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LaPO4 in N2O decomposition and CO oxidation
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Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation Huan Liu, Zhen Ma∗ Shanghai Key Laboratory of Atmospheric Particle Pollution & Prevention (LAP3 ), Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China
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Article history: Received 17 August 2016 Revised 16 November 2016 Accepted 18 November 2016 Available online xxx Keywords: Nanostructures N2 O decomposition CO oxidation Rh LaPO4
a b s t r a c t LaPO4 has been used as a support to make supported catalysts in some investigations, but the effect of different LaPO4 supports on the catalytic performance of supported catalysts has been rarely reported. In this work, conventional hexagonal LaPO4 (LaPO4 –H) was prepared by precipitation, and hexagonal LaPO4 nanowires (LaPO4 –HNW) was prepared by hydrothermal synthesis at 150 °C. Monoclinic LaPO4 nanowires (LaPO4 -MNW-220 and LaPO4 -MNW-900) were prepared by hydrothermal synthesis at 220 °C or calcining LaPO4 –HNW at 900 °C. These samples were used to support Rh2 O3 via impregnation using Rh(NO3 )3 as a precursor. The activities of these supported catalysts in N2 O decomposition and CO oxidation follow the same sequence of Rh2 O3 / LaPO4 –HNW > Rh2 O3 / LaPO4 –H > Rh2 O3 / LaPO4 -MNW220 > Rh2 O3 / LaPO4 -MNW-900, i.e., two hexagonal LaPO4 supports are better than two monoclinic LaPO4 supports in making active catalysts, and hexagonal LaPO4 nanowires are better than hexagonal LaPO4 composed of nanoparticles and short nanorods. The most active catalyst has highly dispersed Rh2 O3 species interacting strongly with the support, more oxygen-adsorption capacity, and more basic sites. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Conventional supported catalysts are prepared by loading metals or metal oxides onto solid supports such as oxides, carbon, and zeolites. Different metal phosphates have different acid–base/redox properties and thermal stability, thus making them suitable as catalysts [1–6] or supports [7–15]. LaPO4 , a typical metal phosphate, can be used for the dehydration of light alcohols [16,17], oxidative dehydrogenation of isobutene [18], and selective alkylation of catechol with methanol [19]. LaPO4 can also be used as a catalyst support. Examples in this regard include Au/LaPO4 [9,10,12,13,20] and Pt/LaPO4 [21] for CO oxidation, Pd/LaPO4 for NO decomposition [22], Rh/LaPO4 for three-way catalysis [23], and Pt/LaPO4 for photocatalytic reduction of CO2 with H2 O [24]. However, the effect of different LaPO4 supports on the catalytic performance of LaPO4 supported catalysts has been rarely reported. Yan et al. found that gold supported onto hexagonal LaPO4 nanoparticles (average particle size: 6–8 nm; BET surface area: 111 m2 /g) is more active than gold supported on monoclinic LaPO4 nanoparticles (average particle size: 10 nm; BET surface area: 55 m2 /g) in CO oxidation [9]. They inferred that Au/hexagonal LaPO4 nanoparticles has stronger Au-LaPO4 interaction, consider-
∗
Corresponding author. Fax: +86 2165643597. E-mail address: [email protected] (Z. Ma).
ing the catalyst’s high activity and thermal stability, and they provided high-resolution TEM images for proving the strong interaction. However, it is not clear whether the support effect (hexagonal support vs. monoclinic support) is general for other LaPO4 -based catalysts and for other reactions. In addition, more experiments are certainly needed to unravel the reasons for the support effect. We revisited the Au/LaPO4 system [20]. Au/commercial LaPO4 prepared by deposition–precipitation with urea was found to be the most active in CO oxidation among Au/metal phosphate (Au/M-P-O, M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts. In addition, Au/hexagonal LaPO4 nanowires developed was found to be much more active than Au/commercial LaPO4 due to the presence of smaller gold particles, a small fraction of Au+ species, and more adsorbed active oxygen species. Although it is clear that hexagonal LaPO4 nanowires are better than hexagonal commercial LaPO4 (composed of LaPO4 nanoparticles and nanorods) in making supported gold catalysts for CO oxidation, it is not clear whether the support effect is general for other LaPO4 -based supported catalysts and for other reactions. In addition, the effect of crystal phases (hexagonal versus monoclinic) of LaPO4 supports on the catalytic performance was not addressed. LaPO4 samples with various crystal phases and morphologies can be prepared via different methods. For instance, hexagonal LaPO4 nanoparticles can be prepared by direct precipitation [25]. Hexagonal LaPO4 nanowires can be prepared via a hydrothermal
http://dx.doi.org/10.1016/j.jtice.2016.11.024 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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process [26]. Monoclinic LaPO4 can be prepared by either heating hexagonal LaPO4 above 800 °C [27] or by hydrothermal synthesis at 220 °C [28]. These synthesis protocols may provide opportunities for investigating the influence of different LaPO4 supports on the performance of LaPO4 -supported catalysts. N2 O is a greenhouse gas and it may also contribute to ozonelayer depletion. Catalysts used for the decomposition (abatement) of N2 O include metal oxides, ion-exchanged zeolites, and supported metal catalysts [29–31]. Rh-based catalysts for N2 O decomposition include Rh-ZSM-5 [32], Rh/MgO [33], Rh/USY [34], Rh/Al2 O3 [35,36], Rh/CeO2 [37,38], Rh/KIT-6 [39], Rh/SBA15 [40,41], and Rh/MCM-41 [42]. Our group developed a series of Rh2 O3 /metal phosphate (Rh2 O3 /M-P-O, M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts for N2 O decomposition [43], and found that Rh2 O3 /hydroxyapatite is the most active among these catalysts and Rh2 O3 /LaPO4 is the second most active. However, these metal phosphates are all from commercial sources, with little control over crystal phase and morphology. Herein, we revisited the Rh2 O3 /LaPO4 system by using four LaPO4 supports prepared by different methods. Difference in catalytic activities in N2 O decomposition and CO oxidation was found, and these catalysts were characterized. 2. Experimental 2.1. Preparation 2.1.1. Hexagonal LaPO4 (LaPO4 –H) 6.92 g La(NO3 )3 ·6H2 O (Aladdin, 99.9%) and 1.84 g NH4 H2 PO4 (Sinopharm, AR) were dissolved in 80 ml deionized water under vigorous stirring for 20 min. The pH value was adjusted to 6 by adding aqueous ammonia (27 wt.%). The mixture was stirred for 6 h. The product was filtered, washed with ethanol, and dried at 80 °C. 2.1.2. Hexagonal LaPO4 nanowires (LaPO4 –HNW) [26] 6.92 g La(NO3 )3 ·6H2 O and 1.84 g NH4 H2 PO4 were dissolved in 80 ml deionized water under vigorous stirring for 20 min. The pH value was adjusted to 1 by adding aqueous ammonia (27 wt.%). The suspension was transferred into a Teflon-lined stainless steel autoclave and heated at 150 °C for 12 h. The product was filtered, washed with ethanol, and dried at 80 °C. 2.1.3. Monoclinic LaPO4 nanowires (LaPO4 -MNW-220 and LaPO4 -MNW-900) Method 1: the process is similar to the preparation of LaPO4 –HNW except that the hydrothermal treatment temperature was 220 °C and the duration of hydrothermal treatment was 18 h [28]. Method 2: LaPO4 –HNW was calcined at 900 °C for 3 h [27]. 2.1.4. Rh2 O3 /LaPO4 catalysts 2 ml Rh(NO3 )3 solution (0.01 g/ml based on Rh) was placed in an agate mortar containing 1.98 g LaPO4 . The mixture was ground till dry under an infrared lamp. The powders were calcined at 500 °C for 3 h. 2.2. Characterization X-ray diffraction (XRD) data were obtained on a MSAL XD2 instrument with Cu Kα radiation. The scanning rate was 8°/min, and the step length was 0.01°. N2 adsorption-desorption experiments were conducted using a Micromeritics Tristar 30 0 0 instrument. The sample was degassed in vacuum at 300 °C for 3 h before analysis. ICP-OES analysis was performed on a Perkin–Elmer OPTIMA 2100 DV optical emission spectrometer. 50 mg of sample
was dissolved in a mixture of 9 ml HCl, 3 ml HNO3 , 1 ml HClO4 , 0.5 ml H2 O2 , and 3 ml HF reagents, and heated at 150 °C for 2–3 h. After that, 3 ml HCl, 1 ml HNO3 , 1 ml HF, and 0.5 ml HClO4 were added again. The mixture was transferred to an autoclave, heated at 180 °C for 4 h, allowed to cool down, and then diluted with distilled water for analysis. TEM images were obtained by a JEM-2011F transmission electron microscope operated at 200 kV. The samples were dispersed in ethanol and placed on a carbon-coated molybdenum grid followed by solvent evaporation. The sizes of RhOx nanoparticles were obtained by measuring 100 particles for each sample, using the DigitalMicrograph software. XPS spectra were recorded on a Perkin–Elmer PHI 50 0 0 C spectrometer with Mg Kα X-ray source. The carbonaceous C1 s line (284.8 eV) was used as the reference to calibrate the binding energies (BE). CASXPS software was used for analysis. CO2 temperature-programmed desorption (CO2 -TPD) experiments were performed on a FINESORB-3010 instrument. 0.15 g of sample was pretreated at 200 °C with He (30 ml/min) for 1 h and then cooled down to 50 °C. The sample was saturated with 5% CO2 /He (40 ml/min) for 1 h and swept by He flow at 50 °C for 3 h. The sample was heated to 600 °C at a rate of 10 °C/min, and the amount of desorbed CO2 was calibrated using 5% CO2 /He with known volume and concentration. O2 temperature-programmed desorption (O2 -TPD) experiments were carried out on a FINESORB-3010 instrument. A sample (0.15 g) was pretreated with He (30 ml/min) at 500 °C for 2 h, cooled to room temperature, and O2 (30 ml/min) was adsorbed at 50 °C for 1 h. Then O2 -TPD was carried out with flowing He (30 ml/min) from 50 to 600 °C at a heating rate of 10 °C/min. Temperature-programmed reduction (H2 -TPR) experiments were carried out on a Micromeritics Auto Chem II instrument. 50 mg of sample was pretreated at 200 °C with He (15 ml/min) for 20 min and then cooled down to 60 °C. The flowing gas was switched to 5% H2 /Ar (50 ml/min), and the sample was swept for 2 h. The catalyst was then heated to 800 °C at a ramping rate of 10 °C/min.
