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Fabrication of Rhodium Nanoparticles with Reduced Sizes: An Exploration of Confined Spaces Li Huang, Zhi-Min Xing, Yu Kou, Li-Ying Shi, Xiao-Qin Liu, Yao Jiang, and Lin-Bing Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04314 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Industrial & Engineering Chemistry Research

Fabrication of Rhodium Nanoparticles with Reduced Sizes: An Exploration of Confined Spaces Li Huang, Zhi-Min Xing, Yu Kou, Li-Ying Shi, Xiao-Qin Liu,* Yao Jiang, and Lin-Bing Sun*

State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

E-mail: [email protected]; [email protected]

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ABSTRACT: The catalytic activity of supported noble metal nanoparticles (NPs) is strongly dependent on their size. However, it is still a pronounced challenge to develop a facile method to reduce the size of noble metal NPs. Here we report a facile but efficient strategy by utilizing confined spaces between templates and silica walls in as-prepared SBA-15 (AS) to regulate the size of a well-known noble metal, rhodium (Rh). Rh NPs formed in confined spaces possess obviously smaller size than their counterparts generated in traditional calcined SBA-15 (CS) without confined spaces. Our results also demonstrate that the obtained materials are highly active in CO oxidation. For a typical material 5.0RhAS containing 5.02 wt% of Rh, the complete conversion of CO is achieved at 70 oC, which is evidently lower than that on its analogue 5.0RhCS (102 oC).

KEYWORDS: Noble metal nanoparticles, Rhodium, Confined spaces, Mesoporous silicas, CO oxidation

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INTRODUCTION Noble metal nanoparticles (NPs) are useful catalysts in multifarious catalytic reactions including hydrogen evolution,1-4 oxidation,5-8 and hydrogenation.9-11 Especially, the well-known noble metal NPs, such as gold (Au),12, 13 rhodium (Rh),14, 15 palladium (Pd),16 and platinum (Pt),17, 18 are widely used in CO oxidation, which is of great importance for the purification of hydrogen used in polymer electrolyte membrane fuel cells, the removal of CO from exhaust streams, to name just a few. Therefore, noble metal NPs have been dispersed on various supports including metal oxides (i.e. Al2O3, CeO2 and ZrO2),11, 19, 20 metal-organic frameworks (MOFs),21 covalent organic frameworks (COFs),22, 23 and mesoporous silicas.24, 25 Numerous investigations have attested that the activity of NPs is forcefully related to their size.14, 26-28 As a result, various methods have been reported to reduce the size of NPs. For example, uniformly dispersed Pd NPs with an average size of around 3.2 nm on a new triptycene polymer (NTP) were obtained by a gas bubbling-assisted membrane reduction (GBMR) method.29 Smaller Rh NPs (1.9-5.1 nm) synthesized following the one-step polyol synthesis method in the presence of sodium citrate were introduced to SBA-15 by nanoparticle encapsulation (NE).30 Uniform Pt NPs with an average size of about 2.0 nm on Al2O3 were fabricated by an area-selective atomic layer deposition (AS-ALD) method.17 By use of chemical fluid deposition (CFD), Rh-Pt NPs with a mean diameter of 6.1 nm were formed on mesoporous silica SBA-15.31 These reports show some interesting approaches for the preparation of noble metal NPs with enhanced catalytic activity, while simplification of the preparation approaches is highly expected. For the formation of noble metals with reduced size, mesoporous silicas, a kind of materials 3



