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Heterostructured Copper-Ceria and Iron-Ceria Nanorods: Role of Morphology, Redox, and Acid Properties in Catalytic Diesel Soot Combustion Putla Sudarsanam, Brendan Hillary, Mohamad Hassan Amin, Nils Rockstroh, Ursula Bentrup, Angelika Brückner, and Suresh K. Bhargava Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03998 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Heterostructured Copper-Ceria and Iron-Ceria Nanorods: Role of Morphology, Redox, and Acid Properties in Catalytic Diesel Soot Combustion Putla Sudarsanam,*,†,‡ Brendan Hillary,† Mohamad Hassan Amin,† Nils Rockstroh,§ Ursula Bentrup,§ Angelika Brueckner,§ Suresh K. Bhargava*,† †

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of

Science, RMIT University, Melbourne, Victoria 3001, Australia §

Leibniz Institute of Catalysis e.V. (LIKAT), Albert-Einstein-Str. 29a, Rostock Germany

‡

Present address: Centre for Surface Chemistry and Catalysis, KU Leuven Celestijnenlaan 200F, 3001 Heverlee, Belgium

ABSTRACT: This work reports the synthesis of heterostructured copper-ceria and iron-ceria nanorods and the role of their morphology, redox, and acid properties in catalytic diesel soot combustion. Microscopy images show the presence of nanocrystalline CuO (9.5 ± 0.5 nm) and Fe2O3 (7.3 ± 0.5 nm) particles on the surface of CeO2 nanorods (diameter is 8.5 ± 2 nm and length within 16–89 nm). In addition to diffraction peaks of CuO and Fe2O3 nanocrystallites, X-ray diffraction (XRD) studies reveal doping of Cu2+ and Fe3+ ions into the fluorite lattice of CeO2, hence abundant oxygen vacancies in the Cu/CeO2 and Fe/CeO2 nanorods, as evidenced by Raman spectroscopy studies. XRD and Raman spectroscopy studies further show substantial perturbations in Cu/CeO2 rods, resulting in an improved reducibility of bulk cerium oxide and formation of abundant Lewis acid sites, as investigated by H2temperature

programmed

reduction

and

pyridine

adsorbed

FT-IR

studies,

respectively. The Cu/CeO2 rods catalyze the soot oxidation reaction at lowest temperatures under both tight contact (Cu/CeO2; T50 = 358 oC, temperature at which 50% soot conversion is achieved, followed by Fe/CeO2; T50 = 368 oC and CeO2; T50 = 433 oC) and loose contact conditions (Cu/CeO2; T50 = 419 oC and Fe/CeO2; T50 = 435 oC). A possible mechanism based on the synergetic effect of redox and acid properties of Cu/CeO2 nanorods was proposed: acid sites can activate soot particles to form reactive carbon species, which are oxidized by gaseous oxygen/lattice oxygen activated in the oxygen vacancies (redox sites) of ceria rods.

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1. INTRODUCTION Nanostructuring and metal/oxide interface engineering are new-fangled parameters for the design of novel, high performing catalysts.1,2 On the one hand, the reduction of catalyst particle size to nanoscale leads to abundant surface active sites, improved specific surface area, and enriched surface redox properties. On the other hand, metal/oxide interfaces provide large amounts of catalytically active sites due to the synergistic interactions of active metals with oxide supports at their interface sites. These implications have generated a tremendous research interest towards designing

heterostructured

catalysts

composed

of

transition

metal

oxide

nanoparticles (e.g., CuO, Fe2O3, MnOx, and NiO) with shape-controlled redox metal oxides (e.g., CeO2 nanorods and CeO2 nanocubes).3-9 These unique catalyst systems can maximize the effects of nanostructuring and metal/oxide interfaces, resulting in improved catalytic efficiencies. Rod shaped nano-CeO2 is a rich redox material, which preferentially exposes a large fraction of reactive (110) and (100) crystal planes.10 These crystal planes can exhibit a high concentration of redox sites (oxygen vacancies and Ce3+ species), depending upon the conditions employed during the reaction.11-13 Therefore, highly efficient and selective catalytically active sites could be formed in these catalyst systems due to synergistic interactions of crystal facets of CeO2 nanorods with transition metal oxide nanoparticles. Cu- and Fe-oxides play a key role as promoter/active phase in a number of industrially important catalytic processes, such as water-gas-shift reaction, FischerTropsch synthesis, CO oxidation, selective oxidation of hydrocarbons, etc.14-18 This wide applicability is due to their lower cost, higher availability, non-toxic nature, and interchangeable oxidation states (Cu: 0, +1, and +2 and Fe: 0, +2, and +3). The aim of this study is therefore to develop heterostructured catalysts composed of Cu- and Fe-oxide nanoparticles dispersed on CeO2 nanorods and to investigate the role of their morphology, redox, and acid properties for catalytic diesel soot oxidation. Soot particulates emitted by diesel exhaust engines are responsible for various health issues particularly in urban regions, including asthma, bronchitis, lung cancer as well as skin cell alterations.19,20 The use of diesel particulate filters (DPFs) to trap soot particles followed by their removal using an appropriate catalyst is a well-advanced technology currently used in diesel engines. This process enables regeneration of DPFs at low temperatures, improving the efficiency of the engine and simultaneously 2 ACS Paragon Plus Environment

