 g of catalyst was dissolved by Aqua Regia (HNO3:HCl, in 1:3 ratio) in a microwave oven. A 3.0 wt.% gold loading was obtained for all the Au/CeO2 and Au/SiO2 samples. 2.2. Characterization of catalyst The physical and chemical characterization of the catalysts was performed by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The catalysts characterization was performed on the freshly prepared SMAD samples and after treating the DP samples at 373 and 573 K, in order to check the stability of the metallic particles towards sintering and the distribution of gold species in different oxidation states. XRD measurements were carried out by a Philips PW 1820 (vertical goniometer) using a Ni-filtered Cu Ka radiation ˚ ). A proportional counter and a 0.05 step size in (l = 1.5418 A 2u were used. The XRD patterns reported in this study resulted from the average of 20 measurements collected for each sample. The assignment of the crystalline phases was based on the JPDS powder diffraction file cards [20]. Gold particle sizes were calculated from the line broadening of the most intense Au reflections by using the Scherrer equation [21]. X-ray absorption spectra were recorded at RT on LIII Au absorption edge (11.920 KeV) at the BL 11.1 beamline of ELETTRA Sincrotrone Trieste S.C.p.A., operating at an energy of 2.4 GeV with a 130 mA storage ring current. The beamline was equipped with a Si (1 1 1) double crystal monochromator with energy resolution 2 10 4. In addition to catalysts samples, XANES spectra were also acquired for Au foil (internal standard reference for the energy calibration) and AuCl3 samples, as structural references for the Au clusters in the support matrix. XAFS measurements of reference samples and Au/SiO2 catalysts were performed in transmission mode by using two ionization chambers filled with a N2/Ar mixture at different composition for the incident I0 and transmitted I1 beam, respectively. Due to the different support nature, Au/ CeO2 samples were investigated in fluorescence mode (seven elements Ge EUROSYS detector). Au/CeO2 and Au/SiO2 samples for analysis were prepared by catalyst suspension in pure ethanol (Aldrich, 99.99%) and subsequent deposition on a suitable membrane. An edge jump of 0.2 in total absorption was obtained for the Au/SiO2 catalyst and, therefore, four
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measurements for each sample were needed to improve the signal-to-noise ratio. For Au/CeO2 catalysts five measurements for each sample were collected. All the spectra were recorded in the 11.700–13.000 KeV energy range. Samples were calibrated using the internal Au foil (LIII edge energy of 11.920 KeV) and normalized to an edge jump of unity. The surface chemical composition of the samples was studied by XPS in an ultrahigh vacuum chamber (base pressure 10 8 Torr). Photoemission spectra were collected by a VG Microtech ESCA 3000 Multilab spectrometer, equipped with a standard Al Ka excitation source (hn = 1486.6 eV), a ninechanneltrons detection system and a hemispherical analyzer operating at a constant pass energy of 20 eV. Gold catalysts were sampled as powders and pressed on a double-side adhesive tape. The binding energy (BE) scale was calibrated by measuring C 1s peak (BE = 285.1 eV) from the surface contamination and the accuracy of the measure was 0.1 eV. A non-linear least-square peak fitting routine was used for the analysis of XPS spectra, separating elemental species in different oxidation states. Relative concentrations of chemical elements were calculated by a standard quantification routine, including Wagner’s energy dependence of attenuation length [22] and a standard set of VG Escalab sensitivity factors. XPS quantification results are affected by a standard error lower than 10%. A Au 4f7/2 binding energy of 84.1 eV was obtained from a pure Au foil and used as the standard reference for metallic Au. 2.3. CO Catalytic oxidation tests The catalytic activities of the samples were compared by measuring CO conversion as a function of temperature (lightoff test). CO oxidation tests were performed in a quartz reactor loaded with 0.05 g powdered catalyst. A gas mixture containing 1% CO and 1% O2, in a balance of He as gas carrier, was fed continuously into the reactor at a total flow rate of 50 ml/min (STP), which corresponds to a weight hourly space velocity (WHSV) of 60.000 ml g 1 h 1. CO conversion was determined by analysing the effluent gases with a UV–IR Analyser (ABB Uras 14). 3. Results and discussion 3.1. Au/SiO2 catalyst XRD diffractograms of Au/SiO2 catalysts in the 348–708 range are shown in Fig. 1. The XRD pattern of the ‘‘as prepared’’ Au/SiO2 (SMAD) sample gives evidence of the presence of very broad diffraction peaks due to the f.c.c. structure of gold. A rough estimate of gold particle size by the Scherrer approximation results in Au clusters with an average ˚ . XRD analysis performed on the DP-prepared diameter of 20 A Au/SiO2 catalysts shows no evidence of metallic gold particles in the Au/SiO2 (DP) 373 K sample, whose pattern is that typical of the amorphous SiO2 support. Calcination at T = 573 K leads to gold particles agglomeration up to an average particle size of ˚ , as derived from the very intense and narrow Au about 150 A
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Fig. 1. XRD patterns of the Au/SiO2 catalysts prepared by the SMAD and DP procedures.