with large surface area, tunable pore structure, and high synthetic controllability, are ideal choice of supports.32-35 Usually, as-prepared mesoporous silicas are subjected to calcination to remove the templates and obtain open channels.36, 37 Precursors are then introduced to open channels followed by a second calcination to produce active species. Actually, Zhu and co-workers realized highly efficient CO2 capture by incorporating amine into as-synthesized mesoporous silica and demonstrated the existence of a special micro-environment between templates and silica walls in as-prepared mesoporous silicas,38 which might provide confined spaces beneficial to the formation of noble metal NPs with reduced size. Sun and co-workers reported highly efficient adsorptive desulfurization performance by the application of the as-prepared SBA-15 as the support to improve the dispersion of copper species.39 Moreover, the existence of plentiful hydroxyl groups in as-prepared mesoporous silicas has been demonstrated.40 Such hydroxyl groups should be able to interact with noble metal NPs and facilitate the reduction of their size.41 Hence, fabrication of noble metal NPs by exploring the confined spaces in as-prepared mesoporous silicas is extremely desirable. Here we report a facile but efficient strategy by utilizing the confined spaces in as-prepared mesoporous silicas to reduce the size of noble metal NPs. As a proof of concept, a well-known noble metal, Rh, was employed to as-prepared SBA-15 (AS, Scheme 1). It is fascinating that the size of Rh NPs can be well regulated by using confined spaces. Rh NPs obtained in AS possess obviously smaller size in comparison with that in calcined SBA-15 (CS). The smaller size of Rh NPs endows them with excellent catalytic activity in CO oxidation. For a typical material 5.0RhAS containing 5.02 wt% of Rh, the complete conversion of CO is realized at a low temperature of 70 oC. This temperature is apparently 4


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lower than that for its counterpart 5.0RhCS prepared from CS (102 oC).

RESULTS AND DISCUSSION The

Rh

precursor

(RhCl3)

was

introduced

to

AS

via

grinding

followed

by

calcination/reduction. The obtained samples were denoted as nRhAS (n is 3.0, 5.0, or 8.0 and represents the weight percent of Rh in the resultant materials). Through a similar method, an equal amount of Rh was introduced to CS and the obtained samples were denoted as nRhCS (n is 3.0, 5.0, or 8.0). Figures 1a and S1 show the low-angle X-ray diffraction (XRD) patterns for the materials RhAS and RhCS. The Rh-containing samples own a strong peak along with two weak ones, which are similar to the pristine mesoporous silica. These diffraction peaks are indexed to (100), (110), and (200) reflections, suggesting a 2D hexagonal pore symmetry.42 Dark-field transmission electron microscopy (TEM) images of the samples 5.0RhAS and 5.0RhCS show clearly the ordered mesoporous structure through the white-dark contrast (Figures S2 and S3). The TEM images thus confirm the low-angle XRD results, pointing out that the periodic mesopores of parent silica is well preserved after the introduction of Rh NPs. All the samples present a broad diffraction peak centered at 23o in wide-angle XRD patterns, which corresponds to amorphous silica walls (Figures 1b and S4).27 At the same time, the Rh-containing samples present three diffraction peaks at 2θ of 41o, 48o, and 70o ascribed to metallic Rh.43 A great difference between the two types of samples is the diffraction peak intensity of metallic Rh. For the samples with the same Rh content, the peak intensity of Rh in RhAS samples is obviously smaller than that in RhCS samples. On the basis of the diffraction peaks, the particle size of Rh can be calculated via the Scherrer equation (Table S1). All the 5



