Preparation of Graphene-supported Pt-Pd Catalyst and Its Catalytic Activity and Mechanism for Hydrogen Generation Reaction†

doi:10.7503/cjcu20170720

Abstract

Abstract:

The graphene-supported nano Pt-Pd bimetallic catalysts with various chemical compositions(Pt/Pd molar ratio: 4∶1, 1∶1, 1∶4) were prepared via simple ethanol reduction method by reflux of graphene oxide(GO) and metallic salts in ethanol/water solvent for 5.5 h at 88 ℃. For comparison, graphene-supported Pt, Pd monometallic catalysts were also synthesized under the same experimental conditions. The morphology of as-prepared catalysts was characterized by XRD and TEM, the results indicate that chemical composition of metallic catalysts shows distinct influence on the particle morphology and size, the Pt-Pd bimetallic catalyst particles disperse on the surface of graphene support most evenly with the average diameter of approximately 5.6 nm when the Pt/Pd molar ratio was set as 1∶1. Their catalytic activities for hydrolysis reaction of alkaline potassium borohydride(pH=12.0) were investigated, the catalytic experiment results demonstrate that the catalytic activities significantly depend on chemical composition of metallic catalyst, among them the Pt-Pd bimetallic catalyst with the molar ratio of 1∶1 displays higher activity than other Pt-Pd bimetallic catalysts(Pt/Pd molar ratios of 1∶4 and 4∶1) and Pt, Pd monometallic catalysts, the hydrogen generation efficiency of graphene-supported nano Pt-Pd(molar ratio 1∶1) bimetallic catalysts is about 4380 mo $l H 2$ ·mo $l M - 1$ ·h^-1, which is higher than graphene-supported Pt catalyst by 52% and approximately as four-fold as graphene-supported Pd catalyst. Investigation on the catalytic durability demonstrated that about 83% of the initial catalytic activity was retained even after three cycles. Catalytic reaction kinetics studies manifest the activation energy of borohydride hydrolysis reaction catalyzed by graphene supported Pt/Pd(molar ratio 1∶1) bimetallic catalyst is about 20.90 kJ/mol, which is distinctly lower than those of the reaction catalyzed by Pt/C and Ru/C catalysts. DFT calculations based on M55 model revealed that the barrier height of rate-determining step catalyzed by Pt25Pd30 are distinctly lower than that catalyzed by Pd55 and Pt55, indicating the graphene supported Pt-Pd(Pt/Pd molar ratio 1∶1) possess more excellent catalytic activity.

Key words: Catalytic hydrogen generation, Pt-Pd bimetallic catalyst, Potassium borohydride, Reaction mechanism

CLC Number:

O643

TrendMD:

LU Lilin,SHU Hongfei,RUAN Zhuhua,NI Jiaqi,ZHANG Haijun. Preparation of Graphene-supported Pt-Pd Catalyst and Its Catalytic Activity and Mechanism for Hydrogen Generation Reaction^†[J]. Chem. J. Chinese Universities, 2018, 39(5): 949.

Figures/Tables 6

Fig.1 Catalytic activities of graphene-supported metallic catalysts for hydrolysis reaction of KBH4(A) and the durability of graphene-supported Pt-Pd[n(Pt)/n(Pd)=1∶1] bimetallic catalyst(B) Note:(A) Catalyst: a. Pt; b. n(Pt)/n(Pd)=4∶1; c. n(Pt)/n(Pd)=1∶1; d. n(Pt)/n(Pd)=1∶4; e. Pd.

Fig.2 Arrhenius plots for the reaction rate of hydrolysis of KBH4 catalyzed by graphene-supported Pt-Pd[n(Pt)/n(Pd)=1∶1] bimetallic catayst

Fig.3 XRD patterns of graphite(a), graphene oxide(b) and the as-prepared graphene-supported Pt-Pd[n(Pt)/n(Pd)=1∶1] bimetallic catayst(c)

Fig.4 TEM images of graphene(A,A'), graphene-supported Pd catalyst(B,B'), graphene-supported Pt-Pd catalysts with Pt/Pd molar ratios of 1∶4(C,C'), 1∶1(D,D'), 4∶1(E,E') and graphene-supported Pt catalyst(F, F') with different magnifications

Fig.5 Optimized geometries of absorption complexes of BH4- on Pt55(A), Pd55(B) and Pt25Pd30(C) with the corresponding absorption energies(kJ/mol)

Fig.6 BLYP/DNP calculated energetic profiles for the rate-determining step in the borohydride hydrolysis catalyzed by Pd55(Ⅰ), Pt55(Ⅱ) and Pt25Pd30(Ⅲ)

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