Insight Investigation of Active Palladium Surface Sites in Palladium

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Energy, Environmental, and Catalysis Applications

The insight investigation of active palladium surface sites in palladium-ceria catalysts for NO + CO reaction Ke Tang, Yuqing Ren, Wei Liu, Jingjing Wei, Jinxin Guo, Shuping Wang, and Yanzhao Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02557 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Materials & Interfaces

The insight investigation of active palladium surface sites in palladium-ceria catalysts for NO + CO reaction Ke Tang1, Yuqing Ren1, Wei Liu1, 2, Jingjing Wei1, Jinxin Guo1, Shuping Wang1, and Yanzhao Yang1* 1

Key Laboratory for Special Fuctional Aggregate Materials of Education Ministry, School of chemistry and Chemical Engineering, Shandong University, Jinan, 250100, P. R. China. 2 School of Resources and Environment, School of University of Jinan, Jinan, 250022, P. R. China.

*Corresponding author: E-mail: [email protected] KEYWORDS: palladium-ceria catalysts, active sites, catalytic process, calcination rate, NO + CO reaction ABSTRACT The palladium species in ceria-based catalysts have a significant influence on their catalytic performance. In this work, the structure evolution of palladium species induced by various calcination rate were investigated and then these calcined catalysts were applied to NO + CO catalytic reaction. Systematic investigations by various measurements demonstrate that the calcination rate and catalytic process play crucial roles on the formation ways of palladium species and identify the forms of active palladium surface sites for NO + CO reaction. Results indicate that the calcination process resulted in the formation of three types of palladium components: PdO interacted with ceria supports (PdOx/Pd-O-Ce cluster), PdO nanoparticles on the surface and Pd2+ ions incorporated into the subsurface lattice (Pd-O-Ce solid solution). It is also proved that the state and distribution of palladium components are dependent on the calcination rate: rapid calcination rate is beneficial for the generation of PdO species (PdOx/Pd-O-Ce and PdO), while slow calcination rate makes contribution to the formation of Pd-O-Ce. Furthermore, the subsequent catalytic process could induce the decomposition of PdOx/Pd-O-Ce and formation more fraction of active Pd species in PdO oxide phase. Based on the catalytic performances we found the superior catalytic properties are detected for catalysts containing 0.5% Pd (0.5% is corresponding to the palladium content in molar ratio) with fast calcination rate. This is due to the synergistic effect of active Pd in PdO decomposed form PdOx/Pd-O-Ce in the catalytic process and the palladium ions in Pd-O-Ce solid solution. INTRODUCTION In decades, increasing attention has been paid to automotive emission conversion, such as nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC). Catalyst converter in emission system and engine control system have been developed to efficiently convert the emissions into environment-friendly exhaust gas including N2, CO2, H2O.1-3 Currently the reaction that NO reduced by CO with the formation of N2 and CO2 has become a hot topic on the basis of the synchronous removal of two

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

types of pollutions.4-6 Three-way catalysts (TWCs), such as noble metal (Pd, Pt, Rh)7-9 and metal oxides (CeO2, La2O3, Al2O3),10-12 have been utilized to achieve this catalytic process. Among these catalysts, Pd/CeO2 catalysts have shown excellent stability against the sintering of precious metal particles and catalytic performance, due to the strong interactions between palladium and ceria supports, the increase in the lattice oxygen storage and mobility of ceria.13, 14 It has been demonstrated that the catalytic performance is determined by the interaction mode between noble metal and supports, such as atomic distribution of noble metal on the surface and in the bulk.15,16 The interaction mode is determined by many factors. For instance, the relationship between palladium contents and the states have been studied to investigate the catalytically active sites.17 Synthesis methods18-21 such as sol-gel method, impregnation, solution-combustion method and deposition-precipitation method could modify the ionic state of palladium. Moreover, the pretreatment condition such as calcination temperature can result a migration of the palladium species from ceria lattice to surface with forming PdO species.22 However, the states of palladium species in the palladium-ceria catalysts can be related to miscellaneous factors, especially the subsequent calcination conditions and catalytic process. Therefore, we utilized the ceria rods with well-defined facets as the supports to trace the structure evolution of palladium species during calcination process with different calcination rate and subsequent catalytic process. The purpose of this work was to detect the relation between palladium species state and catalytic performance of NO + CO reaction, so various calcination strategies were processed on the as-obtained catalysts. A series of physicochemical methods were used to elucidate the structure evolution of active palladium species during the calcination process and catalytic process. Results indicate the calcination rate influences the formation ways of palladium species and the subsequent catalytic process further results a migration of palladium species, which contributes to the efficient species (PdO nanoparticles and Pd-O-Ce solid solution) for the low-temperature catalytic activity. The results of this work can be utilized to build a systematical understanding toward the structure evolution of the active species in palladium-ceria system during calcination and catalytic process, and could inspire the reasonable design of noble metal systems. EXPERIMENTAL SECTION Preparation of CeO2 Nanorods. CeO2 nanorods were synthesized using the solvothermal method.23 Ce(NO3)3.6H2O (0.25 mmol) was dissolved in NaOH (7.45 M, 30 ml) solution under magnetic stirring. After stirring for 30 min, the mixture was transferred into a 40 ml Teflon bottle, and heated to 100 oC for 24 h. The ceria supports were washed with deionized water and ethanol, dried at 60 oC and calcined at 500 oC for 2 h. Preparation of Pd/CeO2 Catalysts. Pd/CeO2 catalysts were prepared by using the deposition-precipitation (DP) method. 0.3 g ceria support was suspended in 15 mL deionized water. The calculated amounts of Pd (II) stock solution (0.0282 M, prepared by dissolving metal chloride in hydrochloric acid solution) were added into the


