Preparation of bimetallic gold catalysts by redox

Catalysis Today 246 (2015) 216–231

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Preparation of bimetallic gold catalysts by redox reaction on oxide-supported metals for green chemistry applications E.A. Redina ∗ , O.A. Kirichenko, A.A. Greish, A.V. Kucherov, O.P. Tkachenko, G.I. Kapustin, I.V. Mishin, L.M. Kustov N.D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, Moscow 119991, Russia

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Article history: Received 2 August 2014 Received in revised form 9 October 2014 Accepted 3 December 2014 Available online 9 January 2015 Keywords: Bimetallic catalysts Redox reaction method Gold nanoparticles Ethanol Glycerol Lactic acid

a b s t r a c t The major problem in successful preparation of bimetallic supported catalysts is the formation of an extended contact area between the monometallic phases. The selective deposition of Au on the surface of nanoparticles of a primary oxide-supported metal has been performed by a redox method that is based on the reduction of the second metal (M2) ions with hydrogen adsorbed on the surface of first metal (M1) or with M1 itself. The gold containing bimetallic catalysts with different atomic ratios and metal combinations Au–M1 (M1 = Pd, Pt, Rh, Ru, and Au–RuOx ) deposited on oxide supports (TiO2 , SiO2 or ␪-Al2 O3 ) have been prepared by this method with special emphasis on the preparation of low-loaded gold containing bimetallic catalysts. The samples were characterized by X-ray diffraction analysis (XRD), scanning transmission electron microscopy (STEM), temperature programmed reduction (TPR) analysis, CO adsorption, X-ray photoelectron spectroscopy (XPS), Diffuse reflectance Fourier-transform IR spectroscopy of adsorbed CO (DRIFTS-CO). By varying the conditions of the preparation procedure, the direct contact area was obtained either between two metal phases (Au/M1) or between metal and a metal oxide phase (Au/M1Ox ) that changed the catalytic properties of primary supported M1. The prepared bimetallic catalysts exhibited the high activity in various reactions: from the Cl-VOC oxidative degradation to the up-to-date “green” reactions aimed at the synthesis of fine chemicals from ethanol and glycerol. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Bimetallic catalysts are widely used for transformation of the natural and industrial feedstock to useful products [1–3]. Improving the catalytic performance (activity, selectivity and durability) and minimizing the noble metal content are the major advances of bimetallic catalysts. A variety of methods has been used successfully to prepare bimetallic catalysts with an enhanced catalytic activity and unique catalytic properties [3,4]. Commonly, the bimetallic nanoparticles are preliminarily synthesized in a stabilized sol and then are supported on a carrier, yet unstable solid solutions are often formed by this way [5,6]. Another drawback is stabilizing agent removal [7–9]. Non-selective deposition of metals (impregnation, co-precipitation methods) leads to formation of the bimetallic particles with the presence of monometallic ones as well. In addition, the structure of bimetallic particles is hard to control [10–12]. Therefore, the major problem in the preparation of supported bimetallic catalysts is the selective formation of an extended

∗ Corresponding author. Tel.: +7 9152694149. E-mail address: [email protected] (E.A. Redina). http://dx.doi.org/10.1016/j.cattod.2014.12.018 0920-5861/© 2014 Elsevier B.V. All rights reserved.

contact area between the monometallic phases [13,14], i.e. a developed contact area between the monometallic particles (clusters, amorphouse species, nanocrystallites). The selective deposition of a metal on the surface of another metal can be performed by redox methods. The proposed redox methods [14] are based on the reduction of the second metal M2 ions either by the pre-supported first metal M1 (direct redox reaction, DRR) [15–18] or by a reducing agent adsorbed on the surface of the first metal, for instance, hydrogen (the redox reaction with adsorbed species, RRA) [19–22]. The outermost electronic configurations, the atomic radii of the metals, and the low preparation temperature seem to be important factors for the different states of these bimetallic catalysts, and M2 can be monolayer dispersed on the surface of M1 or the surface solid solutions, and even the particles of comparable sizes are formed [19,23]. The M2 deposition can be induced with preadsorbed hydrogen under an inert gas (M1 = Pd, Pt) or only under an H2 gas flow (M1 = Rh, M2 = Ge [24]). The high electrochemical potential of M20 /M2n+ does not guarantee the complete deposition. The nature of the oxide support affects the character and completeness of the M2 deposition as well [24]. Prolong treatment in the redox system results in submonolayer adatom structures [25].

