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. 2020 Sep 8;6(9):1617–1627. doi: 10.1021/acscentsci.0c00822

Sinter-Resistant Nanoparticle Catalysts Achieved by 2D Boron Nitride-Based Strong Metal–Support Interactions: A New Twist on an Old Story

Hao Chen †,‡, Shi-Ze Yang , Zhenzhen Yang ‡,§,*, Wenwen Lin †, Haidi Xu §, Qiang Wan ∥,⊥, Xian Suo †,‡, Tao Wang ‡, De-en Jiang ∥, Jie Fu †,*, Sheng Dai ‡,§,*
PMCID: PMC7517410  PMID: 32999937

Abstract

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Strong metal–support interaction (SMSI) is recognized as a pivotal strategy in hetereogeneous catalysis to prevent the sintering of metal nanoparticles (NPs), but issues including restriction of supports to reducible metal oxides, nonporous architecture, sintering by thermal treatment at >800 °C, and unstable nature limit their practical application. Herein, the construction of non-oxide-derived SMSI nanocatalysts based on highly crystalline and nanoporous hexagonal boron nitride (h-BN) 2D materials was demonstrated via in situ encapsulation and reduction using NaBH4, NaNH2, and noble metal salts as precursors. The as-prepared nanocatalysts exhibited robust thermal stability and sintering resistance to withstand thermal treatment at up to 950 °C, rendering them with high catalytic efficiency and durability in CO oxidation even in the presence of H2O and hydrocarbon simulated to realistic exhaust systems. More importantly, our generic strategy offers a novel and efficient avenue to design ultrastable hetereogeneous catalysts with diverse metal and support compositions and architectures.

Short abstract

Construction of strong metal−support interaction (SMSI) based on nanoporous and highly crystalline hexagonal boron nitride (h-BN) is demonstrated via an in situ encapsulation and reduction pathway.

Introduction

Supported metal nanoparticles (NPs) dispersed on support materials are ubiquitous throughout heterogeneous catalysis and are widely employed in the chemical industry for technological applications.1−6 However, owing to their inherent properties characterized by low Tammann temperature and high surface energy, catalytically active metal NPs are thermodynamically unstable and prone to minimize the surface energies by accumulating to large particle size upon exposure to high temperatures during long-term operations.7−9 The deactivation resulting from metal sintering forms the showstopper in design of thermally robust and efficient nanocatalysts in practical applications, such as oxidation of hydrocarbons and CO, selective hydrogenation, and the water–gas shift reaction.10,11 Up to now, limited approaches capable of producing supported metal NPs with high stability under harsh reaction conditions have been established: (1) premodification of supports before deposition of metal NPs;12−15 (2) confinement of metal NPs within channels of nanoporous materials such as zeolite and silica16,17 (however, these two approaches cannot suppress the Brownian-like motion and Ostwald ripening process of metal NPs, leading to growth and coalescence);13,17 (3) encapsulation of the metal NPs by reducible metal oxide overlayers, being recognized as classic strong metal–support interaction (SMSI) and first demonstrated by Tauster et al. in 1970s.18−20 The third strategy is recognized as the key strategy to prevent the sintering of metal NPs via the formation of oxide barriers on the surfaces.11,18,19,21,22 Generally, the SMSI encapsulation state is constructed through dispersion of metal species or NPs on reducible metal oxide carriers and subsequent high-temperature treatment with hydrogen, which causes a reduction of the oxide support to sub-stoichiometric oxygen concentrations and induces thermodynamically favorable migration of the oxide overlayers to the surface of metal NPs.21,23

