Halloysite Nanotube Supported Ru Nanocatalysts Synthesized by the

Article pubs.acs.org/JPCC

Halloysite Nanotube Supported Ru Nanocatalysts Synthesized by the Inclusion of Preformed Ru Nanoparticles for Preferential Oxidation of CO in H2‑Rich Atmosphere Li Wang, Jiuling Chen, Lei Ge, Victor Rudolph, and Zhonghua Zhu* School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

ABSTRACT: The small-sized and well-dispersed Ru nanocatalysts supported on halloysite nanotubes (HNTs) were synthesized by the inclusion of preformed Ru nanoparticles onto HNTs (Ru NPs/HNTs) and employed for the preferential oxidation of CO in H2-rich atmosphere (PROX). Polyol reduction was adopted to prepare Ru nanoparticles, and the synthesis conditions affected the morphology of the resulting nanoparticles. The catalysis results show that the Ru NPs/HNTs present significantly higher CO conversion and CO2 selectivity than the catalyst prepared by traditional wet impregnation. RuCl3 is a better Ru precursor than Ru(acac)3 for synthesizing Ru nanoparticles with higher catalytic reactivity. The catalytic performance of Ru NPs/HNTs can be further enhanced by the reduction pretreatment due to the removal of polyvinylpyrrolidone (PVP) capping on the surface of Ru nanoparticles, and this enhancement is more significant with reduction at 400 °C than at 200 °C. Finally, the characterizations on the used catalysts indicate that the morphology of Ru nanoparticles is maintained after PROX reaction; slight growth of particle size is observed with 200 °C reduction pretreatment, yet Ru nanoparticles lose their original size and shape with 400 °C reduction.

1. INTRODUCTION Halloysite nanotubes (HNTs) are a type of naturally occurring aluminosilicate (Al2(OH)4Si2O5·nH2O) with nanotubular structures. HNTs are widely deposited in soil in wet tropical and subtropical regions, weathered rocks, and soil generated from volcanic ashes, available in abundance in China, Japan, France, Brazil, Belgium, Australia, and New Zealand.1−4 Due to the nanotubular shape of HNTs, they possess highly meso-/ macroscopic pore structure and large specific surface area. The nanotubular structure of HNTs can also provide a confined space for the metal nanoparticles and reactions. Therefore, HNTs can be explored as a support for active phases in catalysis. Nevertheless, despite their significant deposits and considerable interest in a range of other synthetic nanotubular materials such as carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs), there are limited investigations targeted to use the beautiful hollow tubular structure of HNTs in catalysis, with only a few studies reported.3,5−8 In our previous studies, traditional wet impregnation was employed to prepare HNT-supported Ru nanoparticles, which were then applied to catalyze ammonia decomposition and © 2013 American Chemical Society

preferential oxidation of CO in a H2-rich atmosphere (PROX).9,10 It was found that HNT-supported Ru nanoparticles prepared by wet impregnation exhibited broad particle size distribution up to a maximum of over 30 nm, owing to the weak interaction between Ru and HNTs. The big Ru nanoparticles on HNTs showed low catalytic activity. Only at high Ru loading content, a satisfactory catalytic activity can be achieved in ammonia decomposition.9 In the PROX reaction, even with high Ru loading content, insufficient CO removal efficiency was observed because large Ru nanoparticles prepared by wet impregnation favored the oxidation of H2 over that of CO.10 To improve Ru dispersion and enhance the catalytic performance, different procedures should be employed to synthesize HNT-supported Ru nanocatalysts. The solution-phase syntheses succeed in preparing metal colloidal nanoparticles with different sizes, shapes, compositions, and surface structures. A myriad of solution-phase Received: December 19, 2012 Revised: January 24, 2013 Published: January 31, 2013 4141

dx.doi.org/10.1021/jp312491m | J. Phys. Chem. C 2013, 117, 4141−4151

The Journal of Physical Chemistry C

Article

reaction temperature, were varied, to synthesize Ru nanoparticles for loading onto HNTs. The denotations for Ru nanoparticles and their corresponding synthesis conditions were listed in Table 1.

methods have been developed for the synthesis of nanoparticles.11 Among them, polyol reduction is an easy, versatile, and most commonly used procedure, which utilizes highboiling polyalcohol as both solvent and reducing agent to generate metal colloids from an organometallic complex or a metal salt precursor in the presence of a stabilizing agent.12−14 The synthesis conditions in polyol reduction can be finely tuned to generate well-defined shape- and size-controlled metal nanoparticles.14−19 The formed nanoparticles can then be deposited onto supports to prepare 2D or 3D supported catalysts, where the particle morphologies can be maintained.19−21 In this case, the morphologies of supported metal nanoparticles are determined by the nanoparticle synthesis process and therefore are less support-dependent compared to the wet impregnation method. Small and well-dispersed Ru nanoparticles on HNTs, which cannot be obtained by the wet impregnation method, can then be expected by the immobilization of preformed Ru nanoparticles onto HNTs. In this work, Ru nanoparticles were prepared by the polyol reduction method and then deposited onto HNTs with the assistance of sonication. The characterizations on the preformed Ru nanoparticles and the resulting Ru NP/HNT catalysts were conducted. Their catalytic performances in PROX reactions were evaluated and compared with the catalyst counterpart prepared by conventional wet impregnation. The effects of the synthesis conditions for the Ru nanoparticles and the reduction pretreatment for the supported Ru NPs/HNTs on the catalytic performance were also investigated. Finally, the characterizations on the used catalysts were carried out to study the morphology change of Ru nanoparticles during the reduction pretreatment and PROX reaction.

