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Co-existance of various active gold species in Au-mordenite catalyst for CO oxidation
Catalysis Communications 8 (2007) 977–980 www.elsevier.com/locate/catcom
Co-existance of various active gold species in Au-mordenite catalyst for CO oxidation I.V. Tuzovskaya a, A.V. Simakov a, A.N. Pestryakov b,*, N.E. Bogdanchikova a, V.V. Gurin c, M.H. Farı´as a, H.J. Tiznado d, M. Avalos a a
c
CCMC-UNAM, Apdo Postal 2681, Ensenada, BC, Mexico b Tomsk Polytechnic University, Tomsk 634034, Russia Physico-Chemical Research Institute, BSU, Minsk 220080, Belarus d University of California, Riverside, USA
Received 3 October 2006; received in revised form 13 October 2006; accepted 13 October 2006 Available online 19 October 2006
Abstract Different structural and electronic states of gold species in H-mordenite with SiO2/Al2O3 molar ratio 206 and their transformations under redox treatments have been studied by the methods of diffuse reflectance UV–visible spectroscopy and FTIR spectroscopy of adsorbed CO. Different states of ionic and metallic gold were detected in the zeolite channels and on the external surface of the zeolite – Au+ and Au3+ ions, charged clusters Audþ n , and neutral nanoparticles Aum. Catalytic tests of the samples revealed the co-existence of several types of active species of gold in CO oxidation – gold clusters <1.5 nm (responsible for low-temperature activity) and gold nanoparticles (responsible for high-temperature activity). 2006 Elsevier B.V. All rights reserved. Keywords: Gold; Zeolite; Catalyst; Active sites; Oxidation
1. Introduction An intensive study of gold in catalysis for last decade was stimulated by discovery of unique activity of gold nanoparticles in CO oxidation at low temperature [1]. However, the origin of such unusual behavior of gold remains still under debate. A number of gold active sites were proposed: gold-support interface [2], the second layer of gold atoms from the support [3], gold nanoclusters with nonmetallic electronic properties due to a quantum-size effect [4], Au3+ ions [5], etc. The effect of multiplicity of Au active sites in one catalyst and their activation under different conditions revealed in our previous studies [6–9] could partially explain the discrepancy in the nature of gold active sites described in literature. Various research groups could work under *
Corresponding author. Tel.: +7 3822 563 861; fax: +7 3822 563 637. E-mail address: [email protected] (A.N. Pestryakov).
1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.10.014
different conditions activating different sites in the same catalyst. So the question is not what the nature of active sites is in a given sample, but which active site is activated in the specific catalyst under applied conditions. So, in some cases probably the results of different groups do not contradict but supplement each other. In Ref. [7] we revealed the effect of co-existence of at least two types of gold active sites possessing different activity in CO oxidation on Au species supported on silica-poor mordenite. The aim of the present study is to extend the investigation of effect of multiplicity of Au active species to silica-rich mordenite. 2. Experimental AuM206 sample was prepared by ion exchange procedure of protonic mordenite (TOSOH Corporation, Japan, SiO2/Al2O3 molar ratio (MR) = 206) with aqueous [Au(NH3)4](NO3)3 complex at room temperature [10].
