Design and Properties of Confined Nanocatalysts by Atomic Layer

Aug 8, 2017 - Yong Qin received his B.S. degree in Chemistry from Chongqing University in 1996, M.S. degree in Materials Physics and Chemistry from Qi...
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Design and Properties of Confined Nanocatalysts by Atomic Layer Deposition Zhe Gao and Yong Qin* State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan 030001, China CONSPECTUS: Governing the process and outcome of chemical reactions is the most important aim of catalytic chemistry. The confinement of active sites inside nanosized spaces provides a powerful strategy to achieve this goal. Reacting molecules (reactants, intermediates, and products of a reaction) and nanomaterials (metal/metal-oxide nanoparticles) confined inside nanoreactors have been observed to exhibit modified behaviors and properties with respect to their unconfined counterparts. Typically, catalysts confined in zeolites, mesoporous materials, metal−organic frameworks, and nanotubes are obtained by traditional liquid-phase methods. However, excess metals or undesired solvents and other reagents must be removed. It is also difficult to precisely regulate the confined nanostructures and assemble multifunctional sites in the confined nanospaces. Atomic layer deposition (ALD) provides a controllable method to fabricate confined catalysts due to its outstanding advantages. In this Account, we describe our progress in the design and properties of confined nanocatalysts by ALD. ALD is an elegant method to directly deposit highly dispersed metal or oxide species into porous materials, including zeolites and mesoporous materials. We deposited Pt nanoclusters in the micropores of a KL zeolite with precisely controlled size by ALD. We also introduced CoOx nanoclusters into mesoporous SBA-15. We have reported pioneering works on the synthesis of confined nanoparticles with metal-in-nanotube structures by a templateassisted ALD method. Confined Cu nanoparticles were prepared by reducing CuO nanowires coated with Al2O3, TiO2, or alucone layers by ALD. Confined Cu and Au nanoparticles were also prepared starting from the corresponding metal nanowires with the assistance of sacrificial layers produced by ALD. In a more facile strategy, Au nanoparticles confined in Al2O3 nanotubes were produced using a sacrificial template by ALD. Furthermore, we synthesized a multiply confined Ni-based nanocatalyst through a template-assisted ALD method. We assembled multiple interfaces (Ni/Al2O3 and Pt/TiO2) in a confined nanospace for tandem reactions by template-assisted ALD. The synergistic effect of two interfaces enhanced the tandem reaction, and the confined nanospace favored the instant transfer of intermediates between the two interfaces. In addition, porous TiO2 nanotubes with spatially separated Pt and CoOx cocatalysts were also produced by ALD. The confined catalysts can be further treated by ALD. We used ALD to modify the mesoporous SBA-15 support to precisely tune the active species−support interaction. In addition to the support, the confined metal nanoparticles can also be coated with an ultrathin oxide layer by ALD to further improve their catalytic activities. Moreover, the structure and size of the confined nanospace can be tuned precisely by ALD. Overall, ALD has exhibited noteworthy applications in and will provide new opportunities for the design and synthesis of highly effective confined catalysts.

1. INTRODUCTION

The physicochemical properties of reacting molecules can be significantly modified through the confinement effects. Notable effects on the adsorption energies of molecules confined in porous materials, including zeolites, mesoporous materials, and metal−organic framework (MOF) materials, have been observed.2,3 Local concentration enrichment effects inside carbon nanotubes (CNTs) have been reported because of their unique adsorption behavior. 4−6 Locally increased concentrations can enhance chemical reactions, and selective enrichment of one reactant over another can allow for the modulation of product selectivity.5 The contact time of

Catalysis is an important field in chemistry. Governing the process and outcome of chemical reactions has been the most important aim of catalytic chemistry. Confining the active catalyst inside a nanospace provides a powerful strategy for achieving this goal. Reacting molecules and metal/metal-oxide nanoparticles confined inside nanospaces have been observed to exhibit modified behaviors and properties with respect to their unconfined counterparts. Despite the multitude of factors that can affect the reactions, we can classify them into two main groups: the first type of effect acts on the reacting molecules, and the second acts on the confined metal/metal-oxide nanoparticles.1 © 2017 American Chemical Society

Received: May 25, 2017 Published: August 8, 2017 2309

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Accounts of Chemical Research reactants and reaction intermediates with the active sites can be prolonged within a nanospace, leading to better catalytic activities.7,8 Superior diffusion properties of organic gases, water, and other molecules in confined spaces have been reported.9,10 In addition to the confined molecules, the physicochemical properties of the metal or metal-oxide nanoparticles confined in a nanospace can also be modified by the confinement effect. The spatial restriction of metal nanoparticles within a nanosized space can inhibit their sintering, aggregation, detachment, and poisoning, thus prolonging activity.11−13 Electronic interactions exist between the nanoparticles and CNT surfaces because of the unique electronic microenvironments within the CNTs that modulate electron transfer processes.14 The modified electronic structures of the nanoparticles induced by confinement can influence their catalytic performances, as redox reactions involve electron transfer between reactants and catalysts. Nanocatalysts confined in oxide nanotubes also exhibit electronic interactions between the nanoparticles and oxide nanotubes.15,16 Most often, confined catalysts are obtained by the incorporation of active nanoparticles into the confined space through traditional liquid-phase methods, including impregnation, self-assembly, and hydrothermal synthesis.12,14,15 However, through traditional preparation methods, additional steps are needed to remove the excess metals or undesired solvents and other reagents. For nanotube confined catalysts, there are very few options for shell materials, and the thickness of the shell material as well as the size and composition of the confined nanoparticle are hard to regulate precisely. In addition, the introduction and assembly of multifunctional active sites in a confined nanospace remains difficult. Therefore, advanced technologies for the preparation of confined catalysts are needed to solve these challenges.

