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Effect of ceria crystal plane on the physicochemical and catalytic properties of Pd/ceria for CO and propane oxidation Zong Hu, Xiaofei Liu, Dongmei Meng, Yun Guo, Yanglong Guo, and Guanzhong Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02617 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016
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Effect of ceria crystal plane on the physicochemical and catalytic properties of Pd/ceria for CO and propane oxidation Zong Hu, Xiaofei Liu, Dongmei Meng, Yun Guo*, Yanglong Guo, Guanzhong Lu*
Key Laboratory for Advanced Materials and Research Institute of Industrial catalysis, East China University of Science and Technology, Shanghai 200237, P. R. China. * Corresponding Author: Fax: +86-21-64252923, E-mail:
[email protected] (G.Z. Lu);
[email protected] (Y. Guo) ABSTRACT: Ceria nanocrystallites with different morphologies and crystal planes were hydrothermally prepared, and the effects of ceria supports on the physicochemical and catalytic properties of Pd/CeO2 for the CO and propane oxidation were examined. The results showed that, the structure and chemical state of Pd on ceria were affected by ceria crystal planes. The Pd species on CeO2-R (rods) and CeO2-C (cubes) mainly formed PdxCe1-xO2-σ solid solution with -Pd2+-O2--Ce4+linkage. And the PdOx nanoparticles were dominated on the surface of Pd/CeO2-O (octahedrons). For the CO oxidation, the Pd/CeO2-R catalyst showed the highest catalytic activity among three catalysts, its reaction rate reached 2.07 × 10−4 mol g−1Pd s−1 at 50 °C, in which CeO2-R mainly exposed the (110) and (100) facets with the low oxygen vacancy formation energy, strong reducibility and high surface oxygen mobility. TOF of Pd/CeO2-R (3.78 × 10−2 s−1) was much higher than that of Pd/CeO2-C (6.40 × 10−3 s−1) and Pd/CeO2-O (1.24 × 10−3 s−1) at 50 °C, and its Ea was 40.4 kJ/mol. For propane oxidation, the highest reaction rate (8.08 × 10-5 mol g−1Pd s−1 at 300 °C) was obtained over the Pd/CeO2-O catalyst, in which CeO2-O mainly exposed the (111) facet. There are the strong surface Ce-O bonds on the ceria (111) facet, which is in favor of the existence of PdO particles and propane activation. TOF of the Pd/CeO2-O catalyst was highest (3.52 × 10−2 s−1) at 300 °C and its Ea was 49.1 kJ/mol. These results demonstrate the inverse facet sensitivity of ceria for the CO and propane oxidation over Pd/ceria.
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KEYWORDS: Supported Pd catalyst; Ceria crystal plane; CO oxidation; Propane oxidation; Effect of ceria support.
1. INTRODUCTION Noble metal catalysts (such as Pd, Pt, etc.), especially the catalysts of platinum groups, have attracted much attention because they have high catalytic activities for many chemical reactions, for instance, the noble metal supported on alumina catalysts has been widely researched and applied commercially.1-5 As ceria possesses the high oxygen storage capacity (OSC) and a unique redox property by the cycle of Ce4+/Ce3+ redox pairs, the presence of Ce can increase the migration of lattice oxygen in the Ce-included oxide catalysts,6-8 and ceria as the catalytic component plays an important role in the oxidation and VOCs (volatile organic compounds) purification catalysts.9-11 Ceria can be not only used as the active components of catalyst,12-14 but also as the support of Pd (and other noble metals) catalysts for the oxidations of CO and propane and automotive emission control.6,15 Since heterogeneously catalytic reaction occurs on the catalyst surface, the crystal planes exposed on the catalyst surface would affect obviously the catalytic performance, especially for the structure-sensitive catalytic reactions.16-20 Recently, ceria nanocrystals with uniform and well defined morphologies were controllably synthesized, the effect of ceria morphology on its catalytic performance was studied. We have reported the catalytic oxidation of 1,2-dichloroethane and ethyl acetate over ceria nanocrystals with well-defined crystal planes.21 Comparing with nanocubes and nano-octahedrons, ceria nanorods (mainly exposed (110) and (100) facets) showed higher activity due to its smaller crystallite size, more oxygen vacancies, higher OSC and oxygen mobility. Aneggi et al. reported that ceria nanocubes show a higher catalytic performance compared with ceria polycrystalline for soot combustion, and the reason is that ceria nanocubes expose highly reactive (100) facets and ceria polycrystalline exposes the less reactive (111) facets.22 Gianvito et al. studied the facet sensitivity of CeO2 for CO oxidation and C2H2 hydrogenation, and found the (100) plane is optimal for CO oxidation and the (111) plane dominates in C2H2 hydrogenation.23 2 ACS Paragon Plus Environment
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Moreover, the morphology effect of the ceria support on the catalytic properties has also been widely studied.24-27 Huang et al. studied the Ru catalysts supported on ceria with well-defined surface planes for total oxidation of chlorobenzene.28 And found ceria nanorods enclosed with (110) and (100) facets are the most suitable support and the effect of shape/crystal plane of the support is a crucial factor to influence the interaction between Ru species and CeO2 supports. The effects of shape and crystal plane of ceria on the reducibility and activity of CuO/CeO2 catalysts were studied. The results showed that CuO supported on CeO2 rods showed a high surface reducibility and activity for NO + CO at low temperatures, because the synergistic interaction between CuO and ceria nanorods was more strong than that between CuO and CeO2 cubes;20 and for N2O decomposition, CuO supported on ceria nanorod showed the best catalytic activity, because the active Cu pahse could easily form on the (110) and (100) planes.29 Wang et al.30 studied the effects of shape and crystal plane of ceria on the reducibility and activity of Ru/CeO2 catalysts for CO2 methanation, and found Ru/CeO2-NCs{100} catalyst exhibites the highest rate as it possesses the highest concentration of surface oxygen vacancy, which depends on the morphology of CeO2 support. For the Au/CeO2 catalyst used in WGS reaction, when Au was supported on ceria nanorods enclosed by (110) and (100) plans, Au species could be easily activated and stabilized, resulting in the increase in its catalytic activity.19 For the supported Pd catalysts, the effect of the support natures on its catalytic performance is often described by the interaction between the support and Pd or PdO species. Cargnello et al. reported the Pd nanocrystals catalyst supported on ceria for CO oxidation,6 and found that CO oxidation is greatly enhanced at the ceria-Pd interface sites and clarified the pivotal role played by the ceria support. Tan et al. found that Pd0 can be readily formed on the (100) facets of CeO2 cubes, and exhibits the highest activity for the low temperature catalytic oxidation of formaldehyde and the great resistance against moisture; Pd oxide dominated on ceria nanorods has a low activity, due to the high defects and oxygen vacancies on ceria nanorods.31 Colussi et al. used the DFT calculation and HRTEM technique to confirm that the Pd/CeO2 catalyst prepared by solution combustion method is consisted of a reconstructed CeO2(110) surface with Pd2+ ions substituted for Ce4+, thus the ordered Pd-O-Ce moieties has the high performance for methane combustion.32 However, the effects of the 3 ACS Paragon Plus Environment
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crystal planes of supports (especially ceria support) on the physicochemical and catalytic properties of active components are barely reported. Herein, we hydrothermally prepared ceria nanorods (CeO2-R), nanocubes (CeO2-C) and nanooctahedrons (CeO2-O) as the support of Pd catalyst. The CO and propane oxidation were used as the model reactions to test the catalytic performances of Pd/CeO2 catalysts. The results show that the structure and chemical state of Pd on ceria are affected by ceria crystal planes: the Pd species on CeO2-R (rods) and CeO2-C (cubes) can form the surficial PdxCe1-xO2-σ solid solution with -Pd2+-O2--Ce4+- linkage, and the PdOx nanoparticles are dominated on the surface of Pd/CeO2-O (octahedrons). CeO2-R exposed (110) and (100) facets has lower formation energy of oxygen vacancies, and Pd/CeO2-R shows the higher catalytic performance for CO oxidation. And Pd supported on CeO2-O exposed less reactive (111) facet shows the excellent reaction rate for propane oxidation.
