Pr6O11 Catalysts for

Article pubs.acs.org/IECR

Mechanistic Insight into Nanoarchitected Ag/Pr6O11 Catalysts for Efficient CO Oxidation Xinxin Zhang,†,∇ Shiyang Cheng,‡,∇ Wei Zhang,*,†,§,∥ Cai Zhang,† Nicholas E. Drewett,§ Xiyang Wang,⊥ Dong Wang,† Seung Jo Yoo,# Jin-Gyu Kim,# and Weitao Zheng*,† †

State Key Laboratory of Automotive Simulation and Control, Department of Materials Science, Key Laboratory of Mobile Materials MOE, Jilin University, 2699 Qianjin Street, Changchun 130012, China ‡ Guoxuan High-Tech Power Energy Company Ltd., Daihe Road No. 599, Xinzhan District, Hefei 230000, China § CIC Energigune, Albert Einstein 48, 01510 Miñano, Á lava, Spain ∥ IKERBASQUE, Basque Foundation for Science, Bilbao 48013, Spain ⊥ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China # Electron Microscopy Research Center, Korea Basic Science Institute, Daejeon 34133, South Korea S Supporting Information *

ABSTRACT: Ag/Pr6O11 catalysts supported by either Pr6O11 nanorods (Pr6O11-NRs) or nanoparticles (Pr6O11-NPs) were prepared by conventional incipient wetness impregnation. The nanocomposite of Ag/Pr6O11-NRs demonstrated a higher catalytic activity for CO oxidation than Ag/Pr6O11NPs at lower temperatures. This improved performance may be ascribed to the mesoporous features and resultant oxygen vacancies of the Pr6O11 nanorods support, as well as the large surface area and homogeneous loading of Ag species. As a result, 98.7 and 100% CO conversions were achieved at 210 and 240 °C for Ag/Pr6O11-NRs, while Ag/Pr6O11-NPs require a temperature of 320 °C to obtain the 100% CO conversion rate. These findings reveal that Pr6O11-NRs is the preferable support, comparative to Pr6O11-NPs, for Ag/Pr6O11 catalysts, for CO oxidation.

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INTRODUCTION The increase in carbon monoxide (CO) emissions, arising from fossil fuel combustion, has become a key environmental issue. One of the most significant factors behind this is the range of undesirable properties related to CO, e.g., high toxicity (due to the high affinity of CO with hemoglobin) and acting as a precursor for ground level ozone. One approach to mitigating CO production is to employ the use of catalytic oxidation, which efficiently reduces the release of CO directly to the atmosphere.1−5 Currently, many high-performance catalysts have been developed for CO oxidation, such as the frequently utilized family of noble metal based catalysts (e.g., Pd,6−8 Pt,9−12 Au13−15) and the less commonly used non-noble metal catalysts (e.g., Co-, Cu-, Fe-, and Ce-based oxides16−21). Recently, the catalytic performance of inexpensive Ag catalysts for CO oxidation has attracted interest,22−24 with research mostly focusing on the preparation of miscellaneous supports (e.g., transition metal oxides and hydroxides, as shown in Table 1) so as to ensure a highly dispersed distribution with optimal synergy. However, the influence of using rare earth oxides (REOs) as supports on the catalytic performance has rarely been studied. With their electronic configuration, 4fx5d0−16s2, some REs exhibit distinctive redox activity.25 For example, it is © XXXX American Chemical Society

Table 1. Comparison of Catalytic Activities of Different Oxide-Supported Ag Catalysts for CO Oxidation catalyst

Ag (wt %)

T50a (°C)

T100b (°C)

SBET (m2/g)

year

ref

Ag/HMS-450 Ag/CeO2 Ag/Fe2O3 Ag/Co3O4 Ag/Fe2O3 Ag/TiO2 Ag/CeO2 Ag/TiO2 Ag/Pr6O11-NRs

4 5 7 − 20 4 6 2.2 17.9

50 ∼160 − 78 150 >100 220 160 ∼125

105c 200 200 100 275 250 250 310 210d

1112 − 24.3 − 40.86 278 32 45 19

2016 2015 2015 2014 2013 2013 2012 2011 2017

Ag/Pr6O11-NPs

22.5

∼250

330

5.2

2017

30 31 32 18 33 34 24 35 this work this work

a

T50 (°C), temperature for 50% CO conversion. temperature for 100% CO conversion. cT98. dT98.7.

