Solvothermal synthesis and characterization of nanosized zinc aluminate spinel used in iso-butane combustion

Journal of Alloys and Compounds 492 (2010) 500–507

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Solvothermal synthesis and characterization of nanosized zinc aluminate spinel used in iso-butane combustion Wiktoria Staszak, Mirosław Zawadzki ∗ , Janina Okal Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland

a r t i c l e

i n f o

Article history: Received 6 July 2009 Received in revised form 19 November 2009 Accepted 22 November 2009 Available online 26 November 2009 Keywords: Glycothermal synthesis Microwave heating ZnAl2 O4 Pt/ZnAl2 O4 iso-Butane combustion

a b s t r a c t Nanocrystalline (average crystallite size 3 nm), single-phase zinc aluminate with spinel structure (ZnAl2 O4 ) was prepared under solvothermal conditions at medium temperature and pressure (200 ◦ C, 2.5 MPa), and heating time (0.5 h), using zinc acetate and aluminium isopropoxide as precursors, 1,4butanediol as solvent, and by using two different heating methods: microwaves (MW) and conventional. Structural and physicochemical properties of ZnAl2 O4 samples and Pt catalysts supported on them, were investigated using X-ray diffraction (XRD), electron microscopy (HRTEM), infrared spectroscopy (FTIR), H2 chemisorption and N2 adsorption–desorption measurements. Catalytic performances of 0.5 and 1% Pt/ZnAl2 O4 samples were evaluated in the total oxidation of iso-butane reaction. Solvothermally obtained ZnAl2 O4 is characterized by a high specific surface area (above 400 m2 /g) with narrow pore size distributions centered below 5 nm. Additionally, ZnAl2 O4 prepared using MW heating presented superior thermal stability (>900 ◦ C). Pt catalysts supported on such prepared ZnAl2 O4 were found to exhibit good activity in iso-butane combustion, which could be correlated with a high metallic dispersion and low particle sizes. © 2009 Elsevier B.V. All rights reserved.

1. Introduction During the last decades the study of nanomaterials has become a very active area of research because of their unusual properties which are different from those of bulk materials including interesting size-dependent electrical, optical, magnetic, chemical and catalytic properties [1]. Special attention has been focused on the preparation methods because unique properties of nanostructures and resulting applications are intensely related to their morphology, i.e. size and shape of the nanoparticles [2]. Consequently, the development of techniques for controllably preparing nanoparticles with desired shapes, sizes, or forms is very important. At present, there are many physical and chemical methods for the preparation of various inorganic nanomaterials among them solution-phase methods are usually preferred because it is possible to control size and shape of nanoparticles in solution by varying many synthesis parameters such as reaction temperature or time, reaction solution composition, presence of additives including surfactants or templates, etc. [3]. However, most of these liquid-phase methods have some disadvantages, such as complicated process, difficult reaction conditions, necessity of toxic precursors using, difficulties

∗ Corresponding author. E-mail address: [email protected] (M. Zawadzki). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.151

in purification of the final product (e.g. template method), etc. [4,5]. Recently, the solvothermal route in contrast to traditional preparation methods, has become more popular due to its mild conditions and facile process, advantages of environment friendly characteristics and quite low cost as well [6–16]. Solvothermal syntheses usually refer to the synthesis by chemical reactions or transformations of substances in a sealed heated solution above ambient temperature and pressure [17,18]. The specific physicochemical properties of solvents in these conditions can significantly improve the diffusion of chemical species and then markedly advance the nucleation and crystallization of desired phases. The products obtained by solvothermal synthesis usually show very interesting textural properties, superior thermal stabilities, well-controlled chemical composition, and characteristic morphologies which may provide higher potential for catalytic applications as compared to corresponding materials commercially available [19–21]. Solvothermal processes are commonly performed using stainless steel autoclave placed in electric oven. However, this technique usually requires pro-longed reaction time when low temperatures are applied. More recently, conventional solvothermal method is often substituted by the microwaveassisted solvothermal method, which seems to have an advantage of high efficiency and rapid formation of nanoparticles with desired morphology, narrow particle size distribution and slight aggromeration [22–26]. Solvothermal method, with conventional