2. EXPERIMENTAL SECTION 2.1. Catalysts preparation Preparation of ceria nanorods (CeO2-R) and nanocubes (CeO2-C).33 1.736 g of Ce(NO)3·6H2O and 19.2 g of NaOH were dissolved in 10 mL and 70 mL of de-ionized water, respectively. After two solutions were mixed, it was continually stirred for 30 min. Then this mixed solution was transferred into a Teflon-lined stainless steel autoclave, and hydrothermally treated at 373 and 453 K for 24 h to get ceria nanorods (CeO2-R) and nanocubes (CeO2-C), respectively. The formed solids were separated by centrifugation and washed with de-ionized water and ethanol several times, followed by drying at 353 K for 8 h and calcined in air at 673 K for 4 h. Preparation of ceria octahedrons (CeO2-O).34 0.858 g of Ce(NO)3·6H2O and 0.0076 g Na3PO4 were dissolved in 10 mL and 70 mL of de-ionized water, respectively. After two solutions were mixed and it was continually stirred for 30 min, this mixed solution was transferred into a Teflon-lined stainless steel autoclave and hydrothermally treated at 443 K for 10 h. After being cooled to room temperature, the formed solids were separated by centrifugation and washed with 4 ACS Paragon Plus Environment
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de-ionized water and methanol several times, then dried at 353 K for 8 h and calcined in air at 673 K for 4 h. Preparation of the Pd/CeO2 catalysts. Pd/CeO2 was prepared by the wet impregnation method. Typically, the desired amount of Pd(NO3)2 aqueous solution was added dropwise to CeO2 powder, and this slurry was stirred continually for 10 min. After impregnation, the sample was kept at room temperature for 4 h and then dried at 323 K overnight. Finally, it was calcined in air at 673 K for 4 h. The Pd content in the sample was analysed by inductively coupled argon plasma (ICP, TJA IRIS 1000) and shown in Table 1. 2.2. Catalysts characterization Powder X-ray diffraction (XRD) patterns of catalysts were carried out on a Rigaku D/Max-rC diffractometer with Cu Kα radiation (λ = 1.5418Å) operated at 40 kV and 40 mA. The mean crystallite size (D) of sample was calculated by Scherrer equation (D =
kλ β1/2cosθ ) based on the
diffraction peak broadening. Herein, assuming the grain shape is spherical (k = 0.89); β1/2 is the broadening at half the maximum intensity (rad) and θ is the Bragg angle. The surface areas of samples were measured on a Micromeritics ASAP 2400 instrument by the N2 adsorption at 77K and calculated by the Brunauer-Emmett-Teller (BET) method. The samples were degassed at 453K for 12 h before measurement. The scanning electron microscope (SEM) images of samples were obtained on a Hitachi S-3400N microscope operated at 15 kV. The transmission electron microscopy (TEM) images of samples were recorded on a JEOL 2100F electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a VG ESCALAB MK II system equipped with a hemispherical electron energy analyzer. The carbonaceous C1s line (284.6 eV) was used as the reference to calibrate the binding energies (BEs). The Raman spectra were recorded on a Renishaw Raman spectrometer at ambient condition, and the 514 nm line of a Spectra Physics Ar+ laser was used for an excitation. The laser beam intensity and the spectrum slit width were 2 mW and 3.5 cm−1, respectively.
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Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) absorption spectra were recorded on a Nicolet Nexus 670 FT-IR spectrometer equipped with a MCT detector, and the sample cell was fitted with ZnSe windows and a heating chamber can be heated up to 600 °C. The DRIFTS spectra were obtained with a resolution of 4 cm−1 and 64 scans. For CO adsorption, before the testing, the sample was heated at 673K in Ar for 1 h, and then cooled to 323 K; after the feed gas (20 mL/min) of 0.25% CO/Ar was purged through cell for 10 min, 5% O2/Ar (20 mL/min) was used instead of 0.25% CO/Ar. For C3H8 + O2 reaction, before measurement, the sample was heated at 673K in Ar for 1 h, and then cooled to 573 K. After being purged with 0.5% C3H8 + 5% O2/Ar (20 mL/min) for 10 min, the feed gas was changed to 5% O2/Ar (20 mL/min). Temperature-programed reduction (TPR) of the catalyst was carried out on a Micromeritics AutoChem II 2920. For H2-TPR, 50 mg of the sample was used, the heating rate was 10 K/min and the reduction gas was 10% H2/Ar with a flow rate of 40 mL/min. Before measurement, the sample was pretreated at 473 K with 3%O2/He for 30 min, and then cooled to 223 K in He. The hydrogen consumption was monitored by thermal conductivity detector (TCD). For CO-TPR, the reduction gas was 10%CO/He with a flow rate of 40 mL/min. Before measurement, the catalyst (50 mg) was reduced at 473 K with 10%H2/Ar for 30 min and then cooled to 223 K in He. A mass spectrometer (HPR20 QIC) was used to monitor the effluent gas and the signal of CO2 (m/z = 44) was recorded. For C3H8-TPSR, the sample (50 mg) was pretreated with 3%O2/He at 673 K for 30 min, and then cooled to the room temperature. The sample in 10%C3H8/Ar flow (40 mL/min) was heated at 10 K/min. A mass spectrometer (HPR20 QIC) was used to monitor the effluent gas and the MS signal of H2O (m/z = 18), C3H6 (m/z =41), C3H8 (m/z = 43) and CO2 (m/z =44) was recorded. Temperature-programed desorption of O2 (O2-TPD) adsorbed on the sample was carried out on a Micromeritics AutoChem II 2920. The sample (50 mg) was pretreated in 3%O2/He (40 mL/min) at 673 K for 30 min, then cooled to 223 K and kept in 3%O2/He (40 mL/min) for 1 h. After being purged with He for 1h, the sample was heated at a rate of 10 K/min in He of 40 mL/min. 2.3. Evaluation of the catalytic performance
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The kinetic data for the CO oxidation was tested in a fixed-bed reactor at atmospheric pressure, and 10 mg of catalyst diluted with 90 mg inert quartz sand (40-60 mesh) was used. Before the test, the catalysts were pretreated at 200 °C with 10% H2/Ar (40 mL/min) for 30 min. The feed gas was consisted of 1 % CO + 20 % O2/Ar and its flow rate was 50 mL/min. The CO concentration was measured after the catalytic methanation by online gas chromatograph (GC-2060) equipped with a FID, after 60 min of the steady operation at each temperature. The CO conversion (XCO) was calculated by the following equation: XCO =
[CO]in -[CO]out × 100 % [CO]in
where [CO]in and [CO]out are the CO concentration in the inlet and outlet gas, respectively. For propane oxidation, the feed gas was consisted of 0.2 % C3H8 + 2% O2/Ar (100 mL/min) and 20 mg of catalyst diluted with 80 mg inert quartz sand (40-60 mesh) was used. Propane conversion (XC3H8) was calculated by the same equation as CO conversion. The CO (propane) conversion was controlled below 15%. Prior to testing, the effects of the internal diffusion and external diffusion on the reaction have been eliminated by changing the catalyst particle size and feed gas velocity, respectively. For the CO oxidation, the reaction rate, rCO (mol/(gPd·s)), was calculated by the equation of rCO = XCO·VCO/gPd, where XCO is the CO conversion, VCO is the CO gas flow rate (mol/s) and gPd is the mass of Pd (g). For propane oxidation, the reaction rate rC3H8 was calculated by the same equation as the CO oxidation. TOF (s−1) was calculated with TOF = XCO·VCO·NA/N, where NA is Avogadro constant and N is the number of catalytically active sites (N = Nt · DPd). For propane oxidation, TOF was calculated with the same equation as the CO oxidation. The Pd dispersion (DPd) was measured by CO chemical adsorption at 25 °C (CO : Pd = 1 : 1) on a micromeritics AutoChem II 2920 chemical adsorption instrument. The Pd mean particle size (dPd) was calculated by the formula of dPd(nm) = 1.12/DPd,35 which was deduced in Supporting Information. The total length of perimeter of the Pd-Ce interface (I0) was calculated based on the assumption that 7 ACS Paragon Plus Environment