E.A. Redina et al. / Catalysis Today 246 (2015) 216–231

The supported bimetallic catalysts prepared via redox deposition have been studied in the gas-phase reactions of toluene hydrogenation, cyclohexane dehydrogenation, hydrocarbon oxidation, exhaust gas purification and in the liquid-phase reactions (citral hydrogenation, glycerol hydrogenolysis) [14,20,24]. Nevertheless, the catalytic properties of redox prepared bimetallic catalysts on oxide supports are poorly studied, especially in environmental catalysis and catalytic transformations of the biomass derived products. The high standard redox potential of gold makes the redox method the best choice to prepare gold-containing bimetallic catalysts with strong Au–M1 interaction. Despite the reported successful preparation of the supported gold bimetallic catalysts via redox deposition [19,20], the majority of the research groups working on the development of gold-based catalysts used the preparation procedures based on deposition of bimetallic particles from the preliminary synthesized sols. This way excluded strong interaction between the metallic nanoparticles and a support [26] that results in intensive leaching of nanoparticles from the support to the reaction volume during the liquid-phase catalytic processes [27,28], which are the most promising in conversion of natural derived substrates. In our previous studies, we have shown that the redox prepared Au/Pd/TiO2 catalyst is more active and stable in oxidation of sulfur containing volatile organic compounds (SVOC) than the Pd/TiO2 and Au/TiO2 samples [29], and the Au/Ru/Al2 O3 catalyst performs preferential CO oxidation in a mixture with NH3 that is impossible on monometallic samples [30]. Thus obtained Au–CuOx catalytic systems are active in the aerobic gas-phase ethanol selective oxidation to acetaldehyde under very mild conditions [31]. The objective of this work is the elaboration of redox methods for the preparation of oxide-supported M2–M1 (Au–Pd, Au–Pt, Au–Rh, Au–Ru, Au–RuOx ) bimetallic catalysts with different atomic ratios, metal content, and evaluation of their catalytic behavior in the oxidative degradation of dichloromethane and in the oxidation of the platform molecules: ethanol, 1,2-propanediol, glycerol to high-value products. Hydrogenation of glycerol to 1,2-propanediol was also discussed. Especially we were interested in preparation and investigation of the catalysts with low-loadings of the noble metals.

2. Experimental M1 metals were supported as nanoparticles (NPs) on TiO2 (Degussa P25, 45 m2 /g; Acros P25, 61 m2 /g), SiO2 (KSKG, Russia, 300 m2 /g), ␪-Al2 O3 (Russia, 95 m2 /g) by the deposition–precipitation procedure similar to the one described in [29], or by incipient wetness impregnation of the support with an M1 precursor solution followed by thermal decomposition and reduction of the metal precursor [30,31]. The deposition–precipitation procedure included: (1) the preliminary hydrolysis of precious metal complexes in a solution until the formation of polyhydroxocomplexes [32], (2) the deposition of the complexes formed on the support surface in the slurry that is enhanced with alkali addition and heating the slurry, (3) the reduction of deposited hydroxo-compounds with hydrogen. To deposit the Au species, the prepared parent monometallic sample was saturated with H2 , and the required amount of an Au precursor solution (HAuCl4 ) with a definite concentration was added to the monometallic sample. The reduction of M1 catalyst was provided just prior to the hydrogen adsorption and the Au deposition. In some cases, Au deposition followed the M1/support reduction (3) in the slurry (2) without separation of the later from the slurry as the sequential step of M1 preparation (Au/Rh/TiO2 and Au/Pd/TiO2 catalysts). After stirring the slurry for 0.5–2 h under an Ar or H2