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removing soot particles at low temperatures. CeO2-containing materials have been widely used in heterogeneous catalysis, for tasks such as the purification of car exhaust emissions, removal of organics in wastewater, combustion of volatile organic compounds, as well as diesel soot oxidation.21-26 The unique ability of Ce to switch between 4+ and 3+ oxidation states, along with the resulting oxygen vacancy defects which balance the charge in the cerium oxide lattice are sources of active sites for various catalytic applications including diesel soot oxidation. In addition, few studies have investigated the importance of acid properties of the catalysts for soot oxidation.27,28 The acid sites can convert soot particles into active carbon radical cation species, which can be easily oxidized by O2. Although Ce-, Cu- and Fe-oxides show mild acid properties, nanostructuring, shape selectivity, and metal/oxide interfaces in Cu/CeO2 rods and Fe/CeO2 rods may enhance the acid properties that can show a favorable effect in diesel soot oxidation. It is therefore vital to thoroughly investigate both redox and acidic properties of bare CeO2 rods, Cu/CeO2 rods, and Fe/CeO2 rods including their structural and morphology properties under a single roof that can provide new insights into understanding the effects of nanostructuring and metal/oxide interfaces in diesel soot oxidation, a typical heterogeneous catalytic reaction. For this, various analytical techniques, such as powder XRD, Raman spectroscopy, H2-TPR, pyridine adsorbed FT-IR, HRTEM, STEM-EDS mapping, and XPS spectroscopy are used in this study. A possible mechanism based on the synergistic effect of redox and acid properties of the synthesized catalysts is proposed for the oxidation of diesel soot in this work.

2. EXPERIMENTAL SECTION 2.1 Catalyst Preparation. An alkaline hydrothermal method was used to synthesize CeO2 nanorods using NaOH as the base. To synthesize a 1 g batch of CeO2 nanorods, the estimated quantity of cerium precursor (Ce(NO3)36H2O, Aldrich, AR grade) was dissolved in Milli-Q water under stirring conditions for 10 minutes. Then, 6 M of aqueous NaOH solution (60 mL) was added drop-wise to the above cerium solution. The stirring was continued for another 30 min at room temperature. The resultant solution was then transferred into a 100 mL Teflon bottle and sealed in a stainless-steel autoclave. The hydrothermal treatment was conducted at 100 oC for 24 h. After cooling to room temperature, the sample was collected, centrifuged 3 ACS Paragon Plus Environment

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several times with Milli-Q water until pH of the solution reaches to ~7, and then ovendried at 95 oC for 12 h. The dried yellow powder was calcined at 500 oC for 5 h in static air with a heating ramp of 5 oC/min. To synthesize Cu/CeO2 nanorods (10 wt.% Cu with respect to Ce) and Fe/CeO2 nanorods (10 wt.% Fe with respect to Ce), the calculated amounts of Cu(NO3)2·3H2O (Aldrich, AR grade) or Fe(NO3)3·9H2O (Aldrich, AR grade) were dissolved in Milli-Q water at room temperature. The required quantity of finely powdered CeO2 nanorods was then added to the above solution. The excess water was evaporated on a hot plate at approximately 95 oC under stirring conditions. The obtained sample was oven-dried at 95 oC for 12 h and then, calcined at 500 oC for 5 h with a heating ramp of 5 oC/min under static air conditions. 2.2 Catalyst Characterization. 2.2.1. Powder X-ray Diffraction (XRD) Studies. The crystalline nature of the materials is studied using powder XRD analysis on a Rigaku diffractometer with Cu Kα radiation (0.1540 nm). The data were recorded in the 2θ range of 10−90° with a step size of 0.1° and a step time of 1.5 s. The ceria lattice parameter of the samples was estimated by applying a standard cubic indexation method using the intensity of the most prominent (111) peak of the cerium oxide. 2.2.2. Transmission Electron Microscopy (TEM) Studies. The morphology of the materials, particle size, and the crystal phases of the metal oxides was estimated using TEM analysis. For this, a JEOL 2100F operating at 200 kV accelerating voltage, equipped with a Gatan Orius SC1000 charge-coupled device camera is used. The dispersion of copper and iron species on the surface of CeO2 is estimated using an EDS spectrometer (Oxford XMax80T) in a scanning TEM mode. 2.2.3. N2 Adsorption-Desorption Analysis: BET surface area and textural properties of the materials were estimated using N2 adsorption-desorption analysis with ASAP 2020 instrument at liquid nitrogen temperature (-196

o

C). Prior to

analysis, the sample was degassed under vacuum for 45 min at ambient temperature and then fast-mode degassing at 150 oC for overnight. BET surface area of the materials was estimated using desorption data. 2.2.4. X-ray Photoelectron Spectroscopy (XPS) Analysis. The oxidation states of the elements and relative surface concentration of Ce3+ ions in the catalysts are determined using XPS studies performed on a Thermo K-Alpha XPS equipped with 4 ACS Paragon Plus Environment

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an Al Kα radiation (1486.6 eV) X-ray source at a pressure lower than 10−7 Torr. The binding energies of Ce, Cu, O, and Fe were charge-corrected with respect to the adventitious carbon (C 1s) peak at 284.6 eV. 2.2.5. Hydrogen-Temperature Programmed Reduction (H2-TPR) Studies: The H2-TPR studies were conducted on a TPDRO (1100 series) 1100 instrument (Ser.Nr.20022896) equipped with a thermal conductivity detector (TCD). The sample (50 mg) was placed into a quartz tube reactor and heated up to 200 °C at a rate of 10 oC/min in N2 flow (20 mL/min) for 1 h to remove adsorbed hydroxyl species from the catalyst surface. After cooling to 40 oC, the gas flow was switched to 5% H2 in N2 (20 mL/min) and then the temperature was raised to 950 °C at a continuous heating ramp of 10 oC/min. The reactor effluent gas was passed through a molecular sieve trap to remove the produced water and then analysed by gas chromatography (GC) using a TCD detector. 2.2.6. FTIR Spectroscopy of Adsorbed Pyridine Studies: The FT-IR spectroscopy of adsorbed pyridine experiments were carried out on a Bruker Tensor 27 spectrometer equipped with a heatable and evacuable IR cell with CaF2 windows connected to a gas dosing and evacuation system. Approximately 50 mg of sample was pressed into self-supporting wafers with a diameter of 20 mm, which are then pre-treated at 400 °C for 4 h in synthetic air, and subsequent cooling to room temperature. Pyridine was adsorbed at room temperature until saturation of sample surface. The physisorbed pyridine was removed by evacuating the reaction cell. The desorption of pyridine chemisorbed on acid sites was carried out by heating the sample in vacuum to 300 °C and recording spectra every 50 °C. 2.2.7. Raman Studies: The Raman spectra were collected on a Renishaw inVia Raman Microscope using a 633 nm laser with a laser power of 1.6 mW. For each measurement, approximately 5 mg of the catalyst was placed on object slides and measured without any further treatment. To proof the homogeneity of the materials, spectra were acquired at different points of the sample.