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or microwave heating, provides then direct and one step route (post-treatment at high temperatures resulting in sintering of nanoparticles is not required) to obtain nanomaterials with high surface area and pore volume, as well as unique surface properties, which can show superior catalytic performances [27–34]. Zinc aluminate with spinel structure ZnAl2 O4 is well-known wide-band gap semiconductor, which can be used as UVtransparent conductor, sensor, dielectric material, and optical material [35–38]. Besides all these properties, ZnAl2 O4 is also used as catalyst or catalyst support due to its unique combination of desirable properties, such as high thermal stability, high mechanical resistance, and low surface acidity [39–42]. However, ZnAl2 O4 would be attracting more attention for catalytic applications if it would be conveniently prepare as high surface material. Glycothermal methods have great potential for the direct preparation of nanocrystalline mixed-metal oxides [43–47]. Because prefix “solvo” indicates any type of solvent used, including organic media such as glycols, the term “glycothermal” means one type of the solvothermal process. This kind of synthesis was exploring among others by Inoue et al. [48–52] to prepare various inorganic materials, including aluminium-based spinels. In this study, nanocrystalline zinc aluminate showing large specific surface area (up to 450 m2 /g) were prepared by the glycothermal method at medium temperature and pressure (200 ◦ C, 2.5 MPa) using 1,4-butanediol and two different ways of autoclave heating (conventional and microwave), and were employed as supports for platinum catalysts. Physicochemical properties of obtained materials were investigated by XRD, HRTEM, FTIR spectroscopy and N2 physisorption. Zinc aluminate supported platinum catalysts were also characterized by H2 chemisorption and tested for catalytic activities in iso-butane combustion reaction. 2. Experimental 2.1. Preparation of ZnAl2 O4 The experiments involved herein were carried out in a typical stainless steel autoclave with electric heating and in a microwave accelerated reaction system (MW Reactor ERTEC, Poland). The synthesis parameters such as temperature/pressure and hold time were varied systematically in order to estimate the optimum values for the formation of zinc aluminate as single-phase nanomaterial with spinel structure and high specific surface area. ZnAl2 O4 was prepared by the glycothermal method using aluminium isopropoxide AIP (Alfa Aesar) and zinc acetate (POCH Poland) as reactants suspended in 1,4-butanediol (1,4 BDO) (Alfa Aesar). Precursors mixture in molar ratio Zn:Al = 1:2 was put into: • teflon vessel and then placed in autoclave with microwave (MW) heating through 30 min, at 200 ◦ C under autogeneous pressure ∼2.5 MPa (product denoted as A); • glass vessel and placed in autoclave with electric (EC) heating and continuous stirring under the same conditions of pressure, temperature and time of reaction as above (product denoted as B). In both cases, the resulting products were washed several times with using NaNO3 (1 M) solution to remove organic compounds. After centrifugation, the obtained gel was extruded to form wires of about 2 mm in diameter, air-dried overnight, calcinated at 550 ◦ C for 3 h, and then crushed and sieved into the pieces of 0.8–1.2 mm. Additionally, ZnAl2 O4 samples were heated at higher temperatures to study their thermal stability. 2.2. Preparation of Pt/ZnAl2 O4 Pt/ZnAl2 O4 catalysts were prepared by the incipient wetness impregnation method using H2 PtCl6 ·x6H2 O as the platinum precursor. The supports A and B, previously calcined at 550 ◦ C, were impregnated with an aqueous solution containing sufficient amount of Pt to result in 0.5 wt.% Pt (denoted as 0.5%Pt/A or 0.5%Pt/B) and 1 wt.% Pt (denoted as 1%Pt/A or 1%Pt/B) catalysts, respectively. The catalysts were dried in an oven at 100 ◦ C overnight and then heated in air at 550 ◦ C for 3 h. Next, the catalyst sample placed into a quartz tube was heated in purified hydrogen flow (50 cm3 /min), with the heating rate of 4 ◦ C/min, to the temperature of 500 ◦ C and then was maintained for 3 h. After cooling down to room temperature under flowing hydrogen, the catalyst sample was passivated by allowing air to leak slowly into the reduction tube. All characterization studies and activity measurements were carried out ex situ, and reduced catalyst samples were handled in air.

