Hydrogenation of CO2 to methanol over In-doped m-ZrO2: a DFT investigation into the oxygen vacancy size-dependent reaction mechanism,Physical Chemistry Chemical Physics

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Hydrogenation of CO2 to methanol over In-doped m-ZrO2: a DFT investigation into the oxygen vacancy size-dependent reaction mechanism
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2022-09-06 , DOI: 10.1039/d2cp02788g
Jie Yu ₁ , Yabing Zeng ₂ , Wei Lin _{2,

3} , Xin Lu _{1,

3}

Affiliation

Selective methanol synthesis via CO₂ hydrogenation has been thoroughly investigated over defective In-doped m-ZrO₂ using density functional theory (DFT). Three types of oxygen vacancies (Ovs) generated either at the top layer (O1_v and O4_v) or at the subsurface layer (O2_v) are chosen as surface models due to low Ov formation energy. Surface morphology reveals that O1_v has smaller oxygen vacancy size than O4_v. Compared with perfect In@m-ZrO₂, indium on both O1_v and O4_v is partially reduced, whereas the Bader charge of In on O2_v remains almost the same. Our calculations show that CO₂ is moderate in adsorption energy (∼−0.8 eV) for all investigated surface models, which facilitates the formate pathway for both O1_v and O4_v. O2_v is not directly involved in CO₂ methanolization but could readily transform into O1_v once CO₂/H₂ feed gas is introduced. Based on the results, the synthesis of methanol from CO₂ hydrogenation turns out to exhibit conspicuous vacancy size-dependency for both O1_v and O4_v. The reaction mechanism for small-sized O1_v is controlled by both the vacancy size effect and surface reducibility effect. Thus, H₂COO* favors direct C–O bond cleavage (c-mechanism) before further hydrogenation to methanol, which is similar to the defective In₂O₃. The vacancy size effect is more competitive than the surface reducibility effect for large-sized O4_v. Therefore, H₂COO* prefers protonation to H₂COOH before C–O bond cleavage (p-mechanism) which is similar to the ZnO–ZrO₂ solid solution. Furthermore, we also determined that stable-CH₃O*, which is too stable to be hydrogenated, originates from the O1_v surface. In contrast, CH₃O* with similar configuration is allowed to be further converted to methanol on O4_v. Overall, our findings offer a new perspective towards how reaction mechanisms are determined by the size of oxygen vacancies.

中文翻译：

In掺杂m-ZrO2上CO2加氢制甲醇：氧空位尺寸依赖反应机理的DFT研究

已经使用密度泛函理论 (DFT)对有缺陷的 In 掺杂 m-ZrO ₂进行了通过CO ₂加氢的选择性甲醇合成进行了深入研究。由于 Ov 形成能低，选择在顶层（O1_v 和 O4_v）或次表层（O2_v）产生的三种氧空位（Ovs）作为表面模型。表面形态表明O1_v 的氧空位尺寸小于O4_v。与完美的In@m-ZrO ₂相比，O1_v 和O4_v 上的铟部分减少，而O2_v 上In 的Bader 电荷几乎保持不变。我们的计算表明，CO ₂对于所有研究的表面模型，吸附能适中（~-0.8 eV），这有利于 O1_v 和 O4_v 的甲酸盐途径。O2_v 不直接参与CO _{2甲醇化，但一旦引入CO}₂ /H ₂原料气，就可以很容易地转化为O1_v 。基于这些结果，CO ₂加氢合成甲醇对 O1_v 和 O4_v 都表现出显着的空位尺寸依赖性。小尺寸 O1_v 的反应机理受空位尺寸效应和表面还原性效应的控制。因此，在进一步氢化成甲醇之前，H ₂ COO* 有利于直接 C-O 键断裂（c 机制），这类似于有缺陷的 In ₂ O₃ . 对于大尺寸 O4_v，空位尺寸效应比表面还原性效应更具竞争力。因此，_{与 ZnO-ZrO 2}固溶体相似_{，H 2} COO* 在 C-O 键断裂（p 机制）之前更喜欢质子化而不是 H_{2 COOH。}此外，我们还确定了由于太稳定而无法氢化的稳定-CH₃ O* 来源于O1_v 表面。相反，具有相似构型的 CH₃ O* 可以在 O4_v 上进一步转化为甲醇。总体而言，我们的研究结果为反应机制如何由氧空位的大小决定提供了一个新的视角。

更新日期：2022-09-06

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