precursors and byproducts are removed by a purge step. The sequence of surface reactions and purges constitute an ALD cycle. By repeating this process, films can be obtained. In addition to continuous films, uniformly dispersed metal or metal-oxide nanoparticles can also be obtained by ALD, relying on the distribution of surface functional groups and wetting properties of deposited materials.18−21 Because of its outstanding advantages, including precise thickness control at the atomic level, excellent uniformity and step coverage, and extraordinarily good reproducibility, ALD has emerged as a powerful tool for the atomically precise design and synthesis of catalysts.22−28 It has been utilized to synthesize efficient catalysts with improved activity, selectivity, and stability. In addition, a myriad of catalytic structures can be created by ALD. Recently, ALD has been used to obtain a wide range of confined catalysts with enhanced catalytic performance. The confined catalysts prepared by ALD mainly include catalysts confined in nanoporous materials, such as zeolites, mesoporous materials, MOFs, and so forth, as well as catalysts confined in nanotubes. This Account describes our recent work on confined catalysts designed by ALD and on the investigation of their catalytic properties. Two synthetic routes are summarized to fabricate confined catalysts by ALD. First, it is feasible to deposit materials onto various shapes of substrates by ALD, including flat substrates, nanowires, nanoparticles, and even on high-aspect-ratio substrates, such as porous materials.29 Thus, ALD can be used to directly incorporate highly dispersed catalytic nanoparticles into confined nanospaces. Second, we propose a facile and general template-assisted ALD method to obtain confined metal-in-nanotube nanostructures, since the precise thickness control combined with the high step coverage allows us to introduce ALD as an attractive approach for the development of complex nanostructured materials.30,31

2. ATOMIC LAYER DEPOSITION Atomic layer deposition (ALD) is a powerful thin-film or nanoparticle deposition technique similar to chemical vapor deposition, except that the substrate is exposed separately to each gaseous precursor and that the deposition is broken into cycles.17 As shown in Figure 1, ALD is based on a reaction between two or more gaseous precursors, which are pulsed alternately to avoid the presence of gas phase reactions. In this manner, the reactants are kept separated and react with surface functional groups. The process is self-limiting. Unreacted

3. CONFINED CATALYSTS BY ALD 3.1. Catalysts Confined in Porous Materials by ALD

Confining metal nanoparticles into porous materials could provide many advantages, such as high dispersion, improved stability due to metal−support interactions and spatial confinement effects of the nanopores. It is, therefore, highly attractive to develop a tool for uniform and controlled deposition of metal nanoparticles into the channels of porous systems. ALD is an excellent method for fabricating catalysts confined in porous materials through the first synthetic route described above, i.e., directly depositing highly dispersed metal or oxide species into porous materials. Compared with the traditional liquid-phase methods, extra steps required to remove excess metals or undesired solvents and other reagents are not necessary with ALD. For microporous materials with large aspect ratios and small pore sizes, such as zeolites, it is challenging to deposit ALD species uniformly into the porous materials due to the diffusion limitations of precursor molecules. The most commonly reported ALD species deposited inside micropores are alumina or titania. For example, Vandegehuchte et al. reported that ALD Al2O3 could be introduced into the small pores of zeolites to modify their properties.32 We successfully deposited Pt nanoclusters with precisely controlled size (0.8 nm) and high dispersion into the micropores of KL zeolites by ALD (Figure 2).33 N2 sorption isotherms, infrared spectra of adsorbed CO, and density functional theory (DFT) calculations clearly

Figure 1. Schematic representation of an ALD cycle using TiCl4 and H2O as precursors to deposit TiO2. 2310

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dispersion. These catalysts exhibited good stability, which was due to the confinement of the SBA-15 channels. MOFs have been utilized as supports for metal nanoparticles since they provide powerful confinement. Compared to other porous supports, the structures of MOFs, comprising organic linkers and inorganic nodes, are more readily tuned by rational ligand design, as evidenced by their wide applications in catalysis. ALD in MOFs (AIM) offers new routes to tune their surface chemistry for selective guest sorption and to produce designed catalysts. The key conceptual advancement of AIM is that the precursor molecules deposit only at chemically reactive surface sites, and these reactions are self-limiting. Hupp and coworkers fabricated a Zr-based NU-1000 MOF with large 1D hexagonal channels (∼3.1 nm) and successfully realized the ALD-based incorporation of acidic Al3+ and Zn2+ sites, catalytically active cobalt sulfide and Ni and Co ions.37

Figure 2. Schematic illustration of the Pt ALD mechanism on the KL zeolite. Reproduced with permission from ref 33. Copyright 2017 Royal Society of Chemistry.

3.2. Catalysts Confined in Nanotubes by ALD

showed that most of Pt resided in the KL channels, while a small part of Pt was located at the pore openings or the outer surface. The confinement effects inside the micropores of the KL zeolites prevented the agglomeration of the Pt nanoclusters and lead to an electron-enriched state of Pt. For n-heptane reforming reaction, the produced Pt/KL catalyst exhibited high toluene selectivity (67.3%), low methane selectivity (0.9%), and high stability under reaction conditions. For mesoporous supports possessing large channels (>2 nm), metal nanoparticles, such as Ir and Ni nanoparticles, have been successfully deposited into the mesopores by optimizing the ALD process.34,35 We introduced CoOx into mesoporous SBA-15 by ALD (CoOx/SBA-15) recently for the epoxidation of styrene (Figure 3a).36 The presence of CoOx species in