501

2.3. Characterization The physicochemical properties of the resulting samples were characterized by several complementary methods. Powder X-ray diffraction (XRD) patterns were obtained using DRON-3 diffractometer with a Ni-filtered Cu K␣ radiation ( = 1.54056 Å), of which the data were collected in steps of 0.05◦ (2) s−1 at the range 10–80◦ (2). XRD patterns were used to phase identification and average size of crystallite calculation. The average grain size was estimated according to the Scherrer equation (peak positions were calibrated from high purity silicon). High resolution transmission electron microscopy (HRTEM) was employed to study the morphology and microstructure of the samples including heat treatment effect. HRTEM images and selected area electron diffraction (SAED) patterns were recorded with Philips CM20 SuperTwin microscope, which at 200 kV provides 0.25 nm resolution. The specimens for HRTEM were prepared by grinding of the samples in a mortar, dispersing it ultrasonically in methanol and placing a droplet of the suspension onto a copper gird covered with perforated carbon. Infrared measurements were carried out to confirm the samples purity. FTIR spectra were recorded by the BRUKER IFS-88 spectrometer in the region of 4000–400 cm−1 with a spectral resolution of 2 cm−1 . The samples were prepared using the normal KBr disc technique. Nitrogen adsorption–desorption isotherms at temperature of liquid nitrogen were used to determine the textural properties (porosity and specific surface area) of the obtained materials. The measurements were carried out using a Autosorb-1 Quantachrome Instruments automated system. The specific surface area was calculated by the Brunauer–Emmet–Teller (BET) method and pore size distribution was analysed from the desorption branch of the isotherms using the Barret–Joyner–Halenda (BJH) method. Each sample was degassed under vacuum in the Quantachrome system at 200 ◦ C for 2 h prior to N2 physisorption. Hydrogen chemisorption studies were applied to determine platinum dispersion in Pt/ZnAl2 O4 catalysts and Pt metal surface area as well. An appropriate amount (about 1 g) of the catalyst was placed in a quartz cell, degassed for 1 h at room temperature (RT), then re-reduced in static H2 (250 Torr) at 400 ◦ C for 2 h and evacuated at the same temperature for 2 h. Next, the sample was cooled under dynamic vacuum to the adsorption temperature. All chemisorption measurements were performed at RT and at pressure range of 120–280 Torr using the conventional volumetric glass apparatus. At this temperature, the time needed to reach the equilibrium conditions with the gas phase was found to be 60 min for all samples. After the first isotherm was recorded, the sample was outgassed for 30 min at adsorption temperature, to remove weakly chemisorbed hydrogen, and a second isotherm was measured. Extrapolation of the linear portion of both the total and reversible hydrogen isotherms to zero pressure (to correct for reversible adsorption on the support) gave the two uptake values used to calculate irreversible hydrogen adsorption on a reduced platinum surface. The uncertainty of the reported uptakes is ±0.3 ␮mol H2 /g cat. The number of accessible metallic atoms was calculated from the amount of irreversible hydrogen uptake, assuming a chemisorption stoichiometry of one hydrogen atom per surface metal atom [53,54]. 2.4. Catalytic experiments The complete oxidation of iso-butane was studied by obtaining curves of hydrocarbon conversion (X) as a function of temperature (light-off curves). The catalytic reaction was carried out in a fixed-bed flow reactor, made of quartz tubing of 10 mm inner diameter, placed in a programmable furnace, by passing a gaseous mixture of iso-butane and air over 1.2 g catalyst. Measurements were taken as the samples were heated stepwise at the temperature in the range of 25–450 ◦ C. A gaseous mixture of iso-butane and air was fed at a flow rate of 15 l/h and catalysts were packed to a constant volume to give a gas hourly space velocity GHSV of 21,000 h−1 for all studies. The gas flow was adjusted by mass flow controllers and the volumetric ratio used for the catalytic combustion was C4 /air = 1:500. The combustion products were analysed on-line using a gas chromatograph (Chromatron GCHF 18.3) with a flame ionization detector (FID) using nitrogen as the carrier gas. Conversion data were calculated by the difference between inlet and outlet concentrations. Activity measurements were obtained once steady state was attained and the data are an average of at least 2 consistent analyses. The conversion data were reproducible within 5% accuracy.

3. Results and discussion 3.1. ZnAl2 O4 characteristics X-ray diffraction patterns of the as-prepared and after thermal treatment in air samples A and B are shown in Figs. 1 and 2, respectively. In both cases, reaction of the mixture of AIP and zinc acetate in 1,4 BDO yielded the nanocrystalline spinel ZnAl2 O4 powder with no other impurities. For the as-prepared samples, it is well visible (Figs. 1 and 2a) that diffraction peaks are broad indicating low crystallinity and line positions confirm zinc aluminate

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Fig. 3. Correlation between crystallite size and heating temperature for A and B. Fig. 1. XRD patterns of A – ZnAl2 O4 prepared under solvothermal conditions with the use of the MW heating: as-prepared (a) and heated at 300 ◦ C (b), 550 ◦ C (c), 700 ◦ C (d), 900 ◦ C (e) and 1050 ◦ C (f).