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suspension dropwise, while keeping the pH ~9 by Na2CO3 aqueous solution (0.5 M). After 1 h of stirring at room temperature, the collected products were washed with deionized water and ethanol, and dried at 60 oC. The catalysts were firstly calcined at 300 oC for 30 min with different calcination rate (3 or 10 oC min-1) in O2 stream. After that, the catalysts were further treated at 300 oC under synthetic gas (consist of 1% NO-Ar and 1% CO-Ar, same with the gas mixture for NO+CO reaction), followed by purging in a stream of Ar. In this work, the palladium-ceria catalysts were designated as nPdx and nPdsyn x , where n is the palladium content in molar ratio ( [Pd/Ce]mol × 100%, n = 0.5 and 1), x is the calcination rate in O2 stream, oC min-1 (x = 3 and 10), and syn is corresponding to the further treatment in synthetic gas to simulate the reaction atmosphere. Characterization. X-ray diffraction (XRD) was conducted using Cu kα radiation on a Rigaku D/Max 2200PC diffractometer equipped with a graphite monochromator (λ = 0.15418). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was conducted on an IRIS Intrepid II XSP instrument to detect the contents of palladium. Transmission electron microscope (TEM, JEM-1011), high-resolution TEM (HRTEM) with an accelerating voltage of 200 kV and scanning transmission electron microscope (STEM, JEOL-TEM) were used to detect the morphology of the products. The corresponding elemental mapping were carried out on the energy-dispersive X-ray spectroscopy (EDS) with an accelerating voltage of 200 kV under STEM mode. Raman spectra were performed on a LabRAM HR4800 spectrometer in the spectral window from 100 to 1000 cm-1 with a 514 nm laser as an excitation source. The X-ray photoelectron spectroscopy (XPS) characterization was carried out on an ESCALAB 250 X-ray photoelectron spectrometer by using 150W of Al-Kα radiation. All the binding energies were corrected by the C 1s peak at 284.8 eV, and after background subtraction by Shirley mode, the XPS spectra was optimized by Gaussian-Lorentzian method.24, 25 Temperature-programmed reduction by hydrogen (H2-TPR) was performed on a PCA-1200 instrument equipped with a thermal conductivity detector (TCD) to detect the consumption of H2. The palladium-ceria catalysts (50 mg) were first pretreated in pure Ar at room temperature, then switched to 5% H2-Ar gas mixture (30 mL min-1) and heated (10 oC min-1) from room temperature to 600 oC. Catalytic properties. NO+CO reaction on the palladium-ceria catalysts was performed in a fixed-bed reactor by using 50 mg samples in a gaseous mixture of 1200 ppm NO, 1200 ppm CO and Ar in balance at a space velocity (SV) of 60000 mL h-1 g-1cat. The compositions of the outlet gases were quantified online by infrared gas analyzer (Gasboard-3000) with the resolution of 1 ppm. The conversions of NO and CO were calculated as follows. CO conversion (%) =

[ CO ] in vol % − [ CO ] out vol % × 100 [ CO ] in vol %

NO conversion (%) =

[ NO ] in vol % − [ NO ] out vol % × 100 [ NO ] in vol %



RESULTS Catalytic Properties. CeO2 rods exposed with {110} facets as active sites have been proven to be good candidates to build heterogeneous catalytic system.4, 26-28 Here, palladium species were deposited on ceria rods by DP method. It would be promising to study the correlation between the catalytic properties and the structure evolution of palladium species during the calcination and catalytic process. Figure 1 depicts the NO conversion (Figure 1a-d) and CO conversion (Figure 1e-h) profiles of the palladium-ceria catalysts containing 0.5% and 1% Pd (molar ratio) and calcined under different conditions. The results show that the catalysts with a calcination rate 10 oC min-1 under oxygen flow have a better performance compared to those with a 3oC min-1 calcination rate, and the following heat treatment in synthetic gas can further enhance the catalytic property. The complete conversion temperature of catalysts nPd3 (n=0.5 and 1, the palladium content in molar ratio) was around 191 oC for NO and 250 oC for CO, while the corresponding temperature of nPd10 was around 140 oC for NO and 180 oC for CO. At 120 oC, NO conversion of 0.5Pd10 and 1Pd10 were around 50% and 29%, while there were barely NO conversion for 0.5Pd3 and 1Pd3. A remarkable NO conversion at lower temperature was observed for 0.5Pd10, which had a sharp decrease in NO concentration at 111 oC and ended up at 135 oC. 0.5Pd3 catalyst with a starting conversion temperature at 151 oC then reached a complete conversion at 191 oC. When comparing the NO conversion of catalyst 0.5Pd3 with 1Pd3, they had similar T50 (the temperature of 50% conversion, 167 oC for 0.5Pd3 and 164 oC for 1Pd3) and T100 (the temperature of 100% conversion, 191 oC for 0.5Pd3 and 193 oC for 1Pd3). However, at the low-temperature range 119-165 oC, the NO concentration for 1Pd3 decreased remarkably, followed by a slower reaction rate at the range of 165-193 oC. Same phenomenon was observed for catalysts nPd10, where 1Pd10 had a higher NO conversion at the range of 110-116 oC and a lower conversion from 116 oC to 193 oC than 0.5Pd10. Interestingly, 0.5Pd10 and 1Pd10 showed a quite low conversion efficiency of NO at the range of 40-100 oC. For CO conversion, a diminution was detected for nPd3 (196-248 oC for 0.5Pd3 and 193-257 oC for 1Pd3), while nPd10 exhibited S-shape profiles with a maximum of conversion in a short temperature range. The above data demonstrate that the amount of palladium in catalysts with same calcination rate only affect the catalytic activity in low-temperature zone, while changing in the calcination rate from 3 to 10 oC min-1 dramatically enhance the catalytic performance. The superior conversions in NO+CO reaction for catalysts nPd10 demonstrate a typical behavior of the contribution of calcination rate to the catalytic properties. After thermal treatment in O2 atmosphere, a further treatment under synthetic gas atmosphere was performed to study the structure evolution during the catalytic process. As shown in Figure 1 and Table S1, all catalysts show an enhanced catalytic behavior. The best catalytic property was observed for 0.5Pdsyn 10 , which had a starting o o temperature at 72 C for NO, 62 C for CO and ending temperature at 112 oC for NO, 162 oC for CO. For catalysts 1Pd, as the temperature raising to 120 oC, the NO conversion of 1Pdsyn had three-fold increase; the NO conversion of 1Pdsyn reached to 10 3