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atmosphere, the sample was separated by filtering, washed, and dried. The well-known deposition–precipitation with urea (DPU) method [33] was also used to prepare the reference Au/Pt/TiO2 and Au/RuO2 /Al2 O3 samples. The gold containing samples were prepared avoiding their exposure to UV light and kept in darkness. The catalysts were marked as xAu, xM1, or xAu/yM1, where x and y were wt.%. The metal loading was calculated as: m (metal)·m (support)−1 ·100%. Detailed description of the preparation procedures of the parent M1 catalysts and bimetallic samples can be found in supplementary information (S1–S6). Before testing, a sample was treated in addition as described in a test procedure. The phase composition of the catalysts and the particle size of the supported metal were estimated by X-ray diffraction (XRD) analysis. X-ray diffraction patterns were recorded using a DRON-2 diffractometer with Ni-filtered Cu K␣ radiation ( = 0.1542 nm) in a step scanning mode with the counting time of 0.6 s per step. Identification of the phases was performed by comparison of the position and intensity of the peaks with the data from the files of International Center for Diffraction Data. The crystal size of nanoparticles was calculated from X-ray peak broadening (Scherer equation). The morphology of the supported nanoparticles was studied using a Hitachi SU8000 field-emission scanning electron microscope (FE-SEM) or a high-resolution electron microscope JEM-2000FXII at 200 kV (TEM). Images were acquired in the brightfield STEM or TEM mode at the 30 kV accelerating voltage. Before measurements, the samples were mounted on 3 mm copper grids and fixed in a grid holder [34]. At least 10 images for each catalyst sample were used to calculate particle size distribution and average particle size according to the common equation: dNPs = ni di /n, where ni represents the number of particles with diameter di , n is a total number of calculated particles which is equal to 300. The XPS study was used to evaluate the electronic state of the supported metal NPs in the catalysts and surface composition of the catalysts. X-ray photoelectron spectroscopy (XPS) measurements were performed on Gammadata-Scienta SES 2002 with Al K␣ , h = 1486.6 eV X-ray source at 10 kV and 20 mA. The binding energies were calibrated with the C 1s peak (285 eV) and additional spectra calibration with Au 4f7/2 (84.0 eV), Cu 2p3/2 (932.7 eV), and Ag 3d5/2 (368.3 eV). In order to preserve the gold cations from possible reduction, the XPS data were registered at −50 ◦ C. The pressure in the experimental chamber was kept at 3.7 × 10−10 Torr. High-resolution (HR) spectra were recorded with analyzer energy passage 11.75 eV and density of the data collection 0.1 eV. DRIFT spectra were recorded using a NICOLET “Protege” 460 spectrometer in the interval of 6000–400 cm−1 at a resolution of 4 cm−1 (500 scans). The adsorption of CO was performed at room temperature (20 ◦ C) and CO equilibrium pressure of 15 Torr. Before the experiments, the sample was evacuated for 0.5–1 h at the temperature not exceeded that used to treat the catalyst during its preparation, i.e. 20–60 ◦ C. The metal dispersion and the specific metallic surface area were measured by CO adsorption at 25 ◦ C (Pd) or 35 ◦ C (Pt) in a pressure range of 120–900 Torr using an ASAP 2020 unit (Micromeritics) in the laboratory of “ASPECT-association”. The TPR measurements were performed in the lab-constructed flow system. The sample pretreated in an Ar flow was heated from −50 to 850 ◦ C at the rate 10 ◦ C/min in a 4.6% H2 –Ar gas mixture supplied with a space velocity of 30 ml/min. Then the sample was kept at 850 ◦ C until the hydrogen consumption ceased. To compare the intensities of obtained TPR curves these were specified to one basis (1 g of catalyst). The prepared catalysts were tested in the following reactions: (1) oxidative degradation of dichloromethane (DCM); (2) oxidation of ethanol (EtOH); (3) oxidation of 1,2-propanediol (1,2-PD) and glycerol (Gly) to lactic acid (LA); (4) hydrogenation of glycerol (Gly) to 1,2-propanediol (1,2-PD). Oxidative degradation of DCM in

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air was studied at 200–450 ◦ C in the same way as S-VOC removal [29]. The air feed contained 510–580 ppm DCM and 0.25% or 1.6% water vapor. The outlet gas was analyzed with an FTIR analyzer Gasmet Cr-2000 [35] capable of detecting 12 possible products of chlorinated volatile organic compound (Cl-VOC) destruction. The detected major products were CO2 and HCl, and CO was detected at the level below 6 ppm; the CHCl3 , CH3 Cl, C2 Cl4 concentrations were below 1–2 ppm. The aerobic oxidation of ethanol (EtOH) was carried out in a fixed-bed reactor with EtOH:O2 = 0.3:1 (mol.) and a gas hourly space velocity GHSV = 4600 h−1 at different temperatures in the range of 100–300 ◦ C. Prior to the activity test, the catalysts were treated at 300 ◦ C for 40 min in an air/He flow. The liquid-phase oxidation of 1,2-PD and Gly was performed in the batch-mode in alkali media (a solution of 1 M NaOH) under the oxygen pressure of 4 bar or using air as an oxidant under atmospheric pressure. The analysis of the liquid phase was performed by NMR spectroscopy (Bruker Avance II, 300 MHz) with an external standard (sodium benzoate). The experimental error was within 3%. The gas-phase products were detected by GC (Crystall 5000.2, Porapak Q, Russia). Hydrogenation of Gly was carried out in a liquid phase using a lab-constructed autoclave reactor operated in the batch mode. The obtained liquid products were analyzed by GLC with a FFAP (25 mm × 2 mm) capillary column (Crystall 5000.2). Detection of gaseous products was performed using GC (Crystall 5000.2, Porapak Q). The experimental error was within 5%. Composition of the samples prepared, the main features of their preparation procedures and the test reactions are listed in Table 1.

