A Stable Inverse Spinel Oxide Support for ... - ACS Publications

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Room-temperature CO Oxidation over Pt/MgFe2O4: a Stable Inverse Spinel Oxide Support for Preparing Highly Efficient Pt Catalyst Bin Zheng, Shujie Wu, Xuwei Yang, Mingjun Jia, Wenxiang Zhang, and Gang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06501 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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 Materials & Interfaces 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 25

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 Materials & Interfaces

Room-temperature CO Oxidation over Pt/MgFe2O4: a Stable Inverse Spinel Oxide Support for Preparing Highly Efficient Pt Catalyst Bin Zheng, Shujie Wu, Xuwei Yang, Mingjun Jia, Wenxiang Zhang* and Gang Liu* College of Chemistry, Key Laboratory of Surface and Interface Chemistry of Jilin Province, Jilin University, Changchun, 130012, China KEYWORDS: CO oxidation, supported Pt catalyst, MgFe2O4, nanoparticle, O2 activation

ABSTRACT: MgFe2O4 with inverse spinel structure is demonstrated to be an efficient support for constructing practical potential Pt catalyst (Pt/MgFe2O4). The resultant Pt/MgFe2O4 exhibits excellent catalytic behavior in CO oxidation under normal temperature and humidity. TOF calculated based on the content of Pt is 0.131 s-1. The excellent performance of Pt/MgFe2O4 attributes to the presence of surface under-coordinated lattice oxygens on MgFe2O4 support. These oxygens could participate in the initial CO oxidation, and then be recovered under O2 condition. Over this Pt/MgFe2O4 catalyst, CO catalytic oxidation should mainly follow a redox mechanism.

INTRODUCTION

ACS Paragon Plus Environment

1


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 25

Although CO oxidation has been a prototypical reaction in heterogeneous catalysis,1-4 developing practical catalysts satisfying the applications in CO removing at room-temperature (25 ℃) still faces great challenges.5-8 The catalysts should meet the requirements of high activity, high stability in reaction and storage, and resistance to water vapor.9-11 Free of pretreatment is also needed when the catalysts are used in most of commercial devices, such as air cleaner.12-14 Supported Au catalyst was with very high activity in CO oxidation even at very low reaction temperature, which was intensively studied in the fundamental researches.15-17 However, largescale application of supported Au catalyst is still difficult due to the sensitivity to the preparation condition and the deactivation during long-time storage and operation.18-20 Recently, Pt catalysts promoted by the reducible metal oxide supports, especially Pt-FeOx, exhibit promising activities in the field of CO oxidation with or without H2 atmosphere.21-23 The mechanisms of CO oxidation over most of these catalysts begin with O2 activation, either at the surface oxygen vacancy of FeOx or at coordinately unsaturated Fe2+ sites on Pt. Most of the O2 activation sites were produced by pre-treating Pt/FeOx catalysts under H2 atmosphere. These sites suffer instability in air when the catalysts are stored in ambient conditions.24 Constructing efficient iron hydroxide-Pt interface is another strategy for obtaining high performance Pt-based catalysts in room-temperature CO oxidation.25,

26

Our previous work

showed that ferrihydrite (FeO(OH, H2O)n) was one of the most satisfying supports for preparing such Pt catalysts. Hydroxides of ferrihydrite not only influence the construction of Pt-support interface, but also participate in the catalytic process to promote the activation of O2.27 The shortcoming of this catalyst is the poor thermal stability of ferrihydrite support. It could lose a certain amount of activity for Pt/FeOx catalysts during reaction, transportation and storage.24, 28, 29 Recently, based on synthesizing iron-nickel hydroxide-platinum nanoparticles, Zheng and his co-


2

Page 3 of 25

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


workers have reported that incorporating Ni2+ could enhance the long-term stability of iron hydroxide-Pt catalyst in room-temperature CO oxidation. The role of Ni2+ is to stabilize the interface of iron hydroxide-Pt against dehydration.24 In the present work, we tried an alternative route that introducing MgFe2O4, an inverse spinel oxide with relatively high thermal stability, into the preparation of supported Pt catalyst (denoted as Pt/MgFe2O4). Pt/MgFe2O4 not only exhibits high activity in CO oxidation under hydrous condition at normal temperature, but also possesses high stability in a long operation term and storage. TOF calculated based on the content of Pt can reach 0.131 s-1, which is comparable with that of Pt1/FeOx single-atom catalyst (0.136 s-1, ref. 22). In addition, Pt/MgFe2O4 can be directly used in the reaction without any pre-reducing treatment, even for the samples stored in an ambient environment for more than six months. A series of characterizations were adopted to study the physicochemical properties of Pt/MgFe2O4 catalyst. The reaction mechanism over Pt/MgFe2O4 was discussed based on these characterization results and in-situ CO oxidation detected with diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). EXPERIMENTAL SECTION Materials. The main reagents MgSO4, Fe(NO3)3·9H2O and H2PtCl6·6H2O were the productions of Sinopharm Chemical Reagent Co., Ltd. These chemical reagents were employed directly in the experiment without any other treatments. Catalyst Preparation. A solid-state method was carried out to prepare MgFe2O4 support.30 Magnesium sulfate, ferric nitrate, sodium hydroxide and sodium chloride were used as reagents. Firstly, all the above reagents with molar ratio of MgSO4/Fe(NO3)3/NaOH/NaCl=1/2/8/10 were mixed and ground in a mortar for more than 30 min. This process accompanied with a heat release. Then the mixture was placed in a muffle and annealed at 700 ℃ for 3 h. Then the