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the Pd particle is hemisphere and Pd-Ce interface has the circular geometry.36,37 DPd, dPd and I0 values of Pd/CeO2 samples are shown in Table 1. Table 1. The Pd content, Pd dispersion (DPd), mean Pd particle size (dPd) and total length of the perimeter of Pd-Ce interface (I0) of Pd/CeO2 samples. Sample
Pd content by ICP (wt.%)
DPd (%)
dPd (nm)
I0 × 10−9 (m/g)
1.0%Pd/CeO2-R
1.0
58.3
1.92
2.71
1.7%Pd/CeO2-R
1.7
45.2
2.48
2.76
3.0%Pd/CeO2-R
3.0
33.6
3.33
2.69
0.5%Pd/CeO2-C
0.5
48.6
2.30
0.94
1.0%Pd/CeO2-C
1.0
38.8
2.89
1.21
1.6%Pd/CeO2-C
1.6
25.5
4.39
0.83
0.5%Pd/CeO2-O
0.5
40.4
2.77
0.65
1.0%Pd/CeO2-O
1.0
24.4
4.59
0.47
1.5%Pd/CeO2-O
1.5
16.1
6.96
0.31
3. Results and discussions 3.1. Structures and morphologies of samples The XRD patterns of CeO2 and Pd/CeO2 samples are shown in Fig. 1. All the samples display a typical cubic fluoride CeO2 crystal phase (JCPDS 34-0394). The mean crystallite size calculated by Scherrer equation is 9.8 nm for CeO2-R, 30.7 nm for CeO2-C and 44.8 nm for CeO2-O (Table 2). After supporting Pd species, there is no any new diffraction peaks corresponding to PdO or Pd phase in their XRD patterns, and the diffraction peaks of CeO2 are hardly changed. A possible reason is that the palladium species dispersed well on the surface or incorporated into the CeO2 lattice to form a solid solution,38 as well as a limitation of detection because of the low Pd loading. As shown in Table 2, after loading 1 wt.%Pd, the BET surface area of CeO2 sample was reduced obviously, and their grain sizes and lattice parameters were slightly decreased. Since the radius of Pd2+ (r = 0.85Å) is similar to radius of Ce4+ (0.92Å), the Pd ions can enter the ceria lattice. Compared with the lattice parameters of ceria support (Table 2), the lattice parameters of Pd/CeO2 samples slightly decreased, which proves that the PdxCe1-xO2-σ solid solution can be formed in the surface or subsurface of ceria. 8 ACS Paragon Plus Environment
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B
Intensity
A
Intensity
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CeO2-O
Pd/CeO2-O
Pd/CeO2-C
CeO2-C
Pd/CeO2-R
CeO2-R
10
20
30
40
50
60
70
10
80
20
30
40
50
60
70
80
2 Theta (degree)
2 Theta (degree)
Fig. 1. XRD patterns of various (A) CeO2 and (B) 1.0%Pd/CeO2 catalysts.
Table 2. The grain size, lattice parameter, and BET surface area (SBET) of CeO2 and Pd/CeO2 samples.
a
Sample
Exposed plane
SBET (m2/g)
Grain size (nm)a
Lattice parameter (nm)
CeO2-R
110, 100
102
9.8
0.5407
CeO2-C
100
25
30.7
0.5405
CeO2-O
111
10
44.8
0.5402
1.0%Pd/CeO2-R
PdO(100), Pd(111)
88
9.1
0.5402
1.0%Pd/CeO2-C
PdO(001), PdO(100)
18
30.2
0.5396
1.0%Pd/CeO2-O
PdO(001), Pd(111)
6
44.3
0.5401
Calculated by Scherrer equation based on the reflection of fluorite CeO2.
As shown in the TEM and SEM images, ceria nanorods exhibit an average width of 11 nm and length of 150−200 nm (Fig. 2a); ceria nanocubes show well-defined and regular cubic shapes with slightly rounded edges and an edge length of 20−40 nm (Fig.2b); ceria nanooctahedrons are the particles with arris length of 30−60 nm and well-defined surfaces (Fig. 2c). The high-resolution TEM images show that CeO2-R has exposed (110) and (100) crystal planes (Fig. 2b), CeO2-C has selectively exposed (100) planes (Fig. 2d) and CeO2-O has selectively exposed (111) planes (Fig. 2f).
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Fig. 2. TEM, HRTEM and SEM images of (a, b) CeO2-R, (c, d) CeO2-C and (e, f) CeO2-O.
The TEM and HRTEM images of Pd/CeO2 catalysts are shown in Fig. 3. The results show that the morphologies of CeO2 samples are hardly changed after loading Pd, and the Pd or PdO nanoparticles (size of 7−9 nm) attached on the surface of ceria (Fig. 3b, d, f), and their lattice fringe are 5.2 Å, 3.0 Å and 2.3 Å, which are corresponding to PdO (001), PdO (100) and Pd (111) facets, respectively. Note that the 7-9 nm of Pd size observed in Fig.3 is only the size of one particle and not the average size with statistically significant. We have tried to make Pd particle size distribution of three Pd/CeO2 samples by TEM, but the Pd/CeO2-O sample distribution can be made only, because Pd has incorporated into the lattice of ceria, especially for CeO2-R and CeO2-C. Therefore, Pd particle size distribution of CeO2-R and CeO2-C cannot be obtained. The average size of Pd on CeO2-O is 4.41 ±
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0.7 nm (Figure S2), which is in agreement with the Pd particle size (4.59 nm, Table 1) calculated by CO chemisorption.
Fig. 3. TEM and HRTEM images of (a, b) 1.0%Pd/CeO2-R, (c, d) 1.0%Pd/CeO2-C and (e, f) 1.0%Pd/CeO2-O.
Fig. 4 presents the XPS spectra of Pd 3d, Ce 3d and O 1s for Pd/CeO2 samples. The chemical states of Pd species on the catalyst surface are obviously shape-dependent. As shown in Fig.4A, all samples exhibit two Pd 3d5/2 peaks at 336.8 and 337.7 eV, which can be assigned to Pd2+ in PdOx particles or clusters located on CeO2 and in solid solution PdxCe1-xO2-σ at the surface and subsurface, respectively.38-41 Therefore, the positively charged Pd species dominated in three samples with two surface phases. For the Pd/CeO2-R and Pd/CeO2-C samples, the Pd ions atomically dispersed in ceria matrix, and the solid solution phase of PdxCe1-xO2-σ dominated on the surface with the
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(-Pd2+-O2--Ce4+-) linkages.41 And for the Pd/CeO2-O catalyst the PdOx particles or clusters were mainly located on the surface of CeO2-O, and the Pd ions existed (as the Ce-Pd-O sites) in the interface of Pd particle and ceria, because of the strong interaction between the Pd nanoparticles and ceria support.42 The results above indicate that the chemical states of Pd species on the surface of CeO2 and the interaction of Pd with ceria are obviously shape-dependent.
B u'''
A Intensity (a.u.)
Pd/CeO2-R
Intensity (a.u.)
Pd/CeO2-C
Pd/CeO2-O
346
344
u''
u'
u u0 v''' v'' v'
Pd/CeO2-R
v v0
Pd/CeO2-C
Pd/CeO2-O
342
340
338
336
920
334
910
900
890
880
Binding energy (eV)
Binding energy (eV)
C Pd/CeO 2-R
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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α
β
Pd/CeO 2-C
Pd/CeO 2-O
536
534
γ 532
530
Binding energy (eV)
528
526
Fig. 4. XPS spectra of (A) Pd 3d, (B) Ce 3d and (C) O 1s for the 1.0%Pd/CeO2 samples.