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88%, whereas no NO conversion was observed for 1Pd3. Same as the tendency of NO conversion, catalytic activity of nPdsyn in CO conversion enhanced dramatically. T50 x o o and T100 reached to 117 C, 162 C for 0.5Pdsyn and 122 oC, 171 oC for 1Pdsyn 10 10 , compared with 135 oC, 178 oC for 0.5Pd10 and 142 oC, 193 oC for 1Pd10. At the temperature of 160 oC, the CO conversion of 0.5Pdsyn and 1Pdsyn were four-fold and 3 3 two-fold higher than 0.5Pd3 and 1Pd3. These results demonstrate that the catalytic activity is raised after catalytic process, whereas no contribution to catalytic performance exhibited with increasing palladium contents. The results, displayed in Figure 1 and Table S1, indicate that calcination rate and treatment atmosphere have important influences on the active components. Either active species content or the ways they formed contribute to the surface construction, giving rise to different catalytic behaviors.

Figure 1. Catalytic activities of NO + CO reaction over the palladium-ceria catalysts: (a-d) NO conversion, (e-h) CO conversion.

H2-TPR Study. H2-TPR has been widely utilized to study the redox ability, oxygen storage and release property of catalysts.14, 30, 31 Therefore, we performed H2-TPR in the temperature range of 30-600 oC to evaluate the redox-sites changing with different treating method. As depicted in Figure 2 and Table 1, three peaks (α, β, γ) are observed for all the catalysts. On the basis of previous studies,14, 29-31 α peak can be ascribed to the reduction of PdO particles, β peak is corresponding to the reduction of palladium in CeO2 framework (Pd-O-Ce solid solution), and γ peak can be assigned to surface oxygen reduction. In general, the peak positions (α, β, γ peaks) of nPdx show barely difference, indicating similar active species; however, increasing calcination rate from 3 to 10 oC min-1 induces a different behavior in the distribution of the active species in catalysts. For nPd3, the H2 consumption of Pd-O-Ce solid solution and surface oxygen were much higher than nPd10 (e.g., comparison of β and γ peak areas: 0.5Pd3 (144, 289) > 0.5Pd10 (117, 200), 1Pd3 (117, 444) > 1Pd10 (82, 305)), whereas an opposite tendency was detected for PdO species, of which catalysts nPd10 demonstrated a higher H2 consumption than those of nPd3 (e.g., comparison of α peak areas: 0.5Pd10 (1024) > 0.5Pd3 (37), 1Pd10 (170) > 1Pd3 (50)). It is worth noting that higher palladium contents rise the H2 consumption of surface oxygen, whereas the



amount of active oxygen species in Pd-O-Ce and PdO species decrease. These data verify that calcination rate has an important influence on the forming way of palladium species and thus different surface conditions can be generated. The slow calcination rate can generate more Pd-O-Ce solid solution, surface oxygen and less PdO species, whereas the rapid calcination rate can obtain an opposite result. To further investigate the structural evolution during the catalytic process, we carried out H2-TPR measurements on the catalysts after catalytic process. Comparing with the original sample (shown in Figure 2), the α peak positions of nPdsyn shift to higher x temperature; meanwhile, the H2 consumption increase dramatically. After further treatment under synthetic atmosphere, the α peak position of 0.5Pdsyn increased to 86 10 o C with double peak area compared with 0.5Pd10 (peak position is around 74 oC and o peak area is ca. 1024). For catalysts 1Pdsyn n , the α peak position shifted 40 C and the peak area were eight times than before. When comparing the β and γ peaks of nPdsyn x with nPdx, we detected an reverse tendency. Generally, the peak (both β and γ) positions show no big difference for all the catalysts after catalytic process, indicating a similar interfacial bond strength between Pd phase and CeO2 phase. However, the decreased peak areas demonstrate a change of active oxygen in Pd-O-Ce solid solution and surface oxygen during catalytic process. As summarized in Table 1, the β peak areas of 0.5Pd3 and 0.5Pd10 are five-flod higher than 0.5Pdsyn 3 syn and 0.5Pdsyn 10 ; and the γ peak areas of 1Pd3 and 1Pd10 are two-fold higher than 1Pd3 syn and 1Pd10 . Based on these results, we can suppose that during the catalytic process the reducing atmosphere could react with the active oxygen from surface species, resulting in a migration of part of active oxygen from the Pd-O-Ce solid solution and surface oxygen to PdO nanoparticles on ceria supports.

Figure 2. H2-TPR profiles of 0.5Pd with calcination rate (a) 3 oC min-1, (b) 10 oC min-1 and


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1Pd with calcination rate (c) 3 oC min-1, (d) 10 oC min-1. Table 1. Peak position and area of H2-TPR. H2-TPRa Sample

0.5Pd3 0.5Pdsyn 3 0.5Pd10 0.5Pdsyn 10 1Pd3 1Pdsyn 3 1Pd10 1Pdsyn 10 a

α

β

γ

Position (oC)

Area

Position (oC)

Area

Position (oC)

Area

75

37

190

144

438

289

85

935

198

34

430

155

74

1024

188

117

396

200

86

2199

196

26

406

178

54

50

184

117

415

444

89

463

187

61

410

231

81

170

181

82

436

305

123

866

185

26

432

156

Peak position and areas deduced from Figure 3.

