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rich perturbation conditions
Applied Catalysis A, General 587 (2019) 117268
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Three-way catalytic performance of Fe-doped Pd/CeO2-ZrO2 under lean/ rich perturbation conditions Kento Kusatsugua, Yuichiro Nakamuraa,b, Masaaki Hanedaa,b, a b
T
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Advanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu, 507-0071, Japan Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Gokiso-cho, Showaku, Nagoya, 465-8555, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Three-way catalyst Transition metal additives Pd/CeO2-ZrO2 Oxygen storage capacity
Oxygen storage capacity (OSC) of Pd/CeO2-ZrO2 was significantly improved by addition of transition metals (Ni, Fe, Co, Cu, Sn, Ga, In). Reducible transition metal, such as Ni, Fe, Cu, Co and Sn - doped Pd/CZ showed quite higher OSC than the theoretical value assumed the complete redox of Ce4+ ⇔ Ce3+ (250 mmol-O2 mol-CeO2−1), suggesting the participation of transition metal in reversible oxygen release/sorption. Fe and Sn were effective additives to achieve high OSC property. No significant increase in the three-way catalytic activity of Pd/CeO2ZrO2 under the stoichiometric conditions was achieved by the addition of transition metals. However, Ni, Co, Fe, and Ga – doped Pd/CeO2-ZrO2 showed higher TWC performance than Pd/CeO2-ZrO2 under the lean/rich perturbation conditions. From H2-TPR and FT-IR spectroscopy following CO adsorption, the role of Fe additive was concluded to be the creation of easily reducible Pd species interacting with Fe-doped CeO2-ZrO2 and then to be the stabilization of Pd species in the catalytically active reducing state under the reaction conditions.
1. Introduction Three-way catalysts (TWCs) can work simultaneously and efficiently to reduce NO and to oxidize CO and hydrocarbons in a narrow window of air-to-fuel ratio (A/F), close to the stoichiometric point, along with lean/rich fluctuation. Oxygen storage materials such as CeO2 and CeO2-ZrO2 have been used in three-way catalysts (TWCs) to buffer lean/rich fluctuation, resulting in widening the operational A/F window [1–3]. Therefore, the improvement of oxygen storage capacity (OSC) of CeO2-ZrO2 is an important issue to achieve high catalytic performance under actual conditions. Morikawa et al. [4] reported a new concept in high OSC material composed of Al2O3 as a diffusion barrier and CeO2-ZrO2 particles in intervening layers on a nanometer scale, resulting in its high thermal stability. Since oxygen release/storage behavior of CeO2-ZrO2 is related to the creation and migration of oxygen vacancies in the lattice [5,6], the formation of solid-solution by incorporating other oxides into CeO2ZrO2 seems to be favored to improve the OSC properties. Prasad et al. [7] reported the promoting effect of rare earth (RE) elements on the reducibility and CO oxidation activity of CeO2-ZrO2. This was explained by the creation of oxygen defect sites and by the increased oxygen mobility caused by the formation of CexZryREzO2 solid solution. The introduction of reducible cation such as Sn4+ [8], Bi3+ [9], Ti4+ [10] and various transition metal ions [11–16] was also reported to ⁎
effectively promote the OSC property of CeO2-ZrO2. Li et al. [17,18] investigated the transition metal – doped Pd/CeO2-ZrO2 for automotive emission control, and revealed that Fe and Co are effective additives for the enhancement of OSC and that Fe- and Co-doped Pd/CeO2-ZrO2 show higher TWC performance for NO reduction and CO/HC oxidation than non-doped catalyst. In many cases, the TWC performance was evaluated by using a reaction gas at stoichiometry, although characteristic properties of OSC material should efficiently appear under actual lean/rich perturbation conditions. In the present study, we have investigated the effect of transition metal additives (Ni, Fe, Co, Cu, Sn, Ga, In) on the OSC performance of CeO2-ZrO2 and on the three-way catalytic performance of supported Pd catalysts under the dynamic lean/rich perturbation conditions. Since Fe-doped Pd/CeO2-ZrO2 showed higher TWC performance than Pd/ CeO2-ZrO2 under the dynamic conditions, the promoting effect of Fe was also investigated in detail. 2. Experimental Transition metal (Ni, Fe, Co, Cu, Sn, Ga, In: 3.0 mol%) doped CeO2ZrO2 (M/CZ: Ce/Zr = 25/75) was prepared by a coprecipitation method using ammonium cerium(IV) nitrate, zirconium(IV) oxynitrate and transition metal nitrate (Ni, Fe, Co, Cu, Ga, In) or chloride (Sn) as precursor. To a solution of cerium, zirconium and transition metal
Corresponding author at: Advanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu, 507-0071, Japan. E-mail address: [email protected] (M. Haneda).
https://doi.org/10.1016/j.apcata.2019.117268 Received 6 June 2019; Received in revised form 13 September 2019; Accepted 20 September 2019 Available online 22 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis A, General 587 (2019) 117268
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precursors was added an aqueous solution of ammonia as a precipitation agent at room temperature. The precipitate thus obtained was washed with distilled water, dried at 110 °C and then calcined at 800 °C for 3 h in air. In case of Fe/CZ, the Fe content was changed from 0.8 to 6.3 mol%. The samples are expressed as Fe(x)/CZ, where x is the Fe content. The resulting M/CZ powder was then impregnated with a solution of Pd(NO3)2, followed by drying at 110 °C overnight and calcination at 800 °C for 3 h in air. The Pd loading was fixed at 1 wt%. Catalytic activity was evaluated using a fixed-bed flow reactor system by passing a reaction gas mixture containing NO (0.1%), CO (0.3%), C3H6 (0.04%), O2 (0.28%) and H2O (2%) diluted in N2 at a rate of 500 cm3 min−1 over 100 mg of catalyst (SV = ca. 450,000 h−1), which had been pretreated in situ in the flow of reaction gas at 800 °C for 1 h, under static or dynamic conditions. In case of dynamic condition, oxidizing (+0.15% O2) or reducing (+0.3% CO) gases were alternatively introduced to the steady-state reaction gas at a frequency of 0.1 Hz. In brief, an oxidizing gas mixture (0.1% NO + 0.3% CO + 0.04% C3H6 + 0.43% O2 + 2% H2O/N2) for 5 s and a reducing gas mixture (0.1% NO + 0.6% CO + 0.04% C3H6 + 0.28% O2 + 2% H2O/N2) for 5 s were alternatively changed. The activity was measured while raising the temperature from 40 to 800 °C at a rate of 10 °C min−1. The concentrations of NOx, O2, CO and CO2 and the total hydrocarbons in the effluent gas were continuously monitored using an on-line gas analyzer, Horiba, PG-240 and Shimadzu, VMS-1000 F, respectively. The BET surface area of the catalysts was determined by N2 physisorption at liquid nitrogen temperature using BELSORP mini-II, after evacuating the samples at 300 °C for 1 h. X-ray diffraction (XRD) patterns were recorded using a Rigaku MiniFlex diffractometer with Cu Kα radiation at 30 kV and 15 mA. Raman spectra were measured with a Micor-RAM300/NK (Lambda Vision) equipped with a TE-cooled charge coupled device (CCD) detector and a green laser (λ = 532 nm) under the ambient atmosphere. Temperature-programmed reduction by H2 (H2-TPR) measurement was conducted to estimate the reducibility of the catalysts in the same manner as described elsewhere [19]. After the TPR run up to 800 °C, OSC value was evaluated by injecting O2 pulses to the reduced sample at 600 °C. The amount of chemisorbed CO was measured with a pulse method. The sample was first reduced with H2 at 400 °C for 1 h and then cooled to room temperature in flowing He. Several pulses of CO were introduced into the sample until no more adsorption was observed. As for the stoichiometric ratio of CO/Pdss, Rieck and Bell [20,21] determined the stoichiometric ratio of 0.99 ± 0.04 from the amounts of irreversibly adsorbed H2 and CO. Therefore, the Pd dispersion was calculated from the amount of CO chemisorption by assuming a stoichiometric ratio of 1.0 CO/Pds. FT-IR spectra of adsorbed CO, as a probe molecule, were taken with a Nicolet 6700 FT-IR spectrometer at a resolution of 4 cm−1. Prior to each experiment, a self-supporting sample disk of about 20 mg cm−2 placed in an IR cell with CaF2 windows was first pretreated at 600 °C in a flow of reaction gas containing NO, CO, C3H6 and O2 diluted in He at a stoichiometry and then cooled to 50 °C in flowing of He. Observation of adsorbed CO species was carried out in a flow of 0.5% CO/He at 50 °C.
Table 1 Summary of BET surface area, Pd dispersion and OSC of Pd/M/CZ.
