Selective catalytic reduction of nitrogen oxide by ammonia on substituted strontium ferrites

Applied Catalysis A: General 384 (2010) 230–240

Contents lists available at ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Selective catalytic reduction of nitrogen oxide by ammonia on substituted strontium ferrites Marina V. Bukhtiyarova ∗ , Aleksandra S. Ivanova, Elena M. Slavinskaya, Lyudmila M. Plyasova, Vasily V. Kaichev, Pavel A. Kuznetsov Boreskov Institute of Catalysis SB RAS, Pr. Akad. Lavrentieva, 5, 630090 Novosibirsk, Russia

a r t i c l e

i n f o

Article history: Received 19 April 2010 Received in revised form 21 June 2010 Accepted 21 June 2010 Available online 26 June 2010 Keywords: Ferrites XPS NO reduction NH3 -TPD NH3 -TPR

a b s t r a c t Substituted Sr-ferrites Sr1−x Cex Mn6−y Wy Fe4 Al2 O19 (x = 0; 0.2; y = 0; 0.28; 0.56; 0.84) with the components ratio typical for Sr-hexaferrite obtained by co-precipitation and calcined at 700 ◦ C have been characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and adsorption method. Ferrites have been studied in the selective catalytic reduction of NO by ammonia (SCR) using temperature-programmed desorption of ammonia (NH3 -TPD) and temperature-programmed reaction of ammonia (NH3 -TPR) techniques. It was shown that the substituted Sr-ferrites are multiphase. Its specific surface area is 27–59 m2 /g. Surface concentrations of elements in the samples differ from the bulk chemical contents. According to XPS data, the surface of the samples is enriched by strontium. The main components on the surface are in oxidized states: Sr2+ , Mn3+ , Fe3+ , Al3+ , Ce4+ and W6+ . The surface acidity of the samples determined by NH3 -TPD increases from 0.023 to 0.071–0.082 mmol/g when Ce and W ions are introduced in SrMn6 Fe4 Al2 O19 . The most active (XNO ≈ 100%) and selective with respect to N2 catalyst is the Sr0.8 Ce0.2 Mn5.16 W0.84 Fe4 Al2 O19 ferrite obtained by the precipitation of the soluble nitrates of Sr, Fe, Mn, Al, Ce using ammonium paratungstate and ammonium hydrocarbonate as precipitating agents. This sample is characterized by the highest surface acidity, the highest atomic ratio of [Mn]/[Fe] and the lowest oxygen reactivity. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen oxides are one of the major sources of air pollution. They are emitted from many industrial applications such as internal combustion engines, factories of power industry and heat-power engineering, metal manufactures, engineering and chemical industries (such as preparation of nitric acid and explosives). The fuel combustion processes also contribute to total anthropogenic emissions of nitrogen oxides. NOx -containing gases cause the urban smog, acid rains and contribute to the greenhouse effect. Therefore, the utilization of nitrogen oxides is necessary [1]. The European Union has the fourth lowest level of emission limits for gasoline and diesel powered passenger cars in effect from 2005 onward, and recently they have approved Euro V (effective from 2009 to 2011) and Euro VI (effective from 2014 to 2015) regulations, under which NOx emission standards are more stringent (0.175 and 0.08 g km−1 for Euro V and Euro VI, respectively) [2]. In the recent years, many methods for the decision of this problem have been reported. Among them the selective catalytic reduction of NOx by NH3 is the

∗ Corresponding author. Tel.: +7 383 3269772/3063206; fax: +7 383 3308056. E-mail addresses: [email protected], [email protected] (M.V. Bukhtiyarova). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.06.046

most widespread. The reaction proceeds at a moderate temperature [3]: 4NO + 4NH3 + O2 → 4N2 + 6H2 O A great variety of different catalysts have been tested in this reaction. The most active catalyst is commercial V2 O5 /TiO2 promoted with WO3 and/or MoO3 , which is resistant to sulfur compounds [4,5]. Unfortunately, this catalyst is active only in the very narrow temperature range from 300 to 400 ◦ C. Moreover, nitrous oxide is formed at higher temperature [3]: 4NO + 4NH3 + 3O2 → 4N2 O + 6H2 O The main disadvantage of this catalyst is toxicity of V2 O5 and its rather low melting point (ca. 650 ◦ C) that could lead to discharge of catalyst mass [6]. On the other hand, operating temperature range of the diesel engine is 150–450 ◦ C. However, its operating conditions are transitional and brief temperature increase to 600–800 ◦ C is possible. Therefore, it is expedient to carry out treatment of used catalysts at the temperature exceeding operation temperature. Accordingly, V-W-Ti-O catalyst calcined at 700 ◦ C was 2-fold lower active and selective in comparison with fresh one. Hence, the target of the present work was to develop a nonhazardous SCR catalyst that shows high performance as well as physical stability even at elevated temperature.

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

Among the various oxide compositions, hexaaluminate systems of alkaline-earth and rare-earth elements are characterized by high thermal stability [7,8]. Their high thermal stability is related to the unique lamellar structure that consists of Al2 O3 -containing spinel block intercalated by mirror planes, in which large cations (Ba, Sr, La, etc.) are located [9]. At the same time, Mn-containing compositions, in particular MnOx /Al2 O3 or MnOx /TiO2 , show high activity in the SCR, however they also are not very stable [10,11]. Our earlier investigations [12] showed that the hexaaluminate systems are not enough active and selective with respect to N2 in the SCR. Nevertheless, we suppose that the substituted ferrites with the components ratio typical for hexaferrites can be of interest. Authors [13] reported that compositions containing both iron and manganese are active in the SCR. Furthermore, as it was mentioned above, WO3 is an active promoter, providing the increase of activity and selectivity of the catalysts in the SCR. Therefore, the aim of this work was to study physicochemical properties of the strontium ferrites (SrFe12 O19 ) substituted by Mn, Al, Ce and W and to examine its catalytic performance in the selective catalytic reduction of nitrogen oxide by ammonia. 2. Experimental 2.1. Catalyst preparation The samples with the components ratio typical for Srhexaferrite SrMn6 Fe6 O19 , Sr1−x Cex Mn6 Fe4 Al2 O19 (x = 0; 0.2), Sr0.8 Ce0.2 Mn6−y Wy Fe4 Al2 O19 (y = 0.28; 0.56; 0.84), denoted as SM6F, SM6FA2, SM6FA2C0.2 , SM6FA2C0.2 W0.28 , SM6FA2C0.2 W0.56 , SM6FA2C0.2 W0.84 were prepared via the procedure described elsewhere [14]. The procedure based on the aqueous co-precipitation of soluble nitrates of metals (Sr, Ce, Mn, Fe and Al) using NH4 HCO3 as precipitating agent. In case of W-containing samples, the mixed solution of metal nitrates and NH4 HCO3 solution were added to ammonium para-tungstate. The SM6FA2C0.2 W0.84 * sample was prepared by the precipitation of soluble nitrates of the metals using NH4 HCO3 and ammonium para-tungstate as precipitating agents. This procedure was used for studying the influence of the precipitation method of the SM6FA2C0.2 W0.84 ferrite on its properties. The precipitation was carried out at pH 7.3–7.5 and temperature of 70 ◦ C. The slurry was aged at 70 ◦ C for 2 h and then filtered. The obtained precipitate was washed and dried in air. The solid was dried at 110 ◦ C for 12–14 h and then calcined at 700 ◦ C for 4 h in an air flow. 2.2. Catalyst characterization The catalysts were characterized using elemental analysis, Ar adsorption, thermogravimetric and differential thermal analysis, X-ray diffraction, Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy. Elemental analysis was performed using the ICP atomic absorption spectroscopy with an accuracy of 0.01–0.03% [15]. The specific surface area was determined with an accuracy of ±10% by the thermal desorption of argon [16]. Texture properties were derived from low-temperature (−196 ◦ C) nitrogen adsorption isotherms obtained using an ASAP 2400 instrument (Micromeritics). The thermogravimetric and differential thermal analysis was carried out on a NETZSCH STA 449C apparatus. Catalysts were tested over the temperature range from room temperature up to 1400 ◦ C at the heating rate of 10 ◦ C/min in air. The accuracy of determination of weight losses was ±0.5%. XRD studies were performed on a ARL X‘TRA diffractometer using Cu K˛ monochromatic radiation ( = 1.5418 Å). X-ray diffraction patterns were recorded in a step scan mode in the 2 range