Received: Revised: Accepted: Published: A

b

T100 (°C),

June 23, 2017 August 25, 2017 September 8, 2017 September 8, 2017 DOI: 10.1021/acs.iecr.7b02530 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of synthesis routes for Ag/Pr6O11 nanorods and nanoparticles (a). Nitrogen adsorption−desorption isotherms and BJH pore size distribution curves (insets) for Pr6O11-NPs (b) and Pr6O11-NRs (c).

Figure 2. XRD patterns (a), EDS spectra (b), TEM micrographs (c, d), and HRTEM micrographs (e, f) of Ag/Pr6O11-NRs and Ag/Pr6O11-NPs.

temperature.29 This enables fast electron transport between the support and the metallic catalyst, which might be favorable for Ag catalytic activity. Considering the distinctive oxygen and electronic activities of PrOx, it seemed likely that it would be possible to create a catalyst with a high CO conversion rate by coupling Ag nanoparticles to a PrOx support. Since the morphology of the support influences the metal− Pr6O11 interaction, there is a strong link between this and the catalytic performance. In order to investigate this, we have used a wet chemical route to synthesize Ag/Pr6O11-based catalysts

only praseodymium (Pr), terbium (Tb), and cerium (Ce) oxides that exist abundantly in multiple valence states and oxygen-deficient structures.26 Moreover, the oxygen-deficient structure is favorable for fast oxygen ion transport, accounting for high oxygen surface exchange rate, which may consequently facilitate CO oxidization. For example, PrOx has both high oxygen activity and excellent oxygen storage capacity,27,28 which may lead to high catalytic activity for CO oxidization. Furthermore, when compared with other RE elements, PrOx shows comparatively high electronic conductivity at room B

DOI: 10.1021/acs.iecr.7b02530 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 3. XPS analysis of Ag/Pr6O11-NPs and Ag/Pr6O11-NRs for XPS survey scans (a), Pr 3d (b), Ag 3d (c), and O 1s (d).

pores by the impregnated Ag nanoparticles, leading to a reduced nitrogen permeation rate.8 To further verify the introduction of silver particles, X-ray diffraction (XRD) analyses were conducted, as shown in Figure 2a. From this data, diffraction peaks corresponding to facecentered-cubic Pr6O11 were indexed (JCPDS PDF Card No. 42-1121), as well as peaks for Ag (according to PDF Card No. 87-0717). As shown in Figure 2, the morphology and structure of the Ag/Pr6O11 samples were examined by transmission electron microscopy (TEM). When compared with the TEM images of the Pr6O11-NRs and Pr6O11-NPs in Figure S4d,g, it may be seen that the morphologies of both Pr6O11 nanoparticles and nanorods were not affected by the presence of silver. However, the surface of Pr6O11-NRs becomes rougher and their corners become rounder. Spherical Ag nanoparticles were observed in the Ag/Pr6O11-NRs and the Ag/Pr6O11-NPs (see Figure 2c,d). Meanwhile, the lattice fringes of 0.235 nm in the high-resolution TEM (HRTEM) images (Figure 2e,f) correspond to that of Ag(111). Using inductively coupled plasma (ICP) (Thermo-instrument), the silver contents of the Ag/Pr6O11-NRs and NPs were estimated to be ca. 17.9 and 22.5 wt %, respectively. As can be seen from this data, Ag/ Pr6O11-NPs generally exhibit a higher Ag loading concentration than that of Ag/Pr6O11-NRs. Figure 3 shows the X-ray photoelectron spectroscopy (XPS) analysis from the Ag/Pr6O11-NRs and Ag/Pr6O11-NPs. Praseodymium, silver, and oxygen were detected on the sample surface. As seen in Figure 3b, there are four peaks at binding energies of 929.9, 934.2, 948.8, and 954.5 eV. The two XPS peaks located at ∼934.2 and 954.5 eV were assigned to the Pr 3d3/2 and Pr 3d5/2 levels (whose binding energies are given as 933 and 954 eV, respectively36). The shoulder peaks located at ∼929.9 and 948.8 eV, adjacent to the major Pr 3d XPS peaks, may indicate the existence of Pr3+ ions, which would suggest a considerable concentration of oxygen vacancy. The signals at

with either Pr6O11 nanorod (Pr6O11-NRs) or nanoparticle (Pr6O11-NPs) supports. Through the use of a comprehensive set of physical characterizations, the physical properties of the supports are related to their catalytic activity. Finally, Pr6O11NRs were identified as the optimum support of Ag/Pr6O11based catalysts for high catalytic activities in CO oxidation.

