Catalytic performance of the Ce-doped LaCoO3 perovskite nanoparticles

A series of La1-xCexCoO3 perovskite nanoparticles with rhombohedral phases was synthesized via sol–gel chemical process. X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Electron Diffraction Spectroscopy (EDS), Thermogravimetric Analysis (TGA), UV–Vis spectroscopy, Fourier Transform Infrared spectra (FTIR), Nitrogen Adsorption/desorption Isotherm, Temperature Program Reduction/Oxidation (TPR/TPO), X-ray Photoelectron Spectroscopy (XPS) techniques were utilized to examine the phase purity and chemical composition of the materials. An appropriate doping quantity of Ce ion in the LaCoO3 matrix have reduced the bond angle, thus distorting the geometrical structure and creating oxygen vacancies, which thus provides fast electron transportation. The reducibility character and surface adsorbed oxygen vacancies of the perovskites were further improved, as revealed by H2-TPR, O2-TPD and XPS studies. Furthermore, the oxidation of benzyl alcohol was investigated using the prepared perovskites to examine the effect of ceria doping on the catalytic performance of the material. The reaction was carried out with ultra-pure molecular oxygen as oxidant at atmospheric pressure in liquid medium and the kinetics of the reaction was investigated, with a focus on the conversion and selectivity towards benzaldehyde. Under optimum reaction conditions, the 5% Ce doped LaCoO3 catalyst exhibited enhanced catalytic activity (i.e., > 35%) and selectivity of > 99%, as compared to the other prepared catalysts. Remarkably, the activity of catalyst has been found to be stable after four recycles.

With the advancements of science and nanotechnology, researchers have been continuously trying to establish different techniques for benzaldehyde production 1 . Benzaldehyde production is of vital importance from the scientific and industrial point of views. It is one of the most valuable aromatic aldehydes and versatile intermediates used in various chemical industries, such as pharmaceuticals, perfumery, dyestuff, and agrochemical industries. Among the various aspects in the production of benzaldehyde, three features are of utmost importance: (i) environmentally clean and green oxidants, (ii) non-toxic solvents, and (iii) low cost catalysts with high activity and selectivity 2,3 . Previously, the catalyst used for benzaldehyde production/generation were inorganic and organic oxidants, such as chromium trioxide, ammonium permanganate, and tert-butyl hydroperoxide. However, these chemicals are inherently toxic, expensive, corrosive, thus resulting to many environmental issues and concerns 4 . In view of this, enormous efforts have been devoted to develop a more environmentally friendly catalytic systems to decrease the drawbacks of the usual traditional oxidation approaches 5 . Therefore, a much needed alternative shift must be geared towards the use of clean oxidants, such as aqueous H 2 O 2 and molecular O 2 , which has attracted a considerable amount of attention, because of its economic and environmental advantages 6 . Molecular oxygen is the most desirable oxidant because it is cheap, safe, readily available, and produces water as the sole byproduct 7 . Conventionally, many types of organic solvents, such as benzene, chloroform, toluene, acetonitrile, acetone, and xylene are used in benzaldehyde production via in alcohol oxidation 8 .
In an ideal oxidation process for benzaldehyde production, the catalyst should be eco-friendly, can easily be prepared, and has long term stability. In addition, easy product separation, recycling, and high conversion, besides selectivity, are some of its important parameters. Among catalytic oxidation reactions, heterogeneous catalysis has been widely studied, because of its long term stability and excellent performance. Selective oxidation of alcohols to carbonyl compounds over heterogeneous catalysts using molecular oxygen (aerobic oxidation) has

Experimental detail
Synthesis of catalysts. Specifically, La 1-x Ce x CoO 3 (x = 0.0, 0.05, 0.07, and 0.1) nanoparticles were synthesized by a "citrate based" co-precipitation method. In this typical reaction, an equal volume (1:1 molar ratio) of La(NO 3 ) 3 ·7H 2 O (99.99%, BDH Chemicals Ltd, England) and Co(NO 3 ) 2 ·6H 2 O (99.9% E-Merck, Germany) were mixed together in 50 mL Milli Q water solution and kept under magnetic stirring on a hot plate at 80 °C to achieve a homogenous mixture. A hot solution of cerium nitrate was slowly added into the solution mixture. In another solution, 10 wt% excess aqueous solution of citric acid monohydrate (98.5%, E-Merck, Germany) was injected into the solution for complexation. After this, the solution mixture was transferred into a 250 mL round bottle flask fitted with reflux condenser for complete complexation up to 5-6 h. A quantity of NH 4 OH (99.99%, BDH Chemicals Ltd, England) was introduced to hydrolyze the solution for precipitation reaction. Later on, the obtained brown black colored precipitates were separated by centrifugation, washed many times with water to remove excess quantity of ammonium and nitrate ions, and finally dried at 250 °C, and finally calcined in air at 800 °C for 6 h to obtain the perovskites. This procedure was followed for synthesis of various concentrations of Ce-doped La 1-x Ce x CoO 3 perovskites.
