Interface Tailoring of Heterogeneous Catalysts by Atomic Layer

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Interface tailoring of heterogeneous catalysts by atomic layer deposition Bin Zhang, and Yong Qin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02659 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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ACS Catalysis

Interface tailoring of heterogeneous catalysts by atomic layer deposition *

*

Bin Zhang, Yong Qin*

*

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan

030001, China.

Abstract The tailoring of the metal-oxide interface is an important strategy in the design and development of novel catalysts with superior catalytic performance. However, the structure and location of the metal-oxide interface on supported catalysts cannot be well controlled by traditional methods, and the structure-property relation is not clearly understood in most reactions. Therefore, it is highly desirable to develop new methods to precisely tailor the metaloxide interface and thus achieve highly efficient catalysts and a fundamental understanding of the principle of interface catalysis. Atomic layer deposition (ALD), a high-level film-growth technology, is a promising and controllable approach to precisely design and tailor the metaloxide interface on an atomic scale. In this review, we present and discuss a series of recently developed ALD strategies for tailoring the metal-oxide interface of heterogeneous catalysts, such as overcoating, ultrathin modification, area-selective ALD, template-assisted ALD, and

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template- and sacrificial layer-assisted ALD. These methods have been used to develop many catalysts with different structures, such as core-shell structures, inverse oxide/metal structures, oxide–nanotrapped metal structures, porous sandwich structures, multiply confined metal nanoparticles in oxide nanotubes, and multifunctional catalysts with multiple metal-oxide interfaces. Due to its advantages, ALD can be applied to reveal the catalytic mechanism of metal-oxide interfaces by deliberately designing catalysts with a clear structure, even in confined and synergetic environments. In general, the developed ALD approaches provide us with a toolkit for tailoring the metal-oxide interface and designing heterogeneous catalysts. KEYWORDS: atomic layer deposition, metal-oxide interface, overcoating, ultrathin modification, templateassisted ALD method


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1. Introduction Supported metal particles, particularly those using an oxide as a support or promoter, are important materials in heterogeneous catalysis.1-3 Metal-oxide interfaces form when metal clusters/nanoparticles interact with oxides. Since the morphology and electronic state of metal particles are greatly influenced by the formation of a metal-oxide interface, the tailoring of the metal-oxide interface of heterogeneous catalysts is an important strategy to design and develop highly efficient catalysts.4-10 In general, the ratio of metal-oxide interface sites to all surface metal sites on a supported catalyst prepared by traditional methods is low. This ratio increases with a decrease in metal particle size. However, a decrease in metal particle size always results in changes to the electric properties, coordination state and bond distance of the metal.11-13 Moreover, the remarkable performance improvements in catalysts caused by a metal-oxide interface are easily observed only for catalytic reactions in which metal-oxide interface sites show dramatically higher catalytic activity than do other sites, such as CO oxidation14-15 and CO2 hydrogenation16-20. The function of the metal-oxide interface is obscured in most reactions in which metal sites show better catalytic performance. Therefore, tailoring the metal-oxide interface by directly increasing the ratio of interface sites instead of decreasing the metal particle size is preferred when investigating the advantages and mechanisms of metal-oxide interface catalysis. In 1978, Tauster et al. first used the term strong metal-oxide interaction (SMOI) to describe the formation of


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chemical bonds between a noble metal and TiO2 support; such bonds form because oxides migrate onto the surface of metal particles in a high-temperature reducing environment.21 The oxide coverage produces a larger metal-oxide interface with a strong electric effect in the SMOI system. This SMOI phenomenon has since been observed in many metal catalytic systems and has been broadly extended to describe support-induced catalytic activity and selectivity changes in metal-based catalysts.22 Due to the atomic-level precision required for interface catalysis, it is a considerable challenge to precisely tailor metal-oxide interfaces with traditional methods, such as impregnation, co-impregnation, sequential deposition, co-precipitation, the sol-gel method, gas-induced surface segregation, and high-temperature treatment.23-24 Although recently developed nanotechnology can realize the controlled assembly of metal and oxide nanoparticles to produce metal-oxide interfaces,6 the ratio of interface sites is still rather limited. Despite the importance of the metal-oxide interface, our understanding of this interface is still unsatisfactory. The metal nanoparticles and supports produced by conventional methods are structurally non-uniform, and thus, the derived metal-oxide interface structures are not clear.25 To investigate the mechanism of interface catalysis, surface science techniques have been developed for many years to prepare and investigate a series of model catalysts, such as singlecrystal faces of metals decorated with oxides and oxides decorated with metals.26-28 The metaloxide interface in these model catalysts induces an obvious enhancement in catalytic activity and selectivity in some important catalytic reactions, such as CO oxidation,24,


29

CO2

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ACS Catalysis

hydrogenation,16,

30-32

and the water-gas shift reaction33. Recently, analyses based on in

situ/operando spectroscopy techniques together with theoretical calculations have indicated that the metal-oxide interface plays an important role in catalytic activity by providing multiple sites for the adsorption of reaction intermediates.20 However, we cannot neglect the "materials gap" and "pressure gap" between surface science performed under ultra-high vacuum (UHV) conditions and catalysis in the real environment.27, 34-36 The fundamental principle by which the metal-oxide interface governs catalyst behavior is still unknown, especially in terms of interactions with intermediates and their stability.28 Theoretically, the tailoring of the metal-oxide interface is also challenging because of the tremendous differences between metals and oxides in terms of chemical bonds, crystal structure, electronic state, thermal expansion, etc.20 Typical metal-oxide interfaces exhibit unique boundary structures, electric states, and interactions, which are very complex and derive from hybridization, polarization, and charge transfer. A slight change in interface will cause a substantial difference in the physical and chemical properties of the catalyst materials, resulting in changed catalytic performance. Thus, many factors, such as metal particle size, composition, the valence state of interface elements, and microstructures, should be precisely controlled during the preparation of the catalyst. It is highly desirable to develop new methods to precisely tailor the metal-oxide interface and thus achieve highly efficient catalysts and a fundamental understanding of the principle of interface catalysis.


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Atomic layer deposition (ALD) is a high-level film growth technology.37-38 ALD produces films or nanoparticles through self-limiting layer-by-layer chemical reactions between gaseous precursors and the surface of a solid substrate (Figure 1).37, 39 The advantages of the self-limiting nature of ALD make it possible to precisely control the thickness or size of films (or particles) of metals, oxides, sulfides, nitrides, polymers, inorganic-organic hybrid films and other materials with excellent conformity, uniform thickness, high reproducibility and controllable composition. 16, 37, 39-46

Recently, ALD has been developed as an attractive and effective method for the precise

design and synthesis of catalysts with novel performance traits.38, 47 Due to the advantages of ALD, uniform metal-oxide interfaces can be constructed with highly dispersed size-controllable metal particles, alloys or even single atoms on oxide supports or ALD oxide-coated supports. In this review, we will introduce recent developments in tailoring the metal-oxide interface by ALD. First, we will give an overview and discussion of the use of ALD overcoating in tailoring metaloxide interfaces. Second, we will review the approaches used in ultrathin ALD modification to build metal-oxide interfaces in two ways, including processes that use oxide-modified metal particles and metal-modified oxide particles. Third, we will introduce our recent progress in ALD strategies to tailor the metal-oxide interface in confined nanospaces and in synergetic environments. The features and advantages of different ALD approaches will be compared to provide a clear understanding of the interface catalysis mechanism in typical catalytic reactions.


