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Low-temperature CO oxidation over combustion made Fe and Cr doped Co3O4 catalysts: Role of dopant’s nature toward achieving superior catalytic activity and stability Tinku Baidya, Toru Murayama, Parthasarathi Bera, Olga V. Safonova, Patrick Steiger, Nirmal Kumar Katiyar, Krishanu Biswas, and Masatake Haruta J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2017 Downloaded from http://pubs.acs.org on June 24, 2017

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The Journal of Physical Chemistry

Low-Temperature CO Oxidation over Combustion made Fe and Cr Doped Co3O4 Catalysts: Role of Dopant’s Nature toward Achieving Superior Catalytic Activity and Stability Tinku Baidyaa,*, Toru Murayamab,*, Parthasarathi Berac,*, Olga V. Safonovad, Patrick Steigerd, Nirmal Kumar Katiyare, Krishanu Biswase, Masatake Harutab a

Department of Mechanical and Aerospace Engineering, University of California, 9500 Gilman Drive, San Diego, CA 92093, United States of America

b

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan

c

Surface Engineering Division, CSIR−National Aerospace Laboratories, Bengaluru 560017, India

d

Paul Scherrer Institut, 5253 Villigen, Switzerland

e

Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

Abstract Co3O4 with a spinel structure shows unique activity for CO oxidation at low temperature under dry conditions; however active surface is not very stable. In this study, two series of Fe and Cr doped Co3O4 catalysts were prepared by a single-step solution combustion technique. Fe was chosen because of its redox activity corresponding to the Fe2+/Fe3+ redox couple and compared to Cr, which is mainly stable in Cr3+ state. The catalytic activity of new materials for low temperature CO oxidation was correlated to the nature of the dopant. As a function of dopant concentration, the temperature corresponding to the 50% CO conversion (T50) demonstrated significant differences. The maximal activity was achieved for 15% Fe doped Co3O4 with T50 of –85 °C and remained almost constant up to 25% Fe. In the case of Cr, the activity was observed to be maximum for 7% of Cr with T50 of −42 °C and significantly decreased for higher Cr loadings. Similarly, there was a contrasting behavior in catalyst stability too. 100% CO conversion was achieved below −60 °C for 15% Fe/Co3O4 catalyst and remained unchanged even after calcination at 600 °C. In contrast, Co3O4 or 15% Cr/Co3O4 catalysts strongly deactivated after the same treatment. These differences were correlated to the oxidation states,

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coordination numbers, the nature of surface planes and the redox properties. We observed that both Cr and Fe were typically present in +3 oxidation state, occupying octahedral sites in the spinel structure. The catalysts mainly exposed to (111) and (220) planes on the surface. H2-TPR indicated clear differences in the redox activity of materials due to Fe and Cr substitutions. The reducibility of surface Co3+ species remained similar in all Fe doped Co3O4 catalysts in contrast to non-reducible Cr doped analogs, which shifted the reduction temperature to the higher values. As the Fe3+/Fe2+ redox couple partly substituted the Co3+/Co2+ redox couple in the spinel structure, similar bond strength of Fe−O keep redox activity of Co3+ species almost unchanged leading to higher activity and stability of Fe/Co3O4 catalysts for low temperature CO oxidation. In contrast, non-reducible Cr3+ species characterized by strong Cr−O bond substituting active Co3+ sites can make the Cr/Co3O4 surface less active for CO oxidation.

*Corresponding authors: 1. Email: [email protected] Present address: Department of Mechanical and Aerospace Engineering, University of California, 9500 Gilman Drive, San Diego, CA 92093, United States of America 2. E-mail: [email protected] Present address: Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan 3. Email: [email protected] Surface Engineering Division, CSIR−National Aerospace Laboratories, Bengaluru 560017, India

1. Introduction Removal of CO, mainly by oxidation at ambient temperature is useful for in-door air quality control, purification of H2 for fuel cells, pure N2 and O2 production from air in the semiconductor industry, gas sensing of CO traces etc.1−4 The extensive studies of CO oxidation can also be


