On the nature of active gold species in zeolites in CO oxidation

Applied Catalysis A: General 331 (2007) 121–128 www.elsevier.com/locate/apcata

On the nature of active gold species in zeolites in CO oxidation A. Simakov a, I. Tuzovskaya a, A. Pestryakov b,*, N. Bogdanchikova a, V. Gurin c, M. Avalos a, M.H. Farı´as a a

Centro de Ciencias de la Materia Condensada, Universidad Nacional Auto´noma de Me´xico, Ensenada 22800, Mexico b Tomsk Polytechnic University, Tomsk 634050, Russia c Physico-Chemical Research Institute, BSU, Minsk 220080, Belarus Received 13 March 2007; received in revised form 14 July 2007; accepted 26 July 2007 Available online 1 August 2007

Abstract It was disclosed for Au/zeolites that not one, but several active sites can be reactive in CO oxidation on the same catalyst. Au3+ cationic species do not show the activity under studied conditions. Gold nanoparticles (1.5–5 nm) are characterized by activity at high temperature. Gold clusters display activity in low-temperature region, but only after sample pretreatment in He at 500 8C. Nanoparticle active sites are stable during reaction, in contrast with cluster active sites. The last ones lost activity probably due to complete oxidation and can be reactivated by He treatment. This investigation revealed that clusters are the most active, but not stable and are easily oxidized by oxygen, while stable nanoparticles are less active. # 2007 Elsevier B.V. All rights reserved. Keywords: Gold; Zeolite; Active sites; CO oxidation

1. Introduction Small gold particles supported on different oxide carriers are known to catalyze a number of industrially and environmentally important reactions [1]. The nature of the active sites for CO oxidation, which is the most studied reaction over Au-based catalysts, has been a matter of debate, without reaching a general consensus so far [2,3]. However, the nanoparticle size is considered to play an important role by the majority of scientists. For some systems, it is clear that the size of the Au crystallites does not seem to be sufficient to explain the high activity. Thus, proposals have been made that the perimeter or gold– support interface [4], or small Au clusters that have nonmetallic electronic properties due to a quantum-size effect [5], or step sites on the surface and strain defects [6] are more important than size of Au nanoparticles between 2 nm and 5 nm. On the other hand, other groups are convinced that Au3+ is responsible for the high activity in either CO oxidation, hydrogenation of ethene, synthesis of hydrogen peroxide or water–gas shift reaction [7–10]. An intermediate model was proposed: an

* Corresponding author. E-mail address: [email protected] (A. Pestryakov). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.07.039

ensemble of metallic Au atoms and Au cations with hydroxyl ligands [11]. This model suggests that the Au cations must be stable in a reducing environment and in the neighborhood of metallic gold. It was further concluded that Au1+ would be able to satisfy these requirements, rather than Au3+ and Au+-OH has been proposed as the cationic component. It has also been suggested that the active sites for gold catalysis are anionic gold complexes AuO and AuO33 [12]. In literature, there are a few articles containing interesting results: minimum on the middle of light-off curves of CO conversion on Au–support catalysts. The curves have a shape of ‘‘smile’’ or ‘‘camel’’ [2,13]. This effect is not explained in literature. In our previous papers [14–17] we suggested that this effect is concerned with the coexistence of different types of active gold species working in different temperature ranges. We tried to find systems or/and conditions where the effect of multiplicity of active species becomes apparent. The effect of multiplicity of Au active sites in one catalyst and their activation under different conditions in some Au/ zeolites samples could partially explain the discrepancy in the nature of gold active sites described in literature. Various research groups could work under 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

