Ru-modified Au catalysts supported on ceria–zirconia for the selective oxidation of glycerol

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Ru-modified Au catalysts supported on ceria–zirconia for the selective oxidation of glycerol Lidia E. Chinchilla a,∗ , Carol M. Olmos a , Alberto Villa b , Anna Carlsson c , Laura Prati b , Xiaowei Chen a , Ginesa Blanco a , José J. Calvino a , Ana B. Hungría a a

Departamento de Ciencia de los Materiales, Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, 11510 Cádiz, Spain Dipartimento di Chimica, Universita degli Studi di Milano, via Golgi 19, 20133 Milano, Italy c Europe Nano Port, Achtseweg Noord 5, 5651 GG Eindhoven, The Netherlands b

a r t i c l e

i n f o

Article history: Received 9 December 2014 Received in revised form 9 February 2015 Accepted 11 February 2015 Available online xxx Keywords: Supported bimetallic catalysts Ceria–zirconia mixed oxide Glycerol oxidation STEM-XEDS Gold Ruthenium

a b s t r a c t In the present work, a series of supported gold–ruthenium bimetallic catalysts of varying Au:Ru molar ratios were prepared onto ceria–zirconium mixed oxide (Ce0.62 Zr0.38 O2 ) using sequentially depositionprecipitation and impregnation techniques to add gold and ruthenium, respectively. A combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analyses and different transmission electron microscopy techniques as high-resolution transmission electron microscopy (HRTEM), high-angle annular field imaging (HAADF-STEM) and X-ray energy-dispersive spectroscopy (XEDS) were used to characterize the bimetallic catalysts, being their catalytic behaviour tested in the selective oxidation of glycerol to glyceric acid. The analytical electron microscopy results of the bimetallic catalysts are consistent with the low miscibility gap reported between gold and ruthenium in the bulk state. Nevertheless, detailed STEM-XEDS analyses of individual particles provided evidence that both metals display certain tendency to form bimetallic particles. For the glycerol oxidation, the Au:Ru molar ratio is a key parameter to achieve optimal catalytic performance, i.e. the supported bimetallic catalyst with 1Au:0.5Ru ratio was found to be several times more active than monometallic gold and ruthenium catalysts. Furthermore, studies on bimetallic catalyst reduced at increasing temperatures confirm that the presence of small amounts of ruthenium prevents the aggregation of gold particles. These structural findings strongly suggest that if this increased stability is kept under reaction conditions, this could be one of the key factors to explain its exceptional activity. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The selective oxidation of alcohols in liquid phase is a very important transformation from an industrial point of view in the field of chemicals and pharmaceutical intermediates. In particular, glycerol oxidation to high added-value products has raised great interest in the industry over the last years due to the large surplus of glycerol formed as a by-product during the production of biodiesel by transesterification of vegetable oil [1,2]. Regarding the oxidizing agents, progress in oxidation technology has led to the irreversible decline of the so-called stoichiometric oxidants such as hypochlorites, chromates and permanganates, which produce large amounts of hazardous or toxic wastes in favour of cleaner processes carried out under mild conditions using

∗ Corresponding author. E-mail address: [email protected] (L.E. Chinchilla).

molecular oxygen in aqueous solution. Current approaches point to the use of heterogeneous catalysis, involving a solid catalyst which is stirred with liquid reagents in the presence of oxygen, or even better, air. This solution avoids the use of soluble catalysts at large scale and simplifies the separation of catalyst from the products [3,4]. Previous studies have reported high conversion rates for oxidation of alcohols using supported monometallic catalysts based on platinum, palladium, gold and copper among others [4–9]. In certain cases, it has been found that the use of promoters enhances the activity, selectivity and stability of the catalysts [5,10]. Other aspects that influence both the catalytic activity and selectivity vary with the type of alcohol (primary, secondary or tertiary) and reaction conditions: pH, temperature, pressure and the use of an appropriate solvent [6,10,11]. It has been described the use of bimetallic systems in which the incorporation of a second metal, based on the synergy of the two active phases, would modify the active sites to improve the

http://dx.doi.org/10.1016/j.cattod.2015.02.030 0920-5861/© 2015 Elsevier B.V. All rights reserved.

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selectivity for certain products in addition to promote the stability of the system. In this regard, there is outstanding work with Au–Pd [12–18], Au–Cu [19,20] and Au–Pt [16–18] catalysts for which a synergistic effect resulting from the combination of the two metals in both the activity and the stability is demonstrated. The case of Au–Ru bimetallic systems is especially challenging since, despite the limited miscibility of the two metals, an intimate interaction of Au and Ru has been achieved in some cases [2,3,21–23] and, an improved catalytic activity and selectivity has also been observed in parallel. It is worth mentioning that in the Au:Ru system, the details of the preparation routes, the order in which the metals are added and the nature of the support drastically influence the specific characteristics of the final metal particles [3,24]. Regarding the support, although some investigations related to gold catalysts emphasize the advantages of using carbon due to its higher surface area and lower cost [25,26], other researchers have proposed the use of on cerium oxide-based supports, relying on the unique redox properties of this kind of materials [27–30]. In this work, we have prepared a series of bimetallic Au–Ru catalysts supported on a ceria–zirconia mixed oxide which has been previously submitted to a redox treatment in order to improve its reducibility [31]. A thorough characterization of the metallic phase by transmission electron microscopy-related techniques have been performed which provide key information to understand some aspects of the catalytic performance of the prepared systems in the selective oxidation of glycerol.

