The stability and oxidation of supported atomic-size Cu catalysts in reactive environments
The stability and oxidation of supported atomic-size Cu catalysts in reactive environments
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DOI：
10.1063/1.5110300
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发表时间：
2019-08-07
影响因子：
4.4
通讯作者：
Deskins, N. Aaron
中科院分区：
文献类型：
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作者：
Iyemperumal, Satish Kumar;Fenton, Thomas G.;Deskins, N. Aaron
原子尺度的物质（原子和团簇）作为潜在的高活性催化剂受到了广泛的关注。合成这种在合成或反应条件下可能稳定的催化剂是一个挑战。在这项工作中，我们使用密度泛函理论来模拟Cu团簇在TiO2载体上的生长，包括烧结和氧化。由于金属与载体的相互作用，负载铜的氧化优于烧结。从头算热力学计算表明，O-2很容易氧化大多数簇，而H2O是温和的氧化剂。在相关温度下，CO2没有氧化任何簇。热力学表明，铜原子/簇的氧化很容易发生，但动力学分析表明并非如此。TiO2表面对O-2的吸附较弱，大部分氧化团簇也是如此。O-2离解势垒在未氧化的Cu簇上较低，但在单个Cu原子上相当高（1.88 eV）。我们的研究结果表明，由于高扩散势垒（烧结所必需）和高O-2离解势垒（氧化所必需），单个Cu原子在表面上是稳定的。我们所做的实验确实支持表面上有孤铜原子的前提。在不同的热处理环境下，TiO2上沉积了Cu，并去除了任何Cu2+（表明氧化的Cu团簇）。热处理后只存在Cu-0和Cu1+。吸附的铜原子呈+1氧化态。我们的计算和实验表明，Cu1+（单独吸附的Cu原子）是主要的。氧化/扩散动力学而不是热力学限制了Cu的生长/氧化。综上所述，我们发现金属-载体相互作用是合成稳定的原子级催化剂的关键，因为它们可以强烈影响扩散/氧化等关键过程。
Atomic-scale species (atoms and clusters) have attracted much attention as potential highly active catalysts. Synthesizing such catalysts that may be stable under synthesis or reaction conditions is a challenge. In this work, we used density functional theory to model the growth of Cu clusters on the TiO2 support, including sintering and oxidation. Oxidation of supported Cu was preferred over sintering due to metal-support interactions. Ab initio thermodynamics calculations showed that O-2 readily oxidized most clusters, while H2O was a mild oxidant. CO2 did not oxidize any clusters at relevant temperatures. Thermodynamics would suggest that oxidation of Cu atoms/clusters would readily occur, but kinetic analysis suggested otherwise. O-2 adsorption was weak over the TiO2 surface, as well as most oxidized clusters. O-2 dissociation barriers were low over nonoxidized Cu clusters, but quite high (1.88 eV) over single Cu atoms. Our results suggest that lone Cu atoms are stabilized on the surface, due to a high diffusion barrier (necessary for sintering) and a high O-2 dissociation barrier (necessary for oxidation). We performed experiments that indeed support the premise that lone Cu atoms occur on the surface. Cu species were deposited on TiO2, and any Cu2+ species (indicative of oxidized Cu clusters) were removed after thermal treatment in various environments. Only Cu-0 and Cu1+ species existed after thermal treatment. Lone adsorbed Cu atoms had a +1 oxidation state. Combined, our calculations and experiments indicate that Cu1+ species (lone adsorbed Cu atoms) are dominant. The kinetics of oxidation/diffusion rather than thermodynamics limits the growth/oxidation of Cu. In summary, we show that metal-support interactions are key for synthesizing stable atomic-scale catalysts, since they can strongly influence key processes such as diffusion/oxidation.