with the spinel structure (JCPD 82-1036). The dominant peaks at 2 of 31.2, 36.8, 44.8, 49.1, 55.6, 59.3, 65.2, 74.1 and 77.3 can be indexed as (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0) and (5 3 3) diffraction planes, respectively, which are characteristic for spinel structure of zinc aluminate. Thermal stability of ZnAl2 O4 , obtained under solvothermal conditions, was evaluated after heat treatment in air of the as-prepared samples at 300, 550, 700, 900 and 1050 ◦ C for 3 h, respectively. It can be noticed in Figs. 1 and 2, patterns b–f, that increasing calcination temperature from 300 to 1050 ◦ C resulted in a subsequent increase in the degree of crystallinity and in the crystallite size. Single-phase ZnAl2 O4 was also confirmed with no impurities from any residual ZnO or Al2 O3 as observed through other methods, e.g. sol–gel [55] and thermovaporous [56] synthesis. The results of average crystallite size calculation through the Scherrer equation from the line-shape analysis of the (3 1 1) reflection at 2 = 36.8◦ , shows an exponentiallike growth relationship to the calcination temperature. As shown in Fig. 3, the crystallite size increases very slowly with increasing heating temperature and crystal growth becomes sharper at temperatures above 700 ◦ C. The average sizes of ZnAl2 O4 nanoparticles heated in the range of 300–1050 ◦ C are found to vary from

Fig. 2. XRD patterns of B – ZnAl2 O4 prepared under solvothermal conditions with the use of the EC heating: as-prepared (a) and heated at 300 ◦ C (b), 550 ◦ C (c), 700 ◦ C (d), 900 ◦ C (e) and 1050 ◦ C (f).

∼3 to 10.4 nm for sample A and from ∼3 to 16 nm for sample B. It may be generally assumed that zinc aluminate nanocrystallites grow mainly as a result of interfacial reaction. It can be noticed that 1050 ◦ C calcined ZnAl2 O4 sample prepared using microwave heating is characterized by considerable smaller average grain size. These results indicate also that ZnAl2 O4 materials obtained under solvothermal conditions have a much better thermal stability as compared to ZnAl2 O4 obtained through other methods. It has been reported that average crystallite size of zinc aluminate spinel increases from 5 to 30 nm (with increasing calcination temperature to 900 ◦ C) when it was synthesized by sol–gel method [57] or from 2 to 47 nm (with increasing calcination temperature to 1050 ◦ C) when modified sol–gel method was used [58]. Good thermal stability of glycothermally prepared ZnAl2 O4 was also confirmed by N2 adsorption–desorption measurements. Nitrogen adsorption–desorption isotherms were used to study textural properties of glycothermally prepared ZnAl2 O4 samples. Isotherms and corresponding pore size distributions (inset) of the as-prepared samples A and B, and after heat treatment at 550 ◦ C, are shown in Fig. 4. The shape of the isotherms, and the presence of the hysteresis loop (H2 type in accordance with IUPAC classification) at high relative pressure, suggest that all samples under study are basically mesoporous materials. However, in the case of the as-prepared sample A, adsorption takes place mainly at very low

Fig. 4. Nitrogen adsorption–desorption isotherms and corresponding pore size distributions of ZnAl2 O4 samples: as-prepared A (a), as-prepared B (b), B heated at 550 ◦ C (c), and A heated at 550 ◦ C (d).

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Table 1 Textural properties of the as-prepared and air heated ZnAl2 O4 samples. Heating temperature (◦ C)

Sample

As-prepared

300

550

700

Aa

Surface area BET (m2 /g) Pore diameters (nm) Pore volumes (cm3 /g)

449 3.4 0.25

339 3.4 0.23

139 4.3 0.21

133 4.9 0.18

90 5.5 0.17

40 6.5 0.08

Bb

Surface area BET (m2 /g) Pore diameters (nm) Pore volumes (cm3 /g)

410 3.8 0.32

377 3.8 0.29

180 4.8 0.27

151 7.7 0.26

107 7.8 0.28

11 9.0 0.04

a b

900

1050

Microwave heating. Electric heating.

relative pressures what revealed some contribution from microporosity. The pore size distribution analysis of the both samples shows that pore diameters concentrate in the range 2–7 nm with well defined maximum at about 3.4 and 3.8 nm for as-prepared samples A and B, and 4.9 and 4.8 nm for heated at 550 ◦ C, respectively. The results of BET analysis, average pore diameters, and pore volumes for samples A and B: before and after heat treatment up to 1050 ◦ C are summarized in Table 1. Generally, the as-prepared samples have high surface area (449 and 410 m2 /g for sample A and B, respectively) and high pore volume (0.25 and 0.32 cm3 /g for A and B, respectively), which gradually decrease with increasing calcination temperature suggesting that sintering occurs during the heating process corresponding to the increase of crystallite size. However, ZnAl2 O4 calcined at 1050 ◦ C maintained significant value of surface area, i.e. 40 and 11 m2 /g for sample A and B, respectively. Thus, in line with XRD data (Fig. 3) these results indicate also on much better thermal stability of sample prepared using microwave heating. It could be assumed that the nanoporous structure of zinc aluminate samples are formed by the agglomeration of smaller monodispersed particles inherent in the as-prepared samples or larger particles created after subsequent heat treatment. In the first case it forms uniform pores in the micro/mesoporous range, while the second produces random-size pores in the mesoporous range. Fig. 5 shows representative HRTEM images of ZnAl2 O4 samples heated at 550 ◦ C. The primary particles of both samples A and B are spherical in shape and composed of one single crystal domain with the structure of zinc aluminate, as confirmed by distinct lattice spaces of 2.9 and 2.4 nm, which respectively correspond to those of (2 2 0) and (3 1 1) planes of cubic phase ZnAl2 O4 . The nanocrystallinity and crystallography of the 550 ◦ C heated sam-

