SiO2 Catalysts via Galvanic

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Nano-sized, hollow, and Mn doped CeO/SiO catalysts via galvanic replacement: preparation, characterization, and application as highly active catalyst Chunzheng Wu, Zhiya Dang, Mirko Prato, Sergio Marras, Andrea Cerea, Francesco De Angelis, Liberato Manna, and Massimo Colombo ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00380 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Nano Materials

Nano-sized, Hollow, and Mn Doped CeO2/SiO2 Catalysts via Galvanic Replacement: Preparation, Characterization, and Application as Highly Active Catalyst ‡Chunzheng Wu,a d ‡Zhiya Dang,a Mirko Prato,b Sergio Marras,b Andrea Cerea,c Francesco De Angelis,c Liberato Manna,a and Massimo Colombo*a a. Department of Nanochemistry, b. Materials Characterization Facility and c. Department of Plasmon Nanotechnologies, Istituto Italiano di Tecnologia, Via Morego 30, 16163, Genova, Italy d. Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31-I16146, Genova, Italy * E-mail: [email protected]

Keywords: Galvanic Replacement; Ceria; Cerium Oxide; Oxide Catalyst; Porous Oxides; CO oxidation; Soot oxidation

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ABSTRACT: We prepared 20~30 nm hollow CeO2 nanoparticles with 6~9 nm thick porous shells by performing an easy, cost effective and water based galvanic replacement on SiO2 supported Mn3O4 nanoparticles with Ce3+. The low density, defected structure doped with residual Mn and the easily reducible sur-face makes the catalysts highly reactive for both CO oxidation and soot combustion reactions.


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MAIN TEXT Ceria (CeO2) has been widely exploited in applications in biomedicine, fuel cells, and heterogeneous catalysis1 thanks to its ability to store and release oxygen by switching between +4 and +3 oxidation states. As one of the most reactive metal oxides, CeO2 has been intensively studied in automobile exhaust purification, low-temperature water-gas-shift reaction, hydrocarbon reforming, and many other catalytic oxidation reactions.2-3 Generally, the catalytic performance of CeO2 based catalysts could be largely tuned by tailoring their structural and compositional properties, such as the types of facets that are exposed, the porosity, the doping elements, and the synergy with noble metals.1-3 A combination of porous structures with transition metal doping has been reported recently by Lee et al.4 through a pyrolysis of Mn-Ce bimetallic coordination polymers. The obtained nanoporous MnxCe1-xO2 bulk catalyst exhibited an extremely high oxygen storage capacity (OSC) and a superior CO oxidation activity that was ranked among the best reported to date. It is noteworthy that the morphology of the CeO2 catalysts needs to be carefully designed when solid phase reactants (e.g. soot) are involved. For instance,

three-dimensionally ordered macroporous CeO2 based mixed oxides have displayed high

activity for both the CO oxidation and the soot combustion reactions.5-6 The accessibility of the interior sites of macropores for the soot could dramatically enlarge the CeO2-soot contact, which is a prerequisite for this reaction. Nanoporous, hollow structured CeO2 is also expected to be promising for both CO oxidation and soot combustion, due to the low density and high ratio of exterior surface atoms. Various strategies, including the soft/hard templated growth,7 protective etching,8 Ostwald ripening,9 and Kirkendall effect,10 have been adopted to prepare hollow CeO2 spheres. However, all these approaches have delivered CeO2 structures in the size range of 0.1~a few micrometers, with thick shells and with CO oxidation activities at least one order of


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magnitude lower than the nanoporous MnxCe1-xO2 bulk catalysts reported by Lee et al.4 An improvement of the catalytic activity is expected by further lowering the particle size (to ⩽ 100 nm) and shell thickness, and by introducing transition metal dopants, while preventing the aggregation of the NPs. Following these principles, we report here the preparation of Mn-doped, hollow CeO2 NPs with a 20~30 nm diameter and a 6~9 nm-thick porous shell, supported on SiO2, through an insitu galvanic replacement procedure (Scheme 1). The hollow CeO2 NPs were indeed fabricated based on the oxidation of aqueous Ce3+ by Mn(III) species in Mn3O4 NPs, driven by the higher standard reduction potential of Mn3O4/Mn2+ (1.82 V) compared to that of Ce4+/Ce3+ (1.72 V).

