Low temperature catalytic oxidation has been attracted considerable attention during the past decades because of high conversion efficiency, low operating temperature, eco-friendly and economical technology. Catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO2) assumes a key part in the reducing the emissions of CO. Recently, enormous amounts of CO are globally emitted mainly from transportation, power plants along with domestic and industrial activities.3 Low temperature catalytic oxidation of CO is continuously being developed as an important research topic of huge industrial interest in environmental science. The most common catalysts used in the catalytic oxidation of CO are of two kinds, namely those based on noble metals and mixed metal oxides. Noble metals such as Pt, Ru, Pd and Au used as promoter or supported as active sites on reducible oxides such as CeO2, MnO2, TiO2, SnO2 and TiO2 have been considered as the best candidates for low temperature catalytic oxidation of CO and they are able to convert CO to CO2 completely at ambient temperature. However, noble metals are not only expensive but also prone to poison by either the impurities in the feed stream or intermediates formed during the oxidation reaction.10 Therefore, attentions are increasingly being focused on the design and preparation of cheap and active transition-metal based mixed oxide catalysts as alternatives to noble metals.
Copper and cobalt mixed oxides have been found highly active and promising for CO oxidation.12 Over the past few years, several Cu-Co binary oxides such as CuCo2O4 spinel supported on Al2SiO5, Cu0.9Co2.1O4 spinel on stainless steel support, a thin film of CuCo2O4 oxide and CuO/CoO mixed oxides15 that homogeneously dispersed on the stainless steel grid mesh were employed in the oxidation of CO and exhibited very good catalytic performances. According to the reported literature, the presence of mixed structure of copper oxide and cobalt oxide plays a crucial role in the oxidation of CO over Cu-Co mixed oxides. Copper cobalt spinel presents active sites, whereas, cobalt oxide is the center of oxygen supply and transmission in Cu-Co mixed oxides and provides active oxygen to copper cobalt spinel for completely oxidized organic molecules.
Based on the above mentioned facts, it could be expected that if cobalt oxide is replaced with other metal oxides having better oxygen storage capacity and oxygen-supply capacity, the activity of Cu-Co mixed oxides in catalytic oxidation can be further promoted. Iron oxide is known to have attractive oxygen storage capacity and is extensively used in the oxidation reactions as catalyst. Therefore, the introduction of Fe in the Cu-Co mixed oxides is expected to enable the catalyst with better oxygen capacity and high performance for the low temperature catalytic oxidation of CO.
The present work aims to highlight an approach to synthesize functional Cu-Co-Fe ternary oxides thin ?lm using the pulse-spray evaporation chemical vapor deposition (PSE-CVD) route. The morphology, bulk composition, chemical composition, structure and optical properties of ternary oxides were characterized by SEM, EDS, XPS, XRD and UV-Vis spectroscopy, respectively. The catalytic performance of ternary oxides was evaluated against oxidation of CO in a tubular quartz ?ow reactor. The structural features of the ternary oxides were discussed and correlated with its performance.
The surface morphology of Cu-Co-Fe ternary oxides was investigated by SEM. As can be seen in Fig. 1, the ternary oxides dispersed uniformly on the support owning with dome-top-shaped geometric structures. A smooth thin film loosely packed fine columnar grains was observed. Besides the dome-top shapes, a number of hollows were also observed. The hollows surface structure may encompass more physisorbed and chemisorbed surface active oxygen species, which would assist in the catalytic performance. Cu-Co-Fe ternary oxides showed porous and open structures, which could offer predisposed space holding abundant active sites that can be useful for CO oxidation.
The energy dispersive spectrum of the Cu-Co-Fe ternary oxides obtained from the SEM-EDS analysis clearly showed that the sample contained Cu, Co, Fe and O (Fig. 2). The bulk composition of Cu-Co-Fe ternary oxides is listed in Table 1. The EDS analysis showed the signal of Cu, Co, Fe and O which
indicated that the metal oxide species act as active sites in the oxidation reaction exist on the exterior surface of the inert support. Therefore, it can be deduced that the Cu-Co-Fe ternary oxides with nanosize exhibited high specific surface area, might be beneficial to enhance the catalytic performance for oxidation reaction.
