Preparation mL of graphene stock solution was mixed

Preparation of Graphene-Palladium-Platinum Alloy (G-PdPt) Hybrids: In order to prepare G-PdPt hybrid, 30 mL of graphene stock solution was mixed with 10 mL of PdCl2 aqueous solution (0.4 mM) slowly and heated at 90oC for 5 min. After this, sodium citrate aqueous solution (1mL, 0.05882 % (W/V)) was added and it was kept for another 90 min at 90oC. Subsequently, 2.5 mL of PtCl4 aqueous solution (0.05M) was slowly added into the solution followed by the addition of 7.25 mL ascorbic acid aqueous solution (0.1M) and kept in stirring for further 3h at 90oC to complete the reaction. Finally, the product of graphene, decorated with PdPt alloy (designated as G-PdPt (No PVP)) was obtained. The sample was allowed to cool down naturally and collected after centrifuging and washing several times with distilled water to remove any un-reacted chemicals or by-products.

         In order to elucidate the effect of capping agent on the morphology of PdPt nanoparticles, PdPt nanoparticles were prepared in presence of capping agent by following wet chemical approach reported in ref 38. First of all, polyvinylpyrrolidone (PVP) – functionalized graphene has been prepared. Initially, 0.02 g of graphite oxide was dispersed in 30 mL distilled water by ultrasonication for 2h to obtain graphene oxide (GO). Then, 0.160 g of PVP was added to the dispersed (GO) and kept in stirring for 12h. Subsequently, 630 ?L of ammonia (NH4OH) solution and 70 ?L of hydrazine hydrate (N2H4) solution were added into it and kept for stirring of another 15 minutes. Following this, the whole solution was heated at 90oC for 4h and the stable black dispersion of PVP-functionalized graphene was obtained. To decorate the Pd nanoparticles on these PVP-functionalized graphene, 16.6 mL PdCl2 aqueous solution (56.4 mM) and 23.3 mL of formic acid were added into it subsequently and slowly. The solution was kept for stirring at room temperature for 1h to complete the reduction of all the Pd+2 ions and decorated on graphene sheet. Following this, 95 mL of ascorbic acid (AA) aqueous solution (0.2M) and 33.2 mL of K2PtCl4 aqueous solution (0.1M) were then slowly injected into the solution and it was subsequently heated at 90oC for 3h. After cooling down to room temperature naturally, a black colour solution containing PVP-functionalized graphene, decorated with PdPt nanoparticles (designated as G-PdPt (with PVP)) was obtained. Finally, the sample was collected after centrifuging and washing several times with distilled water.

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     In order to probe the formation of PdPt alloy nanoparticles and its decoration on graphene surface, graphene-decorated with Pd (G-Pd) and graphene-decorated with Pt (G-Pt) were also prepared along with G-PdPt alloy nanoparticles hybrids. For the preparation of G-Pd, equal volume (10 mL) of graphene stock solution and PdCl2 aqueous solution (0.4 mM) were mixed together and heated at 90oC for 5 min. After this, sodium citrate aqueous solution (1mL, 0.05882 % (W/V)) was added to it and kept for another 15 min at 90oC. The product G-Pd hybrid was collected by centrifugation. Similarly, G-Pt was also prepared by treating the graphene solution (20mL) with PtCl4 aqueous solution (2.5 mL, 0.05M) and ascorbic acid aqueous solution (7.25 mL, 0.1M) for 15 min at 90oC.

Experimental Procedure for Catalytic Activity of Hybrids:

To determine the catalytic activity of G-PdPt (with PVP) and G-PdPt(No PVP) for oxidation of CO, the reaction process were carried out in a fixed-bed continuous flow quartz reactor tube whose internal diameter is 5mm at temperature range of 30-300oC under atmospheric pressure. 50 mg of each sample has been taken in the reactor and a gaseous mixture of CO (0.982%) and O2 (0.493%) in H2 flow at a space velocity (SV) of 47746 hr-1 has been passed in the fixed-bed flow reactor. The outlet gas mixture was analyzed by an on-line gas chromatography (CIC, Baroda) equipped with a molecular Sieve 5A (for O2 and CO) and a Porapak Q column (for CO2). The catalytic activity for the oxidation of CO with O2 was evaluated by calculating the percentage conversion of CO to CO2