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RESULTS AND DISCUSSION Two different morphologies (nanoparticles and nanorods) of Pr6O11-based supports were prepared (see the Supporting Information for experimental details). Schematics of the overall synthetic methods for fabricating the Ag/Pr6O11 catalysts are shown in Figure 1a. In order to determine the specific surface areas of the samples (so as to verify the effect of specific surface area on the catalytic performance), the isothermal nitrogen adsorption−desorption curves and the corresponding Barrett− Joyner−Halenda (BJH) pore size distribution curves were obtained, as shown in Figure 1b,c. The specific surface area (SBET) of the Pr6O11-based nanorods (Pr6O11-NRs) was calculated to be 35.9 m2 g−1 by the multipoint Brunauer− Emmett−Teller (BET) method. The pore volume (VBJH) was calculated to be 0.2 cm3 g−1 in terms of the isothermal desorption results using the BJH method.36,37 The average pore size was found to be 7.8 nm. The specific surface area, pore volume, and pore size of the Pr6O11-based nanoparticles (Pr6O11-NPs) were calculated to be 10.1 m2 g−1, 0.1 cm3 g−1, and 10.4 nm, respectively. It is commonly accepted that a large surface area (specific surface area) leads to increased catalytic performance, due to an increase in the area of interface between catalyst and reactant.38,39 As a consequence, the porous structure and large surface area of the Pr6O11-NRs are likely to be more favorable for a good catalytic performance. The specific surface area of the Pr6O11 support impregnated with Ag nanoparticles decreased significantly (Table S1). It seems likely that this was the result of partial blocking of the C


Article


Figure 4. H2-TPR profiles of Pr6O11 (a) and Ag/Pr6O11 (b) catalysts.

954.5 and 934.2 eV were assigned to Pr4+.26 From this, we deduce that Pr in the sample exists as a mixture of Pr3+ and Pr4+ (the presence of Pr4+/Pr3+ signals indicates the coexistence of Pr4+ and Pr3+). The XRD index has been utilized to confirm that the major chemical composition of the support is Pr6O11 (see Figure S3a). XPS is in principle more sensitive to the surface of the material. Based on our calculation in terms of XPS results, the Pr4+/Pr3+ ratio is ∼1.5 (see Table S2), which is lower than the theoretical ratio 2 as in Pr6O11. This deviation suggests that on the surface of the support Pr4+ in Pr6O11 is partially reduced to Pr3+ probably because oxygen vacancy segregates preferentially on the surface of the material. This could also be the reason for the fast oxygen exchange of Pr6O11 acting as an effective “oxygen buffer”. On the other hand, the preparation route is also an important factor that strongly affects the Pr4+/Pr3+ ratio.28 The spectra of the Ag 3d bands are shown in Figure 3c. Here it can be seen that the Ag/Pr6O11 catalyst shows two peaks attributed to Ag 3d5/2 at binding energies of 368.5−368.8 and 370.1 eV, which were assigned to Ag+ and Ag0, respectively. The centers of the Ag 3d5/2 peaks are located at 367.1 eV for Ag2O and 368.8−369.2 eV for Ag.40,41 It should be noted that the binding energy of Ag 3d5/2 has undergone a slight positive shift which suggests that Ag+ may have been partially reduced, resulting in a metallic state.42 One other possible reason would be that the binding energy of Ag0 in smaller clusters is higher than that in the bigger nanoparticles.24 Regardless, the results from this analysis indicate that both Ag+ and Ag0 species reside on the surface of the Ag/Pr6O11 catalysts. The coexistence of Ag+ and Ag0 is a result of the incomplete reduction of Ag2O. It has been generally agreed that metallic silver is relatively active to CO oxidation. During the CO oxidation reaction, Ag+ should be reduced to metallic Ag quickly since Ag2O is relatively unstable in this stringent atmosphere.24,32,40,43 Therefore, metallic silver is the major active site for CO oxidation. The presence of metallic silver is in keeping with the observations made regarding the XRD (Figure 2a) and HRTEM (Figure 2e,f) analyses. The XPS clearly indicates the coexistence of Pr4+/Pr3+. According to the literature, PrOx is a conducting material in which electron conducts via small polarons in terms of a hopping mechanism (electron hopping between Pr3+ and Pr4+). As demonstrated by Habibi et al.,44 when the Ag and metal oxide (Ag/ceria) support attach together, the Fermi energy levels are compromised to a new value so that ceria is much easier to reduce (electron transfer between Ag and ceria occurs). Considering the energy level of Pr 4f is much closer to