3


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 4 of 25

calcinated solid was purified by deionized water for several times to eliminate dissoluble ions. Subsequently, the sample was dried at 120 ℃ overnight. A colloid deposition method was carried out to prepare the Pt/MgFe2O4 catalysts. This method was reported previously by our group on the preparation of ferrihydrite supported Pt catalyst.31, 32 At the first step, Pt colloid nanoparticles were synthesized through the reduction of H2PtCl6·6H2O in the presence of ethylene glycol. Then, MgFe2O4 sample was added to the synthesized colloids. The mixture was kept at 80 ℃ with a moderate agitation for about 12 h. All Pt colloids could be loaded on MgFe2O4 surface. The obtained solid was then isolated from the mixture, and completely purified by deionized water to remove chloride ions (detected by AgNO3 reagent). The resultant products were obtained after dried at 100 ℃ for 12 h and then 200 ℃ was employed as treatment temperature. The treatment time was 2 h. The treatment atmosphere was the mixture of O2 and Ar, of which the O2 volume was 20%. The Pt weight content in the final catalysts is 2%. As a reference, α-Fe2O3 support was prepared by thermal-treating ferrihydrite at 700℃ for 3 h. Ferrihydrite was prepared by the method previously reported by our group.27 The 2 wt % Pt/αFe2O3 catalysts were also prepared by the same method as that of Pt/MgFe2O4 catalysts. Catalyst Characterization. Rigaku X-ray diffractometer was used to measure X-ray diffraction (XRD) profiles of sample. It was equipped with Cu Kα radiation (k = 1.5418 A). Thermo Nicolet 6700 spectrometer was adopted to detect the FT-IR data. Firstly, the same weight ratio (1:100) of Pt/MgFe2O4 to KBr and Pt/α-Fe2O3 to KBr was obtained and the mixtures were grounded in a mortar. Second, the same amount (50 mg) of mixture was added into the mould and pressed under the same pressure (10 MPa). The obtained flake was used as final sample for FT-IR measurement. Room temperature LabRAM HR Evolution Raman spectrometer was used to


4

Page 5 of 25

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


record Raman data. It was with the excitation source of argon laser (532 nm). The JEOL JEM2010 electron microscope with operating voltage of 200 kV was employed to obtain the Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images. N2adsorption/desorption isotherms were recorded by Micromeritics analyzer (ASAP 2010N). The model of Brunauer-Emmett-Teller (BET) was used to calculate the surface areas. Thermo ESCA LAB 250 system was used to measure X-ray photoelectron spectroscopy. It was with Mg Kα source of 1254.6 eV. The calibration of XPS spectra was performed through adjusting C 1s peak of each sample to a position of 284.6 eV. Thermo Nicolet 6700 spectrometer was employed to measure in situ DRIFT data. Firstly, the catalyst was treated at room temperature under Ar atmosphere for 20 min. Then 1 vol % CO/Ar and 20 vol % O2/Ar were introduced separately. The resolution and scans of the background subtracted DRIFT spectra were 4 cm-1 and comprise 32, respectively. The TPR/TPO profiles were obtained by Quantachrome ChemBET Pulsar TPR/TPO apparatus with 50 mg sample in each test. Firstly, the catalyst was treated at the temperature of 120℃ under Ar atmosphere (99.99%). The processing time was 30 min. Secondly, 5 vol % H2/Ar or 5 vol % O2/Ar was introduced at room temperature, then the heating procedure was performed with 10 ℃ ·min-1. In addition, in modified TPO experiment, Pt/MgFe2O4 was pretreated in 5 vol % H2 flow at room temperature before the TPO operation. Catalytic Activity Measurement. CO catalytic activity of Pt/MgFe2O4 was measured by an apparatus reported previously.27 During the test, 100 mg 40-60 mesh catalysts have been used. The reaction gas consists of 1 vol % CO, 5 vol % O2, 1.8 vol % H2O and Ar balanced. The space velocity was fixed at 6.0×104 mL·g-1·h-1. TOF was measured based on the content of Pt under a differential model, where 25 mg catalyst was used in the CO oxidation reaction at room-


5


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 6 of 25

temperature. The first 60 min values of CO conversions were employed and averaged. On the basis of the obtained values, the TOF at room temperature was calculated according to eq 1. TOF (s-1) =XcoFco

(1)

where XCO is the CO conversion at 25 ℃, FCO (mol·s-1) stands for the CO flow rate, mcat represents the catalyst weight, XPt is the loaded amount of platinum, and MPt is the molar weight of platinum (195.05 g·mol-1). RESULTS AND DISCUSSION

