SBA-15 System: The Effect of Metal Loading and

Subscriber access provided by NATIONAL UNIV OF SINGAPORE

Article

Bimetallic AgCu/SBA-15 System - The Effect of Metal Loading and Treatment of Catalyst on Surface Properties Joanna Czaplinska, Izabela Sobczak, and Maria Ziolek J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 29 May 2014 Downloaded from http://pubs.acs.org on May 30, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Bimetallic AgCu/SBA-15 System - The Effect of Metal Loading and Treatment of Catalyst on Surface Properties Joanna Czaplinska, Izabela Sobczak*, Maria Ziolek

Adam Mickiewicz University, Faculty of Chemistry, Umultowska 89b, 61-614 Poznan, Poland

ABSTRACT Monometallic (Ag, Cu) and bimetallic (Ag+Cu) catalysts were prepared by metal loading (Ag : Cu = 0.3 and 1.8) on 3-aminopropyl-trimethoxysilane grafted SBA-15 and calcination at 773 K, and either reduction by NaBH4 before calcination or activation in inert gas after calcination. The catalysts treated in this way were fully characterized. Cu/SBA-15 samples contained CuO and oligonuclear [Cuδ+ · · ·Oδ- · · ·Cuδ+]n clusters irrespective of the catalyst treatment. Silver-SBA-15 contained cationic silver in the form of Ag2O which was transformed to metallic Au0 by the reduction with NaBH4 and was not reoxidized during calcination. In bimetallic catalysts, different species were identified depending mainly on Ag:Cu atomic ratio and the post modification treatment. When the excess of copper was applied the core (Ag2O)–shell (CuO) structure of bimetallic phase was formed. If the reduction with NaBH4 was used prior to calcination, the same core–shell structure was present but with higher dispersion of CuO, manifested as a higher basicity of the catalysts revealed as a higher selectivity to acetone in 2-propanol dehydrogenation and to CO2 in methanol oxidation. The use of silver in excess led to the presence of both cationic silver and copper species in calcined AgCu(1)/S(C) material. In the sample reduced with NaBH4 and then calcined (AgCu(1)/S(RC)), metallic copper was partially surrounded by metallic silver. In bimetallic samples Cu-Ag interaction led to the electron transfer from copper to silver species enhancing their redox properties and causing the superior activity in the low temperature total oxidation of methanol to CO2. Keywords: silver, copper, characterization; core-shell structure; oxidation of methanol

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 44

1. INTRODUCTION Nowadays, in the field of heterogeneous catalysis, much attention has been paid to the catalysts containing two metals.1,2 Bimetallic catalysts have attracted increasing attention because of their properties being markedly different from those of either of the constituent metals, and above all, their enhanced catalytic activity, selectivity and stability.3 The most attractive bimetallic catalysts containing noble metals are Au-Pd, Au-Pt, Au-Ag systems supported on various matrices (e.g. TiO2, Fe2O3, MgO, carbons, layered hydroxide, mesoporous silica).2,4-10 The origin of the synergistic effect that exists between two metals is still not fully understood, but is supposed to originate from alloy formation leading to an electronic (ligand) and geometric (ensemble) effects resulting in improved activity of bimetallic catalysts in the oxidation reactions.2 The electronic and geometric modifications can be accomplished by tuning the bimetallic structure, surface composition, particle size and its distribution, all of which are critically dependent on the preparation procedures and postactivation treatments. Very recently, a new bimetallic system of Ag-Cu has been developed.11-13 Silver and copper are metals that for a long time have had many applications in catalytic processes. They have similar chemistries, they are coinage metals found in the same group of the periodic table. However, their catalytic behaviour is quite different. Heterogeneous copper catalysts are well known to be effective in hydrogenation and oxidation, whereas silver catalysts are used mainly in selective oxidation reactions. Lee et al.13 have studied AgCu (core-shell and alloy) bimetallic nanoparticle (NP) systems as new catalysts in oxygen reduction reaction (ORR). It was found that the adsorption sites and the oxygen adsorption energies differed for different NPs configurations. AgCu (alloy) and Cu-NPs exhibited strong adsorption energies and low activation-energy barriers. However, the overly strong oxygen adsorption energy of Cu-NPs hindered the ORR. AgCu-alloy nanoparticle system was proposed as a potential

2 ACS Paragon Plus Environment

Page 3 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


highly efficient catalyst as it showed good catalytic properties and prevented Cu oxidation. On the other hand, silver-modulated SiO2-supported copper catalysts have been studied in selective hydrogenation of dimethyl oxalate to ethylene glycol and a remarkably enhanced performance of bimetallic catalysts for this reaction.12 The improved activity and stability in comparison to monometallic Cu/SiO2 was attributed to the formation of Cu nanoparticles containing Ag nanoclusters on the SiO2 surface. The coherent interactions between Cu and Ag species helped formation of the active Cu+/Cu0 species in a suitable proportion and prevent the transmigration of bimetallic nanoparticles during the hydrogenation process. Mesoporous materials, such as SBA-15, are promising candidates for the support of nanostructured metals because of their narrow pore size distribution, high specific surface area and large pore volume.14 However, the preparation of small-sized metal nanopartciles on “inert” supports such as mesoporous silicas is difficult (because of the lack of anchoring sites).15 From this point of view, silica-supported noble metal catalysts are rather inactive in catalytic processes. Moreover, the noble metal particles on the surface of silica are susceptible to aggregation upon thermal treatment, which causes a dramatic loss of activity. A possible way to enhance the activity and selectivity of the catalyst based on silica is the preparation of bimetallic systems, because the presence of a second metal is able to limit the growth of noble metal nanoparticles and generate synergistic effect between two metals. The aim of this paper is the preparation of bimetallic AgCu/SBA-15 materials in order to obtain new catalysts that can be attractive because of the interactions between Ag and Cu. The focus of this paper is the characterization of AgCu/SBA-15 in comparison to monometallic Cu/SBA-15 and Ag/SBA-15 and determination of the effect of preparation conditions (calcination or reduction and calcination), the metal loading and the pretreatment conditions (the thermal treatment in the reduction conditions) on the physicochemical properties of the catalysts obtained and their catalytic activity.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 44

2. EXPERIMENTAL

2.1. Synthesis of SBA-15 materials. SBA-15 support was prepared according to published procedure.14 Pluronic P123 (Poly(ethylene glycol)-block-Poly(ethylene glycol)block-Poly(ethylene glycol)-block) copolymer was used as a surfactant and TEOS as a source of Si. The reactant mixture consisted of water, 0.2 M hydrochloric acid (performed with 3538% HCl, P.O.Ch.), Pluronic P123 (Sigma-Aldrich) and TEOS (>99,0%, Sigma-Aldrich), with the molar ratio: 1SiO2 : 0.005Pluronic P123 : 1.45HCl : 124H2O. The reagents were not purified before using. After dissolving Pluronic P123 in hydrochloric acid solution, the source of silica was added. The mixture was stirred at 328 K for 8 h and then moved into a PP bottle and heated without stirring at 353 K for 16 h. The solid was filtered, washed with water and finally dried at 333 K for 12 h. The template was removed by calcination at 823 K for 8 h in air in static conditions (temperature rate 6 K min-1).

2.2. Functionalisation of SBA-15 with organosilanes. SBA-15 material was grafted with 3-aminopropyl-trimethoxysilane (APMS) (97%, Sigma-Aldrich) in order to functionalise the support before modification with metals. The grafting was based on the published procedure16 and was as follows: 6 g of the support powder were refluxed in a dry toluene solution (200 mL) containing 15 mL of APMS at 373 K for 18 h. The reagents were not purified before using. The catalyst (NH2-SBA-15) was recovered by filtration followed by washing in dry toluene (200 mL), water (100 mL) and acetonitrile (20 mL). The powder was dried in an oven at 353 K.