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applied conditions. Therefore, in some cases, probably the results of different groups do not contradict but supplement each other. The main determining factors of formation of active gold species are: method of preparation, appropriate metal oxide support and gas pretreatments [18]. Many studies have been devoted to preparation of ultra-fine gold particles on the surface of metal oxides [19,20] and zeolites [21–23] for application in various reactions. 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 particles 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 Si/Al molar ratio [24]. The aim of the present study is the amplification and summarizing of investigation dedicated to the effect of multiplicity of Au active species in mordenites of different type. Obtaining 100% activity at room temperature was not an aim of this study, because at high conversion we cannot investigate the active sites of different nature. It does not matter, if the activity of gold is observed at room temperature or some higher, because for different reactions gold can be an active catalyst in a wide temperature range, up to 500–700 8C (for example, in alcoholselective oxidation [25]). We chose CO oxidation as simple model reaction, but the effect of multiplicity is interesting for understanding a mechanism of catalysis by gold in general. Detailed physicochemical characterization of the studied samples was carried out earlier [26]. These studies revealed that ion exchange of H+-mordenite with [Au(NH3)4](NO3)3 complex at room temperature led to stabilization of two gold species: Aun clusters formed as a result of precursor decomposition and Au3+ cations. Acid site strength did not change significantly the amount of the clusters but influenced the electronic state of gold in these clusters and redox properties of gold ions. Aun clusters are partly or completely reduced species with number of atoms to be n  8 proposed based on the theoretical calculations. They were located inside zeolite channels and are unexpectedly stable up to 500 8C. Thermal treatments with H2 and in air atmosphere leads to reduction of the rest part of Au3+ cations to nanoparticles with size 1–15 nm and located on the external zeolite surface. Easier formation of nanoparticles was observed for mordenites with weak acid sites than for mordenites with strong ones that proves their stabilization on acid centers. 2. Experimental Protonic forms of mordenites (M), produced by TOSOH Corporation, Japan, with a SiO2/Al2O3 molar ratio (MR) 15 and 206 were used. AuM was prepared by the ion exchange procedure with an aqueous solution of [Au(NH3)4](NO3)3 complex synthesized according to Skibsted and Bjerrum [27]. 30 ml of precursor solution with an Au content ca. 0.159 mol/l was added to 3 g of zeolite powder and stirred for 24 h at room temperature. Samples were washed with aqueous ammonia, keeping the same pH (pH  7) as for the ion exchange solution

to avoid hydrolysis of the used complex. The exchanged and washed samples dried in air at room temperature for 48 h were named as AuM15 and AuM206 according to zeolite MR value. It is very important to follow exactly the preparation procedure. [Au(NH3)4]3+ hydrolyzes rapidly in alkaline solution forming an orange-brown explosive precipitate known as fulminating gold [27,28]. It is formed if the ammonia solution is added too fast or in large excess. The preparation has to be done slowly drop by drop and at pH  7. Gold loadings, measured by energy dispersive spectroscopy in a JEOL 5300 scanning electron microscope equipped with a Kevex Superdry detector, were 1.7 and 1.9 wt.% for AuM15 and AuM206, respectively. Carbon monoxide oxidation was carried out in a flow Pyrex microreactor. Samples were packed in a glass reactor (0.1 g) and several of them prior to catalytic runs were pretreated in a molecular oxygen flow (25O2 vol.% in He) or in 100% helium flow with temperature increase from 25 8C to 525 8C following a ramp of 5 8C/min, kept at 500 8C for 1 h and then cooled to 25 8C. Then, the flow was changed for the feed with reaction mixture: 1 vol.% CO and 1 vol.% O2 in helium with the flow rate 40 ml/min. Catalytic tests were carried out with temperature increase and subsequent decrease within range of 25–500 8C following a ramp of 5 8C/min. The products were analyzed by means of a gas chromatograph SRI 8610C equipped with a TCD detector and two columns packed with molecular sieves and silica gel for separation of O2, CO and CO2, respectively. Catalytic activity test was carried out in several subsequent runs. All catalytic measurements were performed with one portion of sample subsequently after the other. FTIR spectra were registered using a Perkin–Elmer 2000 FTIR spectrometer. The initially obtained samples were tested after heating in vacuum at 150 8C for 1 h. To change the electronic state of gold, the catalysts were consistently pretreated in CO or O2 (100 mbar) at 300 8C for 1 h for each pretreatment. 3. Results and discussion First we tested activity in CO oxidation of as-prepared samples without any pretreatment, so the samples contained water easily adsorbed in zeolite. The data obtained during three consequent repetitions of heating within temperature interval 30–270 8C over as-prepared AuM15 and AuM206 samples are presented in Fig. 1. The activity of as-prepared samples in the first run in CO oxidation was negligible. The activity at 270 8C for AuM206 increased consistently with the number of runs, in contrast with AuM15 sample. In our previous part of publication we have described the formation of gold nanoparticles occurred for AuM206 at lower temperature than for AuM15 [26]. In Fig. 2, from the UV–vis spectra of Au-zeolites, we can observe that contribution of different gold species depends significantly on zeolite acidity. Interpretation of the absorption bands has been done in Refs. [14–17]. For both AuM15 and AuM206 samples, the intensity of the peaks corresponding to the Au3+ cations

