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Catalytic combustion of toluene over copper oxide deposited on two types of yttria-stabilized zirconia
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Catalytic combustion of toluene over copper oxide deposited on two types of yttria-stabilized zirconia Anna Białas ∗ , Tomasz Kondratowicz, Marek Drozdek, Piotr Ku´strowski Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 30 October 2014 Received in revised form 22 December 2014 Accepted 4 January 2015 Available online xxx Keywords: ZrO2 –Y2 O3 support Coprecipitation Sol–gel Copper-containing catalysts Volatile organic compounds
a b s t r a c t Yttria-stabilized zirconia (YSZ) was obtained by the coprecipitation at pH 9.0 and by the sol–gel method. These oxide systems crystallized in the tetragonal phase which was revealed by powder X-ray diffraction (XRD). The YSZ supports were mesoporous solids with different kinds of porosity and various BET specific surface areas determined by low-temperature sorption of nitrogen. The incipient wetness technique was used to modify these materials with different amounts of Cu (2.5, 5.0 and 7.5 wt%). The changes in structural and textural properties of the YSZ supports after the CuO introduction were discussed, and the surface composition of the Cu-loaded samples was studied by X-ray photoelectron spectroscopy (XPS). It was found that the intra- and interparticle porosity influences the dispersion of CuO active phase, which is the parameter determining the catalytic activity in the toluene combustion. The material based on coprecipitated YSZ containing 5 wt% of Cu turned out to be the most active catalyst with T50 = 264 ◦ C and the total conversion of toluene observed at 350 ◦ C. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) emitted during processes involving organic chemicals are harmful to organisms and the environment. Various kinds of organic substances belong to this group, e.g. paraffins, olefins, aromatics, ketones, aldehydes, carboxylic acids or chlorinated hydrocarbons [1]. Among the widely encountered VOCs, toluene, emitted during fuel usage and industrial processes, can be listed. This aromatic compound is dangerous for the human nervous system at concentration as low as 100 ppm [2]. If the VOCs’ concentration is low, they can be simply removed by the non-catalytic or catalytic total oxidation [3]. Supported noble metals—particularly platinum or palladium—are the most efficient systems in the VOC’s abatement. On the other hand, such catalysts can be easily poisoned by contaminants and are expensive, and therefore transition metal oxides have been often examined as components of catalysts for the purification of VOCs containing gases [1]. Among them, supported copper and manganese oxides turned out to be the most active materials [4–7]. It was postulated [8] that the adsorption of the toluene molecule is the initial step of toluene oxidation over a CuO–CeO2 /␥-Al2 O3 catalyst. Subsequently, abstraction of hydrogen from the methyl and the phenyl
∗ Corresponding author. Tel.: +48 126632286. E-mail address: [email protected] (A. Białas).
group, abstraction of the carbon atom of the methyl group, and finally, destruction of the aromatic ring, occur. Both lattice and adsorbed oxygen are involved in (i) reoxidation of mildly reduced copper species and (ii) abstraction of hydrogen atoms and scission of C C bonds. It should be noticed that better catalytic performance in the total oxidation of toluene was observed for metal oxides deposited on zirconia than on ␥-alumina [9,10]. This effect was attributed to the formation of active phases exhibiting higher reducibility, because zirconia limits the dispersion of an active phase. Lower reducibility, typical of alumina-supported catalysts, results from strong interactions between an active phase and the support. Larger active phase crystallites facilitate oxygen adsorption on noble metals, which is the rate-determining step in the toluene oxidation occurring according to the Mars van Krevelen or Eley-Rideal mechanisms. Moreover, the presence of oxygen surface vacancies in ZrO2 enables the chemisorption of reactants [9]. Zirconia is also characterized by good mechanical and thermal stability [10,11]. ZrO2 is often more effective in the so-called stabilized form, which contains an addition of another metal oxide (e.g. SiO2 , La2 O3 , CaO, Y2 O3 or Ce2 O3 ) appearing in the fluorite structure and forming a solid solution with zirconia. The presence of additives causes that zirconia crystallizes in the tetragonal phase, exhibiting higher specific surface area than more stable monoclinic ZrO2 . Yttrium was recognized as the best dopant of zirconia in the case of the catalysts for oxidation processes, because it increases the number of available oxygen vacancies, helping to store oxygen and return it at low
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temperatures. The content of 1–10 mol.% of yttria in ZrO2 ensures the highest ion conductivity [11]. The aim of the presented work was the synthesis of zirconia stabilized by 5 mol.% of yttria using two various methods: coprecipitation (at controlled pH of 9.0) and sol–gel. The obtained yttria-stabilized zirconia materials with different structural and textural properties were used as supports for a copper oxide phase. The catalytic activity of the obtained CuO/YSZ materials was studied in the total oxidation of toluene. The influence of the support properties as well as dispersion and content of CuO on the catalytic performance was discussed. 2. Experimental 2.1. Synthesis of catalysts To obtain yttria-stabilized zirconia (5 mol.