Top Catal (2012) 55:782–791 DOI 10.1007/s11244-012-9867-2
ORIGINAL PAPER
Pd (1 wt%)/LaMn0.4Fe0.6O3 Catalysts Supported Over Silica SBA-15: Effect of Perovskite Loading and Support Morphology on Methane Oxidation Activity and SO2 Tolerance L. F. Liotta • G. Di Carlo • G. Pantaleo • J. C. Hernandez Garrido • A. M. Venezia
Published online: 25 July 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Catalysts of palladium (1 wt%) deposited over silica SBA-15 supported LaMn0.4Fe0.6O3 perovskite (with perovskite loading of 10, 30 and 40 wt%), characterized by several techniques (BET, SAXS, XRD, TPR) are tested in the combustion of methane. Bulk LaMn0.4Fe0.6O3 with the corresponding supported Pd catalyst are also considered for comparison purpose. Dispersing LaMn0.4Fe0.6O3 oxide over silica SBA-15 improves the activity of the supported palladium catalysts to an extent depending on the perovskite loading. After ageing at 600 °C for 14 h, Pd catalysts supported over SBA-15 loaded with 30 and 40 wt% of LaMn0.4Fe0.6O3, deactivate less as compared to Pd over bulk perovskite. Moreover, during catalytic tests carried out in the presence of 10 vol. ppm SO2 these catalysts exhibit better sulphur tolerance and higher regeneration
L. F. Liotta G. Pantaleo A. M. Venezia (&) Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, 90146 Palermo, Italy e-mail:
[email protected] L. F. Liotta e-mail:
[email protected] G. Pantaleo e-mail:
[email protected] G. Di Carlo Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, Via Salaria km 29300, 00015 Monterotondo Stazione, Roma, Italy e-mail:
[email protected] J. C. Hernandez Garrido Departamento de Ciencia de los Materiales e Ingenieria Metalurgica y Quimica Inorganica, Facultad de Ciencias, Universidad de Cadiz, Campus Rio San Pedro, 11510 Puerto Real, Ca´diz, Spain e-mail:
[email protected]
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capability as compared to the Pd/LaMn0.4Fe0.6O3. The superior performance of such catalysts is attributed to the good dispersion of the LaMn0.4Fe0.6O3 over the SBA-15, with consequent increase of the perovskite surface area with respect to bulk perovskite. In addition, the porous structure of the silica contributes to a better stabilization of the active species against sintering and acts as a chemical sink during the catalyst exposure to SO2. Keywords Perovskite Pd catalysts Methane oxidation SBA-15 SO2
1 Introduction The use of natural gas, mainly formed by methane, as an alternative fuel for automotive is nowadays increasing due to the much less CO2 emission associated with it, as compared to gasoline and diesel fuels [1, 2]. However, due to the strong greenhouse effect of this gas, about 20 times higher than CO2, it is necessary to remove completely any un-reacted methane from the exhaust gases of the natural gas fuelled vehicles (NGV). To this aim, suitable catalysts are needed. For practical application these catalysts must be very active with feed streams containing low concentration of methane (less than 1,000 ppm) and high concentration of oxygen, at temperature below 600 °C and at high space-velocities. The most active catalysts used for the total oxidation of methane are based on supported noble metals, particularly Pt and Pd. However, they are sensitive to traces of water and sulphur containing molecules and they undergo particle sintering during long-term runs [3]. These limitations can be overcome using thermally stable supports, able to suppress the metal particle growth [4]. As less costly alternative to the noble metals, lanthanum type perovskites (LaBO3 where B is a transition metal) have been
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also explored in hydrocarbon combustion reactions [5–8]. As an advantage, the catalytic properties of these compounds could be tailored using various combinations of metallic elements and partial replacement of La and B cations in order to improve the strength of the perovskite-oxide structure [9]. The coupling of the intrinsically more active noble metal with a perovskite-oxide, characterised by greater oxygen mobility and by redox properties associated with the transition metal, was found to enhance the catalytic activity of the supported metal [10]. Recently, it has been reported that LaMn0.4Fe0.6O3 perovskite catalysts promoted with 2 and 2.5 wt% of Pd exhibited high activity for methane oxidation giving full conversion at 400 °C [11]. The catalyst was the most active within a series of Pd catalysts supported over AMn1-xFexO3 with A = Ba, La, Pr [12]. Moreover, such catalysts maintained relatively high methane conversion even after SO2 treatment. Furthermore a correlation between the sulphur resistance and the perovskite particle size was found with better performance for the smaller perovskite particles. In spite of this, the low specific surface area of typical perovskites hinders the catalytic efficiency and also limits their use as supports. Depositing the perovskites over high surface area oxides can be a convenient way to enhance the catalytic behaviour [8, 13]. Mesoporous silica, characterised by uniform pore sizes, high surface areas and large pore volumes, have been reported as suitable supports of catalytic active species [14–16]. Recent investigation on the catalytic performance of perovskites supported on mesoporous silica have shown a strong influence of the perovskite loading and the support morphology on the catalytic combustion of methane and VOC [8, 17–22]. On these premises, the present work focuses on the effect of the three above cited catalyst components, namely, the noble metal, the perovskite-oxide and the mesoporous support, on the methane oxidation activity. In particular, mesoporous silica SBA-15 loaded with variable amount of LaMn0.4Fe0.6O3 are used as supports for 1 wt% palladium. The obtained catalysts are analysed by BET, SAXS, XRD, TPR, and tested in the catalytic oxidation of methane in lean conditions. Long run activity tests and consecutive tests in absence and in presence of SO2 are carried out. Selected catalysts were also investigated by XRD and FT-IR techniques after catalytic tests.