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Fig. 1. CO conversion vs. temperature over as-prepared AuM15 (left) and AuM206 (right) samples, three subsequent repetitions of runs—RUN1, RUN2 and RUN3 from 30 8C to 270 8C and back to 30 8C.

Fig. 2. UV–vis spectra of AuM15 (A), AuM206 (B) reduced at different temperatures: 1–100 8C; 2–150 8C; 3–200 8C; 4–300 8C; 5–500 8C.

decreased and the peaks at a wavelength of about 550 nm attributed to the plasmon resonance of big gold nanoparticles appeared. However, intensive formation of gold nanoparticles on M206 is observed at lower temperature (200 8C) as compared with M15. Fig. 3 illustrates activity corresponding to RUN4 without temperature ramp at 250 8C for 3 h. One can see that AuM206 catalyst shows no change of activity at 250 8C for relatively long test time (at least 3 h). Thus, an increase of reaction time at 250 8C did not lead to activation of catalyst. Further extension of temperature interval in RUN5 up to 525 8C changed catalytic activity of AuM15 and AuM206 samples in a different manner (Fig. 4). The observed activity of AuM206 at 250 8C in RUN3 is 21%; in RUN4, 17%; in RUN5, 33%. So, AuM206 catalyst was slightly deactivated after RUN3 till 250 8C and then was activated by RUN5 while it was heated

Fig. 3. CO conversion over AuM206 catalyst vs. time in stream at 250 8C.

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Fig. 4. CO conversion vs. temperature over AuM15 (left) and AuM206 (right) samples.

with the reaction mixture up to 500 8C. No difference was observed in the curves for run with raising and following run with decreasing the temperature for AuM206 catalyst, while for AuM15 in the curve for run with decreasing temperature (subsequent after raising temperature), low temperature shift of activity was observed, indicating some transformations of active sites in AuM15 sample. The stabilization of gold species in ionic form seems to be stronger in AuM15 catalyst due to their interaction with stronger acid sites in H-mordenite-15 than with those in H-mordenite-206 [25,26]. So, for the case of AuM15, it is necessary to use a high-temperature treatment in order to convert inactive ionic gold species Au3+ into the active sites. Note that further repetitions of RUN5 did not change significantly the activity level for any of both AuM15 and AuM206 catalysts. This situation implies the formation of the active sites to be completed after the first run carried out up to 500 8C (RUN5).

Sample pretreatment in He results in further changes in catalytic behavior of the tested samples. As presented in Fig. 5a, after sample pretreatment in He at 500 8C for 80 min, the catalytic activity of both samples was improved, particularly in the low-temperature region. For AuM15 one peak with maximum at 200 8C was observed. However, this peak disappeared in the curve of subsequent temperature decrease. Repetition of experiment with pretreatment in He revealed almost reversible sample activation (Fig. 5b). Further experiment without He pretreatment leads to almost complete disappearance of the described activation (Fig. 5c), which, however, was again recovered after He pretreatment (Fig. 5d). So He pretreatment is necessary to activate low-temperature sites. Most complex dynamics of CO conversion with temperature was obtained for AuM206 sample, where several peaks with

Fig. 5. CO conversion vs. temperature over AuM15 four subsequent repetitions of runs from 30 8C to 500 8C and back: (a) after treatment with He at 525 8C for 80 min, (b) treatment with He at 525 8C for 320 min; (c) following run in reaction mixture, (d) treatment with He for 60 min at 525 8C. Dot lines correspond to curves (a).