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explained by the fact that this reaction is structure sensitive and rather simple for mechanistic model studies. The pioneering works by Haruta and coworkers have demonstrated high activity of supported gold nanoparticles for CO oxidation.5−7 However, gold-based catalysts are very costly, and thus, cheaper alternatives with high activity would be highly desirable. Considerable efforts have been concentrated on the development of CO oxidation catalyst based on transition metal oxides such as Co3O4, NiO and a mixture of MnO2 and CuO (called Hopcalite) has been found active at room temperature for CO oxidation in presence of moisture.8–16 Among transition metal oxides, Co3O4 has received the highest interest due to its low-temperature catalytic activity under dry conditions (below 10 ppm H2O).17–20 Co3O4 prepared by different methods have shown different CO oxidation activities due to various factors like surface area, crystal surface morphology and surface structure etc.18, 21−26 Xu and coworkers have prepared Co3O4 nanosheets by an in-situ de-alloying method combined with an oxidation process and the resulting catalyst has demonstrated a full CO conversion at around 110 °C.15 Through a nanocasting process, Tüysüz et al. have synthesized mesoporous Co3O4 that have shown excellent CO oxidation activity at ambient temperature.16 Teng and coworkers have demonstrated that the nature of surface planes, which depends on crystal morphology of shaped Co3O4 nanocrystals, determine the activity for CO oxidation.23 Interestingly, Co3O4 nanorods with predominantly exposed (110) surfaces have exhibited very high activity for CO oxidation at −77 °C under normal conditions.18 The energetics calculation for CO activation on different faces of Co3O4 crystals has established the reactivity order as following: (110) > (100) > (111).25 The variation in reactivity have been correlated to the concentration of Co3+ in those planes. This has promoted the idea of preparation of active catalyst through morphology-controlled synthesis, which ensures exposure of specific



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active planes at the surface. In most cases, however, it is difficult to prepare solid catalysts that have such preferentially exposed active faces, and even if specific morphology is obtained, it may not be stable under reaction conditions or may change to a thermodynamically stable phase. To enhance the stability of active but unstable Co3O4 surfaces, addition of dopants can be beneficial. Although several metal doped Co3O4 catalysts have been employed to achieve improved catalytic activities, stability issue is yet to be pursued seriously. It is obvious that choice of dopant metal play an important role in determining activity and stability of the catalysts. As for example, while Fe doping has improved CO oxidation activity over Co3O4, it has been reduced by Zn and Al substitutions, mainly because of strong covalent nature of Al−O bond in AlO4 motif and enhanced basic properties of Zn passivate the surface due to formation of carbonate species.27−29 The catalytic promotion by indium and bismuth dopants have been reported ut the stability even at moderate temperatures is doubtful as found in our recent results by Bi doping (unpublished).30,31 As the redox characteristics of both Fe and Co are switchable between +3 and +2 oxidation states, Fe is a promising dopant for Co3O4. Therefore, a better understanding of the relation between catalytic property and dopant’s nature can be established if detailed study is carried out on non-redox Cr doped Co3O4 catalysts and compared with the Fe doped analogs. Moreover, Fe and Cr are known to form less stable carbonates, which can minimize the factor of surface passivity. All the Co3O4 based catalysts reported in the literature have been prepared by mild condition followed by calcination at low temperature, mostly at 350 oC. Therefore, it would be interesting to study the activity of the doped Co3O4 catalysts prepared by single-step solution combustion method. Moreover, solution combustion method is known to give highly


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homogeneous solid solutions and may produce active surface due to formation of nano-particles in a non-equilibrium process.32,33 In the present study, two series of Co3O4 oxides doped with Fe and Cr have been prepared by single step solution combustion method. A systematic study of CO oxidation activity as a function of dopant concentration and detailed structural and morphological analysis of the catalysts has been carried out. The catalytic activities have been correlated with the dopant nature related to its intrinsic redox activity.

