Phase & Morphology Engineered Surface Reducibility of MnO 2 Nano-heterostructures: Implications on Catalytic Activity towards CO Oxidation
An understanding of the surface reducibility of a catalytically active oxide support is a prerequisite to understanding its catalytic behavior. In this work, we report that through a stringent control over the phase and morphology of catalytically active MnO2 supports, a control over the surface reducibility can be achieved. Through temperature-programmed reduction (TPR), we prove a higher availability of lattice oxygen for the α-MnO2 phase, compared to that of β-MnO2. Furthermore, by modifying the synthesis method, we could engineer the morphology of the αphase into nanoflowers which in turn leads to a higher surface area, further enhancing the activity. On the engineered support, decoration of Pt nanoparticles can lower the full conversion temperature for CO oxidation at a considerably low temperature.
Introduction<!>Materials<!>Synthesis of α-and β-MnO 2 nanorods<!>Synthesis of α-MnO 2 nanoflowers<!>Nucleation of ultrafine Pt nanoparticles<!>CO oxidation studies<!>Crystallographic and Morphological Characterization<!>X-ray Photoelectron Spectroscopy (XPS)<!>CO-and H 2 -Temperature Programmed Reduction (TPR)<!>α-MnO 2 nanoflowers<!>Surface Area Measurements<!>Pt-decorated MnO 2 supports<!>CO oxidation studies and stability<!>Conclusions
<p>Among heterogeneous catalytic reactions, low-temperature CO oxidation is a critical reaction to overcome the problems of automotive emissions 1 and impure polymer electrolyte membrane fuel cell (PEMFC) feed. 2 Bare as well as supported 3 metal oxides have been extensively investigated in an effort to uncover mechanisms and develop more active catalysts that clearly show the importance of an 'active' support 4 for enhanced catalytic activity. The activity of these active supports is driven by oxygen adsorption 5 onto the sites of oxygen vacancies. 6 Rare-earth metal oxides such as CeO2 have been reported 7 to be highly active supports due to their reducibility, the drawback being that they are not earth-abundant. In this regard, supported 8 and unsupported 9 transition metal oxides (TMO) offer immense potential for catalysis. 10 Along with the presence of oxygen vacancies, they offer a high thermal stability 11 and low cost. The choice of TMO is primarily based on the extent of redox behavior shown by them i.e. the ability of the transition metal to exist in different oxidation states. 12 Among various TMOs, earth-abundant manganese oxide (MnO2) stands out as a possible support due to its interesting redox capability. 13 With Mn existing in multiple oxidation states, the Mn-oxides exhibit a great structural versatility. 14 This property makes them promising candidates for redox catalysis 15 by enhanced interaction with the oxidizing and reducing agents via the involvement of lattice oxygen. 16 It has been discussed in previous reports 17 that by altering the preparation methods, a control on the morphology, phase and crystal orientation of the Mn-oxides can be obtained. MnO2 has been synthesized in many crystal phases 18 , such as α-, β-, δ-and γ. These polymorphs of MnO2 primarily differ in the way the network of MnO6 octahedra 18 are linked in the structure. 19 Structurally, the β-MnO2 exists as a 1 x 1 (2.3 x 2.3 Å) tunnel structure, while the α-MnO2 possesses a 2 x 2 (4.6 x4.6 Å) tunnel structure. 20 These two catalytically important polymorphs, α-MnO2 and β-MnO2, have been reported to show a considerable difference in activities as electrode 17b, 21 for Li/MnO2 batteries and in the catalytic oxidation 22 of volatile organic compounds (VOCs). While there are a few reports 23 on the gas adsorption and CO oxidation mechanism using  and  phases of MnO2, 24 the fundamental reasons for the difference in their catalytic activity has not been discussed in detail. In another report, the electronic properties of -MnO2 has been addressed through simulations 25 .