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ACS Catalysis

Figure 1. A) Schematic illustration of one ALD reaction cycle. B) Overview of materials grown by ALD. Reprinted with permission from ref 39. Copyright 2005 American Institute of Physics. 2. The general ALD approaches to tailor metal-oxide interface

Figure 2. ALD overcoating strategies to build the metal-oxide interface: A) overcoating of ALD oxide on a supported metal catalyst, B) overcoating of ALD oxide on a support followed by metal deposition, and C) overcoating by area-selective ALD. ALD provides atomically precise control of the thickness and composition of the oxide layer. Because the type of substrate and the treatment of substrate can significantly affect the


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ALD surface chemistry, the morphology of ALD layer and the interface structure are closely related to the initial surface state of substrate. Generally, ALD can be utilized to coat metal catalysts, passivation layer-protected metal catalysts, or supports with a uniform thin oxide layer before loading metal nanoparticles (Figure 2). Thus, metal-oxide interfaces can be produced and tailored by ALD in different ways. 2.1 Overcoating by ALD The conformal coating of supported metal nanoparticles by an ALD oxide layer with a thickness of several nanometers can completely cover the particles and build a closed metaloxide interface. In this case, there is no channel or holes for the reactants to access the active sites. Thus, high-temperature treatments in air, O2 or H2 are generally utilized to create pores in the ALD layers by inducing densification and/or crystallization of the ALD oxide layer.48 ALD-Al2O3-overcoated metal catalysts that use trimethylaluminum and water as precursors have been extensively investigated. For example, Al2O3-coated Pd catalysts have been used for the

decomposition

of

methanol,49

oxidative

dehydrogenation

of

ethane,50

selective

decarbonylation of furfural,51 and selective hydrogenation of 1,3-butadiene52. All these reactions showed an increase in catalytic stability and selectivity. However, the enhancement of catalytic performance is usually attributed to the occupation of surface low-coordination sites rather than the formation of interface in early researches. For ALD-Al2O3-coated Pd catalyst, Fourier transform infrared (FTIR) spectroscopy and DFT calculations have indicated that the ALD


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ACS Catalysis

precursors trimethylaluminum and water are preferred for interacting with the Pd nanoparticles at low-coordination surface sites during the initial few ALD cycles (Figure 3).48, 50, 53 When the Pd catalyst coated with a thick ALD Al2O3 layer (45 cycles, 7.7f0.4 nm) was treated at 700 ºC, a porous oxide layer was formed because of the substantial lattice mismatch between Pd and Al2O3, and Al2O3 still preferentially contacted with the low-coordination sites of Pd nanoparticles, just according to IR spectra of CO-chemisorption. Generally, the changes in the IR spectra of COchemisorption are directly related to the adsorption behavior of CO, which intrinsically depends on the physical and chemical parameters of surface metal sites, such as the oxide state, electronic state, length of metal bonds, and coordination number.54-55 Thus, the IR spectra shift of COchemisorption cannot simply be attributed to the decrease of low-coordination sites of Pd after ALD Al2O3 overcoating. The structure of several atomic layers on the surface of metal nanoparticle, which generally provide the catalytic active sites, could be easily altered during initial ALD processes and further high temperature treatments. In principle, chemical bonds forms at the interface between ALD oxide film and metal nanoparticle, independent of the type of oxide. Thus, the SMOI expands to unreducible oxide (such as Al2O3) and metal nanoparticles for ALD-oxide-overcoated metal catalyst. Moreover, a further structural evolution caused by high temperature treatment should not be ignored. On the one hand, the formation of a metal-oxide interface by high-temperature treatment can induce changes in the metal electronic state. On the other hand, new species can be


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formed by reactions at the interface between metal nanoparticles and the overcoating layer after high-temperature treatment. For example, when ALD AlOx overcoating was utilized for Cu catalysts, usually in the form of oxidation, unreducible copper aluminate was formed at the interface between the Al2O3 layer and Cu nanoparticles after annealing at 700 °C.56 This copper aluminate prevented the agglomeration of Cu nanoparticles at the expense the catalytic activity. Similarly, cobalt aluminate was formed for ALD-Al2O3-coated Co/γ-Al2O3 catalyst after calcination.57 This catalyst was not reducible in a hydrogen environment at a temperature up to 800 °C, and even did not show any activity for aqueous-phase hydrogenation. However, the formation of the composite oxide at the interface depends on the treatment temperature and types of metal and ALD-oxide-overcoating layer. When a Cu catalyst was coated by ALD-TiO2 layers, no titanate species but some easily reduced CuOx species were formed at the interface even after high temperature treatment.56 This TiO2-coated Cu-Cr catalyst showed much higher activity than both neat Cu-Cr catalyst and the Al2O3-ALD-overcoated catalyst. The redox properties of Cu were changed slightly after the TiO2 ALD overcoating. However, a subtle electronic interaction was observed between the TiO2 ALD layers and Cu nanoparticles according to X-ray absorption fine structure (XAFS) analysis, indicating the SMOI at the Cu-TiO2 interface. The TiO2 overcoating strategy was also used to enhance the activity, stability and even selectivity in harsh reaction condition for Co/TiO2 catalysts.57-59.


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ACS Catalysis

Figure 3. A) Schematic illustration and TEM images of porous ALD Al2O3 overcoating on Pd nanoparticles. B) Schematic illustration of Al2O3 ALD on a surface coated with Pd nanoparticles. Adapted with permission from ref 53. Copyright 2012 American Chemical Society. Because most of the oxides (Al2O3, SiO2, TiO2, ZrO2, CeO2, etc.) used for supported metal catalysts can be deposited by ALD, it is possible to produce an oxide support with multiple components and further create different types of metal-oxide interfaces on an inexpensive highsurface area support. When metal nanoparticles with an identical size and loading are loaded on ALD oxide layer-modified supports with similar morphology, the effect of different metal-oxide interfaces can be clearly distinguished. For example, two Pt catalysts were prepared by coating 20 ALD cycles of CeO2 or TiO2 on nanoporous spherical Al2O3 particles, followed by the deposition of Pt.60 High-resolution transmission electron microscopy (HRTEM) images showed that the obtained catalysts had a similar morphology, with an identical average particle size of Pt before (2.1 nm) and after (2.9 nm) the aqueous-phase reforming reaction. Therefore, the type of


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Pt-oxide interface was a major variable parameter in these catalysts. In the aqueous-phase reforming reaction of 1-propanol, Pt supported on TiO2-coated Al2O3 produced a higher reaction rate (10.9×105 mol/s/g-cat) than did Pt supported on CeO2-coated Al2O3 (4.1×105 mol/s/g-cat) and Pt/Al2O3 (4.9×105 mol/s/g-cat) without ALD overcoating, indicating that metal-oxide interactions play an important role in catalytic performance. The main problem facing metals supported on ALD oxide-coated supports is the stability of the metal particles and the thinness of the ALD-oxide layer under harsh reaction conditions, such as high temperature, high pressure, and hydrothermal processing conditions.