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copolymer Pluronic P-123 (BASF) is dissolved in 146.8 g de-ionized water and 4.4 g of HCl (37 %) and stirred over night at 35 °C in a 250 mL one neck flask. To this solution 16 g of TEOS (Aldrich 98 %) is quickly added and stirred for 24 h at 35 °C. The milky suspension is dried at 100 °C for 24 h in closed PP bottle. The solid product is filtered, washed with an HCl/water-mixture and calcined at 550 °C for 5 h in air (heating ramp of 1 °C/min). 2.1.2 Synthesis of Bulk Perovskite A reference bulk sample of LaMn0.4Fe0.6O3 is prepared by the conventional citrate-method [10]. La(NO3)35H2O, Mn(NO3)24H2O and Fe(NO3)39H2O, in appropriate amounts to give the nominal composition LaMn0.4Fe0.6O3, are dissolved in water. A solution of citric acid is then added dropwise to the nitrate solution in order to obtain the citrate complex. The molar ratio of citric acid to total (La ? Mn ? Fe) cations is maintained equal to 1.5. Ammonia (25 % v/v) is added slowly until pH 9. The solution is heated up to 150 °C in order to favour the formation of a viscous gel and it is kept at this temperature overnight. Finally, the product is calcined at 750 °C for 5 h in air. 2.1.3 Supported Perovskites Over Mesoporous Silica Oxides LaMn0.4Fe0.6O3 perovskites are supported over silica SBA-15 by incipient wetness impregnation of the supports with an alcoholic solution containing La–Mn–Fe citrate complex precursors. The solution is prepared dissolving in absolute ethanol an appropriate amount of lanthanum, manganese, iron nitrate and citric acid in order to have a nominal perovskite composition LaMn0.4Fe0.6O3. The molar ratio of citric acid to total (La ? Mn ? Fe) cations is fixed to 1.5. After drying at room temperature, the catalysts are calcined at 750 °C for 5 h in air. The resulting samples are labelled as (LaMnFe)x-SBA15 with x corresponding to 10 and 30 and 40 wt% amount of LaMn0.4Fe0.6O3. 2.1.4 Synthesis of Pd-Based Catalysts
2.1 Catalysts Preparation
Pd catalysts are prepared by wet impregnation of the supports with an aqueous solution of Pd(NH3)4(NO3)2 in the appropriate amount to yield a final Pd loading of 1 wt%. The palladium precursor is added to the support under stirring and kept at 60 °C for 1 h under reflux condition. The samples are dried at 120 °C overnight and calcined at 400 °C for 4 h in air.