</p><p>In this work, we have chosen one-dimensional nanostructures of − and −MnO2 as possible catalysts for CO oxidation in an effort to understand the underlying differences in their catalytic behavior in detail. The chosen and -MnO2 nanorods have a tetragonal crystal structure and are oriented along the anisotropic c-axis. We have studied the change in the surface properties with respect to change in the phase of MnO2. X-ray photoelectron spectroscopy (XPS) reveal a significant difference in surface reducibility between the and phases of MnO2. Although XRD patterns reveal phase-pure MnO2 structure in both the cases, XPS shows a large signal (62 %) for Mn +3 in Mn 2p spectrum of -MnO2. On the other hand, there is a stark absence of any Mn +3 signal in case of β-MnO2. This suggests that the surface reducibility of the two phases is significantly different in the two phases. For an in-depth understanding, temperature programmed reduction (TPR) involving H2 and CO was performed, where we observed that α-MnO2 starts reducing at a temperature ̴ 60 °C lower than β-MnO2, confirming the higher surface reducibility of the α-phase. Furthermore, to explore the aspect of surface area, we modified the synthesis procedure to obtain a larger surface area variant of α-MnO2, viz. nanoflowers -leading to a 2-fold increase in the surface area of the material.</p><p>Motivated by the difference in trends of fundamental properties exhibited by the three systems, gas phase CO oxidation was carried out over these surfaces. The full conversion temperature (T100) of CO to CO2 was 240 o C and 220 o C over β-and α-MnO2 nanorods, respectively. The T100 further lowered down to 210 o C for α-MnO2 nanoflowers, making it the best catalyst among all three as anticipated from the microstructural and electronic aspects. A further possibility to improve the catalytic activity is by loading noble metal nanoparticles on the support. 26,26b To prepare a commercially viable catalyst for CO oxidation, ultrafine Pt nanoparticles were nucleated on the surface of MnO2 using a scalable microwave technique 27 . CO oxidation was performed over the catalysts and it was observed that T100 in the light-off curves of Pt-decorated supports α-MnO2 (NR), β-MnO2 (NR) and α-MnO2 (NF) was 110, 135, and 80 o C, respectively. A comparison of the apparent activation energies (Ea) of CO oxidation over the catalysts reveal the trend: Ea(Pt on -MnO2-flower) < Ea(Pt on -MnO2-rod) < Ea(Pt on -MnO2-rod). Based on our results, we propose a mechanism to engineer the catalytic activity using phase and morphology as control handles.</p><!><p>Reagent grade Manganese sulfate monohydrate (MnSO4.H2O), Ammonium persulfate ((NH4)2S2O8) and Ammonium sulfate ((NH4)2SO4) were obtained from Sigma Aldrich, U.S.A. Analytical grade Ethylene glycol (EG) was obtained S.D. Fine Chemicals Ltd., India.</p><!><p>The αand β-MnO2 nanorods were synthesized by modifying an existing literature method 28 . For the preparation of β-MnO2, a clear solution was prepared by dissolving 0.5 mmol reagent grade Manganese sulfate monohydrate (MnSO4.H2O) in 20 mL distilled water. While vigorously stirring the solution, 0.3 mmol reagent grade Ammonium persulfate ((NH4)2S2O8) was added. The solution was transferred to a Teflon-lined steel autoclave and maintained at 150 o C for 12 h. To prepare the α-phase as product, 0.8 mmol of reagent grade Ammonium sulfate ((NH4)2SO4) was also included in the reaction. The hydrothermal reactions yielded black colored precipitates dispersed in a highly acidic (pH=2) environment. The precipitates were then washed with DI water and ethanol repeatedly till final pH of the dispersion was neutral. The catalysts were dried overnight at 70 o C in air to yield black powders. The black powders were calcined at 300 o C for 2 h to remove impurities and yield the final materials to be used as substrates for the nucleation of Pt nanoparticles. The αand β-MnO2 nanorods are designated as α-MnO2 (NR) and β-MnO2 (NR) in the rest of the text.</p><!><p>The precursors (as mentioned above) used for α-MnO2 synthesis were added to 20 mL distilled water and maintained at 150 o C for 30 min under hydrothermal conditions. The obtained black precipitate was subjected to cleaning and drying protocols in a similar fashion as mentioned above. The dried black powder was calcined at 300 o C for 2h. This will be used as a substrate and will further be modified by nucleating Pt nanoparticles on its surface. The α-MnO2 nanoflowers are designated as α-MnO2 (NF) in the rest of the text.</p><!