Figure 4. STEM images: (A, B) Cu/SiO2 overcoated by 30 cycles of ALD aluminum oxide (30ALD-Al2O3/Cu/SiO2). EELS map images: (C) 30ALD-AlOx/Cu/SiO2, (D) 30ALD-


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ACS Catalysis

Al2O3/Cu/SiO2 further coated with 20 cycles of NbOx. Correspondence of colors and elements: yellow, Nb; green, Al; blue, Si; red, Cu. Adapted with permission from ref 61. Copyright 2014 American Chemical Society. ALD oxide overcoating on metal catalysts can create a bifunctional catalyst by coating the metal nanoparticles with multilayer oxides. For example, a metal-acid bifunctional catalyst can be prepared by coating a supported metal catalyst with an acidic ALD oxide layer. When 30 cycles of Al2O3 layering were coated onto the surface of Cu/SiO2, followed by heat treatment in air at 700 °C, the resulting porous Al2O3 layer with ~1 nm pores stabilized the Cu metal nanoparticles against sintering and leaching during the liquid-phase hydrogenation of furfural to furfuryl alcohol (Figure 4A-C).61 The acidity of the AlOx layer catalyzed the etherification of furfuryl alcohol with 1-butanol to form furfuryl butyl ether. Further deposition of the more acidic NbOx oxide on the AlOx overcoating layer followed by calcination at 900 °C (Figure 4D) significantly improved the conversion rate of furfuryl butyl ether. Because the two overcoating strategies, namely, overcoating of ALD oxide on a supported metal catalyst and overcoating of ALD oxide on a support followed by metal deposition, produce metal-oxide interfaces with different morphologies and ratios, the comparison of the two types of catalysts is interesting and clearly reveals the function of interfaces in catalytic reactions. For example, in order to investigate the function of MnO in the Rh-catalyzed conversion of syngas to C2+oxy, ALD was applied to prepare two types of MnO-promoted Rh catalysts: Rh supported on


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MnO-modified SiO2 and MnO-coated Rh/SiO2 catalyst.62 Compared to the Rh/SiO2 catalyst, MnO enhanced the catalytic activity and the C2+oxy selectivity in both MnO-overcoated Rh catalysts. The lack of Rh-Mn alloy or mixed oxide formation and the minimal change in Rh dispersion after MnO modification indicated that the Rh-MnO interface provided highly active sites for both the dissociation of C-O bonds and C2+oxy formation. In theory, a MnO overlayer on the top of the Rh surface can generate more Rh-MnO interface sites. However, the enhancement of the MnO overlayer-modified Rh/SiO2 catalyst was much lower than that obtained when MnO was used as a support layer in a modified catalyst. Further X-ray photoelectron spectroscopy (XPS) and CO-chemisorption analysis suggested that the MnOcoated Rh/SiO2 catalyst did not form stable interface sites. CO adsorption may induce the segregation of MnO layers away from the Rh surface, resulting in the decrease in Rh-MnO interface sites. In contrast, the metal oxide interface sites were more stable when MnO was used as a support layer in modified materials. A strong metal-oxide interaction can improve the stability of the interface. For example, a strong Pt-ZnO interaction was induced in ALD ZnO-coated Pt/Al2O3 catalyst (ZnO/Pt/Al2O3) and in Pt supported on ALD ZnO (6 cycles, 1.3 nm)-coated Al2O3 (Pt/ZnO/Al2O3) (Figure 5).63 The average Pt-Pt bond distance of the Pt/Al2O3 sample was 2.72 Å, which was relaxed to 2.75 Å in the Pt/ZnO/Al2O3 catalyst. Compared to X-ray adsorption near edge spectroscopy (XANES) analysis of the Pt/Al2O3 sample, an obvious shift in the white line to higher photo energy by 2.5


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ACS Catalysis

and 2.2 eV was observed for Pt/ZnO/Al2O3 and ZnO/Pt/Al2O3, respectively, indicating a significant charge transfer from Zn to Pt. Moreover, the Pt nanoparticles contacted both ZnO and Al2O3 in the ZnO/Pt/Al2O3 catalyst. This contact allowed the ZnO/Pt/Al2O3 catalyst to display higher activity and H2 selectivity than did Pt/ZnO/Al2O3 and Pt/Al2O3 in the aqueous-phase reforming of 1-propanol. A strong interaction between metal and oxide was also observed for a Fe oxide-overcoated Pt-based catalyst (Fe2O3/Pt/Al2O3).64 Unlike the behavior in the Pt-ZnO system, the Pt-Fe oxide interaction results in a significant charge transfer from Pt to Fe oxide and a shrinkage in the Pt-Pt bond. The Fe2O3/Pt/Al2O3 catalyst with an optimized ratio of Pt-FeOx sites showed an increase in selectivity in the hydrogenation of cinnamaldehyde to cinnamyl alcohol, which was caused by suppressing the hydrogenation of the C=C bond. The strong metaloxide interaction was also happened in a Fe oxide-overcoated Pd-based catalyst (Fe2O3/Pd/Al2O3),

which provided a significant improvement in both activity and butane

selectivity in the selective hydrogenation of 1,3-butadiene.65 However, the Fe2O3/Pd/Al2O3 catalyst became electron deficient compared to that of Pd/Al2O3 catalyst. Therefore, electron transfer at the metal-oxide interface depends on the type of both the metal and oxide, which is an important factor in determining the adsorption and activation of reactants and intermediates during catalytic reactions.


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Figure 5. A) Preparation of ZnO-promoted Pt catalysts with inverse spatial arrangement; Aberration-corrected HAADF-STEM images of (B) Pt/ZnO/Al2O3 and (C) ZnO/Pt/Al2O3 before reaction; (D, E) Pt/ZnO/Al2O3 after aqueous-phase reforming of 1-propanol carried out at 523 K and 6.4 MPa. Reprinted with permission from ref 63. Copyright 2016 American Chemical Society. ALD oxide-overcoating approaches are generally utilized to enhance the catalytic performance by encapsulating the supported metal nanoparticles or overcoating the support


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ACS Catalysis

followed by deposition of metal. Compared to traditional solution methods, these ALD approaches are more simple, precise and uniform in the generation of metal-oxide interface. The damage to catalyst structure by ALD overcoating is also less than the solution methods with a complex preparation process. The ALD overcoating on the support followed by deposition of metal will be a good strategy to build extensive types of metal-oxide interfaces over cheap and high surface area supports. Moreover, the thin ALD oxide layers are different to the conventional oxide supports, and will perform special electric and adsorptive properties in building metaloxide interface. However, there are some disadvantages that need to be mentioned for the ALD overcoating. For ALD-oxide-overcoated metal catalyst, the creation of pores in the ALD oxide layer at high temperature is necessary to expose the surface metal active sites. Both the ALD process and further heat treatment will lead to the further structure evolution of the metal catalyst at the interface. In some case, inactive spinel or other composite metal oxide species might be formed in the oxide layer-coated metal (Fe, Co, Ni, Cu) oxide nanoparticles or surface-oxidized metal nanoparticles after high temperature treatments. These will significantly affect the interface structure, active site and catalytic performance. Moreover, the stability of the overcoating layer under reaction conditions also needs to be noticed. The interaction between the thin ALD oxide films and the molecules (reactants, solvents and products) may results in further structural changes of the catalyst during reaction. 2.2 Template-assisted ALD