231

from 10◦ to 75◦ with the step of 0.02–0.05◦ and 3–5 s per step depending on the sample crystallinity. The phase identification was performed by comparison of the measured set of the interfacial distances di and the corresponding intensities of the diffraction maximums Ii with that found in the ICDD, PDF-2. FTIR spectra were recorded in the range of 250–2000 cm−1 on a Bomem MB-102 spectrometer. To take the spectra, the samples were prepared by the pelletizing with CsI. X-ray photoelectron spectroscopy was applied for characterization of the surface contents of the elements. XPS measurements were performed on a SPECS’s machine equipped with an X-ray source XR-50M with a twin Al/Ag anode, an ellipsoidal crystal monochromator FOCUS-500, and a hemispherical electron energy analyzer PHOIBOS-150. The core-level spectra were typically obtained using monochromatic Al K˛ radiation (h = 1486.74 eV) and fixed analyzer pass energy of 20 eV under ultrahigh vacuum conditions. During XPS measurements, the static charge was minimized by a flood gun of electrons. For further calibration of the charge shift, C1s peak at 284.8 eV from adventitious hydrocarbon was used. Spectra were background-subtracted using a Shirley fit algorithm [17] and then fitted onto separate components. Doniach–Sanjic symmetric function was applied for peak approximation [18]. To quantify the atomic concentration of the present elements, the cross-sections according to Scofield [19] were used. 2.3. Catalyst activity test The catalyst activity in selective catalytic reduction was tested in accordance with the light-off test at the following experimental conditions [12]: the reaction gas composition: NO – 350 ppm, NH3 – 350 ppm, O2 – 14 vol.%, H2 O – 4.5 vol.%, He – balance; space velocity – 120,000 h−1 ; catalyst volume – 0.25 cm3 ; contact time – 0.03 s; heating rate – 10 ◦ C/min, temperature range – 100–600 ◦ C. The procedure of the reaction mixture analysis was divided into two parts. The first part was GC analysis on nitrogen. The second part was FTIR analysis on nitrogen oxides and ammonia. NH3 -TPD: Before the experiment, 1 g of the sample was trained in a flow of 20 mol%O2 /He from 25 to 600 ◦ C at the heating rate of 10 ◦ C/min and was sustained at 600 ◦ C for 2 h and then cooled to room temperature. Then the catalyst heated to 40 ◦ C was exposed to a flow of 500 ppm NH3 /He (500 ml/min). Ammonia was adsorbed on the catalyst surface that was reflected by drastic decrease of ammonia outlet concentration. Saturation of catalyst by ammonia was detected by assignment of ammonia initial concentration in reactor outlet. The amount of adsorbed ammonia was determined by the area under concentration curve. After saturation of the catalyst by ammonia its feeding was stopped. Helium was passed through the reactor to total disappearance of ammonia in a reaction mixture. Then temperature-programmed heating the catalysts was carried out in a flow of He from 40 to 600 ◦ C at the heating rate of 10 ◦ C/min. TPR-NH3 : After saturation of the catalysts by ammonia, temperature-programmed heating was carried out in a flow of gas mixture containing 500 ppm NH3 /He from 40 to 600 ◦ C at the heating rate of 10 ◦ C/min with following cooling in a flow of helium. The analysis of N2 was carried out by GC and the analysis of NH3 , N2 O, NO, NO2 and O2 was carried out using FTIR spectroscopy. 3. Results 3.1. Phase composition Non-isothermal temperature-programmed treatments of the fresh air-dried SM6F, SM6FA2, SM6FA2C0.2 , SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples resulted in the appearance of exo- and

232

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

Table 1 Exoeffects and endoeffects of ferrites. Sample

Tendo , ◦ C/loss, %

Texo , ◦ C/loss, %

 loss (%)

SM6F

121/5.6; 233/1.86; 388/4.55; 405/3.76; 464/1.44; 543/7.37; 840/1.49; 922/0.28; 986/1.20 124/8.68; 209/4.73; 268/0.98 375/6.03; 385/5.42; 462/2.36; 511/4.39; 856/0.91; 912/0.18; 986/1.09 127/8.52; 209/9.17; 350/3.44; 362/4.83; 469/3.35; 530/1.68; 600/1.09; 854/0.97; 973/0.85 143/7.18; 221/11.09; 350/5.88; 459/5.71; 852/0.59; 977/0.51 125/8.45; 211/7.28; 232/4.95; 338/4.08; 475/2.00; 503/4.45; 983/1.32 122/9.58; 210/6.23; 233/4.10; 359/3.89; 491/3.42; 513/4.08; 982/1.35