Physical Characterization. Figure S3 depicts the diffraction patterns of the palladium-ceria catalysts before (nPdx, Figure S3a) and after (nPdsyn x , Figure S3b) catalytic process. In these two cases, all the XRD patterns can be ascribed to fluorite CeO2 phase (JCPDS 34-0394) without any reflections corresponding to the metal or oxide states of palladium. To further investigate the structural and intrinsic properties of palladium-ceria catalysts, we performed TEM measurements. As shown in Figure S2, the palladium-ceria catalysts were nanorods with length of 50-200 nm and width around 10 nm. The rod-like structure of palladium-ceria catalysts was well-maintained after the catalytic process, which is consistent with the XRD results, and no isolated palladium-related nanostructures were detected (comparing nPdx, Figure S2a-d with nPdsyn x , Figure S2e-h). The results of XRD and TEM reveal that the deposition of palladium on ceria substrate has not altered the main structure. Moreover, it can be concluded that the palladium species are finely-dispersed on the surface before and after catalytic process. To fully get insight into the primary structure of palladium-ceria catalysts, we carried out high-resolution TEM characterization. Figure 3 exhibits the highly crystallized nanostructures with the interplanar distance of 0.32 nm, which is in good agreement with CeO2 (111) facets.23 On the basis of STEM-EDS elemental mapping results, the homogeneous distribution of Ce, Pd, O elements are indicated for 0.5Pdx (x= 3, 10) catalysts. As shown in Figure 3b, no related three-dimensional palladium nanostructures can be observed in the catalysts with slow calcination rate, whereas the flattened nanoparticles (described as PdOx cluster) are detected on the surface (sample 0.5Pd3, the marked area).22, 32 However, a small amount of PdO nanoparticles should not be excluded from the structure since the low palladium contents and high contrast of ceria substrate. As the calcination rate increased to 10 oC min-1, three-dimentional



palladium nanoparticles can be detected (sample 0.5Pd10, Figure 3d). It can be clearly seen from the corresponding EDS-mapping results of 0.5Pd10 that parts of palladium are concentrated more densely on the surface, which can be explained by structure evolution of parts of palladium into the PdO nanoparticles. On this basis, it can be concluded that fast calcination rate has not induced the catastrophic sintering of the palladium on the surface, which indicates the stability of the palladium-ceria catalysts. After catalytic process, from STEM (Figure 3e, g) and TEM images (Figure S2e-h), no separated palladium-related structures in nano-domain range are detected. Additionally, from HRTEM images shown in Figure 3f and h, the fine crystallinity of ceria supports with well-dispersed palladium structures are maintained after catalytic process. Figure 3f exhibits that parts of the flattened palladium nanostrucutres are transformed into three-dimensional nanoparticles, which indicate a structural modification of palladium without surface sintering. Therefore, the results of the physical characterization indicate that slow calcination rate can dissolve more palladium species on the subsurface of ceria substrates (the flattened nanoparticles in 0.5Pd3) compared with the fast calcination rate (the three-dimensional nanoparticles in 0.5Pd10), and the subsequent catalytic process can prompt the generation of three-dimensional structures for the catalyst with low palladium content and slow calcination rate. Additionally, although the structure evolution has been detected for sample 0.5Pd3, the catalytic process has not induced the sintering of palladium species on the surface and ceria-based supports.

Figure 3. STEM (a, c, e, g) and HRTEM (b, d, f, h) of palladium-ceria catalysts: (a, b) 0.5Pd3, the below are the corresponding STEM-EDS elemental mapping images, (c, d) 0.5Pd10, the below are the corresponding STEM-EDS elemental mapping images, (e, f) 0.5Pdsyn and (g, h) 3 . The marked area is corresponding to PdO species. 0.5Pdsyn 10

Vis-Raman has been utilized to get deep insight into the crystal structure of


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palladium-ceria catalysts. The Raman spectra depicted in Figure 4 indicate the typical defect structure of ceria. Based on previous work,20, 33 the main peak centered at 470 cm-1 can be attributed to the symmetrical stretching mode of [Ce-O8] (F2g mode). The band located at 260 cm-1 can be assigned to the transverse second-order acoustic vibrational mode (2TA mode), and the band at 600 cm-1 can be ascribed as the defect-induced mode (D mode).34, 35 At low palladium contents, shown in Figure 5, the increase in calcination rate cause a slightly blue-shift of F2g mode band. Same blue-shift of F2g band was also detected for the catalysts 0.5Pdsyn x . This band shift is due to the generation of the small size of PdO nanoparticles on ceria matrix surface, which has been validated by the HRTEM results (Figure 3b and d). With increasing palladium contents, the shift of F2g band increased from 461 to 470 cm-1 for catalysts nPd3 and 463 to 471 cm-1 for catalysts nPd10, which can be explained by both the increasing amount of PdO nanoparticles on the surface and the high structural defects.20 Furthermore, for catalysts 1Pdx with faster calcination rate and the following catalytic process it did not vary the F2g band. This result indicate that at a high level of palladium contents the Pd species on the surface are mainly in three-dimentional forms instead of the flattened cluster forms. This result also demonstrated that changing calcination rate and the following catalytic process do not destroy the structure of palladium-ceria catalysts, especially the Pd species on the surface, which coincides with the results of XRD and TEM measurements. Therefore, associated with the results of physical characterization, the Raman results may give several points toward the surface changing: formation of flattened nanoclusters (PdOx/Pd-O-Ce) and dissolution of Pd species in ceria supports with slow calcination rate; formation of three-dimensional nanoparticles (PdO) and decomposition of solid solution (Pd-O-Ce) with fast calcination rate and catalytic process.



