Nd Ce Yue nivers ormal d for e–Z chniq 3 cata s the reduction–reoxidation properties of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al2O3 Applied Catalysis A: General 2 catalyst. # 2005 Elsevier B.V. All rights reserved. Keywords: Palladium; Ce–Zr/Al2O3; Rare earths; Thermal stabilization; Methane combustion 1. Introduction In recent years, high-temperature catalytic combustion of methane has proven to be a suitable alternative to conventional flame combustion due to high combustion efficiency and extremely low emission of NOx and unburned hydrocarbons [1–6]. Although methane combustion has been proposed for many industrial applications, such as gas turbines, jet engines, the major difficulty is to develop a practical catalyst and support that has both high-temperature stability and low-temperature activity [7,8]. Palladium-based catalysts are the most effective catalytic systems for methane combustion [9–12]. In practice, alumina is usually adopted as support in order to maintain favorable dispersion of active metal to achieve valid utilization of precious metal. However, alumina supported palladium catalysts are not stable at the high temperature commonly used for methane oxidation. The deactivation of PdO/Al2O3 catalysts is reported to be mainly due to a decrease in the surface area of alumina [13] and to the transformation of PdO to Pd [14] at high temperature. For these reasons, addition of a promoter in order to increase the thermal stability of the catalyst is considered as an attractive alternative to enhance the performance of the catalyst [15–18]. CeO2 is a well known promoter in noble metal based combustion catalysts. But with the temperature improve- ment, especially higher than 1000 8C, CeO2 readily sinters at elevated temperatures resulting in catalyst deactivation. The addition of Zr and especially the formation of Ce–Zr mixed oxide have been found to be effective in preventing the Ce from sintering. Therefore, the using of Ce–Zr mixed oxides as additives in alumina supported noble metal catalysts is of great technological importance [19,20]. Recently we have studied the effects of Ce and Zr modified Pd/Al2O3 catalysts for methane combustion. A synergism between Ce–Zr and Pd leading to a better activity and thermal stability in the methane combustion is observed. In order to further improve the thermal stability, in this paper we research the influence of the addition of rare earths (La, Pr, Nd, Sm and Y) on the methane combustion over Pd/Ce– Zr/Al2O3 catalysts. * Corresponding author. Tel.: +86 571 88273290; fax: +86 571 88273283. E-mail address:
[email protected] (R. Zhou). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.08.002 Effect of rare earths (La, Pr, combustion over Pd/ Baohua Yue a, Renxian Zhou a,*, a Institute of Catalysis, Zhejiang U bDepartment of Chemistry, Zhejiang N Received 16 May 2005; received in revise Abstract The effects of rare earths (La, Pr, Nd, Sm and Y) addition to Pd/C are characterized by BET, XRD, XPS, TEM, TPR, TPO and TPSR te Yor Sm obviously improves the catalytic activity of Pd/Ce–Zr/Al2O and thermal stability. The addition of Y to Pd/Ce–Zr/Al2O3 inhibit , Sm and Y) on the methane –Zr/Al2O3 catalysts juan Wang b, Xiaoming Zheng a ity, Hangzhou 310028, PR China University, Jinhua 321004, PR China m 21 July 2005; accepted 1 August 2005 r/Al2O3 catalyst have been investigated. The supported Pd catalysts ues. Activity tests in methane combustion show that the addition of lyst, and the Pd/Ce–Zr–Y/Al2O3 shows the highest catalytic activity site growth and decomposition of PdO particles and improves the www.elsevier.com/locate/apcata 95 (2005) 31–39 2.4. Temperature-programmed reaction tests The H2-TPR is carried out in a flow system to observe the reducibility of the supported Pd catalysts. Prior to H2-TPR measurement, a 50 mg catalyst is pre-treated in air at 300 8C for 0.5 h. The reduction gas is a gas mixture of 5 vol.