As mentioned in section 1, CNT or oxide-nanotube confined nanocatalysts exhibit impressive catalytic performances due to confinement effects. A wide variety of fabrication methods have been used to incorporate metal/oxide nanoparticles into nanotubes. However, there are few options for shell materials, and the thickness of the shell material as well as the size and composition of the confined nanoparticle are hard to regulate precisely with traditional methods. We have performed pioneering and exploratory works on the synthesis of confined catalysts with metal-in-nanotube structures using ALD. Using the second synthetic route described above, i.e., the templateassisted ALD method, the catalysts with metal-in-nanotube nanostructures can be obtained conveniently. This facile and general ALD method can be easily extended to other confined nanoreactors, and the confined nanostructures can be regulated precisely. First, we successfully synthesized the metal-in-nanotube nanostructures using the template-assisted ALD method. We reported the generation of Cu nanoparticles confined in Al2O3 nanotubes by the reduction of CuO nanowires coated with Al2O3 shells, which were deposited by ALD.38 Here, the CuO nanowires served as self-templates. The formation mechanism was ascribed to Rayleigh instability together with the uniform volume shrinkage created by reduction. By replacing the Al2O3 shell with a porous Al2O3 layer, Cu nanoparticles confined in porous Al2O3 nanotubes were also produced.39 We also demonstrated a general assembly method for the synthesis of various metal nanoparticles confined in nanotubes.40 Two different approaches are shown in Figure 4. The first approach starts with metal nanowires, which are also considered selftemplates (Figure 4a). The wires are coated first with a sacrificial layer and then with a shell layer of different materials by ALD. Then, the sacrificial layer is etched away. Subsequent annealing produces the confined nanoparticles. The second approach is suitable to produce nanoparticles confined in aligned nanotubes (Figure 4b). At first, shell and sacrificial layers are deposited onto the pore walls of anodic alumina (AAO) templates by ALD. Subsequently, metal nanowires are grown inside the pores by electrodeposition. Confined nanoparticles are obtained by annealing after the simultaneous removal of the sacrificial layers and templates. This method is quite general not only for different metals but also for different tube-shell materials given the availability of a wide range of materials that can be deposited by ALD. A more facile strategy is to prepare confined nanoparticles starting from a sacrificial

Figure 3. Schematic diagram of the preparation process for the (a) CoOx/SBA-15 and (b) CoOx/TiO2/SBA-15 catalysts. Reproduced with permission from ref 36. Copyright 2017 Royal Society of Chemistry.

CoOx/SBA-15 was confirmed by inductively coupled plasma atomic emission spectrometry, elemental mapping, and X-ray photoelectron spectroscopy, although these species could not be clearly determined by transmission electron microscopy (TEM) and X-ray diffraction analyses due to the low content and small size. It was concluded that highly dispersed CoOx nanoclusters were produced by ALD. For comparison, the catalysts prepared by incipient wetness impregnation (IWI) were also obtained (CoOx/SBA-15-IWI). The conversion and styrene oxide selectivity after reaction for 6 h were 88.0% and 61.0% for the catalyst with 35 ALD CoOx cycles (35CoOx/ SBA-15), and 65.7% and 47.8% for CoOx/SBA-15-IWI, respectively. Although these two catalysts possessed the same Co content (0.98%), 35CoOx/SBA-15 exhibited better performance than CoOx/SBA-15-IWI, because of its higher 2311

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and an amorphous Al2O3 film were deposited onto CNCs. Then, calcination and reduction treatments produced Ni-inANTs. For the synthesis of Ni-out-ANTs, the deposition sequence of the NiO nanoparticles and Al2O3 film were exchanged. None of the Ni nanoparticles of Ni-in-ANTs were located out of Al2O3 nanotubes (Figure 6). For Ni-in-ANTs, Ni

Figure 4. Schematic of two fabrication approaches for confined nanoparticles. Reproduced with permission from ref 40. Copyright 2008 American Chemical Society. Figure 6. TEM images of (A, B) Ni-in-ANTs and (C) Ni-out-ANTs. (D) Evolution of the cinnamaldehyde conversion with reaction time. (E) Recycling results for the catalysts in the hydrogenation of cinnamaldehyde. Adapted with permission from ref 42. Copyright 2015 Wiley-VCH.

template. For example, Au nanoparticles confined in Al2O3 nanotubes were prepared by this method via annealing carbon nanocoil (CNC) templates coated with sputtered Au films and further deposited Al2O3 by ALD in air.41 The confined nanoparticles mentioned above are too large to be used as efficient catalysts because they originated from nanowires or sputtered films. Based on the template-assisted method, highly efficient confined nanocatalysts were synthesized using ALD to directly produce nanoparticles less than 10 nm in size. For example, we synthesized a multiply confined Nibased nanocatalyst (Ni-in-ANTs) and an unconfined Ni nanocatalyst (Ni-out-ANTs) using CNCs as sacrificial templates (Figure 5).42 For Ni-in-ANTs, NiO nanoparticles

nanoparticles were not only confined in Al2O3 nanotubes but also embedded in the cavities of the Al2O3 interior walls to maximize the metal−support interfaces. Ni-in-ANTs showed greatly improved activity and stability for the hydrogenations of cinnamaldehyde and nitrobenzene compared with Ni-outANTs. The turnover frequency (TOF) values for cinnamaldehyde hydrogenation at an initial time of 60 min were 0.42 and 0.08 s−1 for Ni-in-ANTs and Ni-out-ANTs, respectively. After the catalysts were reused four times, Ni-in-ANTs was relatively stable. However, the conversion for Ni-out-ANTs decreased sharply. The greatly improved performance of Ni-in-ANTs was ascribed to the multiple confinement effects. First, its increased interfacial sites enhanced the hydrogen spillover effect, which exerted a great influence on the hydrogenation activity. Second, the tubular channel structure inhibited the leaching and detachment of the Ni nanoparticles of Ni-in-ANTs. Our template-assisted ALD method is also suitable for other templates, such as CNTs, ZnO nanowires, and so forth. Most importantly, it can be easily extended to other confined nanoreactors because of the wide variety of available materials that can be produced by ALD, such as shell materials (TiO2, SiO2, etc.) and metals (Pt, Pd, Ru, Fe, Co, Cu, etc.). 3.3. Nanotube Confined Catalysts with Multiple Interfaces

Thanks to the excellent advantages of ALD for preparation of complex nanostructures, based on the work of previous studies, we developed a new concept for the introduction and assembly of multiple interfaces in confined nanospaces at the atomic level. With traditional preparation methods, it is hard to precisely control the microstructure of multiple interfaces.