Figure 4. Raman spectra of 0.5Pd with calcination rate (a) 3 oC min-1, (b) 10 oC min-1 and 1Pd with calcination rate (c) 3 oC min-1, (d) 10 oC min-1.


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Figure 5. Magnified Raman spectra of 0.5Pd with calcination rate (a) 3 oC min-1, (b) 10 oC min-1 and 1Pd with calcination rate (c) 3 oC min-1, (d) 10 oC min-1.

XPS Study. The surface composition was investigated by XPS measurement and is shown in Figure S4. The decomposition of Pd3d spectra display the oxidized state of palladium in the catalysts even after the catalytic process (Eb(Pd3d5/2) = 336.0-339.0 eV, the detailed binding energies shown in Table S3). It is worth noting that the proportion of the oxidized palladium on the surface exceeds its original concentration. This result suggests the enrichment of palladium on the surface, which demonstrates a fine dispersity of Pd on the surface based on the positive association between XPS peak intensity and the dispersity of sample. As shown in Figure 6, the deconvolution of Pd3d spectra demonstrate the presence of three forms of palladium in the catalysts. On the basis of previous work,22, 32, 36-38 three existed forms of palladium supported on ceria can be ascribed as: PdO in close contact with substrate (PdOx/Pd-O-Ce, Eb(Pd3d5/2) around 336.0 eV), PdO nanopartcles on the surface (Eb(Pd3d5/2) around 337.0 eV) and the oxidized Pd-O-Ce solid solution (Eb(Pd3d5/2) around 338.0 eV). As summarized in Table 2, the fast calcination rate can stimulate the generation of PdO species on the surface (PdOx/Pd-O-Ce and PdO nanopartcles with Eb(Pd3d5/2) = 336.0 and 337.0 eV), while the slow calcination rate can stimulate an increase of Pd2+ concentration in the solid solution (Pd-O-Ce with Eb(Pd3d5/2) = 338.0 eV). This phenomenon is consistent with the H2-TPR and Raman results. Compared with nPdsyn x , catalysts nPdx are characterized by an unexpected peak located at 336.0 ev corresponding to PdOx/Pd-O-Ce. After catalytic process, nPdsyn show signals of PdO x nanopartcles and Pd-O-Ce solid solution, which is caused by the transformation of PdOx interacted with CeO2 to PdO nanoparticles supported on the surface. Moreover, catalytic process strongly increased the overall peak intensity of Pd3d, which was mainly due to the appearance of Pd2+ ions on the surface (three-dimentional PdO nanoparticles, approved by the HRTEM data shown in Figure 3). For 0.5Pdx catalysts, there were two signals corresponding to palladium in PdOx/Pd-O-Ce and Pd-O-Ce solid solution (for 0.5Pd3, Eb(Pd3d5/2) = 336.2 and 337.7 eV and for 0.5Pd10, Eb(Pd3d5/2) = 336.1 and 337.5 eV). However, in this case it is hard to rule out the presence of PdO nanoparticles with binding energy around 337.0 eV, because a weak signal located at 337.0 eV may be masked by the strong signals around 336.0 and depicted an increase in the fraction of PdO 338.0 eV.34, 36 After catalytic, 0.5Pdsyn x nanoparticles with Eb(Pd3d5/2) = 337.4 eV for 0.5Pdsyn and 337.2 eV for 0.5Pdsyn 3 10 , while accompanying a decrease in the fraction of Pd-O-Ce solid solution with Eb(Pd3d5/2) = 338.2 eV for 0.5Pdsyn and 337.9 eV for 0.5Pdsyn 3 10 . With increasing palladium contents, the fraction of the Pd-O-Ce solid solution with Eb(Pd3d5/2) around 338 eV decreased, whereas the fraction of oxide-like structure of Pd species increased (Table 2). For 1Pdx catalysts, in addition to the PdOx/Pd-O-Ce state, anther peak located around 337.0 eV (the small PdO nanoparticles) was detected. These results indicate that in the original reaction solution, parts of palladium species exist as Pd-O-Ce, and excessive parts of palladium are in PdO forms. For 1Pdsyn x , the increasing tendency was observed for both PdO nanoparticles and Pd-O-Ce solid



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solution (1Pdsyn (48%, 337.2 eV; 52%, 338.1 eV) > 1Pd3 (38%, 337.2 eV; 45%, 338.0 3 syn eV), 1Pd10 (49%, 337.0 eV; 51%, 337.9 eV) > 1Pd10 (48%, 337.5 eV; 26%, 338.9 eV)). As a result, the fraction of the two components turn to be approximately equal. The different tendency in Pd-O-Ce between 0.5Pdsyn and 1Pdsyn can be related to x x efficient dissolution of excessive PdO species in the bulk of CeO2 at a high concentration of palladium. Moreover, it is worth noting that size change of the PdO species on the surface can influence the binding energy of Pd3d (a decrease in the size can cause a decrease of the binding energy).22, 32, 38 In this work, no obvious changes in the binding energy of PdO species are detected (changing value is below 0.5 eV), which mean a homogeneous distribution of PdO species on the surface for all the catalysts.

(c) 0.5Pd10 (d) 0.5Pdsyn (e) 1Pd3 (f) 1Pdsyn (g) Figure 6. Pd3d spectra of (a) 0.5Pd3 (b) 0.5Pdsyn 3 10 3 syn 1Pd10 and (h) 1Pd10 . Table 2. Surface compositions of all the samples calculated from XPS data Proportion (at. %) Sample 0.5Pd3 0.5Pdsyn 3 0.5Pd10 0.5Pdsyn 10 1Pd3 1Pdsyn 3 1Pd10 1Pdsyn 10

Pda/Ceb 1.3 0.8 0.7 0.8 1.4 1.7 1.4 1.4

(PdOx/PdO-Ce)c /Pd 29 -37 -17 -26 --

PdOd/Pd -43 -46 38 48 48 49

Pd-O-Cee/ Pd 71 57 63 54 45 52 26 51

(Os+OHs) /Of 42 40 35 48 45 36 42 30

Ce3+/Ce 33 42 28 42 32 43 31 40

a

Pd = (PdOx/Pd-O-Ce) + PdO + Pd-O-Ce, bCe = Ce3+ + Ce4+, cPdOx/Pd-O-Ce with the signal around 336 eV, dPdO with the signal around 337 eV, ePd-O-Ce with the signal around 338 eV, f O= (Os+OHs) + O2-.