The Ce 3d5/2 and 3d3/2 XPS spectra of the three samples are shown in Fig. 4B. XPS spectra of the Ce 3d core level can be resolved into ten groups.43,44 The five main 3d5/2 features are denoted as v0 (881.7 eV), v (882.9 eV), v’ (885.7 eV), v’’ (889.0 eV) and v’’’ (897.6 eV) components, and the five 3d3/2 features are assigned to u0(899.1 eV), u(901.2 eV), u’(903.0 eV), u’’(907.7 eV) and u’’’(916.8 eV), respectively.44 The amount of surface Ce3+ ion can be determined by CCe3+ = Ce3+/(Ce3+ + Ce4+), where Ce3+ = v0 + v’ + u0 + u’ and Ce4+ = v + v’’ + v’’’ + u + u’’ + u’’’. The Ce3+ concentration can be correlated to the oxygen vacancies on the ceria surface. Among three catalysts, CCe3+ of
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ACS Catalysis
Pd/CeO2-R was the highest value of 27.1%, then Pd/CeO2-C was 26.4% and Pd/CeO2-O was only 18.7%, which agree with the order of formation energy of oxygen vacancies on the ceria crystal facets.45 The O 1s XPS spectra of the three samples are shown in Fig. 4C, in which there are three states of the surface oxygen. The peak at 529.3 eV can be attributed to lattice oxygen (Oα), the peak at 531.2 eV can be assigned to oxygen vacancies (Oβ), and the peak at 533−534 eV can be attributed to other weakly bound oxygen species (Oᵧ). The Oβ/Oα ratio can be used to evaluate the oxygen vacancies concentration. The Oβ/Oα ratio ranks in the following order of Pd/CeO2-R (38.6%) > Pd/CeO2-C (33.1%) > Pd/CeO2-O (24.3%), which is agreed with the results obtained from the Ce 3d XPS spectra. Although the γ peak is broader, the Oᵧ/Oα ratio can be still estimated: Pd/CeO2-O (18.1%) > Pd/CeO2-C (12.9%) > Pd/CeO2-R (7.2%), which shows that Pd/CeO2-O has the highest weakly bound oxygen concentration. Fig. 5 shows the Raman spectra of CeO2 and Pd/CeO2 samples. For the CeO2 samples, four peaks were observed in their Raman spectra, a strong peak at 461 cm−1 and three weak peaks at 260, 598 and 1174 cm−1, which correspond to fluorite F2g mode, second order transverse acoustic (2TA) mode, defect induced (D) mode and second order longitudinal optical (2LO) mode, respectively.21,46 After loading Pd, new peaks were observed at 654 and 835 cm−1 in Raman spectra of Pd/CeO2-R and Pd/CeO2-C samples, and another new peak at 964 cm−1 in the Raman spectrum of the Pd/CeO2-O sample was also observed. The peak at ~654 cm−1 can be ascribed to the vibration mode of Pd-O bond,47,48 and the peaks at 835 and 964 cm−1 can be attributed to neither CeO2 nor Pd species, probably resulting from the interaction between Pd and CeO2. Huang et al. studied the interaction between Ru and CeO2 by Raman spectroscopy and assigned these new bands to the formation of Ru-O-Ce bond.28 Moreover, relating with their Pd 3d XPS spectra mentioned above, we can find, there are two different interactions of Pd and ceria in Pd/CeO2 samples. Therefore, it is reasonable to attribute these new bands to two different kinds of Pd-O-Ce bond. Peak fitting analysis was carried out, and the ratio of I(1174+598)/I(461) that reflects the intrinsic defect concentration was calculated. The I(1174+598)/I(461) ratio of bare CeO2 nanocrystals is decreased in the 13 ACS Paragon Plus Environment
ACS Catalysis
order of CeO2-R (0.09) > CeO2-C (0.06) > CeO2-O (0.04), indicating that ceria nanorods have more oxygen defects than others.49 After Pd was added on CeO2, the ratio of I(1174+598)/I(461) was increased notably and ranked in the order of Pd/CeO2-R (0.21) > Pd/CeO2-C (0.18) > Pd/CeO2-O (0.11). These results show that the presence of Pd species can create oxygen defect on ceria. Combining the results of XPS, XRD and Raman spectroscopy, we can conclude that the presence of Pd species can result in the increase in ceria oxygen defect, and the Pd ions in PdxCe1-xO2-σ solid solution can easily create oxygen vacancies than PdO species on the surface. 461 Intensity (a.u.)
B
Intensity (a.u.)
A
CeO2-O
CeO2-C CeO2-R
200 400 600 800 1000 1200 1400 -1
260
Raman shift (cm )
598
1174
CeO2-O
Intensity (a.u.)
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Pd/CeO2-C Pd/CeO2-O
588 654
Pd/CeO2-R
200 400 600 800 1000 1200 1400 -1 Raman shift (cm )
834
964
1168
Pd/CeO2-O
CeO2-C CeO2-R
200
400
600
800
1000
1200
-1
Raman shift (cm )
1400
1600
Pd/CeO2-C Pd/CeO2-R
200
400
600
800
1000
1200
1400
1600
-1
Raman shift (cm )
Fig. 5. Raman spectra of (A) CeO2 and (B) 1.0%Pd/CeO2 samples.
3.2. Redox properties of the catalysts H2-TPR was used to investigate the reducibility of CeO2 and Pd/CeO2 catalysts, and the results are shown in Fig. 6. In the TPR curves of CeO2 samples, the reduction peak at ~480 °C can be attributed to the reduction of surface oxygen. The hydrogen consumption was calculated and ranked as follows: CeO2-R (1046 µmol/g) > CeO2-C (525 µmol/g) > CeO2-O (43 µmol/g). Ceria-R exhibits the highest reducibility, which is associated with its high oxygen mobility and high specific surface area. The presence of Pd on CeO2 nanocrystals can promote significantly the surface reduction of CeO2 supports. For Pd/CeO2-R, the reduction peak was located at 82 °C (top temperature), and could be attributed to the reduction of PdO species,50 in which the amount of H2 taken up was ~438 µmol/g, and much greater than 82 µmol/g required from PdO to Pd0. The additional H2 consumption is due to the 14 ACS Paragon Plus Environment
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reduction of the surface oxygen of CeO2 around Pd species by the hydrogen spillover from Pd species onto the support. The broad peak at 390 °C should be associated with the reduction of ceria surface oxygen far away Pd species. This reduction temperature was decreased drastically compared with CeO2-R, indicating that this surface oxygen is more active than that on the CeO2-R support, that is to say, the reducibility of surface oxygen is enhanced by the incorporation of Pd. For Pd/CeO2-C, its reduction peak shifted to lower temperature (~0 °C), and the H2 consumption of this peak was ~96 µmol/g and should be a reduction of PdO to Pd0. Like the Pd/CeO2-C sample, there is a reduction peak at ~0 °C in the TPR curve of Pd/CeO2-O, but its H2 consumption is only 32 µmol/g and less than 82 µmol/g required from PdO to Pd0, which means the Pd species are not completely reduced at ~ 0 °C. And the additional peak at 200 °C of Pd/CeO2-O should be a reduction of the Pd species strongly interacted with CeO2-O. The results above show that, the promotional effect of Pd on the surface reduction of CeO2 in Pd/CeO2 depends on the morphology of the CeO2 support; the higher reducibility of Pd/CeO2-R and Pd/CeO2-C compared to Pd/CeO2-O may be due to a stronger interaction between Pd and ceria (as a formation of PdxCe1-xO2-σ solid solution) and/or the higher reducibility of pure CeO2 rods and cubes, and the peaks of hydrogen consumption of Pd/CeO2-C and Pd/CeO2-O shifted at lower temperature, probably due to the presence of surface metallic Pd.