Table 1. Experimental Conditions for Synthesis of Various Ru Nanoparticlesa sample Ru Ru Ru Ru a

NP-A NP-B NP-C NP-D

Ru precursor type

Ru precursor concentration (mM)

final reaction T (°C)

Ru(acac)3 Ru(acac)3 Ru(acac)3 RuCl3

5 5 20 5

180 150 180 180

Note: EG as solvent, Ru/PVP stoichiometric ratio of 1:10.

2.2. Synthesis of Ru NPs/HNTs by Inclusion of Preformed Ru Nanoparticles. To prepare HNT-supported Ru nanoparticles by the inclusion of preformed Ru nanoparticles, HNTs were first added into ethanol solution and sonicated for 100 min to disperse HNTs in ethanol. The diluted Ru nanoparticle ethanol solution was then added into the HNT ethanol solution to achieve a nominal Ru loading of 2 wt %. The mixture was subjected to sonication for 100 min and magnetic stirring for 150 min. Finally, the Ru-x/HNT (x refers to the synthetic conditions shown in Table 1) solid samples were separated from the solution by centrifuge and dried at 70 °C in a vacuum oven overnight. For comparison, Ru/HNTs-WI was synthesized by a traditional wet impregnation method reported previously.10 The Ru loading content was measured to be 2.6 wt % by inductively coupled plasma optical emission spectrometry (ICP-OES). 2.3. Characterizations. The actual Ru loading contents of Ru NPs/HNTs were determined by a Varian Vista Pro ICPOES using a Sturman−Masters spray chamber and a V-groove nebulizer. N2 physisorption isotherms of the samples were obtained using Micromeritics TriStar 3020 at −196 °C. The samples were degassed at 150 °C for 24 h before the measurement. The corresponding specific surface area (Sg) was calculated by the Brunauer−Emmett−Teller (BET) equation at relative pressure (P/Pø) between 0.05 and 0.35. Total pore volume (Vp) was evaluated at relative pressures (P/Pø) close to unity using the Barrett−Joyner−Halenda (BJH) method. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100 microscope with accelerating voltages of 200 kV. For Ru nanoparticles, several drops of particle solution were dripped onto a thin carbon film copper grid. For Ru NPs/ HNTs, the samples were dispersed by sonication in a mixture of ethanol and isopropanol and then deposited on a holey carbon copper grid and dried. The powder X-ray diffraction (XRD) was performed with a Bruker advanced X-ray diffractometer using (40 kV, 30 mA) with Cu Kα (λ = 0.15406 nm) radiation and a graphite monochromator at a scanning rate of 1°/min from 10° to 80°. Thermo gravimetric analysis (TGA) was carried out using a Perkin-Elmer Instruments STA 6000 Thermo Gravimetric Analyzer. The sample was first maintained at 110 °C for 60 min to remove water and then heated from 110 to 900 °C at 10 °C/ min under N2. X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis ULTRA XPS instrument incorporating a 165 mm

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ru Nanoparticle Colloidal Solution. The Ru nanoparticles were synthesized by a polyol reduction method reported previously.15 Either ruthenium(III) acetylacetonate (Ru(acac)3, 97%, Sigma-Aldrich) or ruthenium(III) chloride (RuCl3, ReagentPlus, Sigma-Aldrich) was used as the Ru precursor, polyvinylpyrrolidone (PVP, Mw: 55 000, SigmaAldrich) as the stabilizer, and polyol, including ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and 1,4-butanediol, as both solvent and reducing agent. In a typical synthesis, Ru precursor and PVP with certain Ru/PVP stoichiometric ratio (PVP in terms of repeating unit) were added into 50 mL of polyol at room temperature. The solution was then heated to 80 °C with magnetic stirring and maintained at this temperature for 30 min under Ar to remove water and oxygen. After that, the solution was heated to the final reaction temperature at a rate of 10 °C/min and remained at this temperature for 2 h, still under Ar flow. When the reaction was complete, acetone was added into the solution, and the resulting black suspension was subjected to centrifuge. The precipitated Ru nanoparticles were then separated, collected, and redispersed in ethanol. A set of conditiondependent experiments were carefully carried out, and the resulting Ru nanoparticles were examined under transmission electron microscopy (TEM). The results showed that the type of polyol and the Ru/PVP stoichiometric ratio have little influence on the final size and shape of Ru nanoparticles. Therefore, EG was used as solvent, and the Ru/PVP stoichiometric ratio was fixed at 1:10. The other factors, such as the Ru precursor type, Ru precursor concentration, and final 4142



Article

Figure 1. TEM images of Ru nanoparticles: (a) Ru NP-A; (b) Ru NP-B; (c) Ru NP-C; (d) Ru NP-D. Insets: TEM images at a higher magnification.

Figure 2. Particle size distribution (PSD) of Ru nanoparticles: (a) Ru NP-A; (b) Ru NP-B; (c) Ru NP-C.

4143



Article

Figure 3. TEM images of fresh Ru NPs/HNTs: (a) Ru NP-A/HNTs; (b) Ru NP-B/HNTs; (c) Ru NP-C/HNTs; (d) Ru NP-D/HNTs.

