Surface Engineering Protocol To Obtain an Atomically Dispersed Pt

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A Surface Engineering Protocol to Obtain an Atomically Dispersed Pt/CeO2 Catalyst with High Activity and Stability for CO Oxidation jiayu Chen, Yongjin Wanyan, Jianxin Zeng, Huihuang Fang, Zejun Li, Yongdi Dong, Ruixuan Qin, Changzheng Wu, Deyu Liu, Mingzhi Wang, Qin Kuang, Zhaoxiong Xie, and Lan-Sun Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02613 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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ACS Sustainable Chemistry & Engineering

A Surface Engineering Protocol to Obtain an Atomically Dispersed Pt/CeO2 Catalyst with High Activity and Stability for CO Oxidation Jiayu Chen,† Yongjin Wanyan, † Jianxin Zeng,† Huihuang Fang,† Zejun Li,‡ Yongdi Dong,† Ruixuan Qin,† Changzheng Wu,‡ Deyu Liu,† Mingzhi Wang, † Qin Kuang,*, † Zhaoxiong Xie,*,† and Lansun Zheng† †

State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center

of Chemistry for Energy Materials Department of Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, No.422, Siming South Road, Xiamen, Fujian, 361005, China. ‡

Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative, Innovation

Center of Chemistry for Energy Materials, Hefei Science Center (CAS), CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, No.96, JinZhai Road Baohe District, Hefei, Anhui, 230026, China.

* Correspondence to: [email protected]; zxxie@ xmu.edu.cn

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ABSTRACT: Atomically dispersed metal catalysts often exhibit a superior performance than nanoparticle catalysts in many catalysis processes. However, these so-called ‘single-atom’ catalysts have a consistently low loading density on the support surface and easily aggregate at high temperatures, hindering their practical application. Herein, we demonstrate a facile surface engineering protocol using molecule-surface charge transfer adducts to fabricate highly stable noble metal catalysts with atomic dispersion, using a Pt/CeO2 catalyst as an example. The key of this approach is the generation of an adequate amount of Ce3+ defective sites on the porous CeO2 surface through the adsorption of reductive ascorbic acid molecules and a subsequent surface charge transfer process. Subsequently, noble metal Pt atoms can be well-dispersedly anchored onto the generated Ce3+ sites of porous CeO2 nanorods with a loading density of up to 1.0 wt%. The as-prepared highly dispersed Pt/CeO2 catalyst showed outstanding catalytic activity at near room temperature towards CO oxidation, with excellent stability over several days, which is far superior to the traditional impregnation-prepared catalysts, the activity (complete conversion at 90 °C) of which is severely decayed within a couple of hours. The proposed synthetic route is simple yet versatile, and can therefore be potentially applied to fabricate other supported noble metal catalysts with atomic dispersion.

KEYWORDS: Ascorbic Acid, Surface Engineering, Reductive Site, Atomic Dispersion, Low Temperature CO Oxidation


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INTRODUCTION

Atomically dispersed metal catalysts have attracted increasing interest in the past decade due to their high catalytic activity and high selectivity towards specific reactions as well as their maximum atom utilization efficiency, arising from the highly unsaturated coordination environment and the homogeneity of their active catalytic sties.1-4 Besides their major advantages with regards to performance and cost, such catalysts are deemed as a promising bridge between homogeneous and heterogeneous catalysts to light up the surprising effects from the surface of supports and metal-support interfaces.5 Therefore, much research has focused on the controlled synthesis of atomically dispersed metal catalysts.6-11 Recently, in addition to traditional precipitation and impregnation routes, new synthetic strategies have been proposed, including support-mediated photochemical routes,5,12 metal-organic framework-templated heat decomposition,13-15 and atomic layer deposition,16,17 among others. Nevertheless, despite significant progress being made in this field, further efforts are required to obtain atomically dispersed metal catalysts with tailored properties and performances. Furthermore, as emerging technique catalysts, currently available atomically dispersed metal catalysts have a low surface concentration1,6,7,9,16 and poor stability.18 Atomically dispersed metal catalysts loaded onto the support are prone to aggregation, forming nanoclusters or nanoparticles during pre-processing or catalysis, especially at high loading concentrations, thereby leading to a significant decrease of activity and selectivity.11,19 Additionally, the activity and stability of supported metal catalysts are highly correlated to the oxidation states of the catalytic metal active sites, which often change upon interaction with the support.19-21 However, precisely engineering the metal catalyst-support interface (i.e., controlling the loading status of metal catalysts on the