A Intensity (a.u.)
B
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
CeO2-R
Pd/CeO2-R
Pd/CeO2-C
CeO2-C
Pd/CeO2-O
CeO2-O 0
100
200
300
400
500
600
700
0
100
o
Temperature ( C)
200
300
400
500
600
700
o
Temperature ( C)
Fig. 6. H2-TPR curves of (A) CeO2 and (B) Pd/CeO2 catalysts.
To determine the mobility of oxygen species, O2-TPD-MS was tested and the results are shown in Fig. 7. Generally, the adsorbed oxygen changes in the following processes: O2(ad) → O2−(ad) →
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O−(ad) → O2−(lattice).51 In the O2-TPD spectra of the CeO2-C and CeO2-O samples, the oxygen desorbed peak at ~125 °C should be attributed to the weakly bound surface oxygen, the desorption peaks at 200−500 °C can be related to the chemically adsorbed oxygen species on the vacancies, and the desorption peak at > 500 °C can be attributed to bulk lattice oxygen. After supporting Pd on CeO2-C and CeO2-O, their O2-TPD spectra are hardly changed, except the peak attributed to the weakly bound oxygen shifted to ~112 °C. For CeO2-R, the O2-TPD curve is different from these of CeO2-C and CeO2-O, and its first desorption peak of oxygen is located at 370 °C. After supporting Pd on CeO2-R, a new desorption peak at ~260 °C can be observed, which should not be attributed to the decomposition of PdOx but the chemically adsorbed oxygen. Therefore, the amount of chemically adsorbed oxygen species could be a reflection of the oxygen vacancies on the surface or subsurface of sample and can be enhanced by the presence of Pd especially on CeO2-R. In the O2-TPD spectra of three Pd/CeO2 samples, the peak intensity at 200−500 °C for Pd/CeO2-R was much higher than others, and Pd/CeO2-O has the strongest desorption peak of bulk lattice oxygen.
Pd/CeO2-O
MS Signal (m/z=32)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Pd/CeO2-C Pd/CeO2-R
CeO2-O CeO2-C CeO2-R
0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 7. O2-TPD curves of CeO2 and 1.0%Pd/CeO2 catalysts.
The CO-TPR spectra of Pd/CeO2 catalysts are shown in Fig. 8. Before experiment, the samples were reduced at 200 °C in 10%H2/Ar for 30 min and then cooled to −50 °C in He to remove physisorption oxygen on the surface. An obvious CO2 desorption peak at 110 °C could be observed on the Pd/CeO2-R catalyst, which can be ascribed to the reduction of the oxygen species of -Pd-O-Ce- in an interface between PdO and CeO2.52 The oxygen species in Pd/CeO2-C and Pd/CeO2-O samples were hardly reduced by CO. As shown in Fig. 8, the reducibility of the surface 16 ACS Paragon Plus Environment
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oxygen species by CO was decreased in order of Pd/CeO2-R >> Pd/CeO2-C > Pd/CeO2-O, which is consistent with the order of the oxygen mobility and storage of ceria crystal facets: (110) > (100) > (111).22 These results above indicate that the surface oxygen can be supplied from bulk ceria, especially on Pd/CeO2-R catalyst, and oxygen mobility of the support can enhance its catalytic performance for the CO oxidation.
MS Signal of CO2 (m/z=44)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Pd/CeO2-R Pd/CeO2-C Pd/CeO2-O
-50
0
50
100 o
150
200
Temperature ( C)
Fig. 8. CO2 desorption curves in the CO-TPR process of 1.0%Pd/CeO2 catalysts.
The C3H8-TPSR was also used to investigate the reducibility of surface oxygen in the Pd/CeO2 samples (Fig. 9). The Pd/CeO2 catalyst (50 mg) was pretreated with 3%O2/He at 400 °C for 30 min, and then cooled to the room temperature. The Pd/CeO2 catalyst was heated in 10%C3H8/Ar flow (40 mL/min) at 10 °C/min, and the MS signals of H2O (m/z = 18), C3H6 (m/z =41), C3H8 (m/z = 43) and CO2 (m/z =44) were recorded. Note that the CO2 and C3H8 have the same mass number. Hence the signal of m/z = 43 was used to detect C3H8, and after the signal of m/z =44 subtracting that of m/z = 43, the desorption amount of CO2 could be observed. The C3H8-TPSR for CeO2-R, CeO2-C and CeO2-O was also tested, and the results (Fig. S3) show that no surface reaction product was obtained during the testing at 100−500, which shows that CeO2 is inactive in the atmosphere of propane at < 500 °C. As shown in Fig. 9A, a negative peak at 193 °C in the curves of m/z = 44 and 43 was observed for the Pd/CeO2-R sample, and the peak at 220 °C in the curves of m/z=41 and 18 was also found, meaning the partial oxidation of propane occurred on the surface at this temperature. Based on the strong peak of propene (m/z = 41) at 330 °C and no signal of H2O at m/z = 18, the propane dehydrogenation reaction was taken place. Similar results 17 ACS Paragon Plus Environment
ACS Catalysis
could be observed in the C3H8-TPSR spectra of Pd/CeO2-C sample in Fig. 9B, but these peaks shifted to lower temperatures. For Pd/CeO2-O sample, as shown in Fig. 9C, a strong peak (m/z =44) and a negative peak (m/z = 43) were found at ~157 °C, indicating a total oxidation of propane occurred. And the peak (m/z = 41) at ~324 °C is the most small among three Pd/CeO2 samples. Compared with the O2-TPD spectra, this desorption peak of water corresponds to the consumption of chemically adsorbed oxygen species on the vacancies over Pd/CeO2-R and the consumption of weakly bound oxygen on the surface of Pd/CeO2-C and Pd/CeO2-O, indicating that the weakly bound oxygen can also react with propane. Therefore, both the weakly bound oxygen and chemically adsorbed oxygen species play an important role in the propane oxidation. A
B
1%Pd/CeO2-R
1%Pd/CeO2-C m/z = 44
m/z = 44 m/z = 43 193 330
m/z = 41
220
m/z = 43
MS Signal
MS Signal
161 300 m/z = 41
m/z = 18
m/z = 18
100
200
300
400
o
100
500
200
Temperature ( C)
300
o
400
500
Temperature ( C)
C
1%Pd/CeO2-O m/z = 44
MS Signal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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m/z = 43 157 324 m/z = 41 m/z = 18
100
200
300
o
400
500
Temperature ( C) Fig. 9. C3H8-TPSR curves on (A) 1.0%Pd/CeO2-R, (B) 1.0%Pd/CeO2-C and (C) 1.0%Pd/CeO2-O catalysts.
