- Email: [email protected]
Alkali-promoted V2O5 catalysts for the partial oxidation of H2S to sulphur
Catalysis Today 192 (2012) 28–35
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Alkali-promoted V2 O5 catalysts for the partial oxidation of H2 S to sulphur M.D. Soriano a , J.M. López Nieto a,∗ , F. Ivars a , P. Concepción a , E. Rodríguez-Castellón b a b
Instituto de Tecnología Química, UPV-CSIC, Campus Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain Dept. Química Inorgánica, Facultas de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
a r t i c l e
i n f o
Article history: Received 29 November 2011 Received in revised form 7 February 2012 Accepted 7 February 2012 Available online 19 March 2012 Keywords: Hydrogen sulphide partial oxidation Sulphur Vanadium oxide (V2 O5 V4 O9 ) Alkali metal (lithium sodium potassium caesium)
a b s t r a c t Present paper describes the influence of the incorporation of alkali metal cations (AM = Li, Na, K, Cs; and an AM/V ratio of 0.04) and Na-content (Na/V ratio of 0.02–0.30) in alkali metal promoted V2 O5 catalyst on both the catalyst structure and the catalytic performance in H2 S partial oxidation reactions. The catalytic activity depends on the alkali metal and the amount of alkali metal added, although Nacontaining catalysts seem to be the more active ones. However, selectivity to sulphur higher than 98% is achieved in the main of catalysts when working at reaction temperature lower than 220 ◦ C. According to the characterization results of used catalysts, V4 O9 is selectively formed during the catalytic tests on catalysts presenting V2 O5 crystallites. In catalysts with Na/V ratios higher than 0.04, V4 O9 and Na0.33 V2 O5 are observed, the presence of Na0.33 V2 O5 increasing when increasing the Na/V ratio. Accordingly, V4 O9 and Na0.33 V2 O5 can be proposed as the active and selective crystalline phase in Na-containing catalysts. The role of the presence of V4+− O–V5+ pairs in partial oxidation of H2 S is also discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Vanadium oxides, and their corresponding supported materials, have been extensively studied in the last three decades. The easy conversion among oxides of different stoichiometry and formation of oxygen vacancies enable vanadium oxides to function as interesting catalysts in a number of partial oxidation reactions [1], i.e. selective oxidation of methanol, olefins and alkanes, as well as in the ammoxidation of aromatic hydrocarbons and the selective catalytic reduction of nitric oxide with ammonia [1–5]. More recently, they have been also studied as catalysts in the partial oxidation of H2 S to sulphur [6–15]. It is well known that supported vanadium oxides with V-loading lower than the theoretical monolayer, are active and selective catalysts in partial oxidation of H2 S to sulphur, although they show an important deactivation [6–15]. Recently, it has been suggested that supported vanadium oxide catalysts with high V-loading, and presenting V2 O5 crystallites, could be preferred in the partial oxidation of H2 S at relatively low reaction temperature, since the V2 O5 crystallites are partially reduced forming V4 O9 which seems to be active and selective in this reaction [14]. In fact, results obtained in operando and in situ conditions using Raman and XAS spectroscopies confirmed this behaviour in both unsupported and
supported V2 O5 -containing catalysts [15]. However, the characteristics of V2 O5 crystallites should be tailored in order to improve the catalytic behaviour. In this sense, the catalytic behaviour of supported-vanadium oxide catalysts and the characteristics of surface vanadium species formed (such as, oxidation state, coordination, etc.) change depending on the V-content and the support, although generally only catalysts presenting high V-loading (i.e. presenting V2 O5 crystallites) have catalytic performances similar to those achieved over pure vanadium oxide. On the other hand, doping vanadium oxide catalysts with alkalimetals has been reported that can modify both structural and acid characteristics of catalysts, changing thus their catalytic behaviour [1–5,16–22]. In this paper, a study on the influence of the incorporation of alkali metal cations (Li, Na, K, Cs) on pure V2 O5 catalysts, as well as the influence of Na-content in Na-promoted V2 O5 catalysts, on both the catalyst structure and the catalytic performance in H2 S partial oxidation reactions is shown. The used catalysts will be also studied in order to evaluate changes in the structure and vanadium oxidation state after reaction. 2. Experimental 2.1. Catalyst preparation
∗ Corresponding author. Fax: +34 963877809. E-mail address: [email protected] (J.M. López Nieto). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.02.016
Alkali metal-promoted vanadium oxides were prepared by the impregnation method. The required amount of the alkali nitrate
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
was dissolved in water and then dropwise added to the ammonium metavanadate (NH4 VO3 , Aldrich) and shaking the mixture with a spatula up to obtain an homogeneous paste. The impregnated solid was dried overnight in furnace at 100 ◦ C. Finally, the samples were calcined at 550 ◦ C for 4 h. For comparison, pure vanadium pentoxide was prepared by calcination of ammonium metavanadate at 550 ◦ C for 4 h. The catalysts will be named as VAM-x, where AM is the alkali metal incorporated and x is referred to the alkali-metal/vanadium (AM/V) atomic ratio. Pure V2 O5 will be named as VAM-0. 2.2. Catalytic tests Catalytic tests for the partial oxidation of H2 S to sulphur were carried out in a fixed-bed quartz tubular flow reactor, at atmospheric pressure and in the 180–240 ◦ C temperature range. A catalyst weight of 0.1 g was mixed with silicon carbide as inert to obtain an adequate bed height. A total flow of 130 mL min−1 has been used, with H2 S/Air/He molar ratio of 1.2/5.0/93.8. Analysis of reactants and reaction products was carried out online by gas chromatography using two different chromatographic columns (Molecular Sieve 5 A˚ and Porapak T) [14]. 2.3. Catalyst characterization X-ray diffraction patterns were collected in an Enraf Nonius PSD120 diffractometer with a monochromatic CuK␣1 source operated at 40 keV and 30 mA. Crystalline phases were identified by matching experimental patterns to the JCPDS powder diffraction file. Temperature-programmed reduction (TPR) was carried out in a Micromeritics Autochem 2910 equipped with a TCD detector, using 10% H2 in Ar with a flow rate of 50 mL min−1 . The temperature range explored was from room temperature to 800 ◦ C. The heating rate was 10 ◦ C min−1 for all samples. Details of specific conditions for each sample are given with the TPR profiles. Infrared spectra were recorded at room temperature in the 300–4000 cm−1 region with a Nicolet 205xB spectrophotometer equipped with a data station at a spectral resolution of 1 cm−1 and accumulations of 128 scans. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with non monochromatic Mg K␣ radiation (300 W, 15 kV, 1253.6 eV) for the analysis of core level signals of C 1s, O 1s, V 2p and Na 2p. Spectra were recorded with the constant pass energy values at 29.35 eV, using a 720 m diameter analysis area. The spectrometer energy scale was calibrated by using the Cu 2p3/2 , Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7, 368.3 and 84.0 eV, respectively. During data processing of the XPS spectra, binding energy values were referenced to the C 1s peak (284.8 eV) from the adventitious contamination layer. The PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gauss–Lorentz curves, in order to determine the binding energy of the different element core levels more accurately. The error in BE was estimated to be ca. 0.1 eV. Short acquisition time of 10 min was first used to examine C 1s, V 2p regions in order to avoid, as much as possible, photo-reduction of V5+ species. Satellite subtraction of the O 1s signal was always performed to study the V 2p region. 3. Results and discussion 3.1. Characterization of catalysts Fig. 1 shows the XRD patterns of alkali-metal-promoted (AM/V ratio of 0.04) catalysts (Fig. 1A) and those with different Na-content
29
(Fig. 1B). For comparison, the XRD pattern of pure V2 O5 has been also included (Fig. 1A, pattern e). The peaks at 2 = 15.3, 20.3, 26.2, 31.2, 34.3 and 47.3◦ , in alkali promoted catalysts, can be related to the presence of orthorhombic V2 O5 [JCPDS: 41–1426]. Moreover, the appearance of peaks at 2 = 9.37, 12.16, 26.43, 27.92 and 28.94◦ (in sample VNa-0.04) suggests the presence of Na0.33 V2 O5 [JCPDS: 19-1052], as minority phase. On the other hand, some differences in the intensity of diffractions have been also observed depending on the alkali-metal doping. Thus, a remarkable increase in the intensity of (0 0 1) and (0 0 2) diffraction lines is observed in the Cs-promoted sample with respect to the remaining alkali-doped catalysts, while the intensity of (1 1 0) diffraction lines is considerably higher in Li-, Na- and K-promoted catalysts. From the comparison between the intensity of (0 0 1) and (1 1 0) diffraction lines, I001 /I110 ratios higher than 5 are observed in V2 O5 and VCs-0.04 samples, whereas I001 /I110 higher than 1 (in VLi-0.04 sample) or lower than 1 (in VNa0.04 and VK-0.04) are observed.No apparent change in the I001 /I110 ratio is observed when increasing the Na-loading. However, the formation of Na0.33 V2 O5 [JCPDS: 19-1052], with peaks at 2 = 9.34, 12.16, 18.75, 24.57, 26.40, 27.92, 28.94, 30.60, 32.91, 41.49 and 50.52◦ , is clearly favoured at high sodium-loading. Fig. 2 shows the infrared spectra of alkali metal-promoted catalysts prepared with an alkali-metal/V atomic ratio of 0.04 (Fig. 2A) and Na-promoted catalysts with different Na-contents (Fig. 2B). For comparison, it has been also included the IR spectrum of the pure vanadium oxide based catalyst (VAM-0) (Fig. 2A, spectrum e). Accordingly, V2 O5 shows bands at 1020, 828 cm−1 and 450–650 cm−1 , related to V O stretching vibration, V O V stretching vibration and V O V rocking vibration in V2 O5 , respectively [16–19,23]. A similar spectrum to that of VAM-0 is observed in Li- and Na-promoted catalysts, while a shift of the band at 828 cm−1 to 783–808 cm−1 region is clearly observed for VK-0.04 and VCs-0.04 samples. Moreover, a new band at 965 cm−1 is observed in VCs-0.04 sample, which can be assigned to the formation of vanadates [19]. On the other hand, changes in the intensity of the bands at 450–650 cm−1 , characteristics of V O V asymmetric bending vibrations in V2 O5 structure [17], are also observed depending of the alkali-metal doping. In this way, the intensity of the band at 620 cm−1 decreases in the case of VK-0.04 and VCs-0.04 samples, suggesting small modifications on the structure of pure V2 O5 [21]. The IR spectra of Na-promoted catalysts with different Nacontent also show the characteristics bands of V2 O5 (Fig. 2B). However, new bands appear increasingly as the content of sodium is raised from Na/V atomic ratio of 0.02–0.30. Thus, broad band at 1013 cm−1 (with a shoulder at 1020 cm−1 ), and bands at 994, 963 and 940 cm−1 , which can be assigned to the formation of Na0.33 V2 O5 bronze, are observed [19,21]. The shift of the band maxima associated to the V = O mode from 1020 to 1013 cm−1 suggests the presence of V5+ and V4+ species [20], respectively. The effect of the alkali metal addition to V2 O5 on the reduction profiles in TPR-H2 experiments is presented in Fig. 3A. The reduction profiles change depending on the alkali metal added, suggesting in all cases different reduction steps. Thus, the temperature of the maximum hydrogen consumption in VLi-0.04 and VNa-0.04 samples is higher than that observed in pure vanadium oxide. However, the TPR profiles of VK-0.04 and VCs-0.04 samples suggest that the reducibility of V-atoms is higher (lower temp. of maximum H2 consumption) than that of pure vanadium oxide. A similar behaviour was previously reported by Bentrup et al. [20] during the study of alkali-promoted V2 O5 catalysts. Moreover, the starting point of hydrogen-uptake seems to drop with increasing the size of the alkali metal, as reported previously [20]. On the other hand, the H2 -TPR profiles of catalysts with different content of sodium are shown in Fig. 3B. The catalysts with high Na-loadings (>1%) present two reduction peaks whose maxima
30
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
Fig. 1. XRD patterns of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B): (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.04; (h) VNa-0.10; (i) VNa-0.20; (j) VNa-0.30. Symbols: Na0.33 V2 O5 (), V2 O5 (䊉).
hydrogen consumption appears at 630 and 723 ◦ C. However, the catalysts with low Na-loading (0.5%) present different reduction maximum. The hydrogen consumptions for each sample between 550 and 725 ◦ C during the TPR-H2 experiments are summarized in Table 1. It is observed that the amount of H2 consumption is different depending on the alkali-metal doping. It must be specially indicated that hydrogen consumption lower than that of pure vanadium oxide (5.26 mmol H2 g−1 ) were only observed in Na-doped samples
(3.94 mmol H2 g−1 ), which could be due to the stabilization of V4+ species in Na0.33 V2 O5 bronze crystals observed in these catalysts according to XRD results [20]. However, the hydrogen consumption changes with the Na-loading, showing initially an increase with Na-loading, although the samples with high Na-loading show an opposite trend (Table 1). Table 2 shows the binding energy values of the constituent elements and surface composition, obtained by XPS, of the sodiumcontaining catalysts (prepared with Na/V atomic ratios from 0.04 to
Fig. 2. FTIR spectra of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B): (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.04; (h) VNa-0.10; (i) VNa-0.20; (j) VNa-0.30.
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
31
A
B
e i
d
h
c
b
g
a
f
300
400
500
600
700
800
300
Temperatura, °C
400
500
600
700
800
Temperatura, °C
Fig. 3. H2 -TPR patterns of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B): (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.10; (h) VNa-0.20; (i) VNa-0.30. Table 1 Characteristics of alkali-promoted vanadium oxide catalysts. Sample
Alkali
Atomic ratio
TPR resultsa
AM/V
TCM (◦ C)
H2 -uptake (mmol H2 gcat −1 )
Crystalline phase
VLi-0.04 VNa-0.04 VK-0.04 VCs-0.04
Li Na K Cs
0.04 0.04 0.04 0.04
634 664 595 586
5.24 3.94 5.56 6.91
V2 O5 V2 O5 + Na0.33 V2 O5 V2 O5 V2 O5
VNa-0.02 VNa-0.10 VNa-0.20 VNa-0.30
Na Na Na Na
0.02 0.1 0.2 0.3
639 633 635 640
4.59 4.76 4.25 3.40
V2 O5 V2 O5 + Na0.33 V2 O5 V2 O5 + Na0.33 V2 O5 V2 O5 + Na0.33 V2 O5
VAM-0
–
0
619
5.26
V2 O5
a
Temperature of maximum hydrogen consumption (TCM) and H2 -uptake (in mmol H2 g−1 ) achieved during the TPR results.
0.30). A Na/V ratio on the catalyst surface lower than that observed for bulk can be observed, indicating that part of sodium cations is incorporated in the bulk of catalysts. On the other hand, changes in the oxidation state of vanadium on the catalyst surface are also observed in Na-doped samples. In this way, the observed core level V 2p3/2 signal can be decomposed in two contributions at ca. 516.0 and 517.9 eV, which are assigned to V4+ and V5+ [24,25], respectively. The amount of V4+ species increases from ca. 4% to 16% when increasing the sodium content in catalysts (with Na/V atomic ratios in bulk from 0.04 to 0.20). The O 1s core level signals of the Na doped-samples are very similar and all show two contributions with very similar binding
energies and similar percentages (see Table 2). In addition, these values are also similar to those observed for pure V2 O5 , i.e. bands at 530.4 eV (91%), related to metal oxides, and 532.5 eV (9%), hydroxyl groups. 3.2. Catalytic test in the selective oxidation of H2 S to sulphur Fig. 4 shows the variation of the conversion of H2 S with the time on stream (TOS) during the oxidation of H2 S at 200 ◦ C over alkalipromoted V2 O5 catalysts (with an alkali metal/vanadium atomic ratio of 0.04). For comparison, results achieved over pure V2 O5 (sample VAM-0) have been also included. Initial catalyst decay is
Table 2 XPS results of sodium-promoted vanadium oxide catalysts. Sample
Surface composition
Na/V atomic ratio
O/V/Na atomic ratio
Surface
Bulk
Core level binding energy (eV)
VNa-0.04
71.7/28.0/0.3
0.01
0.04
VNa-0.10
72.5/26.9/0.6
0.02
0.10
VNa-0.20
81.8/17.5/0.7
0.04
0.20
VNa-0.30
72.1/25.8/2.1
0.08
0.30
O 1s 530.3 (87%) 532.6 (13%) 530.6 (87%) 532.1 (13%) 530.2 (89%) 532.3 (11%) 530.4 (87%) 532.4 (13%)
Na 2p
V 2p3/2
1071.1
515.7 (4.4%) 517.4 (95.6%) 516.5 (14.0%) 517.8 (86.0%) 515.9 (16.6%) 517.3 (83.4%) 516.3 (16.8%) 517.7 (83.2%)
1071.4 1070.9 1071.6
32
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
Fig. 4. Variation of the H2 S conversion with the time on stream (TOS) at 200 ◦ C obtained over alkali-promoted vanadium oxide catalysts: VLi-0.04 (); VNa-0.04 (); VK-0.04 (); VCs-0.04 (); VAM-0 (). Reaction conditions: 0.1 g of catalyst; total flow of 130 mL min−1 ; H2 S/air/He molar ratio of 1.2/5.0/93.8 (i.e. H2 S/O2 ratio −1 of 1/0.9) and W/F = 31.2 gcat h mol H2 S−1 .
shown during the first minutes of reaction for almost all the catalysts (Fig. 4), except in the case of Na-doped one, for which no significant decay has been observed. On the other hand, the Na-containing catalyst (VNa-0.04 catalyst) presents a catalytic activity higher to those achieved over the rest of alkali metal doped catalysts or pure V2 O5 . Although this has been discussed later, the different incorporation of alkali metal in the holes of the V2 O5 structure could influence the catalytic performance of alkali-metal promoted catalysts. A clear influence of Na-content (from 0.5 to 6 wt% of Na) on the catalytic activity is observed, independently of the reaction temperature (Fig. 5). Thus, the catalytic activity increases when increasing the sodium content, reaching a maximum for the catalysts with a Na/V ratio of 0.1 (sample VNa-0.1). In samples with higher Na-loadings, the catalytic activity decreases when increasing the sodium content. It can be noticed that no important decay was observed in the catalysts with different Na-content (not shown), although small differences depending on the Na-loading were observed. Sulphur and water were the only reaction products in all studied catalysts, although SO2 was also observed as minority (selectivity lower than 2%) only in the first minutes of reaction. Fig. 6 shows the variation of the conversion of H2 S and the selectivity of sulphur with the temperature of the reaction over
Fig. 6. Variation of the H2 S conversion () and selectivity to sulphur (䊉) with the reaction temperature achieved during the partial oxidation of H2 S over VNa-0.1 catalyst. Reaction conditions: 0.1 g of catalyst; total flow of 130 mL min−1 ; H2 S/air/He molar ratio of 1.2/5.0/93.8; time on stream (TOS) of 120 min.
