Cu-Ce-O catalyst revisited for exceptional activity at low temperature CO oxidation reaction

Surface & Coatings Technology 354 (2018) 313–323

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Cu-Ce-O catalyst revisited for exceptional activity at low temperature CO oxidation reaction Abdallah F. Zedana,b, Amina S. AlJaberb

⁎,1

, Kyriaki Polychronopoulouc,d,

T

⁎⁎,1

, Ayesha Asifc, Siham Y. AlQaradawib,

a

National Institute of Laser Enhanced Sciences, Cairo University, Giza 12613, Egypt Department of Chemistry and Earth Sciences, Qatar University, Doha 2713, Qatar c Department of Mechanical Engineering, Khalifa University of Science, and Technology, Main Campus, Abu Dhabi 127788, United Arab Emirates d Center for Catalysis & Separation, Khalifa University of Science and Technology, Abu Dhabi 127788, United Arab Emirates b

A B S T R A C T

In the present study, CexCu1−xO2−δ (x = 0, 0.3, 0.5, 0.8, 0.9, and 1) catalysts were synthesized using a facile and reproducible combustion method and were evaluated towards the CO oxidation (CO-OX) and CO preferential oxidation (CO-PROX) reactions as inexpensive and active catalytic materials. Pure ceria and copper oxide were compared to CexCu1−xO2−δ mixed metal oxides for benchmarking purposes. The catalysts were prepared using citric acid (C6H8O7) as fuel and nitrate salts of cerium and copper as oxidizers under fuel rich conditions. A variety of techniques including XRD, EDX, BET, SEM, Raman, XPS, H2-TPR and CO2-TPD, were used to analyze the microstructural, thermal and redox properties that may influence CO oxidation performance. A profound effect of Cu content was revealed that not only impacts the structural and redox properties of the catalysts but also affects the catalytic activity. The Ce0.8Cu0.2O2−δ catalyst presented the most promising performance among many similar (Cu-Ce-based catalysts) as well as noble supported catalysts published in the literature with T50 = 62 °C and T100 = 78 °C, an activity that coincides with the availability of labile oxygen species at low temperature (T < 100 °C) and the enhanced CO2 desorption at low temperature (low CO2philicity) of this catalyst.

1. Introduction In the purification process of hydrogen, CO oxidation (COOX) and CO preferential oxidation (CO-PROX) are essential catalytic reactions for reduction of CO content under 10 ppm. The enhanced purity is integral in hydrogen utilization for applications in the automotive industry (three way catalytic converters) and PEM fuel cells [1,2]. Particularly important for fuel cells, where even small CO content can result in poisoning of the Pt catalyst used in fuel cells and such occurrences emphasize the importance of the COOX and CO-PROX processes [3]. The widespread approach of using noble metal catalyst (e.g. Pt, Rh, Pd, Au and Ru) has been proved to be expensive catalysts and so, affordable, alternative ones are in great demand. Cheaper alternative catalysts, include transition metals and lanthanides, such as ceria (CeO2) have been shown to possess good oxidation activity and even more promising activity with the addition of noble metals or base metals, such as Cu [4]. CeO2 has superior oxygen storage capacity (OSC) due to the feasible interchange of Ce4+/Ce3+ species [5]. Nano-sized CeO2 is expected to

have higher abundance of oxygen vacancies, therefore it is anticipated to improve the ultimate redox performance due to size reduction. On the other hand, CeO2 is prone to thermal deactivation leading to degradation of active sites and surface area deterioration [6,7]. In addition, it is well reported that the CuO based catalysts show notable performance in terms of activity and selectivity in the CO oxidation reaction [8], whereas it remains as a low-cost, alternative compared to noble metals. The superior oxidation activity of Cu-doped ceria is associated with synergistic interactions in the Ce-Cu-O system [9,10]. Studies have concluded that the active sites for the oxidation are located at the interface of the two metal oxides. Such active sites are formed by Cu+ species resulting from the reduction of CuO promoted by the adjacent CeO2 [11]. The relative activity towards the competing co-adsorption of CO and H2, determines the catalyst performance during the oxidation process in case of the COPROX reaction [9]. Also, studies in the presence of CO2 and H2O have been reported, where the co-presence of CO2 and H2O introduced a stronger inhibition in the COPROX rather than CO2 alone [12]. Although, literature validates the presence of different active sites for the two-competing species, CO and

⁎

Correspondence to: K. Polychronopoulou, National Institute of Laser Enhanced Sciences, Cairo University, Giza 12613, Egypt. Correspondence to: A. F. Zedan, Department of Mechanical Engineering, Khalifa University of Science, and Technology, Main Campus, Abu Dhabi 127788, United Arab Emirates. E-mail addresses: [email protected] (A.F. Zedan), [email protected] (K. Polychronopoulou). 1 Equal contribution. ⁎⁎

https://doi.org/10.1016/j.surfcoat.2018.09.035 Received 3 August 2018; Received in revised form 14 September 2018; Accepted 15 September 2018 Available online 17 September 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 354 (2018) 313–323

A.F. Zedan et al.

90 °C. The reaction mixture was left under heating and stirring till evaporation of excess water and formation of gel-like material. The gellike material formed was then transferred to a muffle furnace pre-heated to 380 °C and was kept at this temperature for 4 h leading to combustion and formation of spongy solid product. The spongy solid product was calcined in static air at 550 °C for 4 h with heating rate of 5 °C/min. Finally, the resulting solid was ground to fine powder and used as is for characterization and catalytic testing.

H2, the competition between the two species cannot be overlooked. As elegantly stated by Konsolakis in his recent review, there are many questions under study regarding this very popular catalyst, regarding the charge transfer, defects and chemical identity of the active sites and how all these factors impact the interfacial chemistry and activity in COOX, and CO-PROX [13]. Further improvement of the Cu-CeO2 system was reported by Yen et al. where the incorporation of Co and Fe (CuM/CeO2, M = Co, Fe) led to selective, low temperature (40 °C) active catalyst [14]. It has also been found that the preparation method significantly impacts the redox properties of e.g. the Cu-doped CeO2 as it regulates the dispersion of the one oxide in the other or the extent of doping which results in different catalytic performance [8,15,16]. The Ce-Cu-O systems can be synthesized using different preparation methods including, co-precipitation, deposition-precipitation, microwave, flame synthesis pyrolysis, sol-gel etc. [17–20]. In the present study, the citrate-nitrate combustion method was employed to prepare the different mixed oxide catalysts as it is considered a fairly simple and convenient method for preparing various porous materials, nanoparticles and advanced catalysts without the need for post-synthesis processing [21]. The technique makes use of the redox reaction between the fuel (citrate) and the oxidant (metal nitrate) [21]. Synthesis via combustion method produces homogenous morphology containing fewer impurities and larger surface area in comparison to the solid solutions produced using conventional solid-state methods [22]. The enhanced porosity and surface area can potentially lead to much more superior oxidation activity. The enhanced purity and ultrafine nature of the nanosized powders produced make this method highly desirable. In the present study, slightly modified combustion method was employed to synthesize pure ceria along with mixed metal oxides of the general formula CexCu1−xO2−δ, where x = 0, 0.3, 0.5, 0.7, 0.8, 0.9, and 1. The catalysts were prepared using citric acid (C6H8O7) as fuel and nitrates of cerium and copper as oxidizers. A variety of techniques including XRD, EDX, BET, SEM, Raman, XPS, H2-TPR and CO2-TPD, were used to analyze the microstructural, thermal and redox properties that may influence the CO oxidation performance under CO-OX and COPROX conditions.