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Samples were dried at room temperature for 24 h (this type of catalysts was called ‘as-prepared’). UV–visible diffuse reflectance spectra (DRS) were recorded using a CARY 300 SCAN (VARIAN) spectrophotometer. X-ray photoelectron spectroscopy (XPS) study was performed with a photoelectron spectrometer Riber-Cameca Mac-3. Thermo-programmed reduction (TPR) was measured in an AMI-1 Altamira-Instruments. X-ray diffraction (XRD) analysis was carried out with a Philips X’pert diffractometer. Transmission electron microscopy (TEM) analysis was carried out with a JEOL 2010 microscope. Au weight loading after the ion-exchange was evaluated by energy dispersive spectroscopy in a JEOL 5300 scanning electron microscope. Result of this study showed that gold concentration was about 2.0%. FTIR spectra were registered using a Perkin–Elmer 2000 FTIR spectrometer. The initially obtained samples were tested after heating in vacuum at 150 C for 1 h. To change the electronic state of gold the catalysts were consistently treated with CO and O2 (100 mbar) at 300 C for 1 h for each pretreatment. Catalytic activity was carried out in flow micro reactor with the following gas mixture composition: 1 vol.% CO and 1 vol.% O2, balanced with helium. Reagents and products were analyzed in line using a gas chromatograph (SRI Instrument). 3. Results and discussion According to the data of UV–visible DRS (Fig. 1) the ion exchange of Au-complex with H+-mordenite and following calcinations results in appearance of the absorption bands due to different types of gold species: (i) Au3+ and Au+ cations, (ii) few-atomic gold clusters (can be partially charged) and (iii) gold nanoparticles. Interpretation of the optical bands made according to our previous studies [6–9]
Fig. 1. UV–visible spectra of AuM206 calcined at different temperatures: 1–100 C, 2–150 C, 3–200 C, 4–300 C, 5–500 C.
is marked directly in Fig. 1. Increase of calcination temperature leads to decrease of content of gold cations and the noticeable enhancement of the maximum due to nanoparticles, while the contribution of clusters does not change significantly (Fig. 1). TPR data showed that in as-prepared sample all gold ions are reduced within temperature range (100–400 C) and amount of consumed H2 corresponds to 30% of gold content. Therefore, the residual part of gold species (70%) in as-prepared sample is in the reduced form before heating, and according to Fig. 1 they could be assigned to the few-atomic Aun clusters. These clusters, less than 0.7 nm in diameter, may be localized within the mordenite channels having the cross section of channels 0.64 · 0.7 nm. Part of the clusters may be stabilized also on the external surface of zeolite. XPS data confirmed the presence of Au clusters in as/prepared samples along with gold ions with the binding energy of Au 4f7/2 84.3 and 87.1 eV, respectively. XRD and TEM data evidenced the noticeable formation of Au nanoparticles in the size range 1.5–17.0 nm and therefore localized on the external surface after sample calcinations at temperatures higher than 150 C. Catalytic activity of Au-zeolite is displayed by a series of temperature dependencies (light-off curves) of the CO conversion in 2CO + O2 = 2CO2 reaction with both upward and downward runs (Fig. 2). Activity of as-prepared AuM206 samples was not high, but it was stable because both runs practically coincide (Fig. 2a). The sample pretreatment in He flow at 500 C for 80 min results in abrupt catalyst activation (Fig. 2b), and four new peaks (marked by arrows) appeared on the up-warding curve of CO conversion. We emphasize that this activation after He pretreatment was reproducible. However, it disappeared in the downward curve, and the activity becomes even lower at the corresponding temperatures than that for as-prepared sample shown in Fig. 2a. Probably, the shape of light-off curve represents a superposition of several S-shape curves typical for certain active site each. It manifests transformation of active sites with temperatures. This effect of activation by He observed for Au-mordenite could be assigned to removal of carbonate species blocking active sites and/or to redox transformation of active sites. The experiments with sample pretreatment in O2 flow permitted to reveal the role of these factors. A pretreatment in O2 at 525 C should be more effective for removal of any carbonates than in He. Nevertheless, O2 pretreatment at 525 K resulted in disappearance of the low-temperature peak (Fig. 2c) not recoverable even after following re-treatment in He (Fig. 2d). This implies that redox transition of gold species can be responsible for catalyst activation/deactivation at low temperature. Partly (Audþ n ) or completely (Au0m ) reduced gold species are more active than oxidized ones. The low activity of as-prepared sample containing gold ions permits to conclude that gold cations are inactive in this process, and the gold clusters revealed by the optical data for as-prepared samples and samples calcined at low
I.V. Tuzovskaya et al. / Catalysis Communications 8 (2007) 977–980
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Fig. 2. Consequent experiments (a ! b ! c ! d) of CO conversion versus temperature over AuM206: without pretreatment (a), after treatments for 80 min: in He at 500 C (b), in O2 flow at 525 C (c) in He at 500 C (d).