f.c.c. peaks detected in the XRD pattern of the Au/SiO2 (DP) 573 K catalyst. XANES data of the Au/SiO2 samples at RT are reported in Figs. 2 and 3, together with those of Au foil and AuCl3 taken as reference materials. The spectrum of Au0 from gold foil shows a shoulder at an energy 15 eV above the X-ray absorption edge (vertical dashed line in the figures) and intense peaks at 25 and 50 eV, labeled (a), (b) and (c), respectively. The spectrum of Au+3 shown in Fig. 3 is characterized by a very strong feature at 4 eV above the edge, which is absent in metallic gold, due to the complete occupancy of the d states, together with a shoulder at 15 eV and a very broad shoulder at 50 eV. An important evidence from Fig. 2 is represented by the shift and the broadening of the XANES peaks for the Au/SiO2 (SMAD) sample with respect to the pure Au foil. The XANES peaks labeled (a) and (b) in Fig. 2 are characteristic of a pure gold f.c.c. structure ordered up to the third shell. Balerna et al. [23] observed that a shift to higher energies and the broadening of these peculiar peaks occur as the cluster size decreases. In particular, the shift has been attributed to the shortening of the Au–Au first neighbour distance and the broadening to the
Fig. 2. XANES spectra of gold supported catalysts, containing Au metallic species. The energy scale is expressed relatively to the theoretical energy of the Au LIII edge at 11.920 KeV (vertical dashed line).
Fig. 3. XANES spectra of DP-prepared gold supported catalysts, containing oxidized and metallic Au species. The energy scale is expressed relatively to the theoretical energy of the Au LIII edge at 11.920 KeV (vertical dashed line).
