Preparation , Characterization and NO-CO Redox Reaction Studies over Palladium and Rhodium Oxides Supported on Manganese Dioxide

The catalytic activity of PdO/MnO2 and Rh2O3/MnO2 is investigated for NO-CO redox reaction. Supported catalysts are prepared by wet impregnation method. Among the tested catalysts, PdO/MnO2 shows higher activity for this reaction. Active metal dispersion on MnO2 enhances the selectivity for N2 over N2O in this reaction. The XRD substantiate the formation of MnO2 monophasic phase. SEM images show the formation of elongated particles. TEM images indicate nano-size rod-like morphologies. An increase in the catalytic activity is observed on supported Pd and Rh oxides on MnO2. Temperature programmed desorption studies with NO and CO are undertaken to investigate the catalytic surface studies. © 2015 BCREC UNDIP. All rights reserved


Introduction
Oxides of nitrogen (NOx) and carbon monoxide (CO) are unambiguously serious air pollutants and harmful to living beings.NOx is a major cause of acid rain and photochemical smog.Both NO and CO are mostly produced as exhaust gases from motor vehicles and industries.Several methods have been developed for the NO and CO removal to meet the increasingly stringent regulations imposed on exhaust emissions.Fortunately, catalytic method for the redox reaction is available and is considered to be a powerful technique in detoxification of these gases.The efforts of the researchers are directed towards the development of various new catalytic methods to control the emission of these pollutants in the environment [1][2][3][4].Selective catalytic reduction (SCR) of NOx is one of the most efficient and economical technology for the removal of NOx by using reducing agents such as NH3, hydrocarbons, CO and H2 [1][2][3][4][5][6][7][8].CO as a reducing gas offers some distinct advantages of being emitted from the same exhaust of vehicles due to the incomplete combustion of fossil fuels.Lot of research has been focussed on developing catalysts for the reduction of NO by CO which will promote the simultaneous detoxification of these two pollutants [7][8][9][10][11][12].Most of the reported catalysts tested for NO-CO reaction give complete NO conversion at much higher temperatures and hence it is necessary to develop a catalyst which can give complete NO-CO conversion to nitrogen and CO2 at comparatively lower temperatures.
MnO2 with different crystallographic forms and morphologies is one of the most striking materials because of its valuable technological applications in catalysis, electrochemistry, molecular adsorption, waste removal, ion exchange processes and magnetic applications [7,8,[12][13][14][15].It has superior ability to activate and supply oxygen which is helpful in catalytic redox reactions.Nano-sized MnO2 is expected to exhibit good performance because of its large surface area and good activity.The properties of MnO2 are influenced significantly by its structure, morphology and preparative methods.Different types of manganese oxides, doped manganese oxides, mixed manganese oxides, metal supported manganese oxides are widely used for NO reduction and CO oxidation reactions [3][4][5][6][7][8][13][14][15][16].MnO2 exhibits considerable enhance activity when incorporated with some active metals or metal oxides.There are scanty reports of precious metals or metal oxides supported on MnO2 catalysts for lower temperature NO-CO reaction.
The present investigation reports the preparation of palladium and rhodium oxides supported on manganese dioxide by wet impregnation method.The catalysts are characterized by different instrumental techniques and tested for NO -CO redox reactions.Active metal substitution enhances the activity and selectivity for N2 in NO-CO reaction.Chemisorption of CO and NO was studied over these catalysts.

Catalysts Preparation
PdO/MnO2 with 5.9 wt% Pd loading and Rh2O3/MnO2 with 5.7 wt% Rh loading catalysts were prepared by wet impregnation method.The nano-sized pristine MnO2 powder was prepared by dextrose assisted co-precipitation method.Mn(CH3COO)2.4H2O(Sigma-aldrich 99.9%) was dissolved in distilled water and 2% of dextrose solution was added at 100 o C. Precipitation was carried out by dropwise addition of 10% NaOH solution under vigorous stirring.This suspension of manganese hydroxide (pH=9) was subjected to oxidation using 30% H2O2.The precipitate was then filtered, washed and dried at 120 o C for 10 h.MnO2 was mixed with an aqueous solution of appropriate amount of metal ions (PdCl2 and RhCl3 from Sigma Aldrich, 99.9 and 99.98% respectively) under vigorous stirring.Sodium hydroxide (10%) solution was added drop wise with the constant stirring to precipitate supported metals as metal hydroxide.Solution was allowed to stand and then checked for the complete precipitation by adding more NaOH.The precipitate was filtered, washed with distilled water and dried in air.The obtained samples were homogenized well in mortar and pestle and dried at 250 o C for 5 h.