2.2.2. Characterization Scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDX) measurements were carried out using NOVA NANOSEM 450 microscope (FEI, Brno, Czech Republic). The powder sample was sputter coated with gold prior to the SEM analysis, whenever needed. The powder X-ray diffraction (XRD) measurements were carried out at room temperature using a Rigaku MiniFlex II powder diffraction system (Rigaku, Tokyo, Japan) with Cu-Kα1 radiation at 30 kV and 20 mA between 2θ angles of 5° and 80° with a scanning rate of 0.025° per step per second. The identification of crystal structures of prepared materials was done using the database of the Joint Committee on Powder Diffraction Standards-International Center for Diffraction Data (JCPDS-ICCDD) system. The average crystallite size of prepared CuO and CexCu1−xO2-δ solid solution crystallites was estimated from the diffraction patterns using Scherrer formula, D = (k.λ) / (β.cosθ), where D is the mean crystallite size, k is the so-called shape factor (used as 0.9), λ is the wavelength of the X-ray used (1.54056 nm for Cu-Kα1), β is the line broadening and θ is the angle of the X-ray reflection [24]. The analysis was performed using angle and broadening information from the XRD reflection (111) (strongest) that is manifested at low angular value. Lattice strain was calculated by adopting the Williamson-Hall plots (βcosѳ/λ vs. sinѳ/λ) on the (111), (200), (220) and (311) diffraction peaks of the fluorite cubic cell [24]. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a KRATOS AXIS Ultra XPS with a monochromatic Al Kα radiation source (1486.6 eV) in a UHV environment (ca. 5 × 10−9 Torr) (KRATOS Analytical, Manchester, UK). The operating conditions were kept at constant high resolution pass energy of 20 eV, emission current of 10 mA, and anode HT of 15 kV. To subtract the surface charging effect, the C1s peak at 284.8 eV was used for calibration. Raman spectra were recorded using a DXR 2 Raman Microscope (Thermo Fisher Scientific, Madison, WI, USA) equipped with a 780 nm LASER source for excitation. The spectrum acquisition consisted of 20 accumulations with a total acquisition time of 5 min at a spectral resolution of 4 cm−1 and laser power of 5 mW. Porosimetry was carried out using a high-resolution 3Flex Micromeritics (Atlanta, USA) adsorption instrument equipped with high-vacuum system and three 0.1 Torr pressure transducers. Before performing the analysis, the materials were degassed at 150 °C for 4 h to remove any residual moisture. The surface area was calculated by adopting the BET (Brunauer-Emmett-Teller) theory in the relative pressure range of P/Po = 0.04–0.25. Pore size and pore volume were calculated by adopting the BJH method on the desorption segment of the isotherms. H2 temperature-programmed reduction (H2-TPR) experiments were performed using an Autochem 2920, (Micromeritics, Atlanta, USA). In the H2-TPR experiments, a 10 vol%/H2/Ar gas mixture (30 N mL/min) is passed over ~0.2 g of the pre-calcined (20 vol% O2/He, 500 °C, 2 h) material while a temperature ramp of 30 °C/min was applied. The signal was continuously recorded using a Thermal Conductivity Detector (TCD) signal. The material was mounted on a U-shaped quartz sample tube plugged with quartz wool and placed in a flow through reactor. CO2-TPD experiments were performed using Autochem 2920, (Micromeritics, Atlanta, USA). In CO2-TPD experiment, the material was mounted on a U-shaped quartz sample tube plugged with quartz

2. Materials and methods 2.1. Materials Chemicals were purchased from Taufkirchen (Germany) and used without further treatment including cerium (III) nitrate hexahydrate (99% trace metal basis, Sigma-Aldrich), copper (II) nitrate trihydrate (purum, 98%, Sigma-Aldrich) and citric acid (anhydrous, 99.5% GPR, BDH-England). The water used in the preparation of the solutions used was ultrapure (type 1) deionized water (Direct-Q 5UV, Millipore S.A.S., Molsheim, France). 2.2. Methods 2.2.1. Synthesis of CeO2 and CexCux−1O2−δ nanoparticles The different mixed oxide catalysts were prepared by slightly modified two-steps solution combustion synthesis using Ce(NO3)3.6H2O and Cu(NO3)2.3H2O as oxidizers and citric acid (C6H8O7) as a fuel [23]. For the synthesis of CeO2, CuO and CexCux−1O2 mixed oxides, appropriate volumes of 0.5 M stock solutions of cerium nitrate, copper nitrate and citric acid, all dissolved in ultrapure deionized water, were mixed together. For the synthesis of CexCu1−xO2−δ, a suitable amount of copper nitrate was used to obtain catalysts with the general formula CexCu1−xO2−δ, where x = 0, 0.3, 0.5, 0.7, 0.8, 0.9, 1. For all catalysts, the amount of citric acid was chosen so that the mole ratio of the citric acid to the total metal ions is 1.5:1 (slight excess of citric acid). The reaction mixture of the metal nitrate precursors and the fuel were heated under stirring in a beaker placed in a sand-bath over a hotplate at 314

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wool and placed in a flow through reactor. CO2-TPD experiments were carried out by passing a 5 vol%/CO2/Ar gas mixture for 30 min (30 N mL/min) over ∼0.2 g of the pre-calcined (20 vol% O2/He, 500 °C, 2 h) solid while a temperature ramp of 30 °C/min was applied. The TCD signal was recorded continuously.

Table 1 Textural and structural parameters of the pure CeO2 and CuO along with the CexCu1-xO2−δ catalysts prepared by combustion method.

2.2.3. Catalytic activity and stability studies Catalytic CO oxidation experiments were conducted to determine the activity of the different catalysts. Experiments were carried out using a custom-built continuous flow fixed-bed catalytic test reactor as described in our previous work [10,17,18]. The reactor was equipped with a quartz tube (10 mm ID) placed in the middle of a programmable split tube furnace (Lindberg/Blue M Mini-Mite Tube Furnace-Thermo). In all experiments, 50 mg of as-prepared catalyst powder was placed inside a bed of quartz wool that is in contact with a k-type thermocouple to measure the catalyst temperature. The feed gas mixture consisted of 4% CO and 20% O2 in a balance of He and was passed through the catalyst bed at a flow rate of 60 cm3/min (72,000 cm3/ g h−1 WHSV). The flow rate was controlled by a set of digital mass flow controllers (HI-TEC, Model-F-201CV-10K-AGD-22-V, Multi-Bus, DMFC; Bronkhorst). All experiments were carried out in the temperature range of 30 to 500 °C with a ramp rate of 5 °C/min and at ambient pressure. The effluent gas was analyzed using an online multichannel infrared gas analyzer (IR200, Yokogawa, Japan) to simultaneously monitor the CO conversion. The volume percentages of the CO and CO2 gases were determined simultaneously and logged, along with the catalyst temperature during the experiment. The catalytic activity was expressed by the conversion of CO in the effluent gas and indicated as CO fractional conversion. The long-term stability of the selected catalyst was studied at a temperature of 115 ± 5 °C for 72 h under a continuous stream of feed gas.