2.3. Catalytic Diesel Soot Oxidation.

The catalytic efficiency of the prepared

materials was tested for diesel soot oxidation under both loose contact and tight contact conditions. Printex-U (Degussa) was used as the model soot in this study. Its particle size was 25 nm and the specific surface area was 100 m2/g. The catalyst and soot with a weight ratio of 4:1 were ground in a mortar for 10 min to obtain tight 5 ACS Paragon Plus Environment

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contact mixtures. The same weight ratio of catalyst and soot was mixed with a spatula for 5 min to obtain loose contact mixtures. The experiments were conducted in a thermogravimetric analyzer (Mettler Toledo, TGA/SDTA851e), which consisted of heating the catalyst−soot mixtures at a rate of 10

o

C/min from ambient

o

temperature to 950 C with 100 mL/min flow of air. Each test was repeated two times to ensure the reproducibility of the results and the values reported from the data were accurate to within ±3 oC. 3. RESULTS AND DISCUSSIONS 3.1 Morphology and Structural Properties Figure 1 shows HRTEM images of pure CeO2, Cu/CeO2 and Fe/CeO2 samples. Rod shaped CeO2 nanoparticles were formed as shown in Figure 1A-B and Figure S1. The average diameter and length of the CeO2 nanorods were found to be approximately 8.5 ± 2 nm and 16–89 nm, respectively. The lattice fringes can be clearly seen in HRTEM images of CeO2 nanorods (Figure 1A-B). The estimated dspacings are 0.27 and 0.19 nm, which correspond to (100) and (110) crystal facets of cubic structured fluorite CeO2, respectively.6,29 The arrangement of Ce and O in (100) and (110) crystal planes of cubic structured fluorite CeO2 is presented in Figure 1C. Although Cu/CeO2 and Fe/CeO2 samples maintain the shape of CeO2 rods (Figure 1D-E and Figure S2), considerable structural perturbations (stress, step defects, and face truncation) were observed along the edges and corners of the ceria rods. This could be due to strong interactions of Cu-oxide and Fe-oxide with the ceria rods. Cu-oxide and Fe-oxide phases with clearly visible lattice fringes can be seen in Cu/CeO2 (Figure 1D) and Fe/CeO2 (Figure 1E) samples, respectively. The estimated d-spacings of ∼0.252 and 0.289 nm (Figure 1D) are assigned to (111) and (110) crystal planes of CuO phase in Cu/CeO2 nanorods, respectively. The estimated ∼0.205 and 0.252 nm d-spacings are assigned to (101) and (110) crystal planes of Fe2O3 phase in Fe/CeO2 rods, respectively (Figure 1E). The STEM-EDS elemental mapping images clearly show the rod shaped CeO2 and the homogeneous distribution of Cu and Fe species in Cu/CeO2 rods and Fe/CeO2 rods, respectively (Figure 2). The average particle size of CuO and Fe2O3 was estimated to approximately 9.5 ± 0.5 and 7.3 ± 0.5 nm in Cu/CeO2 rods and Fe/CeO2 rods, respectively.


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(A)

(B)

CeO2 nanorod

0.27 nm (100)

CeO2 nanorod

0.19 nm (110)

0.27 nm (100)

0.19 nm (110)

5 nm

(C)

Ce O (110)

(100) (D)

CeO2 nanorod

CuO (111) 0.252 nm

CeO2 nanorod

(E)

CuO (110) 0.289 nm

Fe2O3 (101) 0.205 nm Fe2O3 (110) 0.252 nm Fe2O3 (110) 0.252 nm

Figure 1: (A-B) HRTEM images of CeO2 nanorods, (C) arrangements of Ce and O in (100) and (110) crystal planes of cubic structured fluorite CeO2, (D) HRTEM image of Cu/CeO2 nanorods, and (E) HRTEM image of Fe/CeO2 nanorods. 7 ACS Paragon Plus Environment

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Ce

Cu

Ce

Fe

50 nm

Fe/CeO2

50 nm

Figure 2. STEM-EDS elemental mapping images of Cu/CeO2 and Fe/CeO2 materials. The powder XRD patterns of CeO2 rods, Fe/CeO2 rods, and Cu/CeO2 rods are shown in Figure 3. Various XRD peaks at 2 theta values of ~28.46, 33.01, 47.39, 56.29, 59.05, 69.34, 76.69, 79.01, and 88.42o are noticed in all the materials (Figure 3A). These peaks correspond to (111), (200), (220), (311), (222), (400), (331), (420), and (422) crystal facets of cubic structured fluorite CeO2, respectively.30,31 In addition, few minor peaks are noticed at 2 theta = ~35.59 and 39.19o in Cu/CeO2 rods, which are attributed to a CuO phase.6 As well, Fe/CeO2 rods show various XRD peaks at 2 theta = ~35.81, 41.01 and 54.26o, which indicate the presence of a α-Fe2O3 phase.32,33 The XRD peaks of CeO2 nanorods shift to higher angles after the addition of Cu and Fe (Figure 3B). This indicates that the crystal structure of the cerium oxide is modified in Cu/CeO2 and Fe/CeO2 samples. According to Vegard’s law, if the Ce4+ ions (0.097 nm) are replaced by smaller sized Fe3+ (0.078 nm) and Cu2+(0.073 nm) ions in the cerium oxide lattice, a decrease in the lattice parameter of 8 ACS Paragon Plus Environment