characterizations. The powder samples were characterized XRD with the use of Rigaku D 2,500 diffractometer using the Cu Kα radiation (λ = 0.154 nm, 40 kV, 40 mA). The FTIR spectra were recorded on the Perkin-Elmer 580B IR spectrometer using the KBr pellet technique. UV/Vis spectra were recorded from the Perkin-Elmer Lambda-40 Spectrophotometer. The thermal decompositions of the perovskite precursors were performed simultaneously by thermogravimetric-thermal analysis (TG-DTG) using Mettler Toledo TGA/DSC 1 STAR e thermal analyzer (Switzerland) between 50 and 900 °C at the heating rates of 20 °C min −1 in nitrogen atmosphere at a flow rate of 20 mL min −1 . The morphology was checked using the Field Emission TEM, equipped with EDX (JEM-2100F, JEOL, Japan) operating at an accelerating voltage of 200 kV. The BET surface areas of the calcined catalysts were measured using the Micromeritics TriStar 3000 BET Analyzer, taking a value of 0.162 nm 2 for the cross-sectional area of the N 2 molecule adsorbed at 77 K. The sample degassing was carried out at 300 °C prior to measuring the adsorption isotherms.
The redox properties (H 2 -TPR and O 2 -TPO) were recorded using a chemisorption apparatus (Micromeritics AutoChem II 2920), equipped with a thermal conductivity detector. In this measurement, about 25 mg of sample was loaded in a U shaped quartz tube (6 mm ID). Samples were packed in the tube by quartz wool plugs and a thermocouple is inserted to measure the bed temperature. The samples were initially flushed in Argon at 300 °C for 60 min in order to eliminate the adsorbed water, and then cool to room temperature. H 2 -TPR was performed using a mixture of 10% H 2 /Ar at a flow rate of 20 mL min −1 . The sample tube was heated at the ramping rate Scientific RepoRtS | (2020) 10:15012 | https://doi.org/10.1038/s41598-020-71869-z www.nature.com/scientificreports/ of 10 °C min −1 from 50 to 800 °C. After the reduction, sample was cooled to room temperature; and was then exposed to 10% O 2 /He for the oxidation (O 2 -TPO) at the same operating condition. The XPS spectra were recorded using the PerkinElmer PHI 5000C system equipped with a hemispherical electron energy analyzer. Using the Mg K alpha anode (hv = 1,253.6 eV), the XPS was operated at 15 kV and 20 mA. The binding energy (BE) scale was referenced to the C 1s peak (~ 284.6 eV) arising from adventitious carbon in the sample. catalytic reactions. The liquid-phase aerobic oxidation reaction of benzyl alcohol was carried out at atmospheric pressure in a magnetically stirred three-necked flask equipped with reflux condenser and thermometer. In brief, 200 mg catalyst, 2 mmol benzyl alcohol and 10 mL toluene (solvent) were mixed transferred in 50 mL flask. Prior to the oxidation process, the reaction mixture was purged in Argon gas for 1 h. The mixture was immersed in an oil bath and heated to 100 °C. The oxygen gas was bubbled at a flow rate of 20 mL min −1 into the pre-charged flask to start the reaction. The liquid products were collected every 2 h, separated by centrifuging, and analyzed by Agilent Gas Chromatograph 7890A, equipped with FID and HP-PONA capillary column. Using the equations below, the conversions and selectivities were calculated by the peak area, using Undecane as an internal standard. The recyclability of the highly active catalyst, La 0.95 Ce 0.05 CoO 3 , was also investigated 32 .