temperature (curve 1 in Fig. 1) are also of low activity without He pretreatment at 500 C. FTIR study of CO adsorption (Fig. 3) sheds a light onto the nature of supposed gold species since the absorption bands of CO depend on the type of gold species [7,9]. The intensity of IR-CO adsorption bands in the region 1992–2067 cm1 corresponding to CO adsorbed on metallic gold clusters (probably negatively charged) decreased
Fig. 3. FTIR spectra of CO adsorbed at room temperature over AuM206 after sample pretreatments at 300 C in CO (fine line) and O2 (bold line).
after sample treatment in O2 while 2212 cm1 band assigned to positively charged clusters appeared [7,9]. This effect can be explained by the cluster redox transformations depending on the treatment atmosphere and temperature. These processes could be the reason of the activation and deactivation of sample in CO oxidation. The complex structure of bands can evidence the co-existence of several types of gold clusters slightly different probably in the effective charge, position on support, nuclearity, geometry, etc. Note, that the curves of catalytic activity also reveal a complicated structure (Fig. 2b). Obviously, clusters (<1.5 nm) are more sensitive to oxidation than the nanoparticles (1.5–17 nm) due to the cluster small size and stronger metal-support interaction [11]. Indeed, partial oxidation of gold clusters (appearance of partial charge) occurred after sample treatment at 300 C in O2 as it was shown by IR-spectroscopy (Fig. 3). Therefore, gold clusters and nanoparticles incorporated in silica-rich mordenite can be strongly activated in the reaction of CO oxidation by the pretreatment in He, while gold cations are inactive. Negatively charged gold clusters can be responsible for the low temperature CO oxidation being deactivated by interaction with oxygen at 525 C. Thus, the revealed effect of superposition of several S-shape light-off curves we assign to exhibition of different
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activity by Au clusters and nanoparticles of several types co-existing in one catalyst. A similar effect was observed in Ref. [6,7] for silica-poor mordenite. Acknowledgments The authors would like to express their gratitude to E. Flores, P. Casillas, F. Ruiz, E Aparicio, I. Gradilla, A. Dı´az and J. Peralta for technical assistance in experimental work. This work was supported by CONACYT No 42568Q and by PAPIIT-UNAM grants IN 109003. References [1] M. Haruta, Catal. Today 36 (1997) 153. [2] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [3] D.W. Goodman, Catal. Lett. 99 (1–2) (2005) 1.
[4] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [5] N.A. Hodge, C.J. Kiely, R. Whyman, M.R.H. Siddiqui, G.J. Hutchings, Q.A. Pankhurst, F.E. Wagner, R.R. Rajaram, S.E. Golunski, Catal. Today 72 (2002) 133. [6] A. Simakov, N. Bogdanchikova, I. Tuzovskaya, E. Smoletseva, A. Pestryakov, M. Farias, M. Avalos, in: M.W. McCall, G. Dewar, M.A. Noginov (Eds.), Proceedings of SPIE: Complex Mediums VI: Light and Complexity, vol. 5924, Bellingham, WA, 2005, p. 592410– 592411. [7] E. Smolentseva, N. Bogdanchikova, A. Simakov, A. Pestryakov, I. Tuzovskaya, M. Avalos, M.H. Farı´as, J.A. Dı´az and V. Gurin, Surf. Sci., in press. [8] E. Smolentseva, N. Bogdanchikova, A. Simakov, V. Gurin, M. Avalos, A. Pestryakov, M. Farias, J.A. Diaz, A. Tompos, Int. J. Modern Phys. 19 (2005) 2496. [9] A. Pestryakov, I. Tuzovskaya, E. Smolentseva, N. Bogdanchikova, F. Jentoft, A. Knop-Gericke, Int. J. Modern Phys. B 18 (2005) 2321. [10] L.H. Skibsted, J. Bjerrum, Acta Chem. Scand. A28 (1974) 740. [11] J.T. Miller, A.J. Kropf, Y. Zha, J.R. Regalbuto, L. Delannoy, C. Louis, E. Bus, J.A. van Bokhoven, J. Catal. 240 (2006) 222.
