Architecture controlled PtNi@mSiO₂ and Pt–NiO@mSiO₂ mesoporous core–shell nanocatalysts for enhanced p-chloronitrobenzene hydrogenation selectivity

Abstract

Architecture controlled PtNi@mSiO₂ and Pt–NiO@mSiO₂ mesoporous core–shell nanocatalysts were synthesized for selective p-chloronitrobenzene hydrogenation to p-chloroaniline. Tetradecyl trimethyl ammonium bromide (TTAB) capped PtNi nanoparticles (NPs) were coated by SiO₂ through the hydrolysis of tetraethylorthosilicate. The resultant PtNi@SiO₂ core–shell NPs were calcined to remove TTAB to obtain mesoporous Pt–NiO@SiO₂ core–shell nanocatalysts (Pt–NiO@mSiO₂), which were subsequently reduced by hydrogen to form mesoporous PtNi@SiO₂ core–shell nanocatalysts (PtNi@mSiO₂). The relevant characterizations such as XRD, TEM, H₂-TPR, and BET confirm that the PtNi@mSiO₂ NPs consist of PtNi alloy nanoparticle cores and mesoporous SiO₂ shells while the Pt–NiO@mSiO₂ NPs contain Pt–NiO heteroaggregate nanoparticle cores and mesoporous SiO₂ shells. The catalytic results for selective hydrogenation of p-chloronitrobenzene show that the selectivity of p-chloroaniline formation over the PtNi@mSiO₂ and Pt–NiO@mSiO₂ nanocatalysts is significantly improved relative to that of control Pt@mSiO₂ nanocatalysts. Moreover, the PtNi@mSiO₂ and Pt–NiO@mSiO₂ nanocatalysts demonstrate high stability during multiple cycles of catalytic hydrogenation reactions. The enhanced catalytic performance is ascribed to the metal–metal interaction for the PtNi@mSiO₂ catalysts and metal–oxide interaction for the Pt–NiO@mSiO₂ catalysts.

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1. Introduction

Core–shell nanoparticles (NPs) have received intense interest due to their wide applications in catalysis,^1–5 hydrogen absorption^6,7 and biological sciences.^8,9 Among the various core–shell structures, those with metal nanoparticle cores and mesoporous oxide shells have attracted particular interest in heterogeneous catalysis because the mesoporous oxide shells can easily transport reactants/products and the catalytically active metal cores resist sintering due to encapsulation by oxide shells.¹⁰ For example, mesoporous Pt@SiO₂ core–shell NPs (Pt@mSiO₂) developed by Somorjai and co-workers exhibit thermal stability up to 750 °C in air and catalytic stability for high temperature CO oxidation reaction.¹¹

Currently, various porous metal@oxide core–shell NPs,^12–16 such as Au@mSiO₂^17,18 and Pd@CeO₂^19,20 have been successfully synthesized. Among the oxide shell materials, SiO₂ is widely used as shell materials due to easy availability, high surface area and thermal stability. The Stöber method²¹ and sol–gel process²² are common methods to synthesize metal@mSiO₂ NPs. Our previous work²³ exploited the typical sol–gel method to synthesize size-controlled Pd@mSiO₂ nanocatalysts for catalytic hydrogenation reactions. The Pd@mSiO₂ NPs illustrate high catalytic activity with high catalytic stability for nitrobenzene hydrogenation.

Although Pd@mSiO₂ nanocatalysts demonstrate a good catalytic performance in nitrobenzene hydrogenation,²³ they show poor p-chloroaniline (p-CAN) selectivity in p-chloronitrobenzene (p-CNB) hydrogenation,²⁴ where significant formation of de-chlorination products is observed. To improve selectivity for functionalized nitroaromatics hydrogenation, bimetallic catalysts are developed.^25,26 For example, the poly(N-vinyl-2-pyrrolidone) (PVP) anchored bimetallic Pd–Ru catalysts exhibit p-CAN selectivity of 94% while the major product over PVP–Pd catalysts is de-chlorination product of aniline (AN).²⁷ The selectivity improvement over PVP–Pd–Ru catalysts is ascribed to the synergic effect between Pd and Ru.