RhAS samples show lower size of Rh NPs than corresponding RhCS. Typically, the size of Rh NPs in 5.0RhAS (4.9 nm) is obviously lower than that of its analogue 5.0RhCS (8.1 nm). More bright-field and dark-field TEM images of each sample with different magnification can be seen in Figures 2, S5, and S6. It is Rh NPs that the location of the white specks consistent with where particles are observed in dark field. In addition, we can also observe the corresponding black specks in the bright-field TEM images. The TEM images of each sample gained in both bright- and dark-field present more evidence of smaller Rh sizes in RhAS samples. To clarify this point further, the average size of Rh also estimated from TEM images (Table S1). For the sample 3.0RhAS, the average size of Rh NPs is 2.1 nm, which is obviously lower than that of its analogue 3.0RhCS (4.5 nm). With the increase of Rh loading, the size of Rh NPs increases in both RhAS and RhCS samples. The samples with a Rh loading of ~8.0 wt% show the largest Rh particle size, while Rh NPs formed on AS are still much smaller than that on CS (4.7 nm vs. 7.5 nm). Elemental mapping images were taken as well (Figures S2 and S3). In addition to Si and O derived from mesoporous silica matrix, the element Rh is detected as well. According to the descriptions above, it is safe to say that the utilization of confined spaces in AS reduces the size of Rh NPs successfully. On the contrary, larger size of Rh NPs are produced on the traditional template-free mesoporous silica. N2 adsorption/desorption isotherms at −196 °C of different samples were measured. The nitrogen physisorption data of each sample with adding more data points in the region of the hysteresis loop can be seen in Figure S7. All of the RhAS and RhCS samples demonstrate type-IV isotherms with H1-type hysteresis loops, which are similar to parent mesoporous silica.44 A close look shows that, different from CS, the hysteresis loops of Rh-containing 6


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samples show a “tail” in their isotherms labeled by a down arrow, which is caused by the formation of NPs in the mesopores. And for 3.0 sample, there is a less obvious “tail” on CS sample than on AS due to lower Rh content. With the increase of Rh content, the “tails” in the isotherm of other RhCS samples are more obvious than that of RhAS. This means that the size of Rh NPs in RhAS samples is smaller than that in RhCS, which leads to less blockage of mesopores. This can also be verified by the pore size distributions. The pore size of RhAS samples is generally larger than that of RhCS except 8.0RhCS due to the formation of Rh NPs with smaller size in RhAS (Figure S7 and Table S1). But for the sample 8.0RhCS, the average size of Rh NPs is 7.5 nm which indicates that the particle size of certain amount of Rh is larger than the mesopore diameter (8.1 nm). Some Rh NPs locate outside the mesopores, which results in less decrease of the pore size. Moreover, RhAS samples exhibit larger surface area as compared with their RhCS counterpart with the same Rh content. For example, the surface area of 5.0RhAS is 766 m2·g−1, which is apparently greater than that of 5.0RhCS (623 m2·g−1). These results confirm that the confined spaces in AS favor the formation of Rh NPs with smaller size, which is unattainable for the use of conventional CS without confined spaces. The resultant Rh-containing materials were applied to catalyze the oxidation of CO. CO oxidation is one of the most researched heterogeneous reactions, being significant in the industry, particularly for the purification of hydrogen used in polymer electrolyte membrane fuel cells and the removal of CO from exhaust streams.26, 45 The CO oxidation performance with the same amount of catalysts is illustrated in Figures 3, S8, and S9. On the catalysis of pristine mesoporous silica, no CO was converted at all at the temperature range investigated, 7



owing to the absence of active species. The presence of Rh NPs endows the materials with activity in CO conversion. The catalytic activities of all RhAS samples are better than corresponding RhCS. For 5.0RhAS, the conversion of CO initiates at a low temperature of 37 °C, while the onset temperature over 5.0RhCS is as high as 69 °C. Furthermore, the complete conversion of CO is achieved at 70 oC over 5.0RhAS, which is much lower than that over 5.0RhCS (102 °C). To compare the catalytic activity further, Rh content of each sample was first measured by ICP and shown in Table S1. Then the catalytic reactions were run based on the same amount of Rh rather than the same amount of catalysts, as shown in Figures S10 and S11. The catalytic activities of all RhAS samples are also better than corresponding RhCS. Interestingly, the samples 8.0RhAS and 3.0RhCS have similar particle sizes (Table S1), but the sample 8.0RhAS shows better catalytic activity. This further demonstrates that the presence of template is beneficial to the preparation of catalysts with higher activity in comparison with those without template. When the oxidation rate is quick enough to guide an intense increase of temperature, ignition occurs and then the catalytic light-off phenomenon propagates, permitting the thermal CO oxidation.46 Therefore, the light-off temperature (T50) as a significant index is defined as the temperature for which 50% CO conversion is acquired. The light-off temperature is widely used for the comparison of catalytic activity among different samples. All RhAS samples show an obviously lower light-off temperature as compared with their RhCS analogues (Table S1). For the typical material 5.0RhAS, the light-off temperature is as low as 66 oC. In order to compare the grinding method with the much more common incipient wetness impregnation (IWI) method, we also prepared the sample 5.0RhCS(i) by IWI method and tested the performance of CO 8