the level of O 2p as compared to that of Ce 4f, the electron transfer is expected to be more favorable in the case of Ag/ Pr6O11. The coverage of Ag on Pr6O11 should thus facilitate fast electron transfer so that the electron transfer induced surface CO oxidation reaction may be in turn accelerated. Figure 3d shows O 1s spectra of the two samples. It can be seen that the spectra are characterized by three oxygen species at 528.7−528.9, 530.9, and 532.8−533.4 eV. The peaks at binding energy of 528.7−528.9 eV are assigned to O2− in the Pr6O11 lattice,40,45 while the binding energy at 530.9 eV is associated with the surface adsorbed oxygen species (O−, OH−).26,45 The relative quantity of the surface oxygen may be estimated from the relative area of the subpeak. It can be seen that the relative amount of adsorbed oxygen species on Ag/ Pr6O11-NRs is higher than that on Ag/Pr6O11-NPs. The oxygen vacancies are the congregate location of adsorbed oxygen species, and a mass of adsorbed oxygen species implies a higher oxygen vacancy density.26 The peak located at 532.8−533.4 eV may be ascribed to adsorbed molecular water.46 The hydrogen temperature-programmed-reduction (H2TPR) test was undertaken in order to analyze the reduction behavior of Pr6O11-NPs and Pr6O11-NRs, demonstrating the effect of morphology and specific surface area on the reducibility. Figure 4a shows the TPR profiles of the two Pr6O11 samples with different morphologies. As can be seen from this data, the reduction peaks are centered at 492.2 and 478.7 °C for Pr6O11-NPs and Pr6O11-NRs, respectively. These results indicate the temperature where the partial reduction of Pr4+ to Pr3+occurs.26,47 It should be noted that the reduction of Pr appears to have been more facile with Pr6O11-NRs than with Pr6O11-NPs. This implies that the reduction of Pr is significantly influenced by the morphology or the specific surface area of the oxide (Pr6O11-NPs (SBET = 10.1 m2/g) and Pr6O11-NRs (SBET = 35.9 m2/g)). Figure 4b shows the TPR profiles of Ag/Pr6O11-NPs and Ag/Pr6O11-NRs catalysts. From this, it can be seen that both Ag/Pr6O11 catalysts exhibit two reduction peaks, one in 100−400 °C and one in the 500−800 °C temperature range. The temperatures at which the highest intensities occur are 242.2 °C for Ag/Pr6O11-NRs and 296.1 °C for Ag/Pr6O11-NPs, respectively. These peaks are assigned to the reduction of surface oxygen from Pr6O11 due to interactions with silver species. The peaks at 526 and 545.8 °C are attributed to the reduction of surface-capping oxygen of Pr6O11 without silver.40,47,48 This suggests a strong synergistic interaction between silver and Pr6O11. Finally, the reduced temperature of Ag/Pr6O11-NPs is larger than that of Ag/ D


Article


Figure 5. Performance in catalytic oxidation of CO for several samples (a) and corresponding turnover frequency calculated with respect to the Ag loading (b).