Another way to improve the catalytic selectivity for hydrogenation of functionalized nitroaromatics is to increase the metal–support interaction.^28–30 Corma and co-workers pretreated Pt/TiO₂ catalysts by high temperature H₂ for selective hydrogenation, and the selectivity improvement over pretreated Pt/TiO₂ for functionalized nitroaromatics hydrogenation is ascribed to the formation of close contact Pt/TiO_x structure induced by H₂ treatment.³⁰ Dai and co-workers first transformed AuNi NPs into close contact Au–NiO heteroaggregate nanocatalysts for low temperature CO oxidation, which resist sintering upon calcination at high temperatures.³¹ Our group exploited the same strategy to prepare Pt–M_xO_y/Al₂O₃ (M = Ni, Fe and Co) heteroaggregate nanocatalysts³² and Pd–NiO@mSiO₂ nanocatalysts²⁴ for p-CNB hydrogenation. The Pt–M_xO_y/Al₂O₃ nanocatalysts show enhanced catalytic performance for p-CNB hydrogenation relative to the Pt/Al₂O₃ catalysts, but the catalytic activity decreases dramatically from cycle to cycle. The Pd–NiO@mSiO₂ NPs illustrate the improvement of p-CAN selectivity and stability, but the selectivity is still not satisfied.

These above findings encourage us to combine the concept of mesoporous metal@oxide architecture for catalytic stability improvement with the concept of the bimetallic and metal–oxide heteroaggregate architecture for selectivity improvement to design highly selective and stable nanocatalysts for p-CNB hydrogenation. Here, we first synthesized PtNi@mSiO₂ and Pt–NiO@mSiO₂ mesoporous core–shell nanocatalysts using the sol–gel method, and systematically investigated the effects of active component architectures on the catalytic performance for p-CNB hydrogenation. The schematic demonstration of the syntheses of PtNi@mSiO₂ and Pt–NiO@mSiO₂ nanocatalysts is shown in Scheme 1. The PtNi@mSiO₂ and Pt–NiO@mSiO₂ nanocatalysts illustrate significantly improved p-CAN selectivity with the suppression of the de-chlorination product formation and high stability during cycle-to-cycle hydrogenation reactions due to their unique architectures.

Scheme 1 Schematic demonstration of synthetic procedures for PtNi@mSiO₂, Pt–NiO@mSiO₂ and Pt@mSiO₂ mesoporous core–shell nanocatalysts.

2. Experimental section

Chemicals

Chloroplatinic acid (H₂PtCl₆·6H₂O), nickel(II) acetylacetonate (Ni(acac)₂, 95%), sodium borohydride (NaBH₄, 98%), ammonium hydroxide (NH₄OH, AR), and tetraethylorthosilicate (TEOS, AR) were purchased from Aladdin. Tetradecyl trimethyl ammonium bromide (TTAB, AR) and p-chloronitrobenzene (p-CNB, AR) were purchased from Shanghai Chemical Reagent Company. All reagents were used without further purification.

Synthesis

Synthesis of TTAB-capped PtNi alloy NPs. In a typical synthesis, 8.1 mg of Ni(acac)₂ were dissolved in 85 mL of deionized water under a magnetic stirring in a 250 mL three-necked round-bottomed flask at room temperature. The mixture was then treated by ultrasonic for 5 minutes to totally dissolve Ni(acac)₂. The resultant solution was charged with 3.0 mL of 10.0 mM H₂PtCl₆ aqueous solution and 0.840 g of TTAB. The obtained mixture was stirred at room temperature for about 15 minutes, and 5.0 mL of 528 mM ice-cooled NaBH₄ solution was then injected into the mixture through a syringe. The syringe was kept in the system to release the generated H₂, and then the syringe was pulled out to make the flask a closed system. The above solution was further stirred for 2.5 hours at a low speed at room temperature to obtain TTAB-capped PtNi alloy NPs. Finally, the TTAB-capped PtNi NPs were collected by centrifugation with acetone washing and subsequent drying in air for future characterization.

Synthesis of PtNi@SiO₂ core–shell NPs. PtNi@SiO₂ core–shell NPs with ∼17 nm thickness of silica shells were synthesized by a modified sol–gel method.²⁴ The above as-synthesized PtNi alloy colloid containing 0.0077 mmol Pt and 0.0077 mmol Ni (24.0 mL colloid) was mixed with 120 mL deionized water in a 250 mL three-necked round-bottomed flask with a magnetic stirring at low speeds. The pH value of the solution was adjusted to 10.5, 10.6 or 10.7 by concentrated ammonia aqueous solution (25–28 wt%). After pH adjustment to a certain value, 600 µL of TEOS (2.69 mmol, a TEOS/Pt ratio of 350/1) was injected into the system to coat PtNi alloy NPs with silica layer through the hydrolysis of TEOS. The resultant mixture was further stirred for 1.5 hours at low speeds. Finally, the PtNi@SiO₂ NPs were collected by centrifuging with acetone washing and following drying in air for future use.