180 °C is mostly composed of spherical particles with uniform particle size (Figure 1(a)). The lower reaction temperature of 150 °C could lead to larger nanoparticles and less homogeneous particle size; moreover, some large nanoparticles are in the form of a triangular plate (Ru NP-B in Figure 1(b)). The higher Ru precursor concentration during the synthesis can also enlarge the particle size, as indicated by Ru NP-C in Figure 1(c). Ru NP-D, with the different Ru precursor (RuCl3) applied during synthesis, shows worm-like shape (Figure 1(d)), which is very different from the spherical or triangular plate shape of the other samples. The particle size distributions (PSDs) of Ru nanoparticles are illustrated in Figure 2, obtained from measuring the sizes of around 1000 particles randomly taken from the TEM images of each sample. Due to the fact that the nanoparticles in Ru NP-D are not well-separated, the measurement of their sizes becomes difficult. Also, since their worm-like shape is very different from the spherical or nanoplate shape of other samples, the direct comparison of the particle size between Ru NP-D and other samples cannot be made. Therefore, the PSD of Ru NP-D is not statistically acquired. Comparing the PSD of the three spherical Ru nanoparticles, it is clear that Ru NP-A has the smallest average particle size, 2.9 nm, and the narrowest PSD, with the entirely observed particles falling into the size range of 2−4 nm and 90% of particles presenting a particle size of 2.5− 3.5 nm (Figure 2(a)). In comparison, Ru NP-B has a larger average particle size, 3.5 nm, as well as a much wider PSD (Figure 2(b)). The average particle size of Ru NP-C is similar to that of Ru NP-B, but the PSD of the former is narrower than that of the latter (Figure 2(c)). From the above results, it can be seen that the final reaction temperature and the concentration as well as the type of Ru precursor all affect the morphology of the resulting Ru nanoparticles. First, the higher reaction temperature yields Ru

hemispherical electron energy analyzer. The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 225 W (15 kV, 15 mA). 2.4. Catalytic Activity Tests. PROX reaction was performed in a vertical stainless steel tube reactor (7.5 mm inner diameter), with a catalyst weight of 0.2 g, a feed gas containing 1 vol % CO, 1 vol % O2, 15 vol % CO2, 20% vol % He, balanced in H2, and a detailed procedure reported elsewhere.10 For Ru/HNTs-WI, an in situ reduction using pure H2 at 400 °C was conducted before the catalytic reaction. For Ru NP/HNT series samples, no reduction pretreatment was applied except the part to investigate the effect of reduction pretreatment on the catalytic performance where reduction at 200 or 400 °C in pure H2 was carried out before the catalytic activity tests. CO2 selectivity is defined as the ratio of O2 consumption for CO oxidation over the total O2 consumption. The reaction was performed at atmospheric pressure, and the temperature was measured using a thermocouple inserted into the catalyst bed. Each reaction temperature was maintained for at least 1 h to obtain the steady state. A blank activity test showed that HNT support exhibited no obvious catalytic activity in PROX reaction in the whole investigated temperature range.

3. RESULTS AND DISCUSSION 3.1. Characterization of Ru Nanoparticles. During the nanoparticle synthesis in polyol reduction, the careful manipulation of the experimental conditions can alter the nucleation and growth process of nanoparticles in a controllable way and therefore generate nanoparticles with different sizes and shapes. The morphologies of the Ru nanoparticles prepared under different conditions have been examined by TEM, shown in Figure 1. Ru NP-A synthesized with a low concentration of Ru(acac)3 at a final reaction temperature of 4144



Article

the inclusion of preformed nanoparticles. This method could also provide a common route to deposit uniform-distributed small metal nanoparticles onto the supports like HNTs which present weak interactions with the active phases. 3.2.2. ICP and Nitrogen Physisorption. The actual loading contents of Ru, characterized by ICP, are quite similar, ranging from 1.6 to 1.9 wt % in the Ru NP/HNT series samples (Table 2). Considering the nominal loading rate of 2 wt %, it can be

