ZrO2 catalysts

Accepted Manuscript Title: Low–temperature oxidation of methane on Pd-Sn/ZrO2 catalysts Author: Juan J. Lov´on-Quintana Jo˜ao B.O. Santos Adriana S.P. Lov´on Nath´alia La-Salvia Gustavo P. Valenc¸a PII: DOI: Reference:

S1381-1169(15)30048-0 http://dx.doi.org/doi:10.1016/j.molcata.2015.08.001 MOLCAA 9588

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

28-4-2015 29-6-2015 4-8-2015

Please cite this article as: Juan J.Lov´on-Quintana, Jo˜ao B.O.Santos, Adriana S.P.Lov´on, Nath´alia La-Salvia, Gustavo P.Valenc¸a, Lowndashtemperature oxidation of methane on Pd-Sn/ZrO2 catalysts, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Low–temperature oxidation of methane on Pd-Sn/ZrO2 catalysts Juan J. Lovón-Quintana(1), João B. O. Santos(2), Adriana S. P. Lovón(1), Nathália LaSalvia(1), Gustavo P. Valença(1) (1)

School of Chemical Engineering, University of Campinas, UNICAMP P.O. Box 6066, 13083-970, Campinas-SP, Brazil (2) Department of Chemical Engineering, Universidade Federal de São Carlos, Rodovia Washington Luís, km 235 - SP-310, São Carlos-SP, 13565-905, Brazil  To whom correspondence should be addressed Tel. 55-19-99712-1667

e-mail: [email protected] Graphical abstract

Highlights 

Sn oxide species adsorb O2 acting as an oxygen reservoir



Pd particle size increases with calcination temperature and Sn content



The presence of Sn on Pd based catalysts promotes the methane oxidation reaction



The Sn effects on Pd-Sn-containing catalysts decrease in solids calcined at 1100ºC

Abstract Pd/ZrO2, Pd-Sn/ZrO2 and Sn/ZrO2 were prepared by incipient wetness using Pd(NO3)2.XH2O and SnC4H4O6.XH2O as precursors, dried and calcined in static air at 500ºC, 800ºC or 1100ºC. The influence of Sn species in the oxygen storage capacity, distribution of Pd particle sizes and CH4 oxidation reaction were investigated. Pd particle size was larger on Sn-containing catalysts and showed lower mobility than those in the absence of Sn. Sn oxide species act as an oxygen reservoir that are easily transferred to Pd surfaces promoting the oxidation reaction of methane. For Pd-Sn-containing solids calcined at 1100ºC, the contribution of Sn in the CH4 oxidation reaction is negligible. Keywords:

Pd/ZrO2,

Pd-Sn/ZrO2,

Sn/ZrO2,

impregnation

wetness,

low-

temperature CH4 oxidation 1. Introduction Natural gas combustion processes (e.g., lean-burn natural gas vehicles, thermoelectric power generation) usually release small amounts of unburned methane (CH4), which is a powerful greenhouse gas with a global warming potential 21 times greater than CO2 [1]. Nevertheless, removal requires the use of efficient catalysts for CH4 oxidation that can operate at temperatures of exhaust gases below 500ºC [2]. Catalysts for complete CH4 oxidation at low temperatures (< 500ºC) are usually based on noble metals (e.g., Pd, Pt, Rh, Ru) supported on single or compound oxides, such as Al2O3, SiO2, ZrO2, SnO2, CeO2, TiO2 and others [3-9]. However, Pd-based catalysts are considered to be the most efficient for CH4 oxidation and there is a consensus that the active phase of Pd-based catalysts is

associated to reversible thermal transformation of metallic palladium (Pdº) and oxide palladium species (PdO-PdOx) [10-16]. Furthermore, tin oxide-supported palladium catalysts (Pd/SnO2) and zirconia-supported palladium catalysts (Pd/ZrO2) are promising catalysts for CH4 oxidation at low temperatures in oxygenrich atmospheres [17-19]. The high activity of Pd/SnO2 catalysts, in spite of the low SnO2 surface area (ca. 4 m2/g), is attributed to SnO2, which acts as a reservoir and supplier of oxygen, suppressing the dissociation of palladium oxide (PdO) [20]. While Pd/ZrO2 catalysts are characterized by their high thermal stability of PdO species on the surface of ZrO2 (up to 900°C) in comparison with the decomposition of PdO unsupported (800ºC) and PdO supported on Al2O3 (750ºC) and SiO2 (800ºC) [11,13,21]. The efficiency of Pd catalysts supported on SnO2 or ZrO2 in the CH4 reaction oxidation at low temperatures is a well-known data [14,17-20,22]. However, there is a lack of information related to the oxygen storage capacity of the species of Sn oxide and, consequently, its influence in the methane oxidation reaction. This study provides important information that may help to understand the mechanisms of the surface chemistry involving the possible interaction between Pd and Sn species in the adsorption and mobility of oxygen which promotes the methane oxidation reaction. So, in the current work, samples of Pd, Sn and Pd-Sn supported on ZrO2 were prepared by incipient wetness technique and calcined in static air at 500ºC, 800ºC or 1100ºC. Then, the oxygen storage capacity of these solids was evaluated by adsorption measurements and their influence in the distribution of Pd particle sizes and CH4 oxidation reaction was also investigated. 2. Experimental procedures 2.1. Sample preparation

