The Palladium Nano-catalyst Was Prepared By A Surfactant-assisted Biological Reduction Method

Aqueous Pd nano-catalysts with special shapes were prepared by biological reduction method using plant leaf extracts as the reducing agent,Na₂PdCl₄ solution as the precursor and cetyl trimethylammonium bromide?CTAB?as the shape-directing agent.The effects of the amount of CTAB and extract,the reaction temperature and time on the properties and catalytic performances of aqueous Pd nano-catalysts were investigated by 4×4 orthogonal test to determine the best preparation conditions.The reduction process and mechanism were characterized by UV-Vis and FT-IR analysis.The structure,crystallinity,morphology and particle size of the Pd nano-catalysts were explored by XRD,TEM,SAED analysis.Electro-oxidation of methanol reaction?MOR?and Suzuki coupling reactions were selected to examine the catalytic properties of catalysts.The Pd nanocubes?PdNCs?were prepared by using pine needle extract as reducing agent.The optimum electro-catalytic performance was obtained when the molar ratio of CTAB/Pd²⁺=2,the amount of pine needle extract was 26.67 reduction equivalents,the reduction time was 12 h and the reduction temperature was 55?.The PdNCs consisting of Pd{100}crystal facets had a relatively high purity?85%?and well crystallinity,with an average particle size of 11.18 nm.FT-IR analysis indicated that the flavonoid,ester,terpenoid and polyol in the pine needle extract were the main active reducing substances.The anodic peak current density?I_f?of MOR catalyzed by PdNCs was 3.35 mA cm^-2,which was higher than that of commercial Pd/C.The ratio of current density between anode peak and cathode peak?I_f/I_b?was 1.52,indicating that PdNCs had a good anti-poisoning ability.The activity after 1500 cycles was only decreased by 30.48%,proving the long-term stability of PdNCs.In addition,a 96.92%of catalytic yield of the product 4-methylbiphenyl for the Suzuki coupling reaction of4-bromotoluene with phenylboronic acid was obtained.The Pd truncated octahedrons?PdTOs?were prepared by using firmiana simplex leaf extract as reducing agent.The optimum electro-catalytic performance was obtained when the molar ratio of CTAB/Pd²⁺=1.2,the amount of firmiana simplex leaf extract was 20 reduction equivalents,the reduction time was 8 h and the reduction temperature was 30?.The PdTOs consisting of Pd{100}and Pd{111}crystal facets had a relatively high purity?⁷5%?,with a mean grain diameter of 9.2 nm.FT-IR analysis indicated that the polyol,phenol,protein,ester,ether and flavonoid in the firmiana simplex leaf extract were the main active reducing substances.The protein compounds,with?NH-C?=O groups,played a dual role in both reduction of Pd²⁺ions and prevention of Pd nanoparticles from agglomeration.The I_f of MOR catalyzed by PdTOs was 10.5 mA cm^-2,which was higher than that of PdNPs-0:1 and PdNPs-4:1reference catalysts.The value of I_f/I_b was 2.21,indicating that PdTOs had a good anti-poisoning ability.The activity after 1500 cycles was 71.28%of the initial value,proving the long-term stability of PdTOs.In addition,a 97.29%yield of the product4-methylbiphenyl for the Suzuki coupling reaction was obtained,which was similar to the catalytic result of PdNCs.The Pd cubic octahedrons?PdCOs?were prepared by using prunus padus leaf extract as reducing agent.The optimum electro-catalytic performance was obtained when the molar ratio of CTAB/Pd²⁺=2.4,the amount of prunus padus leaf extract was2.5 reduction equivalents,the reduction time was 6 h and the reduction temperature was 45?.The PdCOs consisting of Pd{100}and Pd{111}crystal facets had mean grain diameter of 23.75 nm,which was much larger than PdNCs and PdTOs.FT-IR analysis indicated that the polyol,polyphenol,carboxylic acid and flavonoid in the prunus padus leaf extract were the main active reducing substances.The morphology of PdCOs prepared under the optimal conditions was the most regular,and the I_f of the MOR was 11.97 mA cm^-2,which was higher than PdCOs prepared at other temperatures.I_f/I_b value was 1.98,close to PdTOs,as they both had Pd{111}crystal facets with high toxicity resistance.Further,PdCOs had good electro-catalytic stability.Due to the large particle size of PdCOs,the yield of 4-methylbiphenyl was 94.53%,which was lower than PdNCs and PdTOs.