Catalyst

BET (m2/ g)

DXRD (nm)

Pore volume (cm3/g)/ Pore Size (nm)

Lattice parameter (Å)

Lattice volume (Å)3

Lattice strain (ԑ)

CeO2

16

35.38

0.0596/ 15

5.448

161.704

0.0174

Ce0.9Cu0.1O2 Ce0.8Cu0.2O2

32

18.33 12.83

5.429 5.417

160.039 163.733

0.0193 0.0208

5.426 5.429 5.403

159.709 160.039 157.747

0.0189 0.0221 0.0133

–

–

–

Ce0.7Cu0.3O2 Ce0.5Cu0.5O2 Ce0.3Cu0.7O2

15

CuO

–

17.97 16.67 15.74 (34.00)* 39.34

0.0875/ 11

0.03828/ 10

to 20 at.% Cu content. This is attributed to lattice distortion (contraction) caused by the incorporation of the smaller Cu dopant (Cu2+ radius of 0.73 Å) into the ceria (Ce4+ radius of 0.97 Å) lattice. This peak shift corroborates for the formation of CexCu1-xO2-δ solid solution. Further increase of copper beyond 20 at.%, caused the diffraction peaks to be shifted to 28.4°, 33°, 47.4° and 56.2° e.g. in the case of Ce0.3Cu0.7O2-δ. These higher values of diffraction angles are accompanied by additional peaks at 32.4o and 35.5o corresponding to CuO indicating formation of separate crystalline phase. The CuO crystalline phase peaks only appear at the significant high copper content (> 20 at.%) implying the formation of a two phases system at such high content of Cu. The peaks corresponding to the CexCu1−xO2−δ solid solution are becoming broader progressively as Cu content increases in the material. This broadening can be attributed to (a) small crystallite size of the CexCu1xO2-δ solid solution in the low Cu contents and (b) the competitive growth between the two phases that of CexCu1−xO2−δ solid solution and CuO, in the high Cu contents. The crystallite size was calculated based on Scherrer formula (see Table 1) and it was found that Cu incorporation at the level of 10 at.% causes a reduction in the crystallite size from 35.4 nm (pure CeO2) to 18.3 nm (Ce0.9Cu0.1O2−δ), while further increase of Cu content maintained the crystallite size reduction. This demonstrates the beneficial role of Cu towards the suppression of ceria/Cu-doped ceria crystal growth and enhancement of its thermal stability. As it is evident from the lattice parameter (Å)/lattice volume (Å)3 values, upon Cu introduction up to 20 at.% the lattice is being subjected into contraction, as expected, whereas for Cu contents higher than 20 at.% the lattice parameter is increased again since part of the additional Cu cannot be hosted into the ceria cubic lattice but it falls out towards formation of a separate CuO phase, a fact that is evident from the XRD patterns (Fig. 1). Raman spectroscopy studies were performed to probe the oxygen sublattice distortions that accompanied the Cu incorporation into ceria lattice. The Raman spectra for pure ceria and the CexCu1-xO2 (x = 0.7, 0.8, 0.9) mixed oxides are presented in Fig. 2. The F2g peak centered at 468.7 cm−1 is characteristic of fluorite-structured CeO2 corresponding to the vibration mode of oxygen around the Ce4+ ions in the fluorite structure (cubic lattice having ideal coordination) [26]. Incorporation of copper in ceria results in red shift of the F2g band to lower values (shown in the Fig. 2a insert). Among the catalysts investigated, the Ce0.8Cu0.2O2-δ one experienced the highest Raman shift to 466.8 cm−1. This shift can be explained by increased lattice strain and phonon confinement due to the addition of Cu2+ ions that results in defects, such as oxygen vacancies and lattice overall distortion/contraction. For every Cu2+ ion introduced into the lattice, one oxygen vacancy is anticipated to be formed. The latter is expected to lead to compressive strains resulting in shorter CeeO bond length and thus more mobile

3. Results and discussion 3.1. Structural studies (XRD, Raman) The XRD patterns of CexCu1−xO2−δ mixed oxides (x = 0, 0.3, 0.5, 0.7, 0.8, 0.9, and 1) along with the pattern of pure CeO2 and CuO, for comparison purposes, are presented in Fig. 1. For pure CeO2, four main peaks are observed at diffraction angles 28.3°, 32.9°, 47.3° and 56.2° which correspond to the characteristic planes (111), (200), (220) and (311) of the fluorite cubic lattice of ceria (JCPDS card: 34–0394) [25]. The diffraction angles values shift to smaller ones of 28.2°, 32.7°, 47.2° and 56.1° when Cu2+ dopant is introduced and this trend is observed up

Fig. 1. XRD profiles of the CexCu1-xO2-δ (x = 0.1, 0.2, 0.3, 0.5, 0.7, 1) materials synthesized using combustion method. XRD patterns for pure CeO2 and CuO are shown for comparison. 315

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phase were observed at higher Cu content. 3.2. Morphological and textural studies The SEM images shown in Fig. 3 present the morphology of pure CeO2 (Fig. 3a and b) and Ce0.8Cu0.2O2−δ (Fig. 3c & d) oxides. Without doping, the pure ceria exhibits a coral reef, spongy morphology with macro porous structure. The presence of large voids is due to the large volume of gases released during the combustion method. Doping with 20 at.% copper leads to agglomeration of nano-particles of CuO which are evident as spherical clusters visible at higher magnification (500 nm) in Fig. 3d, as spot EDX analysis revealed. These results are in agreement with the above presented XRD and Raman results regarding the CuO separate phase presence and the reduction of crystal size that results from the Cu incorporation into ceria lattice. This is in agreement with results of the BET area analysis for CeO2 and Ce0.8Cu0.2O2-δ catalysts, where the doped ceria was found to have double the BET area of pure ceria. It has been reported that in the case of praseodymium-doped ceria (PDC) nanoparticles, prepared by combustion method, spherical nanoparticles of PDC in the 10–25 nm size range, are obtained, which resembles to the CuO agglomerates observed in the SEM images Fig. 3d [15]. EDX results are presented in Fig. 4 and they confirm the consistency between the nominal and the actual values of Ce, and Cu atomic composition for pure CeO2 (Fig. 4a) and Ce0.8Cu0.2O2-δ (Fig. 4b), respectively. The N2 adsorption-desorption isotherms obtained at 77 K are shown in (Fig. 5a) for pure ceria catalyst as well as the two doped catalysts with two distinctly different copper content (Cu-poor and Cu-rich), namely Ce0.8Cu0.2O2−δ and Ce0.3Cu0.7O2−δ. The materials demonstrated characteristics IV isotherms, according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The physical adsorption is characterized by low N2 volumes (i.e. < 10 cm3/ g) for the lower applied P/P0 value (i.e. 6 × 10−3), as clearly observed in Fig. 5a. The small hysteresis loop formed between adsorption and desorption processes at the higher P/P0 range of ~0.5 and ~0.9 is most likely due to the capillary condensation of N2 gas inside the mesopores [28]. Finally, the adsorption isotherm did not show any point of saturation (plateau) at the maximum P/P0 point of ~0.99, but it extended without limit in an almost vertical direction, a trend that is usually linked with the presence of macroporosity (i.e. pore sizes above 50 nm) and/or external rough specific surface area (SSA). The P/Po range of 0.04–0.25 was used to calculate the BET surface area values (m2/g) of the catalysts. Pure ceria had a BET area of 16 m2/ g which improved with the addition of copper as the Ce0.8Cu0.2O2-δ catalyst showed a BET area of 32 m2/g. The 100% increase of specific surface area in the case of Ce0.8Cu0.2O2−δ can be linked with the nanocrystallization of the precursor compound, namely Ce-Cu-nitrate-citrate. It seems that the fuel in this particular mixture of cerium nitrate and copper nitrate was more efficient and rapid towards fine particles production compared to the case of only cerium nitrate present in the solution. It is suggested that the addition of 20 at.% copper results in agglomerated nanoparticles and the crystal sizes of CeO2 and CuO are smaller due to a competitive growth between the two crystalline phases. The smaller crystal size of Ce0.8Cu0.2O2−δ, as this discussed in the XRD section, is in agreement with the higher BET surface area of this material compared to pure CeO2. This result is also in agreement with that of Akkarat et al. [29]. Particularly at lower copper content (10–20 at.%), the uniform morphology and smaller particle sizes of CuO yields high surface area which is anticipated to give rise to good CO conversion performance. On the other hand, higher dopant content (> 20 at. %) led to decrease of BET surface area (15 m2/g) for the Ce0.7Cu0.3O2-δ catalyst. Thus, large CuO agglomerates are anticipated to block the active surface area and reducing the activity of the catalyst dramatically, as it will be discussed later. Also, it has been reported by Cao et al. that flower-like CeO2 synthesized via a surfactant-mediated method has a BET of 34 m2/g, whereas CeO2 shaped as hollow