2. Experimental 2.1. Catalyst preparation A low-surface-area Ce0.62 Zr0.38 O2 oxide provided by Grace Davison (20 m2 g−1 ) was used as support. Prior to deposition of the metallic components, this oxide was submitted to successive ageing cycles consisting of a severe reduction treatment (SR) followed by a mild oxidation step (MO) in order to enhance its reducibility as described in a previous study [31]. In brief, 50 g of Ce0.62 Zr0.38 O2 oxide was first reduced in a flow of pure hydrogen at 950 ◦ C (5 h), flushed with helium at 950 ◦ C (1 h) and then cooled to room temperature (RT) under inert gas flow. After completing this severe reduction step, the support was reoxidized at RT with pulses of helium–oxygen mixture (5%O2 /He) and later on was heated under the same gas flowing at 500 ◦ C for 1 h. In all steps, a flow rate of 500 ml min−1 and a heating rate of 5 ◦ C min−1 were used. The resulting oxide will be further referred to as CZ. Two monometallic 1 wt% ruthenium and 1 wt% gold catalysts were prepared by the classical techniques of incipient wetness impregnation and deposition-precipitation, respectively. The ruthenium catalyst (RuCZ) was prepared adding 1 ml of an aqueous solution 0.225 M of Ru(NO)(NO3 )3 (ACS, 99.98% metal basis, Johnson Matthey) drop by drop over 3 g of CZ, meanwhile thoroughly mixing the whole mass. After impregnation, the mixture was placed in an oven at 110 ◦ C to evaporate the water excess. For the gold catalyst (AuCZ), 13 g of CZ support were suspended into ultrapure water (600 ml) under thorough stirring and heating at 60 ◦ C. Then the pH of the suspension was adjusted to 8 by drop-wise adding a solution of Na2 CO3 (0.05 M). These two parameters, temperature at 60 ◦ C and pH ≈ 8, were controlled during the deposition-precipitation of the gold precursor by using automatic equipment as a thermocouple attached to a heating blanket and a TitriLab 856 Ph Stat Titration Workstation (Radiometer Analytical), respectively. The deposition-precipitation of the gold was performed by slowly adding an aqueous solution 0.002 M of HAuCl4 ·3H2 O (ACS, 99.99% metal basis, Johnson Matthey) to the suspension over a period of

4 h. The solution was aged for 1 h at the same temperature and then allowed to cool down to RT. The obtained precipitates were collected by filtration and washed repeatedly until the filtrate resulted negative in the chlorides test using AgNO3 as reagent. The resultant catalyst powders were then dried in an oven at 110 ◦ C overnight. For the preparation of the bimetallic catalysts, the previously prepared fresh AuCZ solid was impregnated with solutions of the appropriate concentration of Ru(NO)(NO3 )3 dissolved in ultrapure water, to achieve three different Au:Ru molar ratios (1Au:2Ru, 1Au:1Ru and 1Au:0.5Ru). After the impregnation treatment, all the catalysts were dried as explained before. After the drying treatment, all catalysts were activated at 350 ◦ C under pure hydrogen (1 h) and further evacuation in helium at the same temperature for another hour. Before its exposure to the air, the catalysts were pulsed with a helium–oxygen mixture (O2 (5%)/He) at room temperature and then cooled under the same mixture flowing in a cooling bath mixture (liquid N2 /acetone) for 30 min to minimize the effects of uncontrolled exothermic reoxidation of the samples [32]. All the Au-containing catalysts preparations were performed in the absence of light, which is known to decompose the metal precursors [33]. 2.2. Catalyst characterization techniques The metal content of the catalysts was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). 20 mg of each catalyst was dissolved in aqua regia and diluted in ultrapure water to a concentration within the detection range of the instrument before performing the analysis. X-ray powder diffraction (XRD) patterns were recorded on a Bruker AXS, D8 Advance, instrument, with a Cu K radiation source ˚ operating at 40 kV and 40 mA. Scans were collected from (1.5418 A) 10◦ to 100◦ with a 0.05◦ in step size and step time of 30 s. X-ray photoelectron spectroscopy measurements of selected samples were acquired on a Kratos Axis Ultra DLD spectrometer. The samples were mounted as pellets using double-sided adhesive tape. Spectra were recorded by using monochromatized Al K␣ radiation (1486.6 eV) at a power of 150 W, in constant analyser energy (CAE) mode, with pass energy of 20 eV. Surface-charging effects were compensated by using the Kratos coaxial neutralization system. The binding energy (BE) scale was referenced to the Zr 3d5/2 (182.64 eV) component present in the mixed oxide support [34]. Quantitative chemical surface analysis was based on XPS peaks areas determined by XPSMultiQuant v. 7.00.92 software (Dr. M. Mohai, Hungarian Academy of Sciences). Due to the overlap of the C 1s peak (coming from adventitious carbon contamination) with the Ru 3d3/2 peak, for quantitative analyses, the Ru 3p peaks were better chosen, and compared with corresponding Au 4f peaks to estimate Au:Ru molar ratios. Samples for examination by transmission electron microscopy were prepared by depositing a small amount of the dry catalyst powders onto a holey carbon film supported on a 300-mesh copper grid. Electron micrographs of the samples were acquired on a JEOL JEM-2010 FEG transmission electron microscope with a structural resolution of 0.19 nm, operating in TEM and STEM mode at an accelerating voltage of 200 kV. From the HRTEM and STEM-HAADF images, the size of ca. 300 particles was measured in order to get the particle size distribution and the average particle diameter (d). The total metal dispersion (also called fraction exposed) was calculated according to D = Ns /Nt , where Ns is total number of surface metal atoms and Nt is total number of atoms in the metal particle. In order to understand the interaction between gold and ruthenium in bimetallic catalysts, it was necessary to determine the composition of individual particles. For this purpose, detailed STEM-HAADF imaging combined with STEM-XEDS studies were

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Table 1 Chemical composition, average particle size and metal dispersion of the mono- and bimetallic catalysts. Catalyst

Metal loading (wt%)a Au

AuCZ 1:0.5AuRuCZ 1:1AuRuCZ 1:2AuRuCZ RuCZ a b

0.85 0.84 0.84 0.83

Au:Ru (mol:mol)a

Particle size (nm)b

Metal dispersion (%)b

2.4 2.7 3.5 3.0 1.4

41 39 26 17 51

Ru 0.33 0.60 1.06 0.96

– 1:0.7 1:1.4 1:2.5 –

Molar ratio was calculated by weight percentage of metal as determined by ICP-AES. Average metal particle size and metal dispersion determined by TEM by measuring at least 300 particles.

carried out in spot mode. At least 75 individual particles from each bimetallic system were analysed in these studies. The electron probe size used in STEM-XEDS experiments was 0.5 nm and each spectrum was collected in an Oxford Instruments INCA system for live times of 30 s per spot. In order to avoid support contribution to the spectrum, particles were analysed from areas at the edge of the aggregates. The X-ray lines used for quantitative analysis were Au-L (9.71 keV) series and Ru-K (19.29 kV) series. Quantification of the X-ray energy spectrums was performed using the Cliff-Lorimer method [35,36]. The metal composition and size of the particles have been plotted as composition-size diagrams [37]. High-resolution XEDS maps were carried out using an FEI Titan G2 80-200 TEM/STEM with ChemiSTEM Technology operating at 200 kV. The catalytic activity tests were performed in a 30 mL glass reactor, where glycerol 0.3 M and the catalyst (substrate/total metal = 1000 mol/mol) were mixed in distilled water in 10 ml total volume and 4 eq. of NaOH as described elsewhere [3]. The reactor was pressurized at 300 kPa of nitrogen and set to 50 ◦ C. Once this temperature was reached, the gas supply was switched to oxygen and the reaction was initiated by stirring. The pressure was maintained at a constant value by continuously feeding pure oxygen. Glycerol oxidation was monitored periodically by taking aliquots

(0.5 ml) from the reactor. The aliquots were filtered and then analysed by high-performance liquid chromatography (HPLC) using a column (Alltech OA-10308, 300–7.8 mm) with UV and refractive index (RI) detection. An aqueous solution of H3 PO4 (0.1 wt%) was used as eluent. Products were identified by comparison with the original samples. Glycerol and all reaction products were provided from Sigma-Aldrich. 3. Results and discussion 3.1. Characterization Metal content analysis by ICP-AES of the catalysts with molar ratios 1Au:0.5Ru, 1Au:1Ru and 1Au:2Ru shows only slight deviations from the theoretical Au:Ru ratios in all bimetallic catalysts (Table 1). Since the first stage of preparation by depositionprecipitation of gold presented a metal deposition efficiency of about 80% in the CZ support, this may explain the discrepancy noted above. Although the preparation of CZ-supported gold catalysts by deposition-precipitation (DP) has the disadvantage of gold loss during the preparation procedure, we found a good reproducibility and an average value of 0.9–1 wt% gold was deposited on the CZ support. A similar observation has also been reported by other

Fig. 1. HAADF-STEM and HREM images of supported monometallic catalysts: (a, b) AuCZ and (d, e) RuCZ. (c, f) Histograms of the particle size distributions and the average particle size diameter for AuCZ and RuCZ, respectively.

