TiO2 catalysts

Catalysis Today 244 (2015) 146–160

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On the nature of the active Au species: CO oxidation on cyanide leached Au/TiO2 catalysts Patrick Kast 1 , Gabriela Kuˇcerová, R. Jürgen Behm ∗ Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany

a r t i c l e

i n f o

Article history: Received 6 April 2014 Received in revised form 22 May 2014 Accepted 1 June 2014 Available online 28 July 2014 Keywords: Gold catalysis CO oxidation Active Au species Cyanide leaching Au TiO2

a b s t r a c t In order to learn more about the nature of the active Au species for CO oxidation on TiO2 supported Au catalysts, we studied the influence of different gold oxidation states on the CO oxidation activity at 80 ◦ C on structurally well defined Au/TiO2 (P25) catalysts. For that purpose, the metallic Au0 nanoparticles were selectively removed from the catalyst by cyanide leaching, while dispersed ionic Aun+ species would remain on the catalyst. The catalysts were prepared by deposition-precipitation. The composition of the active Au phase before leaching was varied by either using a fresh catalyst or calcining it in an O2 /N2 mixture, and by applying different conditioning procedures after leaching. The size of the gold particles and their chemical state were characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), the CO adsorption and oxidation behavior was evaluated by kinetic measurements and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. In the latter measurements, we did not detect any influence of ionic Au3+ on the reaction. Metallic Au0 nanoparticles, obtained mainly by conditioning in O2 atmosphere at elevated temperatures (400 ◦ C), but also during the CO oxidation reaction on a N2 pre-treated or fresh catalyst, seem to be a requisite for a high CO oxidation activity of Au/TiO2 catalysts. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Metal oxide supported Au catalysts have attracted large interest in recent decades because of their high activity in a variety of oxidation and reduction reactions already at comparatively low temperatures [1–3]. Nevertheless, fundamental mechanistic questions are still unresolved and debated controversially. One of them is the nature of the active Au species in these catalysts, where positively charged Au+3 or Au+1 /Auı+ (see below) species [4–6], negatively charged Auı− species [7,8], metallic Au0 nanoparticles [9–11], or a combination of positively charged and metallic Au species [12–15] were proposed as active Au species. These proposals were based either on a combination of spectroscopic or microscopic data with kinetic measurements [10,11], or on kinetic/spectroscopic measurements performed on Au catalysts where the Au0 nanoparticles were selectively removed by cyanide leaching [4–6,9,16–23]. The actual results of these studies were rather controversial [5,9,17,20]. For example, the activity of a

∗ Corresponding author. E-mail address: [email protected] (R.J. Behm). 1 Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry, Faradayweg 4-6, D-14195 Berlin, Germany. http://dx.doi.org/10.1016/j.cattod.2014.06.021 0920-5861/© 2014 Elsevier B.V. All rights reserved.

Au/CeO2 catalyst in the water gas shift (WGS) reaction was reported to remain unchanged upon leaching, which led the authors to the conclusion that nonmetallic gold species strongly associated with the CeO2 surface are responsible for the activity [4,5]. For a similar type of catalyst and the same reaction, Karpenko et al. observed a significant reduction in activity as well as the presence of both Au3+ and Au0 species after leaching and concluded that metallic particles contribute significantly to the water gas shift activity of these catalysts [18]. Similar results and conclusions were reported by Kim et al., who explicitly stated that the activity of Au0 nanoparticles (in terms of turnover frequencies) is much higher than that of the atomically dispersed ionic Au species [17]. Considering the CO oxidation reaction on metal oxide supported Au catalysts, Allard et al. reported that leaching of a Au/Fe2 O3 catalyst drastically decreased its activity and concluded that the CO oxidation activity is associated with metallic Au nanoparticles [20]. Manzoli et al. demonstrated that Au0 species are much more active for the CO oxidation over Au/CeO2 catalysts than positively charged gold species [19]. Raphulu reported that extended cyanide leaching (1 h) of a Au/TiO2 catalyst which was dried at 120 ◦ C over night prior to leaching, led to a catalyst with no measurable activity (Au loading <0.1%), while for shorter leaching periods the (Au mass normalized) CO oxidation activity was higher than that of the non-leached parent catalyst [16]. He proposed that the

P. Kast et al. / Catalysis Today 244 (2015) 146–160

activity of the catalyst depends on the simultaneous presence of metallic Au0 nanoparticles and positively charged Auı+ species. This is rather similar to the proposal by Bond, who suggested that the active site on Au/TiO2 catalysts consists of an ensemble of Au(OH)3 and metallic gold, and thus requires both Au0 nanoparticles and Au3+ species [3]. Other examples include the methanol steam reforming reaction or the hydrogenation of 1,3-butadiene. For the first reaction, Yi et al. concluded that on nanonastructured Au/CeO2 catalysts positively charged Au ions and clusters (<1 nm, TEM invisible), dispersed mainly on the {1 1 0} faces of ceria nanorods, represent the active Au species, while Au0 nanoparticles (<3 nm) on the {1 0 0} surfaces of ceria nanocubes are inactive [24]. For 1,3-butadiene hydrogenation, Guan et al. reported that after leaching of a Au/CeO2 catalyst, the resulting catalyst with Au0 clusters, formed by reduction of atomically dispersed Au3+ species, exhibits a very high intrinsic activity (normalized to the Au mass), which is at least one order of magnitude higher than that of the nanometer-sized gold particles in the non-leached parent catalyst, and concluded that the dispersed Au0 clusters are the active Au species [21]. A major problem in these measurements on leached catalysts and also a possible reason for the apparent discrepancies between different studies was the wide variety of treatments before and after leaching, which may have considerable consequences on the state of the catalyst at the beginning of the reaction. Furthermore, modifications of the catalyst, e.g., of the Au oxidation state or Au particle size, which may occur during reaction, had hardly been considered. In an effort to obtain a more rational understanding of these effects, we systematically investigated the influence of different Au oxidation states in Au/TiO2 catalysts on the CO oxidation reaction, focusing on the effect of different pre-treatment and conditioning procedures before and after catalyst leaching. For this purpose we prepared a series of Au/TiO2 catalysts by deposition-precipitation, which were then pre-treated/conditioned in different ways. Using either a fresh catalyst or a catalyst calcined in O2 at 400 ◦ C, the metallic Au0 nanoparticles were removed by cyanide leaching, following the procedure introduced by Fu et al. [4]. This allowed us to separate contributions from metallic Au0 nanoparticles from those of Au1+ /Auı+ species, or ionic Au3+ species, on the CO oxidation activity. It should be noted here that Au1+ species, which were initially identified as such due to their shift in Au(4f) binding energy in core level spectra, were later attributed to very small sub-nanometer gold species, where the shift in binding energy was (largely) associated with final state effects [25]. This explanation fits well with recent high resolution scanning transmission electron microscopy (STEM) results [26]. A slight positive charge cannot be ruled out and henceforth these Au species shall be termed as Auı+ species or as sub-nanometer Au clusters. After leaching, the catalysts were either used freshly, after drying in air at room temperature, or exposed to a final conditioning procedure, which involved either drying in N2 at 100 ◦ C (N100), or calcination in O2 at 400 ◦ C (O400), or a combination of both procedures (N100 + O400). The conditioning procedure, which is known to play a vital role for the activity and stability of the catalyst, e.g., by influencing the local structure of the oxide, the nature/abundance of defect sites, and the formation of metallic Au0 nanoparticles, was followed by the reaction measurements. The overall Au loading, the Au0 particle size/size distributions and the relative contents of different gold species were measured at different stages of the sequence of processing steps by inductively coupled plasma optical emission spectroscopy (ICP-OES), by transmission electron microscopy (TEM), and by X-ray photoelectron spectroscopy (XPS), respectively. The influence on the activity and CO adsorption was evaluated in kinetic measurements performed under differential reaction conditions and by in situ diffuse