As depicted in Figure 7 and Table S3, the deconvolution of O1S spectra demonstrate


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two types of oxygen species: lattice oxygen (O2-) at 529.0 eV and surface oxygen, hydroxyl species (Os+OHs) at 531.0 eV.39 The data shown in Table 2 demonstrate that calcination of catalysts with slow rate generated more surface oxygen and hydroxyl species (Os+OHs) compared with the catalysts with fast calcination rate (0.5Pd3 (42%, 531.4 eV) > 0.5Pd10 (35%, 531.2 eV), 1Pd3 (45%, 531.6 eV) > 1Pd10 (44%, 531.8 eV)). After catalysis, for catalyst 0.5Pdsyn 10 , the Os+OHs fraction increased from 35% to 48%, while other catalysts exhibited a decrease (0.5Pd3 (42%, 531.4 eV) > 0.5Pdsyn 3 (36%, 531.2 eV), 1Pd10 (42%, 531.8 (40%, 531.7 eV), 1Pd3 (45%, 531.6 eV) > 1Pdsyn 3 eV) > 1Pdsyn (30%, 531.5 eV)). According to previous work, the surface oxygen is 10 generated with the presence of oxygen vacancy and Ce3+. As shown in the following equation, more generation of Ce3+ means more oxygen vacancy, thus more surface oxygen can be observed. Therefore, further detailed information can be obtained from Ce3d spectra shown in Figure 8 and Table 2. 4Ce4+ + O2- → 4Ce4+ + 2e-/δ δ + 0.5O2 → 2Ce4+ + 2Ce3+ + δ + 0.5O2, δ represent an oxygen vacancy. As shown in Figure 8 and Table S3, all the peaks can be verified based on the previous work: three doublets from CeO2 (v, u, vʹʹ, uʹʹ, vʹʹʹ, uʹʹʹ) and two doublets from Ce2O3 (vo, uo, vʹ, uʹ).40 As summarized in Table 2, similar to the changes of surface oxygen, the nPd3 catalysts demonstrate a larger fraction of Ce3+ than that of nPd10 (0.5Pd3 (33%) > 0.5Pd10 (28%), 1Pd3 (32%) > 1Pd10 (31%)). The further heat treatment of 0.5Pd10 with synthetic gas resulted in a sharp increase in the fraction of (42%) > 0.5Pd10 (28%), compared with Ce3+ compared with other catalysts (0.5Pdsyn 10 syn syn (40%) > 1Pd10 0.5Pd3 (42%) > 0.5Pd3 (33%), 1Pd3 (43%) > 1Pd3 (32%), 1Pdsyn 10 (31%)). It can be expected that an increase in the fraction of surface oxygen after catalytic process could be observed in catalyst 0.5Pd10, due to the largest generation of Ce3+ and oxygen vacancy among all the catalysts.

Figure 7. O1s spectra of (a) 0.5Pd3 (b) 0.5Pdsyn (c) 0.5Pd10 (d) 0.5Pdsyn (e) 1Pd3 (f) 1Pdsyn (g) 3 10 3 syn 1Pd10 and (h) 1Pd10 .



Figure 8. Ce3d spectra of palladium-ceria catalysts after (a) calcination process and (b) catalytic process.

DISCUSSION Structure evolution. The obtained data for the catalysts synthesized by the DP method indicate that parts of palladium speices incorporate into the ceria lattice in the form of solid solution Pd-O-Ce, and the residual palladium is in PdO forms with two kinds of interaction modes with the surface of ceria supports. A fraction of PdO species interact with the CeO2/Pd-O-Ce particles, which drives the assembling of ceria around PdO nanoparticles to form the PdO-CeO2 aggregates. This formation was depicted in the TEM results and further confirmed by TPR, Raman and XPS results. The interaction between PdO and Pd-O-Ce solid solution is highly dependent on the catalytic process, and the distribution of palladium species is influenced by the calcination rate and the following catalytic process. Generally, the results obtained from XRD, Raman, HRTEM and the corresponding EDX-mapping indicate a consistent homogeneous distribution of palladium-ceria system without any sintered Pd species on the surface, which ensures a good interaction between palladium species with ceria supports. Generally, the different catalytic performance can be correlated with the structure evolution of catalysts. Results demonstrate that for the catalysts with slow calcination rate (3 oC min-1) a large fraction of palladium species is incorporated into the ceria supports to form the Pd-O-Ce solid solution, which is supported by the TPR and XPS data (especially for the catalyst 0.5Pd3, TPR and XPS results shown in Figure 2 and 6). This dissolution resulted in the strong interaction between palladium and ceria and the enrichment of Pd2+ ions in the subsurface layers of ceria (increasing peak intensity of Eb(Pd3d5/2) around 338.0 eV, Table 2). For the catalysts with fast calcination rate (10 oC min-1) considerable amount of palladium species in PdO forms was generated on the surface (especially for the catalysts 0.5Pd10 and 1Pd10). The calculated results of TPR and XPS clearly demonstrate that higher contents of Pd species are in PdO phase instead of Pd-O-Ce forms for the catalysts with fast calcination rate than the catalysts with slow calcination rate. It can be proposed that with slow calcination rate the palladium tend to incorporate with ceria supports based on the strong interaction and also the dissolution of PdO into the subsurface lattice, and in the fast calcination rate conditions the adjacent palladium ions have a tendency to aggregate to form PdO