% H2 in N2, which is purified using deoxidizer and silica gel. The temperature of the sample is programmed to rise at a constant rate of 10 8C/min. The amount of H2 uptake during the reduction is measured by a TCD. The effluent H2O formed during H2-TPR is adsorbed with a 5 A molecular sieve. The reoxidation properties of the reduced catalysts are measured by means of O2-TPO. A 100 mg catalyst is used for each measurement. Prior to TPO experiments, the samples are reduced in H2 atmosphere at 500 8C for 1 h. Subsequently, TPO experiments are performed using a gas mixture of 5 vol.% O2 in He with a flow rate of 40 ml/min, increasing the temperature from room temperature to 1000 8C at a heating rate of 20 8C/min. After reaching 1000 8C, the temperature is cooled down to 300 8C at a cold sis A: General 295 (2005) 31–39 2. Experimental 2.1. Catalyst preparation Ce–Zr/Al2O3 and Ce–Zr–M/Al2O3 (M = La, Pr, Nd, Sm and Y) supports are prepared by co-impregnation of pseudo- boehmite (SBET = 218 m 2/g) with an aqueous solution of Ce and Zr nitrates, Ce, Zr and rare earths (La, Pr, Nd, Sm and Y) nitrates, respectively. The samples are dried at 100 8C, and then calcined at 900 8C for 2 h. The total content of Ce and Zr as oxide state (CeO2 + ZrO2) is 18 wt.% and the molar ratio of Ce and Zr is 1:4. The content of rare earths (M) is 1 wt.% for Ce–Zr–M/Al2O3 supports. Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 (M = La, Pr, Nd, Sm and Y) catalysts are prepared by conventional impregnation with an aqueous of H2PdCl4 as metal precursors. The impregnated samples are reduced by hydrazine hydrate, filtered and washed with large amount of water, dried at 100 8C for 12 h and then calcined at 500 8C for 2 h. In order to compare their thermal stability, the catalysts are calcined at 1100 8C for 4 h. The content of Pd for all catalysts is 0.5 wt.%. 2.2. Catalyst characterization The XRD patterns are recorded with a Rigaku D/max- 3BX diffractometer using monochromatic Cu Ka radiation. Specific surface area is obtained from N2 adsorption isotherms (at the liquid nitrogen temperature) with the BET method, using a Coulter OMNISORP-100 apparatus. Prior to adsorption measurements, the samples are degassed under vacuum for 2 h at 200 8C. The size of the metallic particles on the supported Pd catalysts is checked with transmission electron microscopy (TEM) using a JEM-2010 (HR) apparatus operated at 200 KV. X-ray photoelectron spectroscopy (XPS) measurements are performed on a VG ESCALAB 2201-XL spectrometer. Non-monochro Mg Ka radiation are used as a primary excitation. The binding energies are calibrated with the C1s level of adventitious carbon (284.6 eV) as the internal standard reference. 2.3. Catalytic activity tests The reaction of methane combustion is carried out in a conventional flow system under atmospheric pressure. Catalyst (40–60 mesh) is loaded in a quartz reactor, with quartz wool packed at both ends of the catalyst bed. The reaction mixtures containing 1.5 vol.% methane, 6.0 vol.