2.3. Catalytic activity measurements N2 O decomposition was measured in a fixed bed reactor. 0.25 g of catalyst was loaded into a U-shaped quartz tube (7 mm inner diameter). 0.5% N2 O/He was flowed through the catalyst (flow rate: 60 ml/min). The catalyst was maintained at room temperature for 1 h, and the reaction temperature was then increased stepwise and kept at each temperature step for 0.5 h. The effluent gas was analyzed every 10 min using an on-line GC (Agilent 7890A). The N2 O conversion was calculated as ([N2 O]in – [N2 O]out )/[N2 O]in × 100%. Catalytic CO oxidation was evaluated on a FINESORB-3010 instrument at atmospheric pressure. 0.25 g of catalyst was loaded into a U-shaped glass tube for the reaction. The reaction gas was composed of 1% CO in air. The flow rate was 50 ml/min. The catalyst was maintained at room temperature for 1 h, and heated to 200 °C at a ramping rate of 0.5 °C/min. The effluent gas was analyzed periodically (every 10 min) using an on-line gas chromatograph (GC; Agilent 7890A, equipped with a TCD detector) capable of separating CO, CO2 , N2 , and O2 , using He as a carrier gas. The CO conversion was calculated as ([CO]in – [CO]out )/[CO]in × 100%, where [CO]in is the CO concentration at room temperature (at which no reaction occurs), and [CO]out is the CO concentration at an elevated temperature. Here the catalysts used were fine powders. There was no difference in catalytic activity of the fine powder catalyst and a catalyst with grain sizes of 40–60 meshes (Fig. S1).
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Fig. 3. HRTEM images of Rh2 O3 /LaPO4 catalysts.
Fig. 1. XRD patterns of Rh2 O3 /LaPO4 catalysts.
Fig. 2. TEM images of Rh2 O3 /LaPO4 catalysts.
3. Results 3.1. Basic physicochemical properties Fig. 1 shows the XRD patterns of LaPO4 and Rh2 O3 /LaPO4 . LaPO4 –HNW prepared by hydrothermal synthesis at 150 °C and LaPO4 –H prepared by conventional precipitation show the hexagonal phase (PDF no. 46–1439), whereas LaPO4 -MNW-220 prepared by hydrothermal synthesis at 220 °C and LaPO4 -MNW-900 prepared by calcining LaPO4 –HNW at 900 °C show the monoclinic phase (PDF no. 32–0493). Rh2 O3 /LaPO4 samples have XRD patterns similar to their corresponding supports. No peaks due to Rh2 O3 are detected, probably due to the low Rh content (∼1 wt.%) and high dispersion of Rh2 O3 . Fig. 2 shows the TEM images of Rh2 O3 /LaPO4 . Only the LaPO4 supports can be seen, whereas Rh2 O3 species are not seen at this low resolution. Rh2 O3 /LaPO4 –HNW has LaPO4 nanowires (width: 10–30 nm; length: 50–650 nm). Rh2 O3 /LaPO4 –H has irregular aggregates composed of LaPO4 nanoparticles (particles size:
45–100 nm) and short nanorods (width: 4–10 nm; length: 10– 65 nm). Rh2 O3 /LaPO4 -MNW-220 contains LaPO4 nanowires (width: 10–40 nm; length: 60–1200 nm), so does Rh2 O3 /LaPO4 -MNW-900 (nanowires width: 10–50 nm; length: 60–1200 nm). Fig. 3 shows the HRTEM images of Rh2 O3 /LaPO4 , focusing on the details of LaPO4 supports. The interplanar distances of LaPO4 –HNW and LaPO4 –H are 0.619 and 0.616 nm, respectively, corresponding to the (100) plane of hexagonal LaPO4 [26]. Figs. S2 and S3 in the Supporting Information present additional HRTEM images and the corresponding selected-area electron diffraction (SAED) patterns of Rh2 O3 /LaPO4 –HNW and Rh2 O3 /LaPO4 –H, respectively, showing that these LaPO4 supports are hexagonal. On the other hand, the interplanar distance of LaPO4 -MNW-220 is 0.476 nm, corresponding to the (110) plane of monoclinic LaPO4 [28]. The interplanar distance of LaPO4 -MNW-900 is 0.425 nm, corresponding to the (−111) plane of monoclinic LaPO4 [27]. Figs. S4 and S5 present additional HRTEM images and the corresponding selected-area electron diffraction (SAED) patterns of Rh2 O3 /LaPO4 MNW-220 and Rh2 O3 /LaPO4 -MNW-900, respectively, showing that these LaPO4 supports are monoclinic. Fig. 4 shows the TEM images of Rh2 O3 /LaPO4 , focusing on the supported Rh2 O3 species. No Rh2 O3 particles can be observed for Rh2 O3 /LaPO4 –HNW (Fig. 4(a)) and Rh2 O3 /LaPO4 –H (Fig. 4(b)), indicating that Rh2 O3 species are highly dispersed on supports. TEM-mapping images show that the Rh2 O3 species are well dispersed on LaPO4 –HNW (Fig. S6) and LaPO4 –H (Fig. S7). However, Rh2 O3 particles can be seen clearly for Rh2 O3 /LaPO4 -MNW-220 (Figs. 4(c) and S8) and Rh2 O3 /LaPO4 -MNW-900 (Figs. 4(d) and S9). On the basis of 100 particles for each sample, the mean Rh2 O3 particle sizes are estimated to be 1.3 ± 0.7 and 2.0 ± 0.5 nm for Rh2 O3 /LaPO4 -MNW-220 and Rh2 O3 /LaPO4 -MNW-900, respectively. The particle size distributions of these two catalysts are shown in Fig. S10. 3.2. Catalytic performance The catalytic performance of different Rh2 O3 /LaPO4 catalysts was studied using N2 O decomposition (N2 O = N2 + 1/2O2 ) as a probe reaction [29–31]. As shown in Fig. 5, the activities of catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW-900, when comparing the catalysts’ T50 (temperature required for achieving 50% conversion) values (260, 267, 285, and 314 °C, respectively) or N2 O conversions at 275 °C (78%, 66%, 36%, and 17%, respectively). The Rh contents of these catalysts are determined by ICP-OES as 1.04 wt.%, 1.01 wt.%, 0.98 wt.%, and 0.99 wt.%, respectively. Thus, the specific rates of these catalysts at 275 °C are calculated to be
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Fig. 4. TEM images of Rh2 O3 /LaPO4 catalysts, focusing on Rh2 O3 species. (a) Rh2 O3 /LaPO4 –HNW; (b) Rh2 O3 /LaPO4 –H; (c) Rh2 O3 /LaPO4 -MNW-220; (d) Rh2 O3 /LaPO4 -MNW900.
