Selective Passivation of Pt Nanoparticles with Enhanced Sintering

Subscriber access provided by University of Florida | Smathers Libraries

Article

Selective Passivation of Pt Nanoparticles with Enhanced Sintering Resistance and Activity Toward CO oxidation via Atomic Layer Deposition Jiaming Cai, Jie Zhang, Kun Cao, Miao Gong, Yun Lang, Xiao Liu, Sheng-Qi Chu, Bin Shan, and Rong Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00026 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Nano Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Applied Nano Materials

Selective Passivation of Pt Nanoparticles with Enhanced Sintering Resistance and Activity Toward CO oxidation via Atomic Layer Deposition Jiaming Cai,a,# Jie Zhang,b,# Kun Cao,*a Miao Gong,a Yun Lang,b Xiao Liu,a Shengqi Chu,c Bin Shan,b and Rong Chen*a a

State Key Laboratory of Digital Manufacturing Equipment and Technology and School of

Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China. b

State Key Laboratory of Material Processing and Die and Mould Technology and School of

Materials Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China. c

Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, People’s

Republic of China. #

These authors contributed equally to this work.

*E-mail of corresponding author: [email protected], [email protected]

KEYWORDS: Platinum nanoparticles; sintering; metal−oxide interaction; selective passivation; CO oxidation; atomic layer deposition

ACS Paragon Plus Environment

1

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

Page 2 of 40

ABSTRACT

A selective atomic layer deposition method is developed to decorate Pt nanoparticles with nickel oxide (NiOx), resulting in greatly improved catalytic performance. During the initial growth stage, NiOx can be selectively deposited on the low coordinated sites of Pt nanoparticles. The selectivity is realized through intrinsic binding energy differences of Ni precursor on Pt sites, which has been confirmed by FTIR characterizations and density functional theory simulations. The NiOx/Pt/Al2O3 catalysts show enhanced activity towards CO oxidation, which is mainly due to highly active metal-oxide interfaces created. More importantly, the sintering resistance of the composite NiOx/Pt/Al2O3 catalysts has been improved significantly, which can be attributed to the stabilization of volatile atoms at low coordinated sites, and the strong metal-oxide interaction that anchors Pt nanoparticles. This study reveals that selective passivation is an effective method to simultaneously enhance catalytic activity and stability.

.


2

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


1. INTRODUCTION Platinum has been widely used in the automobile converter to catalyze harmful exhausts like carbon monoxide (CO), nitrogen oxides (NOx).1 It is notable for high activity and chemical stability towards these reactions.2,3,4 Yet the dispersed Pt nanoparticles (NPs) working under elevated temperature have a strong sintering tendency that small Pt nanoparticles form larger particles.5 Such phenomenon is even more remarkable as nanoparticles’ size reduces to 1-2 nm or sub nanometer, as smaller particles have higher surface energy.6 Sintering leads to the decrease of total surface area of nanoparticles, thus reducing their catalytic activity significantly.7,8 The sintering of Pt nanoparticles mainly followed with particles migration and Ostwald ripening.9 During Ostwald ripening, smaller particles with low melting point tend to gasify and migrate to larger particles.10,11,12 In this situation, Pt atoms on the low coordinate sites, such as edges and corners are unstable and always become the initial gasification locations at elevated temperature, especially under oxidizing conditions.9,13 The formation of gaseous PtOx species and decompose to other particles results in the increase of Pt particles size.14 It is of great importance to improve sintering resistance and maintain Pt’s catalytic performance for long term usage. Methods are developed to enhance sintering resistance of NPs including interfacial stabilization via oxide support which provides strong metal-support interfacial interactions that can also increase activity,15,16 and encapsulating the nanoparticles with oxide coating layers,17,18 etc. The latter method is particularly effective as it provides physical barriers to anchor NPs from migration. While with this method, the thickness and configuration of the coating layer is extremely important, since thick and continuous protective coating layers would block the access of reactants to the catalysts’ surface sites and lead to activity loss.19 Synthesis methods that enable precisely control of the thickness and porous


3


Page 4 of 40

structures of coating layers are desired. Atomic layer deposition (ALD) is a powerful method to synthesize metal-oxide composite catalysts with motivations of attaining precise control over coating thickness and structure.20,21,22 Oxides fabricated by ALD method have been used as coating layers to stabilize noble metal nanoparticles.23 Such structure in general has exhibited enhanced thermal stability. Meanwhile, oxide coating also causes blockage of vast surface sites. As a precise control and selective method to deposit materials where desired, area-selective ALD (AS-ALD) has been recently developed and utilized in design and synthesis of composite catalysts.24 We demonstrated applications of AS-ALD in composite materials preparation, including patterned oxide like HfO2, ZrO2 and core shell structured Pd@Pt NPs on selfassembled monolayers modified surfaces.25,26,27,28,29 Most recently, CoOx nanotrap and CeOx nanofence structures were formed to stabilize Pt nanoparticles.30,31 It was found that oxide layer with strong metal oxide interactions (SMOI) could anchor Pt particles at high temperature. In these structures, the porous oxide coating layer acted as physical barriers to restrict particles migration and coalescence. In this study, a selective passivation method to directly stabilize the volatile sites on Pt is developed through ALD. NiOx has been utilized to decorate Pt NPs since it demonstrates strong metal oxide interactions with Pt.32,33,34 Highly active Pt-NiOx interfaces are created and the NiOx acts as a promoter for Pt catalysts, thus the catalytic activity towards CO oxidation has been enhanced.35 More importantly, the sintering resistance of NiOx/Pt/Al2O3 catalyst has been improved significantly. It is found that NiOx preferentially initiates its growth on Pt low coordinated sites. The NiOx decoration selectively passivates the unstable sites of Pt which helps to inhibit the Ostwald sintering under oxidizing conditions. Further characterizations prove that the Pt-NiOx interaction helps to maintain Pt at its oxidation states during calcination, thus


