Effect of Zirconium and Cerium Loadings on Aerogel Pd-Based Catalysts for Methane Combustion

Aerogel Pd/(Ce0.33Zr0.66O2)SiO2 catalysts (CeZry) were prepared with variable Ce and Zr loadings (molar ratio Ce/Zr = 1/2) by combining sol-gel and impregnation methods. First, N2-physisorption was used to investigate the texture evolution. Then, H2-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/g, the porosity diameter from 45.7 to 83.6 Å, the Pd particle diameter from 5.2 to 7.0 nm, the CeO2 and ZrO2 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.


Introduction
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 [1]. 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, NO x and unburned hydrocarbons [2]. Palladium oxide is considered to be an effective catalyst in methane oxidation [3] [4] [5] [6]. Unfortunately, the deactivation of the catalyst due to the sintering of palladium during heat treatments is a serious problem for this kind of catalysts [7]. 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 [8]. The ceria-zirconia-supported palladium catalysts showed high activity [9] [10] due to CeO 2 -ZrO 2 redox properties and oxygen storage capacity (OSC) which is better than pure CeO 2 [9] [10] [11] [12]. The addition of zirconium into the cubic CeO 2 framework is effective to prevent cerium sintering, to improve the oxygen mobility in the lattice and to increase thermal stability [13]. Furthermore, the use of SiO 2 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 [14], which prevent CeO 2 depletion through, e.g., reaction with the support giving compounds such as CeAlO 3 formation causing the OSC decrease when Al 2 O 3 is used [15]. However, studies on aerogel SiO 2 as a support for Ce x Zr 1−x O 2 oxide have been scarcely reported, especially when concerning the effect of Ce x Zr 1−x O 2 loading on the Pd/(Ce x Zr 1−x O 2 )SiO 2 catalyst for methane combustion. For these reasons, in the research presented herein, the sol-gel process was used to prepare a SiO 2 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 [17]. However appropriate amounts of Ce and Zr are needed to make the modification efficient [16]. The impregnation method was then used to add palladium and to prepare the Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 catalysts. The effect of Ce and Zr loadings on the texture, the structure and the catalytic activity in methane combustion was then studied.

Catalyst Preparation
The Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 catalysts (CeZry, where y is the Zr loading) were prepared by the impregnation method. The appropriate amounts of palladium acetate and (Ce 0.33 Zr 0.66 O 2 )SiO 2 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).

Catalyst Characterization
The BET specific surface area and the average pore diameter were determined from N 2 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 H 2 using a quartz U-tube reactor, coupled to a thermal conductivity detector (TCD). The catalyst

Nitrogen Physisorption
The N 2 -physisorption results of the Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 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 cm 3 /g, the average pore diameter between 45.7 and 83.6 Å and the BET specific surface area between 615 and 744 m 2 /g, (see Table   1). As can be noted, the surface area of Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 was larger than that of Pd/SiO 2 . These results showed that there is an interaction between CeO 2 -ZrO 2 and SiO 2 , which resulted in an increase in surface area of support, similar phenomenon has been observed in the literature [17]. 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/(Ce 0.33 Zr 0.66 O 2 )SiO 2 catalyst with a relatively high surface area equal to 744 m 2 /g.

Hydrogen Chemisorption
The hydrogen chemisorption analyses were conducted on the Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 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 im- proved by the increasing the ceria-zirconia amounts [18], According to the previous work in our research group [13], 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.

TEM Characterization
The TEM images of the Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 samples are shown in Figure 2.
Homogeneous distribution of the Pd particle size and a good dispersion of the (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 CeO 2 , ZrO 2 or PdO particles, the accurate measurement of the particle size of Pd cannot be achieved [19]. Ye Yuan et al. [20] indicated that most of the Pd particles on a CeO 2 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 CeO 2 and ZrO 2 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. [21]. 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 [13]. 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 Pd 0 is detected on catalyst surface. Ciuparu et al. [22], explained 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.

TPR
The TPR profiles of the Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 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 [19] [23]. 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 [23]. According to the obtained results (Table 3), the introduction of Zr and Ce on the Pd/SiO 2 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 [24].
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 [25].

Catalytic Activity
The methane conversion curves of the Pd/(Ce 0.33 Zr 0.66 O 2 )SiO 2 catalysts prepared with different Zr and Ce loadings are presented in Figure 5. The T 20 , T 50 and T 100 values (respectively temperatures for 20%, 50% and 100% of methane conversion) (Table 4), decreased when the Zr loading increased from 0% to 4% and    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.

Conclusions
The following conclusions can be drawn from this work: