Influence of sodium on activation of gold species in Y–zeolites

Available online at www.sciencedirect.com Catalysis Communications 9 (2008) 1277–1281 www.elsevier.com/locate/catcom Influence of sodium on activation of gold species in Y–zeolites A. Simakov a, I. Tuzovskaya a, N. Bogdanchikova a, A. Pestryakov b,*, M. Avalos a, M.H. Farias a, E. Smolentseva a,b a Centro de Ciencias de la Materia Condensada, Universidad Nacional Autónoma de Mexico, Ensenada, B.C. 22800, Mexico b Tomsk Polytechnic University, Tomsk 634050, Russia Received 27 September 2007; received in revised form 13 November 2007; accepted 19 November 2007 Available online 4 December 2007 Abstract Catalytic tests of the Au–zeolites pure or doped with Na revealed the existence of two types of active sites of gold in CO oxidation – 0 partly charged gold clusters Audþ n (low-temperature activity) and gold nanoparticles Aum (high-temperature activity). The data obtained demonstrates clear direct dependence of gold species reactivity on zeolite acid properties (Si/Al molar ratio and Na addition). Na modified Au–zeolites with different Si/Al molar ratio were found to be activated in CO oxidation in different degrees due to diverse redox state of supported gold species. The revealed effect of activation of one of two active species in CO oxidation ðAudþ n Þ is promising for improvement of activity and stability of the gold–zeolites catalysts. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Gold; Clusters; Nanoparticles; Active sites; CO catalytic oxidation 1. Introduction It is well known that nanosized gold supported on certain metal oxides exhibits high catalytic activity in different reactions, whereas bulk gold and large gold particles (>5 nm) are inactive [1–10]. Although the catalytic activity of gold catalysts in the low-temperature CO oxidation has been intensively studied during the last decade, the nature of the active species is still discussed. Some authors proposed that the reaction takes place at the gold/metal oxide interface and that the metal oxide could act as a source of oxygen [11–13]. Also, the electronic structure of gold in active catalysts is unclear. Different authors assigned gold active sites to Au3+, Au+ ions [14,15] or metallic gold [5,11,16,17]. Gold incorporation into zeolite possessing adjustable acidic properties and regular molecular-size pores in the crystalline lattice provides inclusion of metal ions, which, after subsequent transformations, become ultra fine parti* Corresponding author. Tel.: +7 3822 563 861; fax: +7 3822 563 637. E-mail address: pestryakov2005@yandex.ru (A. Pestryakov). 1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.027 cles and clusters. The contribution, charge and stability of the gold species formed in the zeolite depend on the concentration and strength of acid centers of the zeolite and can be regulated by varying zeolite composition [18–23]. Modification of zeolites by different metal ions is one of the methods of improvement of electronic and catalytic characteristics of supported gold. For example, Fe and Ni oxides stabilize gold clusters in zeolites [24,25]; Na is known as electron-releasing modifier on different supports [26], etc. In our previous studies we have revealed that on Au–zeolites samples several gold active sites can coexist (effect of multiplicity) and have started search of parameters responsible for active sites activation and deactivation [24,25,27,28]. These results show that apparently contradictive data concerning nature of gold active sites published in literature, supplement each other. A multiplicity effect was registered after He pretreatment at 500 °C on Au–mordenite and after addition of Fe3+ and Ni2+ promoters on AuY. The aim of the present work is to extend the search of conditions, where the effect A. Simakov et al. / Catalysis Communications 9 (2008) 1277–1281 Protonic form of Y–zeolites (TOSOH Corporation, Japan) was used. SiO2/Al2O3 molar ratio (MR) of zeolites was 5.6, 14.6 and 40.4. Au–zeolites were prepared by the cation exchange procedure of zeolites with an aqueous solution of [Au(NH3)4](NO3)3 complex synthesized according [29]. 