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maxima at temperatures of 40 8C, 150 8C and 250 8C were detected (Fig. 6a). Figs. 5 and 6 demonstrate that at higher temperatures the catalyst did not suffer critical changes in the structure of the active sites. Indeed, the active sites were deactivated after run at 500 8C. But several sequential post-treatments with He at 500 8C regenerated the low-temperature activity. It means that these active sites are rather stable to aggregation, probably, due to their encapsulation inside zeolite structure. And their deactivation/ reactivation, evidently, is bound up with the changes in electronic state of the active sites. The shape of light-off curve represents a superposition of several (at least two) S-shape curves typical for certain active species (Fig. 5a). This permits us to suggest the coexistence of several types of gold active species in the samples. According to our physicochemical studies of the samples [26], a few different gold states exist in the catalysts, namely Au3+ and Au+ ions, Aund+ clusters and neutral Aum

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nanoparticles. Au3+ seems to be inactive because of low activity of as-prepared samples. The other gold states may be considered as active species in different temperature ranges. In contrast with He pretreatment, sample pretreatment in O2 results in irreversible disappearance of low-temperature activity for both AuM15 and AuM206 catalysts (Fig. 6 b, c). The low-temperature activity could not be recovered by He subsequent pretreatment. Blank activity is also observed on pure zeolites in CO oxidation at high temperatures. However, it did not make significant contribution to total catalytic activity of Au-zeolite samples. For example, at 350 8C, CO conversion on pure zeolites was 22% for M15 and 20–31% for M206 while Auzeolite showed 60% and 90%, accordingly. In total, low-temperature catalytic activity of the samples is not high—40% at maximum. In comparison with TiO2 or Fe2O3, zeolites are not optimal supports for gold. However,

Fig. 6. CO conversion vs. temperature over AuM15 and AuM206 samples three subsequent repetitions of runs from 30 8C to 500 8C and back after pretreatments: in He at 525 8C for 80 min (a), in O2 (air) at 525 8C for 80 min (b), in He at 525 8C for 80 min (c).

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obtaining 100% activity at room temperature was not an aim of this study, because at high conversion we cannot investigate the nature of active sites. In contrast, lower activity allows differentiating several types of active sites of gold in CO oxidation process. It is reasonable to propose that some transformation of the gold species such as reduction, reoxidation or carbon species removal may occur during high-temperature He pretreatment. Activation of catalyst due to resintering is less probable as the process of gold aggregation at high temperatures is irreversible at these conditions [26]. Reduction or oxidation of the species seems to be expected during pretreatment in helium or oxygen, respectively. These suggestions may be confirmed by results of IR-spectroscopy presented in Fig. 7. The IR spectra of the as-prepared samples, evacuated at 150 8C, show a number of absorption bands (a.b.) in four main regions at 1950–2080, 2090–2120, 2125–2140 and >2150 cm 1. According to the literature data [29–34] and our previous studies [35–38], the affiliation of the bands in the first two regions (1950–2080 cm 1 and 2090–2120 cm 1) with carbonyls of the Au0–CO type on metallic gold clusters is obvious. Some authors have also supposed that such clusters with low-frequency signals of adsorbed CO may have some negative charge as a result of metal–support interaction [33,34]. The signals in the two highfrequency ranges (2125–2140 and >2150 cm 1) can be attributed to Aud+–CO and Au+–CO, respectively [28–38]. However, a relatively weak CO adsorption on the pure zeolite attributed to CO adsorbed on Bro¨ensted and Lewis acid and base sites [39,40] or CO condensed in zeolite pores [28] is also