% oxygen and balance of nitrogen are fed to catalyst bed at a space velocity of 18 000 h�1. The methane conversion in the effluent gas is analyzed by on- B. Yue et al. / Applied Cataly32 line gas chromatography. rate of 20 8C/min. Amount of O2 consumption during the O2-TPO is also measured by a thermal conductivity detector (TCD). In order to investigate the relation of methane combustion activity with properties of PdO decomposition and its reoxidation, the temperature-programmed surface reaction is carried out. The experiment is performed in the same apparatus as the catalytic activity tests analysis. For the reaction test, the feed composition is 0.5 vol.% methane, 2 vol.% oxygen and balance of nitrogen. The space velocity is 72 000 h�1. The reaction temperature increases from room temperature to 1000 8C at a heating rate of 20 8C/min. Fig. 1. XRD patterns of the catalysts calcined at 500 8C: (a) Pd/Ce–Zr/ Al2O3; (b) Pd/Ce–Zr–La/Al2O3; (c) Pd/Ce–Zr–Pr/Al2O3; (d) Pd/Ce–Zr–Nd/ Al2O3; (e) Pd/Ce–Zr–Sm/Al2O3; (f) Pd/Ce–Zr–Y/Al2O3. B. Yue et al. / Applied Catalysis A: General 295 (2005) 31–39 33 Table 1 Surface area of the supports and supported Pd catalysts Supports SBET (m 2/g), calcined at 900 8C Catalysts SBET (m 2/g), calcined at 1100 8C Al2O3 98 Pd/Al2O3 25 Ce–Zr/Al2O3 106 Pd/Ce–Zr/Al2O3 49 Ce–Zr–La/Al2O3 111 Pd/Ce–Zr–La/Al2O3 58 Ce–Zr–Pr/Al2O3 115 Pd/Ce–Zr–Pr/Al2O3 54 After reaching 1000 8C, the temperature is cooled down to 300 8C at a cold rate of 20 8C/min. 3. Results and discussion 3.1. Surface characterization of catalysts The XRD patterns of Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/ Al2O3 supports calcined at 500 and 1100 8C are given in Figs. 1 and 2, respectively. From Fig. 1, it can be seen that Fig. 2. XRD patterns of the catalysts calcined at 1100 8C: (a) Pd/Ce–Zr/ Al2O3; (b) Pd/Ce–Zr–La/Al2O3; (c) Pd/Ce–Zr–Pr/Al2O3; (d) Pd/Ce–Zr–Nd/ Al2O3; (e) Pd/Ce–Zr–Sm/Al2O3; (f) Pd/Ce–Zr–Y/Al2O3. the tetragonal and cubic phases of CexZr1�xO2 solid solution are observed in all the samples calcined at 500 8C. The supported CexZr1�xO2 solid solution shows mainly the tetragonal phase due to high Zr loading. The crystalline structure of alumina remains g-alumina phase due to the thermal stabilization of the additions. From Fig. 2, it can be seen that on calcinations at 1100 8C, the peaks of the CexZr1�xO2 solid solution become sharper and more intense, corresponding to an increase in size of the crystalline particle. The presence of Ce–Zr or Ce–Zr–M inhibits the transformation of alumina into the low- surface-area a-phase, which is typical for this tempera- ture. The results show that the presence of Ce–Zr or Ce– Table 2 Data from XPS analysis of the catalysts Sample Calcination temperature (8C) Atomic Ce/Al Pd/Ce–Zr/Al2O3 500 3.1 Pd/Ce–Zr/Al2O3 1100 3.92 Pd/Ce–Zr–Y/Al2O3 500 3.2 Pd/Ce–Zr–Y/Al2O3 1100 3.71 Zr–M obviously improves the thermal stability of the alumina. Table 1 lists the BET surface areas of Al2O3, Ce–Zr/ Al2O3 and Ce–Zr–M/Al2O3 (M = La, Pr, Nd, Sm, Y) supports calcined at 900 8C and supported Pd catalysts calcined at 1100 8C, respectively. From Table 1, it can be seen that after calcinations at 900 8C, the addition of cerium and zirconium oxides to alumina results in an enhanced thermal stability of the supports, which maintains large surface area. The measurements of the BET surface area of the stabilized alumina reflect the significant influence of Ce and Zr up to 1100 8C. The surface area of the non-doped Pd/ Al2O3 undergoes a sharp decrease after calcinations at 1100 8C due to sintering, showing a specific surface area of only 25 m2/g. In contrast, the sample containing Ce and Zr exhibits surface area of 49 m2/g after equivalent thermal treatment. In addition, the