30 ml of [Au(NH3)4](NO3)3 solution (Au content 0.159 mol/l) was added to 3 g of zeolite powder and stirred for 24 h at room temperature. Then, samples were washed with distilled water and dried in air at room temperature. The dried samples were denoted as AuY5, AuY14, AuY40 according to MR of utilized zeolite. Na-modified zeolites (AuY–Na) were obtained by treatment of dried sample with 1 M NaNO3 water solution for 24 h at room temperature. Gold weight loading for the samples evaluated by atomic emission spectroscopy with inductively coupled plasma (AES-ICP) on OPTIMA 4300 DV of Perkin Elmer were ca. 1.3% for all samples. Na content was ca. 0.6% for AuY14 and AuY40 and 1.7% for AuY5. CO oxidation was carried out in a flow reactor. Prior to catalytic runs, freshly prepared sample (0.1 g) packed in a glass U-shape reactor was preheated at 250 °C in oxygen flow for 30 min. Catalytic tests were carried out using gas mixture of UHP grade with composition: 1 vol.% CO and 1 vol.% O2 in He with a flow rate of 40 ml/min. Catalytic runs were performed with temperature increase and decrease within a range of 25–500 °C with a ramp of 5 °C/min. Thermo-programmed reduction (TPR) was measured in an AMI 1 (Altamira-Instruments). To record TPR profile 0.15–0.2 g of sample was heated at 20 °C/min from 25 °C to 550 °C in flowing H2/Ar (1:9) mixture. UV–visible diffuse reflectance spectra were recorded by means of a CARY 300 SCAN (VARIAN) spectrophotometer. The spectra were obtained by subtraction of zeolite spectra from the corresponding Au–zeolite ones. Calcination of Au–zeolite samples before optical study was carried out for 2.5 h at selected temperature (100, 200, 300, 400, 500 °C). 3. Results and discussion Light-off curves of CO oxidation for AuY–zeolites before and after Na modification are presented in Fig. 1. The temperatures of 50% CO conversion for AuY5, AuY14 and AuY40 were 413, 390 and 355 °C, respectively. So, activity increases with Si/Al molar ratio rise. This tendency of dependence of catalytic activity vs. Si/Al molar ratio for AuY–zeolites is similar as that for Au–mordenites [27,28]. CO conversion. parts 2. Experimental 1.0 A Au-HY5.6 Au-HY5.6-Na 0.8 0.6 0.4 0.2 0.0 0 100 200 300 400 1.0 CO conversion. parts of multiplicity of active sites is manifested, by varying zeolite acidity properties (zeolite modification with Na and zeolite SiO2/Al2O3 molar ratio). B Au14 AuY14-Na 0.8 500 0.6 0.4 0.2 0.0 0 100 200 300 400 1.0 CO conversion. parts 1278 C AuY40 AuY40-Na 0.8 500 0.6 0.4 0.2 0.0 0 100 200 300 400 500 o Temperature. C Fig. 1. Light-off curves of CO oxidation over AuY–zeolites with different Si/Al molar ratio before and after addition of Na. The significant difference in shape of light-off curves consisted in the appearance of two temperature regions for AuY14, a low-temperature range (<250 °C) and a high-temperature one (>300 °C). Earlier, we explained this effect for Au–zeolite by the existence of two types of catalytically active gold sites: gold clusters (low-temperature) and gold nanoparticles (high-temperature) [27,28]. Appearance of a low- temperature peak in light-off curves for AuY14 revealed that the activation of gold clusters occurs selectively on Y–zeolite with medium concentration of protons. Hence, the acid properties of AuY14 are optimal for activation of gold clusters in CO oxidation under studied conditions. The position of the low-temperature peak is observed at approximately the same temperature range as for Au samples supported on mordenite-type zeolites with MR equal to 15 and 206. 