Methane is an economical and clean alternative to fuels; it is used to produce energy in gas turbine combustors and as a new energy for vehicles  . The advantage of catalytic combustion of methane is that operates at much lower temperatures than flame combustion, which greatly reduces the noxious emissions of nitrogen oxides, NOx and unburned hydrocarbons  . Palladium oxide is considered to be an effective catalyst in methane oxidation     . Unfortunately, the deactivation of the catalyst due to the sintering of palladium during heat treatments is a serious problem for this kind of catalysts  . Therefore, it is necessary to modify the Pd-base catalysts to improve their thermal stability. Among the parameter which may affect the thermal stability of the catalyst are the support and the additives  . The ceria-zirconia-supported palladium catalysts showed high activity   due to CeO2-ZrO2 redox properties and oxygen storage capacity (OSC) which is better than pure CeO2     . The addition of zirconium into the cubic CeO2 framework is effective to prevent cerium sintering, to improve the oxygen mobility in the lattice and to increase thermal stability  . Furthermore, the use of SiO2 as a support could promote OSC because of its relatively higher dispersive properties due to its relatively high surface area, and because its relatively inert character compared to other supports  , which prevent CeO2 depletion through, e.g., reaction with the support giving compounds such as CeAlO3 formation causing the OSC decrease when Al2O3 is used  . However, studies on aerogel SiO2 as a support for CexZr1−xO2 oxide have been scarcely reported, especially when concerning the effect of CexZr1−xO2 loading on the Pd/(CexZr1−xO2)SiO2 catalyst for methane combustion. For these reasons, in the research presented herein, the sol-gel process was used to prepare a SiO2 aerogel known to have a relatively high specific surface area, which enables relatively better dispersion of cerium and zirconium oxides and lower thickness of the pore walls, which increase oxygen mobility  . However appropriate amounts of Ce and Zr are needed to make the modification efficient  . The impregnation method was then used to add palladium and to prepare the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts. The effect of Ce and Zr loadings on the texture, the structure and the catalytic activity in methane combustion was then studied.
Cerium nitrate Ce(NO3)3・6H2O (Sigma-Aldrich, 99.99%), zirconium(IV) oxynitrate hydrate ZrO(NO3)3・xH2O (Sigma-Aldrich, 99.99%), ethanol C2H5OH (Sigma-Aldrich, ≥99.8%), tetraethyl orthosilicate Si(OC2H5)4 (TEOS) (ACROS, 98%), acetic acid CH3COOH (Sigma-Aldrich, ≥99.7%) and palladium acetate (Pd(OAc)2) (Fluka, 35.5% Pd) are used as chemicals in this work.
2.2. Support Preparation
The (Ce0.33Zr0.66O2)SiO2 support was prepared by sol-gel method with variable Ce and Zr loadings (Zr % = 0, 2, 4 and 8 wt.%) and a fixed molar ratio Zr/Ce = 2. Ce(NO3)3・6H2O, ZrO(NO3)3・xH2O, CH3COOH, Si(OC2H5)4(TEOS), ethanol and deionized water were mixed at 40˚C. The molar ratios of H2O/TEOS = 15 and CH3COOH/TEOS = 1. The obtained sol was maintained under constant stirring until a spongy and transparent gel was formed. The solvent was then removed by evaporation under supercritical conditions of ethanol (Tc = 240.9˚C, Pc = 6.14 MPa). Finally, the obtained solid was calcined at 550˚C for 4 h under oxygen flow (30 mL/min).
2.3. Catalyst Preparation
The Pd/(Ce0.33Zr0.66O2)SiO2 catalysts (CeZry, where y is the Zr loading) were prepared by the impregnation method. The appropriate amounts of palladium acetate and (Ce0.33Zr0.66O2)SiO2 solid were ground in an agate mortar for 10 min. The chosen loading of Pd was 0.5 wt.%. Then, acetone was added (1 mL/g) to obtain a paste which was dried at 60˚C and calcined at 550˚C for 2 h under oxygen flow (30 mL/min).