2. Experimental details Fe doped Co3O4 catalysts were prepared by solution combustion method using cobalt nitrate, ferric nitrate as precursors of Co and Fe, respectively and citric acid as a fuel. For the preparation of 10% Fe/Co3O4, 10 g of Co(NO3)2.6H2O (Acros Organics, 99%), 1.542 g of Fe(NO3)3.9H2O (Sigma-Aldrich, 99.5%) and 6.68 g of citric acid (Macron, 99%) were taken in the molar ratio of 0.9:0.1:0.883. The precursor compounds were dissolved in 30 mL of water in a 300 mL crystallizing dish resulting in a clear solution. The dish was then kept in a furnace preheated to 400 °C. The evaporation led to the dehydration and then the combustion with a flame started yielding the voluminous solid product within a few minutes. Then, the solid product was grinded to a fine powder and calcined in air at 400 oC for 3 h to remove the residual carbon species. These catalysts were named as-prepared/uncalcined samples. Following similar procedure various compositions with different concentration of Fe ranging from 3 at.% to 25 at.% were also prepared. For the preparation of Cr doped Co3O4 catalysts, stoichiometric amounts of Co(NO3)2.6H2O, Cr(NO3)2.6H2O (Alfa Aesar, 98.5%) and citric acid were taken to get Cr concentrations varied from 1 at.% to 33 at.% and similar procedure was followed. For shorter and easier nomenclature, amount of doping (in %) was mentioned in the left side of dopant’s



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atomic symbol. For example: 10 at.% Fe/Co3O4 and 10 at.% Cr/Co3O4 are names as 10Fe and 10Cr, respectively. Nitrogen physisorption isotherms (adsorption–desorption branches) were measured on Quantachrome Autosorb Automated Gas Sorption System (Quantachrome Instruments) at 77 K. The samples were outgassed for 1 h under vacuum at 400 oC before the measurements and the specific surface area (SSA) was determined using the Brunauer Emmett Teller (BET) method. X-ray diffraction (XRD) patterns of the catalysts were obtained with an X-ray diffractometer (BRUKER D2 PHASER) equipped with a monochromator for CuKα radiation at a voltage of 30 kV and a current of 100 mA. Fine powder samples were used for the XRD measurements and were scanned from 2θ = 10 to 70° at the rate of 0.02° s−1. The observed diffraction patterns were identified using the International Centre for Diffraction Data (ICDD) database. Temperature-programmed reduction (TPR) studies of the catalysts were performed with a Quantachrome Instrument (ChemBET-3000 TPR/TPD) to investigate their reduction behaviors. Typically, 25 mg of the sample was placed in a U-shaped quartz tube and ramped from 40 to 700 °C at a 10 °C min−1 rate in a gas mixture containing 3% H2/N2. The consumption of H2 during the reduction was monitored by a thermal conductivity detector (TCD). Prior to a TPR test, the sample was pretreated in oxygen gas flow at 400 °C for 15 min and then cooled down to room temperature in N2 flow. CO2-temperature programmed desorption (TPD) was conducted using a home built test set-up. Prior to the measurements, the samples were pelletized, crushed and sieved to the size fraction of 100–200 micron. Required amounts of sample were loaded into a tubular quartz reactor and sandwiched between two quartz wool plugs. A tubular furnace was used to provide


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heating. A K-type thermocouple centered inside the catalyst bed was used to record the temperature. Prior to CO2 adsorption the catalyst was heated to 400 °C in N2 and cooled down to room temperature (< 28 °C). This pretreatment was followed by 30 min saturation in 10 vol% CO2 at room temperature before rinsing any excess CO2 during 60 min of inert gas stream. The desorption experiments were conducted by ramping the temperature to 400 °C (5 °C min−1) and simultaneously recording CO2 concentration using a transmission infrared spectrometer (NICOLET ANTARIS IGS Analyzer, Thermo Scientific). Total flow rates were kept constant at 15000 mL h−1 g−1 for pretreatment as well as TPD. For transmission electron microscopy (TEM), the material was dispersed in ethanol and few drops were deposited onto a perforated carbon foil supported on a copper grid. TEM investigations were performed on a CM30ST microscope (FEI; LaB6 cathode, operated at 300 kV, point resolution ∼2 Å). The lattice fringe images have been analyzed using software Gatan Digital Micrograph v. 2.31.734.0, where the d-spacing carefully have been calculated after regenerating the image from selected region (marked red square) with help of FFT (Fast Fourier Transform) and selected spots encircled in inset corresponding image. XPS of Fe and Cr doped Co3O4 solid solutions were recorded using a Thermo Scientific Multilab 2000 spectrometer with non-monochromatic Mg Kα radiation (1253.6 eV) and Al Kα radiation (1486.6 eV) as X-ray sources operated at 150 W (12.5 kV and 12 mA). As Fe LMM and Co LMM peaks overlap with Co2p and Fe2p core level peaks, respectively XPS of Fe/Co3O4 catalysts were carried out with Mg Kα radiation. All the spectra were obtained with a pass energy of 40 eV and a step increase of 0.05 eV. The PeakFit v4.11 program was employed for the curvefitting of the Fe2p, Co2p, Cr2p and O1s core level spectra into several components with