1279 A. Simakov et al. / Catalysis Communications 9 (2008) 1277–1281 H2 uptake, µmol H2/g/s 0.6 220 AuY5 AuY5-Na 0.5 0.4 204 0.3 0.2 0.1 0.0 H2 uptake, mkmol H2/g/s 0.6 0 100 200 300 400 500 AuY14 AuY14-Na 0.5 0.4 165 0.3 195 0.2 245 0.1 0.0 0.6 H2 uptake, mkmol H2/g/s Influence of modification with Na on the character of the light-off curves depends on Si/Al molar ratio. For AuY40, no change was observed. For AuY14, after Na addition conversion became ca. 0.06 higher for practically the whole temperature range, with a small increase of intensity of the low-temperature peak. It exhibits that, on AuY14 after modification with Na, both active sites (clusters and larger particles) were slightly activated. The most profound change was found for AuY5 sample, in particular for the up-warding curve when the temperature of 50% CO conversion decreased more then 150 °C. For this sample, CO conversion increased ca. 0.6 at 250 °C and ca. 0.5 at higher temperatures. In this work we maintained the standard conditions of sample preparation in order to keep a similar Au concentration (1.3 wt.%) in all samples. Under these conditions similar Na concentration (0.6 wt.%) was measured for AuY40 and AuY14, but enhanced Na concentration (1.7 wt.%) for AuY5. The influence of MR on the effect of Na addition can be seen by comparing samples supported on Y40 and Y14, where the concentration of Au and Na were similar. For AuY5, a very drastic activation effect was observed after Na addition. For this sample, two parameters (MR and Na concentration) can influence the electronic state of gold clusters. At this moment we cannot answer which parameter is responsible for the drastic activation effect for AuY5. More necessary experiments are in progress [28]. Introduction of sodium into zeolites caused similar changes in light-off curves as sample treatment with He at high-temperature [28] and we suggest that both parameters change the electronic state of gold species. The changes in electronic state of gold after sodium-exchange could be confirmed by TPR data presented in Fig. 2. All TPR profiles for AuY samples are characterized by the presence of non intensive peak at ca. 460 °C, which corresponds probably to reduction of gold ions present in negligible quantities in sodalite cages [27,30]. The profile in the temperature range 100–300 °C could be deconvoluted into two separate peaks with temperature maxima at 180 and 230 °C for AuY40, into three peaks with temperature maxima at 165, 195 and 245 °C for AuY14 and into three peaks with temperature maxima at 204, 220 and 250 °C for AuY5. These peaks could be assigned to the reduction of gold on the external surface and in the big cages of the Y–zeolite [31]. Two low-temperature peaks can be partially due to a stepwise reduction of Au3+ to Au+ and then Au+ to Au0 [32]. Na addition practically does not change TPR profile for AuY40 sample, while remarkable changes profiles of AuY14 and AuY5 samples. As a result of Na addition, the intensity of low-temperature peaks increases; the intensity of peaks at 250 and 460 °C decreases. The observed effects can be explained by different reasons. Sodium cations seem to protect gold species from interaction with zeolite hydroxyls and stabilize gold species in a definite state and on specific sites. On the other hand, Na additives can favor 0 100 200 300 400 500 AuY40 0.5 AuY40-Na 0.4 0.3 180 0.2 230 0.1 0.0 0 100 200 300 400 500 Temperature, oC Fig. 2. TPR profiles of AuY–zeolites with different Si/Al molar ratio before and after addition of Na. Before TPR experiments as-prepared samples were purged with Ar at room temperature only. reduction of gold cations due to their electron-donor action [26]. UV–visible spectroscopy gives information about changes in electronic state of gold in the studied samples. In Fig. 3, the spectra for AuY5 and AuY5–Na during calcination are presented. For samples supported on Y14 and Y40, the spectral behavior was very similar. According to [30–38] peaks at 200–230 nm can be related to Au3+ or Au+. The pronounced maxima in the range 500–550 nm are ascribed to a collective oscillation of conduction electrons in response to optical excitation (to the plasmon resonance of gold metal particles) [33– 38]. The bands at 230–360 nm are in the region of Ligand-to-Metal charge transfer transitions of Au(III) and Au (I) compounds, and, in this region, absorption by few-atomic gold clusters due to intermolecular transitions of electrons is also observed [30,33,35]. 1280 A. Simakov et al. / Catalysis Communications 9 (2008) 1277–1281 0. 