2.4. Catalyst Characterization
The BET specific surface area and the average pore diameter were determined from N2 adsorption-desorption measurements using an automatic Micrometrics ASAP 2020 device (error percentage: 5%). Hydrogen chemisorption measurements were performed at 100˚C in a Micromeritics ASAP 2020C equipment after an in-situ reduction treatment under hydrogen at 300˚C for 2 h. Temperature programmed reduction (TPR) was performed with H2 using a quartz U-tube reactor, coupled to a thermal conductivity detector (TCD). The catalyst (0.05 g) was dried at 250˚C during 0.5 h under argon flow (AGA, 99.99%) and reduced with 10 v/v % H2/Ar flow (30 ml/min) from 25˚C to 400˚C (10˚C/min). TEM studies were performed on a TECNAI G2 instrument operating at 200 kV (error percentage: 2%). The XPS analyses were conducted on calcined samples using a Kratos Analytical AXIS UltraDLD spectrometer (the error percentage was below 1%). The catalytic activity for methane combustion was determined over the calcined sample (0.1 g) in a dynamic micro-reactor.
A flow comprising 1 vol.% methane, 4 vol.% oxygen and balanced with helium was mixed and regulated at a total flow of 100 mL/min. The reactor effluent was then analyzed by a thermal conductivity detector at different reaction temperatures.
The methane conversion and the turnover frequencies (TOF) were calculated by the following equations:
and are respectively, the partial pressures of methane and carbon dioxide.
A: catalyst activity (%), MPd: atomic mass of palladium (106.42 g/mol), l: Pd loading (wt%), DT: total gas flow (L/h), Tr: room temperature, m: catalyst weight (g) and D the dispersion (%).
3. Results and Discussion
3.1. Nitrogen Physisorption
The N2-physisorption results of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts summarized in Table 1 show that the textural properties of the solids were significantly modified when the Ce and Zr loadings were increased. Indeed, the pore volume varied between 0.83 and 1.68 cm3/g, the average pore diameter between 45.7 and 83.6 Å and the BET specific surface area between 615 and 744 m2/g, (see Table 1). As can be noted, the surface area of Pd/(Ce0.33Zr0.66O2)SiO2 was larger than that of Pd/SiO2. These results showed that there is an interaction between CeO2-ZrO2 and SiO2, which resulted in an increase in surface area of support, similar phenomenon has been observed in the literature  . Which suggest that this interaction can enhance also the thermal stability of palladium supported catalysts. In addition, it is important to note, that all the adsorption-desorption isotherms were of type IV in the BDDT classification (Figure 1) and that hysteresis loops apparent on the all isotherms was identified as being of the H1 type (IUPAC), which is characteristic of cylindrical pores. These results suggest that the preparation method adopted in this work is able to produce mesoporous ceria-zirconia-silica supports with improved surface area when the Ce and Zr contents increase. In the present work, 8% of Zr and 6% of Ce allows yielding a Pd/(Ce0.33Zr0.66O2)SiO2 catalyst with a relatively high surface area equal to 744 m2/g.
Table 1. Zr and Ce weight contents, BET surface area SBET (m2/g), average pore diameter Dp (Å), total pore volume Vp (cm3/g), Pd dispersion DH2 (%), average Pd particle diameter dH2 and average CeO2-ZrO2 particle diameter dTEM of the Pd/(Ce0.33Zr0.66O2) SiO2 catalysts.
aFrom N2 chemisorption at 77 K using the BET equation. bTotal pore volume estimated at reduced pressure P/P0 = 0.99, accuracy ± 0.01 cm3/g. cBased on H2 chemisorption measurements. dEstimated according to the TEM images.
Figure 1. (a) Effect of the Zr and Ce loadings on the adsorption-desorption isotherm profile of the Pd/(Ce0.33Zr0.66O2)SiO2 samples. (b) Average pore diameter distributions of the Pd/(Ce0.33Zr0.66O2)SiO2 samples.
Furthermore, we can note that the average pore diameter (Dp) and the total pore volume (Vp) increase with the Ce and Zr loadings up to the CeZr4 catalyst, and then decrease on the CeZr8 sample. The highest Dp and Vp values obtained on the CeZr4 catalyst may affect the catalytic activity through the improvement of the matter and heat transfer limitations.
3.2. Hydrogen Chemisorption
The hydrogen chemisorption analyses were conducted on the Pd/(Ce0.33Zr0.66O2)SiO2 in order to determine the dispersion and the particle sizes of palladium. The obtained results gathered in Table 1, show that the palladium dispersion was slightly improved when the Zr and Ce loadings increased (see Table 1). Indeed, the palladium dispersion increased from 14% on the CeZr0 sample to 18 % on the CeZr8 catalyst. These results suggest that the Pd dispersion could be improved by the increasing the ceria-zirconia amounts  , According to the previous work in our research group  , this results explained by the sintering inhibition of the PdO particles. This improvement of thermal stability could be favored by the enhancement of the PdO-support interaction. In our work, the Zr and Ce loadings seem to be insufficient to improve significantly the Pd dispersion.
