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Enhanced Nickel Catalyzed Methanation Confined under Hexagonal Boron Nitride Shells Lijun Gao, Qiang Fu, Mingming Wei, Yifeng Zhu, Qiang Liu, Ethan Crumlin, Zhi Liu, and Xinhe Bao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02188 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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Enhanced Nickel Catalyzed Methanation Confined under Hexagonal Boron Nitride Shells Lijun Gaoa,b, Qiang Fub*, Mingming Weib, Yifeng Zhub, Qiang Liuc,d, Ethan Crumline, Zhi Liuc,d, Xinhe Baob* a

Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, P.R. China b

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, P.R. China

c

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P.R. China.

d

School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031,P.R. China e

Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA Emails: [email protected]; [email protected]

Abstract Encapsulation of metal nanoparticles with porous oxide shells is a successful strategy to design catalysts with high catalytic performance. We suggest an alternative route to cover metal nanoparticles with two-dimensional (2D) material shells such as hexagonal boron nitride (h-BN), in which active metal components are stabilized by the outer shells and meanwhile catalytic reactions occur at core/shell interfaces through feasible intercalation of the 2D material covers. As an illustration, Ni nanoparticles encapsulated with few-layer h-BN shells were constructed and applied in syngas methanation. Ni@h-BN core-shell nanocatalysts exhibit enhanced methanantion activity, higher resistance to particle sintering, and suppressed carbon deposition and Ni loss in reactions. Surface science studies in h-BN/Ni(111) model systems and chemisorption data confirm the occurrence of methanation reaction on Ni surface under h-BN cover. The confinement effect of h-BN shells improves Ni-catalyzed reaction activity and Ni catalyst stability.

Keywords: hexagonal boron nitride (h-BN), nickel, syngas methanation, core-shell, intercalation

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1. Introduction Supported metal nanoparticle (NPs) are often subject to deactivation during catalytic reactions due to nanoparticle sintering, loss of active components, blocking active sites by poisons, and other factors.1-4 Extensive efforts have been made to find feasible solutions to this long-term issue in heterogeneous catalysis. One of the most efficient is to encapsulate metal NPs with outer shells forming core-shell (metal@shell) nanostructures, in which the active components are well protected inside confined microenvironments.5-10 Obviously, the shell layers need to be porous structures, e.g. mesoporous silica, microporous transition metal oxides, and mesoporous carbon, which allow free access to small part of the active metal surfaces.11-16 Otherwise the metal catalysts are completely shielded from reaction atmospheres and have no activity. Therefore, it remains as a challenge to design core-shell nanocatalysts which can suppress the deactivation while still maintain high active surface area and high reaction activity. Two-dimensional (2D) atomic crystals such as graphene, hexagonal boron nitride (h-BN), and MoS2 can form well-defined interfaces with metal surfaces.17-19 Recent results show that many molecules can intercalate 2D overlayers and get adsorbed on the metal surfaces underneath because of weak interaction of the 2D layered structures with the metal surfaces.20-26 The space between graphene or h-BN overlayers and metals even acts as 2D nanoreactor, in which catalytic reactions occur with enhanced performance due to confinement effect of the top 2D covers.27-30 Inspired by the concept of “Catalysis under 2D cover” suggested by us based on surface science studies31,32 we consider designing core-shell nanocatalysts which consist of metal NPs encapsulated by 2D material shells. The shells prevent the metal nanocatalysts from deactivation and, meanwhile, reactions happen on metal surfaces through intercalation under the 2D material shells. In the present work, we chose Ni catalyzed syngas methanation as a model reaction system. Nickel-based catalysts are widely used in many industrial processes, such as methanation, dry reforming, and steam reforming reactions because of their high activity and low cost.33-37 However, these catalysts are subject to strong deactivations due to carbon deposition, Ni particle sintering, and Ni loss from Ni(CO)4 formation.35,38-41 Here, a simple synthesis strategy was developed to encapsulate Ni NPs with ultrathin h-BN shells. We find that the h-BN shells protect the Ni cores from sintering at high temperatures, suppress carbon deposition, and prevent Ni loss in CO-containing gases. More importantly, surface science measurements and chemisorption results 2


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reveal that highly efficient catalytic processes happen on major part of the Ni surfaces under the h-BN shells. The core/shell interfaces act as confined nanoreactors, which provide intriguing confinement environment for catalysis like hollow channels of carbon nanotubes (CNTs).42 Our results demonstrate that nanocatalysts covered by 2D material shells can present high stability but without much sacrifice of reaction activity, which should be of significance for design of highly stable and efficient core-shell nanocatalysts.

Figure 1. Preparation and characterization of Ni and Ni@h-BN catalysts. a) A scheme illustrating the catalyst preparation processes; b) TEM image of Ni3@(h-BN)1/SiO2 sample, and c) HRTEM images of Ni3@(h-BN)1 sample; d) XRD patterns of NiO/SiO2, Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2-acid catalysts; XPS e) N 1s and f) B 1s spectra of the Ni/SiO2, Ni3@(h-BN)1, and Ni1@(h-BN)3 catalysts, in which SiO2 supports were selectively removed by etching in NaOH solution. 3