649/0.81

28.41

666/1.05

35.80

658/1.40

37.25

653/2.33 632/2.05

33.65 34.50

630/2.03

34.68

SM6FA2 SM6FA2C0.2 SM6FA2C0.2 W0.28 SM6FA2C0.2 W0.84 SM6FA2C0.2 W0.84 *

endoeffects on the TG-DTA curves. The corresponding temperatures are collected in Table 1. When the samples were heated from room temperature up to 1200 ◦ C, they lost from 28% to 35–37% of their weights (Table 1). The endoeffects observed at 128–136 and 219–266 ◦ C were originated from the dehydration and the decomposition of ammonia salts of the corresponding components, respectively. According to [20–22], for the SM6F and SM6FA2 samples, the endoeffects located at 375–405 and 462–464 ◦ C can be related to the decomposition of FeCO3 to Fe2 O3 and MnCO3 to MnO2 , respectively (Table 1). The next sets of peaks at 511–543 ◦ C are indicative for transformation of MnO2 to Mn2 O3 [21,22]. Probably, endoeffects at 840–856 and 912–922 ◦ C are originated due to the decomposition of strontium carbonates [23] and Sr-containing compounds, respectively (Table 1). Endoeffects observed at 980–995 ◦ C can be related both to transition Mn2 O3 → Mn3 O4 [22] and to formation of spinel (Mnx Fe1−x )3 O4 [24]. The introduction of cerium in the SM6FA2 sample leads to change of TG-DTA profile: endoeffect at 600 ◦ C appears and intensity of the peak at 854 ◦ C decreases. These changes indicate that the presence of cerium seems to contribute to the formation of Sr,Cecontaining compound (endoeffect at 600 ◦ C) [25] with decreasing amount of SrCO3 (Table 1). The further promoting SM6FA2C0.2 sample with tungsten oxide does not lead to significant change of TG-DTA profile (Table 1). Probably, it is caused by the fact that strontium interacts with tungsten oxide forming Sr–W-containing compound. Its amount increases with increasing WO3 content. This assumption is confirmed by XRD data indicated below. It should be noted that the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples differing by the preparation method exhibit identical behavior of thermal transformation (Table 1).

Taking into account the fact that exoeffect relating to formation of the hexaferrite phase is observed at 760 ◦ C [26], the exoeffects at 650–697 ◦ C (Table 1) are probably caused by the crystallization of mixed oxide SrMn3 O6 . This suggestion is confirmed by XRD data presented below. The XRD data (Table 2, Fig. 1) show that substituted Sr-ferrites calcined at 700 ◦ C are multiphase. The SM6F sample contains the SrCO3 , ␣-Fe2 O3 and Mn2 O3 phases (Fig. 1). Result obtained agrees with the data [27]: Ba-ferrite calcined at 700 ◦ C is also multiphase

Table 2 Phase composition of ferrites calcined at 700 ◦ C. Sample

Phase composition

SBET , m2 /g

SM6F

␣-Fe2 O3 SrCO3 Mn2 O3

27

SM6FA2

␣-Fe2 O3 Mn2 O3 Uncertain phases

59

SM6FA2C0.2

␣-Fe2 O3 Mn2 O3 Mn3 O4

55

SM6FA2C0.2 W0.28

␣-Fe2 O3 Mn2 O3 Mn3 O4

35

SM6FA2C0.2 W0.84

␣-Fe2 O3 Mn2 O3 SrWO4

32

SM6FA2C0.2 W0.84 *

␣-Fe2 O3 Mn2 O3 SrWO4

39

Fig. 1. XRD patterns of the samples calcined at 700 ◦ C (A): SM6F (a), SM6FA2 (b), SM6FA2C0.2 (c), SM6FA2C0.2 W0.84 (d), SM6FA2C0.2 W0.84 (e), SM6FA2C0.2 W0.84 (f). For comparison the XRD patterns of SM6F calcined at 700 and 800 ◦ C are presented in (B). Symbols: (+) SrWO4 , (m) Mn2 O3 , (v) ␣-Fe2 O3 , (*) SrCO3 , (ˆ) Mn3 O4 , (o) SrMn3 O6 , (s) SrFe12 O19 .

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

233

Fig. 2. Pore size distribution for (a) SM6FA2C0.2 and (b) SM6FA2C0.2 W0.84 *.

and contains the Fe2 O3 phase. The hexaferrite phase BaFe12 O19 is formed only after treatment of the sample at 900 ◦ C. Therefore, the SM6F sample was calcined at 800 ◦ C. It is seen (Fig. 1b) that crystallization of the SrFe12 O19 and SrMn3 O6 phases occurs at this temperature. For the SM6FA2 sample not all phases were identified (Table 2, Fig. 1). The introduction of cerium leads to disappearance of the SrCO3 phase and to appearance of the additional Mn3 O4 phase (Fig. 1), probably, containing iron ions. Absence of SrCO3 phase can be related to the formation of the SrCeO3 phase [28,29]. Amount of SrCeO3 seems to be insignificant and the phase is not registered by XRD. Further introduction of tungsten in the SM6FA2Ce0.2 sample contributes to the formation of the SrWO4 phase (Fig. 1) and to disappearance of the Mn3 O4 phase, whereas the ␣-Fe2 O3 and Mn2 O3 phases remain, however, its crystallinity increases. Thus, the introduction of Ce and/or W in the substituted Sr-ferrites leads to change of the phase composition and crystallinity. 3.2. FTIR spectroscopy Fig. 2 shows FTIR spectra of the SM6F, SM6FA2, SM6FA2C0.2 and SM6FA2C0.2 W0.84 samples calcined at 700 ◦ C. The absorbance bands at 323, 481 and 571 cm−1 relating to ␣-Fe2 O3 [30,31] and the bands at 857 and 1457 cm−1 absorbance bands corresponding to vibrations of CO3 2− in SrCO3 [31] are observed for the SM6F sample (Fig. 2a, curve 1). Furthermore, the 531, 571 and 667 cm−1 bands being also observed in FTIR spectra testify the Mn2 O3 phase [32,33]. According to [34], an additional absorbance band at 1609 cm−1 can be related to molecules of adsorbed water. The further introduction of aluminum, cerium and tungsten ions to the SM6F sample practically does not affect the positions of

absorbance bands (Fig. 2a and b), which are typical for ␣-Fe2 O3 , Mn2 O3 , SrCO3 and adsorbed water. Moreover, for the W-containing samples, the additional absorbance bands at 413, 818 and 912 cm−1 are characteristic of the SrWO4 phase [35]. Thus, the results of FTIR are in a good agreement with TG-DTA and XRD data. 3.3. Texture The specific surface area (SBET ) depends on the nature and ratio of the components. One can see (Table 2) that SBET of the samples calcined at 700 ◦ C ranged from 27 to 59 m2 /g. The introduction of aluminum in the samples promotes increase in surface area, probably, as a result of formation of highly dispersed aluminates which are not registered by XRD. The further introduction of cerium and tungsten ions leads to gradual decrease of SBET , since, as it was mentioned above, the crystallinity of the samples increases. However, the change of the preparation method allows one to increase the specific surface area from 32 to 39 m2 /g for the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples, respectively. The changes in the textural characteristics were investigated with the SM6FA2C0.2 and SM6FA2C0.2 W0.84 * samples calcined at 700 ◦ C. Fig. 3 shows that the samples differing in ratio and nature of the components exhibit the mono-mesopore pore size distribution. At the same time, textural characteristics depend on the sample composition. Specific surface area, pore volume and the mean pore diameter of the SM6FA2C0.2 sample are 51 m2 /g, 0.25 cm3 /g and 194 Å, respectively (Table 3). The introduction of tungsten in the sample leads to decrease in SBET (33 m2 /g), pore volume (0.19 cm3 /g) and to increase in the mean pore diameter (224 Å). Thus, the concerned ferrites calcined at 700 ◦ C exhibit

Fig. 3. FTIR spectra of the samples (a) SM6F (1) and SM6FA2 (2) calcined at 700 ◦ C, (b) SM6FA2 (1), SM6FA2C0.2 (2) and SM6FA2C0.2 W0.84 (3) calcined at 700 ◦ C.