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the resultant Ce−O−Fe and Ce-O-Cu solid solutions must be observed (i.e., lattice contraction) compared with that in pure CeO2.21,34,35 To understand this, the ceria lattice parameter of the samples was estimated with the help of most intense ceria (111) peak using the following equation: a = √(h2+k2+l2) (λ/2SinѲ)

(1)

Where ‘a’ is cerium oxide lattice parameter, ‘λ’ is the wavelength of the X-rays, ‘Ѳ’ is the Braggs angle (radians) and (hkl) are miller indices. Smaller lattice parameters were found for Cu/CeO2 nanorods (0.5338 nm) and Fe/CeO2 (0.5383 nm) nanorods compared with that of the bare CeO2 nanorods (0.5427 nm), confirming the incorporation of Cu2+ and Fe3+ ions into the cerium oxide lattice, respectively. As shown in Figure 3B, peak shifting was more pronounced in the Cu/CeO2 rods, indicating the more significant effect of the doped Cu ions in modifying the crystal structure of ceria compared with Fe in the Fe/CeO2 rods. This could be due to smaller ionic size of Cu2+ and/or doping of a larger amount of Cu2+ into the lattice of CeO2 rods. As evidenced from TEM (Figure 1) and XRD (Figure 3) studies, the Cu/CeO2 and Fe/CeO2 samples contain nanosized CuO and Fe2O3 particles on the surface of CeO2 rods as well as incorporated Cu2+ and Fe3+ ions into the CeO2 rods, revealing the formation of heterostructured copper-ceria and iron-ceria nanorods in this study. N2 adsorption-desorption isotherms of bare CeO2 rods, Cu/CeO2 rods and Fe/CeO2 rods are shown in Figure S3. All materials show type 4 isotherm with H3 hysteresis loop. This type of hysteresis loop is generally associated with aggregates of plate-like particles, giving to slit-shaped pores. The BET surface area of the bare CeO2 is found to be 89 m2/g, which is slightly decreased in the case of Cu/CeO2 rods (79 m2/g) and Fe/CeO2 rods (82 m2/g). This could be due to agglomeration of Cuand Fe-oxides on CeO2 rods and/or penetration of Cu- and Fe-oxide particles into the slit-shaped pores of the CeO2 rods. The actual loading of Cu and Fe was determined by ICP-MS analysis and a 9.6 and 9.8% loading of Cu and Fe with respect to Ce, respectively, was found for the Cu/CeO2 rods and Fe/CeO2 rods.


Langmuir

(A)

CeO2 nanorods Fe/CeO2 nanorods Cu/CeO2 nanorods

Intensity (a.u.)

CeO2 CuO Fe2O3

30

40

50

60

70

80

90

o

Two theta ( )

(B)

CeO2 nanorods Fe/CeO2 nanorods Cu/CeO2 nanorods

Intensity (a.u.)

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

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Peak shifting is 0.49

Peak shifting is 0.25

26

28

30

32

34

o

Two theta ( ) Figure 3. A) Powder XRD patterns of CeO2 rods, Fe/CeO2 rods and Cu/CeO2 rods and B) the enlarged view of the XRD spectra in the range of 26-35o. XPS analysis has been undertaken to estimate oxidation state of the elements present on the surface of the materials. The Cu 2p and Fe 2p XPS spectra of Cu/CeO2 rods and Fe/CeO2 rods, respectively, are shown in Figure S4. Two peaks are noticed at ca. 933.34 and 953.28 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. The estimated splitting between these two peaks is 19.94 eV, indicating the presence of Cu2+ species in Cu/CeO2 rods.36,37 In addition, two shake-up 10 ACS Paragon Plus Environment

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satellites are noticed at ∼941.84 eV and ∼962.13 eV that can occur only for Cu in +2 oxidation state. This confirms that copper nanoparticles are in +2 oxidized state in Cu/CeO2 nanorods, supporting the observations made from TEM (Figure 1) and XRD (Figure 3) studies. The Fe 2p XPS spectrum of Fe/CeO2 sample shows four peaks including Fe 2p3/2 at 710.94 eV, Fe 2p1/2 at 723.84 eV, and two satellite peaks at 717.43 and 732.65 eV (Figure S4).38 These peaks indicate the presence of Fe3+ in Fe/CeO2 rods, in line with the TEM (Figure 1) and XRD (Figure 3) studies. The complex Ce 3d spectra of the materials are shown in Figure 4A. All the samples show six bands at 881.9, 888.5, 897.7, 900.1, 906.6, and 916.0 eV, which corresponds to surface Ce4+ species with the 3d104f0 electronic state.39 Any XPS peaks between 880 and 881 eV characteristic for Ce3+ were not obtained.40 Hence, the prepared materials only contain surface Ce4+ species. A close observation of Figure 4A reveals that all the peaks of bare CeO2 rods are slightly shifted to higher binding energies after the addition of Cu to CeO2, which is not observed in the case of Fe/CeO2 rods. This indicates the presence of strong interactions in Cu/CeO2 sample through Cu-O-Ce linkage, which can influence the chemical environment of the cerium oxide in the respective samples. To determine the different types of oxygen species present in the synthesized catalysts, the O 1s XPS analysis was performed and the data is presented in Figure 4B. Two different O 1s peaks are clearly noticed in all samples. Generally, the lower binding energy band noticed at 528.8–529.1 eV corresponds to lattice oxygen species (such as O2− species) of the metal oxides.41-43 The shoulder peak observed at a higher binding energy (531.72– 532.31 eV) indicates the presence of chemisorbed oxygen species, such as OH−, CO32−, O22− and O− on the catalyst surface.44 As observed in Ce 3d XPS spectra (Figure 4A), here also a slight shifting of O 1s bands of bare CeO2 rods towards higher binding energies is observed after the addition of Cu to CeO2. Since Cu (1.90 eV) is high electronegative metal compared to Fe (1.83 eV), Cu can strongly affect the surrounding environment of Ce–O bond, resulting in the peak shifting of Ce 3d and O 1s bands in Cu/CeO2 material.