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2 Results and Discussion 2.1 Construction and characterization of Ni@h-BN catalysts. A simple preparation process was developed to prepare Ni@h-BN core-shell nanocatalysts (Figure 1a). First, silica support was impregnated with Ni(NO3)2 solution and then calcined at 350 oC forming NiO/SiO2 samples. Further reduction in H2 at 500 oC produced Ni/SiO2 catalyst with loading of 20 wt. %. Alternatively, the NiO/SiO2 sample was impregnated with boric acid (H3BO3) solution. Ni@h-BN/SiO2 catalysts were obtained after consecutive treatments in H2 at 500 oC and in NH3 at 850 oC. During the reduction in H2 up to 500 oC NiO was reduced to metallic nickel and boric acid was decomposed to vitreous boron oxide layer on the catalyst surface. Upon further high temperature treatment in NH3, we suggest that boron oxide on top of Ni nanoparticles may be transformed to boron nitride with the aid of catalysis of nickel.43 Molecular ratio of Ni to H3BO3 has been varied between 3:1 and 1:3. For example, Ni3@(h-BN)1/SiO2 and Ni1@(h-BN)3/SiO2 catalysts were prepared. In case that the Ni1@(h-BN)3/SiO2 catalysts were leached in 0.5 M HNO3 solution the treated catalysts were denoted as Ni1@(h-BN)3/SiO2-acid. Transmission electron microscopy (TEM) images of a typical Ni3@(h-BN)1/SiO2 sample show uniform distribution of Ni@h-BN NPs with an average diameter between 8 and 12 nm (Figure 1b). High resolution TEM (HRTEM) analysis displayed in Figure 1c indicates that most of metal NPs are encapsulated by few-layer graphitic shells with a layer spacing of 3.5 Å, which are close to the (002) interplanar distance in the h-BN structure.44 The inset image indicates that the NPs exhibit a layer spacing around 2.0 Å, in agreement with the (111) plane of metallic Ni.14 The Ni1@(h-BN)3/SiO2 catalyst contains similar core-shell nanostructures but the Ni cores were covered with much thicker h-BN overlayers (Figure S1(a, b)). Most Ni NPs in the Ni/SiO2 catalyst were covered with amorphous overlayers which were due to oxidation of particle surfaces in air (Figure S1(c, d)). The two Ni@h-BN/SiO2 catalysts have similar particle size as the Ni/SiO2 catalyst even though they both have been treated at a much higher temperature (850 vs. 500 o

C). The metallic Ni phase in the Ni@h-BN/SiO2 catalysts has been further confirmed by X-ray

diffraction (XRD) measurements (Figure 1d). In the Ni/SiO2 catalyst additional weak diffraction peaks at 37.2o and 62.8o from NiO phase were present, which suggest that part of Ni NPs were oxidized. Figures 1e and 1f display X-ray photoelectron spectroscopy (XPS) N 1s and B 1s spectra 4


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acquired from the Ni/SiO2, Ni3@(h-BN)1, and Ni1@(h-BN)3 catalysts. Strong N 1s signals located at 397.8 eV in the Ni@h-BN samples are characteristic for h-BN.45,46 In the corresponding B 1s spectra strong components have been identified at 190.4 eV which are from B atoms in h-BN.45,46 The atomic ratio of B/N calculated from the N 1s and B 1s components is around 1:1, which confirms the formation of h-BN in the Ni@h-BN catalysts and is consistent with the HRTEM results. In the B 1s spectra another major component was observed at 192.2 eV, which is between the binding energy positions from h-BN (190.3 eV) and B2O3 (193.5 eV).47 We infer that this signal may be attributed to unreduced boron oxide species anchored on oxide support or BNxOy species at defect sites of h-BN overlayers.48,49 XPS B 1s spectra indicated that 64% and 56 % B 1s signals are from boron oxides for the Ni3@(h-BN)1 and Ni1@(h-BN)3 samples, respectively. XPS Ni 2p spectra from these catalysts indicate the dominant metallic Ni phase in the Ni@h-BN catalysts but NiOx in the Ni/SiO2 catalysts (Figure S2). On the basis of TEM, XRD, and XPS results we conclude that Ni@h-BN core-shell nanostructures consisting of Ni cores and h-BN shells have been constructed.

Figure 2. Chemical and thermal stability of the Ni and Ni@h-BN catalysts. a) Ni loadings in the as-prepared Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2 catalysts and the catalysts treated in 0.5 M HNO3 solution and in 1.0 MPa syngas at 150 oC, respectively. The Ni loadings (wt. %) were determined by EDX. b) Ni particle sizes measured by XRD in the Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2-acid catalysts before and after treatments in H2 at 850 oC for 10 h. 2.2 Chemical and thermal stability of Ni@h-BN nanocatalysts. It is well known that Ni catalysts used in methanation reactions suffer from metal loss through formation of volatile nickel carbonyls (Ni(CO)x, x = 1-4).39,40 To study stability of Ni NPs in CO-containing gases, the Ni/SiO2, 5