Synthesis of Pt–NiO@mSiO₂ and PtNi@mSiO₂ mesoporous core–shell nanocatalysts. Pt–NiO@mSiO₂ mesoporous core–shell nanocatalysts were obtained through calcinations of the above as-synthesized PtNi@SiO₂ NPs in a muffle furnace at 500 °C for 3.0 hours with a ramping rate of 4 °C min⁻¹. PtNi@mSiO₂ mesoporous core–shell nanocatalysts were obtained by H₂ reduction of Pt–NiO@mSiO₂ nanocatalysts with a H₂ flow rate of 60 mL min⁻¹ for 2.0 hours at 500 °C with a ramping rate of 5 °C min⁻¹ in a fixed bed reactor.

Synthesis of control Pt@mSiO₂ mesoporous core–shell nanocatalysts. The similar synthetic procedure of Pt–NiO@mSiO₂ nanocatalysts was employed to prepare Pt@mSiO₂ nanocatalysts with ∼17 nm silica shells. H₂PtCl₄ aqueous solution (5.2 mL, 10.0 mM), 0.840 g of TTAB and 85 mL of deionized water were mixed under a magnetic stirring in a 250 mL three-necked round-bottomed flask at room temperature. After stirring for 15 minutes, ice-cooled NaBH₄ (9.0 mL, 250 mM) aqueous solution was injected into the flask. The syringe was kept in the system to release the generated H₂, and then the syringe was pulled out to make the flask a closed system. The above solution was further stirred for 8.0 hours at a low speed at room temperature to obtain TTAB-capped Pt NPs. The as-synthesized TTAB-capped Pt colloids (24.0 mL) containing 0.0134 mmol Pt were mixed with 120 mL of deionized water in a 250 mL three-necked round-bottomed flask under a magnetic stirring. A certain amounts of concentrated ammonia solution (25–28 wt%) was used to adjust pH value of the solution to 10.9. TEOS (600 µL, 2.69 mmol, a TEOS/Pt ratio of 200/1) was then injected into the solution, and the solution was further stirred for 1.5 hours. The particles were collected by centrifugation, and were further calcined to obtain Pt@mSiO₂ nanocatalysts. The collection and calcination procedures are same as those of Pt–NiO@mSiO₂ nanocatalysts.

Characterization

X-ray diffraction (XRD) patterns of various samples were recorded at a Bruker D8 Advance X-ray diffractometer with a Cu Kα radiation in the 2θ range from 10° to 80°. The transmission electron microscopy (TEM) images of the samples were obtained using a JEOL 2100 transmission electron microscope. The thermal property of the samples was measured by using a Pyris Diamond thermogravimetric analyzer (TGA) in the temperature range from 30 °C to 800 °C at a heating rate of 5 °C min⁻¹ with a gas flow rate of 50 mL min⁻¹ in an air atmosphere. The fourier transform infrared (FT-IR) spectra of the samples were recorded at a Bruker Tensor 27 spectrophotometer. H₂ temperature-programmed reduction (H₂-TPR) was performed on a FINESORB3010 instrument. The Pt–NiO samples obtained by calcination of PtNi NPs at 500 °C was pretreated at 200 °C in Ar for 1.0 hour prior to H₂-TPR measurement, and further reduced by 1/10 H₂/Ar flow (v/v, 50 mL min⁻¹) from room temperature to 800 °C at a ramping rate of 5 °C min⁻¹. Hydrogen consumption was detected by a thermal conductivity detector (TCD). The actual metal contents of nanocatalysts were identified using a PE Optima 2100DV inductive coupled plasma optical emission spectrometer (ICP-OES). The Brunauer–Emmett–Teller (BET) surface area, pore size distribution and the adsorption/desorption isotherms of various samples were measured by N₂ adsorption at 77 K, using a Micromeritics ASAP-2020 M automatic specific surface area and porous physical adsorption analyzer.

Catalytic hydrogenation of p-chloronitrobenzene with H₂

Hydrogenation of p-CNB with H₂ over different Pt-containing mesoporous core–shell nanocatalysts was carried out at ambient H₂ pressure. Pt–NiO@mSiO₂ nanocatalysts (0.186 g, 0.72 wt% of Pt), 0.6 g of p-CNB and 24 mL of absolute ethanol were mixed in a 50 mL three-necked round-bottomed flask at room temperature under a magnetic stirring. The flask was first purged with high purity H₂ three times to get rid of air, and then the hydrogenation reactions were carried out with a vigorous magnetic stirring under H₂ ambient pressure and a defined temperature for a desired time. The liquid products were collected by centrifuging to remove the catalysts and analyzed online by a gas chromatography equipped with a flame ionization detector. For p-CNB hydrogenation over PtNi@mSiO₂ and Pt@mSiO₂ catalysts, 0.179 g of PtNi@mSiO₂ (0.75 wt% of Pt) and 0.098 g of Pt@mSiO₂ nanocatalysts (1.37 wt% of Pt) were used to ensure the same molar number of Pt used in hydrogenation over all three catalysts. The reaction conditions were same as those in hydrogenation over Pt–NiO@mSiO₂ catalysts.