In this respect, despite intensive study on the subject of SMSI, four issues must still be addressed. They are as follows: (1) Limited preparation methodologies restrict the scope of supports applicable in this approach to mainly reducible metal oxides including TiO2,24 V2O3,25 Nb2O5,26 Ta2O5,26 and CeO2.27 Although types of support in SMSI have been further extended to phosphates,28 hydroxyapatite,11 ZnO,29 zeolite,9 and SiO2 in composite supports,23 metal-free carriers are still highly desired and are a big challenge. (2) The architectures of the present support in SMSI are nonporous, presenting adverse effects to mass transfer in heterogeneous catalytic processes.2 Nanoporous counterparts would be preferred that would rely on the introduction of novel carrier types. The work from Xiao’s group achieving encapsulation of metal NPs within the nanoporous zeolite architecture shed light on the progress in this aspect.9 (3) So far, only a few SMSI-based heterogeneous nanocatalysts (e.g., Au NPs) have shown good sintering resistance to calcination at temperatures of 500–600 °C,13,30,31 and the fabrication of metal nanocatalysts capable of withstanding thermal treatment temperature above 600 °C have rarely been reported.11,32,33 A sacrificial coating strategy was demonstrated by our group for the preparation of ultrastable Au nanocatalysts, taking advantage of the unique coating chemistry of dopamine or surfactant.10,34 However, the efficiency of this strategy can only be maintained with thermal treatment at 500–600 °C, and slight sintering appeared at 700 °C and worsened at 800 °C. Zhang et al. demonstrated the fabrication of an ultrastable nanocatalyst via localizing the Au NPs in the interfacial regions of hydroxyapatite and TiO2, exhibiting sintering resistance and high performance after calcination at 800 °C.11 The production of sintering resistant nanocatalysts at even higher thermal treatment temperatures (>800 °C) would dramatically propel the development of stable and sustainable catalysts for a variety of catalytic transformations under harsh conditions. For example, in CO oxidation, catalysts used in modern vehicles have to withstand temperatures above 800 °C, and it is particularly challenging to stabilize nanoparticles at such a high temperature. (4) The “reversible” nature of the current reducible metal oxide supports, which were retreated from the metal NPs upon reoxidation by H2O or O2,35 limits their practical applications in the exhaust gas treatment (e.g., oxidation of CO and methane) in the presence of O2, H2O (typically 5–15 vol %), and hydrocarbons (0.25 vol %), causing severe deactivation.36,37 To offer a practically applicable solution, supports should be able to form a “permanent” SMSI effect with the metal NPs and withstand harsh oxidative conditions. Satisfying solutions to the issues mentioned above rely on the development of a novel fabrication approach and the introduction of appropriate light element-derived materials as supports, which would propel further advancement in the field of SMSI-based heterogeneous catalysis.

To this end, herein, a simple yet highly efficient strategy is demonstrated for the construction of nanoporous and highly crystalline two-dimensional (2D) hexagonal boron nitride (h-BN)-derived SMSI heterogeneous catalysts via an in situ encapsulation and reduction pathway (Figure 1). The SMSI-based catalysts with h-BN as support (i.e., Pd/h-BN-SMSI) were fabricated through a simple two-step thermal treatment approach in one pot under a nitrogen atmosphere using NaBH4, NaNH2, and PdCl2 as the starting materials. The as-prepared materials demonstrated these unique properties and outstanding catalytic performances: (1) The involvement of a metal-free 2D material (h-BN) as support in SMSI-based nanocatalyst was realized for the first time. (2) The nanoporous architecture of h-BN was well retained, rendering the accessibility of small gas molecules to the metal center. (3) The uniformly dispersed metal NPs in the as-prepared nanocatalysts were covered or surrounded by h-BN nanosheets and capable to withstand high thermal treatment procedure at up to 950 °C. (4) The desirable structural patterns and properties of the as-prepared nanocatalysts provide high catalytic efficiency, high thermal stability, and sintering resistance in CO oxidation for a long reaction period—even in the presence of O2, H2O, and hydrocarbon simulated to realistically achievable conditions in the exhaust system. (5) The high efficiency enables this in situ encapsulation approach to be extended to fabricate Pd, Pt, and Au-involved SMSI nanocatalysts from commercial precursors with composite supports composed of h-BN with γ-Al2O3, zeolite, or SiO2. Hence, our generic strategy offers a novel and efficient avenue to design ultrastable hetereogeneous catalysts with diverse metal and support compositions and architectures. This finding leads to the genesis of ultrastable metal nanoparticle catalysts with a metal-free carrier exhibiting nanoporous architecture, sinter resistance at up to 950 °C, and the capability to withstand harsh oxidative conditions simulated to practical applications, unlocking new opportunities to design thermally stable and robust heterogeneous catalysts.

Figure 1.

Figure 1

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Schematic diagram for the fabrication of h-BN-based SMSI with NaBH4, NaNH2, and PdCl2 as the starting materials.

Results

Synthetic Approach and Characterization of h-BN-Derived SMSI Nanomaterial with Pd NPs

2D nanomaterials represent an emerging class of nanomaterials that possess sheet-like structures. Related research has grown exponentially in the fields of material science and nanotechnology.38,39 Along with the development of graphene, the 2D hexagonal boron nitride (h-BN) has attracted tremendous attention owing to the exceptional properties and immense application potentials.40−42 We envisaged that h-BN could be a promising candidate as a carrier in SMSI considering its unique properties, as follows: (1) The high thermal stability and oxidation-resistant properties of h-BN (>800 °C in air) have been witnessed by its application in fabricating nanoscale devices capable of operating under harsh conditions and serving as oxidation-resistant and high-performance coatings.40 In addition, controlled exfoliation of h-BN could be realized by treating h-BN to 800 °C in air and immediately immersing it in liquid nitrogen (−195 °C), fabricating few-layered h-BN nanosheets with well-reserved structures.43 (2) When used as a support, h-BN possesses the ability to modulate the interfacial electronic effect on metal NPs, leading to enhanced catalytic activity.44−47 (3) h-BN exhibits nanoporous architecture with high surface areas.48,49 However, as a nonredox active support, h-BN-derived SMSI materials cannot be fabricated through the traditional impregnation/high-temperature treatment pathway. Thus, a key to success is rational design of the synthetic methodology.