3.3. TEM Characterization
The TEM images of the Pd/(Ce0.33Zr0.66O2)SiO2 samples are shown in Figure 2. Homogeneous distribution of the Pd particle size and a good dispersion of the
(a) (b) (c) (d) (e)
Figure 2. TEM images of the CeZr0 (a), CeZr2 (b), CeZr4 (c), CeZr8 (d) and EDXA of the CeZr4 catalysts (e).
palladium particles are shown on CeZr0 micrograph. According to the particle size distribution, the average Pd particle size is about 6.9 ± 2 nm (Figure 2(a)), which in a good agreement with data obtained from hydrogen chemisorption results. For the CeZr2 CeZr4 and CeZr8 micrographs, it is found therein that the morphologies of these catalysts are similar with each other. A careful inspection reveals the existence of crystals with different sizes, dispersed over an amorphous matrix with different contrasts. The average particle sizes of these crystals are respectively about 12, 32 and 68 nm. It is very difficult to know if the observed contrast is associated to CeO2, ZrO2 or PdO particles, the accurate measurement of the particle size of Pd cannot be achieved  . Ye Yuan et al.  indicated that most of the Pd particles on a CeO2 support could not be observed even when HR-TEM was utilized. But according to the chemisorption results the Pd particle size did not exceed 7 nm, which does not make such particles visible on the TEM images. For this reason, the CeO2 and ZrO2 identification can be based on their particle growth. Contrarily, to the Pd particles which are more dispersed. These results are in agreement with those of Y. T. Kim et al.  . The existence of palladium particles is confirmed by EDX analysis (Figure 2(e)), which confirms the existence of Pd, Ce and Zr. This result suggests that metallic oxides are well dispersed  .
XPS results indicated that palladium is only present as PdO(Pd2+) species for all catalysts. The Pd3d5/2 binding energies for the all Pd/(Ce0.33Zr0.66O2) SiO2 catalysts fall in the range of 336.4 - 337.0 eV (Table 2). Generally, the Pd3d5/2 binding energy for Pd0 and Pd2+ are about 334.9 ± 0.1 eV and 336.4 ± 0.1 eV, respectively  . Here, the Pd3d5/2 binding energies on CeZr8 CeZr4, CeZr2 and CeZr0 samples are respectively 336.8, 337.0, 336.5 and 336.4 eV (Figure 3). According to the literature, values between 336.8 and 337.0 eV are slightly too high for indicating the presence of pure PdO(Pd2+, 336.4 ± 0.1eV). This increase can be related to the interaction between PdO and the CeZr-O2/SiO2 support  . Furthermore, the relatively high Pd3d5/2 BE on the CeZr4 catalysts (337.0 eV) indicates that optimum amounts of Zr(4 wt.%) and Ce(3 wt.%) are needed to enhance the interaction between Pd species and the support. According to the XPS results, no Pd0 is detected on catalyst surface. Ciuparu et al.  , explained
Table 2. Pd binding energies and surface atomic composition of the Pd/(Ce0.33Zr0.66O2) SiO2 catalysts.
Figure 3. XPS patterns of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts: (a) CeZr2, (b) CeZr4 and (c) CeZr8.
this result by the phase with lower surface tension tends to encapsulate the phase with higher surface tension. Knowing that oxides have lower surface tension than metals, so the absence of metallic Pd contribution may be explained encapsulated by PdO oxides.
Furthermore, it is important to note that the surface loadings of palladium remains quasi-constant (about 0.02%) with the increase of Ce and Zr amount.
The TPR profiles of the Pd/(Ce0.33Zr0.66O2)SiO2 catalyst are shown in Figure 4. The negative signals observed below 50˚C, designated as peak α and peak β correspond respectively to the reduction of the larger PdO species dispersed on the catalyst surface and to the reduction of the smaller PdO species having higher interaction with the support   . Therefore, they are more stable than the larger PdO species. However, the positive signal, designated as peak γ is attributed to the decomposition of the PdHx species formed during the PdO reduction  . According to the obtained results (Table 3), the introduction of Zr and Ce on the Pd/SiO2 catalyst affects the PdOx reducibility. This reducibility increased in the following order: CeZr0 < CeZr8 < CeZr2 < CeZr4. Consequently, the introduction of ceria and zirconia improved the PdO redox property  . However, an optimum amounts equal to 4 wt.% of Zr and 3 wt.% of Ce are needed to maximize the PdO reducibility which is a key factor determining the oxidation performance of Pd-based catalysts  .