4

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


inhibits the gasification of PtOx species. The NiOx/Pt/Al2O3 catalyst is found to be stable under oxidative calcination environment up to 750 °C. Compared with physical isolations, the selective passivation method makes it possible to stabilize the volatile low coordinate sites directly by suitable NiOx decoration layer. Selective passivation keeps most of Pt’s active surface sites exposed and improves catalytic activity and thermal stability simultaneously. 2. EXPERIMENTAL SECTION 2.1. Catalysts synthesis. The substrates for catalysts synthesis and catalytic test are Si wafers. For TEM test, ultrathin carbon films and Si3N4 grids are used. 2 nm of Al2O3 thin films are firstly deposited on all the substrates. Pt ALD is performed by sequential exposure of MeCpPtMe3 (Strem Chemicals, 99%) and ozone (O3) carried out at 300 °C. The Pt ALD sequence is consisted of a 1.6 s pulse of MeCpPtMe3 and a 2 s pulse of O3. The 8 s purge of N2 is introduced between the precursor pulses to remove the residual precursors. The bare Pt catalyst is denoted as Pt/Al2O3. Nickle oxide ALD is carried out at 150 °C. The bis-(cyclopentadienyl) nickel (Ni(CP)2) and O3 are used as precursors. The pulse time of Ni(Cp)2 and O3 is 2 s and 5 s, respectively. The N2 purge time between each precursors pulse is 8 s. The Pt coated with NiOx catalyst is labeled as NiOx/Pt/Al2O3. 2.2 Catalysts characterization. Field Emission Transmission electron microscopy (FTEM, Tecnai G2 30) is performed to characterize the morphology and crystal structure of catalysts. The composition and binding energies are examined by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 photoelectron spectroscope (Thermo Fisher Scientific Inc.) using the Al Kα radiation. The thickness of the ALD films is analyzed by spectroscopic ellipsometry (SE, J. A. woollam M2000). Cauchy mode is applied to fit the film thickness of NiOx and Al2O3. The Pt


5


Page 6 of 40

film thickness is obtained from fitting through B-spline and Drude-Lorentz oscillator modeling. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of CO adsorption on catalysts are collected by wide band mercury cadmium telluride (MCT-A) detector, which is deployed on a Nicolet iS50 FTIR spectrometer from Thermo Fisher Scientific. The samples are firstly pretreated at 300 °C by 50 sccm of O2 to remove possible organic species in ALD process. Then the background spectra are collected in 50 sccm N2 flow at 30 °C. CO adsorption spectra are collected after 50 sccm 1% CO treatment and following 50 sccm N2 flow to remove physical adsorbed CO. The X-ray absorption fine structure (XAFS) spectra are performed at the 1W1B beamline of Beijing synchrotron radiation facility. The Pt L3-edge, Ni Kedge spectra are collected in grazing angle reflection mode. The XAFS data are processed by Demeter program.36 The Pt k2-weighted Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra of samples are fitted with Pt foil and PtO2 modes. The parameters such as coordinated number (N), bond length (R, Å), Debye-Waller factor (σ2, Å2) and shift in the edge energy (∆E0, eV) are fitted. The fitting R range is set from 1 Å to 3 Å which is effective bond length range for Pt-Pt and Pt-O. 2.3 Catalytic activity testing towards CO oxidation. The CO oxidation catalytic activities are tested with a planar model catalyst test system mentioned in previous work.37 The reactant gas is consisted of 0.5% CO and 4.5% O2 balanced with Ar and constantly passes through the reactor at 5 sccm. The outlet gas composition after reaction is analyzed by feeding the gas through a capillary positioned about 5 mm above the sample. A mass spectrometer (MS, AMETEK Dycor System 200 LCD) is used to detect the CO, CO2 and O2 signals through the capillary. The MS signal is converted to a total conversion yield (%) using calibrated CO2 cracking pattern to obtain the correct CO content at m/Z=28. The heating of the planar model catalyst test system is


6

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


controlled using a programmable logic controller (PLC). The isotope-labelling experiment is carried out with 18O2 and C16O as the reactant gases. Firstly, the NiOx/Pt/Al2O3 catalyst is treated by 50 sccm of 1%

16

O2 at 400 ºC to convert all the O element in the catalyst into

16

O. The CO

oxidation is carried out using 1% 18O2 and 0.5% C16O, the outlet gas in measured by recording the signals of m/e = 44 and 46 (C16O2 and C16O18O) . 2.4 Density functional theory (DFT) calculations. The DFT calculations are performed using the first-principles plane-wave pseudopotential formulation38,39,40 as implemented in the Vienna ab-initio Simulation Package (VASP). The exchange-correlation functional is Perdew-BurkeErnzerhof (PBE)41 with the generalized gradient approximation (GGA). The cutoff energy of 400 eV for the plane-wave basis, and k-mesh of 3×3×1 for different Pt surfaces are applied to insure the energy convergence is 1 meV and the residual force acting on each atom is less than 0.05 eV/Å. A 4×4, 4×4, 2×5 supercell with a 5-, 5-, and 10-layer is respectively used for the Pt(111), Pt(100), and Pt(211) slab, and the top 3, 3, and 5 layer are allowed to relax. To eliminate interactions between the Pt layer and its periodic images, we use a vacuum distance larger than 17 Å for the supercell geometry. 3. RESULTS AND DISCUSSION A schematic diagram of the selective passivation method is shown in figure 1. Pt NPs without protection layer suffer rapid sintering during calcination, which results in increasing Pt size and activity loss. With selective atomic layer deposition of NiOx on Pt, the NiOx can initiate its growth on Pt low coordinated sites and to passivate these surface sites. As a result, NiOx stabilizes the volatile low coordinated sites directly to prevent sintering through Ostwald ripening. On the other hand, the strong metal-oxide interaction between NiOx and Pt also anchors


7


Page 8 of 40

Pt NPs from particles migration. This selective decoration structure increases the stability of Pt NPs and keeps most of Pt’s surface sites exposed. Moreover, the NiOx decoration layers also provide large amount of Pt-NiOx interfaces thus promote the CO oxidation activity.

Figure 1. The schematic diagram of nickel oxide decorated Pt nanoparticles via selective ALD method and the binding energies of Ni(Cp)2 precursor located on the Pt (111), (100) facets and Pt (211) stepped regions. The calculation results indicate that NiOx precursor preferentially initiate growth on the low coordinated sites of Pt nanoparticles. In order to investigate the chemisorption of Ni(Cp)2 precursor on Pt nanoparticles, DFT simulations are performed to predict adsorption energies of Ni(Cp)2 on Pt. The slab model is utilized in the DFT calculations to represent the small particles because the primary difference that leads to adsorption energy differences in nanoparticles is the coordination number effect42. The nanoparticle size effect is secondary where the adsorption energy changes as a function of cluster size, and asymptotically approach the respective bulk slab limit43. Some related works studied the selective ALD growth of oxides on metal such as Pd, Pt also adopted similar modeling method to calculate the adsorption energy of precursors on the nanoparticles. The


8

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


calculated results are consistent with the experiments44. Thus, the slab approach is a reasonable model to study and compare the selective growth. The Pt(111) and Pt(100) represent the facets, and Pt (211) represents the stepped regions. Adsorption energy (Ead) of Ni(Cp)2 on different sites is defined by the formula: Ead= ENi(Cp)2+surface- ENi(Cp)2 - Esurface Where ENi(Cp)2+surface is the total energy of the precursors adsorbed on the surface, ENi(Cp)2 and Esurface are the energies of the precursor and the clean surfaces, respectively. The results of the DFT calculations are listed in table 1, with the corresponding mode structures plotted in figure 1. From the table below, the value of Ead is in the order of: Pt(111) > Pt(100) > Pt(211), which indicates that Ni(Cp)2 prefers to adsorb at the Pt’s low coordinated sites that possess the lowest energy, followed with flat facets of Pt(100) and Pt(111). The DFT calculations verify that during NiOx ALD process, Ni(Cp)2 preferentially chemisorbed on low coordinated sites which results in the selective passivation structure. In the early stage NiOx mainly deposited on the low coordinated sites of Pt. Table1. Adsorption energies of Ni(Cp)2 on the Pt(111), Pt(100) and Pt(211) surfaces. The unit of all the values is eV.