The construction of Pd/h-BN-SMSI was conducted by thermal treatment of the starting material mixtures composed of NaBH4 (3 g), NaNH2 (3 g), and PdCl2 (0.03 g) under N2 atmosphere at 500 °C for 1 h to form a homogeneous molten phase (Figure 1, step I). Subsequently, the temperature was raised to 850 °C and maintained for 2 h until the B–N bond formed, constructing the h-BN skeleton, and reduction of the Pd(II) species in situ to Pd NPs in the presence of plenty of hydrides (Figure 1, step II). After removing the residual salts by water washing procedure and drying under vacuum, the Pd/h-BN-SMSI was obtained with a high mass yield of 89%, and ∼2 g material was obtained in one batch. The Pd content in Pd/h-BN-SMSI was 1.50 wt %, which was determined by inductively coupled plasma–optical emission spectroscopy (ICP-OES) analysis. The essence of our methodology lies in the adoption of inorganic metal salt mixtures composed of NaNH2 and NaBH4 as precursors for the construction of the h-BN skeleton, as good solvent for homogeneous dispersion of the noble metal species (e.g., PdCl2) in their molten state, and as reductant for the positive noble metal ions in the presence of plenty of hydrides to form metal NPs in situ.

High-resolution transmission electron microscopy (HR-TEM) and high-angle annular darkfield scanning transmission electron microscope (HAADF-STEM) images of Pd/h-BN-SMSI revealed the atomic-scale details of the growth of h-BN layers around the Pd NPs, as shown in Figure 1 (see the green dashed box on the right), which exhibited that the Pd NP located at the edge of the material was surrounded by layered h-BN with thickness of ∼2.5 nm composed of seven sheets with an interlayer distance of 3.6 Å (Figure 2A). Pd NPs covered by h-BN layers were also observed in the middle position of the material (Figure 2B). For the as-prepared Pd/h-BN-SMSI, Pd NPs possessing a distribution of 5.8 ± 0.7 nm were uniformly surrounded and covered by the h-BN nanosheets (Figure 2C). The formation of h-BN skeleton was revealed by the Fourier transform infrared (FTIR) spectra (Figure 2D), which showed two characteristic peaks at ∼1352.3 and 778.4 cm–1, being assigned to in-plane B–N stretching and out-of-plane bending vibrations, respectively. X-ray photoelectron spectroscopy (XPS) further verified the construction of the h-BN skeleton by the presence of signals for B–N bonds with binding energy (BE) = 190.5 eV in B 1s and BE = 398.1 eV in N 1s spectra (Figure 2E). In situ reduction of Pd(II) species was confirmed by the signals in Pd 3d XPS spectrum with two peaks of BE = 335.8 eV (3d5/2) and 341.0 eV (3d3/2) corresponding to Pd NPs.50

Figure 2.

Figure 2

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Characterization of the as-prepared Pd/h-BN-SMSI material. (A,B) HR-TEM and (C) HAADF-STEM images of the Pd/h-BN-SMSI material. Scale bar: 5 nm for (A) and (B), and 50 nm for (C). The areas highlighted by blue dashed circle in (B) are Pd NPs covered by h-BN layers. The inset diagram in (C) shows the particle size distribution of Pd NPs. (D) FTIR spectrum. (E) XPS spectra (B 1s, N 1s, and Pd 3d). (F) PXRD pattern of Pd/h-BN-SMSI. (G) DRIFT spectra of CO adsorption on Pd/h-BN-SMSI and Pd/h-BN. (H) N2 adsorption and desorption isotherm curves at 77 K. (I) Pore size distribution curve obtained from the adsorption branches using NLDFT method.