Fig. 5. N2 O conversions over Rh2 O3 /LaPO4 catalysts as a function of reaction temperature.
Fig. 6. CO conversions over Rh2 O3 /LaPO4 catalysts as a function of reaction temperature.
241, 210, 118, and 55 mmol/gRh /h, respectively. The corresponding LaPO4 supports show no activity below 400 °C. The most active Rh2 O3 /LaPO4 –HNW was subjected to additional testing in N2 O decomposition. As shown in Fig. S11, at a reaction temperature of 290 °C, the initial N2 O conversion reaches 96 %, consistent with the N2 O conversion at 290 °C in Fig. 5. The N2 O conversion decreases slowly to 61% after 120 h on stream. TEM image (Fig. S12) of Rh2 O3 /LaPO4 –HNW collected after stability test still shows no obvious Rh2 O3 particles on the support. The reason for the decrease in activity may be the accumulation of oxygen on the catalyst during the reaction, as confirmed by O2 -TPD data described later. We additionally tested the influence of co-fed O2 and/or H2 O on catalytic activity. As shown in Fig. S13, the presence of 5% O2 in the reaction mixture inhibits the activity less obviously than the presence of 2% H2 O. Both inhibiting effects are reversible,
i.e., the N2 O conversion restores after the retraction of O2 or H2 O (Fig. S14). These catalysts were tested in CO oxidation, a conventional and sensitive probe reaction [44–46]. As shown in Fig. 6, the activities of catalysts still follow the sequence of Rh2 O3 /LaPO4 –HNW (T50 = 36 °C) > Rh2 O3 /LaPO4 –H (T50 = 70 °C) > Rh2 O3 /LaPO4 -MNW220 (T50 = 114 °C) > Rh2 O3 /LaPO4 -MNW-900 (T50 = 118 °C). The shapes of conversion curves of Rh2 O3 /LaPO4 -MNW-220 and Rh2 O3 /LaPO4 -MNW-900 are somewhat different from those of Rh2 O3 /LaPO4 –H and Rh2 O3 /LaPO4 –HNW, but this feature is reproducible (Figs. S15 and S16). However, Rh2 O3 /LaPO4 –HNW deactivates quickly during the stability test conducted at 55 °C (Fig. S17), probably due to the strong adsorption of CO2 or the buildup of carbonate species on basic sites. The corresponding LaPO4 supports are inactive below 200 °C (data not shown).
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Fig. 7. CO2 -TPD profiles of Rh2 O3 /LaPO4 catalysts.
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Fig. 8. O2 -TPD profiles of Rh2 O3 /LaPO4 catalysts.
3.3. Additional characterization of LaPO4 and Rh2 O3 /LaPO4 catalysts After demonstrating that the catalytic activities of the catalysts are quite different, we then present data from in-depth characterization of Rh2 O3 /LaPO4 catalysts and relevant supports in order to unravel the reasons for the difference in catalytic activity. Fig. S18 shows the Rh 3d XPS spectra of Rh2 O3 /LaPO4 catalysts. There are only two peaks at approximately 306–311 and 311–318 eV, assigned to Rh3+ 3d5/2 and Rh3+ 3d3/2 [47,48], respectively. The data confirm that the Rh species exist as Rh2 O3 . Fig. S19 shows the O 1 s XPS spectra of Rh2 O3 /LaPO4 catalysts. The most intense component at ca. 531.1 eV corresponds to lattice oxygen and a shoulder peak at ca. 532.5 eV is attributed to hydroxyl groups [24,49]. The relative proportions of hydroxyl groups (among all oxygen species) in Rh2 O3 /LaPO4 –HNW, Rh2 O3 /LaPO4 –H, Rh2 O3 /LaPO4 -MNW-220, and Rh2 O3 /LaPO4 -MNW-900 are similar (18.2%, 18.5%, 18.3%, and 18.9%, respectively). Fig. 7 shows the CO2 -TPD profiles of LaPO4 and Rh2 O3 /LaPO4 . These supports all exhibit a CO2 desorption peak ranging from ∼130 to ∼450 °C. The amounts of basic sites, determined from the peak areas and calibration, for these supports follow the sequence of LaPO4 –HNW (377 μmol/g) > LaPO4 –H (303 μmol/g) > LaPO4 MNW-220 (63 μmol/g) > LaPO4 -MNW-900 (30 μmol/g), and the amounts of basic sites for supported catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW (293 μmol/g) > Rh2 O3 /LaPO4 –H (252 μmol/g) > Rh2 O3 /LaPO4 -MNW-220 (54 μmol/g) > Rh2 O3 /LaPO4 MNW-900 (27 μmol/g). The data indicate that the basicity of catalysts mainly originates from the supports, and the loading of Rh2 O3 onto LaPO4 supports may decrease the number of basic sites to some extent. Fig. 8 shows the O2 -TPD profiles of LaPO4 and Rh2 O3 /LaPO4 . O2 starts to desorb from LaPO4 –HNW, LaPO4 –H, LaPO4 -MNW-220, and LaPO4 -MNW-900 at 265, 289, 212, and 166 °C, respectively. The peak areas of these supports decrease following the sequence of LaPO4 –HNW > LaPO4 –H > LaPO4 -MNW-220 > LaPO4 -MNW900. The supporting of Rh species increases the peak areas for all these catalysts. The peak areas of supported catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 MNW-220 > Rh2 O3 /LaPO4 -MNW-900. Fig. 9 shows the H2 -TPR profiles of Rh2 O3 /LaPO4 . These catalysts show a single peak corresponding to the reduction of Rh2 O3 to metallic Rh [50,51]. The reduction temperature of catalysts decreases following the consequence of Rh2 O3 /LaPO4 –HNW (228 °C) > Rh2 O3 /LaPO4 –H (218 °C) > Rh2 O3 /LaPO4 -MNW-220 (205 °C) >
Fig. 9. H2 -TPR profiles of Rh2 O3 /LaPO4 catalysts.
Rh2 O3 /LaPO4 -MNW-900 (194 °C). The data provide a key piece of evidence for the strong interaction between Rh2 O3 and LaPO4 for the most active catalysts. 4. Discussion According to XRD (Fig. 1) and TEM (Fig. 2) data, LaPO4 –HNW prepared by hydrothermal synthesis at 150 °C exhibits hexagonal LaPO4 nanowires. LaPO4 –H prepared by precipitation exhibits hexagonal nanoparticles and short nanorods. Both LaPO4 -MNW220 and LaPO4 -MNW-900 exhibit monoclinic nanorods. The catalytic activities of Rh2 O3 /LaPO4 catalysts in N2 O decomposition and CO oxidation follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900 (Figs. 5 and 6), i.e., (1) hexagonal LaPO4 is better than monoclinic LaPO4 in making active Rh2 O3 /LaPO4 catalysts; (2) hexagonal LaPO4 nanowires are better than hexagonal LaPO4 nanoparticles and short nanorods prepared by precipitation in making active Rh2 O3 /LaPO4 catalysts. The activity trend is in line with the findings that: (1) Au/hexagonal LaPO4 is more active than Au/monoclinic LaPO4 in CO oxidation [9]; (2) Au/hexagonal LaPO4 nanowires is more active than Au/commercial LaPO4 (composed of LaPO4 nanoparticles and short nanowires) for CO oxidation [20].