cluster size distribution [23]. From XANES data, we can conclude that the Au/SiO2 (SMAD) sample contains typical f.c.c. gold clusters dispersed in the silica matrix. Due to the ˚) shortening of the lattice constant (from 4.079 to 4.030 A derived from XRD data in Fig. 1, the gold clusters dimension is probably defined in the nanometric range, in good agreement with Balerna et al. [23]. XANES of the Au/SiO2 (DP) 373 K sample shown in Fig. 3 clearly indicate the simultaneous presence of Au+3 and Au0 species. The oxidized species are identified by the diagnostic intense peak at 4 eVand the metallic Au species by the presence of type f.c.c. features that match peaks (b) and (c) in Fig. 3, characteristic of the reference Au foil XANES spectrum. The surface chemical composition of the catalysts has been investigated by XPS and a summary of the quantitative results is reported in Table 1. Samples of pure Au foil, SiO2 and CeO2 supports were also analysed and taken as reference. The Au/Si atomic ratio values are in good agreement with that expected for the bulk composition of samples with a 3.0 wt.% Au content (Au/Si = 0.02). The Au 4f spectra of all the investigated Au/SiO2 samples are shown in Fig. 4 and a shift of the spectrum of the DP-prepared sample towards higher BE values can be noticed. This evidence is confirmed by the curvefitting of the Au 4f core-level spectra by two spin-orbit splitted Au 4f7/2 and Au 4f5/2 components (DE = 3.7 eV), showing increasing BE values that correspond to a different chemical state of gold particles in the samples. In the Au/SiO2 (SMAD) sample the Au 4f7/2 photoelectronic peak is located at BE = 84.2 eV and this value is typical of pure metallic Au species [24]. The Au 4f spectrum of Au/SiO2 (DP) 373 K catalyst shown in Fig. 4b consists of two components located at BE = 84.2 and 86.3 eV, which can be assigned to metallic and oxidized gold species, namely Au0 and Au+3 ions [9,13,24]. In the Au/SiO2 (DP) 573 K sample the Au 4f7/2 peak detected at BE = 84.5 eV shown in Fig. 4c can be attributed to the presence of Au0 species on the surface [9,13,24]. The relative surface distribution of gold species in the catalysts is shown in Table 2, as determined by the curve-fitting of the Au 4f photoelectronic
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313
Table 1 Relative surface chemical composition of the samples Sample
Au 4f
Si 2p
O 1s
Ce 3d
Au/Si
O/Si
Au/Ce
O/Ce
SiO2 support Au/SiO2 (SMAD) Au/SiO2 (DP) 373 K Au/SiO2 (DP) 573 K CeO2 support Au/CeO2 (SMAD) Au/CeO2 (DP) 373 K Au/CeO2 (DP) 573 K
– 0.6 0.7 0.4 – 0.8 2.6 1.5
29.8 30.7 30.6 30.6 – – – –
70.2 68.7 68.7 69.0 60.9 62.7 56.8 59.4
– – – – 39.1 36.5 40.6 39.1
– 0.020 0.023 0.013 – – – –
2.4 2.2 2.3 2.3 – – – –
– – – – – 0.022 0.064 0.038
– – – – 1.6 1.7 1.4 1.5
Elemental concentrations are expressed as atomic percentage (atom.%). The Au/Si and Au/Ce atomic ratio values expected for the bulk composition of samples with a 3.0 wt.% Au loading are 0.02 and 0.05, respectively.
peak. Only metallic gold particles were detected by XPS in the SMAD-prepared Au/SiO2 sample, thus confirming XANES results (see Fig. 2) and the XRD pattern of small Au clusters (see Fig. 1). On the surface of the Au/SiO2 (DP) 373 K samples a major fraction of oxidized Au species were detected by XPS, as reported in Table 2. The presence of gold in an oxidized state in the Au/SiO2 (DP) 373 K catalyst is confirmed by XANES spectra in Fig. 3. The XRD pattern in Fig. 1 reveals that the size of metallic gold is below the XRD detection limit, since no diffraction peaks attributable to f.c.c. gold particles are evidenced. The change in gold species from oxidic to metallic phase in the bulk of the Au/SiO2 (DP) sample upon thermal
Table 2 Relative surface distribution of gold species in the samples, as derived from the curve-fitting of the Au 4f photoelectron peak Sample
Au 4f7/2 BE (eV)
Oxidation state
Peak area (%)
Au foil Au/SiO2 (SMAD) Au/SiO2 (DP) 373 K
84.1 84.2 84.2 86.3 84.5 84.4 84.5 85.8 84.2
Au 0 Au 0 Au 0 Au+3 Au 0 Au 0 Au 0 Au+1 Au 0
100 100 40 60 100 100 90 10 100
Au/SiO2 (DP) 573 K Au/CeO2 (SMAD) Au/CeO2 (DP) 373 K Au/CeO2 (DP) 573 K
(1.9) (2.0) (2.3) (2.2) (2.0) (2.0) (2.0) (2.0) (1.7)
The FWHM values are reported in parentheses. Concentrations are expressed as percentage of the total area of the Au 4f peak (taken as 100%).