147

reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. Finally, we will briefly discuss the implications of these findings on the reaction mechanism and the nature of the active Au species for CO oxidation under present reaction conditions. 2. Experimental 2.1. Catalyst preparation The Au/TiO2 parent catalysts were prepared by a depositionprecipitation method. The TiO2 (P25, Evonik) support material was suspended in water at 60 ◦ C, subsequently the gold precursor (HAuCl4 ·3H2 O, 99.5%, Merck) was added drop-wise under stirring, while keeping the pH value between 5 and 6 by addition of 0.1 M NaOH solution. After 40 min, the Au/TiO2 catalyst was filtered and thoroughly washed with distilled water (1000 ml). Finally, the catalyst was dried in vacuum over night (‘fresh catalyst’). For removal of the metallic gold, the catalysts were leached in an aqueous solution of 2 wt.% NaCN (J.T.Baker) under O2 addition and constant stirring for 30 min at pH 12. After filtration, the catalysts were thoroughly washed and then dried in vacuum over night, similar to the procedure after catalyst preparation. Different conditioning procedures were applied before the kinetic and in situ infrared measurements. For further drying, the fresh catalysts were flushed for 5 min with 20 N ml/min of N2 , heated up to 100 ◦ C in 10 min and then dried for 15 h under a N2 stream, where the N2 was passed over a water-filter (CP17973 GC–MS filter, Varian Inc.) (N100). For calcination, the catalyst was heated up to 400 ◦ C in N2 , calcined for 30 min in 20 N ml of 10% O2 in N2 , and then cooled down in N2 to the reaction temperature (O400). 2.2. Characterization The amount of Au in the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Horiba Jobin Yvon Inc.). Transmission electron microscopy (TEM) measurements were performed on a Philips CM 20 microscope (200 keV). To statistically evaluate the mean Au particle size, at least 400 particles were characterized for each sample and the dispersion was calculated. X-ray photoelectron spectroscopy (XPS) was applied to determine the Au content of the surface region and the oxidation state of the gold species, using a PHI 5800 ESCA system (Physical Electronics). To remove charging effects, all binding energies were calibrated to that of the Ti(2p3/2 ) signal fixed to 459.2 eV, the literature value for anatase and rutile [27]. Peak-fitting was performed using public software (“XPSPEAK 4.1”, by R. Kwok) after a Shirley-type background subtraction. The relative abundances of ionic Au3+ and Au␦+ species as well as metallic Au0 nanoparticles were determined from the Au(4f) peak. For fitting of the Au(4f) signal, special care had to be taken, since on the leached catalysts the amounts of remaining Au and hence the Au related signal intensities were very low. Surface charging effects, differently charged (Au3+ , Au␦+ , Au0 ) and differently sized (Au0 , Au␦+ ) gold species as well as an additional background feature in the Au(4f) energy region (Fig. 5), which was suspected to be a combined excitation of a Ti(3s) and a TiO2 plasmon resonance [28], had to be taken into account. The energy separations between metallic Au0 and Au␦+ and between Au0 and Au3+ signals were kept constant at 0.6 eV and 1.9 eV, respectively. The Gauss–Lorentzian relationship was fixed at 20% for all Au species. Spin-orbital-splitting for the Au(4f) orbital was fixed at 3.67 eV and the 4f5/2 : 4f7/2 intensity ratio was set to 3:4 [27]. The peak width (FWHM) was kept near 0.9, 2.3 and 1.9 eV for Au0 , Au3+ and Au␦+ signals, respectively. Small changes in the parameters had only negligible effects on the intensity ratios. For

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reasonable fits of the Au(4f) signals of the low Au loading catalysts, the plasmon peak described above at 87.0 eV with a fixed FWHM of 5 eV and a constant intensity ratio to the Ti(2p) signal of about 0.01 had to be included as well. This plasmon signal was not included in the fit of the parent catalyst 1, due to the much higher Au intensity (15 times higher) in comparison to the plasmon intensity. The Au(4f) fits allowed us to identify the influence of the different conditioning treatments and the exposure to the CO oxidation reaction atmosphere on the gold species. 2.3. Activity measurements For the kinetic measurements, we used a quartz tube micro reactor (i.d. 4 mm) located in a ceramic tube furnace and connected to a NiCr–Ni (type K) thermocouple for temperature control. The measurements were performed at atmospheric pressure at 80 ◦ C. To work under differential reaction conditions (conversion below 20%), typically 60 mg catalyst powder were diluted with ␣-Al2 O3 , which is inactive for the CO oxidation reaction under present reaction conditions. The catalyst bed was fixed with quartz wool from both ends. As reaction gases, we used high purity gases from Westphalen (CO 4.7, N2 6.0, H2 5.0, and O2 5.0). A reaction gas mixture of 1% CO, 1% O2 and 98% N2 was adjusted by mass-flow-controllers. All reaction measurements were carried out with a total gas flow of 60 N ml/min. The reaction gases were dried by passing over an interconnected CP17973 GC–MS filter (Varian Inc.). The analysis of reaction gases was performed by on-line gas chromatography (GC, 86.10 HT, Dani), using H2 as a carrier gas. 2.4. Infrared measurements In situ infrared (IR) investigations were performed in a reaction cell for diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Harricks, HV-DR2), employing a Magna 560 spectrometer (Nicolet 6700 FTIR spectrometer, Thermo Scientific) equipped with a liquid N2 -cooled MCT narrow band detector. For more detailed information about the setup we refer to reference [29]. In the measurements, the gas flows were controlled in the same way as for the activity measurements (Hastings HFC-202 mass-flow-controllers), using a total flow of 60 N ml/min. About 15 mg of pure catalyst were placed onto a bed of 50 mg of pure ␣-Al2 O3 . The conditioning procedures prior to the infrared measurements were the same as for the kinetic measurements. Typically, 400 scans (acquisition time 4 min 8 s) at a nominal resolution of 8 cm−1 were co-added for one spectrum. The intensities were evaluated in Kubelka–Munk units, which are linearly related to the adsorbate concentration on the catalyst surface [30] (for exceptions see Ref. [31]). Background subtraction was performed using a spectrum recorded in a stream of N2 at the reaction temperature, directly after the catalyst conditioning. To correct for variations in the reflectivity of the respective catalysts, the spectra were scaled to similar intensities at 2600 cm−1 , where the background intensity does not interfere with any other signals and does not change during the reaction. The gas phase CO signal was removed by subtraction of the corresponding spectral region (2040–2240 cm−1 ) from spectra recorded under the same reaction conditions on an inert ␣-Al2 O3 support in CO containing atmosphere from those obtained on the Au/TiO2 catalyst. 3. Results and discussion 3.1. Characterization of Au/TiO2 catalysts Two parent catalysts 1 and 2, with higher (catalyst 1, 3.96 wt.%) and lower Au concentration (catalyst 2, 0.66 wt.%), respectively, were prepared. Parent catalyst 1 was leached either as fresh catalyst, directly after drying in air, yielding catalyst 1a, or after

Table 1 Au content of Au/TiO2 catalysts determined by ICP-OES. Catalyst

Conditioning

Residual Au (wt.%)

Na (wt.%)

