Mechanism of Methanol Synthesis from CO2 Hydrogenation over Rh16/In2O3 Catalysts: A Combined Study on Density Functional Theory and Microkinetic Modeling
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摘要: 本研究采用密度泛函理论 (DFT) 和微动力学模型分析了 Rh16/In2O3 催化剂上二氧化碳 (CO2) 氢化成甲醇 (CH3OH) 的情况；研究了 Rh16/In2O3 界面上 H2 的自发解离和 CO2 的有效吸附，其中， In2O3 中的氧空位提供了有利的效果。此外，Bader 电荷分析显示 Rh16 上带有轻微的正电荷，这对于理解催化剂的电子特性和活性非常重要。证实了RWGS+CO-Hydro 途径是甲醇合成的主要途径，其特点是经过一系列中间转化：CO2*→COOH*→CO*+OH*→HCO*→CH2O*→CH2OH*→ CH3OH*。在不同温度 (373−873K) 和压力 (10−2−103 bar) 下进行的反应速率控制程度分析 (DRC) 揭示了两个关键的动力学现象：在较低温度和较高压力下，转化步骤 CO* + H* → HCO * 显着影响总体反应速率；而在较高温度下，CH2O* + H* → CH3O* 的步骤占主导地位。Abstract: In this investigation, the hydrogenation of carbon dioxide (CO2) to methanol (CH3OH) over a Rh16/In2O3 catalyst is meticulously analyzed through the application of Density Functional Theory (DFT) and microdynamics modeling. The research focuses on the spontaneous dissociation mechanisms of H2 and CO2 at the Rh16/In2O3 interface, with a special emphasis on the role of oxygen vacancies in In2O3 which enhance adsorption processes. Bader charge analysis revealed a marginal positive charge on Rh16, elucidating critical insights into the electronic characteristics and catalytic activity of the system. The study establishes the RWGS+CO-Hydro pathway as the predominant mechanism for methanol synthesis, characterized by a sequential transformation of intermediates: CO2*→COOH*→CO*+OH*→HCO*→CH2O*→CH2OH*→ CH3OH*. Further, Degree of Reaction Rate Control (DRC) analysis conducted across a range of temperatures (373−873K) and pressures (10−2−103 bar) identified two principal kinetic phenomena: at lower temperatures coupled with higher pressures, the conversion of CO* + H* to HCO* significantly impacts the overall rate of reaction; conversely, at higher temperatures, the step from CH2O* + H* to CH3O* is found to dominate.
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图 2 （a）由20个Rh原子组成的Rh棒状模型，将Rh模型加载到有缺陷的In2O3表面上，In2O3表面上的四个 O 原子与四个 Rh 原子相互作用，导致 Rh 以 Rh16 的形式结合；（b）Rh16/In2O3基底：侧视图（上）、俯视图（下）；（c）Rh16/In2O3（110） 模型及Rh16 /In2O3模型中的差分电荷分布图,模型（上），差分电荷分布图（下）
Figure 2 2(a) Rh rod-shaped model composed of 20 Rh atoms, the Rh model is loaded onto the defective In2O3 surface, the four O atoms on the In2O3 surface interact with the four Rh atoms, resulting in Rh in the form of Rh16 combine; combine (b) Rh16/In2O3 substrate: side view (top), top view (bottom); (c) Differential charge distribution diagram in Rh16/In2O3(110) model and Rh16/In2O3 model, model (top), differential charge distribution diagram (bottom)
图 3 Rh16/In2O3中反应物质的稳定吸附结构及其吸附位置
Figure 3 Stable adsorption structure and adsorption position of reactive substances in Rh16/In2O3
图 4 Rh16附近二氧化碳分子周围的电荷分布
Figure 4 Charge distribution around carbon dioxide molecules near Rh16
图 5 CO2到甲醇反应路径，黑色为HCOO路径，绿色代表COOH路径，蓝色代表RWGS路径，其他颜色代表分支反应路径
Figure 5 Reaction pathways from CO2to methanol Black represents the formate (HCOO) pathway, green represents the carboxyl (COOH) pathway, blue represents the RWGS pathway, and other colors represent branch reaction pathways
图 6 Rh16/In2O3催化剂上 HCOO 路径合成甲醇的势能曲线
Figure 6 Potential energy curve of methanol synthesis via HCOO pathway over Rh16/In2O3 catalyst
图 7 CO2甲醇的加氢作用，HCOO通道主自由基反应的初始、过渡和最终状态的优化结构
Figure 7 Hydrogenation of CO2methanol, optimized structures of the initial, transition and final states of the main radical reaction of the HCOO channel
图 8 CO2加氢生成甲醇羧基途径的势能面（PES）
Figure 8 Potential energy surface (PES) of the CO2 hydrogenation to methanol carboxyl pathway