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Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2 catalysts were treated in 1.0 MPa syngas (H2/CO = 3:1) at 150 oC for 3 h. Optical microscopy (OM) and energy-dispersive X-ray spectroscopy (EDX) characterizations (Figure 2a and Figure S3) show that all Ni species have been removed from the Ni/SiO2 catalyst. Under the same treatment condition Ni loadings in the Ni3@(h-BN)1/SiO2 and Ni1@(h-BN)3/SiO2 catalysts only decrease by 6 wt. % and 1 wt. %, respectively (Figure 2a). The contrast results from Ni/SiO2 and Ni@h-BN/SiO2 catalysts suggest that h-BN shells on Ni NPs effectively prevent Ni loss in syngas atmosphere. The three catalysts were treated in a 0.5 M HNO3 solution at room temperature for 12 h. Both OM and EDX measurements indicate that all Ni species have been removed from the Ni/SiO2 and Ni3@(h-BN)1/SiO2 catalysts by the acid leaching treatments (Figure 2a). It is interesting to note that major part of Ni components in the Ni3@(h-BN)1/SiO2 catalyst still remain in the sample after the syngas treatment. The different leaching results in syngas atmosphere and in acid solution suggest that defect sites should be present in the h-BN shells such that acid solution can penetrate through the h-BN shells and dig out Ni cores completely. In contrast, formation of Ni(CO)x compounds and their desorption from h-BN/Ni interfaces may be suppressed by h-BN shells. For the Ni1@(h-BN)3/SiO2 catalyst the major part of Ni (14 wt. %) is left in the acid leached catalyst, in which most Ni NPs should be completely protected by thick h-BN shells. The Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(BN)3/SiO2-acid catalysts were subject to high temperature (850 oC) treatment in H2 for 10 h and then characterized by XRD and TEM (Figures S4 and S5). Figure 2b displays the average Ni particle sizes in the as-prepared and treated catalysts, which were calculated from XRD patterns. The as-prepared Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2-acid catalysts have the similar Ni particle sizes of 9.0, 8.8, and 8.7 nm, respectively, which change to 23.0, 11.2, and 10.2 nm after the thermal treatments. It is expected to see strong sintering of supported Ni NPs without any protection.50 In contrast, Ni NPs encapsulated with h-BN shells present much higher sintering resistance. Apparently, the h-BN shells provide spatial restriction on the Ni nanoparticles which prevent nickel aggregation at high temperature (850 oC). It should be noted that the Ni@h-BN catalysts treated in H2 at high temperatures still have the h-BN shells (Figure S6). The acid and syngas leaching experiments confirm that most Ni NPs are covered by h-BN shells in the Ni@(h-BN) catalysts, in which the Ni3@(h-BN)1/SiO2 catalyst consists of defective 6


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and few-layer h-BN shells while Ni NPs are encapsulated by multilayer and compact h-BN shells in the Ni1@(h-BN)3/SiO2 catalyst. The stability tests suggest that Ni NPs encapsulated with h-BN shells present high chemical and thermal stability under harsh treatment conditions.

Figure 3. Syngas methanation reactions over the Ni/SiO2 and Ni@h-BN/SiO2 catalysts. a) CO conversions of the reactions at 0.1 MPa between 300 and 600 oC. b) CO conversions of the reactions at 1.0 MPa over the catalysts treated in the reaction gas at 150 oC for 3 h first. WHSV = 1650 L·g-1Ni·h-1. 2.3 Syngas methanation reaction activity. The Ni/SiO2 and Ni@h-BN/SiO2 catalysts were used in the syngas methanation reaction. Figure 3a displays CO conversions with a reaction pressure of 0.1 MPa and a high weight hourly space velocity (WHSV) of 1650 L∙g-1Ni∙h-1. For comparison thermodynamic equilibrium data were also included. The Ni3@(h-BN)1/SiO2 catalyst exhibits slightly higher CO conversions than the Ni/SiO2 catalyst in the whole temperature range even though Ni NPs in the former catalyst were covered with few-layer h-BN shells. They both exhibit quite high activities in comparison with other high-performance Ni-based catalysts in the reaction.33-35 In contrast, the Ni1@(h-BN)3/SiO2-acid catalyst presents much lower activity and the maximum CO conversion was only 20% at 600 oC. The methanation reaction was also carried out at 1.0 MPa and the similar reaction result was observed (Figure S7). In order to study the effect of surface BOx species on the methanation activity, the Ni3@(h-BN)1/SiO2 catalyst was washed by water, which can remove boron oxide on the surface, and the washed catalyst presents the similar activity as the original one (Figure S8). Moreover, to explore the effect of defects in the h-BN shells on the reactivity, the Ni1@(h-BN)3/SiO2-acid catalyst was further treated in NH3 at 850 oC for 1 h (denoted as Ni1@(h-BN)3/SiO2-acid-NH3) and XPS measurements indicate that the amount of 7


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defects

have

been

strongly

decreased

(Figure

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S9).

Methanation

reactions

on

the

Ni1@(h-BN)3/SiO2-acid-NH3 catalyst show even lower activity (Figures 3a, Figure S7). Overall, Ni NPs with defective and few-layer h-BN shells present enhanced methanation reaction activity, while decreased activity was observed over the Ni NPs covered by thick h-BN shells with less defects. We also tested our catalysts in low temperature reaction, which is a challenge for Ni-catalyzed syngas conversion reactions.1 The catalysts were treated in 1.0 MPa syngas at 150 oC for 3 h and then methanation reactions were further tested up to 700 oC (Figure 3b). The Ni/SiO2 catalyst presents no activity at all within the reaction temperature regime. However, activity of the Ni3@(h-BN)1/SiO2 and Ni1@(h-BN)3/SiO2-acid catalysts only decrease slightly after the pretreatment at low temperature in the reaction gas. Combined with the syngas leaching experiment shown in Figure 2a, we confirm that the h-BN shells prevent the loss of active Ni component in the syngas atmosphere and the Ni@h-BN catalysts can be applied in the methanation reactions at a low temperature regime (from room temperature to 250 oC).

Figure 4. Comparative studies in carbon deposit on Ni/SiO2 and Ni@h-BN/SiO2 catalysts. a) CO conversions of the syngas methanation reaction versus stream on time over the Ni/SiO2 and Ni3@(h-BN)1/SiO2 catalysts at 375 oC, 0.1 MPa with WHSV at 1650 L·g-1Ni·h-1. b) Raman spectra of

the

Ni/SiO2

and

Ni3@(h-BN)1/SiO2

catalysts

after

8


the

lifetime

test.

c)