treatment is clearly indicated both by the XRD and XANES data in Figs. 1–3. Calcination of the Au/SiO2 (DP) sample at T = 573 K results in a thermal reduction and sintering of gold nanoclusters. The catalytic performances of silica-supported gold nanoparticles in CO oxidation are shown in Fig. 5, where the CO conversion is reported as a function of temperature (light-off test). All the investigated samples supported on an ‘‘inert’’ oxide like silica (i.e. Au/SiO2 catalysts) exhibit a poor catalytic activity, regardless the preparation procedure. Despite the small size of metallic gold particles (few nm) supported on silica, CO oxidation catalysis does not occur at low temperature as desired, since a 50% value of conversion is only attained well
Fig. 4. XPS curve-fitting of the Au 4f photoelectron peak in the Au/SiO2 (SMAD); Au/SiO2 (DP) 373 K and Au/SiO2 (DP) 573 K catalysts, (a), (b) and (c), respectively. The scatter points refer to the raw data, while the solid line to the curve-fitting results.
Fig. 5. CO conversion (%) as a function of temperature for all the investigated gold catalysts.
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beyond T = 500 K. The particle size does not seem to be a sufficient factor to obtain an efficient gold catalyst in the low temperature CO oxidation and this result argues against an electronic effect in small Au particles as a major factor contributing to the activity of gold catalysts. A little or no relationship between catalytic activity and particle size has already been claimed in the literature [25,26] and some other effects such as the metal-support interaction and the oxidation state of gold have in turn been considered as determining factors. The simultaneous presence of metallic and oxidized gold in the form of predominant Au+3 ions and minor Au0 species on the surface of the Au/SiO2 (DP) 373 K sample, as derived by XPS, does not improve the catalytic activity, as shown in Fig. 5. In order to further investigate the gold particle size effect, a ˚ Au/SiO2 sample with gold nanoclusters even smaller than 20 A was prepared by SMAD, after a convenient optimisation of the technique for the tailoring of the size of metallic nanoparticles [18]. In Fig. 6 the XRD pattern of this Au/SiO2 sample and that of the pure SiO2 support are reported. For a best detection of the f.c.c. Au clusters, the difference spectrum, shown at the bottom of the figure, was obtained by using a smoothed curve of the SiO2 diffractogram. The determination of the Au clusters dimensions was performed by using the Scherrer equation, including two distinct contributions for the Au (1 1 1) and Au (2 0 0) peaks, respectively, in a fitting procedure. Upon subtraction of a smooth background to the XRD Au/SiO2 ˚ was found. It is worth raw data, a clusters dimension of 15–20 A noting that this is the first experimental evidence of such small supported Au nanoparticles prepared by the SMAD technique. As already found for the Au/SiO2 (SMAD) shown in Fig. 1, a contraction of the Au lattice constant is detected, which is now ˚ ). Nevertheless, even more pronounced (from 4.079 to 4.000 A even if gold particles are tailored in one nanometer size, the catalytic performance of this Au/SiO2 sample in the low temperature CO oxidation is totally negligible (not shown in Fig. 5).
Fig. 6. XRD patterns of an Au/SiO2 sample prepared by the SMAD technique and pure SiO2 support. The difference spectrum at the bottom was obtained by using a smoothed curve of the SiO2 pattern.
Fig. 7. XRD patterns of the Au/CeO2 catalysts prepared by the SMAD and DP procedures. The inset refers to the presence of the Au (1 1 1) peak among the diffraction peaks of the ceria support, marked with a star.