Cycle-to-cycle experiments for PtNi@mSiO₂ and Pt–NiO@mSiO₂ nanocatalysts were performed to investigate the catalytic stability. In p-CNB hydrogenation over Pt–NiO@mSiO₂ nanocatalysts, 0.186 g of nanocatalysts, 0.6 g of p-CNB and 24 mL of absolute ethanol were mixed in a 50 mL three-necked round-bottomed flask at room temperature under a vigorous magnetic stirring. The hydrogenation reactions were carried out for 2.0 hours under ambient H₂ pressure at 45 °C. After catalytic reactions, the nanocatalysts were collected by centrifugation with absolute ethanol washing twice and drying at 75 °C in an oven for 1.0 hours. The dried nanocatalysts were then used in the next cycle experiment. Due to the catalyst loss in the recovery process, the weight of p-CNB and ethanol in the next cycle decreased accordingly to keep the ratios of p-CNB/catalyst and p-CNB/solvent same as those in the first cycle. In cycle-to-cycle hydrogenation experiments over PtNi@mSiO₂ nanocatalysts, 0.179 g of catalysts were used in the first cycle, and other reaction conditions were same to those of Pt–NiO@mSiO₂ catalysts. Five cycles of hydrogenation were carried out for both catalysts due to the catalyst loss.

3. Results and discussion

In the syntheses of Pt–NiO@mSiO₂ and PtNi@mSiO₂ mesoporous core–shell NPs, TTAB-capped PtNi alloy NPs were first synthesized by reduction of Pt and Ni precursors with NaBH₄ in the presence of TTAB. The PtNi@SiO₂ NPs were then prepared by coating TTAB-capped PtNi NPs with SiO₂ through the hydrolysis of TEOS using TTAB as structure directing agents. The as-synthesized PtNi@SiO₂ NPs were calcined at 500 °C to remove TTAB to obtain mesoporous Pt–NiO@mSiO₂ nanocatalysts, and subsequently reduced by H₂ at 500 °C to obtain mesoporous PtNi@mSiO₂ nanocatalysts. The real Pt and Ni contents in final products were identified by ICP-OES.

TEM images of various samples are shown in Fig. 1. PtNi NPs in Fig. 1a basically display a spherical shape while individual Pt NPs in Fig. 1b exhibit an irregular shape. The lattice spacing of PtNi NPs in the insert of Fig. 1a is 0.267 nm, which is consistent with the lattice spacing of tetragonal PtNi alloy (110) planes. Fig. 1c–e show TEM images of PtNi@SiO₂ NPs. The core–shell structure with PtNi nanoparticle cores and silica shells is clearly observed in these figures. The thicknesses of silica shells of PtNi@SiO₂ NPs synthesized at the pH values of 10.5 in Fig. 1c, 10.6 in Fig. 1d and 10.7 in Fig. 1e are ∼16 nm, ∼17 nm and ∼18 nm, respectively. In addition, some empty silica particles are observed at a lower pH value (Fig. 1c) while some particles with multiple cores are present at a higher pH value (Fig. 1e). This phenomenon is similar to the observation in our previous work,²⁴ confirming that fast hydrolysis at higher pH values results in the formation of core–shell NPs with multiple cores and slow hydrolysis at lower pH values results in the formation of empty silica particles.

Fig. 1 TEM images of different NPs showing (a), TTAB-capped PtNi NPs, and the insert is the HRTEM image of one particle showing a lattice spacing of 0.267 nm; (b), TTAB-capped Pt NPs; (c), PtNi@SiO₂ NPs synthesized at a pH value of 10.5 and a TEOS/Pt ratio of 350/1; (d) PtNi@SiO₂ NPs synthesized at a pH value of 10.6 and a TEOS/Pt ratio of 350/1; (e), PtNi@SiO₂ NPs synthesized at a pH value of 10.7 and a TEOS/Pt ratio of 350/1; (f), Pt–NiO@mSiO₂ nanocatalysts obtained after 500 °C calcination of PdNi@SiO₂ and the insert is the enlarged image of one particle; (g), PtNi@mSiO₂ nanocatalysts obtained after 500 °C calcination and following 500 °C H₂ reduction of PtNi@SiO₂. (h), Pt@SiO₂ NPs synthesized at pH value of 10.9 and a TEOS/Pt ratio of 200/1; (i), Pt@mSiO₂ nanocatalysts obtained after 500 °C calcination of Pt@SiO₂. Scale bars of (a) and (b) are 20 nm; scale bar in the insert of (a) is 2 nm; scale bars of (c)–(i) are 50 nm; scale bar in the insert of (f) is 20 nm.