1 1a 1b 2

– Leaching O400, leaching –

3.96 0.56 0.01 0.66

<0.01 <0.01 <0.01 <0.01

subsequent O400 conditioning, yielding catalyst 1b. Catalyst 2 was prepared for comparison with the leached catalyst 1a, with a similarly low Au loading. The different leaching and conditioning steps are evident also from the schematic illustration in Fig. 8, which will be shown later. 3.1.1. Elemental analysis The actual gold loadings of the two parent catalysts and the remaining gold contents after the leaching procedures, as determined by ICP-OES analysis, are summarized in Table 1. We also give the amount of remaining sodium, to illustrate the completeness of the sample washing procedure. Very small sodium contents (below 0.01 wt.%) were found for all samples investigated. Cyanide leaching was reported to remove about 90% of the metallic gold, without affecting oxidic gold species, due to their different redox standard-potential [5]. Upon CN− leaching of the fresh catalyst 1, the gold content was reduced from 3.96 to 0.56 wt.%, which is a reduction to about 1/7 of the initial gold loading (catalyst 1a). A significantly lower gold content (0.01 wt.%) was found upon CN− leaching of parent catalyst 1, if this was calcined prior to the leaching process (catalyst 1b). This is consistent with the reports that leaching selectively removes metallic Au0 , which is suspected to be the dominant species after O400 calcination (more details see below). 3.1.2. Transmission electron microscopy measurements The mean Au particle sizes of the different catalysts, for different preparation conditions, and the resulting Au particle dispersions are summarized in Table 2. The Au mean particle size was evaluated for the catalyst prior to and after the final conditioning, in some cases also after the CO oxidation reaction. Fig. 1 shows representative TEM images and the corresponding particle size distributions. The O400 calcined parent catalyst 1 has an average particle size of 3.9 ± 1.1 nm, which is close to previous reports for similar catalysts [32]. The leached catalyst 1a was measured without further conditioning, before and after 16 h of CO oxidation reaction (80 ◦ C, 1% CO, 1% O2 , rest N2 ) in order to investigate the possible Au particle growth during the reaction. The Au particles on the pure leached catalyst are finely dispersed, with an average Au particle diameter of 1.2 ± 0.4 nm. During the CO oxidation reaction, the particles are growing, yielding a final Au mean particle size of 2.1 ± 0.8 nm. The average Au particle size of the leached catalyst 1a after subsequent O400 calcination is 2.6 ± 0.7 nm. This Au particle size is rather similar to that of the parent catalyst 2 with comparable Au loading after O400 conditioning (see below). Table 2 Mean Au particle size and Au dispersion of the Au/TiO2 catalysts after different conditioning/reaction procedures. Catalyst 1 1a 1a 1a 1b 2

Conditioning

Au mean particle size (nm)

Dispersion (%)

O400 – O400 16 h CO oxid. O400 O400

3.9 ± 1.1 1.2 ± 0.4 2.6 ± 0.7 2.1 ± 0.8 – 2.0 ± 0.4

25.4 77.2 33.7 41.4 – 55.2

P. Kast et al. / Catalysis Today 244 (2015) 146–160

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Fig. 1. TEM images and Au particle size distributions for different Au/TiO2 catalysts (see panels). (a) Parent catalyst 1 after O400 conditioning, (b) parent catalyst 2 after O400 conditioning, (c, d) leached catalyst 1a fresh, and after subsequent O400 conditioning, (e) leached catalyst 1a after 1000 min CO oxidation (1% CO, 1% O2 , balance N2 , at 80 ◦ C), (f) leached catalyst 1b after subsequent O400 conditioning.

The TEM analysis of catalyst 1b showed no Au particles on the support surface after additional O400 conditioning in the first measurement. Since the kinetic measurements revealed some residual activity for the catalyst, a second set of images was recorded, which resolved a few remaining Au particles with sizes in the range of 4–8 nm. No dispersion could be calculated due to the low number of particles (therefore also no TOF number could be determined in the kinetic data). A possible explanation for the existence of a few remaining Au0 particles could be a higher local concentration of ionic gold Au species after leaching in certain regions, which resulted in a few Au0 particles during the final O400 calcination process. The TEM analysis of the O400 calcined catalyst 2 reveals an average Au particle diameter of 2.0 ± 0.4 nm.

The results of the TEM analysis can be summarized as follows: (i) for the present synthesis procedure, the Au mean particle size and hence the Au dispersion depends on the total concentration of gold, with higher concentration yielding larger Au NPs; (ii) the as-prepared fresh catalyst (without any conditioning) shows a narrow Au particle size distribution with very small Au particles (not shown), (iii) the leached catalyst 1b (O400 + leached) did, with few exceptions, not exhibit visible Au particles in the TEM images, (iv) O400 calcination, either of the fresh catalyst or after leaching, leads to the reduction and agglomeration of the ionic Au3+ species to form homogeneously distributed Au0 nanoparticles. (v) Similar effects, but less pronounced, are observed upon exposure of the fresh, non-calcined catalyst to the CO oxidation reaction atmosphere, while after a preceding O400

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Intensity / cps

1000 cps

1000 cps

a)

250 cps

c)

50 cps

1000 min CO oxidation

-

CN leaching

cat 1a

280

284

288 280 284 Binding energy / eV

288

292

Fig. 2. XP spectra of the C(1s) region of the leached catalyst 1a after O400 conditioning (left) and after 1000 min CO oxidation (1% CO, 1% O2 , balance N2 , reaction at ) fit, ( ) background, ( ) ubiquitous 80 ◦ C) (right). (—) raw signal, ( , ) carbonates. carbon, (

Intensity / cps

cat 1

80 b)

85

90

95 250 cps

O400

80 d)

85

90

95 50 cps

O400

conditioning procedure, the CO oxidation reaction does not lead to further growth of the Au NPs [33]. 3.1.3. XPS analysis 3.1.3.1. C(1s), O(1)s and Ti(2p) regions. The spectra recorded in the O(1s) and Ti(2p) regions did not show any differences for all catalysts investigated and are therefore not shown. The binding energy (BE) of the O(1s) peak was 530.0 eV, with a shoulder at about 532.0 eV, where the latter is most likely due to surface hydroxyl groups [34] and/or surface carbonate species [35] (see also Section 3.3). In the C(1s) region, two signals are resolved, one at 284.8 eV, which is assumed to be due to ubiquitous carbon [27,35], and a second one at 288.8 eV, which represents carbonate/carboxylate surface species. C1(1s) XP spectra of catalyst 1a, recorded before and after CO oxidation (BE normalized to the Ti2p3/2 signal at 459.2 eV) are shown in Fig. 2. After CO oxidation, the carbonate/carboxylate species related C(1s) intensity had increased by about 100%. The shoulder at 286.5 eV and the peak at 288.6 eV are expected to result from different carbonate/carboxylate species [36]. The content of the carbonate species relative to the total carbon amount was 6.8 and 12.6% before and after the reaction, respectively. 3.2. Au(4f) region XP spectra in the Au(4f) region obtained for the catalysts 1 and 1a before and after O400 calcination are shown in Fig. 3. It is clearly visible that the Au3+ content disappears during the O400 calcination process, while the metallic Au0 content grows. Leaching affects metallic gold and small gold species (Auı+ ) in a similar way, while it does not change the amount of ionic Au3+ gold species. The Au(4f5/2 ) binding energy of 83.9 eV for metallic Au0 nanoparticles is in good agreement with literature data [27,37]. For non-conditioned Au/TiO2 catalysts, other authors reported a broad Au(4f5/2 ) peak, which consists of contributions from differently sized small Au particles with different binding energies, due to particle size effects [25,32,38]. The peak shift of the Au0 signal to lower binding energy by about 0.1 eV upon O400 calcination (Fig. 3a and c) also resembles previous reports [25], and is attributed to particle size effects. Comparing two catalysts with similar Au loading, one prepared by leaching a higher loaded catalyst (catalyst 1a) and a freshly prepared low Au loaded catalyst (parent catalyst 2), reveals a significantly lower fraction of metallic species in the leached catalyst. This can be understood by the selective removal of metallic

cat 1-O400

80

85

90

95 85 80 Binding energy / eV

cat 1a-O400

90

95

Fig. 3. XP spectra of the Au(4f) region of parent catalyst 1: a) as prepared, b) after O400 conditioning, c) after cyanide leaching (catalyst 1a) d) leached catalyst 1a ) fit, ( ) backafter subsequent O400 conditioning. (—) raw signal, ( ) Au0 , ( ) Auı+ , ( ) Au3+ , ( ) TiO2 plasmon. ground, (

