Bimetallic Co and Mn Supported on Hydroxyapatite Catalyst for Carbon Monoxide Oxidation at Lower Temperature

The series of bimetallic Co and Mn supported on hydroxyapatite catalyst were prepared by successive deposition method and examined for CO oxidation. The CO oxidation activity was compared with monometallic Mn/HAp and Co/HAp. The catalysts are characterized in detail and correlated to the oxidation activity. The XRD, XPS and TPR characterization showed the presence of more facile Co 2+ , Mn 3+ and adsorbed oxygen due to the interaction between Mn and Co. The 0.4 mol Mn and 0.1 mol Mn deposited on HAp showed formation of maximum active species. The maximum CoO species was observed over bimetallic catalyst compared to the monometallic catalyst. These active lower the activation energy require for CO and oxygen. These species were responsible for the oxidation of CO at lower temperature compared to the remaining catalyst.


Introduction
The CO is most pernicious and toxic gases emitted from industrial and vehicle exhaust. The abatement of CO emission from this exhaust is done by using various noble and non-noble metal supported on Ce, Al, HAp etc. catalysts [ ]. Furthermore, preferentially exposed (110) plane of Co 3 O 4 nanorods was active for CO oxidation [Jansson et al. 2002;Jiang et al. 2011;Nguyen et al. 2015]. This plane enriched with Co 3+ species and was responsible for the exceptional catalytic activity for low temperature CO oxidation [Binning et al. 1982;Xie et al. 2009;Iablokov et al. 2015].
Moreover, molecular oxygen was activated into oxygen superoxide ion (O 2 -) by surface oxygen vacancies, which actively couples with CO molecule adsorbed on cobalt cations of Co 3 O 4 to form product CO 2 [Yu et al. 2009]. The CoO showed weaker metal-oxygen bond among all transition metal oxides. CoO can also be generated through a thermal-driven phase transition [Henrich et al. 1996]. However, CO oxidation using cobalt oxide showed deactivation due to the presence of water/carbonate species on the surface [Mutuberría et al. 1993;Cunningham et al. 1994;Jansson et al. 2001; Thormählen et al. 2001;Grillo et al. 2004]. The deactivation of catalyst could be inhibited by addition of second metal. The Co-Ce-Co and Mn mixed oxide studied for ethanol, butanol and toluene oxidation [Aguilera et al. 2011]. Mn showed preferential oxidation of CO oxidation at 170 ºC due to the rapid removal of carbonate species [Qiang et al. 2010]. However, complete CO oxidation at lower temperature (< 100 ºC) using bimetallic Co and Mn supported on HAp has need to explore in detail with kinetic parameters. Furthermore, the Mn showed presence of Mn 2+ , Mn 3+ and Mn 4+ species having facile redox properties [More et al. 2019]. The bimetallic Mn and Co forms amorphous phase and presence of the higher concentration of active oxygen species [Bulavchenko et al. 2020;Zhang et al. 2018]. Moreover, Hydroxyapatite (HAp) having formula Ca 10 (PO 4 ) 6 (OH) 2 abundance in the nature and is being most studied compound from the apatite class. HAp shows high chemical, thermal stability and weak solubility. It crystallizes in the hexagonal structure in P63/m space group [Henrich et al. 1996;Liu et al. 2012]. HAp is reported for rapid removal of carbonate species from the catalyst surface which was leads to the availability of active sites for CO oxidation [Kunfeng et al. 2013]. In present study, low temperature CO oxidation has been studied with active component modi cation of Mn/HAp by addition of Co. The CO oxidation activity of Co/Mn/HAp has been studied with detailed characterizations.

Catalyst synthesis
The listed chemicals and reagents were used without further puri cation for synthesis of bimetallic Mn  The monometallic 0.5 mol of Mn or Co deposited on HAp using above method and labelled as Mn/HAp or Co/HAp.

Catalytic activity testing
The series of bimetallic Co and Mn supported on HAp were examined for low temperature CO oxidation. The catalytic activity was performed under atmospheric pressure by using tubular down ow reactor located in an electrical furnace for gas space hourly velocity (GHSV) of 50,000 mL.g -1 .h -1 . 0.5 g catalyst was diluted with 2.0 g silicon carbide. The catalyst was preheated for 1 h at 400 o C with 10% O 2 and helium. The composition of reaction feed was 300 ppm CO, 5% O 2 with He balance. This reaction mixture was passed over the catalyst. The catalytic performances were evaluated in the temperature range of 30-400 o C @ 2 o C.min -1 . The micro-GC (Agilent 490) equipped with thermal conductivity detector (TCD) having three columns Porapack Q, MS-5A and CP Sil 5CB. The gaseous feed was analysed by using these columns.
Following formulae was used to calculate carbon monoxide conversion from the inlet and outlet CO concentration.
Where, and are initial and nal concentration of carbon monoxide respectively. According to following equation rate of carbon monoxide (r CO ) was calculated [Mutuberría et al. 1993 The apparent activation energy was calculated by using slope of Arrhenius plot ln(r) Vs reciprocal of temperature (Fig S 1 ).

Catalyst Characterizations
The powder X-ray diffraction (PXRD) studies of the catalyst were performed on the Shimadzu XRD 6100 instrument having Ni-lter and Cu Kα radiation (λ=1.54 Å, 40kV, 30mA). The XRD patterns were recorded at scan rate of 4 o min -1 with 0.02 o step size. Smart Sorb 92/93 instrument from Smart Instrument Co. was used to study nitrogen adsorption. The catalyst was preheated at 250 o C in the environment of N 2 purging for 90 minutes followed by N 2 adsorption. The temperature programmed reduction (H 2 -TPR) study is carried out to analyse the surface reducibility of the catalyst. H 2 -TPR study was carried out on Micromeritics AutoChem II equipped with thermal conductivity detector (TCD). In H 2 -TPR analysis, pretreated samples were heated with a gas ow of 5% H 2 in Ar @ 20 mL.min -1 with heating rate 5 o C.min -1 from ambient temperature to 800 o C. The Kratos Analytical instrument-Model AXIS supra with Monochromatic (AlKα) 600 X-ray source (1486.6 eV) was used to performed X-ray photoelectron spectroscopy (XPS). The background correction was done by C1s peak at 284.8eV.

Results And Discussion
CO oxidation activity of bimetallic Co x /Mn 0.5−x /HAp catalysts.
The CO oxidation activity and activation energy of bimetallic Co x /Mn 0.5−x /HAp are shown in Fig. 1 and

Conclusion
The bimetallic Mn and Co supported on HAp catalyst were prepared by successive deposition method and investigated for CO oxidation. The bimetallic catalyst showed higher CO oxidation activity at lower temperature compared to the monometallic Mn/HAp and Co/HAp. 0.4 mol Co and 0.1 mol Mn supported on HAp showed presence of higher concentration of active species on catalyst surface and lower activation energy for CO oxidation at lower temperature. The formation of higher adsorbed oxygen species and Co 2+ , Mn 3+ species observed over catalyst surface. The CoO is weaker bond and therefore form a more labile oxygen. All these species were responsible for oxidation of CO at lower temperature.