><p>The various MnO2 samples prepared as described were used as substrates to load ultra-fine Pt nanoparticles. For this, 50 mg of the respective MnO2 powders were dispersed in 15 mL ethylene glycol (EG). The homogeneity of the dispersion was ensured by sonicating the mixture for 15 min. An appropriate amount of H2PtCl6 salt corresponding to 2 wt% of Pt was added and the mixture was sonicated further for 10 min. The solution was then subjected to microwave for 5 min (300 W, 100 o C) under vigorous stirring. The black precipitate obtained after the reaction was washed with DI water to remove the solvent residues and then subjected to air drying at 70 o C. The 2 wt% Pt-decorated as-prepared α-MnO2 (NR), β-MnO2 (NR) and α-MnO2 (NF) are designated as Pt/α-MnO2 (NR), Pt/β-MnO2 (NR) and Pt/α-MnO2 (NF). Characterization X-ray diffraction was used to get an insight on the phase purity and crystal structures of the catalysts. X-Ray Diffraction (XRD) patterns of the powder catalysts were collected using Rigaku X-ray diffractometer (Cu Kα (λ = 1.5418 Å) as source). Data were collected in a fixedtime mode with a 0.02 step size and a 2 s/point. The patterns were recorded from 10° to 90° in the 2θ range. The overview of morphology and size of the nanomaterials was obtained using a Zeiss Ultra 555 field-emission scanning electron microscope (FESEM) at an accelerating voltage of 3 kV. The samples were prepared by dispersing the catalyst powder in ethanol and sonicating it to form a homogenous suspension. The suspension was then drop-casted on a silicon wafer to make samples for imaging. TEM images and selected area electron diffraction (SAED) patterns were recorded using a FEI TecnaiT20 transmission electron microscope operating at an accelerating voltage of 200 kV. The high resolution TEM (HRTEM) images were recorded using a TecnaiF30 transmission electron microscope operating at an accelerating voltage of 300 kV. To prepare the samples for TEM imaging, a very dilute suspension of catalyst powders in ethanol was drop-casted on carbon grids. The grids were dried at 70 o C and kept under vacuum for 5 h before imaging. The elemental surface composition of the catalysts was analyzed by X-ray photoelectron spectroscopy (XPS) spectra collected on an AXIS ULTRA instrument with monochromated Al Kα (1486.6 eV) as the source. The binding energy was referenced against C 1s (284.0 eV) line. The sample for XPS analysis was prepared by pasting and pressing the powder on a carbon tape. The Brunauer-Emmett-Teller (BET) surface area of the catalysts was measured by the nitrogen gas adsorption-desorption method at 77 K using a Quantachrome surface area analyzer model Autosorb IQ. The samples were heated at 120 o C for 1 h in helium atmosphere prior to the surface area measurements.</p><p>H2 and CO-TPR (Temperature Programmed Reduction) was performed in a Quantachrome instrument. A 0.25 g of sample was packed between two glass wool plugs and placed in a quartz reactor inside a furnace. The sample was pre-heated at 120 o C for 1.5 h in nitrogen atmosphere. After the furnace cooled down to room temperature, 5% H2/N2 and 10% CO/N2 was passed over the sample for H2-TPR and CO-TPR, respectively. The temperature was then ramped at a rate of at 10 o C min -1 till 600 o C. H2 and CO consumption was determined by a TCD detector; with H2O being trapped in a dry ice cooled trap. The peaks obtained were analyzed and reported.</p><!><p>A packed bed reactor was employed to perform CO oxidation experiments over the catalysts. For each measurement, the catalyst powder (100 mg) was mixed with 100 mg silica gel and loaded into the center of the quartz reactor (i.d. 10 mm) along with a thermocouple. The catalyst bed length was kept constant as 3 mm for all the experiments. The quartz reactor is placed inside a furnace and the temperature of the furnace was controlled using a PID controller. A gas mixture of 2 vol % CO (Chemix Gases, Bangalore), 2 vol % O2 (Chemix gases, Bangalore), and the rest as N2 (Chemix Gases, Bangalore) was passed over the catalyst bed. The total flow rate was maintained constant at 100 mL min −1 for all the experiments. For each cycle, the reactor was heated from room temperature to the temperature of full conversion (T100) with a fixed stepwise heating rate. Gas injection and data acquisition was performed multiple times to ensure the stable signal for conversion at each temperature. The reactor was cooled down to room temperature after each cycle and the reaction was repeated. The gas hourly space velocity (GHSV) of 25500 h -1 was maintained as constant for all the experiments. The temperature (±2) of the furnace was controlled using PID controller. A sample loop placed inside the autosampler controlled and injected a fixed amount of feed gas mixture into the reactor each time the injection was made. The outlet gases from the reactor were analyzed using a gas chromatograph (Mayura Analytical, Bangalore) equipped with flame ionization detector (FID).</p><!