Figure 1. XRD profiles of (a) MgFe2O4 and (b) Pt/MgFe2O4. XRD patterns of MgFe2O4 and Pt/MgFe2O4 were presented in Figure 1. It should be noted that MgFe2O4 support was prepared with an anneal temperature of 700 ˚C. Six relatively obvious diffraction peaks can be observed, which located at 2θ = 30.1, 35.5, 43.1, 53.5, 57.0 and 62.6⁰. They are the characteristic peaks of the cubic spinel structure (JCPDS No. 36-0398) with the space group of Fd3m, and can be attributed to (220), (311), (400), (422), (511) and (440) planes of MgFe2O4, respectively. It shows that the MgFe2O4 sample possesses well-defined spinel phase. After loading Pt colloid nanoparticles, the Pt/MgFe2O4 sample still maintain the spinel


6

Page 7 of 25

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


structure and no diffraction peaks of Pt can be observed. This result suggests that Pt possesses small particle size and Pt particles disperse uniformly on MgFe2O4 surface.

Figure 2. Raman spectra of (a) MgFe2O4 and (b) Pt/MgFe2O4.

Figure 3. FT-IR spectra of (a) MgFe2O4 and (b) Pt/MgFe2O4. Figure 2 shows the Raman spectra of MgFe2O4 and Pt/MgFe2O4. A1g, Eg and 3T2g Raman modes can be found from the spectra. The modes locate around 690 and 309 cm-1 have symmetries A1g and Eg, respectively. The Raman modes found around 205, 466, and 535 cm-1 has the 3T2g symmetry. A1g mode can be split into two types: one peak at 715 cm-1 is attributed to


7


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 8 of 25

Mg2+: A1g (Mg), and the other peak at 670 cm-1 is assigned to Fe3+: A1g (Fe). The A1g(Fe) and A1g (Mg) Raman modes are related to the stretching vibrational modes of tetrahedral Fe and Mg, respectively.33, 34 According to the spectra, tetrahedral sites are mainly occupied by Fe3+ ion, which could reflect the inverse spinel structural feature of MgFe2O4. And the structure has no change before and after loading Pt nanoparticles. As for FT-IR spectra (Figure 3), the peaks of 578 cm-1 and 437 cm-1 are ascribed to vibration of tetrahedral and octahedral species, respectively.35, 36 The broad bands at around 3400 cm-1 originate from the stretching vibration of surface -OH on MgFe2O4.37,

38

The intensity of these signals is much lower than that of

ferrihydrite, but has no obvious change after loading Pt colloid nanoparticles. Combined with the results of XRD and Raman data, one can find that MgFe2O4 support is very stable during the preparation of Pt/MgFe2O4. This phenomenon is quite different from the case of using ferrihydrite as a support. Our previous work showed that ferrihydrite crystal phase partly changed from ferrihydrite to Fe3O4 and α-Fe2O3 during the loading of Pt nanoparticles (see in Figure S1). The amount of surface -OH over ferrihydrite decreased significantly during this process (see in Figure S2).27


8

Page 9 of 25

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 4. TEM images of Pt/MgFe2O4 (a, b), Pt colloid nanoparticles (c) and HRTEM image of Pt/MgFe2O4 (d). The TEM and HRTEM images of Pt/MgFe2O4 are shown in Figure 4. It can be observed clearly that the size of Pt nanoparticles is about 3-4 nm. These small particles highly dispersed on MgFe2O4 surface (Figure 4a and 4b). The lattice fringe spacing (d = 0.22 nm) can be attributed to the cubic Pt (111) plane (Figure 4c).39,

40

Comparing with the Pt colloid

nanoparticles (Figure 4d), the particle size of Pt has no obviously change after loading them on the surface of MgFe2O4. The BET surface area of Pt/MgFe2O4 is about 31 m2·g-1 detected by N2adsorption measurement.

Figure 5. Pt 4f, Fe 2p, Mg 2p and O 1s XPS spectra of Pt/MgFe2O4.