ples are proven by selected area diffraction analysis. Corresponding SAED patterns, shown in the inset of Fig. 5, randomly taken from these particles exhibit broad diffraction rings with d-spacings about 0.291, 0.244, 0.203, 0.158 and 0.143 nm, which are well consistent with the reflection peaks in Figs. 1 and 2, indicating at once on low crystallinity and/or small particle sizes of obtained samples. The particle sizes were calculated on the basis of size measurements of at least 200 particles for each sample. The measurements were done manually using the ImageJ program [59]. The mean particle size, estimated from Gaussian fit of the histograms of particle size distribution (not shown), is in good agreement with that one obtained from XRD data (Fig. 3) and amount to 3.5 nm for sample A and 3.1 nm for sample B. TEM observations of samples heated at higher temperatures (not shown) revealed gradually increase in particle sizes and effects of agglomeration (nanoparticles became agglomerated together to form more or less porous irregular networks). The corresponding particle size distributions become broader than that for the as-prepared and 550 ◦ C heated samples, and SAED patterns showed that particles are well crystallized (narrow diffraction rings match the XRD peaks exactly). Typical transmittance FTIR spectra of ZnAl2 O4 spinel, heated at 550 and 1050 ◦ C, are shown in Fig. 6. Only spectra of one sample (A) are presented because sample B exhibited very similar spectra. The bands at low energy (400–1000 cm−1 ) are related to the stretching mode (peaks at 669 and 560 cm−1 ) and to the bending mode (peak at 500 cm−1 ) of Al–O in octahedral coordination state (AlO6 octahedral units). In accordance with Preudhomme and Tarte [60] and Lehmann and Hesselbarth [61], these bands are characteristic for zinc aluminate spinel structure. No peaks suggesting any deformation or the presence of inversion in the zinc aluminate spinel

Fig. 5. HRTEM images of ZnAl2 O4 sample A (a) and B (b) after heat treatment at 550 ◦ C; inset shows the corresponding SAED patterns.

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Fig. 6. FTIR spectra of ZnAl2 O4 sample A after heating at 550 ◦ C (a) and 1050 ◦ C (b).

structure (e.g. related to Al3+ in a tetrahedral coordination) are observed. The observed peaks, weakly defined for the 550 ◦ C heated samples, become more sharper with the increasing of heat treatment temperature what is in agreement with XRD results. Similar dependence was reported by Duan et al. [62] during preparation of ZnAl2 O4 using sol–gel method when broad and sharp Al–O bands were obtained for the as-formed and heated at high temperatures samples, respectively. Additionally, in both spectra the bands at 3440 and 1630 cm−1 are present, which can be assigned to vibration mode of chemically bonded hydroxyl groups [63] and to the deformation vibration of water (ıOH ) molecule [64], respectively. It should be noticed that spectrum of 550 ◦ C heated sample exhibits at 1350–1550 cm−1 weak vibration modes of the groups originating from the organic compounds; these bands disappears after high temperature treatment. 3.2. Characteristics and catalytic properties of ZnAl2 O4 –supported Pt catalysts The XRD patterns of the all reduced Pt catalysts, shown in Fig. 7, contain only reflections from zinc aluminate indicating that Pt forms very small particles dispersed on the support A and B. Moreover, metal content is not high enough to be detected by XRD

Fig. 7. XRD patterns of Pt/ZnAl2 O4 catalysts: 1%Pt/A (a), 1%Pt/B (b), 0.5Pt/A (c), and 0.5%Pt/B (d).

technique as was already reported by Valenzuela et al. [65] for the 0.5 wt.% Pt and 1.1 wt.% Pt catalysts supported on ZnAl2 O4 . We suppose that in our case, reductive thermal treatment at 500 ◦ C do not lead to the formation of Pt–Zn alloys. Previously, Pakhomov et al. [66] found by XRD studies that such alloys were formed at 575 ◦ C as a result of reduction of ZnO impurities in the support. Valenzuela et al. [67] also found that H2 treatment promoted Alx Zny crystallization but at much higher temperature (800 ◦ C) than we used during catalyst reduction (500 ◦ C). Small platinum particle sizes in the Pt/ZnAl2 O4 catalysts should be confirmed by electron microscopy observations. Figs. 8 and 9 show typical HRTEM images of the 0.5%Pt supported on A and B, and 1%Pt supported on A and B, respectively. Unfortunately, for all samples only some dark spots with diameter below 1 nm are visible, which may be assigned to platinum particles but the absence of any lattice fringes made univocal confirmation of this supposition impossible. It can be also noticed that after Pt supporting and heating in reducing atmosphere at 500 ◦ C, support particles became sharper and more angular in shape but average particle size was unchanged. The physicochemical properties of all Pt/ZnAl2 O4 catalysts are summarized in Table 2. As can be seen, the BET surface areas of the Pt catalysts are decreased as compared to the support A and B suggesting some pore blockage by platinum nanoparticles. However, the 0.5%Pt and 1%Pt/A catalysts show smaller differences between the total pore volumes of

