Department of Chemistry , Yale University , New Haven , Connecticut 06520-8107 , United States.
Department of Chemistry , University of Washington , Seattle , Washington 98195-1700 , United States.
J Am Chem Soc. 2019 Sep 25;141(38):15390-15402. doi: 10.1021/jacs.9b07986. Epub 2019 Sep 13.
Cobalt phosphide (CoP) is one of the most promising earth-abundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of H atoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and H-atom transfer reactions with phenoxy radicals. Reactive H atoms were even present on the as-prepared materials. On the basis of the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H atoms on CoP was estimated to be roughly 51-66 kcal mol (Δ° ≅ 0 to -0.7 eV vs H). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H-flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive H atoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a roadmap to examine hydrogen on other materials.
磷化钴 (CoP) 是最有前途的贵金属催化剂替代品之一，可用于析氢反应 (HER)。HER 的关键是 H 原子的结合。虽然理论研究计算了 H 原子与过渡金属磷化物表面结合的优先位置和能量，但直接的实验研究却很少。在此，我们描述了磷化钴结合 H 的化学计量和热化学测量。我们研究了两种介观磷化钴颗粒，一种是经过酸性预处理后显示出磷化物表面的颗粒，另一种是胶体磷化钴纳米颗粒。用 H 处理会向 CoP 引入大量反应性 H，约为每个 CoP 单元 0.2 H，大约每个 Co 或 P 表面原子 1 H。这是通过炔烃氢化和与苯氧自由基的 H 原子转移反应来定量的。甚至在未经预处理的材料中也存在反应性 H 原子。根据 CoP 与各种提供和接受分子 H 的试剂的反应性，估计 H 原子在 CoP 上的结合自由能分布大致为 51-66 kcal mol（Δ°≈0 至-0.7 eV 相对于 H）。原位 X 射线吸收光谱初步表明了氢化 CoP 的结构，在 H 流下显示出晶格轻微膨胀和 Co 的有效核电荷没有明显变化。这些结果提供了一个关于催化活性 CoP 的新图景，其中含有大量的反应性 H 原子。这对于其催化和电催化性能可能具有根本意义。此外，这里开发的方法为研究其他材料上的 H 提供了一个路线图。