Samples of Pd/ZrO2, Sn/ZrO2 and Pd-Sn/ZrO2 were prepared by incipient wetness using aqueous solutions of palladium (II) nitrate hydrate, Pd(NO3)2.XH2O (Aldrich, 41.9%Pd) and tin (II) tartrate hydrate, SnC4H4O6.XH2O (Alfa Aesar, 43.7%Sn) as precursors. Prior to the impregnation of the metal precursors, pure zirconia (ZrO2, RC-100P-DKK) anteriorly calcined at 900ºC was dried in static air at 120ºC for 24 h. Pd/ZrO2 and Sn/ZrO2 solids were prepared by incipient wetness followed by drying in static air at 120ºC for 24 h and calcined in static air at 500ºC, 800ºC or 1100ºC for 12 h. Pd-Sn/ZrO2 solids were prepared by consecutive wetness impregnation. Firstly, the Sn solution was added to oxide support (ZrO2), dried in static air at 120ºC for 24 h and moderately calcined at 500ºC for 4 h. Then, the Pd solution was added to the Sn-containing zirconia, dried in static air at 120ºC for 24 h and calcined at 500ºC, 800ºC or 1100ºC for 12 h. 2.2. Characterization The amount of Pd and Sn on Pd-Sn/ZrO2 samples was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) in a Varian Vista-MPX. The textural properties of the catalysts were measured by N2 adsorption at 196ºC in a Micromeritics ASAP 2010 analyzer. Previously, the samples (ca. 800 mg) were degassed for 4 h at 350ºC. Surface area was calculated by the Brunauer-Emmett-Teller (BET) equation in a relative pressure range 0.02 < P/Po < 0.3. Total pore volume and average pore diameter were determined by the amount of N2 adsorbed at P/Po ≈ 0.99 and by the Barrett-Joyner-Halenda (BJH) method, respectively. The crystalline phases of the samples were determined by powder X-ray diffraction (XRD) in a Philips X’Pert X-ray diffractometer. XRD data were collected

in a step scan mode with a CuK1 radiation (λ = 0.154056 nm), intensity of 40 kV and current of 40 mA. The diffraction pattern was measured between 10º and 70º at 2 with a step size of 0.02º and a time step of 1.8 s. For the peaks identification was used to JCPDS database. The morphology of the solids was studied by transmission electron microscopy (TEM) in a Philips CM200 instrument operating at 200 kV coupled to an energy-dispersive spectroscopy detector (EDS, PGT Princeton Gamma Tech, Model Prism). The samples were prepared by the immersion of a carbon grid in a suspension of solids (ca. 1.5 mg) in ethanol (10 cm3) and sonicated for 10 min. The grid was then dried at room temperature. The TEM micrographs were obtained in the bright field imaging mode. The reducibility of the catalysts surface was determined by temperatureprogrammed reduction (TPR) in a Quantachrome Instrument (Model ChemBet 3000) equipped with a thermal conductivity detector. Prior to analysis, ca. 100 mg was packed into a quartz cell, heated for 1 h at 300ºC under a He stream and then cooled to room temperature. Afterwards, a mixture of 5% H2 balanced with He was then fed into the cell at a flow rate of 60 cm3/min. The system was heated from room temperature to 620ºC, increasing the temperature linearly at a rate of 5ºC/min. The adsorption properties of the solids were evaluated by O2 adsorption, O2 titration with H2 and H2 adsorption [23-25] in a Micromeritics ASAP 2010C analyzer, using H2 (White-Martins, 99.999%) and O2 (Air-Liquide, 99.999%) as adsorbates. Prior to the adsorption measurements, a mass of catalysts from 0.2 to 1.0 g was reduced at 400ºC for 1 h in H2 with a flow rate of 30 cm3/min. The sample was then degassed at 110-5 mmHg at 410ºC for 2 h and cooled to

analysis temperature (100ºC). For each analysis, two isotherms were obtained. The difference between the two isotherms and extrapolation to zero pressure was used as a measure of the amount of irreversibly adsorbed gas. 2.3. Catalytic oxidation of methane The catalytic tests were carried out in a U-shaped reactor made of Pyrex glass (13 mm ) installed in a tubular furnace connected to a temperature programmer (Edgcon 5P, EDG Equipment). The gases used were N2 (White Martins, 99.999%), synthetic air 20%O2 – 80%N2 (White Martins, 99.997%) and CH4 (White Martins, 99.995%) and were used as received with no further purification. The reaction products were analyzed in a Hewlett-Packard 6890 gas chromatograph equipped with a CarboxenTM 1000 column (4.6 m  3.2 mm ) and a thermal conductivity detector (TCD). The mass of catalysts (20 - 30 mg) was mixed with pure ZrO2 until completing 200 mg and placed in the reactor. Before the catalytic testing, the reactor was flushed with N2 at a flow rate of 100 cm3/min and room temperature for 1 h. Then, the temperature was increased to 325ºC under N2 flow at a rate of 10ºC/min and kept at this temperature for 30 min. After that, the reaction was initiated by switching from the purge gas to the reaction mixture (2% CH4/synthetic air) at a flow rate of 100 cm3/min. The reaction was maintained for 24 h up to the equilibrium was reached. After that, the reactor was cooled to 200ºC, and on the sequence, the temperature was raised to 500ºC at a rate of 2ºC/min. Then, the reactor was cooled again to 200ºC. During this process, the reaction rate was monitored by chromatographic analysis every 10 min. The apparent activation energies (Ea) were determined by the Arrhenius equation in the temperature range where the CH4 oxidation reaction takes place in the kinetic regime and diffusional

regime. The transition temperatures were obtained from the intersection generated by the Arrhenius equations in kinetic and diffusional regimes. In addition, the turnover rate (TOR) was also determined. 3. Results and discussion 3.1. X-ray diffraction The physicochemical properties and the XRD patterns of the solids are shown in Table 1 and Figures 1-3, respectively. The pure ZrO2 showed a uniform monoclinic crystalline phase according to the XRD lines obtained at 2θ = 28.1º and 31.4º (PDF # 78-1807) (Figure 1a) and BET surface area of ca. 20 m2/g. After the impregnation of Pd and Sn precursors in ZrO2 and subsequent calcination in static air at 500ºC and 800ºC, the ZrO2 crystalline structure (Figures 1b, 1d, 1f) did not change and the BET surface area of the resulting solids were similar to pure ZrO2. However, for the samples calcined at 1100ºC, a small percentage (2%) of the crystalline phase of ZrO2 was tetragonal (Figures 1c, 1e, 1g), according to the XRD line at 2θ = 30.2º (PDF # 50-1089) and the BET surface area decreased to ca. 5 m2/g due to sintering of the crystalline particles.