The surface composition and chemical states of Cu-Co-Fe ternary oxides were investigated by XPS, as shown in Figs. 3 and 4. The BE for Cu2p, Co2p, Fe 2p and O1s as well as the atomic percentage are summarized in Table 1. The Cu-Co-Fe ternary oxides displayed two main peaks in the Cu2p spectrum at 931.9 and 952.0 eV that were attributed to the Cu2p3/2 and Cu2p1/2, respectively. Strong satellite peaks were observed at lower and higher binding energy which unambiguously revealed the presence of Cu2+ on the surface of the ternary oxides. These species could be formed by the reaction sequence Cu?Cu_2 O?CuO at the surface. The Co2p spectrum of Cu-Co-Fe ternary oxides showed two main peaks at 779.1 eV and 794.5 eV, ascribed to Co2p3/2 and Co2p1/2 spin-orbital peaks, respectively. As reported in the literature,19,20 the value of spin-orbit splitting for Co3+ and mixed-valance Co3O4 is 15.0 eV and 15.1?15.3 eV, respectively. The spin-orbit splitting value of Co2p was ~15.0 eV which showed the characteristic of Co3+. The first peak at 779.1 eV could be assigned to Co3+, and the peak at 794.5 eV could be ascribed to the Co3+. The Fe2p spectrum of Cu-Co-Fe ternary oxides showed a broad peak at ~711.7 eV, which agrees well with the reported value for Fe2O3.21 The satellite peak at 718.0 eV was consistent with available literature, which also indicated that Fe had a 3+ oxidation state in the ternary oxide.22 In the Cu-Co-Fe ternary oxides, the valence state of Fe was expected to be 3+ if Fe substituted for Co. The presence of Fe3+ ions in the ternary oxides on the surface of ternary oxides could result in availability of excess oxygen species for the oxidation reaction.The O1s spectra of the prepared oxides could be deconvoluted into two components (Fig. 4). The peak at 529.6 eV and 531.2 eV corresponded to the surface lattice oxygen (OLatt) and the adsorbed oxygen (OAds) of the ternary oxides, respectively.
The lattice and absorbed oxygen exhibited high ability to attack an organic molecule in the region of its highest electron density due to their strong electrophilic nature and caused the oxidation of carbon skeleton. Moreover, the electrophilic oxygen species were observed to be in charge of the complete oxidation of CO and availability of excessive oxygen species was expected to be favorable for catalytic oxidation of CO. The prepared ternary material could release oxygen at relatively low temperature easier than other materials counterparts (Table 2) that led to oxygen species to move from lattice to surface, which in turn could facilitate the task for CO to be completely oxidized when reacted with active oxygen species on surface at low temperature.
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The crystal-like arrangement and purity of the Cu-Co-Fe ternary oxides were analyzed by XRD, as presented in Fig. 5. There was no characteristic peak of CuO, Co3O4 and Fe2O3 phases detected in the XRD spectrum of the ternary oxides. As shown earlier by EDS and XPS results, Cu, Co and Fe species were presented by the ternary oxides. Therefore, it revealed the amorphous structure of the Cu-Co-Fe ternary oxides. According to literature, this phenomenon commonly occurred in the multicomponent complex mixed oxides where the crystallization energy was found considerably higher than the thermal energy available at operating temperature. Therefore, it is expected that the synergetic effects among Cu, Co and Fe species as well as amorphous structure of the Cu-Co-Fe ternary oxides could play crucial roles in the catalytic performance for CO oxidation.