><p>The crystallographic characterization of the as-prepared MnO2 was carried out by X-ray diffraction (XRD). The XRD pattern (Figure 1a), obtained from MnO2 synthesized hydrothermally in presence of ammonium persulfate (AP) and ammonium sulfate (AS) can be indexed to the tetragonal phase of α-MnO2 (JCPDS: 44-0141) with space group: I4/m and lattice constants a = 9.81 Å and c = 2.84 Å. Figure 1b shows the XRD pattern obtained from the MnO2 prepared without the addition of ammonium sulfate (AS). The peaks, in this case can be indexed to the tetragonal phase of β-MnO2 (JCPDS: 24-0735), with the space group P4/mnm and lattice constants a = 4.39 Å and c = 2.87 Å. Notably, absence of any other impurity peaks confirms the phase purity of the synthesized materials.</p><p>Scanning electron microscopy was carried out to investigate the morphology of the synthesized materials. The micrographs (Supporting Information, Figure S1) show a uniform distribution of the α-MnO2 and β-MnO2 with a nanorod (NR) morphology. Taking the phase and the morphology into consideration, the αand β-MnO2 samples are designated as α-MnO2 (NR) and β-MnO2 (NR), respectively in the rest of the text.</p><p>Low magnification bright-field (BF) TEM images of α-MnO2 NR and β-MnO2 NR are shown in Figure 1c and 1d, respectively. The selected area electron diffraction (SAED) patterns from single α-MnO2 (NR) (Figure 1e) with zone axis [1-10], and that of β-MnO2 (NR) (Figure 1f) oriented along the [001] zone axis shows the single crystalline nature of the nanorods.</p><p>Furthermore, the high-resolution transmission electron micrograph (HRTEM) of α-MnO2 (Supporting Information, Figure S2a) exhibits fringes corresponding to (110) lattice planes. The FFT pattern obtained from the image shows the [002] direction as the growth direction of the rod. Similarly, the growth axis of β-MnO2 nanorods (Supporting Information, Figure S2b) was also found to be [002]. Having a tetragonal crystal structure, the inherent anisotropy in <c>direction leads the growth of the rods in that direction. Considering the c-axis as the growth direction, the possible low index faceting surfaces that bound the rods can be the (100)/(110) planes. This microstructural information has important implications for catalytic activity of the hybrid as discussed in the following sections.</p><!><p>Detailed XPS of the as-prepared samples were carried out to probe the nature of the catalyst surface. The evaluation of the Mn 2p spectra obtained from XPS is used to explain the surface reducibility of these oxides. As shown in Figure 2b, the deconvoluted Mn 2p spectra in the case of β-MnO2 (NR) shows an asymmetric peak at a binding energy of 642.1 eV (Mn 2p3/2) indicating the presence of Mn 4+ . The binding energy shifts to a lower value (MnO2 > Mn2O3 > Mn3O4) with decreasing oxidation state of Mn in the oxide. In case of α-MnO2 NRs (Figure 2a), the de-convolution of Mn 2p spectra shows a clear presence of Mn 4+ and Mn 3+ at the binding energies 642.1 eV and 641.2 eV, respectively. This presence of Mn 3+ along with Mn 4+ in case of α-MnO2 (NR) suggests that the manganese species on the surface are partially reduced.</p><p>To account for the reducibility of manganese oxide, Mn 3s spectra should also be considered. The quantification of Mn 3s multiplet splitting (ΔEMn3s) offers detailed understanding of Mn chemical states. A splitting of 4.48 eV (Figure 2d) is shown in Mn 3s spectra of β-MnO2 which matches with the reported ΔEMn3s values for MnO2 (4.4-4.5 eV). It is discussed in previous reports 23 that the value of ΔEMn3s shifts towards higher binding energies when MnO2 is reduced to other oxides. The Mn 3s spectra of α-MnO2 NRs (Figure 2c) show ΔEMn3s values of 4.54 eV. This higher value of ΔEMn3s compared to β-MnO2 suggests that in case of α-MnO2, there are more reduced states at the surface. It is interesting to note that there are no extra peaks in the XRD pattern of α-MnO2 (Figure 1a), indicating that the higher oxidation state (+3 state) may not be due to the presence of Mn2O3 (up to the limits of detectability of XRD technique). However, the Mn 2p spectra for α-MnO2 (Figure 2a) show a distinct presence of Mn 3+ . These observations indicate that the Mn 3+ is possibly localized on the surface of -MnO2 without the formation of lower oxides.</p><p>The calculations of relative surface concentrations of Mn 3+ and Mn 4+ are explained in the Supporting Information. The quantification of the Mn 4+ :Mn 3+ ratio is an important a priori information for the catalytic activity of the material. As can be seen from the pie charts shown in Figure 2, the -MnO2 NRs have a significant amount of Mn 3+ , compared to -MnO2 NRs where Mn 3+ is strikingly absent. The observation of the reduced states of Mn only in -MnO2 as-well emphasizes that the electronic structure of the Mn-oxides can be tuned through a stringent control of crystallographic phase during the synthesis.