Table 1 Physicochemical and catalytic properties of the palladium-tin catalysts supported on zirconia

Sample

Calcination temperature ºC

Physisorption analyses S BET m 2/g

Pore Volume cm3/g

Pore Size nm

Chemisorption analyses QO 10-6 a mol/g

TH 0 -6 b QH 10 -6 c mol/g mol/g

Pd Particle size nm

CH 4 oxidation reaction Kinetic regime Ea d kJ/mol

TOR325ºC s-1

Diffusional regime Transition Temperature Ea d TOR450ºC ºC kJ/mol s-1

ZrO2

900

19.6

0.13

26

0.1

0.1

-

-

-

-

-

-

-

4.0%Pd/ZrO2

500

18.9

0.14

28

33.9

88.5

16.9

12

75

0.47

367

30

2.34

4.0%Pd/ZrO2

800

19.8

0.16

31

26.0

65.6

13.3

16

72

0.33

389

39

2.36

3.9%Pd/ZrO2

1100

4.4

0.02

25

5.7

11.8

1.5

141

82

0.66

387

49

6.60

2.8%Pd–2.3%Sn/ZrO2

500

20.9

0.14

26

146.5

137.9

4.3

34

68

1.41

388

30

8.05

2.8%Pd–2.3%Sn/ZrO2

800

20.1

0.16

31

102.8

99.4

3.0

49

68

1.36

397

41

9.88

2.8%Pd–2.2%Sn/ZrO2

1100

5.4

0.04

27

6.7

12.0

1.9

79

68

0.55

420

51

5.05

500

21.2

0.13

24

80.1

38.0

-

-

-

-

-

-

-

2.6%Sn/ZrO2

800

19.4

0.17

34

49.1

21.0

-

-

-

-

-

-

-

2.5%Sn/ZrO2

1100

4.6

0.03

26

2.0

0.8

-

-

-

-

-

-

-

0.8%Pd/ZrO2

800

22.4

0.18

31

19.4

47.2

10.0

4

85

0.29

369

44

2.32

0.8%Pd–0.9%Sn/ZrO2

800

21.3

0.15

27

56.1

61.7

2.0

23

67

0.45

445

38

4.35

1.0%Sn/ZrO2

800

22.5

0.17

27

25.3

11.3

-

-

-

-

-

-

-

0.7%Pd–2.5%Sn/ZrO2

800

21.1

0.14

26

88.0

86.8

1.2

30

77

1.07

389

41

8.85

3.6%Pd–0.9%Sn/ZrO2

800

19.6

0.18

34

54.8

96.2

5.8

32

68

0.47

434

37

4.29

1.6%Pd–1.1%Sn/ZrO2 800 18.3 0.13 27 88.7 95.8 Oxygen adsorption in moles of O2 per gram of catalyst. b Oxygen titration with hydrogen in moles of H2 per gram of catalyst. c Hydrogen adsorption in moles of H2 per gram of catalyst. d Apparent activation energy determined by a Arrhenius plot (Ln TOR vs. 1/T).

3.1

28

78

0.39

432

43

4.83

2.6%Sn/ZrO2

a

Figure 1. XRD patterns of palladium-tin catalysts supported on zirconia.

Figure 2. XRD patterns of catalysts: (a) 4.0%Pd/ZrO2-500ºC; (b) 3.6%Pd0.9%Sn/ZrO2–800ºC; (c) 3.9%Pd/ZrO2–1100ºC; and, (d) 2.8%Pd2.2%Sn/ZrO2-1100ºC.

Figure 3. XRD patterns of catalysts: (a) 2.8%Sn/ZrO2-500ºC; (b) 0.7%Pd2.5%Sn/ZrO2–800ºC; (c) 2.8%Pd-2.2%Sn/ZrO2–1100ºC; and, (d) 2.5%Sn/ZrO2-1100ºC. In regard to Pd-Sn containing solids calcined at 500ºC and 800ºC, the XRD lines of PdO and SnOx were not identified, either because the oxide particles are very small or because they form non-crystalline PdO and SnOx clusters. Actually, as noted in previous works is very difficult to get XRD signals in metallic or metallic oxides catalysts supported on ceramic oxides with low concentration of active phase (e.g., <3%Pd) [14]. Nevertheless, after calcination at 1100ºC, the Pd-containing catalysts suffered a decomposition of PdO and at the same time happened the sintering of the support oxide and segregation of the palladium metallic particles. As a result, it was possible to observe a very weak signal of XRD at 2 = 46.8º which corresponds to the metallic Pd with a preponderance of (200) crystal planes (PDF # 01-1201) (Figure 2a-d). The Pd (200) surface crystal planes have a structure similar to PdO that presents a tetragonal structure with the Pd+2 ions (as well as the O2- ions) located at the corner of the tetrahedrons. Thus, during the PdO decomposition at 1100ºC, the