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support) is difficult to achieve with the currently available technologies regardless of the synthetic route; nevertheless, its importance in promoting catalytic performance has been wide acknowledged.1,11,18,22-24 Herein, we propose a new strategy based on a facile surface engineering protocol using a molecule-surface charge transfer adduct to fabricate atomically dispersed noble metal catalysts, using the Pt/CeO2 catalyst as a proof of concept. Pt is highly active for many oxidation reactions, including CO oxidation,25,26 CO preferential oxidation reaction in H2-rich gases,22,27-29 and threeway catalysis.21,30-32 These reactions usually proceed at an elevated temperature (typically ~80 °C for preferential oxidation reaction and ~350 °C or higher for three-way catalysis), and thus it is crucial for Pt-based catalysts to avoid deactivation due to agglomeration of Pt atoms on the support during catalysis observed at these temperatures.33 Because of its excellent oxygen capacities, abundant surface oxygen vacancies, and easy shuttle between the 3+ and 4+ oxidation states, ceria (CeO2) is considered a promising support for atomically dispersed metal catalysts for oxidation reactions.11,19,24,23,34 In our proposed synthetic route, a large number of Ce3+ sites are pre-reduced on the surface of a porous CeO2 support through the adsorption of the reducing reagent L-ascorbic acid (AA),35 which enables the trapping of Pt atoms, preventing their migration and aggregation. The use of such a pre-reduced support allows the loaded Pt species to remain isolated and sinter-resistant even at loading densities of ca. 1 wt% after the calcination treatment. As expected, the as-synthesized highly dispersed Pt/CeO2 catalyst displays a superior catalytic activity and stability in the CO oxidation reaction compared to catalysts prepared through traditional impregnation methods.

Experimental Section


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Chemicals. Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99.8%), sodium hydroxide (NaOH, 99.8%), 0.196 mmol L-1 hydro chloroplatinic acid (H2PtCl6, 99.8%) , and L-(+)-ascorbic acid (AA, 99.8%) were purchased from commercial suppliers (Alfa Aesar and Sinopharm Chemical Regent Co., Ltd.) and used as received without further purification. Synthesis of porous CeO2 nanorods. Porous CeO2 nanorods were prepared based on the method reported in Mai’s study.31 In a typical experiment, Ce(NO3)3·6H2O (1.736 g, 4 mmol) and NaOH (19.2 g, 0.48 mol) were mixed first, and then dissolved into distilled water (80 mL). After the mixture was vigorously stirred for 30 minutes, the resulting solution was transferred to a Teflon-lined stainless steel autoclave (100 mL) and kept at 100 °C for 12 hours. The products were collected by centrifugation at 8,000 rpm, and washed several times with distilled water. Finally, the products were dried under 60 °C over night and annealed at 400 °C for 1 hour. Synthesis of the CeO2-AA-Pt-cal catalyst. First, porous CeO2 nanorods (500 mg) were dispersed in distilled water (175 mL), and AA (176 mg, 1 mmol) was added, followed by vigorous stirring for 3 hours at room temperature. The resulting suspension was divided into six equal parts, and the solid particles in each part was collected by centrifugation at 8,000 rpm, washed four times with distilled water (30 mL) each time, and dried under 60 °C overnight. The product obtained at this step was denoted as CeO2-AA. Second, CeO2-AA (140 mg) was redispersed in distilled water (55 mL) and H2PtCl6 solution (0.372 mL, 10 mg mL−1) was added. After being stirred for 3 hours at 30 °C, the products was collected by centrifugation at 8,000 rpm, washed two times with distilled water, and dried under vacuum at room temperature. The product obtained at this step was denoted as CeO2-AA-Pt. Next, the CeO2-AA-Pt was calcined at 300 °C under static air for 1 hour, and the final product was denoted as CeO2-AA-Pt-cal.


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Synthesis of CeO2-IMP-Pt catalyst. The catalyst CeO2-IMP-Pt was prepared by traditional incipient wetness impregnation. Briefly, porous CeO2 nanorods (280 mg) was wet with 0.5 mL distilled water, and H2PtCl6 solution (0.744 mL, 10 mg mL-1) was added, followed by vigorous stirring for 3 hours at room temperature. After that, the products was dried under vacuum at room temperature and then treated with 5 % H2/Ar at 250 °C for 1 hour. For comparison, the reference catalyst CeO2-IMP-Pt-cal was specifically prepared by further calcining the CeO2IMP-Pt catalyst at 300 °C under static air for 1 hour. Catalytic measurement of catalyst. The catalytic activity of catalysts towards CO oxidation was performed in a continuous flow reactor. Before fed into the reactor, the reaction gases, CO (99.99%) (0.5 mL min-1), O2 (99.99%) (8 mL min-1) and N2 (99.99%) balance passed above a H2SO4 aqueous solution held in a bottle at a total gas flow of 50 mL min-1, by which the relative humidity (RH) was controlled to 50 ± 5%. 100 mg of catalysts were set in a fixed-bed flow reactor made of glass with an inner diameter of 2.5 mm. Steady-state catalytic activity was measured with the reaction temperature rising from room temperature to a completely CO conversion temperature at interval of 10 °C. The effluent gas was analyzed on-line by an onstream gas chromatograph (Ramiin GC 2060) equipped with a TDX-01 column. We measured the specific rate and turnover frequency (TOF) under different conditions, in which 20 mg of catalysts (GHSV = 150000h-1) and 20 mg SiO2 were physically mixed and set in the fixed-bed flow reactor to maintain a low CO conversion. Characterization of samples. The compositions of products were determined by powder Xray diffraction (XRD) on a Rigaku D/max X-ray diffractometer (Cu Ka radiation, 0.15418 nm). The morphology and crystal structure of products were observed by high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100) with an acceleration voltage of 200 kV. High-