Pd/CZ Pd/Ni/CZ Pd/Fe/CZ Pd/Co/CZ Pd/Cu/CZ Pd/Sn/CZ Pd/Ga/CZ Pd/In/CZ
BET surface area (m2 g−1)
Pd dispersion (CO/Pd)
OSC at 600 °C μmol-O2·g-cat−1
mmol-O2·mol-CeO2−1
33 24 30 11 13 37 26 32
0.25 0.29 0.18 0.14 0.03 0.23 0.14 0.17
443.8 558.2 582.9 557.3 549.6 583.4 494.4 504.5
212.8 271.2 284.7 271.1 267.0 289.5 238.4 246.3
Fig. 1. XRD patterns of Pd/M/CZ. (a) Pd/CZ, (b) Pd/Ni/CZ, (c) Pd/Fe/CZ, (d) Pd/Co/CZ, (e) Pd/Cu/CZ, (f) Pd/Sn/CZ, (g) Pd/Ga/CZ, (h) Pd/In/CZ.
metal - doped CeO2-ZrO2. Since the ionic radius of transition metals (0.055 nm for Co3+, 0.055 nm for Fe3+, 0.062 nm for Ga3+, 0.069 nm for Ni2+, 0.073 nm for Cu2+, 0.080 nm for In3+, 0.081 nm for Sn4+) employed here is smaller than that of Ce4+ (0.097 nm) and Zr4+ (0.084 nm) [25], Ce4+ and Zr4+ ions in CeO2-ZrO2 were partially substituted with smaller transition metal ions to form solid solutions. Since a broadening of XRD peak at 2θ = ca. 34.5°, which can be assigned to the (200) plane, was also observed for Pd/M/CZ, especially in case of Pd/Co/CZ (Fig. 1(d)) and Pd/Cu/CZ (Fig. 1(e)), the presence of transition metal ions causes a partial segregation of different phases. No XRD peaks ascribed to Pd species (Pd0 or PdO) were detected in the XRD patterns of Pd/M/CZ because of high dispersion state of Pd species (see in Table 1) and/or its low content. Due to low sensitivity of the XRD techiques to the crystal symmetry, Raman spectra were measured for Pd/M/CZ samples in the range of 100–800 cm−1. As illustrated in Fig. 2(a), Pd/CZ gave six Raman bands, attributed to tetragonal phase [24,26,27], at 149, 257, 320, 465, 630 and 662 cm−1. This is in accordance with the XRD results given in Fig. 1. It appears that the addition of transition metals into CeO2-ZrO2 did not cause a significant change in the characteristic features of Raman bands. Combining with the XRD result that no segregation of XRD peaks was observed for Pd/M/CZ, it can be concluded that all the Ce and M species are incorporated into the ZrO2 phase to form homogeneous Ce – Zr – M solid solution. As seen in Figs. 2(c) and (e), Fe - and Cu - doped CeO2-ZrO2 gave very broad and weak bands. Raman spectra of disordered phases are known to show less-resolved broad lines
3. Results and discussion 3.1. Physico-chemical properties of transition metal - doped Pd/CeO2-ZrO2 Table 1 summarizes the BET surface area of Pd/M/CZ samples. Although no significant change in the BET surface area was observed by the addition of Fe, In and Sn into CeO2-ZrO2, Ni, Cu, Co and Ga additives caused a decrease in the BET surface area. The surface area of Pd/M/CZ was in the range of 10–40 m2 g−1. Fig. 1 shows the XRD patterns of Pd/M/CZ samples. Distinct XRD peaks ascribed to a solid solution of CeO2 and ZrO2 with a tetragonal t phase [22–24] were detected for all the samples. It should be noted that a shift of XRD peaks to higher angles was recognized for transition 2
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M/CZ irrespective of transition metal additives, although Pd/Co/CZ showed slightly lower CO oxidation activity. As for the C3H6 oxidation (Fig. 3(B)), the activity of Pd/Cu/CZ was quite lower than other Pd/M/ CZ catalysts, especially in the temperature range above 300 °C. In accordance with its low C3H6 oxidation activity, Pd/Cu/CZ did not well catalyze NO reduction in the low temperature region below 500 °C (Fig. 3(C)). It is of interest that Pd/In/CZ and Pd/Sn/CZ were also not effective catalyst for NO reduction, although their C3H6 conversion efficiencies seem to be high enough. This suggests that In and Sn additives increase the undesirable C3H6 oxidation by O2, resulting in low efficiency of C3H6 utilization for NO reduction. As seen in Fig. 3, the light-off temperature for CO oxidation was lower than that for NO reduction and C3H6 oxidation. Similar behavior of CO, NO and HC conversions on Pd/CeO2-ZrO2 based catalysts was reported by Shen et al. [34,35] and Guo et al. [36]. They proposed the reaction scheme that NO is preferentially reduced with hydrocarbons on Pd sites and CO is oxidized by O2 at Pd – CeO2-ZrO2 interface. On the other hand, Muraki et al. [37] reported that NO reduction over Pd catalyst is significantly inhibited by hydrocarbons under rich conditions because of its too strong adsorption on Pd. They also revealed that the hydrocarbon self-poisoning is efficiently interrupted by periodic lean/ rich operation. This suggests that hydrocarbon is oxidized by O2. Di Monte et al. [38] investigated the adsorption behavior of CO/NO containing gas over Pd/Ce0.6Zr0.4O2/δ-Al2O3 by in situ FT-IR spectroscopy, and then proposed that NO redaction and CO oxidation proceeds simultaneously and consecutively via the redox Ce4+/Ce3+ couple at Pd Ce0.6Zr0.4O2 interface. On the other hand, Lin et al. [39] reported the surface chemistry under reaction condition by in situ DRIFTS study that CO is strongly adsorbed on Pd sites dispersed on Ce0.67Zr0.33O2/Al2O3, resulting in the inhibition of NO reduction. In the present study, the strong adsorption of CO and C3H6 was not observed from the shape of conversion efficiency curves (Figs. 3(A) and (B)). Therefore, the finding that the light-off temperature for CO oxidation was lower than that for NO reduction and C3H6 oxidation suggests the low efficiency of CO utilization for NO reduction. Although further detail experiments are necessary, NO seems to be mainly reduced by C3H6 on Pd/M/CZ. To gain information on the additive effect of transition metal, the relationship between the Pd dispersion and the light-off temperature for NO reduction, T50%-NO, at which the conversion reaches 50%, was examined. As shown in Fig. 4, the NO reduction activity of Pd/M/CZ was not directly related to the Pd dispersion, suggesting that transition metal additive participates as catalytically active component in the TWC reaction. This consideration is also supported by the fact that Sn-, In- and Cu-doped Pd/CZ showed quite low NO reduction activity (Fig. 3(C)) while no significant difference in the light-off performance for CO oxidation was observed (Fig. 3(A)). Among the catalysts tested here, Fe- and Ga-doped Pd/CZ showed comparative catalytic performance with Pd/CZ for the TWC reactions (Fig. 3). Unfortunately, no promoting effect of transition metal additives was observed for the TWC reaction under stoichiometric conditions.
Fig. 2. Raman spectra of Pd/M/CZ. (a) Pd/CZ, (b) Pd/Ni/CZ, (c) Pd/Fe/CZ, (d) Pd/Co/CZ, (e) Pd/Cu/CZ, (f) Pd/Sn/CZ, (g) Pd/Ga/CZ, (h) Pd/In/CZ.
resulting from the order defects [28–31]. The addition of Fe and Cu may cause a formation of defect sites in the lattice of CeO2-ZrO2 solid solution. In Table 1 are summarized OSC values evaluated at 600 °C by pulsing O2 after the H2-TPR. It appears that the addition of transition metal caused an increase in the OSC value of Pd/CeO2-ZrO2. This is probably because the creation and migration of oxygen vacancies in the CeO2ZrO2 lattice was promoted by the substitution of Ce4+ and Zr4+ ions with transition metal ions. Since the active component of OSC is CeO2, the maximum OSC value is evaluated to be 250 mmol-O2·mol-CeO2−1 by assuming a complete redox cycle of CeO2: 2CeO2 ↔ Ce2O3 + 1/2O2. In addition, the oxygen uptake for the oxidation of Pd to PdO should be also allowed in OSC value. The OSC values of Pd/M/CZ normalized by molar concentration of CeO2 were calculated by subtracting the oxygen uptake for Pd oxidation to PdO from the total OSC evaluated. As seen in Table 1, reducible transition metal, such as Ni, Fe, Cu, Co and Sn, doped Pd/CZ samples were found to possess quite higher OSC than 250 mmol-O2·mol-CeO2−1, suggesting the participation of transition metal in reversible oxygen release/sorption. On the other hand, the OSC values of Ga and In, which are non-reducible additives, - doped Pd/CZ was less than 250 mmol-O2 mol-CeO2−1. Fe and Sn were found to be effective additives to achieve high OSC property. High OSC performance of Fe-doped Pd/CZ can be explained by the structure disorder as seen in Raman spectra (Fig. 2(c)). However, the effect of Sn can not be explained by the structure disorder. Taking into account the fact that Sn ion has both tetravalent (+4) and divalent (+2) states, high OSC value of Sn-doped Pd/CZ would be due to the redox of Sn2+/Sn4+ [8,32,33].