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Fig. 2. HAADF-STEM images of supported bimetallic catalysts: (a, d) 1:0.5AuRuCZ, (b, e) 1:1AuRuCZ and (c, f) 1:2AuRuCZ. (g, h and i) XEDS spectra from individuals metal particles arrowed in white in panels d, e and f, respectively.

authors, who suggest that the loss of gold during preparations by DP on ceria–zirconium oxides could possibly corresponds to a fraction of weakly bonded gold species that would be removed during the filtering and washing steps [38]. Representative HREM and STEM-HAADF images from monometallic and bimetallic catalysts are shown in

Figs. 1 a, b, d, e and 2a–f, respectively. Analysis of the particle size distribution from such micrographs (Figs. 1c, f and 4a, d, g) show, in general, a very narrow distribution with most of the particles sizes ranging between 0.5 and 5 nm. The average particle diameter in the pure gold and ruthenium catalysts is about 2.4 and 1.4 nm, respectively. In the case of the

Table 2 Binding energy values of gold and ruthenium phases on the mono- and bimetallic catalysts as determined by XPS measurements. Catalyst

Peak designation Au 4f7/2 BE

AuCZ 1:0.5AuRuCZ 1:1AuRuCZ RuCZ a b

84.3 84.4 84.3

a

Relative % of Ru species Ru 3d5/2

b

Ru 3d5/2 b

Low BE

High BE

Low BE

High BE

281.5 281.6 281.6

282.5 282.5 282.7

– 59.7 73.6 72.1

– 40.3 26.4 27.9

The XPS Au 4f7/2 binding energy is consistent with zero-valent Au. Peaks designated as low BE are assigned to RuO2 and those referred as high BE could correspond to RuO3 or very small RuO2 nanoparticles.

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*

5

0.62Zr0.38O2

* Ce

f

*

Intensity (a.u.)

*

e

* *

1:2AuRuCZ

Ru (100) Au (111)

*

**

*

*

d

1:1AuRuCZ

c

1:0.5AuRuCZ RuCZ

b

AuCZ

a

CZ 10

40

2θ (degree)

38

100 34

70

42

44

Fig. 3. X-ray diffraction patterns of the mono- and bimetallic catalysts. *Fluorite-type cubic structure of ceria–zirconia mixed oxide. The insets show enlarged diagrams of the 2 range of: (a) CZ, (b) AuCZ, (c) RuCZ, (d) 1:0.5AuRuCZ, (e) 1:1AuRuCZ and (f) 1:2AuRuCZ.

bimetallic catalysts, the average particle size is slightly larger than that of the corresponding monometallic ones, being in all cases in the 2.7–3.5 nm range. Analysis of the particle size distribution for all bimetallic catalysts shows that despite the fact that the main contribution to the particle population comes from particles smaller than 5 nm, there are important divergences in the fraction of particles larger than 5 nm between the different catalysts. In the case of 1:0.5AuRuCZ, a small fraction of ca. 3% of particles between 5 and 6 nm was found, while the 1:1AuRuCZ catalyst showed a fraction of ca. 13% corresponding to intermediate size particles between 5 and 15 nm. Finally, the 1:2AuRuCZ catalysts contains ca. 8% of particles in the size range of 5–20 nm and presents also a fraction less than 1% of particles somewhat larger than 20 nm. At first sight, this statistical analysis seems to suggest that the formation of larger particles (>5 nm) in the bimetallic catalyst correlates with the ruthenium content, an increasing ruthenium loading resulting in more intense sintering of particles during the activation process. In fact, the analysis of the percentage of exposed metal (Table 1) also shows a progressive decrease with increasing ruthenium loading. On the other hand, it should be noted that the effect of the addition of ruthenium in the 1:0.5AuRuCZ catalyst has negligible effects on both the metal dispersion and the average particle size when compared to the monometallic gold catalyst. The XRD diagrams corresponding to the supported monometallic and bimetallic catalysts exhibit the diffraction peaks expected for the fluorite-type structure of the support, but no metallic peaks from metallic gold or ruthenium were found (Fig. 3). The only exception is the 1:2AuRuCZ sample, which presents a weak intensity reflection at 2 = 38.3◦ that could be assigned to the presence of a fraction of larger metallic particles. However, no specific metal

phase could be identified because the gold peak locates at 38.2◦ and ruthenium peak at 38.4◦ largely overlap. On the basis of the above characterization results, we can suggest that the activation treatment at 350 ◦ C applied to the monometallic and bimetallic catalysts leads to systems of well dispersed metal particles on the CZ support, however the bimetallic catalyst with the highest metal content (1:2AuRuCZ) was eventually more sensitive to the activation treatment than those with lower metal contents. Determining the contribution of gold and ruthenium to the particle population of the bimetallic catalysts is a key factor to understand the possible interaction between both metal components. In this context, a careful STEM-XEDS analysis of at least 75 individual particles for each catalyst was performed. As an example, X-ray signals collected from small particles are shown in Fig. 2g–i. These results were essential to find a relationship between size and composition of the metal particles. As representative examples, Fig. 4b, e, h shows particle size-composition diagrams obtained from the bimetallic catalysts. The 1:0.5AuRuCZ catalyst shows a very narrow particle size distribution with sizes less than 6 nm, which were found to be predominantly gold-rich in the composition range from 0 at.% Ru up to 20 at.% Ru. In this sample, the largest contribution to the particles population comes from monometallic gold particles (Fig. 4c), a minor fraction, about 2% of particles in the size range close to d ≈ 2 nm contained only Ru and approximately 10% of particles showed the presence of both metals. Analytical electron microscopy studies for the 1:1AuRuCZ catalyst (Fig. 4e, f) show that the largest contribution to the particle population, which amounted up to 46% comes from particles containing both metals. These bimetallic entities were found to be predominantly gold rich (2–40 at.% Ru range) with sizes between 3