9


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 10 of 25

XPS was performed to investigate the surface chemical states of Pt/MgFe2O4 (Figure 5). The spectrum of Pt 4f could be splitted into Pt2+ (72.4, 75.6 eV) and Pt0 (71.3, 74.7eV), respectively.26, 41, 42 The calculated molar ratio of Pt0/Pt2+ is about 0.79. The presence of large amount of Pt2+ suggests that an interaction formed between Pt nanoparticles and MgFe2O4 in the preparation process. During the reaction process, certain amount of Pt with positive value could decrease the adsorption strength of CO on the Pt sites and accelerate the reaction rate.22, 32 In the spectrum of Fe 2p, Fe 2p3/2 peak can be fitted into two bands located at about 710.7 eV and 712.4 eV. They can be ascribed to Feo3+ (Fe3+ in octahedral sites) and Fet3+ (Fe3+ in tetrahedral sites), respectively.43-45 Almost all Fe species present with the value of +3.46, 47 The contributions of Feo3+ and Fet3+ were calculated to be 51.6% and 48.4%, respectively. As for the spectrum of Mg 2p, Mg2+ ions located at both octahedral sites (Mgo2+) and tetrahedral sites (Mgt2+) were detected.45, 48, 49 The contribution of Mgo2+ and Mgt2+ is presented by the constant Mgo2+/Mgt2+ ratio of 1.72. These results suggested that the surface phase of MgFe2O4 also mainly presents with inverse spinel structure, which is consistent with its bulk structural features. Two deconvoluted peaks can be obtained from O1s spectrum of Pt/MgFe2O4, which represent two different types of surface oxygen species. One is oxygen in the spinel structure (centered at 530.2 eV), and the other is under-coordinated lattice oxygen (at 531.5 eV).50, 51 It suggests that large amount of structural defects present on the surface of Pt/MgFe2O4. The contribution of surface -OH to the O1s spectrum should be quite limited. As shown in FTIR spectra (Figure 3), very few -OH groups remain on the surface of MgFe2O4 due to its high preparation temperature of 700 ℃.


10

Page 11 of 25

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 6. (A) CO catalytic oxidation activities of (a) Pt/MgFe2O4 and (b) Pt/α–Fe2O3. α–Fe2O3 was prepared by thermal-treating ferrihydrite at 700 ˚C for 3 h. (B) CO conversion versus time on stream on Pt/MgFe2O4 in the presence of water vapor. Fixed-bed reactor was adopted to measure the CO catalytic oxidation over Pt/MgFe2O4. The space velocity was fixed at 6.0×104 mL·g-1·h-1 with gas mixture of CO (1 vol %), H2O (1.8 vol %) and O2 (5 vol %). These mixtures were balanced with Ar. Figure 6A shows curves of the CO conversion depended on the reaction temperature. Pt/MgFe2O4 could achieve CO complete conversion at 25 ℃, exhibiting a high catalytic activity. This performance can be maintained during the whole test period (at least 100 h, Figure 6B). When the space velocity was increased


11


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 12 of 25

to 9.0×104 mL·g-1·h-1, a little decrease of CO conversion can be observed. But it still could maintain at more than 80% (Figure S3), confirming the high stability of Pt/MgFe2O4. TOF of Pt/ MgFe2O4 can reach 0.131 s-1 calculated based on the content of Pt, which is even comparable with that of Pt1/FeOx single-atom catalyst (0.136 s-1, ref. 22). In addition, the performance of Pt/MgFe2O4 stored in ambient environment for more than six months was also investigated. The activity of Pt/MgFe2O4 is still as high as the fresh one. Under the dry reaction condition, a different result can be observed (Figure S4). The room temperature activity of Pt/MgFe2O4 is a little lower than that under the water vapor condition. This result suggested that H2O should participate in the reaction process, which might quite similar to that of iron oxide supported Pt catalysts reported previously.24, 32 As a reference, Pt/α-Fe2O3 was also prepared with the same colloid deposition method. αFe2O3 support was obtained by thermal-treating ferrihydrite at 700 ˚C for 3 h (Figure S5), which is similar to the preparation of MgFe2O4. Figure 6A shows that the activity of Pt/α-Fe2O3 at room-temperature is much lower than that of Pt/MgFe2O4. From FTIR spectra, it can be observed that α-Fe2O3 has similar amount of surface -OH as MgFe2O4 (Figure S6). The much lower catalytic performance of Pt/α-Fe2O3 suggested that surface -OH on these two samples has little effect on the catalytic activity. Therefore, it can be deduced that the reaction mechanism over Pt/MgFe2O4 should be different from that of iron hydroxide-Pt or ferrihydrite supported Pt catalyst. The -OH groups in the latter two catalysts usually participate in the activation of molecular oxygen. Zheng’s work showed that the activities of these catalysts decrease significantly after 10 h reaction due to the amount of surface -OH groups decreased during the reaction process.24


12

Page 13 of 25

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. In situ DRIFT spectra depended on time over Pt/MgFe2O4 catalysts (a) under 1 vol % CO/Ar atmosphere, and (b) switched to 20 vol % O2/Ar atmosphere at 25℃. The interval time was 40 s.

Scheme 1. Proposed CO oxidation mechanism over Pt/MgFe2O4. In this case, the high catalytic performance of Pt/MgFe2O4 in CO oxidation might correlate with the presence of surface under-coordinated lattice oxygen. For exploring the probable CO oxidation process on Pt/MgFe2O4, in-situ DRIFT spectra were employed (Figure 7). Detailed procedure is as follows: Pt/MgFe2O4 was first preprocessed in a flow of Ar to eliminate the