2.3. Modification of NH2-SBA-15 with metals (Ag, Cu). To obtain Ag-SBA-15 and Cu-SBA-15 catalysts, the functionalised NH2-SBA-15 material was stirred for 4 h in 25 cm3 of alcohol (ethanol, 96% - POCH S.A.) solution of silver nitrate (AgNO3, ≥ 99.8% Sigma Aldrich) (3 wt.% of Ag as assumed) or copper nitrate (Cu(NO3)2, 99.99% – Sigma


Page 5 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Aldrich) (1 or 6 wt. % of Cu as assumed) at room temperature (RT). The solid was recovered by filtration and washed with 80 cm3 of ethanol. The Ag-SBA-15 and Cu-SBA-15 samples were obtained after drying at 373 K and calcination at 773 K for 4 h. The metal content was analysed by ICP-MS. The preparation procedure of bimetallic system AgCu-SBA-15 was as follows: 25 cm3 of alcohol (ethanol) solution of silver nitrate (3 wt.% of Ag as assumed) and 25 cm3 of ethanol solution of copper nitrate (1 or 6 wt. % of Cu as assumed) were prepared separately and then mixed for 5 min in one flask. Then SBA-15 after grafting (NH2-SBA-15), was stirred for 4 h in the solution prepared (RT). The solid was recovered by filtration and washed with 80 cm3 of ethanol. The AgCu-SBA-15 sample was dried at 373 K and calcined at 773 K for 4 h. The metal content was analysed by ICP-MS. The catalysts obtained were labelled as Ag/S(C), Cu(1)/S(C), Cu(6)/S(C) and AgCu(6)/S(C), AgCu(1)/S(C). Parts of Ag-SBA-15, Cu-SBA-15 and AgCu-SBA-15 samples were reduced with NaBH4 (Sigma-Aldrich). For this purpose the solid samples containing Ag, Cu or Ag and Cu, obtained after filtration and washing with ethanol, were stirred additionally with 60 cm3 of 0.1 M NaBH4 ethanol solution at RT. After 40 min, the solids were recovered again by filtration and washed with ethanol, dried at 373 K and calcined at 773 K for 4 h for the removal of amino-organosilane. The catalysts obtained were labelled as Ag/S(RC), Cu(1)/S(RC), Cu(6)/S(RC)

and

AgCu(6)/S(RC), AgCu(1)/S(RC), where S stands for SBA-15; (C) – calcination, (RC) – reduction and calcination; (1) and (6) – 1 wt.% and 6 wt.% of copper in the synthesis solution, respectively.

2.4. Samples characterization. The materials prepared were characterized using XRD, N2 adsorption/desorption, TEM, ICP, UV-Vis, XPS, H2-TPR and test reaction (2propanol decomposition).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 44

2.4.1. XRD. XRD measurements were carried out on a Bruker AXS D8 Advance diffractometer with Cu Kα radiation (λ = 0.154 nm), with a step size of 0.02° and 0.05° in the small-angle (0.6-8°) and wide-angle (6-60°) range, respectively. X-ray diffraction line broadening analysis was used for characterizing supported Ag and CuO particles. The Scherrer formula was applied to estimate the average particle size (the size error ± 0.5 nm).

2.4.2. N2 adsorption/desorption. The N2 adsorption/desorption isotherms were obtained in Quantachrome Instruments autosorb iQ2 at 77 K. The samples were pre-treated in situ under vacuum at 573 K. The surface area was calculated by the BET method, whereas the volume and diameter of mesopores were determined by BJH method.

2.4.3. Transmission electron microscopy (TEM). For transmission electron microscopy (TEM) measurements the powders were deposited on a grid covered with a holey carbon film and transferred to JEOL 2000 electron microscope operating at 80 kV. EDS elemental mapping analyses were obtained by an Transmission Electron Microscope Hitachi HT7700 equipped with energy-dispersive X-ray spectrometer at an acceleration voltage of 100 kV.

2.4.4. UV-Vis spectroscopy. UV-Vis spectra were recorded using a Varian-Cary 300 Scan UV-Visible spectrophotometer. Sample powders were placed into the cell equipped with a quartz window. The spectra were recorded in the range from 800 to 180 nm. Spectralon was used as the reference material. The measurements were performed for fresh samples (after calcination), samples pretreated in the flow of nitrogen (40 cm3/min) for 2 h at 673 K.

2.4.5. Temperature-programmed reduction (TPR). The temperature-programmed reduction (TPR) of the samples was carried out using H2/Ar (10 vol.%) as a reducing agent


Page 7 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(flow rate = 40 cm3 min-1). Each sample (0.02 g) was packed in a quartz tube, treated in a flow of helium at 673 K for 2 h and cooled to room temperature (RT). Then the sample was heated at a rate of 10 K min-1 to 1173 K in the presence of the reducing mixture. Hydrogen consumption was measured by a thermal conductivity (TCD) detector.

2.4.6. XPS. The XPS spectra of calcined samples were taken on an ESCALAB-210 (VG Scientific–England)

photoelectron

spectrometer

equipped

with

a

monochromatic

microfocused Al Kα X-ray source (1486.6 eV). Binding energies were referenced to the C1s peak from the carbon surface deposit at 284.6 eV.

2.4.7. Test reaction - 2-propanol decomposition. The 2-propanol dehydration and dehydrogenation was performed using a microcatalytic pulse inserted between the sample inlet and the column of a CHROM-5 chromatograph. A portion of 0.05 g of the granulated catalyst was activated at 673 K for 2 h under helium flow (40 cm3 min-1). The 2-propanol (Chempur Poland) conversion was studied at various temperatures (423-573 K) using 3 µl pulses of alcohol under helium flow (40 cm3 min-1). The substrate was vaporized before being passed through the catalyst bed with the flow of helium carrier gas. The products such as propene, 2-propanone (acetone) and diisopropyl ether were identified by CHROM-5 gas chromatograph online with a microreactor. The reaction mixture was separated on a 2-m column filled with Carbowax 400 loaded on Chromosorb W (80–100 mesh) at 363 K in helium flow (40 cm3 min-1) and detected by TCD.

2.5. Catalytic activity - Methanol oxidation. The methanol oxidation reaction was performed in a fixed-bed flow reactor. A portion of 0.04 g of each catalyst of the size fraction of 0.5 < ∅ < 1 mm was placed in the reactor. The samples were activated in argon flow (40 cm3 min-1) at 673 K for 2 h (the rate of heating was 15 K/min). Then, the temperature was decreased to that of the reaction. The reactant mixture of Ar/O2/MeOH (88/8/4 mol %) was supplied at the rate of 40 cm3 min-1. Methanol (Chempur, Poland) was 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 44

introduced to the flow reactor by bubbling argon gas through a glass saturator filled with methanol. The reactor effluent was analyzed using two online gas chromatographs. One chromatograph, GC 8000 Top equipped with a capillary column of DB-1, operated at 313 K – FID detector was applied for analyses of organic compounds and the second, GC containing Porapak Q and 5A molecular sieves columns for analyses of O2, CO2, CO, H2O and CH3OH, had a TCD detector. The columns in the second chromatograph with TCD were heated according to the following program: 5 min at 358 K, temperature increase to 408 K (heating rate 5 K/min), 4 min at 408 K, cooling down to 358 K (for the automatic injection on the column with 5A), 10 min at 358 K, temperature increase to 408 K (heating rate 10 K/min), 11 min at 408 K. Argon was applied as a carrier gas. The outlet stream line from the reactor to the gas chromatograph was heated at about 373 K to avoid condensation of the reaction products.

3. RESULTS AND DISCUSSION Two series of bimetallic catalysts were prepared. In both, the initial content (in the solution) of silver was the same, 3 wt %. In the first series the excess of copper was applied (6 wt %), whereas in the second the excess of silver was used (3 wt % of Ag and 1 wt % of Cu). The bimetallic solutions were used for modification of amino grafted SBA-15 molecular sieves. The atomic Ag:Cu ratio in the final materials was 1.8 for AgCu(1)/S(C) and AgCu(1)/S(RC) and 0.22 and 0.19 for AgCu(6)/S(C) and AgCu(6)/S(RC), respectively (Table 1). Both series were treated in two different manners: i) calcination at 773 K, ii) reduction with NaBH4 and next calcination at 773 K. Moreover, calcined materials were activated at 673 K in the inert gas flow and the changes in the composition of modifiers were detected.