3.2.2. Lean/rich perturbation conditions As given in Table 1, the addition of transition metal caused an enhancement of OSC property of CeO2-ZrO2. Especially, the reducible transition metal is of interest to participate in reversible oxygen release/sorption. Therefore, Pd/M/CZ with high OSC property is expected to show high TWC performance under the lean/rich perturbation conditions. The TWC performance of Pd/M/CZ, with exception of Pd/Cu/CZ, Pd/In/CZ and Pd/Sn/CZ, which showed low NO reduction activity under stoichiometric conditions (Fig. 3(C)), was evaluated during the lean (+0.15% O2)/rich (+0.3% CO) perturbation at a frequency of 0.1 Hz. Fig. 5(A) shows the temperature dependency of the conversion efficiency of NO over Pd/M/CZ. It appears that the addition of transition metal caused a significant enhancement of NO reduction activity of Pd/
3.2. Three-way catalytic performance of transition metal - doped Pd/CeO2ZrO2 3.2.1. Stoichiometric conditions Fig. 3 shows the temperature dependence of the conversion efficiencies of NO, CO and C3H6 over Pd/M/CZ. Here, the reproducibility of catalytic activity data, for instance, light-off activity expressed as T50, was found to be fairly good with an experimental error of less than 5% for each experiment in this study. As can be seen in Fig. 3(A), no significant difference in the CO conversion efficiency was observed for Pd/ 3
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Fig. 3. Change in the conversion efficiencies of (A) CO, (B) C3H6 and (C) NO as a function of temperature for NO-CO-C3H6-O2 reaction under stoichiometric conditions.
high NO reduction activity under the lean/rich perturbation conditions. Fig. 5(B) shows the relationship between the light-off temperature (T50%) for NO reduction and CO and C3H6 oxidation over Pd/M/CZ. It is of interest that the light-off temperature for NO reduction was linearly correlated with that for C3H6 oxidation, while not with for CO oxidation, suggesting high utilization of C3H6 for NO reduction compared with that of CO. This is in accordance with the insights obtained for three-way catalytic reactions under stoichiometric conditions, as mentioned above. Considering the three-way catalytic performance of Pd/ M/CZ, Pd/Fe/CZ seems to be the most effective catalyst. 3.3. Effect of Fe loading on the activity of Pd/Fe/CZ 3.3.1. Three-way catalytic activity Since Pd/Fe/CZ was found to be the highest active three-way catalyst, the effect of Fe loading was investigated. As given in Fig. 6(A), the NO conversion efficiency on Pd/Fe/CZ under stoichiometric condition was clearly decreased with an increase in Fe loading, although the lightoff temperature was slightly lowered by addition of 0.8–1.6 mol% Fe. The addition of too much amount of Fe (6.3 mol%) caused a significant decrease in the NO reduction activity. Pd/CZ was found to be the highest active catalyst for the NO reduction under stoichiometric
Fig. 4. Relationship between Pd dispersion and T50% for NO reduction over Pd/ M/CZ.
CZ. Taking into account the fact that no increase in the NO reduction activity was observed under the stoichiometric condition (Fig. 3(C)), the improved OSC properties of Pd/M/CZ seems to be responsible for
Fig. 5. (A) Change in NO conversion as a function of temperature for NO-CO-C3H6-O2 reaction under lean/rich perturbation conditions. (B) Relationship between T50% value for NO reduction and T50% value for CO and C3H6 oxidation. 4
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Fig. 6. Change in NO conversion as a function of temperature for NO-CO-C3H6-O2 reaction over Pd/Fe/CZ with different Fe loading under (A) stoichiometric and (B) lean/rich perturbation conditions.
Various kinds of OSC measurement methods were reported so far. For example, Yao et al. [43] proposed the method to measure the dynamic OSC, where O2 and CO pulses were alternatively introduced into the catalyst. Madier et al. [44] compared OSC and OSCC (cumulative oxygen storage capacity) of CexZr(1-x)Om (x = 0.15–0.68), where OSC was evaluated from the amount of CO consumption or CO2 formation and O2 consumption by introducing each CO and O2 pulses, respectively, and OSCC was calculated from their total amount after a few pulses. They found that the reduction of CexZr(1-x)Om by CO gradually proceeds with one by one of pulses, while the oxidation of reduced CexZr(1-x)Om rapidly takes place by introducing one pulse of O2. Gong et al. [45,46] measured the dynamic OSC of Pd/Rh/CeZrOx by alternatively introducing reductant (60 s) and O2 (60 s) gases, and proposed a dual-site OSC model (surface and sub-surface CeO2) from the difference in the rate for oxygen uptake in flowing O2 after the reduction with H2, CO or CH4. They also found that the oxygen consumption behavior is different depending on the reductant. Therefore, it seems to be important to understand the reduction behavior in order to confirm the relevant of the OSC with the catalytic activity of CeO2-based materials. As mentioned above, the addition of Fe into CZ caused an improvement of the OSC of Pd/CZ, leading us to the expectation that the reducibility of Pd species can be altered by using Fe/CZ as a support. Then, H2-TPR measurement was performed for Pd/Fe/CZ with different Fe loading, where the H2-TPR profile was obtained after the pretreatment of consecutive reduction and reoxidation at 600 °C. This is because the lattice strain of Ce-O-Zr is stabilized via the consecutive reduction and reoxidation [47–49]. As seen in Fig. 7(a), a strong H2 consumption peak at 140 °C along with a shoulder peak at 97 °C were observed for Pd/CZ. The latter peak (ca. 100 °C) would be ascribed to the reduction of PdO, while the former (ca. 140 °C) is due to the reduction of Pd – CZ interface. Unfortunately, contrary to the expectation mentioned above, the addition of Fe into CZ did not cause a significant change in the reducibility of PdO species. Namely, no shift of H2 consumption peak at 97 °C was observed. As seen in Fig. 6(B), no significant difference in the temperature at which NO reduction initiates was observed for each Pd/Fe/CZ catalyst under lean/rich perturbation conditions. Therefore, the light-off of the TWC reaction would be
condition. On the other hand, the NO reduction activity of Pd/CZ under the lean/rich perturbation conditions was clearly enhanced by the addition of small amount (0.8 mol%) of Fe (Fig. 6(B)). Although no significant change in the NO reduction activity of Pd/Fe/CZ was observed when Fe loading was increased to 1.6 mol%, further increase in the Fe loading up to 6.3 mol% caused a decrease in the NO reduction activity. The Pd/Fe(6.3)/CZ showed lower NO reduction activity than Pd/CZ, suggesting that the presence of excess Fe additive causes a negative effect. The optimum loading of Fe was found to be 0.8–1.6 mol%. In Table 2 are summarized the BET surface area of and Pd dispersion in Pd/Fe/CZ with different Fe loading. It appears that both BET surface area and Pd dispersion were monotonically decreased with an increase in Fe loading. Since a significant decrease in Pd dispersion was observed for Pd/Fe(6.3)/CZ, this would be responsible for the low NO reduction activity. On the other hand, the improved NO reduction activity of Pd/ Fe/CZ with 0.8–3.0 mol% Fe doping can not be explained by BET surface area and Pd dispersion. 3.3.2. Effect of OSC and reducibility of Pd/Fe/CZ on TWC activity As given in Table 2, the OSC value of Pd/Fe/CZ was linearly increased with an increase in Fe loading. This would be explained by the participation of Fe species in reversible oxygen release/sorption. Fe2O3 is known to be consecutively reduced to Fe(0) by H2 in the temperature range of 300–800 °C via the formation of Fe(II) species [40–42]. In this study, H2-TPR of Pd/Fe/CZ was first performed up to 800 °C, and then OSC was evaluated by introducing O2 pulses at 600 °C. Therefore, Fe species incorporated into CZ is suspected to be reduced to Fe(0). Here, the OSC values originated from CZ without the participation of Fe(III)/ Fe(0) redox, as expressed in terms of OSC_Pd-CZ*, were recalculated by subtracting the oxygen uptake for the oxidation of Fe(0) to Fe(III) from the total OSC evaluated. As also summarized in Table 2, the OSC_Pd-CZ* values of Pd/Fe/CZ were still higher than the total OSC of Pd/CZ, suggesting that Fe species can improve the mobility and activity of oxygen species in the lattice of CeO2-ZrO2. It is of interest that only the Pd/Fe(6.3)/CZ catalyst, which was the lowest active catalyst for NO reduction activity (Fig. 6), gave higher OSC_Pd-CZ* value than the other catalysts. Too high OSC may cause a negative effect on the three-way catalytic performance of supported Pd. Table 2 Summary of BET surface area, Pd dispersion and OSC of Pd/Fe/CZ.