2.1.1 Synthesis of SBA-15 Mesoporous Silica
2.2 Characterization
Pure siliceous SBA-15 is synthesized following the procedure reported by Choi et al. [23]. Accordingly, 8.1 g of the triblock
The effective chemical composition of the catalysts is determined by inductively coupled plasma optical emission
2 Experimental
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spectroscopy (ICP-OES) with Activa (Horiba Jobin–Yvon) instrument, after dissolving the powders in aqua regia and HF. The specific surface areas are determined by the BET equation from nitrogen adsorption isotherms at -196 °C using Sorptomatic 1900 (Carlo Erba) instrument. The mean pore size diameters are calculated by BJH method applied to the desorption curve [24]. Small-angle X-ray scattering (SAXS) experiments are performed on a Bruker Nanostar SAXS System equipped ˚ ). with 2D detector using Cu Ka radiation (k = 1.5418 A The SAXS measurements are collected in the range 0.2–4.7° 2h using a 0.02° step size and a counting time of 1.3 s per step. Powder X-ray diffraction patterns are recorded at room temperature using a Bruker D5000 diffractometer with a copper target tube and a diffracted beam graphite mono˚ ). chromator selective for Cu Ka radiation (k = 1.5418 A The assignment of the various crystalline phases is based on the JPDS powder diffraction file cards [25]. Mean crystallite sizes are calculated from the line broadening of the most intense reflection using the Scherrer equation [26]. Scanning Transmission Electron Microscopy (STEM) images are acquired in a JEOL2010F electron microscope operating at 200 kV using a JEOL high angle annular dark field detector (HAADF). X-ray energy dispersive spectroscopy (XEDS) analyses are recorded using an X-max silicon drift detector and an electron probe with a 0.5 nm diameter. For the electron microscopy studies, samples are deposited onto carbon copper grids. Temperature programmed reduction (TPR) experiments are carried out with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD). The gas mixture with composition 5 % H2 in Ar (50 mL/min) is used to reduce the samples (100 mg), heating from room temperature to 1,000 °C at the rate of 10 °C/min. Before starting the TPR analyses, the catalysts are pretreated at 350 °C with a flowing gas mixture of 5 % O2 in He (50 mL/min) in order to purge the surface, then cooling down under Ar . FT-IR spectra of the samples are recorded in absorbance by means of an FT-IR 5300 Jasco spectrophotometer using the KBr wafer technique. The resolution is 4 cm-1. 2.3 Catalytic Tests The catalytic activity in methane combustion is tested in a tubular U shaped reactor with an inner diameter of 12 mm, electrically heated in a furnace. The catalyst powder (sieved fraction between 180 and 250 l) is diluted 1:2 with inert SiC, in order to avoid thermal gradients. Prior to the catalytic testing, the samples are treated ‘‘in situ’’ under flowing O2 (5 vol. % in He, 50 mL/min) at 350 °C for 1/2 h and in He during cooling at 200 °C. The standard
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reagent gas mixture consisting of 0.3 vol. % of CH4 ? 2.4 vol. % O2 in He is led over the catalyst (50 mg) at a flow rate of 50 mL/min (STP), equivalent to a weight hourly space velocity (WHSV) of 60,000 mL/g/h. Activities are measured by increasing the temperature from 200 to 600 °C (by steps of 50 °C, hold time 45 min). Longterm activity tests are carried out flowing over the catalysts the reactant mixture at 600 °C for 14 h. The inlet and outlet gas compositions are analysed by on line mass quadrupole (ThermostarTM, Balzers), following the evolution of the species, CH4, CO, CO2, H2, H2O, O2. Moreover, the concentrations of CO, CO2 and CH4 species are checked by IR analysers (ABB Uras 14), calibrated in the range 0–3,000 ppm for CO, 0–10,000 ppm for CO2 and 0–30,000 ppm for CH4. The effect of the SO2 poisoning is investigated by adding to the reactants 10 vol. ppm of SO2. In order to mimic severe sulphur poisoning, the catalysts are also treated overnight at 350 °C with a flowing gas mixture of 10 vol.ppm of SO2 in He. Between consecutive runs the samples are cooled down in He atmosphere. The reaction products of methane oxidation are exclusively CO2 and H2O. Carbon balance is close to ±5 % in all the catalytic tests.