234

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

Table 3 Texture characteristics. Sample

SM6FA2C0.2 SM6FA2C0.2 W0.84 *

T (◦ C) of calcination

700 700

Texture characteristics SBET , m2 /g

Vpore , cm3 /g

dpore , Å

51 33

0.25 0.19

194 224

mono-mesopore texture, the average pore diameter is about (200 ± 20) Å. 3.4. Surface distribution and states of various elements in ferrites XPS data indicate that the surface concentrations of elements in the SM6F, SM6FA2, SM6FA2C0.2 , SM6FA2C0.2 W0.28 , SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples differ from the bulk chemical contents (Table 4). One can see, the surface of all samples is enriched by Sr. The atomic ratios of [Sr]/[Fe] and [Sr]/[Mn] are significantly higher than stoichiometric values. Sr segregation can be explained by the formation of surface carbonates, oxides, and hydroxides. Indeed, XPS indicates the presence of surface carbonates in the samples. Two features at 284.8 and 289.2 ± 0.3 eV are observed in the C1s spectra (not shown here). The first one corresponds to carbon in hydrocarbon impurities, whereas the second one – to carbon in carbonate groups [36]. Except for the SM6F and SM6FA2C0.2 W0.28 samples, the atomic ratios of [Mn]/[Fe] are also higher than stoichiometric values (Table 4). This ratio is lower in 2 times than bulk ratio for the SM6F sample, and is comparable for the SM6FA2C0.2 W0.28 one. Moreover, the introduction of additives such as Al, Ce, W in the ferrites leads to enrichment of the sample surface by aluminum since the atomic ratios of [Sr]/[Al] and [Mn]/[Al] are significantly lower than stoichiometric values (Table 4). The formation of Sr and/or Mn aluminates on the sample surface cannot be excluded [12]. According to XPS, the surface ratios of [Mn]/[Al], [Mn]/[Sr], [Mn]/[W] and [Mn]/[Ce] are lower than the bulk ratios (Table 4). On one hand, this can be defined by segregation effects of Al, Sr and W followed by formation of Al2 O3 , SrCO3 and WO3 on the surface. On the other hand, lower values of the atomic concentrations of manganese and iron can arise from the formation of big particles of Mn2 O3 and Fe2 O3 . A distinctive feature of W-containing samples is that the atomic ratio of [Sr]/[W] decreases with increasing the tungsten amount (Table 4) and practically coincides with a stoichiometric value for the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples. The atomic ratio of [Ce]/[Mn] are the same for the SM6FA2C0.2 , SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples (Table 4). It increases with the introduction of the slight amount of tungsten

to the composition of SM6FA2C0.2 W0.28 . Totality of the results obtained gives grounds to assume that the observed changes in atomic ratios of the components are caused by formation of the SrWO4 phase. Its amount increases with enhancement of tungsten content that agrees with XRD data. It is interesting to note that the SM6FA2C0.2 sample is characterized by lowered Sr concentration on the surface. Slightly lower values of the atomic ratios of [Sr]/[Al], [Sr]/[Mn] and [Sr]/[Fe] are observed. Fig. 4a shows the Sr3d spectra of the samples concerned. The Sr3d spectrum is known to represent as the unresolved spin–orbit Sr3d5/2 –Sr3d3/2 doublet (intensity ratio 3:2 and splitting of ca. 1.79 eV). As it can be noted the Sr3d spectra of the samples, except for the SM6FA2 sample, are described by one doublet of Sr3d5/2 –Sr3d3/2 , with the Sr3d5/2 line being located at binding energy of 133.3–133.4 eV. The Sr3d spectrum of the SM6FA2 sample is approximated by two doublets with the Sr3d5/2 binding energies of 132.1–132.6 and 133.6–134.1 eV. According to literature data [37,38], the binding energies for the strontium ions in SrO, Sr(OH)2 and SrCO3 are in the range of 131.7–132.4, 132.8 and 133.4–133.8 eV, respectively. The Sr3d5/2 binding energies of La1−x Srx CrO3 mixed oxides and La0.6 Sr0.4 MnO3 are 131.5–131.7 and 132.1 eV, respectively [36,39]. For the SM6FA2 sample, the literature data allow assignment of the first doublet to Sr2+ in the ferrite structure, and the second one – to hydroxocarbonate phase [12,36]. Taking into account low concentration of carbonate groups, it can be supposed that for the other samples strontium represents predominantly as Sr2+ in the mixed oxides. Indeed, the relative intensity of the C1s peak at 289.0 eV corresponds to the molar ratio [Sr]/[CO3 ] of 1.8 and 2.4 (but not 3 as should be for carbonates) for the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples, respectively. Moreover, the Sr3d5/2 binding energies of 134.3–134.8 eV are typical for some mixed oxides [39,40]. The Fe2p spectra contain the spin–orbit Fe2p3/2 –Fe2p1/2 doublet with maximum intensities at 711.3 and 724.7 eV (Fig. 4b). Iron in FeO, Fe3 O4 , Fe2 O3 and FeOOH is characterized by the Fe2p3/2 binding energies in the range of 709.3–709.7, 709.7–710.6, 710.4–711.2 and 710.2–711.05 eV, respectively [41,42]. Furthermore, the Fe2p spectra of the samples are characterized by additional weak «shakeup satellite» at 719 eV, which together with the Fe2p3/2 binding energy values are used for identification of the chemical state of iron. For example, the feature which discriminate the Fe2p spectrum of FeO from the spectrum of Fe2 O3 is the presence of the weak «shake-up satellites» localized between 6 and 8 eV above the basic Fe2p3/2 and Fe2p1/2 photoelectron lines [41,42]. The satellite structure is absent in the case of Fe3 O4 . Hence, the weak satellite at 719 eV, which is observed in the Fe2p spectra of the samples, as well as the values of the Fe2p3/2 binding energy indicate the formation

Table 4 Atomic concentration of elements on ferrite surface determined by XPS. Sample

Method

[Sr]/[Fe]

[Sr]/[Mn]

[Mn]/[Fe]

[Sr]/[Al]

[Mn]/[Al]

[Sr]/[W]

[Ce]/[Mn]

[W]/[Mn]

SM6F

XPS C.A.

0.27 0.17

0.45 0.17

0.59 1.0

– –

– –

– –

– –

– –

SM6FA2

XPS C.A.

0.53 0.25

0.50 0.17

1.0 1.5

0.25 0.50

0.48 3.00

– –

– –

– –

SM6FA2C0.2

XPS C.A.

0.71 0.20

0.42 0.13

1.65 1.5

0.22 0.40

0.53 3.00

– –

0.15 0.03

– –

SM6FA2C0.2 W0.28

XPS C.A.

0.81 0.20

0.64 0.15

1.27 1.36

0.29 0.40

0.45 2.72

1.55 2.86

0.19 0.04

0.41 0.05

SM6FA2C0.2 W0.84

XPS C.A.