Table 3 Surface elemental composition of AuRuCZ catalyst as determined by XPS. Catalyst

1:0.5AuRuCZ 1:1AuRuCZ

Surface concentration (at.%)

Atomic ratio of Au:Ru

Ce

Zr

O

Au

Ru

25.5 25.5

12.8 13.1

58.6 58.8

1.1 0.6

2.0 2.0

1:1.8 1:3.3

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Fig. 4. Particle size distribution for the supported bimetallic catalysts: (a) 1:0.5AuRuCZ, (d) 1:1AuRuCZ and (g) 1:2AuRuCZ; (b, e, h) Plots of particle composition versus size and (c, f, i) relative frequency by particle metal composition (gold, gold–ruthenium and ruthenium) are also presented.

and 10 nm. The portion of monometallic gold particles, which presented a relative frequency of 33%, exhibited a broad size range up to 12 nm but mainly correspond to particles in the 3–6 nm range. The remaining analysed particles, 21%, were present in the form of monometallic ruthenium, with diameters in the size range between 1 and 3 nm. The situation for the 1:2AuRuCZ catalyst was quite different as can be seen in Fig. 4h, i. A remarkable feature in this case is that the metal distribution splits into two groups: an overwhelming majority of small particles (smaller than 6 nm) were found to be ruthenium-rich with composition in the 85–100 at.% Ru range and the second fraction is presented as larger particles (above 9 nm) particles depicting gold rich compositions in the 0–30 at.% Ru range. These results evidence a clear relationship between the higher gold content and the larger particle size in the particle compositiondistribution map of the 1:2AuRuCZ sample. A representative STEM-HAADF image of a bimetallic particle is shown in Fig. 5. Note that the image exhibits a higher intensity area, which corresponds to a gold-rich part (Fig. 5b), and a lower intensity zone in which ruthenium was clearly detected (Fig. 5a). As a whole, this particular particle was found to be Au-rich (80 at.%).

Further particles analysed also evidenced that ruthenium was not homogeneously distributed inside the bimetallic particles. These results suggest that the bimetallic entities are formed as a result of the aggregation of both metals, having been such type of entities previously defined in the literature [23,39]. The resulting particle size composition diagrams plotted as a function of size shown of the bimetallic systems present a portion of bimetallic Au-rich particles, typically larger than the Ru-rich nanoparticles. On the other hand, in the present bimetallic catalysts, monometallic particles appear to be dominant, being increased the frequency of ruthenium monometallic particles with the ruthenium content. To obtain average information about the chemical composition of the surface, a complementary characterization study by XPS was performed. The observed Au 4f7/2 and Ru 3d5/2 binding energies for the monometallic references and bimetallic 1:0.5AuRuCZ and 1:1AuRuCZ catalysts are included in Table 2. In all the samples, the Au 4f7/2 photoelectron peak is located at BE values between 84.3 and 84.4 eV characteristic of metallic gold [40]. The shift of the Au 4f7/2 core-level peak to higher energies, relative to the bulk value of 84.0 eV, has been attributed to particle size effect [41]. In the case of ruthenium, the Ru 3d5/2 peak is significantly shifted

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Fig. 5. HAADF image (left) and the corresponding (right) XEDS spectra of 1:1AuRuCZ catalyst.

to higher binding energies, 281.5–281.6 and 282.5–282.7 eV for the two different species detected (Fig. 6), with respect to that expected for metallic ruthenium (279.7–280.2 eV). This indicates that ruthenium-oxidized species are present in the analysed surface layer. According to literature, the component at lower binding energy, 281.5 eV, could be assigned to RuO2 [42], whereas the peak

at higher binding energy, 282.7 eV, can be assigned to ruthenium in a higher oxidation state, e.g. as RuO3 , or to very small RuO2 nanoparticles [43]. It is well known that metallic ruthenium is readily oxidized even at room temperature. Since the XPS analysis involved transfer of the sample through air before analysis, it is very likely that the observed oxidized Ru species come from the transformation of metallic ruthenium [44]. Quantification of the Ru XPS signal indicates that the predominant surface ruthenium species in the prepared catalysts is RuO2 . The values of the surface Au:Ru atomic ratio of the bimetallic catalysts obtained from this quantitative analysis of the XP spectra, summarized in Table 3, are below the nominal values (Table 1). This suggests significant ruthenium enrichment on the surface. To confirm these XPS observations and to evaluate more accurately the element distribution in the 1:1AuRuCZ catalyst, detailed high-resolution STEM/XEDS maps were acquired (Fig. 7). For example, the X-ray maps of the Au L␣ and Ru K␣ signals shown in Fig. 7c and d, respectively, exhibit an aggregate containing a large number of small particles of ruthenium and an only one gold-rich particle of about 12 nm. This observation indicates that the ruthenium surface enrichment observed by XPS must result from the occurrence of a very large fraction of highly dispersed ruthenium particles covering very efficiently large areas of the support surface, while gold tends to agglomerate into much larger particles. In summary, combining deposition-precipitation and incipient wetness impregnation methods, a series of bimetallic catalysts can be prepared on which small ruthenium-rich nanoparticles coexist with larger gold-rich entities and, more interestingly, with a considerable amount of particles containing both metals. XPS analysis of those particles suggests that there is no any electronic modification in gold or ruthenium, which maintain in a state similar to that of the monometallic references. The largest amount of bimetallic particles is achieved in the system with intermediate ruthenium content. Moreover, increasing the ruthenium loading appears to have the undesired parallel effect of increasing the size of the gold nanoparticles.