13


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 14 of 25

adsorptive H2O and O2. The measured spectrum of preprocessed Pt/MgFe2O4 was used as the background. Then the Pt/MgFe2O4 spectra versus time under different conditions were recorded. After injection of CO, one band centered at 2067 cm-1 can be found from the spectrum of Pt/MgFe2O4. It originates from CO linear adsorption on Pt. The appearance of the above peak accompanies with a consumption peak in the range of 3700 to 3300 cm-1. According to the ref. 52, the band in this range should be a characterization of the redox between Fe3+ and Fe2+ of MgFe2O4 supports. It reflects that lattice O of MgFe2O4 should participate in the CO oxidation. After shifting to O2/Ar flow, the peaks related to redox of MgFe2O4 disappear. This result shows that lattice O can be recovered during this process. Combined with the corresponding literatures,20,

22, 53

a redox mechanism can be proposed for CO oxidation over Pt/MgFe2O4

catalyst. The characterization results of TPO and TPR could indirectly support this conclusion. As shown in Figure S7A, a small H2 consumption peak at low temperature (below 100 ℃) can be observed in the H2-TPR profile of Pt/MgFe2O4. This result can reflect that Pt/MgFe2O4 is reducible at low temperature, which could directly correlate with its catalytic performance of CO oxidation at room temperature. Based on this result, the TPO measurement was carried out over Pt/MgFe2O4 with and without pretreatment with 5 vol % H2 flow at room temperature. This pretreatment aims to imitate the state of the catalyst after reacted with CO. Comparing these two profiles (Figure S7B), it shows that the TPO peak area of H2-treated sample is obviously larger than that of untreated one, and the center of the peak shifts a little to the low temperature. Although these oxidation peaks cannot correlate well with the catalytic performance of Pt/MgFe2O4 due to the different test temperature, at least, the changes of the peak can reflect that a redox cycle could occur in the reaction. Combined with the results of in-situ DRIFT, it could


14

Page 15 of 25

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


be confirmed that a redox mechanism should occur in CO oxidation over Pt/MgFe2O4 catalyst. According to these results, a plausible reaction process of CO oxidation on Pt/MgFe2O4 was proposed (Scheme 1). The lattice oxygens of MgFe2O4 react with CO adsorbed on Pt nanoparticles and form oxygen vacancy near these Pt nanoparticles. The surface oxygen vacancy can adsorb and dissociate the O2 reactants. Another two adsorbed CO can further react with the adsorbed O2, forming a new vacancy again and then continue another catalytic cycle. Besides, it should be noted that the inverse spinel structure of MgFe2O4 plays an important role in activation of molecular oxygen at room-temperature. Very recently, Ding and his co-workers have reported that Co-Fe based spinel with inverse structure exhibit an excellent activity in oxygen reduction reaction (ORR). With DFT calculation, they demonstrated that the adsorbed O2 was more active on inverse spinel than that on normal one. 54 In our case, Raman and XPS results show that both the bulk and surface of MgFe2O4 possess inverse spinel structure. Mg2+ ions locate at both octahedral sites and tetrahedral sites. This inverse spinel structure might facilitate the activation and cleavage of O-O bonds, which may account for the high catalytic activity of Pt/MgFe2O4 in room-temperature CO oxidation. CONCLUSION The supported Pt catalyst with high activity and stability for CO oxidation at room temperature was prepared with MgFe2O4 as a support. The lattice O of MgFe2O4 could participate in the CO oxidation, and then recovered in the flow of O2. A redox mechanism should occur over Pt/MgFe2O4 catalyst in the CO oxidation. This work may open a new way for preparing highly efficient Pt catalysts for other oxidation reactions. ASSOCIATED CONTENT


15


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 16 of 25

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. It includes XRD patterns and DRIFT spectra of ferrihydrite and Pt/ferrihydrite samples; Conversion of CO versus time on stream over Pt/MgFe2O4 under humidity; CO catalytic oxidation activities of Pt/MgFe2O4 and Pt/α–Fe2O3 under the dry condition; XRD patterns and FTIR spectra of α-Fe2O3 and Pt/α-Fe2O3 samples; TPR and TPO profiles of Pt/MgFe2O4 catalyst.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Gang Liu), [email protected] (Wenxiang Zhang) Tel: (+86)431-85155390 Fax: (+86)431-88499140 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the National Science Foundation of China (Grant No. 21473073, 21473074 and 20973080) and "13th Five-Year" science and technology research of the Education Department of Jilin Province (2016403) for the financial support. Bin Zheng gratefully acknowledges Graduate Innovation Fund of Jilin University (2016161) for the support of this work.