Due to their perfect core–shell structure and acceptable silica shell thickness for catalysis, the PtNi@SiO₂ NPs synthesized at a pH value of 10.6 and a TEOS/Pt ratio of 350/1 were chosen for subsequent treatment under different conditions. Fig. 1f shows the TEM image of Pt–NiO@mSiO₂ nanocatalysts, which is obtained by calcination of PtNi@SiO₂ NPs at 500 °C. The mesoporous silica shells are clearly observed in the enlarged TEM image in the insert. Fig. 1g shows the TEM image of PtNi@mSiO₂ nanocatalysts obtained by reduction of Pt–NiO@mSiO₂ by H₂ at 500 °C. As shown in Fig. 1g, the mesoporous core–shell structures are intact, confirming the structure maintenance after subsequent H₂ reduction at 500 °C. The TEM images of Pt@SiO₂ NPs before and after calcination at 500 °C are illustrated in Fig. 1h and i, respectively. The Pt@mSiO₂ nanocatalysts were synthesized by calcination of as-synthesized Pt@SiO₂ NPs at 500 °C. To ensure that the silica shell thickness of Pt@mSiO₂ NPs is close to those of Pt–NiO@mSiO₂ and PtNi@mSiO₂ NPs, the synthetic parameters of a pH value of 10.9 and a TEOS/Pt ratio of 200/1 were used to obtain Pt@SiO₂ NPs with ∼17 nm silica shell.

The size distributions of various NPs are shown in Fig. 2. The average particle sizes of PtNi NPs in Fig. 2a, the cores of PtNi@SiO₂ NPs in Fig. 2b, the cores of Pt–NiO@mSiO₂ nanocatalysts in Fig. 2c and the cores of PtNi@mSiO₂ nanocatalysts in Fig. 2d are 4.4, 4.3, 4.4 and 4.2 nm, respectively. The close particle sizes are also found for Pt NPs (∼5.0 nm size in Fig. 2e) and the Pt cores of Pt@mSiO₂ nanocatalysts (∼5.1 nm Pt particle size in Fig. 2f), confirming that the cores of metal@mSiO₂ structures can resist sintering due to the encapsulation of metal particles by oxide shells.

Fig. 2 Size distributions of PtNi, Pt, and Pt-containing cores of different core–shell NPs showing (a), PtNi NPs; (b), PtNi@SiO₂ NPs; (c), Pt–NiO@mSiO₂ nanocatalysts; (d), PtNi@mSiO₂ nanocatalysts; (e), Pt NPs; (f), Pt@mSiO₂ nanocatalysts.

XRD patterns of various samples are illustrated in Fig. 3. As shown in Fig. 3a, the broad diffraction at 2θ degree of 41.8° is in good agreement with the (111) diffraction of tetragonal PtNi phase, suggesting the formation of PtNi alloy. Due to their small size, the PtNi alloy NPs (4.4 nm) illustrate the weak (111) diffractions without any diffractions at high 2θ degrees. After coating by silica, the PtNi@SiO₂ NPs in Fig. 3b show broad amorphous silica diffraction at 2θ degree around 22°, and the (111) diffraction of PtNi alloy is very weak due to the low crystallinity of PtNi cores in the PtNi@SiO₂ NPs. After calcination of PtNi@SiO₂ NPs at 500 °C, the Pt diffractions of Pt–NiO@mSiO₂ nanocatalysts in Fig. 3c is clearly observed, and the diffractions of NiO of Pt–NiO@mSiO₂ is not observed possibly due to the formation of amorphous NiO phase. The existence of Ni in the Pt–NiO@mSiO₂ nanocatalysts is evidenced by ICP-OES analysis, in which the ratio of Pt/Ni is close the 1/1 Pt/Ni ratio of the starting materials. Fig. 3d illustrates the XRD pattern of PtNi@mSiO₂ nanocatalysts obtained by reduction of Pt–NiO@mSiO₂ nanocatalysts with H₂ at 500 °C. The PtNi@mSiO₂ nanocatalysts exhibit weak PtNi diffractions after reduction with H₂. However, the diffractions are slightly shifted to low 2θ degrees, possibly due to the formation of PtNi alloy with composition ingredients inside the particles. The XRD patterns of Pt@mSiO₂ nanocatalysts are shown in Fig. 3e. The apparent Pt diffractions in Fig. 3e confirm that the major phase of Pt cores still maintain a metallic state upon 500 °C calcination in air, which is consistent with the literature.³³

Fig. 3 XRD patterns of different samples showing (a), PtNi alloy NPs; (b), PtNi@SiO₂ NPs; (c), Pt–NiO@mSiO₂ nanocatalysts; (d), PtNi@mSiO₂ nanocatalysts; (e), Pt@mSiO₂ nanocatalysts. Vertical lines indicating PtNi (JCPDS 03-065-2797), Ni (JCPDS 03-065-2865), and Pt (JCPDS 01-071-6560).