Notably, the BE of Pd NPs in the Pd/h-BN-SMSI material shifted to a higher value compared with the pure Pd NPs with BE = 335.08 eV (3d5/2),51 indicating the interaction of Pd NPs with the h-BN cover, which was in accordance with the previous reports. That is, for h-BN supported metal NPs, electrons migrated from the metal center to the interfacial region and accumulated mainly below the electron-deficient B atoms.46 In addition, the Pd content measured by XPS was 0.56 wt % (i.e., Pd NPs covered by a relatively thin h-BN layer), much lower than that obtained by ICP-OES (1.50 wt %), indicating that most of the Pd NPs were immersed and surrounded within the h-BN sheets. The powder X-ray diffraction (PXRD) pattern of Pd/h-BN-SMSI revealed the periodic architecture of the h-BN layer/support, which matched well with a typical hexagonal structure (Figure 2F).38,52 A strong diffraction (002) peak at 2θ = 26.54° was observed, corresponding to the interplanar distance of 3.6 Å, in accordance with the TEM result (Figure 2A). The relatively weak diffraction peaks at 2θ = 42.07° and 54.51° were assigned to (100) and (004) crystal planes of h-BN, respectively (JCPDS card no. 01-073-2095),43 suggesting the formation of highly crystalline h-BN layers. In addition, the diffraction peaks for Pd NPs were located at 2θ = 39.90° and 46.36°, corresponding to (111) with interplanar distance of 2.2 Å, as shown in the TEM image (Figure 2A) and (200) crystal planes, respectively (JCPDS card no. 89-4897).53

For comparison, a post-loaded catalyst (denoted as Pd/h-BN) was fabricated via dispersion of PdCl2 on the as-prepared h-BN supports49 by deposition–precipitation, which was then treated under N2 atmosphere with the identical procedure. The PXRD pattern of Pd/h-BN revealed that the h-BN support maintained good crystallinity (Figure S1A). However, compared with Pd/h-BN-SMSI obtained through in situ reduction, the relative intensities of the peaks corresponding to (111) and (200) crystal planes of Pd NPs were obviously increased, indicating that the Pd NPs size was much larger (∼25 nm), as shown in the TEM images (Figure S1B). In addition, the Pd NPs were bare without surrounded or covered h-BN nanosheets (Figure S1C). Notably, h-BN was a nonredox support, and no migration of the h-BN overlayer to the Pd NPs surface can be induced during the thermal treatment procedure as that for the reducible metal oxide carriers.22 The result indicated that the adoption of inorganic metal salt mixtures for the homogeneous dispersion of Pd(II) species in the molten phase, and the in situ reduction strategy in this work was key to ensure the stabilization of Pd NPs and in situ encapsulation by h-BN layers to form h-BN-based SMSI material. The dispersion degrees of Pd NPs in Pd/h-BN-SMSI and Pd/h-BN are calculated according to the average particle size (Daver) of Pd NPs measured by both TEM and XRD (dispersion = 1.12/Daver) (Table S1).54,55 The results exhibited that the dispersion degree of Pd NPs in Pd/h-BN-SMSI was 13.17% (XRD) and 19.31% (TEM), which was much higher than that in Pd/h-BN (∼5%), benefiting from the small particle size endowed by the SMSI effect during the thermal treatment under harsh conditions.

The SMSI usually resulted in the suppression of adsorption of small molecules, such as CO, on the surface of metal NPs, which was generally regarded as a result of the physical coverage of adsorption sites by the support layer.28 As shown in Figure 2G, CO adsorption by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements at room temperature for both Pd/h-BN-SMSI and Pd/h-BN samples were performed. For the Pd/h-BN-SMSI material, the bands at 2117 and 2171 cm–1 were attributed to gaseous CO, and the band at 2046 cm–1 was assigned to linearly adsorbed CO.56 Comparatively, in the DRIFTS result of Pd/h-BN, besides linearly adsorbed CO, additional bands at 1914 and 1955 cm–1 appeared, ascribed to two types of bridged adsorption of CO species on the bare Pd NPs,56 which completely disappeared for Pd/h-BN-SMSI. The presence of linear CO adsorption for Pd/h-BN-SMSI resulted from the nanoporous architecture of h-BN layer,49 supplying channels for CO molecules to go through and interact with the inner Pd NPs. However, encapsulation with several layers of h-BN on Pd NPs made the space insufficient for the adsorption of bridged CO species, while in Pd/h-BN, there were no barriers around the rare Pd NPs. This agreed well with HR-TEM results showing that Pd NPs in Pd/h-BN-SMSI were completely covered (Figure 2A). The obviously restricted CO adsorption performance of Pd/h-BN-SMSI was also verified by the CO temperature-programmed desorption (CO-TPD) tests with significantly diminished intensity compared with that of Pd/h-BN (Figure S2).57