CO

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temperature-programmed dissociation over the Ni/SiO2 and Ni3@(h-BN)1/SiO2 catalysts. d) Temperature-programmed hydrogenation over the Ni/SiO2 and Ni3@(h-BN)1/SiO2 catalysts after carbonization by 5% CO-He at 375 oC for 1 h. 2.4 Syngas methanation reaction stability. The resistance for carbon deposition over the Ni and Ni@h-BN catalysts was investigated by performing the methanation reaction at 375 oC and 0.1 MPa, in which strong coke formation often occurs on Ni catalysts. The CO conversions from the Ni/SiO2 and Ni3@(h-BN)1/SiO2 catalysts with the time on stream were shown in Figure 4a. The initial CO conversions were 90% and 88% on the Ni3@(h-BN)1/SiO2 and Ni/SiO2 catalysts, respectively. After reaction for 28 h CO conversion from the Ni3@(h-BN)1/SiO2 catalyst decreased slightly to 88 %. In contrast, the Ni/SiO2 sample showed a remarkable loss of the activity and CO conversion was 82% at the end. After the reactions the two catalysts were characterized by Raman. Strong D band at 1350 cm-1 and G band at 1586 cm-1 originating from surface carbon were observed on the used Ni/SiO2 catalyst, which, however, cannot be detected on the used Ni3@(h-BN)1/SiO2 catalyst (Figure 4b). TEM images from the two used catalysts (Figure S10) show that the agglomeration of the Ni particles was much stronger in the Ni/SiO2 catalyst than the Ni3@(h-BN)1/SiO2. Furthermore, filamentous carbon nanostructures can be seen in the Ni/SiO2 catalyst but not much in the Ni3@(h-BN)1/SiO2 catalyst. All the characterization results confirm that the coke formation has been strongly suppressed on the Ni3@(h-BN)1/SiO2 catalysts in the methanation reaction. To understand such a difference, temperature-programmed dissociation of CO and subsequent temperature-programmed hydrogenation (TPH) were carried out on the two catalysts. A main peak at 428 oC and a small shoulder peak around 540 oC were observed on the Ni/SiO2 catalyst, which can be attributed to CO dissociation on Ni through CO disproportionation and carbon deposit.1,38 In the temperature-programmed dissociation profile of the Ni3@(h-BN)1/SiO2 catalyst, the main CO dissociation peak is now around 557 oC (Figures 4c). The CO dissociation on Ni has been weakened by the h-BN shells because of the confinement effect of the h-BN cover.28,29 The TPH profile of the Ni/SiO2 catalyst with carbon deposit presents one main peak of CH4 formation at 500 oC, while the h-BN covered Ni sample shows one main peak at 480 oC and another small peak at 210 oC (Figures 4d). Compared to the Ni/SiO2 catalyst, the Ni3@(h-BN)1/SiO2 catalyst exhibits higher CO dissociation temperature but lower temperature for hydrogenation of the surface carbon, which can 9


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be attributed to the higher resistance for coke formation on the Ni@h-BN catalyst than the Ni catalyst. 2.5 Discussions. The reaction data show that the Ni NPs encapsulated with few-layer h-BN shells demonstrate improved methanation performance in the reaction activity and stability compared to the pure Ni catalyst. Then, a natural question arose is whether the reaction happens on Ni surface under the h-BN shells or on the h-BN surface. To address this issue, we construct a h-BN/Ni(111) model surface and study the surface chemistry. Monolayer (1 ML) h-BN overlayers were grown on a clean Ni(111) surface through chemical vapor deposition (CVD) process.51 The surface was exposed to CO gas with increasing pressure and simultaneously investigated by near-ambient pressure XPS (NAP-XPS)52,53 (Figure 5). The as-prepared 1 ML h-BN/Ni(111) surface presents N 1s and B 1s signals located at 398.8 and 190.6 eV, respectively, which are similar to the previous reports on h-BN layers grown on metal surfaces.29,51 When exposed to 1 × 10-6 Torr CO no any change has been observed by in-situ NAP-XPS, which suggests that the h-BN overlayer has blocked CO adsorption on the Ni surface under high vacuum condition and also excludes the possibility of CO adsorption on the h-BN surface. Big changes in the NAP-XPS spectra were observed under 0.1 Torr CO exposure condition (Figure 5). A strong O 1s signal at 531.1 eV appears, which can be attributed to CO adsorbed on Ni(111) surface.20 Meanwhile, the main peak positions of N 1s and B 1s spectra shift to 396.4 and 188.7 eV, showing the binding energy shifts of -2.4 and -1.9 eV, respectively. All the changes in the XPS spectra provide a clear evidence of CO intercalation at the h-BN/Ni interface, in which intercalated CO molecules decouple h-BN overlayer from Ni surface and cause the negative B 1s and N 1s binding energy shifts.54 In-situ NAP-XPS studies confirm that CO molecules can diffuse underneath the h-BN cover and adsorb onto the Ni surface in millibar CO, which is similar to CO intercalation reactions observed on other graphene/metal and h-BN/metal surfaces.20,26,28,29 In typical catalytic reactions the gas pressure is often 1 bar or even higher, which is several orders higher than that required for the CO intercalation. Therefore, it is well expected that CO molecules can diffuse into the h-BN/Ni interfaces and react on the Ni surfaces in the real catalytic reactions.