TG/DTA analysis of PtNi@SiO₂ NPs and FT-IR spectra of PtNi@SiO₂ NPs before and after calcination at 500 °C are shown in Fig. 4. From the left panel of Fig. 4, it is clear observed that TTAB starts to remove at ∼130 °C and is completely removed at ∼450 °C. Therefore, the calcination at 500 °C is applied to remove TTAB in PtNi@SiO₂ NPs to obtain Pt–NiO@mSiO₂ nanocatalysts. In the right panel of Fig. 4, the absorption peaks of antisymmetric and symmetric stretching modes of methylene of TTAB³⁴ in PtNi@SiO₂ NPs before calcination are observed at 2922 cm⁻¹ and 2856 cm⁻¹, respectively. In contrast, these absorption peaks in the PtNi@SiO₂ NPs after calcination at 500 °C disappear, confirming the absence of TTAB in the Pt–NiO@mSiO₂ nanocatalysts.

Fig. 4 TG/DTA curves of as-synthesized PtNi@SiO₂ NPs (left panel) and FT-IR spectra of PtNi@SiO₂ NPs before and after 500 °C calcination (right panel).

H₂-TPR curve of Pt–NiO NPs obtained by calcination of PtNi NPs at 500 °C is shown in Fig. 5. Since XRD patterns of Pt–NiO@mSiO₂ in Fig. 3c and Pt@mSiO₂ in Fig. 3e have confirmed the metallic state of Pt species and it is well known that Pt is in metallic state upon calcination at high temperatures, the hydrogen absorption in Fig. 5 is mainly due to the reduction of NiO. However, the hydrogen absorption around room temperature in Fig. 5 could be from the reduction of surface Pt oxides,³⁵ which is formed due to the high activity of surface Pt atoms. Moreover, the presence of Pt in Pt–NiO will facilitate the reduction of NiO by hydrogen spillover effect, resulting in the reduction of NiO at lower temperatures.³⁶ From the Fig. 5, it is concluded that the complete reduction of NiO with H₂ is around 500 °C, and thus the reduction of Pt–NiO@mSiO₂ nanocatalysts at 500 °C is applied to obtain PtNi@mSiO₂ nanocatalysts.

Fig. 5 H₂-TPR of Pt–NiO NPs obtained by calcination of PtNi NPs at 500 °C.

The textural properties of Pt–NiO@mSiO₂, PtNi@mSiO₂ and Pt@mSiO₂ nanocatalysts were characterized by N₂ adsorption/desorption experiments. Fig. 6 presents the N₂ adsorption/desorption isotherms and the pore size distributions of different Pt-containing nanocatalysts. The typical type IV isotherms³⁷ are observed for all samples, confirming the presence of mesopores resulted from TTAB removal at 500 °C calcination. Moreover, the insert in Fig. 6 suggests that the pore size of mesopores in these samples is around 2 nm with some large pores possibly formed by connection of particles. Table 1 shows BET surface areas, pore volumes and pore sizes of Pt-containing samples. These materials exhibit large BET surface areas, further confirming the thermal stability upon high temperature treatment.

Fig. 6 N₂ adsorption/desorption isotherms and pore size distribution (inset) showing (a), Pt@mSiO₂ nanocatalysts; (b), Pt–NiO@mSiO₂ nanocatalysts; (c), PtNi@mSiO₂ nanocatalysts.

Table 1 BET surface areas, pore volumes, and pore sizes of Pt@mSiO₂, Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts

Samples	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
Pt@mSiO2	656	1.1	2.0
Pt–NiO@mSiO2	786	1.2	2.0
PtNi@mSiO2	753	1.0	2.0

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The actual metal loadings by ICP-OES measurement for Pt–NiO@mSiO₂, PtNi@mSiO₂, and Pt@mSiO₂ nanocatalysts are presented in Table 2. The real Pt loadings of Pt@mSiO₂, Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts are 1.37 wt%, 0.72 wt%, and 0.75 wt%, respectively, while the real Ni loadings in Pt–NiO@mSiO₂ and PtNi@mSiO₂ are 0.17 wt% and 0.19 wt%, respectively. According to the ICP-OES measurements, the molar ratios of Pt/Ni in the Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts are 1/1.27 and 1/1.19, respectively, which is close to the starting 1/1 ratio.