Pt(111)

Pt(100)

Pt(211)

ENi(Cp)2+surface

−599.18

−590.93

−699.46

ENi(Cp)2

−129.42

−129.42

−129.42

Esurface

−467.68

−459.18

−567.67

Ead

−2.08

−2.33

−2.37


9


Page 10 of 40

Spectroscopic ellipsometer is applied to analyze the growth rate of NiOx on Pt surface and Al2O3 substrate. NiOx hardly grows on Al2O3 surface, while a liner growth of NiOx on Pt without nucleation delay is observed. The growth rate is approximately 0.12±0.01 Å/cycle (shown in figure S1). The Pt surface is catalytically active to dissociate ozone into active oxygen that can initiate NiOx growth, which may be responsible for the faster growth rate on Pt than on Al2O3.45 TEM characterizations are carried out to reveal the structures of NiOx/Pt/Al2O3 catalysts. Pt nanoparticles are fabricated with 20 cycles, the clear lattice fringes with the interplanar spacing of 0.23nm correspond with Pt(111) facets (figure 2a). The size distribution shows that the Pt nanoparticles mainly located between 0.5 nm to 2.5 nm with average size of 1.44±0.03 nm. The surface area of Pt in this planar model catalyst system is calculated based on the density of Pt, and it is about 8.05*1017 Pt atoms/m2 on the substrate. After NiOx deposition on Pt nanoparticles, the oxide layer is also crystalline as can be visualized under TEM. The NiOx coating layer with interplanar spacing of 0.208 nm is in correspondence with the NiO(200) facet. By adjusting the NiOx ALD cycles, the thickness of coating layers can be precisely controlled. Figure 2b, c, d show the NiOx/Pt/Al2O3 catalysts’ morphology with increasing NiOx ALD cycles of 10, 50 and 200, respectively. It is also confirmed that NiOx prefers to nucleate on Pt NPs rather than on Al2O3 substrate during the nucleation stage. It’s worth noting that in the initial growth (10 cycles), the NiOx is hardly to be found and the lattice fringe mainly belongs to that of Pt. With increasing NiOx ALD cycles, NiOx grows thicker around Pt and the crystalline NiOx species can be observed. The NiOx deposition on Pt makes it possible to form active Pt-NiOx interfaces and the elaborate decoration structure anchors Pt NPs on the substrate. The nucleation of NiOx on Pt forms discontinuous coating layer in the initial growth stage. With increasing of NiOx ALD


10

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


cycles, the NiOx layer begins to grow around the Pt NPs and finally form core shell structures as shown in figure 2d.

Figure 2. The TEM images of Pt nanoparticles coated with NiOx layer, the ALD cycles of NiOx are (a) 0 cycles, (b) 10 cycles, (c) 50 cycles, (d) 200 cycles, respectively. TEM characterizations reveal only the local morphology information of NiOx growth on Pt surface. To explore the overall decoration structure of NiOx on Pt, and the structural evolution with increasing ALD cycles, FTIR of CO chemisorption on Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts has been performed (figure S2). It is found that no CO chemisorption peaks for pure NiOx sample. For pure Pt sample, two main peaks are observed, the peak located at 2072 cm-1 with a wide shoulder can be assigned to linearly adsorbed CO molecules on Pt atoms. Another weak peak at 1840 cm-1 is attributed to bridge-bonded CO species.46 After NiOx coating, the intensity of the linear CO on Pt decreased with increasing NiOx ALD cycles, which shows that NiOx has been deposited on the Pt surface.47 While CO chemisorption peaks on Pt still exist after


11


Page 12 of 40

NiOx coating, indicating that the Pt surface is not fully covered and remains accessible to CO. The discrete NiOx coating layer is helpful to accelerate the CO oxidation reaction as highly active NiOx/Pt interfaces are formed. Atomic scale information of NiOx coating structure on Pt can be obtained from the analyses of linear adsorption peaks fitting curves (figure 3). A linear adsorption peak can be divided into three peaks, which can be assigned to CO adsorbs on Pt(111) facet, Pt(100) facet and Pt’s low coordinated sites such as edge and corner.48,49 The position of CO chemisorption’s vibrational frequency on different Pt’s sites are also calculated(figure S3) which agrees with the experimental results. After 5 ALD cycles of NiOx deposited on Pt, the percentage of low coordinated Pt sites decreases from 53% to 25%, suggesting that these low coordinated sites are firstly to be covered (figure 3b). After 10 cycles, most of the low coordinated sites are covered (3% left from the fitting results in figure 3f). At this stage NiOx begins to deposit on Pt (100) facets, as the CO chemisorption intensity on Pt(100) begins to decrease. This selective deposition sequence is also in correspondence with DFT calculations. The low coordinated sites on Pt have been reported as unstable volatile sites during the sintering process, the direct passivation of these volatile sites is beneficial to improve the sintering resistance during Ostwald ripening while keeps most of Pt active terrace sites exposed.


12

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


Figure 3. The fitting curves of FTIR CO liner adsorption peak for Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts. The ALD cycles of NiOx is (a) 0 cycles, (b) 5 cycles, (c) 10 cycles, (d) 20 cycles, (e) 50 cycles, respectively, (f) the three divided peak’s percentage of different NiOx coating cycles. XPS is conducted to study the chemical states of Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts (figure S4). Pt, Ni and O elements are clearly observed in the spectra survey scan (figure S4a). Pt 4f, O 1s and Ni 2p spectra are displayed in figure S4b, c, d, respectively. The Pt 4f7/2 and Pt 4f5/2 peaks appear at 70.9 eV and 74.2 eV represent the metallic phase show that Pt is mainly at Pt0 state.50 The shift fitted Pt 4f peaks at 72.5 eV and 76.1 eV can be ascribed to Pt2+ species originated from Pt bonded with NiOx through Pt-O.51 The 529.1 eV and 530.8 eV peaks of O 1s are corresponded with that of lattice oxygen (O2−) and near surface adsorbed oxygen and oxygen vacancies (O22−, O−, etc.), respectively.52 The ratio of lattice oxygen to adsorbed oxygen is about 1:1 according to the calculation of the peak area. Two peaks are observed at the Ni 2p3/2 XPS spectra, the peak located at 854.0 eV belongs to that of NiO and the other peak represent Ni2O3 species in the catalysts.53 Ni 2p spectra show that Ni is in a mixed oxidation state in the catalyst.