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angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and element mapping analyses were carried out on a FEI TECNAI F30 microscopy operated at 300 kV. Cs-corrected HAADF-STEM images was carried out by Titan Cubed Themis G2 300. All TEM samples were prepared by depositing a drop of diluted suspension in distilled water on a copper grid coated with carbon film. Thermogravimetric (TG) Analysis of products was carried out by STD Q600 in air at heating rate of 10 °C min-1 and ultraviolet and visible spectrum (UVVis spectrum) was measured by Varian Cary 5000 UV-Vis-NIR Spectrophotometer at a diffusereflection mode. The surface structure of products was analyzed by X-ray photoelectron spectroscopy (XPS), which was carried out in a UHV system using a monochromatised Al Kα radiation (hν=1486.6 eV), and the binding energies were calibrated with respect to the signal for adventitious carbon (binding energy of 284.6 eV). Normal XPS was measured by PHI Quantum2000 and ex-situ XPS was measured by Qtac-100 LEISS-XPS. The precise contents of Pt element in the samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Baird PS-4). The X-ray absorption fine structure (XAFS) measurements were performed in fluorescence mode at the 1W1B beamline of the Beijing Synchrotron Radiation Facility. Si (111) double-crystal monochromator was calibrated by Pt foil and the XAFS data of Pt foil was collected for the reference spectrum. All the spectra were recorded in ambient conditions. The XAFS data analysis was processed with the programs of Athena and Artemis, and Demeter package was used in this work. In-situ DRIFTS study. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was recorded on a Thermo Nicolet 6700 equipped with an in situ reaction chamber, which is covered by ZnSe filters. Before CO adsorption, the cell was heated to 300 °C for 1 hour. After cooled to room temperature, the chamber was purged with Ar (99.99%) gas flow at a flow rate of


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40 mL min-1 for 20 min and then a background spectrum was taken. For CO adsorption step, the CO (99.99%) gas was controlled by mass flow meters and fed at a flow rate of 40 mL min-1 for 3 minutes. The spectra were recorded at room temperature by accumulating 256 scans at a spectra resolution of 4 cm-1. After CO adsorption, the gas flow was switched to 40 mL min-1 Ar, and the spectra were recorded in desired times. CO-temperature programmed desorption (CO-TPD) study. CO-TPD was carried out by AutoChem 2920. Pt/CeO2 catalyst was pretreated at same condition as the pretreatment of CO oxidation. Briefly, 100 mg CeO2-AA-Pt was heat at 300 °C in a static air for 1 hour to form CeO2-AA-Pt-cal, and then cooled down to room temperature. The CO gas flow (20 mL min-1) was introduced into the sample chamber for 5 minutes after the He gas flow (40 mL min-1) blew for 5 minutes. After full adsorption of CO, the He gas flow (40 mL min-1) blew the chamber for 30 min to remove CO gas. The chamber temperature was raised at the rate of 10 °C min-1, meanwhile, the signal was recorded by a TCD detector. H2-temperature programmed reduction (H2-TPR) study. H2-TPR was carried out by a homemade equipment. Pt/CeO2 catalyst was pretreated with 5% O2/Ar. Briefly, 25 mg Pt/CeO2 was heat to 200 °C under a 5% O2/Ar gas flow (30 mL min-1) for 30 min, and then cooled down to 16 oC. Then He gas flow (30 mL min-1) was blew into the sample chamber for 30 minutes. After that, the 5% H2/Ar gas flow (30 mL min-1) was introduced into the chamber and temperature was raised at the rate of 10 °C min-1, meanwhile, the signal was recorded by a TCD detector. RESULTS AND DISCUSSION


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Figure 1. (a) Schematic illustration of the L-ascorbic acid (AA)-assisted reduction synthesis of the atomically dispersed Pt/CeO2 catalyst (CeO2-AA-Pt-cal). (b) UV-Vis spectra and corresponding photographs (insets) of CeO2 and CeO2-AA. (c) XPS spectra and corresponding fitting curves of Ce 3d in CeO2 and CeO2-AA. The blue and green lines are corresponding to the fitting curves for Ce4+ and Ce3+ components, respectively. Figure 1a illustrates our proposed synthetic route for the atomically dispersed Pt/CeO2 catalyst. The route involves three main steps, namely the pre-reduction of the CeO2 support with AA (step-1), the reduction of H2PtCl6 precursors on the support surface (step-2), and calcination treatment at 300 °C in air (step-3). For convenience, the original CeO2 support and corresponding products at every step are denoted as CeO2, CeO2-AA, CeO2-AA-Pt, and CeO2AA-Pt-cal, in sequence. Highly porous CeO2 nanorods with a large surface area (85.9 m2·g-1) were chosen as the support in order to provide abundant adsorption sites for the active species (Figure S1, Supporting information). Further, centrifugation and re-dispersion were repeatedly conducted to remove excess AA molecules from the CeO2 support (Figure S2), and the presence of