Figure 5. Preparation processes of the multiply confined Ni nanocatalyst (Ni-in-ANTs) and unconfined Ni nanocatalyst (Ni-outANTs). Reproduced with permission from ref 42. Copyright 2015 Wiley-VCH. 2312

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catalysts with one interface (Al−Pt/Ti and Al/Ni−Ti), Nimodified Al−Pt/Ti (Al−(mNi)Pt/Ti), Pt-modified Al/Ni−Ti (Al/Ni(nPt)−Ti) (m and n are the ALD cycle numbers of Ni and Pt, respectively), a physical mixture of the confined catalysts with single interfaces (Al/Ni−Ti + Al−Pt/Ti), and a physical mixture of the unconfined catalysts with single interfaces (Ni/Al + Pt/Ti) were also prepared. The catalytic activities of these catalysts for the nitrobenzene hydrogenation using N2H4·H2O as a hydrogen source were much lower than that of Al/Ni−Pt/Ti (Figure 8C). Two reasons contributed to the high efficiency of the confined tandem catalyst. First, the confined nanospace favored the instant transfer of intermediates between the two interfaces. Second, the synergistic effect of the two interfaces facilitated the tandem reaction. Recently, we prepared a novel CoOx/TiO2/Pt photocatalyst also by template-assisted ALD for efficient photocatalytic hydrogen production (Figure 9).44 First, Pt nanoclusters and

We synthesized a confined tandem catalyst with multiple interfaces (Ni/Al2O3 and Pt/TiO2) using template-assisted ALD (Figure 7).43 First, an Al2O3 layer and NiO nanoparticles

Figure 7. Schematic illustration of the synthesis procedure of the confined tandem catalyst with both Ni/Al2O3 and Pt/TiO2 interfaces (Al/Ni−Pt/Ti) and semisectional views of the confined catalysts with single interfaces (Al/Ni−Ti and Al−Pt/Ti). Reproduced with permission from ref 43. Copyright 2016 Wiley-VCH.

were deposited on a CNC sacrificial template. Second, a sacrificial polyimide layer was coated. Finally, Pt nanoparticles and a TiO2 layer were deposited. After calcination and reduction treatments, the confined tandem catalyst (Al/Ni− Pt/Ti) was obtained. High-angle annular dark field scanning TEM (HAADF-STEM) image of the Al/Ni−Pt/Ti catalyst (Figure 8A) provided a clear contrast of the inner Al2O3

Figure 9. Synthetic processes for the porous TiO2 nanotube confined Pt photocatalyst (TiO2/Pt) and porous TiO2 nanotubes with spatially separated Pt and CoOx cocatalysts (CoOx/TiO2/Pt). Adapted with permission from ref 44. Copyright 2017 Wiley-VCH.

a TiO2 film were deposited onto a CNC template. Then, the template was removed by calcination, obtaining a TiO2/Pt photocatalyst with a porous hollow structure. Finally, CoOx nanoclusters were deposited onto TiO2/Pt, producing a CoOx/ TiO2/Pt photocatalyst. The Pt nanoclusters were confined in the inner surface of TiO2 nanotubes, acting as electron collectors and active sites for the reduction reaction, while the CoOx nanoclusters were located on the outer surface of TiO2 nanotubes, acting as hole collectors and active sites for the oxidation reaction. With the ultralow contents of noble Pt (0.046 wt %) and CoOx (0.019 wt %) deposited by one ALD cycle each, the CoOx/TiO2/Pt photocatalyst exhibited a remarkably high hydrogen production rate of 275.9 μmol h−1, which was almost five times higher than that of the bare porous TiO2 nanotubes (56.5 μmol h−1) and was clearly higher than those of the porous TiO2 nanotubes decorated with the CoOx cocatalyst (CoOx/TiO2, 7.6 μmol h−1) and the monometallic TiO2/Pt photocatalyst (185.9 μmol h−1) (Figure 10). In addition, the hydrogen production rates for the Pt−CoOx dual cocatalysts distributed either on the inner (TiO2/Pt−CoOx) or on the outer surfaces (Pt−CoOx/TiO2) of the nanotubes were determined to be 131.7 and 73.7 μmol h−1, respectively. The high dispersion of Pt and CoOx nanoclusters, the large specific surface area of the porous TiO2 nanotubes, and the synergetic effect of the Pt−CoOx dual cocatalysts contributed to the excellent photocatalytic activity of CoOx/TiO2/Pt.