nanoparticles with smaller size and narrower PSD. This is commonly observed in the nanoparticle polyol synthesis,15,17,19 probably because a balance between the metal precursor reduction and crystal growth is reached at high reaction temperature, resulting in uniform nanocrystal size.19 In addition, the significant proportion of triangular nanoplates, a shape controlled by growth kinetics, in Ru NP-B indicates that the slow reduction at low temperature brings the reaction into the kinetically controlled regime.22 Second, at a given reaction temperature, the sizes of Ru nanoparticles can be controlled by changing the concentration of the Ru precursor. Larger Ru nanoparticles are formed with higher initial precursor concentration, similar to the previous studies on Ru and Rh nanoparticle synthesis.14,15,19 In terms of obtaining enlarged Ru nanoparticles while maintaining their monodispersity, it can be seen that the increase of Ru precursor concentration is a better way compared to the reduction of reaction temperature. Finally, Ru nanoparticles synthesized from Ru(acac)3 or RuCl3 present very different morphologies, indicating the type of Ru precursor largely defines the final morphology of Ru nanoparticles. On one hand, the anionic ligand in the precursor, such as halides and acac−, can act as a surface adsorbent, selectively adsorb on the specific crystal facet, and define the nanoparticle morphology.11 On the other hand, the stability of the metal complex can influence the reduction as well as the particle growth rate and change the morphology of the resulting nanoparticles.14 RuCl3 is less stable than Ru(acac)3;19 therefore, the former can be reduced at lower temperature or higher reduction rate than the latter. The easy reduction of RuCl3 may contribute to the poorly defined shape of Ru nanoparticles in Ru NP-D because Ru(0) atoms can be produced in a large amount from the fast reduction of RuCl3, leading the crystal growth to fail to form well-defined shape in a controlled manner. In summary, the Ru nanoparticles with controlled sizes can be obtained by polyol reduction via modifying the synthesis conditions. Nevertheless, it failed to acquire Ru nanoparticles with well-defined and uniform shapes by the one-step polyol process. Many efforts have been made to achieve the shape control on Ru nanoparticles; however, it remains a grand challenge to date.11 3.2. Characterization of Fresh Ru NPs/HNTs. 3.2.1. Transmission Electron Microscopy. The synthesized Ru nanoparticles have been loaded onto HNTs; their TEM images are shown in Figure 3. It can be seen that in all samples the Ru nanoparticles are well dispersed on HNTs as individual nanoparticles, with no aggregation observed. Nevertheless, if no sonication is adopted during the loading process, Ru nanoparticles are found to be deposited on HNTs in the form of bunches of particle aggregates (TEM images not shown here). Therefore, sonication is an effective way to facilitate the uniform dispersion of the preformed Ru nanoparticles on HNTs. Moreover, most Ru nanoparticles are deposited on the external surface of HNTs, and the nanochannels of HNTs are mostly empty. This may be due to the capping agent on the surface of Ru nanoparticles hindering the capillary inclusion of Ru nanoparticles into the small nanochannels of HNTs. A comparison between the HNT-supported Ru nanoparticles (Figure 3) and their original colloidal nanoparticles (Figure 1) reveals that the shape and size of the Ru nanoparticles have been maintained after deposition onto HNTs. Small-sized and uniform-dispersed Ru nanoparticles on HNTs, which cannot be achieved by the traditional wet impregnation, are obtained by

Table 2. Specific Surface Areas and Pore Volumes of HNTs and Ru/HNT Catalysts sample

actual Ru loading (wt %)

Sg (m2/g)

Vp (cm3/g)

HNTs Ru NP-A/HNTs Ru NP-B/HNTs Ru NP-C/HNTs Ru NP-D/HNTs

1.9 1.7 1.6 1.8

24.3 23.7 23.3 23.9 23.1

0.140 0.116 0.111 0.115 0.116

seen that the inclusion of preformed Ru nanoparticles is an efficient way to load Ru nanocatalysts with little Ru loss. The nitrogen physisorption isotherms of HNT support and various Ru NP/HNT catalyst samples are presented in Figure 4. All

Figure 4. Nitrogen adsorption isotherms of HNTs and various Ru NPs/HNTs (filled symbols, adsorption isotherm; opened symbols, desorption isotherm).

four catalyst samples exhibit isotherms similar to that of HNTs; therefore, the pore structure of HNTs is maintained after Ru nanoparticle loading. S g and V p calculated from the corresponding isotherms are shown in Table 2. The comparison of Sg and Vp between HNTs and catalysts shows that the loading of preformed Ru nanoparticles has a negligible effect on the surface area but leads to a decrease in pore volume due to the blockage of pores by the Ru nanoparticles. The four catalyst samples with different preformed Ru nanoparticles exhibit similar Sg and Vp, indicating the size and shape of Ru nanoparticles has little influence on the pore structure of HNT support. 4145



Article

3.2.3. Thermal Gravimetric Analysis. Thermal gravimetric analysis (TGA) was conducted to investigate the thermal stability of HNTs and Ru NPs/HNTs, as well as the decomposition of PVP capping on the surface of Ru nanoparticles. The TG curves and the corresponding differential thermogravimetric (DTG) profiles are shown in Figure 5.

temperature. Therefore, the thermal treatment could be an effective way to remove PVP capping on the surface of Ru nanoparticles and expose the active sites of Ru for catalytic reactions. The major weight loss at 400−600 °C is assigned to the dehydroxylation of structural AlOH groups of halloysite.4,25 Our previous studies showed that the tubular structure of HNTs could be maintained despite the structural dehydroxylation, and the catalytic performance would not be affected by the dehydroxylation of HNT support.9 3.3. Catalytic Activities. 3.3.1. Effect of Preparation Method. The catalytic performance over the HNT-supported Ru catalysts in PROX reaction was illustrated in terms of CO conversion and CO2 selectivity, shown in Figure 6. Wet

Figure 5. TG (a) and DTG (b) profiles of HNTs and various Ru NPs/HNTs.