addition of alkaline earths can further improve the thermal stability of the Pd/Ce–Zr/Al2O3 catalyst. Table 2 presents the values of the surface atomic ratios, as well as the values of surface Pd/PdO molar ratio determined from XPS. For Pd/Ce–Zr/Al2O3 catalyst, the lower Zr/Al atomic ratio with respect to the bulk value suggests an insertion of Zr into the carriers’s structure in this system. The insertion implies a deep interaction between Al2O3 and Zr, which could then explain the alumina thermal stabilization. After the addition of Y, the Zr/Al atomic ratio slightly increases due to that the alumina cationic vacancies are already occupied by Y. While the lower XPS Zr/Al and Y/Al Ce–Zr–Nd/Al2O3 118 Pd/Ce–Zr–Nd/Al2O3 55 Ce–Zr–Sm/Al2O3 116 Pd/Ce–Zr–Sm/Al2O3 56 Ce–Zr–Y/Al2O3 116 Pd/Ce–Zr–Y/Al2O3 60 atomic ratios with respect to the bulk value also suggest a deep interaction between Al2O3 and Zr–Y. This is confirmed by BET data where the addition of Y further improves the thermal stability of the Pd/Ce–Zr/Al2O3 support. In addition, ratios (�102) Pd/PdO ratio Zr/Al Y/Al Pd/Al 6.0 – 3.70 0 5.84 – 3.22 0.57 6.29 0.57 3.77 0 6.2 1.27 3.47 0.48 sis A: the surface concentration of Pd is much higher than that derived from the bulk value, indicating that a preferential deposit of palladium species takes place on the outer surface of catalyst particles. It can be seen that the Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–Y/Al2O3 catalysts calcined at 500 8C do not present any metallic Pd contribution in the XPS spectra, suggesting that CeO2-based systems can stabilize PdO through an interaction via surface oxygen [21]. The total surface concentration of palladium species decreases after calcinations at 1100 8C, which is indicative of dissociation of PdO and agglomeration to large metallic Pd clusters. As shown in Table 2, the Pd/PdO ratios in the catalyst obviously increase after calcined at 1100 8C for the Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–Y/Al2O3 catalysts. Comparing the Pd/PdO ratios in Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–Y/Al2O3 catalysts cal- cined at 1100 8C, it can be seen that the ratio become greatly lower in the latter. The introducing of Y into CeO2 lattices B. Yue et al. / Applied Cataly34 Fig. 3. Light-off curves of methane oxidation over Pd/Ce–Zr/Al2O3 and Pd/ Ce–Zr–M/Al2O3 catalysts calcined at 500 8C. promoted the oxygen transfer from the bulk to the support surface and from the support to the Pd particles. The increase in oxygen mobility can facilitate the maintenance of PdO. 3.2. Catalytic activity tests Fig. 3 shows the results of methane combustion activity over Pd/Al2O3, Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 catalysts calcined at 500 8C. The presence of Ce–Zr or Ce– Zr–M negatively affects the catalytic activity of the Pd/ Al2O3 catalyst. A large difference can be seen in the catalytic activity between Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 catalysts. The addition of Y, Sm or Nd obviously improves the activity of Pd/Ce–Zr/Al2O3 catalyst for methane combustion. The catalyst, added with Pr, exhibits almost the same activity as that of Pd/Ce–Zr/Al2O3. However the catalytic activity of Pd/Ce–Zr/Al2O3 catalyst is deteriorated by the addition of La. The results imply the possibility of unfavorable interaction between the metals after the addition of La, which would result in the deterioration of the catalytic activity of Pd/Ce–Zr/Al2O3 catalyst. Fig. 4 shows the results of the catalytic combustion of methane over