3 Results and Discussion The N2 adsorption/desorption isotherms of the pure SBA15 and the corresponding supported perovskites are shown in Fig. 1. The isotherms are of type IV, with hysteresis loops characteristic of mesoporous compounds. According to the IUPAC classification, the hysteresis loop of these materials are classified as H1. As observed in the figure, deposition of the perovskite produces a decrease and a shift to low pressure of the hysteresis loop. In Table 1 the textural properties obtained from the N2 physisorption measurements are listed along with the perovskite crystallite sizes obtained from XRD measurements. In accord with the literature [18, 23], the SBA-15 is characterized by high surface area and mean pore diameter of 7.5 nm. From the isotherms and the resulting data in Table 1 it is clear that the deposition of LaMn0.4Fe0.6O3 perovskite causes a progressive decrease of the surface area and pore volume and a reduction in the pore diameter of the SBA-15 host material. The surface area of the bulk perovskite is characteristic of this type of oxides when prepared by the citrate-method, affording larger area as compared to other perovskite preparation method [6, 8]. The SAXS patterns of SBA-15 and corresponding LaMn0.4Fe0.6O3 supported perovskites are displayed in Fig. 2. Three well-resolved peaks at 2h = 0.98, 1.7, and 1.9° corresponding to the (100), (110), and (200) diffraction peaks, characteristics of SBA-15 hexagonal ordered
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Fig. 1 Nitrogen adsorption/desorption isotherms of SBA-15 and supported LaMn0.4Fe0.6O3 perovskites
Fig. 2 Small angle X-ray diffraction patterns of SBA-15 and supported LaMn0.4Fe0.6O3 perovskites
Table 1 Textural and structural properties of the different supports Sample
BETa (m2/g)
dbp (nm)
Vcp (cm3/g)
SBA-15
591
7.5
0.77
(LaMnFe)10-SBA-15
364
7.2
0.82
n.d.
(LaMnFe)30-SBA-15
244
6.6
0.36
n.d.
(LaMnFe)40-SBA-15
167
5.7
0.25
25
21
3.0
0.04
55
LaMn0.4Fe0.6O3
ddperovskite (nm)
n.d. not detected a
The BET values were calculated using the range 0.05–0.2 p/p0
b
The mean pore diameter was determined using the BJH model
c
The pore volume was determined considering the range p/p0 until to 0.98
d
Perovskite particle size was determined by XRD measurements
structure, are visible in the pattern of the pure silica oxide. Deposition of LaMn0.4Fe0.6O3 (10 wt%) produces a shift of the peaks towards higher angles, together with a decrease of the peak intensities. These changes agree with the presence of perovskite inside the support pores with consequent decrease of the pore size, in accord with the data in Table 1. The peaks almost disappear in the 40 wt% perovskite loaded sample suggesting, for this particular case, deterioration of the ordered mesoporous structure. XRD patterns of Pd/LaMn0.4Fe0.6O3 and Pd/supported perovskites along with the ICSD references, PdO (no. 29281) and LaFeO3 cubic perovskite (29118) are shown in Fig. 3. The presence of a single phase with cubic perovskite structure is detected for Pd/LaMn0.4Fe0.6O3. By applying the Scherrer equation to the main reflection peaks, an average crystallite size of 55 nm is estimated. The XRD patterns of the SBA-15 samples with 10 and 30 wt% loading of LaMn0.4Fe0.6O3, do not contain perovskite phase related peaks. Only for the higher
Fig. 3 XRD patterns of Pd/supported perovskite catalysts: a Pd/ (LaMnFe)10-SBA-15, b Pd/(LaMnFe)30-SBA-15, c Pd/(LaMnFe)40SBA-15, d PdO ICSD reference (no. 29281), e Pd/LaMn0.4Fe0.6O3, f LaFeO3 perovskite ICSD reference (no. 29118)
40 wt% loading, peaks of the perovskite phase are observed with an average oxide crystallite size of *25 nm. A broad peak attributed to highly dispersed PdO is visible at 2h *34° only in the XRD patterns of the amorphous supported perovskites. The absence of such peak in the patterns of Pd/ LaMn0.4Fe0.6O3 and Pd/(LaMnFe)40-SBA-15 may be due to particle sizes below the detection limit of the diffraction technique (\3 nm). STEM images and XEDS analyses of selected zones of Pd/LaMn0.4Fe0.6O3 and Pd/(LaMnFe)30-SBA-15 catalysts are given in Figs. 4 and 5 respectively. For the former sample, perovskite crystallites of size ranging between 20 and 100 nm are observed. The mean perovskite particle
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Fig. 4 Representative STEM images and XEDS analysis of a selected zone of Pd/ LaMn0.4Fe0.6O3 catalyst