3.1. Chemical composition of the samples. Table 1 presents the metal loadings in SBA-15 catalysts studied. Silver loading in Ag-SBA-15 materials is as assumed (3 wt.%). It 8 ACS Paragon Plus Environment

Page 9 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shows that the modification using grafting with organosilanes is effective for silver incorporation under the conditions used in this work. The amount of copper introduced depends on the metal loading. The modification with 1 wt. % of Cu allows the full copper incorporation, whereas if 6 wt. % of Cu was used, the amount of copper in Cu-SBA-15 samples was lower than assumed (4.7-4.8 wt. %). It is important to stress that the loading of silver in bimetallic SBA-15 samples depends on the content of copper. When the higher amount of Cu was used (6 wt %) silver incorporation into the SBA-15 material was much lower (1.6 – 1.8 wt %) than assumed (3 wt %), whereas the loading of copper was similar as in monometallic materials (4.9-5.0 wt. %). It indicates that the presence of an excess of copper cations in the solution does not allow the incorporation of all silver cations. The use of lower amount of copper (1 wt. %) allows incorporation of 3 wt.% of silver, i.e. all Ag is introduced into the final material.

3.2. Texture/structure characterisation. Textural parameters calculated from N2 adsorption/desorption isotherms of all the samples studied are summarized in Table 1. The surface area of mesoporous SBA-15 support is relatively high and reaches ca. 700 m2/g. These values significantly decrease for the samples modified with Ag, Cu or Ag and Cu. The decrease in pore volumes is also observed. The decrease in the surface area and mesopore volume of the samples modified with metals when compared with the pristine support suggests the collapse of some mesopores or pore blocking as a result of NaBH4 treatment (Table 1). Nitrogen adsorption isotherms (Fig. SI1 - SI) are typical of SBA-15 and according to IUPAC classification they are type IV.17 The isotherms of reduced materials are characterized by low volume of nitrogen adsorption. All samples studied gave well defined XRD patterns (Fig. SI2 - SI) in the small angle range with a main peak at 2θ = 1◦, which indicates the presence of cylindrical hexagonally arranged mesopores.14,18 These cylindrical tubes also show a long range ordering which is



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 44

demonstrated in XRD patterns by two additional peaks in the region 1–3◦. The exception is AgCu(1)/S(RC) whose XRD pattern shows much less pronounced reflections in the range 1-3 indicating some disordering of hexagonal pores. The above conclusions are supported by TEM microphotographs depicted in Fig. 1, which indicate the presence of parallel channels in the structure of all samples studied. Moreover, TEM micrographs (Fig. 1 and Fig. SI3 - SI) show the presence of metal particles located partly in the channels and partly on the surface of SBA-15 .

3.3. Surface characterization – the state of silver and copper. The state of metals (Ag, Cu) in the SBA-15 catalysts prepared was studied by XRD, EDS, UV-Vis, H2TPR and XPS spectroscopy depending on the conditions of pre-treatment of the samples. EDS mapping of copper and silver performed for bimetallic samples AgCu/S (Fig. SI4 - SI) shows the distribution of both elements which indicates the presence of Ag and Cu in the neighborhood. The concentration of metals is in line with the amount of Cu and Ag introduced into the samples (Table 1). It can be a sign of the presence of bimetallic phase which is confirmed by the results described below.

3.3.1. The calcined samples – XRD and UV-Vis studies. XRD patterns in the wide-angle range were examined in order to observe the presence of copper (bulk CuO, Cuo) and silver (Ag2O, Ag0) species on the surface of calcined catalysts. As Fig. 2 shows, the reflexes assigned to CuO at 2Θ = 35.5˚, 38.6˚ and 48.7˚

JCPDS 45-0937, 19

are present in the

diffractograms of both types of copper containing catalysts: bimetallic AgCu(6)/S(C) and monometallic Cu(6)/S(C) with a higher amount of copper. The reflections from CuO are not observed for samples containing 1 wt. % of copper. No crystalline silver phases (metallic or oxides) are observed in the wide-angle XRD patterns of non reduced mono-and bimetallic catalysts containing silver. However, one should remember about the XRD analysis limitations. This method gives reliable results for relatively large crystals and high enough 10 ACS Paragon Plus Environment

Page 11 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentration of species. Therefore, UV–vis spectroscopy was applied as a complementary technique to identify silver and copper species. In Uv–vis spectra of Cu/S(C) and AgCu(6)/S(C) samples shown in Fig. 3, two main absorption bands at ca. 230 and 255 nm are observed. According to literature, the bands in the range 220-260 nm are assigned to the charge transfer transition between the ligand O2- and metal centre Cu2+ of copper oxide (O2→ Cu2+) (LMCT - ligand to metal charge transfer).20-23 The other copper species, oligonuclear [Cuδ+· · ·Oδ-· · ·Cuδ+]n clusters give rise to the band in the region 400-600 nm. 24,25

In the spectra of Cu(6)/S(C) and AgCu(1)/S(C) this band is hidden in the tail of the

spectrum. The band in the range between 400-600 nm, from linear oligonuclear [Cuδ+· · ·Oδ-· · ·Cuδ+]n cluster-like chains possibly inserted into mesoporous channels, is well visible in the spectrum of Cu(1)/S(C). The spectrum of Ag/S(C) show a strong absorbance in the 200-260 nm range (two bands at. 223 and 254 nm), revealing the existence of isolated silver cationic species (charge transfer 4d10→ 4d95s1 for Ag+ highly dispersed on the support and cationic Agnδ+ clusters, respectively.26-28 Moreover, a low-intensity band at ca. 400 nm is observed. It is well known that a surface plasmon band of spherical silver nanoparticles appears at around 400 nm region.29,30 It indicates that Ag/S(C) after calcination contains mainly cationic silver and a small amount of metallic silver nanoparticles. Interestingly, in the spectrum of bimetallic AgCu(6)/S(C), the band at 254 nm shifts to 260 nm. The shift observed is a result of Ag-Cu interactions. The XRD and UV-Vis results indicate, that in AgCu/S(C) samples silver is present mainly in the cationic forms, whereas copper in the form of CuO and [Cuδ+· · ·Oδ-· · ·Cuδ+]n clusters. The types of Cu and Ag species on the surface of SBA-15 change after the reduction of catalysts. Fig. 2 presents the X-ray diffraction patterns of SBA-15 materials reduced with NaBH4 and then calcined. Contrary to Ag/S(C), the presence of Ag-metal particles on the surface of Ag/S(RC) can be postulated on the basis of the XRD reflections observed at 2Θ = 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 44

38˚and 44.3˚. These reflections correspond to the (111) and (200) lattice planes of the cubic structure of Ag, respectively.31,32 The results obtained indicate that there is significant difference in the oxidation states of metals (Ag and Cu) depending on the loading of copper in bimetallic SBA-15. In XRD pattern of AgCu(6)/S(RC) containing 6 wt. % of copper only low intensity shoulder at 38˚ from Ag0 is visible. In contrast, the high intensity reflections at 2Θ = 38˚and 44.3˚ assigned to metallic silver are observed for AgCu(1)/S(RC). It suggests that copper in the form of CuO (the reflections at 2Θ = 35.5˚ and 38.6˚) covers silver cationic species like in core-shell preventing silver reduction in AgCu(6)/S(RC). This effect does not occur in AgCu(1)/S(RC). Interestingly, samples Cu(6)/S(C) and Cu(6)/S(RC) show similar diffraction patterns indicating the presence of CuO on the surface of both catalysts. Moreover, the reflection at 2Θ = 36.8˚ assigned to Cu2O appears for Cu(6)/SBA-15(RC). However, it is important to note that the intense reflection at 2Θ = 43.2˚ assigned to metallic copper is visible in XRD pattern of samples Cu(6)/NH2-S(R) and AgCu(6)/NH2-S(R) (NH2 means the amine loading) reduced before calcination (after drying – Fig. 2). It indicates that Cu0 formed during the reduction process is oxidized to Cu2O and CuO on calcination in air at 773 K. The patterns recorded for samples Cu(1)/S(RC) and AgCu(1)/S(RC) low loaded with copper species are different. In them metallic copper is present on the surface (the reflections at 2Θ = 43.2˚) after reduction and calcination. The results obtained suggest that metallic copper formed after reduction is easier oxidized on calcination in the materials with higher metal loading. The UV-Vis spectra confirm the XRD results obtained for the samples containing silver. The band at ca. 560 nm assigned to metallic copper33 is visible in UV-Vis spectra of Cu(1)/S(RC) sample. The UV-Vis spectra of Ag/S(RC) and AgCu(1)/S(RC) show the characteristic intense ultraviolet-visible band at ca. 400 nm typical of metallic silver (Fig. 3). Moreover, for AgCu(1)/S(RC) the bands at ca. 225 and 260 nm indicate the presence of Cu2+, 12 ACS Paragon Plus Environment