Figure 8. (A) HAADF-STEM image of Al/Ni−Pt/Ti. (B) EDX elemental mapping for the boxed area in (A). (C) Catalytic activity of different catalysts for nitrobenzene hydrogenation using N2H4·H2O as a hydrogen source. Adapted with permission from ref 43. Copyright 2016 Wiley-VCH.

nanotube, the Ni nanoparticles supported homogeneously on the surface of the Al2O3 nanotube, the void space, and the TiO2 nanotube. Elemental mapping (Figure 8B) using energydispersive X-ray (EDX) spectroscopy clearly revealed the existence of Al, Ni, Pt, and Ti. Ni nanoparticles were dispersed on the outer surface of the Al2O3 nanotubes (Ni/Al2O3 interface), and Pt nanoparticles were confined in the TiO2 nanotubes (Pt/TiO2 interface). For comparison, the confined 2313

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Figure 11. Structure diagrams of (A) Pt nanoparticles confined in Al2O3 nanotubes (Pt-in-ANTs) and (B) confined Pt nanoparticles further coated with an ultrathin Al2O3 layer (xAl−Pt-in-ANTs). (C) The conversions of 4-nitrophenol over the reaction time for Pt-inANTs and xAl−Pt-in-ANTs. Adapted with permission from ref 46. Copyright 2016 Wiley-VCH.

Figure 10. Photocatalytic hydrogen evolution over different photocatalysts. Reproduced with permission from ref 44. Copyright 2017 Wiley-VCH.

4. FURTHER MODIFICATION OF CONFINED CATALYSTS AND TUNING OF CONFINED SPACE BY ALD In terms of maximizing the metal−support interactions, if the enhanced performance of the confined catalyst is caused only by the effect of spatial restriction, this confined structure is not the most ideal. The confined catalysts can be further treated by ALD. For this purpose, the modification of confined catalysts and tuning of confined space by ALD have been explored. The porous supports can be further modified by ALD.45 We used ALD to modify mesoporous supports through the first route (deposition of ALD species into the porous materials directly) to tune the active species−support interaction precisely. As mentioned above, we prepared CoOx/SBA-15 by directly introducing CoOx species into SBA-15 by ALD, which exhibited high catalytic performance for styrene epoxidation. Before the CoOx ALD process, TiO2 was coated on SBA-15 with different ALD cycles, and then, CoOx species were introduced, obtaining a CoOx/TiO2/SBA-15 catalyst (Figure 3b). The catalyst modified with five TiO2 ALD cycles (35CoOx/5TiO2/SBA-15) exhibited higher selectivity for the epoxide (67.5%) than the unmodified catalyst (35CoOx/SBA15, 61.0% selectivity). Moreover, 35CoOx/5TiO2/SBA-15 also displayed higher stability. This improvement of catalytic performance was due to the appropriate interaction between CoOx and the support, which was modified precisely by the ALD TiO2 modification.36 ALD ultrathin coating has been demonstrated to be a powerful tool for the modification of metal nanoparticles.23 The metal nanoparticles confined in nanotubes can also be coated with an ultrathin oxide layer by ALD (Figure 11).46 This was achieved by sequential deposition of an ultrathin Al2O3 coating, Pt nanoparticles, and a thick Al2O3 film on a CNC template by ALD, followed by the removal of the sacrificial template. For the first few cycles of ALD, the Al2 O 3 coating was discontinuous on the metal nanoparticles, and thus, more Pt−Al2O3 interfacial sites were obtained. The confined catalysts

(Pt-in-ANTs) and the ultrathin-Al2O3-coated catalysts (xAl− Pt-in-ANTs, x = number of ALD cycles for Al2O3) were evaluated for their catalytic performances in the hydrogenation of 4-nitrophenol. The ultrathin-coated catalysts with Al2O3 coatings in the range of 1−2 cycles showed higher activities than that of Pt-in-ANTs. 2Al−Pt-in-ANTs exhibited the highest activity, proving the beneficial effect of maximizing the interfacial sites. The TOF values are 147.2 and 18.3 s−1 for 2Al−Pt-in-ANTs and Pt-in-ANTs, respectively. Upon increasing the number of Al2O3 cycles (3−5), the catalysts exhibited even lower activities than Pt-in-ANTs because most of the surfaces of the metal particles were covered by Al2O3, and it was difficult for the reactants to access the active surface. Furthermore, the size and shape of the confined nanospace can also be precisely tuned depending on the advantages of ALD. For porous materials, such as AAO and mesoporous materials, their pore sizes can be straightforward tuned by depositing a film onto their inner walls to a demanded size by ALD.47 If the confined metal-in-nanotube structures are produced starting from a nanowire template, it is feasible to tune the confined nanospace with the assistance of a sacrificial layer deposited by ALD, as shown in Figure 4. The confined space corresponding to the initial sacrificial layers can be easily controlled by the number of ALD cycles.40 The size of the metal particles obtained here is determined by the diameter of starting metal nanowires. If too large, the confined particles may not be suitable as efficient catalysts. However, this template method assisted with a sacrificial layer provides an efficient way to precisely tune the structure of confined nanospace. Another example is to create a crescent-shaped nanospace by the controlled removal of a ZnO sacrificial layer, which was deposited by ALD onto the pore walls of an AAO template. The size and structure of the nanospace rely on the thickness of the initial sacrificial ZnO layer and can thus be conveniently controlled by varying the number of ZnO ALD cycles.31 We can choose different types of sacrificial materials 2314

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Accounts of Chemical Research for the creation of nanospace, such as ZnO, Al2O3, and polyimide, depending on different metal and nanotube materials. ZnO and Al2O3 deposited by ALD can be selectively removed by wet-chemical etching. For oxidation-resistant metals like Pt, a polymer film, such as polyimide, can be used as a sacrificial layer.

structured materials by atomic layer deposition and their applications in heterogeneous catalysis. Yong Qin received his B.S. degree in Chemistry from Chongqing University in 1996, M.S. degree in Materials Physics and Chemistry from Qingdao University of Science and Technology in 2001, and PhD degree in Environmental Science from Ocean University of China in 2005. In July 2004, he joined Prof. Xin Jiang’s group as a Research Associate at University of Siegen, Germany. In April 2007, he transitioned to Max Planck Institute of Microstructure Physics − Halle (MPI-Halle), Germany, working as a Postdoctoral Research Associate with Prof. Ulrich Goesele and Prof. Mato Knez. In November 2011, he moved to Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, China and set up a new research group supported by the “Hundred Talents Program” of the Chinese Academy of Science. His research interests mainly include atomic layer deposition, heterogeneous catalysis, and nanomaterials.