Pd/CZ Pd/Fe(0.8)/CZ Pd/Fe(1.6)/CZ Pd/Fe(3.0)/CZ Pd/Fe(6.3)/CZ a
BET surface area (m2 g−1)
Pd dispersion (CO/Pd)
OSC at 600 °C (μmol-O2·g-cat−1)
OSC_Pd-CZ* at 600 °Ca (μmol-O2·g-cat−1)
33 33 32 30 24
0.25 0.24 0.23 0.18 0.05
443.8 504.1 505.2 582.9 722.8
443.8 482.2 470.8 498.2 544.4
OSC_Pd-CZ* was recalculated by subtracting the oxygen uptake for the oxidation of Fe(0) to Fe(III) from the total OSC evaluated. 5
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Table 3 TOF of NO reduction for NO-CO-C3H6-O2 reaction over Pd/Fe/CZ under lean/ rich perturbation conditions. TOF (mol of NO reacted·min−1·Pd-atoms−1)
Pd/CZ Pd/Fe(0.8)/CZ Pd/Fe(1.6)/CZ Pd/Fe(3.0)/CZ Pd/Fe(6.3)/CZ
260 °C
280 ºC
0.77 1.50 1.45 1.21 2.37
1.82 3.19 3.30 3.02 3.99
Fig. 7. H2-TPR profiles of Pd/Fe/CZ with different Fe loading. (a) Pd/CZ, (b) Pd/Fe(0.8)/CZ, (c) Pd/Fe(1.6)/CZ, (d) Pd/Fe(3.0)/CZ and (e) Pd/Fe(6.3)/CZ.
related to the reducibility of PdO species. On the other hand, the H2 consumption peak due to the reduction of Pd – CZ interface was slightly shifted to lower temperature by the addition of Fe (0.8–3.0 mol%). This indicates that the presence of small amount of Fe can slightly improve the reducibility of Pd – CZ interface. On the other hand, as seen in Fig. 7(e), Pd/Fe(6.3)/CZ, which is a less active catalyst than Pd/CZ (Fig. 6(B)), gave the H2 consumption peak at 148 °C in the H2-TPR profile, suggesting the formation of slightly irreducible Pd – CZ interface. This is probably due to too strong interaction of Pd with Fe(6.3)/CZ, which possesses high OSC. Since three-way catalytic reactions under lean/rich perturbation conditions was reported to proceed via the redox of Pd and CZ [50], the formation of slightly irreducible Pd – CZ interface in Pd/Fe(6.3)/CZ might be responsible for low TWC activity. Unfortunately, the markedly improved TWC activity of Pd/Fe/CZ with 0.8–3.0 mol% Fe doping can not be explained by a slight promotion of the reducibility of Pd – CZ interface, suggesting the presence of other factors affecting the TWC activity.
Fig. 8. FT-IR spectra of CO species adsorbed on (a) Pd/CZ, (b) Pd/Fe(0.8)/CZ, (c) Pd/Fe(1.6)/CZ, (d) Pd/Fe(3.0)/CZ and (e) Pd/Fe(6.3)/CZ.
In order to confirm the effect of the improved OSC properties of CZ by Fe additive (Table 2) on the surface valence state of Pd, FT-IR spectra of CO species adsorbed on Pd/Fe/CZ catalysts, which were pretreated with the stoichiometric reaction gas, were measured to gain information on the surface valence state of Pd. As seen in Fig. 8(a), the exposure of CO to Pd/CZ treated with the reaction gas caused an appearance of two IR bands at 2151 and 2091 cm−1, which can be ascribed to CO species linearly boned on Pdδ+ and Pd0, respectively [26,27]. This suggests that PdO supported on CZ can not be completely reduced to Pd metal, resulting in the presence of Pd particles with cationic and metallic state. On the other hand, Pd/Fe/CZ with 0.8–3.0 mol% of Fe loadings showed very similar IR spectrum: namely, three distinct IR bands at 2142, 2086 and 1972 cm−1 were observed. These bands can be assigned to CO species linearly boned on Pdδ+ and Pd0 and bridgebonded on Pd0, respectively [26,27]. It should be noted that Pd/Fe/CZ gave strong IR bands due to CO species adsorbed on Pd0 sites, compared with those on Pdδ+ sites. A similar surface valence state of Pd supported on Fe/CZ with 0.8–3.0 mol% of Fe loadings would be responsible for the similar TOF values of NO reduction (Table 3). In contrast, relatively strong IR band due to CO species adsorbed on Pdδ+ was observed for Pd/CZ, resulting in low TOF value. It is of interest that Pd/Fe(6.3)/CZ, which showed lower TWC activity under the lean/rich perturbation conditions than Pd/Fe(0.8–3.0)/CZ (Fig. 6(B)), showed very similar IR spectrum with that for Pd/CZ, suggesting the similarity of surface valence state of Pd. These results suggest that Pd species
3.3.3. Effect of surface valence state of Pd on TWC activity Since the catalytically active species of Pd/Fe/CZ for NO−CO-C3H6O2 reaction must be Pd, it would be important to understand the effect of OSC on the chemical state of Pd under the reaction conditions. In order to gain information leading to consider the chemical state of Pd supported on Fe/CZ, turnover frequency (TOF), expressed as moles of NO reacted per mole of surface Pd atoms per minute, of NO reduction was evaluated under the lean/rich perturbation conditions at 260 and 280 °C. As summarized in Table 3, Pd/CZ gave the smallest value of TOF, while the highest TOF was obtained for Pd/Fe(6.3)/CZ. It should be noted that Pd/Fe/CZ with 0.8–3.0 mol% Fe doping, which showed the improved NO reduction activity, gave similar TOF values. These results suggest that the catalytically active state of Pd is different depending on the Fe loading. In other words, the chemical state of Pd may be altered depending on the OSC of Fe/CZ. Machida et al. [51] investigated the role of OSC in the stabilization of Pd metal during the TWC reaction over Pd/CZ under fluctuating A/F ratios, and found the importance of the oxygen migration at the three-phase boundary, where Pd, CZ and the gas phase meet. This suggests that the OSC property of CZ plays an important role in controlling the chemical state of Pd under the reaction conditions. 6
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supported on CZ and Fe(6.3)/CZ are stabilized not only in reducing but also in oxidizing state during the TWC reaction, while those on Fe (0.8–3.0)/CZ stabilized in reducing state. Namely, the presence of relatively large portion of Pdδ+ species in Pd/Fe(6.3)/CZ is suspected to be due to too strong interaction between Pd and Fe(6.3)/CZ with high OSC. Although Pd/Fe(6.3)/CZ (Fig. 8(e)) showed similar IR spectrum obtained for Pd/CZ (Fig. 8(a)), the former catalyst gave higher TOF than the latter one (Table 3). A new active sites, which can not be clarified by FT-IR spectroscopy, may be created via too strong Pd – Fe/ CZ, and then may participate in the TWC reaction. In conclusion, the role of Fe additive is to create the easily reducible Pd species interacting with Fe-doped CeO2-ZrO2 and then to stabilize the surface valence state in catalytically active reducing state.
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4. Conclusions The additive effect of transition metal (Ni, Fe, Co, Cu, Sn, Ga, In) on the catalytic activity of Pd/CeO2-ZrO2 for NO−CO-C3H6-O2 reaction under stoichiometric and lean/rich perturbation conditions was investigated. Oxygen storage capacity (OSC) of Pd/CeO2-ZrO2 was efficiently improved by the addition of transition metal. Especially, Fe and Sn were effective additives to achieve high OSC property. Although the addition of transition metal caused an increase in the OSC, the threeway catalytic activity of Pd/CeO2-ZrO2 under the stoichiometric conditions was not clearly improved by the addition of transition metal. On the other hand, the addition of Ni, Co, Fe, and Ga into Pd/CeO2-ZrO2 caused a significant enhancement of three-way catalytic activity under the lean/rich perturbation conditions. H2-TPR measurements of Fe – doped Pd/CeO2-ZrO2 with different Fe content revealed that easily reducible Pd species interacting with Fe-doped CeO2-ZrO2 are created by the addition of small amount of Fe (0.8–3.0 mol%). In contrast, the addition of excess Fe led to the formation of a little irreducible Pd species, resulting in the low three-way catalytic activity. FT-IR spectroscopy following CO adsorption also revealed that the valence state of Pd species interacting with Fe-doped CeO2-ZrO2 is stabilized in the reducing state under the reaction conditions. The promoting effect of Fe was explained by the creation of easily reducible Pd species, which are stabilized in the catalytically active reducing state under the reaction conditions, interacting with Fe-doped CeO2-ZrO2. References [1] R.Di Monite, J. Kašpar, Top. Catal. 28 (2004) 47–57. [2] H. Hirata, Catal. Surv. Asia 18 (2014) 128–133. [3] T. Montini, M. Melchionna, M. Monai, P. Fornasiero, Chem. Rev. 116 (2016) 5987–6041. [4] A. Morikawa, T. Suzuk, T. Kanazawa, K. Kikuta, A. Suda, H. Shinjo, Appl. Catal. B 78 (2008) 210–221. [5] A. Trovarelli, Catal. Rev. Sci. Eng. 38 (1996) 439–520. [6] D. Duprez, C. Descorme, A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, 2013, pp. 343–375. [7] D.H. Prasad, S.Y. Park, H.I. Ji, H.R. Kim, J.W. Son, B.K. Kim, H.W. Lee, J.H. Lee, J. Phys. Chem. C 116 (2012) 3467–3476. [8] K. Yasuda, A. Yoshimura, A. Katsuma, T. Masui, N. Imanaka, Bull. Chem. Soc. Jpn. 85 (2012) 522–526.