Co-existance of various active gold species in Au-mordenite catalyst for CO oxidation I.V. Tuzovskaya a, A.V. Simakov a, A.N. Pestryakov b,*, N.E. Bogdanchikova a, V.V. Gurin c, M.H. Farı´as a, H.J. Tiznado d, M. Avalos a a
c
CCMC-UNAM, Apdo Postal 2681, Ensenada, BC, Mexico b Tomsk Polytechnic University, Tomsk 634034, Russia Physico-Chemical Research Institute, BSU, Minsk 220080, Belarus d University of California, Riverside, USA
Received 3 October 2006; received in revised form 13 October 2006; accepted 13 October 2006 Available online 19 October 2006
Abstract Different structural and electronic states of gold species in H-mordenite with SiO2/Al2O3 molar ratio 206 and their transformations under redox treatments have been studied by the methods of diffuse reflectance UV–visible spectroscopy and FTIR spectroscopy of adsorbed CO. Different states of ionic and metallic gold were detected in the zeolite channels and on the external surface of the zeolite – Au+ and Au3+ ions, charged clusters Audþ n , and neutral nanoparticles Aum. Catalytic tests of the samples revealed the co-existence of several types of active species of gold in CO oxidation – gold clusters <1.5 nm (responsible for low-temperature activity) and gold nanoparticles (responsible for high-temperature activity). 2006 Elsevier B.V. All rights reserved. Keywords: Gold; Zeolite; Catalyst; Active sites; Oxidation
1. Introduction An intensive study of gold in catalysis for last decade was stimulated by discovery of unique activity of gold nanoparticles in CO oxidation at low temperature [1]. However, the origin of such unusual behavior of gold remains still under debate. A number of gold active sites were proposed: gold-support interface [2], the second layer of gold atoms from the support [3], gold nanoclusters with nonmetallic electronic properties due to a quantum-size effect [4], Au3+ ions [5], etc. The effect of multiplicity of Au active sites in one catalyst and their activation under different conditions revealed in our previous studies [6–9] could partially explain the discrepancy in the nature of gold active sites described in literature. Various research groups could work under *
Corresponding author. Tel.: +7 3822 563 861; fax: +7 3822 563 637. E-mail address: [email protected] (A.N. Pestryakov).
1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.10.014
different conditions activating different sites in the same catalyst. So the question is not what the nature of active sites is in a given sample, but which active site is activated in the specific catalyst under applied conditions. So, in some cases probably the results of different groups do not contradict but supplement each other. In Ref. [7] we revealed the effect of co-existence of at least two types of gold active sites possessing different activity in CO oxidation on Au species supported on silica-poor mordenite. The aim of the present study is to extend the investigation of effect of multiplicity of Au active species to silica-rich mordenite. 2. Experimental AuM206 sample was prepared by ion exchange procedure of protonic mordenite (TOSOH Corporation, Japan, SiO2/Al2O3 molar ratio (MR) = 206) with aqueous [Au(NH3)4](NO3)3 complex at room temperature [10].
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Samples were dried at room temperature for 24 h (this type of catalysts was called ‘as-prepared’). UV–visible diffuse reflectance spectra (DRS) were recorded using a CARY 300 SCAN (VARIAN) spectrophotometer. X-ray photoelectron spectroscopy (XPS) study was performed with a photoelectron spectrometer Riber-Cameca Mac-3. Thermo-programmed reduction (TPR) was measured in an AMI-1 Altamira-Instruments. X-ray diffraction (XRD) analysis was carried out with a Philips X’pert diffractometer. Transmission electron microscopy (TEM) analysis was carried out with a JEOL 2010 microscope. Au weight loading after the ion-exchange was evaluated by energy dispersive spectroscopy in a JEOL 5300 scanning electron microscope. Result of this study showed that gold concentration was about 2.0%. FTIR spectra were registered using a Perkin–Elmer 2000 FTIR spectrometer. The initially obtained samples were tested after heating in vacuum at 150 C for 1 h. To change the electronic state of gold the catalysts were consistently treated with CO and O2 (100 mbar) at 300 C for 1 h for each pretreatment. Catalytic activity was carried out in flow micro reactor with the following gas mixture composition: 1 vol.% CO and 1 vol.% O2, balanced with helium. Reagents and products were analyzed in line using a gas chromatograph (SRI Instrument). 3. Results and discussion According to the data of UV–visible DRS (Fig. 1) the ion exchange of Au-complex with H+-mordenite and following calcinations results in appearance of the absorption bands due to different types of gold species: (i) Au3+ and Au+ cations, (ii) few-atomic gold clusters (can be partially charged) and (iii) gold nanoparticles. Interpretation of the optical bands made according to our previous studies [6–9]
Fig. 1. UV–visible spectra of AuM206 calcined at different temperatures: 1–100 C, 2–150 C, 3–200 C, 4–300 C, 5–500 C.