Fig. 8. HRTEM images of 0.5%PtA (a) and 0.5%Pt/B (b) catalysts; arrows indicate small platinum particles.

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Fig. 9. HRTEM images of 1%Pt/A (a) and 1%Pt/B (b) catalysts; arrows indicate small platinum particles.

Table 2 Physicochemical properties of Pt catalysts supported on ZnAl2 O4 . Catalyst

Surface area BET (m2 /g)

Total pore volume (m3 /g)

Avg. pore diameter (nm)

Irreversible H2 uptake at room temperature (␮mol/g cat.)

Dispersiona (H/Pt)

d (chem)b (nm)

d (XRD) (nm)

Avg. size of support particlesc (nm)

0.5%Pt/A 0.5%Pt/B 1%Pt/A 1%Pt/B

98 122 101 125

0.17 0.19 0.17 0.15

6.5 5.5 5.6 4.2

13.0 12.0 22.5 20.5

0.92 0.88 0.88 0.80

1.2 1.3 1.3 1.4

Not visible Not visible Not visible Not visible

5.6 5.3 5.8 4.3

a b c

Dispersion of Pt – number of hydrogen atoms chemisorbed to the total number of Pt atoms in the given catalyst. Calculated average size of Pt particles based on irreversible H2 chemisorption results and using the formula d (nm) = 1.13/D. Average size of support particles from TEM observations.

the support and of the supported catalysts, indicating that much of Pt nanoparticles may be located on the external surfaces. Sirikajorn et al. reported similar effect for Pd catalysts supported on zinc aluminate [40]. Hydrogen chemisorption measurements were used for the determination of the Pt dispersion in all Pt/ZnAl2 O4 catalysts. Fig. 10 presents typical isotherms of H2 chemisorption obtained at RT on the 0.5%Pt and 1%Pt/A catalysts. Similar isotherms were obtained on the catalysts supported on B sample. The total H2 uptake, as

Fig. 10. Typical isotherms of H2 chemisorption at room temperature on 0.5%Pt/A (a and b) and 1%Pt/A catalysts (c and d). Full and empty symbols represent total and reversible adsorption, respectively.

well as reversible H2 uptake was higher on the 1%Pt/A catalyst as compared to that on the 0.5%Pt/A catalyst. The difference between the total and the reversibly hydrogen adsorption yields the amount of irreversibly chemisorbed hydrogen, which after extrapolation to zero pressure, gives the capacity of a hydrogen monolayer. It can be noted that the adsorption of hydrogen on ZnAl2 O4 support occurred through physical adsorption, because the adsorption capacity at zero pressure, determined by extrapolating the isotherms (not shown) was ∼0 ␮mol H2 /g ZnAl2 O4 . The values of the irreversibly chemisorbed hydrogen on the all Pt/ZnAl2 O4 studied catalysts are shown in Table 2. The dispersion of metallic phase in the Pt/ZnAl2 O4 catalysts was estimated using the assumption of 1 H per surface Pt atom. The range of indicated dispersion, D, ca. 0.92–0.88 for the both 0.5%Pt catalysts, corresponds to crystallite sizes of 1.2–1.3 nm using the relationship d (nm) = 1.13/D [68], thus giving diffraction peaks too broad to be distinguished from the zinc aluminate spinel background by typical XRD patterns, which is consistent with our measurements (Fig. 7). Accordingly, for the both 1%Pt catalysts, the range of dispersion ca. 0.88–0.80, corresponds to crystallite sizes of 1.3–1.4 nm. Also, in this case the diffraction peaks of Pt are too broad to be observed easily by XRD method. It means that for our Pt/ZnAl2 O4 catalysts, for which metal content is low (0.5–1 wt.%), the hydrogen chemisorption measured by the volumetric technique remains the base method. Moreover, the high capacity of Pt to hydrogen chemisorption suggest that in our Pt/ZnAl2 O4 catalysts, reduced in hydrogen at 500 ◦ C, strong interaction between metal phase and the support is rather negligible. In opposite, AguilarRios et al. [69,70] have been reported that the platinum metal in the low loaded Pt/ZnAl2 O4 catalyst undergoes in a strong metal support interaction, which cause very low H2 chemisorption capacities. The authors concluded that the H2 chemisorption method is

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Fig. 11. iso-Butane conversion over 0.5% Pt/ZnAl2 O4 catalysts; inset shows the Arrhenius plots.