3.3. Effects of ceria facet on the catalytic activities of Pd/CeO2 The catalytic performances of Pd/CeO2 catalysts were examined for the CO oxidation and propane oxidation, and the results are shown in Figs. 10 and 11 and their reaction rates, apparent activation energies (Ea) and TOFs are listed in Table 3. For the CO oxidation (Fig. 10), the catalysts were 18 ACS Paragon Plus Environment
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pretreated at 200 °C in 10% H2/Ar (40 mL/min) for 30 min. The catalytic activities of Pd/CeO2 samples at 50 °C are ranked in the order of Pd/CeO2-R (rCO = 2.07 × 10−4 molCO gPd−1 s−1) > Pd/CeO2-C (2.30 × 10−5) > Pd/CeO2-O (3.00 × 10−6), which is consistent with the order of Pd-O-Ce reducibility and surface oxygen mobility for three samples. Therefore, among three CeO2 supports, ceria-R is the best support for Pd/CeO2 catalysts for CO oxidation, since its weak surface Ce-O bond and strong reducibility. TOF of Pd/CeO2-R catalyst for the CO oxidation at 50 °C is 3.78 × 10−2 s−1, and is higher than that of Pd/CeO2-C (6.40 × 10−3 s−1) and Pd/CeO2-O (1.24 × 10−3 s−1). Arrhenius plots of ln r verses 1/T for Pd/CeO2 catalysts are showed in Fig. 10B, and the apparent activation energy (Ea) for Pd/CeO2-R, Pd/CeO2-C, and Pd/CeO2-O is 40.4, 42.6 and 45.4 kJ/mol, respectively. A -1
-1
Pd/CeO2-R
Pd/CeO2-R
10
7 Pd/CeO2-C
6 5
7
-1
15
5 0
B
-1
20
ln(r x 10 )(molCO gPd s )
8
5
25
Rate (molCOgPd s ) x10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Pd/CeO2-C
20
30
40
50
Pd/CeO2-O
60
4
Pd/CeO2-O
3
70
2.9
o
Temperature ( C)
3.0
3.1
3.2
3.3
3.4
1000/T(K)
Fig. 10. The catalytic activities of (A) Pd/CeO2 samples for the CO oxidation and (B) ln r as a function of 1/T. (The feed gas was 1% CO + 20% O2/Ar, and GHSV=15000 mL h-1 g-1).
For the propane combustion (Fig. 11), the catalytic activities of Pd/CeO2 samples at 300 °C are ranked in the order of Pd/CeO2-O (rC3H8 = 8.08 × 10−5 molCO gPd−1 s−1) > Pd/CeO2-C (9.43 × 10−6) > Pd/CeO2-R (3.83 × 10−6), which is different from the situation above for the CO oxidation. Relating to the O2-TPD and C3H8-TPSR results, we can see, both weakly bound oxygen and chemically absorbed oxygen can take part in the reaction, indicating that the role of surface oxygen mobility of the CeO2 support may not be crucial for propane oxidation, but rather the chemical adsorption and activation of propane may be important for propane oxidation. TOF of the Pd/CeO2-O catalyst for the propane oxidation at 300 °C is 3.52 × 10−2 s−1, and is higher than that of Pd/CeO2-C (2.59 × 10−3 s−1) and Pd/CeO2-R (6.97 × 10−4 s−1). Arrhenius plots of ln r verses 1/T for Pd/CeO2 catalysts are showed 19 ACS Paragon Plus Environment
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in Fig. 11B, and the apparent activation energy (Ea) for Pd/CeO2-R, Pd/CeO2-C, Pd/CeO2-O was 58.2,
12
10
-1
Pd/CeO2-O
8 6 4 Pd/CeO2-C
2 0
280
B
-1
A
9
-1 -1
12
ln(r x 10 )(molC3H8 gPd s )
5
53.6 and 49.1 kJ/mol respectively, in which Ea over the Pd/CeO2-O catalyst is the lowest.
Rate (molC3H8 gPd s ) x 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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290
300
Pd/CeO2-R
310
320
330
11
Pd/CeO2-O
10 Pd/CeO2-C
9 Pd/CeO2-R
8
1.64 1.66 1.68 1.70 1.72 1.74 1.76 1.78 1.80 1.82
o
Temperature ( C)
1000/T(K)
Fig. 11. The catalytic activities of (A) Pd/CeO2 samples for the C3H8 oxidation and (B) ln r as a function of 1/T. (The feed gas was 0.2% C3H8 + 2% O2/Ar, and GHSV=30000 mL h-1 g-1). Table 3. Catalytic activities of Pd/CeO2 catalysts for CO and C3H8 oxidation. CO oxidation at 50 °C
C3H8 oxidation at 300 °C
Catalyst
r ×105 (mol gPd−1 s−1)
Ea (kJ/mol)
TOF × 103 (s−1)
r ×106 (mol gPd−1 s−1)
Ea (kJ/mol)
TOF × 103 (s-1)
1.0%Pd/CeO2-R
20.7 ± 0.8
40.4 ± 2.3
37.8 ± 0.4
3.83 ± 0.04
58.2 ± 4.8
0.697 ± 0.025
1.0%Pd/CeO2-C
2.3 ± 0.06
42.6 ± 2.7
6.40 ± 0.5
9.43 ± 0.09
53.6 ± 3.1
2.59 ± 0.06
1.0%Pd/CeO2-O
0.3 ± 0.03
45.4 ± 2.6
1.24 ± 0.4
80.8 ± 0.02
49.1 ± 1.4
35.2 ± 0.1
It is well known that the particle size of the noble metal (NM) components can influence the catalytic performances of the supported NM catalysts.6,36,37 Hence, the effect of the Pd particle size (dPd) for the Pd/CeO2 catalysts with different Pd loadings on the specific reaction rate (R), per meter of perimeter of the Pd-Ce interface, was investigated for CO oxidation at 50 °C and propane oxidation at 300 °C. The specific rate (mol m−1 s−1) is based on the total length of the perimeter of the Pd-Ce interface (I0) (Table 1), which assumed that Pd particles are hemisphere and Pd-Ce interface has the circular geometry. As shown in Fig. 12A, the effect of the Pd particle size (dPd) on the specific reaction rate of CO conversion at 50 °C, the specific rate (RCO) ranks in the order of Pd/CeO2-R > Pd/CeO2-C > Pd/CeO2-O and is linearly decreased with an increase in the Pd particle size, which shows that CO
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oxidation over Pd/CeO2 catalysts is dependent on Pd particle size and the reactivity of active sites based on the Pd-Ce interface depends on the Pd particle size.6 For the propane oxidation, the contrary results to the CO oxidation were found in Fig. 12B. RC3H8 of Pd/CeO2-O is largest, and Pd/CeO2-R is smallest. And the RC3H8 is increased with an increase in the Pd particle size, which also suggest that the reactivity of active sites based on the Pd-Ce interface is related to the Pd particle size. Therefore, both CO oxidation and propane oxidation over the Pd/CeO2 catalysts are size dependent for the Pd particles. For the same reaction, the morphology of support ceria would affect obviously the catalytic activity of the Pd particles. For the different reactions, the effect of Pd particle size on the specific reaction rate (R) is different or may be contrary.
13
A Pd/CeO2-R
B 160
Pd/CeO2-C
10 Pd/CeO2-O
120 80
3
dPd(nm)
4
5
6
7
Pd/CeO2-C
40 0
2
Pd/CeO2-O
-1
-1
-1
-1
RC3H8 (mol m s ) x 10
15
200
100
RCO (mol m s ) x 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Pd/CeO2-R 2
3
4
5
dPd(nm)
6
7
Fig. 12. Effect of the Pd particle size (dPd) on the specific reaction rate (R, per meter of perimeter of the Pd-Ce
interface) of CO or propane conversion over the Pd/CeO2 catalysts for (A) CO oxidation at 50 °C and (B) propane oxidation at 300 °C. For CO oxidation, the samples were pretreated at 200 °C with 10% H2/Ar (40 mL/min) for 30 min before the test.
For comparison, the catalytic activities of Pd/CeO2 catalysts reported in literatures for CO (and propane) oxidation are also showed in Table 4 (and Table 5). As shown in Tables 4 and 5, the effect of ceria crystal plane on the catalytic performance of the Pd/CeO2 catalyst is apparent for CO and propane oxidation. Meanwhile, the catalysts stabilities were tested and showed in Fig.S1. For CO oxidation, the deactivation was found on all the three catalysts, which in agreement with the reported results.53 However, for propane oxidation the catalytic activities of the three samples are hardly changed after 30 h of reaction at 300 °C.
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Table 4. Catalytic activities of Pd/CeO2 catalysts for CO oxidation at low temperature. React.temp. (°C)
rCO (mmol g-1Pd s-1)
TOF (s-1)
Ref.