0.79 0.20

0.62 0.19

1.29 1.08

0.31 0.40

0.51 2.16

0.97 0.95

0.16 0.05

0.63 0.19

SM6FA2C0.2 W0.84 *

XPS C.A.

0.83 0.20

0.51 0.19

1.61 1.08

0.27 0.40

0.53 2.16

0.95 0.95

0.14 0.05

0.54 0.19

C.A.: chemical analysis.

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

235

Fig. 4. (a) Sr3d core-level XPS spectra. (1) SM6FA2C0.2 ; (2) SM6FA2C0.2 W0.28 ; (3) SM6FA2C0.2 W0.84 ; (4) SM6FA2C0.2 W0.84 *; (5) SM6FA2; (6) SM6F. All spectra normalize to the intensity of the corresponding Mn2p spectra; (b) Fe2p core-level XPS spectra. (1) SM6FA2C0.2 ; (2) SM6FA2C0.2 W0.28 ; (3) SM6FA2C0.2 W0.84 ; (4) SM6FA2C0.2 W0.84 *; (5) SM6FA2; (6) SM6F; (c) Mn2p3/2 core-level XPS spectra. (1) SM6FA2C0.2 ; (2) SM6FA2C0.2 W0.28 ; (3) SM6FA2C0.2 W0.84 ; (4) SM6FA2C0.2 W0.84 *; (5) SM6FA2; (6) SM6F; (d) Al2p core-level XPS spectra. (1) SM6FA2C0.2 ; (2) SM6FA2C0.2 W0.28 ; (3) SM6FA2C0.2 W0.84 ; (4) SM6FA2C0.2 W0.84 *; (5) SM6FA2; (e) Ce3d core-level XPS spectra. (1) SM6FA2C0.2 ; (2) SM6FA2C0.2 W0.28 ; (3) SM6FA2C0.2 W0.84 ; (4) SM6FA2C0.2 W0.84 *; (f) W4f core-level XPS spectra. (1) SM6FA2C0.2 W0.28 ; (2) SM6FA2C0.2 W0.84 ; (3) SM6FA2C0.2 W0.84 *.

of the Fe3+ ions. Recently it has been shown [43,44] that the Fe2p3/2 binding energy of the LaFeAl11 O19 hexaaluminate is 711.4–711.8 eV that is also typical for Fe3+ . The Mn2p3/2 spectra of the samples under studies shown in Fig. 4c exhibit an asymmetric shape. Therefore, two peaks were applied for the approximation of experimental curves. The main peak corresponds to the Mn2p3/2 line. The second less-intensive peak shifted to the higher binding energies by 2.4 eV is originated due to the multiplet splitting [45]. The Mn2p3/2 binding

energy value of 641.8–641.9 eV is characteristic of Mn3+ ions. The similar values of 641.0–641.5 eV have been observed for Mn3+ in LaMnO3 , and La1−x Srx MnO3 oxides [46,47]. According to [48], manganese in MnO, Mn2 O3 and MnO2 is characterized by the Mn2p3/2 binding energies in the range of 640.6–641.7, 641.5–641.9 and 642.2–642.6 eV, respectively. The Al2p binding energy for the samples concerned is 73.9–74.1 eV that corresponds to Al3+ ions (Fig. 4d). For comparison, aluminum in ␣-Al2 O3 , AlOOH and LaFeMnx Al11−x O19−ı is charac-

236

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

Fig. 5. Temperature dependences of the NH3 (a) and NO (b) conversion for the ferrite catalysts.

terized by the Al2p binding energies of 74.2, 73.9 and 74.6–74.8 eV, respectively [45,49]. Fig. 4e shows the Ce3d spectra of the Ce-containing samples. Ce is mainly in the Ce4+ state. This fact is confirmed by high intensity of the u peak at 917 eV [50,51]. This peak is absent in the Ce3d spectrum of Ce3+ compounds. The u line position is 917.6–917.7 eV. For CeO2 this peak located at 916.7 eV [51]. The W4f spectrum for the samples contains the W4f7/2 –W4f5/2 spin–orbit doublet (Fig. 4f). The W4f7/2 binding energy lies in the range of 35.4–35.5 eV, which is typical for W6+ [52]. Tungsten in WO3 is characterized by the W4f7/2 binding energy of 35.5 eV. 3.5. Catalytic activity tests The activity of the samples in the selective catalytic reduction of NO by NH3 and N2 concentration depend on the nature and ratio

of the components and the preparation method (Figs. 5 and 6). The obtained results show that temperatures of total conversion of ammonia are different for all samples (Fig. 5a) and range from 385 to 490 ◦ C. Among the Mn-substituted ferrites differing by the Mn loading the highest NO conversion of 88% is achieved over the SrMn6 Fe6 O19 sample. According to [53], Fe–Mn-oxides with ratio Mn:Fe = 1:1 are active in the SCR. Probably, the highest activity of the SrMn6 Fe6 O19 sample is caused by such ratio of Mn and Fe ions as well. The introduction of Al in the SrMn6 Fe6 O19 sample is accompanied by the decrease of NO conversion to 75%, that, probably, is related to change of the Mn: Fe ratio and the phase composition (Fig. 1). It is known [54] that the substitution of Ce for La in La1−x Cex SrNiO4 could further enhance its activity in NO decomposition. Thus, the SM6FA2 sample was promoted by Ce and W ions. The introduction of Ce (SM6FA2C0.2 ) and Ce–W

Fig. 6. Temperature dependences of the N2 (a), N2 O (b) and NO2 (c) concentration for the ferrite catalysts.

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

237

Fig. 7. TPD-NH3 of the ferrite catalysts (a) SM6F, (b) SM6FA2C0.2 W0.84 , (c) SM6FA2C0.2 W0.84 * calcined at 700 ◦ C.

(SM6FA2C0.2 W0.28 ) in the SM6FA2 sample leads to decrease of NO conversion to 53% (Fig. 5b). Only increasing the tungsten loading SM6FA2C0.2 W0.28 → SM6FA2C0.2 W0.56 → SM6FA2C0.2 W0.84 allowed us to increase NO conversion 53% → 59% → 82%. Moreover, the change of the preparation method of the SM6FA2C0.2 W0.84 * sample promotes practically complete NO conversion (XNO = 98%). It should be noted that the introduction of tungsten influences NO conversion and a temperature window (taken as 50% NO conversion). It can be seen from Fig. 5b the temperature window was broadened by the addition of WO3 in the SM6FA2C0.2 sample from 113 to 195 ◦ C. The temperature window was broadened up to 250 ◦ C with the change of the preparation method of the SM6FA2C0.2 W0.84 * sample. The NO conversion profiles show a maximum at 240–300 ◦ C and then a decrease above this temperature (Fig. 5b). As it has been shown earlier [12,55,56], reduction in NO conversion at high temperatures (>300 ◦ C) can be related to side reactions of the ammonia oxidation to N2 , N2 O and NO2 . The results obtained in present work confirm this hypothesis (Fig. 6). Besides the main reaction product (N2 ), the formation of N2 O and NO2 is also detected. One can see (Fig. 6a) that the most selective sample with respect to N2 is SM6FA2C0.2 W0.84 *. The concentration of nitrogen for this sample was about 263 ppm. Decreasing in the N2 concentration is accompanied by increase of the amounts of N2 O and NO2 . Fig. 6b shows that appreciable amounts of N2 O are formed at temperatures above 150–250 ◦ C. The highest concentration of N2 O (153 ppm) is formed over the SM6F sample, and the lowest concentration (48 ppm) – over the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples. NO2 is also formed in the SCR (Fig. 6c). It should be noted that the highest NO2 concentration of 316 ppm is observed over the SM6FA2 sample. However, the further introduction of Ce and W ions leads to decrease of its concentration to 95–110 ppm.