Au0 nanoparticles during leaching, leaving the Au3+ content at a value typical for a non-conditioned higher loaded Au/TiO2 catalyst (for a more detailed discussion see below). For both catalysts, the Au0 content increases significantly upon calcination (O400) and becomes the dominant species, mainly on the expense of the Au3+ species (see Fig. 4, Table 3). In order to identify Au(4f) contributions for the catalyst 1b (O400-leached) with its extremely low Au content, we performed a careful measurement of the Au(4f) energy region on the pure P25 support for a precise determination of the background intensity, where the P25 material was conditioned in the same way as the catalyst 1b. As evident from the spectra in Fig. 5, there is essentially no difference between the Au(4f) spectrum of catalyst 1b and that of P25, confirming the result of the ICP-OES analysis also for the surface region. As shown in Table 3, the BEs of the different gold species on the catalysts investigated stay more or less constant for the various catalysts. This table also summarises the integrated peak areas for the contribution of the Au species in different oxidation states as well as the Au/Ti and “plasmon”/Ti intensity ratios (the latter one also compared to the pure P25 support). The results discussed so far can be understood in a consistent picture. The fresh, non-conditioned catalyst is characterized by significant amounts of ionic Au3+ species and a broad Auı+ peak (see also Section 1). Interestingly, the relative amount of Auı+ species of close to 10% of the total Au content in the non-conditioned catalysts 1 and 2 seems to be independent of the gold loading. During O400

P. Kast et al. / Catalysis Today 244 (2015) 146–160

a)

b)

200 cps c)

100 cps

Intensity / cps

200 cps

151

80

85

90

80 85 90 95 Binding energy / eV

95

80

85

90

95

Fig. 4. XP spectra of the Au(4f) region of low Au loading catalysts. a) Parent catalyst 2 as prepared (left panel), b) parent catalyst 2 after O400 conditioning (middle panel), c) ) fit, ( ) background, ( ) Au0 , leached catalyst 1a after 1000 min CO oxidation (1% CO, 1%O2 , balance N2 , reaction at 80 ◦ C) (right panel). (—) raw signal, ( ) AAuı+ , ( ) Au3+ , ( ) TiO2 plasmon. (

20 cps

Intensity / cps

20 cps

80

85

90

80 85 95 Binding energy / eV

90

95

Fig. 5. XP spectra of the Au(4f) region of the leached catalyst 1b after O400 conditioning (left) and of the P25 support after a similar leaching procedure.

calcination, Au3+ species are thermally reduced and transformed into small Auı+ entities and (mainly) into metallic Au0 nanoparticles, reaching a metallic gold content of 74–84%, both for the leached and the non-leached catalysts. No more ionic Au3+ species are present after O400 treatment and the content of the small Au␦+

particles is considerably reduced because of thermal sintering of these very small particles during calcination [25,26,39,40]. Gold is essentially completely removed if the catalyst was O400 calcined before leaching, since ionic Au3+ species were completely reduced during calcination, predominantly to metallic Au0 nanoparticles, and accessible to leaching. If the catalyst was not O400 calcined before leaching, a considerable fraction of the Au species on the catalyst was present as ionic Au3+ species and therefore not accessible to leaching. Interestingly, however, also the absolute Au3+ amount seems to be reduced by the leaching procedure, as evident from comparison of the Au3+ related intensities in catalyst 1 and catalyst 1a. Apparently, leaching reduces the Au3+ intensity to about 40% of its initial amount. This can be understood by considering that subsequent room temperature drying of the catalyst, after the leaching procedure, will already cause partial reduction of the Au3+ species and convert them into Auı+ and Au0 species. Similar effects are observed also for the fresh catalyst, where drying leads already to a distinct reduction of the Au3+ species (see Table 3 and Figs. 3 and 4, as well as Ref. [38]). Comparison of the Au/Ti ratios for the non-leached catalyst 1 and the leached catalyst 1a shows the same trend as the results of the ICP analysis, namely a decay of the relative amount of Au to 1/7. Hence, the results of bulk analysis and surface analysis are in complete agreement, indicating that there are no significant amounts of bulk Au species that are inaccessible to leaching.

Table 3 XPS results for different Au/TiO2 catalysts used in this study. Catalyst

Conditioning after leaching

BE (Au0 ) (eV)

BE (Auı+ ) (eV)

BE (Au3+ ) (eV)

1 1 1a 1a 1a 1b 1b 2 2 Support

– O400 – O400 CO ox – O400 – O400 –

83.9 83.9 83.9 83.9 83.9 – – 84.0 84.0 –

84.5 84.5 84.5 84.5 84.5 – – 84.6 84.6 –

85.8 85.8 85.8 85.8 85.8 – – 85.9 85.9 –

I(Au)/I(Ti) 0.125 0.130 0.019 0.020 0.020 – – 0.023 0.018 –

I(Au0 )/I(Auı+ ) 1.595 2.833 0.724 2.805 1.26 – – 0.258 5.329 –

I(Au0) /I(Au3+ ) 6.590 – 1.377 – – – – 1.915 – -

I(Au0 ) (%)

I(Auı+ ) (%)

I(Au3+ ) (%)

I(plasmon)/I(Ti)

47.6 73.9 32.2 73.7 55.66 0.0 0.0 18.5 84.2 ––

42.8 26.1 44.4 26.3 44.44 0.0 0.0 71.8 15.8 –

9.6 0.0 23.4 0.0 0 0.0 0.0 9.7 0.0 –

– – 0.010 0.010 0.010 0.013 0.010 0.010 0.010 0.014

39.2 – 2.71 – – – – – 37 103 3.8/1.4 – 0.7/0.26 0.27/0.28 106.2/39.8 40.7/42.1

– – 113/78 53.1/36.6 91 – 0.005/0.004 47/37

– – 98/62 68.1/45.4 65 – 0.008/0.005 69.6/45.4

– – 347/347 81/81 116 0.003/0.003 23.3/27.1

–

4.67 5.58 – 1.42 1.42 – – – – – – 721/546 – – – – – 77/58 56 564 501 63 31 185 2.5/1.4 0.3/1.9 0.5/2.5 3.3/2.1 8.9/2.7 –

2 2

1b

1b

1b

N100, O400 – N100 O400 N100, O400 –, reconditioned N100, reconditioned O400, reconditioned N100 + O400, reconditioned N100, O400 N100 1 1a 1a 1a 1a 1b

1.29/0.74 0.072/0.41 0.063/0.32 0.32/0.2 0.85/0.26 0.0012/0.0022

Relative activity after 660 min/ % Initial/final TOF after 660 min/s−1 Initial/final rate after 660 min/ 10−4 mol gcat −1 s−1 Initial/final rate after 660 min/ 10−4 mol gAu −1 s−1 Conditioning

3.3.1. Effect of pre-treatment before Au leaching The effect of leaching and the different pre-treatment procedures is most simply illustrated by comparing the reaction behavior of catalyst 1 and of the leached catalysts 1a and 1b, all after subsequent N100 + O400 conditioning (see Fig. 6). The latter two catalysts differ by their pre-treatment before leaching, which involved only drying in air at room temperature before leaching for catalyst 1a, and O400 calcination before leaching for catalyst 1b. In both cases, the general reaction characteristics resemble those of the nonleached parent catalyst with their typical deactivation. Because of the loss of Au upon leaching, the absolute activity decreased for the latter two catalysts, to values only somewhat lower for catalyst 1a, and for values at the detection limit for catalyst 1b. Normalized to the amount of Au still present on the sample, however, the activity of the leached catalysts is much higher than that of the non-leached catalyst 1, with 150 × 10−4 mol gAu −1 s−1 for catalyst 1a and, slightly lower, 50 × 10−4 mol gAu −1 s−1 for catalyst 1b. The deactivation in these cases was 69% for catalyst 1a and 9% for catalyst 1b after 660 min on stream. It should be noted,

Table 4 Reaction rates during the CO oxidation reaction over studied catalysts at 80 ◦ C.