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Template-assisted ALD can combine the advantages of ALD and template method to realize the synthesis of the low-dimensional nanomaterials and complex nanostructures,66 such as nanotubes,67-69 non-close-packed inverse opal,70 3D network of metal nanowires,71 3D porous nanostructured metal,72 nanoparticle or nanochains embedded in nanotubes,73-75 crescent-shaped half-nanotubes,76 nanoporous gyroid,77 and tube-in-tube structure78. It provides a bottom-up approach to construct catalyst with uniform nanostructures and metal-oxide interface. The intrinsic properties of the metal-oxide interface are obscured in most metal-based catalysts, because surface metal sites are generally dominant in catalytic reactions. It is highly desirable to reveal the intrinsic activity of the metal-oxide interface or distinguish the difference between metal sites and metal-oxide interface sites in catalytic reactions. Thus, it is necessary to maximize the ratio of the metal-oxide interface by overcoating the metal sites with porous oxide films to build a catalyst with single metal-oxide interface sites. By using the template-assisted ALD overcoating method, we can easily prepare a porous sandwich catalyst in which metal nanoparticles directly interact with two identical porous oxide layers. Because all the surface metal sites contact the same porous oxides, the metal-oxide interfaces are uniform and dominant in the sandwich structure. For example, porous sandwich TiO2/Pt/TiO2 catalysts were prepared by a a template-assisted ALD method by sequentially depositing a TiO2 inner layer (42 nm), Pt nanoparticles, and a TiO2 outer layer (≥ 4.2 nm) on a carbon nanofiber template, followed by thermal treatment in air at 500 °C to remove the template and produce nanopores in the TiO2


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ACS Catalysis

layer (Figure 6A).79 Dominant Pt−TiO2 interface sites were realized in the sandwich structure because all the Pt nanoparticles interacted with the two porous TiO2 layers (Figure 6C and D). The surface Pt species at the Pt−TiO2 interface exhibited electron-rich properties, according to XPS analysis (Figure 6E). However, there was little difference between the Pt/TiO2 and TiO2/Pt/TiO2 sandwich catalysts in terms of the bulk atoms of Pt nanoparticles, based on the XAFS results (Figure 6F). Therefore, the formation of a Pt−TiO2 interface only affected the surface electric state of the Pt nanoparticle in the sandwich catalyst. The TiO2/Pt/TiO2 catalyst demonstrated high selectivity and stability in the semihydrogenation of various alkynes to olefins and selective hydrogenation of an α,β-unsaturated aldehyde to an α,β-unsaturated alcohol by preventing the further hydrogenation of C=C double bonds (Figure 6G). In contrast, Pt/TiO2 catalysts without an outer TiO2 layer or without the complete coverage of a porous TiO2 outer layer (< 4.2 nm) had low selectivity in the semihydrogenation of alkynes. Therefore, the selectivity of semihydrogenation strictly depends on the formation of a Pt−TiO2 interface to remove free surface-metal sites. A series of controlled experiments performed by changing the type of outer oxide layer further verified the importance of the metal-oxide interface in semihydrogenation. When an Al2O3 film was used as the outer layer (Al2O3/Pt/TiO2), the selectivity of olefins was very low. We further coated a TiO2 layer onto a Al2O3/Pt/TiO2 catalyst to prepare a TiO2/Al2O3/Pt/TiO2 catalyst, which had the same pore structure as that of the TiO2/Pt/TiO2 sandwich catalyst. However, the selectivity of olefins over the TiO2/Al2O3/Pt/TiO2 catalyst was still low, thus excluding the influence of pores on selectivity.


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Figure 6. (A) Schematic illustration of the synthesis process of porous TiO2/Pt/TiO2 sandwich nanostructures by the template-assisted ALD method. TEM images of (B) Pt/TiO2 (Pt loaded on the outer surface of a TiO2 tube), (C) the porous sandwich catalyst 30TiO2/Pt/TiO2 and (D) 50TiO2/Pt/TiO2. (E) XPS analysis. (F) Fourier-transform k3-weighted EXAFS spectra. (G)


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Influence of the number of ALD cycles of an outer TiO2 layer on the semihydrogenation of phenylacetylene to olefin. Adapted with permission from ref 79. Copyright 2017 American Chemical Society. This template-assisted ALD method can be utilized for the preparation of other types of sandwich catalyst with one single metal−oxide interface. By changing the number of ALD cycles of the outer oxide layer, three types of model catalysts, an ultrathin oxide modified metal, an overcoated catalyst, and a sandwich catalyst with only one single metal−oxide interface, can be synthesized. These model catalysts can be applied to reveal the function of surface metal sites, metal−oxide interface sites, and the synergy between surface metal sites and the interface in many reactions. Furthermore, a tandem catalyst with two metal-oxide interfaces can be easily realized by covering metal nanoparticles with two different porous oxide layers in the style of a sandwich structure. 2.3 Area-selective ALD Area-selective ALD (AS-ALD) is a promising method of constructing unique nanostructures by precisely controlling the growth of certain components on a specific type of site.80-81 Recently, AS-ALD was used to tailor the metal-oxide interface via the selective deposition of ALD oxide on a specific metal surface.82 Thus, typical metal-oxide interface sites can be generated.


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Figure 7. a) Schematic for the preparation of Co3O4 nanotrap-anchored Pt nanoparticles on Al2O3 supports based on AS-ALD. TEM images for b), c) Pt/Al2O3; d), e) Co3O4/Pt/Al2O3 (Co3O4 nanotrap-anchored Pt nanoparticles); and f), g) Co3O4@Pt/Al2O3 (Co3O4-overcoated Pt nanoparticles). Reprinted with permission from ref 83. Copyright 2016 WILEY-VCH. In most AS-ALD processes, organic ligands are used as a metal surface passivation layer to prevent oxide deposition during the ALD process. For example, Pt nanoparticles anchored to Co3O4 nanotraps were prepared by the AS-ALD method (Figure 7).83 1-Octadecanethiol (ODT) molecules were used as a blocking agent on the surface of the Pt nanoparticles to prevent Co3O4 deposition, thereby realizing the selective deposition of Co3O4 onto the Al2O3 support. After