3.2. Au/CeO2 catalyst In the XRD pattern of the Au/CeO2 (SMAD) sample shown in Fig. 7 only reflections of the fluorite structured CeO2 are evidenced and no diffraction peaks of gold are found. The presence of metallic gold in the sample can be inferred by the XANES spectrum in Fig. 2. On the surface of the Au/CeO2 (SMAD) catalyst gold is in the zerovalent state, as detected by the Au 4f7/2 photoelectron peak at BE = 84.4 eV, shown in Fig. 8a. As expected, the SMAD synthesis allows the formation of very small metallic gold clusters, not detectable by XRD, in good agreement with literature data [13,18]. Also in the Au/ CeO2 (DP) 373 K diffractogram reported in Fig. 7 no traces of gold f.c.c. particles are detected along the peaks of the pure ceria support. Still the XANES spectrum in Fig. 3 indicates that metallic gold species are present in the sample, evidently as very small particles undetectable by XRD, due to the higher scattering factor of ceria with respect to silica. The XPS spectrum in the Au 4f core-level region shown in Fig. 8b can be deconvoluted by two components at BE = 84.5 and 85.8 eV, which can be assigned to Au0 and Au+1species, respectively [9,13,24]. On the surface of the Au/CeO2 (DP) 373 K catalyst a small fraction of cationic gold species are found, together with the most abundant neutral gold particles, as indicated by the relative distribution of gold species in Table 2. The classical deposition–precipitation method is known to provide both metallic and cationic gold species [11,27]. Nevertheless, Au/ CeO2 samples prepared by the DP method may result in substantially different Au oxidation state, if small differences in the experimental procedure are introduced. In our previous work, we prepared a 3 wt.% Au/CeO2 (DP) 393 K catalyst, which exhibited only ionic gold species on the surface, and we found a high catalytic activity in CO oxidation below T = 273 K [13]. In the present case, the use of NaOH instead of Na2CO3, as precipitation agent, and a prolonged aging period of the solution at higher temperature (24 h at 353 K) resulted in reduced gold nanoparticles in the sample, predominantly as Au0 and partially as Au+1 species.
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Fig. 8. XPS curve-fitting of the Au 4f photoelectron peak in the Au/CeO2 (SMAD); Au/CeO2 (DP) 373 K and Au/CeO2 (DP) 573 K catalysts, (a), (b) and (c), respectively. The scatter points refer to the raw data, while the solid line to the curve-fitting results.
The agglomeration of these very small metallic gold particles occurs upon calcination at T = 573 K, as shown by the presence of the Au (1 1 1) reflection at 2u = 38.38 in the XRD pattern of Au/CeO2 (DP) 573 K catalyst (see the inset in Fig. 7). A rough estimate of gold particle size by the Scherrer approximation indicates gold clusters with an average diameter ˚ . The presence of metallic gold in the Au/CeO2 (DP) of 150 A 573 K catalyst is confirmed by the XANES data reported in Fig. 2 and by the Au 4f7/2 photoelectron peak detected by XPS at BE = 84.2 eV, as shown in Fig. 8c and in Table 2. Catalytic results of the Au/CeO2 samples in the reaction of CO oxidation are reported in Fig. 5. Better performances in the low temperature range are displayed by the ceria-supported gold catalysts with respect to the silica ones, thus confirming the importance of the nature of support for gold nanoparticles, well assessed in the literature [1]. The presence of very small pure metallic gold particles in the Au/CeO2 (SMAD) sample is not a key-factor in the low temperature CO catalysis, as already ˚ gold particles size. A found for the Au/SiO2 samples with 20 A significant improvement in the light-off temperature is obtained by using the Au/CeO2 (DP) 373 K catalyst, which reaches a 50% CO conversion at T = 250 K. This evidence may be ascribed to Au+1 species in the sample that were detected by the XPS along with the most abundant neutral gold particles (see