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Gaussian peaks after linear background subtraction. Peak positions, spin–orbit splitting, doublet intensity ratios, and full width at half maximum (FWHM) were fixed as given in the literature. X-ray absorption spectra at Cr and Fe K-edges were measured at SuperXAS beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland) using the Si(111) channel cut monochromator. The rejection of higher harmonics and focusing were achieved by a Sicoated collimating mirror at 2.8 mrad and a rhodium-coated toroidal mirror at 2.8 mrad, respectively. The size of the X-ray beam on the sample was 1.5 mm by 0.5 mm in horizontal and vertical directions, respectively, with a total intensity of about 5 × 1011 ph/s. The optimal amount of each sample was mixed with cellulose and pressed into pellets. The X-ray absorption spectra were measured in transmission mode using ionization chambers as the detectors. Reference foils of Cr (K edge at 5989 eV) and Fe (K edge at 7112 eV) were measured simultaneously with the samples for precise energy calibration. The spectra were reduced following the standard procedures in the Demeter program package.34 Extended X-ray absorption fine structure (EXAFS) spectra at Cr K-edge were fitted in R-space between 1 and 5 Å using k weighting of 3 in the range between 3−12 Å−1. EXAFS spectra at Fe K-edge were fitted in R-space between 1 and 5 Å using k weighting of 3 in the range between 3−10 Å−1. Amplitude reduction factor (S02) was fitted using Fe and Cr reference foils. Catalytic activity was tested through CO oxidation reaction. The catalyst (0.15 g) was placed in a fixed bed reactor and 1 vol% CO in air was flowed (50 mL min−1). H2O concentration in the flow gas was monitored by a dew-point meter, and the concentrations were 15−50 ppm at T50 (temperature for 50% CO conversion) in all experiments. T50 or T100 indicate a temperature for 50% or 100% CO conversion to compare the catalytic activity of the samples.


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3. Results and discussion The catalytic CO conversion curves as a function of temperature for CO + 1/2O2 → CO2 reaction over Fe and Cr doped Co3O4 catalysts are shown in Fig. 1 (a,b) keeping pure Co3O4 as a reference. The results indicate a systematic change of activity with gradually increasing concentration of Fe and Cr in Co3O4. Pure Co3O4 demonstrates the poorest activity among all the catalysts, but it is significantly improved due to doping of Fe and Cr. By comparing CO conversion in the equivalent compositions of both catalysts series at any temperature, it becomes evident that Fe doped Co3O4 is more active than its Cr doped analogs (see Figs. 1 a, b). The activity of Fe/Co3O4 catalysts increases with an increase in Fe content up to 15% and then it remains almost constant up to 25% Fe content, especially for (100) > (111) correlating to the concentration of Co3+ ions and low-coordinated oxygen species.25 However, in the present study, (111) surface in Fe doped Co3O4 catalyst shows high catalytic activity similar to other catalysts prepared by other ambient synthesis method containing more active (100) or (110) surfaces.18,19 Similarly, the TEM analysis of 7% Cr/Co3O4 crystallites shows (220) and (111) as major crystal planes (Fig. S3a, a′). Detailed XPS studies of Fe and Cr doped Co3O4 catalysts can provide very useful information on the elemental oxidation states of the catalysts. Survey spectra of all the catalysts show the presence core level and Auger peaks of respective elements, as shown for 7% Fe/Co3O4 and 5% Cr/Co3O4 catalysts in Fig. S4. XPS of Fe2p core level region in Fe doped Co3O4 catalysts with different Fe concentrations are presented in Fig. 8. Fe2p core level spectra are found to be broad which indicates that Fe is in multiple oxidation states. All the Fe2p core level spectra are curve-fitted into several component peaks. In Fig. 9, typical curve-fitted Fe2p core