VNa-0.1. The catalytic activity increases and the selectivity to sulphur decreases when increasing the reaction temperature. Thus, no formation of SO2 is observed at temperature below 200 ◦ C, but its formation increases at high reaction temperatures. 3.3. Characterization of catalysts after catalytic reaction It has been reported that in catalysts based on V2 O5 , this phase is selectively transformed to V4 O9 during the catalytic tests for partial oxidation of H2 S [14,15]. For this reason, it is interesting to study the nature of crystalline phases in alkali-promoted V2 O5 based catalysts after the catalytic tests. Fig. 7 shows the XRD pattern of alkali metal-promoted V2 O5 (Fig. 7A) and Na-promoted catalysts (Fig. 7B). Diffraction peaks at 2 = 10.8, 13.8, 21.6, 27.7, 33.9, 35.5, 41.2, 54.2 and 56.4◦ are observed in all the used catalysts, which indicate the presence of V4 O9 [JCPDS: 23-720]. However no characteristics diffraction peaks related to V2 O5 phase are observed. This suggests a complete transformation of V2 O5 to V4 O9 during the catalytic tests. In addition, changes in the relative intensity of (2 0 0) and (2 0 2) diffraction lines corresponding to V4 O9 are also observed, depending on the alkali metal added. Thus, an I200 /I202 ratio of ca. 3 is observed for pure vanadium oxide, while this is ca. 1 for the Na-containing samples. On the other hand, no changes in the characteristic diffraction peaks of Na0.33 V2 O5 phase was observed, which suggest that this crystalline phase is not reduced during the catalytic tests. Fig. 8 shows the IR spectra of used catalysts. For comparison, it has been also included the spectrum of a synthesized V4 O9 , which was previously characterized by other techniques [15]. Used pure vanadium oxide catalyst (VAM-0 sample) shows similar bands than those observed for V4 O9 sample, at 1028, 980, 946, 924, 881, 848, 730, 544, 467 and 413 cm−1 , in agreement with the spectrum of V4 O9 previously reported by other authors [26,27]. In the case of Na-containing catalysts, they also present IR spectra similar to that of V4 O9 . Moreover, the sample with higher Na-content shows a shoulder at 993 cm−1 (Fig. 8, spectrum c), which should be related to the presence of Na-containing bronze as indicated previously in the IR spectra of fresh catalysts (Fig. 2B, spectra i and j). 3.4. General remarks
Fig. 5. Variation of the H2 S conversion with the Na/V ratio of catalysts in Na-doped samples, at 200 ◦ C () or 180 ◦ C (䊉). Reaction conditions: 0.1 g of catalyst; total flow of 130 mL min−1 ; H2 S/air/He molar ratio of 1.2/5.0/93.8; time on stream (TOS) of 120 min.
The characterization results of fresh catalysts suggest some modification of the physico-chemical characteristics depending on the alkali metal added. In this way, the incorporation of K or Cs
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
33
Fig. 7. XRD patterns of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B) after catalytic test: (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.10; (h) VNa-0.20; (i) VNa-0.30. Symbols: Na0.33 V2 O5 (), V4 O9 (♦).
Fig. 8. FTIR spectra of used catalysts: (a) VAM-0; (b) VNa-0.04; (c) VNa-0.20; (d) V4 O9 .
in V2 O5 leads to some changes in the V O V stretching vibration and a V O V rocking vibration, while the crystalline arrangement present in these catalysts remains unchanged. In addition, the reducibility of the main V-species is higher than that observed in pure V2 O5 , since the temperature of maximum hydrogen consumption appears at lower temperature. On the contrary, no changes in the V O V stretching/rocking vibrations of V2 O5 are apparently observed in Li- or Na-doped samples. It is interesting to indicate that the incorporation of Li and Na favours changes in face orientation of V2 O5 crystallites, presenting a ratio between the intensity of (0 0 1) and (1 1 0) diffraction lines, i.e. I001 /I110 , near to 1, different to those observed for V2 O5 and VCs-0.04 samples (I001 /I110 ratios higher than 5). Moreover, formation of Na0.33 V2 O5 is favoured in the Na-doped catalyst. A
higher amount of Na0.33 V2 O5 has been observed as increasing the Na-loading. Therefore, the catalysts containing sodium forms Nacontaining vanadium bronzes, Na0.33 V2 O5 , in which the alkali metal atom occupies interstitial positions in hosts between V O V layers of the vanadium pentoxide [16]. At this point, it should be commented that Martin et al. reported the formation of K- or Cscontaining bronzes, while they do not observed the formation of similar bronzes in Li-containing V2 O5 catalysts [21]. However, they used alkali metal sulphate as precursors, which could modify the incorporation of alkali metal in the V2 O5 structure. In this way, sulphate anions were detected as traces in their catalysts. In an opposite trend to that observed for K- or Cs-doped catalysts, the reducibility of the main V-species in Li- and Na-doped vanadium oxides is lower than that observed in pure V2 O5 , since the temperature of maximum hydrogen consumption appears at higher temperature. It seems in agreement to the formation of bronzes in Li- and Na-doped catalysts, which favours the presence of V4+ species stabilized into the crystalline structure. Maybe, this fact could justify the lower selectivity to SO2 (<2%) observed in the latter catalysts (results not shown) during the initial time of reaction with respect to remaining catalysts (≥2%). However, it must be indicated that differences in the selectivity to SO2 among alkalidoped catalysts are not significant after ca. 1 h of reaction; that means, after the period in which the catalyst is stabilized, with selectivities to sulphur of ca. 100%. On the other hand, the catalytic activity trend for the catalysts with different alkali-metal (prepared with alkali-metal/V atomic ratio of 0.04) has been observed to decrease as follows: Na > Cs ∼ K ∼ Li ∼ V2 O5 . We must indicate that an initial decay of the catalytic activity is observed in almost all the catalysts, although it is stabilized after ca. 1 h. Only in the case of Cs-containing catalyst, is observed an opposite behaviour, with an initial improvement of the catalytic activity in a similar period of time. Therefore, the catalytic behaviour of V2 O5 for the partial oxidation of H2 S to sulphur can be modified by incorporating an alkali metal, although the catalytic activity depends on the type and/or content of the alkali metal.
34
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
Scheme 1. Scheme of the evolution of the crystallographic faces of V2 O5 and V4 O9 .
3.4.1. On the formation and role of V4 O9 The characterization results of used catalysts confirm the partial reduction of vanadium and the consequent formation of V4 O9 during the catalytic test, independently of the alkali metal added. This partial reduction of the catalyst apparently occurs as a consequence of the low re-oxidation rate in the reaction conditions used here (i.e. 200 ◦ C and a H2 S/air ratio of 0.2). In this sense, it has been reported that V4 O9 can be prepared by reduction of V2 O5 using reducing agents such as carbon, sulphur, hydrogen, SO2 or organic compounds at temperatures higher than 300 ◦ C [28–33]. In our case, V4 O9 is selectively obtained during the reaction conditions at low temperature, i.e. 200 ◦ C [15]. Accordingly, this can be an interesting method for preparation this non stoichiometric metal oxides. It has been proposed that V4 O9 could be a slightly deformed superstructure of V2 O5 , since V4 O9 grows topotactically on V2 O5 single crystals [31]. This crystalline phase has been also reported as active and selective in partial oxidation of hydrocarbons [32–36]. It must be taken into account that V4 O9 could be reduced to VO2 when working at higher reaction temperatures [28–31]. In the case of used catalysts, an I200 /I202 ratio of ca. 3 for the diffractions related to V4 O9 was observed in pure vanadium oxide catalyst, while this intensity ratio was approximately 1 for the LiNa- and K-containing catalysts. It should be reminded that a similar trend was observed for the I001 /I110 ratio of the diffractions related to V2 O5 structure in the case of the fresh catalysts. This could suggest a relation between the (0 0 1)/(1 1 0) intensity ratio in pure or alkali-promoted V2 O5 and the (2 0 0)/( 20 2) intensity ratio in V4 O9 crystallites formed after the catalytic tests associated to different crystal face orientations growth as indicated in Scheme 1. It this way, it has been suggested that some catalytic properties in vanadium oxides could show structure sensitivity, as the V2 O5 and its derivatives form anisotropic lattices [1]. However, this is not apparently the case for the partial oxidation of H2 S. In fact, the intensity ratio between the (2 0 0) and (2 0 2) diffraction lines for V4 O9 , i.e. I200 /I202 , changes with the alkali metal added, while only slight changes are observed in their catalytic behaviour. This is the case of K- and Na-doped catalysts (with an alkalimetal/V ratio of 0.04), which show similar DRX patterns of fresh (I(001) /I(110) ratio) and used catalysts (I(200) /I(202) ratio), but they present an opposite trend in catalytic activity. Therefore, it should be concluded that, for these catalysts, this reaction seems not to be structure-sensitive.