PdO could be easily transformed into metallic Pd crystallites, exhibiting preferentially (200) planes without large changes in the crystalline structure, such as those having predominantly (111) planes usually found after thermal pre-treatment in a reducing atmosphere, which are constituted by dense atomic structures (face-centered cubic) forming hexagonal arrangements with shorter interatomic distances [11,26,27]. Furthermore, on the Sn-containing samples sintered at 1100ºC, it was also possible to observe a very weak signal of XRD at 2 = 26.6º (Figure 3a-d), which corresponds to the most stable (110) planes of the rutile structure of SnO2 (PDF # 01-0657). This result suggests that during the sintering process of the Sn-containing samples at 1100ºC, the particles of SnO2 over the ZrO2 surfaces were segregated and remaining stable. 3.2. Transmission electron microscopy The TEM micrographs for the Pd/ZrO2, Sn/ZrO2 and Pd-Sn/ZrO2 samples calcined at 500ºC and 1100ºC with magnifications varying from 70,000× to 135,000× are shown in Figures 4 to 6. The dark areas correspond to regions of high atomic density and the light areas to regions of low atomic density, with Sn > Pd > Zr.

Figure 4. TEM micrographs: (a) 4.0%Pd/ZrO2 calcined at 500ºC; and, (b) 3.9%Pd/ZrO2 calcined at 1100ºC. For the sample 4.0%Pd/ZrO2 calcined at 500ºC (Figure 4a), it was very difficult to observe individual particles of the Pd species, suggesting that the Pd supported on ZrO2 was completely oxidized, thus producing small and/or noncrystalline PdO clusters [14,21]. Nevertheless, for the sample 3.9%Pd/ZrO2 calcined at 1100ºC, it was possible to distinguish the contour images of the metallic Pd particles with sizes ranging from 10 to 100 nm (Figure 4b) as obtained in previous works [13,21]. The micrographs for the Sn-containing samples 2.6%Sn/ZrO2 (Figure 5a) and 2.5%Sn/ZrO2 (Figure 5b) calcined at 500ºC and 1100ºC show well-defined images. The Sn was identified as SnO2 in the form of darker particles than those of ZrO2. In the samples calcined at 500ºC, SnO2 particles showed irregular hemispherical shapes with sizes varying from 25 to 50 nm. In the samples calcined at 1100ºC, SnO2 particles were in the size range of 50 to 200 nm and ZrO2 formed agglomerated particles with clear contours. However, no dilution or fusion of SnO2 into ZrO2 was observed.

Figure 5. TEM micrographs: (a) 2.6 %Sn/ZrO2 calcined at 500ºC; and, (b) 2.5%Sn/ZrO2 calcined at 1100ºC. The particle sizes range for SnO2 supported on the 2.6%Sn/ZrO2 and 2.5%Sn/ZrO2 samples calcined at 500ºC and 1100ºC was the same as that observed for the 2.8%Pd-2.3%Sn/ZrO2 and 2.8%Pd-2.2%Sn/ZrO2 samples calcined at the same temperatures, respectively (Figure 6). Likewise, the characteristics observed for the PdO on the 4%Pd/ZrO2 catalysts calcined at 500ºC (Figure 4a) were similar to those of the 2.8%Pd-2.3%Sn/ZrO2 catalysts calcined at the same temperature (Figure 6a). However, the 3.9%Pd/ZrO2 and 2.8%Pd-2.2%Sn/ZrO2 samples calcined at 1100ºC (Figure 6b) in addition to agglomerated large metallic Pd particles also had a small fraction of metallic Pd with particle sizes below 10 nm. The formation of Pd particles size inferior to 10 nm may be due to the presence of SnO2, as previously suggested by Eguchi and Arai [19] on Pd/SnO2 samples. Nevertheless, it was not possible to obtain evidences about the formation of Pd-Sn alloys.

Figure 6. TEM micrographs: (a) 2.8%Pd–2.3%Sn/ZrO2 calcined at 500ºC; and, (b) 2.8%Pd-2.2%Sn/ZrO2 calcined at 1100ºC. 3.3. Temperature-programmed reduction The TPR profiles for the solids Pd-Sn/ZrO2, previously calcined in static air at 800ºC for 12 h, with a Pd content varying from 0% to 4% and Sn content varying from 2.6% to 0% are shown in Figure 7. The negative peak observed between 60ºC and 90ºC corresponds to H2 desorption as a result of the reduction and adsorption of H2 at room temperature, which increases with increasing of Pd load. These observations are consistent with those in previous works. PdO can be easily reduced in H2 at low temperatures with the subsequent adsorption of H2 on metallic Pd species, thus forming different hydride species such as α-H-Pd, β-H-Pd and γ-H-Pd. These hydride species can be decomposed by the thermal desorption of H2 between –100ºC and 400ºC [28, 29].

. Figure 7. TPR profiles of Pd-Sn/ZrO2 samples calcined in static air at 800ºC for 12 h. Feed gas: 5.0%H2–95%He; heating rate: 5ºC/min. The peak observed above 450ºC corresponds to SnO2 reduction, which also increases with increasing of Sn content. SnO2 is more stable and can be reduced in a H2 atmosphere between 450ºC and 800ºC, from Sn+4 to Sn+2, with the reduction to Snº kinetically slower [30,31]. Furthermore, it has been suggested that the external surface layers of SnO2 particles, when exposed to a reducing atmosphere (e.g., 5 %H2 at 200ºC), can be partially reduced to SnOx (1 ≤ x < 2) without the migration of monoxides species to the internal layers of the SnO2 particles [32]. In this study, the reduction peak of SnO2 observed between 450ºC and