Results and discussion
physiochemical properties of perovskite. The XRD patterns and calculated lattice parameters of the pre-calcined powder are presented in Fig. 1 and Table 1 23,33 . The XRD peak broadening suggests that the as-prepared perovskites are well-crystalline and small in grain sizes (~ 100 nm), although, weak intensity of CeO 2 peak is also observed for 7% and 10% Ceria dopings. Since CeO 2 peak is not seen in the low concentration (5%) doping of ceria, it shows that low concentration can be accommodated in LaCoO 3 framework 31,34 . On the addition of Ce 3+ ions into the LaCoO 3 structure, the reflection lines are slightly shifted towards higher 2θ angles, signifying decreases in the lattice parameters. Additionally, substitution of smaller radius Ce 3+ (1.34 Å) ions at a site into LaCoO 3 crystal distorts the crystal lattice due to a reduction in crystal spacing distance 35,36 . The observed discrepancy in lattice parameters may be due to ion-radius mismatch on the substitution of smaller radius ion at the La 3+ site, which is in good agreement with the earlier reports (Table 1) 37,38 . The cell parameter "a", for example, reduces with increasing Ce 3+ content, as the ionic radius of the Ce 3+ ion is smaller than that of the La 3+ ion 30,39,40 . The structural alteration of the rhombohedral phase of the perovskite phase is a function of the size of replacing lanthanides, which generate the oxygen vacancies because of lattice distortion 41 . The cation replacement at the La 3+ sites leads to different ion and vacancy ordering. This can result in deviations in the catalytic activities. It is observed that the activity of the as-prepared catalysts has improved after increasing the substitution concentration.
The TEM-EDX of La 0.95 Ce 0.05 CoO 3 nanoparticles is shown Fig. 2. As shown in Fig. 2a, the product is nanocrystal with 135-200 nm wide and shows rope-like structure in which Ce doped with uncontrolled size. Moreover, EDX spectrum of La 0.95 Ce 0.05 CoO 3 clearly shows the peaks of the expected elements, such as La Co, Ce, and O throughout the whole structure, which is well-matched with the chemical composition of La 0.95 Ce 0.05 CoO 3 nanoparticles (Fig. 2b). This indicates the successful substitution of the trivalent Ce 3+ ions into the perovskite crystal lattice.
The surface chemistry of the as-designed perovskites plays a vital role in their applications in catalysis, especially under harsh environmental conditions. In Table 2, we summarized the results of the BET scan of LaCoO 3  www.nature.com/scientificreports/ and their Ce 3+ substituted perovskites. The specific surface areas pore sizes, and pore volumes were calculated from the respective nitrogen adsorption isotherm. On the textural properties, such as specific surface areas, pore sizes, and pore volumes summarized in Table 2, the as-synthesized LaCoO 3 has BET surface area of 3.9 m 2 g −1 .
This slight improvement in the surface area could be due to the formation of La-Ce-Co-O network after the introduction of ceria in the crystal lattice. La 0.95 Ce 0.05 CoO 3 has BET surface area of 4.7 m 2 g −1 . However, upon further addition of cerium ion from 5, to 7, and finally to 10%, the surfaces of the catalysts decreased from 4.4 to 4.3 m 2 g −1 .
The as-synthesized powders were thermally analyzed by TG-DTG measurements. All thermograms shown in Fig. 3 show similarities in shape and illustrated three-step decomposition. In case of LaCoO 3 perovskite, 6% weight loss was observed in the temperature range from 0 to 395 °C. The initial weight loss is attributed to the removal of surface-adsorbed water molecules and organic moieties. The second weight loss at ~ 6% in the temperature range 395-727 °C is attributed to the removal of crystalline water molecules. A sluggish weight loss of ~ 3% is observed in between 727 and 900 °C temperature, which is assigned to the burning or combustion of lattice oxygen with airborne carbonates to form the perovskite structure. Such observations are in accord with previous literature reports [42][43][44][45][46] . It is worth noting that, in increasing the Ce ion substitution concentration in perovskite lattice, the quantity of weight loss has also increased as seen in the TGA data of La 0.90 Ce 0.10 CoO 3 . We expected this since increasing the doping concentration in perovskite lattice would also increase the lattice distortion within the crystal matrix, because of the reduced bond distance. As verified from the XRD data, increasing doping creates a large number of oxygen vacancies, which decompose or burn at a higher temperature. All thermograms reveal similar thermal decomposition temperatures. Except in the differences in weight losses, this may be due to the surface attached water molecules and organic moieties, which alter the crystallinity of the materials.