AA

residues

was

confirmed

by

Fourier

transform

infrared

spectroscopy

and

thermogravimetric analysis (Figure S3). Interestingly, the surface oxidation state change of the support was reflected in an observed color change of the CeO2 nanorod solution before and after modification with AA, from an initial light-yellow (CeO2) color to a subsequent brownish-


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yellow (CeO2-AA) (Figure 1b, insets). Moreover, UV-Vis spectra reveal that the absorption of CeO2-AA in the range of 350~750 nm is significantly higher than that of the blank CeO2 (Figure 1b). The color change suggests that the surface state of CeO2 nanorods has changed due to the AA modification. Furthermore, XPS analysis (Figure 1c) reveal that two different kinds of Ce species can be detected in the Ce 3d XPS spectra of CeO2 and CeO2-AA. The position peak around 883.4 eV, 902 eV, 889.31 eV, 908.23 eV, 899.1 eV and 917.42 eV can be assigned to Ce (IV) species (fitting with blue line), while 882.2eV, 900.8 eV, 886.1 eV and 904.49 eV can be assigned to Ce (III) species (fitting with green line).36 According to the fitting results (Table 1), the proportion of Ce3+ species was increased from 27.3% (CeO2) to 36.8% (CeO2-AA) after the porous CeO2 nanorods were modified with AA. This indicates that a large number of Ce3+ are generated in the CeO2 nanorods due to the modification with AA. Moreover, Raman spectra (Figure S4) of the samples revealed that the CeO2-AA exhibited higher relative intensity of the band assigned to the oxygen vacancies. The abundance of Ce3+ sites in the CeO2-AA support provided a greater number of sites with a weak reducing ability to atomically anchor Pt catalyst atoms compared to the original CeO2 nanorods. The XPS spectrum analysis with regard to the CeO2-AA-Pt confirms that the Pt species loaded onto the CeO2 nanorods exhibit a oxidation state of Pt2+ (Figure S5), which indicates Pt species are not just physically adsorbed on the ceria surface. Finally, after loading, the Pt catalyst was activated by calcination at 300 °C under static air before its application. Calcination removed residual AA molecules and greatly reinforced the interaction between the reduced Pt catalysts and the CeO2 support, thus improving the stability of the Pt catalyst at high loading amounts. Of note, the proportion of Ce3+ sites in the CeO2 support was reduced to around 24.1% after the calcination under static air, which recovered the level for the raw CeO2 nanorods (Figure S6). To illustrate the advantages of our prepared Pt/CeO2


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catalyst, the most common catalyst, synthesized by traditional incipient wetness impregnation and calcination with 5% H2/Ar at 250 °C (denoted as CeO2-IMP-Pt), was chosen as reference in our study (see experimental section for synthetic details). According to ICP-AES analysis, the loading amounts of Pt in the above two catalysts were 1.0 wt% for CeO2-AA-Pt-cal and 1.2 wt% for CeO2-IMP-Pt. We also noticed that the amount of Pt loaded on the original CeO2 support without the assistance of AA was very low (ca. 0.01 wt%), which proved that the reduced surface state of CeO2 support is indeed conducive to anchoring active species onto the support.

Table 1. The proportions of Ce3+ and Ce4+species in different samples, which are simulated from XPS data. Sample

Ce3+ species

Ce4+ species

CeO2

27.3 %

72.7 %

CeO2-AA

36.8 %

63.2 %

CeO2-AA-Pt

34.5%

65.5%

CeO2-AA-Pt-cal

24.1%

75.9%

The proportion of Ce3+ species and Ce4+ species was calculated through equation (1), shows as fellow:

P( Ce3+ ) =

Area

( Ce3+ )

Area

( Ce3+ )

+ Area

( Ce4+ )

× 100%

(1)

P is the percentage of Ce species; Area is the fitting area of Ce species.


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Figure. 2 (a) Cs-corrected HAADF-STEM, (b) HAADF-STEM and (c-e) corresponding elemental mappings image of CeO2-AA-Pt-cal. (f,g) HAADF-STEM and (h-j) corresponding elemental mappings image of CeO2-IMP-Pt. The loading status of Pt catalysts on the support in the two Pt/CeO2 catalysts was firstly compared by TEM. In the bright-field TEM and high-resolution TEM images, both CeO2-AA-Ptcal and CeO2-IMP-Pt seemed apparently indistinguishable from the original porous CeO2 nanorods, with no metallic nanoclusters or nanoparticles being observed (Figure S7, and Figure S8). However, no diffraction peaks assigned to metallic Pt were detected in the powder X-ray diffraction patterns of either catalyst (Figure S9). To reveal true loading information of Pt in the two catalysts, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were used, wherein the large Z difference between particles and matrix provides an ideal contrast for tomography, and corresponding elemental mappings were recorded. For CeO2-AA-Pt-cal, no evident Pt nanoclusters or nanoparticles were observed on the support in the HAADF-STEM mode (Figure S10a). By using Cs-corrected HAADF-STEM