Page 13 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ag+ and Agn clusters.28 It is also important to note that for Ag/S(RC), in comparison to Ag/S(C), the intensity of the band at 223 nm (Ag+) decreases, whereas that of the band at 254 nm (Agnδ+) is still present. The band at ca. 400 nm is not observed for bimetallic sample AgCu(6)/S(RC) confirming core-shell structure formation. The size of silver nanoparticles and CuO crystallites in SBA-15 catalysts was estimated by XRD using the Scherrer formula (Table 1).34 The effect of reduction process as well as the presence of bimetallic phase is evidenced. The largest CuO crystallites are present on the surface of bimetallic AgCu(6)/S(C). They are much larger (52 nm) than those on the surface of Cu(6)/S(C) (35 nm). It can suggest that CuO species agglomerate around silver species forming large crystallites (core-shell structure). The reduction with NaBH4 followed by calcination leads to much lower (ca. 20 nm) CuO particles formed by the oxidation of metallic copper. The presence of bimetallic phase has insignificant influence on the size of silver nanoparticles.

3.3.2. The samples after temperature treatment in the flow of inert gas –UVVis and H2-TPR studies. UV-Vis and H2-TPR studies of the catalysts after heating at 673 K in the flow of inert gas were performed in order to determine the metal state in the samples before testing their catalytic activity. These results reveal the changes in copper and silver species on the catalysts surface upon thermal activation. Figure 4 shows UV-Vis spectra recorded. The results obtained indicate that copper species in the calcined SBA-15 materials are stable and that temperature treatment in the reducing conditions (inert gas flow) does not influence the oxidation state of copper. The UV-Vis spectra of Cu(6)/S(C) and AgCu(6)/S(C) after thermal activation in the inert gas show no change in the position of the bands in the comparison with the spectra of the catalysts after calcination. On the other hand, the silver present in Ag/S(C) and AgCu(1)/S(C) is sensitive to the reduction conditions during activation. Significant changes are visible in the spectrum of these silver catalysts. The band 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 44

at about 400 nm appears indicating the presence of metallic silver. The UV-Vis results indicate that activation at 673 K in a flow of inert gas influences only silver in monometallic Ag/S(C) and in AgCu(1)/S(C) with low loading of copper, in which silver cations are partly reduced to metallic form. Such behaviour is not observed for AgCu(6)/S(C), which means that the addition of 6 wt. % of copper to the silver catalyst and the formation of the core-shell structure prevents silver reduction under heating in the presence of inert gas. This confirms that copper oxide in AgCu(6)/S(C) completely covers the phase of silver species (mainly Ag2O oxides as identified by (H2-TPR)). The activation in the inert gas of Ag/S(RC) sample leads to an increase in the intensity of the band from metallic silver suggesting the reduction of the metal. More information on the copper and silver state on the surface of the catalysts after activation come from the H2-TPR results (Fig. 5). The results of temperature-programmed reduction are in agreement with UV-Vis results (Fig. 4). H2-TPR profiles of Ag-containing monometallic SBA-15 materials (C and RC) do not show any peaks characteristic of the reduction of silver species confirming that Ag after temperature activation in the inert gas is present in the form of silver metallic particles. However, it is important to note that in bimetallic AgCu(6)/S(C), silver is present as Ag2O. The reduction profile of this sample is characterized by a peak at 417 K contributing to the dispersed Ag2O phase.35 Moreover, H2TPR studies allow identification of copper species in SBA-15 materials. According to literature the following reduction processes should be considered:36-38 CuO + H2 → Cu0 + H2O

(H2-TPR peak in the 440 - 480 K range)

(1)

Cu2+ + ½ H2 → Cu+ + H+

(> 480 K)

(2)

Cu+ + ½ H2 → Cu0 + H+

(> 600 K)

(3)

The TPR profiles of Cu(6)/S(C), Cu(6)/S(RC), AgCu(6)/S(C) and AgCu(6)/S(RC) present well distinguished peaks at a temperature below 900 K, characteristic of copper reduction. 14 ACS Paragon Plus Environment

Page 15 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The low temperature peaks at 460-480 K noted in the profiles of all samples containing copper are assigned to the reduction of CuO phase, while the peaks at 489 (Cu(6)/S(C)) and that at ca. 590 K (Cu(6)/S(RC) and AgCu(6)/S(RC)) originate from the reduction of Cu2+ cations (in oligonuclear [Cuδ+ · · ·Oδ- · · ·Cuδ+]n clusters) and Cu+ (in Cu2O), respectively. H2-TPR measurements show a marked effect of the SBA-15 reduction with NaBH4 on the reducibility of CuO oxides. The calcination following the reduction leads to a significant increase in Tmax of the peak assigned to CuO reduction (from 465 to 482 K) in AgCu(6)/S samples indicating much more difficult reduction of copper in this sample. Moreover, in the samples subjected to calcination the reduction temperature of CuO in bimetallic AgCu(6)/S(C) material is lower than that in Cu(6)/S(C) (465 and 473 K, respectively) indicating the easier reducibility of CuO species caused by the presence of silver.

3.3.3. X-ray photoelectron spectroscopy (XPS). More precise information on the oxidation states of copper and silver species can be obtained from XPS study. XPS technique allows us to distinguish the Cu2+ isolated species from Cu+ and Cu0. The Cu2+ has mainly d9 character while Cu+ and Cu0 species have filled d levels. It is necessary to note that the Cu+ ion is difficult to distinguish from zero-valence copper by XPS, but Cu2+ species can be easily identified. XP spectra in the binding energies region characteristic of copper species are shown in Figure 6. The spectra of Cu(6)/S(RC), AgCu(6)/S(C) and AgCu(6)/S(RC) reveal the similar shapes with two main peaks cantered at ca. 933.5 eV and 953.5. eV (for the latter sample at 934.6 and 954.4 eV) assigned to Cu2+ 2p3/2 and 2p1/2, respectively, along with two shake-up satellite peaks at ~943.2 and ~962.5 eV. It is known that these satellite signals are attributed to an electron transfer from a ligand orbital to a d orbital of the metal.23 This transition is impossible for Cu+ and Cu0 species that have filled d levels, but is characteristic of bivalent copper.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 44