3.2. Catalytic properties

Fig. 6. Ru 3d and C 1s photoemission spectra of: (a) 1:1 AuRuCZ; (b) 1:0.5 AuRuCZ and (c) RuCZ catalysts. Light grey peaks correspond to C 1s, whereas dark and medium grey correspond to Ru 3d in low and high oxidation state, respectively.

It has been shown in earlier investigations of glycerol oxidation in the presence of noble metal-supported catalysts that the reaction conditions, such as temperature, oxidant pressure, time of reaction, nature of organic substrate and base concentration may modify both activity and selectivity [26,45,46]. When the target product is a carboxylic acid, precise control of the alkaline pH is critical to achieve good selectivity [47]. Prati et al. demonstrated that supported monometallic gold and bimetallic Au–Ru nanoparticles on

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Fig. 7. Representative STEM-XEDS map from 1:1AuRuCZ catalyst. (a) HAADF image and the corresponding (b) Ce L␣, (c) Au L␣ and (d) Ru K␣ XEDS maps. (e) Overlay map of the Ce, Au and Ru chemical distribution.

100

AuCZ 1:0.5AuRuCZ

80

1:1AuRuCZ 1:2AuRuCZ

Conversion (%)

activated carbon can be selective to carboxylic acids for the oxidation of glycerol under relatively mild conditions (glycerol 0.3 M in water solvent, metal/alcohol = 1/1000 mol/mol, pO2 = 300 kPa, T = 50 ◦ C, and 4 moles of NaOH) [3,48]. Particularly, our preliminary experiments using AuCZ or 1:0.5AuRuCZ catalysts indicated that base is required, both catalysts being found inactive in the absence of NaOH. This behaviour has been attributed to the fact that both Au and Ru are not able to perform the initial dehydrogenation (H-abstraction) step in the glycerol oxidation. According to the literature, the oxidation mechanism proceeds via dehydrogenation process in aqueous solution and subsequently involve hydride abstraction and subsequent oxidation by O2 [18,26,43,49]. We then determined the activity of the bimetallic and monometallic catalysts in the presence of NaOH under mild conditions as is described elsewhere [3]. The results shown in Fig. 8 are of considerable interest and their main features are: (a) the bimetallic 1:0.5AuRuCZ catalyst exhibited exceptionally high activity compared to the rest of catalysts. In addition, this catalyst reached the maximum conversion (≈100%) after 2 h, whereas for the monometallic gold and ruthenium catalysts used as references the maximum conversions were 79 and 28%, respectively, after 3 h of reaction. (b) Clearly ruthenium itself does not have the ability to promote the glycerol

RuCZ 60

40

20

0 0.25

0.5

1

2

3

Reaction time (h) Fig. 8. Reaction profiles of the liquid-phase glycerol oxidation as a function of time in the presence of the series supported bimetallic catalysts with different Au:Ru ratios in comparison with monometallic Au and Ru catalysts. Reaction conditions: 0.3 M glycerol solution, 4 eq. of NaOH, glycerol/metal = 1000 mol/mol, glycerol/NaOH = 1; T = 50 ◦ C; pO2 = 300 kPa.

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9

3000

Turnover frequency (h-1)

2500 2000 1500 1000 500 0 0

43

58 Ruthenium atomic (%)

71

100

Fig. 9. Specific activity for glycerol oxidation over a series of the supported monoand bimetallic catalysts as a function of ruthenium content.