Fig. 12. iso-Butane conversion over 1% Pt/ZnAl2 O4 catalysts; inset shows the Arrhenius plots.

not adequate to measurement of the Pt particle size for this type of the catalyst. Bocanegra et al. [71] have found by using the XRD analysis of the 1 wt.% Pt/ZnAl2 O4 catalyst (reduced at 500 ◦ C) that the Pt–Zn alloy was formed, although very low intensities of the diffraction peaks were obtained. This alloy appears to be inactive for the hydrogen chemisorption, dehydrogenation of paraffins [66], and for the oxidation of iso-butene [72]. The catalytic properties of Pt/ZnAl2 O4 catalysts were investigated for iso-butane combustion and the results are shown in Figs. 11 and 12 and summarized in Table 3. On all samples, the iso-butane oxidation starts at low temperatures, below 100 ◦ C, but the conversion increases slowly up to about 250 ◦ C when light-off curves become more sharper reaching the value of 50% and 100% conversion in the range 322–355 ◦ C and 425–444 ◦ C, respectively.

Table 3 Temperatures required for 50% and 100% conversion of iso-butane, turnover frequencies TOF and apparent activation energies Ea for Pt catalysts supported on ZnAl2 O4 . Catalyst

T50% (◦ C)

T100% (◦ C)

TOFa (s−1 )

Ea (kcal/mol)

0.5%Pt/A 0.5%Pt/B 1%Pt/A 1%Pt/B

322 336 335 355

425 439 430 444

0.004262 0.003311 0.001803 0.001457

9.3 11.9 10.5 10.7

a

Molecules of iso-butane reacted per surface Pt per second at 300 ◦ C.

Fig. 13. iso-Butane conversion over 1% Pt/ZnAl2 O4 catalysts at 330 ◦ C as a function of time on stream.

Similar butane light-off performances were reported for other supported platinum catalysts, and T50% = 370 ◦ C [73] or 280–290 ◦ C [74,75] were found for Pt/MgO and Pt/Al2 O3 , respectively, while for Pt supported on zeolites the enhanced activity was observed (T50% < 200 ◦ C) [73]. It can be noticed that for the same metal concentration, the Pt supported on zinc aluminate prepared with the use of the MW heating reaches 50% conversion at lower temperature as compared to the catalyst, which support was obtained with the use of the EC heating. Moreover, the light-off curves for iso-butane combustion shift to a little higher temperature with increasing Pt content from 0.5 to 1 wt.% for both support (A and B). Inset in Figs. 11 and 12 presents the Arrhenius plots, the effect of temperature on the rate of iso-butane oxidation, obtained on the studied catalysts. Turnover frequencies values (TOF, molecules of iso-butane reacted per surface Pt per second) used in the Arrhenius plots were obtained from the activity data showed in Figs. 11 and 12 and H2 chemisorption results, by assuming that under conditions of excess oxygen iso-butane combustion over Pt is first order in respect to C4 H10 and zero order in respect to oxygen. The apparent activation energies Ea , calculated from the Arrhenius plots for total iso-butane combustion amount to 9.3 kcal/mol for 0.5%Pt/A and 11.9 kcal/mol for 0.5%Pt/B catalyst, while for 1%Pt supported on A or B catalysts are the same and amount to 10.5–10.7 kcal/mol (Table 3). Reported activation energies for light alkane oxidation over other Pt catalysts range from 8.8 to 29 kcal/mol [76]. It can be also seen from Table 3 that when activity is expressed as TOF, 0.5%Pt/A catalyst is about 3 times more active than 1%Pt catalyst supported on B. Therefore, the potential of improving the catalytic properties of zinc aluminate using the solvothermal assisted-microwave method seems to be promising. The positive effect of microwave heating during solvothermal synthesis on catalytic properties of obtained materials was already reported for other mixed-metal oxides. Microwave prepared La–Ce–Mn–O [77] and La–Ag–Mn–O–perovskites [78] were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than that ones prepared using electric heating. Microwave-solvothermal process presented also advantages in higher photocatalytic activity for the preparation of Bi2 WO6 [27], ZnWO4 [79] or nitrogen doped titania [34] in comparison to conventional solvothermal process. The evolution of the catalytic activity for iso-butane oxidation reaction as a function of time on stream was also measured for all studied catalyst samples, at constant temperature. Catalyst stability tests were carried out at temperatures which resulted in conversion less than 50%, providing a more sensitive indication