3.4.2. On the importance of vanadium bronzes Changes in physico-chemical and catalytic behaviour of these catalysts seem to be related to the ionic radius of the alkali-metal ˚ Na (0.98 A), ˚ K (1.33 A), ˚ added, which increases as follows: Li (0.68 A), ˚ In this way, the behaviour of vanadium oxide based Cs (1.67 A). catalysts with Na+ seems to be different to that observed for catalysts with smaller cations (Li+ ) or bigger cations (K+ or Cs+ ) (Fig. 4). This difference could be explained by considering the formation of vanadium bronzes favoured with the presence of cations with intermediate radius, as in the case of Na-containing catalysts. In fact, the vanadium bronze Na0.33 V2 O5 is observed in the latter catalyst (with alkali metal/vanadium ratio of 0.04), while similar compounds are not observed in K- or Cs-containing samples. The presence of Na0.33 V2 O5 bronze in Na-containing catalysts seems to be an important aspect in partial oxidation of H2 S, since these catalysts presented significantly improved catalytic behaviour for this reaction than the remaining alkali-doped catalysts (Fig. 4). The amount of Na0.33 V2 O5 increases with the Na/V ratio, while the amount of V2 O5 (in fresh catalysts) and V4 O9 (in used catalysts) decreases. According to the catalytic results for the catalysts with different Na-content (Fig. 5), it is clear that the Na-containing vanadium bronze could be proposed as active and selective in the partial H2 S oxidation. The catalytic activity of Nadoped catalysts presents a maximum at a Na/V ratio of 0.10 (near to the theoretical Na/V ratio of 0.165, necessary for achieving pure Na0.33 V2 O5 ), although small differences are observed in all the Na/V ratios studied. According to the characterization and catalytic results, both V4 O9 and Na0.33 V2 O5 can be proposed as active and selective phases in the partial oxidation of H2 S. Note that as in V4 O9 , Na0.33 V2 O5 crystals present V5+ and V4+ sites. In this way, XPS results (Table 2) show that the concentration of V4+ species increases on the surface of Na-promoted fresh catalysts as increasing the Na-content, suggesting that a higher amount of Na0.33 V2 O5 favours a higher formation of V4+ species in the catalyst. It has been proposed that the stabilization of V4+ oxidation state could improve the redox properties of vanadium oxides catalysts [21]. This is in good agreement to the results presented here in which a higher V4+ /V5+ ratio is observed in Na-promoted catalysts and with the fact that V4+ O V5+ pairs as in V4 O9 are active and selective in H2 S partial oxidation reaction to sulphur [14,15]. However, this seems to be more adequately obtained when incorporating Na rather than K or Cs, probably because, in our preparation conditions, the formation of K- or Cs- bronzes is less favoured. On the other hand, no changes in the structure of Na0.33 V2 O5 crystals are observed after the catalytic test which suggests that this is stable under the reaction conditions here used. Thus, it has been proposed that below 200 ◦ C the formation of SO2 is slow compared with the oxidation of H2 S into sulphur, which seems to favour the high selectivity to sulphur in these catalysts [37]. However, above 300 ◦ C two aspects should be considered: (i) cleavage of S S bonds occurs readily and an alternative route wherein SO2 acts as intermediate begins to contribute to the overall reaction rate [37]; (ii) a relatively higher reoxidation of vanadium species, favouring a higher concentration of V5+ species which can favours a higher formation of SO2. However, these aspects could be less favoured on Na-containing vanadium bronzes, since they show a less capacity for reoxidation than V2 O5 .
4. Conclusions In conclusion, this paper shows a study on the influence of the addition of alkali metal to vanadium oxide on both the
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
physico-chemical characteristics and the catalytic properties of modified vanadium oxide catalysts in partial oxidation of H2 S. According to results shown it can be concluded that: - The incorporation of alkali metal to pure V2 O5 , with an alkali metal/vanadium ratio of 0.04, especially the sodium-doped one, favours an increase in the catalytic activity for partial oxidation of H2 S, with high selectivity to sulphur (>98%). - In all cases, V4 O9 is the main crystalline phase observed in catalysts after the catalytic test. Moreover, the incorporation of alkali metal to V2 O5 , especially in the case of Na-doped samples, changes the distribution of crystal faces in both fresh (V2 O5 ) and used (V4 O9 ) catalysts. - The formation of more oxidized compounds (i.e. SO2 ) as minority reaction products seems to be more favoured in the first hour of the catalytic tests, i.e. over oxidized catalysts, but it decays with the time on stream, in parallel with the partial reduction of V2 O5 to V4 O9 . However, when increased the temperature of the reaction (temperature higher than 200 ◦ C) the formation of SO2 is favoured. - In samples with Na/V ratios higher than 0.1 is favoured the formation of Na0.33 V2 O5 bronze, which can be suggested as active and selective in the partial transformation of H2 S into sulphur. - The presence of V5+ O V4+ pairs (as in V4 O9 and Na0.33 V2 O5 ) instead of V5+ O V5+ pairs (as in V2 O5 ) favours a higher selectivity to the partial oxidation product (i.e. sulphur). - One particularity of V4 O9 (and also the Na0.33 V2 O5 bronze) could be its relative stability under the reaction conditions used here, i.e. 200 ◦ C. Acknowledgements Financial support from DGICYT in Spain through Project CTQ2009-14495 and Project MAT2009-10481 and FEDER funds are gratefully acknowledged. MDS thanks a fellowship from the Universidad Politécnica of Valencia. References [1] J. Haber, Catal. Today 142 (2009) 100–113.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15]
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
35
B. Gryzbowska-Swierkosz, Top. Catal. 21 (2002) 35–46. F. Cavani, F. Trifiró, Catal. Today 36 (1997) 431–439. J.M. López Nieto, Top. Catal. 41 (2006) 3–15. J. Haber, M. Witko, R. Tokarz, Appl. Catal. A: Gen. 157 (1997) 3–22. M.Y. Shin, Ch.M. Nam, D.W. Park, J.Sh. Chung, Appl. Catal. A: Gen. 211 (2001) 213–225. K.T. Li, T.Y. Chien, Catal. Lett. 57 (1999) 77–80. K.T. Li, M.Y. Hyang, W.D. Cheng, Ind. Eng. Chem. Res. 35 (1996) 621–626. K.T. Li, Z.H. Chi, Appl. Catal. B: Environ. 31 (2001) 173–182. K.T. Li, Z.H. Chi, Appl. Catal. A: Gen. 206 (2001) 197–203. D.W. Park, B.H. Byung, W.D. Ju, M.I. Kim, K.H. Kim, H.C. Woo, Korean J. Chem. Eng. 22 (2005) 190–195. K.V. Bineesh, D.R. Cho, S.Y. Kim, B.R. Jermy, D.W. Park, Catal. Commun. 9 (2008) 2040–2043. M.I. Kim, W.D. Ju, K.H. Kim, D.W. Park, S.S. Hong, Stud. Surf. Sci. Catal. 159 (2006) 225–228. M.D. Soriano, J. Jiménez-Jiménez, P. Concepción, A. Jiménez-López, E. Rodríguez-Castellón, J.M. López Nieto, Appl. Catal. B: Environ. 92 (2009) 271–279. J.P. Holgado, M.D. Soriano, J. Jiménez-Jiménez, P. Concepción, A. Jiménez-López, A. Caballero, E. Rodríguez-Castellón, J.M. López Nieto, Catal. Today 155 (2010) 296–301. D.V. Fikis, K.W. Heckley, W.J. Murphy, R.A. Ross, Can. J. Chem. 56 (1978) 3078–3083. D.V. Fikis, W.J. Murphy, R.A. Ross, Can. J. Chem. 57 (1979) 2464–2469. S. Takenaka, T. Tanaka, T. Yamakazi, T. Funabiki, S. Yoshida, J. Phys. Chem. B 101 (1997) 9035–9040. T. Ono, Y. Tanaka, T. Takeuchi, K. Yamamoto, J. Mol. Catal. A 159 (2000) 293–300. U. Bentrup, A. Martin, G.U. Wolf, Thermochim. Acta 398 (2003) 131–143. A. Martin, U. Bentrup, G.U. Wolf, Appl. Catal. A: Gen. 227 (2002) 131–142. M. Witko, R. Grybos, R. Tokarz-Sobieraj, Top. Catal. 38 (2006) 105–115. G. Busca, J.C. Lavalley, Spectrochim. Acta A 42 (1986) 443–445. L.E. Briand, O.P. Tkachenko, M. Guraya, X. Gao, I.E. Wachs, W. Grünert, J. Phys. Chem. B 108 (2004) 4823–4830. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Standard Spectra for Identification and Interpretation of XPS Data, Perkin Elmer, Eden Prairie, MN, 1992. R. Nilsson, T. Lindblad, A. Andersson, J. Catal. 148 (1994) 501–513. M. Nohair, D. Aymes, P. Perriat, B. Gillot, Vib. Spectrosc. 9 (1995) 181–190. R.J. Tilley, B.G. Hyde, J. Phys. Chem. Solids 31 (1970) 1613–1619. K.A. Wilhelmi, K. Waltersson, Acta Chem. Scand. 24 (1970) 3409–3411. F. Theobald, J. Bernard, C.R. Acad. Sci. Paris Sr. C 268 (1969) 60–63. G. Grymonprez, L. Fiermans, J. Vennik, Acta Cryst. A33 (1977) 834–837. S. Yamazaki, Ch. Li, K. Ohoyama, M. Nishi, M. Ichihara, H. Ueda, Y. Ueda, J. Solid State Chem. 183 (2010) 1496–1503. R.D. Srivastava, A.B. Stiles, G.A. Jones, J. Catal. 77 (1992) 192–199. J.L. Seoane, P. Boutry, R. Montarnal, J. Catal. 63 (1980) 182–190. A. Andersson, J.O. Bovin, P. Walter, J. Catal. 98 (1986) 204–220. A. Legouri, T. Baird, J.R. Fryer, J. Catal. 140 (1993) 173–183. M. Steijns, F. Derks, A. Verloop, P. Mars, J. Catal. 42 (1976) 87–95.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Alkali-promoted V2 O5 catalysts for the partial oxidation of H2 S to sulphur M.D. Soriano a , J.M. López Nieto a,∗ , F. Ivars a , P. Concepción a , E. Rodríguez-Castellón b a b
Instituto de Tecnología Química, UPV-CSIC, Campus Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain Dept. Química Inorgánica, Facultas de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
a r t i c l e
i n f o
Article history: Received 29 November 2011 Received in revised form 7 February 2012 Accepted 7 February 2012 Available online 19 March 2012 Keywords: Hydrogen sulphide partial oxidation Sulphur Vanadium oxide (V2 O5 V4 O9 ) Alkali metal (lithium sodium potassium caesium)