10


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Figure 5. CO adsorption on Ni surfaces covered by h-BN overlayers. In-situ NAP-XPS a) O 1s, b) N 1s, and c) B 1s spectra from the 1 ML h-BN/Ni(111) model catalyst exposed to CO with various partial pressures at room temperature. d) CO TPD profiles of the Ni/SiO2, Ni3@(h-BN)1/SiO2 and Ni1@(h-BN)3/SiO2-acid nanocatalysts. Temperature-programmed desorption of CO (CO-TPD) was conducted on the Ni/SiO2 and Ni@h-BN/SiO2 catalysts (Figure 5d). Both Ni/SiO2 and Ni3@(h-BN)1/SiO2 catalysts present strong CO desorption peaks, which indicate the feasible CO adsorption on Ni surfaces. However, no TPD peaks were observed on the Ni1@(h-BN)3/SiO2-acid catalyst. The chemisorption of H2 was performed on the Ni/SiO2 and Ni@h-BN/SiO2 catalysts at 40 oC as well. The H2 uptake amounts were 669, 301, and 9 µmol∙g-1Ni for the Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2-acid catalysts, respectively (Table 1). The little H2 adsorption and CO desorption on the Ni1@(h-BN)3/SiO2-acid catalyst can be attributed to the thick h-BN shells which block molecule adsorption on the Ni NPs. In contrast, the Ni3@(h-BN)1/SiO2 catalyst presents significant H2 and CO adsorption, which is due to H2 and CO intercalation at the h-BN/Ni interface of the Ni@h-BN nanocatalysts and similar to the findings on the h-BN/Ni(111) model surface. Based on these results we suggest that reactants including CO and H2 diffuse underneath ultrathin h-BN shells and methanation reactions occur at the h-BN/Ni interfaces (Scheme 1). The intercalation reactions at the h-BN/metal interfaces may rely on the size of the intercalants since the intercalation channels become limited for large molecules such that reactions involving big molecules are inhibited.31 To confirm this hypothesis, a contrast experiment was carried out by performing hydrogenation of 5-hydroxymethylfurfural (HMF) on the three catalysts at 120 oC and 4 MPa (Table 1).55 The two Ni@h-BN/SiO2 catalysts exhibit much lower activity than the pure Ni catalyst. The very low activity of HMF hydrogenation on the Ni3@(h-BN)1/SiO2 catalyst is contrast 11


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with the enhanced methanation reaction on the same catalyst, which again suggest that reactions over the Ni@h-BN catalysts have to be facilitated by intercalation of reactants under the h-BN shells (Scheme 1). Table 1. H2 chemisorption at 40 oC and catalytic activity for CO methanation and HMF hydrogenation reactions over Ni/SiO2, Ni3@(h-BN)1/SiO2, and Ni1@(h-BN)3/SiO2-acid catalysts. H2 Chemisorption

CO Methanationa)

HMF Hydrogenationb)

[µmol·g-1Ni]

Conversion [%]

Conversion [%]

Ni/SiO2

669

79.2

56.8

Ni3@(h-BN)1/SiO2

301

90.8

1.5

Ni1@(h-BN)3/SiO2-acid

9

1.4

0.5

Catalyst

a)

CO methanation reaction condition: 300 oC, 0.1 MPa with WHSV = 1650 ml·g-1Ni·h-1; b)HMF

hydrogenation reaction condition: 120 oC, 4.0 MPa, 5 h.

Scheme 1. Schematic illustrations of syngas methanation and HMF hydrogenation reactions over a) Ni/SiO2, b) Ni3@(h-BN)1/SiO2, and c) Ni1@(h-BN)3/SiO2-acid catalysts. Cyan ball: Ni; grey ball: C; Red ball: O; blue ball: N; brown ball: B; white ball: H; yellow ball: Si. Our results reveal important role of defects in the h-BN shells during the molecule intercalation and methanation reaction. Defects are often present in the 2D material overlayers grown on metal surfaces, which include island edges, domain boundaries, wrinkles, and point defects.56,57 These defects provide the diffusion channels for reactant adsorption underneath the h-BN shells and product desorption from the h-BN/Ni interfaces. The acid-leaching experiment and XPS measurements indicate the presence of defects in the few-layer h-BN shells. Ni NPs covered with the few-layer h-BN shells demonstrate enhanced methanation reaction. In contrast, the 12


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ACS Catalysis

Ni1@(h-BN)3/SiO2-acid catalyst having thick h-BN shell does not adsorb CO and H2 strongly and has quite low methanation activity (Figure 3a and Figures 5d). The further treatment of the Ni1@(h-BN)3/SiO2-acid catalysts in NH3 at 850 oC lowers the activity significantly because of a decrease in the defect density (Figures 3a and Figures S7). All the results substantiate that the defect sites are critical in the molecule intercalation.

3. Conclusions We established a simple synthesis method to prepare Ni@h-BN core-shell nanocatalysts, in which Ni nanoparticles were covered by h-BN shells with controlled thickness. In comparison with the pure Ni NPs, Ni NPs covered with few-layer h-BN shells exhibit enhanced methanation activity, higher resistance to particle sintering, suppressed carbon deposition, and less Ni loss. Surface science measurements on h-BN/Ni(111) model surface and chemisorption results on Ni@h-BN nanocatalysts confirm the feasible CO and H2 adsorption on Ni surfaces covered with ultrathin h-BN overlayers through molecule intercalation at the h-BN/Ni interfaces. The methanation reactions are catalyzed by Ni surfaces under the h-BN covers. We attribute the enhanced activity and stability to the confinement effect of the h-BN shells. These results help us to tailor the catalytic performance of metal nanocatalysts through encapsulation with 2D material shells.