The bandgap energy (Eg) of the Cu-Co-Fe ternary oxides was evaluated by UV–vis spectroscopy. As shown in Fig. 6a, a clear absorbance was detected in the area of 350–750 nm. It was observed that the absorption steadily decreased as the wavelength extended towards the visible region, which was found to be in good agreement with pervious results. The obtained absorption spectrum was used to assess the direct Eg from the Tauc equation: ?h? = A(h? – Eg)n, where h? is the photon energy, Eg represent bandgap, A is a constant relying on the refractive index and ? is the absorption coef?cient. The consistent Tauc plot of h? vs (?h?)2 for the Cu-Co-Fe ternary oxides is shown in Fig. 6b. In the present study, Cu-Co-Fe oxides tended to have two bandgap energies. The bandgap energies for Cu-Co-Fe ternary oxides were obtained at 1.50 and 2.05 ± 0.05 eV. The bandgap energies attained with Cu–Co-Fe oxide shifted to lower values, contrasted with the comparing estimations of pure cobalt oxide (2.15 eV), copper oxide (2.16 eV) and iron oxide (2.18 eV). The bandgap energies Eg1 and Eg2 can be ascribed to the O2–octahedral ions (mainly Fe3+ and Co3+) and O2–Cu2+ charge transfer, respectively. Moreover, numerous factors could affect the Eg of metal oxides such as flaws, charged impurities, disorder at the grain boundaries and etc..32 Jibril et al33 reported a strong correlation between the catalytic behavior and bandgap of the metal oxides in oxidation reactions. The availability of excessive lattice oxygen at the surface of Cu-Co-Fe ternary oxides revealed by XPS and low Eg might be determinant in the catalytic performance and play vital part in the reducibility of Cu-Co-Fe ternary oxides.
The catalytic performances of the Cu-Co-Fe ternary oxides grown on SSGM were investigated for complete CO oxidation at atmospheric pressure. The light-off curve of CO conversion over Cu-Co-Fe ternary oxides is shown in Fig. 7. Interestingly, the deposition of Cu, Co and Fe species on the SSGM provided suitable active sites on the surface via synergetic effects, which resulted in a boost in the CO conversion to CO2. Notably, it was observed that the CO conversion became visible at about 140 °C and whole conversion was attained at 234 °C over Cu-Co-Fe ternary oxides. Only CO2 was observed and no any other byproduct was detected in the CO oxidation over Cu-Co-Fe ternary oxides.
Compared with the individual single metal oxides, mixed metal oxides and noble metals reported for CO oxidation (Table 2) in the literature, the Cu-Co-Fe ternary oxides exhibited better catalytic activity. The complete CO conversion was achieved at much lower temperature about 100 to 200 °C compared to the reported metal oxide catalysts, even a high GHSV of 75,000 mL g-1 h-1 was used. Thus, the introduction of iron oxide in Cu-Co mixed oxides significantly influenced the physiochemical properties of the obtained ternary oxides by providing effectual synergetic effects and improved the catalytic performance (Table 2). For the CO oxidation over metal oxides, several parameters of metal oxides such as the structure, oxidation state of the metal species, surface metal content, bandgap energy and oxygen mobility could affect the catalytic performance. In this study, the physicochemical properties of Cu-Co-Fe ternary oxides were also correlated with the catalytic performance to gain further insights into the possible reason for high performance. In general, it is acknowledged that when oxidation reactions occur by following the Mars-van-Krevelen mechanism, the performance of metal oxides depends on their oxygen mobility. The reactions involve alternative oxidation and reduction of oxides on surface along with the formation of surface oxygen vacancies (as a fundamental step) and its replacement with oxygen in the gaseous phase. The other observation has been reported in the catalytic oxidation process, the abundance of oxygen adsorbed on the surface of metal oxides might play a decisive part.36 It is probable that a Mars-van-Krevelen mechanism can be applied to the total oxidation of CO on Cu-Co-Fe ternary oxides, having lattice and absorbed oxygen as an active oxygen species. The hollow dome top-like morphology and low bandgap energy of ternary oxides could adsorb more oxygen and expose more surface area with easy oxygen mobility which probably made the oxidation occur at low temperature.34 Moreover, the synergetic effects of metal species and their reducibility played a great role in influence the catalytic performance and redox properties of Cu-Co-Fe ternary oxides for the oxidation of CO.
A general review of appendix about particular total oxidation of CO over metal oxides con?rmed that only few works investigated the durability of the metal oxides 46. The deactivation of metal oxides considered as a parameter of great significance for industrial application and it recently achieved much more interest of the researcher. Therefore, the catalyst stability and effect of reaction time on the catalytic performance of Cu-Co-Fe ternary oxides were investigated by performing the CO oxidation reaction for 30 h at 215 °C and the obtained results are shown in Fig. 8. The results showed that the Cu-Co-Fe ternary oxides had good stability during reaction for 30 h, without any decrease in CO conversion. The stability of Cu-Co-Fe ternary oxides was also observed better than Co-promoted Pt/Al2O3 catalysts which reported the lost of 20% conversion in 20 h. Therefore, the stability of Cu-Co-Fe ternary oxides could be related to the promoting effect of iron species which empowered copper and cobalt species in amorphous structure to resist against complete reduction.