13


Page 14 of 40

Catalytic performance of NiOx/Pt/Al2O3 catalysts is tested towards CO oxidation as a probe reaction. Figure 4a shows the light off curves of NiOx/Pt/Al2O3 with different cycles of NiOx. For pure NiOx sample, the CO conversion curve is almost the same to the blank Si substrate, which signifies that NiOx film alone cannot catalyze CO. Compared with pure Pt sample, the T50 (defined as the temperature corresponding to 50% of the maximum conversion) of NiOx/Pt/Al2O3 catalysts decreases with a certain range of NiOx coating. The lowest T50 is 184 ºC, which decreases about 33 ºC compared with the pure Pt sample, 10 cycles of NiOx coating is the optimal value. As shown in FTIR spectra, 10 cycles coating reveals that most of the low coordinated sites have been covered while leaves large amount of terrace sites exposed. When the ALD cycles of NiOx increase to 20, the intensity of low coordinate sites keeps the same, however, NiOx mainly deposit on Pt facets. With NiOx ALD cycles increase from 10 to 20, the amount of Pt-NiOx interfaces is similar, while part of the active Pt terrace sites have been covered leading to the decrease of catalytic activity. After 200 cycles NiOx ALD, the NiOx/Pt/Al2O3 catalyst is still active towards CO oxidation which indicates that the porous coating structure still exists. From the above analysis, Pt surfaces serve as the reactive sites in the beginning. With NiOx growing on Pt low coordinated sites, the interfaces between Pt and NiOx are created and accelerate the CO oxidation reaction. With NiOx further growing on Pt facets, the exposed Pt sites decrease, resulting in decrease of catalytic activity for the NiOx ALD cycles above 100. The catalytic performance evolution indicates that the CO oxidation is a structural sensitive reaction, and the precisely control of coating structure is crucial to optimize the activity. Figure 4b presents the Arrhenius curves of the Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts. The activation energies of these catalysts (Ea) are consistent with the catalytic activities. Ea of 10 cycles NiOx coated Pt catalyst (88.79 KJmol-1) is much lower than that of pure Pt (146.66 KJmol-


14

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


1

). However, Ea doubles when Pt is coated by 100 or 200 cycles of NiOx. Reaction order test of

10 cycles NiOx coated Pt is carried and shown in figure 4c. For Pt/Al2O3 catalyst, the reaction orders are −0.95 in CO and 0.78 in O2, indicating that the adsorption of CO and O2 on Pt NPs are two competed processes. The strong binding of chemisorbed CO molecules on Pt NPs inhibits the O2 activation on the Pt surface, which limits the Pt catalytic activities.54 The CO poisoning effect is a main factor that increases the reaction barrier of Pt catalysts toward CO oxidation. In the case of the NiOx/Pt/Al2O3, the reaction orders are −0.40 in CO and 0.42 in O2. The chemisorbed CO molecules on the metal can react with O2 provided by the lattice oxygen of NiOx, thus the CO on the metal is unable to suppress the rate of O2 adsorption onto NiOx.55 Compared with Pt/Al2O3, the increasing reaction order of CO and decreasing reaction order of O2 for NiOx/Pt/Al2O3 sample indicate weaker CO adsorption energies and lower O2 activation barriers. These can be attributed to the orbital hybridization and synergistic effects between NiOx and Pt.32 To further study the role of Pt-NiOx interfaces during CO oxidation, isotope-labelling experiment with C16O and 18O2 has been performed. The entire O element in the catalyst is firstly converted into 16O using 16O2 flow pretreatment. Then after CO oxidation reaction with C16O and 18

O2 molecules, the signal of C16O2 is observed in the outlet gas as shown in figure S5, which

indicates that the lattice oxygen in NiOx participates in the CO oxidation reaction that promotes the activity.56,57


15


Page 16 of 40

Figure 4. (a) CO oxidation curves, (b) Arrhenius curves of Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts, (c) reaction order test and (d) Fourier-transformed k3x data of Pt L3-edge EXAFS for Pt and 10 cycles NiOx coated Pt samples. The X-ray absorption fluorescence spectroscopy (XAFS) of Pt L3-edge is carried to study the fine structure of NiOx/Pt/Al2O3 catalysts. The Fourier transform extended X-ray absorption fine structure (EXAFS) spectra (figure 4d) indicate an enrichment of Pt-O bonds (the peak located at 1-2 Å) in NiOx/Pt/Al2O3 sample compared to Pt/Al2O3, which is consistent with XPS results. Meanwhile, the fitted Fourier transform EXAFS spectra (The fitting results are shown in figure S6) show that the fitted Pt-Pt bond lengths (table S1) of NiOx/Pt/Al2O3 and pure Pt are much


16

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


shorter than that of perfect Pt foil, which is due to orbital hybridizations of the Pt NPs.58 The NiOx/Pt/Al2O3 catalyst exhibits the shortest Pt-Pt bonds, which may be caused by the composite nanostructure formed and the strong interaction at the NiOx/Pt interfaces.

Figure 5. TEM images of (a) Pt/Al2O3 and (b, c) NiOx/Pt/Al2O3 nanoparticles after calcination, (d) particles size distribution. The calcination is 750 ºC in air for 2 h.


17


Page 18 of 40

Calcination of NiOx/Pt/Al2O3 catalysts is carried to test the catalytic thermal stability. The morphology of Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts after 750 ºC oxidative calcination are characterized with TEM (shown figure 5). Obvious sintering can be observed for pure Pt sample (figure 5a). The average size of Pt NPs increases from ~2.0 nm to ~15.5 nm. Particles with diameter larger than 20 nm can also be observed. On the contrary, no sintering phenomenon happens to NiOx/Pt/Al2O3 catalysts after calcination treatment. The average size of Pt NPs of the composite catalysts remains ~2 nm. It is noticeable that the decoration with few NiOx ALD cycles (10 cycles, Figure 5b) can achieve similar stability compared with thick NiOx coating (50 cycles, figure 5c). The size distribution of Pt nanoparticles coated with 10 and 50 cycles NiOx after calcination (NiOx coating layer is not calculated in the particle size) is similar for the two samples (Figure S7), which supports that the selective blocking of low coordinated sites can effectively stabilize Pt NPs. Figure 5d is the particles size distribution of Pt/Al2O3 and NiOx/Pt/Al2O3 (10 cycles NiOx) nanoparticles after calcination. For pure Pt nanoparticles, the catalytic activity starts to decrease as the sintering phenomenon is observed at 750 ºC. The particle size distribution of NiOx coated Pt nanoparticles remains the same, nevertheless, and the catalytic activity maintains. NiOx/Pt/Al2O3 fabricated with 10 ALD cycles of NiOx as coating layer shows both enhanced thermal stability and activity as well. The FTIR test of CO chemisorption is carried on Pt/Al2O3, NiOx/Pt/Al2O3 after calcination (shown in figure S8). It is found that after calcination, the peak of linearly adsorbed CO on Pt decreases significantly and bridged peak of CO disappears, indicating that the total amount of chemisorbed CO chemisorption decreases seriously after calcination treatment. It may be due to the decrease the total surface area for CO chemisorption caused by the sintering of Pt nanoparticles. On the contrary, the intensity of linearly adsorbed CO peak even increases for NiOx/Pt/Al2O3 catalysts,