is marked directly in Fig. 1. Increase of calcination temperature leads to decrease of content of gold cations and the noticeable enhancement of the maximum due to nanoparticles, while the contribution of clusters does not change significantly (Fig. 1). TPR data showed that in as-prepared sample all gold ions are reduced within temperature range (100–400 C) and amount of consumed H2 corresponds to 30% of gold content. Therefore, the residual part of gold species (70%) in as-prepared sample is in the reduced form before heating, and according to Fig. 1 they could be assigned to the few-atomic Aun clusters. These clusters, less than 0.7 nm in diameter, may be localized within the mordenite channels having the cross section of channels 0.64 · 0.7 nm. Part of the clusters may be stabilized also on the external surface of zeolite. XPS data confirmed the presence of Au clusters in as/prepared samples along with gold ions with the binding energy of Au 4f7/2 84.3 and 87.1 eV, respectively. XRD and TEM data evidenced the noticeable formation of Au nanoparticles in the size range 1.5–17.0 nm and therefore localized on the external surface after sample calcinations at temperatures higher than 150 C. Catalytic activity of Au-zeolite is displayed by a series of temperature dependencies (light-off curves) of the CO conversion in 2CO + O2 = 2CO2 reaction with both upward and downward runs (Fig. 2). Activity of as-prepared AuM206 samples was not high, but it was stable because both runs practically coincide (Fig. 2a). The sample pretreatment in He flow at 500 C for 80 min results in abrupt catalyst activation (Fig. 2b), and four new peaks (marked by arrows) appeared on the up-warding curve of CO conversion. We emphasize that this activation after He pretreatment was reproducible. However, it disappeared in the downward curve, and the activity becomes even lower at the corresponding temperatures than that for as-prepared sample shown in Fig. 2a. Probably, the shape of light-off curve represents a superposition of several S-shape curves typical for certain active site each. It manifests transformation of active sites with temperatures. This effect of activation by He observed for Au-mordenite could be assigned to removal of carbonate species blocking active sites and/or to redox transformation of active sites. The experiments with sample pretreatment in O2 flow permitted to reveal the role of these factors. A pretreatment in O2 at 525 C should be more effective for removal of any carbonates than in He. Nevertheless, O2 pretreatment at 525 K resulted in disappearance of the low-temperature peak (Fig. 2c) not recoverable even after following re-treatment in He (Fig. 2d). This implies that redox transition of gold species can be responsible for catalyst activation/deactivation at low temperature. Partly (Audþ n ) or completely (Au0m ) reduced gold species are more active than oxidized ones. The low activity of as-prepared sample containing gold ions permits to conclude that gold cations are inactive in this process, and the gold clusters revealed by the optical data for as-prepared samples and samples calcined at low
I.V. Tuzovskaya et al. / Catalysis Communications 8 (2007) 977–980
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Fig. 2. Consequent experiments (a ! b ! c ! d) of CO conversion versus temperature over AuM206: without pretreatment (a), after treatments for 80 min: in He at 500 C (b), in O2 flow at 525 C (c) in He at 500 C (d).