Langmuir

(A) Cu/CeO2

(B)

Cu/CeO2 Fe/CeO2 CeO2

Intensity (a.u.)

Fe/CeO2 CeO2

Intensity (a.u.)

880

890

900

910

525

920

Binding energy (eV)

530

535

540

Binding energy (eV)

Figure 4. (A) Ce 3d XPS spectra and (B) O 1s XPS spectra of pure CeO2 rods, Fe/CeO2 rods, and Cu/CeO2 rods.

(A) 100 T100 80

Soot oxidation (%)

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

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60

T50

40

Cu/CeO2 rods Fe/CeO2 rods CeO2 rods

20 0 280

320

360

400

440

480

520

560

600

o

Temperature ( C)

(B)

Figure 5. A) Soot conversion (%) versus temperature (oC) under tight contact conditions and (B) T50 and T100 values for CeO2, Cu/CeO2 rods, and Fe/CeO2 rods. 12 ACS Paragon Plus Environment

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3.2. Catalytic Soot Oxidation Results Figure 5A shows the oxidation of soot as a function of temperature over pure CeO2 rods, Fe/CeO2 rods and Cu/CeO2 rods under tight contact conditions. As shown in Figure 5A, soot oxidation starts in the range of 305-310 oC for all the catalysts and is finished at approximately ∼579 oC for pure CeO2 rods. Among the catalysts tested, the Cu/CeO2 rods catalyze the soot oxidation reaction at lowest temperatures. We estimated T50 and T100 values, temperatures at which 50 and 100% of soot are oxidized, respectively, to clearly understand the role of the catalyst in the soot oxidation and the results are given in Figure 5B. Pure CeO2 rods catalyze 50 and 100% soot oxidation at 433 oC (T50) and 579 oC (T100), respectively. The achieved T50 value of CeO2 nanorods for the oxidation of diesel soot is not only very low compared with conventional CeO2 (T50 = 601 oC), but also with well-established CeZr-O (T50 = 522 oC) and Ce-La-O (T50 = 467 oC) solid solutions.25 This indicates the significance of shape-controlled ceria nanorods in catalyzing soot oxidation at lower temperatures. It is interesting to highlight here that the observed T50 value of pure CeO2 rods is within the exhaust temperature range (200-500 oC) of diesel exhaust engines,20 indicating the practical application of ceria nanorod based catalysts in diesel soot oxidation. Very high temperatures (T50 = 607 oC) are needed to oxidize the soot under blank conditions (Figure S5). This indicates the necessity of the catalyst for the oxidation of soot at low temperatures. The addition of Cu- and Feoxide nanoparticles to CeO2 nanorods led to a huge shift in the T50 and T100 values towards lower temperatures (Figure 5B). Approximately 75 and 179 oC temperature differences were found in the T50 and T100 values of Cu/CeO2 rods and pure CeO2 rods, respectively. A small difference (10 oC) in T50 values of Cu/CeO2 (358 oC) and Fe/CeO2 (368 oC) samples is found. However, the Cu/CeO2 rods catalyze 100% soot oxidation at very low temperature (400 oC) compared with that of Fe/CeO2 rods which catalyze 100% soot oxidation at 440 oC. This clearly highlights the high efficiency of Cu/CeO2 rods in catalyzing soot oxidation reaction at lower temperatures. We have also carried out soot oxidation (tight contact mode) over Cu/CeO2 particles in which 10% of Cu is impregnated on CeO2 particles synthesized by a conventional precipitation method24 and the data is presented in Figure S6. It was found that CuO/CeO2 rods show a higher catalytic efficiency (T50 = 358 oC and T100 = 400 oC) compared with that of CuO/conventional CeO2 catalyst (T50 = 383


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o

C and T100 = 451

o

C). This observation reveals the morphology-dependent

performance of the Cu/CeO2 catalyst in the oxidation of diesel soot.

(A) 100

Soot oxidation (%)

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

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T100

80

Cu/CeO2 rods Fe/CeO2 rods

60

T50

40 20 0 300

350

400

450

500

550

600

o

Temperature ( C)

(B)

Figure 6. (A) Soot conversion (%) versus temperature (oC) under loose contact conditions and (B) T50 and T100 values for Cu/CeO2 rods and Fe/CeO2 rods. Investigating the soot oxidation reaction under loose contact mode provides a good understanding into the catalysts’ efficiency under practical conditions. Hence, we have studied the catalytic performance of Cu/CeO2 rods and Fe/CeO2 rods under loose contact mode and the results are shown in Figure 6. Both Cu/CeO2 and Fe/CeO2 catalysts show a positive soot conversion trend with increasing temperature: soot oxidation starts in the range of ∼310 oC for both catalysts and is completed at 620 oC for Fe/CeO2 rods (Figure 6A). Under loose contact also, a small 14 ACS Paragon Plus Environment