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level spectra of 7% and 20% Fe/Co3O4 catalysts are displayed. Fe2p3/2 peaks observed at 709.8 and 711.3 eV correspond to Fe2+ and Fe3+ species, respectively present in the Fe/Co3O4 catalyst.39−41 Fe2+ and Fe3+ species in Fe2p core level spectra are distinguished by their respective satellite peak positions. For Fe2+ species, intense satellite peak is situated around 4.5−5.5 eV above the main peak and a weak satellite peak related to Fe3+ species is observed around 7−9 eV above the main peak.42 In Fig. 9, observation of both satellite peaks confirms that Fe2+ and Fe3+ are present in the catalyst. It has been evaluated from XPS that Fe/Co3O4 catalysts contain mainly Fe3+ species along with certain amount of Fe2+ species. Surface concentrations of Fe2+ species vary from 24 to 26% in all the catalysts. This results correlate well with Fe K-edge XANES data demonstrating mainly presence of Fe3+ in all doped samples. Co2p core level spectra of these catalysts show that Co is present in both Co2+ and Co3+ states. The relative concentrations of Co3+ in the Fe/Co3O4 catalysts are found to be similar for all compositions (57 to 62%) as presented in Table S1. Similar ratios of Co3+/Co2+ in the Fe doped Co3O4 samples explain constant high catalytic activity at higher Fe concentrations. XPS of Cr2p core levels in Cr doped Co3O4 catalysts with different Cr concentrations are presented in Fig. 10. Cr2p core level spectra are observed to have a long tail in the higher binding energy region indicating that different Cr species are present in the catalysts which are resolved by curve-fitting. In Fig. 11, typical curve-fitted Cr2p core level spectra of 3% and 15% Cr/Co3O4 catalysts are shown. Observed Cr2p3/2,1/2 peaks around 576.2 and 585.9 eV with 9.7 eV spin-orbit separation correspond to Cr3+ species.43,44 Weak component peaks at 578.6 and 588.3 eV with 9.7 eV spin-orbit separation can be attributed to either Cr3+ species in Cr(OH)3 or Cr6+ species.45,46 Possibility of formation of hydroxide species may be more as component peak related to hydroxide species is observed in O1s core level spectra (discussed later). It has been


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noticed that surface concentrations of hydroxide/Cr6+ is observed to be in 10−23% in these catalysts. Cr6+ content is less in the catalysts with lower Cr concentration whereas it is slightly more in higher Cr concentrations. However, it is evident from XPS studies that Cr3+ species are the major species in all the catalyst. Spectral envelopes of Co2p core level spectra of Cr doped Co3O4 catalysts are broad which indicates that Co is present in different oxidation states. All the Co2p spectra are curve-fitted into sets of spin-orbit doublets along with associated satellite peaks. Accordingly, Co2p3/2,1/2 peaks at 779.6 and 794.4 eV with spin-orbit separation of 14.8 eV correspond to Co3+, whereas peaks at 781.3 and 796.8 eV with 15.5 eV spin-orbit separation are attributed to Co2+ species.47−50 Binding energy peaks related to oxidized Co2p3/2,1/2 species are located at 780.5 ± 1.5 and 796.0 ± 1.5 eV, respectively. Previous studies have reported that differences in the 2p peak positions between Co2+ and Co3+ vary from 0.1 to 1.5 eV making it difficult to use the primary 2p peaks for distinguishing between Co2+ and Co3+. Similar to Fe2p core level spectra these two oxides can be distinguished, however, by the differences in the satellite features. The 2p spectrum of Co2+ contains prominent satellite peaks 4−6 eV higher in energy than the primary peaks. On the other hand, Co3+ species is recognized by the presence of a weak satellite peak at 9−10 eV higher binding energy than the main peak. In the Cr/Co3O4 catalysts, the Co2p3/2 peak around 781.3 eV along with the satellite peak at 784.4 eV are attributed to Co2+ species, whereas the 2p3/2 peak around 779.6 eV with the satellite peak at 789.7 eV are ascribed for Co3+ species. These satellite features arise from shake-up phenomenon in which the excitation of unpaired valence electrons increases the number of relaxed final states. It has been observed that Co2+ concentration increases with the increase in Cr concentration in Co3O4 matrix. This is because of the substitution with Cr3+ species into Co3+ sites of the Co3O4 substrate. Changes in Co2+ and Co3+ concentrations in Cr/Co3O4 catalysts evaluated from XPS



