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Support Morphology-dependent Catalytic Activity of Pd/CeO2 for Formaldehyde Oxidation Hongyi Tan, Jin Wang, Shuzhen Yu, and Kebin Zhou Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01264 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 6, 2015

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Environmental Science & Technology

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Support Morphology-dependent Catalytic Activity

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of Pd/CeO2 for Formaldehyde Oxidation

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Hongyi Tan, Jin Wang, Shuzhen Yu, and Kebin Zhou*

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School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

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Beijing 100049, P.R. China

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Keywords: Formaldehyde (HCHO), Catalytic oxidation, Pd/CeO2, Morphology-dependent,

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Metal-support interaction

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Abstract: To eliminate indoor formaldehyde (HCHO) pollution, Pd/CeO2 catalysts with

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different morphologies of ceria support were employed. The palladium nanoparticles loaded on

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{100}-faceted CeO2 nanocubes exhibited much higher activity than those loaded on {111}-

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faceted ceria nanooctahedrons and nanorods (enclosed by {100} and {111} facets). The HCHO

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could be fully converted into CO2 over the Pd/CeO2 nanocubes at a GHSV of 10 000 h-1 and a

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HCHO inlet concentration of 600 ppm at ambient temperature. The prepared catalysts were

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characterized by a series of techniques. The HRTEM, ICP-MS and XRD results confirmed the

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exposed facets of the ceria and the sizes (1-2 nm) of the palladium nanoparticles with loading

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amounts close to 1%. According to the Pd 3d XPS and H2-TPR results, the status of the Pd-

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species was dependent on the morphologies of the supports. The {100} facets of ceria could

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maintain the metallic Pd species rather than the {111} facets, which promoted HCHO catalytic

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combustion. The Raman and O 1 s XPS results revealed that the nanorods with more defect sites

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and oxygen vacancies were responsible for the easy oxidation of the Pd-species and low catalytic

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activity.

23 24 25 26 27 28 29 30 31 32 33 34 35 36


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Introduction:

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Formaldehyde (HCHO) is considered to be a major toxic indoor pollutant that can be

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sourced from various wood-based materials, insulation materials, adhesives and coatings.1 Long-

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term exposure to formaldehyde may induce eye, nose, and skin irritation, pulmonary diseases or

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even nasopharyngeal cancer.2 For the sake of improving human health, significant efforts have

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been made to eliminate indoor HCHO pollution. Among the numerous HCHO purity

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techniques,3-6 catalytic oxidation has proven to be an efficient and promising method.7-9 To

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achieve HCHO purification at ambient temperature, the supported noble catalysts (Pt, Au, Pd, Rh

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and Ag) are more favorable.

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received much attention in recent years due to their high efficiency.15-18 However, the high prices

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of these precious metals limit their widespread application, and there is still a large demand for

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the development of low cost catalysts with good activity at room temperature.

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Particularly, the Pt- and Au- based supported catalysts have

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Among the noble metals, palladium is less expensive and more abundant compared with

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gold and platinum.19 The palladium-based catalysts have been demonstrated to be very

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competent at catalyzing the oxidation of various compounds, including methane, benzene, ethyl

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acetate and other volatile organic compounds.20-22 In the early research, however, the activity of

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Pd-based catalysts toward the catalytic oxidation of HCHO does not stand out in comparison

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with the other noble metals.13 Most recently, He et al. found that the addition of sodium ions to

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Pd/TiO2 led to completely catalytic oxidation of HCHO at ambient temperature, and the

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negatively charged Pd-species played a key role in the significant activity promotion.23

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Therefore, to improve the performance of palladium catalysts in the catalytic oxidation of

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HCHO, it seems crucial to understand the relationship between the structure of the catalysts and

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the chemical state of the Pd-species and to stabilize the Pd-species in its metallic form.