5. CONCLUSIONS AND PROSPECTS Confined nanocatalysts tend to show unique catalytic properties. ALD provides a promising method to precisely design and controllably synthesize confined catalysts. Highly dispersed catalytic nanoparticles can be confined into nanoporous materials by ALD. ALD is also a powerful tool to fabricate confined nanostructures with the assistance of sacrificial templates. Furthermore, ALD is also an excellent method to precisely modify nanoporous supports and confined metal nanoparticles at the atomic level. In recent years, nonflat surfaces have also emerged as candidates for supports to design partially confined catalysts using ALD.27,28 Additionally, ALD can be used to synthesize other types of confined catalysts. For example, Sun et al. developed confined Pt-based catalysts with enhanced electrochemical performances by the locationselective deposition of metal oxide using ALD.48,49 The obtained confined catalysts by ALD have shown considerably improved activities, selectivities, and even changed reaction pathways for tandem catalysis. However, it should be mentioned that there are still some challenges. It is challenging to deposit ALD species uniformly into the porous materials possessing too large aspect ratios and small pore sizes, such as microporous zeolites, due to the diffusion limitation of precursor molecules. The deposition depths of ALD metal/ metal-oxide species into porous materials are limited. Moreover, the related characterizations of confined nanocatalysts are also more difficult and critical due to obstruction of the porous materials or the shell materials of nanotubes compared with unconfined nanocatalysts. In addition, the theoretical understanding of the relationships between the confinement effects and the nanospace structures needs to be further investigated (e.g., by DFT). Thanks to the advantages of ALD, it is possible to easily control the structure of a confined nanospace at the atomic level by the number of ALD cycles. Overall, ALD has exhibited noteworthy applications in and will provide new opportunities for the design and synthesis of highly effective confined catalysts.

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ACKNOWLEDGMENTS We appreciate the financial support of National Natural Science Foundation of China (21403272 and 21673269). REFERENCES

(1) Miners, S. A.; Rance, G. A.; Khlobystov, A. N. Chemical reactions confined within carbon nanotubes. Chem. Soc. Rev. 2016, 45, 4727− 4746. (2) Canivet, J.; Fateeva, A.; Guo, Y.; Coasne, B.; Farrusseng, D. Water adsorption in MOFs: fundamentals and applications. Chem. Soc. Rev. 2014, 43, 5594−5617. (3) Gounder, R.; Iglesia, E. The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions. Chem. Commun. 2013, 49, 3491−3509. (4) Guan, J.; Pan, X.; Liu, X.; Bao, X. Syngas Segregation Induced by Confinement in Carbon Nanotubes: A Combined First-Principles and Monte Carlo Study. J. Phys. Chem. C 2009, 113, 21687−21692. (5) Zhang, H.; Pan, X.; Han, X.; Liu, X.; Wang, X.; Shen, W.; Bao, X. Enhancing chemical reactions in a confined hydrophobic environment: an NMR study of benzene hydroxylation in carbon nanotubes. Chem. Sci. 2013, 4, 1075−1078. (6) Chen, Z.; Guan, Z.; Li, M.; Yang, Q.; Li, C. Enhancement of the Performance of a Platinum Nanocatalyst Confined within Carbon Nanotubes for Asymmetric Hydrogenation. Angew. Chem., Int. Ed. 2011, 50, 4913−4917. (7) Chen, W.; Fan, Z. L.; Pan, X. L.; Bao, X. H. Effect of confinement in carbon nanotubes on the activity of Fischer−Tropsch iron catalyst. J. Am. Chem. Soc. 2008, 130, 9414−9419. (8) Yue, H. R.; Zhao, Y. J.; Zhao, S.; Wang, B.; Ma, X. B.; Gong, J. L. A copper-phyllosilicate core-sheath nanoreactor for carbon-oxygen hydrogenolysis reactions. Nat. Commun. 2013, 4, 2339. (9) Stallmach, F.; Gröger, S.; Künzel, V.; Kärger, J.; Yaghi, O. M.; Hesse, M.; Müller, U. NMR Studies on the Diffusion of Hydrocarbons on the Metal-Organic Framework Material MOF-5. Angew. Chem., Int. Ed. 2006, 45, 2123−2126. (10) Liu, X.; Pan, X.; Zhang, S.; Han, X.; Bao, X. Diffusion of Water Inside Carbon Nanotubes Studied by Pulsed Field Gradient NMR Spectroscopy. Langmuir 2014, 30, 8036−8045. (11) Liu, L.; Diaz, U.; Arenal, R.; Agostini, G.; Concepcion, P.; Corma, A. Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D. Nat. Mater. 2017, 16, 132−138. (12) Otto, T.; Zones, S. I.; Iglesia, E. Challenges and strategies in the encapsulation and stabilization of monodisperse Au clusters within zeolites. J. Catal. 2016, 339, 195−208. (13) Wang, P.; Wang, H.; Chen, X.; Liu, Y.; Weng, X.; Wu, Z. Novel SCR catalyst with superior alkaline resistance performance: enhanced self-protection originated from modifying protonated titanate nanotubes. J. Mater. Chem. A 2015, 3, 680−690.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Qin: 0000-0002-5567-1464 Notes