Such characteristic XP spectra containing shakeup satellite peaks were identified for the samples (monometallic reduced and calcined as well as both bimetallic) in which an excess of copper was used in relation to silver species. We assigned them to CuO species identified earlier by UV-Vis, XRD, H2-TPR techniques. BE of Cu 2p3/2 peak is shifted to a higher value after the reduction of the samples. It has been stressed in literature that such a shift of Cu 2p3/2 peak towards high energy in the CuO-loaded mesoporous silica may result from better dispersion of copper oxide on mesoporous silica.39,40 Taking into account the values of BE for CuO species in the bimetallic systems, it can be concluded that CuO is better dispersed when reduction with NaBH4 was applied before calcination (a higher BE for AgCu(6)/S(RC) than that for AgCu(6)/S(C)). The XP spectra of three other samples presented in Figure 6 significantly differ from those described above. The shake-up satellite peaks are less intense for Cu(6)/S(C) material and are not present for the two others. There is an agreement that the presence of shake-up peak and the higher values of Cu2p3/2 BE (933 – 934 eV) are two major characteristic features of Cu2+ ions, while lower Cu2p3/2 BE (932–933 eV) and the absence of shake-up peak are typical of reduced copper species.41-43 The XP spectrum of Cu(6)/S(C) exhibits two peaks at 932.4 and 952.3 eV, which are attributable to Cu 2p3/2 and Cu 2p1/2 levels of reduced copper species.41 These peaks are more intense than those corresponding to Cu2+ ions, that was also agreed as a characteristic feature of the reduced copper species. One can postulate the origin of the presented peaks to BE in linear oligonuclear [Cuδ+ · · ·Oδ- · · ·Cuδ+]n clusters. It is in line with the finding from UV-Vis study. Similar species can be postulated for bimetallic calcined sample in which the excess of silver is present (AgCu(1)/S(C)). As reported in literature XPS peaks at 367.9–368.1 eV and 373.9–374.1 eV are typical of metallic Ag 3d5/2 and Ag 3d3/2, respectively.12,41 Moreover, the Ag 3d5/2 BE values of Ag2O and AgO range from 367.6 eV to 367.7 eV and 367.2 eV to 367.4 eV, respectively. These


Page 17 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reports are in contrast to the typical positive core level BE shifts of metal cations in ionic materials. Another literature source mentioned the binding energy in the range 368.6-368.1eV as characteristic of metallic silver (Ag 3d5/2) and 367.1 eV, which is characteristic of Ag2O species.44 The position of XPS peaks depends on the nature of the support and the surrounding of metal species. Therefore, for bimetallic catalysts one can expect a change in peaks position. Huang et al. have observed, the Ag 3d5/2 peak of the as-reduced bimetallic catalysts shifted to relatively higher BE values, as compared with the monometallic Ag/SiO2 catalyst.12 Taking into account the literature search and the results of UV-Vis, XRD, and H2TPR study presented in this paper, one can propose the following interpretation of the XP spectra presented in Figure 7. Calcined silver containing SBA-15 exhibits two Ag 3d5/2 peaks at 366.5 eV and 367.8 eV, coming from Ag2O and Ag0 species, respectively. The domination of cationic silver indicated by UV-Vis and XRD results is confirmed by this XP spectrum. The reduced and calcined Ag/S(RC) sample exhibits the XP spectrum typical of metallic silver (Ag 3d5/2 and Ag 3d3/2 at 367.92 and 373.90 eV, respectively). The presence of an excess of copper species in AgCu(6)/S(C) and AgCu(6)/S(RC) materials causes a shift of BE to higher values (368.4 and 370.2 eV in AgCu(6)/S(C) for cationic and metallic silver, respectively; and 367.9 and 370.4 eV in AgCu(6)/S(RC) for cationic and metallic silver, respectively). In both samples cationic silver species dominates. In contrast, the excess of silver in bimetallic catalysts leads to the appearance of only one silver species: cationic silver characterized by BE at 367.3 and 373.4 eV for AgCu(1)/S(C) and metallic silver characterized by BE at 368.4 and 374.4 eV for AgCu(1)/S(RC). The most important conclusion found from XPS study is the existence of synergetic effect between copper and silver. The increase of BE of Cu 2p3/2 and Cu 2p1/2 in AgCu(6)/S(C) in relation to Cu(6)/S(C) (Fig. 6) indicates the electron transfer from copper to silver. BE growth for Ag 3d5/2 and Ag 3d3/2 in bimetallic samples is caused by the acceptation



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 44

of this electron. This let us to the conclusion that synergism between copper and silver arises from electronic effect.

3.4. Surface characterization – Acidity/basicity of the samples. The acidity/basicity of the catalysts applied in this work was determined in the test reaction of 2propanol dehydration. The 2-propanol decomposition is a test reaction for characterisation of acidic (Brønsted or Lewis) and/or basic properties of solids.45 Dehydration of alcohol to propene and/or di-isopropyl ether requires acidic centres (Lewis or BrØnsted), whereas the dehydrogenation to acetone occurs on the basic sites. It is noteworthy that ether production requires the presence of pairs of Lewis acid-base centres. Some authors46 have reported that acetone formation takes place on redox centres. As shown in Table 2, the activity of SBA-15 support in i-PrOH decomposition at 573 K is very low, but it is clearly evidenced that the only reaction product is propene formed in the dehydration process. It indicates the acidic properties of the sample. However, taking into account the very low activity of SBA-15, the acidity of the pristine support is low. The modification with metals increases the activity (the increase in activity with increasing total metal loading is observed) and changes the acid-base properties of SBA-15 surface. Dehydrogenation towards acetone occurs after modification of mesoporous support with silver and copper. This behaviour is the most pronounced for the samples containing higher loading of Cu (ca. 80-90% selectivity to acetone). It indicates basic and redox properties of the mono- and bimetallic copper catalysts. Basic (Lewis) centres originate from the oxygen in CuO or [Cuδ+· · ·Oδ-· · ·Cuδ+]n clusters, whereas copper ions are responsible for redox activity. In case of silver modification basicity comes from Ag2O and redox character is characteristic for Ag0.

3.4. Catalytic activity - Methanol oxidation. Methanol (MeOH) oxidation can lead to various products depending on the nature of catalysts and the temperature of reaction. 18 ACS Paragon Plus Environment

Page 19 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47-50

Formaldehyde, methyl formate (HCOOCH3), methylal ((CH3O)2CH2) and carbon oxides

are the main products of the oxidation. Formaldehyde (FA) is nowadays the most desirable industrial product, because it is an important intermediate in the synthesis of many chemicals. Methanol oxidation is not only the target process but also the test reaction for acidic-basic and redox centres.47, 50-52 Therefore, analysis of the reaction products allows concluding about the surface properties of the catalysts. As summarized in ref.47, the formation of dimethyl ether or carbon oxides with high selectivity points to highly acidic or highly basic character of catalysts, respectively. The mild oxidation products of methanol (formaldehyde, methyl formate, methylal) indicate bi-functional acid-base character of catalysts. The results of methanol oxidation on SBA-15 catalysts studied in this work are displayed in Tables 3 and 4. A comparison of activity and selectivity in the reaction performed at 523 K (Table 3) shows very high (97- 99 % of methanol conversion) methanol total oxidation to CO2 over three catalysts: Ag/S(C), Ag/S(RC) and AgCu(1)/S(C). All of them contain metallic silver accessible for the reagents. It should be pointed out that as a result of activation (673 K, in the inert gas) of Ag/S(C) material containing silver oxide in as made sample, the surface species change to metallic silver (as documented above). So, metallic silver plays a crucial role in the total oxidation of methanol. However, when bimetallic sample containing an excess of silver (AgCu(1)/S(RC) is reduced by NaBH4 prior to calcination and both metallic phases are formed (Ag0 and Cu0) the activity of the catalyst is very low (2 % of methanol conversion). The strong Ag0 – Cu0 interaction causes a drop in the catalytic activity of metallic silver. Copper oxide species (CuO and oligonuclear [Cuδ+ · · ·Oδ- · · ·Cuδ+]n clusters) exhibit acidic-basic properties which are required for the high selectivity to formaldehyde and methyl formate in methanol oxidation. As Lewis acid sites (LAS) (Cu cationic species) are relatively strong, some of formaldehyde (FA) molecules do not desorb and interact with the other methanol molecules towards methyl formate (MF). So, the presence of MF in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 44