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oxide phase in order to construct a stable surface structure by reducing the surface energy. Moreover, the different distribution of Pd species with different calcination rate induce different behavior in the surface oxygen in palladium-ceria catalysts. From the calculated results (γ peak in TPR shown in Table 1 and Os+OHs species with binding energy around 531.0 eV in O1s spectra shown in Table 2), slow calcination rate is great benefit to the formation of surface oxygen. On the basis of the different ionic radius between Pd2+ and Ce4+,13 incorporation of Pd ions into the CeO2 lattice can induce high structural defects, which facilitate the generation of the oxygen vacancy and Ce3+. Based on these results and the positive relationship between the surface oxygen and oxygen vacancy, with slow calcination rate more Pd species dissolve in ceria supports to form Pd-O-Ce solid solution, and thus more surface oxygen species can be generated. XPS data depicted the appearance of specific peak with Eb(Pd3d5/2) around 336.0 eV for catalysts nPdx, whereas for catalysts nPdsyn no signals located around 336.0 eV x were detected. It should be noted that the value 336.0 eV is quite low compared with the binding energy of PdO phase around 337.0 eV (reduction value of the binding energy is around 1 eV).32, 36 According to the previous work,37, 38 this peak can been assigned to PdOx clusters, which directly interacts with ceria supports (it is worth noted that the value of x is below 1, which means that the state value is between Pd0 and Pd2+). HRTEM also detected structure evolution of Pd species from PdOx/Pd-O-Ce nanoclusters to three-dimentional PdO nanoparticles through the catalytic process. On the basis of the obtained data, several changes of surface structure were detected in HRTEM, XPS and TPR measurements during catalytic process. From XPS analysis, a shift of the Pd3d toward high binding energy from 336.0 to 337.0 eV was detected, which resulted from a transition of Pd species from the interaction state with ceria supports to form the PdO structures. As depicted in Table S3, this transition also leads to a banding shift of Ce3d doublet (v and u) toward higher banding energy (the shift value reached approximately to 2 eV). Moreover, as evidenced in TPR results, the peak position of α peak (PdO species) shifted to the high temperature range. From the calculated data of TPR and XPS, the fraction of active oxygen species in Pd-O-Ce solid solution and surface oxygen decreased (β and γ peak in TPR) along with an increase in the proportion of Ce3+ (Ce3d spectra in XPS). On this basis, we can draw several points toward the evolution of the oxygen species in active components during the catalytic process. Initially, the synthetic gas reacted with the active oxygen species in PdOx/Pd-O-Ce, which could result in the decomposition of PdOx. Then, the active oxygen species in surface oxygen and Pd-O-Ce transformed to Pd on the surface, which leads to the formation of PdO nanoparticles. Simultaneously, the consumption of oxygen species in ceria matrix under the reduction atmosphere facilitated the generation of oxygen vacancies due to the redox property of Ce3+/Ce4+, which was beneficial for the transformation of oxygen species to the stable active components. Therefore, on the basis of the obtained results and above discussion, we can make several conclusions for the catalysts structure evolutions with different pretreating conditions. For the catalysts with low Pd contents, 0.5Pd3 possesses considerable



amount of ceria phase incorporated with palladium ions in the subsurface compared with 0.5Pd10, which leads to a tendency for the adjacent Pd2+ ions to form aggregation, resulting in the generation of cluster on the surface. The formation of Pd-O-Ce solid solution can induce a strong distortion of the subsurface lattice,41 which can result in the releasing of palladium ions from subsurface to surface to form the stable structure during catalytic process. Therefore, for catalysts 0.5Pdsyn and 0.5Pdsyn there is a 3 10 remarkable increase of the proportion of PdO nanoparticles along with the decrease of Pd-O-Ce solid solution. Increase of the palladium contents leads to enrichment of PdO species on the surface. After catalytic process, a remarkable increase of palladium species in solid solution is observed, which result from parts of the decomposed PdO species dissolve in the ceria phase to form a more stable structure based on the strong interaction of palladium with ceria. Correlation of catalytic properties with catalyst structure. Based on the discussion and the corresponding catalytic results, certain conclusions can be made to correlate the catalytic property toward NO+CO model reaction with the palladium species and catalyst structures. As depicted in Figure S1, pure rod-like catalyst is inactive toward the NO+CO reaction at the temperature range 0-300 oC. Introducing the palladium into ceria system dramatically enhanced the catalytic performance. However, differences in the active components of catalysts resulted from different treating conditions lead to different catalytic properties. Results depicted in Figure 1 and Table S1 demonstrate catalyst 0.5Pd10 with fast calcination rate and low palladium concentration show the low-temperature activity regardless of the following catalytic process. From the discussion in physical characterization, the obtained high-performance in NO+CO reaction can be correlated with the calcination rate, palladium contents and active sites. The obtained results and the corresponding discussion indicate that at fast calcination rate palladium preferably distribute on the surface with the formation of PdO species (parts of that have strongly anchored on the surface as PdOx/Pd-O-Ce), and the residual parts of palladium incorporate with ceria matrix to form Pd-O-Ce solid solution. As depicted in TPR data, the concentration of the active sites related to the palladium on the surface is relatively high in the 0.5Pd10 case, which result in the dramatic catalytic property. With increasing contents of palladium, the complete conversion temperature slightly shifted to high temperature range, whereas at the low temperature range the catalytic activity was higher than the catalysts with low palladium concentration. As discussed above, increasing palladium contents induces an extensive increase in the concentration of PdO species. The excessive PdO species result in a huge consumption of NO at the beginning of the catalytic process, and with continued catalytic process the proportion of surface active sites sharply decrease based on the decomposition of PdO species, which lead to a decrease of the catalytic performance in the high temperature range. As discussed above, the catalytic process further stimulated different structure evolution on the foundation of the differences of surface structure resulted from the calcination rate and palladium contents. Generally, the resulted transformation of PdOx/Pd-O-Ce to PdO produces an increase of the amount of the surface active sites,