W. Staszak et al. / Journal of Alloys and Compounds 492 (2010) 500–507

of changes of the catalyst performance with the time on stream. Because obtained results are very similar, for example only isobutane conversion over 0.5%Pt/A sample as a function of time on stream at 300 ◦ C is shown in Fig. 13. It is observed that iso-butane conversion does not change after 6 h time on-line indicating on high stability of the catalyst under reaction conditions. 4. Conclusions In this study, properties of zinc aluminate obtained under solvothermal conditions using two different heating methods: microwave and conventional were investigated. It was found that there are some differences between samples prepared using MW (sample A) and electric heating (sample B), especially in average crystallite size, textural properties (specific surface area and porosity) and thermal stability. Generally, in both cases nanocrystalline (d ∼ 3 nm) zinc aluminate with spinel structure and high specific surface area (above 400 m2 /g for the as-prepared samples) with narrow pore size distributions centered below 5 nm was obtained. Thermal stability at high temperatures was significant better for sample A than B: after heating at 1050 ◦ C, average crystallite size and specific surface area amount to 10 nm and 40 m2 /g, respectively, in opposite to 16 nm and 11 m2 /g for sample B. Some differences between Pt catalysts supported on both samples are also found, mainly in Pt dispersion and the catalytic activity towards the iso-butane oxidation reaction. 0.5%Pt supported on ZnAl2 O4 prepared using MW heating showed the best catalytic performance among all catalysts investigated, thought catalytic activity (expressed in T50% ) of others was only insignificantly lower. Good catalytic performances of studied samples seem to arise from a high metallic dispersions and low particle sizes. Based on our catalytic and characterization data it was revealed that Pt supported on zinc aluminate spinel prepared under glycothermal conditions using either microwave or electric heating may be appropriate as catalysts for combustion of dilute iso-butane. Acknowledgements The authors are very grateful to Prof. J. Baran for FTIR measurements, Mrs. L. Krajczyk for TEM studies and Mrs. A. Cielecka for adsorption measurements. References [1] J.N. Park, J. Joo, S.G. Kwon, Y.J. Jang, T.H. Hyeon, Angew. Chem. Int. Ed. 46 (2007) 4623. [2] Y.W. Jun, J.S. Choi, J.W. Cheon, Angew. Chem. Int. Ed. 45 (2006) 3414. [3] L.B. Cushing, V.L. Kolesnichenko, Ch.J. O’Connor, Chem. Rev. 104 (2004) 3893. [4] G. Garnweitner, M. Niederberger, J. Mater. Chem. 18 (2008) 1171. [5] K. Holmberg, J. Colloid Interf. Sci. 274 (2004) 355. [6] G. Demazeau, J. Mater. Chem. 9 (1999) 15. [7] S. Feng, R. Xu, Acc. Chem. Res. 34 (2001) 239. [8] R.I. Walton, Chem. Soc. Rev. 31 (2002) 230. [9] Y. Gao, H. Niu, Ch. Zeng, Q. Chen, Chem. Phys. Lett. 367 (2003) 141. [10] K. Tang, Y. Qian, J. Zeng, Adv. Mater. 15 (2003) 448. [11] Ch. An, Y. Jin, K. Tang, Y. Qian, J. Mater. Chem. 13 (2003) 301. [12] M. Rajamathi, R. Seshardi, Curr. Opin. Solid State Mater. Sci. 6 (2002) 337. [13] W. Tu, H. Liu, J. Mater. Chem. 10 (2000) 2207. [14] S. Gao, J. Zhang, Y. Zhu, C.-M. Che, New J. Chem. 24 (2000) 739. [15] K. Byrappa, T. Adschiri, Prog. Cryst. Growth Ch. 53 (2007) 117. [16] S.H. Yu, J. Ceram. Soc. Jpn. 109 (2001) S65. [17] A. Rabenau, Angew. Chem. Int. Ed. Engl. 24 (1985) 1026. [18] K. Byrappa, I. Yoshimura, M. Yoshimura, Handbook of Hydrothermal Technology, SciTech Publishing Incorporated, 2001. [19] H. Kominami, K. Yukishita, T. Kimura, M. Matsubara, K. Hashimoto, Y. Kera, B.O. Ohtani, Top. Catal. 47 (2008) 155. [20] S. Konar, A. Clearfield, Inorg. Chem. 47 (2008) 3489. [21] T. Thongtem, A. Phuruangrat, S. Thongtem, Mater. Lett. 60 (2006) 3776. [22] S. Verma, P. Joy, Y.B. Khollam, H.S. Potdar, S.B. Deshpande, Mater. Lett. 58 (2004) 1092.

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