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technique, we can see the Pt catalysts were atomically dispersed throughout the CeO2 support in CeO2-AA-Pt-cal (Figure 2a and Figure S10b-c), with no apparent agglomeration after calcination at 300 °C. In contrast, some nanoclusters of 1-2 nm, observed through their brighter contrast in the HAADF-STEM mode, frequently appeared on the support in the CeO2-IMP-Pt sample (as marked by arrows) (Figure 2f and Figure S10d-f), despite Pt signals uniformly distributes on the whole nanorods in the element mapping (Figure. 2g-j). This result confirms that atomic dispersion at high loading densities is difficult to achieve in supported Pt catalysts synthesized by the traditional impregnation method. Previous studies have shown that reducible oxides like CeO2, TiO2, and Fe2O3 are suitable for the loading of atomically dispersed metal catalysts, as naturally occurring surface defects (i.e., reduced metal sites) do not only serve as anchoring sites for metal clusters or even single atoms, but also help to stabilize them on supports by forming metal-oxygen-support bonds.1,21,22 Clearly, the high dispersion of Pt in CeO2-AA-Pt-cal is related to the large amount of Ce3+ sites generated on the support surface due to the adsorption of AA.


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Figure. 3 In situ DRIFTS of CO adsorption and desorption on (a) CeO2-AA-Pt-cal and (b) CeO2IMP-Pt. (c) The k2-weighted Fourier transform EXAFS spectra of CeO2-AA-Pt-cal, CeO2-IMPPt, and bulk Pt foil at the Pt L3-edge. (d) The normalized XANES spectra at the Pt L3-edge of CeO2-AA-Pt-cal, CeO2-IMP-Pt, and bulk Pt foil. The differences in the Pt dispersion status between the two Pt/CeO2 catalysts was further verified by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption and desorption. For the CeO2-AA-Pt-cal (Figure 3a), three bands associated with CO were observed; the bands at 2171.5 and 2117.5 cm-1 were attributed to the CO gas, and that at 2090.5 cm-1 was attributed to the linearly (on-top) bonded CO on Pt ionic species.1,11 No signals were observed in the region 1800-2000 cm-1, in which bands ascribed to CO adsorbed on bridge and three-fold hollow sites are observed.1,36 Furthermore, through the flow of Ar gas, the two peaks assigned to the CO gas gradually decayed until they disappeared. In contrast, the band


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position of CO adsorbed onto the CeO2-AA-Pt-cal at ~2090.5 cm-1 remained largely unchanged even after sustained Ar flow, indicating that Pt catalysts are dispersed on the CeO2 support in the form of single atomic sites in the CeO2-AA-Pt-cal catalyst.1,11,24 For the CeO2-IMP-Pt catalyst (Figure 3b), the band assigned to the CO linearly adsorbed on Pt(0) was observed at ~ 2069.3 cm-1,1,37 in addition to the bands assigned to the linearly bonded CO on Pt ionic species (2086.6 cm-1) and to CO gas (2171.5 and 2117.5 cm-1). Following Ar gas flow and CO gas replacement, the band at ~2069.3 cm-1 gradually red shifted to 2048.1 cm-1 due to the linear absorbance of CO on the small sized Pt nanoparticles on CeO2.1,38,39 The band at ~2086.6 cm-1 also experienced a slight red shift to ~2080.6 cm-1 following Ar flow. Thus, the DRIFTS results for CeO2-AA-Pt-cal and CeO2-IMP-Pt are consistent with the dispersion status of Pt on the CeO2 support observed in the HAADF-STEM images. On the other hand, the bands between 2300 and 2400 cm-1, attributed to CO2, exhibited similar trends on the two catalysts during Ar flow. The concentration of CO2 increased continuously and reached a maximum at 10 minutes after the CO flow stopped, followed by a decline to a normal background level. The increasing CO2 signal could be attributed to the direct reaction between CO and the lattice oxygen in the Pt/CeO2 catalyst via the well-known Mars−van Krevelen mechanism.2,22,24,33,40-43 Of note, the CeO2-AA-Pt-cal presented a stronger relative intensity of CO2 signals than CeO2-IMP-Pt at the maximum CO2 signal after the CO flow stopped, suggesting a better CO activation ability of CeO2-AA-Pt-cal compared to CeO2-IMP-Pt. Table 2 EXAFS parameters of Pt foil, CeO2-AA-Pt-cal, CeO2-IMP-Pt and CeO2-IMP-Pt-cal. Sample

Shell

CN

R (Å)

σ2 (Å2)

∆E0 (eV)