FTIR spectra of the as-synthesized perovskites are illustrated in Fig. 4. FTIR spectra of all perovskites exhibit diffused band at around 3,450 cm −1 along with two low-intensity infrared absorption bands located at 1,480 and 1,360 cm −1 , which are assigned to the O-H stretching, bending and scissoring vibrational modes of physically adsorbed H 2 O molecules over the exterior of perovskites 42 . The observed strong intensity infrared band at low frequency (below 700 cm −1 ) is ascribed to the symmetric vibrational modes of La-Ce-Co-O network 47 . Clearly observed from the FTIR spectra of the doped sample, the infrared absorption band at lower frequency are progressively shifted from 572 cm −1 to the higher frequency (590 cm −1 ) when increasing the Ce ions doping quantity. Based on the force constant phenomenon, the Ce 3+ ion (1.12) is more electronegative than their respective La 3+ ion (1.10), it, therefore, implies that substitution of Ce can shift the La-O bonding towards longer frequency by forming the Ce-O bonds 48 .
The optical absorption spectra were measured to determine the optical properties, such as the energy band gab (E g ) of the perovskites materials. It is commonly known that cobalt and ceria ions reveal strong absorption in UV/Vis region. As demonstrated in Fig. 5, all perovskites exhibited broad absorption features in the whole UV/Visible region, even to extended some portion of the near infrared region, because of charge transfer band from O 2 to 2p orbitals 49 . The E g values are normally estimated by fitting the absorption data into Tauc formula by extrapolating the linear portions of the curves to absorption to zero 22 . The experimentally calculated E g values of all samples are found to be 1.13, 0.86, 0.79 and 0.71 eV for LaCoO 3 , La 0.95 Ce0.05CoO 3 , La 0.93 Ce 0.07 CoO 3 and La 0.90 Ce 0.10 CoO 3 , respectively, as shown in Fig. 6. It is observed from the figure that the E g values has gradually reduced by when increasing the amount of the doping Ce ions. It is known that the valence band of LaCoO 3 consists of O 2p charge transfer band, whereas the conduction band is made up of Ce (4f → 5d) orbitals 49 . The hybridization, therefore, occurs between Ce 5d and oxygen (2p) orbitals due to the similarity in their energy and spatial overlaps. Additionally, the enhancement in the electronegativity of the Ce metal increases the magnitude of metal-oxygen hybridization, resulting into a shift of the metal 5d orbitals and oxygen 2p orbital which are closer in energies 48 . It is also known that Ce has high electronegativity than that of La, so that in the substitution of Ce ion quantity in LaCoO 3 , an additional energy level below the conduction band is created and thus lowers      Figure 7a illustrates the reduction profiles of LaCoO 3 and their Ce doped perovskite materials. Two reduction peaks can be identified for all samples, indicating a stepwise reduction of perovskites and their derivatives. An observed low-temperature peak in between 260 and 460 °C, whereas high-temperature peaks were observed between 475 and 650 °C temperature ranges. At lower temperature H 2 -consumption peak is attributed to the reduction of Co 3+ to Co 2+ and the reduction of the excess O 2 species and the chemisorbed oxygen on the surface of the catalysts, while reduction peak observed at high temperature (595 °C), is ascribed to the reduction of Co 2+ into metallic cobalt (Co 0 ), and the reduction of chemisorbed oxygen caused by lattice defects, as reported in literature 34,[51][52][53][54] . With the introduction of 5% Ce, the H 2 -consumption peaks progressively shifted towards lower temperature. This could be due to the insertion of Ce ion, which significantly accelerate the reduction of lattice oxygen species, thus resulting to the weakening of Co-O bond 21,34,[55][56][57] . As shown in Fig. 7a, increasing the ceria content from 5 to 10%, shifts both the reduction peaks shifted towards higher temperatures. The shifting of reduction peaks at higher temperature, when increasing the cerium content, may be caused by the existence of Ce ions in the framework, which provides electrons to hydrogen and thus delay the reduction of Co species, in accordance with the literature 31,34,57 . Additionally, it indicates that incorporation of large amount of Ce ions into the LaCoO 3 crystal lattice enhances the reduction of Co species 34 . Furthermore, it also suggests that substitution of Ce ions into the LaCoO 3 crystal lattice could have promoted the formation of structural defects. Temperature program oxidation study was performed to explore the catalytic oxidation and the oxygen species adsorbed on the surface of catalysts. As shown in Fig. 7b, the reverse transformations of Co oxidation states are observed by the TPO analysis. It shows an observed peak at low temperature in between 110 and 365 °C, typically assigned to the transition of metallic Co 0 to Co 2+ , while the peak in between 623 and 790 °C shows the oxidation from Co 2+ to Co 3+ . These results are in good agreement with reported literature [58][59][60] .