Catalysts Characterization
X-ray powder diffraction measurement was carried out on a RIGAKU diffractometer, using Cu Kα radiation (λ = 1.5418Å, filtered through Ni filter) to investigate the structure as well as phase of the prepared catalysts.The surface morphology was determined with JSM-5800LV Scanning Electron Microscope (SEM).The Transmission electron microscope (TEM) images were recorded using PHILIPS CM 200 electron microscope with a resolution of 2.4 Å.The BET surface area was measured by nitrogen adsorption at liquid Nitrogen temperature using SMART SORB-91 surface area analyzer.NO and CO TPD studies of the samples were carried out using Quantachrome Autosorb iQ.For TPD studies 5% of gas (CO or NO) in N2 was passed over 0.2 g of catalyst at 100 o C for 30 mins, then cooled and flushed with N2 and sample was subjected to TPD analysis using TCD detector.FTIR spectra of NO adsorbed samples were recorded at room temperature using Shimadzu IR Prestige 21 spectrometer.

Catalytic Activity Tests
The catalytic tests for NO reduction by CO were performed in a continuous flow, fixed bed glass reactor.The catalyst powder weighing 0.9 g was supported between glass wool plugs in a glass reactor which was placed in an electric furnace.Catalytic activity was determined using a feed gas composition of 5% NO and 5% CO in 90% argon and passed over the catalyst at a rate of 5000 ml h -1 .The catalysts were heated in N2 atmosphere at 100 o C for 20 min prior to the reaction.The feed gases and the products were analyzed employing an online Gas Chromatograph with molecular sieve 13X and Porapak Q columns with H2 as a carrier gas.

Catalysts Characterization
Figure 1a shows the XRD pattern of pristine MnO2.The MnO2 shows diffraction features characteristic of tetragonal MnO2 phase, which is in agreement with the reported literature data (ICDD card 44-0141).PdO/MnO2 and Rh2O3/MnO2 catalysts show diffraction pattern similar to pristine MnO2 (Figures 1b and 1c The topography and morphological features of the samples are investigated by SEM. Figure 2 shows the SEM images of (a) PdO/MnO2 and (b) Rh2O3/MnO2 catalysts.The SEM images show the aggregates of fine particles.The aggregates have moderately elongated rod-like morphology.The representative TEM images are given for pristine MnO2 and Rh2O3/MnO2 catalysts (Figures 2c and 2d).From Figure 2c, it can be seen that MnO2 catalyst shows rod-like nano particles with thickness less than 10 nm.Similar features are shown by Rh2O3/MnO2 catalyst (Figure 2d).Inset in both the TEM images illustrate electron diffraction patterns.The electron diffraction patterns show clear concentric ring patterns indicating polycrystalline nature of the samples.These results are further supported by noisy X-ray diffraction pattern indicating nano-crystalline nature of these samples.
The BET surface area of all the catalysts is determined using nitrogen adsorption at boiling liquid nitrogen temperature.The BET surface areas of MnO2, PdO/MnO2 and Rh2O3/MnO2 are found to be 62, 65 and 60 m 2 g -1 respectively.Current preparation procedure was successful in getting uniform and high surface area catalysts.