16

Page 17 of 25

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


REFERENCES (1) Freund, H. J.; Meijer, G.; Scheffler, M.; Schlogl, R.; Wolf, M. CO Oxidation as a Prototypical Reaction for Heterogeneous Processes. Angew. Chem., Int. Ed. 2011, 50, 1006410094. (2) Xu, L. S.; Zhang, W. H.; Zhang, Y. L.; Wu, Z. F.; Chen, B. H.; Jiang, Z. Q.; Ma, Y. S.; Yang, J. L.; Huang, W. X. Oxygen Vacancy-Controlled Reactivity of Hydroxyls on an FeO(111) Monolayer Film. J. Phys. Chem. C 2011, 115, 6815-6824. (3) Wu, Z. L.; Jiang, D. E.; Mann, A. K. P.; Mullins, D. R.; Qiao, Z. A.; Allard, L. F.; Zeng, C. J.; Jin, R. C.; Overbury, S. H. Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO2 Supported Au25(SCH2CH2Ph)18 Nanoclusters. J. Am. Chem. Soc. 2014, 136, 6111-6122. (4) Tang, L. H.; Wei, J. K.; Liu, F.; Qiao, B. T.; Pan, X. L.; Li, L.; Liu, J. Y.; Wang, J. H.; Zhang, T. Strong Metal-Support Interactions between Gold Nanoparticles and Nonoxides. J. Am. Chem. Soc. 2016, 138, 56-59. (5) Guan, H. L.; Lin, J.; Qiao, B. T.; Yang, X. F.; Li, L.; Miao, S.; Liu, J. Y.; Wang, A. Q.; Wang, X. D.; Zhang, T. Catalytically Active Rh Sub-Nanoclusters on TiO2 for CO Oxidation at Cryogenic Temperatures. Angew. Chem., Int. Ed. 2016, 55, 2820-2824. (6) Xie, X. W.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. J. Low-temperature Oxidation of CO Catalysed by Co3O4 Nanorods. Nature 2009, 458, 746-749. (7) Li, N.; Chen, Q. Y.; Luo, L. F.; Huang, W. X.; Luo, M. F.; Hu, G. S.; Lu, J. Q. Kinetic Study and the Effect of Particle Size on Low Temperature CO Oxidation over Pt/TiO2 Catalysts. Appl. Catal., B 2013, 142, 523-532.


17


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 18 of 25

(8) Xu, X. J.; Fu, Q.; Guo, X. G.; Bao, X. H. A Highly Active “NiO-on-Au” Surface Architecture for CO Oxidation. ACS Catal. 2013, 3, 1810-1818. (9) Ma, Z.; Yin, H. F.; Dai, S. Performance of Au/MxOy/TiO2 Catalysts in Water-Gas Shift Reaction. Catal. Lett. 2009, 136, 83-91. (10) Xu, H.; Fu, Q.; Yao, Y. X.; Bao, X. H. Highly Active Pt–Fe Bicomponent Catalysts for CO Oxidation in the Presence and Absence of H2. Energy Environ. Sci. 2012, 5, 6313-6320. (11) Lin, J.; Wang, A. Q.; Qiao, B. T.; Liu, X. Y.; Yang, X. F.; Wang, X. D.; Liang, J. X.; Li, J.; Liu, J. Y.; Zhang, T. Remarkable Performance of Ir1/FeOx Single-Atom Catalyst in Water Gas Shift Reaction. J. Am. Chem. Soc. 2013, 135, 15314-15317. (12) Bekyarova, E.; Fornasiero, P.; KasÏpar, J.; Graziani, M. CO Oxidation on Pd/CeO2±ZrO2 Catalysts. Catal. Today 1998, 45, 179-183. (13) Shelef, M.; McCabe, R. W. Twenty-Five Years after Introduction of Automotive Catalysts: What Next? Catal. Today 2000, 62, 35–50. (14) Twigg, M. V. Progress and Future Challenges in Controlling Automotive Exhaust Gas Emissions. Appl. Catal., B 2007, 70, 2-15. (15) Haruta, M. Gold as a Novel Catalyst in the 21st Century: Preparation, Working Mechanism and Applications. Gold Bull. 2004, 37, 1–2. (16) Ma, Z.; Dai, S. Development of Novel Supported Gold Catalysts: A Materials Perspective. Nano Res. 2010, 4, 3-32.


18

Page 19 of 25

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


(17) Chen, S. L.; Luo, L. F.; Jiang, Z. Q.; Huang, W. X. Size-Dependent Reaction Pathways of Low-Temperature CO Oxidation on Au/CeO2 Catalysts. ACS Catal. 2015, 5, 1653-1662. (18) Fujitani, T.; Nakamura, I. Mechanism and Active Sites of the Oxidation of CO over Au/TiO2. Angew. Chem. Int. Ed. 2011, 50, 10144-10147. (19) Zanella, R.; Rodríguez-González, V.; Arzola, Y.; Moreno-Rodriguez, A. Au/Y-TiO2 Catalyst: High Activity and Long-Term Stability in CO Oxidation. ACS Catal. 2012, 2, 1-11. (20) Liu, K.; Wang, A. Q.; Zhang, T. Recent Advances in Preferential Oxidation of CO Reaction over Platinum Group Metal Catalysts. ACS Catal. 2012, 2, 1165-1178. (21) Fu, Q.; Li, W. X.; Yao, Y. X.; Liu, H. Y.; Su, H. Y.; Ma, D.; Gu, X. K.; Chen, L. M.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. H. Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328, 1141-1144. (22) Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nat. Chem., 2011, 3, 634-641. (23) 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. (24) Chen, G. X.; Zhao, Y.; Fu, G.; Duchesne, P. N.; Gu, L.; Zheng, Y. P.; Weng, X. F.; Chen, M. S.; Zhang, P.; Pao, C. W.; Lee, J. F.; N. F. Zheng. Interfacial Effects in Iron-Nickel Hydroxide-Platinum Nanoparticles Enhance Catalytic Oxidation. Science 2014, 344, 495-499.