620ºC for the Pd-Sn-containing solids is slightly displaced to lower temperatures for samples with Pd content. The change in the reduction temperature suggests that the surfaces of Pd enable H to access the partial reduction of SnO2. However, no peaks were observed between 100ºC and 450ºC as reported by Takeguchi et al. [30]. The latter authors suggest that a broad and small peak developed at around 200ºC or the appearance of multiple peaks in the temperature range from 100 to 500°C can be attributed to reduction of PdO in PdO-SnO2 species. Thus, the results obtained in this work suggest that there is an interaction between Pd-Sn species. Nevertheless, no claims can be made about the distance between the Pd and SnO2 particles, nor can be inferred that there is a formation of a Pd-Sn alloy. 3.4. Chemisorption experiments O2 adsorption As shown in Table 1, the adsorption of O2 on the pure oxide support of ZrO2 was negligible. The amount of O2 adsorbed on solids calcined at 500ºC decreased in the following order: 2.8%Pd-2.3%Sn/ZrO2 > 2.6%Sn/ZrO2 > 4.0%Pd/ZrO2. The same tendency was observed for solids calcined at 800ºC with high Pd and Sn loads (2.8%Pd-2.3%Sn/ZrO2 > 2.6% Sn/ZrO2 > 4.0%Pd/ZrO2) and with low Pd and Sn loads (0.8%Pd-0.9%Sn/ZrO2 > 1.0%Sn/ZrO2 > 0.8%Pd/ZrO2). For the 3.9%Pd/ZrO2, 2.8%Pd-2.2%Sn/ZrO2 and 2.5%Sn/ZrO2 samples calcined at 1100ºC, O2 adsorption decreased drastically (below 5%) in relation to the same solids calcined between 500ºC and 800ºC. In all cases, the increase in calcination temperature resulted in a decrease in the amount of O2 adsorbed. In the samples calcined at 1100ºC, this low O2 adsorption confirms that the Pd oxide species (e.g., PdO) were decomposed,

thus forming large metal Pd particles, and that the oxide support was sintered, as previously observed by XRD and N2 adsorption at -196°C. When the values of O2 adsorption on Pd/ZrO2, Sn/ZrO2 and Pd-Sn/ZrO2 are compared, it is possible to observe that the Sn-containing solids adsorb more O2. Earlier studies have demonstrated that the surfaces of SnO2 partially reduced to SnOx (1≤x<2) at 200ºC can be re-oxidized [32]. Likewise, at temperatures below 150ºC, the SnO2 surface adsorbs non dissociated oxygen (as charged O2-1 ions), while at temperatures above 150–200ºC, dissociated oxygen can be adsorbed (as charged O-1 or O-2 ions) [33,34]. Thus, in this work, the O2 adsorption on the Sn-containing solids may be due both to the adsorption of molecular oxygen and to the reconstruction of the SnO2 surfaces partially reduced to SnOx (1≤x<2) during the thermal pre-treatment step of the solids performed at a reducing atmosphere (H2, 400ºC) before the measurement of O2 adsorption. Furthermore, the solids Pd-Sn/ZrO2 adsorbed O2 at an equivalent or greater amount corresponding to the sum of the amount of O2 adsorbed by Pd/ZrO2 and Sn/ZrO2. This finding suggests that the volume of O2 adsorbed on the solids Pd-Sn/ZrO2 is due to the individual contribution of Pd and Sn. For example, the amount of O2 adsorbed on 0.8%Pd-0.9%Sn/ZrO2 was close to the sum of the amount of O2 adsorbed on 0.8%Pd/ZrO2 and that on 1.0%Sn/ZrO2. The

same

was

observed

for

the

amount

of

O2

adsorbed

on

3.6%Pd-0.9%Sn/ZrO2, which was equivalent to the sum of the amount of O2 adsorbed on 4.0%Pd/ZrO2 and that on 1.0%Sn/ZrO2. However, the adsorption of O2 on Pd-Sn/ZrO2 samples happened irrespective of any interaction between Pd and Sn or any formation of a Pd-Sn alloy.

Based on these latest observations and according to the steps involving the O2 adsorption measurements, the following equations can be written for Pd surfaces: Pre-treatment (H2, 400ºC): PdsOx’ + ½(2x’+r)H2 → PdsHr + x’H2O

(Eq. 1)

O2 adsorption: PdsHr + ¼(2x+r)O2 → PdsOx + ½rH2O

(Eq. 2)

where Pds is the number of Pd atoms exposed; x’ is the number of oxygen atoms adsorbed on the Pd surface after calcination and cooling at room temperature; x is the number of oxygen atoms chemisorbed at an analysis temperature of Qo; and r is the number of hydrogen atoms adsorbed on the Pd surface after reduction and degasification at an analysis temperature of Qo. For the SnO2 surfaces, the following equations can be written: Pre-treatment (H2, 400ºC): SnO2 + (2-x’’)H2 → SnOx + (2-x’’)H2O

(Eq. 3)

O2 adsorption: SnOx’’ + ½[(2-x’’) + w]O2 + * → SnO2 + Ow*

(Eq. 4)

where x’’ is the number of oxygen atoms in the SnOx species (1≤x’’<2); w is the number of oxygen atoms adsorbed on the active sites located near or over the SnO2 surface at an analysis temperature of Qo; and * is the active sites located near or over the SnO2 surface. It is important to note that the above equations were written assuming that there is no interaction between the Pd and Sn particles and that the equation parameters (x, x’, x”, r, w) depend on the surface properties of the solids and the measurement conditions used during O2 adsorption. As a consequence, the direct determination of all parameters is not possible. Oxygen titration with H2