Hence, the analysis of the obtained results shows that the highest activity and N2 concentration in the SCR of NO by ammonia is achieved over the ferrites promoted by tungsten. It can be supposed that the introduction of tungsten in the ferrites leads to change the surface acidity. Ammonia adsorption depends on the strength and concentration of Lewis acid sites [57]. 3.6. NH3 -TPD Temperature-programmed desorption of ammonia was carried out over the SM6F, SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples differing by the activity in the SCR of NO with ammonia and obtained N2 concentration. The NH3 -TPD profiles are shown in Fig. 7. The NH3 -TPD profile of the samples is characterized by an asymmetric peak in the temperature range of 40–400 ◦ C. The first maximum is observed at about 100 ◦ C for all samples and the second maximum – at 160–180 ◦ C for the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples. According to [58], ammonia desorbed at low temperatures may be assigned to physically adsorbed NH3 , while chemisorbed NH3 is responsible for ammonia desorption at higher temperatures. A quantitative analysis showed that the amount of adsorbed ammonia is 0.088 and 0.124 mmol/g for the SM6F and SM6FA2C0.2 W0.84 , SM6FA2C0.2 W0.84 * samples, respectively. The amount of desorbed NH3 increases in the order: SM6F (0.023 mmol/g) < SM6FA2C0.2 W0.84 * (0.071 mmol/g) < SM6FA2C0.2 W0.84 (0.082 mmol/g). This points out that the introduction of tungsten ions in the Sr-ferrite increases the surface acidity by about 3.5–4-fold. However, it should be noted that the SM6FA2C0.2 W0.84 * sample is characterized by lower acidity in comparison with the SM6FA2C0.2 W0.84 sample. The change of the preparation method of the SM6FA2C0.2 W0.84 * sample decreases the intensity of the first peak which corresponds to the amount

238

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

Fig. 8. TPR-NH3 of the ferrite catalysts (a) SM6F, (b) SM6FA2C0.2 W0.84 , (c) SM6FA2C0.2 W0.84 * calcined at 700 ◦ C.

of weakly adsorbed ammonia and increases the quantity of chemisorbed NH3 molecules (Fig. 7b and c). The weakly bonded oxygen species is present on the surface of the SM6F sample. This fact is confirmed by the formation of significant amounts of ammonia oxidation products such as N2 , N2 O, NO and NO2 (Fig. 7a), whereas its concentration does not exceed 10 ppm over the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples (Fig. 7b and c). Thus, the ferrites differ by surface acidity and different oxygen species, which determine its reactivity. 3.7. NH3 -TPR Reactivity of ammonia was characterized using temperatureprogrammed reaction of ammonia with catalyst surface. The NH3 -TPR profiles of the SM6F (Fig. 8a), SM6FA2C0.2 W0.84 (Fig. 8b) and SM6FA2C0.2 W0.84 * (Fig. 8c) samples show that reaction of ammonia with catalyst surface is accompanied by the change of its concentration. Ammonia concentration shows a maximum at ∼85, 57 and 60 ◦ C for the corresponding samples, and then a decrease practically to zero at 600 ◦ C. The formation of N2 , N2 O and NO is observed, while NO2 is absent. According to [57], ammonia reacts with Lewis acid sites forming NH2 and N2 H4 . These species reacting with lattice oxygen can be converted to ammonia oxidation products such as N2 , N2 O and NO [56]. Analysis of the obtained results shows (Fig. 8) that the temperature of the initial formation and the behavior of the concentration profiles of the products depend on the catalyst nature. The temperature of the initial formation of the products increases in the order: SM6F → SM6FA2C0.2 W0.84 → SM6FA2C0.2 W0.84 *. For the SM6F and SM6FA2C0.2 W0.84 * samples this temperature, in particular, temperature of N2 formation, differs about 6-fold. On one hand, this is evidence of the fact that the interaction of ammonia with catalyst surface proceeds with different oxygen species. On the other

hand, the surface of the SM6F sample contains the most reactive oxygen, since the ammonia oxidation products are formed at the lower temperature. 4. Discussion It is established that activity of the substituted Sr-ferrites with the components ratio typical for hexaferrites in the SCR depends on nature and ratio of the components, which determine phase composition and surface state of the catalyst components. For Sr-ferrite systems preparation method adopted here allows us to obtain hexaferrite phase after calcination of the samples at 800 ◦ C. At the same time, the systems calcined at 700 ◦ C are multiphase, that is confirmed by TG-DTA, XRD and FTIR data (Table 2). Except ␣-Fe2 O3 and Mn2 O3 phases, which are observed in all samples, Sr-containing phases are also present. Its structure is determined by nature and content of introduced additive. For the sample SM6F this is SrCO3 , which disappears after further introduction of Al, Ce and W in the sample. Phases which are not identified on this stage are also formed. Probably, they are in a highly dispersed state. The introduction of tungsten in the sample results in formation of SrWO4 . The amount of the SrWO4 phase increases (intensity of line at ≈2 = 47.49◦ ) with increasing tungsten content. It should be noted that the preparation method does not influence the phase composition of the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples (Fig. 1). The phase transformations are accompanied by change of the surface properties of the samples. According to XPS data, surface of the SM6F sample is enriched by strontium due to its segregation. Sr segregation is most probably explained by formation of surface carbonates. As a result formation of reactive oxygen occurs. Introduction of Al, Ce and W in the SM6F sample leads to enrichment of the surface by aluminum, probably, due to formation of surface