The N2 sorption isotherm of Pd/h-BN-SMSI showed combined features of type I and type IV with two evident steep steps and a pronounced hysteresis loop (Figure 2H), indicating the coexistence of micro- and mesopores. Based on the calculation from the nonlocal density functional theory (NLDFT) method (Figure 2I), micropores at ∼1.2 nm and mesopores at ∼2.6 nm coexisted. The Brunauer–Emmett–Teller (BET) surface area of Pd/h-BN-SMSI was shown to be 110 m2 g–1 together with a total pore volume of 0.0787 cm3 g–1. The nanoporous architecture was in accordance with the DRIFTS result of CO adsorption (Figure 2G). On the other hand, for catalytic materials, this high surface area and the presence of mesopores could improve their reactivity due to an enhanced mass transfer effect. Notably, for h-BN derived from NaBH4 and NaNH2, only micropores in the range of 1–2 nm existed.49 The introduction of additional mesopores in Pd/h-BN-SMSI was probably caused by the formation of Pd NPs between the interlayer space of h-BN sheets, leading to irregular stacking mode in local areas. The interaction between h-BN with the most exposed Pd (111) surface was explored via first-principles density functional theory (DFT) calculation (see Figure S3 for details). In order to detect the local environment of Pd species in Pd/h-BN-SMSI in detail, X-ray absorption spectroscopy (XAS) measurements were collected at the Pd K-edge. The X-ray absorption near-edge structure (XANES) region of the XAS spectrum provides information about the oxidation state of Pd. The Pd K-edge absorption edge position for Pd/h-BN-SMSI was similar to that of Pd metal (see Figure S4 for details), both of which were at lower photon energies than PdO, indicating that the Pd atoms in Pd/h-BN-SMSI were in the Pd0 valence state. The absence of Pd–O signal also verified the complete encapsulation of Pd NPs by uniformly distributed h-BN nanosheets.

Fabrication and Characterization of Other SMSI-Based Nanocatalysts with Composite Supports

Composite supports derived from the nanoscale architecture design could bring unique and tunable properties to the nanomaterials with enhanced catalytic performance.11,58 For example, localizing the Au NPs in the interfacial regions of TiO2 and hydroxyapatite could lead to ultrastable Au nanocatalyst with good performance after calcination at 800 °C.11 Zhang et al. demonstrated the good performance of Au NPs with SiO2 and TiO2 composite supports in small gas molecule-involved conversions.23

To investigate the generality and efficiency of the in situ encapsulation procedure demonstrated herein in affording metal NP catalysts with high thermal stability and sintering resistance, commercial metal NPs (e.g., Pd, Pt, and Au) supported by nonreducible carriers (e.g., γ-Al2O3, zeolite, and SiO2) were taken as precursors to produce nanocatalysts with composite supports. First, the thermal treatment behavior of γ-Al2O3-supported Pd NPs (denoted as Pd/γ-Al2O3) was investigated. As shown by the HAADF-STEM image (Figure 3A), direct calcination of the Pd/γ-Al2O3 up to 850 °C under nitrogen atmosphere caused severe sintering of the Pd NPs with mean particle size of ∼30.3 nm in the resultant Pd/γ-Al2O3-850. Comparatively, thermal treatment of the mixtures composed of Pd/γ-Al2O3, NaNH2, and NaBH4 up to 850 °C led to the formation of an h-BN layer on the surface of Pd NPs, which prevented the sintering; highly dispersed Pd NPs with a particle size centered at ∼2.3 nm were obtained in h-BN/Pd/γ-Al2O3-850 (Figure 3B).

Figure 3.

Figure 3

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HAADF-STEM images and corresponding PXRD patterns of various nanocatalysts prepared by direct calcination (denoted as M/support-T) or calcination in the presence of NaNH2 and NaBH4 (denoted as h-BN/M/support-T), where M is metal center, support is the carriers in the precursor and T is the calcination temperature. (A–C) Pd/γ-Al2O3-850 and h-BN/Pd/γ-Al2O3-850. (D–F) Pt/γ-Al2O3-950 and h-BN/Pt/γ-Al2O3-950. (G–I) Au/γ-Al2O3-850 and h-BN/Au/γ-Al2O3-850. (J–L) Au/MCM-22-850 and h-BN/Au/MCM-22-850. (M–O) Au/SiO2-850 and h-BN/Au/SiO2-850.

The thermal stability of commerical Pt NPs supported on γ-Al2O3 was much better than that of Pd NPs. High dispersity of the Pt NPs could be maintained by direct calcination of Pt/γ-Al2O3 up to 850 °C (Figure S5). However, further increasing the calcination temperature to 950 °C caused severe sintering of Pt NPs with mean size of >150 nm (Figure 3D and Figure S6). Comparatively, even at a calcination temperature as high as 950 °C, the post-encapsulation process was proven to be efficient for stabilizing the Pt NPs with a much smaller mean particle size being maintained (∼13.2 nm) in the obtained h-BN/Pt/γ-Al2O3-950 (Figure 3E). In the PXRD patterns of the Pd and Pt nanocatalysts as stated above, the intensities of the peaks corresponding to the (111) crystal plane of metal NPs in the h-BN-coated materials were much smaller compared with that in materials with bare metal NPs (Figure 3C and F). In addition, characteristic peaks for the (002) and (100) crystal planes of h-BN were all presented.