Scheme 1. Schematic representation of the present work. Colloidal MnO NPs were deposited on SiO2 and calcined in air to obtained uniform, ligands free, SiO2 supported Mn3O4 NPs. The reaction between the supported Mn3O4 NPs and an aqueous solution of Ce3+ resulted in the formation of Mn doped, hollow CeO2 NPs, accordingly to a galvanic replacement mechanism. The galvanic replacement reaction, has been intensively studied in the preparation of hollow structured bimetallic NPs11 while it is still widely unexplored in the synthesis of mixed metal oxide NPs.12 The replacement of Mn3O4 with Ce3+ has been attempted in our previous work13 as well as in a recent publication by Lee et al.14 Despite the successful replacement of Mn3O4 with CeO2 reported in both publications, in one case the hollowing process did not work13, while in


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the other case it resulted in a collapsed aggregate of CeO2 NPs14. The SiO2 supported hollow CeO2 NPs that we obtained here were doped with Mn residuals, and were characterized by a high density of uncoordinated sites, stepped surfaces and defects, their surfaces can be easily reduced by CO, and exhibited high catalytic activities in both the CO oxidation and soot combustion reactions. Multiple characterization techniques and experimental conditions were used to confirm the galvanic replacement process and to understand the property of the as-synthesized hollow CeO2/SiO2. Three increasing amounts of Ce3+ were added to the starting MnOx/SiO2 sample dispersed in water, as detailed in the experimental section. The ICP analysis (Table 1) highlighted a corresponding increase of the Ce content in the final product, accompanied by a progressive decrease of the Mn concentration. At the highest Ce3+ concentration (20 times the stoichiometric Mn amount in the MnOx/SiO2 sample), a Ce/(Ce+Mn) molar ratio of 94% was measured in the final product (MnOx/SiO2-20Ce sample). Increasing the reaction time from 90 min to 6 h for the lowest Ce3+ concentration (5 times the stoichiometric Mn amount in the MnOx/SiO2 sample), led to similar results: a Ce/(Ce+Mn) molar ratio of 96% was measured in the reaction product (MnOx/SiO2-5Ce-6h sample), showing that the replacement reaction was kinetically controlled under our reaction conditions.

Table 1. Composition and crystal sizes of the original MnOx/SiO2 and Au-MnOx/SiO2 samples and of the Ce replaced ones, obtained from the ICP, XPS and XRD measurements.

MnOx/SiO2

Au wt.%

Mn wt.%

Ce wt.%

Mn+Ce mmol/50 mg

-

9.6(5)

-

0.088

Ce/(Mn+ Surface Crystal size Ce) ratio Ce ratio (nm) [b] at.% at.% [a] -

-

14.5 ± 0.5


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(Mn3O4)

MnOx/SiO2 -5Ce

13.5 ± 0.5 (Mn3O4) -

7.8(2)

6.1(6)

0.093

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50 ± 1 5.5 ± 0.5 (CeO2) 11.5 ± 0.5 (Mn3O4)

MnOx/SiO2 -10Ce

-

MnOx/SiO2 -20Ce

-

0.5(0)

16.0(9)

0.062

94

95 ± 1

4.5 ± 0.5 (CeO2)

MnOx/SiO2 -5Ce-6h

-

0.2(8)

16.4(7)

0.061

96

-

5.5 ± 0.5 (CeO2)

Au0.8(7) Mn3O4/SiO2

3.9(6)

-

0.036

-

-

4.5 ± 0.5 (Au)

Au@hCeO2/SiO2

0.0(9)

8.5(3)

0.031

98

-

4.5 ± 0.5 (Au)

2.3(6)

13.1(8)

0.068

75

85 ± 1 4.5 ± 0.5 (CeO2)

0.7(4)

[a] calculated based on the data from XPS measurement. [b] calculated using Scherrer equation based on the data from XRD measurement.

According to the stoichiometry of the galvanic replacement reaction (i.e. Ce3+ + Mn(III) → CeO2↓ + Mn2+), and considering the ratio of reactive Mn(III) to unreactive Mn(II) in the Mn3O4 phase, we expected a maximum decrease in the molar amount of Mn+Ce of 33%. This would result from the complete replacement of Mn(III) by Ce3+, and from the complete dissolution of the residual Mn(II). In agreement with the galvanic replacement mechanism, and considering the presence of a residual amount of manganese, we observed a loss of 30 % of the Mn+Ce molar amount in the MnOx/SiO2-20Ce. The XRD results (Figure S2) highlighted the progressive increase of the cubic CeO2 phase with the increase either of the Ce3+ amount or of the reaction time. The tetragonal Mn3O4 crystal phase (i.e. the only crystalline phase in the starting sample),


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progressively decreased, until it disappeared in the MnOx/SiO2-20Ce sample. The crystal sizes calculated based on the Scherrer equation clearly exhibited the dissolution of the Mn3O4 NPs (14.5 → 11.5 nm, Table 1), while the presence of the CeO2 crystals with an almost constant size (4.5

SiO2 Catalysts via Galvanic

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