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oxidation, as shown in Figure S12 and Table S1. However, the light-off temperature of 5.0RhCS(i) which contained the same Rh content is 87 oC. These results are well related to the particle size of Rh as shwon in Table S1. The catalytic performances of our materials are also compared with some typical noble metal catalysts reported in literature (Table S2). For Rh NPs on different supports, namely TiO2, AlPO4 and γ-Al2O3, the light-off temperature was -76, 140, and 147 °C, respectively. Noble metals other than Rh were used for CO oxidation as well. Five Pd-containing catalysts, Pd/CeO2, Pd/Ce-MOF, Pd/La-Al2O3, Pd/NTP, and Pd/MgO, give a light-off temperature of 58, 79, 110, 147, and 175 °C, respectively. Au NPs supported on FeOx, TiO2, and ZIF-8, exhibit severally a light-off temperature of -40, 75, and 170 °C. For Pt NPs supported on PbTiO3, CeO2 and Al2O3, the respective light-off temperature is 46, 130, and 120 °C. Apparently, RhAS samples exhibit superior activity in CO oxidation to their RhCS counterparts. Furthermore, the activity of 5.0RhAS is higher than that of some reported catalysts containing Rh as well as Pd, Au, and Pt. In order to compare the intrinsic activities of the samples accurately, corresponding turnover frequency (TOF) values representing the number of CO molecules transformed over per rhodium site and within per minute were calculated on the basis of the same conditions (temperature, etc.), as shown in Table S1. The common and correct definition of TOF is number of reaction events per reaction site. In order to make the TOF calculation comparable, we calculated TOF at 65 °C.47 Differences in observed TOF are likely related to the catalysts having different number of sites available for reaction. Becuase the higher activity of RhAS samples in comparison with their RhCS counterparts, the TOFs for RhAS samples should be larger. Unfortunately, no CO is converted at 65 oC over RhCS samples, and thus no TOFs can be 9



calculated for RhCS samples. We also tried other temperatures to calculate TOFs, while no proper temperatures can be found to obtain meanful values for all samples. To explore the formation mechanism of Rh NPs with reduced size in AS, the Rh-containing samples before calcination were examined by TG. As shown in Figures 4 and S13, the decomposition of template P123 in as-prepared mesoporous silica takes place at 170 °C, which is visibly lower as compared with that of pure P123 (around 210 oC).48 This is because the siliceous framework is able to catalyze the decomposition of organic template.49 However, it is noteworthy that the decomposition of P123 occurs at 263 oC in 5.0RhAS, which is even higher in contrast to the decomposition temperature of pure template P123. The decomposition temperatures of RhAS samples with other Rh content show similar results. To clarify this point further, a mechanical mixture of Rh-containing precursor and AS was prepared and denoted as 5.0RhAS(m). Obviously, Rh precursor cannot be introduced into mesopores by mechanical mixture. The TG results in Figure S14 indicate that the decomposition temperature of P123 in 5.0RhAS(m) is the same as that in AS. This implies that Rh precursor outside the mesopores has no effect on the decomposition of P123, thus confirming that Rh precursor is introduced into mesopores in our materials. In addition, we measured N2 adsorption-desorption isotherms of AS before and after introducing Rh precursor (Figure S15). Some uptake and hysteresis can be observed in the isotherm of AS, due to the existence of confined spaces.40 A “tail” appears in hysteresis at low pressures after the introduction of Rh precursor. More importantly, the pore size decreases gradually with the increase of Rh amount. These results give direct evidences that Rh precursor was introduced into mesopores. Otherwise, the overall N2 uptake might decrease due to Rh introduction while 10