In contrast to the observed O2 adsorption (Qo), H2 consumption during the measurement of oxygen titration with H2 (TH) was higher on Pd/ZrO2 than on Sn/ZrO2. In this case, the higher consumption of H2 in TH over Pd/ZrO2 was attributed to the H2 consumed by the reduction of PdsOx species and selective adsorption of H2 over the atoms exposed to the Pd metallic. However, for Sn/ZrO2 the amount of H2 consumed in TH was low because the H2 may have only been consumed to remove the excess of O2 adsorbed near or on the surface of SnO2. This is possibly due to the stability of the SnO2 surfaces in the conditions used during the measurement of TH [30,31]. Thus, H2 consumption during the measurement of TH over the Pd surfaces can be expressed by: O2 titration: PdsOx + ½(2x+z)H2 → PdsHz + xH2O

(Eq. 5)

and over the SnO2 surfaces can be expressed by: O2 titration: Ow* + wH2 → wH2O + *

(Eq. 6)

where z is the number of hydrogen atoms chemisorbed at an analysis temperature of TH. The other symbols are equal to those in Eqs. 1 to 4. Nevertheless, the H2 consumed in TH over the Pd-Sn/ZrO2 catalysts was higher than that in Pd/ZrO2 and Sn/ZrO2 (Table 1). Similar to the observed measurement of O2 adsorption, the high consumption of H2 on the Pd-Sn/ZrO2 catalysts during the measures of TH was attributed to the individual contributions of Pd and Sn. H2 adsorption The amounts of H2 adsorbed (QH) on the 4.0%Pd/ZrO2 solids calcined at 500ºC and 800ºC were similar. For the sintered solid of 3.9%Pd/ZrO2 calcined

at 1100ºC, the amount of H2 adsorbed decreased drastically. The presence of Sn in the Pd-containing catalysts resulted in a decrease in the amount of H2 adsorbed (e.g., 4.0%Pd/ZrO2 > 3.6%Pd-0.9%Sn/ZrO2 and 0.8%Pd/ZrO2 > 0.8%Pd-0.9%Sn/ZrO2). For the pure ZrO2 and Sn/ZrO2 solids, no uptake of H2 was observed [30,35,36]. In previous works, in catalysts of Pd oxide species supported on pure SnO2, it has been observed that H2 reduces Pd+2 to Pdo followed by H2 adsorption, preferentially on the surface of Pdo even at room temperature with a H2/Pd weight of 0.3% [30]. However, according to Aduriz et al. [35], in Pd-Sncontaining catalysts supported on Al2O3 with a strong chemical interaction between Pd oxide species and SnO2 (e.g., the formation of Pd-Sn alloys), even at low concentrations of Pd (0.09–0.11%) and Sn (0.02–0.11%), Sn may inhibit H2 adsorption on the surfaces of the metallic Pd. Contrary, when the formation of Pd-Sn alloys is not observed (e.g., in catalysts prepared from chloride precursors), neither the decrease in H2 adsorption nor the loss of the catalytic activity of palladium is observed. In this work, using TEM, XRD and TPR, a strong interaction or formation of Pd-Sn alloys or no dilution or fusion of SnO2PdO into ZrO2 was not evidenced. This finding suggests that the inferior adsorption of H2 obtained on the solids Pd-Sn/ZrO2 compared to the solids Pd/ZrO2 occurred because of the higher segregation of the Pd particles in the presence of Sn. This effect was more significant with an increase in calcination temperature during the preparation of the catalysts. Thus, considering that H2 adsorption takes place only on Pd surfaces, the steps of pre-treatment and H2 adsorption performed during the measurement of QH over Pd surfaces can be expressed by Eqs. 1 and 7:

H2 adsorption: PdsHr + ½(y-r)H2 → PdsHy

(Eq. 7)

where y is the number of hydrogen atoms chemisorbed at an analysis temperature of QH. The other symbols are equal to those in Eqs. 1 to 6. Based on the results above, the particle size of Pd (Table 1) was estimated from the H2 adsorption measurements. The ratio of hydrogen atoms adsorbed to palladium atoms exposed (H/Pd) was assumed to be ≈ 1 [25,37]. The palladium average particle sizes were determined by dPd (nm) = 112/[(MPd.2QH)/W Pd×100], where MPd, molecular weight of Pd (g/mol); QH, H2 adsorption (mol/g); W Pd, Pd content (w/w) in fraction units. This equation assumes spherical shaped particles with a density surface of 1.27×1019 Pd atoms per square meter [38,39]. For all Pd-containing catalysts, the Pd metallic particle size increases at high calcination temperature [4,40]. The size of Pd particles on the Pd-Sn/ZrO2 samples was higher than that on the Pd/ZrO2 samples. However, the Pd particles on the Pd-Sn/ZrO2 samples showed lower segregation or sintered less with the increase in calcination temperature than those on the Pd/ZrO2 samples. 3.5. Catalytic oxidation of methane In Table 1 are shown the apparent activation energies (EA) determined in the kinetic and diffusional regimes and the transition temperature obtained between the kinetic and diffusional regimes for the Pd/ZrO2 and Pd-Sn/ZrO2 catalysts. For all catalysts, the experimental data of EA for the kinetic regime (68–85 kJ/mol) and for the diffusional regime (30–50 kJ/mol) as well as the transition temperature (350–450ºC) are in good agreement with those found in the literature such as 71–95 kJ/mol [40-42], 23–45 kJ/mol and 350–450ºC [42], respectively. However, the Pd-Sn-containing catalysts showed lower values of