temperature (curve 1 in Fig. 1) are also of low activity without He pretreatment at 500 C. FTIR study of CO adsorption (Fig. 3) sheds a light onto the nature of supposed gold species since the absorption bands of CO depend on the type of gold species [7,9]. The intensity of IR-CO adsorption bands in the region 1992–2067 cm1 corresponding to CO adsorbed on metallic gold clusters (probably negatively charged) decreased
Fig. 3. FTIR spectra of CO adsorbed at room temperature over AuM206 after sample pretreatments at 300 C in CO (fine line) and O2 (bold line).
after sample treatment in O2 while 2212 cm1 band assigned to positively charged clusters appeared [7,9]. This effect can be explained by the cluster redox transformations depending on the treatment atmosphere and temperature. These processes could be the reason of the activation and deactivation of sample in CO oxidation. The complex structure of bands can evidence the co-existence of several types of gold clusters slightly different probably in the effective charge, position on support, nuclearity, geometry, etc. Note, that the curves of catalytic activity also reveal a complicated structure (Fig. 2b). Obviously, clusters (<1.5 nm) are more sensitive to oxidation than the nanoparticles (1.5–17 nm) due to the cluster small size and stronger metal-support interaction [11]. Indeed, partial oxidation of gold clusters (appearance of partial charge) occurred after sample treatment at 300 C in O2 as it was shown by IR-spectroscopy (Fig. 3). Therefore, gold clusters and nanoparticles incorporated in silica-rich mordenite can be strongly activated in the reaction of CO oxidation by the pretreatment in He, while gold cations are inactive. Negatively charged gold clusters can be responsible for the low temperature CO oxidation being deactivated by interaction with oxygen at 525 C. Thus, the revealed effect of superposition of several S-shape light-off curves we assign to exhibition of different
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activity by Au clusters and nanoparticles of several types co-existing in one catalyst. A similar effect was observed in Ref. [6,7] for silica-poor mordenite. Acknowledgments The authors would like to express their gratitude to E. Flores, P. Casillas, F. Ruiz, E Aparicio, I. Gradilla, A. Dı´az and J. Peralta for technical assistance in experimental work. This work was supported by CONACYT No 42568Q and by PAPIIT-UNAM grants IN 109003. References [1] M. Haruta, Catal. Today 36 (1997) 153. [2] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [3] D.W. Goodman, Catal. Lett. 99 (1–2) (2005) 1.
[4] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [5] N.A. Hodge, C.J. Kiely, R. Whyman, M.R.H. Siddiqui, G.J. Hutchings, Q.A. Pankhurst, F.E. Wagner, R.R. Rajaram, S.E. Golunski, Catal. Today 72 (2002) 133. [6] A. Simakov, N. Bogdanchikova, I. Tuzovskaya, E. Smoletseva, A. Pestryakov, M. Farias, M. Avalos, in: M.W. McCall, G. Dewar, M.A. Noginov (Eds.), Proceedings of SPIE: Complex Mediums VI: Light and Complexity, vol. 5924, Bellingham, WA, 2005, p. 592410– 592411. [7] E. Smolentseva, N. Bogdanchikova, A. Simakov, A. Pestryakov, I. Tuzovskaya, M. Avalos, M.H. Farı´as, J.A. Dı´az and V. Gurin, Surf. Sci., in press. [8] E. Smolentseva, N. Bogdanchikova, A. Simakov, V. Gurin, M. Avalos, A. Pestryakov, M. Farias, J.A. Diaz, A. Tompos, Int. J. Modern Phys. 19 (2005) 2496. [9] A. Pestryakov, I. Tuzovskaya, E. Smolentseva, N. Bogdanchikova, F. Jentoft, A. Knop-Gericke, Int. J. Modern Phys. B 18 (2005) 2321. [10] L.H. Skibsted, J. Bjerrum, Acta Chem. Scand. A28 (1974) 740. [11] J.T. Miller, A.J. Kropf, Y. Zha, J.R. Regalbuto, L. Delannoy, C. Louis, E. Bus, J.A. van Bokhoven, J. Catal. 240 (2006) 222.