18

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


which may be caused by the re-expose of Pt surface after calcination.59 After 750 ºC calcination for 2 h, the catalytic performance of pure Pt decreases obviously. However, for all NiOx coated samples, the activity remains even after 750 ºC calcination treatment (Figure 6). T50 of 10 cycles NiOx/Pt/Al2O3 sample is 70 ºC lower than that of Pt/Al2O3 after calcination, indicating the outstanding thermal stability of NiOx/Pt/Al2O3 catalysts. Compared with thicker coating layer, 10 cycles NiOx coating can achieve similar thermal stability (The decrease of T50 is 21 ºC and 20 ºC for 10 cycles NiOx and 50 cycles NiOx coating, respectively) which can be assigned to the selective and direct blocking of unstable sites.

Figure 6. Catalytic activity of Pt/Al2O3 and NiOx/Pt/Al2O3 towards CO oxidation after calcination treatment. XAFS is carried out to investigate the origin of the thermal stability for NiOx/Pt/Al2O3 catalysts. Figure 7a shows Pt L3-edge XANES spectra for Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts after 750 ºC aging in air for 2 h. The absorption intensity of the white line reflects the vacancy in the 5d orbital of Pt atoms.60 The white line intensity of pure Pt is quite similar with that of Pt foil, suggesting that Pt is mainly in the Pt0 (metal species) state after aging. On the other hand, the


19


Page 20 of 40

white line intensity of NiOx/Pt/Al2O3 is between Pt foil and PtO2, suggesting that the Pt has a higher oxidation state in the aged NiOx/Pt/Al2O3 sample. Generally, it is reported that PtO2 decomposes to Pt metal under oxidizing conditions at around 600 ºC and above according to the thermodynamic phase diagram.61 It is found that Pt is in the Pt0 state in pure Pt after aging at 750 ºC in the oxidizing atmosphere. On the other hand, the NiOx coating can stabilize a high oxidation state of Pt after aging. It is suggested that the strong Pt-NiOx interaction in the NiOx/Pt/Al2O3 contribute to the stabilization of high-oxidation state of Pt under the oxidizing condition. Fourier transform EXAFS spectra of the catalysts after calcination are shown in figure 7b to investigate the interaction between Pt and NiOx after calcination. For the aged Pt sample, the fitted Pt-Pt bond lengths (The fitting results are shown in figure S9) are closed to that of Pt foil. The FT spectrum of NiOx/Pt/Al2O3 after aging differs from both Pt foil and PtO2 sample. The fitted Pt-Pt bond lengths are much shorter than that of Pt foil, which suggests that strong interaction between Pt and NiOx maintains after calcination. It's worth noting that the oxidative state of NiOx is stable during the calcination process (normalized Ni K-edge XANES spectra in figure S10), where the spectra before and after calcination are almost the same. This phenomenon differs from oxides like Co3O4 whose oxidation state influenced significantly by oxidation calcination, thus cause the catalytic activity loss of Co3O4/Pt.32 The interaction between Pt and NiOx and the stability of NiOx in oxidizing environment keep the activity of NiOx/Pt/Al2O3 after aging. XPS characterizations of Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts in oxidative and inert environment are conducted to evidence the stabilization of the high-oxidation state upon NiOx coating. For pure Pt sample (figure 7c), the peak intensity decreases strongly when annealing in


20

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


O2 flow，which can be assigned to the gasification of Pt to PtOx species and carried off by O2 flow. It also infers that the volatile PtOx species are formed in oxidation environment, and the readsorption and decomposition of PtOx species on other Pt NPs cause sintering.13,14 The CO oxidation test after annealing in oxidative and inert environment (figure S11) also supports this deduction. The XPS results for calcinated NiOx/Pt/Al2O3 sample are shown figure 7d, the peak intensity remains and it is found that Pt2+/Pt0 increases (Pt2+/Pt0 = 0.131 in He and 0.244 in O2) which is in agreement with the XANES results (figure S12). The XPS and XANES results indicate that Pt species has been partly oxidized in oxidizing conditions, and NiOx helps to stabilize the PtOx spices at their higher oxidation states to improve the sintering resistance.

Figure 7. (a) Pt L3-edge XANES spectra, (b) Fourier-transformed k3x data of Pt L3-edge EXAFS for Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts after 750 ºC aging in air for 2h, XPS of Pt 4d spectra


21


Page 22 of 40

for (c) Pt and (d) NiOx/Pt/Al2O3 catalysts after 750 ºC aging in absence of O2 (He environment) and presence of O2 for 2h. Compared with supported Pt catalysts, the selective decoration structure can directly stabilize the unstable sites (low coordinated sites) of Pt thus improves the thermal stability of Pt greatly. The comparison of coated catalysts and supported catalysts is carried and shown in Figure S13. The T50 increases 44 oC (from 177 oC to 224 oC) after calcination which is much larger than that of NiOx decorated Pt nanoparticles (about 20 oC). At the same time, if 10 cycles NiOx is deposited on supported Pt catalysts to form NiOx/Pt/NiOx structure, the catalytic activity can be further increased as more metal-oxide interfaces created (Figure S13). 4. CONCLUSIONS NiOx decorated Pt nanoparticles catalysts have been synthesized via selective ALD method. In the initial growth stage, NiOx can be selectively deposited on the low coordinated sites of Pt NPs. The selectivity is realized through intrinsic binding energy differences of Ni precursor on Pt sites. The growth behavior is supported by FTIR analysis and DFT simulations. The NiOx/Pt/Al2O3 catalysts exhibit both enhanced catalytic activity and thermal stability compared with Pt/Al2O3 towards CO oxidation. The enhanced catalytic activity is mainly due to highly active oxide-metal interfaces created. The promoted sintering resistance ability of the composite catalyst can be attributed to the stabilization of volatile atoms at low coordinated sites and the strong metal-oxide interaction that inhibits sintering of particles. ASSOCIATED CONTENT Supporting Information Available: Growth rate for NiOx on Pt and Al2O3. FTIR spectra of the catalysts before and after calcination. DFT calculation of CO vibration frequency on Pt’s


