Aerogel Pd/(Ce 0.33Zr 0.66O 2)SiO 2 catalysts (CeZry) were prepared with variable Ce and Zr loadings (molar ratio Ce/Zr = 1/2) by combining sol-gel and impregnation methods. First, N 2-physisorption was used to investigate the texture evolution. Then, H 2-chimisorption and TEM were performed to study the effect on particle dispersion. After, TPR was used to determine the catalyst reducibility. Furthermore, XPS characterization was done to identify the palladium oxidation state and to evaluate the Pd-support interaction. Finally, the prepared catalysts were tested in methane combustion to assess their catalytic activity. The obtained results showed that, when the Zr and Ce loadings are varied between 0% and 8% and between 0% and 6% respectively, the BET surface area was increased from 615 to 744 m 2/g, the porosity diameter from 45.7 to 83.6 Å , the Pd particle diameter from 5.2 to 7.0 nm, the CeO 2 and ZrO 2 particle size from 0 to 68 nm, the reduction temperature shift reached 16 °C, the Pd binding energy shift attained 0.6 eV, but an optimum amounts of Zr (4 wt.%) and Ce (3 wt.%) are needed to maximize the PdO reducibility and to enhance the catalytic activity. In effect, 100% conversion of methane was reached at around 415 °C on the CeZr4 catalyst.
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 [
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.
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).
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).
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:
Conversion ( % ) = P CO 2 P CH 4 + P CO 2 × 100
P CH 4 and P CO 2 are respectively, the partial pressures of methane and carbon dioxide.
TOF = A × M Pd l × D
A = P CO 2 × D T × 273 22.4 × T r × m
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 (%).
The N2-physisorption results of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts summarized in
Sample | Zr wt.% | Ce wt.% | SBETa m2/g | Dpa (Å) | Vpb (cm3/g) | DH2c (%) | dH2(Pd)c (nm) | dTEM(CeO2-ZrO2)d (nm) |
---|---|---|---|---|---|---|---|---|
CeZr8 | 8 | 6 | 744 | 58.9 | 1.13 | 18 | 5.2 | 68 |
CeZr4 | 4 | 3 | 734 | 83.6 | 1.68 | 17 | 5.6 | 32 |
CeZr2 | 2 | 1.5 | 696 | 60.6 | 1.18 | 16 | 5.7 | 12 |
CeZr0 | 0 | 0 | 615 | 45.7 | 0.83 | 14 | 7.0 | - |
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.
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.
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
The TEM images of the Pd/(Ce0.33Zr0.66O2)SiO2 samples are shown in
palladium particles are shown on CeZr0 micrograph. According to the particle size distribution, the average Pd particle size is about 6.9 ± 2 nm (
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 (
Sample | BE Pd 3d3/2 (eV) | BE Pd 3d5/2 (eV) | Oxidation state | Pd (%) |
---|---|---|---|---|
CeZr8 | 342.1 | 336.8 | Pd2+ | 0.02 |
CeZr4 | 342.3 | 337.0 | Pd2+ | 0.02 |
CeZr2 | 341.4 | 336.5 | Pd2+ | 0.02 |
CeZr0 | 341.4 | 336.4 | Pd2+ | 0.02 |
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
The methane conversion curves of the Pd/(Ce0.33Zr0.66O2)SiO2 catalysts prepared with different Zr and Ce loadings are presented in
Sample | T(˚C) | T(˚C) | T(˚C) |
---|---|---|---|
CeZr8 | 23 | 49 | 64 |
CeZr4 | 16 | 37 | 58 |
CeZr2 | 18 | 43 | 60 |
CeZr0 | 32 | 47 | 65 |
Sample | T20(˚C) | T50(˚C) | T100(˚C) | TOFa(h−1) |
---|---|---|---|---|
CeZr8 | 331 | 362 | 462 | 875 |
CeZr4 | 317 | 342 | 415 | 1588 |
CeZr2 | 329 | 358 | 425 | 1243 |
CeZr0 | 450 | - | - | 157 |
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. [
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
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.
Sadouki, K., Fessi, S., Ksibi, Z., Capron, M., Dumeignil, F. and Ghorbel, A. (2018) Effect of Zirconium and Cerium Loadings on Aerogel Pd-Based Catalysts for Methane Combustion. Advances in Materials Physics and Chemistry, 8, 105-119. https://doi.org/10.4236/ampc.2018.83008