The Au NPs are a challenge in constructing SMSI in previous investigations owing to the low surface energy and work function of Au, the weak ability to dissociate molecular H2 for activating the oxide surface, and the sintering of Au NPs before the formation of SMSI during the thermal treatment process at high temperature.2,23 Using γ-Al2O3 supported Au NPs as the starting material,59 direct calcination of the material under nitrogen atmosphere up to 850 °C led to severe sintering of the Au NPs in Au/γ-Al2O3-850, and the mean particle size reached ∼47.4 nm (Figure 3G). Comparatively, by coating h-BN on the surface of Au NPs during thermal treatment with the same heating condition, the mean particle size significantly decreased to ∼13.6 nm in h-BN/Au/γ-Al2O3-850 (Figure 3H). The difference was clearly shown by the PXRD patterns in which the intensity of the peak corresponding to the (111) crystal plane of Au NPs was much stronger for Au/γ-Al2O3-850 than that for h-BN/Au/γ-Al2O3-850 (Figure 3I). In addition, the peaks for the (002) and (100) crystal planes of h-BN were exhibited in the PXRD pattern of h-BN/Au/γ-Al2O3-850. Au NPs supported by other carriers were also employed as the precursors to investigate the support adaptability. The zeolite framework MCM-22 with lamellar frameworks has been considered a potential solid acid catalyst, and its surface properties can be modified by doping it with various metal species in the extra positions of the framework.60 The unique properties of MCM-22 render it a good support to stabilize metal NPs under harsh reaction conditions.

As evidenced in our calcination procedure of up to 850 °C, Au NPs with an average particle size of ∼21.3 nm and reasonable dispersity could be maintained (Figure 3J). However, the dispersity and particle size were further improved in the post-encapsulation process in the presence of NaNH2 and NaBH4; ultrasmall Au NPs with mean particle size of only ∼6.5 nm were achieved (Figure 3K), and hence almost no signal for a (111) crystal plane of Au NPs in PXRD pattern was observed (Figure 3L). A silica supported Au nanocatalyst (denoted as Au/SiO2) was also taken as the precursor, considering the easy preparation, wide application, and nonreducible property of silica. Not surprisingly, severe sintering of the Au NPs was observed by direct calcination of the Au/SiO2 nanocatalyst at up to 850 °C under nitrogen atmosphere, verifying by the large mean size of Au NPs in the resultant Au/SiO2-850 (32.6 nm) (Figure 3M). Comparatively, the thermal treatment of the mixtures composed of Au/SiO2, NaNH2, and NaBH4 up to 850 °C led to the formation of encapsulated Au NPs (denoted as h-BN/Au/SiO2-850) with high dispersity and uniform distribution, and the mean size of the Au NPs maintained at around 7.2 nm (Figure 3N).

Transmission electron microscopy (TEM) analyses of h-BN/Au/SiO2-850 showed that the silica-supported Au NPs were encapsulated by the in situ formed h-BN layer, and the interface between Au/h-BN, Au/SiO2, and h-BN/SiO2 was clearly observed (Figure 4A). The h-BN layer was homogeneously covered on the surface of the whole silica support (Figure 4B). The h-BN nanosheets were highly crystalline with an interlayer distance of 3.6 Å, which was in accordance with the previous reports.47,49 In addition, the interlayer distance of Au NPs was 2.2 Å (Figure 4C). The elemental mapping analysis of the obtained h-BN/Au/SiO2-850, including Au center, B and N from h-BN layer, and Si and O from the silica support, exhibited that both the original silica support and the post-encapsulated h-BN layer were uniformly distributed at the position of Au NPs (Figure 4D–I). Furthermore, the post-encapsulation strategy developed herein was verified to be highly efficient to prevent sintering of Au NPs with a calcination temperature up to 950 °C, maintaining the small particle size and high dispersity of the Au NPs within the supports (Figure S7). FTIR spectra of h-BN-coated Au nanocatalysts fabricated by the post-encapsulation procedure with different precursors, as well as the Pd and Pt nanocatalysts, all exhibited characteristic peaks for B–N bending and stretching in h-BN frameworks (Figure S8). Other characterizations of Au/SiO2, Au/SiO2-850, and h-BN/Au/SiO2-850 (XRD, XPS, and catalytic reduction of nitrobenzene) as discussed in detail in Figure 3O, and Figure S9 further proves this structure of SMSI for the h-BN/Au/SiO2-850.

Figure 4.

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TEM images and elemental mapping of h-BN/Au/SiO2-850. (A–C) TEM images of different regions in h-BN/Au/SiO2-850. Scale bar: 20 nm for A, 10 nm for B, and C. (D–I) Elemental mapping of h-BN/Au/SiO2-850, including Au, B, N, Si, and O. The red dashed circles highlighted the position of Au NPs. Scale bar: 20 nm.

Therefore, the encapsulation strategy realized by an in situ coating of the metal NPs with an h-BN layer proved capable of producing a supported metal nanocatalyst with high dispersity and sintering resistance at temperatures up to 950 °C using metal NPs (e.g., Pd, Pt, and Au) supported by different carriers (e.g., γ-Al2O3, zeolite, and SiO2) as the precursors. The simplicity, high efficiency, good performance, and wide application of this strategy will unlock new opportunities to fabricate ultrastable metallic nanocatalyst in heterogeneous catalysis.

Thermal Stability and Catalytic Durability of Pd/h-BN-SMSI in CO Oxidation under Simulated Practical Conditions

Pd species have demonstrated unique catalytic efficiency in CO oxidation;61 the as-prepared Pd/h-BN-SMSI catalyst was subjected to CO oxidation in order to better evaluate its catalytic performance, particularly its catalytic durability and thermal stability under simulated practical conditions for automotive emission abatement in the presence of O2, H2O, and hydrocarbon.61−63 The present generation of catalysts for emission treatment is mainly composed of metallic NPs such as Pt, Pd, and Rh supported on aluminum oxide in the presence of other oxide promoters.61 It is known that metal NPs are mobile during the high-temperature treatment procedure and agglomerate into large particles, leading to loss of their catalytic efficiency.64 Heterogeneous metallic catalysts involving SMSI provided an effective strategy to prevent the sintering of metal NPs by forming oxide barriers on the surface.11,21,22 Up to now, the main challenge has been that H2O and hydrocarbons in the exhaust gas could cause severe deactivation to the known catalysts.65,66 The as-prepared Pd/h-BN-SMSI material in this work obtained via high temperature (850 °C) treatment was a promising candidate as efficient catalysts in CO oxidation for practical applications. First, the catalytic activity of Pd/h-BN-SMSI toward CO oxidation was evaluated with feed gas composed of 1 vol % CO balanced with dry air, and the light-off curve exhibited that the Pd/h-BN-SMSI catalysts became active at 140 °C and achieved 50% CO conversion at 187 °C and almost complete CO conversion at 235 °C (Figure 5A). The long-term catalytic stability of the Pd/h-BN-SMSI catalysts was further studied at 185 °C, and the CO conversion was controlled below 50% to avoid the activity saturation. Under these conditions, slight sintering of metal NPs or accumulation of carbonates on the surface of the metal NPs would lead to decreased catalytic activity. As shown in Figure 5B, for Pd/h-BN-SMSI, negligible deactivation in performance with ∼43% CO conversion was observed over 50 h on stream. The XRD pattern of the used Pd/h-BN-SMSI showed no difference compared with the fresh one (Figure S10A). HAADF-STEM (Figure S10B,C) and TEM images (Figure S10D) of the catalyst used exhibited highly dispersed and small Pd NPs, which were still covered and surrounded by h-BN support.

Figure 5.

Figure 5

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CO oxidation performance of Pd/h-BN-SMSI under simulated practical conditions. (A) CO light-off curves of Pd/h-BN-SMSI catalysts under different conditions. (B) Catalytic stability of Pd/h-BN-SMSI with feed gas composed of 1 vol % CO balanced with dry air. (C) Catalytic stability of Pd/h-BN-SMSI with feed gas containing 5 vol % H2O. (D) Catalytic stability of Pd/h-BN-SMSI with feed gas containing 5 vol % H2O and 0.1 vol % propene.