The Au mass normalized reaction rate (CO2 formation), deactivation and the TOF for CO oxidation on the differently conditioned catalysts were measured/calculated to determine the influence of the gold oxidation state on the CO oxidation activity. The evolution of the catalytic activities of the different catalysts with time on stream is presented in Figs. 6 and 7, the resulting values for the initial and final activity, after 660 min (1000 min) on stream as well as the corresponding deactivation are summarized in Table 4. The reaction behavior of the two parent catalysts treated by standard conditioning (N100 + O400) can be taken as basis for the subsequent discussion of leaching and pre-treatment/conditioning effects. The activity of the parent catalyst 1 drops about exponentially from 33 × 10−4 (initial activity) to 19 × 10−4 mol gAu s−1 after 660 min on stream. The low-loaded parent catalyst 2, pre-treated in the same way, exhibits rather similar reaction characteristics, but in this case the Au mass normalized activity is higher, with an initial activity of 106 × 10−4 mol gAu s−1 and a final activity of 40 × 10−4 mol gAu s−1 after 660 min on stream. The higher activity of the latter catalyst under these reaction conditions goes along with a larger tendency for deactivation (44% for catalyst 1, 63% for catalyst 2). The general reaction characteristics of the two catalysts closely resemble those reported earlier for the same type of catalyst, and also the activities are of similar order of magnitude [33]. Possible reasons for the higher activity and higher tendency for deactivation of the lower loaded catalyst 2 compared to that of catalyst 1 will be discussed later.

Initial/final rate after reconditioning after 1000 min/10−4 mol gAu −1 s−1

3.3. Kinetic measurements

32.6/18.6 12.9/72.7 11.3/56.9 56.6/35.8 152/46.5 10.6/19.6

Initial/final rel. activity after reconditioning after 1000 min/%

N Au perimeter/ 10−18 g−1

Initial rate correlated to N Au perimeter/ 10−18 mol NAu perimeter −1 −1 s )

To test the influence of the reaction atmosphere (1% CO, 1% O2 , 80 ◦ C) on the composition of the Au species on a non-conditioned, leached catalyst, we characterized catalyst 1a after 1000 min on stream by XPS (see Fig. 4c). No Au3+ species are detected anymore after the reaction. The ratio of metallic Au0 to Au␦+ species is 1.25:1, equivalent to 55.6% Au0 content. As expected, this is more than observed in the untreated leached catalyst 1a (0.72:1), but significantly less than in the same catalyst 1a after O400 calcination (2.8:1). Hence, exposure to a 1:1 mixture of CO to O2 in N2 at 80 ◦ C leads to a complete reduction of the ionic Au3+ species in a Au/TiO2 catalyst, similar as O400 calcination, but growth of the small sub-nanometer Auı+ species is much less pronounced than during O400 calcination. These conclusions are supported also by preliminary in situ X-ray absorption near edge spectroscopy (XANES) data recorded on the non-conditioned catalyst 1b, which indicated a rapid change from an ionic to a Au0 state upon exposure to reaction atmosphere.

6.98 2.31 – 39.86 107 –

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0 800 1000 1200

Time/min Fig. 6. Evolution of the Au mass-specific CO oxidation rate (upper panels (a) and (b)), turn-over frequency (TOF) (middle panels (c) and (d)), and the relative activity (bottom panels (e) and (f)) during 1000 min of CO oxidation (1% CO, 1% O2 , balance N2 , reaction at 80 ◦ C) on different Au/TiO2 catalysts. Left panels a, c, e: Activity/deactivation behavior of parent catalysts 1 and 2 after different treatments: ( ) catalyst 1 after standard conditioning (N100 + O400), ( ) catalyst 2 after standard conditioning (N100 + O400), ( ) catalyst 2 after N100 conditioning. Right panels b, d, f: Activity/deactivation behavior of leached catalyst 1a after different conditioning procedures: ( ) after standard conditioning (N100 + O400), ( ) after O400 conditioning, ( ) after N100 conditioning, () without any conditioning.

however, that due to the extremely small Au content of catalyst 1b (0.01 wt.% Au) and the very low absolute rate (mol gcat s−1 , see Table 4), these numbers for the latter catalyst have to be taken more qualitatively. The differences in the initial rates for the above catalysts can be explained by the difference in the Au mean particle sizes and their distribution of the particular catalysts, which also determines the number of Au atoms at the perimeter sites (Nperimeter Au , see Table 4), which are considered to represent the active sites under these reaction conditions [41]. While catalyst 1 shows a mean Au particle size of 3.9 nm, with the main fraction of Au nanoparticles (NPs) between 2 and 6 nm, catalyst 1a shows a broad Au size distribution with a significant fraction of Au NPs below 3 nm, and catalyst 2 shows a narrow size distribution of Au NPs between 1 and 3 nm. The initial rate scaled to Nperimeter Au decreases in the order catalyst 1a > catalyst 2 > catalyst 1 (107 > 39 > 7 10−18 mol NAu perimeter Au −1 s−1 ) and/or the same order for the turn over frequencies. Using the TOFs for comparison (8.9 > 3.8 > 2.5 s−1 ), the differences are less pronounced, since the fraction of surface Au atoms increases faster with decreasing particle size than the number of perimeter Au atoms. At this point we cannot say whether the differences between different catalysts

represent a real particle size effect or whether they result from the presence of remaining sub-nanometer Au particles. For other conditioning procedures, in particularly using the fresh catalyst or N100 drying, the effects of pre-treatment (before leaching) and conditioning (after leaching) can be separated less clearly, they will therefore be discussed below. 3.3.2. Effect of conditioning after Au leaching The effect of different conditioning procedures after leaching shall be first illustrated on the catalyst 1a (see Fig. 6b). After drying in air at room temperature (fresh catalyst 1a-0) or after N100 drying (catalyst 1a-N100), we obtained rather similar reaction characteristics. The initial activity was rather low (12.9 × 10−4 and 11.3 × 10−4 mol gAu −1 s−1 for the catalysts 1a0 and 1a-N100, respectively), then it steadily increased during the time on stream to finally reach values of 72.7 × 10−4 and 56.9 × 10−4 mol gAu −1 s−1 . Replacing the N100 drying by an O400 calcination step after leaching leads to a change in both initial activity and deactivation behavior. Now the initial activity is significantly higher (56.6 × 10−4 mol gAu −1 s−1 ), and the activity decreased by ∼37% during the first 200 min. Finally, after the standard conditioning (N100 + O400), we obtained a comparable

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80

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40

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0 recalcination (O400)

800

600

400

200

0 0

400

800

1200 Time / min

1600

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Fig. 7. Evolution of the Au mass-specific CO oxidation rate (upper panel) and the relative activity (bottom panel) during 1000 min of CO oxidation (1% CO, 1% O2 , balance N2 , reaction at 80 ◦ C) on the leached Au/TiO2 catalysts 1b: ( ) catalyst 1b after standard conditioning (N100 + O400), ( ) after N100 conditioning, ( ) after O400 conditioning, and () without any conditioning. Recalcination (O400) after about 1000 min of CO oxidation.

reaction behavior, but with a significantly higher initial activity of 47 × 10−4 mol gAu −1 s−1 , which also decayed about exponentially to 37 × 10−4 mol gAu −1 s−1 (deactivation of 9%) after 660 min on stream. Similar trends were obtained for the leached catalyst 1b (see Fig. 7). It should be noted, that in this case the catalyst was already O400 calcined before leaching, hence there are almost no Au3+ species left after pre-treatment, resulting in a very low final Au loading after leaching. We expect, however, that the small amount of Au remaining after leaching is also composed of the three different Au species obtained for catalyst 1a (Au3+ , Auı+ , Au0 ), but at much smaller absolute concentrations and possibly also with different contributions of the individual species. For the air dried catalyst (catalyst 1b-0) and for the N100 dried catalyst (catalyst 1b-N100) we again obtained an activation period during the CO oxidation reaction. The increase in activity and also the final activity, however, were much lower than seen for catalyst 1a, with the activity increasing from 10.6 × 10−4 to 19.6 × 10−4 mol gAu −1 s−1 for catalyst 1b-0 and from 23.3 × 10−4 to 27.1 × 10−4 mol gAu −1 s−1 , for catalyst 1b-N100. Also for the O400 calcined and the N100 + O400 conditioned catalysts 1b, the

trends were comparable to those of catalysts 1a, with a distinct deactivation and a higher initial activity. For the O400 calcined catalyst, the initial activity was about two times higher than for parent catalyst 1 (69.6 × 10−4 mol gAu −1 s−1 ) and it decayed by 35% over 660 min on stream (final activity 45.4 × 10−4 mol gAu −1 s−1 ). A higher activity than that of the non-calcined catalysts, though lower than that of the O400 treated catalyst 1b, was obtained after standard conditioning (catalyst 1b-N100 + O400), with initial (final) rates of 47 × 10−4 (37 × 10−4 ) mol gAu −1 s−1 and a deactivation of 20%. The similar trends obtained for the differently conditioned catalysts 1b compared to the catalysts 1a indicate that our above assumption that all three Au species, though in very low concentrations, are present also on catalyst 1b after leaching, is reasonable. Similar as for catalyst 1a, conditioning after leaching results in characteristic differences in the reaction behavior, in particular in the activation/deactivation behavior. Differences in absolute numbers of the initial/final activities and of the (de-)activation during time on stream are most likely related to different relative contributions of the respective Au species after leaching on catalysts 1a and 1b. The conclusions on the conditioning effects after leaching stated above are supported also by the rather similar effects observed after