The authors declare no competing financial interest. Biographies Zhe Gao received his B.S. degree in Material Science and Engineering from Tianjin University in 2006. After he obtained his PhD degree in Material Physics and Chemistry from Shanghai Institute of Ceramics, Chinese Academy of Sciences, he joined the Institute of Coal Chemistry, Chinese Academy of Sciences in 2011. He is currently an Associate Professor working primarily on the synthesis of nano2315

DOI: 10.1021/acs.accounts.7b00266 Acc. Chem. Res. 2017, 50, 2309−2316

Article

Accounts of Chemical Research (14) Pan, X. L.; Bao, X. H. The Effects of Confinement inside Carbon Nanotubes on Catalysis. Acc. Chem. Res. 2011, 44, 553−562. (15) Yang, X.; Yu, X.; Long, L. Z.; Wang, T. J.; Ma, L. L.; Wu, L. P.; Bai, Y.; Li, X. J.; Liao, S. J. Pt nanoparticles entrapped in titanate nanotubes (TNT) for phenol hydrogenation: the confinement effect of TNT. Chem. Commun. 2014, 50, 2794−2796. (16) Pappas, D. K.; Boningari, T.; Boolchand, P.; Smirniotis, P. G. Novel manganese oxide confined interweaved titania nanotubes for the low-temperature Selective Catalytic Reduction (SCR) of NOx by NH3. J. Catal. 2016, 334, 1−13. (17) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (18) Wang, G.; Ran, G.; Wan, G.; Yang, P.; Gao, Z.; Lin, S.; Fu, C.; Qin, Y. Size-Selective Catalytic Growth of Nearly 100% Pure Carbon Nanocoils with Copper Nanoparticles Produced by Atomic Layer Deposition. ACS Nano 2014, 8, 5330−5338. (19) Yang, P.; Tong, X.; Wang, G.; Gao, Z.; Guo, X.; Qin, Y. NiO/ SiC Nanocomposite Prepared by Atomic Layer Deposition Used as a Novel Electrocatalyst for Nonenzymatic Glucose Sensing. ACS Appl. Mater. Interfaces 2015, 7, 4772−4777. (20) Zhang, J.; Chen, C.; Chen, S.; Hu, Q.; Gao, Z.; Li, Y.; Qin, Y. Highly dispersed Pt nanoparticles supported on carbon nanotubes produced by atomic layer deposition for hydrogen generation from hydrolysis of ammonia borane. Catal. Sci. Technol. 2017, 7, 322−329. (21) Dasgupta, N. P.; Meng, X.; Elam, J. W.; Martinson, A. B. F. Atomic Layer Deposition of Metal Sulfide Materials. Acc. Chem. Res. 2015, 48, 341−348. (22) Lu, J.; Elam, J. W.; Stair, P. C. Synthesis and Stabilization of Supported Metal Catalysts by Atomic Layer Deposition. Acc. Chem. Res. 2013, 46, 1806−1815. (23) O’Neill, B. J.; Jackson, D. H. K.; Lee, J.; Canlas, C.; Stair, P. C.; Marshall, C. L.; Elam, J. W.; Kuech, T. F.; Dumesic, J. A.; Huber, G. W. Catalyst Design with Atomic Layer Deposition. ACS Catal. 2015, 5, 1804−1825. (24) Wegener, S. L.; Marks, T. J.; Stair, P. C. Design Strategies for the Molecular Level Synthesis of Supported Catalysts. Acc. Chem. Res. 2012, 45, 206−214. (25) Lu, J. L.; Fu, B. S.; Kung, M. C.; Xiao, G. M.; Elam, J. W.; Kung, H. H.; Stair, P. C. Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition. Science 2012, 335, 1205−1208. (26) Cheng, N.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, B.; Li, R.; Sham, T.-K.; Liu, L.-M.; Botton, G. A.; Sun, X. Platinum single-atom and cluster catalysis of the hydrogen evolution reaction. Nat. Commun. 2016, 7, 13638. (27) Singh, R.; Bapat, R.; Qin, L.; Feng, H.; Polshettiwar, V. Atomic Layer Deposited (ALD) TiO2 on Fibrous Nano-Silica (KCC-1) for Photocatalysis: Nanoparticle Formation and Size Quantization Effect. ACS Catal. 2016, 6, 2770−2784. (28) Bayal, N.; Singh, R.; Polshettiwar, V. Nanostructured Silica− Titania Hybrid using Dendritic Fibrous Nanosilica as a Photocatalyst. ChemSusChem 2017, 10, 2182−2191. (29) Detavernier, C.; Dendooven, J.; Pulinthanathu Sree, S.; Ludwig, K. F.; Martens, J. A. Tailoring nanoporous materials by atomic layer deposition. Chem. Soc. Rev. 2011, 40, 5242−5253. (30) Wang, G.; Gao, Z.; Tang, S.; Chen, C.; Duan, F.; Zhao, S.; Lin, S.; Feng, Y.; Zhou, L.; Qin, Y. Microwave Absorption Properties of Carbon Nanocoils Coated with Highly Controlled Magnetic Materials by Atomic Layer Deposition. ACS Nano 2012, 6, 11009−11017. (31) Qin, Y.; Pan, A.; Liu, L.; Moutanabbir, O.; Yang, R. B.; Knez, M. Atomic Layer Deposition Assisted Template Approach for Electrochemical Synthesis of Au Crescent-Shaped Half-Nanotubes. ACS Nano 2011, 5, 788−794. (32) Vandegehuchte, B. D.; Thybaut, J. W.; Detavernier, C.; Deduytsche, D.; Dendooven, J.; Martens, J. A.; Sree, S. P.; Korányi, T. I.; Marin, G. B. A Single-Event MicroKinetic assessment of n-alkane hydroconversion on ultrastable Y zeolites after Atomic Layer Deposition of alumina. J. Catal. 2014, 311, 433−446.