It can be seen that the TG curves of four Ru NP/HNT samples are quite similar; however, their weight loss is significantly larger than that of HNT support. During the temperature range between 200 and 400 °C, an average weight loss of 3.0% occurs in the catalyst samples, compared to that of 1.2% in the support. A more obvious difference between the catalysts and support can be observed in the DTG curves, where a broad DTG peak only exists in the catalyst samples. There is another major weight loss for all samples in the temperature range of around 400−600 °C, with the corresponding DTG peak at around 510−520 °C. The weight loss of the Ru NP/HNT samples in the temperature range of 200−400 °C can be attributed to the degradation or decomposition of PVP capping on the surface of Ru nanoparticles. The pure PVP starts to decompose at temperatures above 300 °C;23 however, the noble metal nanoparticles can play a role of catalyst to accelerate the decomposition of adsorbed PVP and lower its decomposition temperature to more than 100 °C.16,23,24 The decomposition of PVP appears to be a slow and gradual process since the corresponding DTG peak is quite broad. After 460 °C, the DTG curves of Ru NP/HNT samples are coincident with that of HNTs, indicating the full PVP degradation at that

Figure 6. CO conversion (a) and CO2 selectivity (b) for the PROX reaction with increasing reaction temperature over Ru/HNTs-WI by wet impregnation and Ru NP/HNT series samples by the inclusion of preformed Ru nanoparticles: catalyst amount, 0.2 g; reaction gas flow rate, 63 mL/min; reaction gas composition, 1 vol % CO, 1 vol % O2, 15 vol % CO2, and 20 vol % He, balanced in H2.

impregnation resulted catalyst has significantly lower catalytic performance than the catalysts prepared by the inclusion of preformed Ru nanoparticles onto HNTs. Ru/HNTs-WI can only reach a maximal CO conversion rate of 55% at 123 °C, compared to 72%, 77%, 79%, and 82% for Ru NP-A/HNTs, Ru NP-B/HNTs, Ru NP-C/HNTs, and Ru NP-D/HNTs, respectively, at a similar reaction temperature. The CO2 selectivity of Ru/HNTs-WI is also significantly lower than that of Ru NP/HNT series samples; at most reaction temperatures, Ru/HNTs-WI exhibit CO2 selectivities of just half of that over Ru NP-D/HNTs. 4146



Article

catalytic reactivities of PROX reaction compared to Ru/HNTsWI, their catalytic performances still have room for improvement. Reduction in H2 at elevated temperature is then employed as pretreatment before PROX reaction. Ru NP-B/ HNTs and Ru NP-D/HNTs are taken as representatives to investigate the effect of reduction pretreatment on the catalysis. The CO conversion and CO2 selectivity over these two catalysts after reduction pretreatment at 200 and 400 °C are shown in Figure 7; for comparison, the catalytic performance of the corresponding catalyst without reduction is also included.

Ru/HNTs-WI (2.6 wt %) have higher Ru loading content than the Ru NP/HNT series samples (1.6−1.9 wt.%), yet the former presents significant lower catalytic reactivity than the latter. The intrinsic properties of HNT-supported Ru nanoparticles prepared by different methods are the determining factor of their catalytic performance. From our previous studies, it has been known that the interaction between Ru and the surface of HNTs is very weak; therefore, the Ru nanoparticles deposited onto HNTs by wet impregnation are large in size. A broad particle size distribution, with most Ru nanoparticles in the range of 10−20 nm and the largest ones over 30 nm, has been observed in Ru/HNTs-WI.9,10 By comparison, the Ru nanoparticles are much smaller in the Ru NP/HNT series samples; Ru nanoparticles bigger than 5 nm are seldom observed in those samples (Figure 3). The Ru nanoclusters with different sizes exhibit different catalytic performances in PROX reaction due to their inherent characteristics. It has been found in our previous studies that the smaller Ru nanoparticles favor the oxidation of CO over that of H2,10,26 which is also consistent with the findings from other researchers.27−29 Therefore, the small Ru nanoparticles in the Ru NP/HNT series samples present higher CO2 selectivities than that of the large Ru nanoparticles in Ru/HNTs-WI, leading to superior CO removal efficiency of the former. In summary, the inclusion of preformed nanoparticles onto support is a better route to prepare HNT-supported Ru catalysts than wet impregnation since the former route can obtain small-sized and uniformdispersed Ru nanoparticles, which present high catalytic selectivity in PROX reaction. 3.3.2. Effect of Preformed Ru Nanoparticles. Figure 6 shows that four Ru NP/HNT samples exhibit a similar reactivity trend with the increase of reaction temperatures. Both CO conversion and CO2 selectivity increase first with temperature up to around 105−120 °C and then decrease with further temperature increment, indicating the enhancement of H2 oxidation at higher temperature, as also observed by other researchers.27,29−31 Despite the similar trend, the synthesis conditions for the Ru nanoparticles give rise to a difference in the catalytic performance of the resulting catalysts. Ru NP-D/ HNT presents the highest CO conversion and CO2 selectivity in the whole reaction temperature range of 80−170 °C. The other three catalysts show similar values in both CO conversion and CO2 selectivity. It is clear that the Ru precursor for the synthesis of Ru nanoparticles is the critical factor to determine the catalytic performance of Ru NPs/HNTs. Ru nanoparticles synthesized from RuCl3 show significantly higher catalytic activity and selectivity than those synthesized from Ru(acac)3. Several reasons may contribute to the effect of the Ru precursor on the catalytic performance, such as the impurities and residues from the precursor, as well as the particle shape difference resulting from different precursors. Other factors for the Ru nanoparticle synthesis, including Ru precursor concentration and final reduction temperature, though affecting the size and shape of the resulting nanoparticles, have little influence on their overall catalytic reactivity. The reason could be that the morphology difference between those samples is not enough to observe the particle size effect as well as the shape effect on the catalytic performance. To investigate these effects, monodispersed metal nanoparticles with homogeneous shapes and distinct size differences should be synthesized. 3.3.3. Effect of Reduction as Pretreatment. Despite the fact that Ru NPs/HNTs exhibit significant enhancement on the

Figure 7. CO conversion and CO2 selectivity for PROX reaction with increasing reaction temperature over Ru NP-B/HNTs (a) and Ru NPD/HNTs (b) without or with reduction at different temperatures as pretreatment (filled symbols, CO conversion; open symbols, CO2 selectivity); catalyst amount, 0.2 g; reaction gas flow rate, 63 mL/min; reaction gas composition, 1 vol % CO, 1 vol % O2, 15 vol % CO2, and 20 vol % He, balanced in H2.

