Theoretical assessments of Pd–PdO phase transformation and its impacts on H 2 O 2 synthesis and decomposition pathways
Theoretical assessments of Pd–PdO phase transformation and its impacts on H 2 O 2 synthesis and decomposition pathways
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Pd→PdO相变的理论评估及其对H 2 O 2 合成和分解途径的影响
DOI：
10.1039/d3cy00404j
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发表时间：
2023
影响因子：
5
通讯作者：
Kwon, Stephanie
中科院分区：
文献类型：
--
作者：
Vyas, Manasi;Fajardo-Rojas, Fernando;Gómez-Gualdrón, Diego A.;Kwon, Stephanie
由O2和H2直接合成双氧水提供了一条绿色途径来生产双氧水，这是一种流行的工业氧化剂。在这里，我们从理论上研究了钯的氧化态、配位环境和颗粒大小对初级过氧化氢选择性的影响，通过计算形成过氧化氢(通过OO*还原；KO-H)和分解O2O*(通过O-O裂解；KO-O)的速率常数的比率来评估。对于Pd金属，在300K时，Ko-H/Ko-O比从Pd(111)的10−4下降到Pd13团簇的10−10，表明随着Pd颗粒尺寸的减小，H_2O_2的选择性变差，H_2O_2的初级选择性总体较低。随着氧化学势的增加和金属形成表面氧化物和体氧化物，晶格O原子对Pd-Pd系综位的扰动导致选择性大大高于1。例如，在300K时，随着Pd(111)分别氧化为Pd5O4/Pd(111)和PdO(100)，KO-H/KO-O比从10−4显著增加到10 9到10 16。相反，对于持续包含更多金属化的、配位不足的Pd-Pd系综位的表面和体氧化物，例如PdO(101)/Pd(100)和PdO(101)，没有观察到这样的选择性增强。当较小的Pd纳米颗粒完全氧化时，这些Pd-Pd系综也不存在，这表明较小的PdO团簇对过氧化氢合成具有更高的选择性。一次H_2O_2选择性的这些趋势与H_2O_2通过O-O键断裂分解速率的趋势成反比，这表明具有高H_2O_2选择性的催化剂也会阻碍H_2O_2的分解。用从头算热力学方法估算了Pd、PdO/Pd和PdO在O2、H2O2/H2O和O2/H2环境中的热力学有利相。这些结果结合在一起表明，在较低的氧化学势下，较小的Pd纳米颗粒更容易被氧化，在此基础上，它们比较大的Pd颗粒更有选择性地合成过氧化氢。
The direct synthesis of H2O2 from O2 and H2 provides a green pathway to produce H2O2, a popular industrial oxidant. Here, we theoretically investigate the effects of Pd oxidation states, coordination environments, and particle sizes on primary H2O2 selectivities, assessed by calculating the ratio of rate constants for the formation of H2O2 (via OOH* reduction; kO–H) and the decomposition of OOH* (via O–O cleavage; kO–O). For Pd metals, the kO–H/kO–O ratio decreased from 10−4 for Pd(111) to 10−10 for the Pd13 cluster at 300 K, indicating poorer H2O2 selectivity as Pd particle size decreases and low primary selectivities for H2O2 overall. As the oxygen chemical potential increases and metals form surface and bulk oxides, the perturbation of Pd–Pd ensemble sites by lattice O atoms results in selectivities that become dramatically higher than unity. For instance, at 300 K, the kO–H/kO–O ratio increases significantly from 10−4 to 109 to 1016 as Pd(111) oxidizes to Pd5O4/Pd(111) and to PdO(100), respectively. In contrast, such selectivity enhancements are not observed for surface and bulk oxides that persistently contain rows of more metallic, undercoordinated Pd–Pd ensemble sites, such as PdO(101)/Pd(100) and PdO(101). These Pd–Pd ensembles are also absent when smaller Pd nanoparticles fully oxidize, indicating that smaller PdO clusters can be more selective for H2O2 synthesis. These trends for primary H2O2 selectivities were found to inversely correlate with trends for H2O2 decomposition rates via O–O bond cleavage, demonstrating that catalysts with high primary H2O2 selectivity can also hinder H2O2 decomposition. Ab initio thermodynamic calculations are used to estimate the thermodynamically favored phase among Pd, PdO/Pd and PdO in O2, H2O2/H2O, and O2/H2 environments. These results are combined to show that smaller Pd nanoparticles are more prone to be oxidized at lower oxygen chemical potentials, upon which they become more selective than larger Pd particles for H2O2 synthesis.