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analysis are summarized in Table S1. Typical curve-fitted Co2p spectra of 5% Cr/Co3O4 and 33% Cr/Co3O4 catalysts are given in Fig. S5. The decreasing trend of Co3+/Co2+ ratio in the Cr/Co3O4 catalysts goes in parallel with decreasing trend of activity for CO oxidation. O1s core level spectra of Fe and Cr doped Co3O4 catalysts have also been recorded. Spectral characteristics of O1s core levels are broad suggesting that several oxygen component species are present in the catalysts. Spectra are decomposed into different peaks by curve-fitting. Typical curve-fitted O1s core level spectra of 10% Fe and 10% Cr doped Co3O4 catalysts are presented the Fig. S6. Component peak at 530.0 eV is assigned for O2- species and peaks around 531.6 stands for hydroxyl species.51 Weak peak with binding energy above 533.0 eV is related to adsorbed water species present in the catalysts. However, in all the catalysts, oxide species is found to be the major species in comparison with other two species. The reducibility of Fe and Cr doped Co3O4 catalysts determine the nature of catalytic activity in these materials. Fig. 12(a,b) presents H2-TPR profiles of Fe and Cr doped Co3O4 samples with reference to pure Co3O4. The reduction may occur in two consecutive steps: Co3+ (Co3O4) → Co2+ (CoO) and CoO → Co (metal). However, since redox reaction involving Co3+ ↔ Co2+ couple on the surface determines the catalytic activity, hydrogen uptake corresponding to the lowest temperature region below 200 oC is discussed here. The huge signal above 250 oC is related to bulk reduction and has hardly any role in catalytic activity. As can be seen in the Fig. 12a that reduction starts at 90 °C in pure Co3O4 and it gets shifted to 140 °C in all the Fe doped analogs up to 25% substitution, indicating similar nature of the reducing species. Previous study has shown that Fe3+ species in Fe3O4 gets reduced only above 200 °C despite having similar structure to Co3O4 and therefore, reduction is not related to low coordinated surface


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species alone.52 This indicates that low temperature peak occurs due to the reduction of Co3+ species in the Fe doped oxides. The preferential reduction of Co3+ in comparison with Fe3+ species can probably be related to higher reduction potential of the former ion than the later. Since Fe is relatively less reducible, the low temperature peak intensity is expected to decrease at higher Fe concentrations, rather the constant peak height can be related to higher surface area accessing more Co3+ species. On the other hand, in Cr/Co3O4 catalysts, the low temperature reduction peak is consistently shifted to higher temperature with increasing Cr concentrations, i.e from 105 °C in pure Co3O4 to 215 °C in 25% Cr doped sample. As Cr3+ is non-redox in nature, the presence of Cr3+ reduces the surface concentration of Co3+ and also makes it Co3+ less reducible, leading to lower activities of Cr doped catalysts. Now, the difference in catalytic activities due to different dopants in Co3O4 can be correlated to the reducibility discussed above. As Cr3+ is non-reducible in nature, it hinders the removal of oxygen sharing with Co3+ through Cr3+−O−Co3+ linkage. On the other hand, reducible Fe3+ can share the charge created due to oxygen removal from the adjacent site and therefore, it cannot modify Co3+ species. Although, reduction with H2 occurs at higher temperatures as compared to CO oxidation, H2-TPR obviously indicates the trend in reducibility of surface species which can be correlated to the trend in the CO oxidations in Fe and Cr doped Co3O4. There may be two reasons for this difference. The H2/N2 mixture used in the H2 TPR must be containing more moisture as compared with dry condition of CO oxidation, shifting reduction of Co3+ in the sensitive surface of Co3O4 based catalysts. Moreover, CO is easily adsorbed on transition metal ion like Co3+ due to synergic Pi-bond formation as compared with H2 adsorption through σ bond donation. The basic character of the catalyst surface generally affects the catalytic activities due to formation of carbonate species preventing CO adsorption. Fig. 13 (a,b) shows temperature