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Metal-oxide interactions are of growing importance with respect to supported noble metal

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catalysts. The charge redistribution and mass transport processes at the interface may happen

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because of the contact between a metal nanoparticle and the oxide.24 The morphology of an

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oxide support, including crystal planes exposed on the surface and crystallinity, is an important

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structural aspect in the process. Therefore, morphology-controlling strategies have become a new

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method for tuning the catalytic performance of oxide-incorporating catalysts.25, 26 Ceria (CeO2) is

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one of most important oxides in heterogeneous catalysis and is widely used as the support

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because of its redox performance.27-30 Various uniform and well-defined morphologies of ceria

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have been synthesized to explore the relationship between morphology and catalytic

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performance in a series of catalytic reactions.31, 32 In 2008, when applying Au/CeO2 catalysts to

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the water-gas shift reaction, Si et al. found that the valence of gold varied with the shape of the

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ceria supports. The chemical state of the active component could be affected when being loaded

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on different crystal planes.33 Similar phenomena were observed when platinum or ruthenium was

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supported on ceria with different morphologies.34-36 The morphological effect of the ceria on the

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supported Pd catalysts, to the best of our knowledge, has seldom been studied. Hence, it is worth

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applying the Pd/CeO2 catalyst to the catalytic oxidation of HCHO and trying to tune its catalytic

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activity with different morphologies of ceria.

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In this work, we prepared different Pd/CeO2 catalysts by employing different morphologies

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of ceria (nanocubes, nanooctahedrons, and nanorods). The catalytic performances for HCHO

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oxidation were found to be quite different with these three catalysts. The palladium nanoparticles

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showed the best redox ability with the ceria nanocubes, and the activity was much higher than

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for the other two catalysts, achieving full combustion of 600 ppm of HCHO at a gas hourly space

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velocity (GHSV) of 10 000 h-1 at ambient temperature. The relationship between the catalytic


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performance and the morphology of the ceria support was elucidated by analyzing the

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characterization results.

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Experimental Section All the materials are analytically pure and were used as received without further purification in this study.

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Preparation of the Ceria Supports. All of the ceria materials were prepared via the

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hydrothermal method. The preparation of CeO2 nanocrystals exposing different facets followed

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the method used in our previous studies.32, 37 For ceria nanorods, 1.5 g of Ce(NO)3·6H2O was

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dissolved in 20 ml deionized water, and a proper amount of 10% NaOH solution was added.

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Then, all of the slurry was rapidly transferred into a 50 ml autoclave, which was filled with water

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up to 80% of its total volume to give a final NaOH concentration of approximately 2 M. The

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autoclave was heated at 120 °C in the oven for 12 h. The final product was collected by filtration,

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washed with deionized water to remove any possible ionic remnants, and then dried and calcined

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at 350 °C for 4 h. To obtain CeO2 nanooctahedrons, 6 g of Ce(NO)3·6H2O was dissolved in 30

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ml water, and 10 ml solution containing 0.01 g NaOH was added under vigorous stirring. The

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stock solution was stirred vigorously for approximately 10 min and then sealed in a 50 ml

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autoclave. Hydrothermal treatment was conducted at 180 °C for 12 h. The final product was

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collected by filtration, washed with deionized water and then dried and calcined at 350 °C for 4

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h. The preparation of CeO2 nanocubes followed the same procedure as nanooctahedrons except

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that the dosage of Ce(NO)3·6H2O was 1 g, 8 g of NaOH was used, and the reaction temperature

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was set to 200 °C for 24 h.


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Loading of the Palladium Nanoparticles. CeO2 powder (0.3 g of nano-rods, octahedrons,

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and cubes) was dispersed in 100 ml deionized water and sonicated for 10 min, then 0.005 g of

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PdCl2 was added and stirred for an hour. After that, the pH of the slurry was adjusted to neutral

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with NaOH. After that, 2 ml of an aqueous solution containing 0.01 g NaBH4 was added into the

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suspension as reducing agent under stirring after aging. The obtained precipitates were washed

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with deionized water and ethanol 3–4 times and dried at 60 °C overnight. The obtained

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palladium supported CeO2 nanorods, octahedrons, and cubes are denoted as Pd/Rod, Pd/Oct, and

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Pd/Cube, respectively.