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

SrAl2 O4 . For the W-containing samples, atomic ratio of [Sr]/[W] in surface layer decreases with increasing tungsten content and it practically coincides with stoichiometric value due to SrWO4 formation. Thus, introduction of Al, Ce and W promotes decrease in amount of surface carbonates and amount of reactive oxygen. Based on the binding energy data of XPS it can be concluded that oxidation state of the main components are Sr2+ , Ce4+ , Mn3+ , Fe3+ , W6+ and Al3+ . Practically complete NO conversion is achieved over Wcontaining samples which are most active catalysts among all ferrites. At the same time N2 concentration increases from 148 to 223 ppm with increasing tungsten loading in the order: SM6FA2C0.2 W0.28 → SM6FA2C0.2 W0.56 → SM6FA2C0.2 W0.84 . The most selective catalyst is the SM6FA2C0.2 W0.84 * sample prepared by precipitation of solution of metal nitrates using NH4 HCO3 and ammonium para-tungstate as precipitating agents. N2 concentration is 263 ppm over this catalyst. The ammonia TPD profiles showed that NH3 could adsorb on a variety of acid sites of the catalysts and they desorbed over a wide temperature range. The ammonia adsorbed species include a physically adsorbed NH3 and chemisorbed NH3 . Besides, introduction of tungsten in the Sr-ferrites promotes increase in surface acidity in about 3.5–4 times. By comparing surface acidity of the Sr-ferrites (via NH3 -TPD and SCR performance) we can conclude that the surface acidity has no bearing on NO conversion and N2 concentration. It means that not only surface acidity influence SCR activity. The phase composition, surface ratio of [Mn]/[Fe], presence of reactive oxygen and specific surface area can also determine activity of the catalysts in the SCR. The results of catalytic activity test evidence that NO conversion practically does not depend on the phase composition of the samples. It is known [6,59] that maximum NO conversion for Mn2 O3 and Fe2 O3 oxides is 75% and 43%, respectively. According to XRD, the SM6FA2C0.2 W0.84 and SM6FA2C0.2 W0.84 * samples differing by the preparation method have the same phase composition, but amount of the Mn2 O3 and Fe2 O3 phases is higher for the SM6FA2C0.2 W0.84 * sample that is confirmed by intensities of the peaks (Fig. 1a). At the same time, the SM6FA2C0.2 W0.28 sample is the least active (Fig. 5b), however, it contains the same phases practically in the same amount. Our previous work [12] showed that N2 concentration of the substituted aluminates depends on the amount of Mn3+ ions. The higher being amount of Mn3+ ions, the higher N2 concentration. The atomic ratio of [Mn]/[Fe] of the samples concerned increases in the following order: 0.59 (SM6F) → 1.0 (SM6FA2) → 1.27 (SM6FA2C0.2 W0.28 ) → 1.29 (SM6FA2C0.2 W0.84 ) → 1.61 (SM6FA2C0.2 W0.84 *) → 1.65 (SM6FA2C0.2 ) (Table 4). N2 concentration increases in the same order, except for the SM6FA2C0.2 sample (Fig. 6a). For this sample atomic ratio of [Mn]/[Fe] is comparable with value of atomic ratio obtained on the most selective SM6FA2C0.2 W0.84 * sample. Hence, in case of the Sr-ferrites this dependence is confirmed. NH3 -TPR data show that lattice oxygen of the catalysts can oxidize adsorbed ammonia to N2 , N2 O and NO. Formation temperature of the ammonia oxidation products depends on reactivity of oxygen species. It is shown that ammonia oxidation products are formed at lower temperature on the surface of the SM6F sample containing the most reactive oxygen. The formation of the traces of these products over W-containing Sr-ferrites can be related to presence of the strongly bonded oxygen species which does not react with adsorbed ammonia in the investigated temperature range. It should be noted that there is no direct relation between specific surface area and activity of the samples. One can see that SBET increases in the order (m2 /g): SM6F (27) < SM6FA2C0.2 W0.84 (32) < SM6FA2C0.2 W0.84 * (39) < SM6FA2 (59). However, this sequence does not correlate with NO conversion (Fig. 5b), max-

239

imum NO conversion is achieved over the SM6FA2C0.2 W0.84 * sample. Hence, the high value of NO conversion and N2 concentration using substituted Sr-ferrites can be achieved by simultaneous regulation of surface acidity of the catalysts, its redox properties and reactivity of lattice oxygen. 5. Conclusions Series of the Sr1−x Cex Mn6−y Wy Fe4 Al2 O19 (x = 0; 0.2; y = 0; 0.28; 0.56; 0.84) ferrites with components ratio typical for hexaferrites was prepared by the co-precipitation of soluble nitrates of corresponding metals with NH4 HCO3 as a precipitating agent at a fixed temperature and pH. The samples were characterized by different physicochemical methods and were tested in the selective catalytic reduction of nitric oxide by ammonia. The Sr0.8 Ce0.2 Mn5.16 W0.84 Fe4 Al2 O19 * sample was prepared by the coprecipitation method of solution of corresponding nitrates with NH4 HCO3 and ammonium para-tungstate as a precipitating agents for studying the influence of the preparation method on its properties. Some specific conclusions can be drawn as follows: • The phase composition of the samples depends on the nature and ratio of the components. The ferrites calcined at 700 ◦ C are multiphase, the hexaferrite phase is not formed. The preparation method of the Sr0.8 Ce0.2 Mn5.16 W0.84 Fe4 Al2 O19 * sample does not influence the phase composition. • The specific surface area of the samples calcined at 700 ◦ C lies in the range of 27–59 m2 /g. The introduction of aluminum ions promotes an increase of SBET , whereas the introduction of Ce and W ions leads to its decrease. The change of the preparation method of the Sr0.8 Ce0.2 Mn5.16 W0.84 Fe4 Al2 O19 * sample slightly increases the specific surface area. • The samples surface is enriched with strontium. Taking into account low concentration of the carbonate groups it can be supposed that Sr is predominantly localized on the surface as Sr2+ in the mixed oxides. The main components on the ferrite surface are in the oxidized states: Mn3+ , Fe3+ , Ce4+ and W6+ . • The activity of the ferrites and obtained N2 concentration in the selective catalytic reduction of nitrogen oxide by ammonia depends on the nature and ratio of the catalyst components and the preparation method. It was shown that maximum NO conversion and N2 concentration are achieved over Sr0.8 Ce0.2 Mn5.16 W0.84 Fe4 Al2 O19 *. It was established that the activity of the catalysts in the SCR and N2 concentration are determined by the surface acidity, surface atomic ratio of [Mn]/[Fe] and the presence of reactive oxygen. Acknowledgements The authors wish to thank kindly collaborators from Boreskov Institute of Catalysis: G.S. Litvak for TG-DTA measurements, I.Yu. Molina for taking X-ray diffraction patterns and G.N. Kustova for the experimental support in the FTIR measurements. References [1] G. Qi, R.T. Yang, J. Catal. 217 (2003) 434–441. [2] S. Roy, A. Baiker, Chem. Rev. 109 (2009) 4054–4091. [3] F. Eigenmann, M. Maciejewski, A. Baiker, Appl. Catal. B: Environ. 62 (2006) 311–318. [4] G.L. Bauerle, S.C. Wu, K. Nobe, Ind. Eng. Chem. Prod. Res. Dev. 17 (1978) 123–128. [5] W.C. Wong, K. Nobe, Ind. Eng. Chem. Prod. Res. Dev. 25 (1986) 179–186. [6] N. Apostolescu, B. Geiger, K. Hizbullah, M.T. Jan, S. Kureti, D. Reichert, F. Schott, W. Weisweiler, Appl. Catal. B: Environ. 62 (2006) 104–114. [7] J. Wang, Zh. Tian, J. Xu, Yu. Xu, Zh. Xu, L. Lin, Catal. Today 83 (2003) 213–222. [8] G. Groppi, C. Cristiani, P. Forzatti, Appl. Catal. B: Environ. 35 (2001) 137–148.