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difference in T50 values of Fe/CeO2 rods (435 oC) and Cu/CeO2 rods (419 oC) is found, while exhibiting a considerable difference in T100 values of Fe/CeO2 rods (620 oC) and Cu/CeO2 rods (540 oC) for the soot oxidation. This observation indicates the high catalytic performance of Cu/CeO2 rods in the oxidation of diesel soot under both loose and tight contact conditions. However, these values are a lot higher than those obtained under tight contact mode (Figure 5). The reason is that tight contact conditions enable greater contact between the soot particles and catalyst particles, resulting in higher soot removal efficiencies. The significance of the present work is highlighted by comparing the soot oxidation efficiencies of Cu/CeO2 rods with that of Cu/CeO2 cubes under loose contact mode. The soot oxidation profile of Cu/CeO2 cubes as a function of temperature is shown in Figure S7. The detailed structural, textural, and morphology properties of Cu/CeO2 cubes can be found in our recent article.4 It was found that the Cu/CeO2 rods exhibit 50 and 100% soot conversions at much lower temperatures (T50 = 419 oC and T100 = 540 oC) compared with Cu/CeO2 cubes (T50 = 465 oC and T100 = 680 oC) as can be seen in Figure 7. In particular, there is a huge 140 oC difference in the T100 values of Cu/CeO2 rods (T100 = 540 oC) and Cu/CeO2 cubes (T100 = 680 oC). The high catalytic performance of Cu/CeO2 rods can be explained by various factors, which include the presence of preferentially exposed crystal planes, increased BET surface area and altered redox properties. It is a welldocumented fact that CeO2 rods preferentially expose (110) and (100) crystal planes, while CeO2 cubes expose (100) crystal planes.9,10 As shown in Figure 1C, the (110) crystal plane contains a high concentration of surface exposed oxygen species, which can participate in the soot oxidation under high temperature reaction conditions. Another key factor is that the BET surface area of Cu/CeO2 rods (79 m2/g) is much higher than the corresponding cubic sample (36 m2/g)4, hence the high catalytic performance of Cu/CeO2 rods in soot oxidation. Hence, the shape of the CeO2 and the resulting properties play a crucial role in the efficiency of Cu/CeO2 catalysts for the oxidation of soot. These results reveal the novelty of the present work in the design of promising catalysts for the oxidation of soot at lower temperatures. To understand the catalyst stability, we have performed recycling experiments for the soot oxidation using high performing Cu/CeO2 rods under tight contact conditions and the results are shown in Figure 8. After each test, the soot 15 ACS Paragon Plus Environment

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was mixed with the collected catalyst with a weight ratio of 1:4 for further recyclability tests. As shown in Figure 8, the Cu/CeO2 rods exhibit an insignificant deactivation after two recycles with a small decrease in T50 (around 15 oC). This demonstrated that the CuO/CeO2 rods could maintain a stable active structure during the soot oxidation reaction.

Figure 7. Estimated T50 and T100 soot conversion values (loose contact mode) for Cu/CeO2 rods and Cu/CeO2 cubes.

100

Soot conversion (%)

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

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80 60

T50

40 o

20 0

Fresh (T50 = 358 C) o 1st cycle (T50 = 362 C) o 2nd cycle (T50 = 373 C)

280 320 360 400 440 480 520 560 o

Temperature ( C) Figure 8. Recyclability of Cu/CeO2 rods in the oxidation of soot under tight contact conditions.


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3.3. Redox and Acid Properties: Different types of mechanisms have been suggested for catalytic diesel soot oxidation. Liu et al. reported that the acid properties of the catalyst play a key role in the oxidation of soot.27,28 Acid sites can convert soot particles into active carbon cation radicals. These cation radical intermediates are highly active and can be oxidized by molecular oxygen at lower temperatures. In contrast, Soler et al. highlighted the importance of redox sites (oxygen vacancies) of ceria-based materials in soot oxidation.23 A two-way cooperative mechanism of soot oxidation is proposed: 1) oxygen vacancies are created at the ceria–soot interface due to the reduction of ceria by carbon soot and 2) gas phase O2 is activated by the oxygen vacancies present in cerium oxide, creating active oxygen species, which will oxidize the soot particles. Katta et al. also reported the importance of oxygen vacancies in the formation of a greater number of active oxygen species for soot oxidation.25 It seems that both redox and acid properties of the catalysts are important for efficient removal of soot at low temperatures. To understand this, we have undertaken Raman spectroscopy (to estimate oxygen vacancies), H2-TPR (to estimate redox properties), and FTIR spectra of adsorbed pyridine (to estimate acid sites) studies and the obtained results are thoroughly discussed in the following sections. Raman spectra of CeO2 rods, Fe/CeO2 rods, and Cu/CeO2 rods are shown in Figure 9. An intense band centered at ~462 cm-1 is noticed for all the samples (Figure 9A), which is attributed to F2g vibrational mode of fluorite CeO2.45 This band is due to the symmetrical stretching vibration of the O atoms around the Ce atoms in fluorite CeO2. In addition, Fe/CeO2 rods exhibit few minor bands centered at ~224, 240, 292, 407, 654 and 1309 cm−1, which can be assigned to hematite Fe2O3, in line with the XRD results (Figure 3).33,46 As well, Cu/CeO2 rods show two bands at ~292 and 341 cm−1, indicating the presence of CuO, supporting the observations made in the XRD studies (Figure 3).47 As shown in Figure 9B, the F2g band of Cu/CeO2 rods is quite different compared with that of Fe/CeO2 rods and bare CeO2 rods. It is broadened and shifted from 462 (bare CeO2 rods) to 458 cm−1 (Cu/CeO2 rods), indicating a modification of the cerium oxide lattice in Cu/CeO2 rods. The shift is attributed to a change of the Ce–O vibration frequency, due to the incorporation of Cu2+ ions into the lattice of the ceria rods as evidenced by XRD studies (Figure 3). Although some Fe3+ ions are doped, too, into the ceria lattice of Fe/CeO2 rods as observed by XRD studies (Figure 3), their influence appears to be less significant 17 ACS Paragon Plus Environment

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since no visible shift, but only a slight broadening of the F2g band of Fe/CeO2 rods was observed compared to bare CeO2 rods. Two broad bands are noticed at ~606 and 1154 cm-1 for Cu/CeO2 rods. These bands are related to the formation of oxygen vacancies in the CeO2 lattice.47,48 In contrast, Fe/CeO2 rods exhibit only one rather narrow Raman peak at about ~604 cm-1. No Raman bands of oxygen vacancies are seen in bare CeO2 rods. These observations could suggest that Cu/CeO2 rods exhibit a greater number of oxygen vacancies compared with Fe/CeO2 rods. This difference can be understood from the mechanism of substitution of Ce4+ by trivalent (Fe3+) and divalent (Cu2+) cations in the cerium oxide lattice. CeO2

CeO2

(B)

Fe/CeO2

Fe/CeO2

Cu/CeO2

Cu/CeO2

Fe2O3 CuO

Ov

200

400

600

CeO2

Ov

Intensity (a.u.)