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Catalytic Activity Test. The catalytic activity test for the oxidation of formaldehyde over

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the catalysts was conducted in a continuous flow fixed-bed quartz tubular reactor (4 mm internal

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diameter) at atmospheric pressure. 100 mg of catalyst was sandwiched between two quartz wool

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layers in the micro-reactor. Before the test, the catalyst was reduced under a hydrogen

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atmosphere at 200 °C for an hour and stored in the air after cooling in case the catalysts were

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oxidized during drying process. The standard feed gas contained 600 ppm of HCHO, 20% O2,

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and N2 comprised the balance. Gaseous HCHO was generated by flowing N2 over

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paraformaldehyde, which was placed in a thermostatic water bath. The concentration of HCHO

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was determined by the flow rate of N2 and the temperature of the water bath. The total flow rate

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was 45 ml min-1, corresponding to a gas hourly space velocity (GHSV) of 10 000 h-1. Addition of

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moisture was achieved by another stream of air flowed through a water bubbler in a thermostatic

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water bath, and the relative humidity was tuned by the temperature of the water bath ranged of

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10 to 40 oC. The streams of moisture and HCHO were rapidly mixed before catalytic reaction

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and the total humidity was calculated with saturated vapor pressure of water. The total flow rate

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was raised to 60 ml min-1, corresponding to a gas hourly space velocity (GHSV) of 15 000 h-1.


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The temperature of the reactor stayed at the room temperature under different moisture

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conditions proved that the temperature of water bath has little effect on the reaction temperature.

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The generated CO2 resulting from HCHO oxidation was monitored by an on-line SP-2100 gas

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chromatograph equipped with a FID detector. The conversion of formaldehyde is expressed as

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[CO2]/[CO2]*, where [CO2]* is the concentration of CO2 in the effluent when HCHO is oxidized

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completely and [CO2] is the concentration of CO2 at various temperatures. The initial

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concentration of HCHO is determined by external standard method. A CO2 standard curve was

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created using the different CO2 concentrations. And the concentration of CO2 when HCHO is

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oxidized completely is 600 ppm according to the standard curve.

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Catalysts Characterization. The size and morphology of the catalysts were characterized

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with a FEI Tecnai F20 high-resolution transmission electron microscope (HRTEM). The powder

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X-ray diffraction (XRD) patterns were obtained on an MSAL-XD2 X-ray diffractometer with

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Ni-filtered Cu Kα radiation (λ=0.1541 nm), and the data were recorded at a scan rate of 4

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degrees min-1. The elemental analysis was obtained with a Varian Vista MPX Inductively

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Coupled Plasma Optical Emission Spectrometer (ICP-OES). The Brunauer−Emmett−Teller

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(BET) surface area was evaluated from nitrogen adsorption data recorded using a Gemini V

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Micromeritics (U.S.A). X-ray photoelectron spectroscopy (XPS) measurements were performed

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with an ESCALAB 250 Xi photoelectron spectroscope. The samples were reduced under a

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hydrogen atmosphere at 200 °C for an hour and stored in the air after cooling before XPS test. H2

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temperature-programmed reduction (TPR) was conducted with a ChemiSorb 2720 apparatus

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equipped with a TCD detector. Before the H2-TPR analysis, the samples were treated in pure

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oxygen at 200 °C for an hour. TPR was performed by heating the catalysts (approximately 50

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mg) from approximately 0 to 850 °C in a 5 vol% H2-Ar mixture with a flow rate of 25 mL min-1.