Fig. 2. Typical Raman spectra in the spectral region of the F2g mode of CeO2 for (a) CexCu1−xO2−δ mixed oxides synthesized by combustion method. Insert is a magnification of the Raman spectrum in the 450–550 cm−1 area. (b) Oxygen vacancies region (500–650 cm−1).

oxygen vacancies [2,17]. Zhou et al. attributed the superior activity of Cu doped ceria, in comparison to doping with other transition metals, on the Raman results that indicated the highest shift of F2g peaks to lower wavenumbers [27]. This shift was attributed to formation of Ce3+ species, in the CeO2 lattice, which accompanies the oxygen vacancies formation [27]. The reduction of the crystal size (due to shorter CeeO bond) causes Raman peaks to red shift with the addition of Cu. These Raman results support the XRD results, previously presented, where the cell contraction was observed through the diffraction peaks shift at higher angles as the Cu content increased. The presence of a very weak signal in the 520–590 cm−1 (see Fig. 2b) range is the Raman range attributed to the oxygen vacancies that arise when cations of lower charge (e.g. herein Cu2+) are introduced in the ceria lattice, substituting Ce4+ ions. Though due to the very weak signal no much elaboration can be done for the data presented in Fig. 2b. In the case of CexCu1-xO2 (x = 0.8, 0.7) mixed oxides a small peak at 298 cm−1 is emerging, indicating the presence of the separate CuO phase (Fig. 2a). CuO crystallizes in the monoclinic lattice and as such has three Raman active modes (Ag + 2Bg) which correspond to the CueO stretching and bending vibration. The CuO presence confirms the formation of a two phases system (solid solution and CuO) as the Cu content becomes higher (Cu-rich catalysts). These results are in agreement with XRD results, presented above, where peaks corresponding to separate CuO 316

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Fig. 3. SEM microphotographs of (a, b) CeO2 and (c, d) Ce0.8Cu0.2O2-δ, synthesized by combustion method. The arrows indicate the CuO phase as identified using EDX spot analysis.

nanospheres exhibits a value BET of 72 m2/g [17]. In terms of the average pore diameter based on the BET theory (D = 4 V/A, where V: Volume and A: surface area measured) CeO2, Ce0.8Cu0.2O2-δ and Ce0.3Cu0.7O2-δ catalysts had pore diameters of ~15 nm, 11 nm and 10 nm, respectively, where with the Cu content increase, which is accompanied by a transition to a two phases system, the pore diameter was found to decrease. These values of pore diameter are in good agreement with the nanosized Cu-CeO2 system prepared using precipitation method reported by Su et al. [30] and they are much smaller compared to the 2D and 3D Cu-CeO2 catalysts prepared via mesoporous silica templates.

peak at 933.5 eV shifts at 933.7 eV, a result that supports the change in the electronic environment of Cu with the transition from the one phase to the two phases system. The O 1s core spectra have broad peaks, consisted of one at ~529 eV and a shoulder at ~530.6–531.6 eV as shown in Fig. 6c and Table 3. The one at lower binding energy corresponds to lattice oxygen [32,33], whereas the peak at higher binding energy (530.6–531.6 eV), as shown in Table 3, is associated with the presence of carbonate, hydroxyl species or polarised O2– ions adjacent to the O vacancies [32,33]. It is important to notice here that the O1s peak that corresponds to the lattice oxygen is appeared shifted in higher binding energy in the doped ceria catalysts compared to pure ceria and this can be linked with the difference in the electronegativity of Cu and Ce ions that affects the chemical environment of lattice oxygen. In the case of Ce0.8Cu0.2O2-δ catalysts the peak shift was −0.3 eV, whereas no additional peak shift was noticed in the case of the Ce0.7Cu0.3O2 catalyst. This can be due to the segregation of the additional Cu into a CuO separate phase (in agreement with XRD and Raman studies). The H2-TPR profiles of pure ceria and of the doped catalysts Ce0.8Cu0.2O2−δ and Ce0.3Cu0.7O2-δ are presented in Fig. 7. H2-TPR experiment provides insight information on the reducibility of oxygen species on the surface/subsurface (low/medium temperature regime), and the bulk of the catalyst. The mobility of oxygen species is key step towards the formation of oxygen vacancies and fundamental parameter for the CO oxidation reaction as it will be discussed later (section 3.5). The two doped catalysts were selected based on their apparent difference in the copper content. Ce0.8Cu0.2O2−δ has the most intense peak at around 175 °C and a shoulder peak around 75 °C. The Ce0.8Cu0.2O2−δ catalyst exhibits the best reducibility compared to the rest catalysts in the temperature regime below 100 °C. In the case of Cu-rich catalyst, Ce0.3Cu0.7O2−δ, the higher copper content increases the reducibility of the catalyst even more as it has the highest intensity signal at around 200 °C, whereas in the temperature range of 150–250 °C it has the highest reducibility among the studied catalysts. In general, pure ceria exhibits less reducibility when compared to that of the doped catalysts,

3.3. Surface chemistry of the catalysts The XPS core level spectra were obtained over pure CeO2 and CexCu1−xO2−δ (x = 0.7, 0.8 and 1) catalysts and are presented in Fig. 6. Core level spectra for Ce 3d shows seven noticeable components labelled in terms of u and v representing the transitions associated with Ce 3d3/2 and Ce 3d5/2, respectively. The strong peaks u‴ (916.5 eV), u′ (900.6 eV), v‴ (898.2 eV), v″ (888.9 eV), and v′ (882.4 eV) are associated with the Ce4+ oxidation state, whereas the remaining less intense peaks indicate the presence of the Ce3+ oxidation state. The predominance of Ce4+ species agrees with the XRD and Raman analysis. The binding energy of the u‴ (peak at 916.5 eV) is shifted to lower binding energies in the case of Ce0.8Cu0.2O2−δ and Ce0.7Cu0.3O2−δ catalysts compared to pure ceria, demonstrating the impact of the Cu dopant into the ceria chemical environment. The Cu 2p core level spectra in copper-ceria mixed oxide feature peaks at 933.7 eV, and 953.6 eV (Fig. 6b) which correspond to Cu 2p3/2 and Cu 2p1/2, respectively, whereas the shake-up peak around 940–945 eV (max at 942 eV) (Table 3) indicates the existence of Cu2+ species [31], which is in agreement with a similar study where the shake-up peak at 943.8 eV was obtained for doped ceria [27]. As we move from the Ce0.8Cu0.2O2-δ catalyst to the Ce0.7Cu0.3O2-δ one, the 317

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Fig. 4. Energy Dispersive X-ray analysis performed over (a) pure CeO2, and (b) Ce0.8Cu0.2O2–δ.