% of Y2 O3 ), two methods were applied: coprecipitation (c) and sol–gel (s). In the coprecipitation procedure, zirconyl chloride octahydrate (23.85 g) and yttrium(III) nitrate hexahydrate (2.99 g) (both supplied by Sigma-Aldrich) were dissolved together in deionised water (50 mL). This solution was added dropwise to another solution (370 mL) containing 1.2 wt% of ammonia (Lach-ner) under stirring till achieving pH 9.0. The obtained precipitation was stirred for 30 min, then filtered and washed a few times with deionised water to remove chloride and ammonium ions. In the sol–gel synthesis, the two separate solutions containing (I) 2.99 g of yttrium(III) nitrate hexahydrate in 1-propanol (50.0 mL, Sigma-Aldrich) and (II) 24.26 g of zirconium(IV) propoxide in 1propanol (100.4 mL, Sigma-Aldrich) were simultaneously added drop by drop (for 30 min) into the mixture of 1-propanol (240 mL) and deionised water (3 mL) under stirring. The obtained sol was heated to 70 ◦ C and kept at this temperature for 6 h. The final suspension was stirred for further 16 h at room temperature. Subsequently, the precipitation was filtered and washed a few times with deionised water. Both obtained materials (cZrY and sZrY) were dried overnight and then calcined at 600 ◦ C for 6 h. The calcination temperature was achieved at a rate of 5 ◦ C/min. Such synthesized supports were subsequently modified with an aqueous solution of copper(II) nitrate using the incipient wetness technique to obtain catalysts containing 2.5, 5.0 and 7.5 wt% of Cu after calcination at 500 ◦ C (achieved at a heating rate of 5 ◦ C/min) for 3 h. The synthesized samples are denoted as xCu/cZrY or xCu/sZrY, where x expresses the intended copper loading. 2.2. Characterization The crystallographic structure of the samples was determined with powder X-ray diffraction by means of a Philips X’pert APD ˚ The XRD patdiffractometer with a Cu radiation ( = 1.540560 A). terns were recorded in the 2 range of 2–64◦ with a step of 0.02◦ . Textural properties of the supports and catalysts were determined with low-temperature sorption of nitrogen using an ASAP 2020 sorptometer (Micromeritics). Before the measurements, the samples were outgassed at 350 ◦ C under vacuum for 5 h. The X-ray photoelectron spectra were recorded on a Prevac photoelectron spectrometer equipped with a hemispherical VG SCIENTA R3000 analyser. The spectra were measured using a monochromatised aluminum Al K␣ source (E = 1486.6 eV) and a low-energy electron flood gun (FS40A-PS) to compensate the charge on the surface of nonconductive samples. The base pressure in the analysis chamber during the measurements was 5 × 10−9 mbar. Spectra were recorded with a constant pass energy of 100 eV. The binding energies were referenced to C 1s core level
(Eb = 285.0 eV). The composition and chemical surrounding of the sample surface were investigated on the basis of the areas and binding energies of Cu 2p, Zr 3d, Y 3d, O 1s and C 1s photoelectron peaks. The fitting of high-resolution spectra was provided through the CasaXPS software. 2.3. Catalytic tests The synthesized materials were tested as catalysts in the total oxidation of toluene. 0.1 g of a catalyst (with particle size between 160 and 315 m) was placed on quartz wool in a quartz flow microreactor. The reaction temperature was maintained by a thermoregulator connected with a thermocouple placed in a catalyst bed. Prior to a catalytic test, the catalyst was outgassed at 500 ◦ C in flowing air (82 mL/min) for 30 min. Then the reactor was cooled down to 200 ◦ C and toluene dosing from a saturator (kept at 0 ◦ C) began with an additional stream of air (18 mL/min), connected to the main airstream in front of the reactor, giving the total air flow of 100 mL/min. Toluene concentration in the reaction mixture passing by the catalyst bed was equal to 1000 ppm. The catalytic tests were carried out at 200, 250, 275, 300, 325, 350, 400, 450 and 500 ◦ C. These temperatures were attained at a rate of 10 ◦ C/min and kept for 65 min. At each temperature, three analyses of reaction products were conducted using a Bruker 450-GC gas chromatograph equipped with three columns (Molecular Sieve 5A for separation of O2 , N2 and CO, Porapak S for separation of CH4 , CO2 and H2 O, and Chromosorb WAW-DMCS for separation of aromatic compounds), a thermal conductivity detector, two flame ionization detectors as well as a methaniser. Toluene conversion (XT ) was calculated from the following formula: XT =
nT,0 − nT × 100% nT,0
where nT,0 and nT are numbers of moles of toluene present in the inlet and outlet stream. 3. Results and discussion To check the crystallographic structure of yttrium-doped zirconia supports, their powder X-ray diffraction patterns were recorded (Fig. 1). For the coprecipitated sample, the reflections at 2 equal to 30.26◦ , 35.14◦ , 50.39◦ , 59.92◦ , 62.86◦ and 74.07◦ , whereas for the sol–gel support at 30.34◦ , 35.12◦ , 50.52◦ , 60.01◦ , 62.96◦ and 74.19◦ are observed. These reflections are ascribed to (1 0 1), (1 1 0), (1 1 2), (2 1 1), (2 0 2) and (2 2 0) planes, respectively, of ZrO2 in the tetragonal form [JCPDS 79-1764]. In the XRD patterns, no reflections characteristic of yttrium oxide are visible. It can be caused by the low concentration of added yttrium, or as described in the literature yttrium is a structural dopant forming a solid solution with zirconia and enforcing tetragonal structure of YSZ [11,12]. Depending on the yttrium content, YSZ can form tetragonal phase (t) (for an oxide system containing 0–3.0 mol.% Y2 O3 ), which is transformable into the monoclinic structure, or the metastable tetragonal phase (t ) (at a higher yttrium concentration—3.0–6.5 mol.% Y2 O3 ) which is non-transformable. √ These two phases can be distinguished by calculating the c/a · 2 parameter. √For the pure zirconia tetragonal phase [JCPDS 79-1764], the c/a · 2 value is 1.020, considered to be the low-stabilized t phase. The a and c lattice parameters determined for the cZrY (3.600 and 5.142) and sZrY (3.611 and√5.110) samples indicate that in the case of these supports the c/a · 2 values are equal to 1.010 and 1.001, respectively, which confirm the incorporation of yttrium into the ZrO2 structure forming the highly stabilized t phase [13]. The impregnation of the yttria-stabilized zirconia supports with various amounts of copper nitrates and their subsequent calcination resulted in the appearance of a new phase beside the