oxidation, however when a small amount of ruthenium was added to AuCZ, the catalyst showed an increased activity compared to AuCZ. (c) The bimetallic 1:1AuRuCZ catalyst exhibit intermediate performance, however its activity is lower compared to the reference monometallic AuCZ. (d) In contrast to the behaviour observed for 1:0.5AuRuCZ and 1:1AuRuCZ bimetallic catalysts, 1:2AuRuCZ shows much poorer glycerol oxidation activities. For a more reasonable comparison, the effect of the Au:Ru molar ratio on catalytic activity is presented in Fig. 9, where the reaction rate per surface metal atom (turnover frequency, TOF) of glycerol oxidation at 0.5 h is plotted as function of ruthenium content for all catalysts investigated. The observed dependence of TOFs with the ruthenium content indicates that the addition of small amounts of ruthenium on AuCZ catalysts has a substantial effect on the activity for the oxidation of glycerol. In particular, the order of activities is: 1:0.5AuRuCZ > AuCZ > 1:1AuRuCZ > RuCZ > 1:2AuRuCZ. The comparison of the TOFs reported in Fig. 9 together with the conversion profiles shown in Fig. 8 demonstrates that glycerol oxidation is strongly favoured with the presence of gold and ruthenium as components of supported bimetallic catalyst at some specific Au:Ru molar ratio (1:0.5AuRuCZ catalyst). In contrast, further increase in ruthenium content results in a decrease in the specific activity. It should be noted that the activity drastically decreased and the TOF varied from 2995 h−1 in 1:0.5AuRuCZ to 574 h− 1 over 1:1AuRuCZ and 217 h−1 over 1:2AuRuCZ. In terms of selectivity, all catalysts investigated presented high selectivity (≥65%) to glyceric acid. For example, the selectivity of AuCZ and 1:0.5AuRuCZ, which presented higher activity, is shown in detail in Fig. 10, whereas a comparison of total conversion and evolution of different products are plotted as function of reaction time. It can be seen that as conversion increased, the selectivity to glyceric acid was almost constant. However, with 1:0.5AuRuCZ, the selectivity to glyceric acid is slightly lower than with AuCZ, and after reaching full conversion (at 2 h) a small degradation was observed at the expense of tartronic acid. Looking at the conversion profiles (Fig. 8) and the metal distribution (Fig. 4) for bimetallic catalysts, some other consideration can be underlined: although the 1:0.5AuRuCZ catalyst presents the lower percentage of bimetallic particles, this sample presented a remarkable activity. The TEM observations point out that this catalyst was more effective for maintaining a narrow particle size distribution and metal dispersion compared to the other bimetallic systems. The presence of new active sites such as small bimetallic entities among active small gold particles on the bimetallic 1:0.5AuRuCZ catalyst results in an increased catalytic activity for the glycerol oxidation. Accordingly, one may ascribe the enhanced activity of the bimetallic catalyst to the interaction of gold and ruthenium components,

Fig. 10. Evolution of glyceric acid, glycolic acid, formic acid, lactic acid and tartronic acid selectivities also is shown glycerol conversion during the reaction in the presence of the (a) AuCZ, (b) 1:0.5AuRuCZ and (c) 1:0.5AuRuCZ-R500 catalysts. Reaction conditions: 0.3 M glycerol solution, 4 eq. of NaOH, glycerol/metal = 1000 mol/mol, glycerol/NaOH = 1; T = 50 ◦ C; pO2 = 300 kPa.

arising from their intimate contact in the bimetallic particles and also to the particle size effect. Therefore, changes in activity can be attributed to a geometric effect rather than an electronic interaction between the two metals. Additionally, the presence of a high number of very small ruthenium particles all over the support surface prevents the agglomeration of gold particles, this improving the performance of the bimetallic catalysts in terms of stability. 3.3. Effect of the reduction treatment at higher temperatures Since the 1:0.5AuRuCZ catalyst presented the best performance among the bimetallic catalysts, it was interesting to study its stability against sintering, so was investigated the effect of the reduction treatment at higher temperature (500 ◦ C). In order to correlate the catalytic performance with structural changes, the bimetallic catalyst was characterized and compared with the monometallic references submitted to the same reduction treatment consisting

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Fig. 11. (a) HAADF-STEM and (b) HREM images of 1:0.5AuRuCZ-R500 catalyst, (c) particle size distribution, (d) composition versus particle size for individual particles and (e) relative frequency by particle metal composition.

Fig. 12. Micrographs of supported monometallic catalysts reduced at 500 ◦ C: (a) AuCZ-R500 and (b) RuCZ-R500. (c, d) Particle size distributions and the average particle size diameter are also reported.

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100

AuCZ 1:0.5 AuRuCZ

80

Total Converison (%)