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ACS Catalysis

removing the ODT blocking agents via calcination in air, the Pt sites were re-exposed, and PtCo3O4 interfaces were created. STEM images showed that the Pt nanoparticles were surrounded by Co3O4 nanotraps (Figure 7e), inducing an increase in Pt2+ species at the Pt-Co3O4 interface, according to XPS and XAFS results. Compared to the Pt/Al2O3 catalyst, the AS-ALD Co3O4modified Pt/Al2O3 catalyst demonstrated an obvious decrease in CO oxidation temperature and activation energy (Ea) due to the formation of more of the Pt-Co3O4 interface. Orbital hybridization and synergistic effects between Co3O4 and Pt at the interface were responsible for the enhanced catalytic activity by reducing the CO adsorption energy and O2 activation barrier. AS-ALD can also be realized by directly depositing oxide on a metal catalyst. The reactivity of precursors directly influences the deposition behavior of oxides on metal surfaces. In the growth of Al2O3 on a metal surface using trimethylaluminum and water as precursors, the Al2O3 is prone to nucleation on the edge and low-coordination sites of metals such as Pt or Pd in the first few cycles. However, Al2O3 will rapidly form a continuous film and cover the entire surface of the substrate upon increasing the number of ALD cycles. In contrast, CeOx nanofences around Pt nanoparticles were formed when using tetrakis(2,2,6,6-tetramethyl-3,5-heptanedione) cerium(IV) [Ce(thd)4] and O3 as precursors at 150 °C.84 CeOx species selectively nucleated on Pt(111) but remained intact on Pt(100) during the initial growth stage even after a number of cycles (< 200); furthermore, these species produced an adequately thick film by horizontal epitaxy on Pt(111). The Pt(111) facet provided active sites for selective O3 adsorption and the


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further oxidation of [Ce(thd)4]. The nanofence structure exposed both active facets and more of the active Pt-CeO2 interface for CO oxidation and enhanced the catalytic activity. AS-ALD is an attractive strategy for tailoring metal-oxide interfaces. The deposition of oxide on different types of unique metal sites can be realized by choosing different passivation molecules or precursors. However, there are very few examples of the construction of a metaloxide interface by the selective deposition of metal on unique sites. It is highly desirable to develop better AS-ALD strategies for the tailoring of metal-oxide interfaces. 2.4 Overcoating by molecular layer deposition When organic molecules with multiple functional groups are used as precursors, the deposition process can be termed molecular layer deposition (MLD), which produces polymers or organic-inorganic hybrid films through self-limiting surface reactions similar to ALD and can be considered as a subset of ALD.37, 85-86 The obtained hybrid films can be converted into porous oxide films after removing the organic components, and the pore size of the films can be tailored by changing the size of the organic components or applying heat treatment.26,27,87 Because the hybrid films have a regular structure, uniform interconnected porous structures are produced after removing the organic components. For example, alucone films, produced with trimethylaluminum and ethylene glycol (EG) as precursors, can be transformed into porous Al2O3 films with 0.6 nm micropores.85, 88 Al2O3 films with a pore size of 0.8 nm can be produced by annealing an MLD film deposited by a three-step A-B-C process using trimethylaluminum,


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ACS Catalysis

ethanolamine and maleic anhydride as precursors.89 However, since the size of micropores is close to that of the organic reactants, the functions of micropores and the metal-oxide interface cannot be easily distinguished. On the other hand, the remaining carbon in the porous films can occupy and even poison the catalytic active sites. A high calcination temperature (>500 ºC) is still needed.90 To tailor the metal-oxide interface by MLD, it is advisable to use easily removable organic components. Copper-based catalysts have a broad range of applications in industrial catalysis, photocatalysis, and electrocatalysis.91-95 It is a considerable challenge to increase the stability and activity of Cu nanoparticles because they are unstable and prone to agglomeration or leaching during preparation and reaction.96 The tailoring of the Cu-oxide interface provides a chance to enhance the thermal stability, selectivity and catalytic efficiency of Cu-based catalysts. We designed a new A-B-C-B four-step MLD process to deposit Zn-polyurea hybrid films, using Zn(Et)2 (A), 1,4-phenylene diisocyanate (PPDI, B), and ethylenediamine (C) as precursors.97 This Zn-polyurea hybrid layer could uniformly overcoat Cu precursors loaded on carbon nanotubes (CNTs). High dispersion of Cu and ZnO nanoparticles with a close interface interaction was obtained after calcination at 300 ºC in air and further reduction with H2/N2 (Figure 8A and 8B). In contrast, the size distribution of the Cu nanoparticles on the reduced Cu/CNTs catalyst was broad, ranging from 2 to 300 nm. Thus, the Zn MLD shells interacted with Cu precursors, prevented the agglomeration of Cu particles during reduction, and further


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promoted the generation of more Cu-ZnO interface sites (Figure 8D). For comparison, ALDZnO, using Zn(Et)2 and water as precursors, was also coated on the Cu/CNTs. However, large ZnO nanoparticles (11-18 nm) covered the Cu active sites after reduction, which induced a low content of copper surface and Cu-ZnO interface sites. According to XPS and auger electron spectroscopy (AES) results, the strong interaction at the Cu and ZnO interface resulted in electron transfer from ZnO to Cu and the formation of new Cu0Zn sites (or Cu-ZnO interface sites). The critical role of the Cu-ZnO interface was embodied in the catalytic performance. Compared to Cu/CNTs and ALD ZnO-coated Cu/CNTs, the MLD ZnO-coated Cu/CNTs exhibited remarkable enhancements in efficiency, selectivity, and stability in levulinic acid hydrogenation due to the formation of more Cu/ZnO interface sites (Cu0Zn) (Figure 8E, F). Moreover, an increase in the number of MLD cycles can be utilized to tailor the Cu0Zn interface sites with an optimized ratio.


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ACS Catalysis

Figure 8. A) Preparation process of the Cu-ZnO catalyst through Zn-polyurea MLD hybrid filmassisted routes; B) Cu-ZnO-80MLD/CNTs (Zn-polyurea MLD hybrid film-overcoated Cu/CNTs) before calcination; C) TEM and D) HRTEM of Cu-ZnO-120MLD/CNTs reduced at 300 °C in H2/N2; E) Correlation between the TOF of GVL yield and the ratio of Cu0Zn/(Cu0 + Cu0Zn + Cu+); (F) The stability of the catalysts in the hydrogenation of levulinic acid (240 °C, 1 MPa, H2/N2 = 0.5). Adapted with permission from ref 97. Copyright 2015 American Chemical Society. Compared to the ALD coating strategy, the MLD strategy enables the tailoring of the metaloxide interface via overcoating metal nanoparticles with porous oxide layers or well-dispersed oxide additives with mild post-treatment processes. An easily decomposed organic component in the MLD film, such as polyurea, can further reduce the treatment temperature to prevent the sintering of small metal clusters and produce more metal-oxide interface sites. The pore size of the oxide layer or particle size of the oxide additive, together with the ratio of the metal-oxide interface, can be changed by tailoring the length of the organic components and the thickness of the MLD hybrid layers, respectively. However, the MLD coating strategy has not been extensively researched due to the rarity or complexity of the production of hybrid films via the MLD method. The structural diversity of MLD films makes MLD increasingly attractive in designing new catalysts by tailoring the metal-oxide interface. 2.5 Ultrathin modification by ALD


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When several-atom-thick ALD layers are coated on the surface of a substrate, the coating process is called ultrathin modification. Two types of ultrathin modification, namely, the deposition of oxide on metal nanoparticles and the deposition of metal on oxide nanoparticles, have been utilized to tailor metal-oxide interfaces (Figure 9).