3.6. Catalytic Activity
The methane conversion curves of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts prepared with different Zr and Ce loadings are presented in Figure 5. The T20, T50 and T100 values (respectively temperatures for 20%, 50% and 100% of methane conversion) (Table 4), decreased when the Zr loading increased from 0% to 4% and
Figure 4. TPR profiles of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts.
Table 3. Reduction temperatures of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts.
Figure 5. Effect of the Zr and Ce loadings on the activity of Pd/(Zr0.66Ce0.33O2)SiO2 catalysts.
Table 4. Light-off temperature and turnover number at 325˚C of the Pd/(Ce0.33Zr0.66O2) SiO2 catalysts.
aTurnover frequency (TOF) at 325˚C.
then increased with the further Zr loading increase to 8%. Thus the presence of cerium and zirconium oxides would obviously improve the thermal stability of PdO phase of the Pd/SiO2 catalysts and enhance the catalytic activity. This is in agreement with our previous research. In fact, I. B. Said et al.  show that the addition of Ce and Zr to the Pd/MCM-41 enhance the catalytic activity for methane combustion, and an appropriate amount of these oxides can improve significantly its catalytic activity. In our case, this optimum corresponds to 4% of Zr and 3% of Ce. Considering the similar textural properties of CeZr4 and CeZr2 samples, the differences in the catalytic performance is mainly dependent on the reduction and structural properties. The TPR results show that improved reducibility is obtained with increasing the doping amount of Zr in the following order: CeZr0 < CeZr2 < CeZr4. Furthermore these results suggest that the activity of the catalytic sites is enhanced by the improvement of the PdO reducibility. In addition, according to the XPS results, the increase in Zr and Ce amounts stabilizes palladium particles in PdO state and allows a strong interaction of PdO with the support, which enhances the re-oxidation properties of PdO.
Moreover, in the case of CeZr8 catalyst, the increase of Ce and Zr amount decrease the catalytic activity compared to that of CeZr4 and CeZr2 samples. As can be noted, similar dispersion and textural propriety are obtained for all catalysts. Thus, the reason how could mainly explain the activity drop is the Pd reducibility.
The turnover number frequency (TOF) has been calculated to investigate the effect of Ce and Zr introduction in the activity per site. As summarized in Table 4. it is clearly observed that the CeZr4 sample exhibit a relatively high TOF (1588 h−1), in the combustion of methane at 325˚C, followed by CeZr2 (1243 h−1), CeZr8 (875 h−1) and CeZr0 (157 h−1). From the obtained results, the TOF evolution can explain the activity decreases. The TOF is inversely proportional to the reduction temperature order. The higher the reduction temperature, the lower is the turnover number. As a conclusion, optimum values of Ce and Zr should be reached to maximize activity in methane combustion.
The following conclusions can be drawn from this work:
・ The impregnation of the (Ce0.33Zr0.66O2)SiO2 aerogel support with palladium and the optimization of the Ce and Zr contents enable producing mesoporous Pd/(Ce0.33Zr0.66O2)SiO2 catalysts with relatively high specific surface areas;
・ The palladium dispersion is improved when the Zr and Ce loading are increased on the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts;
・ With the preparation method adopted in this work, relatively small particles of CeO2 and ZrO2 oxides, are obtained on the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts.
・ The Pd3d5/2 photopeak binding energy increased with the Ce and Zr amount increase. This seems to be due to a strong interaction between PdO and CeO2-ZrO2;
・ The addition of Ce and Zr to Pd/SiO2 (CeZr0) improves the PdO redox property and an optimum amounts of Zr and Ce is observed, for which the PdO reducibility is maximized; The improvement of the Pd/(Ce0.33Zr0.66O2)SiO2 reducibility is most likely responsible for the observed catalytic activity enhancement in the methane combustion reaction.
Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord-Pas de Calais and FEDER are acknowledged for supporting and funding partially this work.