</p><!><p>Motivated by the difference in surfaces as observed in XPS, Temperature programmed reduction (TPR) reactions were carried out to probe its effect on the adsorption of the reactant gases and to study the reducibility of the catalysts. The H2 consumption profile of α-and β-MnO2 (NR) plotted with respect to temperature is shown in Figure 3b. In case of β-MnO2, the reduction starts This indicates that α-MnO2 shows a greater ability to release the lattice oxygen compared to β-MnO2 (NR). The H2 TPR profile of β-MnO2 displays a two-step reduction showing one peak 324 o C and the other at 521 o C. This can be accounted for the conversion of MnO2 to intermediate reduced oxides 29 , eventually leading to the formation of MnO (Supporting Information, Figure S3) Interestingly, there are no separate peaks seen in H2-TPR profile of α-MnO2 (NR). The presence of a broad peak (319-520 o C) might be because of multiple reduction steps taking place simultaneously. It is to be noted that the initiation of reduction of MnO2 was observed to occur at a much lower temperatures compared to the existing reports 30 .</p><p>The behavior of MnO2 catalysts under CO atmosphere was evaluated using CO-TPR. The CO consumption profiles are shown in Figure 3a. Further insights into the CO-TPR profile of α-MnO2 is gained by analyzing the reaction intermediates at different temperatures. The reaction was stopped at various intervals and the collected materials were subjected to XRD analysis (for details, see SI. Clearly, as the temperature ramps up, MnO2 reduces to a mixture of MnO2 and Mn3O4 (200 o C). The mixed phases are then reduced to pure Mn3O4 (after 300 o C) which further transforms to MnO (600 o C). After 600 o C, the CO-TPR plot shows a flat signal which suggests that there is no further oxidation occurring. This also suggests that the materials possess a continuous reduction capability.</p><p>Under CO atmosphere, the temperature at which α-MnO2 starts losing lattice oxygen is observed to be ~80 o C. An increase in the temperature in the CO consumption profile is observed in case of β-MnO2, the threshold temperature being ~139 o C. This indicates that under CO atmosphere, the ability of α-MnO2 to release its lattice oxygen is found to be ~50 o C lower than that of β-MnO2. The experimental results are consistent with the theoretical studies reporting low oxygen vacancy formation energies 31,32 for the (100) and (110) surfaces of α-and β-MnO2. Our TPR experiments coupled with the XPS data conclusively suggests that the α-phase possesses a higher reducibility.</p><!><p>It is known that an increase in surface area of a material can render it greater number of catalytically active sites. After observing that the α-phase is more reducible, we devised a way to engineer the surface area of the α-MnO2 NR by reducing the duration of synthesis to 30 min.</p><p>It is evident from SEM micrographs and bright-field TEM images shown in Figure 4a and 4b that the flower-like morphology is derived from multiple sheets stacked in a random order. The SAED pattern obtained from the -MnO2 nanoflowers (Figure 4c) exhibit a ring diffraction pattern, indicating its polycrystalline nature. The XRD pattern was indexed to the tetragonal αphase of MnO2 (Figure S4 in SI). The XPS analysis of the Mn 2p (Figure S5 in SI) indicates the presence of Mn 3+ along with Mn 4+ . The quantitative analysis was done, and it was observed that ~67 % of Mn exists in 3+ oxidation state. This observation confirms that we were able to synthesize a higher surface area variant of the more reducible phase i.e. α-phase of MnO2.</p><!><p>The specific surface area calculated by BET measurements of α-MnO2 (NR) and β-MnO2 (NR) was found to be 54 and 45 m 2 g -1 , respectively. The specific surface area of α-MnO2 was measured as 118 m 2 g -1 in nanoflower (NF) morphology. The reason of such enhanced surface area of the NFs can be attributed to their large 2-D sheet like morphology, along with their polycrystalline nature.</p><!><p>It has been widely studied that the interaction of reducible oxides with noble metal particles further enhances the catalytic activity of the material. By using an ultra-fast microwave technique, we were able to decorate MnO2 supports with 2 wt.