(33) Xu, D.; Wu, B.; Ren, P.; Wang, S.; Huo, C.; Zhang, B.; Guo, W.; Huang, L.; Wen, X.; Qin, Y.; Yang, Y.; Li, Y. Controllable deposition of Pt nanoparticles into a KL zeolite by atomic layer deposition for highly efficient reforming of n-heptane to aromatics. Catal. Sci. Technol. 2017, 7, 1342−1350. (34) Kim, D. H.; Seo, H. O.; Jeong, M.-G.; Kim, Y. D. Reactivity and Stability of Ni Nanoparticles Supported by Mesoporous SiO2 and TiO2/SiO2 for CO2 Reforming of CH4. Catal. Lett. 2014, 144, 56−61. (35) Vuori, H.; Silvennoinen, R. J.; Lindblad, M.; Ö sterholm, H.; Krause, A. O. I. Beta Zeolite-Supported Iridium Catalysts by Gas Phase Deposition. Catal. Lett. 2009, 131, 7−15. (36) Li, Y.; Zhao, S.; Hu, Q.; Gao, Z.; Liu, Y.; Zhang, J.; Qin, Y. Highly efficient CoOx/SBA-15 catalysts prepared by atomic layer deposition for the epoxidation reaction of styrene. Catal. Sci. Technol. 2017, 7, 2032−2038. (37) Islamoglu, T.; Goswami, S.; Li, Z.; Howarth, A. J.; Farha, O. K.; Hupp, J. T. Postsynthetic Tuning of Metal−Organic Frameworks for Targeted Applications. Acc. Chem. Res. 2017, 50, 805−813. (38) Qin, Y.; Lee, S.-M.; Pan, A.; Gösele, U.; Knez, M. RayleighInstability-Induced Metal Nanoparticle Chains Encapsulated in Nanotubes Produced by Atomic Layer Deposition. Nano Lett. 2008, 8, 114−118. (39) Qin, Y.; Yang, Y.; Scholz, R.; Pippel, E.; Lu, X.; Knez, M. Unexpected Oxidation Behavior of Cu Nanoparticles Embedded in Porous Alumina Films Produced by Molecular Layer Deposition. Nano Lett. 2011, 11, 2503−2509. (40) Qin, Y.; Liu, L.; Yang, R.; Gösele, U.; Knez, M. General Assembly Method for Linear Metal Nanoparticle Chains Embedded in Nanotubes. Nano Lett. 2008, 8, 3221−3225. (41) Qin, Y.; Vogelgesang, R.; Eßlinger, M.; Sigle, W.; van Aken, P.; Moutanabbir, O.; Knez, M. Bottom-Up Tailoring of Plasmonic Nanopeapods Making Use of the Periodical Topography of Carbon Nanocoil Templates. Adv. Funct. Mater. 2012, 22, 5157−5165. (42) Gao, Z.; Dong, M.; Wang, G.; Sheng, P.; Wu, Z.; Yang, H.; Zhang, B.; Wang, G.; Wang, J.; Qin, Y. Multiply Confined Nickel Nanocatalysts Produced by Atomic Layer Deposition for Hydrogenation Reactions. Angew. Chem., Int. Ed. 2015, 54, 9006−9010. (43) Ge, H.; Zhang, B.; Gu, X.; Liang, H.; Yang, H.; Gao, Z.; Wang, J.; Qin, Y. A Tandem Catalyst with Multiple Metal Oxide Interfaces Produced by Atomic Layer Deposition. Angew. Chem., Int. Ed. 2016, 55, 7081−7085. (44) Zhang, J.; Yu, Z.; Gao, Z.; Ge, H.; Zhao, S.; Chen, C.; Chen, S.; Tong, X.; Wang, M.; Zheng, Z.; Qin, Y. Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx Cocatalysts Produced by Atomic Layer Deposition for Photocatalytic Hydrogen Production. Angew. Chem., Int. Ed. 2017, 56, 816−820. (45) Feng, H.; Elam, J. W.; Libera, J. A.; Setthapun, W.; Stair, P. C. Palladium Catalysts Synthesized by Atomic Layer Deposition for Methanol Decomposition. Chem. Mater. 2010, 22, 3133−3142. (46) Wang, M.; Gao, Z.; Zhang, B.; Yang, H.; Qiao, Y.; Chen, S.; Ge, H.; Zhang, J.; Qin, Y. Ultrathin Coating of Confined Pt Nanocatalysts by Atomic Layer Deposition for Enhanced Catalytic Performance in Hydrogenation Reactions. Chem. - Eur. J. 2016, 22, 8438−8443. (47) Li, F.; Li, L.; Liao, X.; Wang, Y. Precise pore size tuning and surface modifications of polymeric membranes using the atomic layer deposition technique. J. Membr. Sci. 2011, 385, 1−9. (48) Cheng, N.; Banis, M. N.; Liu, J.; Riese, A.; Li, X.; Li, R.; Ye, S.; Knights, S.; Sun, X. Extremely Stable Platinum Nanoparticles Encapsulated in a Zirconia Nanocage by Area-Selective Atomic Layer Deposition for the Oxygen Reduction Reaction. Adv. Mater. 2015, 27, 277−281. (49) Cheng, N.; Norouzi Banis, M.; Liu, J.; Riese, A.; Mu, S.; Li, R.; Sham, T.-K.; Sun, X. Atomic scale enhancement of metal-support interactions between Pt and ZrC for highly stable electrocatalysts. Energy Environ. Sci. 2015, 8, 1450−1455.

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DOI: 10.1021/acs.accounts.7b00266 Acc. Chem. Res. 2017, 50, 2309−2316