The preparation of Cu-Co-Fe ternary oxides was achieved by pulsed-spray evaporation chemical vapor deposition (PSE-CVD), and details of experimental setup and procedure can be found in our previous works. In the present work, the precursors of copper acetylacetonate (Cu(acac)2), cobalt acetylacetonate (Co(acac)2) and iron acetylacetonate (Fe(acac)3) were individually dissolved in ethanol as a feedstock. By adjusting and optimizing the molar ratios of metallic elements: Cu/Co/Fe = 50/25/25, the blended feedstock was obtained, which was kept under atmospheric pressure at room temperature, during the deposition process. The PSE output was obtained with the time and the opening frequency of the valve at 1.0 ms and 4 Hz, respectively. The mixed feedstock was introduced into evaporation chamber as a fine spray at 200 °C and flow rate of N2 and O2 were 0.25 and 0.5 standard L/min., respectively. Bare glass (BG), stainless-steel (SS) and stainless-steel grid mesh (SSGM) substrates were used and heated to 320 °C using a ?at resistive heater, during deposition. The entire pressure was set aside at around 2.0 kPa in the deposition chamber.
Several techniques were used to characterize the Cu-Co-Fe ternary oxides. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were carried out using Hitachi SU 8020 field-emission scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical compositions and ionic states of the obtained samples by using AXIS ULTRA DLD (Shimadzu Kratos) with pass energy of 80 eV and analysis voltage of 15 kV. The C1s peak at 284.8 eV was used as the reference to calibrate binding energies (BE). X-Ray Diffraction (XRD) technique was employed to collect patterns in 2? = 20-80°, by using a Phillips X’Pert Pro MDR diffractometer with PW3830 X-ray generator (Cu K? radiation, ? = 0.154056 nm). Cary 5000 UV-Vis-NIR spectrophotometer (range = 175-3300 nm) was used to evaluate the optical properties.
The catalytic performance of the Cu-Co-Fe ternary oxides was evaluated against the CO oxidation in a continuous flow fixed bed quartz reactor. The setup has been described in detail elsewhere. The reaction mixture containing 1% CO and 20% O2 diluted in Ar, fed into the reactor with total flow rate of 15 mL/min and gas hourly space velocity (GHSV) of 75,000 mL g-1 h-1. The temperature of the reactor was raised up with an incline of 5 °C/min utilizing an advanced electrical heater. Moreover, the durability of as-prepared samples was analyzed under the same experimental conditions at 75% fuel conversion (215 °C) for 30 h. The exhaust gas was analyzed in terms of compsoition with an online FTIR spectrometer in the range of 400-4000 cm-1, associated with a homemade KBr-transmission cell.
The novel introduction of Fe in the Cu-Co mixed oxides was achieved by PSE-CVD method. The active sites dispersion, structure, morphology, composition, cationic rearrangement and optical bandgap energy of the Cu-Co-Fe ternary oxides were found be quite different from the individual single or binary oxides. The catalytic performance indicated that the complete oxidation was accomplished at much lower temperature about 100-200 °C over Cu-Co-Fe ternary oxides than other reported metal oxide catalysts and non-coated mesh. The excellent catalytic performance was attributed to the amorphous structure, hollow dome top-like morphology, cationic rearrangement of Co3+, Cu2+ and Fe3+, excessive availability and easy mobility of lattice and adsorbed oxygen species at surface and lower optical bandgap energy of the Cu-Co-Fe ternary oxides, which provided the suitable active sites for the oxidation reaction. Moreover, the Cu-Co-Fe ternary oxides revealed good stability during oxidation reaction for 30 h, without any decrease in CO conversion. The remarkable catalytic performance and stability of Cu-Co-Fe ternary oxides might make it possible to use as an efficient heterogeneous catalyst for the oxidation of CO at industrial scale. These findings open up a new avenue for a new generation ternary oxides thin film prepared by PSE-CVD, due to energy saving, high efficiency and eco-friendly features.