Synthesis of Ce_1−xPd_xO_2−δ Solid Solution in Molten Nitrate

Abstract

CeO₂-based solid solutions in which Pd partially substitutes for Ce attract considerable attention, owing to their high catalytic performances. In this study, the solid solution (Ce_1−xPd_xO_2−δ) with a high Pd content (x ~ 0.2) was synthesized through co-precipitation under oxidative conditions using molten nitrate, and its structure and thermal decomposition were examined. The characteristics of the solid solution, such as the change in a lattice constant, inhibition of sintering, and ionic states, were examined using X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM−EDS), transmission electron microscopy (TEM)−EDS, and X-ray photoelectron spectroscopy (XPS). The synthesis method proposed in this study appears suitable for the easy preparation of CeO₂ solid solutions with a high Pd content.

1. Introduction

Platinum group metals (PGMs) supported on oxides are widely used as catalysts. Among these systems, Pd particles supported on ceria (CeO₂) are used for the oxidation of carbon monoxide and hydrocarbons such as methane [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. However, catalysts with a better performance are needed for environmental protection. In addition, it is important to reduce the use of Pd, because of its limited availability. Therefore, catalysts with better performance are being developed with a focus on their microscopic structures, active sites, and thermal stability. Acting as a promoter, CeO₂ not only facilitates the catalytic reactions through the storage and release of oxygen, but also prevents Pd particles from sintering. In addition, the chemical interaction between Pd and CeO₂ has attracted much attention in recent years; thus, current studies are focusing not only on Pd particles on CeO₂, but also on solid solutions in which Pd ions partially substitute for Ce⁴⁺ in CeO₂ [1].

Pd-doped CeO₂ can be obtained through various routes, as reviewed in Ref. [1]. The most common method is impregnation, in which Pd is dispersed on a porous oxide support [2,3,4]. In the case of the synthesis of solid solutions, combustion processes [4,5,6,7,8,9] and co-precipitation in aqueous solution [10,11,12] were found to be effective. Water-in-oil microemulsion [13,14], sonochemical synthesis [15], plasma arc [16] and ultrasonic spray pyrolysis [17] methods were employed in special cases.

In a previous study [18], we applied the synthesis method of Adams’ catalyst (platinum oxide, PtO₂) [19,20] to obtain powders consisting of a PtO₂−CeO₂ solid solution. Because this method enables the co-precipitation of oxide particles at a moderately high temperature under oxidative conditions in molten nitrate, it successfully produced a homogeneous oxide with high Pt content. In addition, it was found that the thermal decomposition of the PtO₂−CeO₂ solid solution generated Pt₃O₄ particles, which were not formed by the decomposition of bare PtO₂. In this study, the co-precipitation approach was applied to the synthesis of PdO−CeO₂ powders to obtain a solid solution (Ce_1−xPd_xO_2−δ) with a high Pd content. The structure and thermal decomposition of the solid solution were examined and compared with those of bare PdO and CeO₂ prepared in the same manner.

2. Results and Discussion

2.1. Crystal Structures of PdO−CeO₂ Powders

The molar ratios of Ce to Pd in the PdO−CeO₂ powders prepared in this study were 1, 2, 3, and 4. The PdO−CeO₂ powder with a Ce/Pd molar ratio of y is hereafter referred to as PdCey. The X-ray diffraction (XRD) patterns of the prepared PdO and PdO−CeO₂ powders are shown in Figure 1a–e. For comparison, the XRD pattern of a CeO₂ sample synthesized by the same method is shown in Figure 1f. Figure 1a confirms that the synthesized PdO has a tetragonal structure (ICDD No. 00-041-1107). The XRD patterns of PdCe1 and PdCe2 in Figure 1b,c show peaks corresponding to both PdO and CeO₂. On the other hand, as shown in Figure 1d,e, the patterns of PdCe3 and PdCe4 do not display peaks related to PdO, and thus only the fluorite structure of CeO₂ can be detected. The disappearance of PdO phase indicates that almost entire Pd in PdCe3 and PdCe4 is incorporated into CeO₂ phase. The peaks of the CeO₂ in these PdO−CeO₂ powders are located at lower angles compared to those of CeO₂ reported in the literature (a = 5.411 Å, ICDD No. 00-034-394) and of the prepared CeO₂, as shown in Figure 1f. The peak shift reflects the lattice expansion of the CeO₂-based fluorite structure, due to the formation of a Pd-substituted CeO₂ solid solution (Ce_1−xPd_xO_2−δ). Using Bragg’s law, the lattice constant of PdCe4 can be estimated to be 5.450 Å.