P. Kast et al. / Catalysis Today 244 (2015) 146–160

applying different conditioning procedures to fresh catalysts, after synthesis. It is well known that the activity of Au/TiO2 catalysts which are exposed to reaction conditions without a preceding conditioning step increases with time, which was related to a gradual reduction and Au0 particle formation during the reaction [25]. Similar results are obtained also in the present study for catalyst 2. When exposing the fresh catalyst 2 only to a N100 drying step rather than the standard conditioning procedure (N100 + O400), the initial activity is much lower (40.7 × 10−4 mol gAu −1 s−1 ) than in the latter case (106 × 10−4 mol gAu −1 s−1 ) and increases with time. Different from the leached catalysts 1a and 1b, however, the activation period is much shorter and the maximum activity (52 × 10−4 mol gAu −1 s−1 ) was reached already after 44 min on stream. Afterwards the activity decreased by 20% to 42.1 × 10−4 mol gAu −1 s−1 after 660 min, which was only slightly above the initial activity. For comparison, the standard pretreatment (N100 + O400) of catalyst 2 resulted in an initial (final) activity of 106 (40) × 10−4 mol gAu −1 s−1 (63% deactivation). In an earlier study on the effect of leaching on the CO oxidation reaction on 120 ◦ C dried Au/TiO2 , Raphulu observed an increasing CO oxidation activity with decreasing amount of Au (increased leaching) [16]. On the other hand, a severely leached catalyst containing only 0.06%Au loading was found to be inactive. The author believed that both the ionic and the metallic gold species were necessary for the high activity of the catalysts. Overall, the effects of final conditioning after leaching are at least qualitatively similar to the effects induced by conditioning a fresh Au/TiO2 catalyst, with the only difference that the initial state (composition of the Au phase) after synthesis and after leaching are quantitatively different. While after synthesis and drying in air the Au3+ species are already partly converted into Au␦+ species and Au0 nanoparticles, this has happened only to a lower extent on the leached catalysts. This means, that after the corrosive removal of Au0 nanoparticles and Au␦+ species, the fraction of remaining ionic Au3+ is much higher than on the fresh catalyst after preparation. 3.3.3. Effect of re-conditioning after deactivation Finally, the beneficial effect of the O400 conditioning on the catalyst activity was tested on catalysts which were exposed previously to a CO oxidation reaction atmosphere for 660 min. The O400 re-conditioning was carried out for the four 1b catalysts which were conditioned differently after leaching and whose initial reaction behavior was discussed in the last section. All re-conditioned catalysts showed an increase of the initial activity as compared to the final activity in the preceding CO oxidation reaction. This increase was most pronounced for samples which were only air dried or N100 dried after leaching. For these two catalysts, the initial activities after re-conditioning exceeded the corresponding activities at the onset of the preceding CO oxidation reaction procedure, after the first conditioning procedure. On the other hand, for the catalysts which where calcined (O400 or N100 + O400) after leaching, reconditioning leads to almost the same initial activity as obtained at the onset of the preceding CO oxidation run. Focusing on the effects of O400 re-conditioning, these can be summarized in a consistent picture: (i) The deactivation during CO oxidation under present reaction conditions is reversible, which is incompatible with a deactivation mechanism dominated by Au particle sintering; but consistent with reversible deactivation by accumulation of surface species such as surface carbonates (see Section 3.3). (ii) In addition to removing reaction inhibiting surface species, the O400 calcination step can transform existent Au3+ /Au␦+ species into more active Au0 nanoparticles, which are essential for catalysts highly active for reaction under present reaction conditions (T = 80 ◦ C). Catalysts which were exposed

155

to reaction conditions without a preceding calcination step, either after synthesis and/or after leaching, can in addition to the activation during time on stream, be further activated by O400 re-conditioning. This is evident by the much higher reaction rate upon re-conditioning as compared to the activity at the beginning of the reaction. (iii) Deactivation and re-activation of the Au/TiO2 catalyst after leaching resembles the tendencies observed for a non-leached catalyst after identical procedures for initial conditioning and for re-activation [25,38], although in the latter case activation is generally less pronounced, which we tentatively attribute to higher Au0 in the latter catalysts due to a higher Au loading. This also underlines our above conclusion of rather similar reaction and deactivation characteristics of leached catalysts and non-leached catalysts that were not exposed to O400 calcination. The consistent data obtained for the reaction behavior after reconditioning provide strong evidence for the conclusions derived from the reaction behavior of these catalysts (see last section) that after O400 calcination and subsequent leaching small amounts of Au3+ , Au␦+ species and possibly very few Au0 nanoparticles are left on the catalyst, which can be transformed into more active Au0 nanoparticles by calcination, either directly after leaching or during re-conditioning, or by exposure to the reaction atmosphere. This is illustrated schematically in Fig. 8. Because of the very low Au loading of the catalyst 1b after leaching (0.01 wt.%), this information is not accessible for that catalyst by spectroscopic techniques (see Fig. 5), but can be derived from the kinetic measurements. 3.4. DRIFTS measurements The state of the catalysts after leaching and subsequent conditioning as well as the accumulation of adsorbed CO and other adsorbed reaction intermediates and side products on the different leached Au catalysts during CO oxidation were characterized by in situ diffuse reflectance IR spectroscopy (DRIFTS). The conditioning and reaction conditions were identical to those applied in the kinetic measurements described above. First we will present and discuss raw spectra recorded directly after standard conditioning (N100 + O400) of the parent catalyst 1 (Fig. 9a, top spectrum), parent catalyst 2 (Fig. 9a, middle spectrum), leached catalyst 1b (Fig. 9a, bottom spectrum) to illustrate the effect of leaching on the surface composition of the catalysts after standard conditioning. The spectra in this first set look very similar. They are characterized by a broad hump in the range 1400–3500 cm−1 , which results from the frequency dependent reflectivity of the catalyst material, and by absorption bands at 3724, 3675 and 3620 and 1544/1435 and 1366 cm−1 , which are indicative of surface hydroxyl groups with different adsorption configurations [42], and bidentate as well as monodentate surface carbonate species, respectively. The presence of formate species on the surface is also indicated by bands at 2965/2926 cm−1 . Overall, the spectra indicate that cyanide leaching (catalyst 1b) has little influence on the surface composition of the catalyst after subsequent standard conditioning. Next we compare the surface composition of the leached catalyst 1a with its higher residual Au content compared to catalyst 1b (see above) after different subsequent conditioning procedures (Fig. 9b), non-conditioned (Fig. 9b, top spectrum), after N100 conditioning (Fig. 9b, middle), after standard conditioning (N100 + O400) (Fig. 9b, bottom) to elucidate typical differences between the surfaces (note that for spectrum of the N100 treated catalyst 1a (Fig. 9b, middle) a residual CO signal is present, which was removed for background subtraction in Fig. 11b). For these catalysts, the general appearance of the spectra resembles that of the first set

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Fig. 8. Schematic illustration of the composition of the Au phase on the Au/TiO2 catalysts after the respective processing steps (see figure). The absolute amount of Au (Au loading in wt.%) and the contents of the ionic Au3+ species, sub-nanometer Auı+ and Au0 nanoparticles, as taken from Tables 1 and 3, are reflected by the respective symbols (see figure), where 1 point stands for ∼0.1 wt.% (Au0 : ∼0.2 wt.%). Only for the leached catalysts 1b (indicated by ‘Au/TiO2 ’), where the remaining Au content is about 0.01 wt.% and therefore well below the value of 0.1 wt.% given above, the symbols just illustrate the composition of the Au phase.