Pd/Al2O3, Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/ Al2O3 catalysts calcined at 1100 8C. From the results we can see that the activity of Pd/Al2O3 undergoes a sharp decrease after calcined at 1100 8C, while the activity of Pd/Ce–Zr/ Al2O3 and Pd/Ce–Zr–M/Al2O3 catalysts shows slightly decrease. The addition of rare earths except Nd obviously improves the activity of Pd/Ce–Zr/Al2O3 catalyst for methane combustion. In particular, in the presence of Y, the T90% only increases 5 8C after calcined at 1100 8C. Fig. 5 gives the TEM picture of Pd/Ce–Zr/Al2O3, Pd/Ce– Zr–Y/Al2O3 catalyst calcined at 500 and 1100 8C, respec- tively. From Fig. 5, it can be seen that after calcined at 1100 8C, the sintering of PdO and support occurs obviously, and the particle size of PdO and support becomes larger for General 295 (2005) 31–39 Fig. 4. Light-off curves of methane oxidation over Pd/Ce–Zr/Al2O3 and Pd/ Ce–Zr–M/Al2O3 catalysts calcined at 1100 8C. Pd/Ce–Zr/Al2O3 catalyst. This is due to the higher pre- treatment temperature leading to a decrease in surface area, which causes poor dispersion of palladium. But for Pd/Ce– Zr–Y/Al2O3 catalyst, the change of the particle size of PdO and support is not obvious before/after calcined at 1100 8C. This indicates that the addition of Y would inhibit the sintering of PdO particles in Pd/Ce–Zr–Y/Al2O3 catalyst because of increasing the thermal stability of Ce–Zr/Al2O3, and improves the thermal stability of the supported Pd catalyst. 3.3. Temperature-programmed reduction H2-TPR profiles of the Pd/Al2O3, Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 catalysts calcined at 500 8C are shown in Fig. 6 and values of the H2 consumption and peak temperature maxima are shown in Table 3. From Fig. 6 and Table 3, it can be seen that palladium oxide supported on Al2O3 is more easily reduced and H2-TPR profile of Pd/ Al2O3 catalyst exhibits a hydrogen consumption peak B. Yue et al. / Applied Catalysis A: General 295 (2005) 31–39 35 500 8 Fig. 5. The TEM photograph of the catalysts: (a) Pd/Ce–Zr/Al2O3 calcined at at 500 8C; (d) Pd/Ce–Zr–Y/Al2O3 calcined at 1100 8C. around 10 8C representing the reduction of PdO species and a negative peak at about 90 8C which is generally attributed to the decomposition of palladium hydride[22], while H2- TPR profile of Pd/Ce–Zr/Al2O3 catalyst exhibits three hydrogen consumption peaks at 35 8C (peak a), 45 8C (peak b), 102 8C (peak g) and a negative peak at about 90 8C. The positive peaks are attributed to consumption of hydrogen on the reduction of PdO species, which indicates variation in the distribution of PdO with support composition. According to the previous work in our research group, the TPR profiles for Pd/Ce–Zr catalysts show that the PdO is reduced at 60– 90 8C [23]. So we suggest that the peak a is the reduction of the PdO species finely dispersed on alumina-rich grains and the peak b is the reduction of the PdO species finely dispersed on Ce–Zr-rich grains. There have been analogy Table 3 H2 consumption and temperature of TPR peaks for the catalysts calcined at 500 Catalysts a Peak b Peak H2 consumption (mmol/gcat) Peak temperature (8C) H2 consum (mmol/gca Pd/Al2O3 46.1 � 0.88 11 Pd/Ce–Zr/Al2O3 24.5 � 0.49 35 13.5 � 0.3 Pd/Ce–Zr–La/Al2O3 37.6 � 0.65 39 – Pd/Ce–Zr–Pr/Al2O3 28.5 � 0.58 35 3.65 � 0.0 Pd/Ce–Zr–Nd/Al2O3 3.5 � 0.09 15 28.8 � 0.5 Pd/Ce–Zr–Sm/Al2O3 5.2 � 0.11 18 28.2 � 0.5 Pd/Ce–Zr–Y/Al2O3 12.2 � 0.28 17.5 21.8 � 0.4 C; (b) Pd/Ce–Zr/Al2O3 calcined at 1100 8C; (c) Pd/Ce–Zr–Y/Al2O3 calcined reports in the H2-TPR profiles of Pd/TiO2–Al2O3 catalysts by Wang et