R-factor

Pt foil

Pt-Pt

12a

2.76 ± 0.01

0.004 ± 0.001

7.4 ± 0.7

0.004


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CeO2-AA-Pt-cal

Pt-O

3.3 ± 0.5

1.98 ± 0.01

0.001 ± 0.002

8.7 ± 1.6

0.009

CeO2-IMP-Pt

Pt-O

2.7 ± 0.5

2.02 ± 0.02

0.002 ± 0.003

13.3 ± 2.1

0.019

CeO2-IMP-Pt-cal

Pt-O

3.5 ± 0.5

2.01 ± 0.02

0.002 ± 0.002

10.4 ± 1.8

0.014

This pattern was fixed during fitting process. Extended X-ray absorption fine structure spectra (EXAFS), which can provide the valence

and the coordination structure of Pt on the CeO2 support, were used for the further characterization of atomically dispersed catalyst. No Pt-Pt bond was detected in either CeO2-AAPt-cal or CeO2-IMP-Pt (Figure 3c). Based on the EXAFS curve fitting (Figure S11), notable peaks were observed in the region from 1 to 2 Å attributed to a Pt-O contribution in the two Pt/CeO2 catalysts, considerably different from that in the bulk Pt foil. Table 2 summarizes the coordination number and length of the Pt-O bond in the two catalysts. The Pt-O coordination number in CeO2-AA-Pt-cal was approximately 3.3, larger than in CeO2-IMP-Pt (2.7), while the distance between Pt and O was 1.98 Å, slightly shorter than in CeO2-IMP-Pt (2.02 Å) and the bulk value.29,44 These observations indicate a strong interaction between Pt and the CeO2 surface in CeO2-AA-Pt-cal. The difference in the Pt-O distance from the bulk value is likely due to the decreasing bond length with under-coordinated oxygen atoms. According to Avakyan et al.,29 such a short Pt-O bond (1.98 Å) could be assigned to an α-PtO2 monolayer formed on the surface, indicating that Pt is bonded to three oxygen atoms on the CeO2 surface. Of note, when CeO2-IMP-Pt was calcined in air (i.e., CeO2-IMP-Pt-cal), the Pt-O coordination number was increased to 3.5, which is very close to that of CeO2-AA-Pt-cal. Based on the CO-probe DRIFTS result, we speculate that, for CeO2-IMP-Pt, small sized Pt nanoparticles that are formed during heat treatment in H2/Ar are easily converted into PtOx species (no Pt-Pt bond) with high oxidation states under air conditions, leading to a decrease in catalytic activity.19,24,33 The


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normalized X-ray absorption near-edge structure spectra of the two Pt/CeO2 catalysts and the reference sample show that the white line intensity reflects the valence state of Pt in the different catalysts (Figure 3d). It is clear that both CeO2-AA-Pt-cal and CeO2-IMP-Pt have a larger white line intensity than Pt foil, which suggests that the Pt species in these two catalysts carry a positive charge.1,29

Figure 4. (a) Curves of CO conversion over different Pt/CeO2 catalysts (gas hourly space velocity (GHSV) = 30,000 h-1). (b) Curves of CO conversion over two different Pt/CeO2 catalysts (GHSV = 150,000 h-1). (c) Arrhenius plots for the CO oxidation over Pt/CeO2 catalysts. To assess the catalytic performance of the CeO2-AA-Pt-cal and CeO2-IMP-Pt samples, CO oxidation as a model oxidation reaction was performed at a gas hourly space velocity (GHSV) of 30,000 h-1, using CeO2-IMP-Pt-cal (Figure S12) and blank CeO2 as references (Figure 4a). The CeO2 nanorods alone exhibited poor catalytic activity for CO oxidation, while CO oxidation over both CeO2-AA-Pt-cal and CeO2-IMP-Pt commenced at near room temperature (30 °C). Compared to the CeO2-IMP-Pt, the CeO2-AA-Pt-cal led to a higher conversion at temperatures above 50 °C. The temperature at which complete conversion of CO was achieved over CeO2AA-Pt-cal was only 60 °C, compared to 90 °C over CeO2-IMP-Pt and 240 °C over blank CeO2. Of note, the complete conversion temperature of CO dramatically increased to 180 °C after the CeO2-IMP-Pt being thermally treated in air similar to CeO2-AA-Pt-cal, which indicates that the


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CeO2-IMP-Pt is not sinter resistant under oxidization atmosphere. The decrease of catalytic activity over CeO2-IMP-Pt-cal might be attributed to the formation of stable PtOx species with a high oxidation state.33 A kinetic study of Pt/CeO2 during CO oxidation was performed at a GHSV of 150,000 h-1 under 15% CO conversion. The activity of CeO2-IMP-Pt was slightly higher than that of CeO2-AA-Pt-cal below 60 °C (Figure 4b), gradually increasing with increasing temperature. Meanwhile, the activity of CeO2-AA-Pt-cal increased drastically with the increasing temperature. According to the Arrhenius equation (2), we calculated the apparent activation energies (Ea) of the two catalysts through the first five temperature plots (Figure 4c) to be approximately 59.68 KJ mol-1 for CeO2-AA-cal and 32.04 KJ mol-1 for CeO2-IMP-Pt. Additionally, the specific rate and turnover frequency (TOF) based on metal loading amount was also calculated for comparison (Table S1). At a low temperature (40 °C), CeO2-IMP-Pt exhibited a higher activity than CeO2-AA-Pt-cal, with a calculated TOF of 0.0030 s-1 for CeO2-IMP-Pt and 0.0023 s-1 for CeO2-AA-Pt-cal. Further, at a higher temperature (80 ºC), CeO2-AA-Pt-cal was even more active relative to CeO2-IMP-Pt (TOF 0.029 s-1 and 0.012 s-1, respectively). Based on the Arrhenius equation (∆ln(k)∝ Ea∆T), our result shows that the catalytic activity of atomically dispersed Pt catalyst is more sensitive to temperature than CeO2-IMP-Pt, requiring a higher apparent activation energy to enhance the change of reaction rate under the same change of temperature.