The structure of Ce_1−xPd_xO_2−δ has been examined in several studies. For example, Gulyaev et al. reported the lattice constant of a solid solution obtained by the oxidation of a Pd−Ce−C composite prepared by the plasma arc method [16]. To compare the result of this study with those available in the literature [4,5,6,11,12,13,14,15,16,17], the reported lattice constants are plotted against the composition in Figure 2. The mole fraction of Pd in PdCe4 is 0.2, which is higher than that reported in most previous studies. Figure 2 shows a positive correlation between the mole fraction of Pd and the lattice constant of Ce_1−xPd_xO_2−δ.

A substitution of Pt for Ce in CeO₂ results in a decrease in its lattice constant, as confirmed in a previous study [18]. This can be explained in terms of ionic radii, because Pt²⁺ and Pt⁴⁺ (0.80 and 0.63 Å, respectively [21]) are smaller than Ce⁴⁺ (0.97 Å [21]). On the other hand, the increase in lattice constant upon substituting Pd for Ce cannot be explained by the difference in their ionic radii, because Pd²⁺ and Pd⁴⁺ (0.86 and 0.62 Å, respectively [21]) are smaller than Ce⁴⁺, similar to Pt. Therefore, the increase in lattice constant might be explained in several different ways. First, the substitution of Pd²⁺ for Ce⁴⁺ gives rise to an oxygen vacancy accompanied by a Ce³⁺ ion; thus, the increase in lattice constant can be attributed to the larger ionic radius of Ce³⁺ (1.14 Å [21]) than that of Ce⁴⁺ [15]. In addition, the removal of oxygen caused by the substitution of Pd²⁺ for Ce⁴⁺ might enhance the cation−cation repulsion [17]. On the other hand, some previous studies [13,14,17] ascribed the change in lattice constant to the small particle size, because the lattice constant of CeO₂ increases to more than 5.6 Å in the nanoparticle form [22,23]. Gulyaev et al., who proposed that CeO₂ can contain water molecules in the vicinity of Pd substituting for Ce, also explained the expansion of the lattice in terms of the sizes of the particles [24]. In the case of the present study, it can be assumed that the grain size of the Ce_1−xPd_xO_2−δ solid solution is similar to that of CeO₂ prepared by the same method; hence, the increase in lattice constant is likely to be caused not by the grain size, but by the substitution of Pd for Ce.

2.2. Thermal Decomposition of PdO−CeO₂

The decomposition temperature of bare PdO to Pd in air was reported to be 800 °C [25]. In the case of the PdO−CeO₂ solid solution, the formation of Pd by thermal decomposition started at 825 °C in Ref. [13]. The XRD patterns of PdO after heating are shown in Figure 3. Figure 3a shows that heating at 800 °C for 2 h does not lead to the decomposition of PdO, and the crystallinity of this phase is improved compared with that before heating (Figure 1a). Heating at 850 and 900 °C results in the decomposition of PdO and formation of metallic Pd (ICSD No. 01-071-3757), as shown in Figure 3b,c, respectively. Figure 4 shows the XRD patterns of PdCe4 after heating. Heating at 800 °C for 2 h does not induce changes in the XRD pattern, as shown by Figure 4a. Heating at 850 °C generates small peaks that denote the formation of metallic Pd (Figure 4b). This shows that the thermal decomposition of PdO is hindered by the formation of a solid solution with CeO₂, as reported in a previous study [13]. The XRD pattern obtained after heating at 900 °C (Figure 4c) shows the peaks of the Pd phase more clearly. A small peak at 33.9°, which is visible in Figure 4b,c but not clear in Figure 4a, might be attributed to the PdO formed by oxidation of Pd during cooling.

2.3. Microscopic Observation

Figure 5a shows a scanning electron microscopy (SEM) image of the PdO powder, which appears as an aggregate of particles smaller than 1 μm. Figure 5b displays the SEM image of the powder obtained by heating PdO at 900 °C for 2 h, which consists of sintered micrometre-sized Pd particles.

Figure 6a shows a SEM image of the prepared PdCe4 sample, which is an aggregate of particles smaller than 1 μm. As shown by the elemental mapping in Figure 6b,c, obtained by energy-dispersive X-ray spectroscopy (EDS), these particles include both Pd and Ce. A transmission electron microscopy (TEM) image with higher magnification is shown in Figure 6d, with the Ce/Pd atomic ratios obtained by EDS. The ratios of Ce to Pd are close to the expected value (i.e., 4), although some nonuniformity is present.