4. Experimental 4.1 Synthesis of Ni/SiO2 and Ni@h-BN/SiO2 catalysts. Ni NPs supported on commercial silica (Qingdao ocean chemical company) were prepared by an impregnation method using Ni(NO3)2∙6H2O as the precursor. The nominal loading of Ni was maintained at 20 wt. %. The fresh samples were dried in an oven at 120 oC overnight, and then calcined at 350 oC for 4 h (donated as NiO/SiO2). The NiO/SiO2 samples were impregnated with 0.1 M boric acid solution at 40 oC. The molecular ratios of Ni to H3BO3 were between 3:1 and 1:3. Then the samples were consecutively treated in H2 (70 mL∙min-1) at 500 oC for 2 h with 3 oC∙min-1 and in NH3 (70 mL∙min-1) at 850 oC for 1 h with 5 oC∙min-1 (denoted as Ni3@(h-BN)1/SiO2 and Ni1@(h-BN)3/SiO2). The NiO/SiO2 catalysts were treated in H2 at 500 oC for 2 h and labeled as Ni/SiO2 catalyst. 4.2

Synthesis

of

Ni3@(h-BN)1,

Ni1@(h-BN)3,

Ni1@(h-BN)3/SiO2-acid,

and

Ni1@(h-BN)3/SiO2-acid-NH3 catalysts. The Ni3@(h-BN)1/SiO2 and Ni1@(h-BN)3/SiO2 catalysts 13


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were soaked in 0.5 M NaOH solution at 40 oC for 1 h, and then washed by deionized water (donated as Ni3@(h-BN)1 and Ni1@(h-BN)3). The Ni1@(h-BN)3/SiO2 catalyst was leached by 0.5 M HNO3 at room temperature overnight, and then washed thoroughly by deionized water (donated as Ni1@(h-BN)3/SiO2-acid). The Ni1@(h-BN)3/SiO2-acid catalysts were further treated in NH3 (70 mL∙min-1) at 850 oC for 1 h with 5 oC∙min-1 (donated as Ni1@(h-BN)3/SiO2-acid-NH3). 4.3 Materials characterization. XRD patterns were collected on a Empyrean diffractometer using a Cu ka (λ = 1.5406 Å) radiation source and scanning rate of 12 oC∙min-1. TEM images were recorded on the Hitachi HT 7700 microscope operated at an acceleration voltage of 100 KV, and HRTEM images were acquired on JEM-2100 microscope operated at an accelerating voltage of 200 KV or FET Tecnai F30 microscope at an accelerating voltage of 300 KV. The sample compositions were analyzed by EDX attached to the scanning electron microscope instrument (Phenom SEM). Raman measurements were undertaken on a LabRam HR 800 instrument with a 532 nm excitation laser. XPS measurements were performed using a Thermo Scientific ESCALAB 250Xi spectrometer using an Al Ka X-ray source and pass energy of 20 eV. The C 1s peak located at 284.5 eV was used to calibrate binding energy positions. NAP-XPS experiments were performed at beamline 9.3.2 of the Advanced Light Source in Lawrence Berkeley National Laboratory with a specially designed photoemission spectrometer which can be operated at near ambient pressure. 4.4 Surface adsorption and desorption. Temperature-programmed dissociation was carried out in a home-made micro-reactor equipped with a mass spectrometer (OMNI STAR). The catalysts were firstly reduced in H2 (50 mL∙min-1) at 500 oC for 2 h and then cooled down to room temperature in He. The reduced catalysts were heated in 5% (v/v) CO-He (40 mL∙min-1) from room temperature to 750 oC with a ramping rate of 5 oC∙min-1. The products were analyzed by an on-line mass spectrometer. The reduced catalysts were exposed to 5% (v/v) CO-He (80 mL∙min-1) at 375 oC for 1 h and then cooled down to room temperature in He. The carbonized catalysts were heated in 10% (v/v) H2-He (30mL∙min-1) from room temperature to 900 oC with a ramping rate of 5 oC∙min-1. The product was analyzed by an on-line mass spectrometer. H2 pulse chemisorption was done using a Micromeritics Chemisorption Analyzer (Auto Chem II 2920). The samples were pretreated in 10% (v/v) H2-Ar (50 mL∙min-1) at 500 oC for 2 h and then changed to He (50 mL∙min-1) to purge the chemisorbed hydrogen on samples at 500 oC (for Ni/SiO2 catalysts) or 700 oC (for Ni@(h-BN)/SiO2 catalysts) for 1 h. After cooling down to room temperature, the samples were exposed to H2 pulses 14


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ACS Catalysis

consisting of 10% (v/v) H2 balanced with Ar (50 mL∙min-1). H2 concentration was measured using a thermal conducting detector (TCD). CO temperature-programmed desorption was carried out on the Auto Chem II 2920. After the pretreatment (the same as H2 chemisorption), the sample was adsorbed by CO at -50 oC for 1 h (50 mL∙min-1) and then purged by He (50 mL∙min-1) at 30 oC for 1 h. After that, the samples were heated in He (30 mL·min-1) from 30 to 700 oC with 5 oC·min-1. 4.5 Syngas methanation reaction. Syngas methanation reactions were carried out in a fixed bed reactor equipped with a stainless steel tube lined with a quartz inner. For a typical test, a certain amount of catalysts (20-40 mesh) were loaded in the tube reactor and reduced in H2 (80 mL∙min-1) at 500 oC for 2 h with a heating rate of 5 oC∙min-1. After cooling down to the starting reaction temperature in pure H2, the gas was switched to syngas (a gas mixture containing H2/CO = 3/1, 5% Ar balanced). WHSV was 1650 L∙g-1Ni∙h-1. The reactions were conducted in the temperature range 300-700 oC with an interval of 50 oC at 0.1 MPa or 1.0 MPa. Lifetime test was carried out at 375 oC, 0.1 MPa. The products and reactants in the gas phase were detected online using an Agilent 6890 GC instrument. Ar, CO, CH4 and CO2 were analyzed using a TDX-01 column with a TCD. CH4 and other hydrocarbons were monitored using a HP-POLOT Q column with a flame ionization detector (FID). 5% Ar in the syngas was used as an internal standard for calculation. 4.6 HMF (5-hydroxymethylfurfural) hydrogenation. The reaction was carried out in a 100 mL tank reactor. Prior to the reaction, the catalysts were reduced in a quartz tube under H2 flow at 500 oC for 2 h and protected by 1.4-dioxane. The reactor was fed with HMF (0.75 g), 1, 4-dioxane (35 mL), and pre-reduced catalysts (0.06 g), and then sealed and purged by H2 for a while. After that, the reactor was filled with 4 MPa H2 and heated to 120 oC maintaining for 5 h. After the reaction, the reactor was quenched in ice-water, and then the liquid and gas products were analyzed by a GC equipped with a FID detector having a capillary column (J&W DB-WAX). The conversion was determined by calibrated area normalization.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 21373208, No. 91545204, No. 21321002, and No. 11227902), and Ministry of Science and Technology of China (No. 2016YFA0200200, No. 2013CB834603 and No. 2013CB933100), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. 15