a b s t r a c t Present paper describes the influence of the incorporation of alkali metal cations (AM = Li, Na, K, Cs; and an AM/V ratio of 0.04) and Na-content (Na/V ratio of 0.02–0.30) in alkali metal promoted V2 O5 catalyst on both the catalyst structure and the catalytic performance in H2 S partial oxidation reactions. The catalytic activity depends on the alkali metal and the amount of alkali metal added, although Nacontaining catalysts seem to be the more active ones. However, selectivity to sulphur higher than 98% is achieved in the main of catalysts when working at reaction temperature lower than 220 ◦ C. According to the characterization results of used catalysts, V4 O9 is selectively formed during the catalytic tests on catalysts presenting V2 O5 crystallites. In catalysts with Na/V ratios higher than 0.04, V4 O9 and Na0.33 V2 O5 are observed, the presence of Na0.33 V2 O5 increasing when increasing the Na/V ratio. Accordingly, V4 O9 and Na0.33 V2 O5 can be proposed as the active and selective crystalline phase in Na-containing catalysts. The role of the presence of V4+− O–V5+ pairs in partial oxidation of H2 S is also discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Vanadium oxides, and their corresponding supported materials, have been extensively studied in the last three decades. The easy conversion among oxides of different stoichiometry and formation of oxygen vacancies enable vanadium oxides to function as interesting catalysts in a number of partial oxidation reactions [1], i.e. selective oxidation of methanol, olefins and alkanes, as well as in the ammoxidation of aromatic hydrocarbons and the selective catalytic reduction of nitric oxide with ammonia [1–5]. More recently, they have been also studied as catalysts in the partial oxidation of H2 S to sulphur [6–15]. It is well known that supported vanadium oxides with V-loading lower than the theoretical monolayer, are active and selective catalysts in partial oxidation of H2 S to sulphur, although they show an important deactivation [6–15]. Recently, it has been suggested that supported vanadium oxide catalysts with high V-loading, and presenting V2 O5 crystallites, could be preferred in the partial oxidation of H2 S at relatively low reaction temperature, since the V2 O5 crystallites are partially reduced forming V4 O9 which seems to be active and selective in this reaction [14]. In fact, results obtained in operando and in situ conditions using Raman and XAS spectroscopies confirmed this behaviour in both unsupported and
supported V2 O5 -containing catalysts [15]. However, the characteristics of V2 O5 crystallites should be tailored in order to improve the catalytic behaviour. In this sense, the catalytic behaviour of supported-vanadium oxide catalysts and the characteristics of surface vanadium species formed (such as, oxidation state, coordination, etc.) change depending on the V-content and the support, although generally only catalysts presenting high V-loading (i.e. presenting V2 O5 crystallites) have catalytic performances similar to those achieved over pure vanadium oxide. On the other hand, doping vanadium oxide catalysts with alkalimetals has been reported that can modify both structural and acid characteristics of catalysts, changing thus their catalytic behaviour [1–5,16–22]. In this paper, a study on the influence of the incorporation of alkali metal cations (Li, Na, K, Cs) on pure V2 O5 catalysts, as well as the influence of Na-content in Na-promoted V2 O5 catalysts, on both the catalyst structure and the catalytic performance in H2 S partial oxidation reactions is shown. The used catalysts will be also studied in order to evaluate changes in the structure and vanadium oxidation state after reaction. 2. Experimental 2.1. Catalyst preparation
∗ Corresponding author. Fax: +34 963877809. E-mail address: [email protected] (J.M. López Nieto). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.02.016
Alkali metal-promoted vanadium oxides were prepared by the impregnation method. The required amount of the alkali nitrate
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
was dissolved in water and then dropwise added to the ammonium metavanadate (NH4 VO3 , Aldrich) and shaking the mixture with a spatula up to obtain an homogeneous paste. The impregnated solid was dried overnight in furnace at 100 ◦ C. Finally, the samples were calcined at 550 ◦ C for 4 h. For comparison, pure vanadium pentoxide was prepared by calcination of ammonium metavanadate at 550 ◦ C for 4 h. The catalysts will be named as VAM-x, where AM is the alkali metal incorporated and x is referred to the alkali-metal/vanadium (AM/V) atomic ratio. Pure V2 O5 will be named as VAM-0. 2.2. Catalytic tests Catalytic tests for the partial oxidation of H2 S to sulphur were carried out in a fixed-bed quartz tubular flow reactor, at atmospheric pressure and in the 180–240 ◦ C temperature range. A catalyst weight of 0.1 g was mixed with silicon carbide as inert to obtain an adequate bed height. A total flow of 130 mL min−1 has been used, with H2 S/Air/He molar ratio of 1.2/5.0/93.8. Analysis of reactants and reaction products was carried out online by gas chromatography using two different chromatographic columns (Molecular Sieve 5 A˚ and Porapak T) [14]. 2.3. Catalyst characterization X-ray diffraction patterns were collected in an Enraf Nonius PSD120 diffractometer with a monochromatic CuK␣1 source operated at 40 keV and 30 mA. Crystalline phases were identified by matching experimental patterns to the JCPDS powder diffraction file. Temperature-programmed reduction (TPR) was carried out in a Micromeritics Autochem 2910 equipped with a TCD detector, using 10% H2 in Ar with a flow rate of 50 mL min−1 . The temperature range explored was from room temperature to 800 ◦ C. The heating rate was 10 ◦ C min−1 for all samples. Details of specific conditions for each sample are given with the TPR profiles. Infrared spectra were recorded at room temperature in the 300–4000 cm−1 region with a Nicolet 205xB spectrophotometer equipped with a data station at a spectral resolution of 1 cm−1 and accumulations of 128 scans. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with non monochromatic Mg K␣ radiation (300 W, 15 kV, 1253.6 eV) for the analysis of core level signals of C 1s, O 1s, V 2p and Na 2p. Spectra were recorded with the constant pass energy values at 29.35 eV, using a 720 m diameter analysis area. The spectrometer energy scale was calibrated by using the Cu 2p3/2 , Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7, 368.3 and 84.0 eV, respectively. During data processing of the XPS spectra, binding energy values were referenced to the C 1s peak (284.8 eV) from the adventitious contamination layer. The PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gauss–Lorentz curves, in order to determine the binding energy of the different element core levels more accurately. The error in BE was estimated to be ca. 0.1 eV. Short acquisition time of 10 min was first used to examine C 1s, V 2p regions in order to avoid, as much as possible, photo-reduction of V5+ species. Satellite subtraction of the O 1s signal was always performed to study the V 2p region. 3. Results and discussion 3.1. Characterization of catalysts Fig. 1 shows the XRD patterns of alkali-metal-promoted (AM/V ratio of 0.04) catalysts (Fig. 1A) and those with different Na-content
29
(Fig. 1B). For comparison, the XRD pattern of pure V2 O5 has been also included (Fig. 1A, pattern e). The peaks at 2 = 15.3, 20.3, 26.2, 31.2, 34.3 and 47.3◦ , in alkali promoted catalysts, can be related to the presence of orthorhombic V2 O5 [JCPDS: 41–1426]. Moreover, the appearance of peaks at 2 = 9.37, 12.16, 26.43, 27.92 and 28.94◦ (in sample VNa-0.04) suggests the presence of Na0.33 V2 O5 [JCPDS: 19-1052], as minority phase. On the other hand, some differences in the intensity of diffractions have been also observed depending on the alkali-metal doping. Thus, a remarkable increase in the intensity of (0 0 1) and (0 0 2) diffraction lines is observed in the Cs-promoted sample with respect to the remaining alkali-doped catalysts, while the intensity of (1 1 0) diffraction lines is considerably higher in Li-, Na- and K-promoted catalysts. From the comparison between the intensity of (0 0 1) and (1 1 0) diffraction lines, I001 /I110 ratios higher than 5 are observed in V2 O5 and VCs-0.04 samples, whereas I001 /I110 higher than 1 (in VLi-0.04 sample) or lower than 1 (in VNa0.04 and VK-0.04) are observed.No apparent change in the I001 /I110 ratio is observed when increasing the Na-loading. However, the formation of Na0.33 V2 O5 [JCPDS: 19-1052], with peaks at 2 = 9.34, 12.16, 18.75, 24.57, 26.40, 27.92, 28.94, 30.60, 32.91, 41.49 and 50.52◦ , is clearly favoured at high sodium-loading. Fig. 2 shows the infrared spectra of alkali metal-promoted catalysts prepared with an alkali-metal/V atomic ratio of 0.04 (Fig. 2A) and Na-promoted catalysts with different Na-contents (Fig. 2B). For comparison, it has been also included the IR spectrum of the pure vanadium oxide based catalyst (VAM-0) (Fig. 2A, spectrum e). Accordingly, V2 O5 shows bands at 1020, 828 cm−1 and 450–650 cm−1 , related to V O stretching vibration, V O V stretching vibration and V O V rocking vibration in V2 O5 , respectively [16–19,23]. A similar spectrum to that of VAM-0 is observed in Li- and Na-promoted catalysts, while a shift of the band at 828 cm−1 to 783–808 cm−1 region is clearly observed for VK-0.04 and VCs-0.04 samples. Moreover, a new band at 965 cm−1 is observed in VCs-0.04 sample, which can be assigned to the formation of vanadates [19]. On the other hand, changes in the intensity of the bands at 450–650 cm−1 , characteristics of V O V asymmetric bending vibrations in V2 O5 structure [17], are also observed depending of the alkali-metal doping. In this way, the intensity of the band at 620 cm−1 decreases in the case of VK-0.04 and VCs-0.04 samples, suggesting small modifications on the structure of pure V2 O5 [21]. The IR spectra of Na-promoted catalysts with different Nacontent also show the characteristics bands of V2 O5 (Fig. 2B). However, new bands appear increasingly as the content of sodium is raised from Na/V atomic ratio of 0.02–0.30. Thus, broad band at 1013 cm−1 (with a shoulder at 1020 cm−1 ), and bands at 994, 963 and 940 cm−1 , which can be assigned to the formation of Na0.33 V2 O5 bronze, are observed [19,21]. The shift of the band maxima associated to the V = O mode from 1020 to 1013 cm−1 suggests the presence of V5+ and V4+ species [20], respectively. The effect of the alkali metal addition to V2 O5 on the reduction profiles in TPR-H2 experiments is presented in Fig. 3A. The reduction profiles change depending on the alkali metal added, suggesting in all cases different reduction steps. Thus, the temperature of the maximum hydrogen consumption in VLi-0.04 and VNa-0.04 samples is higher than that observed in pure vanadium oxide. However, the TPR profiles of VK-0.04 and VCs-0.04 samples suggest that the reducibility of V-atoms is higher (lower temp. of maximum H2 consumption) than that of pure vanadium oxide. A similar behaviour was previously reported by Bentrup et al. [20] during the study of alkali-promoted V2 O5 catalysts. Moreover, the starting point of hydrogen-uptake seems to drop with increasing the size of the alkali metal, as reported previously [20]. On the other hand, the H2 -TPR profiles of catalysts with different content of sodium are shown in Fig. 3B. The catalysts with high Na-loadings (>1%) present two reduction peaks whose maxima
30
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
Fig. 1. XRD patterns of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B): (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.04; (h) VNa-0.10; (i) VNa-0.20; (j) VNa-0.30. Symbols: Na0.33 V2 O5 (), V2 O5 (䊉).