240

M.V. Bukhtiyarova et al. / Applied Catalysis A: General 384 (2010) 230–240

[9] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal. A: Gen. 234 (2002) 1–23. [10] W.S. Kijlstra, D.S. Brands, H.I. Smit, E.K. Poels, A. Bliek, J. Catal. 171 (1997) 219–230. [11] G. Qi, R.T. Yang, Appl. Catal. B: Environ. 44 (2003) 217–225. [12] M.V. Bukhtiyarova, A.S. Ivanova, L.M. Plyasova, G.S. Litvak, V.A. Rogov, V.V. Kaichev, E.M. Slavinskaya, P.A. Kuznetsov, I.A. Polukhina, Appl. Catal. A: Gen. 357 (2009) 193–205. [13] X. Zhang, X. Li, J. Wu, R. Yang, Zh. Zhang, Catal. Lett. 130 (2009) 235–238. [14] G. Groppi, C. Cristiani, P. Forzatti, J. Catal. 168 (1997) 95–103. [15] W.J. Price, Analytical atomic-absorption spectroscopy, New York (1976). [16] S. Lowell, J.E. Shields, M.A. Thomas, M. Thommes, Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Springer, Netherlands, 2006. [17] D.A. Shirley, Phys. Rev. B 5 (1972) 4709–4714. [18] S. Doniach, M. Sanjic, J. Phys. C 3 (1970) 285–291. [19] J.H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 129–137. [20] N.-A.M. Deraz, G.A. El-Shobaky, Thermochim. Acta 375 (2001) 137–145. [21] L. Biernacki, S. Pokrzywnicki, J. Therm. Anal. Calorim. 55 (1999) 227–232. [22] W.M. Shaheen, M.M. Selim, J. Therm. Anal. Calorim. 59 (2000) 961–970. [23] Z. Xiujing, F. Yan, H. Qing, C. Gang, Chin. J. Mater. Res. 22 (2008) 539–544. [24] S. Guillemet-Fritsch, A. Navrotsky, P. Tailhades, H. Coradin, M. Wang, J. Solid State Chem. 178 (2005) 106–113. [25] A.N. Shirsat, K.N.G. Kaimal, S.R. Bharadwaj, D. Das, J. Solid State Chem. 177 (2004) 2007–2013. [26] W. Roos, J. Am. Ceram. Soc. 63 (1980) 601–603. [27] A. Favre, N. Guilhaume, J.-M.M. Millet, M. Primet, Catal. Lett. 49 (1997) 207–211. [28] T. Scharbon, A.S. Nowick, Solid State Ionics 35 (1989) 189–194. [29] R.C.T. Slade, S.D. Flint, N. Singh, Solid State Ionics 82 (1995) 135–141. [30] S. Onari, T. Ari, K. Kudo, Phys. Rev. B 16 (1977) 1717–1721. [31] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Mir, Moscow, 1991. [32] D.P. Das, K.M. Parida, Appl. Catal. A: Gen. 324 (2007) 1–8. [33] M.R. Morales, B.P. Barbero, L.E. Cadús, Fuel 87 (2008) 1177–1186. [34] S. Valange, A. Beauchaud, J. Barrault, Z. Gabelica, M. Daturi, F. Can, J. Catal. 251 (2007) 113–122. [35] T. Thongtem, S. Kaowphong, S. Thongtem, Appl. Surf. Sci. 254 (2008) 7765–7769.

[36] K. Rida, A. Benabbas, F. Bouremmad, M.A. Pena, A. Merthinez-Arias, Catal. Commun. 7 (2006) 963–968. [37] M.I. Sosulnikov, Yu.A. Teterin, J. Electron Spectrosc. Relat. Phenom. 59 (1992) 111–126. [38] J.-C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys. 2 (2000) 1319–1324. [39] K. Horiba, A. Chikamatsu, H. Kumigashira, M. Oshima, H. Wadati, A. Fujimori, M. Lippmaa, M. Kawasaki, H. Koinuma, J. Electron Spectrosc. Relat. Phenom. 155–158 (2007) 375–378. [40] S.N. Shkerin, M.V. Kuznetsov, N.A. Kalashnikova, Russ. J. Electrochem. 39 (2003) 591–599. [41] N.C. McIntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521–1529. [42] P.C.J. Graat, M.A.J. Somers, Appl. Surf. Sci. 100–101 (1996) 36–40. [43] Z. Xu, L. Zhao, F. Pang, L. Wang, C. Niu, J. Nat. Gas Chem. 16 (2007) 60–63. [44] X. Ren, J. Zheng, Y. Song, P. Liu, Catal. Commun. 9 (2008) 807–810. [45] H.W. Nesbitt, D. Banerjee, Am. Mineral. 83 (1998) 305–315. [46] E. Regan, T. Groutso, J.B. Metson, R. Steiner, B. Ammundsen, D. Hassell, P. Pickering, Surf. Interface Anal. 27 (1999) 1064–1068. [47] S. Ponce, M.A. Pena, J.L.G. Fierro, Appl. Catal. B: Environ. 24 (2000) 193–205. [48] V. Di Castro, G. Polzonetti, J. Electron Spectrosc. Relat. Phenom. 48 (1989) 117–123. [49] N. Kosova, E. Devyatkina, A. Slobodyuk, V. Kaichev, Solid State Ionics 179 (2008) 1745–1749. [50] E. Wuilloud, B. Delley, W.-D. Schneider, Y. Baer, Phys. Rev. Lett. 53 (1984) 202–205. [51] H. Borchert, Yu.V. Frolova, V.V. Kaichev, I.P. Prosvirin, G.M. Alikina, A.I. Lukashevich, V.I. Zaikovskii, E.M. Moroz, S.N. Trukhan, V.P. Ivanov, E.A. Paukshtis, V.I. Bukhtiyarov, V.A. Sadykov, J. Phys. Chem. B 109 (2005) 5728–5738. [52] Y. Baek, K. Yong, J. Phys. Chem. C 111 (2007) 1213–1218. [53] R.Q. Long, R.T. Yang, R. Chang, Chem. Commun. (2002) 452–453. [54] J.J. Zhu, D.H. Xiao, J. Li, X.G. Yang, Y. Wu, J. Mol. Catal. A: Chem. 234 (2005) 99–105. [55] Zh. Wu, R. Jin, Y. Liu, H. Wang, Catal. Commun. 9 (2008) 2217–2220. [56] Q. Zhao, J. Xiang, L. Sun, S. Su, S. Hu, Energy Fuels 23 (2009) 1539–1544. [57] G. Ramis, L. Yi, G. Busca, J. Catal. 157 (2) (1995) 523–535. [58] R.Q. Long, R.T. Yang, J. Catal. 207 (2002) 158–165. [59] F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Appl. Catal. B 3 (1994) 173–189.