22

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


different sites. XPS of NiOx/Pt/Al2O3. Results of isotope-labelling experiment. Fitting of EXAFS spectra for catalysts before and after calcination. Pt size distribution after calcination with 10 cycles and 50 cycles NiOx coating. Pt L3-edge and Ni K-edge XANES spectra of NiOx/Pt/Al2O3 before and after calcination. CO oxidation curves for Pt/Al2O3 after calcination in different environment. Thermal stability of NiOx supported Pt catalyst and NiOx coated Pt catalyst. The Supporting Information is available free of charge e via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by National Basic Research Program of China (2013CB934800), the National Natural Science Foundation of China (51575217, 51702106, 51572097), the Hubei Province Funds for Distinguished Young Scientists (2015CFA034) and the China Postdoctoral Science Foundation (Grant 2017M622433). Rong Chen acknowledges the Thousand Young Talents Plan, the Recruitment Program of Global Experts and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13017). The authors would also like


23


Page 24 of 40

to acknowledge the technology support by Analytic Testing Center and Flexible Electronics Research Center of HUST.

REFERENCES (1) Zheng, B.; Wu, S.; Yang, X.; Jia, M.; Zhang, W.; Liu, G., Room Temperature CO Oxidation over Pt/MgFe2O4: A Stable Inverse Spinel Oxide Support for Preparing Highly Efficient Pt Catalyst. ACS Appl. Mater. Interfaces, 2016, 8, 26683-26689. (2) Huang, M.; Zhang, J.; Wu, C.; Guan, L., Pt Nanoparticles Densely Coated on SnO2-Covered Multiwalled Carbon Nanotubes with Excellent Electrocatalytic Activity and Stability for Methanol Oxidation. ACS Appl. Mater. Interfaces, 2017, 9, 26921-26927. (3) Wang, J.; Wang, Z.; Liu, C., Enhanced Activity for CO Oxidation over WO3 Nanolamella Supported Pt Catalyst. ACS Appl. Mater. Interfaces, 2014, 6, 12860-12867. (4) Zarraga-Colina, J.; Nix, R. M., Fabrication of Model Pt-ceria Catalysts and an Analysis of Their Performance for CO Oxidation. Surf. Sci. 2006, 600, 3058-3071. (5) Asoro, M. A.; Kovar, D.; Shao-Horn, Y.; Allard, L. F.; Ferreira, P. J., Coalescence and Sintering of Pt Nanoparticles: in situ Observation by Aberration-corrected HAADF STEM. Nanotechnology 2010, 21, 025701. (6) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T., Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Accounts. Chem. Res. 2013, 46, 1740-1748. (7) Li, W.-Z.; Kovarik, L.; Mei, D.; Engelhard, M. H.; Gao, F.; Liu, J.; Wang, Y.; Peden, C. H. F., A General Mechanism for Stabilizing the Small Sizes of Precious Metal Nanoparticles on Oxide Supports. Chem. Mater. 2014, 26, 5475-5481.


24

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


(8) Xiong, H.; Peterson, E.; Qi, G.; Datye, A. K., Trapping Mobile Pt Species by PdO in Diesel Oxidation Catalysts: Smaller Is Better. Catal. Today 2016, 272, 80-86. (9) Hansen, T. W.; Delariva, A. T.; Challa, S. R.; Datye, A. K., Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald Ripening? Accounts. Chem. Res. 2013, 46, 17201730. (10) Simonsen, S. B.; Chorkendorff, I.; Dahl, S.; Skoglundh, M.; Sehested, J.; Helveg, S., Direct Observations of Oxygen-induced Platinum Nanoparticle Ripening Studied by In Situ TEM. J. Am. Chem. Soc. 2010, 132, 7968-7975. (11) Simonsen, S. B.; Chorkendorff, I.; Dahl, S.; Skoglundh, M.; Sehested, J.; Helveg, S., Ostwald Ripening in a Pt/SiO2 Model Catalyst Studied by in situ TEM. J. Catal. 2011, 281, 147155. (12) Benavidez, A. D.; Kovarik, L.; Genc, A.; Agrawal, N.; Larsson, E. M.; Hansen, T. W.; Karim, A. M.; Datye, A. K., Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts. ACS Catal. 2012, 2, 2349-2356. (13) Shi, W.; Li, J.; Polsen, E. S.; Oliver, C. R.; Zhao, Y.; Meshot, E. R.; Barclay, M.; Fairbrother, D. H.; Hart, A. J.; Plata, D. L., Oxygen-promoted Catalyst Sintering Influences Number Density, Alignment, and Wall Number of Vertically Aligned Carbon Nanotubes. Nanoscale 2017, 9, 5222-5233. (14) Porsgaard, S.; Merte, L. R.; Ono, L. K.; Behafarid, F.; Matos, J.; Helveg, S.; Salmeron, M.; Cuenya, B. R.; Besenbacher, F., Stability of Platinum Nanoparticles Supported on SiO2/Si(111): A High-Pressure X-ray Photoelectron Spectroscopy Study. ACS Nano 2012, 6, 10743-10749.


25


Page 26 of 40

(15) Park, J. Y.; Somorjai, G. A., Hot Electron Surface Chemistry at Oxide–Metal Interfaces: Foundation of Acid-base Catalysis. Catal. Lett. 2016, 146, 1-11. (16) Park, J. Y.; Baker, L. R.; Somorjai, G. A., Role of Hot Electrons and Metal–Oxide Interfaces in Surface Chemistry and Catalytic Reactions. Chem. Rev. 2015, 115, 2781-2817. (17) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A., Thermally Stable Pt/mesoporous Silica Core-shell Nanocatalysts for High-temperature Reactions. Nat. Mater. 2009, 8, 126-131. (18) Moon, S. Y.; Naik, B.; Jung, C.-H.; Qadir, K.; Park, J. Y., Tailoring Metal-oxide Interfaces of Oxide-encapsulated Pt/silica Hybrid Nanocatalysts with Enhanced Thermal Stability. Catal. Today 2016, 265, 245-253. (19) Ma, Z.; Brown, S.; Howe, J. Y.; Overbury, S. H.; Dai, S., Surface Modification of Au/TiO2 Catalysts by SiO2 via Atomic Layer Deposition. J. Phys. Chem. C 2008, 112, 9448-9457. (20) 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. (21) George, S. M., Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111-131. (22) Lee, K., Kim, D. H., Parsons, G. N., Free-Floating Synthetic Nanosheets by Atomic Layer Deposition, ACS Appl. Mater. Interfaces, 2014, 6, 10981-10985. (23) Lu, J.; Elam, J. W.; Stair, P. C., Synthesis and Stabilization of Supported Metal Catalysts by Atomic Layer Deposition. Accounts. Chem. Res. 2013, 46, 1806-1815. (24) 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, 52425253.