al. [24]. For the peak g, it is suggested that the stable PdO species are present on the Pd/Ce–Zr/Al2O3 catalyst due to the interaction of PdO and ZrO2 [25,26]. In addition, it can be seen that the peak a shifts to lower temperature after the addition of Nd, Sm and Y. The amount of H2 consumption increases in the order: Pd/Ce–Zr–Nd/ Al2O3 < Pd/Ce–Zr–Sm/Al2O3 < Pd/Ce–Zr–Y/Al2O3. The addition of La or Pr has no obvious influence on the peak a. The results show that the addition of Nd, Sm or Y to Pd/ Ce–Zr/Al2O3 increases the reducibility of PdO dispersed on alumina-rich grains. It is interesting that the temperature and the amount of H2 consumption could be correlated with the catalytic activity. From the results of Fig. 3, we find that the trend of the peak a temperature decreasing is similar to that 8C g Peak ption t) Peak temperature (8C) H2 consumption (mmol/gcat) Peak temperature (8C) 1 45 8.5 � 0.22 102 – 7.3 � 0.21 105 9 43 14.8 � 0.36 98.5 9 43 15.1 � 0.37 98.5 9 45 12.5 � 0.29 99 4 47.5 13.8 � 0.32 102.5 sis A: General 295 (2005) 31–39 Fig. 7. TPR profiles of the catalysts calcined at 1100 8C: (A) Pd/Al2O3; (B) Pd/Ce–Zr/Al O ; (C) Pd/Ce–Zr–Nd/Al O ; (D) Pd/Ce–Zr–Y/Al O . B. Yue et al. / Applied Cataly36 Fig. 6. TPR profiles of the catalysts calcined at 500 8C: (A) Pd/Al2O3; (B) Pd/Ce–Zr/Al2O3; (C) Pd/Ce–Zr–La/Al2O3; (D) Pd/Ce–Zr–Pr/Al2O3; (E) Pd/Ce–Zr–Nd/Al2O3; (F) Pd/Ce–Zr–Sm/Al2O3; (G) Pd/Ce–Zr–Y/Al2O3. of their oxidation activities increasing. This suggests that the catalytic activity may be related to the reducibility of PdO species finely dispersed on alumina-rich grains, and the higher the reducibility of the PdO species, the higher the catalytic activities. The reduction peak appears at the lowest temperature for Pd/Al2O3 among the catalysts, and the catalyst is the most active. The TPR profiles of Pd/Al2O3, Pd/Ce–Zr/Al2O3, Pd/Ce– Zr–Nd/Al2O3 and Pd/Ce–Zr–Y/Al2O3 catalysts calcined at 1100 8C are shown in Fig. 7. The results show that hydrogen consumption peak and hydrogen desorption peak in all catalysts become obviously sharper, indicating that the sintering phenomena of PdO species occurs after calcined at 1100 8C. The hydrogen consumption peak in Pd/Al2O3 catalyst shifts to higher temperature. For Pd/Ce–Zr/Al2O3, Pd/Ce–Zr–Nd/Al2O3 and Pd/Ce–Zr–Y/Al2O3 catalysts, they only exhibit one peak of hydrogen consumption in the range of 0–70 8C, and area of peak g also decrease obviously and its peak temperature also shift to higher temperature. This suggests that PdO species on the surface of Ce–Zr/Al2O3 or Ce–Zr–M/Al2O3 supports could be migrated and decom- posed into metallic Pd due to weakening the interaction of PdO and Ce–Zr oxides or ZrO2 at high temperature of 1100 8C [26]. A part of PdO species dispersed on Ce–Zr-rich grains and had strong interaction with ZrO2 may be moved on alumina-rich grains. The reduction temperature of PdO species in the catalysts decreases in the order: Pd/Al2O3 > Pd/Ce–Zr/Al2O3 > Pd/Ce–Zr–Nd/Al2O3 > Pd/ Ce–Zr–Y/Al2O3. This order is also in accordance with that of their oxidation activities decreasing. 