of heating the catalysts at 500 ◦ C in a flow of H2 (5%) in Ar for 1 h and then a further evacuation at the same temperature in steady flow of helium for another hour. The obtained catalysts were coded as usual followed by “-R500”. Representative HAADF-STEM and HRTEM images of the 1:0.5AuRuCZ-R500 sample are shown in Fig. 11. As can be seen in the histogram (Fig. 11c), the thermal treatment causes only some broadening of the particle size distribution and increment of the mean particle size to 3.3 nm compared with 2.7 nm size of the precursor (Fig. 4a, 1:0.5AuRuCZ catalyst). STEM-XEDS analysis of individual particles (Fig. 11d, e) reveals an increment of the bimetallic particles frequency, with most of the particles in the 0–40 at% Ru range. Interestingly, the metal particles of the 1:0.5AuRuCZ-R500 catalysts were smaller than those on AuCZ-R500 (Fig. 12). When comparing the size evolution of the metallic particles depending on their composition in the bimetallic system before (Fig. 4c) and after (Fig. 11d) the reduction treatment, it is clear that the size of gold particles increase, whereas the ruthenium monometallic and gold–ruthenium bimetallic particles growth proceed marginally. Fig. 12 summarized the TEM results of the monometallic reference catalysts submitted to a reduction pretreatment at 500 ◦ C. In the case of the RuCZ-R500 catalyst, small changes in the metal dispersion (41%) and the mean particle size (1.7 nm) strongly evidence higher thermal stability of the supported ruthenium catalyst compared to the gold reference, since what is observed that the gold dispersion decreased until 16% and mean particle size grow up to 3.8 nm under the same reduction conditions. On the other hand, the decrease in the metallic dispersion caused by particle agglomeration after reduction at 500 ◦ C was much smaller in the bimetallic 1:0.5AuRuCZ-R500 system than the AuCZ catalyst. In effect, the bimetallic catalyst still shows a significant metal dispersion, about 30%, after reduction at high temperature, which clearly proves a better stability. It is important to notice that the metal dispersion loss over the 1:0.5AuRuCZ-R500 was alleviated by the presence of a high number of very small ruthenium nanoparticles, probably due to the formation of bimetallic entities more stable against sintering. For the 1:0.5 AuRuCZ-R500 catalyst, the XPS analysis reveals that the Au 4f7/2 binding energy shifts slightly to lower energies, 84.3 eV, with respect to the precursor 1:0.5AuRuCZ (at 84.4 eV). As discussed in Section 3.1, these values are characteristic of nanometre-sized metallic gold particles. In the case of Ru 3d3/2 , the binding energy values were found similar to those of 1.0.5AuRuCZ, reported in Table 2 (at 281.5 and 282.5), indicating the presence of oxidized ruthenium species, such as RuO2 and RuO3. Finally, the Au content observed by XPS decreased very slightly, down to 0.9 at.%, and the Ru content was about 2.0 at%, resulting in a Au:Ru atomic ratio of 1:2.2, i.e. higher than the 1:1.8 atomic ratio shown by the catalyst before the reduction treatment (Table 2). This finding indicates an increase in the amount of ruthenium exposed on the surface, probably due to the slight agglomeration of gold particles mentioned earlier. When the catalytic activity of the bimetallic system reduced at 500 ◦ C (1:0.5AuRuCZ-R500) is studied (Fig. 13), it is very interesting to note that, although it has a slight loss of activity compared to its counterpart reduced at 350 ◦ C, it is still more active than the monometallic references and the other bimetallic systems treated at 350 ◦ C. In this context, it is likely that the decline in activity found in the 1:0.5AuRuCZ-R500 could be caused by the change in the gold particle size observed by STEM-XEDS analysis of individual particles. The comparison of the turnover frequency (TOF) calculated after 0.5 h of reaction for 1:0.5AuRuCZ-R500 (2367 h−1 ) with the data showed in Fig. 9 corresponding to the monometallic and bimetallic systems treated at 350 ◦ C confirms the synergistic effect of the presence of a small amount of ruthenium in contact with the gold nanoparticles for the oxidation of glycerol even when some agglomeration occurs after the sample has been subjected to

11

1:0.5 AuRuCZ-R500 RuCZ

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Reaction time (h) Fig. 13. Reaction profiles of the liquid-phase glycerol oxidation as a function of time in the presence of supported bimetallic 1:0.5AuRuCZ catalysts reduced at different temperatures (350 and 500 ◦ C) in comparison with monometallic Au and Ru catalysts. Reaction conditions: 0.3 M glycerol solution, 4 eq of NaOH, glycerol/metal = 1000 mol/mol, glycerol/NaOH = 1; T = 50 ◦ C; pO2 = 300 kPa.

treatment at a higher temperature. In terms of selectivity, during the initial stage, the production of glyceric acid was lower compared with other catalysts (Fig. 10) and high amounts of tartronic acid were detected followed by glycolic acid. However, after 0.5 h of reaction, the selectivity to glyceric acid increased up to 76% and it was observed constant until full conversion of glycerol. From the selectivity point of view, comparison of the product distribution after 0.5 h of glycerol oxidation was obtained better result using the 1:0.5AuRuCZ-R500 catalyst.

4. Conclusions The activity for the oxidation of glycerol of a monometallic gold catalyst supported on ceria–zirconia mixed oxide prepared by deposition precipitation has been improved by adding a small loading of ruthenium by incipient wetness impregnation. Gold and ruthenium have been found to be effective catalysts to promote the selective formation of glyceric acid from glycerol. Combining STEM-HAADF imaging and XEDS analysis was found that the composition of Au:Ru nanoparticles varied with size in a systematic way, such that the smallest particles tended to be Ru-rich, while the largest particles were Au-rich. The as-prepared bimetallic system, 1:0.5AuRuCZ, is composed mainly of gold nanoparticles with sizes under 6 nm with a percentage (10%) of bimetallic entities showing quite small size as well. This system has shown a much higher reaction rate per surface metal atom than the monometallic references and the other bimetallic systems with higher ruthenium content. This synergistic effect between the gold nanoparticles and small amounts of ruthenium is maintained even when the system is reduced at higher temperatures as 500 ◦ C, demonstrating a good thermal stability of the bimetallic catalyst, which can be of great importance for its resistance during reaction and to enhance recyclability.

Acknowledgments Funding from the Junta de Andalucía (Project FQM-3994), MINECO/FEDER Project (MAT2013-40823-R) and CSD2009-00013 are gratefully acknowledged. X.C. and A.B.H. thank Ramon y Cajal Program.

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Please cite this article in press as: L.E. Chinchilla, et al., Ru-modified Au catalysts supported on ceria–zirconia for the selective oxidation of glycerol, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.02.030