26

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


(25) Chen, R.; Bent, S. F., Chemistry for Positive Pattern Transfer Using Area-selective Atomic Layer Deposition. Adv. Mater. 2006, 18, 1086-1090. (26) Chen, R.; Bent, S. F., Highly Stable Monolayer Resists for Atomic Layer Deposition on Germanium and Silicon. Chem. Mater. 2006, 18, 3733-3741. (27) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F., Self-assembled Monolayer Resist for Atomic Layer Deposition of HfO2 and ZrO2 High-κ Gate Dielectrics. Appl. Phys. Lett. 2004, 84, 4017-4019. (28) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F., Investigation of Self-assembled Monolayer Resists for Hafnium Dioxide Atomic Layer Deposition. Chem. Mater. 2005, 17, 536-544. (29) Cao, K.; Zhu, Q.; Shan, B.; Chen, R., Controlled Synthesis of Pd/Pt Core Shell Nanoparticles Using Area-selective Atomic Layer Deposition. Sci. Rep. 2015, 5, 8470. (30) Liu, X.; Zhu, Q.; Lang, Y.; Cao, K.; Chu, S.; Shan, B.; Chen, R., Oxide-Nanotrap-Anchored Platinum Nanoparticles with High Activity and Sintering Resistance by Area-Selective Atomic Layer Deposition. Angew. Chem. Int. Edit. 2017, 56, 1648-1652. (31) Cao, K.; Shi, L.; Gong, M.; Cai, J.; Liu, X.; Chu, S.; Lang, Y.; Shan, B.; Chen, R., Nanofence Stabilized Platinum Nanoparticles Catalyst via Facet-Selective Atomic Layer Deposition. Small 2017, 13, 1700648. (32) An, K.; Alayoglu, S.; Musselwhite, N.; Plamthottam, S.; Melaet, G.; Lindeman, A. E.; Somorjai, G. A., Enhanced CO Oxidation Rates at the Interface of Mesoporous Oxides and Pt Nanoparticles. J. Am. Chem. Soc. 2013, 135, 16689-16696. (33) Han, S. W.; Kim, D. H.; Jeong, M.-G.; Park, K. J.; Kim, Y. D., CO Oxidation Catalyzed by NiO Supported on Mesoporous Al2O3 at Room Temperature. Chem. Eng. J. 2016, 283, 992-998.


27


Page 28 of 40

(34) Jeong, M.-G.; Kim, I. H.; Han, S. W.; Kim, D. H.; Kim, Y. D., Room Temperature CO Oxidation Catalyzed by NiO Particles on Mesoporous SiO2 Prepared via Atomic Layer Deposition: Influence of Pre-annealing Temperature on Catalytic Activity. J. Mol. Catal. A: Chem. 2016, 414, 87-93. (35) Mohamed, Z.; Dasireddy, V. D. B. C.; Singh, S.; Friedrich, H. B., The Preferential Oxidation of CO in Hydrogen Rich Streams over Platinum Doped Nickel Oxide Catalysts. Appl. Catal. B: Environ. 2016, 180, 687-697. (36) Ravel, B.; Newville, M., ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537-541. (37) Cao, K.; Liu, X.; Zhu, Q.; Shan, B.; Chen, R., Atomically Controllable Pd@Pt Core-Shell Nanoparticles towards Preferential Oxidation of CO in Hydrogen Reactions Modulated by Platinum Shell Thickness. ChemCatChem 2016, 8, 326-330. (38) Yang, C.; Balakrishnan, N.; Bhethanabotla, V'; Joseph, B.; Interplay between Subnanometer Ag and Pt Clusters and Anatase TiO2 (101) Surface: Implications for Catalysis and Photocatalysis. J. Phys. Chem. C 2017, 118, 4702-4714. (39) Kvashnin, A. G.; Oganov, A. R.; Samtsevich, A. I.; Allahyari, Z., Computational Search for Novel Hard Chromium-Based Materials. J. Phys. Chem. Lett. 2017, 8, 755-764. (40) Liu, Q.; Li, J.; Tong, X.; Zhou, G., Enhancing Dissociative Adsorption of Water on Cu(111) via Chemisorbed Oxygen. J. Phys. Chem. C 2017, 121, 12117-12126. (41) Balout, H.; Roques, J.; Gautheron, C.; Tassan-Got, L., Computational Investigation of Interstitial Neon Diffusion in Pure Hematite. Comp. Mater. Sci. 2017, 128, 67-74.


28

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


(42) Allian, A. D.; Takanabe, K.; Fujdala, K. L.; Hao, X.; Truex, T. J.; Cai, J.; Buda, C.; Neurock, M.; Iglesia, E., Chemisorption of CO and Mechanism of CO Oxidation on Supported Platinum Nanoclusters. J. Am. Chem. Soc. 2011, 133, 4498-4517. (43) Laletina, S. S.; Mamatkulov, M.; Shor, E. A.; Kaichev, V. V.; Genest, A.; Yudanov, I. V.; Rösch, N., Size-Dependence of the Adsorption Energy of CO on Pt Nanoparticles: Tracing Two Intersecting Trends by DFT Calculations. The Journal of Physical Chemistry C 2017, 121, 17371-17377. (44) Lu, J.; Liu, B.; Guisinger, N. P.; Stair, P. C.; Greeley, J. P.; Elam, J. W., First-Principles Predictions and in Situ Experimental Validation of Alumina Atomic Layer Deposition on Metal Surfaces. Chemistry of Materials 2014, 26, 6752-6761. (45) Weber, M. J.; Mackus, A. J. M.; Verheijen, M. A.; van der Marel, C.; Kessels, W. M. M., Supported Core/Shell Bimetallic Nanoparticles Synthesis by Atomic Layer Deposition. Chem. Mater. 2012, 24, 2973-2977. (46) Yajima, T.; Uchida, H.; Watanabe, M., In-situ ATR-FTIR Spectroscopic Study of Electrooxidation of Methanol and Adsorbed CO at Pt-Ru Alloy. J. Phys. Chem. B 2004, 108, 2654-2659. (47) Lu, J.; Low, K.-B.; Lei, Y.; Libera, J. A.; Nicholls, A.; Stair, P. C.; Elam, J. W., Toward Atomically-precise Synthesis of Supported Bimetallic Nanoparticles Using Atomic Layer Deposition. Nat. Commun. 2014, 5, 3264. (48) Kobayashi, S.; Kaneko, S.; Ohshima, M.-a.; Kurokawa, H.; Miura, H., Effect of Iron Oxide on Isobutane Dehydrogenation over Pt/Fe2O3-Al2O3 Catalyst. Appl. Catal. A: Gen. 2012, 417418, 306-312.