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Figure 5. (a) Time on stream curves of the Pt/CeO2 catalysts at a gas hourly space velocity (GHSV) of 30,000 h-1. The reaction temperature was fixed at 60 °C for CeO2-AA-Pt-cal and 90 °C for CeO2-IMP-Pt. (b) Catalytic cycles of CeO2-AA-Pt at a GHSV of 50,000 h-1. (c) Catalytic cycles of CeO2-IMP-Pt at a GHSV of 30,000 h-1. (d) Comparison of CO conversion obtained on CeO2-IMP-Pt (GHSV = 30,000 h-1) and CeO2-AA-Pt-cal (GHSV = 50,000 h-1) at 80 °C in cyclic tests. As mentioned above, the main issue hindering the application of highly dispersed metal-based catalysts is their poor stability in catalytic processes, especially at elevated temperatures. Therefore, the stability of CeO2-AA-Pt-cal and CeO2-IMP-Pt in catalytic conditions was assessed herein using time on stream curves (Figure 5a). The conversion of CO over CeO2-AA-Pt-cal at 60 °C was retained at 100% within the 50 hours of testing. However, 100% activity of CeO2IMP-Pt at 90 °C could only be maintained for 30 minutes, decaying thereafter. After 10 hours, CO conversion over CeO2-IMP-Pt decreased to only 30%.


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Further, the durability of the two Pt/CeO2 catalysts was assessed by cyclic catalytic experiments. Of note, to ensure the same activity of the two catalysts at the start of testing, the GHSV over CeO2-AA-Pt-cal was increased to 50,000 h-1, while that over CeO2-IMP-Pt remained at 30,000 h-1. Due to the increase in GHSV, the complete conversion temperature of CO over CeO2-AA-Pt-cal increased to 80 °C during the first cycle, but the activity of CeO2-AA-Pt-cal showed no signs of decay in the following five cycles (Figure 5b). In sharp contrast, the conversion of CO over CeO2-IMP-Pt at 80 °C was close to 100% during the first cycle, but declined to 38% in the sixth cycle (Figure 5c). To intuitively display the difference of the two catalysts with regards to catalytic durability, the CO conversion over the two catalysts at 80 °C was compared in the cyclic measurements (Figure 5d). The activity decay of CeO2-IMP-Pt during CO oxidation is caused by the formation of stable but inactive PtOx species under catalytic conditions.23,33 Through the traditional impregnation method, the activity of Pt/CeO2 catalysts can be improved by treating the catalysts in a reducing atmosphere, especially in a high temperature H2 atmosphere.45 However, such a reducing treatment leads to the agglomeration of metallic Pt, leaving it subject to easier deactivation under oxidization atmospheres.24 Another reason of decayed activity in Pt/CeO2 is the surface adsorbed carbonate or carboxylate species.37,40,46 There is a smaller amount of carbonate bidentates in the CeO2-AA-Pt-cal than CeO2-IMP-Pt (Figure S13), which contributes to its enhanced stability.


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Figure 6. (a) XPS spectra and corresponding fitting curves of Pt 4f in the CeO2-AA-Pt-cal and CeO2-IMP-Pt. (b) XPS spectra and corresponding fitting curves of Pt 4f in the CeO2-AA-Ptused, CeO2-IMP-Pt-used and CeO2-IMP-Pt-cal, and the -used sample representing that Pt/CeO2 is measured after time on stream test. The valence states of Pt in the two Pt/CeO2 catalysts was further confirmed by XPS analysis. Pt 4f XPS spectra of CeO2-AA-Pt-cal appeared double peaks assigned to Pt 4f7/2 and Pt 4f5/2 being centered at 72.80 and 76.15 eV, respectively (Figure 6a, top). The binding energies of Pt 4f peaks were obviously higher than that of metallic Pt, but were close to that of Pt2+ species;43 this indicates that Pt supported on the CeO2 nanorods mainly exists in the Pt2+ oxidation state in the CeO2-AA-Pt-cal, which may be due to the electron transfer from Pt atoms to CeO2 arising from the interfacial effect.20,47 Different from CeO2-AA-Pt-cal, the Pt species of the freshly prepared CeO2-IMP-Pt are mainly Pt(0), but the metallic Pt species easily changed to Pt2+ when the catalysts are exposure to air for a long time (Figure 6a, middle/bottom). To explore the changes of oxidation state of Pt catalysts during the CO oxidation, the Pt 4f XPS spectra of the two Pt/CeO2 catalysts after the time on stream test were measured likewise. For the CeO2-AA-Pt-cal (Figure 6b, top), Pt(0) species located at 70.76 eV and 74.11 eV can be detected after time-onstream test, which means that the Pt(II) in CeO2-AA-Pt-cal was partially reduced to Pt(0). By contrast, the Pt species in CeO2-IMP-Pt (Figure 6b, middle) presented a higher oxide state (Pt(IV)) after the test. These results indicate that in an oxidation environment at elevated temperatures, Pt2+ species in the CeO2-IMP-Pt are easy to oxidize to a higher oxidation state. As a proof, the proportion of the Pt4+ component was found to significantly increase when CeO2IMP-Pt was calcined at 300 °C under static air for 1 hour (Figure 6b, bottom). Based on the above results, it can be deduced that the oxidation of Pt catalysts deposited on the CeO2 is the main reason for decreased catalytic activity of the catalyst CeO2-IMP-Pt.