the CO conversion rate is 78.2% at 150 °C. However, when Ag is not homogeneously dispersed in Ag/Pr6O11-NRs catalysts (Figure S5a), the CO conversion rate is only ∼10% at 150 °C. When Pr6O11-NPs are used as support, Ag is better dispersed (Figure 2d), giving rise to a 100% CO conversion at 330 °C. When the silver is seriously agglomerated (Figure S5b), 100% CO conversion occurs at a slightly higher temperature (ca. 380 °C). The results of catalytic performance demonstrate that the homogeneous loading of Ag species was beneficial to harvest a better catalytic activity. In short, the best CO oxidation activity was obtained from Ag/Pr6O11-NRs, with a 100% CO conversion at 240 °C. A comparison between the literature and the Pr6O11 nanocomposites here, shown in Table 1, demonstrates that the two Ag/Pr6O11 nanocomposites prepared in this work show much higher activity. This is particularly noticeable for Ag/ Pr6O11-NRs, which are more active than most of the other supported Ag catalysts.22,24,33−35 The initial step for silver catalyzed CO oxidation occurs when oxygen chemically absorbs on silver.49,50 Then CO reacts with the chemisorbed oxygen. The large quantities of oxygen vacancies in the PrOx nanorods, together with their high specific surface area, promote the chemisorption of gaseous oxygen. It has been suggested that the reaction kinetics are proportional to the oxygen partial pressure, but independent of the CO partial pressure.49 As the following two reactions occur simultaneously, PrOx = PrOx−n + 0.5nO2 and CO + 0.5O2 = CO2, PrOx nanorods may serve as an effective “oxygen buffer” by continuously absorbing and releasing active gaseous oxygen for the CO oxidation, which preferentially occurs on Ag nanoparticles. Thus, the use of Ag impregnated PrOx materials kinetically enhances the oxidization reaction. As previously discussed, the presence of Pr3+ and Pr4+ redox couple suggests there is an electron transfer from the Ag particles to the Pr6O11 supports. The electron transfer between Ag and the O2 molecule weakens the O−O bonding. As a result, the species of O2− were formed concomitantly with the improved oxygen activation. This indicates that the interface between Ag and PrOx is crucial for enhancing the catalytic oxidation of CO. Thus, the excellent activity of Ag/Pr6O11-NRs may be attributed to the nature of this material, e.g., its large surface area, the uniformly dispersed Ag species, and the mesoporous structure of the Pr6O11-NRs support.

Pr6O11-NRs, indicating that the Ag/Pr6O11-NRs are more likely to exhibit higher catalytic activity. An examination of CO conversion percentage as a function of reaction temperature, shown in Figure 5a, reveals that there is a strong link between the morphology of the praseodymium oxide and its catalytic activity. The Pr6O11-NRs become active at 260 °C, which is a slight improvement comparative to the Pr6O11-NPs (becoming active at 275 °C). This improved performance is likely due to the Pr6O11-NRs possessing a higher surface area. However, the Pr6O11-NPs only achieved 50% CO conversion at 350 °C. By comparison, the Ag/Pr6O11 catalysts show superior catalytic performance (50 °C vs 260 °C for NRs and 150 °C vs 275 °C for NPs) with the temperature for the onset of activation effectively lowered. Finally, it should be noted that the Ag/Pr6O11-NRs also show superior catalytic activity in comparison to Ag/Pr6O11-NPs. To gain insights into the intrinsic activities of the Ag/Pr6O11 catalysts, the corresponding turnover frequency (TOF) with respect to the Ag loading in CO oxidation (Figure 5b) was calculated. The TOF values of Ag/Pr6O11 catalysts in our work are lower than those of Ag/CeO2 composite catalysts.45 The TOF values over Ag/Pr6O11-NRs and Ag/Pr6O11-NPs at 150 °C were 0.0038 and 0.00013 molCO·molAg−1·s−1, respectively. The Ag/Pr6O11NRs catalysts displayed higher TOFs than Ag/Pr6O11-NPs and the results were consistent with the CO conversion percentage curves (Figure 5a). This can be explained by the high specific surface area of Pr6O11-NRs compared with Pr6O11-NPs, as more anchoring sites are provided for the nucleation of silver nanoparticles. After loading the silver nanoparticles, the surface lattice oxygen can be activated to produce oxygen vacancies, promote surface reduction, and promote catalytic activity.45 Moreover, the high specific surface area of the Pr6O11-NRs facilitates a uniform distribution of the silver nanoparticles. It appears from the TEM (in Figure 2c,d) that silver disperses more homogeneously on the praseodymium oxide nanorods than on the praseodymium oxide nanoparticles. It is estimated that the size distribution of Ag nanoparticles locates within the range 10.8−12.4 nm and the mean particle size of silver is 11.8 nm. In contrast to the TEM micrographs in Figure 2c,d, the Ag/Pr6O11 series catalyst in Figure S5 suffers from serious aggregation of silver. In order to evaluate the effect of the degree of dispersion of silver on the catalytic performance, the corresponding catalytic performance has been conducted, as shown in Figure S5a,b. It was found that when Ag is homogeneously distributed on Ag/Pr6O11-NRs (Figure 2c), E