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Three-way catalytic performance of Fe-doped Pd/CeO2-ZrO2 under lean/ rich perturbation conditions Kento Kusatsugua, Yuichiro Nakamuraa,b, Masaaki Hanedaa,b, a b
T
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Advanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu, 507-0071, Japan Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Gokiso-cho, Showaku, Nagoya, 465-8555, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Three-way catalyst Transition metal additives Pd/CeO2-ZrO2 Oxygen storage capacity
Oxygen storage capacity (OSC) of Pd/CeO2-ZrO2 was significantly improved by addition of transition metals (Ni, Fe, Co, Cu, Sn, Ga, In). Reducible transition metal, such as Ni, Fe, Cu, Co and Sn - doped Pd/CZ showed quite higher OSC than the theoretical value assumed the complete redox of Ce4+ ⇔ Ce3+ (250 mmol-O2 mol-CeO2−1), suggesting the participation of transition metal in reversible oxygen release/sorption. Fe and Sn were effective additives to achieve high OSC property. No significant increase in the three-way catalytic activity of Pd/CeO2ZrO2 under the stoichiometric conditions was achieved by the addition of transition metals. However, Ni, Co, Fe, and Ga – doped Pd/CeO2-ZrO2 showed higher TWC performance than Pd/CeO2-ZrO2 under the lean/rich perturbation conditions. From H2-TPR and FT-IR spectroscopy following CO adsorption, the role of Fe additive was concluded to be the creation of easily reducible Pd species interacting with Fe-doped CeO2-ZrO2 and then to be the stabilization of Pd species in the catalytically active reducing state under the reaction conditions.
1. Introduction Three-way catalysts (TWCs) can work simultaneously and efficiently to reduce NO and to oxidize CO and hydrocarbons in a narrow window of air-to-fuel ratio (A/F), close to the stoichiometric point, along with lean/rich fluctuation. Oxygen storage materials such as CeO2 and CeO2-ZrO2 have been used in three-way catalysts (TWCs) to buffer lean/rich fluctuation, resulting in widening the operational A/F window [1–3]. Therefore, the improvement of oxygen storage capacity (OSC) of CeO2-ZrO2 is an important issue to achieve high catalytic performance under actual conditions. Morikawa et al. [4] reported a new concept in high OSC material composed of Al2O3 as a diffusion barrier and CeO2-ZrO2 particles in intervening layers on a nanometer scale, resulting in its high thermal stability. Since oxygen release/storage behavior of CeO2-ZrO2 is related to the creation and migration of oxygen vacancies in the lattice [5,6], the formation of solid-solution by incorporating other oxides into CeO2ZrO2 seems to be favored to improve the OSC properties. Prasad et al. [7] reported the promoting effect of rare earth (RE) elements on the reducibility and CO oxidation activity of CeO2-ZrO2. This was explained by the creation of oxygen defect sites and by the increased oxygen mobility caused by the formation of CexZryREzO2 solid solution. The introduction of reducible cation such as Sn4+ [8], Bi3+ [9], Ti4+ [10] and various transition metal ions [11–16] was also reported to ⁎
effectively promote the OSC property of CeO2-ZrO2. Li et al. [17,18] investigated the transition metal – doped Pd/CeO2-ZrO2 for automotive emission control, and revealed that Fe and Co are effective additives for the enhancement of OSC and that Fe- and Co-doped Pd/CeO2-ZrO2 show higher TWC performance for NO reduction and CO/HC oxidation than non-doped catalyst. In many cases, the TWC performance was evaluated by using a reaction gas at stoichiometry, although characteristic properties of OSC material should efficiently appear under actual lean/rich perturbation conditions. In the present study, we have investigated the effect of transition metal additives (Ni, Fe, Co, Cu, Sn, Ga, In) on the OSC performance of CeO2-ZrO2 and on the three-way catalytic performance of supported Pd catalysts under the dynamic lean/rich perturbation conditions. Since Fe-doped Pd/CeO2-ZrO2 showed higher TWC performance than Pd/ CeO2-ZrO2 under the dynamic conditions, the promoting effect of Fe was also investigated in detail. 2. Experimental Transition metal (Ni, Fe, Co, Cu, Sn, Ga, In: 3.0 mol%) doped CeO2ZrO2 (M/CZ: Ce/Zr = 25/75) was prepared by a coprecipitation method using ammonium cerium(IV) nitrate, zirconium(IV) oxynitrate and transition metal nitrate (Ni, Fe, Co, Cu, Ga, In) or chloride (Sn) as precursor. To a solution of cerium, zirconium and transition metal
Corresponding author at: Advanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu, 507-0071, Japan. E-mail address: [email protected] (M. Haneda).
https://doi.org/10.1016/j.apcata.2019.117268 Received 6 June 2019; Received in revised form 13 September 2019; Accepted 20 September 2019 Available online 22 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
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precursors was added an aqueous solution of ammonia as a precipitation agent at room temperature. The precipitate thus obtained was washed with distilled water, dried at 110 °C and then calcined at 800 °C for 3 h in air. In case of Fe/CZ, the Fe content was changed from 0.8 to 6.3 mol%. The samples are expressed as Fe(x)/CZ, where x is the Fe content. The resulting M/CZ powder was then impregnated with a solution of Pd(NO3)2, followed by drying at 110 °C overnight and calcination at 800 °C for 3 h in air. The Pd loading was fixed at 1 wt%. Catalytic activity was evaluated using a fixed-bed flow reactor system by passing a reaction gas mixture containing NO (0.1%), CO (0.3%), C3H6 (0.04%), O2 (0.28%) and H2O (2%) diluted in N2 at a rate of 500 cm3 min−1 over 100 mg of catalyst (SV = ca. 450,000 h−1), which had been pretreated in situ in the flow of reaction gas at 800 °C for 1 h, under static or dynamic conditions. In case of dynamic condition, oxidizing (+0.15% O2) or reducing (+0.3% CO) gases were alternatively introduced to the steady-state reaction gas at a frequency of 0.1 Hz. In brief, an oxidizing gas mixture (0.1% NO + 0.3% CO + 0.04% C3H6 + 0.43% O2 + 2% H2O/N2) for 5 s and a reducing gas mixture (0.1% NO + 0.6% CO + 0.04% C3H6 + 0.28% O2 + 2% H2O/N2) for 5 s were alternatively changed. The activity was measured while raising the temperature from 40 to 800 °C at a rate of 10 °C min−1. The concentrations of NOx, O2, CO and CO2 and the total hydrocarbons in the effluent gas were continuously monitored using an on-line gas analyzer, Horiba, PG-240 and Shimadzu, VMS-1000 F, respectively. The BET surface area of the catalysts was determined by N2 physisorption at liquid nitrogen temperature using BELSORP mini-II, after evacuating the samples at 300 °C for 1 h. X-ray diffraction (XRD) patterns were recorded using a Rigaku MiniFlex diffractometer with Cu Kα radiation at 30 kV and 15 mA. Raman spectra were measured with a Micor-RAM300/NK (Lambda Vision) equipped with a TE-cooled charge coupled device (CCD) detector and a green laser (λ = 532 nm) under the ambient atmosphere. Temperature-programmed reduction by H2 (H2-TPR) measurement was conducted to estimate the reducibility of the catalysts in the same manner as described elsewhere [19]. After the TPR run up to 800 °C, OSC value was evaluated by injecting O2 pulses to the reduced sample at 600 °C. The amount of chemisorbed CO was measured with a pulse method. The sample was first reduced with H2 at 400 °C for 1 h and then cooled to room temperature in flowing He. Several pulses of CO were introduced into the sample until no more adsorption was observed. As for the stoichiometric ratio of CO/Pdss, Rieck and Bell [20,21] determined the stoichiometric ratio of 0.99 ± 0.04 from the amounts of irreversibly adsorbed H2 and CO. Therefore, the Pd dispersion was calculated from the amount of CO chemisorption by assuming a stoichiometric ratio of 1.0 CO/Pds. FT-IR spectra of adsorbed CO, as a probe molecule, were taken with a Nicolet 6700 FT-IR spectrometer at a resolution of 4 cm−1. Prior to each experiment, a self-supporting sample disk of about 20 mg cm−2 placed in an IR cell with CaF2 windows was first pretreated at 600 °C in a flow of reaction gas containing NO, CO, C3H6 and O2 diluted in He at a stoichiometry and then cooled to 50 °C in flowing of He. Observation of adsorbed CO species was carried out in a flow of 0.5% CO/He at 50 °C.
Table 1 Summary of BET surface area, Pd dispersion and OSC of Pd/M/CZ.