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and thereby the enhanced catalytic performance can be obtained. From the calculated TPR results, the concentration of active oxygen species in PdO dramatically increase after the catalytic process, which stimulate the catalytic performance of all the catalysts. The best catalytic performance and characteristic in low temperature range was observed in catalyst 0.5Pdsyn 10 . The initial temperature of NO+CO reaction for all the catalysts shifted to the lower region, and the complete conversion was achieved rapidly. The enhanced catalytic performance can be attributed to the increased active sites on the surface and also the preserved surface texture with well-dispersed palladium species. From the above discussion, increasing palladium contents resulted in the excessive PdO species on the surface, which covered the surface active sites. However, in addition to the transformation of PdOx/Pd-O-Ce to PdO, the catalytic process also facilitates the dissolution of parts of the excessive PdO species into the supports lattice, which exposes more active sites for the reaction. Therefore, the superior catalytic performance can be obtained based on these factors. Correlated with the structure evolution, we can draw several points toward the relationship between catalytic properties and catalyst structure. The low temperature activity can be attributed to the surface PdO nanoparticles. Although for the catalysts with high catalytic activity the active oxygen species in Pd-O-Ce solid solution depicted in TPR data is extremely low when comparing with PdO species, the contribution of solid solution to the high temperature activity can not be ruled out in the case with slow calcination rate. Catalysts with slow calcination rate, which possess a greater fraction of active sites in Pd-O-Ce solid solution compared with PdO species, achieved the complete conversion at high temperature around 190 oC. At the temperature region of 110-200 oC, the Pd-O-Ce solid solution turned to be catalytically active. Thus, it is hard to eliminate the contribution of solid solution to catalytic activity and also even to the low temperature activity. On this basis, the peculiarity of Pd species (PdOx/Pd-O-Ce clusters, PdO nanoparticles and Pd-O-Ce solid solution) can correlate the catalytic properties with structure evolution induced by the calcination rate and catalytic process. CONCLUSIONS In conclusion, we have utilized the deposition-precipitation method to fabricate the palladium-ceria system with highly dispersed Pd species both on the surface and subsurface. Systematic researches have been carried out to demonstrate that the formation ways of palladium species as well as the distributions were considerably dependent on the calcination process and catalytic process, and thereby different catalytic behaviors has been observed. Results demonstrate that the calcination process after deposition-precipitation synthesis result in three different interaction between palladium components and ceria supports: PdO closely contacted with substrate (PdOx/Pd-O-Ce cluster), PdO nanoparticles on the surface and Pd ions incorporated into the subsurface lattice (Pd-O-Ce solid solution), and the following catalytic process cause the decomposition of PdOx/Pd-O-Ce cluster with generating more fraction of PdO naoparticles. It is proved that slow calcination rate is benefit to the formation of Pd-O-Ce components,



and fast calcination rate can generate more PdO species (PdOx/Pd-O-Ce and PdO). On these basis, the catalytic process can further facilitate the formation of PdO active components. Owing to the structural differences, the excellent catalytic properties was detected on the catalysts 0.5Pd with fast calcination rate (the 100% NO conversion temperature for 0.5Pd10 and 0.5Pdsyn was 135 and 112 oC, respectively). Therefore, it 10 can be concluded that the two active components, PdO nanoparticles and Pd-O-Ce solid solution, are responsible for the high catalytic activity. Supporting Information TEM and catalytic performance of pure nanorods, TEM and XRD patterns of samples, error analysis of XPS data, palladium contents, catalytic activity data and XPS binding energy of catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *E-mail: [email protected]; Fax: +86-531-88564464; Tel: +86-531-88362988. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of China (grant nos 21476129 and 21703120) and Shandong Province (ZR2017MB044 and ZR2017BB004). REFERENCES (1) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts. Catal. Rev. 2004, 46, 163-245. (2) Granger, P.; Parvulescu, V. I. Catalytic NOx abatement systems for mobile sources: from three-way to lean burn after-treatment technologies. Chem. Rev. 2011, 111, 3155-3207. (3) Gandhi, H. S.; Graham, G. H.; McCabe, R. W. Automotive Exhaust Catalysis. J. Catal. 2003, 216, 433-442. (4) Tang, K.; Liu, W.; Li, J.; Guo, J. X.; Zhang, J. C.; Wang, S. P.; Niu, S. L.; Yang, Y. Z. The effect of exposed facets of ceria to the nickel species in nickel-ceria catalysts and their performance in a NO + CO reaction. ACS Appl. Mater. Interfaces 2015, 7, 26839-26849. (5) Liotta, L. F.; Pantaleo, G.; Di Carlo, G.; Marci, G.; Deganello, G. Structural and Morphological Investigation of a Cobalt Catalyst Supported on Alumina-Baria: Effects of Redox Treatments on the Activity in the NO Reduction by CO. Appl. Catal., B 2004, 52, 1-10. (6) Srinivasan, A.; Depcik, C. Review of chemical reactions in the NO reduction by CO on rhodium/alumina catalysts. Catal. Rev. 2010, 52, 462-493. (7) Szailer, T.; Kwak, J. H.; Kim, D. H.; Hanson, J. C.; Peden, C. H. F.; Szanyi, J. Reduction of stored NOx on Pt/Al2O3 and Pt/BaO/Al2O3 catalysts with H2 and CO. J. Catal. 2006, 239, 51-64. (8) Almusaiteer, K. A.; Chuang, S. S. C. Infrared characterization of Rh surface states


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Insight Investigation of Active Palladium Surface Sites in Palladium

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