Ea in the kinetic regime than the Pd-containing catalysts. These tendencies favor the CH4 oxidation reaction on Pd-Sn/ZrO2 catalysts and suggest an interaction or possible synergy between Pd and Sn. Nevertheless, Ea in the diffusional regime showed no clear differences among the catalysts containing Pd or Pd-Sn and calcined at the same temperature, probably, because the chemical reactions above the transition temperature are preferably governed by heat and mass transfer phenomena. In Figures 8 and 9 are shown the profiles of TOR in terms of the temperature reaction, temperature calcination and solids composition. For the Pd/ZrO2 and Pd-Sn/ZrO2 catalysts, the reaction rate of CH4 oxidation increased with a rise in the temperature of the reaction from 200ºC to 500ºC, and no hysteresis was observed when the reaction temperature decreased from 500ºC to 200ºC. The Sn/ZrO2 solids showed a small rate of CH4 oxidation above 400ºC, probably due to the partial reduction of SnO2 to SnOx (1≤x<2). But, the surfaces of SnOx did not demonstrate the same ability as those of the Pd surface for the mechanisms of the dissociative adsorption of reducing gases (e.g., H2), being its capacity to catalyze the CH4 oxidation reaction completely limited. The rates of CH4 oxidation on pure oxide support (ZrO2) between 200ºC and 500ºC were practically zero. These results demonstrate that CH4 oxidation took place preferentially on the active sites supplied by the surface of the Pd species (Pdº/PdO-PdOx,) formed during the course of the reaction [10-16].

Figure 8. TOR as a function of the reaction temperature of CH4 oxidation for Pd/ZrO2 and Pd-Sn/ZrO2 calcined at 500ºC, 800ºC and 1100ºC. In previous studies were also observed that the TOR for the CH4 oxidation over Pd is not sensitive to the structure of the catalyst [40,43,44]. The latter means that the CH4 oxidation reaction is independent of the particle size and of the crystal structure of palladium. However, the reaction of CH4 oxidation on the catalysts proposed in this work showed some tendencies on the presence of Sn and with the increasing of the calcination temperature. In the kinetic regime, the catalysts Pd-Sn/ZrO2 calcined between 500ºC and 800ºC showed higher values of TOR than Pd/ZrO2 calcined at the same range of temperatures. For example, from Table 1 it can be seen that on the catalysts 2.8%Pd-2.3%Sn/ZrO2 calcined at 500ºC or 800ºC, the TOR was higher (3 to 4 times) than that on 4.0%Pd/ZrO2 calcined at the same temperatures. Similar trends were observed in catalysts

with close loads of Pd and with or without Sn calcined at the same temperature. Thus, the TOR values for 0.8%Pd-0.9%Sn/ZrO2, and 0.7%Pd-2.5%Sn/ZrO2 were 2 to 4 times higher than those for 0.8%Pd/ZrO2. When the Pd/ZrO2 and Pd-Sn/ZrO2 catalysts were calcined at 1100ºC, even presenting different Pd particle sizes, the solids did not shown a significant difference in TOR value. For this latter case, the Sn contribution was negligible on the CH4 oxidation reaction over catalysts calcined at higher temperatures due to the sintering of the oxide support and increase in the segregation of Sn and Pd on the surface of ZrO2. In the diffusional regime, the TOR values increased significantly, maintaining the tendencies observed in the kinetic regime, despite the methane oxidation reaction is happening in conditions where the phenomena of heat and mass transfer are important. Another particularity is also shown in Figure 9. The solids with different contents of Pd and Sn calcined at the same temperature (800ºC) showed three different groups. The first had high TOR, corresponding to the solids with a high content of Sn (2.3%Sn). The second group was observed for solids containing 1%Sn. The third group had low TOR, which corresponded to the solids without Sn. As observed on the measures of oxygen adsorption (Table 1), the presence of Sn on the catalysts Pd/ZrO2 contributes to the increase in the amount of adsorbed oxygen available for the reaction. This effect is more significant for solids calcined at temperatures ≤ 800ºC. However, the surfaces of SnO2 do not show specific sites for the CH4 oxidation reaction. Therefore, we can infer that the higher values of TOR obtained for the solids Pd-Sn/ZrO2 with respect to the solids Pd/ZrO2 were caused by the species of Sn acting as a stock of oxygen that is easily transferred to the surfaces of Pd, as also observed for the

catalysts Pd/ZrO2 doped with cerium oxide, CeO2 [2,45].

Figure 9. TOR as a function of the reaction temperature of CH4 oxidation for Pd/ZrO2 and Pd-Sn/ZrO2 calcined at 800ºC.

4. Conclusions The oxygen storage capacity, distribution of Pd particle sizes and CH4 oxidation reaction were evaluated on Pd/ZrO2 and Pd-Sn/ZrO2 catalysis after calcination in static air at 500ºC, 800ºC or 1100ºC. In the catalysts containing Pd and Sn species, the Pd particles were larger and showed less mobility with respect to those supported on ZrO2 in the absence of Sn. However, no strong interaction or formation of Pd-Sn alloys or no dilution or fusion of SnO2-PdO into ZrO2 was evidenced. The Sn-containing solids, calcined at temperatures ≤ 800ºC, showed a high capacity of O2 adsorption behaving like an oxygen

reservoir that could be easily transferred to the surfaces of Pd increasing the catalytic activity of Pd-Sn/ZrO2 catalysts in the CH4 oxidation. For Pd/ZrO2 and Pd-Sn/ZrO2 catalysts calcined at 1100ºC, the reaction rates were similar. In this condition, the contribution of Sn on the reaction of CH4 oxidation is negligible.

Acknowledgments Financial support from the CNPq, National Council of Science and Technology, Brazil.