reaction products means the participation of LAS in the reaction pathway. Although both samples, AgCu(6)/S(C) and AgCu(6)/S(RC), have the same core (Ag2O) – shell (CuO) structure, the reduced sample is more active because the dispersion of CuO species is higher, as documented above. With increasing reaction temperature (Table 4) the methanol conversion increases and the selectivity to CO2 becomes more important. Carbon dioxide can be produced by a parallel or consecutive oxidation. In the consecutive process the basic oxygen plays an important role. It comes from copper-oxide species. Therefore, the bimetallic catalyst (AgCu(6)/S(C) in which the core (Ag2O) – shell (CuO) phase is present shows high selectivity (80 %) to carbon dioxide at 623 K and 71 % conversion of methanol. However, the highest total oxidation activity is characteristic for the catalysts containing metallic silver. The most spectacular results were obtained for bimetallic AgCu(1)/S(C) exhibiting very high total oxidation of methanol (96 % conversion and 99 % selectivity to CO2) at 423 K. This material contains metallic silver, CuO and oligonuclear [Cuδ+ · · ·Oδ- · · ·Cuδ+]n clusters, so high activity can be explained by Cu-Ag interaction which leads to the electron transfer from Cu2+ to Ag0 enhancing redox properties of the system. The formation of dimethyl ether in methanol oxidation over all the catalysts studied is negligible indicating the domination of redox (basic) character of the materials prepared over acidic properties, although Lewis acidity is important in the formation of middle oxidation products. Moreover, it is worth of notice that the size of metal nanoparticles on the surface of SBA15 catalysts does not play an important role in MeOH oxidation. It is in line with our earliest studies.53-54

4. CONCLUSIONS


Page 21 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Surface properties of all samples prepared depend on the composition of the materials and the post modification treatment. The species formed in monometallic SBA-15 materials are determined by the preparation method. Scheme 1 shows the pathways for the formation of copper species identified in this study. Amino grafted SBA-15 anchors copper cations which are stabilized by the lattice oxygen. The calcination at 773 K removes propylamine and gives rise to linear oligonuclear [Cuδ+ · · ·Oδ- · · ·Cuδ+]n clusters inserted into mesoporous channels and bulky CuO oxide (Cu(6)/S(C) sample). If, before the removal of amine the reduction with NaBH4 is performed, all copper-oxo species are reduced to metallic copper. The calcination at 773 K of reduced material leads to the oxidation of metallic copper and the formation of CuO (Cu(6)/S(RC) sample). Similarly like copper modified materials, silver containing SBA-15, prepared by calcination of anchored silver species exhibits mainly cationic silver in the form of Ag2O (Ag/S(C) material). The reduction of anchored silver species with NaBH4 before calcination causes the formation of silver metallic clusters which are not reoxidized during calcination at 773 K. Thus Ag/S(RC) sample contains mainly metallic silver species. The introduction of both, copper and silver species significantly changes the surface properties by the formation of different species, depending mainly on the Ag : Cu atomic ratio in the mixture used for the modification, and the post modification treatment. When the excess of copper is applied (Ag:Cu = 0.3) the core–shell structure of bimetallic phase is formed where the core contains cationic silver (Ag2O) and it is surrounded by CuO crystallites (see Scheme 2). If the reduction by NaBH4 is applied prior to calcination, the same core–shell structure is formed but a higher dispersion of CuO species is obtained (AgCu(6)/S(RC) sample). It is manifested by a higher basicity of the catalysts demonstrated by the higher selectivity to acetone in 2-propanol dehydrogenation and to CO2 in methanol oxidation. The shell of CuO protects the core silver cationic species from the reduction by activation of the material with heating in the inert gas.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 44

The use of an excess of silver in the mixture applied for modification (Ag:Cu =1.8) totally changes the types of species formed on SBA-15 surface. Calcined AgCu(1)/S(C) material contains both silver and copper cations, whereas the sample reduced by NaBH4 and next calcined (AgCu(1)/S(RC)) reveals the presence of both metallic silver and metallic copper clusters (Scheme 2). The Cu0 species formed by the reduction with NaBH4 is better stabilized when an excess of Ag is used and therefore is not oxidized to CuO during calcination at 773 K. It can be supposed that metallic copper is partially surrounded by metallic silver clusters. Interestingly, the activation of calcined AgCu(1)/S(C) at 673 K in the presence of inert gas causes the reduction of cationic silver to metallic one but CuO is not reduced under these conditions (Scheme 2). The interaction between CuO and silver species takes place, as concluded from a shift of H2-TPR maximum from CuO reduction and a shift of BE in the XP spectra. Cu-Ag interaction leads to the electron transfer from copper to silver species, enhancing redox properties of the system. Such behaviour leads to the superior activity in low-temperature total oxidation of methanol to CO2. AUTHOR INFORMATION *Corresponding author:

Izabela Sobczak, E-mail: [email protected], Adam Mickiewicz University, Faculty of Chemistry, Umultowska 89b, 61-614 Poznan, Poland Telephone: +48 61 8291305

ACKNOWLEDGEMENTS Financial support from the National Science Centre in Poland (Grant No. DEC2013/10/ST5/00642) is gratefully acknowledged.


Page 23 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information N2 ads./des isotherms, XRD diffractograms in small-angle range, TEM images and EDS mapping of SBA-15 materials. This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1) Bracey, Ch.L.; Ellis, P.R.; Hutchings, G.J. Application of Copper–Gold Alloys in Catalysis: Current Status and Future Perspectives, Chem. Soc. Rev. 2009, 38, 2231-2243. (2) Wang, A.; Liu, X.Y.; Mou, C.-Y.; Zhang, T. Understanding the Synergistic Effects of Gold Bimetallic Catalysts, J. Catal. 2013, 308, 258-271. (3) Ferrando, R.; Jellinek, J.; Johnson, R.L. Application of Copper–Gold Alloys in Catalysis: Current Status and Future Perspectives, Chem. Rev. 2008, 108, 846-890. (4) Hermans, S.; Deffernez, A.; Devillers M., Au–Pd/C Catalysts for Glyoxal and Glucose Selective Oxidations, Appl. Catal. A., 2011, 395, 19-27. (5) Dimitratos, N.; Lopez-Sanchez, J.A.; Anthonykutty, J.M.; Brett, G.; Carley, A.F.; Tiruvalam, R.C.; Herzing, A. A.; Kiely, Ch. J.; Knight, D.W.; Hutchings, G.J. Oxidation of Glycerol Using Gold–Palladium Alloy-Supported Nanocrystals, Phys. Chem. Chem. Phys. 2009, 11, 4952-4961. (6) Dimitratos, N.; Porta, F.; Prati, L. ; Villa, A. Synergetic Effect of Platinum or Palladium on Gold Catalyst in the Selective Oxidation of D-Sorbitol, Catal. Lett. 2005, 99, 181-185. (7) Enache, D.I.; Edwards, J.K.; Landon, P.; Solsona-Espriu, B.; Carley, A.F.; Herzing, A.A.; Watanabe, M.; Kiely, C.J.; Knight, D.W.; Hutchings, G.J. Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Au-Pd/TiO2 Catalysts, Science 2006, 311, 362-365. (8) Shi, Y.; Yang, H.; Zhao, X.; Cao, T.; Chen, J.; Zhu, W.; Yu, Y.; Hou, Z. Au–Pd