hydrogen consumption appears at 630 and 723 ◦ C. However, the catalysts with low Na-loading (0.5%) present different reduction maximum. The hydrogen consumptions for each sample between 550 and 725 ◦ C during the TPR-H2 experiments are summarized in Table 1. It is observed that the amount of H2 consumption is different depending on the alkali-metal doping. It must be specially indicated that hydrogen consumption lower than that of pure vanadium oxide (5.26 mmol H2 g−1 ) were only observed in Na-doped samples
(3.94 mmol H2 g−1 ), which could be due to the stabilization of V4+ species in Na0.33 V2 O5 bronze crystals observed in these catalysts according to XRD results [20]. However, the hydrogen consumption changes with the Na-loading, showing initially an increase with Na-loading, although the samples with high Na-loading show an opposite trend (Table 1). Table 2 shows the binding energy values of the constituent elements and surface composition, obtained by XPS, of the sodiumcontaining catalysts (prepared with Na/V atomic ratios from 0.04 to
Fig. 2. FTIR spectra of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B): (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.04; (h) VNa-0.10; (i) VNa-0.20; (j) VNa-0.30.
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
31
A
B
e i
d
h
c
b
g
a
f
300
400
500
600
700
800
300
Temperatura, °C
400
500
600
700
800
Temperatura, °C
Fig. 3. H2 -TPR patterns of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B): (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.10; (h) VNa-0.20; (i) VNa-0.30. Table 1 Characteristics of alkali-promoted vanadium oxide catalysts. Sample
Alkali
Atomic ratio
TPR resultsa
AM/V
TCM (◦ C)
H2 -uptake (mmol H2 gcat −1 )
Crystalline phase
VLi-0.04 VNa-0.04 VK-0.04 VCs-0.04
Li Na K Cs
0.04 0.04 0.04 0.04
634 664 595 586
5.24 3.94 5.56 6.91
V2 O5 V2 O5 + Na0.33 V2 O5 V2 O5 V2 O5
VNa-0.02 VNa-0.10 VNa-0.20 VNa-0.30
Na Na Na Na
0.02 0.1 0.2 0.3
639 633 635 640
4.59 4.76 4.25 3.40
V2 O5 V2 O5 + Na0.33 V2 O5 V2 O5 + Na0.33 V2 O5 V2 O5 + Na0.33 V2 O5
VAM-0
–
0
619
5.26
V2 O5
a
Temperature of maximum hydrogen consumption (TCM) and H2 -uptake (in mmol H2 g−1 ) achieved during the TPR results.
0.30). A Na/V ratio on the catalyst surface lower than that observed for bulk can be observed, indicating that part of sodium cations is incorporated in the bulk of catalysts. On the other hand, changes in the oxidation state of vanadium on the catalyst surface are also observed in Na-doped samples. In this way, the observed core level V 2p3/2 signal can be decomposed in two contributions at ca. 516.0 and 517.9 eV, which are assigned to V4+ and V5+ [24,25], respectively. The amount of V4+ species increases from ca. 4% to 16% when increasing the sodium content in catalysts (with Na/V atomic ratios in bulk from 0.04 to 0.20). The O 1s core level signals of the Na doped-samples are very similar and all show two contributions with very similar binding
energies and similar percentages (see Table 2). In addition, these values are also similar to those observed for pure V2 O5 , i.e. bands at 530.4 eV (91%), related to metal oxides, and 532.5 eV (9%), hydroxyl groups. 3.2. Catalytic test in the selective oxidation of H2 S to sulphur Fig. 4 shows the variation of the conversion of H2 S with the time on stream (TOS) during the oxidation of H2 S at 200 ◦ C over alkalipromoted V2 O5 catalysts (with an alkali metal/vanadium atomic ratio of 0.04). For comparison, results achieved over pure V2 O5 (sample VAM-0) have been also included. Initial catalyst decay is
Table 2 XPS results of sodium-promoted vanadium oxide catalysts. Sample
Surface composition
Na/V atomic ratio
O/V/Na atomic ratio
Surface
Bulk
Core level binding energy (eV)
VNa-0.04
71.7/28.0/0.3
0.01
0.04
VNa-0.10
72.5/26.9/0.6
0.02
0.10
VNa-0.20
81.8/17.5/0.7
0.04
0.20
VNa-0.30
72.1/25.8/2.1
0.08
0.30
O 1s 530.3 (87%) 532.6 (13%) 530.6 (87%) 532.1 (13%) 530.2 (89%) 532.3 (11%) 530.4 (87%) 532.4 (13%)
Na 2p
V 2p3/2
1071.1
515.7 (4.4%) 517.4 (95.6%) 516.5 (14.0%) 517.8 (86.0%) 515.9 (16.6%) 517.3 (83.4%) 516.3 (16.8%) 517.7 (83.2%)
1071.4 1070.9 1071.6
32
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
Fig. 4. Variation of the H2 S conversion with the time on stream (TOS) at 200 ◦ C obtained over alkali-promoted vanadium oxide catalysts: VLi-0.04 (); VNa-0.04 (); VK-0.04 (); VCs-0.04 (); VAM-0 (). Reaction conditions: 0.1 g of catalyst; total flow of 130 mL min−1 ; H2 S/air/He molar ratio of 1.2/5.0/93.8 (i.e. H2 S/O2 ratio −1 of 1/0.9) and W/F = 31.2 gcat h mol H2 S−1 .
shown during the first minutes of reaction for almost all the catalysts (Fig. 4), except in the case of Na-doped one, for which no significant decay has been observed. On the other hand, the Na-containing catalyst (VNa-0.04 catalyst) presents a catalytic activity higher to those achieved over the rest of alkali metal doped catalysts or pure V2 O5 . Although this has been discussed later, the different incorporation of alkali metal in the holes of the V2 O5 structure could influence the catalytic performance of alkali-metal promoted catalysts. A clear influence of Na-content (from 0.5 to 6 wt% of Na) on the catalytic activity is observed, independently of the reaction temperature (Fig. 5). Thus, the catalytic activity increases when increasing the sodium content, reaching a maximum for the catalysts with a Na/V ratio of 0.1 (sample VNa-0.1). In samples with higher Na-loadings, the catalytic activity decreases when increasing the sodium content. It can be noticed that no important decay was observed in the catalysts with different Na-content (not shown), although small differences depending on the Na-loading were observed. Sulphur and water were the only reaction products in all studied catalysts, although SO2 was also observed as minority (selectivity lower than 2%) only in the first minutes of reaction. Fig. 6 shows the variation of the conversion of H2 S and the selectivity of sulphur with the temperature of the reaction over
Fig. 6. Variation of the H2 S conversion () and selectivity to sulphur (䊉) with the reaction temperature achieved during the partial oxidation of H2 S over VNa-0.1 catalyst. Reaction conditions: 0.1 g of catalyst; total flow of 130 mL min−1 ; H2 S/air/He molar ratio of 1.2/5.0/93.8; time on stream (TOS) of 120 min.