3. Results and discussion 3.1. Au/M1/support systems 3.1.1. Catalyst preparation and characterization The preparation procedures have been optimized for different Au/M1 pairs based on the studies of the effects of the metal nature and loading, the temperature of M1 deposition and reduction, the slurry and solution concentration, the duration and atmosphere of stirring, the Au/M1 ratio on the completeness of deposition, on the metal dispersion and redox properties, as well as on the catalytic behavior. While supporting M1, the increase in temperature from 60 to 80 ◦ C resulted in complete deposition of Pd, Pt, and Rh precursors at pH = 6.8–7.1. The supported Pd and Rh precursor species were completely reducible with hydrogen even in the slurry at RT or at 60–80 ◦ C, whereas complete reduction to metallic Pt and Ru required heating to 200 and 300 ◦ C, respectively, so the reduction of M1 can be provided either in slurry or in dry (for more details of M1 deposition onto the support and Pt/TiO2 TPR study see Sup. Info. S1–S6). The TPR studies confirmed the presence of palladium as metallic species Pd0 in the samples Pd/TiO2 reduced in the slurry at room temperature, as well as hydrogen consumed with Pd in the form of ␤-PdH. The TPR curves of reduced Pd/TiO2 catalyst (Fig. 1) exhibited two maxima that can be attributed to the Pd induced TiO2 reduction at 250–600 ◦ C [36], as well as the minimum at 60 ◦ C that has been commonly attributed to elaboration of H2 due to ␤-PdH decomposition [37]. The deposition of Au from the precursor solution was provided by the redox reaction with preadsorbed hydrogen either in one-step mode in the slurry after M1 reduction (Au/Pd and Au/Rh samples) or by hydrogen preadsorbed on dry catalyst (Au/Pt and Au/Ru samples), an Au concentration in solution being varied (Table 1). Hydrogen adsorption was performed at room temperature until its consumption ceased, different duration being required depending on a system. To deposit Au on the Pt or Ru surface, the M1/support sample, directly after reduction, was cooled down to RT under the hydrogen

Fig. 1. TPR curves of the TiO2 and 1Pd/TiO2 samples.

flow and kept at this temperature for 30 min with the hydrogen flowing. Then the hydrogen flow was switched off, and an appropriate amount of the HAuCl4 solution with a definite concentration was added to the sample without contact with air. The reduction of Au3+ to Au0 was achieved by preadsorbed hydrogen under the H2 atmosphere (see Sup. Inf. S1–S6). To study the effect of concentration of HAuCl4 solution on the 1Au/1Pt catalyst properties, two different samples of this composition were prepared using the solutions of different concentration. The 1Au/1Pt-D sample was prepared using diluted 3.37·10−3 M HAuCl4 aqueous solution, and the 1Au/1Pt-C sample was obtained from more concentrated 8.16·10−3 M HAuCl4 aqueous solution. The blank preparation, in which the HAuCl4 solution was added to the TiO2 sample after TiO2 treatment at 200 ◦ C under an H2 flow, has shown no Au0 deposition on thus treated TiO2 surface or on the quartz reactor walls. It clearly indicated that no redox reaction occurred without M1 supported on the carrier, and suggested that Au deposited directly on the M1 surface or the M1/support contact perimeter rather than on the support surface. For the M1/support systems reduced in the slurry, HAuCl4 solution was added to the slurry, and the Au redox deposition was carried out under an H2 atmosphere. The use of the H2 atmosphere during stirring of the slurry after the HAuCl4 addition allowed reaching the complete Au deposition even at the high Au:M1 = 1 atomic ratio in 30–60 min. Switching from the H2 flow to the Ar flow, as described in [21], resulted in incomplete Au deposition (94%) and further Pd leaching while washing the catalysts with water (see Supp. Inf. S4–S6). 3.1.1.1. STEM/TEM. The STEM images of the selected monometallic Pt/TiO2 sample and corresponding Au/Pt/TiO2 samples prepared by a redox method are presented in Fig. 2. The STEM images of the Pt/TiO2 catalysts (Fig. 2a) provide evidence for the isolated Pt-NPs of the 3D shape that are clearly recognized, and their size distribution is extremely narrow and uniform. The mean particle size was 2 nm. Redox deposition of Au on the 0.025Au/1Pt and 0.05Au/1Pt samples from a 0.31 to 0.51 mM HAuCl4 solution did not change the particle size distribution. Alternatively, the particle size growth was observed in the samples with the high Au:Pt ratio and high Au loading, the size distribution being dependent on the


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Table 1 The features of bimetallic catalysts prepared by RR method and the processes in which they have been studied. Catalyst composition

M1 (wt.%)

Initial dM1 (nm)

3

0.85a

Au/Pd/TiO2

Au/Pt/TiO2

Au/Rh/TiO2

Au/RuOx /Al2 O3

Au/Ru/Al2 O3

Au/CuOx /SiO2

0.5