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Results and Discussion

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Crystal and structural properties. Figure 1a shows the typical low-resolution HRTEM

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image of the as-obtained Pd/Cube, with the average sizes of the ceria nanocubes approximately

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50 nm. The high-resolution image in Figure 1b shows the clear (200) lattice fringe with an

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interplanar spacing of 0.27 nm, revealing that the CeO2 nanocubes are mainly enclosed by the

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{100} planes.38 Figure 1c shows that sizes of the CeO2 nanooctahedrons are between 40–60 nm,

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and the high-resolution image (Figure 1d) shows a clear (111) lattice fringe with an interplanar

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spacing of 0.31 nm, implying the nanooctahedrons are mainly enclosed by {111} facets. Figure

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1e shows the low-resolution image of the Pd/Rod, and the ceria nanorods have a less uniform

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diameter between 10–20 nm and lengths between 100–300 nm. Based on the interplanar spacing,

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the high-resolution image results in Figure 1f suggests that the predominantly exposed planes are

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both {100} and {111} planes, which is consisted with our previous results.32 The lattice fringe is

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less clear in the nanorods, which may be due to a poorer crystallinity and crystal defects on its

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surface. In addition, the palladium nanoparticles are noted with white arrows in the HRTEM

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images of the three samples. The hemispheric palladium nanoparticles are homogeneously

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dispersed on the surface of the CeO2 supports. The sizes of the nanoparticles are similar, all

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approximately 2 nm.

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Figure 1. HRTEM images of (a, b) Pd/Cube, (c, d) Pd/Oct, (e, f) Pd/Rod with low and high

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resolution, respectively.

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Figure 2 shows the XRD patterns of the Pd/CeO2 catalysts. The feature peaks are observed

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at 2θ= 28.6, 33.1, 47.5, 56.3, 59.1, 69.4 and 76.7, which correspond to the (111), (200), (220),

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(311), (222), (400), (331) plane diffraction patterns of the fluorite structure of CeO2 (JCPDS 34-

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0394). However, no feature peaks corresponding to the palladium species can be found in the

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XRD patterns, confirming that Pd nanoparticles should be small in size and highly dispersed on

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the surface of all three CeO2 supports.39 The broadening of the diffraction peak ascribed to the

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nanorods distinctly indicates its poorly crystalline nature, with agrees with the results shown in

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the HRTEM image.

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Figure 2. XRD patterns of Pd/Cube, Pd/Oct and Pd/Rod.

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BET surface areas of the catalysts are listed in Table 1. The specific area of Pd/Rod is 83.1

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m2/g, which is 3–4 times that of Pd/Cube and Pd/Oct (18.4 and 29.9 m2/g, respectively). The

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palladium loading content was measured with ICP-OES. As shown in Table 1, the palladium

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contents in these three samples are all similar to our desired 1 wt% palladium loading. Combined

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with the HRTEM results, it can be concluded that the physical states of Pd nanoparticles in the

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three catalysts are similar.

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Table 1. Specific area and the palladium content of different Pd/CeO2 catalysts Sample

Pd/Cube

Pd/Oct.

Pd/Rod

BET area (m2/g)

18.4

29.9

83.1

Pd content (wt%)

0.91

0.94

0.96

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Activity Test. The catalytic activities of formaldehyde oxidation on the three Pd/CeO2 catalysts

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were evaluated with an air stream containing 600 ppm HCHO at a GHSV of 10 000 h-1, and the

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temperature dependence of formaldehyde conversion is shown in Figure 3. The oxidation of

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HCHO starts slightly higher than the room temperature over the Pd/Rod catalyst, it accelerates

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quickly after 50 °C, and a complete conversion is achieved at 90 °C. The Pd/Oct catalyst

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performs much more efficiently than Pd/Rod. The “light-off” temperature of Pd/Oct is much

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lower than that of Pd/Rod, which is approximately 5 °C. The conversion of HCHO reached 45%

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at room temperature and was completed at 60 °C. When compared to the Pd/Oct, the Pd/Cube

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shows an even higher catalytic activity. The HCHO is fully converted into water and carbon

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dioxide at 20 °C. A 35% conversion of HCHO can be achieved even at 0 °C.