Table 2 The real loadings of Pt and Ni in Pt-containing core–shell mesoporous nanocatalysts by ICP-OES

Sample	Pt loading (wt%)	Ni loading (wt%)
Pt@mSiO2	1.37	NA
Pt–NiO@mSiO2	0.72	0.17
PtNi@mSiO2	0.75	0.19

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p-CNB hydrogenation with H₂ was carried out under ambient H₂ pressure and reaction temperature of 25–55 °C. Different weights of core–shell nanocatalysts with different Pt loadings were applied to ensure the same molar number of Pt used in hydrogenation. The major products are p-CAN with some de-chlorination products including AN and p-aminophenol (p-AP).

Table 3 summarizes the comparison of catalytic performance over Pt@mSiO₂, Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts with a SiO₂ shell thickness of ∼17 nm. The Pt@mSiO₂ nanocatalysts exhibit a 97.8% of p-CNB conversion with a 77.6% of p-CAN selectivity while the selectivity of de-chlorination product of AN is 20.2%. The 77.6% of p-CAN selectivity over Pt@mSiO₂ is similar to those of reported Pt colloid and carbon nanotube supported Pt catalysts at ambient H₂ pressure.^25,38 In contrast, the Pt–NiO@mSiO₂ nanocatalysts illustrate an improved p-CAN selectivity of 95.3% with a slightly decreased p-CNB conversion. The PtNi@mSiO₂ nanocatalysts maintain the high p-CNB conversion with a significantly improved p-CAN selectivity of 97.1%. This study clearly demonstrates that the PtNi bimetallic and Pt–NiO heteroaggregate structures could significantly enhance the p-CAN selectivity, and basically maintain the high catalytic activity of Pt catalysts for p-CNB hydrogenation.

Table 3 Product distributions in p-CNB hydrogenation over Pt-containing core–shell mesoporous nanocatalysts with a SiO₂ shell thickness of ∼17 nm^a

Catalyst	Conv. (%)	Selectivity (%)
		p-CAN	AN	p-AP	Others
a Reaction conditions: reaction temperature-45 °C; ambient H2 pressure; p-CNB-0.6 g; reaction time-2.0 hours; ethanol-24 mL; Pt@mSiO2-0.098 g (Pt loading-1.37 wt%); Pt–NiO@mSiO2-0.186 g (Pt loading-0.72 wt%); PtNi@mSiO2-0.179 g (Pt loading-0.75 wt%); speed of agitation-500 rpm.
Pt@mSiO2	97.8	77.6	20.2	0	2.2
Pt–NiO@mSiO2	87.9	95.3	4.7	0	0.0
PtNi@mSiO2	96.1	97.1	1.3	0.6	1.0

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Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts were chosen for cycle-to-cycle hydrogenation experiments. In the experiments, no additional fresh catalysts were introduced into the system, and due to the loss of catalysts in the recovery process the weights of p-CNB and ethanol decreased to keep the same ratios of p-CNB/catalyst and p-CNB/solvent as those in the first cycle. Table 4 summarizes the catalytic results in five cycles of reactions. The Pt–NiO@mSiO₂ nanocatalysts illustrate a good catalytic stability with an average 88.8% of p-CNB conversion and 96.7% of p-CAN selectivity. The PtNi@mSiO₂ nanocatalysts show an even better result with an average 97.0% of p-CNB conversion and 97.6% of p-CAN selectivity.

Table 4 Cycle-to-cycle reactions of p-CNB hydrogenation over Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts^a

Cycle index	Pt–NiO@mSiO2 catalyst				PtNi@mSiO2 catalyst
	Weight (g)		Conv. (%)	p-CAN selectivity (%)	Weight (g)		Conv. (%)	p-CAN selectivity (%)
	Catalysts	p-CNB			Catalysts	p-CNB
a Reaction conditions: reaction temperature-45 °C; atmospheric H2 pressure; reaction time-2.0 hours; speed of agitation-500 rpm; 24 mL of ethanol was used as solvent in the first cycle, and the ratio of ethanol/p-CNB in the following cycles was kept same as that of the first cycle.
1	0.186	0.600	87.9	95.3	0.179	0.600	96.1	97.1
2	0.153	0.495	85.1	98.2	0.141	0.473	91.9	96.3
3	0.124	0.400	92.6	96.9	0.119	0.399	99.7	97.6
4	0.106	0.342	88.2	95.4	0.097	0.326	100	98.3
5	0.086	0.277	90.1	97.5	0.070	0.236	97.4	98.5