1%CO-20%O2/Ar
50
0.21
0.04
This work
0.97%Pd/CeO2-C
1%CO-20%O2/Ar
50
0.02
0.006
This work
0.99%Pd/CeO2-O
1%CO-20%O2/Ar
50
0.003
0.001
This work
0.2%PdO/CeO2
0.95%CO-1.75%O2/N2
140
0.11
0.036
48
2.5%Pd/CeO2 1%PCS-5 (PdO-CeO2/SiO2) 0.5%Pd/CeO2 (small Pd partcles)
1%CO-20%O2/N2
20
-
0.024
54
1%CO-1% O2/N2
150
-
0.03
55
1%CO-4%O2/N2
80
-
~0.01
6
Catalyst
Reaction gases
1.00%Pd/CeO2-R
Table 5. Catalytic activities of Pd/CeO2 catalysts for propane combustion. GHSV (mL h-1 gcat-1)
T50 (°C)
Ref.
1%C3H8-5%O2/Ar
60000
401
This work
0.97%Pd/CeO2-C
1%C3H8-5%O2/Ar
60000
336
This work
0.99%Pd/CeO2-O
1%C3H8-5%O2/Ar
60000
268
This work
0.5%Pd/CeO2
0.5%C3H8-5%O2/N2
15000
~375
56
1.01%Pd/CeO2(Kanto Chem. Co.)
1 mol%C3H8-5 mol%O2/He
60000
~255
15
0.98%Pd/CeO2(Rhodia chimie)
1 mol%C3H8-5 mol%O2/He
60000
~345
15
Catalyst
Reaction gases
1.00%Pd/CeO2-R
Fig. 13 shows the DRIFT spectra of CO adsorbed on three Pd/CeO2 samples at 50 °C. The bands at 2300‒2400 cm‒1 are associated to the absorption vibration of gas CO2, and the bands at 1800‒2300 cm‒1 are associated to the CO adsorption. For the Pd/CeO2-R sample, the adsorption band at 2090 and 2117 cm‒1 can be associated to the IR absorption of CO linearly adsorbed on Pd0 and Pd+, respectively. For the Pd/CeO2-C sample, there were three kinds of absorption band of CO adsorbed at 2100, 1957 and 1851 cm‒1, which are corresponding to the linear (Pd0-CO), isolated bridged (Pd0-CO-Pd0) and triply bridged ((Pd0)2-CO-Pd0) carbonyls, respectively.57 For the Pd/CeO2-O catalyst, there were two kinds of CO adsorption band at 2096 and 2173 cm−1, which are ascribed to the CO linearly adsorbed on Pd0 and Pd2+, respectively. These results above show that CO mainly adsorbed on Pd0, and Pd2+ ions can be readily reduced by CO at 50 °C. The fact that the bands of CO adsorbed on Pd2+ can be observed in the IR spectra of Pd/CeO2-O, suggests that the Pd2+ ions are more stable on the (111) facet of CeO2-O, which are consistent to the H2-TPR results.
22 ACS Paragon Plus Environment
8 6
Pd/CeO2-R
0.05
2090 2117
1583 1643
1025
Kubelka-Munk
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Fig. 13. DRIFT spectra of Pd/CeO2-R(a), Pd/CeO2-C(b) and Pd/CeO2-O(c) at 50 °C in the feed gas (20 mL/min) of 0.25% CO/Ar (1), and then 5% O2/Ar (2) with the time (min).
It is well-known that, the absorption peaks of carbonate and related species are located at 1000~1800 cm−1. For the Pd/CeO2-R sample, the bands of chemisorbed CO at 1025, 1267 and 1635 cm−1 fit to the formed bidentate carbonate, and the bands at 1396 and 1583 cm−1 fit to the unidentate carbonate. Like the Pd/CeO2-R catalyst, the unidentate carbonate species (bands at 1391 and 1581 cm−1) are present in the IR species of the Pd/CeO2-C sample. For the Pd/CeO2-O sample, only traces of unidentate carbonate (band at 1566 cm−1) are observed, which are different from the situations of 23 ACS Paragon Plus Environment
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Pd/CeO2-R and Pd/CeO2-C samples. These results show that carbonate species are more easily formed on the (110) and (100) facets of CeO2 than on less reactive (111) facet of CeO2. It has been well accepted that the CO oxidation on Pd/CeO2 catalyst followed the Mars-van Krevelen mechanism:17,48 (1) chemisorption of CO on reduced Pd, (2) the migration of the chemisorbed CO to the interface of Pd and CeO2, (3) reaction between the chemisorbed CO at interface and lattice oxygen, and (4) refill of oxygen vacancies by gas oxygen. To further investigate the role of surface carbonate species in the CO oxidation, CO was shut down after 10 min of CO flowing, and oxygen was purged instead of CO (Fig. 13). The results show that the IR absorption peaks at 1000 ~1800 cm−1 were hardly changed, that is to say, most of carbonate species are stable and inert during the reaction, and CO may directly react with lattice oxygen to form gaseous CO2, which means the mobility of lattice oxygen and defect sites of ceria support would be crucial factors for CO oxidation. Therefore, we can easily understand the reason that Pd/CeO2-R and Pd/CeO2-C have the high reaction rates for CO oxidation, although carbonate species are easily formed on their surfaces. The DRIFTS spectra of propane oxidation on ceria supports at 300 °C are showed in Fig. 14. It can be found that the absorption bands at 2966 and 2929 cm−1 are the characteristics of the C-H vibration of gaseous propane.58,59 The band at ~2840 and 2721 cm−1 can be assigned to the stretching vibrations of C-H bond, which result from the CH2(ads) and CH3(ads) species,60,61 and show that propane can adsorbed on CeO2 especially on CeO2-R and CeO2-C at 300 °C. The bands at 1548 and 1355 cm−1 fit to the asymmetric and symmetric stretching vibrations of carboxylate groups, and the bands at 1371 cm-1 can be attributed to δ(CH).61-63 These absorption peaks at 1350~1550 on CeO2-C are weaker than these on CeO2-R. For the CeO2-O support, only trace of carboxylate and other species was detected in its DRIFTS spectra, due to its high formation energy of oxygen vacancy and strong Ce-O bond on its (111) facet. The IR vibration absorption peaks of gas CO2 at 2300‒2400 cm‒ 1
were not found among the three ceria samples, which mean the adsorbed propane cannot be totally
oxidized to CO2 at 300 °C over ceria support. After 10 min of the C3H8 + O2 reaction, C3H8 was shut down, that is, only 5% O2/Ar passed through the IR cell (Fig. 14). The results show that the intensity 24 ACS Paragon Plus Environment
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of the all peaks decreased rapidly or gradually for CeO2-O and CeO2-C, and only the absorption peaks on CeO2-R were hardly changed even after 30 min, which show that there is a strong interaction between adsorbed propane and ceria support. This is attributed to the more oxygen vacancies and coordinatively unsaturated sites on ceria nanorods. CeO2-R
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Fig. 14. DRIFT spectra of CeO2-R(a), CeO2-C(b) and CeO2-O(c) at 300 °C in the feed gas (20 mL/min) of 0.5% C3H8 + 5% O2/Ar (1), and then 5% O2/Ar (2) with the time (min).
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Fig. 15. DRIFT spectra of 1.0%Pd/CeO2-R(a), 1.0%Pd/CeO2-C(b) and 1.0%Pd/CeO2-O(c) at 300 °C in the feed gas (20 mL/min) of 0.5% C3H8 + 5% O2/Ar (1), and then 5% O2/Ar (2) with the time (min).