XPS analysis was performed to determine the valence states of the elements in the as-prepared perovskites. The wide spectra of LaCoO 3 , 5, 7 and 10% doped Ce ions are shown in Fig. 8. The results reveal the existence of La (3d), Co (2p), C (1s) and O (1s) atoms in perovskites. A strong peak below 300 eV was observed for the adventitious carbon on the surface materials 49,61 . Two strong intensity peaks at 840.9 and 858.6 eV are assigned the spin-orbit splitting of 3d 5/2 and 3d 3/2 of the La(III) ions in all perovskites 61,62 . The XPS spectrum of LaCoO 3 illustrates three prominent peaks along with shake-up satellites at the binding energies of (780, 784.2), (789.9, 792), and (795, 799 eV) and ascribed to Co 2p 3/2 , Co 2p 1/2 , respectively. This indicates that Co ion in LaCoO 3 is mainly in the trivalent state [63][64][65] . The occurrence of shake-up satellite peaks at 784.2, 789.9, 792, and 799 eV verifies the characteristic peaks of Co 2+ ion. These results are well consistent with the previous literature reports 39,65 . Figure 8 has clearly revealed the influence of ceria doping, that is, in the insertion of ceria in perovskite lattice, the binding energies are progressively shifted towards the lower sides. Figure 9 demonstrates the narrow-range Co 2p XPS spectra of the LaCoO 3 and other perovskites. It is observed that the binding energy of Co 2p 3/2 in La 0.95 Ce 0.05 CoO 3 perovskite is shifted to lower binding energy at 778, with respect to the LaCoO 3 sample. It could be due to some amounts of Co 3+ ions being transformed into Co 4+ state, after the incorporation of Ce ions in the perovskite lattice. We believe Ce ion is in the tetravalent state, because of its high stability, then in their trivalent state. Furthermore, to maintain the electronic balance, after the replacement of trivalent La by higher valence Ce 4+ , trivalent Co is bound to transform into a higher valence 66 . Similarly, in the La 0.93 Ce 0.07 CoO 3 perovskite, peaks are shifted to lower side, implying that substitution of Ce ion induces some amount of Co 3+ to transform into Co 4+ in the samples, which is analogous to the literature reported for Co 2 O 3 65,66 . It, therefore, verifies that the main valence of Co ions is, indeed, trivalent 67 . Figure 10 demonstrates the narrow range O1s XPS spectra of the LaCoO 3 and other perovskites. XPS spectra of O1s reveal two dominant components of different types of oxygen species. The XPS spectra of LaCoO 3 exhibited a low rising peak between 523 and 529 eV, which is attributed to the lattice oxygen molecules. Another   www.nature.com/scientificreports/ larger specific surface area, which can significantly enhance the adsorption capability of oxygen. As a result, the La 0.95 Ce 0.05 CoO 3 perovskite possess higher catalytic activity due to the high surface area and high mobility of the oxygen adsorbed vacancies. This will be further discussed below.