size, determined by STEM, is 63 nm in fair agreement with the XRD derived diameter. Due to the low contrast with the perovskite support, Pd particles are hardly detectable. For Pd/(LaMnFe)30-SBA-15 sample, the presence of ordered channels typical of the SBA-15 is clearly visible in Fig. 5a, confirming that perovskite deposition onto the support does not destroy the porous structure of SBA-15. The magnification of Fig. 5a reveals aggregate of the perovskite composition hosted inside the mesopores. The size of these aggregates (*8 nm) is roughly similar to the inner diameter of the mesopores (7.5 nm). In accord with the lack of XRD perovskite crystalline peaks the objects observed in the STEM images are likely aggregates of amorphous particles. The chemical composition in terms of Pd, La, Mn, Fe, derived by XEDS analyses, is in good agreement with the chemical values derived by ICP-OES analysis of the samples. The reduction properties of the supported Pd catalysts are investigated by TPR. In Fig. 6 the H2-TPR profiles of Pd/LaMn0.4Fe0.6O3 and Pd/(LaMnFe)30-SBA-15 and corresponding perovskite supports are shown. The TPR profile of the bulk LaMn0.4Fe0.6O3 is characterized by a first broad peak, centered at 500 °C, followed by a second one, with an onset at temperature higher than 700 °C. According to the literature [27, 28], the peak at 500 °C is assigned to the reduction of Mn4? to Mn3?. At temperature higher than 700 °C reduction of Mn3? to Mn2? along with reduction of the iron species, Fe3? and Fe2?, to Fe0 occurs [28]. Supporting the perovskite on the SBA-15 produces some changes in the TPR pattern of the supported oxide. Indeed, as indicated by the profile of the sample (LaMnFe)30-SBA15, a first reduction peak appears at 320 °C with a shoulder at 255 °C, the peak at 500 °C is still present, while the
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high-temperature peak is centred at *830 °C. The increased reducibility of the supported perovskite is attributed to the lower crystallinity of the sample as compared to the bulk oxide. Moreover, the deposition of palladium enhances the reducibility of the perovskite. Indeed, in the supported palladium catalysts, the peak in the range of 300–500 °C shifts towards lower temperature, at around 150–170 °C [27]. Such peak due to the reduction of Mn4? to Mn3? catalyzed by metallic palladium, contains also a contribution from the reduction of palladium oxide [10]. On both supports the high-temperature region of the TPR pattern is essentially unchanged. The methane conversion curves of SBA-15 supported Pd/LaMn0.4Fe0.6O3 catalysts and of the reference Pd/bulk perovskite are shown in Fig. 7a. According to the curves, depositing the perovskite over mesoporous SBA-15 enhances the palladium methane conversion as compared to the palladium over the bulk perovskite, with a shift of the conversion curve to lower temperature. The best catalyst is the Pd/(LaMnFe)30-SBA-15. The activity in terms of methane conversion decreases in the following order: Pd/(LaMnFe)30-SBA-15 slightly [ Pd/(LaMnFe)40-SBA15 [ Pd/(LaMnFe)10-SBA-15 [ Pd/LaMn0.4Fe0.6O3. It should be remarked that the Pd catalysts over the silica supported perovskites achieve full and stable methane conversion in the investigated range of temperature. On the contrary, Pd over the unsupported perovskite gives a maximum of 95 % methane conversion at 550 °C then decreasing to 90 % at 600 °C. In order to study the stability of Pd/LaMn0.4Fe0.6O3 and Pd over the SBA-15 supported perovskites, selected catalysts, after the first catalytic run, are aged under the reaction mixture at 600 °C for 14 h. The corresponding
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Fig. 5 a, b Representative STEM images and XEDS analysis of selected zones of Pd/ (LaMnFe)30-SBA-15 catalyst
conversions as a function of time are given in Fig. 7b. Overall quite stable conversions are observed for all the analyzed samples. Conversions close to 99–97 % are registered for Pd supported over SBA-15 loaded with 30 and 40 wt% of perovskite. A slight deactivation occurs for Pd/ LaMn0.4Fe0.6O3. The samples, after being cooled down, undergo a second run for which the corresponding conversion curves are given in Fig. 7c. Pd/(LaMnFe)30SBA-15 during the second cycle is more active than Pd/(LaMnFe)40-SBA-15 in turn performing better than Pd/bulk perovskite. For an easier comparison, the temperatures of 50 % methane conversion (T50) during the first and second run are summarized in Table 2 where the calculated differences between the T50 are also given.