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Figure 3. HCHO conversion over Pd/Cube, Pd/Oct and Pd/Rod samples at different

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temperatures. Reaction conditions: 600 ppm of HCHO, 20% O2, N2 balance, GHSV 10 000 h-1.


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The specific reaction rate of formaldehyde oxidation in terms of consumed amount of

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HCHO per gram of palladium per second was measured. The effects of internal diffusion and

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external diffusion were eliminated by tuning the amount of catalysts and diluting with quartz

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sand. At room temperature, the specific reaction rate for Pd/Oct and Pd/Cube is 6.0 and 21.6

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µmol g(Pd)-1 s-1, respectively, and the Pd/Rod showed no activity at this temperature. This result

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indicates that the morphology of the CeO2 supports has a significant effect on the rate of

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formaldehyde oxidation. For comparison, we calculated the apparent reaction rate of some

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catalysts that can realize the total removal of formaldehyde at room temperature in the literature.

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The apparent reaction rate was 11.2 µmol g(Pt)-1 s-1 for Pt/f-SiO2,40 10.78 µmol g(Au)-1 s-1 for

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Au/FeOx,41 and 1.5 µmol g(Pd)-1 s-1 for Pd/TiO2,42 respectively. And under our experiment

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condition, the apparent reaction rate of Pd/Cube is 13.6 µmol g(Pd)-1 s-1. These results suggest the

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Pd/Cube can be an inexpensive and efficient catalyst for indoor formaldehyde removal.

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Figure 4. Pd 3d XPS spectra of Pd/Cube, Pd/Oct and Pd/Rod samples.


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XPS Analysis of Pd Species. Within the different substrates, the active chemical state of the Pd

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species may vary.42, 43 To investigate the chemical states of the Pd elements on the prepared

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catalysts surfaces and determine the relationship with their catalytic activities, XPS analyses of

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Pd 3d were performed. The obtained spectra are summarized in Figure 4, and the binding energy

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peaks were deconvoluted according to the literature.44 It can be found that the Pd/Cube displays

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two Pd 3d5/2 peaks at 335.89 and 336.98 eV, the typical characteristics of metallic Pd and Pd

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oxide, respectively. In the case of Pd/Oct, the two peaks of Pd 3d5/2 shift to higher binding

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energy, at 336.10 and 337.00 eV, respectively. For Pd/Rod, the binding energy of Pd 3d5/2 is

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337.39 eV, which can be attributed to Pd oxide. The calculation results shows that approximately

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54% of Pd species on the Pd/Cube are in the metallic state, and on the Pd/Oct, this value

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decreases to 27%, whereas all the Pd species in Pd/Rod are in oxide form. These results reveal

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that the palladium of Pd/Cube is easier to keep metallic state in air and more electron rich than

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that in other two samples, which may be due to the metal-support interaction in Pd/CeO2.45 It is

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well known that the formation of metallic Pd species can promote the activation of oxygen

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species because the O2 adsorption is enhanced by the donation of electrons from the metal to the

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antibonding п* orbital of O2.46, 47 Hence, the percentage of metallic Pd species depending on the

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morphology of CeO2 is vital to the low temperature catalytic oxidation of HCHO. High

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percentages of metallic Pd species lead to more efficient catalytic activity at ambient temperature.

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Reducibility of Catalysts. To explore the probable reason for the different chemical states of Pd

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species, H2-TPR was employed to further investigate the redox ability of Pd species on the

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Pd/CeO2 catalysts. Figure 5 shows the H2-TPR profiles of the three Pd/CeO2 catalysts. All of

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these profiles show two reduction peaks in the temperature region of 0–850 °C. The peaks at


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high temperature regions (> 600 °C) can be attributed to the reduction of lattice oxygen of bulk

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CeO2 (not shown), while the peaks at low temperatures can be attributed to the reduction of Pd

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species in the oxide state. As shown in Figure 5, the intense reduction peaks of the Pd species

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center at 31, 40 and 75 °C for the Pd/Cube, Pd/Oct and Pd/Rod, respectively. This result