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Table 1 Characteristics of the Cu-containing catalysts supported on ZrO2 –Y2 O3 . Catalyst
Mean size of CuO crystallites (nm)
Textural parameters BET surface area (m2 /g)
Total pore volume (cm3 /g)
T50 in toluene combustion (◦ C)
cZrY 2.5Cu/cZrY 5.0Cu/cZrY 7.5Cu/cZrY
– n.m. 15.9 20.1
62 56 59 50
0.10 0.10 0.10 0.08
509 278 264 269
sZrY 2.5Cu/sZrY 5.0Cu/sZrY 7.5Cu/sZrY
– 16.9 20.1 21.1
35 25 22 25
0.05 0.04 0.04 0.04
525 294 292 292
Fig. 1. X-ray diffraction patterns of the Cu-containing catalysts supported on ZrO2 –Y2 O3 synthesized by (A) coprecipitation and (B) sol–gel methods (—CuO).
unchanged support. The reflections at 2 equal to 32.5◦ , 35.6◦ , 38.8◦ , 49.0◦ , 61.6◦ , 66.3◦ and 68.1◦ are attributed to (1 1 0), (−1 1 1), (1 1 1), (−2 0 2), (−1 1 3), (−3 1 1) and (2 2 0) planes, respectively, of the copper oxide phase [JCPDS 05-0661]. The mean size of CuO crystallites (D) was calculated from the Scherrer equation: D=
K · ˇ · cos
where K is the shape factor (=0.9), the X-ray wavelength used for the measurements, ˇ the width (FWHM) of the CuO (1 1 1) line and the Bragg angle. The mean size of CuO crystallites increases with the copper loading, from 15.9 to 20.1 nm for 5.0 and 7.5 wt% of copper deposited on the coprecipitated support and from 16.9 to 21.1 nm for 2.5–7.5 wt% of copper deposited on the support obtained by the sol–gel technique (Table 1). At the lowest copper loading (2.5 wt%), the crystalline copper oxide is not observed on the coprecipitated support (Fig. 1A). It could suggest that the surface area of the coprecipitated support is higher than the sol–gel one, and the CuO phase exhibits higher dispersion in the case of catalysts based on cZrY. The textural properties of the samples were examined by lowtemperature sorption of nitrogen. The obtained isotherms indicate that the supports contain different kinds of pores (Fig. 2). The
Fig. 2. Isotherms of N2 adsorption collected at −196 ◦ C for the Cu-containing catalysts supported on ZrO2 –Y2 O3 synthesized by (A) coprecipitation and (B) sol–gel methods.
observed isotherm shape and hysteresis loop confirm that in the cZrY sample intraparticle mesopores with wide pore size distribution are dominant. In contrast, in the sZrY support, the contribution of this type of mesopores is significantly smaller. This is confirmed by lower amount of nitrogen adsorbed at relative pressures in the range of 0.5–0.8. The porosity of the sZrY sample is mainly formed by the voids present among support particles (interparticle porosity). The presence of such pores is confirmed by a rapid increase in
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nitrogen adsorption at the highest relative pressures. It indicates that the sZrY support consists of smaller particles, which could be expected for a material obtained by the sol–gel method. During the deposition of copper oxide, porosity of both supports did not undergo significant changes. Nevertheless, a smaller amount of adsorbed nitrogen indicates that the formed CuO crystallites block partially the mesopores of a support. This explains differences in textural properties of the preparations (e.g. specific surface area and total pore volume) demonstrated in Table 1. The BET specific surface area of cZrY and sZrY supports is equal to 62 and 35 m2 /g, respectively. In the case of the cZrY mesoporous support, pore blocking depends on the amount of deposited CuO. For the samples containing 2.5 and 5.0 wt% of Cu SBET decreases by 5–10% and for the 7.5Cu/cZrY sample by 20%. For the sZrY support, a significant drop in surface area (by about 30%) is observed even after the deposition of the smallest amount of copper and almost does not change with raising copper content. The similar tendency is observed for the total pore volume of the samples. In the case of the coprecipitated support, the introduction of small amounts of copper oxide does not influence the pore volume. Only for the
7.5Cu/cZrY sample, a 20% drop of pore volume is observed. Opposite to it, for the sol–gel support, the lowest amount of copper oxide causes a 20% decrease in pore volume which remains constant for all copper loadings. This effect can be explained by almost total blocking intraparticle pores (partly confirmed by isotherms). Thus, for the sZrY catalysts CuO is mainly deposited on the outer surface not influencing significantly the material porosity. The surface composition of the samples containing 5 wt% of Cu was investigated by X-ray photoelectron spectroscopy (XPS). The collected XPS spectra of Cu 2p, Zr 3d, Y 3d and O 1s core levels are shown in Fig. 3, whereas the calculated surface compositions are summarized in Table 2. The binding energy of the Cu 2p3/2 peak at 934.1 eV and the characteristic shake-up peak at a binding energy of about 944 eV display the presence of Cu2+ species [14]. A higher concentration of Cu is found on the surface of the sZrY support, which confirms the observations based on the N2 adsorption isotherms. For the examined samples, two kinds of zirconium species were revealed (Fig. 3B) with a binding energy of Zr 3d5/2 at 182.0–182.1 eV (Zr4+ ions in pure ZrO2 ) and 183.5 eV (zirconium species bound to a more electron-attractive
Fig. 3. XPS spectra of (A) Cu 2p, (B) Zr 3d, (C) Y 3d and (D) O 1s core levels for the 5.0Cu/cZrY and 5.0Cu/sZrY catalysts.