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CONCLUSIONS

AUTHOR INFORMATION

Corresponding Authors

A series of Ag/Pr6O11 catalysts have been successfully prepared by the impregnation method and the following activity order for the CO oxidation was observed: Ag/Pr6O11-NRs > Ag/ Pr6O11-NPs > Pr6O11-NRs > Pr6O11-NPs. The excellent catalytic properties of Pr6O11-NRs (compared to Pr6O11-NPs) may be attributed to the significantly increased surface area and the formation of surface oxygen vacancies. Finally, it was found that the choice of support morphology significantly affects the interaction between oxygen vacancies and Ag−Pr6O11 in the Ag/Pr6O11 catalysts, which may be related to the observed differences in performance.

*E-mail: [email protected] *E-mail: [email protected]. ORCID

Wei Zhang: 0000-0002-6414-7015 Author Contributions ∇

X.Z. and S.C.: These two authors contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China (2016YFA0200400), the NSFC (51372095), the Special Funding for Academic Leaders, the Open Foundation from State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2016-09) at Jilin University, and the KBSI (Korea Basic Science Institute) grant to H.-S. Kweon (T37210).

EXPERIMENTAL SECTION Synthesis of Pr6O11. Praseodymium(III) nitrate hexahydrate (Aladdin, 99.9%) was dissolved in deionized water to form praseodymium nitrate solution (0.1 M), to which an NaOH (Beijing Chemical Works, ≥96.0%) aqueous solution (5 M) was added dropwise under stirring until praseodymium hydroxide precipitated completely. The mixture was then stirred for another 0.5 h at room temperature. Finally, the mixture was transferred to an autoclave with a polytetrafluoroethylene (PTFE) lining. The autoclave was then tightly sealed and heated in an oven (180 °C) for 45 h, after which the autoclave was cooled to room temperature naturally. The product was washed with distilled water to remove residual metal ions and then dried at 60 °C in air for 12 h. Finally, the acquired absinthe-green powder was calcined in a muffle oven at designated temperatures (400, 500, 600, 700, and 800 °C, respectively) for 2 h to synthesize the Pr6O11 nanorods. The synthesis procedure for Pr6O11 nanoparticles was undertaken via a facile precipitation−deposition method. The preparation method of the praseodymium oxide nanoparticles is similar to that of praseodymium oxide nanorods, with the only difference being that the nanoparticles do not require further hydrothermal treatment. Synthesis of Ag/Pr6O11 Catalysts. Ag/Pr6O11 catalysts were prepared by a conventional wet chemical method. The asprepared Pr6O11 nanorods or nanoparticles were then employed as the support for Ag catalyst. The preparative experiments were carried out in the absence of light. First, the Ag/Pr6O11 catalysts were prepared by dispersing AgNO3 in distilled water, followed by the addition of ca. 0.1 g of praseodymium oxide during vigorous stirring. After this, the sample was centrifuged, and then treated with an aqueous NaBH4 solution to obtain the silver nanoparticles. Finally, the compound was annealed at 300 °C for 1 h in air for another reduction.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02530. TG/DTA curves of Pr(OH)3 nanorods; detailed analysis of XRD and FTIR spectra; SEM, TEM, HRTEM, and selected area electron diffraction (SAED) of Pr(OH)3NRs, Pr6O11-NRs, and Pr6O11-NPs; Gaussian distribution of mean particle size of silver; specific surface area of Pr6O11 supports and Pr6O11 supported Ag samples; fitting data of XPS over Ag/Pr6O11-NRs and Ag/Pr6O11NPs catalysts (PDF) F


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Pr6O11 Catalysts for

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