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The Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts not only exhibit high activity and high selectivity, but also illustrate good catalytic stability during multiple cycles of catalytic reactions. The selectivity enhancement over PtNi@mSiO₂ is ascribed to the synergistic effect between Pt and Ni, which has been documented in other systems.³⁹ The selectivity enhancement over Pt–NiO@mSiO₂ can be assigned to the strong interaction between Pt and NiO, and the literature has reported that the metal–oxide strong interaction can transform non-selective catalysts into selective catalysts in high temperature H₂ pretreated Pt/TiO₂ catalysts for hydrogenation of functionalized nitroaromatics.³⁰ In our previous work of Pt–M_xO_y/Al₂O₃ (M = Ni, Co and Fe) for p-CNB hydrogenation,³² the Pt–M_xO_y/Al₂O₃ catalysts illustrate high activity and high p-CAN selectivity at elevated H₂ pressures, but shows a poor catalytic stability, in which the p-CNB conversion is 35.8% after 4 cycles of reaction. In this study, the stability enhancement over PtNi@mSiO₂ and Pt–NiO@mSiO₂ nanocatalysts for p-CNB hydrogenation is due to the mesoporous core–shell structures, which can easily transport reactants/products and prevent active components from sintering and dissolution.

The effects of reaction time on p-CNB conversion and p-CAN selectivity over Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts at 45 °C are shown in Fig. 7. In the left panel of Fig. 7, it is observed that p-CNB conversions over Pt–NiO@mSiO₂ nanocatalysts increase from 41% at 0.5 hours to 100% at 3.0 hours and p-CAN selectivity increases from 87.6% at 0.5 hours to 100% at 3.0 hours. Similarly, the p-CNB conversion and p-CAN selectivity over PtNi@mSiO₂ nanocatalysts follows the similar trends. The p-CAN selectivity increases with the p-CNB conversion increasing, and this phenomenon is consistent with the literature.^40–42 The effects of reaction temperatures on catalytic performance over Pt–NiO@mSiO₂ nanocatalysts are illustrated in Fig. 8. It is obviously seen that p-CNB conversion increase with reaction temperatures increasing and p-CAN selectivity increase with p-CNB conversions increasing. The p-CNB conversions increase from 60.2% at 25 °C to 93.7% at 55 °C, and p-CAN selectivity accordingly increases from 86.3% at 25 °C to 98.3% at 55 °C.

Fig. 7 Effect of reaction time on catalytic performance over Pt–NiO@mSiO₂ (left panel) and PtNi@mSiO₂ nanocatalysts (right panel). Reaction conditions: reaction temperature-45 °C; ambient H₂ pressure; Pt–NiO@mSiO₂-0.186 g; PtNi@mSiO₂-0.179 g; p-CNB-0.6 g; 24 mL of ethanol; speed of agitation-500 rpm.

Fig. 8 Effect of temperatures on catalytic performance over Pt–NiO@mSiO₂ nanocatalysts. Reaction conditions: ambient H₂ pressure; Pt–NiO@mSiO₂-0.186 g; p-CNB-0.6 g; 24 mL of ethanol; reaction time-2.0 hours; speed of agitation-500 rpm.

4. Conclusions

In summary, Pt–NiO@mSiO₂ and PtNi@mSiO₂ mesoporous core–shell nanocatalysts were successfully synthesized for selective p-CNB hydrogenation. The relevant characterizations confirm that the Pt–NiO@mSiO₂ nanocatalysts consist of Pt–NiO heteroaggregate nanoparticle cores and mesoporous silica shells, and the PtNi@mSiO₂ nanocatalysts contain PtNi alloy nanoparticle cores and mesoporous silica shells. Both Pt–NiO@mSiO₂ and PtNi@mSiO₂ show superior catalytic performance relative to Pt@mSiO₂. The p-CAN selectivity over Pt–NiO@mSiO₂ and PtNi@mSiO₂ is significantly enhanced with maintaining the high activity of Pt catalysts. Moreover, the Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts show high catalytic stability during cycle-to-cycle experiments. The catalytic performance enhancement over Pt–NiO@mSiO₂ and PtNi@mSiO₂ nanocatalysts is ascribed to their unique architectures, in which the bimetallic PtNi and Pt–NiO heteroaggregate structures enhance the p-CAN selectivity and mesoporous core–shell structure enhance the catalytic stability. Due to mesoporous core–shell structures, the catalytically active cores (PtNi alloy or Pt–NiO heteroaggregate NPs) are protected by mesoporous silica shells and the core–shell structures prevent the cores from sintering and dissolution, thus improving their catalytic stability and efficiency. The synthetic method could be extended to synthesize other mesoporous core–shell structures. Importantly, this study provides a catalyst design using the mesoporous core–shell structure to improve the catalytic stability and bimetallic or metal–oxide heteroaggregate structures to improve catalytic hydrogenation selectivity, and could be extended to other catalytic systems.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

S. Zhou thanks the Ministry of Science and Technology of China (Grant no. 2012DFA40550) for financial supports. H. Yu thanks the financial support from Ningbo Municipal Natural Science Foundation (Grant No. 2013A610038). H. Liu thanks Hainan University for financial supports of development plan of outstanding graduate thesis.

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