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Table 2 Comparison of surface composition (at.%) for the ZrO2 –Y2 O3 -based catalysts containing 5 wt% of Cu. Catalyst
5.0Cu/cZrY 5.0Cu/sZrY
Cu 2p
Zr 3d
Cu2+
ZrO2
Zrı+
Y 3d Y2 O3
Y3+ –OH− /CO3 2−
O 1s Lattice O2−
OH− /CO3 2−
H2 O
6.6 7.8
23.7 21.1
2.2 4.1
2.5 1.7
0.7 0.5
41.1 38.5
9.5 15.6
1.0 1.1
oxide), 531.5 eV (oxygen atoms in hydroxyl and carbonate groups) and 533.9–534.1 eV (oxygen in adsorbed water) [11,17]. The copper-containing YSZ samples were tested as catalysts in the total oxidation of toluene. The cZrY and sZrY supports exhibit low catalytic activity attaining 50% toluene conversion (T50 ) at 509 and 525 ◦ C, respectively. The deposition of copper oxide results in almost twice lowering T50 (cf. Table 1). For the cZrY-supported samples, T50 is achieved at 264 ◦ C over the most efficient catalyst (5.0Cu/cZrY). In the presence of this catalyst, toluene is totally combusted at 350 ◦ C (Fig. 4). The 2.5Cu/cZrY and 7.5Cu/cZrY samples exhibit lower activity with T50 equal to 278 and 269 ◦ C, respectively. The catalytic activity of all sZrY-supported samples is similar with T50 at about 290 ◦ C and almost total toluene conversion at 450 ◦ C. The catalysts supported on coprecipitated yttria-stabilized zirconia turned out to be more efficient systems compared to the Cu/sZrY samples. It is most likely that this effect corresponds to the copper dispersion as well as the accessibility of active sites. In the case of the sZrY catalysts, copper oxide crystallites are mainly deposited on the outer surface of the support with a limited size. Thus, the dispersion of an active phase is poor and does not change with an increase in CuO content causing comparable catalytic activity for all CuO/sZrY materials. Higher porosity of the coprecipitated support results in its more expanded surface area which enables better dispersion of the active phase, and therefore cZrY samples are more efficient catalysts. The highest reaction rate is observed for the 5.0Cu/cZrY sample probably due to the highest content of accessible copper sites related to the CuO content and its dispersion.
4. Conclusions
Fig. 4. Conversion of toluene vs. reaction temperature achieved over the Cucontaining catalysts supported on ZrO2 –Y2 O3 synthesized by (A) coprecipitation and (B) sol–gel methods.
species and/or partially reduced Zrı+ sites) [15]. The participation of non-stoichiometric oxide is larger in the catalyst based on the sol–gel support. Nevertheless, Zr4+ ions from lattice oxide play a dominant role in both measured samples. Two different chemical environments were found for the yttrium atoms present in the studied samples (Fig. 3C). The Y 3d5/2 peaks at 156.9–157.0 and 158.2–158.3 eV are assigned to Y3+ in the Y2 O3 lattice and surrounded by hydroxyl/carbonate groups, respectively [16]. Both metallic components of the supports are identified at the Y/(Y + Zr) atomic ratio of 10.9 (5.0Cu/cZrY) and 7.9 (5.0Cu/sZrY). Taking into account the expected value of this ratio (9.5), one can notice that the surface of 5.0Cu/cZrY is enriched in Y, whereas 5.0Cu/sZrY exhibits deficit of this element. The O 1s spectrum (Fig. 3D) can be deconvoluted into three peaks at 529.7–529.8 eV (oxygen from lattice
Yttria-containing zirconia systems obtained by the coprecipitation and sol–gel methods crystallized in the metastable non-transformable tetragonal phase (t ). They were mesoporous solids with intraparticle porosity dominant in the cZrY sample and a significant contribution of interparticle voids in the sZrY preparation, which resulted in almost twice higher BET specific surface area of the coprecipitated system. In the case of the cZrY support, the deposition of CuO occurred in the pores, leading to a high dispersion of active phase. On the other hand, larger crystallites of CuO were formed on the sZrY material, where the deposition of the active phase occurred mainly on the outer surface. The differences in the CuO dispersion influenced the catalytic performance in the total oxidation of toluene. The Cu/cZrY catalysts appeared to be more active compared to the Cu/sZrY ones. The most active catalyst (5.0Cu/cZrY) allowed to achieve T50 at 264 ◦ C and totally combust toluene at 350 ◦ C.