3.4. CO oxidation catalytic studies

particularly in the lower temperature regime (< 300 °C). According to literature, H2-TPR profile of pure ceria presents peaks at around 247 °C corresponding to sub surface reduction [32,33] and two additional peaks at higher temperatures of 467 °C and 1000 °C are attributed to formation of non-stoichiometric oxides of Ce [34,35]. Incorporating copper results in a decrease of reduction temperature because of a spillover process that promotes reduction of metal and support simultaneously [8]. This is in harmony with the literature where it has been reported that dopants incorporation into ceria lattice greatly facilitates the reducibility of the latter. Peaks at lower temperatures e.g. around 80–160 °C may be due to well-dispersed, easy to reduce copper species whereas those at higher temperatures, around 200–400 °C correspond to reduction of bulk copper oxide which cannot be involved in PROX reaction [8]. For pure bulk CuO it has been mentioned in the literature that the reduction happened at 334 °C [36]. In the insert (Fig. 6), a magnification of the reduction profiles in the 200–400 °C region is presented, where the much more intense reduction peak for the Ce0.3Cu0.7O2-δ can be seen. This peak corresponds to the CuO phase reduction of this catalyst. Based on the results presented in Fig. 7, it can be seen that only Ce0.3Cu0.7O2−δ catalyst showed a reduction peak at T ≥ 200 °C confirming XRD results for the presence of bulk CuO, whereas the Ce0.8Cu0.2O2-δ catalyst is the only one which possess mobile oxygen species at T < 100 °C. The easiness of reduction for the latter catalyst is also boosted due to the higher surface area (32 m2/g) compared to the rest which facilitates the hydrogen/oxygen diffusion.

CO oxidation activity, in terms of CO conversion, was studied and light-off curves were obtained over all the catalysts in the temperature range of 25–500 °C (shown in Fig.8a). Comparing the values of T50 (T50 is the temperature at which 50% CO conversion is achieved) the order of activity is as follows: Ce0.5Cu0.5O2-δ > Ce0.8Cu0.2O2-δ > Ce0.7Cu0.3O2-δ > Ce0.9Cu0.1O2-δ > Ce0.3Cu0.7O2-δ > CuO > CeO2-δ. However, examining the T80, T90 and T100 (TX is the temperature at which X% CO conversion is achieved) performance indicators (see Table 2), the superior performance of Ce0.8Cu0.2O2-δ which exhibited full fractional CO conversion at the lowest temperature (T100 = 78 °C) can be seen. This was significantly lower than the T100 values for the other catalysts which were all ≥133 °C. In general, doped catalysts exhibited significantly better activity than the pure ceria with all the catalysts exhibiting CO full conversion under 200 °C (Table 2). The results for the Ce0.5Cu0.5O2 catalyst indicate suppression of activity once 80% CO conversion was reached, similar behavior as the one observed in the CuO. The lower activity after T80 of ceria doped with higher Cu content, namely 50 and 70 at.%, could be attributed to the presence of CuO as separate phase, as it was confirmed by the XRD results, which antagonize the beneficial role of the solid solution during the CO oxidation reaction. The lower activity after T80 can be due to the formed carbonates for the following reason: The CO2-TPD results (Fig.10) showed that most of the adsorbed CO2 is being desorbed in the temperatures lower than 200 °C, whereas the maximum of the peak is at 318

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Fig. 5. (a) N2 adsorption-desorption isotherms at 77 K and (b) BJH pore size distribution from the desorption segment of the isotherm, obtained over CeO2 and CexCu1−xO2 (x = 0.8, 0.2).

around 100 °C, close to the T50, this means that carbonates are formed even at lower conversions than 80%. It can be assumed that at XCO = 80% the carbonates accumulated on the surface are more in population and maybe more resistant to thermal decomposition. The latter is due to the anticipated bigger size of carbonates formed at a higher temperature (e.g. T80 vs. T50). The poisonous role of carbonates in the CO-OX reaction has been reported in many publications [37–39]. The most promising catalyst of this study presents the highest red shift of the F2g Raman peak (see Table 2) compared to pure CeO2 and this supports the contraction of the M-O bond and the expansion of the M-[Vo]. The latter implies a higher mobility for the oxygen vacancies. This catalyst also outperforms similar CexCu1−xO2−δ compositions reported in the literature (see Fig. 9). Some representative, literature reported, catalysts with similar (CexCu1−x-based) or different (CexM1−x-based, M: La, Hf, Zr, Sm) compositions are presented in Fig. 8. It can be seen that for example, in the case of CuCeO2−δ microspheres prepared by sol-gel method using a polymer as soft template reported by Zhou et al. [27] the T100 reported to be 300 °C, whereas CeCu-O reported by Gurbani et al. [8] had their T50 at 137 °C. In another study, where CuO was supported on CeO2 using impregnation and different Cu precursors were studied, the optimum conditions were achieved at T50 = 115 °C [40], whereas Ce0.8Cu0.2O2 catalyst in the present study had a T50 and T100 of 62 °C and 78 °C, respectively. It is also very interesting to notice that this catalyst overperforms even noble metal catalysts, such as Pd supported on ceria-zirconia

Fig. 6. XPS high-resolution spectra of (a) Ce 3d, (b) Cu 2p and (c) O 1s of the different CexCu1-xO2-δ (x = 0.2, 0.5, 0.8, 1) catalysts prepared using the combustion methods.

(T50 = 120 °C) [41], or Pt supported on TiO2 [42] as well as other compositions, such as Ce-La-O (T50 = 362 °C), Ce-Hf-O (T50 = 300 °C), and Ce-Zr-O (T50 = 285 °C) [43–45]. Though it is noteworthy that there are reports in the literature, where Cu-CeO2 nanosized catalyst presented T50 of 35 and 45 °C [30] for the CO oxidation reaction. Though templates were used for their synthesis, such as mesoporous silicas three dimensional KIT-6 and two dimensional SBA-15 towards formation of an ordered pore structure. The advantage of the present study catalyst is not only in the low temperature of CO oxidation obtained but 319

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Table 2 Catalytic performance of the CexCu1−xO2 catalysts in the CO oxidation reaction (4%CO/20%O2/He). In parenthesis the activity under PROX conditions (4%CO/20%O2/40%H2/He) is presented.