Figure 9. The two strategies for ALD ultrathin modification. The ultrathin modification of metal nanoparticles is an effective way to upgrade traditional industrial catalysts by forming more metal-oxide interfaces.98 Because the oxide generally acts as a support for the dispersion of metal in traditional catalysts, ultrathin modification results in an inverse oxide/metal catalyst.98 In reducible oxide-supported metal catalysts, modification of the metal nanoparticles is frequently observed in catalysts with SMOI after high-temperature reduction.21 However, the direct modification of metal with an ultrathin oxide layer is a challenge. In recent years, ALD has been proved to be a more straightforward way to realize the preparation of inverse oxide/metal catalysts.


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Generally, because of the incomplete coverage of ALD precursors on the metal surface, the oxide forms islands instead of a uniform layer on the metal surface when a small number of ALD cycles are used. For example, the ALD of Al2O3 using trimethylaluminum and water as precursors on Pd nanoparticles proceeds by a self-poisoning, self-cleaning process.53 In the first few cycles, Al2O3 is discretely grown on highly active sites on the Pd surface, which results in a porous structure of the Al2O3 shell at the initial depositing stage (< 8 cycles, < 1 nm). Similar behavior was also observed for TiO2 ALD on Au catalysts.99-100 When an ultrathin ALD oxide layer is deposited on oxide-supported metal catalysts, the surface of the metal and support, as well as the interface between the metal and oxide support, is partially covered. For example, the Au/TiO2 interface shows high activity in CO oxidation, while Au metal sites have no or low activity. When Au/TiO2 was modified by an amorphous thin ALD SiO2 layer, the catalytic activity was decreased due to the formation of low-activity Au-SiO2 interface sites that were formed by occupying the surface Au and more active Au-TiO2 interface sites.101 In contrast, the catalytic performance of Au/SiO2 and Au/Al2O3 catalysts was remarkably enhanced after ultrathin ALD TiO2 modification, although the number of Au surface sites was decreased (Figure 10).99 These results provided direct evidence that the number of AuTiO2 interface sites (or the total area of interface sites) plays a key role in catalytic activity in CO oxidation. Moreover, the catalytic performance of Au-TiO2 interface sites changed with the size of Au nanoparticles. Because a decrease in metal particle size can result in an increase in Au


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dispersion and the ratio of the metal-oxide interface, smaller Au nanoparticles (2.9 ± 0.6 nm)modified by ALD TiO2 showed a higher CO oxidation activity than did other Au catalysts with larger Au particles.100

Figure 10. A) Schematic model of the precise tuning of the interface between Au and a TiO2 ALD overcoat. B) HRTEM images of unsupported Au nanocrystals with 20 cycles of TiO2 overcoating (TiO2 islands formed on the Au nanocrystals). Adapted with permission from ref 99. Copyright 2016 American Chemical Society.


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Figure 11. TEM, HRTEM, and HAADF-STEM images, and particle size histograms of different samples: 20Pt/CNTs (A and D, prepared by depositing 20 cycles of ALD Pt on CNTs); 10Fe2O320Pt/CNTs (B and E, prepared by the sequential deposition of 20 cycles of ALD Pt and 10 cycles of ALD Fe2O3 on CNTs); and 20Pt-50Fe2O3/CNTs (C and F, prepared by the sequential deposition of 50 cycles of ALD Fe2O3 and 20 cycles of ALD Pt on CNTs). Reprinted with permission from ref 102. Copyright 2016 American Chemical Society. Generally, the metal-oxide interface forms at the cost of metal surface sites through ALD overcoating or modification. A decrease in metal particle size, even to several atoms or a single atom, is an alternative choice to increase the utilization of metal. However, the preparation of catalysts with a high loading of metal clusters or single atoms is a challenge due to the instability


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of such catalysts.103-104 ALD can produce single atoms or clusters in the initial several cycles.16, 105-106

If small oxide nanoparticles are first deposited onto a high-surface area support, we can

realize the further deposition of single metal atoms or clusters with a high loading. Correspondingly, oxide nanoparticles modified with an ultrathin metal layer can achieve a high metal utilization and produce maximized metal-oxide interface sites by tailoring the number of ALD cycles. We prepared Fe2O3-modified Pt nanoparticles (Fe2O3-Pt/CNTs) and Pt-modified Fe2O3 nanoparticles (Pt-Fe2O3/CNTs) by changing the deposition sequence of Fe2O3 and Pt onto an inert surface of CNTs without further treatment.102 HAADF-STEM images showed that the less electron-dense iron oxide with a thickness of a few atoms was loaded on the surface of Pt nanoparticles in 10Fe2O3-20Pt/CNTs (Figure 11E) and Pt atoms were selectively loaded on the surface of Fe2O3 nanoparticles in 20Pt-50Fe2O3/CNTs (Figure 11F). A significant charge transfer from Pt to Fe was observed, according to XPS analysis. As is typical, 10Fe2O3-20Pt/CNTs and 20Pt-50Fe2O3/CNTs demonstrated the same binding energy with Pt, Fe and O in the XPS results, indicating the formation of similar Pt-Fe(3+)OH interfaces. Both catalysts with dominant PtFe(3+)OH interfaces showed a high selectivity for imine (-C=N-) in the reductive coupling of nitrobenzene and aldehyde. In contrast, CNTs-supported Pt catalysts, even with an optimized size of Pt clusters, showed a low selectivity for imine due to the overhydrogenation of imine (C=N-) to amine (-C-NH-). The Pt-Fe(3+)OH interface weakens the Pt-H bond and induces a


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ACS Catalysis

lower coverage of hydrogen on the catalytic surface, which suppresses the further hydrogenation of the C=N bond of imine. The differences between ultrathin ALD Pt-modified Fe2O3 and ultrathin Fe2O3-modified Pt nanoparticles are mainly in the catalytic activity and the utilization of Pt. The 20Pt-50Fe2O3/CNTs showed a significantly higher conversion rate of imine and TOF (27 mmol·gCat-1·h-1, TOF = 1071 h-1) than did the 10Fe2O3-20Pt/CNTs (10 mmol·gCat-1·h-1, TOF = 224 h-1). 3. Tailoring of the metal-oxide interface in confined nanospaces Confined nanospaces have been observed to significantly enhance the catalytic performance and stability of metal nanoparticles due to the confinement effect.107-108 In fact, the loading of metal nanoparticles into porous oxides is commonly applied in industry. Both the confinement effect and the metal-oxide interface play important roles in this process. However, the tailoring of metal-oxide interfaces in confined spaces is a challenge. Recently, ALD was proved to be a highly effective method to directly deposit highly dispersed metal or oxide species into complex porous 3D materials.47, 109-110 Therefore, ALD can be utilized to precisely engineer the catalytic sites (including metal-oxide interface sites) in confined spaces by regulating the particle size, pore size, and morphology of the confined space.


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Figure 12. A) Schematic illustration of the preparation process of multiply conﬁned Ni nanocatalyst (Ni-in-ANTs) and unconﬁned Ni nanocatalyst (Ni-out-ANTs). TEM images of (B, C) Ni-in-ANTs and (D) Ni-out-ANTs. (E) Evolution of the cinnamaldehyde conversion with reaction time in the hydrogenation reaction. (F) Recycling results for the two catalysts. Adapted with permission from ref 75. Copyright 2015 WILEY-VCH.