Acknowledgments This work was supported by the National Science Center under the grant no. 2013/11/B/ST5/01550. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract No. POIG.02.01.00-12-023/08).
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Catalytic combustion of toluene over copper oxide deposited on two types of yttria-stabilized zirconia Anna Białas ∗ , Tomasz Kondratowicz, Marek Drozdek, Piotr Ku´strowski Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 30 October 2014 Received in revised form 22 December 2014 Accepted 4 January 2015 Available online xxx Keywords: ZrO2 –Y2 O3 support Coprecipitation Sol–gel Copper-containing catalysts Volatile organic compounds
a b s t r a c t Yttria-stabilized zirconia (YSZ) was obtained by the coprecipitation at pH 9.0 and by the sol–gel method. These oxide systems crystallized in the tetragonal phase which was revealed by powder X-ray diffraction (XRD). The YSZ supports were mesoporous solids with different kinds of porosity and various BET specific surface areas determined by low-temperature sorption of nitrogen. The incipient wetness technique was used to modify these materials with different amounts of Cu (2.5, 5.0 and 7.5 wt%). The changes in structural and textural properties of the YSZ supports after the CuO introduction were discussed, and the surface composition of the Cu-loaded samples was studied by X-ray photoelectron spectroscopy (XPS). It was found that the intra- and interparticle porosity influences the dispersion of CuO active phase, which is the parameter determining the catalytic activity in the toluene combustion. The material based on coprecipitated YSZ containing 5 wt% of Cu turned out to be the most active catalyst with T50 = 264 ◦ C and the total conversion of toluene observed at 350 ◦ C. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) emitted during processes involving organic chemicals are harmful to organisms and the environment. Various kinds of organic substances belong to this group, e.g. paraffins, olefins, aromatics, ketones, aldehydes, carboxylic acids or chlorinated hydrocarbons [1]. Among the widely encountered VOCs, toluene, emitted during fuel usage and industrial processes, can be listed. This aromatic compound is dangerous for the human nervous system at concentration as low as 100 ppm [2]. If the VOCs’ concentration is low, they can be simply removed by the non-catalytic or catalytic total oxidation [3]. Supported noble metals—particularly platinum or palladium—are the most efficient systems in the VOC’s abatement. On the other hand, such catalysts can be easily poisoned by contaminants and are expensive, and therefore transition metal oxides have been often examined as components of catalysts for the purification of VOCs containing gases [1]. Among them, supported copper and manganese oxides turned out to be the most active materials [4–7]. It was postulated [8] that the adsorption of the toluene molecule is the initial step of toluene oxidation over a CuO–CeO2 /␥-Al2 O3 catalyst. Subsequently, abstraction of hydrogen from the methyl and the phenyl
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group, abstraction of the carbon atom of the methyl group, and finally, destruction of the aromatic ring, occur. Both lattice and adsorbed oxygen are involved in (i) reoxidation of mildly reduced copper species and (ii) abstraction of hydrogen atoms and scission of C C bonds. It should be noticed that better catalytic performance in the total oxidation of toluene was observed for metal oxides deposited on zirconia than on ␥-alumina [9,10]. This effect was attributed to the formation of active phases exhibiting higher reducibility, because zirconia limits the dispersion of an active phase. Lower reducibility, typical of alumina-supported catalysts, results from strong interactions between an active phase and the support. Larger active phase crystallites facilitate oxygen adsorption on noble metals, which is the rate-determining step in the toluene oxidation occurring according to the Mars van Krevelen or Eley-Rideal mechanisms. Moreover, the presence of oxygen surface vacancies in ZrO2 enables the chemisorption of reactants [9]. Zirconia is also characterized by good mechanical and thermal stability [10,11]. ZrO2 is often more effective in the so-called stabilized form, which contains an addition of another metal oxide (e.g. SiO2 , La2 O3 , CaO, Y2 O3 or Ce2 O3 ) appearing in the fluorite structure and forming a solid solution with zirconia. The presence of additives causes that zirconia crystallizes in the tetragonal phase, exhibiting higher specific surface area than more stable monoclinic ZrO2 . Yttrium was recognized as the best dopant of zirconia in the case of the catalysts for oxidation processes, because it increases the number of available oxygen vacancies, helping to store oxygen and return it at low
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temperatures. The content of 1–10 mol.% of yttria in ZrO2 ensures the highest ion conductivity [11]. The aim of the presented work was the synthesis of zirconia stabilized by 5 mol.% of yttria using two various methods: coprecipitation (at controlled pH of 9.0) and sol–gel. The obtained yttria-stabilized zirconia materials with different structural and textural properties were used as supports for a copper oxide phase. The catalytic activity of the obtained CuO/YSZ materials was studied in the total oxidation of toluene. The influence of the support properties as well as dispersion and content of CuO on the catalytic performance was discussed. 2. Experimental 2.1. Synthesis of catalysts To obtain yttria-stabilized zirconia (5 mol.