3.4. Temperature-programmed oxidation In order to investigate the PdO–Pd transformation properties during raising and lowering the temperature, O2-TPO experiments are carried out. Fig. 8 shows the results of TPO experiment over the Pd/Al2O3, Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 catalysts calcined at 500 8C and values 2 3 2 3 2 3 Fig. 8. TPO profiles of the catalysts calcined at 500 8C: (A) Pd/Al2O3; (B) Pd/Ce–Zr/Al2O3; (C) Pd/Ce–Zr–La/Al2O3; (D) Pd/Ce–Zr–Pr/Al2O3; (E) Pd/Ce–Zr–Nd/Al2O3; (F) Pd/Ce–Zr–Sm/Al2O3; (G) Pd/Ce–Zr–Y/Al2O3. of the O2 consumption and peak temperature maximum are shown in Table 3. TPO profile of Pd/Al2O3 catalyst shows two negative peaks (named as first and third peaks, respectively) representing oxygen desorption during raising the temperature in the range of 30–1000 8C and a positive peak (named as fourth peak) representing oxygen consump- tion during lowering the temperature. The negative peak at about 100 8C corresponds to desorption of adsorption oxygen species, while the negative peak at about 820 8C is attributed to the decomposition of PdO species. The positive peak during lowering the temperature is assigned to the reoxida- position and its reoxidation. 3.5. Temperature-programmed surface reaction In order to investigate the relation of methane combustion with properties of PdO decomposition and its reoxidation, CH4/O2-TPSR experiments of Pd/Al2O3, Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–Y/Al2O3 catalysts calcined at 1100 8C are B. Yue et al. / Applied Catalysis A: General 295 (2005) 31–39 37 at 500 8C d peak Fourth peak onsumption ol/gcat) Peak temperature (8C) O2 consumption (mmol/gcat) Peak temperature (8C) .5 � 0.3 860 15.9 � 0.31 675 .1 � 0.33 860 15.9 � 0.31 675 .2 � 0.32 885 16 � 0.31 700 .1 � 0.32 875 15.8 � 0.3 675 .9 � 0.32 875 16 � 0.32 675 .9 � 0.32 880 16.1 � 0.33 710 .3 � 0.34 885 16.1 � 0.33 710 tion of metallic Pd. However, oxygen consumption peak during TPO from 30 to 800 8C has been not obtained, indicating metallic Pd species finely dispersed on Al2O3 have been oxidized into PdO because of its high activity at the balance of TPO beginning. TPO profiles of Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 catalyst displays an additional oxygen consumption peak (named as second peak) at about 350 8C besides oxygen consumption/desorption peaks obtained in Pd/Al2O3 catalyst. However, no oxygen consumption peak is observed in the range of 30–300 8C and the area of PdO decomposition peak is obviously larger than that of oxygen consumption peak at 350 8C, while the amount of H2 consumption of g peak (Fig. 7) is about double of that of O2 consumption of second peak. We suggest that the positive peak at about 350 8C is attributed to the oxidation of stable metallic Pd species obtained in the TPR profiles (peak g), while Pd species finely dispersed on alumina-rich grains and Ce–Zr-rich grain have also been oxidized at the balance of TPO beginning. In the presence of cerium and zirconium oxides, temperatures of decomposition of PdO and reoxidation of metallic Pd both shift higher temperature than the corresponding temperatures observed on Pd/Al2O3 catalyst, indicating that the presence of Ce–Zr in Pd/Al2O3 would improve the thermal stability of PdO. Temperatures of decomposition of PdO and reoxidation of metallic Pd can further shift to higher temperature after the addition of rare earths, In particular, in the presence of La or Y, the temperature of decomposition of PdO shifts about 50 8C above the corresponding temperature observed on Pd/Ce–Zr/Al2O3 catalyst (Table 4). Fig. 9 shows O2-TPO profiles of the Pd/Al2O3, Pd/Ce–Zr/ Al2O3, Pd/Ce–Zr–Nd/Al2O3 and Pd/Ce–Zr–Y/Al2O3 cata- Table 4 O2 consumption and temperature of TPO peaks for the catalysts calcined Catalysts Second peak Thir O2 consumption (mmol/gcat) Peak temperature (8C) O2 c (mm Pd/Al2O3 – – �21 Pd/Ce–Zr/Al2O3 4.6 � 0.16 355 �22 Pd/Ce–Zr–La/Al2O3 4.2 � 0.13 350 �22 Pd/Ce–Zr–Pr/Al2O3 6.9 � 0.23 350 �22 Pd/Ce–Zr–Nd/Al2O3 7.2 � 0.26 340 �21 Pd/Ce–Zr–Sm/Al2O3 