Subsequently, the catalytic behaviors of Pd/h-BN-SMSI at realistically achievable conditions were evaluated. (1) Water is usually thought to accelerate the sintering of metal particles.11,67,68Figure 5A showed light-off curves for CO oxidation in the presence of 1 vol % CO and 5 vol % H2O to provide a test of water tolerance. The Pd/h-BN-SMSI catalyst was able to achieve 50% and 100% CO conversion at 180 and 220 °C, respectively, which was somewhat enhanced compared to that obtained under neat conditions. The stability test for moisture demonstrated good performance of Pd/h-BN-SMSI over 50 h, maintaining 57% CO conversion at 185 °C (Figure 5C), indicating a limited effect of H2O upon the stability of Pd/h-BN-SMSI catalyst. (2) Introduction of hydrocarbons was another issue causing severe inhibition of metal NP catalysts in CO oxidation through adsorption competition on active metal sites.66 To study the inhibitory effects of hydrocarbons during CO oxidation, propene (0.3 vol %) was involved during the evaluation process as a model hydrocarbon. As expected, CO oxidation activity of Pd/h-BN-SMSI was stable in the presence of propene (Figure 5A), which was able to reach a 47% CO conversion at 185 °C under simulated exhaust conditions and gas hourly space velocity (GHSV) = 150 000 mL gcat–1 h–1 in the presence of common inhibitors including 5 vol % H2O and 0.3 vol % propene (Figure 5D). These results demonstrated the excellent chemical and thermal stability of the Pd/h-BN-SMSI catalyst at realistically achievable conditions in the exhaust system, maintaining the size of their metallic NPs and exhibiting high sintering resistant capabilities thanks to the SMSI endowed by the particular nanoarchitecture and chemical/thermal stability of the h-BN support. The catalyst deactivated soon when there is no BN protection as shown in the Figure S11. To highlight the role of the BN protection layer for the stability of catalyst, we measured the CO oxidation combined with propene, H2O, and toluene as shown below (Figure S12). The above results unambiguously show that our h-BN/Pd/γ-Al2O3-850 and h-BN/Pt/γ-Al2O3-950 catalysts are highly active and extremely stable at elevated temperatures even in the presence of propene, H2O, and toluene, while the Pd/γ-Al2O3-850 and Pt/γ-Al2O3-950 were deactivated soon under the propene, H2O, and toluene. Such catalyst characteristics imply a great potential for practical applications. As shown in Figure S12, the CO conversion for the Pd/γ-Al2O3-850 and Pt/γ-Al2O3-950 decreased from 100% to almost 0 within a test time of only 10 h. Our h-BN/Pd/γ-Al2O3-850 and h-BN/Pt/γ-Al2O3-950, however, exhibited a much better durability with almost no decrease of CO conversion in a test time of 40 h, suggesting excellent stability under practical application conditions.

To further prove the role of BN for the protection of noble metal nanoparticles in catalysts, we added the hydrogenation of phenylacetylene by Pd/h-BN, Pd/γ-Al2O3-850, Pt/Al2O3-950, Pd/h-BN-SMSI, h-BN/Pd/γ-Al2O3-850, and h-BN/Pt/γ-Al2O3-950. As shown in Figure S13, it was shown that Pd/h-BN, Pd/γ-Al2O3-850, and Pt/Al2O3-950 did not give any hydrogenation activity because the nanoparticles of Pt and Pd grew larger than 25 nm under high temperature treatment, which deactivated them for the hydrogenation of phenylacetylene. However, for Pd/h-BN-SMSI, h-BN/Pd/γ-Al2O3-850, and h-BN/Pt/γ-Al2O3-950, the particle size of Pt and Pd particles in these catalysts were maintained below 10 nm by the protection of BN layers under high temperature; thus, they showed a high activity for the hydrogenation of phenylacetylene, and the selectivity of styrene was higher than 90%. The existence of the conversion of phenylacetylene and yield of styrene indicates that the catalytic interfaces are not totally buried.

Conclusion

In conclusion, to address the challenging issues in the present SMSI-derived nanocatalysts, including restriction of supports to reducible metal oxides, nonporous architecture, sintering by thermal treatment at >800 °C, and the unstable nature under oxidative conditions, a simple yet highly efficient strategy is demonstrated for the construction of nanoporous and highly crystalline h-BN-derived SMSI nanocatalysts via an in situ encapsulation and reduction pathway with inorganic salt mixtures as the starting materials. The as-prepared nanocatalysts were composed of uniformly dispersed metal NPs covered by h-BN nanosheets capable of withstanding high thermal stability at up to 950 °C and demonstrated high catalytic efficiency, high thermal stability, and sintering-resistance in CO oxidation under realistically achievable conditions. This protocol could dramatically expand the scope of carriers and fabrication approaches in SMSI-involving catalysts and shed light on the design of ultrastable and robust heterogeneous catalysts for practical applications.

Acknowledgments

H.C., Z.Y. and S.D. were sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science program. J.F. was supported by the National Natural Science Foundation of China (No. 21706228, 21978259), Zhejiang Provincial Natural Science Foundation of China (No. LR17B060002) and the Fundamental Research Funds for the Central Universities.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.0c00822.

  • Experimental Procedures, Figures S1–S8, and Table S1 (PDF)

Author Contributions

# S.D. conceived the research idea. H.C., Z.Y., J.F., and S.D. designed the experiments. H.C. and Z.Y. performed all the experiments and analyzed all the data. S.Y. obtained HADDF-STEM images. Q.W. and D.J. carried out the density functional theory (DFT) calculation. All authors discussed the results and commented on the manuscript. H.C., Z.Y., and S.D. cowrote the paper. H.C., and S.Y. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

oc0c00822_si_001.pdf (880.5KB, pdf)

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