% of Y2 O3 ), two methods were applied: coprecipitation (c) and sol–gel (s). In the coprecipitation procedure, zirconyl chloride octahydrate (23.85 g) and yttrium(III) nitrate hexahydrate (2.99 g) (both supplied by Sigma-Aldrich) were dissolved together in deionised water (50 mL). This solution was added dropwise to another solution (370 mL) containing 1.2 wt% of ammonia (Lach-ner) under stirring till achieving pH 9.0. The obtained precipitation was stirred for 30 min, then filtered and washed a few times with deionised water to remove chloride and ammonium ions. In the sol–gel synthesis, the two separate solutions containing (I) 2.99 g of yttrium(III) nitrate hexahydrate in 1-propanol (50.0 mL, Sigma-Aldrich) and (II) 24.26 g of zirconium(IV) propoxide in 1propanol (100.4 mL, Sigma-Aldrich) were simultaneously added drop by drop (for 30 min) into the mixture of 1-propanol (240 mL) and deionised water (3 mL) under stirring. The obtained sol was heated to 70 ◦ C and kept at this temperature for 6 h. The final suspension was stirred for further 16 h at room temperature. Subsequently, the precipitation was filtered and washed a few times with deionised water. Both obtained materials (cZrY and sZrY) were dried overnight and then calcined at 600 ◦ C for 6 h. The calcination temperature was achieved at a rate of 5 ◦ C/min. Such synthesized supports were subsequently modified with an aqueous solution of copper(II) nitrate using the incipient wetness technique to obtain catalysts containing 2.5, 5.0 and 7.5 wt% of Cu after calcination at 500 ◦ C (achieved at a heating rate of 5 ◦ C/min) for 3 h. The synthesized samples are denoted as xCu/cZrY or xCu/sZrY, where x expresses the intended copper loading. 2.2. Characterization The crystallographic structure of the samples was determined with powder X-ray diffraction by means of a Philips X’pert APD ˚ The XRD patdiffractometer with a Cu radiation ( = 1.540560 A). terns were recorded in the 2 range of 2–64◦ with a step of 0.02◦ . Textural properties of the supports and catalysts were determined with low-temperature sorption of nitrogen using an ASAP 2020 sorptometer (Micromeritics). Before the measurements, the samples were outgassed at 350 ◦ C under vacuum for 5 h. The X-ray photoelectron spectra were recorded on a Prevac photoelectron spectrometer equipped with a hemispherical VG SCIENTA R3000 analyser. The spectra were measured using a monochromatised aluminum Al K␣ source (E = 1486.6 eV) and a low-energy electron flood gun (FS40A-PS) to compensate the charge on the surface of nonconductive samples. The base pressure in the analysis chamber during the measurements was 5 × 10−9 mbar. Spectra were recorded with a constant pass energy of 100 eV. The binding energies were referenced to C 1s core level
(Eb = 285.0 eV). The composition and chemical surrounding of the sample surface were investigated on the basis of the areas and binding energies of Cu 2p, Zr 3d, Y 3d, O 1s and C 1s photoelectron peaks. The fitting of high-resolution spectra was provided through the CasaXPS software. 2.3. Catalytic tests The synthesized materials were tested as catalysts in the total oxidation of toluene. 0.1 g of a catalyst (with particle size between 160 and 315 m) was placed on quartz wool in a quartz flow microreactor. The reaction temperature was maintained by a thermoregulator connected with a thermocouple placed in a catalyst bed. Prior to a catalytic test, the catalyst was outgassed at 500 ◦ C in flowing air (82 mL/min) for 30 min. Then the reactor was cooled down to 200 ◦ C and toluene dosing from a saturator (kept at 0 ◦ C) began with an additional stream of air (18 mL/min), connected to the main airstream in front of the reactor, giving the total air flow of 100 mL/min. Toluene concentration in the reaction mixture passing by the catalyst bed was equal to 1000 ppm. The catalytic tests were carried out at 200, 250, 275, 300, 325, 350, 400, 450 and 500 ◦ C. These temperatures were attained at a rate of 10 ◦ C/min and kept for 65 min. At each temperature, three analyses of reaction products were conducted using a Bruker 450-GC gas chromatograph equipped with three columns (Molecular Sieve 5A for separation of O2 , N2 and CO, Porapak S for separation of CH4 , CO2 and H2 O, and Chromosorb WAW-DMCS for separation of aromatic compounds), a thermal conductivity detector, two flame ionization detectors as well as a methaniser. Toluene conversion (XT ) was calculated from the following formula: XT =
nT,0 − nT × 100% nT,0
where nT,0 and nT are numbers of moles of toluene present in the inlet and outlet stream. 3. Results and discussion To check the crystallographic structure of yttrium-doped zirconia supports, their powder X-ray diffraction patterns were recorded (Fig. 1). For the coprecipitated sample, the reflections at 2 equal to 30.26◦ , 35.14◦ , 50.39◦ , 59.92◦ , 62.86◦ and 74.07◦ , whereas for the sol–gel support at 30.34◦ , 35.12◦ , 50.52◦ , 60.01◦ , 62.96◦ and 74.19◦ are observed. These reflections are ascribed to (1 0 1), (1 1 0), (1 1 2), (2 1 1), (2 0 2) and (2 2 0) planes, respectively, of ZrO2 in the tetragonal form [JCPDS 79-1764]. In the XRD patterns, no reflections characteristic of yttrium oxide are visible. It can be caused by the low concentration of added yttrium, or as described in the literature yttrium is a structural dopant forming a solid solution with zirconia and enforcing tetragonal structure of YSZ [11,12]. Depending on the yttrium content, YSZ can form tetragonal phase (t) (for an oxide system containing 0–3.0 mol.% Y2 O3 ), which is transformable into the monoclinic structure, or the metastable tetragonal phase (t ) (at a higher yttrium concentration—3.0–6.5 mol.% Y2 O3 ) which is non-transformable. √ These two phases can be distinguished by calculating the c/a · 2 parameter. √For the pure zirconia tetragonal phase [JCPDS 79-1764], the c/a · 2 value is 1.020, considered to be the low-stabilized t phase. The a and c lattice parameters determined for the cZrY (3.600 and 5.142) and sZrY (3.611 and√5.110) samples indicate that in the case of these supports the c/a · 2 values are equal to 1.010 and 1.001, respectively, which confirm the incorporation of yttrium into the ZrO2 structure forming the highly stabilized t phase [13]. The impregnation of the yttria-stabilized zirconia supports with various amounts of copper nitrates and their subsequent calcination resulted in the appearance of a new phase beside the
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Table 1 Characteristics of the Cu-containing catalysts supported on ZrO2 –Y2 O3 . Catalyst
Mean size of CuO crystallites (nm)
Textural parameters BET surface area (m2 /g)
Total pore volume (cm3 /g)
T50 in toluene combustion (◦ C)
cZrY 2.5Cu/cZrY 5.0Cu/cZrY 7.5Cu/cZrY
– n.m. 15.9 20.1
62 56 59 50
0.10 0.10 0.10 0.08
509 278 264 269
sZrY 2.5Cu/sZrY 5.0Cu/sZrY 7.5Cu/sZrY
– 16.9 20.1 21.1
35 25 22 25
0.05 0.04 0.04 0.04
525 294 292 292
Fig. 1. X-ray diffraction patterns of the Cu-containing catalysts supported on ZrO2 –Y2 O3 synthesized by (A) coprecipitation and (B) sol–gel methods (—CuO).
unchanged support. The reflections at 2 equal to 32.5◦ , 35.6◦ , 38.8◦ , 49.0◦ , 61.6◦ , 66.3◦ and 68.1◦ are attributed to (1 1 0), (−1 1 1), (1 1 1), (−2 0 2), (−1 1 3), (−3 1 1) and (2 2 0) planes, respectively, of the copper oxide phase [JCPDS 05-0661]. The mean size of CuO crystallites (D) was calculated from the Scherrer equation: D=
K · ˇ · cos
where K is the shape factor (=0.9), the X-ray wavelength used for the measurements, ˇ the width (FWHM) of the CuO (1 1 1) line and the Bragg angle. The mean size of CuO crystallites increases with the copper loading, from 15.9 to 20.1 nm for 5.0 and 7.5 wt% of copper deposited on the coprecipitated support and from 16.9 to 21.1 nm for 2.5–7.5 wt% of copper deposited on the support obtained by the sol–gel technique (Table 1). At the lowest copper loading (2.5 wt%), the crystalline copper oxide is not observed on the coprecipitated support (Fig. 1A). It could suggest that the surface area of the coprecipitated support is higher than the sol–gel one, and the CuO phase exhibits higher dispersion in the case of catalysts based on cZrY. The textural properties of the samples were examined by lowtemperature sorption of nitrogen. The obtained isotherms indicate that the supports contain different kinds of pores (Fig. 2). The
Fig. 2. Isotherms of N2 adsorption collected at −196 ◦ C for the Cu-containing catalysts supported on ZrO2 –Y2 O3 synthesized by (A) coprecipitation and (B) sol–gel methods.
observed isotherm shape and hysteresis loop confirm that in the cZrY sample intraparticle mesopores with wide pore size distribution are dominant. In contrast, in the sZrY support, the contribution of this type of mesopores is significantly smaller. This is confirmed by lower amount of nitrogen adsorbed at relative pressures in the range of 0.5–0.8. The porosity of the sZrY sample is mainly formed by the voids present among support particles (interparticle porosity). The presence of such pores is confirmed by a rapid increase in
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nitrogen adsorption at the highest relative pressures. It indicates that the sZrY support consists of smaller particles, which could be expected for a material obtained by the sol–gel method. During the deposition of copper oxide, porosity of both supports did not undergo significant changes. Nevertheless, a smaller amount of adsorbed nitrogen indicates that the formed CuO crystallites block partially the mesopores of a support. This explains differences in textural properties of the preparations (e.g. specific surface area and total pore volume) demonstrated in Table 1. The BET specific surface area of cZrY and sZrY supports is equal to 62 and 35 m2 /g, respectively. In the case of the cZrY mesoporous support, pore blocking depends on the amount of deposited CuO. For the samples containing 2.5 and 5.0 wt% of Cu SBET decreases by 5–10% and for the 7.5Cu/cZrY sample by 20%. For the sZrY support, a significant drop in surface area (by about 30%) is observed even after the deposition of the smallest amount of copper and almost does not change with raising copper content. The similar tendency is observed for the total pore volume of the samples. In the case of the coprecipitated support, the introduction of small amounts of copper oxide does not influence the pore volume. Only for the
7.5Cu/cZrY sample, a 20% drop of pore volume is observed. Opposite to it, for the sol–gel support, the lowest amount of copper oxide causes a 20% decrease in pore volume which remains constant for all copper loadings. This effect can be explained by almost total blocking intraparticle pores (partly confirmed by isotherms). Thus, for the sZrY catalysts CuO is mainly deposited on the outer surface not influencing significantly the material porosity. The surface composition of the samples containing 5 wt% of Cu was investigated by X-ray photoelectron spectroscopy (XPS). The collected XPS spectra of Cu 2p, Zr 3d, Y 3d and O 1s core levels are shown in Fig. 3, whereas the calculated surface compositions are summarized in Table 2. The binding energy of the Cu 2p3/2 peak at 934.1 eV and the characteristic shake-up peak at a binding energy of about 944 eV display the presence of Cu2+ species [14]. A higher concentration of Cu is found on the surface of the sZrY support, which confirms the observations based on the N2 adsorption isotherms. For the examined samples, two kinds of zirconium species were revealed (Fig. 3B) with a binding energy of Zr 3d5/2 at 182.0–182.1 eV (Zr4+ ions in pure ZrO2 ) and 183.5 eV (zirconium species bound to a more electron-attractive
Fig. 3. XPS spectra of (A) Cu 2p, (B) Zr 3d, (C) Y 3d and (D) O 1s core levels for the 5.0Cu/cZrY and 5.0Cu/sZrY catalysts.