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programmed desorption (TPD) profiles using acidic CO2 as adsorbent for Fe and Cr doped Co3O4 samples. The intensity of corresponding CO2 desorption peak indicates that larger amount of CO2 has been adsorbed on Fe doped Co3O4 as compared to the Cr doped analogs. Interestingly, while concentration of desorbed CO2 decreases with increasing Cr concentration, the opposite trend is observed with the Fe substitutions. Moreover, CO2 desorption maxima in the Fe doped samples occur at higher temperature above ∼100 oC as compared to Cr doped samples having CO2 desorption peak at 65 oC. This means that Fe doped Co3O4 samples form more stable carbonate species as compared with Cr doped analogs. Moreover, the observations clearly indicate that Fe center creates basic sites and gets easily blocked with adsorbed CO2. The increasing CO oxidation activity with Fe concentrations in Fe/Co3O4 samples despite increasing CO2 coverage indicate that Fe site does not take part in CO adsorption leading to oxidation. On the other hand, decrease in CO oxidation activity with increasing Cr doping indicates that Cr is not involved in CO oxidation. Further, CO2 desorption decreases with decreasing Co3+ concentrations in Cr doped catalysts (see XPS section, Table S1). Therefore, it can be concluded that Co3+ acts as active site for CO adsorption as well as CO oxidation reaction in presence of both type of dopant cations.

4. Conclusions Homogeneous Fe and Cr doped Co3O4 solid solutions with spinel structure have been prepared by single-step solution combustion method. The catalytic CO oxidation activity (T50) of Co3O4 behaved differently with concentration of the dopants. The Fe/Co3O4 catalysts have achieved maximum activity on 15% Fe/Co3O4 with T50 at −85 °C and activity remains similar up to 25% Fe substitution. On the contrary, incorporation of Cr3+ decreases the catalytic activity gradually


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up to 33% substitution, after passing through the maximum activity for 7% Cr with T50 at −42 °C. The stability of Fe doped Co3O4 catalyst is superior as compared with pure Co3O4 or Cr/Co3O4 catalysts. Detailed characterizations have indicated that factors like oxidation state and local coordination of the dopants and major surface planes in the doped catalysts are almost similar in both the catalysts. Therefore, the difference in catalytic activity and stability have been correlated with the change in the Co3+−O bonding influenced by the intrinsic redox nature of the dopants. While temperature of surface Co3+ reduction remained unaffected by the presence of Fe, in contrary, the same increases with increasing Cr concentration in Co3O4. This means that Co3+−O bond remains almost unaffected by the adjacent Fe3+−O bonds, possibly due to similar bond strength. On the other hand, stronger Cr−O bond makes adjacent Co3+−O bond stronger due to the non-redox nature of Cr3+ ion. Therefore, depending on intrinsic redox property of the dopants, catalytic activity in Co3O4 can be tuned accordingly. These materials may find interesting applications in various selective oxidations due to availability of reactive oxygen species at low temperatures. Since moisture affects the catalytic sites, further study is required in this direction.

Supporting Information Table S1. Crystallite sizes calculated using Scherrer formula, BET surface area and Surface concentrations of Co3+ in Fe/Co3O4 and Cr/Co3O4 catalysts; Table S2. Local coordination of Cr and Fe in Co3O4 structure analyzed by Cr K-edge and Fe K-edge EXAFS; Figure S1. Elemental maps of Co (Green) and Fe (Red) obtained by TEM analysis in 15%Fe/Co3O4; Figure S2. XRD patterns of as-prepared and calcined 15% Fe/Co3O4 catalysts; Figure S3. TEM images of as-



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prepared 7%Cr/Co3O4 in different locations; Figure S4. Survey spectra of (a) 7% Fe/Co3O4 (MgKα radiation) and (b) 5% Cr/Co3O4 catalysts (AlKα radiation); Figure S5. Curve-fitted XPS of Co2p core levels of (a) 3% Cr/Co3O4 and (b) 33% Cr/Co3O4 catalysts; Figure S6. Curve-fitted XPS of O1s core levels of (a) 10% Fe/Co3O4 and (b) 10% Cr/Co3O4 catalysts.