VNa-0.1. The catalytic activity increases and the selectivity to sulphur decreases when increasing the reaction temperature. Thus, no formation of SO2 is observed at temperature below 200 ◦ C, but its formation increases at high reaction temperatures. 3.3. Characterization of catalysts after catalytic reaction It has been reported that in catalysts based on V2 O5 , this phase is selectively transformed to V4 O9 during the catalytic tests for partial oxidation of H2 S [14,15]. For this reason, it is interesting to study the nature of crystalline phases in alkali-promoted V2 O5 based catalysts after the catalytic tests. Fig. 7 shows the XRD pattern of alkali metal-promoted V2 O5 (Fig. 7A) and Na-promoted catalysts (Fig. 7B). Diffraction peaks at 2 = 10.8, 13.8, 21.6, 27.7, 33.9, 35.5, 41.2, 54.2 and 56.4◦ are observed in all the used catalysts, which indicate the presence of V4 O9 [JCPDS: 23-720]. However no characteristics diffraction peaks related to V2 O5 phase are observed. This suggests a complete transformation of V2 O5 to V4 O9 during the catalytic tests. In addition, changes in the relative intensity of (2 0 0) and (2 0 2) diffraction lines corresponding to V4 O9 are also observed, depending on the alkali metal added. Thus, an I200 /I202 ratio of ca. 3 is observed for pure vanadium oxide, while this is ca. 1 for the Na-containing samples. On the other hand, no changes in the characteristic diffraction peaks of Na0.33 V2 O5 phase was observed, which suggest that this crystalline phase is not reduced during the catalytic tests. Fig. 8 shows the IR spectra of used catalysts. For comparison, it has been also included the spectrum of a synthesized V4 O9 , which was previously characterized by other techniques [15]. Used pure vanadium oxide catalyst (VAM-0 sample) shows similar bands than those observed for V4 O9 sample, at 1028, 980, 946, 924, 881, 848, 730, 544, 467 and 413 cm−1 , in agreement with the spectrum of V4 O9 previously reported by other authors [26,27]. In the case of Na-containing catalysts, they also present IR spectra similar to that of V4 O9 . Moreover, the sample with higher Na-content shows a shoulder at 993 cm−1 (Fig. 8, spectrum c), which should be related to the presence of Na-containing bronze as indicated previously in the IR spectra of fresh catalysts (Fig. 2B, spectra i and j). 3.4. General remarks
Fig. 5. Variation of the H2 S conversion with the Na/V ratio of catalysts in Na-doped samples, at 200 ◦ C () or 180 ◦ C (䊉). Reaction conditions: 0.1 g of catalyst; total flow of 130 mL min−1 ; H2 S/air/He molar ratio of 1.2/5.0/93.8; time on stream (TOS) of 120 min.
The characterization results of fresh catalysts suggest some modification of the physico-chemical characteristics depending on the alkali metal added. In this way, the incorporation of K or Cs
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
33
Fig. 7. XRD patterns of alkali-promoted vanadium oxide with different alkali metal (A) and with different content of sodium (B) after catalytic test: (a) VLi-0.04; (b) VNa-0.04; (c) VK-0.04; (d) VCs-0.04; (e) VAM-0; (f) VNa-0.02; (g) VNa-0.10; (h) VNa-0.20; (i) VNa-0.30. Symbols: Na0.33 V2 O5 (), V4 O9 (♦).
Fig. 8. FTIR spectra of used catalysts: (a) VAM-0; (b) VNa-0.04; (c) VNa-0.20; (d) V4 O9 .
in V2 O5 leads to some changes in the V O V stretching vibration and a V O V rocking vibration, while the crystalline arrangement present in these catalysts remains unchanged. In addition, the reducibility of the main V-species is higher than that observed in pure V2 O5 , since the temperature of maximum hydrogen consumption appears at lower temperature. On the contrary, no changes in the V O V stretching/rocking vibrations of V2 O5 are apparently observed in Li- or Na-doped samples. It is interesting to indicate that the incorporation of Li and Na favours changes in face orientation of V2 O5 crystallites, presenting a ratio between the intensity of (0 0 1) and (1 1 0) diffraction lines, i.e. I001 /I110 , near to 1, different to those observed for V2 O5 and VCs-0.04 samples (I001 /I110 ratios higher than 5). Moreover, formation of Na0.33 V2 O5 is favoured in the Na-doped catalyst. A
higher amount of Na0.33 V2 O5 has been observed as increasing the Na-loading. Therefore, the catalysts containing sodium forms Nacontaining vanadium bronzes, Na0.33 V2 O5 , in which the alkali metal atom occupies interstitial positions in hosts between V O V layers of the vanadium pentoxide [16]. At this point, it should be commented that Martin et al. reported the formation of K- or Cscontaining bronzes, while they do not observed the formation of similar bronzes in Li-containing V2 O5 catalysts [21]. However, they used alkali metal sulphate as precursors, which could modify the incorporation of alkali metal in the V2 O5 structure. In this way, sulphate anions were detected as traces in their catalysts. In an opposite trend to that observed for K- or Cs-doped catalysts, the reducibility of the main V-species in Li- and Na-doped vanadium oxides is lower than that observed in pure V2 O5 , since the temperature of maximum hydrogen consumption appears at higher temperature. It seems in agreement to the formation of bronzes in Li- and Na-doped catalysts, which favours the presence of V4+ species stabilized into the crystalline structure. Maybe, this fact could justify the lower selectivity to SO2 (<2%) observed in the latter catalysts (results not shown) during the initial time of reaction with respect to remaining catalysts (≥2%). However, it must be indicated that differences in the selectivity to SO2 among alkalidoped catalysts are not significant after ca. 1 h of reaction; that means, after the period in which the catalyst is stabilized, with selectivities to sulphur of ca. 100%. On the other hand, the catalytic activity trend for the catalysts with different alkali-metal (prepared with alkali-metal/V atomic ratio of 0.04) has been observed to decrease as follows: Na > Cs ∼ K ∼ Li ∼ V2 O5 . We must indicate that an initial decay of the catalytic activity is observed in almost all the catalysts, although it is stabilized after ca. 1 h. Only in the case of Cs-containing catalyst, is observed an opposite behaviour, with an initial improvement of the catalytic activity in a similar period of time. Therefore, the catalytic behaviour of V2 O5 for the partial oxidation of H2 S to sulphur can be modified by incorporating an alkali metal, although the catalytic activity depends on the type and/or content of the alkali metal.
34
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
Scheme 1. Scheme of the evolution of the crystallographic faces of V2 O5 and V4 O9 .
3.4.1. On the formation and role of V4 O9 The characterization results of used catalysts confirm the partial reduction of vanadium and the consequent formation of V4 O9 during the catalytic test, independently of the alkali metal added. This partial reduction of the catalyst apparently occurs as a consequence of the low re-oxidation rate in the reaction conditions used here (i.e. 200 ◦ C and a H2 S/air ratio of 0.2). In this sense, it has been reported that V4 O9 can be prepared by reduction of V2 O5 using reducing agents such as carbon, sulphur, hydrogen, SO2 or organic compounds at temperatures higher than 300 ◦ C [28–33]. In our case, V4 O9 is selectively obtained during the reaction conditions at low temperature, i.e. 200 ◦ C [15]. Accordingly, this can be an interesting method for preparation this non stoichiometric metal oxides. It has been proposed that V4 O9 could be a slightly deformed superstructure of V2 O5 , since V4 O9 grows topotactically on V2 O5 single crystals [31]. This crystalline phase has been also reported as active and selective in partial oxidation of hydrocarbons [32–36]. It must be taken into account that V4 O9 could be reduced to VO2 when working at higher reaction temperatures [28–31]. In the case of used catalysts, an I200 /I202 ratio of ca. 3 for the diffractions related to V4 O9 was observed in pure vanadium oxide catalyst, while this intensity ratio was approximately 1 for the LiNa- and K-containing catalysts. It should be reminded that a similar trend was observed for the I001 /I110 ratio of the diffractions related to V2 O5 structure in the case of the fresh catalysts. This could suggest a relation between the (0 0 1)/(1 1 0) intensity ratio in pure or alkali-promoted V2 O5 and the (2 0 0)/( 20 2) intensity ratio in V4 O9 crystallites formed after the catalytic tests associated to different crystal face orientations growth as indicated in Scheme 1. It this way, it has been suggested that some catalytic properties in vanadium oxides could show structure sensitivity, as the V2 O5 and its derivatives form anisotropic lattices [1]. However, this is not apparently the case for the partial oxidation of H2 S. In fact, the intensity ratio between the (2 0 0) and (2 0 2) diffraction lines for V4 O9 , i.e. I200 /I202 , changes with the alkali metal added, while only slight changes are observed in their catalytic behaviour. This is the case of K- and Na-doped catalysts (with an alkalimetal/V ratio of 0.04), which show similar DRX patterns of fresh (I(001) /I(110) ratio) and used catalysts (I(200) /I(202) ratio), but they present an opposite trend in catalytic activity. Therefore, it should be concluded that, for these catalysts, this reaction seems not to be structure-sensitive.