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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ICP data show that the actual Rh content of these catalysts are 1.04 wt.%, 1.01 wt.%, 0.98 wt.%, and 0.99 wt.%, respectively, all closed to 1 wt.%. The XPS data confirm that the Rh species exist as Rh2 O3 (Fig. S18) and the relative proportions of surface hydroxyls (among oxygen species) of these catalysts are similar to each other (Fig. S19). Therefore, the difference in catalytic activity of different Rh2 O3 /LaPO4 catalysts is not due to the Rh content, oxidation state of Rh species, and relative proportion of hydroxyl group of these catalysts. As revealed by TEM data (Fig. 4), Rh2 O3 species are invisible on LaPO4 –HNW and LaPO4 –H, whereas they (with average sizes of 1.3 ± 0.7 and 2.0 ± 0.5 nm, respectively) are seen on LaPO4 MNW-220 and LaPO4 -MNW-900 (Figs. S8–S10). The size of Rh2 O3 species may influence the catalytic activity of these catalysts, in the sense that highly dispersed Rh2 O3 species (invisible in TEM images) display higher activity than Rh2 O3 nanoparticles (with average sizes of 1.3 ± 0.7 and 2.0 ± 0.5 nm). The specific surface areas of catalysts follow the sequence of Rh2 O3 /LaPO4 –H (98.8 m2 /g) > Rh2 O3 /LaPO4 –HNW (54.4 m2 /g) > Rh2 O3 /LaPO4 -MNW-220 (32.1 m2 /g) > Rh2 O3 /LaPO4 -MNW-900 (25.0 m2 /g). The better dispersion (i.e., smaller sizes) of Rh2 O3 species on hexagonal versus monoclinic LaPO4 supports may be due to the larger specific surface areas of hexagonal LaPO4 supports. Because monoclinic supports, i.e., LaPO4 -MNW-220 and LaPO4 MNW-900, were prepared by hydrothermal synthesis at a high temperature (220 °C instead of 150 °C) or by calcining LaPO4 –HNW at a high temperature (900 °C), it is not surprising that monoclinic LaPO4 supports have lower surface areas than hexagonal LaPO4 supports prepared under milder conditions [9]. As shown in the TEM images of Fig. 2, LaPO4 -MNW-220 exhibits nanowires (width: 10–40 nm; length: 60–1200 nm), so does LaPO4 -MNW-900 (width: 10–50 nm; length: 60–1200 nm). LaPO4 –HNW exhibits nanowires (width: 10–30 nm; length: 50–650 nm), whereas LaPO4 –H is composed of nanoparticles and short nanorods. These TEM data indicate that, monoclinic LaPO4 nanowires are generally longer and wider than hexagonal LaPO4 , which corresponds to the lower surface areas of the former supports. Although in general specific surface areas of Rh2 O3 /LaPO4 catalysts correlate with the size of Rh2 O3 species and thus with the catalytic activity, specific surface area may not be the sole factor that determines the activity, because although Rh2 O3 /LaPO4 –H has a larger specific surface area (98.8 m2 /g) than Rh2 O3 /LaPO4 –HNW (54.4 m2 /g), it is less active than the latter (Figs. 5 and 6). Certainly, LaPO4 nanowires are better than LaPO4 nanoparticles and short nanorods in making active Rh2 O3 /LaPO4 catalysts. This conclusion is consistent with the finding that Au/hexagonal LaPO4 nanowires (specific surface area: 60.8 m2 /g) is more active than Au/commercial LaPO4 (specific area: 95.6 m2 /g; composed of LaPO4 nanoparticles and nanorods) for CO oxidation [9]. CO2 -TPD data (Fig. 7) show that the amounts of basic sites of catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900, correlating with the activity sequence. It has been reported that the basic property of a catalyst is beneficial for catalytic N2 O decomposition [52] and CO oxidation [21]. However, it must be emphasized that basicity per se does not guarantee high activity: LaPO4 supports also have ample basic sites (Fig. 7), but they are not active for N2 O decomposition and CO oxidation. It seems that Rh2 O3 species highly dispersed on LaPO4 supports with ample basic sites exhibit the highest activity. O2 -TPD data (Fig. 8) reveal that the O2 desorption peak areas of catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900, correlating with the activity sequence. Asano et al. found that the K-doped Co3 O4 catalysts with higher activity for N2 O decomposition adsorbed more oxygen [53]. Zhang et al. [54] reported
that O2 desorption areas in O2 -TPD data can be used as an indicator for the relative amount of active sites. Fino and co-workers pointed out that the active sites for N2 O decomposition and oxygen adsorption/desorption sites are basically the same [55]. Data from the influence of co-fed O2 (Figs. S3 and S4) also imply that O2 competes with N2 O on active sites of the catalyst. In addition, catalysts that can adsorb more oxygen may have high activity in CO oxidation [20]. H2 -TPR data (Fig. 9) show that reduction peak temperature of catalysts decreases following the consequence of Rh2 O3 /LaPO4 –HNW (228 °C) > Rh2 O3 /LaPO4 –H (218 °C) > Rh2 O3 /LaPO4 -MNW-220 (205 °C) > Rh2 O3 /LaPO4 -MNW-900 (194 °C), correlating with the activity trend. The reduction temperature sequence implies the strength of interfacial interaction between Rh2 O3 and LaPO4 support because if Rh2 O3 species interact very strongly with LaPO4 support, then they are more difficult to be reduced. According to the H2 -TPR data (Fig. 9), the strengths of interfacial interaction between Rh2 O3 and LaPO4 on catalyst follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW-900, correlating with the activity sequence. The trend is also consistent with the size of Rh2 O3 species supported on LaPO4 supports (Fig. 4) because a strong interaction between Rh3+ precursor/intermediate and LaPO4 may keep the Rh2 O3 species (produced by thermal decomposing of Rh3+ precursor/intermediate) as small as possible during calcination. In turn, the small size of Rh2 O3 species may indicate the strong Rh–LaPO4 interaction. Kim et al. [56] reported that the catalytic activity in N2 O decomposition over Rh catalysts is dependent on the interaction between Rh and the support material. Bueno-Lopez et al. [37] demonstrated that the excellent activity of Rh2 O3 /CeO2 is due to its high redox potential and the interaction between Rh2 O3 and CeO2 . The most active Rh2 O3 /LaPO4 –HNW is compared with other Rh-based catalysts. As shown in Table S1, the specific rate of Rh2 O3 /LaPO4 –HNW in N2 O decomposition at 275 °C is 241 mmol/gRh /h, higher than those of RhOx /HAP (52 or 205 mmol/gRh /h, depending on Rh content) [43], Rh2 O3 /mesoporous CoOx -Al2 O3 (136 mmol/gRh /h) [57], Rh2 O3 /mesoporous Al2 O3 (57 mmol/gRh /h) [57], Rh2 O3 /γ -Al2 O3 (32 mmol/gRh /h) [38], Rh2 O3 /SBA-15 (0 mmol/gRh /h) [41], Rh2 O3 /Al-MCM-41 (95 mmol/gRh /h) [42], and Rh2 O3 /KIT-6 (0 mmol/gRh /h) [58], although lower than that of Rh2 O3 /CeO2 (932 mmol/gRh /h) [37]. As shown in Table S2, the specific rate of Rh2 O3 /LaPO4 –HNW in CO oxidation at 50 °C is 453 mmol/gRh /h, higher than those of Rh2 O3 /γ -Al2O3 (0 mmol/gRh /h) [59], Rh2 O3 /mesoporous Al2 O3 (10 mmol/gRh /h) [57], Rh2 O3 /mesoporous MnOx -Al2 O3 (19 mmol/gRh /h) [57], and Rh2 O3 /CeO2 (78 mmol/gRh /h) [60], but it is difficult to compare Rh2 O3 /LaPO4 –HNW with Rh/TiO2 (>321 mmol/gRh /h) because the latter shows 100% CO conversion below 50 °C [61]. Rh2 O3 /LaPO4 –HNW is also more active than Rh2 O3 /HAP (30 mmol/gRh /h) developed in our previous work [57].
5. Conclusions Four LaPO4 supports were synthesized by different preparation methods or under different preparation conditions. Rh2 O3 was loaded onto these supports via impregnation. The catalysts were tested in N2 O decomposition and CO oxidation. The catalytic activities of these catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900. The catalytic performance correlates with the size of Rh2 O3 species and the interaction between Rh2 O3 species and LaPO4 supports. The most active Rh2 O3 /LaPO4 –HNW catalyst has highly dispersed Rh2 O3 species, the strongest Rh2 O3 –LaPO4 interaction, as well as the largest amounts of basic sites and O2 -adsorption sites.