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indicates the reducibility of the Pd species of different catalysts follows the trend: Pd/Cube >

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Pd/Oct > Pd/Rod. The Pd species on the CeO2 nanocubes are easier to reduce, which benefits the

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maintenance of its metallic state and its HCHO catalytic oxidation activity. The hydrogen

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consumption of the reduction peaks at low temperatures was calculated, while the theoretical

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value of hydrogen consumption on Pd/CeO2 was also calculated based on the assumption of PdO

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formation from palladium concentration in ICP-OES results (Table 1). The actual hydrogen

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consumption amounts of Pd/Cube and Pd/Oct are 8.19 and 13.61 µmol, respectively, which is

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close to the theoretical value 4.29 and 4.62 µmol. However, the actual hydrogen consumption of

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Pd/Rod is 72.40 µmol, much higher than the theoretical value of 4.62 µmol, which implies

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significant surface oxygen species reduction together with the reduction of Pd species. This

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phenomenon may be due to the strong palladium and ceria nanorods interaction, oxygen transfer

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and reduction enhancement allowed by the defects in the ceria rods crystallites.48

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Considering most of the characteristics of Pd/Cube and Pd/Oct are similar, the differences

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in redox ability of Pd species should be attributed to the different facets to which they are

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exposed. That is to say, the Pd species have more favorable redox properties on the {100} facets

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of ceria than on the {111} facets, and are able to maintain the metallic state on the {100} facets,

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which leads to more efficient oxidation activity.


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Figure 5. H2-TPR profiles of Pd/Cube, Pd/Oct and Pd/Rod samples.

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Effect of Ceria Surface Defects. If one solely ascribes the different chemical states of the Pd

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species to the exposed facets, it is difficult to explain the fact that the Pd/Rod catalyst which

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exposes both the (100) and (111) facets showed the poorest activity. To further clarify how the

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morphology of CeO2 affects the redox property of the supported palladium species, Raman

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spectra were collected to study the ceria surface defects. Figure 6 displays the visible Raman

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spectra (514 nm) of the catalysts. To facilitate observation, the data of Pd/Rod were 4 times

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magnified. A distinct F2g symmetry mode of the CeO2 phase centers at approximately 462 cm-1,

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which can be attributed to symmetrical stretching of the Ce-O vibrational unit in 8-fold

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coordination.49 The peaks at 598 and 1179 cm-1 are attributed to the defect induced mode (D) and

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the second-order longitudinal optical mode (2 LO), respectively. The peak intensity of defect

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induced mode depends on the presence of some defects and is enhanced when oxygen vacancies

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are present in the ceria lattice.50, 51 The degree of defect sites on CeO2 can be determined from

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the relative intensity of D/F2g.52 The intensity ratios of Pd/Cube and Pd/Oct are quite close,

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which are 0.024 and 0.026, respectively. However, the intensity ratio of Pd/Rod is 0.188, much


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larger than the other two catalysts, revealing that the Pd/Rod has the most intrinsic defect sites

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and the oxygen vacancies are abundant.

282 283

Figure 6. Raman spectra of Pd/Cube, Pd/Oct and Pd/Rod samples.

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It is well known that the oxygen vacancies are conducive to absorbing oxygen species.53, 54

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The surface oxygen species were further characterized by the O 1 s XPS analysis. Figure 7

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shows the deconvoluted O 1 s XPS spectra of the Pd/CeO2 catalysts. The O 1 s XPS data exhibit

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a main peak at the binding energy range of 529.30–529.44 eV and a smaller shoulder peak at the

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binding energy range of 531.03–531.30 eV. The common main peak can be ascribed to the

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lattice oxygen of bulk CeO2 (Olat), and the shoulder peak can be attributed to surface

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chemisorbed oxygen (Osur), which contains O-, O2-, O22-, etc. The ratios of Osur/(Olat + Osur) are