Pd/CZ Pd/Ni/CZ Pd/Fe/CZ Pd/Co/CZ Pd/Cu/CZ Pd/Sn/CZ Pd/Ga/CZ Pd/In/CZ
BET surface area (m2 g−1)
Pd dispersion (CO/Pd)
OSC at 600 °C μmol-O2·g-cat−1
mmol-O2·mol-CeO2−1
33 24 30 11 13 37 26 32
0.25 0.29 0.18 0.14 0.03 0.23 0.14 0.17
443.8 558.2 582.9 557.3 549.6 583.4 494.4 504.5
212.8 271.2 284.7 271.1 267.0 289.5 238.4 246.3
Fig. 1. XRD patterns of Pd/M/CZ. (a) Pd/CZ, (b) Pd/Ni/CZ, (c) Pd/Fe/CZ, (d) Pd/Co/CZ, (e) Pd/Cu/CZ, (f) Pd/Sn/CZ, (g) Pd/Ga/CZ, (h) Pd/In/CZ.
metal - doped CeO2-ZrO2. Since the ionic radius of transition metals (0.055 nm for Co3+, 0.055 nm for Fe3+, 0.062 nm for Ga3+, 0.069 nm for Ni2+, 0.073 nm for Cu2+, 0.080 nm for In3+, 0.081 nm for Sn4+) employed here is smaller than that of Ce4+ (0.097 nm) and Zr4+ (0.084 nm) [25], Ce4+ and Zr4+ ions in CeO2-ZrO2 were partially substituted with smaller transition metal ions to form solid solutions. Since a broadening of XRD peak at 2θ = ca. 34.5°, which can be assigned to the (200) plane, was also observed for Pd/M/CZ, especially in case of Pd/Co/CZ (Fig. 1(d)) and Pd/Cu/CZ (Fig. 1(e)), the presence of transition metal ions causes a partial segregation of different phases. No XRD peaks ascribed to Pd species (Pd0 or PdO) were detected in the XRD patterns of Pd/M/CZ because of high dispersion state of Pd species (see in Table 1) and/or its low content. Due to low sensitivity of the XRD techiques to the crystal symmetry, Raman spectra were measured for Pd/M/CZ samples in the range of 100–800 cm−1. As illustrated in Fig. 2(a), Pd/CZ gave six Raman bands, attributed to tetragonal phase [24,26,27], at 149, 257, 320, 465, 630 and 662 cm−1. This is in accordance with the XRD results given in Fig. 1. It appears that the addition of transition metals into CeO2-ZrO2 did not cause a significant change in the characteristic features of Raman bands. Combining with the XRD result that no segregation of XRD peaks was observed for Pd/M/CZ, it can be concluded that all the Ce and M species are incorporated into the ZrO2 phase to form homogeneous Ce – Zr – M solid solution. As seen in Figs. 2(c) and (e), Fe - and Cu - doped CeO2-ZrO2 gave very broad and weak bands. Raman spectra of disordered phases are known to show less-resolved broad lines
3. Results and discussion 3.1. Physico-chemical properties of transition metal - doped Pd/CeO2-ZrO2 Table 1 summarizes the BET surface area of Pd/M/CZ samples. Although no significant change in the BET surface area was observed by the addition of Fe, In and Sn into CeO2-ZrO2, Ni, Cu, Co and Ga additives caused a decrease in the BET surface area. The surface area of Pd/M/CZ was in the range of 10–40 m2 g−1. Fig. 1 shows the XRD patterns of Pd/M/CZ samples. Distinct XRD peaks ascribed to a solid solution of CeO2 and ZrO2 with a tetragonal t phase [22–24] were detected for all the samples. It should be noted that a shift of XRD peaks to higher angles was recognized for transition 2
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M/CZ irrespective of transition metal additives, although Pd/Co/CZ showed slightly lower CO oxidation activity. As for the C3H6 oxidation (Fig. 3(B)), the activity of Pd/Cu/CZ was quite lower than other Pd/M/ CZ catalysts, especially in the temperature range above 300 °C. In accordance with its low C3H6 oxidation activity, Pd/Cu/CZ did not well catalyze NO reduction in the low temperature region below 500 °C (Fig. 3(C)). It is of interest that Pd/In/CZ and Pd/Sn/CZ were also not effective catalyst for NO reduction, although their C3H6 conversion efficiencies seem to be high enough. This suggests that In and Sn additives increase the undesirable C3H6 oxidation by O2, resulting in low efficiency of C3H6 utilization for NO reduction. As seen in Fig. 3, the light-off temperature for CO oxidation was lower than that for NO reduction and C3H6 oxidation. Similar behavior of CO, NO and HC conversions on Pd/CeO2-ZrO2 based catalysts was reported by Shen et al. [34,35] and Guo et al. [36]. They proposed the reaction scheme that NO is preferentially reduced with hydrocarbons on Pd sites and CO is oxidized by O2 at Pd – CeO2-ZrO2 interface. On the other hand, Muraki et al. [37] reported that NO reduction over Pd catalyst is significantly inhibited by hydrocarbons under rich conditions because of its too strong adsorption on Pd. They also revealed that the hydrocarbon self-poisoning is efficiently interrupted by periodic lean/ rich operation. This suggests that hydrocarbon is oxidized by O2. Di Monte et al. [38] investigated the adsorption behavior of CO/NO containing gas over Pd/Ce0.6Zr0.4O2/δ-Al2O3 by in situ FT-IR spectroscopy, and then proposed that NO redaction and CO oxidation proceeds simultaneously and consecutively via the redox Ce4+/Ce3+ couple at Pd Ce0.6Zr0.4O2 interface. On the other hand, Lin et al. [39] reported the surface chemistry under reaction condition by in situ DRIFTS study that CO is strongly adsorbed on Pd sites dispersed on Ce0.67Zr0.33O2/Al2O3, resulting in the inhibition of NO reduction. In the present study, the strong adsorption of CO and C3H6 was not observed from the shape of conversion efficiency curves (Figs. 3(A) and (B)). Therefore, the finding that the light-off temperature for CO oxidation was lower than that for NO reduction and C3H6 oxidation suggests the low efficiency of CO utilization for NO reduction. Although further detail experiments are necessary, NO seems to be mainly reduced by C3H6 on Pd/M/CZ. To gain information on the additive effect of transition metal, the relationship between the Pd dispersion and the light-off temperature for NO reduction, T50%-NO, at which the conversion reaches 50%, was examined. As shown in Fig. 4, the NO reduction activity of Pd/M/CZ was not directly related to the Pd dispersion, suggesting that transition metal additive participates as catalytically active component in the TWC reaction. This consideration is also supported by the fact that Sn-, In- and Cu-doped Pd/CZ showed quite low NO reduction activity (Fig. 3(C)) while no significant difference in the light-off performance for CO oxidation was observed (Fig. 3(A)). Among the catalysts tested here, Fe- and Ga-doped Pd/CZ showed comparative catalytic performance with Pd/CZ for the TWC reactions (Fig. 3). Unfortunately, no promoting effect of transition metal additives was observed for the TWC reaction under stoichiometric conditions.
Fig. 2. Raman spectra of Pd/M/CZ. (a) Pd/CZ, (b) Pd/Ni/CZ, (c) Pd/Fe/CZ, (d) Pd/Co/CZ, (e) Pd/Cu/CZ, (f) Pd/Sn/CZ, (g) Pd/Ga/CZ, (h) Pd/In/CZ.
resulting from the order defects [28–31]. The addition of Fe and Cu may cause a formation of defect sites in the lattice of CeO2-ZrO2 solid solution. In Table 1 are summarized OSC values evaluated at 600 °C by pulsing O2 after the H2-TPR. It appears that the addition of transition metal caused an increase in the OSC value of Pd/CeO2-ZrO2. This is probably because the creation and migration of oxygen vacancies in the CeO2ZrO2 lattice was promoted by the substitution of Ce4+ and Zr4+ ions with transition metal ions. Since the active component of OSC is CeO2, the maximum OSC value is evaluated to be 250 mmol-O2·mol-CeO2−1 by assuming a complete redox cycle of CeO2: 2CeO2 ↔ Ce2O3 + 1/2O2. In addition, the oxygen uptake for the oxidation of Pd to PdO should be also allowed in OSC value. The OSC values of Pd/M/CZ normalized by molar concentration of CeO2 were calculated by subtracting the oxygen uptake for Pd oxidation to PdO from the total OSC evaluated. As seen in Table 1, reducible transition metal, such as Ni, Fe, Cu, Co and Sn, doped Pd/CZ samples were found to possess quite higher OSC than 250 mmol-O2·mol-CeO2−1, suggesting the participation of transition metal in reversible oxygen release/sorption. On the other hand, the OSC values of Ga and In, which are non-reducible additives, - doped Pd/CZ was less than 250 mmol-O2 mol-CeO2−1. Fe and Sn were found to be effective additives to achieve high OSC property. High OSC performance of Fe-doped Pd/CZ can be explained by the structure disorder as seen in Raman spectra (Fig. 2(c)). However, the effect of Sn can not be explained by the structure disorder. Taking into account the fact that Sn ion has both tetravalent (+4) and divalent (+2) states, high OSC value of Sn-doped Pd/CZ would be due to the redox of Sn2+/Sn4+ [8,32,33].