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation Huan Liu, Zhen Ma∗ Shanghai Key Laboratory of Atmospheric Particle Pollution & Prevention (LAP3 ), Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 17 August 2016 Revised 16 November 2016 Accepted 18 November 2016 Available online xxx Keywords: Nanostructures N2 O decomposition CO oxidation Rh LaPO4
a b s t r a c t LaPO4 has been used as a support to make supported catalysts in some investigations, but the effect of different LaPO4 supports on the catalytic performance of supported catalysts has been rarely reported. In this work, conventional hexagonal LaPO4 (LaPO4 –H) was prepared by precipitation, and hexagonal LaPO4 nanowires (LaPO4 –HNW) was prepared by hydrothermal synthesis at 150 °C. Monoclinic LaPO4 nanowires (LaPO4 -MNW-220 and LaPO4 -MNW-900) were prepared by hydrothermal synthesis at 220 °C or calcining LaPO4 –HNW at 900 °C. These samples were used to support Rh2 O3 via impregnation using Rh(NO3 )3 as a precursor. The activities of these supported catalysts in N2 O decomposition and CO oxidation follow the same sequence of Rh2 O3 / LaPO4 –HNW > Rh2 O3 / LaPO4 –H > Rh2 O3 / LaPO4 -MNW220 > Rh2 O3 / LaPO4 -MNW-900, i.e., two hexagonal LaPO4 supports are better than two monoclinic LaPO4 supports in making active catalysts, and hexagonal LaPO4 nanowires are better than hexagonal LaPO4 composed of nanoparticles and short nanorods. The most active catalyst has highly dispersed Rh2 O3 species interacting strongly with the support, more oxygen-adsorption capacity, and more basic sites. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Conventional supported catalysts are prepared by loading metals or metal oxides onto solid supports such as oxides, carbon, and zeolites. Different metal phosphates have different acid–base/redox properties and thermal stability, thus making them suitable as catalysts [1–6] or supports [7–15]. LaPO4 , a typical metal phosphate, can be used for the dehydration of light alcohols [16,17], oxidative dehydrogenation of isobutene [18], and selective alkylation of catechol with methanol [19]. LaPO4 can also be used as a catalyst support. Examples in this regard include Au/LaPO4 [9,10,12,13,20] and Pt/LaPO4 [21] for CO oxidation, Pd/LaPO4 for NO decomposition [22], Rh/LaPO4 for three-way catalysis [23], and Pt/LaPO4 for photocatalytic reduction of CO2 with H2 O [24]. However, the effect of different LaPO4 supports on the catalytic performance of LaPO4 supported catalysts has been rarely reported. Yan et al. found that gold supported onto hexagonal LaPO4 nanoparticles (average particle size: 6–8 nm; BET surface area: 111 m2 /g) is more active than gold supported on monoclinic LaPO4 nanoparticles (average particle size: 10 nm; BET surface area: 55 m2 /g) in CO oxidation [9]. They inferred that Au/hexagonal LaPO4 nanoparticles has stronger Au-LaPO4 interaction, consider-
∗
Corresponding author. Fax: +86 2165643597. E-mail address: [email protected] (Z. Ma).
ing the catalyst’s high activity and thermal stability, and they provided high-resolution TEM images for proving the strong interaction. However, it is not clear whether the support effect (hexagonal support vs. monoclinic support) is general for other LaPO4 -based catalysts and for other reactions. In addition, more experiments are certainly needed to unravel the reasons for the support effect. We revisited the Au/LaPO4 system [20]. Au/commercial LaPO4 prepared by deposition–precipitation with urea was found to be the most active in CO oxidation among Au/metal phosphate (Au/M-P-O, M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts. In addition, Au/hexagonal LaPO4 nanowires developed was found to be much more active than Au/commercial LaPO4 due to the presence of smaller gold particles, a small fraction of Au+ species, and more adsorbed active oxygen species. Although it is clear that hexagonal LaPO4 nanowires are better than hexagonal commercial LaPO4 (composed of LaPO4 nanoparticles and nanorods) in making supported gold catalysts for CO oxidation, it is not clear whether the support effect is general for other LaPO4 -based supported catalysts and for other reactions. In addition, the effect of crystal phases (hexagonal versus monoclinic) of LaPO4 supports on the catalytic performance was not addressed. LaPO4 samples with various crystal phases and morphologies can be prepared via different methods. For instance, hexagonal LaPO4 nanoparticles can be prepared by direct precipitation [25]. Hexagonal LaPO4 nanowires can be prepared via a hydrothermal
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Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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process [26]. Monoclinic LaPO4 can be prepared by either heating hexagonal LaPO4 above 800 °C [27] or by hydrothermal synthesis at 220 °C [28]. These synthesis protocols may provide opportunities for investigating the influence of different LaPO4 supports on the performance of LaPO4 -supported catalysts. N2 O is a greenhouse gas and it may also contribute to ozonelayer depletion. Catalysts used for the decomposition (abatement) of N2 O include metal oxides, ion-exchanged zeolites, and supported metal catalysts [29–31]. Rh-based catalysts for N2 O decomposition include Rh-ZSM-5 [32], Rh/MgO [33], Rh/USY [34], Rh/Al2 O3 [35,36], Rh/CeO2 [37,38], Rh/KIT-6 [39], Rh/SBA15 [40,41], and Rh/MCM-41 [42]. Our group developed a series of Rh2 O3 /metal phosphate (Rh2 O3 /M-P-O, M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts for N2 O decomposition [43], and found that Rh2 O3 /hydroxyapatite is the most active among these catalysts and Rh2 O3 /LaPO4 is the second most active. However, these metal phosphates are all from commercial sources, with little control over crystal phase and morphology. Herein, we revisited the Rh2 O3 /LaPO4 system by using four LaPO4 supports prepared by different methods. Difference in catalytic activities in N2 O decomposition and CO oxidation was found, and these catalysts were characterized. 2. Experimental 2.1. Preparation 2.1.1. Hexagonal LaPO4 (LaPO4 –H) 6.92 g La(NO3 )3 ·6H2 O (Aladdin, 99.9%) and 1.84 g NH4 H2 PO4 (Sinopharm, AR) were dissolved in 80 ml deionized water under vigorous stirring for 20 min. The pH value was adjusted to 6 by adding aqueous ammonia (27 wt.%). The mixture was stirred for 6 h. The product was filtered, washed with ethanol, and dried at 80 °C. 2.1.2. Hexagonal LaPO4 nanowires (LaPO4 –HNW) [26] 6.92 g La(NO3 )3 ·6H2 O and 1.84 g NH4 H2 PO4 were dissolved in 80 ml deionized water under vigorous stirring for 20 min. The pH value was adjusted to 1 by adding aqueous ammonia (27 wt.%). The suspension was transferred into a Teflon-lined stainless steel autoclave and heated at 150 °C for 12 h. The product was filtered, washed with ethanol, and dried at 80 °C. 2.1.3. Monoclinic LaPO4 nanowires (LaPO4 -MNW-220 and LaPO4 -MNW-900) Method 1: the process is similar to the preparation of LaPO4 –HNW except that the hydrothermal treatment temperature was 220 °C and the duration of hydrothermal treatment was 18 h [28]. Method 2: LaPO4 –HNW was calcined at 900 °C for 3 h [27]. 2.1.4. Rh2 O3 /LaPO4 catalysts 2 ml Rh(NO3 )3 solution (0.01 g/ml based on Rh) was placed in an agate mortar containing 1.98 g LaPO4 . The mixture was ground till dry under an infrared lamp. The powders were calcined at 500 °C for 3 h. 2.2. Characterization X-ray diffraction (XRD) data were obtained on a MSAL XD2 instrument with Cu Kα radiation. The scanning rate was 8°/min, and the step length was 0.01°. N2 adsorption-desorption experiments were conducted using a Micromeritics Tristar 30 0 0 instrument. The sample was degassed in vacuum at 300 °C for 3 h before analysis. ICP-OES analysis was performed on a Perkin–Elmer OPTIMA 2100 DV optical emission spectrometer. 50 mg of sample
was dissolved in a mixture of 9 ml HCl, 3 ml HNO3 , 1 ml HClO4 , 0.5 ml H2 O2 , and 3 ml HF reagents, and heated at 150 °C for 2–3 h. After that, 3 ml HCl, 1 ml HNO3 , 1 ml HF, and 0.5 ml HClO4 were added again. The mixture was transferred to an autoclave, heated at 180 °C for 4 h, allowed to cool down, and then diluted with distilled water for analysis. TEM images were obtained by a JEM-2011F transmission electron microscope operated at 200 kV. The samples were dispersed in ethanol and placed on a carbon-coated molybdenum grid followed by solvent evaporation. The sizes of RhOx nanoparticles were obtained by measuring 100 particles for each sample, using the DigitalMicrograph software. XPS spectra were recorded on a Perkin–Elmer PHI 50 0 0 C spectrometer with Mg Kα X-ray source. The carbonaceous C1 s line (284.8 eV) was used as the reference to calibrate the binding energies (BE). CASXPS software was used for analysis. CO2 temperature-programmed desorption (CO2 -TPD) experiments were performed on a FINESORB-3010 instrument. 0.15 g of sample was pretreated at 200 °C with He (30 ml/min) for 1 h and then cooled down to 50 °C. The sample was saturated with 5% CO2 /He (40 ml/min) for 1 h and swept by He flow at 50 °C for 3 h. The sample was heated to 600 °C at a rate of 10 °C/min, and the amount of desorbed CO2 was calibrated using 5% CO2 /He with known volume and concentration. O2 temperature-programmed desorption (O2 -TPD) experiments were carried out on a FINESORB-3010 instrument. A sample (0.15 g) was pretreated with He (30 ml/min) at 500 °C for 2 h, cooled to room temperature, and O2 (30 ml/min) was adsorbed at 50 °C for 1 h. Then O2 -TPD was carried out with flowing He (30 ml/min) from 50 to 600 °C at a heating rate of 10 °C/min. Temperature-programmed reduction (H2 -TPR) experiments were carried out on a Micromeritics Auto Chem II instrument. 50 mg of sample was pretreated at 200 °C with He (15 ml/min) for 20 min and then cooled down to 60 °C. The flowing gas was switched to 5% H2 /Ar (50 ml/min), and the sample was swept for 2 h. The catalyst was then heated to 800 °C at a ramping rate of 10 °C/min.