% Pt. The XRD patterns of Pt/α-MnO2 NR, Pt/β-MnO2 NR and Pt/α-MnO2 NF showed no extra peaks that could be addressed to Pt. This suggests that the amount of Pt of the surface is very less and the very small size of Pt particles results in peak broadening. TEM bright field images (Figure 5) reveal the presence of uniformly dispersed ultra-fine Pt nanoparticles on the surface of MnO2. The XPS spectra of the Pt-decorated samples are provided in Figure S6 in SI.</p><!><p>The bare MnO2 as well as the Pt-decorated MnO2 supports were tested for their activity in CO oxidation. The activities of α-and β-MnO2 NRs for CO oxidation are shown in Figure 6. The temperature of full conversion (T100) and 50% conversion (T50) for β-MnO2 (NR) are observed as ~240 o C and ~180 o C, respectively. The values of T100 and T50 over α-MnO2 (NR) are observed as ~220 o C and ~156 o C, respectively. This comparative study of CO oxidation activity over αand β-MnO2 NR supports the observations from XPS and TPR studies suggesting that the surface reducibility plays a major role in catalytic activity. Further, CO oxidation was performed over α-MnO2 (NF) and the activity was observed to be higher than α-MnO2 (NR). Interestingly, the T100 and T50 for α-MnO2 (NF) are observed as 190 o C and ~154 o C, respectively. To enhance the CO oxidation activity of the bare MnO2 substrates, 2 wt.% Pt was decorated as ultra-fine particles. These Pt-decorated oxide supports were also tested for CO oxidation. The light-off curves shown in Figure 6   For further insight into the rate of reaction, the kinetic regime in the differential reactor was determined by varying the catalyst weight in the reactor. The variation of W/FCO (W=weight of catalyst and FCO=molar flow rate of CO) with fractional conversion of CO over all the catalysts is shown in Figure S7 in SI. A change in slope was observed above 40% CO conversion, indicating the change into diffusion regime. Rate of reaction was determined from the slope of the initial region (kinetic regime) and is calculated by using the following formula / ( / )</p><p>Here, rco is the rate of reaction calculated from the slope of the linear region; XCO is the fractional conversion of CO, W is the weight of the catalyst loaded and FCO is the molar flow rate of CO. Furthermore, an Arrhenius plot (Figure 7a) was used to calculate the apparent activation energies (Eapp) of all the catalysts. The Eapp for α-MnO2 (NR), β-MnO2 (NR) and α-MnO2 (NF) were found to be 58 ± 1, 87 ± 2 and 41 ± 1 kJ mol -1 . Similar calculations were performed on Pt-decorated supports and the apparent activation energies (Ea) of CO oxidation were calculated using an Arrhenius plot (Figure 7b). The calculated apparent activation energies of Pt/α-MnO2 (NR), Pt/β-MnO2 (NR) and Pt/α-MnO2 (NF) were 38.1 ± 2, 43.2 ± 1 and 28.2 ± 1 kJ mol -1 , respectively. The thus prepared Pt/α-MnO2 (NF) shows the best activity for CO oxidation. There is a limited literature on Pt-MnO2 hybrids in literature. The values of T100 and Eapp obtained for CO oxidation over Pt/α-MnO2 (NF) as well as α-MnO2 (NF) is found to be significantly lower than the values reported in literature 33 .</p><p>It is important to check the catalyst for their stability under the reaction conditions for determining its practical applicability. The stability of Pt-supported catalysts was checked by continuing the experiment at the temperature of full conversion (T100) for a period of 24 h. It is shown (Supporting Information, Figure S8) that the activity of α-and β-MnO2 (NR) was stable till 16 h and decreased by 4% till 24 h. It is also observed that there is a slight reduction (6%) in the activity of α-MnO2 (NF) after 24 h at T100.</p><!><p>In summary, we show that through a stringent control over phase and morphology, surface reducibility of catalytic 1-D MnO2 support can be engineered. The phase-dependent difference of the surface-reducibility can be observed in photoelectron spectroscopy. Controlled TPR experiments under reducing atmosphere affirms higher accessibility of lattice oxygen in case of the α-MnO2, in agreement with the XPS results. Furthermore, a morphology-controlled fabrication of α-MnO2 flowers with higher surface area yields a better catalytic activity for CO oxidation reaction. With an understanding of these two factors, viz. surface reducibility and specific surface area, we were able to engineer MnO2-Pt nanohybrids, exhibiting a very low temperature for full CO conversion, as low as ~90 o C, with remarkable cyclic stability. Thus, this study proposes a bottom-up approach to engineer catalysts using surface reducibility and surface area as two handles of controlling their activities.</p>