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Table 2). The higher catalytic performances of the Au/CeO2 samples with respect to the Au/SiO2 catalysts can be attributed to a better stabilization of the Au+1 species by cerium cations. The influence of the addition of cerium oxide on the surface electronic states of supported gold has been studied in the literature [28]. The addition of CeO2 was found to stabilize the ionic states of supported gold and to increase the effective charge of gold ions [28]. The stabilization of gold ionic states in Au–Ce samples was attributed to the direct influence of CeO2 on the one-charged Au+1 cations, which had proven to be the active sites in the partial oxidation of alcohols, as M+1 cations of the Cu subgroup metals [29]. The co-existence of gold metallic and cationic species with hydroxyl ligands has been suggested as a chemical model of the active site in CO oxidation catalysis. Nevertheless, controversial reports are found in the literature, claiming the catalytic effectiveness of Au+1 rather then Au+3 cationic species [30,31]. In order to corroborate the role of Au+1–OH species as active sites, the presence of Au+1 cations in an active catalyst was determined by direct spectroscopic evidence, while the participation of hydroxyl groups to the reaction was inferred by the effect of water vapour [31]. In a different article, gold cations supposed as Au+3 were proposed as a ‘‘chemical glue’’ for binding gold particles to the support and preventing their sintering, while the presence of gold atoms was considered necessary as a site for CO chemisorption [30]. With respect to the various and controversial literature reports concerning the nature of the active site in CO catalytic oxidation, our experimental evidences seem to corroborate the hypothesis of the promoting effect of Au+1 ions, probably in the form of AuO species, as derived by theoretical calculations [14]. The comparison between Au/SiO2 and Au/CeO2 catalysts highlights the important role of Au+1 species, which are better stabilized by suitable cations from a rare earth oxide support like CeO2 [28,29]. The presence of pure metallic gold nanoparticles (size dimension in the range: 1–6 nm) has proven once again not to be the major requisite for the low temperature catalytic oxidation of CO to CO2. Moreover, also the morphology of the support in terms of porosity should be taken into account to explain the better catalytic performances of the Au/CeO2 (DP) 573 K sample with respect to the Au/ CeO2 (SMAD) catalyst. The small sized Au nanoparticles obtained by the SMAD technique may be entrapped inside the porous structure of the support, thus decreasing the surface Au/ Ce ratio (see Table 1). It is worth noticing that in the Au/CeO2 (DP) catalysts the Au/Ce atomic ratio values reported in Table 1 are in close agreement with that expected for the bulk composition of samples with a 3.0 wt.% Au loading (Au/ Ce = 0.05). 4. Conclusions The structural and surface analyses evidenced the presence of small gold metal crystallites (cluster size 2 nm) in the case of the SMAD-prepared samples and oxidized gold species in the case of the DP catalysts. By comparing gold nanoparticles supported on ceria, the presence of Au+1 species seems to play
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an important role in catalysis. By comparing the activities of different catalysts, the presence of non-metallic Au species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. Pure nanosized metallic gold particles exhibit a worse catalytic performance, both on a ‘‘reducible’’ support like ceria and on an ‘‘inert’’ support like silica. The particle size is not an essential pre-requisite to obtain efficient gold catalysts in the low temperature CO oxidation. The metal-support interaction may account for the higher activity of Au/CeO2 (DP) samples with respect to Au/SiO2 (DP) catalysts that can be ascribed to a better stabilization of Au+1 ions by the cerium oxide, possibly as AuO species. The lower activity of the Au/CeO2 (SMAD) sample with respect to Au/CeO2 (DP) 573 K catalyst, both containing pure metallic gold nanoparticles, can be attributed to the embedding of the small gold particles within the porous structure of the support. Acknowledgments Financial support from the European Community (Grant COST, Project D 15), from CNR (Consiglio Nazionale delle Ricerche) and from the University of Palermo (‘‘Fondi Ricerca Scientifica’’) are gratefully acknowledged. References [1] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319. [2] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [3] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [4] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [5] L. Guczi, G. Peto¨, A. Beck, K. Frey, O. Geszti, G. Molnar, C. Daro´czi, J. Am. Chem. Soc. 125 (2003) 4332. [6] M.S. Chen, D.W. Goodman, Science 306 (2004) 252.
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