Pd/(LaMnFe)30-SBA-15 exhibits the minimum deactivation with DT50 = 40 °C in comparison with Pd/LaMn0.4Fe0.6O3 with DT50 = 79 °C. XRD patterns of the SBA-15 supported and unsupported Pd catalysts after the catalytic sequence are shown in Fig. 8. Contrary to the Pd/(LaMnFe)30-SBA-15 sample for which no perovskite peaks are detected, sintering of perovskite phase occurs in Pd/LaMn0.4Fe0.6O3 with the XRD derived mean crystallite size of 71 nm. With respect to PdO phase, a hardly discernible narrow peak of low intensity is present in the patterns of Pd/LaMn0.4Fe0.6O3, whereas a very broad feature due to highly dispersed PdO species is observed in the XRD pattern of the aged Pd/ (LaMnFe)30-SBA-15. According to these results it can be
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Fig. 6 H2-TPR profiles of Pd supported perovskites. For comparison, the TPR curves of the corresponding Pd-free LaMn0.4Fe0.6O3 perovskites are shown
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stated that the morphology of the SBA-15 with its large channels allows the confinement of the perovskite aggregates inside the pores, avoiding particle growth during the reaction cycles. In addition PdO species interacting with the perovskite are also stabilized against particle sintering. In order to study the catalyst tolerance against sulfur poisoning, methane oxidation is carried out while cofeeding SO2 (10 ppm). Subsequently, catalytic tests with SO2-free reactants and also after SO2 overnight treatment are consecutively performed in order to investigate the catalyst regeneration capability. In Table 3 the T50 values of the different catalytic runs are reported. In Figs. 9, 10, and 11 the catalytic behaviors of Pd/(LaMnFe)30-SBA-15, Pd/(LaMnFe)40-SBA-15 and Pd/LaMn0.4Fe0.6O3 during consecutive runs are reported. By comparing the first run without and with SO2 (see first columns in Tables 2, 3; Figs. 7a, 9, 10, and 11), in the presence of the sulfur compound, whereas Pd catalysts over the SBA-15 loaded perovskites deactivate just partially, Pd/LaMn0.3Fe0.6O3
Fig. 7 a Methane conversion versus temperature over supported Pd catalysts. b Long run activity test at 600 °C for 14 h over supported Pd catalysts. c Methane conversion versus temperature over supported Pd samples aged during long run at 600 °C
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Table 2 Temperature of 50 % of methane conversion (T50) registered during two consecutive catalytic runs II cycle (°C)
DT (II–I) cycle (°C)
Sample
I cycle (°C)
Pd/(LaMnFe)10-SBA-15
358
–
–
Pd/(LaMnFe)30-SBA-15
327
367
40
Pd/(LaMnFe)40-SBA-15 Pd/LaMn0.4Fe0.6O3
334 373
381 452
47 79
Fig. 9 Methane conversion versus temperature over Pd/(LaMnFe)30-SBA-15 for different runs in different conditions
Fig. 8 XRD patterns of selected catalysts after ageing at 600 °C for 14 h under methane oxidation mixture
undergoes a strong deactivation with a drastic drop of the activity at 600 °C. In the consecutive second SO2-free run, Pd/(LaMnFe)30-SBA-15 maintains its activity at lowtemperature and partially regenerates above 400 °C, whereas Pd/(LaMnFe)40-SBA-15 and Pd/LaMn0.4Fe0.6O3 become even less active as compared to the previous run with SO2. To mimic severe poisoning an isothermal treatment is carried out at 350 °C by flowing 10 ppm of SO2/He over the samples. Thereafter three catalytic tests labeled as runs III, IV and V are performed. Both palladium catalysts supported over perovskite loaded SBA-15 are strongly poisoned by SO2 after overnight treatment. However, at variance with the Pd/LaMn0.3Fe0.6O3 sample, showing permanent deactivation, Pd/(LaMnFe)40-SBA-15 and to a
Table 3 Temperature of 50 % of methane conversion (T50) registered during consecutive catalytic runs with and without SO2