It can be seen from Figure 7 that the reduction pretreatment increases both CO conversion and CO2 selectivity, especially at high reaction temperatures. Moreover, the enhancement is more significant with 400 °C reduction compared to that with 200 °C reduction. Taking a reaction temperature of 170 °C for example, the CO conversion goes up from 46% without reduction to 70% and 89% with 200 and 400 °C reduction, respectively, over Ru NP-B/HNTs and increases from 53% to 75% and 100% accordingly over Ru NP-D/HNTs. It is noteworthy that a full CO conversion can be reached at 170 °C on Ru NP-D/HNTs after 400 °C reduction. The CO2 selectivity proceeds in parallel to CO conversion, presenting a similar enhancement trend to that of CO conversion. Also, it 4147



Article

some triangular nanoplates, while Ru NP-D/HNTs still present a worm-like shape. The careful examination on the particle size also reveals that negligible particle growth occurs during PROX reaction for both catalyst samples. After 200 °C reduction and PROX reaction, there is still no obvious change in the shape of Ru particles for both samples, although slight enlargement of their particle size can be observed (Figures 9(b) and 10(b)). Nevertheless, if reduction is conducted at 400 °C to pretreat the samples, both the particle shape and size change dramatically after PROX reaction. For Ru NP-B/HNTs, the PSD becomes very ununiform; most nanoparticles grow to 6−9 nm in elongated shape, but small nanoparticles with the original spherical shape and size of 3−4 nm still exist (Figure 9(c)). Ru NP-D/HNTs exhibit elongated nanoparticles and small spherical nanoparticles after 400 °C reduction and PROX reaction (Figure 10(c)), similar to Ru NP-B/HNTs, even though their fresh counterparts show very different morphologies. In summary, the PROX reaction itself cannot change the size and shape of Ru nanoparticles; the reduction pretreatment at 200 °C leads to a slight particle growth, but the 400 °C reduction can totally alter the shape and PSD of the nanoparticles in Ru NPs/HNTs. Several previous studies have investigated the effect of the catalytic process on the shape and size of metal nanoparticles. In the arene hydrogenation and electron transfer reaction, the size and shape of metal nanoparticles remain unchanged.35,36 Both reactions are conducted at room temperature; nevertheless, if harsh reaction conditions are employed, drastic changes in both particle size and shape have been observed.36 It is worth noting that these reactions are all catalyzed by the colloidal metal nanoparticles, and no support is involved. In our study, the shape and size of Ru nanoparticles supported on HNTs remain unchanged after PROX reaction. Considering the highest reaction temperature of up to 170 °C, the support could provide an extra stabilization effect on the nanoparticles together with the capping agents to prevent their morphology change during the reaction. The 200 °C reduction pretreatment can trigger the particle growth, but only to a limited extent. This is because only a small amount of PVP capping on the surface of Ru nanoparticles is removed during 200 °C reduction, and the remaining PVP can still stabilize the nanoparticles from aggregation. When the reduction temperature increases to 400 °C, severe nanoparticle aggregation occurs since the complete PVP capping removal at that temperature destabilizes the nanoparticle. The close-by nanoparticles aggregate together and form one large nanoparticle in the elongated shape. It was previously reported that Rh nanoparticles capped by PVP presented a slight particle growth after heating at 250 °C, but with 450 °C heating, the average particle size increased significantly,24 consistent with our findings. Combined with the catalysis results, it can be seen that the reduction pretreatment at 200 °C can increase the catalytic activity, while the nanoparticle size only increases to a limited extent. With 400 °C reduction, the Ru nanoparticles lose their original size and shape, but a larger enhancement on their catalytic reactivity has been obtained. The XRD patterns of Ru NP-B/HNTs and Ru NP-D/HNTs after 400 °C reduction and PROX reaction are shown in Figure 11. There exist typical diffraction peaks of 7 Å dehydrate halloysite Al2Si2O5(OH)4 with JCPDS Card No. 29-148737,38 and impurities like cristobalite (JCPDS Card No. 39-1425) and quartz (JCPDS Card No. 82-0511); nevertheless, no diffraction

can be noted that HNT-supported Ru nanoparticles synthesized by the inclusion of preformed Ru nanoparticles and pretreated by reduction exhibited PROX catalytic performance comparable to, if not higher than, those Ru nanoparticles supported on commercial Al2O3,32 SiO2,27,33 and Yttriastabilized zirconia (YSZ).34 Therefore, this synthetic procedure together with reduction pretreatment could provide a route to realize the practical application of natural HNTs as catalyst supports. The TGA results show that the PVP capping on the surface of Ru nanoparticles starts to decompose at 200 °C (Figure 5), and therefore maintaining the reduction at 200 °C for some time can help to remove PVP from the Ru surface via decomposition. At a higher reduction temperature of 400 °C, PVP decomposes to a larger extent. XPS was carried out to confirm the PVP removal during the reduction pretreatment, shown in Figure 8. Fresh Ru NP-D/HNTs present a clear