图 9 Rh16/In2O3上CO2加氢合成甲醇，对 COOH 通道各个自由基反应的初始、过渡和最终状态进行了优化结构，其余分支反应如图7所示
Figure 9 Methanol is synthesized from CO2 hydrogenation on Rh16/In2O3. The initial, transition and final states of each free radical reaction in the COOH channel are optimized. The remaining branch reactions are shown in Figure 7
图 10 在Rh16/In2O3催化剂上，RWGS+CO-二氧化碳加氢成甲醇的势能面（PES）
Figure 10 Potential energy surface (PES) of RWGS+CO-carbon dioxide hydrogenation to methanol on Rh16/In2O3 catalyst
图 11 对RWGS + CO-HydrO通道的二氧化碳加氢反应合成甲醇的单个原始反应的初始、过渡和最终状态的优化结构，其余分支反应见图7
Figure 11 Optimized structures of the initial, transition and final states of a single original reaction for the synthesis of methanol from carbon dioxide hydrogenation of the RWGS + CO-HydrO channel. The remaining branch reactions are shown in Figure 7
图 12 不同压力和温度下的整体反应速率
Figure 12 Overall reaction rate at different pressures and temperatures
图 13 （a）、 （b）、 （c）不同反应步骤的 DRC 动力学曲线作为温度和压力的函数（d） 反应步骤的 DRC 与温度曲线
Figure 13 (a), (b), (c) DRC kinetic curves for different reaction steps as a function of temperature and pressure(d) DRC vs. temperature curves of reaction steps
表 1 五种Rh金属团簇的功函数
Table 1 Work functions of seVeral Rh metal clusters
Model Diameter/nm Number of atoms Work function Rh13 0.6 13 4.039 Rh43 1.0 43 4.130 Rh55 1.2 55 4.177 Rh165 1.8 165 4.317 Rh(rods) − 16 4.418 
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表 2 Rh16/In2O3上的吸附能、吸附位和反应种类的结构参数
Table 2 Structural parameters of adsorption energies, adsorption sites and reaction types on Rh16/In2O3
Specie Eads/eV Site Bond length (Å) and bond angle (°) CO2 −1.16 interface d(Rh−O)=2.126；d(In−O)= 2.247；
d(Rh−C)=1.973；∠Oa−C−Ob=121.9°H(1/2 H2) −0.68 Rh metal d(Rh−H)=1.709/1.772 H2O −0.83 In2O3 interface d(In−O)= 2.352 HCOOH −0.94 In2O3 interface d(In−O)=2.314 CH2O −1.51 Rh metal d(Rh−O)=2.016；d(Rh−C)=2.149/2.124 CH3OH −1.43 In2O3 interface d(In−O)= 2.264；d(O−H)= 1.577 CO −2.57 Rh metal d(Rh−C)=1.964/1.982
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表 3 HCOO 途径中甲醇合成所涉及的基本步骤的反应能ΔE 和势垒Eb
Table 3 Reaction energy ΔE and potential barrier Eb of the basic steps involved methanol synthesis in the HCOO pathway
Elementary reaction step Eb/eV △E/eV CO2* + H* → HCOO* + * 0.69 −0.86 HCOO* + H* → H2COO* + * 1.72 0.17 H2COO* + H* → H2COOH* + * 0.97 0.41 H2COOH* + *→CH2O*+ OH* 0.79 −0.06 HCOO* + H* → HCOOH* + * 1.15 0.14 HCOOH* + * → HCO* + OH* 0.54 −1.02 HCO* + H* → CH2O* + * 1.10 −0.06 CH2O* + H* → CH3O* + * 1.07 0.57 CH3O* + H* → CH3OH* + * 0.30 0.10 CH2O* + H* → CH2OH* + * 1.02 0.20 CH2OH* + H* → CH3OH* + * 0.37 0.10
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表 4 COOH途径中甲醇合成所涉及的基本步骤的反应能 ΔE 和势垒Eb
Table 4 Reaction energies ΔE and potential barriers Eb for the basic steps involved in methanol synthesis in the COOH pathway
Elementary reaction step Eb/eV △E/eV CO2* + H* → COOH* + * 0.99 −0.70 COOH* + H* → HCOOH* + * 0.88 0.14 HCOOH* + *→HCO* + OH* 0.68 −1.01 COOH* + * → CO* + OH* 0.51 −1.88 HCO* + H* → CH2O* + * 1.10 −0.06
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表 5 RWGS + CO-Hydro 通道中甲醇合成所涉及的基本步骤的反应能 ΔE和能垒 Eb
Table 5 Reaction energy ΔE and energy barrier Eb for the basic steps involved in methanol synthesis in the RWGS +CO-Hydro channel
Elementary reaction step Eb/eV △E/eV CO2* + H* → COOH* + * 0.99 −0.70 COOH* + * → CO* + OH* 0.51 −1.88 CO* + H* → HCO* + * 1.37 −1.01 HCO* + H* → CH2O* + * 1.10 −0.06 CO2* + * → CO* + O* 2.18 −1.18 CO* +O* → C* + O* 2.94 0.72
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