CeO2 Ce0.9Cu0.1O2 Ce0.8Cu0.2O2 Ce0.7Cu0.3O2 Ce0.5Cu0.5O2 Ce0.3Cu0.7O2 CuO

T50 (°C)

T80 (°C)

T90 (°C)

T100 (°C)

F2g (cm−1)

373 75 (111) 62 (100) 66 (97) 60 (95) 78 (105) 148

407 76 (129) 63 (112) 69 (109) 69 (106) 81 (121) 189

423 85 (136) 64 (118) 82 (113) 97 (111) 102 (128) 268

450 133 (148) 78 (125) 143 (122) 143 (122) 182 (140) 424

~468.7 ~466.8 ~466.6 ~467.2

should be interpreted with cautiousness, as comparison in terms of conversion performance and intrinsic reactivity should be carried out to demonstrate the superiority or not of the present catalysts taking into account literature studies. Though in most of the literature reports T50 values are only provided and thus is was used as a performance indicator. PROX results are presented in Fig. 8b for the best performing catalysts (for CO oxidation in absence of hydrogen), namely the CexCu1−xO2 mixed oxides (x = 0.3, 0.5, 0.7, 0.8 and 0.9) in the presence of 40% hydrogen. Hydrogen can influence the reaction in two ways. The inhibiting effect of H2 is due to competitive adsorption of CO2 and H2 which results in lower activity [46]. Additionally, H2 can be oxidized producing H2O, which promotes the WGS reaction, contributing to an additional route during CO2 production, particularly for T > 200 °C. Another possible effect of hydrogen presence is the formation of oxo-hydroxy species which promote and facilitate the CO oxidation [47]. From the results obtained, it is evident that the values of T50, T80 and T90 have increased under PROX reaction conditions and this results in less abrupt increase of activity with respect to temperature. However, for higher copper content (> 20 at.%) the final temperature at full conversion T100 for PROX is much lower value than the COOX case, indicating the positive impact of hydrogen addition. The T100 for PROX over Ce0.9Cu0.1O2-δ and Ce0.8Cu0.2O2-δ was increased from 133 to 148 °C and from 78 to 125 °C, respectively. The general order of activity with respect to T50 changed only slightly Ce0.5Cu0.5O2δ > Ce0.7Cu0.3O2−δ > Ce0.8Cu0.2O2-δ > Ce0.3Cu0.7O2δ > Ce0.9Cu0.1O2−δ but the differences in their T50 are very small (2–6 °C). The Ce0.5Cu0.5O2-δ and Ce0.7Cu0.3O2−δ catalysts showed the best performance with T100 of 122 °C. In a study comparing the performance of transition metal doped ceria as composite and microsphere structures, CuxCeO2−x (10 at.%) microspheres with H2 oxidation indicated the best performance with T50 and T100 equal to 150 and 300 °C, respectively [27] which is significantly higher than the results of the poorest performing Ce0.9Cu0.1O2−δ (T50 and T100 equal to 111 and 148 °C). The best T100 value reported by Zhou et al. [27] are still much higher than all the catalysts in this report, which supports the superiority of the present catalysts. In another study by Zhu et al. [48] the best performing catalyst was the one with 20 mol% copper contents (CuCeO2-20) which had T50 of 77 °C with a relatively high space velocity of 260,000 mL h−1 gcat−1, compared to the one used in the present study. The surface of the most promising performing Ce0.8Cu0.2O2−δ catalyst (based on the T100) was titrated with CO2 and temperature programmed desorption study was performed (CO2-TPD) towards exploring the CO2-philicity of the surfaces (see Fig. 10). The latter property is of great importance for the CO-oxidation catalysts as CO2 is a reaction product. The tendency of the surface to easily release the CO2 could imply a highly active surface, whereas the opposite scenario would corroborate for a surface having its active sites rather blocked from the adsorbed CO2. The CO2-TPD profile for the Ce0.8Cu0.2O2−δ catalyst has a peak around 100 °C and a shoulder at 200 °C. Peaks at temperatures under 200 °C are associated with the presence of weak

Fig. 7. H2-TPR profiles obtained over the pure CeO2 and CexCu1−xO2 catalysts synthesized using combustion method.

Fig. 8. Light-off curves of CO oxidation measured for different CexCu1−xO2 catalysts in (a) absence of H2 and (b) presence of hydrogen in the feed gas under WHSV of 60,000 cm3 g−1 h−1.

also in the simplicity of the synthesis method, where no templates/ polymeric matrices used, and thus no complication in the purity of the final catalyst was introduced. Needless to mention the low cost and stability of the catalyst in stream, as will be discussed later. It has to be mentioned that the comparison with the literature as presented in Fig. 8 320

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Table 3 Binding energies (eV) of the constituent elements based on the XPS studies. Catalysts

CeO2 Ce0.8Cu0.2O2 Ce0.7Cu0.3O2

Binding energy (eV) Ce3d5/2

Cu2p3/2

O1s

882.5, 889.2 882.4, 889 882.6, 889.4

931.4 (940.1)* 933.5 (942.2) 933.7 (942.9)

528.9, 530.6 529.2, 531.3 529.2, 531.6

*Values in parenthesis are the shake-up satellite peak.

Fig. 11. Stability test performed over the Ce0.8Cu0.2O2−δ catalysts for 72 h on stream at 115 °C.

Ce0.8Cu0.2O2-δ catalyst) and this suggests the increased likelihood of CO2 residency on the surface since it takes higher temperatures to be removed. For the most promising catalyst (based on the T100), Ce0.8Cu0.2O2−δ, Fig.11 shows the long term catalytic stability at 115 °C for 72 h. The catalyst exhibited 100% conversion for 72 h continuously within the prescribed temperature window. This could be attributed to the fact that Ce0.8Cu0.2O2-δ catalyst does not suffer from extensive accumulation of nano agglomerates of CuO as evident from XRD results and SEM studies. This particular catalyst also presented superior reducibility (H2TPR studies) and CO2-TPD results and unblocked active sites in this temperature range.

Fig. 9. Comparison of CO oxidation performance (expressed as T50, °C) of the best catalytic system of this work with catalysts reported in the literature. (1): CuCeO2 microsphere [27], (2) Cu-Ce-O [8], (3) CuO [18], (4) CeO2 [41], (5) CeSm-O [41], (6) Ce-Zr-O [44], (7) Pd/Ce-Zr [42], (8) CeeFe [43], (9) CeeHf [45], (10) Ce-Sm-O [17], (11) Ce-Zr-O [45], (12) Cu-Ce-O.

3.5. Mechanistic aspects It is well known that the CO oxidation reaction over ceria-based oxides it follows the Mars–van Krevelen mechanism. The sequence of steps involved are the following: (1) CO from the gas phase is being adsorbed onto the surface towards formation of monodentate/bidentate species, (2) oxygen from the gas phase is adsorbed onto the vacancies (□) towards formation of superoxide anions (3) oxygen vacancies are formed as a result of the reaction of the lattice oxygen (Olattice) with the carbonate species. Following the formation of an oxygen vacancy site, the coordination of metal sites (e.g. Ce, Cu) is decreased thus making them more active. (4) The vacant sites are filled by the gaseous oxygen in the feed gas [50]. The desorption of oxygen follows the sequence: O2 > O2– > O– > Olattice2− [51,52]. In the CO oxidation process, the carbonates decomposition is a key step which requires activation and dissociation of the oxygen from the gas phase, Along the same lines, hydroxyl groups have been reported as detrimental contributors to the formation of bicarbonates during the reaction [9], the formation of the latter being accompanied by faster CO oxidation rather than the formation of the carbonates. Oxygen vacancies, on the other hand, they offer an energy favorable pathway for diffusion of atomic oxygen. It can be stated that the coral reef morphology of the Ce0.8Cu0.2O2 catalyst facilitates the lattice oxygen diffusion and hence the carbonates decomposition. Based on the H2-TPR pattern of the Ce0.8Cu0.2O2−δ catalyst, a low temperature peak at < 100 °C (70 °C) can be seen as well as the peaks at 150 °C and 220 °C, respectively. The peak below 100 °C, demonstrates the presence of mobile oxygen species in the surface of the particular catalyst. The mobile oxygen species can be partially due to the increased lattice strain of this catalyst (Table 1) which shows the presence of elongated (weaker) M-O bonds in the case of this catalyst. When the Cu content increases, the reduction profile is being shifted to higher temperatures (case of Ce0.3Cu0.7O2−δ catalyst) which reflects the increased temperature that is needed to create mobile oxygen species. More labile oxygen species at lower temperatures dictates the formation

Fig. 10. CO2-TPD profiles obtained over the Ce0.8Cu0.2O2−δ and Ce0.3Cu0.7O2 catalysts.

basic sites, peaks in the range of 200–450 °C correspond to moderate basic sites and peaks at 450 °C and higher are linked to strong basic sites [49]. The results suggest that the Ce0.8Cu0.2O2−δ catalyst has an abundance of weak basic sites and so the release of CO2 at lower temperatures indicates that active sites are unblocked from the CO2 product relatively easily and thus they become free for CO adsorption in the desired temperature range. At the same time, the Ce0.3Cu0.7O2-δ catalyst presents a rather broader CO2-TPD profile (compared to the 321

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of more oxygen vacancies at these temperatures which in turn increases the possibility of having O2 (from the gas phase) adsorption, dissociation and activation on the existent vacant sites. The latter activated oxygen species can react with carbonates towards O2(g) and CO2(g) formation (decomposition of carbonates).