Figure 8. XPS spectra of HNTs and Ru NP-D/HNTs without reduction or with reduction at 200 and 400 °C.

nitrogen peak, which is attributed to the PVP capping on the surface of Ru nanoparticles since HNTs do not present this peak. The nitrogen peak shows a lower intensity after 200 °C reduction, yet it vanishes after 400 °C reduction, which verifies the PVP removal during the reduction pretreatment and a more thorough PVP removal with 400 °C reduction. The removal of PVP leads to the exposure of active sites originally blocked by this capping agent and thus improves the catalytic activity of Ru nanoparticles. Since the PVP removal is more complete with 400 °C reduction than with 200 °C reduction, more active sites on Ru nanoparticles are exposed, and more significant enhancement in the catalytic performance has been obtained with the former pretreatment. 3.4. Characterization of Used Ru NPs/HNTs. The size and shape of metal nanoparticles could change during the course of their catalytic function. Therefore, the used catalysts were examined under TEM to observe the possible morphology change after PROX reaction as well as the reduction pretreatment, as shown in Figures 9 and 10. The used samples without reduction pretreatment (Figures 9(a) and 10(a)) show particle morphology similar to the fresh samples (Figure 3(b) and (d)); Ru NP-B/HNTs maintain its spherical shape, with 4148



Article

Figure 9. TEM images of used Ru NP-B/HNTs after PROX reaction: (a) without reduction pretreatment; (b) with 200 °C reduction as pretreatment; (c) with 400 °C reduction as pretreatment.

Figure 10. TEM images of used Ru NP-D/HNTs after PROX reaction: (a) without reduction pretreatment; (b) with 200 °C reduction as pretreatment; (c) with 400 °C reduction as pretreatment.

TGA results. Even though some large Ru nanoparticles are observed under TEM in Ru NPs/HNTs after 400 °C reduction, they are not detectable by XRD. In comparison,

peaks corresponding to the Ru phase are observed. The halloysite phase is still presented, indicating that the structural dehydroxylation has not occurred yet at 400 °C, consistent with 4149



Article

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by an Australian Research Council Discovery Project, and author Li Wang acknowledges financial support from a CSC scholarship from China. Li Wang would also like to thank Prof. Gabor Somorjai, Dr. Kwangjin An, and Dr. Selim Alayoglu for their kind help and support during her exchange period at the University of California, Berkeley.

■

Figure 11. XRD patterns of used Ru NP-B/HNTs and Ru NP-D/ HNTs after 400 °C reduction followed by PROX reaction.

Ru/HNT-WI with similar or smaller Ru loading content exhibits intensive Ru diffraction peaks.9 Therefore, even with some sintering and aggregation during the reduction at 400 °C, the HNT-supported Ru nanoparticles by the inclusion of preformed nanoparticles are much better dispersed compared to those prepared by wet impregnation.

4. CONCLUSIONS Polyol reduction can be employed to synthesize small and uniform Ru nanoparticles, and the synthesis conditions, such as reduction temperature, precursor type, and concentration, affect the size and shape of the resulting nanoparticles. The morphology of these preformed Ru nanoparticles can be maintained after deposition onto HNTs. In PROX reaction, the HNT-supported Ru nanocatalysts by the inclusion of preformed Ru nanoparticles (Ru NPs/HNTs) present significantly higher CO conversion and CO2 selectivity than their counterparts by traditional wet impregnation (Ru/HNTsWI). This is because the small-sized and uniform-dispersed Ru nanoparticles in Ru NPs/HNTs favor the oxidation of CO over that of H2 compared to the large nanoparticles in Ru/HNTsWI, leading to high CO2 selectivity and CO removal efficiency of the former. It is also interesting to note that RuCl3 was a better Ru precursor in polyol reduction than Ru(acac)3 for synthesizing Ru nanoparticles with higher catalytic reactivity. The reduction pretreatment can lead to the removal of PVP capping on the surface of Ru nanoparticles and thus the exposure of active sites originally blocked by the capping agent, therefore resulting in an enhancement of the catalytic performance over Ru NP/HNT series samples. This enhancement is more significant with 400 °C reduction compared to that with 200 °C reduction; nevertheless, Ru nanoparticles aggregate together and form big nanoparticles in elongated shapes after 400 °C reduction, while only slight growth of particle size is observed after 200 °C reduction. If no reduction pretreatment is conducted, the morphology of Ru nanoparticles could be maintained after PROX reaction. To conclude, the inclusion of preformed nanoparticles is an effective route to synthesize HNT-supported metal catalysts with small size and high catalytic performance. The application of natural HNTs, with the advantages of low price and abundant resource, in catalysis can then be realized.

