Bhattacharjee S, Gupta K, Jung N, Yoo S J, Waghmare U V, Lee S C
Indo-Korea Science and Technology Center (IKST), Bangalore, India.
Phys Chem Chem Phys. 2015 Apr 14;17(14):9335-40. doi: 10.1039/c4cp05557h. Epub 2015 Mar 11.
Oxidation of Co at the surface poses a major problem in the cyclable use of CoPt, a cost-effective catalyst for proton exchange membrane fuel cells. This can be alleviated by attaching a ligand selectively to Co-sites to stop its oxidation without compromising the catalytic activity. Here, we present a comparative analysis of adsorption of NH3 on the (0001) surface of Co in the HCP structure and (111) surfaces of Pt and CoPt alloy in the FCC structure, using first-principles density functional theoretical calculations. While NH3 binds more strongly with the Pt surface than with the Co surface, we find that its binding with the Co atom is stronger than that with the Pt atom on the surface of the CoPt alloy. Our analysis of the charge density and electronic structure shows how this originates from (a) the electron transfer from the minority spin d-band of Co to Pt, and (b) shift in the energy of d-bands and the magnetic moments of Co atoms on the surface of the CoPt alloy relative to those on the (0001) surface of Co. Hybridization of the d-states of Co in CoPt with pz states of N in NH3 used to stop Co oxidation also results in improving the charge transfer from Co to Pt that is relevant to the catalytic activity of CoPt. We finally present the analysis of how the interaction of NH3 with the CoPt surface can be tuned with strain.
在质子交换膜燃料电池的一种经济高效催化剂CoPt的可循环使用中，Co在表面的氧化是一个主要问题。通过将配体选择性地附着在Co位点上以阻止其氧化而不损害催化活性，可以缓解这一问题。在此，我们使用第一性原理密度泛函理论计算，对HCP结构的Co的(0001)表面以及FCC结构的Pt和CoPt合金的(111)表面上NH₃的吸附进行了比较分析。虽然NH₃与Pt表面的结合比与Co表面的结合更强，但我们发现它与CoPt合金表面上的Co原子的结合比与Pt原子的结合更强。我们对电荷密度和电子结构的分析表明，这源于(a)从Co的少数自旋d带向Pt的电子转移，以及(b)相对于Co的(0001)表面，CoPt合金表面上Co原子的d带能量和磁矩的变化。用于阻止Co氧化的CoPt中Co的d态与NH₃中N的pz态的杂化也导致改善了与CoPt催化活性相关的从Co到Pt的电荷转移。我们最后分析了如何通过应变来调节NH₃与CoPt表面的相互作用。