[15] K. Polychronopoulou, M. Abi Jaoudé, Nano-architectural advancement of CeO2driven catalysis via electrospinning, Surf. Coat. Technol. 350 (2018) 245–280. [16] K. Polychronopoulou, C. M. Kalamaras, A. M Efstathiou, Ceria-based materials for hydrogen production via hydrocarbon steam reforming and water-gas shift reactions, Recent Patents Mater. Sci. 4 (2) (2011) 122–145. [17] K. Polychronopoulou, et al., Tailoring the efficiency of an active catalyst for CO abatement through oxidation reaction: the case study of samarium-doped ceria, J. Environ. Chem. Eng. 6 (1) (2018) 266–280. [18] A. Zedan, N. Allam, S. Alqaradawi, A study of low-temperature CO oxidation over mesoporous CuO-TiO2 nanotube catalysts, Catalysts 7 (5) (2017) 129. [19] Reto Strobel, Alfons Baiker, S.E. Pratsinis, Aerosol flame synthesis of catalysts, Adv. Powder Technol. 17 (5) (2006) 457–480. [20] Christel Laberty-Robert, Jeffrey W. Long, Erik M. Lucas, Katherine A. Pettigrew, Rhonda M. Stroud, Michael S. Doescher, Debra R. Rolison, Chem. Mater. 18 (1) (2006) 50–58. [21] C. Esther Jeyanthi, R. Siddheswaran, Pushpendra Kumar, M. Karl Chinnu, K. Rajarajan, R. Jayavel, Investigation on synthesis, structure, morphology, spectroscopic and electrochemical studies of praseodymium-doped ceria nanoparticles by combustion method, Mater. Chem. Phys. 151 (2015) 22–28. [22] C. Peng, Y. Whang, Z.W. Cheng, X. Cheng, J. Meng, Nitrate-citrate combustion synthesis and properties of Ce1-xCaxO2-x solid solutions, J. Mater. Sci: Mater. Electron. 13 (2002) 757–762. [23] A. Varma, et al., Solution combustion synthesis of nanoscale materials, Chem. Rev. 116 (23) (2016) 14493–14586. [24] Yoshio Waseda, Eiichiro Matsubara, Kozo Shinoda, X-ray Diffraction Crystallography, (2011) (ISBN 978-3-642-16635-8). [25] Thadathil S. Sreeremya, Asha Krishnan, Kottayilpadi C. Remani, Kashinath R. Patil, Dermot F. Brougham, and Swapankumar Ghosh, ACS Appl. Mater. Interfaces 7, 16, 8545–8555. [26] V.G. Keramidas, W.B. White, Raman spectra of oxides with the fluorite structure, J. Chem. Phys. 59 (1973) 1561. [27] L. Zhou, X. Li, Z. Yao, Z. Chen, M. Hong, R. Zhu, Y. Liang, J. Zhao, Transition-metal doped ceria microspheres with nanoporous structures for CO oxidation, Sci. Rep. 6 (1) (2016). [28] J. Rouquerol, F. Rouquerol, P. Llewellyn, G. Maurin, K.S. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, 2014. [29] W. Kongsi Wongkaew, P. Limsuwan, Physical properties and selective CO oxidation of Coprecipitated CuO/CeO2 catalysts depending on the CuO in the samples, Adv. Mater. Sci. Eng. 2013 (2013). [30] Yunfei Su, Lingfeng Dai, Qingwen Zhang, Yunzhen Li, Jiaxi Peng, Ren'an Wu, Weiliang Han, Zhicheng Tang, Yi Wang, Fabrication of Cu-doped CeO2 catalysts with different dimension pore structures for CO catalytic oxidation, Catal. Surv. Jpn. 20 (2016) 231–240. [31] A. Osherov Zhu, M.J. Panzer, Surface chemistry of electrodeposited Cu2O films studied by XPS, Electrochim. Acta 111 (2013) 771–778. [32] A. Pfau, K.D. Schierbaum, The electronic structure of stoichiometric and reduced CeO2 surfaces: an XPS UPS and HREELS study, Surf. Sci. 321 (1994) 71–80. [33] M.A. Henderson, C.L. Perkins, M.H. Engelhard, S. Thevuthasan, C.H.F. Peden, Redox properties of water on the oxidized and reduced surfaces of CeO2 (111), Surf. Sci. 526 (2003) 1–18. [34] B.M. Reddy, L. Katta, G. Thrimurthulu, Novel nanocrystalline Ce1−xLaxO2 (x = 0.2) solid solutions: structural characteristics and catalytic performance, Chem. Mater. 22 (2010) 467–475. [35] M.A. Ebiad, D.R. Abdel-Hafiz, R.A. Elsalamony, L.S. Mohamed, Ni supported high surface area CeO2–ZrO2 catalysts for hydrogen production from ethanol steam reforming, RSC Adv. 2 (2012) 8145–8156. [36] G. Avgouropoulos, T. Ioannides, Selective CO oxidation over CuO-CeO2 catalysts prepared via the urea–nitrate combustion method, Appl. Catal. 244 (2003) 155–167. [37] https://doi.org/10.1021/cm502106v, Chem. Mater. [38] E.C. Njagi, C.-H. Chen, H. Genuino, H. Galindo, H. Huang, S.L. Suib, Appl. Catal. B Environ. 99 (2010) 103–110. [39] G. Xia, Y. Yin, W. Willis, J. Wang, S. Suib, J. Catal, Appl. Catal. B Environ. 188 (2016) (1999) 292–304 185, 91–105., 78 in A. Elmhamdi et al.. [40] Shuaishuai Sun, Dongsen Mao, Jun Yu, Zhiqiang Yang, Guanzhong Lu, Zhen Ma, Catal. Sci. Technol. 5 (2015) 3166–3181. [41] K. Kuntaiah, P. Sudarsanam, B.M. Reddy, A. Vinu, Nanocrystalline Ce1−xSmxO2−δ (x = 0.4) solid solutions: structural characterization versus CO oxidation, RSC Adv. 3 (2013) 7953–7962. [42] Hisahiro EinagaNarihiro Urahama, Akihiro TouYasutake Teraoka, CO oxidation over TiO2-supported Pt–Fe catalysts prepared by coimpregnation methods, Catal. Lett. 144 (10) (2014) 1653–1660. [43] P. Sudarsanam, B. Mallesham, D. Naga Durgasri, B.M. Reddy, Physicochemical characterization and catalytic CO oxidation performance of nanocrystalline Ce–Fe mixed oxides, RSC Adv. 4 (2014) 11322–11330. [44] Yi Liu, Cun Wen, Yun Guo, Guanzhong Lu, Yanqin Wang, Modulated CO oxidation activity of M-doped ceria (M = Cu, Ti, Zr, and Tb): role of the Pauling electronegativity of MJ, Phys. Chem. C 114 (21) (2010) 9889–9897. [45] B.M. Reddy, G. Thrimurthulu, L. Katta, Design of Efficient CexM1−xO2−δ (M = Zr, Hf, Tb and Pr) nanosized model solid solutions for CO oxidation, Catal. Lett. 141 (2011) 572–581. [46] Benedetto Di, G. Landi, L. Lisi, G. Russo, Role of CO2 on CO preferential oxidation over CuO/CeO2 catalyst, Appl. Catal B. 142–143 (2013) 169–177. [47] S. Rossignol, F. Sandrine, L. Morphin, V. Piccolo, J.-L. Caps, Rousset, selective oxidation of CO over model gold-based catalysts in the presence of H2, J. Catal. 230

