Abstract
The interest in single-atom catalysts (SACs) is increasing, as these materials have the ultimate level of catalyst utilization, while novel reactions where SACs are used are constantly being discovered. However, to properly understand SACs and to further improve these materials, it is necessary to consider the nature of active sites under operating conditions. This is particularly important when SACs are used as electrocatalysts due to harsh experimental conditions, including extreme pH values or high anodic and cathodic potential. In this contribution, density functional theory-based thermodynamic modelling is used to address the nature of metal centers in SACs formed by embedding single metal atoms (Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) into graphene monovacancy. Our results suggest that none of these SAC metal centers are clean at any potential or pH in the water thermodynamic stability region. Instead, metal centers are covered with Hads, OHads, or Oads, and in some cases, we observed the restructuring of the metal sites due to oxygen incorporation. Based on these findings, it is suggested that setting up theoretical models for SAC modelling and the interpretation of ex situ characterization results using ultra-high vacuum (UHV) techniques requires special care, as the nature of SAC active sites under operating conditions can significantly diverge from the basic models or the pictures set by the UHV measurements.
Highlights
IntroductionSingle-atom catalysts (SACs) present the ultimate limit of catalyst utilization [1,2,3]
Single-atom catalysts (SACs) present the ultimate limit of catalyst utilization [1,2,3].Since virtually every atom possesses catalytic function, even SACs based on Pt-group metals are attractive for practical applications
We investigate model SACs consisting of single metal atoms (Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) that have been embedded into a single-vacancy graphene site
Summary
Single-atom catalysts (SACs) present the ultimate limit of catalyst utilization [1,2,3]. Since virtually every atom possesses catalytic function, even SACs based on Pt-group metals are attractive for practical applications. SACs can be modeled relatively as the single-atom nature of active sites enables the use of small computational models that can be treated without any difficulties. A combination of experimental and theoretical methods is frequently used to explain or predict the catalytic activities of SACs or to design novel catalytic systems. As the catalytic component is atomically dispersed and is chemically bonded to the support, in SACs, the support or matrix has an important role as the catalytic component. One single atom at two different supports will never behave the same way, and the behavior compared to a bulk surface will be different [1,2,3]
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