(Fig. 9a), but the bands characteristic for adsorbed species are significantly more intense, at least for the non-conditioned and the N100 conditioned catalyst. In particular the bands arising from surface formates (2965/2926 cm−1 and 1366 cm−1 ) and bidentate carbonates (1544/1435 cm−1 ) as well as the signals for the adsorbed water (1620 cm−1 ), and of the OH stretching mode of adsorbed water molecules interacting by hydrogen bonds with surface OH groups (2500–3300 cm−1 ) [43] are much more intense than after N100 + O400 calcination. The spectrum recorded after standard (N100 + O400) conditioning, in contrast, closely resembles the spectra in Fig. 9a. This illustrates that the adsorbed species present after leaching and also after N100 drying, are largely removed by the calcination procedure. These findings on the non-conditioned catalyst are in good agreement with the observations reported by Denkwitz et al. for a fresh, non-conditioned Au/TiO2 catalyst, where significant amounts of formates and water were reported to be present on the fresh catalyst [25]. Next we focus on the accumulation of adsorbed species during the CO oxidation reaction and discuss differences between leached and non-leached systems and different conditioning. Selected spectra recorded 0.5, 3, 9, 15, 40, 300, 660, and 990 min (from bottom to top) after starting the CO oxidation reaction are presented in Figs. 10 and 11. It should be noted that these spectra are background corrected, using the spectra in Fig. 9 as background. In Fig. 10, we followed the reaction on different catalysts, the parent catalysts 1 and 2 and the leached catalyst 1b, after standard conditioning (N100 + O400), in Fig. 11 we compare the build-up of adsorbed species on the leached catalyst 1a after different conditioning procedures. The figures show the full spectrum (bottom) as

well as detail spectra of the regions characteristic for adsorbed CO (top right) and for O–H stretch vibrations (top left). The evolution of the bands related to the (further) accumulation of adsorbed species on a non-leached catalyst is illustrated in Fig. 10a and b for parent catalysts 1 and 2 with different Au loadings. The spectra are characterized by the development of distinct bands at 2121–2128 and 2174 cm−1 , which are related to CO adsorbed on Au0 nanoparticles and on Ti4+ sites [44,45], a double band related to gas phase CO2 , a set of bands at 3724, 3675 and 3620 cm−1 related to surface hydroxyl groups, a band at ∼1620 cm−1 arising from the HOH vibration of adsorbed water, and finally a set of bands in the region 1200–1590 cm−1 related to different vibration of surface carbonate species. The characteristic bands and their wave numbers are listed in Table 5. The signals for gaseous CO2 appeared about instantaneously and then continued to grow in intensity for about 40 min, before they started to decrease again. Similarly to the CO2 behavior, the intensity for the adsorbed COad (on Au0 nanoparticles) appeared instantaneously and also started to slightly decrease after 40 min on stream. The weak band for CO adsorbed on Ti4+ sites was found for all catalysts, also catalyst 1a (Fig. 10c), after standard conditioning (N100 + O400). After about 40 min, an additional peak developed at 1616–1621 cm−1 , reflecting the accumulation of adsorbed water. The intensity of the water signal increased with ongoing reaction, while the intensity of the OH band at 3725 cm−1 decreased to about 30% of the initial intensity after 1000 min on stream. This indicates OH consumption and H2 O formation during the reaction, which was also observed [33,46] or suggested [47] previously on similar conditioned Au/TiO2 (P25) catalysts. In addition to the water band at 1610 cm−1 , a broad

P. Kast et al. / Catalysis Today 244 (2015) 146–160

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Fig. 9. Upper panel (a) DRIFT spectra of different catalysts taken directly after standard conditioning (N100 + O400): parent catalyst 1 (top), parent catalyst 2 (middle), and leached catalyst 1b (bottom). Lower panel (b) DRIFT spectra of the leached catalyst 1a after different conditioning procedures: non-conditioned (top), after N100 conditioning (middle), after standard conditioning (N100 + O400) (bottom).

absorption feature developed in the range 2500–3300 cm−1 , which started to increase after about 10 min on stream. This feature is generally attributed to OH vibration of adsorbed water. Different types of carbonates (bidentate, monodentate carbonate, see Table 5) appeared from the very beginning of the CO oxidation reaction, which grow significantly in intensity during the time on stream. The different surface species and their evolution during time on stream observed here closely resemble findings reported in earlier studies on CO oxidation on calcined Au/TiO2 catalysts [25,32,48]. The peaks evolving during the reaction on the leached catalyst 1b (Fig. 10c), after standard conditioning, closely resemble those developing during CO oxidation on non-leached Au/TiO2 catalysts (Fig. 10a and b). Due to the very low Au concentration in the leached

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catalyst 1b, no CO adsorbed on Au species is observed. The formation of water does not differ significantly on the different catalysts, while the build-up of surface carbonate species (bidentate, monodentate carbonate, frequencies see Table 5) varies significantly between the different catalysts, being most pronounced on catalyst 1 and least effective on catalyst 1b. Hence, the build-up of carbonates can be correlated with the tendency for CO2 formation, which due to the very low Au loading on the leached catalyst 1b is close to negligible on that catalyst. In total, the formation of surface species on the leached and then N100 + O400 calcined catalyst 1b closely resembles the behavior of parent catalysts after similar treatment. The same is true also for the N100 + O400 calcined catalyst 1a (see below). This provides further evidence that after N100 + O400 conditioning the leached catalysts are essentially identical to similarly treated non-leached catalysts with comparable Au loading and Au particle size distribution. Next we compare the build-up of adsorbed species on the leached catalyst 1a after different conditioning procedures (Fig. 11). The COad peak assigned to CO adsorbed on metallic Au species appears between 2116 and 2119 cm−1 on the catalyst 1a, independent of conditioning (Fig. 11). For the non-conditioned catalyst 1a, (Fig. 11a), the CO adsorption band has a significant shoulder at 2140 cm−1 , which may be attributed to adsorption on positively charged Au3+ or Au␦+ species, based on previous reports where bands observed in the 2155–2130 cm−1 region have been attributed to CO on positively polarized gold particles [49,50]. This would also be in agreement with our findings from the Au(4f) XPS data. On all catalysts, the CO–Au0 band position shows a red shift with increasing time on stream. Conditioning dependent differences appear, however, in the time dependence of the COad band intensity, which will be discussed in the next paragraph. Conditioning dependent differences are observed also for the other surface and product species. For the leached catalyst 1a after subsequent standard conditioning (N100-O400, Fig. 11c), we find a sharp CO2 peak at 2348 cm−1 instead of the characteristic double peak for gaseous CO2 . The origin for the different shape of the CO2 peak can only be speculated upon. Possibly, it results from adsorbed CO2 , reflecting a stronger bond of CO2 on this catalyst. Interestingly, the concentration of CO2 increases for both non-conditioned and the N100 treated catalysts 1a with time on stream (Fig. 11b), where the former reaches about steady state after 300 min and the latter increases only slightly after that time. This is in good agreement with the activation period observed in the kinetic measurements. In contrast to the CO2 band, the COad band intensity first increases in intensity and then, after reaching its maximum after about 5 min on stream for the non-conditioned and 10–15 min for the N100 treated catalyst, decreases again, reaching final intensities of about 35% of the initial intensity for the non-conditioned and 25% for the N100 conditioned catalyst. The apparent discrepancy between the activation behavior seen in the kinetic measurements and in the CO2 band intensities on the one hand and the decrease in the COad intensity on the other hand can at least qualitatively be explained

Table 5 Different types of surface species observed in the study. Wave number (cm−1 )