Figure 7a shows a SEM image of the PdCe4 sample after heating at 900 °C for 2 h, while Figure 7b,c show the elemental distributions of Pd and Ce, respectively. Although the XRD data confirmed that the heat treatment leads to the formation of a metallic Pd phase, the Pd particles formed on CeO₂ are not visible in the magnified SEM image. The results of TEM observation and EDS analysis are shown in Figure 7d. Although it is difficult to distinguish Pd particles from CeO₂ in the image, the EDS analysis confirms the existence of Ce- and Pd-rich particles. The particles have sizes of several tens of nanometres, and are thus much smaller than the Pd particles obtained upon heating PdO, shown in Figure 5b. Therefore, CeO₂ hinders the sintering of Pd particles at high temperature, as reported in a previous study [26].

2.4. Oxidation States

The oxidation states of Pd and Ce in the prepared powders before and after heating were examined by X-ray photoelectron spectroscopy (XPS). The Pd3d spectra of PdO and PdCe4 before and after heating are compared in Figure 8. The Pd3d spectrum of PdO (Figure 8a) shows peaks at binding energies (E_b) of 336.7 and 342.1 eV, which are close to the Pd3d_5/2 and Pd3d_3/2 energies of PdO reported in literature [27,28]. Another small satellite peak is observed around 339 eV [27]. After heating PdO at 900 °C for 2 h, new peaks are detected at lower E_b, indicating the formation of metallic Pd (Figure 8b). The spectrum was separated into peaks corresponding to metallic Pd (E_b = 335.1 eV for Pd3d_5/2 and 340.3 eV for Pd3d_3/2), according to the literature [27,28]. Although the XRD pattern in Figure 3c shows that the whole PdO was converted to metallic Pt, the XPS data suggest that a large fraction of Pd²⁺ ions (82%, according to the peak areas) remain on the surface. This is probably due to surface oxidation during cooling from 900 °C to room temperature in air.

The Pd3d spectrum of PdCe4 shows main peaks at 337.8 and 343.1 eV, whose binding energies are higher than peaks of the PdO, as indicated by a blue line in Figure 8c. This result agrees with earlier research on Pd-substituted CeO₂ solid solutions [5,6,7,8,9,10,16,17,29,30]. Because Pd ions in CeO₂ have binding energies similar to those of Pd⁴⁺ in PdO₂ (337.9 eV for Pd3d_5/2 [31]), some studies assumed that Pd⁴⁺ ions exist as a solid solution [6,8,9]. Other researchers ascribed these peaks to Pd²⁺ in CeO₂ [29,30]. Therefore, in Figure 8c, we tentatively indicate Pd in CeO₂ as Pdⁿ⁺. According to the peak areas, 92% of the Pd species are present as Pdⁿ⁺ on the surface of PdCe4, while 8% forms PdO. The Pd3d spectrum of PdCe4 after heating (Figure 8d) displays new peaks corresponding to metallic Pd formed by thermal decomposition and PdO produced by surface oxidation of Pd during cooling. The atomic percentages of Pdⁿ⁺, Pd²⁺, and Pd were estimated to be 50%, 40% and 10%, respectively.

The Ce3d spectra of PdCe4 are compared with those of CeO₂ in Figure 9. Figure 9a,b show the spectra of CeO₂ synthesized in molten nitrate and that of CeO₂ after calcination at 600 °C, respectively. The Ce3d spectrum of CeO₂ is known to include ten peaks, six of which can be ascribed to Ce⁴⁺ (v, v″, and v‴ for Ce⁴⁺3d_5/2, u, u″, and u‴ for Ce⁴⁺3d_3/2), while the other four correspond to Ce³⁺ (v₀ and v′ for Ce³⁺3d_5/2, u₀ and u′ for Ce³⁺3d_3/2) [32]. The Ce3d spectra of CeO₂ obtained in this study indicate that Ce⁴⁺ coexists with Ce³⁺, as represented by red and yellow lines in Figure 9a,b, and the percentage of Ce³⁺ is 20−30%. On the other hand, the Ce3d spectra of PdCe4 do not show the presence of Ce³⁺ before and after heating. Hence, we tentatively separate the spectra into Ce⁴⁺ peaks, as shown in Figure 9c,d. As discussed in Section 2.1, some researchers ascribed the large lattice constant of Pd-substituted CeO₂ to Ce³⁺. The XPS data of our study, however, suggest that the substitution of Pd for Ce stabilizes Ce⁴⁺. In contrast, the substitution of Pt for Ce in CeO₂ did not lead to such effects in our previous study [18]. Therefore, it seems necessary to propose a new explanation for the structure of the solid solution. For example, the substitution of Ce⁴⁺ by two Pd²⁺ ions, which was proposed by Su et al. for the surface of Pd-doped CeO₂ [33], might explain the lattice expansion.