19


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 20 of 25

(25) Li, S. Y.; Liu, G.; Lian, H. L.; Jia, M. J.; Zhao, G. M.; Jiang, D. Z.; Zhang, W. X. LowTemperature CO Oxidation over Supported Pt Catalysts Prepared by Colloid-Deposition Method. Catal. Commun. 2008, 9, 1045-1049. (26) Liu, L. Q.; Zhou, F.; Wang, L. G.; Qi, X. J.; Shi, F.; Deng, Y. Q. Low-Temperature CO Oxidation over Supported Pt, Pd Catalysts: Particular Role of FeOx Support for Oxygen Supply during Reactions. J. Catal. 2010, 274, 1-10. (27) Zheng, B.; Liu, G.; Geng, L. L.; Cui, J. Y.; Wu, S. J.; Wu, P.; Jia, M. J.; Yan, W. F.; Zhang, W. X. Role of the FeOx Support in Constructing High-Performance Pt/FeOx Catalysts for LowTemperature CO Oxidation. Catal. Sci. Technol. 2016, 6, 1546-1554. (28) Qiao, B. T.; Deng, Y. Q. Highly Effective Ferric Hydroxide Supported Gold Catalyst for Selective Oxidation of CO in the Presence of H2. Chem. Commun. 2003, 2192-2193. (29) Z Zhao, K. F.; Tang, H. L.; Qiao, B. T.; Li, L.; Wang, J. H. High Activity of Au/γ-Fe2O3 for CO Oxidation: Effect of Support Crystal Phase in Catalyst Design. ACS Catal. 2015, 5, 35283539. (30) Liu, Y. L.; Liu, Z. M.; Yang, Y.; Yang, H. F.; Shen, G. L.; Yu, R. Q. Simple Synthesis of MgFe2O4 Nanoparticles as Gas Sensing Materials. Sens. Actuators, B 2005, 107, 600-604. (31) An, N. H.; Li, S. Y.; Duchesne, P. N.; Wu, P.; Zhang, W. L.; Lee, J. F.; Cheng, S.; Zhang, P.; Jia, M. J.; Zhang, W. X. Size Effects of Platinum Colloid Particles on the Structure and CO Oxidation Properties of Supported Pt/Fe2O3 Catalysts. J. Phys. Chem. C 2013, 117, 21254-21262. (32) Li, S. Y.; Jia, M. J.; Gao, J.; Wu, P.; Yang, M. L.; Huang, S. H.; Dou, X. W.; Yang, Y.; Zhang, W. X. Infrared Studies of the Promoting Role of Water on the Reactivity of Pt/FeOx


20

Page 21 of 25

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


Catalyst in Low-Temperature Oxidation of Carbon Monoxide. J. Phys. Chem. C 2015, 119, 2483-2490. (33) Nakagomi, F.; da Silva, S. W.; Garg, V. K.; Oliveira, A. C.; Morais, P. C.; Franco Jr, A. Influence of the Mg-Content on the Cation Distribution in Cubic MgxFe3-xO4 Nanoparticles. J. Solid State Chem. 2009, 182, 2423-2429. (34) Chandradass, J.; Jadhav, A. H.; Kim, K. H.; Kim, H. Influence of Processing Methodology on the Structural and Magnetic Behavior of MgFe2O4 Nanopowders. J. Alloys Compd. 2012, 517, 164-169. (35) Pradeep, A.; Priyadharsini, P.; Chandrasekaran, G. Sol-Gel Route of Synthesis of Nanoparticles of MgFe2O4 and XRD, FTIR and VSM Study. J. Magn. Magn. Mater. 2008, 320, 2774-2779. (36) Shen, Y.; Wu, Y. B.; Li, X. Y.; Zhao, Q. D.; Hou, Y. One-Pot Synthesis of MgFe2O4 Nanospheres by Solvothermal Method. Mater. Lett. 2013, 96, 85-88. (37) Raja, K.; Ramesh, P. S.; Geetha, D. Structural, FTIR and Photoluminescence Studies of Fe Doped ZnO Nanopowder by Co-Precipitation Method. Spectrochim. Acta, Part A 2014, 131, 183-188. (38) Parker, S. F. The Role of Hydroxyl Groups in Low Temperature Carbon Monoxide Oxidation. Chem. Commun. 2011, 47, 1988-1990. (39) Wang, H. L.; Krier, J. M.; Zhu, Z. W.; Melaet, G.; Wang, Y. H.; Kennedy, G.; Alayoglu, S.; An, K.; Somorjai, G. A. Promotion of Hydrogenation of Organic Molecules by Incorporating