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neither the shape of isotherm nor pore size would change. Owing to the energy barrier supplied by confined spaces, larger Rh particles that commonly generated through the traditional approach are downsized. Hence, confined spaces constructed by template and silica walls provide a desired interspace for the fabrication of Rh NPs with reduced size. Simultaneously, IR data imply that RhAS samples have more surface hydroxyls in contrast to RhCS samples on the silica walls, as pointed out by the inset view of the 960 cm-1 region (Figures S16 and S17).39, 50 Maybe there are fewer surface hydroxyls interacting with the templating agent because the salt disrupts the interaction, but more surface hydroxyls have been preserved due to the introduction of precursor salt. In contrast to RhAS, the RhCS samples are subjected to more calcination, and hydroxyl groups are consumed during calcination. It is evidenced by DFT simulations that the hydroxyl groups can interact with noble metal NPs and beneficial to their dispersion on the surface of silica.41 Therefore, higher hydroxyl density in AS is conducive to reduce the size of Rh NPs. In terms of the aforementioned discussion, one can say that the smaller size of Rh NPs formed in RhAS samples is attributed to the confined spaces between template and siliceous framework as well as the strong interaction of Rh NPs with abundant hydroxyl groups in as-prepared samples.

CONCLUSIONS A simple but proficient strategy has been designed to reduce the size of noble metal Rh NPs by utilizing the confined spaces. Rh NPs formed in as-prepared mesoporous silica with confined spaces present obviously smaller size than their counterparts created in calcined mesoporous silica. The reduced size of Rh NPs is ascribed to the confined micro-environment between template and siliceous framework as well as the plentiful hydroxyl groups on the 11



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surface. It is also evidenced that the resultant RhAS materials show outstanding performance in CO oxidation. The catalytic performance over RhAS materials is obviously better than that over their RhCS analogues and the better catalytic activity occurs in RhAS samples when two types of samples have the similar size. The present strategy might open up an avenue for the fabrication of new functional materials by use of confined spaces. These active species with reduced particle size or enhanced dispersion could vary from various noble metals to metal oxides and even composites, which is extremely desirable for the catalysis but difficult to realize by traditional approaches.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Physicochemical

properties,

catalytic

performance,

XRD,

TEM,

N2

adsorption/desorption isotherms, conversion plot, TG, and IR results of different samples; and their catalytic activity (PDF)

AUTHOR INFORMATION

Corresponding Author *Prof. Xiao-Qin Liu: [email protected]. *Prof. Lin-Bing Sun: [email protected].

Notes The authors declare no competing financial interest. 12


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21676138, 21722606, and 21576137) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Scheme 1.. Formation of (a) Rh NPs in Conventional Mesopores of CS and (b) Rh NPs in Confined Spaces between the Template P123 and Silica Walls in AS.

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Figure 1. (a) Low-angle and (b) wide-angle XRD patterns of the samples CS, 5.0RhAS, and 5.0RhCS.

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Figure 2. (a, c, e, g) Bright-field and (b, d, f, h) dark-field TEM images of the samples (a, b, c, d) 5.0RhAS and (e, f, g, h) 5.0RhCS with different magnification.

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Figure 3. The conversion plot showing the CO oxidation activity of the samples CS, 5.0RhAS, and 5.0RhCS.

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Figure 4. (a) TG and (b) DTG curves of the samples AS, 5.0RhAS, and 5.0RhCS before calcination.

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Table of Contents

A facile but efficient strategy is developed by utilizing the confined spaces between templates and silica walls in template-containing mesoporous silica to reduce the size of Rh nanoparticles. The obtained Rh nanoparticles possess obviously smaller size than their counterparts generated in traditional template-free mesoporous silica, which endows them with high activity in CO oxidation.

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Fabrication of Rhodium Nanoparticles with ... - ACS Publications

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