observed in this region, so an exact interpretation of the absorption bands is sometimes complicated. FTIR spectra of Au/mordenite samples exhibit a rich set of signals that is not typical for CO adsorbed on supported gold catalysts, especially for the region <2100 cm 1. Carbon monoxide is very weakly adsorbed on metallic gold (and on silver as well) because of some features of s–p binding in M0– CO for Ag and Au in comparison to other noble metals (Pt, Pd, Ru, Rh, Cu) [41]. Only highly dispersed gold clusters or atoms can be sites for CO adsorption; on the gold ionic states CO adsorption is much stronger. Evidently, the rich number of CO absorption bands is caused by the peculiarities of this type of zeolite support—the possibility to (i) produce metal nanoparticles with rather narrow size distribution, as well as ions isolated in the zeolite structure, and (ii) to form two different types of supported particles (inside zeolite channels and on the external surface of zeolite) [26]. The contribution of absorption bands of CO adsorbed on the pure mordenites complicates the interpretation of some signals; however, a number of conclusions can be made on the basis of the obtained results. After heating the catalysts with CO, the intensity of the signals in the low-frequency ranges increases abruptly. Reoxidation of the samples with O2 leads to decrease in the signals of the neutral complexes and rise of Au+–CO bands. It is important to note that, according to FTIR measurements as well as other spectroscopic data [26], soft redox treatments do not change significantly the contribution of gold charged clusters Aund+. These states remain in the samples due to good stabilization in the zeolite structure and strong metal-support interaction. Charged clusters Aund+ are formed under both

Fig. 7. FTIR spectra of CO adsorbed over mordenites and Au-mordenites after redox pretreatments.

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reducing and oxidizing treatments of the catalysts as a result of partial reduction of gold ions or partial oxidation of metallic particles. Strong redox treatments (>300 8C) cause almost complete reduction or oxidation of gold species. So, we can attribute the disappearance of the low-temperature activity after strong oxidizing treatment with O2 at 500 8C to the process of gold clusters oxidation (effective charge increase). Obviously, low activity at RT of the as-prepared samples is caused by the same reason—ionic gold Au3+ is not active in lowtemperature CO oxidation. Otherwise, thermal treatments in He create the conditions of soft thermoreduction of gold ions in inert medium. The enhancement of catalytic activity after treatment with He could be assigned to transformation of gold ions and/or oxides to Aund+ or Aun0 clusters. However, we cannot exclude another factor, which could be responsible for catalyst activation—thermodesorption of the carbonate species blocking the catalyst active sites. The experiments with sample pretreatment in O2 flow permitted to reveal the role of these factors. A pretreatment in O2 at 525 8C should be more effective for removal of any carbonates than in He. Nevertheless, O2 pretreatment at 525 8C resulted in disappearance of the low-temperature peak (Fig. 6) not recoverable even after following re-treatment in He. This implies that redox transition of gold species can be responsible for catalyst activation/deactivation at low temperature. Comparison of physicochemical and catalytic results permits to conclude that partly reduced gold species (Aud+, 0 < d  1) are more active than the oxidized ones (Au3+). Partial effective charge d+ on gold species seems to be important for low-temperature CO oxidation. According to literature data [11,42–44] and our previous studies [14–17], gold nanoparticles Aum (>1 nm) are not active in the lowtemperature region. However, they may be responsible for hightemperature CO oxidation (>400 8C). Thus, zeolite supports due to their unique structure allowed us to observe the effect of multiplicity of active sites of gold catalysts. Of course, on zeolites the active species of gold at lower temperature are very sensitive to treatments and are easily deactivated. However, these active species can be stabilized using modifying additives of Fe or Ni oxides. As we demonstrated in articles [16,17] CO conversion on Au/Fe/ mordenite can reach 50–60% at RT along with apparent effect of two-range activity. The obtained data can be one of possible reasons of the discrepancy in the literature concerning the nature of active species showing that in one Au catalyst several species can be active under different conditions. The effect of multiplicity can also explain unusual shape of the light-off curves in some studies [2,33,45]. 4. Conclusions 1. The activity of Au/mordenite catalysts without any treatment is characterized by normal S-shape activity dependence versus temperature while volcano shape dependence appeared additionally after He treatment manifesting the existence of several types of active species in CO oxidation.

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