(A)

Intensity (a.u.)

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

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800

1000 1200 1400 -1 Raman shift (cm )

420 440 460 480 500 -1 Raman shift (cm )

Figure 9. A) Raman spectra of CeO2 rods, Cu/CeO2 rods, and Fe/CeO2 rods and B) enlarged view of the Raman spectra (410-500 cm-1). As shown in Eq. 2, the substitution of Ce4+ by Fe3+ leads to the formation of a radical oxygen anion species (O•).19 This anion radical is highly active and eventually loses an electron, resulting in the formation of oxygen vacancies (Vo), along with the transformation of Ce4+ to Ce3+ to balance the charge in the cerium oxide lattice (Eq. 3). Therefore, an oxygen vacancy is formed in the cerium oxide lattice when doped with a Fe3+ ion. [2Ce4+, O2, O2]

[Ce4+, Fe3+, O•, O2]

(2)

[Ce4+, Fe3+, O•, O2]

[Ce3+, Fe3+, O2, Vo] + ½ O2

(3)


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By contrast, the substitution of Ce4+ with divalent Cu2+ leads to a spontaneous oxygen vacancy (Vo) formation as shown in Eq. 4.19 [Ce4+, O2]

[Cu2+, Vo] + ½ O2

(4)

The formed [Cu2+, Vo] site can induce the transformation of adjacent Ce4+ to Ce3+, resulting in the formation of another oxygen vacancy as shown in Eq. 5. Therefore, Cu2+-doped CeO2 can exhibit a higher number of oxygen vacancies compared with that of Fe3+-doped CeO2, which may be deduced from Raman studies (Figure 9) in the present work. [2Ce4+, O2, O2, Cu2+, Vo]

[2Ce3+, O2, Cu2+, 2Vo] + ½ O2

Intensity (a.u.)

3000 2500 2000 1500

(C)

1000

(5)

200 150 100 50 400

600

800

Temperature (oC)

500

Intensity (a.u.)

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

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0 800 600 400

(B)

200 0 500

Fe2O3

Fe3O4

Fe3O4

Fe

400 300 200

(A)

α

100

β

0

200

400

600

800 o

Temperature ( C)

Figure 10. H2-TPR studies of (A) CeO2 rods, (B) Fe/CeO2 rods, and (C) Cu/CeO2 rods (inset: enlarged view of H2-TPR graph of Cu/CeO2 rods in the range of 4001000 oC).


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The results of H2-TPR analysis, performed to better understand the reducible behavior of the materials, are shown in Figure 10. Pure CeO2 rods show two reduction peaks centered at ~522 and ~853 oC.49,50 The lower temperature peak (~522 oC) corresponds to the reduction of ceria surface oxygen (α), while the peak at a higher temperature (~853 oC) indicates the reduction of ceria bulk oxygen (β). By contrast, Fe/CeO2 rods show four reduction peaks centered at ∼375, 515, 664 and 852 oC. XRD (Figure 3) and XPS (Figure S4) studies revealed the presence of a Fe2O3 phase in the Fe/CeO2 rods. Pure Fe2O3 usually undergoes a two-step reduction, i.e., Fe2O3 → Fe3O4 and Fe3O4 → Fe(0) at ∼385 and 590

o

C,

respectively.49,51 Therefore, by comparing the reduction profiles of pure CeO2 rods and Fe/CeO2 rods, it was obvious that the peaks noticed at ∼515 and 852 oC correspond to the reduction of surface ceria (α) and bulk ceria (β), respectively. The remaining two peaks noticed at ∼375 and 664 oC correspond to Fe2O3 → Fe3O4 and Fe3O4 → Fe(0) transitions, respectively. The Cu/CeO2 rods show an unresolved peak in the temperature range of 110–280 °C as well as two minor peaks at approximately 535 and 852 oC. The unresolved peak is fitted with two peaks, centered at ∼153 and 192 oC: the lower temperature peak can be assigned to the reduction of well-dispersed CuO species that have strong interactions with CeO2 and the high temperature peak corresponds to the reduction of slightly larger CuO crystals having weak interactions with CeO2.51-53 It was clear that the reduction behavior of CuO dominates in Cu/CeO2 rods, hence only low intensity reduction peaks for the cerium oxide are noticed, in line with the literature reports.50 To understand this, the reduction profile of Cu/CeO2 rods is magnified in the range of 400-1000 oC as shown in Figure 10. It was found that the Cu/CeO2 rods show a relatively high intensity peak ratio of bulk ceria reduction to surface ceria reduction compared with that in pure CeO2 and Fe/CeO2 rods (inset, Figure 10). This indicates that Cu/CeO2 nanorods show an improved reduction of bulk oxygen of cerium oxide. This is due to the substantial perturbations occurred in the cerium oxide lattice (Figure 3 and 9) and the presence of abundant oxygen vacancies (Figure 9), which can enhance the mobility of bulk oxygen and hence, facile reduction of bulk oxygen of cerium oxide in Cu/CeO2 nanorods (Figure 10).54


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Figure 11. FTIR spectra of adsorbed pyridine on CeO2, Fe/CeO2, and Cu/CeO2 materials.