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XDB17020200). This work was also partially supported by CAS-Shanghai Science Research Center, Grant No: CAS-SSRC-YH-2015-01. The Advanced Light Source and beamlines 9.3.2 are supported by the Director, Office of Science, Office of Basic Energy Sciences, and the Division of Chemical Sciences, Geosciences and Biosciences of the U.S. Department of Energy under contract No. DE-AC02-05CH11231.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM, XRD, XPS data of catalysts, photographs of catalysts, syngas methanation reaction data (PDF)

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References: (1) Bartholomew, C. H. Appl. Catal. A 2001, 212, 17-60. (2) Forzatti, P.; Lietti, L. Catal. Today 1999, 52, 165-181. (3) Moulijn, J. A.; van Diepen, A. E.; Kapteijn, F. Appl. Catal. A 2001, 212, 3-16. (4) Hansen, T. W.; DeLaRiva, A. T.; Challa, S. R.; Datye, A. K. Acc. Chem. Res. 2013, 46, 1720-1730. (5) Wei, S.; Wang, Q.; Zhu, J.; Sun, L.; Lin, H.; Guo, Z. Nanoscale 2011, 3, 4474-4502. (6) De Rogatis, L.; Cargnello, M.; Gombac, V.; Lorenzut, B.; Montini, T.; Fornasiero, P.

ChemSusChem 2010, 3, 24-42. (7) Gawande, M. B.; Goswami, A.; Asefa, T.; Guo, H.; Biradar, A. V.; Peng, D.-L.; Zboril, R.; Varma, R. S. Chem. Soc. Rev. 2015, 44, 7540-7590. (8) Tian, H.; Li, X.; Zeng, L.; Gong, J. ACS Catal. 2015, 5, 4959-4977. (9) Zhang, Q.; Lee, I.; Joo, J. B.; Zaera, F.; Yin, Y. Acc. Chem. Res. 2013, 46, 1816-1824. (10) Li, S.; Boucheron, T.; Tuel, A.; Farrusseng, D.; Meunier, F. Chem. Comm. 2014, 50, 1824-1826. (11) Lu, J.; Fu, B.; Kung, M. C.; Xiao, G.; Elam, J. W.; Kung, H. H.; Stair, P. C. Science 2012, 335, 1205-1208. (12) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Nat. Mater. 2009, 8, 126-131. (13) Ma, Z.; Dai, S. ACS Catal. 2011, 1, 805-818. (14) Fu, T.; Wang, M.; Cai, W.; Cui, Y.; Gao, F.; Peng, L.; Chen, W.; Ding, W. ACS Catal. 2014, 4, 2536-2543. (15) Cui, X.; Li, Y.; Bachmann, S.; Scalone, M.; Surkus, A.-E.; Junge, K.; Topf, C.; Beller, M. J.

Am. Chem. Soc. 2015, 137, 10652-10658. (16) Wu, Z.; Lv, Y.; Xia, Y.; Webley, P. A.; Zhao, D. J. Am. Chem. Soc. 2012, 134, 2236-2245. (17) Edwards, R. S.; Coleman, K. S. Acc. Chem. Res. 2013, 46, 23-30. (18) Geim, A. K.; Grigorieva, I. V. Nature 2013, 499, 419-425. (19) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R.; Liang, L.; Louie, S. G.; Ringe, E.; Zhou, W.; Kim, S. S.; Naik, R. R.; Sumpter, B. G.; Terrones, H.; Xia, F.; Wang, Y.; Zhu, J.; Akinwande, D.; Alem, N.; Schuller, J. A.; 17


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Schaak, R. E.; Terrones, M.; Robinson, J. A. ACS Nano 2015, 9, 11509-11539. (20) Wei, M.; Fu, Q.; Yang, Y.; Wei, W.; Crumlin, E.; Bluhm, H.; Bao, X. J. Phys. Chem. C 2015, 119, 13590-13597. (21) Zhang, H.; Fu, Q.; Cui, Y.; Tan, D.; Bao, X. J. Phys. Chem. C 2009, 113, 8296-8301. (22) Feng, X.; Maier, S.; Salmeron, M. J. Am. Chem. Soc. 2012, 134, 5662-5668. (23) Mu, R.; Fu, Q.; Jin, L.; Yu, L.; Fang, G.; Tan, D.; Bao, X. Angew. Chem. Int. Ed. 2012, 51, 4856-4859. (24)Duong, D. L.; Han, G. H.; Lee, S. M.; Gunes, F.; Kim, E. S.; Kim, S. T.; Kim, H.; Ta, Q. H.; So, K. P.; Yoon, S. J. Nature 2012, 490, 235-239. (25) Politano, A.; Cattelan, M.; Boukhvalov, D. W.; Campi, D.; Cupolillo, A.; Agnoli, S.; Apostol, N. G.; Lacovig, P.; Lizzit, S.; Farías, D. ACS Nano 2016, 10, 4543-4549. (26) Grånäs, E.; Andersen, M.; Arman, M. A.; Gerber, T.; Hammer, B.; Schnadt, J.; Andersen, J. N.; Michely, T.; Knudsen, J. J. Phys. Chem. C 2013, 117, 16438-16447. (27) Schriver, M.; Regan, W.; Gannett, W. J.; Zaniewski, A. M.; Crommie, M. F.; Zettl, A. ACS