■

REFERENCES

(1) Du, M.; Guo, B.; Jia, D. Polym. Int. 2010, 59 (5), 574−582. (2) Levis, S. R.; Deasy, P. B. Int. J. Pharm. 2002, 243 (1−2), 125− 134. (3) Machado, G. S.; Castro, K.; Wypych, F.; Nakagaki, S. J. Mol. Catal. A: Chem. 2008, 283 (1−2), 99−107. (4) Joussein, E.; Petit, S.; Churchman, J.; Theng, B.; Righi, D.; Delvaux, B. Clay Miner. 2005, 40 (4), 383−426. (5) Antill, S. J. Aust. J. Chem. 2003, 56 (7), 723−723. (6) Barrientos-Ramirez, S.; Ramos-Fernandez, E. V.; Silvestre-Albero, J.; Speulveda-Escribano, A.; Pastor-Blas, M. M.; Gonzalez-Montiel, A. Microporous Mesoporous Mater. 2009, 120 (1−2), 132−140. (7) Liu, P.; Zhao, M. F. Appl. Surf. Sci. 2009, 255 (7), 3989−3993. (8) Shchukin, D. G.; Sukhorukov, G. B.; Price, R. R.; Lvov, Y. M. Small 2005, 1 (5), 510−513. (9) Wang, L.; Chen, J.; Ge, L.; Zhu, Z.; Rudolph, V. Energy Fuels 2011, 25 (8), 3408−3416. (10) Wang, L.; Chen, J.; Rudolph, V.; Zhu, Z. Adv. Powder Technol. 2012, 23 (4), 465−471. (11) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48 (1), 60−103. (12) Fievet, F.; Lagier, J. P.; Blin, B.; Beaudoin, B.; Figlarz, M. Solid State Ionics 1989, 32−33 (Part 1 (0)), 198−205. (13) Fievet, F.; Lagier, J.; Figlarz, M. MRS Bull. 1989, 14, 29−34. (14) Biacchi, A. J.; Schaak, R. E. ACS Nano 2011, 5 (10), 8089− 8099. (15) Joo, S. H.; Park, J. Y.; Renzas, J. R.; Butcher, D. R.; Huang, W.; Somorjai, G. A. Nano Lett. 2010, 10 (7), 2709−2713. (16) Rioux, R. M.; Song, H.; Grass, M.; Habas, S.; Niesz, K.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. Top. Catal. 2006, 39 (3− 4), 167−174. (17) Yan, X.; Liu, H.; Liew, K. Y. J. Mater. Chem. 2001, 11 (12), 3387−3391. (18) Zawadzki, M.; Okal, J. Mater. Res. Bull. 2008, 43 (11), 3111− 3121. (19) Zhang, Y.; Grass, M. E.; Habas, S. E.; Tao, F.; Zhang, T.; Yang, P.; Somorjai, G. A. J. Phys. Chem. C 2007, 111 (33), 12243−12253. (20) Lee, I.; Delbecq, F.; Morales, R.; Albiter, M. A.; Zaera, F. Nat. Mater. 2009, 8 (2), 132−138. (21) Lee, I.; Zaera, F. J. Catal. 2010, 269 (2), 359−366. (22) Washio, I.; Xiong, Y.; Yin, Y.; Xia, Y. Adv. Mater. 2006, 18 (13), 1745−1749. (23) Du, Y. K.; Yang, P.; Mou, Z. G.; Hua, N. P.; Jiang, L. J. Appl. Polym. Sci. 2006, 99 (1), 23−26. (24) Borodko, Y.; Lee, H. S.; Joo, S. H.; Zhang, Y.; Somorjai, G. J. Phys. Chem. C 2010, 114 (2), 1117−1126. (25) Yuan, P.; Southon, P. D.; Liu, Z. W.; Green, M. E. R.; Hook, J. M.; Antill, S. J.; Kepert, C. J. J. Phys. Chem. C 2008, 112 (40), 15742− 15751. (26) Wang, L.; Chen, J.; Patel, A.; Rudolph, V.; Zhu, Z. Appl. Catal. A: Gen. 2012, 447−448, 200−209. (27) Chin, S. Y.; Alexeev, O. S.; Amiridis, M. D. J. Catal. 2006, 243 (2), 329−339. (28) Son, I. H.; Shamsuzzoha, M.; Lane, A. M. J. Catal. 2002, 210 (2), 460−465.

AUTHOR INFORMATION

Corresponding Author

*Fax: 61 7 3365 4199. E-mail: [email protected]. 4150



Article

(29) Chin, S. Y.; Alexeev, O. S.; Amiridis, M. D. Appl. Catal. A: Gen. 2005, 286 (2), 157−166. (30) Echigo, M.; Tabata, T. Catal. Lett. 2004, 98 (1), 37−42. (31) Avgouropoulos, G.; Manzoli, M.; Boccuzzi, F.; Tabakova, T.; Papavasiliou, J.; Ioannides, T.; Idakiev, V. J. Catal. 2008, 256 (2), 237− 247. (32) Kim, Y. H.; Park, E. D. Appl. Catal. B: Environ. 2010, 96 (1−2), 41−50. (33) Kim, Y. H.; Yim, S.-D.; Park, E. D. Catal. Today 2012, 185 (1), 143−150. (34) Kim, Y. H.; Park, E. D.; Lee, H. C.; Lee, D. Appl. Catal. A: Gen. 2009, 366 (2), 363−369. (35) Schulz, J.; Roucoux, A.; Patin, H. Chem.Eur. J. 2000, 6 (4), 618−624. (36) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109 (26), 12663−12676. (37) Tierrablanca, E.; Romero-García, J.; Roman, P.; Cruz-Silva, R. Appl. Catal. A: Gen. 2010, 381 (1−2), 267−273. (38) Luo, P.; Zhao, Y.; Zhang, B.; Liu, J.; Yang, Y.; Liu, J. Water Res. 2010, 44 (5), 1489−1497.

4151


Halloysite Nanotube Supported Ru Nanocatalysts Synthesized by the

Recommend Documents