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ACS Catalysis

The tailoring of the metal-oxide interface in oxide nanotubes is an alternative strategy. With the assistance of one-dimensional templates, ALD can be used to design and prepare threedimensional (3D) complex nanostructures with precisely controlled morphology, composition and physicochemical properties.66,

68, 111

Recently, we developed a template-assisted ALD

method to synthesize oxide nanotube-confined metal nanoparticles with more metal-oxide interfaces.111 For example, multiply confined Ni nanoparticles embedded in Al2O3 nanotubes (Ni-in-ANTs) were prepared by sequentially depositing NiO nanoparticles and an Al2O3 layer on a carbon nanofiber template, followed by oxidation in air to remove the template and then reduction in H2 at 550 °C (Figure 12A).75 HRTEM images showed that the Ni nanoparticles were not only confined in the Al2O3 nanotubes but also embedded in the inner walls of the Al2O3 nanotubes (Figure 12B, C). For a clear comparison, Ni nanoparticles supported on the outer wall of ANTs (Ni-out-ANTs), which possess a similar size, metal loading of Ni and BET surface area, were also prepared by switching the deposition sequence of NiO and Al2O3 (Figure 12D). The confined Ni-in-ANTs catalyst showed a remarkably higher turnover frequency (TOF, 0.42 s-1) than that of Ni-out-ANTs (0.08 s-1) in cinnamaldehyde hydrogenation, although Ni-in-ANTs displayed a smaller number of accessible Ni surface sites (110.7 mmol g-Ni) than did Ni-outANTs (135.0 mmol g-Ni). The Ni-in-ANTs also showed enhanced stability (Figure 12E, F). The XAFS spectra and H2-TPR results indicated that the confinement of Al2O3 cavities created more Ni-Al2O3 interfacial sites, which enhanced the hydrogen spillover effect. Generally, this


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enhancement of hydrogen spillover provides more active hydrogen and thereby accelerates the hydrogenation reaction. Since the formation of metal-oxide interfaces induces a high activity in hydrogenation, it is highly desirable to further increase the amount of metal-oxide interfaces in confined spaces. We proposed a graceful strategy to maximize the metal-oxide interface ratio by modifying confined metal nanoparticles with an ultrathin oxide layer.112 This modification was realized based on a modified template-assisted ALD method, namely, by sequentially depositing ultrathin oxide, metal nanoparticles, and a thick oxide layer on a carbon nanofiber template, followed by air oxidation to remove the template (Figure 13). Taking a Pt/Al2O3 catalyst as example, an ultrathin Al2O3 (1~2 cycles)-coated Pt catalyst confined in Al2O3 nanotubes showed a significant improvement in the hydrogenation of 4-nitrophenol compared to the performance of the confined Pt catalyst. The TOF increased from 18.3 s-1 to 147.2 s-1 after 2 cycles of Al2O3 modification (2Al-Pt-in-ANTs). Further increases in the number of ALD cycles of the ultrathin Al2O3 layer resulted in a decrease in activity due to the occupation of the surface active sites. The decrease in the chemisorption of CO and H2 over the Al2O3-modified Pt-in-ANTs confirmed the formation of more metal-oxide interfaces.


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ACS Catalysis

Figure 13. Schematic illustration of the preparation process for Pt nanoparticles confined in Al2O3 nanotubes (Pt-in-ANTs) and ultrathin Al2O3 (x ALD cycles) modified Pt-in-ANTs (xAlPt-in-ANTs) by ALD. Reprinted with permission from ref 112. Copyright 2016 WILEY-VCH. 4. Tailoring of metal-oxide interfaces in synergetic environments The design of tandem reactions with high efficiency, good selectivity of target products, and excellent molecular efficiency is a great challenge in catalytic chemistry. Tandem catalysts that are produced by combining different catalytic functional sites and can perform with high efficiency and selectivity in one-pot reactions can significantly decrease costs by avoiding additional and costly synthesis and purification steps.113-115 Tandem catalysts provide a synergetic environment for complex multi-step reactions. However, it is a challenge to tailor many factors of tandem catalysts, such as the intrinsic properties of the active sites, the distance between active sites, and the transmission paths of intermediates, in a synergetic environment.


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Recently, ALD has shown advantages in tailoring metal-oxide interfaces in a synergetic environment. Tandem catalysts with novel performance in multi-step reactions can be designed and synthesized by integrating different functional metal-oxide interfaces in one catalyst.6 However, it is difficult to tune the composition, spatial distribution of active sites and microstructure of tandem catalysts at an atomic scale by traditional methods. The outstanding advantages of ALD in the synthesis of uniform metal clusters and thin films with precise control of size and film thickness make it feasible to design and synthesize multifunctional tandem catalysts with multiple metal-oxide interfaces. Recently, we developed two types of multifunctional catalysts via a template-assisted ALD method. We used ALD assisted by templates and a sacrificial layer to synthesize a tandem catalyst with two metal-oxide interfaces in a tube-in-tube structure.78 The two interfaces were spatially isolated on the outer surface of the inner oxide nanotube and the inner surface of the outer oxide nanotube of the tube-in-tube structure. Typically, such tandem catalysts with both a Ni-Al2O3 interface and Pt-TiO2 interface (Ni/Al-Pt/Ti) were synthesized by sequentially depositing an Al2O3 layer (150 cycles, 15 nm), Ni nanoparticles (200 cycles, 6.3 nm), a polyimide layer (sacrificial layer, 100 cycles), Pt nanoparticles (10 cycles) and a TiO2 layer (300 cycles, 12 nm) on a carbon nanofiber template, followed by air oxidation and H2/Ar reduction (Figure 14). The polyimide sacrificial layer made it possible to build nanospaces between the two metal-oxide


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ACS Catalysis

interfaces, and the distance between the two interfaces could be controlled by changing the thickness of the sacrificial layer. In clear contrast to the Al2O3 nanotube, the supported Ni nanoparticles on the Al2O3 nanotube, the void space, and the TiO2 nanotube in the tube-in-tube Ni/Al-Pt/Ti catalyst were clearly observed in the HAADF-TEM image and the energy-dispersive X-ray spectrometry (EDX) map (Figure 15A, and B). For comparison, corresponding tube-intube catalysts with only one interface (Al-Pt/Ti and Al/Ni-Ti), ultrathin Ni-modified Pt/TiO2 (Al(mNi)Pt/Ti), ultrathin Pt-modified Al/Ni/Al2O3 (Al/Ni(nPt)-Ti) (m and n represent the number of ALD cycles), and unconfined catalysts (Ni/Al and Pt/Ti) were also prepared. The Ni/Al-Pt/Ti catalyst showed a dramatically higher activity than those of the other catalysts and the physical mixtures of catalysts with single interfaces (Al-Pt/Ti + Al/Ni-Ti and Ni/Al + Pt/Ti) in the tandem hydrogenation of nitrobenzene to aniline using N2H4 H2O as a hydrogen source (Figure 15C). In our results, Ni/Al2O3 (Al/Ni-Ti and Ni/Al) catalysts were efficient catalysts for the decomposition of hydrazine hydrate to hydrogen, while Pt/TiO2 catalysts (Al-Pt/Ti and Pt/Ti) could catalyze nitrobenzene hydrogenation at room temperature (Figure 15D and E). When the Pt-TiO2 interface was replaced by a Pt-Al2O3 interface in the tandem catalyst, a considerable decrease in catalytic activity was observed. These results and further control experiments and characterizations (XAFS, H2-TPD, and H2-chemisorption) suggested that the high efficiency of the tube-in-tube tandem catalyst derived from the synergy of the two metal-oxide interfaces and the instant transfer of active intermediates (such as active hydrogen) between the two interfaces, which changed the reaction paths and rate-determining step (Figure 15F).