6.2 � 0.2 345 �21 Pd/Ce–Zr–Y/Al2O3 6.8 � 0.25 350 �22 lysts calcined at 1100 8C. From Fig. 9, it can be seen that oxygen desorption peak at about 100 8C disappears, and peak of PdO decomposition becomes obviously sharper for Pd/Al2O3 catalyst, indicating that amount of bigger PdO or Pd particles increases after calcined at 1100 8C and ability of adsorbing surface oxygen decreases. For Pd/Ce–Zr/Al2O3 and Pd/Ce–Zr–M/Al2O3 catalysts, area of the oxygen desorption peak at about 100 8C and the oxygen consump- tion peak at about 350 8C obviously decreases and also the latter shifts to higher temperature (about 500 8C). In addition, the temperatures of PdO decomposition and its reoxidation during raising and lowering the temperature shift to lower temperature for all the catalysts after calcined at 1100 8C, but the presence of Ce–Zr or Ce–Zr–M in Pd/ Al2O3 obviously raises the temperatures of PdO decom- Fig. 9. TPO profiles of the catalysts calcined at 1100 8C: (A) Pd/Al2O3; (B) Pd/Ce–Zr/Al2O3; (C) Pd/Ce–Zr–Nd/Al2O3; (D) Pd/Ce–Zr–Y/Al2O3. decomposition of PdO and reoxidation of Pd into PdO Ce–Zr/Al2O3 catalyst. sis A: General 295 (2005) 31–39 (B) Pd/Ce–Zr/Al2O3; (C) Pd/Ce–Zr–Y/Al2O3. carried out (see Fig. 10). The results show that the presence of Ce–Zr or Ce–Zr–Y in Pd/Al2O3 obviously decreases the beginning temperature of methane combustion. The order of the beginning temperature is in accordance with that of their oxidation activities increasing. However, it is interesting that there is a trend of CH4 oxidation activity decreasing in the range of 725–900 8C during raising the temperature and in the range of 850–575 8C during lowering the temperature. Comparing the TPSR profile with the TPO of the Pd/Al2O3 catalyst (Fig. 8), it may be seen that the activity decrease in the heating process is related with the decomposition of PdO into the less active Pd. A strong drop of activity during the cooling-down step is due to that Pd is still at the metallic state, and the activity increases until PdO reforms. At the same time, we can also see that for Pd/Ce–Zr/Al2O3 and Pd/ Ce–Zr–Y/Al2O3 catalysts, the level of CH4 oxidation activity decreasing during raising the temperature is not obvious and the reaction temperature of their oxidation activities recovering during lowering the temperature remarkably increases, indicating that the presence of Ce– Zr or Ce–Zr–Y in Pd/Al2O3 improves thermal stability of PdO and reoxidation property of Pd, thus increasing their activities of CH4 oxidation and thermal stability of catalyst. B. Yue et al. / Applied Cataly38 Fig. 10. TPSR profiles of the catalysts calcined at 1100 8C: (A) Pd/Al2O3; 4. Conclusions The effects of rare earths (La, Pr, Nd, Sm and Y) addition to Pd/Ce–Zr/Al2O3 catalysts have been investigated through different characterization techniques and methane combus- tion tests. The results show that the presence of Ce–Zr or Ce– Zr–M (M = La, Pr, Nd, Sm and Y) greatly improves the thermal stability of alumina. 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Yue et al. / Applied Catalysis A: General 295 (2005) 31–39 39 Effect of rare earths (La, Pr, Nd, Sm and Y) on the methane �combustion over Pd/Ce-Zr/Al2O3 catalysts Introduction Experimental Catalyst preparation Catalyst characterization Catalytic activity tests Temperature-programmed reaction tests Results and discussion Surface characterization of catalysts Catalytic activity tests Temperature-programmed reduction Temperature-programmed oxidation Temperature-programmed surface reaction Conclusions Acknowledgements Reference