3.2.2. Lean/rich perturbation conditions As given in Table 1, the addition of transition metal caused an enhancement of OSC property of CeO2-ZrO2. Especially, the reducible transition metal is of interest to participate in reversible oxygen release/sorption. Therefore, Pd/M/CZ with high OSC property is expected to show high TWC performance under the lean/rich perturbation conditions. The TWC performance of Pd/M/CZ, with exception of Pd/Cu/CZ, Pd/In/CZ and Pd/Sn/CZ, which showed low NO reduction activity under stoichiometric conditions (Fig. 3(C)), was evaluated during the lean (+0.15% O2)/rich (+0.3% CO) perturbation at a frequency of 0.1 Hz. Fig. 5(A) shows the temperature dependency of the conversion efficiency of NO over Pd/M/CZ. It appears that the addition of transition metal caused a significant enhancement of NO reduction activity of Pd/
3.2. Three-way catalytic performance of transition metal - doped Pd/CeO2ZrO2 3.2.1. Stoichiometric conditions Fig. 3 shows the temperature dependence of the conversion efficiencies of NO, CO and C3H6 over Pd/M/CZ. Here, the reproducibility of catalytic activity data, for instance, light-off activity expressed as T50, was found to be fairly good with an experimental error of less than 5% for each experiment in this study. As can be seen in Fig. 3(A), no significant difference in the CO conversion efficiency was observed for Pd/ 3
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Fig. 3. Change in the conversion efficiencies of (A) CO, (B) C3H6 and (C) NO as a function of temperature for NO-CO-C3H6-O2 reaction under stoichiometric conditions.
high NO reduction activity under the lean/rich perturbation conditions. Fig. 5(B) shows the relationship between the light-off temperature (T50%) for NO reduction and CO and C3H6 oxidation over Pd/M/CZ. It is of interest that the light-off temperature for NO reduction was linearly correlated with that for C3H6 oxidation, while not with for CO oxidation, suggesting high utilization of C3H6 for NO reduction compared with that of CO. This is in accordance with the insights obtained for three-way catalytic reactions under stoichiometric conditions, as mentioned above. Considering the three-way catalytic performance of Pd/ M/CZ, Pd/Fe/CZ seems to be the most effective catalyst. 3.3. Effect of Fe loading on the activity of Pd/Fe/CZ 3.3.1. Three-way catalytic activity Since Pd/Fe/CZ was found to be the highest active three-way catalyst, the effect of Fe loading was investigated. As given in Fig. 6(A), the NO conversion efficiency on Pd/Fe/CZ under stoichiometric condition was clearly decreased with an increase in Fe loading, although the lightoff temperature was slightly lowered by addition of 0.8–1.6 mol% Fe. The addition of too much amount of Fe (6.3 mol%) caused a significant decrease in the NO reduction activity. Pd/CZ was found to be the highest active catalyst for the NO reduction under stoichiometric
Fig. 4. Relationship between Pd dispersion and T50% for NO reduction over Pd/ M/CZ.
CZ. Taking into account the fact that no increase in the NO reduction activity was observed under the stoichiometric condition (Fig. 3(C)), the improved OSC properties of Pd/M/CZ seems to be responsible for
Fig. 5. (A) Change in NO conversion as a function of temperature for NO-CO-C3H6-O2 reaction under lean/rich perturbation conditions. (B) Relationship between T50% value for NO reduction and T50% value for CO and C3H6 oxidation. 4
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Fig. 6. Change in NO conversion as a function of temperature for NO-CO-C3H6-O2 reaction over Pd/Fe/CZ with different Fe loading under (A) stoichiometric and (B) lean/rich perturbation conditions.
Various kinds of OSC measurement methods were reported so far. For example, Yao et al. [43] proposed the method to measure the dynamic OSC, where O2 and CO pulses were alternatively introduced into the catalyst. Madier et al. [44] compared OSC and OSCC (cumulative oxygen storage capacity) of CexZr(1-x)Om (x = 0.15–0.68), where OSC was evaluated from the amount of CO consumption or CO2 formation and O2 consumption by introducing each CO and O2 pulses, respectively, and OSCC was calculated from their total amount after a few pulses. They found that the reduction of CexZr(1-x)Om by CO gradually proceeds with one by one of pulses, while the oxidation of reduced CexZr(1-x)Om rapidly takes place by introducing one pulse of O2. Gong et al. [45,46] measured the dynamic OSC of Pd/Rh/CeZrOx by alternatively introducing reductant (60 s) and O2 (60 s) gases, and proposed a dual-site OSC model (surface and sub-surface CeO2) from the difference in the rate for oxygen uptake in flowing O2 after the reduction with H2, CO or CH4. They also found that the oxygen consumption behavior is different depending on the reductant. Therefore, it seems to be important to understand the reduction behavior in order to confirm the relevant of the OSC with the catalytic activity of CeO2-based materials. As mentioned above, the addition of Fe into CZ caused an improvement of the OSC of Pd/CZ, leading us to the expectation that the reducibility of Pd species can be altered by using Fe/CZ as a support. Then, H2-TPR measurement was performed for Pd/Fe/CZ with different Fe loading, where the H2-TPR profile was obtained after the pretreatment of consecutive reduction and reoxidation at 600 °C. This is because the lattice strain of Ce-O-Zr is stabilized via the consecutive reduction and reoxidation [47–49]. As seen in Fig. 7(a), a strong H2 consumption peak at 140 °C along with a shoulder peak at 97 °C were observed for Pd/CZ. The latter peak (ca. 100 °C) would be ascribed to the reduction of PdO, while the former (ca. 140 °C) is due to the reduction of Pd – CZ interface. Unfortunately, contrary to the expectation mentioned above, the addition of Fe into CZ did not cause a significant change in the reducibility of PdO species. Namely, no shift of H2 consumption peak at 97 °C was observed. As seen in Fig. 6(B), no significant difference in the temperature at which NO reduction initiates was observed for each Pd/Fe/CZ catalyst under lean/rich perturbation conditions. Therefore, the light-off of the TWC reaction would be
condition. On the other hand, the NO reduction activity of Pd/CZ under the lean/rich perturbation conditions was clearly enhanced by the addition of small amount (0.8 mol%) of Fe (Fig. 6(B)). Although no significant change in the NO reduction activity of Pd/Fe/CZ was observed when Fe loading was increased to 1.6 mol%, further increase in the Fe loading up to 6.3 mol% caused a decrease in the NO reduction activity. The Pd/Fe(6.3)/CZ showed lower NO reduction activity than Pd/CZ, suggesting that the presence of excess Fe additive causes a negative effect. The optimum loading of Fe was found to be 0.8–1.6 mol%. In Table 2 are summarized the BET surface area of and Pd dispersion in Pd/Fe/CZ with different Fe loading. It appears that both BET surface area and Pd dispersion were monotonically decreased with an increase in Fe loading. Since a significant decrease in Pd dispersion was observed for Pd/Fe(6.3)/CZ, this would be responsible for the low NO reduction activity. On the other hand, the improved NO reduction activity of Pd/ Fe/CZ with 0.8–3.0 mol% Fe doping can not be explained by BET surface area and Pd dispersion. 3.3.2. Effect of OSC and reducibility of Pd/Fe/CZ on TWC activity As given in Table 2, the OSC value of Pd/Fe/CZ was linearly increased with an increase in Fe loading. This would be explained by the participation of Fe species in reversible oxygen release/sorption. Fe2O3 is known to be consecutively reduced to Fe(0) by H2 in the temperature range of 300–800 °C via the formation of Fe(II) species [40–42]. In this study, H2-TPR of Pd/Fe/CZ was first performed up to 800 °C, and then OSC was evaluated by introducing O2 pulses at 600 °C. Therefore, Fe species incorporated into CZ is suspected to be reduced to Fe(0). Here, the OSC values originated from CZ without the participation of Fe(III)/ Fe(0) redox, as expressed in terms of OSC_Pd-CZ*, were recalculated by subtracting the oxygen uptake for the oxidation of Fe(0) to Fe(III) from the total OSC evaluated. As also summarized in Table 2, the OSC_Pd-CZ* values of Pd/Fe/CZ were still higher than the total OSC of Pd/CZ, suggesting that Fe species can improve the mobility and activity of oxygen species in the lattice of CeO2-ZrO2. It is of interest that only the Pd/Fe(6.3)/CZ catalyst, which was the lowest active catalyst for NO reduction activity (Fig. 6), gave higher OSC_Pd-CZ* value than the other catalysts. Too high OSC may cause a negative effect on the three-way catalytic performance of supported Pd. Table 2 Summary of BET surface area, Pd dispersion and OSC of Pd/Fe/CZ.