3. Materials and Methods

PdO−CeO₂ powders with various molar ratios were prepared by a procedure similar to that reported in a previous study of PtO₂−CeO₂ [18] based on Ref. [34]. For the preparation of the PdO powder, 1.0 g of PdCl₂ (99.0%, Kishida Chemical Co., Ltd., Japan), 10 g of NaNO₃ (99%, Kishida Chemical Co., Ltd., Japan) and 5 mL of reagent water with an electrical conductivity below 0.5 mS∙m⁻¹ were mixed in a quartz crucible (15 mL capacity, 34 mm outer diameter, 42 mm height). The crucible containing the mixture was heated with a resistance furnace under gradually increasing temperature. An evaporation of water was apparently completed after heating for 40 min, leaving a dark brown powder. The powder became liquid at around the melting point of NaNO₃ (308 °C), and then generated a reddish-brown gas (NO₂) for a short time. After heating for 2 h, the temperature reached 500 °C and then the crucible was removed from the furnace. After cooling naturally, water was poured onto the crucible to dissolve residual NaNO₃ and NaCl. The obtained mixture was filtered (filter paper 5C, Advantec Toyo Kaisha, Ltd., Japan) to collect an insoluble brown powder, which was shown to be PdO.

To prepare PdO−CeO₂ powders, Ce(NO₃)₃∙6H₂O (98%, Kishida Chemical Co., Ltd., Japan) was mixed with PdCl₂ and NaNO₃ in water and heated following the same procedure described above. For example, PdCe4 was synthesized from 0.15 g of PdCl₂ and 1.48 g of Ce(NO₃)₃∙6H₂O. Bare CeO₂ was also prepared by the same method reported in Ref. [18].

A portion of the prepared oxide powders (PdO and PdCe4) was heated in air to examine their thermal decomposition at 800, 850, and 900 °C for 2 h.

The phases formed in the prepared powders and after the heat treatment were examined with an X-ray diffractometer (X’Pert PRO Alpha-1, PANalytical B.V., The Netherlands), using Cu-Kα emission and a θ-2θ geometry.

The morphology of the particles was examined using a scanning electron microscope (JSM-6510LA, JEOL, Japan) equipped with an energy dispersive X-ray spectrometer (JED-2300, JEOL, Japan). Higher magnification images were obtained using a transmission electron microscope (JEM-2100, JEOL, Japan) operated at 200 kV. For the TEM observation, a specimen was ground with a mortar and pestle, and then suspended in ethanol. A small drop of the suspension was placed onto a collodion-coated Cu grid and dried before observation.

The oxidation states of Pd and Ce in the prepared powders before and after heating were examined with an XPS instrument (1600E, ULVAC PHI Inc., Japan), using Mg-Kα radiation operated at 15 kV and 400 W. Argon sputtering, which is widely used for depth profiling in XPS measurements, was not employed here because it might reduce Pd and Ce to lower oxidation states [35,36]. The experimental data were analysed with the MultiPak ver. 6.2.1 software, to fit the spectra with Gaussian-Lorentzian peaks after Shirley background subtraction. A C1s peak of a contamination carbon was used as an internal standard.

4. Conclusions

Ce_1−xPd_xO_2−δ solid solution with a high Pd content (x ~ 0.2) was synthesized in molten nitrate. The lattice constant of CeO₂ increased upon the substitution of Pd for Ce, in agreement with the literature. The formation of a solid solution hindered the formation and sintering of Pd particles during heating, and thermal decomposition thus resulted in Pd particles of much smaller size than those obtained from thermal decomposition of bare PdO. The synthesis method proposed in this study appears suitable for the easy preparation of CeO₂ solid solutions with high Pd content. In addition, the XPS analysis indicated that the formation of a solid solution did not result in increased Ce³⁺ amounts and stabilized the Ce⁴⁺ ions in CeO₂.