2.3. Catalytic activity measurements N2 O decomposition was measured in a fixed bed reactor. 0.25 g of catalyst was loaded into a U-shaped quartz tube (7 mm inner diameter). 0.5% N2 O/He was flowed through the catalyst (flow rate: 60 ml/min). The catalyst was maintained at room temperature for 1 h, and the reaction temperature was then increased stepwise and kept at each temperature step for 0.5 h. The effluent gas was analyzed every 10 min using an on-line GC (Agilent 7890A). The N2 O conversion was calculated as ([N2 O]in – [N2 O]out )/[N2 O]in × 100%. Catalytic CO oxidation was evaluated on a FINESORB-3010 instrument at atmospheric pressure. 0.25 g of catalyst was loaded into a U-shaped glass tube for the reaction. The reaction gas was composed of 1% CO in air. The flow rate was 50 ml/min. The catalyst was maintained at room temperature for 1 h, and heated to 200 °C at a ramping rate of 0.5 °C/min. The effluent gas was analyzed periodically (every 10 min) using an on-line gas chromatograph (GC; Agilent 7890A, equipped with a TCD detector) capable of separating CO, CO2 , N2 , and O2 , using He as a carrier gas. The CO conversion was calculated as ([CO]in – [CO]out )/[CO]in × 100%, where [CO]in is the CO concentration at room temperature (at which no reaction occurs), and [CO]out is the CO concentration at an elevated temperature. Here the catalysts used were fine powders. There was no difference in catalytic activity of the fine powder catalyst and a catalyst with grain sizes of 40–60 meshes (Fig. S1).
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Fig. 3. HRTEM images of Rh2 O3 /LaPO4 catalysts.
Fig. 1. XRD patterns of Rh2 O3 /LaPO4 catalysts.
Fig. 2. TEM images of Rh2 O3 /LaPO4 catalysts.
3. Results 3.1. Basic physicochemical properties Fig. 1 shows the XRD patterns of LaPO4 and Rh2 O3 /LaPO4 . LaPO4 –HNW prepared by hydrothermal synthesis at 150 °C and LaPO4 –H prepared by conventional precipitation show the hexagonal phase (PDF no. 46–1439), whereas LaPO4 -MNW-220 prepared by hydrothermal synthesis at 220 °C and LaPO4 -MNW-900 prepared by calcining LaPO4 –HNW at 900 °C show the monoclinic phase (PDF no. 32–0493). Rh2 O3 /LaPO4 samples have XRD patterns similar to their corresponding supports. No peaks due to Rh2 O3 are detected, probably due to the low Rh content (∼1 wt.%) and high dispersion of Rh2 O3 . Fig. 2 shows the TEM images of Rh2 O3 /LaPO4 . Only the LaPO4 supports can be seen, whereas Rh2 O3 species are not seen at this low resolution. Rh2 O3 /LaPO4 –HNW has LaPO4 nanowires (width: 10–30 nm; length: 50–650 nm). Rh2 O3 /LaPO4 –H has irregular aggregates composed of LaPO4 nanoparticles (particles size:
45–100 nm) and short nanorods (width: 4–10 nm; length: 10– 65 nm). Rh2 O3 /LaPO4 -MNW-220 contains LaPO4 nanowires (width: 10–40 nm; length: 60–1200 nm), so does Rh2 O3 /LaPO4 -MNW-900 (nanowires width: 10–50 nm; length: 60–1200 nm). Fig. 3 shows the HRTEM images of Rh2 O3 /LaPO4 , focusing on the details of LaPO4 supports. The interplanar distances of LaPO4 –HNW and LaPO4 –H are 0.619 and 0.616 nm, respectively, corresponding to the (100) plane of hexagonal LaPO4 [26]. Figs. S2 and S3 in the Supporting Information present additional HRTEM images and the corresponding selected-area electron diffraction (SAED) patterns of Rh2 O3 /LaPO4 –HNW and Rh2 O3 /LaPO4 –H, respectively, showing that these LaPO4 supports are hexagonal. On the other hand, the interplanar distance of LaPO4 -MNW-220 is 0.476 nm, corresponding to the (110) plane of monoclinic LaPO4 [28]. The interplanar distance of LaPO4 -MNW-900 is 0.425 nm, corresponding to the (−111) plane of monoclinic LaPO4 [27]. Figs. S4 and S5 present additional HRTEM images and the corresponding selected-area electron diffraction (SAED) patterns of Rh2 O3 /LaPO4 MNW-220 and Rh2 O3 /LaPO4 -MNW-900, respectively, showing that these LaPO4 supports are monoclinic. Fig. 4 shows the TEM images of Rh2 O3 /LaPO4 , focusing on the supported Rh2 O3 species. No Rh2 O3 particles can be observed for Rh2 O3 /LaPO4 –HNW (Fig. 4(a)) and Rh2 O3 /LaPO4 –H (Fig. 4(b)), indicating that Rh2 O3 species are highly dispersed on supports. TEM-mapping images show that the Rh2 O3 species are well dispersed on LaPO4 –HNW (Fig. S6) and LaPO4 –H (Fig. S7). However, Rh2 O3 particles can be seen clearly for Rh2 O3 /LaPO4 -MNW-220 (Figs. 4(c) and S8) and Rh2 O3 /LaPO4 -MNW-900 (Figs. 4(d) and S9). On the basis of 100 particles for each sample, the mean Rh2 O3 particle sizes are estimated to be 1.3 ± 0.7 and 2.0 ± 0.5 nm for Rh2 O3 /LaPO4 -MNW-220 and Rh2 O3 /LaPO4 -MNW-900, respectively. The particle size distributions of these two catalysts are shown in Fig. S10. 3.2. Catalytic performance The catalytic performance of different Rh2 O3 /LaPO4 catalysts was studied using N2 O decomposition (N2 O = N2 + 1/2O2 ) as a probe reaction [29–31]. As shown in Fig. 5, the activities of catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW-900, when comparing the catalysts’ T50 (temperature required for achieving 50% conversion) values (260, 267, 285, and 314 °C, respectively) or N2 O conversions at 275 °C (78%, 66%, 36%, and 17%, respectively). The Rh contents of these catalysts are determined by ICP-OES as 1.04 wt.%, 1.01 wt.%, 0.98 wt.%, and 0.99 wt.%, respectively. Thus, the specific rates of these catalysts at 275 °C are calculated to be
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Fig. 4. TEM images of Rh2 O3 /LaPO4 catalysts, focusing on Rh2 O3 species. (a) Rh2 O3 /LaPO4 –HNW; (b) Rh2 O3 /LaPO4 –H; (c) Rh2 O3 /LaPO4 -MNW-220; (d) Rh2 O3 /LaPO4 -MNW900.