8 0.8 AuY5-Na AuY5 0. 7 0.7 5 Absorbance, a.u. 0. 6 3 6 0.6 4 0. 5 0.5 3 0. 4 0.4 0. 3 6 1 0. 2 4 5 0.3 2 0.2 2 1 0. 1 0. 0 200 0.1 300 400 500 600 700 800 0.0 200 300 400 500 600 700 800 Wavelength, nm Fig. 3. UV–vis spectra of AuY5 before and after Na modification, calcined in air at temperatures: 100 (1), 150 (2), 200 (3), 300 (4), 500 °C for 1 h (5), and at 500 °C for 3 days (6). From Fig. 3, it can be seen that after modification of the samples with sodium, the thermostability of the system changes. After sodium addition, the temperature of appearance of nanoparticles was decreased from 200 °C to 150 °C. Apparently the replacement of zeolite protons with sodium cations promotes easier reduction of ionic gold due to the absence of the electron-seeking effect of the neighboring protons. Sodium has an electron-donating property [26] that favors easier reduction of gold ions. The obtained optical results agree with results of TPR. After calcination of AuY5 at 500 °C for 3 days we can observe a decrease of the plasmon peak intensity, the appearance of a shoulder at ca. 600 nm and a simultaneous notable increase of intensity in region of Au3+ cations. This situation evidences partial oxidation of Au nanoparticles of definite size (corresponding to plasmon at 540 nm), while under these conditions Au nanoparticles with plasmon peak at 600 mn do not change. For the sodium-exchanged sample after calcination of the sample at 500 °C for 3 days, no significant changes were observed. Hence, the replacement of protons for sodium decreases the ability of gold nanoparticles to be oxidized. These results agree with a high ability of protons (Brønsted acid sites) to oxidize the metal particles. Obtained results extend conditions where the effect of multiplicity of active sites on one gold catalyst (revealed in our previous works for Au–zeolites) is manifested. In previous studies, a multiplicity effect was registered after He pretreatment at 500 °C on Au–mordenite and after addition of Fe3+ and Ni2+ promoters on AuY and Au– mordenite. In this work, AuY samples exhibited this effect after sodium modification and MR increase. Na addition leads to easier reducibility of oxidized states of gold. With increase MR (OH group concentration in zeolite), the multiplicity effect and reducibility of gold species after Na addition was better observed. It is suggested that the change of state of gold by Fe, Ni, Na addition and He pretreatment brings to change of gold cluster charge, which influences their activity. Optimum charge is necessary for maximum activity of Audþ n clusters, which are active at low-temperatures, but are very sensitive. Extreme sensitivity of clusters to additives and pretreatments, their easy activation and deactivation are due to their very small size. It is possible to suggest that the sensitivity of gold catalysts to additives and pretreatments described in the literature is due to sensitivity of small clusters (61 nm), which are the active sites at low-temperature. On the contrary, the nanoparticles are only spectators at low-temperature. Small clusters very often accompany nanoparticles, but nanoparticles are easier to register than clusters, therefore very often the study of clusters does not receive sufficient attention of researchers. The revealed effect of activation of one of two active species in CO oxidation (Audþ n clusters) on AuY–Na is promising for improvement of activity and stability of the Au– zeolites catalysts. Acknowledgements The authors would like to express their continued appreciation to Dr. Natalia Boldyreva, Pedro Casillas, Israel Gradilla, Juan Antonio Peralta, Margot Sainz, Eloisa Aparicio and Eric Flores for their highly professional and substantial technical support on the work presented in this paper. This research was supported by CONACYT, Mexico through grant No 42658Q and by PAPIIT-UNAM, Mexico through grant IN 120706. References [1] G.C. Bond, C. Louis, D.T. Thompson, Catalysis by Gold, Imperial College Press, London, 2006. A. 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