large extent the Pd/(LaMnFe)30-SBA-15 recover their activity during the subsequent IV and V runs. The deactivation of the catalysts in the presence of SO2 is attributable to the adsorption of the sulfur compound over the active perovskite and palladium oxide [11, 29–31]. However the presence of the mesoporous silica affects the extent of the chemisorption. In analogy with what observed in a previous study on the effect of SO2 over high surface area SiO2 supported palladium, due to the particular morphology, the SBA-15 would act as a sponge for the SO2 molecules limiting the poisoning of the active palladium and perovskite species [32]. The SO2, weakly adsorbed in the silica support, would easily be removed in the subsequent SO2-free run [32, 33]. In the absence of mesoporous silica, during the run with SO2, the Pd/ LaMn0.4Fe0.6O3 is deactivated mainly by the chemisorption of the sulfur molecule over the perovskite and then, above 500 °C, as shown in Fig. 11, by the chemisorptions over the palladium. The subsequent SO2-free run would still deactivate the catalyst due to the continuous spill-over of the SO2 from the perovskite to the palladium. Moreover under the severe poisoning treatment, the complete saturation of the active sites with SO2 and the sintering of the perovskite phase produce the irreversible loss of activity. An intermediate situation is occurring for perovskite loading of 40 wt%. In this case as observed in Fig. 10 and summarized in Table 3, the beneficial effect of the
Catalyst
T50 (°C) I cycle with 10 ppm SO2 (°C)
T50 (°C) II cycle (°C)
T50 (°C) III cycle after pretr. with SO2 (°C)
T50 (°C) IV cycle (°C)
T50 (°C) V cycle (°C)
Pd/LaMn0.4Fe0.6O3
458
[600
[600
[600
[600
Pd/(LaMnFe)30-SBA-15
384
381
519
420
390
Pd/(LaMnFe)40-SBA-15
378
492
552
503
491
123
790
Fig. 10 Methane conversion versus temperature over Pd 1 %/ (LaMnFe)40-SBA-15 for different runs in different conditions
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Fig. 12 FTIR spectra of Pd-based catalysts after five catalytic cycles with/without SO2
tolerance to sulfur poisoning and easier regeneration is then due to the effect of the non sulfating support like SiO2 and also to the confinement of the active sites formed by palladium and perovskite oxides within the porous structure of the support. Such confinement has a twofold effect, the protection of the active species from SO2 poisoning and the increased resistance towards particle sintering during reaction.
4 Conclusions
Fig. 11 Methane conversion versus temperature over Pd/LaMn0.4Fe0.6O3 for different runs in different conditions
mesoporous support is partially lost. Even if the SO2 scavenger activity of the SBA-15 is still operative, the perovskite, present as larger crystallites outside the porous structure, would easily chemisorb the SO2 molecules, therefore producing the detrimental spill-over effect of a sulfating support [32]. Moreover, the easier sintering of the active species inhibits the complete recovery of the activity after subsequent SO2-free runs. The sulfation of the perovskite is confirmed by the FTIR spectra of Pd/LaMn0.4Fe0.6O3 and Pd/(LaMnFe)30SBA-15, obtained after the consecutive five runs, shown in Fig. 12. Differently from the flat spectrum of the SBA-15 supported sample, the spectrum of Pd/LaMn0.4Fe0.6O3 contains broad bands in the 1,300–900 cm-1 range attributable to sulfate species [34–36]. In accord with the presented results, the improved catalytic activity of Pd/(LaMnFe)30-SBA-15 with its superior
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Depositing LaMn0.4Fe0.6O3 perovskite over the high surface area silica SBA-15 improves the methane oxidation activity of the supported Pd catalysts, as compared to Pd/ LaMn0.4Fe0.6O3, with an optimum of performance in correspondence of 30 wt% loading. This catalyst exhibits also a superior tolerance to SO2 poisoning. On the bases of the structural and spectroscopic characterization the improved catalytic performance is attributed to the higher dispersion of the active sites and to their confinement into the support porous channels limiting their particle growth during catalytic reactions. The sulphur tolerance is attributed to the SO2 scavenger ability of the mesoporous support. For perovskite loading above 30 wt%, the beneficial effect of the high surface area support decreases, due to the formation of large LaMn0.4Fe0.6O3 crystallites outside the SBA-15 pores and the consequent sintering during reaction. Acknowledgments Support by the European Community, COST Action CM0903 and by ‘‘PROGETTO EFOR—ENERGIA DA FONTI RINNOVABILI (2011–2013)’’ is acknowledged. Authors are sincerely grateful to Prof. A. Corma of Instituto de Tecnologia Quimica, UPV-CSIC (Valencia) for fruitful and stimulating discussions.
Top Catal (2012) 55:782–791 Dr. Francesco Giordano (ISMN-CNR, Palermo) is gratefully acknowledged for XRD measurements.
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