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 44

Nanoparticles on Layered Double Hydroxide: Highly Active Catalyst for Aerobic Oxidation of Alcohols in Aqueous Phase, Catal. Commun. 2012, 18, 142-146. (9) Cao, E.; Sankar, M.; Nowicka, E.; He, Q.; Morad, M.; Miedziak, P.J.; Taylor, S. H.; Knight, D.W.; Bethell, D.; Kiely, Ch.J.; et al. Selective Suppression of Disproportionation Reaction in Solvent-Less Benzyl Alcohol Oxidation Catalysed by Supported Au-Pd Nanoparticles, Catal. Today 2013, 203,146 –152. (10) Yang, L.; Qi, M.; Jin, M. Fabrication of SBA-15 Supported Ag@Au–Ag MetalCore/Alloy-Shell Nanoparticles for CO Oxidation, Cryst.Eng.Comm. 2013, 15, 2804-2808. (11) Behnajady, M.A.; Eskandarloo, H. Silver and Copper Co-Impregnated onto TiO2-P25 Nanoparticles and Its Photocatalytic Activity, Chem. Engin. J. 2013, 228,1207–1213. (12) Huang, Y.; Ariga, H.; Zheng, X.; Duan, X.; Takakusagi, S.; Asakura, K.; Yuan, Y. Silver-Modulated SiO2-Supported Copper Catalysts for Selective Hydrogenation of Dimethyl Oxalate to Ethylene Glycol, J. Catal. 2013, 307, 74–83. (13) Shin, K.; Kim, D.H.; Lee, H.M. Catalytic Characteristics of AgCu Bimetallic Nanoparticles in the Oxygen Reduction Reaction, ChemSusChem 2013, 6, 1044-1049. (14) Zhao, D.; Feng, J.; Huo, Q.; Malosh, N.; Fredrickson, G.H.; Chmelka, B.F.; Stucky, G.D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores, Science 1998, 279, 548-552. (15) Liu, X.; Wang, A.; Yang, X.; Zhang, T.; Mou, C.Y.; Su, D.S.; Li, J. Synthesis of Thermally Stable and Highly Active Bimetallic Au−Ag Nanoparticles on Inert Supports, Chem. Mater. 2009, 21, 410-418. (16) Zhang, X.; Sau Man Lai, E.; Martin-Aranda, R.; Yeung, K.L. An Investigation of Knoevenagel Condensation Reaction in Microreactors Using a New Zeolite Catalyst, Appl. Catal. A: Gen. 2004, 261, 109-118. (17) Sing, K.S.; Everett, D.H.; Haul, R.A.; Moscou, L.; Pierotti, R.A.; Rouquerol, J.;


Page 25 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Siemieniewska, T. Reporting Physisorption Data for Gas/Solid Systems, Pure Appl. Chem. 1985, 57, 603-619. (18) Benamor, T.; Vidal, L. Influence of Synthesis Parameters on the Physico-Chemical Characteristics of SBA-15 Type Ordered Mesoporous Silica, Micropor. Mesopor. Mater. 2012, 153, 100-114. (19) Tsoncheva, T.; Rosenholm, J.; Linden, M. Iron and Copper Oxide Modified SBA-15 Materials as Catalysts in Methanol Decomposition: Effect of Copolymer Template Removal, Appl. Catal. A: Gen. 2007, 318, 234-243. (20) Gallo, A.; Tsoncheva, T.; Marelli, M.; Mihaylov, M.; Dimitrov, M.; Dal Santo, V., Hadjiivanov, K. Size Controlled Copper Nanoparticles Hosted in Mesoporous Silica Matrix: Preparation and Characterization, Appl. Catal. B: Environmental 2012, 126, 161– 171. (21) Kong, A.; Wang, H.; Yang, X.; Hou, Y.W.; Shan, Y.K. A facile direct route to synthesize large-pore mesoporous silica incorporating high CuO loading with special catalytic property, Micropor. Mesopor.Mater. 2009,118, 348–353. (22) Lu, X.; Yuan, Y.; Copper-Containing Hexagonal Mesoporous Silicas for Styrene Epoxidation Using Tert-Butylhydroperoxide, Appl. Catal. A. Gen. 2009, 365, 180–186. (23) Chanquia, C.M.; Sapag, K.; Rodriguez-Castellon, E.; Herrero, E.R.; Eimer, G.A. Nature and Location of Copper Nanospecies in Mesoporous Molecular Sieves, J. Phys. Chem. C 2010, 114, 1481-1490. (24) Jiang, Y.; Gao, Q. Preparation of Cu2+/+–VSB-5 and Their Catalytic Properties on Hydroxylation of Phenol, Mater. Lett. 2007, 61, 2212-2216. (25) Chanquía, C.M.; Cánepa, A. L.; Bazán-Aguirre, J.; Sapag, K.; Rodríguez-Castellón, E.; Reyes, P.; Herrero, E.R.; Casuscelli, S.G.; Eimer, G.A. Copper-Containing Spherical Mesoporous

Silicates

Prepared

by

Template-Ion

Exchange:

A

Multitechnique

Characterization and Oxidation Properties, Micropor. Mesopor. Mater. 2012, 151, 2–12.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 44

(26) Zhang, X.; Qu, Z.; Yu, F.; Wang, Y. High-Temperature Diffusion Induced High Activity of SBA-15 Supported Ag Particles for Low Temperature CO Oxidation at Room Temperature, J. Catal. 2013, 297, 264–271. (27) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters, Springer-Verlag, New York, 1995 (28) Miao, S.; Wang, Y.; Ma, D.; Zhu, Q.; Zhou, S.; Su, L.; Tan, D.; Bao, X. Effect of Ag+ Cations on Nonoxidative Activation of Methane to C2 Hydrocarbons, J. Phys. Chem. B 2004, 108, 17866-17871. (29) Suber, L.; Sondi, I.; Matijević, E.; Goia, D. V. Preparation and the Mechanisms of Formation of Silver Particles of Different Morphologies in Homogeneous Solutions, J. Colloid Interface Sci. 2005, 288, 489-495. (30) Dikovska, A.O.; Alexandrov, M.T.; Atanasova, P.L.; Tsankov, N.T.; Stefanov, P.K. Silver Nanoparticles Produced by PLD in Vacuum: Role of the Laser Wavelength Used, Appl. Phys. A 2013, 113, 83-88. (31) Zhang, X.; Qu, Z.; Li, X.; Zhao, Q.; Zhang, X.; Quan, X. In-Situ Synthesis of Ag/SBA-15 Nanocomposites by the “pH-Adjusting” Method, Mat. Lett. 2011, 65, 1892–1895. (32) Huang X.; Yang, M.; Wang, G.; Zhang, X. Effect of Surface Properties of SBA-15 on Confined Ag Nanomaterials via Double Solvent Technique, Micropor. Mesopor. Mater. 2011, 144, 171-175. (33) Pestryakov, A.N.; Petranovskii, V.P.; Kryazhov, A.; Ozhereliev, O.; Pfänder, N.; Knop-Gericke, A. Study of Copper Nanoparticles Formation on Supports of Different Nature by UV–Vis Diffuse Reflectance Spectroscopy, Chem. Phys. Lett. 2004, 385, 173–176. (34) Bergeret, G.; Gallezot, P. in Handbook of Heterogeneous Catalysis; G. Ertl, H. Knozinger, J. Weitkamp, Eds.; VCH: Weinheim, 1997; Vol. 2, pp. 439-464.


Page 27 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Luo, M.; Yuan, X.; Zheng, X. Catalyst Characterization and Activity of Ag–Mn, Ag–Co and Ag–Ce Composite Oxides for Oxidation of Volatile Organic Compounds, Appl. Catal. A: Gen. 1998, 175,121-129. (36) Guidry, T.F.; Price, G.L. The Conversion of 1-Propanamine on Copper-Containing MFI and BEA Zeolites Prepared by Aqueous and Vapor Ion-Exchange, J. Catal. 1999, 181,16-27. (37) Torre-Abreu, C.; Ribeiro, M.F.; Henriques, C.; Delahay, G. Characterisation of CuMFI Catalysts by Temperature Programmed Desorption of NO and Temperature Programmed Reduction. Effect of the Zeolite Si/Al Ratio and Copper Loading; Appl. Catal. B: Environ. 1997, 12, 249-256. (38) Sarkany, J.; d’Itri, J.L.; Sachtler, W.M. Redox Chemistry in Excessively Ion-Exchanged Cu/Na-ZSM-5, Catal. Lett. 1992, 16, 241-249. (39) Wu, Z.; Wang, Y.; Huang, W.; Yang, J.; Wang, H.; Xu, J.; Wei, Y.; Zhu J., Formation of Cubic Ia3d Silicas and Metal Oxide-Loaded Silicas Using a Triblock Copolymer (EO20PO70EO20)Acetate Mixture as Structure Director in Aqueous Solution, Chem. Mater. 2007, 19, 1613–1625.

