Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States.
J Am Chem Soc. 2023 Mar 8;145(9):5114-5124. doi: 10.1021/jacs.2c11323. Epub 2023 Feb 27.
Palladium-silver-based alloy catalysts have a great potential for CO-free hydrogen production from formic acid for fuel cell applications. However, the structural factors affecting the selectivity of formic acid decomposition are still debated. Herein, the decomposition pathways of formic acid on Pd-Ag alloys with different atomic configurations have been investigated to identify the alloy structures yielding high H selectively. Several PdAg surface alloys with various compositions were generated on a Pd(111) single crystal; their atomic distribution and electronic structure were determined by a combination of infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). It was established that the Ag atoms with Pd neighbors are electronically altered, and the degree of alteration correlates with the number of nearest Pd. Temperature-programmed reaction spectroscopy (TPRS) and DFT demonstrated that the electronically altered Ag domains create a new reaction pathway that selectively dehydrogenates formic acid. In contrast, Pd monomers surrounded by Ag are demonstrated to have a similar reactivity compared to pristine Pd(111), yielding CO and HO in addition to the dehydrogenation products. However, they bind to the produced CO weaker than pristine Pd, demonstrating an enhancement in resistance to CO poisoning. This work therefore shows that surface Ag domains modified by interaction with subsurface Pd are the key active sites for selective decomposition of formic acid, while surface Pd atoms are detrimental to selectivity. Hence, the decomposition pathways can be tailored for CO-free H production on Pd-Ag alloy systems.
钯银基合金催化剂在燃料电池应用中具有从甲酸中无 CO 生产氢气的巨大潜力。然而，影响甲酸分解选择性的结构因素仍存在争议。在此，研究了具有不同原子构型的 Pd-Ag 合金上甲酸的分解途径，以确定产生高 H 选择性的合金结构。在 Pd(111)单晶上生成了几种具有不同组成的 PdAg 表面合金；通过红外反射吸收光谱 (IRAS)、X 射线光电子能谱 (XPS) 和密度泛函理论 (DFT) 的组合确定了它们的原子分布和电子结构。结果表明，具有 Pd 相邻原子的 Ag 原子在电子上发生了变化，并且变化程度与最近 Pd 的数量相关。程序升温反应光谱 (TPRS) 和 DFT 表明，电子修饰的 Ag 域创建了一种新的反应途径，可选择性地使甲酸脱氢。相比之下，被 Ag 包围的 Pd 单体与原始 Pd(111)相比具有相似的反应性，除了脱氢产物外，还生成 CO 和 HO。然而，它们与生成的 CO 的结合比原始 Pd 弱，表明对 CO 中毒的抵抗力增强。因此，本工作表明，与次表面 Pd 相互作用修饰的表面 Ag 域是甲酸选择性分解的关键活性位，而表面 Pd 原子不利于选择性。因此，可以对 Pd-Ag 合金体系进行 CO 免费 H 生产的分解途径进行定制。