21


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

Page 22 of 25

Iron into Platinum Nanoparticle Catalysts: Displacement of Inactive Reaction Intermediates. ACS Catal. 2013, 3, 2371-2375. (40) Zhu, H. Y.; Wu, Z. L.; Su, D.; Veith, G. M.; Lu, H. F.; Zhang, P. F.; Chai, S. H.; Dai, S. Constructing Hierarchical Interfaces: TiO2-Supported PtFe-FeOx Nanowires for Room Temperature CO Oxidation. J. Am. Chem. Soc. 2015, 137, 10156-10159. (41) Yang, Z. Z.; Zhang, N.; Cao, Y.; Gong, M. C.; Zhao, M.; Chen, Y. Q. Effect of Yttria in Pt/TiO2 on Sulfur Resistance Diesel Oxidation Catalysts: Enhancement of Low-Temperature Activity and Stability. Catal. Sci. Technol. 2014, 4, 3032-3043. (42) Ahmadi, R.; Amini, M. K.; Bennett, J. C. Pt-Co Alloy Nanoparticles Synthesized on SulfurModified Carbon Nanotubes as Electrocatalysts for Methanol Electrooxidation Reaction. J. Catal. 2012, 292, 81-89. (43) Zhou, Z. P.; Zhang, Y.; Wang, Z. Y.; Wei, W.; Tang, W. F.; Shi, J.; Xiong, R. Electronic Structure Studies of the Spinel CoFe2O4 by X-Ray Photoelectron Spectroscopy. Appl. Surf. Sci. 2008, 254, 6972-6975. (44) Ran, F. Y.; Tsunemaru, Y.; Hasegawa, T.; Takeichi, Y.; Harasawa, A.; Yaji, K.; Kim, S.; Kakizaki, A. Valence Band Structure and Magnetic Properties of Co-Doped Fe3O4 (100) films. J. Appl. Phys. 2011, 109, 123919. (45) Yan, Z. K.; Gao, J. M.; Li, Y.; Zhang, M.; Guo, M. Hydrothermal Synthesis and Structure Evolution of Metal-Doped Magnesium Ferrite from Saprolite Laterite. RSC Adv. 2015, 5, 9277892787.


22

Page 23 of 25

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


(46) Li, F.; Liu, X. F.; Yang, Q. Z.; Liu, J. J.; Evans, D. G.; Duan, X. Synthesis and Characterization of Ni1-xZnxFe2O4 Spinel Ferrites from Tailored Layered Double Hydroxide Precursors. Mater. Res. Bull. 2005, 40, 1244-1255. (47) Guo, S.; Zhang, G. K.; Guo, Y. D.; Yu, J. C. Graphene Oxide-Fe2O3 Hybrid Material as Highly Efficient Heterogeneous Catalyst for Degradation of Organic Contaminants. Carbon 2013, 60, 437-444. (48) Ardizzone, S.; Bianchi, C. L.; Fadoni, M.; Vercelli, B. Magnesium Salts and Oxide: an XPS Overview. Appl. Surf. Sci., 1997, 119, 253-259. (49) Zhang, H.; Qi, R.; Evans, D. G.; Duan, X. Synthesis and Characterization of a Novel Nanoscale Magnetic Solid Base Catalyst Involving a Layered Double Hydroxide Supported on a Ferrite Core. J. Solid State Chem. 2004, 177, 772-780. (50) Kim, J. G.; Pugmire, D. L.; Battaglia, D.; Langell, M. A. Analysis of the NiCo2O4 Spinel Surface with Auger and X-Ray Photoelectron Spectroscopy. Appl. Surf. Sci. 2000, 165, 70-84. (51) Tudorache, F.; Popa, P. D.; Dobromir, M.; Iacomi, F. Studies on the Structure and Gas Sensing Properties of Nickel-Cobalt Ferrite Thin Films Prepared by Spin Coating. Mater. Sci. Eng. B 2013, 178, 1334-1338. (52) Li, L.; Wang, A. Q.; Qiao, B. T.; Lin, J.; Huang, Y. Q.; Wang, X. D.; Zhang, T. Origin of the High Activity of Au/FeOx for Low-Temperature CO Oxidation: Direct Evidence for a Redox Mechanism. J. Catal. 2013, 299, 90-100.


23


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 24 of 25

(53) Bliem, R.; Hoeven, J. V. D.; Zavodny, A.; Gamba, O.; Pavelec, J.; Jongh, P. E. D.; Schmid, M.; Diebold, U.; Parkinson, G. S. An Atomic-Scale View of CO and H2 Oxidation on a Pt/Fe3O4 Model Catalyst. Angew. Chem., Int. Ed. 2015, 54, 13999-14002. (54) Wu, G. P.; Wang, J.; Dong, W.; Nie, Y.; Li, L.; Qi, X. Q.; Chen, S. G.; Wei, Z. D. A Strategy to Promote the Electrocatalytic Activity of Spinels for Oxygen Reduction by Structure Reversal. Angew. Chem., Int. Ed. 2016, 55, 1340-1344.


24

Page 25 of 25

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


Table of Contents Graphic


25

A Stable Inverse Spinel Oxide Support for ... - ACS Publications

Recommend Documents