Acknowledgement We thank Dr. Frank Krumeich for the electron microscopy study, which has been performed at EMEZ (Electron Microscopy ETH Zurich).

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Figures

100

100 (b)

(a) 80

80 Conversion/%

Conversion/%

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60

40

20

0Fe0Fe 7Fe7Fe 15Fe 15Fe 25Fe 25Fe

0 -100 -80 -60 -40 -20 0 20 Temperature/ oC

40 60

60

40 0Cr 0Cr 3Cr 3Cr 7Cr 7Cr 15Cr15Cr 33Cr33Cr

20

0 -80

-60 -40 -20 0 20 o Temperature/ C

40

60

Figure 1. CO conversion as a function of Temperature over (a) Fe/Co3O4 and (b) Cr/Co3O4 catalysts with various compositions.


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-100

-80 M= Fe M= Cr

-60 o

T50 / C

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-40

-20

0 0

5

10

15 20 25 x% M Co3O4

30

35

40

Figure 2. T50 for CO conversion vs Concentration of Fe and Cr dopants in Co3O4 lattice.



100

100

(b)

(a)

15Cr uncalcined un

80 Conversion/%

80 Conversion/%

60

40

20

15Fe uncalcined

15Cr 500 5 oC 15Cr 600 6 oC

60

40

20

15Fe 500oC o

15Fe 600 C

0 -100

-50

0 50 Temperature/ oC

0 -100

100

-50


100

100 (c) Co3Co O4 uncalcined 3O4 fresh

80

o

o

Co3O Co 500 C 4 500 3O4 C o

Co3O C o Co 4 600 3O4 600 C

Conversion/%

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60

40

20

0 -100

-50


100

Figure 3. CO conversion as a function of Temperature over (a) 15%Fe/Co3O4, (b) 15%Cr/Co3O4 and (c) pure Co3O4 in fresh and calcined at 500 and 600 oC.


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Figure 4. XRD patterns of (a) Fe/Co3O4 catalysts: (i) Co3O4, (ii) 3Fe, (iii) 10Fe (iv) 15Fe, (v) 20Fe and (vi) 25Fe; (b) Cr/Co3O4 catalysts: (i) 7Cr, (ii) 10Cr, (iii) 15Cr, (iv) 20Cr and (v) 33Cr.



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Figure 5. (a) Cr K-edge XANES of Cr doped Co3O4 containing 3, 7 and 10% of Cr in comparison with the Cr foil ; (b) Fe K-edge XANES of Fe doped Co3O4 containing 3, 15 and 25% of Fe in comparison to the Fe foil.


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Figure 6. (a) Fit of the Cr K-edge EXAFS spectra between 1 and 5 Å for Cr doped Co3O4 containing 3, 7 and 10 % of Cr ; (b) Fit of the Fe K-edge EXAFS spectra between 1 and 5 Å for Fe doped Co3O4 containing 3, 10, 15 and 25% of Fe.



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Figure 7. TEM images of as-prepared 15% Fe/Co3O4 in fresh (a,a′) and calcined (b, b′) catalysts.


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Figure 8. XPS of Fe2p core levels in Fe/Co3O4 catalysts: (a) 3Fe, (b) 7Fe, (c) 10Fe, (d) 15Fe, (e) 20Fe and (f) 25Fe.



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Figure 9. Curve-fitted XPS of Fe2p core levels in (a) 7% and (b) 20% Fe/Co3O4 catalysts.


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Figure 10. XPS of Cr2p core levels in Cr/Co3O4 catalysts: (a) 3Cr, (b) 5Cr, (c) 7Cr, (d) 10 Cr, (e) 15Cr and (f) 33Cr.



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Figure 11. Curve-fitted XPS of Cr2p core levels in (a) 3% and (b) 15% Cr/Co3O4 catalysts.


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Figure 12. H2-TPR profiles for (a) Fe/Co3O4 and (b) Cr/Co3O4 catalysts.



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Figure 13. CO2-TPD profiles for (a) Cr/Co3O4 and (b) Fe/Co3O4 catalysts.


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