Aerobic oxidation of benzyl alcohol. The prepared catalysts were tested for oxidation of benzyl alcohol to benzaldehyde. The results are shown in Table 3. Benzaldehyde is the main product, with a negligible amount of benzoic acid as a by-product. It was found that the un-doped perovskite, i.e., LaCoO 3 starts off with a benzyl alcohol conversion of 11% within 2 h. However when the reaction was continued for 12 h, and it yielded 30% conversion of benzyl alcohol with the specific activity of 0.25 mmol g −1 h −1 . It is also interesting to make a further comparison of the three catalysts with different cerium contents. Figure 11 shows that insertion of Ce into LaCoO 3 perovskite lattice remarkably improves the catalytic activities, and maintains the high selectivity towards benzaldehyde. On substitution of 5% Ce ions into LaCoO 3 perovskites a significant enhancement in catalytic performance was observed such as catalyst yielded a 40% conversion product with the specific activity of 0.33 mmol g −1 h −1 . It is worth noting that the addition of cerium ions into the perovskite lattice influence the surface vacancies, which significantly enhanced the synergistic effect between the Ce 3+/4+ and Co 2+/3+ ions. It is well-known fact that Ce 3+/4+ exhibited reversible oxidation states, resulting cerium ions significantly improve the catalytic activity of the catalyst. It is also evident from XPS analysis Co existed in trivalent state and generally Ce exist in tetravalent state, so that to keep the charge equilibrium in perovskite oxide, Ce should promote the excessive oxide anion on the surface and, thereby, oxygen is easily delivered from the surface for CO oxidation. Although, XPS analysis illustrate enrichment of Co at the surface, which stimulates the oxygen storage capacity. However, when the percentage composition of Ce is further increased in the perovskite system, the catalytic performance decreased. It could be due to the low quantity of Ce 3+/4+ ions accommodate within the lattice and enhanced the active sites, whereas high quantity doping surplus within the lattice and suppressed the active sites of the catalyst. Similar trend is observed in the case of the surface area, which decreases from 4.732 to 4.271 m 2 g −1 . This indicates that the Ce atom in the perovskite could have hindered the active site of the perovskite leading to depreciation of catalytic performance and of the surface area of the catalysts. These observed results indicated that doping of ceria ions played a crucial role in the improvement of catalytic activity, because of cerium ions induces high oxygen ion mobility, resulting it increase the redox characteristics of the perovskites as verified from XPS analysis.
The catalyst recovery has significant importance both from the industrial and academic point of view. In this work, the recyclability of La 0.95 Ce 0.05 CoO 3 for the oxidation of benzyl alcohol via molecular O 2 was also studied under optimized conditions. Upon addition of fresh toluene, the mixture was filtered to recover the catalyst by a simple filtration process. The filtered catalyst was washed successively with toluene and dried at 100 °C for 12 h (Fig. 12). The catalyst was used for several times and was found that the catalytic performance has depreciated by 1.55% in the first reuse. However further reuse leads to further loss in catalytic activity And upon 4 times of reuse, the catalyst yielded a 31.7% conversion of benzyl alcohol after 12 h of reaction time, which is 7.44% less Table 3. Effect on the catalytic properties with the incorporation of Ce in the perovskite system La 1-x Ce x CoO 3 . Reaction conditions: 2 mmol of benzyl alcohol, calcination temperature at 300 ºC, oxygen with rate 20 mL min −1 , 0.3 g of catalyst, 10 mL of toluene, reaction temperature at 100 ºC, and 12 h of reaction time.
Scientific RepoRtS | (2020) 10:15012 | https://doi.org/10.1038/s41598-020-71869-z www.nature.com/scientificreports/ than the first time use of the catalyst, indicating that the catalyst is marginally stable for re-use, and can be further modified to increase its catalytic performance and reusability.

conclusions
A series of cerium doped LaCoO 3 perovskites were successfully prepared by sol-gel chemical process. In the comparative results, the substitution of Ce ions in perovskite lattice has remarkably improved the crystallinity, thermal, optical, surface properties and redox properties of the perovskites. The TPR/TPO study has verified that the low concentration doping of Ce ion enhances the reducing ability of the LaCoO 3 perovskite at low temperature. When the resulting catalysts were tested in the liquid-phase aerobic oxidation of benzyl alcohol to benzaldehyde using molecular O 2 as a green oxidant, the catalytic activity tests show that the activity of catalyst depends strongly on the percentage doping of cerium. In these newly designed perovskites, La 0.95 Ce 0.05 CoO 3 , the perovskite has demonstrated an excellent catalytic activity towards benzyl alcohol oxidation with high selectivity The catalyst can be recycled several times without any loss in conversion and selectivity. This, therefore, suggests its reusability and stability. Easy product recovery and recycling efficiency along with high selectivity of this material could be useful for the synthesis of different chemicals under eco-friendly conditions.