DFT法研究HCOOH在Pd/WC(0001)上的分解机理

doi:10.1016/S1872-2067(19)63463-7

摘要/Abstract

摘要： 作为便携式电子设备的动力源，直接甲酸燃料电池（DFAFC）具有燃料跨界范围小、电动势大、甲酸无毒、低温下功率密度大等优点，因而引起了人们的极大兴趣.DFAFC商业化的主要挑战之一是阳极电催化剂材料的高成本和低CO耐受性.阳极通常需要高负载的贵金属电催化剂（Pt或Pd）氧化甲酸（HCOOH）以获得所需的电能.完全电氧化甲酸在Pt和Pd表面上会产生强吸附的CO，从而降低了Pt或Pd催化剂的活性.Pt和Pd储量少且价格昂贵，减少Pt和Pd含量且保持催化性能的燃料电池催化剂一直是研究者的奋斗目标.
本文用周期性密度泛函理论（DFT）系统地研究了WC负载的单分子层Pd（Pd/WC（0001））催化剂对甲酸的分解机理，这可为所需的反应路径设计、筛选催化剂提供指导.
Trans-HCOOH通过C-H，O-H，C-O键的活化发生分解.关于吸附，确定了可能反应中间体的最稳定吸附构型.trans-HCOOH，HCOO，mHCOO，cis-COOH，trans-COOH，CO，H₂O，OH和H的吸附过程是化学吸附，而cis-HCOOH和CO₂与Pd/WC（0001）表面的相互作用较弱，是物理吸附.此外，提出了trans-HCOOH分解的不同途径来探索分解机理.trans-HCOOH中O-H，C-H和C-O键的活化能垒分别为0.61，0.77和1.05eV，O-H键断裂的能垒最小，则trans-HCOOH优先通过O-H键断裂生成HCOO.双齿HCOO是HCOOH分解的主要中间体，它可以转变为单齿HCOO，这条路线生成CO₂的能垒比双齿HCOO的低0.04eV.CO₂是HCOO主要解离产物，这一步是总反应的决速步骤.对于cis-COOH和trans-COOH，CO是其主要解离产物.此外，trans-HCOOH也能直接生成CO，但克服的能垒较大.在Pd/WC（0001）表面上分解trans-HCOOH的最有利途径是HCOOH→HCOO→CO₂，其中HCOO脱氢形成CO₂的步骤是速率决定步骤.
本文提供了HCOOH在Pd/WC（0001）表面上分解的活性中间体、能垒和机理的推测，CO形成主要是通过cis-COOH、trans-COOH及HCO的分解，CO₂的形成主要是通过HCOO的分解，CO₂占主导.该结论与Pd（111）面上甲酸分解结果一致，说明WC作为Pd载体没有改变Pd对甲酸的催化性能，但降低了Pd的使用量.
综上，本文阐明了WC负载单分子层Pd催化剂上甲酸催化分解机理，得出甲酸分解的最佳反应路径，为直接甲酸燃料电池设计低贵金属含量、高活性的负载型Pd催化剂提供了理论指导；可用于预测不同载体负载Pd催化剂的性能，大大减少实验成本，以验证提出的实验假设.

关键词: 密度泛函理论, 甲酸, 直接甲酸燃料电池, WC(0001)负载单分子层Pd, 分解机理

Abstract: In pursuit of low-cost direct formic acid fuel cells, tungsten carbide (WC) supported Pd catalyst is considered as an ideal candidate for efficient decomposition of formic acid due to low Pd utilization and excellent performance. Herein, different adsorption configurations and active sites of the intermediates, involved in the HCOOH decomposition, on WC(0001)-supported Pd monolayer (Pd/WC(0001)) surface investigated by using density functional theory. The results reveal that trans-HCOOH, HCOO, cis-COOH, trans-COOH, HCO, CO, H₂O, OH and H exhibit chemisorption on Pd/WC(0001) surface, whereas cis-HCOOH and CO₂ exhibit weak interactions with Pd/WC(0001) surface. In addition, the minimum energy pathways of HCOOH decomposition are analyzed to generate CO and CO₂ due to the fracture of C-H, H-O and C-O bonds. The adsorbed HCOOH, HCOO, mHCOO, cis-COOH and trans-COOH configurations exhibit dissociation rather than desorption. CO formation occurs through the decomposition of cis-COOH, trans-COOH and HCO, whereas the CO₂ formation happens due to the decomposition of HCOO. It is found that the most favorable pathway for HCOOH decomposition on Pd/WC(0001) surface is HCOOH→HCOO→CO₂, where the formation of CO₂ from HCOO dehydrogenation determines the reaction rate. Overall, CO₂ is the most dominant product of HCOOH decomposition on Pd/WC(0001) surface. The presence of WC, as monolayer Pd carrier, does not alter the catalytic behavior of Pd and significantly reduces the Pd utilization.

Key words: Density functional theory, Formic acid, Direct formic acid fuel cells, WC(0001)-supported Pd monolayer, Decomposition mechanism

张金花, 佘远斌. DFT法研究HCOOH在Pd/WC(0001)上的分解机理[J]. 催化学报, 2020, 41(3): 415-425.

Jinhua Zhang, Yuanbin She. Unveiling the decomposition mechanism of formic acid on Pd/WC(0001) surface by using density function theory[J]. Chinese Journal of Catalysis, 2020, 41(3): 415-425.

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