Species

Catalyst

Reference

3724, 3675 and 3620 2500–3300 2881–2953 1616–1630 1527–1586 1431–1439 1401–1428 1369–1381 1333–1347 1222–1242

OH stretching vibration of OH groups on different sites OH of water molecules interacting by hydrogen bonds with surface OH groups CH of formates OH OH bending vibration of adsorbed water C O of bidentate carbonate, the bridging O atoms bonded to two Ti atoms C O of bidentate carbonate with the bridging O atoms bonded to the same metal atom sCOO of monodentate carbonate species C O of formates asCOO of monodentate carbonate species asCOO < of bidentate carbonates

all catalysts 1,2, 1b, and 1a after N100 + O400 1a, non-conditioned 1,2, 1b, and 1a after N100 + O400 all catalysts all catalysts 1a after 100 1a, non-conditioned all catalysts all catalysts

[42] [43] [51] [25,35,45] [25,35,45] [51,52] [25,45,52] [51,53] [25,45,52,54] [35]

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Fig. 10. Sequence of DRIFT spectra recorded during CO oxidation (1% CO, 1% O2 , balance N2 , reaction at 80 ◦ C, 990 min) on various catalysts: (a) parent catalyst 1, (b) parent catalyst 2, (c) leached catalyst 1b after subsequent standard conditioning (N100 + O400). Bottom: full spectrum, top right: OCO region, top left: O–H region. The spectra displayed (bottom to top) were recorded after about 0.5 min, 5 min, 10 min, 15 min, 40 min, 300 min, 660 min and 990 min.

by the competition between deactivation of the reaction on the Au0 nanoparticles and activation of the overall reaction because of the ongoing conversion of Au3+ species into Auı+ and Au0 species on these catalysts. In contrast to the build-up of adsorbed water and the correlated consumption of surface hydroxyls on the N100 + O400 calcined catalysts (Fig. 10a–c and Fig. 11c), we find no signals for adsorbed water (1616 cm−1 ) during the CO oxidation reaction on the nonconditioned or N100 conditioned catalysts 1a. Also the broad water feature between 2500 and 3300 cm−1 did not form, and no OH consumption (see Table 5) was observed. Since these features are observed again on the same catalyst 1a after standard conditioning (N100 + O400), this cannot result from a difference in the catalysts, but must be related to the calcination treatment. On the other

a)

0.2 a.u.

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b)

2131

hand, we know from the initial spectra (Fig. 9) that there are some of surface hydroxyl species present on the non-calcined catalyst. Obviously, on these catalysts it is not or not easily possible to oxidatively convert surface hydroxyl groups during the reaction into adsorbed water. Similar observations were reported by Denkwitz et al. for CO oxidation on non-conditoned Au/TiO2 catalyst [25]. They found already in the background spectra significant concentrations of isolated OH groups (3724/3675 cm−1 ), and of adsorbed water (1630 cm−1 , 2500–3300 cm−1 ). The intensity of the water signals did not increase with time but rather decreased slightly, in good agreement with our findings. Finally, also for the non-conditioned or N100 dried catalyst 1a significant amounts of surface carbonate species are formed. Different from our previous interpretation that the formation of surface

0.2 a.u. 0.02 KMU

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2146

2125-2129

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0.02 KMU

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Fig. 11. Sequence of DRIFT spectra recorded during CO oxidation (1% CO, 1% O2 , balance N2 , reaction at 80 ◦ C, 990 min) on catalyst 1a after different conditioning procedures: (a) non-conditioned, (b) after N100 conditioning, (c) after standard conditioning (N100 + O400). Bottom: full spectrum, top right: OCO region, top left: O–H region. The spectra displayed (bottom to top) were recorded after about 0.5 min, 5 min, 10 min, 15 min, 40 min, 300 min, 660 min and 990 min.

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carbonates is largely responsible for the deactivation of the Au/TiO2 catalysts, these catalysts do not show any deactivation, but instead reveal an ongoing increase in activity. The apparent difference can be tentatively explained in terms of a picture where the formation of carbonate species does indeed lead to deactivation but where this effect is overcompensated by an activation due to conversion of Au3+ and Au␦+ species into more active metallic Au0 clusters and nanoparticles. In total, the IR spectra discussed in this section confirm the findings from previous sections that the cyanide leached Au/TiO2 catalysts behave rather similar to the non-leached catalysts, if similar conditioning procedures are applied before and after leaching, and when considering the lower Au content and differences in the Au0 particle size distribution. This is not only true for the deactivation and re-activation behavior, but also for the accumulation of adsorbed reactants (CO) and reaction side products.

a fresh catalyst or of a freshly leached catalyst. Accordingly, the active species for CO oxidation under these conditions are mainly metallic Au0 nanoparticles. A positive role of sub-nanometer Au clusters on the CO oxidation reaction cannot be excluded. Hence, the CO oxidation activity of leached Au/TiO2 catalysts is largely determined by the Au loading and Au0 particle size distribution after leaching and subsequent conditioning. 5. The deactivation and re-activation behavior of leached catalysts closely resembles that of the non-leached catalysts after similar conditioning, with a build-up of surface carbonates as main reason for deactivation and their thermal decomposition upon re-activation by O400 calcination. For non-calcined catalysts, deactivation was overcompensated by activation due to the conversion of ionic Au3+ species into significantly more active metallic Au0 nanoparticles and sub-nanometer Au clusters.

4. Conclusion

Overall, the results of this study underline the importance of well defined procedures before and after leaching, which together with the different duration (extent) of the leaching procedure, explains at least partly the variation between different studies. Comparing different oxide support materials, a second important aspect is the stabilization of dispersed ionic Au3+ species and Au clusters, which is different for different oxide materials. It is, e.g., much stronger for CeO2 than for TiO2 .

Based on the results of a detailed kinetic and in situ DRIFTS study on the effects of cyanide leaching and different catalyst treatment procedures before and after leaching, complemented by ex situ characterization of the catalysts by TEM, XPS and ICP-OES, we arrived at the following conclusions on the effect of catalyst leaching/conditioning and on the role of the different Au species (ionic Au3+ species, Au␦+ species, metallic Au0 nanoparticles) in the CO oxidation reaction over Au/TiO2 supported catalysts at 80 ◦ C (see also Fig. 8): 1. Treatment of the fresh catalyst before leaching largely determines the extent of Au removal, since it determines the content of Au0 nanoparticles attacked by cyanide leaching. After high temperature calcination (O400), which essentially completely reduces the ionic Au species in the fresh Au/TiO2 catalysts and converts them into Au0 nanoparticles, residual Au traces after leaching are minimal (here 0.01 wt.%). For fresh catalysts, not exposed to calcination, the relative amount of Au0 clusters and nanoparticles is considerably lower, with the exact amount depending of the specific treatment after synthesis and prior to leaching, and accordingly the residual Au contents are significantly higher. 2. Conditioning after leaching affects the relative amounts of the different Au species on the Au/TiO2 catalyst, ionic Au3+ species, sub-nanometer Au clusters, or metallic Au0 nanoparticles in the same way as conditioning after synthesis, and allows to systematically vary the Au composition. Due to the almost complete removal of metallic Au0 nanoparticles, it is possible to prepare catalysts with significantly higher Au3+ content than obtained after synthesis by deposition–precipitation procedures. Subsequent conditioning lowers the amount of ionic Au3+ species, N100 + O400 calcination leads to a complete reduction of Au3+ species into sub-nanometer Au clusters and Au0 nanoparticles, which is identical to the state reached after calcination of a fresh catalyst with similar Au loading. 3. Similar to calcination, also CO oxidation (80 ◦ C reaction temperature) affects the composition of the catalyst, converting ionic Au species into sub-nanometer Au clusters and metallic Au0 nanoparticles. On the other hand, the relative amount of sub-nanometer Au clusters (Au␦+) increases slightly during the reaction. 4. Despite the lower Au dispersion, Au/TiO2 catalysts containing more metallic Au0 clusters and nanoparticles are significantly more active for CO oxidation under these conditions than catalysts containing higher amounts of ionic Au3+ species, as evidenced by the pronounced activation period for reaction of

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