References [1] R.E. Hayes , Chem. Eng. Sci., 59 (2004) 4073-4080. [2] P. Gélin, M. Primet, Appl. Catal. B, 39 (2002) 1-37. [3] R.B. Anderson, K.C. Stein, J.J. Feenan, L.J.E. Hofer, J. Ind. Eng. Chem., 53 (1961) 809-812. [4] R.F. Hicks, H. Qi, M.L. Young, R.G. Lee, J. Catal., 122 (1990) 280-294. [5] S.H. Oh, P.J. Mitchell, R.M. Siewert, J. Catal., 132 (1991) 287-301. [6] Y. Mizushima, M. Hori, Appl. Catal. A, 88 (1992) 137-148. [7] R. Burch, P.K. Loader, Appl. Catal. B, 5 (1994) 149-164. [8] G. Pecchi, P. Reyes, R. Zamora, T. López, R. Gómez, J. Chem. Technol. Biotechnol., 80 (2005) 268-272. [9] X. Fan, F. Wang, T. Zhu, H. He, J. Environ. Sci., 24 (2012) 507-511. [10] S. Colussi, A. Trovarelli, E. Vesselli, A. Baraldi, G. Comelli, G. Groppi, J. Llorca, Appl. Catal. A, 390 (2010) 1-10. [11] A. Baylet, P. Marécot, D. Duprez, P. Castellazzi, G. Groppi, P. Forzatti, Phys. Chem. Chem. Phys., 13 (2011) 4607-4613. [12] N.M. Kinnunen, J.T. Hirvi, T. Venalainen, M. Suvanto, T.A. Pakkanen, Appl. Catal. A, 397 (2011) 54-61. [13] R.J. Farrauto, M.C. Hobson, T. Kennely, E.M. Walterman, Appl. Catal., 81 (1992) 227-237. [14] K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, J. Catal., 179 (1998) 431-442.

[15] R. Burch, F.J. Urbano, Appl. Catal. A, 124 (1995) 121-138. [16] S. Yang, A. Maroto-Valiente, M.I. Benito-Gonzales, I. Rodriguez-Ramos, A. Guerrero-Ruiz, Appl. Catal. B, 28 (2000) 223-233. [17] H. Widjaja, K. Sekisawa, K. Eguchi, Chem. Lett., 27 (1998) 481-482. [18] K. Sekizawa, H. Widjaja, S. Maeda, Y. Ozawa, K. EGUCHI, Catal. Today, 59 (2000) 69-74. [19] K. Eguchi, H. Arai, Appl. Catal. A, 222 (2001) 359-367. [20] H. Widjaja, K. Sekisawa, K. Eguchi, Bull. Chem. Soc. Jpn., 72 (1999) 313320. [21] N.M. Rodriguez, S.G. Oh, R.A. Dalla Betta, R.T.K. Baker, J. Catal., 157 (1995) 676-686. [22] W. Lin, L. Lin, Y.X. Zhu, Y.C. Xie, K. Scheurell, E. Kemnitz, Appl. Catal. B, 57 (2005) 175-181. [23] J.E. Benson, H.S. Hwang, M. Boudart, J. Catal., 30 (1973) 146-153. [24] D.J. O’Rear, D.G. Löffler, M. Boudart, J. Catal., 121 (1990) 131-140. [25] G. Prelazzi, M. Cerboni, G. Leofanti, J. Catal., 181 (1999) 73-79. [26] W. Juszczyk, Z. Karpinski, D. Lomot, J. Pielaszek, J. Catal., 220 (2003) 299-308. [27] E. Garbowski, C. Feumi-Jantou, N. Mouaddib, M. Primet, Appl. Catal. A, 109 (1994) 277-291. [28] V. Ragaini, R. Giaanantonio, P. Magni, L. Lucarelli, G. Leofanti, J. Catal., 146 (1994) 116-125. [29] C.W. Chou,S.J. Chu, H.J. Chiang, C.Y. Huang, C. J. Lee, S.R. Sheen, T. P. Perng, C.T. Yeh, J. Phys. Chem. B, 105 (2001) 9113-9117. [30] T. Takeguchi, O. Takeoh, S. Aoyama, J. Ueda, R. Kikuchi, K. Eguchi, Appl. Catal. A, 252 (2003) 205-214. [31] P.W. Park, H.H. Hung, D.W. Kim, M.C. Kung, J. Catal., 184 (1999) 440454. [32] G.B. Hoflund, Chem. Mater., 6 (1994) 562-568. [33] S.C. Chang, J. Vac. Sci. Technol., 17 (1980) 366-369. [34] A. Gurlo, Chem. Phys. Chem., 7 (2006) 2041-2052. [35] H.R. Aduriz, P. Bodnariuk, B. Coq, F. Figueras, J. Catal., 119 (1989) 97107.

[36] D.L. Hoang, H. Lieske, Catal. Lett., 27 (1994) 33-42. [37] F. Hicks, Q.J. Yen, A.T. Bell, J. Catal., 89 (1984) 498-510. [38] P.C. Aben, J. Catal., 10 (1968) 224-229. [39] M. Boudart, H. S. Hwang, J. Catal., 39 (1975) 44-52. [40] F.H. Ribeiro, M. Chow, R.A. Dalla Betta, J. Catal., 146 (1994) 537-544. [41] Y.Y. Yao, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 293-298. [42] C. F. Cullis, B.M. Willat, J. Catal., 83 (1983) 267-285. [43] G. Zhu, J. H, D.Y. Zemlyanov, F.H. Ribeiro, J. Am. Chem. Soc., 126 (2004), 9896-9897. [44] C.Q. Lv, K.C. Ling, G.C. Wang, J. Chem. Phys., 131 (2009) 144704. [45] R.M. Heck, R.J. Farrauto, Appl. Catal. A, 221 (2001) 443-457.