4. Conclusions In the present study, a series of CexCu1−xO2−δ catalysts was prepared using the combustion method. Among them, the superiority of the Ce0.8Cu0.2O2−δ mixed oxide catalyst is demonstrated for the CO oxidation and CO-PROX reactions with the extraordinary performance of T50 and T100 at 62 and 78 °C, respectively. The particular catalyst exhibited a coral reef morphology with large voids as a result of the excessive volume of gases released from the precursor compound. The particular catalyst presents mobile oxygen species at low temperature (T < 100 °C) (H2-TPR studies) and a very low affinity for CO2 (CO2TPD studies), the latter being product of the reaction. These features along with the high surface area (32 m2/g) contribute to the exceptional activity, which outperforms other Cu-Ce-based catalysts, as well as noble (Pd, Pt) metal supported systems already reported, the latter by 50% and 57%, respectively. Acknowledgements This work was made possible by the grant number NPRP 8-1912-1354 from the Qatar National Research Fund (a member of Qatar Foundation). K. Polychronopoulou would like to acknowledge the financial support from Abu Dhabi Educational Council for supporting this research through the ADEC 2015 Award for Research Excellence. References [1] A. Mishra, R. Prasad, A review on preferential oxidation of carbon monoxidein hydrogen rich gases, Bull. Chem. React. Eng. Catal. 6 (1) (2011) 1–14. [2] K. Polychronopoulou, A.F. Zedan, M.S. Katsiotis, M.A. Baker, A.A. AlKhoori, Siham Y. AlQaradawi, S.J. Hinderd, Saeed AlHassan/, Rapid microwave assisted sol-gel synthesis of CeO2 and CexSm1-xO2 nanoparticle catalysts for CO oxidation, J. Mol. Catal. A Chem. 428 (2017) 41–55. [3] P.P. Du, W.W. Wang, C.J. Jia, Q.S. Song, Y.Y. Huang, R. Si, Effect of strongly bound copper species in copper–ceria catalyst for preferential oxidation of carbon monoxide, Appl. Catal. A Gen. 518 (2016) 87–101. [4] R. Prasad, Pratichi Singh, A review on CO oxidation over copper chromite catalyst, Catal. Rev. Sci. Eng. 54 (2) (2012) 224–279. [5] M. Pudukudy, Z. Yaakob, Catalytic aspects of ceria–zirconia solid solution: part-I: an update in the synthesis, properties and chemical reactions of ceria zirconia solid solution, Der Pharma Chem 6 (2014) 188–216. [6] K. Polychronopoulou, K. Giannakopoulos, A.M. Efstathiou, Tailoring MgO-based supported Rh catalysts for purification of gas streams from phenol, Appl. Catal. B Environ. 111 (2012) 360–375. [7] K.C. Petallidou, K. Polychronopoulou, S. Boghosian, S. Garcia-Rodriguez, A.M. Efstathiou, Water–gas shift reaction on Pt/Ce1–xTixO2−δ: the effect of Ce/Ti Ratio, J. Phys. Chem. C 117 (48) (2013) 25467–25477. [8] A. Gurbani, J. Ayastuy, M. González-Marcos, M. Gutiérrez-Ortiz, CuO-CeO2 catalyst synthesized by various methods: comparative study of redox properties, Int. J. Hydrog. Energy 35 (2010) 11582–11590. [9] D. Gamarra, A. López Cámara, M. Monte, S.B. Rasmussen, L.E. Chinchilla, A.B. Hungría, G. Munuera, N. Gyorffy, Z. Schay, V.C. Corberán, Preferential oxidation of CO in excess H2 over CuO/CeO2 catalysts: characterization and performance as a function of the exposed face present in the CeO2 support, Appl. Catal. B Environ. 130–131 (2013) 224–238. [10] M. AlKetbi, K. Polychronopoulou, Abdallah.F. Zedan, V. Sebastián, Mark A. Baker, A. AlKhoori, M.A. Jaoude, O. Alnuaimi, Steve S. Hinder, Anjana Tharalekshmy, A.S. AlJaberi, Mater. Res. Bull. 108 (2018) 142–150. [11] Arantxa Davó-Quiñonero, Miriam Navlani-García, Dolores Lozano-Castelló, Agustín Bueno-López, James A. Anderson, Role of hydroxyl groups in the preferential oxidation of CO over copper oxide–cerium oxide catalysts, ACS Catal. 6 (2016) 1723–1731. [12] Y. Liu, Q. Fub, M.F. Stephanopoulos, Preferential oxidation of CO in H2 over CuOCeO2 catalysts, Catal. Today 93–95 (2004) 241–246. [13] M. Konsolakis, The role of copper–ceria interactions in catalysis science: recent theoretical and experimental advances, Appl. Catal. B Environ. 198 (2016) 49–66. [14] Hoang Yen, Yongbeom Seo, Serge Kaliaguine, Freddy Kleitz, Tailored mesostructured copper/ceria catalysts with enhanced performance for preferential oxidation of CO at low temperature, Angew. Chem. Int. Ed. 51 (2012) 12032.

322

Surface & Coatings Technology 354 (2018) 313–323

A.F. Zedan et al.

[50] T.X.T. Sayle, S.C. Parker, C.R.A. Catlow, The role of oxygen vacancies on ceria surfaces in the oxidation of carbon monoxide, Surf. Sci. 316 (1994) 329–336. [51] Satyapaul A. Singh, Giridhar Madras, Detailed mechanism and kinetic study of CO oxidation on cobalt oxide surfaces, Appl. Catal. A Gen. 504 (2015) 463. [52] T. Baidya, A. Gupta, P.A. Deshpandey, G. Madras, M.S. Hedge, High oxygen storage capacity and high rates of CO oxidation and NO reduction catalytic properties of Ce1−xSnxO2 and Ce0.78Sn0.2Pd0.02O2-δ, J. Phys. Chem. C 113 (2009) 4059.

(2005) 476. [48] Jingkang Zhu, Qiuming Gao, Zhi Chen, Preparation of mesoporous copper cerium bimetal oxides with high performance for catalytic oxidation of carbon monoxide, Appl. Catal. B Environ. 81 (2008) 236–243. [49] P. Kumar, V.C. Srivastava, I.M. Mishra, Synthesis and characterization of Ce–La oxides for the formation of dimethyl carbonate by transesterification of propylene carbonate, Catal. Commun. 60 (2015) 27–31, https://doi.org/10.1016/j.catcom. 2014.11.006.

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