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approximately 34%, 31% and 43% for Pd/Cube, Pd/Oct and Pd/Rod, respectively. The

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percentage of Osur on the Pd/Rod catalyst is higher than that of Pd/Cube and Pd/Oct. This

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phenomenon implies that the Pd/Rod exhibited a higher concentration of chemisorbed oxygen,

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which may due to the higher proportion of Ce3+ in the ceria nanorods generated more oxygen

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vacancies (Figure S1). According to the literature, the oxygen vacancies on the ceria can

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generate adsorbed atomic oxygen.55 The adsorbed oxygen can greatly influence the oxidation


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and reduction properties of the supported Pd-species; the metallic palladium can be oxidized and

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subsequently stabilized by the oxygen vacancies,27, 28 resulting in deactivation of the Pd/Rod

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catalyst.

300 301

Figure 7. O 1 s XPS spectra of Pd/Cube, Pd/Oct and Pd/Rod samples.

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Stability of Pd/Cube Catalyst. The durability and humidity resistance of the catalysts are very

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important properties in their practical applications. The catalytic performance of the Pd/Cube

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was measured by a long isothermal test at the room temperature. With the same reaction

305

conditions, no obvious activity loss is observed and approximately 100% HCHO conversion is

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maintained during the 12 h test as shown in the Figure 8a. For the different catalysts of HCHO

307

catalytic oxidation, the effect of moisture on catalytic activity may be positive or negative which

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is attributed to desorption of products, competitive adsorption of water and reactant56 or catalytic

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effect of surface hydroxide radical.57 Figure 8b shows the effect of humidity on catalyst

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performance of Pd/Cube. An additional stream of moisture was added to the system. With the

311

elevated GHSV (15000 h-1), the HCHO supply was also increased to maintain the concentration


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of HCHO at 600 ppm. As seen, the presence of water has little effect on the activity of the

313

catalyst. The Pd/Cube catalyst is robust within a relative humidity range of 0–70 %.

314 315

Figure 8. (a) Durability test of Pd/Cube. Reaction conditions: 600 ppm of HCHO, 20% O2, N2

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balance, GHSV 10 000 h-1, at room temperature; (b) Catalytic activity of Pd/Cube under different

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humidity conditions. Reaction conditions: 600 ppm of HCHO, 20% O2, N2 balance, GHSV 15

318

000 h-1 at room temperature.

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In summary, this work demonstrates that palladium nanoparticles supported on ceria

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nanocubes can be promising catalysts for practical application to the ambient catalytic oxidation

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of formaldehyde. This catalyst can reliably convert formaldehyde into water and carbon dioxide

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with high efficiency and has great resistance against moisture. The palladium in the metallic state

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is crucial for the low temperature catalytic oxidation of formaldehyde. Compared to the {111}


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facets, which is exposed on ceria nanooctahedrons, exposing the {100} facets on the ceria

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nanocubes increases the reducibility of the palladium species. Meanwhile, the degree of

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crystallinity also has a significant effect on the catalytic activity. The defects and oxygen

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vacancies on ceria nanorods are favored by the palladium oxide thus hindering the process of

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catalytic oxidation. This work confirms the importance of support morphology on the catalytic

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oxidation of formaldehyde with noble metal based catalysts.

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ASSOCIATED CONTENT

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Supporting Information.

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XPS Analysis of Cerium Species. This material is available free of charge via the Internet at http://pubs.acs.org.

335 336

AUTHOR INFORMATION

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Corresponding Author

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*Address: School of Chemistry and Chemical Engineering, University of Chinese Academy of

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Sciences, Beijing 100049, P.R. China. Phone and Fax number: 86-10-88256940. E-mail:

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[email protected]

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Author Contributions

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All authors have given approval to the final version of the manuscript.

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Notes

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The authors declare no competing financial interests.


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ACKNOWLEDGMENT

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This work was supported by the National Natural Science Foundation of China (21473199 and

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U1162113) and the Beijing Municipal Science & Technology Commission.

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REFERENCES

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