Fig. 5. N2 O conversions over Rh2 O3 /LaPO4 catalysts as a function of reaction temperature.
Fig. 6. CO conversions over Rh2 O3 /LaPO4 catalysts as a function of reaction temperature.
241, 210, 118, and 55 mmol/gRh /h, respectively. The corresponding LaPO4 supports show no activity below 400 °C. The most active Rh2 O3 /LaPO4 –HNW was subjected to additional testing in N2 O decomposition. As shown in Fig. S11, at a reaction temperature of 290 °C, the initial N2 O conversion reaches 96 %, consistent with the N2 O conversion at 290 °C in Fig. 5. The N2 O conversion decreases slowly to 61% after 120 h on stream. TEM image (Fig. S12) of Rh2 O3 /LaPO4 –HNW collected after stability test still shows no obvious Rh2 O3 particles on the support. The reason for the decrease in activity may be the accumulation of oxygen on the catalyst during the reaction, as confirmed by O2 -TPD data described later. We additionally tested the influence of co-fed O2 and/or H2 O on catalytic activity. As shown in Fig. S13, the presence of 5% O2 in the reaction mixture inhibits the activity less obviously than the presence of 2% H2 O. Both inhibiting effects are reversible,
i.e., the N2 O conversion restores after the retraction of O2 or H2 O (Fig. S14). These catalysts were tested in CO oxidation, a conventional and sensitive probe reaction [44–46]. As shown in Fig. 6, the activities of catalysts still follow the sequence of Rh2 O3 /LaPO4 –HNW (T50 = 36 °C) > Rh2 O3 /LaPO4 –H (T50 = 70 °C) > Rh2 O3 /LaPO4 -MNW220 (T50 = 114 °C) > Rh2 O3 /LaPO4 -MNW-900 (T50 = 118 °C). The shapes of conversion curves of Rh2 O3 /LaPO4 -MNW-220 and Rh2 O3 /LaPO4 -MNW-900 are somewhat different from those of Rh2 O3 /LaPO4 –H and Rh2 O3 /LaPO4 –HNW, but this feature is reproducible (Figs. S15 and S16). However, Rh2 O3 /LaPO4 –HNW deactivates quickly during the stability test conducted at 55 °C (Fig. S17), probably due to the strong adsorption of CO2 or the buildup of carbonate species on basic sites. The corresponding LaPO4 supports are inactive below 200 °C (data not shown).
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Fig. 7. CO2 -TPD profiles of Rh2 O3 /LaPO4 catalysts.
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Fig. 8. O2 -TPD profiles of Rh2 O3 /LaPO4 catalysts.
3.3. Additional characterization of LaPO4 and Rh2 O3 /LaPO4 catalysts After demonstrating that the catalytic activities of the catalysts are quite different, we then present data from in-depth characterization of Rh2 O3 /LaPO4 catalysts and relevant supports in order to unravel the reasons for the difference in catalytic activity. Fig. S18 shows the Rh 3d XPS spectra of Rh2 O3 /LaPO4 catalysts. There are only two peaks at approximately 306–311 and 311–318 eV, assigned to Rh3+ 3d5/2 and Rh3+ 3d3/2 [47,48], respectively. The data confirm that the Rh species exist as Rh2 O3 . Fig. S19 shows the O 1 s XPS spectra of Rh2 O3 /LaPO4 catalysts. The most intense component at ca. 531.1 eV corresponds to lattice oxygen and a shoulder peak at ca. 532.5 eV is attributed to hydroxyl groups [24,49]. The relative proportions of hydroxyl groups (among all oxygen species) in Rh2 O3 /LaPO4 –HNW, Rh2 O3 /LaPO4 –H, Rh2 O3 /LaPO4 -MNW-220, and Rh2 O3 /LaPO4 -MNW-900 are similar (18.2%, 18.5%, 18.3%, and 18.9%, respectively). Fig. 7 shows the CO2 -TPD profiles of LaPO4 and Rh2 O3 /LaPO4 . These supports all exhibit a CO2 desorption peak ranging from ∼130 to ∼450 °C. The amounts of basic sites, determined from the peak areas and calibration, for these supports follow the sequence of LaPO4 –HNW (377 μmol/g) > LaPO4 –H (303 μmol/g) > LaPO4 MNW-220 (63 μmol/g) > LaPO4 -MNW-900 (30 μmol/g), and the amounts of basic sites for supported catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW (293 μmol/g) > Rh2 O3 /LaPO4 –H (252 μmol/g) > Rh2 O3 /LaPO4 -MNW-220 (54 μmol/g) > Rh2 O3 /LaPO4 MNW-900 (27 μmol/g). The data indicate that the basicity of catalysts mainly originates from the supports, and the loading of Rh2 O3 onto LaPO4 supports may decrease the number of basic sites to some extent. Fig. 8 shows the O2 -TPD profiles of LaPO4 and Rh2 O3 /LaPO4 . O2 starts to desorb from LaPO4 –HNW, LaPO4 –H, LaPO4 -MNW-220, and LaPO4 -MNW-900 at 265, 289, 212, and 166 °C, respectively. The peak areas of these supports decrease following the sequence of LaPO4 –HNW > LaPO4 –H > LaPO4 -MNW-220 > LaPO4 -MNW900. The supporting of Rh species increases the peak areas for all these catalysts. The peak areas of supported catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 MNW-220 > Rh2 O3 /LaPO4 -MNW-900. Fig. 9 shows the H2 -TPR profiles of Rh2 O3 /LaPO4 . These catalysts show a single peak corresponding to the reduction of Rh2 O3 to metallic Rh [50,51]. The reduction temperature of catalysts decreases following the consequence of Rh2 O3 /LaPO4 –HNW (228 °C) > Rh2 O3 /LaPO4 –H (218 °C) > Rh2 O3 /LaPO4 -MNW-220 (205 °C) >
Fig. 9. H2 -TPR profiles of Rh2 O3 /LaPO4 catalysts.
Rh2 O3 /LaPO4 -MNW-900 (194 °C). The data provide a key piece of evidence for the strong interaction between Rh2 O3 and LaPO4 for the most active catalysts. 4. Discussion According to XRD (Fig. 1) and TEM (Fig. 2) data, LaPO4 –HNW prepared by hydrothermal synthesis at 150 °C exhibits hexagonal LaPO4 nanowires. LaPO4 –H prepared by precipitation exhibits hexagonal nanoparticles and short nanorods. Both LaPO4 -MNW220 and LaPO4 -MNW-900 exhibit monoclinic nanorods. The catalytic activities of Rh2 O3 /LaPO4 catalysts in N2 O decomposition and CO oxidation follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900 (Figs. 5 and 6), i.e., (1) hexagonal LaPO4 is better than monoclinic LaPO4 in making active Rh2 O3 /LaPO4 catalysts; (2) hexagonal LaPO4 nanowires are better than hexagonal LaPO4 nanoparticles and short nanorods prepared by precipitation in making active Rh2 O3 /LaPO4 catalysts. The activity trend is in line with the findings that: (1) Au/hexagonal LaPO4 is more active than Au/monoclinic LaPO4 in CO oxidation [9]; (2) Au/hexagonal LaPO4 nanowires is more active than Au/commercial LaPO4 (composed of LaPO4 nanoparticles and short nanowires) for CO oxidation [20].