Nano 2013, 7, 5763-5768. (28) Yao, Y.; Fu, Q.; Zhang, Y. Y.; Weng, X.; Li, H.; Chen, M.; Jin, L.; Dong, A.; Mu, R.; Jiang, P.; Liu, L.; Bluhm, H.; Liu, Z.; Zhang, S. B.; Bao, X. Proc. Natl. Acad. Sci. USA 2014, 111, 17023-17028. (29) Zhang, Y.; Weng, X.; Li, H.; Li, H.; Wei, M.; Xiao, J.; Liu, Z.; Chen, M.; Fu, Q.; Bao, X.

Nano Lett. 2015, 15, 3616-3623. (30) Zhang, Y.; Fu, Q.; Cui, Y.; Mu, R.; Jin, L.; Bao, X. Phys. Chem. Chem. Phys. 2013, 15, 19042-19048. (31) Fu, Q.; Bao, X. Chin. J. Catal. 2015, 36, 517-519. (32) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Nat Nanotech. 2016, 11, 218-230. (33)Wang, C.; Zhai, P.; Zhang, Z.; Zhou, Y.; Zhang, J.; Zhang, H.; Shi, Z.; Han, R. P. S.; Huang, F.; Ma, D. J. Catal. 2016, 334, 42-51. (34) Liu, Q.; Gao, J.; Zhang, M.; Li, H.; Gu, F.; Xu, G.; Zhong, Z.; Su, F. RSC Adv. 2014, 4, 16094-16103. (35) Gao, J.; Liu, Q.; Gu, F.; Liu, B.; Zhong, Z.; Su, F. RSC Adv. 2015, 5, 22759-22776. 18


Page 18 of 21

Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(36) Besenbacher, F.; Chorkendorff, I.; Clausen, B.; Hammer, B.; Molenbroek, A.; Nørskov, J. K.; Stensgaard, I. Science 1998, 279, 1913-1915. (37) Courson, C.; Makaga, E.; Petit, C.; Kiennemann, A. Catal. Today 2000, 63, 427-437. (38) Mirodatos, C.; Praliaud, H.; Primet, M. J. Catal. 1987, 107, 275-287. (39) Munnik, P.; Velthoen, M. E. Z.; de Jongh, P. E.; de Jong, K. P.; Gommes, C. J. Angew.

Chem. Int. Ed. 2014, 53, 9493-9497. (40) Mihaylov, M.; Hadjiivanov, K.; Knözinger, H. Catal. Lett. 2001, 76, 59-63. (41) Fu, Y.; Wu, Y.; Cai, W.; Yue, B.; He, H. Sci. Chin. Chem. 2015, 58, 148-155. (42) Pan, X.; Bao, X. Acc. Chem. Res. 2011, 44, 553-562. (43) Li, J.; Gao, L. J. Mater. Chem. 2003, 13, 628-630. (44) Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. Adv. Mater. 2009, 21, 2889-2893. (45) Zhang, C.; Fu, L.; Zhao, S.; Zhou, Y.; Peng, H.; Liu, Z. Adv. Mater. 2014, 26, 1776-1781. (46) Suzuki, S.; Molto Pallares, R.; Orofeo, C. M.; Hibino, H. J. Vac. Sci. Technol. B 2013, 31, 041804. (47) Guimon, C.; Gonbeau, D.; Pfister-Guillouzo, G.; Dugne, O.; Guette, A.; Naslain, R.; Lahaye, M. Surf. Interface Anal. 1990, 16, 440-445. (48)Montero, I.; Galán, L.; Osòrio, S. P.; Martínez-Duart, J. M.; Perrière, J. Surf. Interface Anal. 1994, 21, 809-813. (49) Das, M.; Basu, A. K.; Ghatak, S.; Joshi, A. G. J. Eur. Ceram. Soc. 2009, 29, 2129-2134. (50) Park, J. C.; Bang, J. U.; Lee, J.; Ko, C. H.; Song, H. J. Mater. Chem. 2010, 20, 1239-1246. (51) Yang, Y.; Fu, Q.; Li, H.; Wei, M.; Xiao, J.; Wei, W.; Bao, X. ACS Nano 2015, 9, 11589-11598. (52) Starr, D.; Liu, Z.; Hävecker, M.; Knop-Gericke, A.; Bluhm, H. Chem. Soc. Rev. 2013, 42, 5833-5857. (53)Grass, M. E.; Karlsson, P. G.; Aksoy, F.; Lundqvist, M.; Wannberg, B.; Mun, B. S.; Hussain, Z.; Liu, Z. Rev. Sci. Instrum. 2010, 81, 053106. (54) Yang, Y.; Fu, Q.; Wei, M.; Bluhm, H.; Bao, X. Nano Res. 2015, 8, 227-237. (55) Kong, X.; Zhu, Y.; Zheng, H.; Li, X.; Zhu, Y.; Li, Y.-W. ACS Catal. 2015, 5, 5914-5920. (56) Banhart, F.; Kotakoski, J.; Krasheninnikov, A. V. ACS Nano 2011, 5, 26-41. (57) Huang, P. Y.; Ruiz-Vargas, C. S.; van der Zande, A. M.; Whitney, W. S.; Levendorf, M. P.; 19


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Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y.; Park, J.; McEuen, P. L.; Muller, D. A.

Nature 2011, 469, 389-392.

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Enhanced Nickel-Catalyzed Methanation Confined ... - ACS Publications

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