After supporting Pd on CeO2, the DRIFT spectra of three 1.0%Pd/CeO2 samples during the surface reaction of C3H8 + O2 at 300 °C are showed in Fig. 15. For the Pd/CeO2-R catalyst, the bands at 1541, 1353 and 1419 cm−1 can be attributed to the vibration absorptions of carboxylate groups,61,62 and the bands at 1288 cm−1 assigned to the CH= deformation vibration.61 In the IR spectra of the Pd/CeO2-C sample, besides these peaks mentioned above, the band at ~3650 cm−1 can be assigned to the terminal Ce-OH groups.61 The band at 1623 cm−1 resulted from the C=C stretching, and the bands at 1457 and 26 ACS Paragon Plus Environment
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1384 cm−1 are associated to the asymmetric and symmetric –CH3 vibration, respectively.62 For the Pd/CeO2-O sample, no obvious absorption peak was found at 1250-1550 cm−1, that is to say, the carboxylate species are unstable on Pd/CeO2-O. The peaks attributed to CO2 at 2300‒2400 cm‒1 can found on the Pd/CeO2-C and Pd/CeO2-O catalysts, and on the Pd/CeO2-R only very small peaks existed, meaning that the propane can be totally oxidized over these Pd/CeO2 catalysts at 300 °C, and the activation of propane on Pd is the crucial factor for total oxidation of propane, compared with the DRIFTS spectra of propane oxidation on ceria supports. After 10 min of the reaction of C3H8 + O2, C3H8 was shut down and only 5% O2/Ar passed through the IR cell. As shown in Fig. 15, the intensity of carboxylate (at 1541 cm−1 and 1353 cm−1) and other related species (at 1457 cm−1 and 1288 cm−1) decreased gradually on Pd/CeO2-R and Pd/CeO2-C with the reaction time, indicating that they may be the intermediate species during propane oxidation, and can be removed from the surface immediately once propane was removed in the feed gas. Note that there are the absorption peaks of gas CO2 at 2300‒2400 cm‒1 on Pd/CeO2-C and Pd/CeO2-O, but not on Pd/CeO2-R, which show that the propane adsorbed species (carboxylates etc.) on Pd/CeO2-C and Pd/CeO2-O was total oxidized to CO2 and the one on Pd/CeO2-R was not oxidized to CO2 but rather desorbed from the surface. Therefore, the possible reaction mechanism of propane oxidation over Pd/CeO2 can be described as follows: (1) the activation and dissociated adsorption of propane on the surface of Pd or ceria support, (2) the migration of the fragments (as CH2- ,CH3- species) adsorbed to the Pd-Ce interface, (3) the reaction between the fragments and adsorbed oxygen on the Pd-Ce interface, (4) the fast decomposition of carboxylate adsorbed, and (5) the continual formation of adsorbed oxygen on CeO2 from gases oxygen. A similar mechanism was also put forward over other catalyst systems.56,64 Since ceria supports possess different morphologies and crystal planes, the properties of Pd/CeO2 catalysts for propane oxidation are affected by the crystal planes of ceria. For instance, the formation energy of oxygen vacancy is varied in the order of (110) < (100) < (111) for ceria,65 because the Ce-O bonds are less relaxing on the (111) facet. Therefore, the carboxylates and other related species on the (111) facet are unstable, and the high catalytic performance of Pd/CeO2-O for propane 27 ACS Paragon Plus Environment
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oxidation may be associated with less oxygen defects or vacancies on the surface of CeO2-O support and the easy decomposition of carboxylates and desorption of CO2 on the catalyst. Although ceria nanorods have the highest oxygen mobility and high OSC, carboxylates or other adsorbed species on the surface of CeO2-R support were hardly removed, thus may hinder the reaction. It was reported that the C-H cracking is the rate-determining step in propane oxidation and the C-H bond breaking ability can be enhanced by the presence of Pd particles on ceria support.56 The defects and oxygen vacancies on ceria nanorods are in favor of the presence of Pd2+ species and the formation of PdxCe1-xO2-σ solid solution with -Pd2+-O2--Ce4+- linkage. The Pd/CeO2-O catalyst has the highest ratio of PdO/PdxCe1-xO2-σ among three catalysts, which mean more active sites existed on Pd/CeO2-O sample. Therefore, the Pd/CeO2-O catalyst has the more ability for breaking the C-H bond of propane than Pd/CeO2-R.
4. CONCLUSIONS In summary, three different shapes of nano-ceria were prepared and Pd was deposited on them by the wet impregnation method. It was found that the morphology of the ceria support strongly influences the catalytic performances of Pd/CeO2 catalysts. Ceria nanorods (CeO2-R) is the most suitable support of Pd/CeO2 catalysts for CO oxidation, and the Pd/CeO2-O (nanooctahedrons) catalyst showed the excellently catalytic performance for propane oxidation. The inverse structure sensitivity of ceria support for CO and propane oxidation can be attributed to the nature of the exposed facets and to the different interactions of Pd-O-Ce in the interface between the CeO2 and Pd crystallites. Ceria nanorods (CeO2-R) mainly enclosed by (110) and (100) facets, and has the most low formation energy of oxygen vacancy, strong reducibility and high surface oxygen mobility among three ceria supports. As shown in XPS spectra, two types of Pd species exist on the surface of Pd/CeO2 catalysts: PdO nano-crystallites and PdxCe1-xO2-σ solid solution. The Pd2+ ions on CeO2-R and CeO2-C (nanocube) mainly formed PdxCe1-xO2-σ solid solution with -Pd2+-O2--Ce4+- linkage, and could be readily reduced by CO at ambient temperature and are more stable on the (111) facet of 28 ACS Paragon Plus Environment
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CeO2. Meanwhile, the presence of Pd species can result in the increase in ceria oxygen defects, and the Pd ions in PdxCe1-xO2-σ solid solution can easily create oxygen vacancies than PdO species. For the CO oxidation, TOF of the Pd/CeO2-R catalyst was 3.78 × 10−2 s−1 at 50 °C and higher than that of Pd/CeO2-C (6.40 × 10−3 s−1) and Pd/CeO2-O (1.24 × 10−3 s−1), and its Ea was 40.4 kJ/mol. Therefore, the mobility of lattice oxygen and oxygen defects of Pd/CeO2-R is crucial for CO oxidation. The PdOx nanoparticles were dominated on the surface of Pd/CeO2-O (nanooctahedrons), due to the strong surface Ce-O bond on ceria (111) facet. For propane oxidation, C-H cracking is the rate determining step, which can be enhanced by the existence of Pd particles on ceria. The Pd/CeO2-O catalyst has the highest ratio of PdO/PdxCe1-xO2-σ among three catalysts, which mean more active sites existed on Pd/CeO2-O sample. Therefore, the Pd/CeO2-O catalyst has the more ability for breaking the C-H bond of propane than Pd/CeO2-R. And the carboxylates etc. related species on the (111) facet are unstable and easily decompose, and formed CO2 can fast desorb on the Pd/CeO2-O catalyst. For propane oxidation, TOF of the Pd/CeO2-O catalyst was 3.52 × 10−2 s−1 at 300 °C, and much higher than that of Pd/CeO2-C (2.59 × 10−3 s−1) and Pd/CeO2-R (6.97 × 10−4 s−1) catalysts, and its Ea was 49.1 kJ/mol. These results show the inverse facet sensitivity of ceria for the CO and propane oxidation over Pd/ceria, and provide the better understanding of relationship between the morphology of the oxide support and catalytic performance of supported Pd catalysts, or say, the effect of the support crystal planes on the catalytic performance of the supported Pd catalysts.
ASSOCIATED CONTENT * Supporting Information The Pd particle size calculated by CO chemisorption, the catalytic stabilities of Pd/CeO2 samples for CO and propane oxidation (Figure S1), TEM images of 1.0%Pd/CeO2-O and its Pd particle distribution (Figure S2), and C3H8-TPSR curves on CeO2 samples (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Authors *G.Z. Lu: fax, +86-21-64252923; e-mail,
[email protected]. *Y. Guo: e-mail,
[email protected]. Notes The authors declare no competing financial interest.
ACKNOWLEDGEMENTS This project was financially supported by the National BasicResearch Program of China (2013CB933201), the National NaturalScience Foundation of China (21273150), the national high technology research and development program of China (2011AA03A406, 2012AA062703), the Fundamental Research Funds for the Central Universities.
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ACS Catalysis
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ACS Catalysis
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