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Figure 14. Schematic illustration of the synthesis procedure for tandem catalyst with both Ni/Al2O3 and Pt/TiO2 interfaces by ALD assisted with a template and a sacrificial layer. Reprinted with permission from ref 78. Copyright 2016 WILEY-VCH.


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ACS Catalysis

Figure 15. A) HAADF-STEM image of an Al/Ni-Pt/Ti tandem catalyst. B) EDX elemental mapping of Al/Ni-Pt/Ti for the boxed area in (A). C) Catalytic activity of nitrobenzene hydrogenation using N2H4·H2O as hydrogen source. D) Catalytic activity of H2 generation from N2H4·H2O decomposition. E) Catalytic activity of the hydrogenation of nitrobenzene. F) Illustration of tandem catalysis on an Al/Ni-Pt/Ti catalyst in ethanol aqueous solution. Adapted with permission from ref 78. Copyright 2016 WILEY-VCH.


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Figure 16. A) Schematic illustration of the synthesis process for TiO2/Pt and spatially separated CoOx/TiO2/Pt photocatalysts with spatially separated Pt and CoOx cocatalysts by templateassisted ALD. B) and C) The Photocatalytic hydrogen evolution from a 15 vol% methanol–water solution over different photocatalysts (0.035 g) under UV-light irradiation (35 mW/cm2). Adapted with permission from ref 69. Copyright 2017 WILEY-VCH. We also utilized template-assisted ALD to synthesize a multifunctional tubular catalyst in which two types of metals were separately located on the inner and outer surfaces of an oxide nanotube.69 For example, a CoOx/TiO2/Pt photocatalyst was synthesized by sequentially depositing Pt clusters and a TiO2 layer on carbon nanofiber templates, followed by air oxidation


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to remove the template and the deposition of CoOx (Figure 16A). The CoOx was deposited after removing the template to prevent the formation of cobalt titanate. The photogenerated electrons and holes on the porous TiO2 nanotubes flowed inward and outward to Pt electron collector sites for the reduction reaction and to CoOx hole collector sites for the oxidation reaction. This CoOx/TiO2/Pt catalyst achieved a remarkably higher hydrogen production rate (275.9 µmol h-1) from photochemical water splitting than those of bare TiO2 nanotubes (56.5 µmol h-1), CoOxdecorated TiO2 nanotubes (CoOx/TiO2, 7.6 µmol h-1), Pt-decorated TiO2 nanotubes (TiO2/Pt, 185.9 µmol h-1), and TiO2 nanotubes with both Pt and CoOx loaded on either the inner surface or the outer surface (Figure 16B and C). 5. Conclusions and Perspectives The tailoring of metal-oxide interfaces is an important strategy in the design of novel catalysts with high catalytic activity, selectivity and stability. ALD provides a promising and controllable approach to precisely design and tailor the metal-oxide interface at an atomic scale. In this review, we presented and discussed a series of recently developed ALD strategies for tailoring the metal-oxide interface of heterogeneous catalysts, such as ALD overcoating, ultrathin modification, area-selective ALD, template-assisted ALD, and template- and sacrificial layer-assisted ALD. These methods have enabled the development of many new metal-based catalysts with different structures, such as core-shell structures, inverse oxide/metal structures, metal-in-oxide nanotraps, porous sandwich structures, multiply confined metal particles in oxide


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nanotubes, tube-in-tube nanostructures with multiple metal-oxide interfaces, and porous oxide nanotubes with two spatially separated interfaces. Excellent catalytic performance can be obtained by meeting the specific requirements of different reactions by precisely tailoring the metal-oxide interface in different structures. Furthermore, ALD has shown its potential in revealing the function and catalytic mechanism of metal-oxide interfaces during catalytic reaction. Since factors such as the size of nanoparticles, pore structure, content and dispersion can be easily controlled by ALD, it is possible to individually investigate the effect of single factors. The developed ALD approaches provide a toolkit for the design of highly efficient catalysts. It is feasible to tailor metal-oxide interfaces in confined spaces and build multifunctional catalysts in a synergetic environment, as well as to further reveal the synergetic effect of different active sites in tandem catalysis. This process is also helpful in building models for theoretical calculations due to the clear structure of the interfaces on an atomic level. However, there are still challenges in tailoring metal-oxide interfaces by ALD because of the structural complexity of such interfaces. It is highly desirable to design and synthesize heterogeneous catalysts by combining ALD and other advanced nanofabrication techniques. In addition, advanced in situ/operando experimental techniques could be utilized together with theoretical calculations to reveal catalytic processes on the metal-oxide interface and intermediate transfer processes between different functional active sites. These approaches can


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accelerate the development of heterogeneous catalysis and promote the design of new catalyst systems for high-efficiency and high-selectivity chemical reactions in the future. Author Information Bin Zhang obtained his B.S. degree in Applied Chemistry in 2008 from Shaanxi Normal University. After he received his Ph.D. degree in Chemical Engineering and Technology in 2013 from the University of Chinese Academy of Science, he became a researcher at the Institute of Coal Chemistry, Chinese Academy of Sciences, in 2013. He is currently an associate professor working primarily on atomic layer deposition, multifunctional catalysts and heterogeneous catalysis. Yong Qin obtained his B.S. in Chemistry from Chongqing University in 1996, M.S. in Materials Physics and Chemistry in 2001 from the Qingdao University of Science and Technology, and Ph.D. degree in Environmental Science in 2005 from the Ocean University of China. From 2004 to 2007, he was a postdoctoral fellow and research associate in Prof. Xin Jiang’s group at the University of Siegen. After postdoctoral work with Prof. Ulrich Goesele and Prof. Mato Knez at the Max Planck Institute of Microstructure Physics−Halle (MPI-Halle), he was appointed to full professor and group leader in 2011 at the Institute of Coal Chemistry, Chinese Academy of Sciences, supported by the “Hundred Talents Program” of the Chinese Academy of Science. His research interests focus on atomic layer deposition, nanomaterials and heterogeneous catalysis.


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Corresponding Author *[email protected]

ACKNOWLEDGMENT

We appreciate financial support from the National Key R&D Program of China (2017YFA0700101), the National Natural Science Foundation of China (21673269, 21872160, U1832208), the Youth Innovation Promotion Association of CAS (2017204), the Shanxi Science and Technology Department, and the Department of Human Resource and Social Security of Shanxi Province.

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Interface Tailoring of Heterogeneous Catalysts by Atomic Layer

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