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Figure 7. CO-TPD curves of CeO2-AA-Pt-cal and CeO2-IMP-Pt. The adsorption of CO on the catalysts was analyzed by the CO temperature programmed desorption technique. The adsorption behavior of CO on CeO2-AA-Pt-cal and CeO2-IMP-Pt at temperatures above 250 °C were similar (Figure 7), likely due to the thermally stable carbonate species on the CeO2 nanorods.40 However, the adsorption of CO below 250 °C, which is mainly attributed to the absorption of CO onto the Pt species and Pt-CeO2 interface, were distinct in intensity and position between the two Pt/CeO2 catalysts.30,42,48 CeO2-IMP-Pt exhibited two desorption peaks, indicating the presence of two different sites for CO adsorption; this is consistent with the result of in situ CO-probed DRIFTS. The peaks at 109.6 °C and 157.5 °C corresponded to CO bonded on Pt (II) and metallic Pt, respectively. However, for the CeO2-AAPt-cal, only one desorption peak was observed at 123.3 °C, corresponding to the linearly bonded CO on Pt, with a 2.6-fold intensity compared to that of CeO2-IMP-Pt. Thus, the atomically dispersed Pt sites in the CeO2-AA-Pt-cal catalyst are capable of absorbing more CO through the linearly bonded mode. These results are in good agreement with the observed CO oxidation


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performance, since a larger CO adsorption amount and a weaker Pt-CO binding bound to cause a high catalysis rate. The H2-TPR technique was used to reveal the surface redox properties of the two Pt/CeO2 catalysts (Figure S14). The surface of CeO2-IMP-Pt is prone to be reduced at low temperature, while the major reduction peak appeared at 277.5 °C in CeO2-AA-Pt-cal. The results presented here are consistent with previous studies.49-50 This indicated a more stable surface in CeO2-AA-Pt-cal, which could contribute to its stability. CONCLUSION In summary, we successfully synthesized an atomically dispersed Pt catalyst supported on porous CeO2 nanorods with a high loading capacity (up to 1 wt%) through a simple AA-assisted reduction route. Following pre-modification with a trace amount of AA, the surface state of the CeO2 support was changed from an oxidation state to a reduction state, generating abundant surface defects (i.e., Ce3+) acting as active sites for Pt atom anchoring and stabilization. CO oxidation measurements revealed that the as-prepared catalysts exhibited superior catalytic activity (complete conversion at 60 °C) and long-term durability (no decay for 50 hours) compared to a catalyst obtained by the traditional impregnation method. Through the use of multiple surface analysis techniques, including in situ CO-DRIFTS, CO temperature programmed desorption, XPS, and XAFS spectra, we showed that the significantly enhanced catalytic activity and sinter-resistant ability of the prepared catalyst are closely associated with the formation of isolated Pt sites with a very short Pt-O length due to the strong interaction between Pt atoms and the support. The proposed synthetic route is simple and efficient, and can be potentially extended to fabricate other atomically dispersed noble metal catalysts. ASSOCIATED CONTENT


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Supporting Information. Characterization of samples, including TEM images, XRD, N2 adsorption–desorption isotherms, FT-IR and TGA analysis; Raman spectra; H2-TPR profiles; Fitting details of EXAFS. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Correspondence to: [email protected]; [email protected]; Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2017YFA0206801), the National Basic Research Program of China (2015CB932301), the National Natural Science Foundation of China (21333008, 21671163, 21773190 and 21721001) and the Fundamental Research Funds for the Central Universities (20720160026). And we also thank Prof. Wei Xi and Prof. Zhiming Zhang at Tianjing University of technology for their help in Cs-corrected TEM characterization and the XAFS station of Beijing Synchrotron Radiation Facility for the help with the project. REFERENCES (1)

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TABLE OF CONTENTS GRAPHIC

Atomically dispersed Pt/CeO2 catalyst for CO oxidation was fabricated by a surface engineering protocol based on molecule-surface charge transfer adduct.


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Surface Engineering Protocol To Obtain an Atomically Dispersed Pt

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