Pd/CZ Pd/Fe(0.8)/CZ Pd/Fe(1.6)/CZ Pd/Fe(3.0)/CZ Pd/Fe(6.3)/CZ a
BET surface area (m2 g−1)
Pd dispersion (CO/Pd)
OSC at 600 °C (μmol-O2·g-cat−1)
OSC_Pd-CZ* at 600 °Ca (μmol-O2·g-cat−1)
33 33 32 30 24
0.25 0.24 0.23 0.18 0.05
443.8 504.1 505.2 582.9 722.8
443.8 482.2 470.8 498.2 544.4
OSC_Pd-CZ* was recalculated by subtracting the oxygen uptake for the oxidation of Fe(0) to Fe(III) from the total OSC evaluated. 5
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Table 3 TOF of NO reduction for NO-CO-C3H6-O2 reaction over Pd/Fe/CZ under lean/ rich perturbation conditions. TOF (mol of NO reacted·min−1·Pd-atoms−1)
Pd/CZ Pd/Fe(0.8)/CZ Pd/Fe(1.6)/CZ Pd/Fe(3.0)/CZ Pd/Fe(6.3)/CZ
260 °C
280 ºC
0.77 1.50 1.45 1.21 2.37
1.82 3.19 3.30 3.02 3.99
Fig. 7. H2-TPR profiles of Pd/Fe/CZ with different Fe loading. (a) Pd/CZ, (b) Pd/Fe(0.8)/CZ, (c) Pd/Fe(1.6)/CZ, (d) Pd/Fe(3.0)/CZ and (e) Pd/Fe(6.3)/CZ.
related to the reducibility of PdO species. On the other hand, the H2 consumption peak due to the reduction of Pd – CZ interface was slightly shifted to lower temperature by the addition of Fe (0.8–3.0 mol%). This indicates that the presence of small amount of Fe can slightly improve the reducibility of Pd – CZ interface. On the other hand, as seen in Fig. 7(e), Pd/Fe(6.3)/CZ, which is a less active catalyst than Pd/CZ (Fig. 6(B)), gave the H2 consumption peak at 148 °C in the H2-TPR profile, suggesting the formation of slightly irreducible Pd – CZ interface. This is probably due to too strong interaction of Pd with Fe(6.3)/CZ, which possesses high OSC. Since three-way catalytic reactions under lean/rich perturbation conditions was reported to proceed via the redox of Pd and CZ [50], the formation of slightly irreducible Pd – CZ interface in Pd/Fe(6.3)/CZ might be responsible for low TWC activity. Unfortunately, the markedly improved TWC activity of Pd/Fe/CZ with 0.8–3.0 mol% Fe doping can not be explained by a slight promotion of the reducibility of Pd – CZ interface, suggesting the presence of other factors affecting the TWC activity.
Fig. 8. FT-IR spectra of CO species adsorbed on (a) Pd/CZ, (b) Pd/Fe(0.8)/CZ, (c) Pd/Fe(1.6)/CZ, (d) Pd/Fe(3.0)/CZ and (e) Pd/Fe(6.3)/CZ.
In order to confirm the effect of the improved OSC properties of CZ by Fe additive (Table 2) on the surface valence state of Pd, FT-IR spectra of CO species adsorbed on Pd/Fe/CZ catalysts, which were pretreated with the stoichiometric reaction gas, were measured to gain information on the surface valence state of Pd. As seen in Fig. 8(a), the exposure of CO to Pd/CZ treated with the reaction gas caused an appearance of two IR bands at 2151 and 2091 cm−1, which can be ascribed to CO species linearly boned on Pdδ+ and Pd0, respectively [26,27]. This suggests that PdO supported on CZ can not be completely reduced to Pd metal, resulting in the presence of Pd particles with cationic and metallic state. On the other hand, Pd/Fe/CZ with 0.8–3.0 mol% of Fe loadings showed very similar IR spectrum: namely, three distinct IR bands at 2142, 2086 and 1972 cm−1 were observed. These bands can be assigned to CO species linearly boned on Pdδ+ and Pd0 and bridgebonded on Pd0, respectively [26,27]. It should be noted that Pd/Fe/CZ gave strong IR bands due to CO species adsorbed on Pd0 sites, compared with those on Pdδ+ sites. A similar surface valence state of Pd supported on Fe/CZ with 0.8–3.0 mol% of Fe loadings would be responsible for the similar TOF values of NO reduction (Table 3). In contrast, relatively strong IR band due to CO species adsorbed on Pdδ+ was observed for Pd/CZ, resulting in low TOF value. It is of interest that Pd/Fe(6.3)/CZ, which showed lower TWC activity under the lean/rich perturbation conditions than Pd/Fe(0.8–3.0)/CZ (Fig. 6(B)), showed very similar IR spectrum with that for Pd/CZ, suggesting the similarity of surface valence state of Pd. These results suggest that Pd species
3.3.3. Effect of surface valence state of Pd on TWC activity Since the catalytically active species of Pd/Fe/CZ for NO−CO-C3H6O2 reaction must be Pd, it would be important to understand the effect of OSC on the chemical state of Pd under the reaction conditions. In order to gain information leading to consider the chemical state of Pd supported on Fe/CZ, turnover frequency (TOF), expressed as moles of NO reacted per mole of surface Pd atoms per minute, of NO reduction was evaluated under the lean/rich perturbation conditions at 260 and 280 °C. As summarized in Table 3, Pd/CZ gave the smallest value of TOF, while the highest TOF was obtained for Pd/Fe(6.3)/CZ. It should be noted that Pd/Fe/CZ with 0.8–3.0 mol% Fe doping, which showed the improved NO reduction activity, gave similar TOF values. These results suggest that the catalytically active state of Pd is different depending on the Fe loading. In other words, the chemical state of Pd may be altered depending on the OSC of Fe/CZ. Machida et al. [51] investigated the role of OSC in the stabilization of Pd metal during the TWC reaction over Pd/CZ under fluctuating A/F ratios, and found the importance of the oxygen migration at the three-phase boundary, where Pd, CZ and the gas phase meet. This suggests that the OSC property of CZ plays an important role in controlling the chemical state of Pd under the reaction conditions. 6
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supported on CZ and Fe(6.3)/CZ are stabilized not only in reducing but also in oxidizing state during the TWC reaction, while those on Fe (0.8–3.0)/CZ stabilized in reducing state. Namely, the presence of relatively large portion of Pdδ+ species in Pd/Fe(6.3)/CZ is suspected to be due to too strong interaction between Pd and Fe(6.3)/CZ with high OSC. Although Pd/Fe(6.3)/CZ (Fig. 8(e)) showed similar IR spectrum obtained for Pd/CZ (Fig. 8(a)), the former catalyst gave higher TOF than the latter one (Table 3). A new active sites, which can not be clarified by FT-IR spectroscopy, may be created via too strong Pd – Fe/ CZ, and then may participate in the TWC reaction. In conclusion, the role of Fe additive is to create the easily reducible Pd species interacting with Fe-doped CeO2-ZrO2 and then to stabilize the surface valence state in catalytically active reducing state.
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4. Conclusions The additive effect of transition metal (Ni, Fe, Co, Cu, Sn, Ga, In) on the catalytic activity of Pd/CeO2-ZrO2 for NO−CO-C3H6-O2 reaction under stoichiometric and lean/rich perturbation conditions was investigated. Oxygen storage capacity (OSC) of Pd/CeO2-ZrO2 was efficiently improved by the addition of transition metal. Especially, Fe and Sn were effective additives to achieve high OSC property. Although the addition of transition metal caused an increase in the OSC, the threeway catalytic activity of Pd/CeO2-ZrO2 under the stoichiometric conditions was not clearly improved by the addition of transition metal. On the other hand, the addition of Ni, Co, Fe, and Ga into Pd/CeO2-ZrO2 caused a significant enhancement of three-way catalytic activity under the lean/rich perturbation conditions. H2-TPR measurements of Fe – doped Pd/CeO2-ZrO2 with different Fe content revealed that easily reducible Pd species interacting with Fe-doped CeO2-ZrO2 are created by the addition of small amount of Fe (0.8–3.0 mol%). In contrast, the addition of excess Fe led to the formation of a little irreducible Pd species, resulting in the low three-way catalytic activity. FT-IR spectroscopy following CO adsorption also revealed that the valence state of Pd species interacting with Fe-doped CeO2-ZrO2 is stabilized in the reducing state under the reaction conditions. The promoting effect of Fe was explained by the creation of easily reducible Pd species, which are stabilized in the catalytically active reducing state under the reaction conditions, interacting with Fe-doped CeO2-ZrO2. References [1] R.Di Monite, J. Kašpar, Top. Catal. 28 (2004) 47–57. [2] H. Hirata, Catal. Surv. Asia 18 (2014) 128–133. [3] T. Montini, M. Melchionna, M. Monai, P. Fornasiero, Chem. Rev. 116 (2016) 5987–6041. [4] A. Morikawa, T. Suzuk, T. Kanazawa, K. Kikuta, A. Suda, H. Shinjo, Appl. Catal. B 78 (2008) 210–221. [5] A. Trovarelli, Catal. Rev. Sci. Eng. 38 (1996) 439–520. [6] D. Duprez, C. Descorme, A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, 2013, pp. 343–375. [7] D.H. Prasad, S.Y. Park, H.I. Ji, H.R. Kim, J.W. Son, B.K. Kim, H.W. Lee, J.H. Lee, J. Phys. Chem. C 116 (2012) 3467–3476. [8] K. Yasuda, A. Yoshimura, A. Katsuma, T. Masui, N. Imanaka, Bull. Chem. Soc. Jpn. 85 (2012) 522–526.
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