(40) Kong, A.; Wang, H.W.; Yang, X.; Hou, Y.W.; Shan,Y.K. A Facile Direct Route to Synthesize Large-Pore Mesoporous Silica Incorporating High CuO Loading with Special Catalytic Property, Micropor. Mesopor. Mater. 2009, 118, 348–353. (41) Tsoncheva, T.; Issa, G.; Blasco, T.; Dimitrov, M.; Popova, M.; Hernandez, S.; Kovacheva, D.; Atanasova, G.; Lopez-Nieto, J.M. Catalytic VOCs Elimination Over Copper and Cerium Oxide Modified Mesoporous SBA-15 Silica, Appl. Catal. A: Gen. 2013, 453,1–12. (42) Wagner, C.D.; Riggs, W.M.; Davis, L.E.; Moulder, J.F.; Muilenberg, G.E. Handbook of X-Ray Photoelectron Spectroscopy, PerkinElmer Corp., Palo Alto, CA, 1978. (43) Maluf, S.S.; Nascente, P.A.; Assaf, E.M. CuO and CuO–ZnO Catalysts Supported on CeO2 and CeO2–LaO3 for Low Temperature Water–Gas Shift Reaction, Fuel Process. Technol. 2010, 91, 1438–1445.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 44

(44) Citrin, P.H.; Wertheim, G.K.; Baer, Y. Surface-Atom X-Ray Photoemission from Clean Metals: Cu, Ag, and Au, Phys. Rev. B 1983, 27, 3160–3175. (45) Gervasisni, A.; Fenyvesi, J.; Auroux, A. Study of the Acidic Character of Modified Metal Oxide Surfaces Using the Test of Isopropanol Decomposition, Catal. Lett. 1997, 43, 219-228. (46) Lahousse, C.; Bachelier, J.; Lavalley, J.C.; Lauron-Pernot, H.; Le Govic, A. M. Validity of Using Isopropanol Decomposition as a Test-Reaction for the Characterization of Metal Oxides Basicity; Comparison with Results Obtained from Methylbutynol Decomposition, J. Mol. Catal. 1994, 87, 329-332. (47) Tatibouet, J.M. Methanol Oxidation as a Catalytic Surface Probe, Appl. Catal. A: Gen. 1997, 148, 213-252. (48) Pestryakov, A.N.; Lunin, V.V.; Bogdanchikova, N.; Temkin, O.N.; Smolentseva, E. Active States of Gold in Small and Big Metal Particles in CO and Methanol Selective Oxidation, Fuel 2013, 110, 48-53. (49) Rousseau, S.; Marie, O.; Bazin, P.; Daturi, M.; Verdier, S.; Harlé, V. Investigation of Methanol Oxidation over Au/Catalysts Using Operando IR Spectroscopy: Determination of the Active Sites, Intermediate/Spectator Species, and Reaction Mechanism, J. Am. Chem. Soc. 2010, 132, 10832-10841. (50) Deo, G., Wachs, I.E., Haber, J. Supported Vanadium Oxide Catalysts: Molecular Structural Characterisation and Reactivity Properties, Crit. Rev. Surf. Chem. 1994, 4, 141187. (51) Deo, G., Wachs, I.E. Reactivity of Supported Vanadium Oxide Catalysts: The Partial Oxidation of Methanol, J. Catal. 1994, 146, 323-334.


Page 29 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(52) Busca, G., Elmi, A.S., Forzatti, P. Mechanism of Selective Methanol Oxidation Over Vanadium Oxide-Titanium Oxide Catalysts: A FT-IR and Flow Reactor Study, J. Phys. Chem. 1987, 91, 5263-5269. (53) Sobczak, I., Szrama, K., Wojcieszak, R., Gaigneaux, E.M., Ziolek, M. CuxCryOz Mixed Oxide as a Promising Support for Gold – The Effect of Au Loading Method on The Effectiveness in Oxidation Reactions, Catal. Today 2012, 187, 48-55. (54) Kaminski, P., Sobczak, I., Decyk, P., Ziolek, M., Roth,W.J., Campo, B., Daturi, M. Zeolite MCM-22 Modified with Au and Cu for Catalytic Total Oxidation of Methanol and Carbon Monoxide, J. Phys. Chem. C 2013, 117, 2147-2159.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 30 of 44

Table 1. The metal loading, the size of crystallites and texture parameters of the catalysts Catalysts

Ag Cu Ag, Cu, Ag:Cu, Ag:Cu The size of wt. % wt. % wt. % wt. % as crystallites, as assumed as assumed ICP ICP assumed ICP XRD, nm

SBA-15 Ag/S(C) Cu(1)/S(C) Cu(6)/S(C) AgCu(1)/S(C) AgCu(6)/S(C) Ag/S(RC) Cu(1)/S(RC) Cu(6)/S(RC) AgCu(1)/S(RC)

3 3 3 3 3

1 6 1 6 1 6 1

3 3 1.8 3 3

1 4.8 1 4.9 1 4.7 1

1.80 0.30 1.80

1.80 0.22 1.80

AgCu(6)/S(RC)

3

6

1.6

5.0

0.30

0.19

35(CuO) 17(Ag0) 58(CuO) 23(Ag0) 21(CuO) 28(Ag0), 34 (Cu0) 20(CuO)

Surface Average pore Average pore area volume diameter, m2/g BJH, (ads.) BJH, (ads) cm3g-1 nm 690 0.52 5.2 370 0.38 5.6 450 0.45 5.7 360 0.40 5.1 290 0.41 5.8 370 0.38 5.6 160 0.24 5.3 100 0.16 5.9 160 0.25 5.3 30 0.05 5.8 150

0.23

5.3

30


Page 31 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 2. The results of 2-propanol decomposition carried out at 573 K Catalyst

i-PrOH conv. [%]

Selectivity [%] towards propene

S

1

100

0

Ag/S(C)

14

71

29

Cu(1)/S(C)

4

55

45

Cu(6)/S(C)

65

8

92

AgCu(1)/S(C)

12

20

80

AgCu(6)/S(C)

86

18

82

Ag/S(RC)

34

2

98

Cu(1)/S(RC)

1

29

71

Cu(6)/S(RC)

65

1

99

AgCu(1)/S(RC)

1

8

92

AgCu(6)/S(RC)

77

0

100

acetone

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 32 of 44

Table 3. Catalytic activity and selectivity of SBA-15 type catalysts in the methanol oxidation reaction (523 K) Selectivity [%] Catalyst

MeOH MeOH conv. rate conv. HCHO [mmol/g/min] [%]

HCOOCH3

CH3OCH3

CH3OCH2OCH3

CO2

Ag/S(C)

97

17.3

traces

2

-

-

98

Cu(6)/S(C)

8

2.9

29

52

0.5

0.5

18

AgCu(6)/S(C)

7

1.3

30

55

0.5

0.5

14

Ag/S(RC)

99

17.7

-

-

-

-

100

Cu(6)/S(RC)

3

0.5

32

40

-

-

28

AgCu(6)/S(RC)

12

2.1

14

36

-

-

50

Cu(1)/S(C)

3

0.5

44

46

traces

1

9

Cu(1)/S(RC)

4

0.7

55

traces

traces

AgCu(1)/S(C)

99

17.7

-

traces

-

AgCu(1)/S(RC)

2

0.4

27

53

45 -

100 20

32


Page 33 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 4. Catalytic activity and selectivity carried out at different temperatures on the selected calcined SBA-15 type catalysts (473-623 K) Selectivity [%] Catalyst

Temp. [K]

MeOH MeOH conv. conv. rate [%] [mmol/g/min] HCHO

HCOOCH3

CH3OCH3

CH3OCH2OCH3

CO2

Ag/S(C)

373

3

0.5

-

93

-

-

7

Ag/S(C)

423

8

2.9

-

88

-

-

12

Ag/S(C)

473

93

16.6

traces

5

-

-

95

Ag/S(C)

523

97

17.3

traces

2

-

-

98

Cu(6)/S(C)

473

SBA-15 System: The Effect of Metal Loading and

Recommend Documents