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Table 2 Comparison of surface composition (at.%) for the ZrO2 –Y2 O3 -based catalysts containing 5 wt% of Cu. Catalyst
5.0Cu/cZrY 5.0Cu/sZrY
Cu 2p
Zr 3d
Cu2+
ZrO2
Zrı+
Y 3d Y2 O3
Y3+ –OH− /CO3 2−
O 1s Lattice O2−
OH− /CO3 2−
H2 O
6.6 7.8
23.7 21.1
2.2 4.1
2.5 1.7
0.7 0.5
41.1 38.5
9.5 15.6
1.0 1.1
oxide), 531.5 eV (oxygen atoms in hydroxyl and carbonate groups) and 533.9–534.1 eV (oxygen in adsorbed water) [11,17]. The copper-containing YSZ samples were tested as catalysts in the total oxidation of toluene. The cZrY and sZrY supports exhibit low catalytic activity attaining 50% toluene conversion (T50 ) at 509 and 525 ◦ C, respectively. The deposition of copper oxide results in almost twice lowering T50 (cf. Table 1). For the cZrY-supported samples, T50 is achieved at 264 ◦ C over the most efficient catalyst (5.0Cu/cZrY). In the presence of this catalyst, toluene is totally combusted at 350 ◦ C (Fig. 4). The 2.5Cu/cZrY and 7.5Cu/cZrY samples exhibit lower activity with T50 equal to 278 and 269 ◦ C, respectively. The catalytic activity of all sZrY-supported samples is similar with T50 at about 290 ◦ C and almost total toluene conversion at 450 ◦ C. The catalysts supported on coprecipitated yttria-stabilized zirconia turned out to be more efficient systems compared to the Cu/sZrY samples. It is most likely that this effect corresponds to the copper dispersion as well as the accessibility of active sites. In the case of the sZrY catalysts, copper oxide crystallites are mainly deposited on the outer surface of the support with a limited size. Thus, the dispersion of an active phase is poor and does not change with an increase in CuO content causing comparable catalytic activity for all CuO/sZrY materials. Higher porosity of the coprecipitated support results in its more expanded surface area which enables better dispersion of the active phase, and therefore cZrY samples are more efficient catalysts. The highest reaction rate is observed for the 5.0Cu/cZrY sample probably due to the highest content of accessible copper sites related to the CuO content and its dispersion.
4. Conclusions
Fig. 4. Conversion of toluene vs. reaction temperature achieved over the Cucontaining catalysts supported on ZrO2 –Y2 O3 synthesized by (A) coprecipitation and (B) sol–gel methods.
species and/or partially reduced Zrı+ sites) [15]. The participation of non-stoichiometric oxide is larger in the catalyst based on the sol–gel support. Nevertheless, Zr4+ ions from lattice oxide play a dominant role in both measured samples. Two different chemical environments were found for the yttrium atoms present in the studied samples (Fig. 3C). The Y 3d5/2 peaks at 156.9–157.0 and 158.2–158.3 eV are assigned to Y3+ in the Y2 O3 lattice and surrounded by hydroxyl/carbonate groups, respectively [16]. Both metallic components of the supports are identified at the Y/(Y + Zr) atomic ratio of 10.9 (5.0Cu/cZrY) and 7.9 (5.0Cu/sZrY). Taking into account the expected value of this ratio (9.5), one can notice that the surface of 5.0Cu/cZrY is enriched in Y, whereas 5.0Cu/sZrY exhibits deficit of this element. The O 1s spectrum (Fig. 3D) can be deconvoluted into three peaks at 529.7–529.8 eV (oxygen from lattice
Yttria-containing zirconia systems obtained by the coprecipitation and sol–gel methods crystallized in the metastable non-transformable tetragonal phase (t ). They were mesoporous solids with intraparticle porosity dominant in the cZrY sample and a significant contribution of interparticle voids in the sZrY preparation, which resulted in almost twice higher BET specific surface area of the coprecipitated system. In the case of the cZrY support, the deposition of CuO occurred in the pores, leading to a high dispersion of active phase. On the other hand, larger crystallites of CuO were formed on the sZrY material, where the deposition of the active phase occurred mainly on the outer surface. The differences in the CuO dispersion influenced the catalytic performance in the total oxidation of toluene. The Cu/cZrY catalysts appeared to be more active compared to the Cu/sZrY ones. The most active catalyst (5.0Cu/cZrY) allowed to achieve T50 at 264 ◦ C and totally combust toluene at 350 ◦ C.
Acknowledgments This work was supported by the National Science Center under the grant no. 2013/11/B/ST5/01550. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract No. POIG.02.01.00-12-023/08).
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