3.4.2. On the importance of vanadium bronzes Changes in physico-chemical and catalytic behaviour of these catalysts seem to be related to the ionic radius of the alkali-metal ˚ Na (0.98 A), ˚ K (1.33 A), ˚ added, which increases as follows: Li (0.68 A), ˚ In this way, the behaviour of vanadium oxide based Cs (1.67 A). catalysts with Na+ seems to be different to that observed for catalysts with smaller cations (Li+ ) or bigger cations (K+ or Cs+ ) (Fig. 4). This difference could be explained by considering the formation of vanadium bronzes favoured with the presence of cations with intermediate radius, as in the case of Na-containing catalysts. In fact, the vanadium bronze Na0.33 V2 O5 is observed in the latter catalyst (with alkali metal/vanadium ratio of 0.04), while similar compounds are not observed in K- or Cs-containing samples. The presence of Na0.33 V2 O5 bronze in Na-containing catalysts seems to be an important aspect in partial oxidation of H2 S, since these catalysts presented significantly improved catalytic behaviour for this reaction than the remaining alkali-doped catalysts (Fig. 4). The amount of Na0.33 V2 O5 increases with the Na/V ratio, while the amount of V2 O5 (in fresh catalysts) and V4 O9 (in used catalysts) decreases. According to the catalytic results for the catalysts with different Na-content (Fig. 5), it is clear that the Na-containing vanadium bronze could be proposed as active and selective in the partial H2 S oxidation. The catalytic activity of Nadoped catalysts presents a maximum at a Na/V ratio of 0.10 (near to the theoretical Na/V ratio of 0.165, necessary for achieving pure Na0.33 V2 O5 ), although small differences are observed in all the Na/V ratios studied. According to the characterization and catalytic results, both V4 O9 and Na0.33 V2 O5 can be proposed as active and selective phases in the partial oxidation of H2 S. Note that as in V4 O9 , Na0.33 V2 O5 crystals present V5+ and V4+ sites. In this way, XPS results (Table 2) show that the concentration of V4+ species increases on the surface of Na-promoted fresh catalysts as increasing the Na-content, suggesting that a higher amount of Na0.33 V2 O5 favours a higher formation of V4+ species in the catalyst. It has been proposed that the stabilization of V4+ oxidation state could improve the redox properties of vanadium oxides catalysts [21]. This is in good agreement to the results presented here in which a higher V4+ /V5+ ratio is observed in Na-promoted catalysts and with the fact that V4+ O V5+ pairs as in V4 O9 are active and selective in H2 S partial oxidation reaction to sulphur [14,15]. However, this seems to be more adequately obtained when incorporating Na rather than K or Cs, probably because, in our preparation conditions, the formation of K- or Cs- bronzes is less favoured. On the other hand, no changes in the structure of Na0.33 V2 O5 crystals are observed after the catalytic test which suggests that this is stable under the reaction conditions here used. Thus, it has been proposed that below 200 ◦ C the formation of SO2 is slow compared with the oxidation of H2 S into sulphur, which seems to favour the high selectivity to sulphur in these catalysts [37]. However, above 300 ◦ C two aspects should be considered: (i) cleavage of S S bonds occurs readily and an alternative route wherein SO2 acts as intermediate begins to contribute to the overall reaction rate [37]; (ii) a relatively higher reoxidation of vanadium species, favouring a higher concentration of V5+ species which can favours a higher formation of SO2. However, these aspects could be less favoured on Na-containing vanadium bronzes, since they show a less capacity for reoxidation than V2 O5 .
4. Conclusions In conclusion, this paper shows a study on the influence of the addition of alkali metal to vanadium oxide on both the
M.D. Soriano et al. / Catalysis Today 192 (2012) 28–35
physico-chemical characteristics and the catalytic properties of modified vanadium oxide catalysts in partial oxidation of H2 S. According to results shown it can be concluded that: - The incorporation of alkali metal to pure V2 O5 , with an alkali metal/vanadium ratio of 0.04, especially the sodium-doped one, favours an increase in the catalytic activity for partial oxidation of H2 S, with high selectivity to sulphur (>98%). - In all cases, V4 O9 is the main crystalline phase observed in catalysts after the catalytic test. Moreover, the incorporation of alkali metal to V2 O5 , especially in the case of Na-doped samples, changes the distribution of crystal faces in both fresh (V2 O5 ) and used (V4 O9 ) catalysts. - The formation of more oxidized compounds (i.e. SO2 ) as minority reaction products seems to be more favoured in the first hour of the catalytic tests, i.e. over oxidized catalysts, but it decays with the time on stream, in parallel with the partial reduction of V2 O5 to V4 O9 . However, when increased the temperature of the reaction (temperature higher than 200 ◦ C) the formation of SO2 is favoured. - In samples with Na/V ratios higher than 0.1 is favoured the formation of Na0.33 V2 O5 bronze, which can be suggested as active and selective in the partial transformation of H2 S into sulphur. - The presence of V5+ O V4+ pairs (as in V4 O9 and Na0.33 V2 O5 ) instead of V5+ O V5+ pairs (as in V2 O5 ) favours a higher selectivity to the partial oxidation product (i.e. sulphur). - One particularity of V4 O9 (and also the Na0.33 V2 O5 bronze) could be its relative stability under the reaction conditions used here, i.e. 200 ◦ C. Acknowledgements Financial support from DGICYT in Spain through Project CTQ2009-14495 and Project MAT2009-10481 and FEDER funds are gratefully acknowledged. MDS thanks a fellowship from the Universidad Politécnica of Valencia. References [1] J. Haber, Catal. Today 142 (2009) 100–113.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15]
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
35
B. Gryzbowska-Swierkosz, Top. Catal. 21 (2002) 35–46. F. Cavani, F. Trifiró, Catal. Today 36 (1997) 431–439. J.M. López Nieto, Top. Catal. 41 (2006) 3–15. J. Haber, M. Witko, R. Tokarz, Appl. Catal. A: Gen. 157 (1997) 3–22. M.Y. Shin, Ch.M. Nam, D.W. Park, J.Sh. Chung, Appl. Catal. A: Gen. 211 (2001) 213–225. K.T. Li, T.Y. Chien, Catal. Lett. 57 (1999) 77–80. K.T. Li, M.Y. Hyang, W.D. Cheng, Ind. Eng. Chem. Res. 35 (1996) 621–626. K.T. Li, Z.H. Chi, Appl. Catal. B: Environ. 31 (2001) 173–182. K.T. Li, Z.H. Chi, Appl. Catal. A: Gen. 206 (2001) 197–203. D.W. Park, B.H. Byung, W.D. Ju, M.I. Kim, K.H. Kim, H.C. Woo, Korean J. Chem. Eng. 22 (2005) 190–195. K.V. Bineesh, D.R. Cho, S.Y. Kim, B.R. Jermy, D.W. Park, Catal. Commun. 9 (2008) 2040–2043. M.I. Kim, W.D. Ju, K.H. Kim, D.W. Park, S.S. Hong, Stud. Surf. Sci. Catal. 159 (2006) 225–228. M.D. Soriano, J. Jiménez-Jiménez, P. Concepción, A. Jiménez-López, E. Rodríguez-Castellón, J.M. López Nieto, Appl. Catal. B: Environ. 92 (2009) 271–279. J.P. Holgado, M.D. Soriano, J. Jiménez-Jiménez, P. Concepción, A. Jiménez-López, A. Caballero, E. Rodríguez-Castellón, J.M. López Nieto, Catal. Today 155 (2010) 296–301. D.V. Fikis, K.W. Heckley, W.J. Murphy, R.A. Ross, Can. J. Chem. 56 (1978) 3078–3083. D.V. Fikis, W.J. Murphy, R.A. Ross, Can. J. Chem. 57 (1979) 2464–2469. S. Takenaka, T. Tanaka, T. Yamakazi, T. Funabiki, S. Yoshida, J. Phys. Chem. B 101 (1997) 9035–9040. T. Ono, Y. Tanaka, T. Takeuchi, K. Yamamoto, J. Mol. Catal. A 159 (2000) 293–300. U. Bentrup, A. Martin, G.U. Wolf, Thermochim. Acta 398 (2003) 131–143. A. Martin, U. Bentrup, G.U. Wolf, Appl. Catal. A: Gen. 227 (2002) 131–142. M. Witko, R. Grybos, R. Tokarz-Sobieraj, Top. Catal. 38 (2006) 105–115. G. Busca, J.C. Lavalley, Spectrochim. Acta A 42 (1986) 443–445. L.E. Briand, O.P. Tkachenko, M. Guraya, X. Gao, I.E. Wachs, W. Grünert, J. Phys. Chem. B 108 (2004) 4823–4830. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Standard Spectra for Identification and Interpretation of XPS Data, Perkin Elmer, Eden Prairie, MN, 1992. R. Nilsson, T. Lindblad, A. Andersson, J. Catal. 148 (1994) 501–513. M. Nohair, D. Aymes, P. Perriat, B. Gillot, Vib. Spectrosc. 9 (1995) 181–190. R.J. Tilley, B.G. Hyde, J. Phys. Chem. Solids 31 (1970) 1613–1619. K.A. Wilhelmi, K. Waltersson, Acta Chem. Scand. 24 (1970) 3409–3411. F. Theobald, J. Bernard, C.R. Acad. Sci. Paris Sr. C 268 (1969) 60–63. G. Grymonprez, L. Fiermans, J. Vennik, Acta Cryst. A33 (1977) 834–837. S. Yamazaki, Ch. Li, K. Ohoyama, M. Nishi, M. Ichihara, H. Ueda, Y. Ueda, J. Solid State Chem. 183 (2010) 1496–1503. R.D. Srivastava, A.B. Stiles, G.A. Jones, J. Catal. 77 (1992) 192–199. J.L. Seoane, P. Boutry, R. Montarnal, J. Catal. 63 (1980) 182–190. A. Andersson, J.O. Bovin, P. Walter, J. Catal. 98 (1986) 204–220. A. Legouri, T. Baird, J.R. Fryer, J. Catal. 140 (1993) 173–183. M. Steijns, F. Derks, A. Verloop, P. Mars, J. Catal. 42 (1976) 87–95.