29


Page 30 of 40

(49) Hoxha, F.; Schimmoeller, B.; Cakl, Z.; Urakawa, A.; Mallat, T.; Pratsinis, S. E.; Baiker, A., Influence of Support Acid–base Properties on the Platinum-catalyzed Enantioselective Hydrogenation of Activated Ketones. J. Catal. 2010, 271, 115-124. (50) Zhang, K.; Yue, Q.; Chen, G.; Zhai, Y.; Wang, L.; Wang, H.; Zhao, J.; Liu, J.; Jia, J.; Li, H., Effects of Acid Treatment of Pt-Ni Alloy Nanoparticles@Graphene on the Kinetics of the Oxygen Reduction Reaction in Acidic and Alkaline Solutions. J. Phys. Chem. C 2011, 115, 379389. (51) Kolobov, N. S.; Svintsitskiy, D. A.; Kozlova, E. A.; Selishchev, D. S.; Kozlov, D. V., UVLED Photocatalytic Oxidation of Carbon Monoxide over TiO2 Supported with Noble Metal Nanoparticles. Chem. Eng. J. 2017, 314, 600-611. (52) Feng, Z.; Wang, J.; Liu, X.; Wen, Y.; Chen, R.; Yin, H.; Shen, M.; Shan, B., Promotional Role of La Addition in the NO Oxidation Performance of a SmMn2O5 Mullite Catalyst. Catal. Sci. Technol. 2016, 6, 5580-5589. (53) Diegle, R. B.; Sorensen, N. R.; Clayton, C. R.; Helfand, M. A.; Yu, Y. C., An XPS Investigation into the Passivity of an Amorphous Ni-20P Alloy. J. Electrochem. Soc. 1988, 135, 1085-1092. (54) Berlowitz, P. J.; Peden, C. H. F.; Goodman, D. W., Kinetics of CO Oxidation on Single Crystal Pd, Pt and Ir. J. Phys. Chem. 1988, 92, 5213-5221. (55) Cargnello, M.; Doan-Nguyen, V. V. T.; Gordon, T. R.; Diaz, R. E.; Stach, E. A.; Gorte, R. J.; Fornasiero, P.; Murray, C. B., Control of Metal Nanocrystal Size Reveals Metal-Support Interface Role for Ceria Catalysts. Science 2013, 341, 771-773.


30

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


(56) Fu, Q.; Li, W. X.; Yao, Y.; Liu, H.; Su, H. Y.; Ma, D.; Gu, X. K.; Chen, L.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X., Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328, 1141-1144. (57) Sun, Y.-N.; Giordano, L.; Goniakowski, J.; Lewandowski, M.; Qin, Z.-H.; Noguera, C.; Shaikhutdinov, S.; Pacchioni, G.; Freund, H.-J., The Interplay between Structure and CO Oxidation Catalysis on Metal-Supported Ultrathin Oxide Films. Angew. Chem. Int. Edit. 2010, 49, 4418-4421. (58) Gatla, S.; Aubert, D.; Agostini, G.; Mathon, O.; Pascarelli, S.; Lunkenbein, T.; Willinger, M. G.; Kaper, H., Room-Temperature CO Oxidation Catalyst: Low-Temperature Metal–Support Interaction between Platinum Nanoparticles and Nanosized Ceria. ACS Catal. 2016, 6, 61516155. (59) Lu, J.; Fu, B.; Kung, M. C.; Xiao, G.; 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. (60) Mansour, A. N.; Cook, J. W.; Sayers, D. E., Quantitative Technique for the Determination of the Number of Unoccupied d-electron States in a Platinum Catalyst Using the L2,3 x-ray Absorption Edge Spectra. J. Phys. Chem. 1984, 88, 2330-2334. (61) Nagai, Y.; Hirabayashi, T.; Dohmae, K.; Takagi, N.; Minami, T.; Shinjoh, H.; Matsumoto, S., Sintering Inhibition Mechanism of Platinum Supported on Ceria-based Oxide and Pt-oxide– Support Interaction. J. Catal. 2006, 242, 103-109.


31


Page 32 of 40

TOC:


32

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


Figure 1.The schematic diagram of nickel oxide decorated Pt nanoparticles via selective ALD method and the binding energies of Ni(Cp)2 precursor located on the Pt (111), (100) facets and Pt(211) stepped regions. The calculation results indicate that NiOx precursor preferentially initiate growth on the low coordinated sites of Pt nanoparticles. 47x26mm (300 x 300 DPI)



Figure 2. The TEM images of Pt nanoparticles coated with NiOx layer, the ALD cycles of NiOx are (a) 0 cycles, (b) 10 cycles, (c) 50 cycles, (d) 200 cycles, respectively. 84x84mm (300 x 300 DPI)


Page 34 of 40

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


Figure 3. The fitting curves of FTIR CO liner adsorption peak for Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts. The ALD cycles of NiOx is (a) 0 cycles, (b) 5 cycles, (c) 10 cycles, (d) 20 cycles, (e) 50 cycles, respectively, (f) the three divided peak’s percentage of different NiOx coating cycles. 39x18mm (300 x 300 DPI)



Figure 4. (a) CO oxidation curves, (b) Arrhenius curves of Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts, (c) reaction order test and (d) Fourier-transformed k3x data of Pt L3-edge EXAFS for Pt and 10 cycles NiOx coated Pt samples. 66x52mm (300 x 300 DPI)


Page 36 of 40

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


Figure 5. TEM images of (a) Pt/Al2O3 and (b, c) NiOx/Pt/Al2O3 nanoparticles after calcination, (d) particles size distribution. The calcination is 750 ºC in air for 2 h. 82x79mm (300 x 300 DPI)



Figure 6. Catalytic activity of Pt/Al2O3 and NiOx/Pt/Al2O3 towards CO oxidation after calcination treatment. 61x44mm (300 x 300 DPI)


Page 38 of 40

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


Figure 7. (a) Pt L3-edge XANES spectra, (b) Fourier-transformed k3x data of Pt L3-edge EXAFS for Pt/Al2O3 and NiOx/Pt/Al2O3 catalysts after 750 ºC aging in air for 2h, XPS of Pt 4d spectra for (c) Pt and (d) NiOx/Pt/Al2O3 catalysts after 750 ºC aging in absence of O2 (He environment) and presence of O2 for 2h. 64x49mm (300 x 300 DPI)



Table of Contents 44x31mm (300 x 300 DPI)


Page 40 of 40

Selective Passivation of Pt Nanoparticles with Enhanced Sintering

Recommend Documents