The acidic properties of bare CeO2 rods, Cu/CeO2 rods and Fe/CeO2 rods were investigated by recording the FTIR spectra of adsorbed pyridine, as shown in Figure 11. Generally, pyridine adsorbed on Brønsted acid sites (PyH+) shows characteristic bands at 1540–1548 cm−1 and 1635–1640 cm−1, while Lewis acid sites (L-Py) are characterized by bands at 1445–1460 cm−1 and 1610–1620 cm−1.40,55,56 It can be seen from the Figure 11 that all samples show two prominent peaks centered at 1440 and 1596 cm−1, which can be assigned to pyridine interacts with OH groups present on catalysts surface via hydrogen bonding (hb-Py).57 In addition to hb-Py peaks, Cu/CeO2 nanorods show a prominent band at 1608 cm−1 as well as a shoulder band at 1447 cm−1, while only a shoulder band is noticed at 1604 cm−1 for Fe/CeO2 nanorods. These peaks correspond to pyridine adsorbed on Lewis acid sites (L-Py). Since bare CeO2 does not show any bands of Lewis acid sites, the observed Lewis acid sites in Fe/CeO2 and Cu/CeO2 samples are created by Fe and Cu species, respectively. It was obvious from Figure 11 that Cu/CeO2 nanorods show high intensity Lewis acid sites bands. In contrast, Fe/CeO2 nanorods show high intensity hb-Py peaks. This indicates that pyridine interacts mainly via OH 21 ACS Paragon Plus Environment

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groups on the surface of Fe/CeO2 rods.57 In addition, the position of the Lewis acid sites band is found at higher wavenumbers (1608 cm-1) for Cu/CeO2 nanorods compared with that of Fe/CeO2 nanorods (1604 cm-1). This indicates that Cu/CeO2 nanorods contain stronger Lewis acid sites because the strength of Lewis sites depends on the position of the band in the range of 1590-1630 cm−1: the more Lewis acid site band is shifted to higher wavenumbers, the stronger is the corresponding Lewis sites.40 It was therefore clear from Raman (Figure 9), H2-TPR (Figure 10), and FTIR spectra of adsorbed pyridine (Figure 11) studies that Cu/CeO2 rods exhibit abundant oxygen vacancies, facile reduction of bulk ceria, and a greater number of stronger Lewis acid sites, respectively. Based on these results, a co-operative mechanism involving redox and acid sites of the Cu/CeO2 rods was proposed for the oxidation of soot. As shown in Figure 12, the gaseous phase oxygen can be adsorbed on the oxygen vacancies of the ceria rods (Figure 9), resulting in the formation of activated oxygen species. The activated oxygen species spill over to the soot adsorbed on the catalyst surface. The presence of strong acid sites in Cu/CeO2 rods can activate the soot particles into the reactive carbon radical cation intermediates (Figure 11). These radical cations are oxidized by activated oxygen species, resulting in the efficient removal of soot at lower temperatures. Since Cu/CeO2 rods exhibit facile reducibility of bulk ceria (Figure 10), the generated oxygen vacancy will be refilled by bulk oxygen of the cerium oxide that will eventually spill over to the soot, hence promoting the oxidation of soot. This work, therefore, highlights that in addition to the morphology of CeO2 as Cu/CeO2 rods exhibit higher activity in soot oxidation compared to Cu/CeO2 cubes (Figure 7), the redox and acid properties of the catalysts play a key role in the efficient removal of soot at lower temperatures.


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Figure 12: Probable mechanism for the soot oxidation over Cu/CeO2 nanorods.

4. CONCLUSIONS In summary, heterostructured copper-ceria and iron-ceria nanocatalysts were developed for the oxidation of diesel soot at lower temperatures. Activity studies revealed that the Cu/CeO2 rods show the best catalytic performance in soot oxidation followed by Fe/CeO2 rods and then bare CeO2 rods under both tight and loose contact conditions. A significant difference of 75 and 179 oC was found in the T50 and T100 values of Cu/CeO2 rods and bare CeO2 rods, respectively, in tight contact soot oxidation studies. As well, there is a huge 140 oC difference between the T100 value of Cu/CeO2 rods (540 oC) and Cu/CeO2 cubes (680 oC) in loose contact conditions. This observation indicates that the shape of CeO2 plays a significant role in tuning the efficiency of the Cu/CeO2 catalyst for the oxidation of soot. The Cu/CeO2 rods show a considerable recyclability in the oxidation of soot and a small decrease in T50 (around 15 oC) was found after two recycles. This indicates that the CuO/CeO2 rods could maintain a stable active structure during the soot oxidation reaction. Characterization studies revealed that Cu/CeO2 rods contain abundant oxygen vacancies, facile reduction of bulk ceria, and a larger number of 23 ACS Paragon Plus Environment

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acid sites. These improved properties are due to the substantial perturbations that resulted from the incorporation of Cu2+ ions into the lattice of the CeO2 rods. It was found that both redox (oxygen vacancies) and acid properties of the catalysts play a favorable role in soot oxidation: oxygen vacancies of the ceria rods assist to the formation of active oxygen species, while acid sites promote the conversion of soot particles into active cation radicals, which are efficiently oxidized by active oxygen species.

■ ASSOCIATED CONTENT Supporting Information TEM images, Cu 2p XPS, and Fe 2p XPS spectra of the catalysts as well as the soot oxidation results for blank, Cu/conventional CeO2, and Cu/CeO2 nanocubes. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (P.S.). Phone: +32 1632 6836 *E-mail: [email protected] (S.K.B.). Phone: +61 3 9925 2330

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The authors duly acknowledge the RMIT Microscopy and Microanalysis Facility (RMMF) for providing access to the instruments used in this study.

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Vanelderen,

P.;

Snyder,

B.

E. R.

Tsai,

M.-L.;

Hadt,

R.

G.;

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TOC Graphic

Heterostructured CuO/CeO2 Catalyst CeO2 nanorod

CuO

CuO

10 nm 100

Soot conversion (%)

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

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CuO o

T100

179 C

80 60

o

75 C

T50

40

Cu/CeO2 rods CeO2 rods

20 0

300 350 400 450 500 550 600

Temperature (oC)

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