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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ICP data show that the actual Rh content of these catalysts are 1.04 wt.%, 1.01 wt.%, 0.98 wt.%, and 0.99 wt.%, respectively, all closed to 1 wt.%. The XPS data confirm that the Rh species exist as Rh2 O3 (Fig. S18) and the relative proportions of surface hydroxyls (among oxygen species) of these catalysts are similar to each other (Fig. S19). Therefore, the difference in catalytic activity of different Rh2 O3 /LaPO4 catalysts is not due to the Rh content, oxidation state of Rh species, and relative proportion of hydroxyl group of these catalysts. As revealed by TEM data (Fig. 4), Rh2 O3 species are invisible on LaPO4 –HNW and LaPO4 –H, whereas they (with average sizes of 1.3 ± 0.7 and 2.0 ± 0.5 nm, respectively) are seen on LaPO4 MNW-220 and LaPO4 -MNW-900 (Figs. S8–S10). The size of Rh2 O3 species may influence the catalytic activity of these catalysts, in the sense that highly dispersed Rh2 O3 species (invisible in TEM images) display higher activity than Rh2 O3 nanoparticles (with average sizes of 1.3 ± 0.7 and 2.0 ± 0.5 nm). The specific surface areas of catalysts follow the sequence of Rh2 O3 /LaPO4 –H (98.8 m2 /g) > Rh2 O3 /LaPO4 –HNW (54.4 m2 /g) > Rh2 O3 /LaPO4 -MNW-220 (32.1 m2 /g) > Rh2 O3 /LaPO4 -MNW-900 (25.0 m2 /g). The better dispersion (i.e., smaller sizes) of Rh2 O3 species on hexagonal versus monoclinic LaPO4 supports may be due to the larger specific surface areas of hexagonal LaPO4 supports. Because monoclinic supports, i.e., LaPO4 -MNW-220 and LaPO4 MNW-900, were prepared by hydrothermal synthesis at a high temperature (220 °C instead of 150 °C) or by calcining LaPO4 –HNW at a high temperature (900 °C), it is not surprising that monoclinic LaPO4 supports have lower surface areas than hexagonal LaPO4 supports prepared under milder conditions [9]. As shown in the TEM images of Fig. 2, LaPO4 -MNW-220 exhibits nanowires (width: 10–40 nm; length: 60–1200 nm), so does LaPO4 -MNW-900 (width: 10–50 nm; length: 60–1200 nm). LaPO4 –HNW exhibits nanowires (width: 10–30 nm; length: 50–650 nm), whereas LaPO4 –H is composed of nanoparticles and short nanorods. These TEM data indicate that, monoclinic LaPO4 nanowires are generally longer and wider than hexagonal LaPO4 , which corresponds to the lower surface areas of the former supports. Although in general specific surface areas of Rh2 O3 /LaPO4 catalysts correlate with the size of Rh2 O3 species and thus with the catalytic activity, specific surface area may not be the sole factor that determines the activity, because although Rh2 O3 /LaPO4 –H has a larger specific surface area (98.8 m2 /g) than Rh2 O3 /LaPO4 –HNW (54.4 m2 /g), it is less active than the latter (Figs. 5 and 6). Certainly, LaPO4 nanowires are better than LaPO4 nanoparticles and short nanorods in making active Rh2 O3 /LaPO4 catalysts. This conclusion is consistent with the finding that Au/hexagonal LaPO4 nanowires (specific surface area: 60.8 m2 /g) is more active than Au/commercial LaPO4 (specific area: 95.6 m2 /g; composed of LaPO4 nanoparticles and nanorods) for CO oxidation [9]. CO2 -TPD data (Fig. 7) show that the amounts of basic sites of catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900, correlating with the activity sequence. It has been reported that the basic property of a catalyst is beneficial for catalytic N2 O decomposition [52] and CO oxidation [21]. However, it must be emphasized that basicity per se does not guarantee high activity: LaPO4 supports also have ample basic sites (Fig. 7), but they are not active for N2 O decomposition and CO oxidation. It seems that Rh2 O3 species highly dispersed on LaPO4 supports with ample basic sites exhibit the highest activity. O2 -TPD data (Fig. 8) reveal that the O2 desorption peak areas of catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900, correlating with the activity sequence. Asano et al. found that the K-doped Co3 O4 catalysts with higher activity for N2 O decomposition adsorbed more oxygen [53]. Zhang et al. [54] reported
that O2 desorption areas in O2 -TPD data can be used as an indicator for the relative amount of active sites. Fino and co-workers pointed out that the active sites for N2 O decomposition and oxygen adsorption/desorption sites are basically the same [55]. Data from the influence of co-fed O2 (Figs. S3 and S4) also imply that O2 competes with N2 O on active sites of the catalyst. In addition, catalysts that can adsorb more oxygen may have high activity in CO oxidation [20]. H2 -TPR data (Fig. 9) show that reduction peak temperature of catalysts decreases following the consequence of Rh2 O3 /LaPO4 –HNW (228 °C) > Rh2 O3 /LaPO4 –H (218 °C) > Rh2 O3 /LaPO4 -MNW-220 (205 °C) > Rh2 O3 /LaPO4 -MNW-900 (194 °C), correlating with the activity trend. The reduction temperature sequence implies the strength of interfacial interaction between Rh2 O3 and LaPO4 support because if Rh2 O3 species interact very strongly with LaPO4 support, then they are more difficult to be reduced. According to the H2 -TPR data (Fig. 9), the strengths of interfacial interaction between Rh2 O3 and LaPO4 on catalyst follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW-900, correlating with the activity sequence. The trend is also consistent with the size of Rh2 O3 species supported on LaPO4 supports (Fig. 4) because a strong interaction between Rh3+ precursor/intermediate and LaPO4 may keep the Rh2 O3 species (produced by thermal decomposing of Rh3+ precursor/intermediate) as small as possible during calcination. In turn, the small size of Rh2 O3 species may indicate the strong Rh–LaPO4 interaction. Kim et al. [56] reported that the catalytic activity in N2 O decomposition over Rh catalysts is dependent on the interaction between Rh and the support material. Bueno-Lopez et al. [37] demonstrated that the excellent activity of Rh2 O3 /CeO2 is due to its high redox potential and the interaction between Rh2 O3 and CeO2 . The most active Rh2 O3 /LaPO4 –HNW is compared with other Rh-based catalysts. As shown in Table S1, the specific rate of Rh2 O3 /LaPO4 –HNW in N2 O decomposition at 275 °C is 241 mmol/gRh /h, higher than those of RhOx /HAP (52 or 205 mmol/gRh /h, depending on Rh content) [43], Rh2 O3 /mesoporous CoOx -Al2 O3 (136 mmol/gRh /h) [57], Rh2 O3 /mesoporous Al2 O3 (57 mmol/gRh /h) [57], Rh2 O3 /γ -Al2 O3 (32 mmol/gRh /h) [38], Rh2 O3 /SBA-15 (0 mmol/gRh /h) [41], Rh2 O3 /Al-MCM-41 (95 mmol/gRh /h) [42], and Rh2 O3 /KIT-6 (0 mmol/gRh /h) [58], although lower than that of Rh2 O3 /CeO2 (932 mmol/gRh /h) [37]. As shown in Table S2, the specific rate of Rh2 O3 /LaPO4 –HNW in CO oxidation at 50 °C is 453 mmol/gRh /h, higher than those of Rh2 O3 /γ -Al2O3 (0 mmol/gRh /h) [59], Rh2 O3 /mesoporous Al2 O3 (10 mmol/gRh /h) [57], Rh2 O3 /mesoporous MnOx -Al2 O3 (19 mmol/gRh /h) [57], and Rh2 O3 /CeO2 (78 mmol/gRh /h) [60], but it is difficult to compare Rh2 O3 /LaPO4 –HNW with Rh/TiO2 (>321 mmol/gRh /h) because the latter shows 100% CO conversion below 50 °C [61]. Rh2 O3 /LaPO4 –HNW is also more active than Rh2 O3 /HAP (30 mmol/gRh /h) developed in our previous work [57].
5. Conclusions Four LaPO4 supports were synthesized by different preparation methods or under different preparation conditions. Rh2 O3 was loaded onto these supports via impregnation. The catalysts were tested in N2 O decomposition and CO oxidation. The catalytic activities of these catalysts follow the sequence of Rh2 O3 /LaPO4 –HNW > Rh2 O3 /LaPO4 –H > Rh2 O3 /LaPO4 -MNW-220 > Rh2 O3 /LaPO4 -MNW900. The catalytic performance correlates with the size of Rh2 O3 species and the interaction between Rh2 O3 species and LaPO4 supports. The most active Rh2 O3 /LaPO4 –HNW catalyst has highly dispersed Rh2 O3 species, the strongest Rh2 O3 –LaPO4 interaction, as well as the largest amounts of basic sites and O2 -adsorption sites.
Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024
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H. Liu, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–8
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Please cite this article as: H. Liu, Z. Ma, Effect of different LaPO4 supports on the catalytic performance of Rh2 O3 /LaPO4 in N2 O decomposition and CO oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.11.024