NO Reduction by CO
Catalytic NO reduction by CO over different catalysts as a function of temperature is presented in Figure 3.The results show that the products of NO reduction by CO for all samples are N2 and CO2.N2O formation is observed at lower temperature (Figure 4).From the figure it is clear that the supported catalysts show much higher activity than pristine MnO2.PdO/MnO2 shows higher activity than Rh2O3/MnO2 catalyst.Catalysts such as Pd/Al2O3, Rh/Al2O3 and Rh/AlPO4 reported in literature exhibit lower activity for NO reduction by CO, exhibiting conversion at higher temperatures [17][18]  o C while that for Rh2O3/MnO2 is observed at 190 o C. Thus, introduction of small amount of PdO and Rh2O3 over MnO2 greatly improves the catalytic activity of NO conversion and drastic decrease in temperature is observed for total conversion.It is a well-established fact that at low temperature surface is predominantly covered with CO and increase in temperature will favor reduction of active catalytic sites by desorption of CO2 with lattice oxygen participation.Reduced catalytic surface is prone for NO adsorption which will get reoxidised by dissociative desorption of NO.Pd and Rh are well known for NO reduction by CO over inert support like Al2O3 [17,18].Synergetic effect of these highly active metals on active support as MnO2 enhances their catalytic activity.
CO conversion occurs in-line with NO conversion.The higher activity is shown by PdO/MnO2, indicating that the presence of PdO fastens the catalytic reaction of NO reduction by CO.The activity for redox reactions were repeated three times with PdO/MnO2 catalyst and the results were found to be satisfactory with no considerable decrease in the activity.Copyright © 2015, BCREC, ISSN 1978-2993 higher than that of pristine MnO2 which indicates there is increase in number of active sites for CO adsorption that is needed for the reductive CO2 desorption from the oxide surface.
Desorption of NO over the catalyst is rather found complex in nature as observed from the Figure 6.MnO2 shows two distinct desorption peaks with peak maxima at 200 and 340 o C respectively.Former signifies a weak adsorption while the later suggests a strong adsorption.In case of supported catalysts, three distinct desorption maxima are observed.These peaks at higher temperature are due to stronger adsorption, indicative of more than one binding mode of NO with the catalyst.Since it is difficult to comment on the binding modes of NO solely based on TPD profile, FTIR spectrum of chemisorbed NO is recorded for all the catalysts along with the pristine MnO2 with and without NO adsorption.Pristine MnO2 show no prominent absorption in the region 1640 -1250 cm -1 as observed from the Figure 7. IR spectra shows characteristic v(MN-O) vibration in the range 1640 -1250 cm -1 for the NO treated catalysts signifying NO adsorption [19].For pristine MnO2, NO chemisorption shows only one major peak at 1627 cm -1 , which is assigned to the terminal v(MN-O).Apart from this, intensities of other peaks in the range 1550-1250 cm -1 are found to be very weak, indicating only one major mode of adsorption.Bands at around 1630 cm -1 can be attributed to the M-NO -species [20].IR data of the Rh supported catalysts shows peaks at 1381 and 1334 cm   are observed for Pd [21].Rh and Pd are electron rich transition elements compared to Mn.They are expected to lower the NO bond order by forming M-NO -, that can in turn facilitate the dissociative desorption of NO, thus increasing the activity.

Conclusion
The supported catalysts PdO/MnO2 and Rh2O3/MnO2 were prepared by wet impregnation method.The TEM and SEM images confirmed the nano-size formation of these catalysts along with elongated rod-like morphologies.Catalytic NO reduction by CO was performed on all the prepared catalysts.CO conversion was obtained in-line with NO conversion.The supported catalysts showed higher activity than pristine MnO2.PdO/MnO2 gave higher activity than Rh2O3/MnO2 for the catalytic reaction studied.Selectivity for N2 over N2O was also enhanced after dispersion of Pd and Rh on MnO2 in NO reduction reaction.Since the surface area of catalysts is uniform, enhancement in activity is attributed to an increase in active sites for the catalytic reaction.CO TPD confirmed an increased quantum of CO adsorption on supporting Pd and Rh.NO desorption profile over the supported catalysts showed multiple desorption peaks suggesting different modes of strong and weak adsorption.NO desorption studies were found to be in compliance with the FTIR, suggests multiple binding modes of NO and insights the formation of M-NO -species.In supported catalysts, forma-tion of M-NO -species considerably lowered bond order of N-O which boosts the oxidative dissociative desorption of N2.
Figure1ashows the XRD pattern of pristine MnO2.The MnO2 shows diffraction features characteristic of tetragonal MnO2 phase, which is in agreement with the reported literature data (ICDD card 44-0141).PdO/MnO2 and Rh2O3/MnO2 catalysts show diffraction pattern similar to pristine MnO2 (Figures1b and 1c).Both Pd and Rh oxides are present over MnO2 support.Low intensity peaks with broader

3. 4 .
Temperature Program Desorption of CO and NO CO desorption profiles are depicted in Figure 5.All catalysts show a typical single broad CO desorption peak.In MnO2, maxima for CO desorption at 173 o C is observed, which is comparatively at a lower temperature as compared to the Rh (210 o C) and Pd (254 o C) supported catalysts.Quantum of CO chemisorbed over PdO/MnO2 and Rh2O3/MnO2 is also found to be Bulletin of Chemical Reaction Engineering & Catalysis, 10 (1), 2015, 101 -1 [v(ONO)].Whereas peaks at 1381 and 1265 cm -1 [v(ONO)]