Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

Abstract

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

Limits of Detection for EXAFS Characterization of Heterogeneous Single-Atom Catalysts

“…Indeed, this is what occurs at the extreme of fitting the Pt 14 O 37 cluster with the SAC model: bond distances are contracted by almost 0.2 Å and σ 2 factors are unreasonably pushed toward an upper limit of 20 × 10 –3 Å 2 (Figure S7). Aside from the poor fit to the actual data (Figure S6), such physically unjustifiable parameters should cause researchers to question their model . However, when the SAC is the dominant structure generating the EXAFS, even with significant (<40%) contributions from the oxidized Pt cluster, all fitted parameters are reasonable: changes in bond lengths are all within 0.05 Å of the input model values and σ 2 values vary sensibly between 1 × 10 –3 and 10 × 10 –3 Å 2 (Figure S7).…”

“…The ability to perform in situ or operando measurements is critically important, as changes in active-site structures of single-atom species can occur under reaction conditions. − Information about the metal oxidation state, electronic properties, and bonding geometry is often gleaned from distinct electronic transitions in the X-ray absorption near-edge spectroscopy (XANES) region. Analysis of the extended X-ray absorption fine structure (EXAFS) region provides information about the local coordination environment around the absorbing atom, including the types, number, and bond distances of nearest neighbor atoms (within ∼5 Å), and thus is often used to assess the potential existence of clusters in a sample. − XAS in general, and EXAFS in particular, has the potential to provide information about the local structure of single-atom active sites that are inaccessible by other techniques, resulting in some proclaiming EXAFS the pinnacle of characterization methods for SACs. , …”

Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT

“…16−18 (c) Is there any trend in CO binding behavior that directly or indirectly depends on catalyst loading, method of preparation, and surface area of the resulting catalysts with metals from different rows in the periodic table? Hence, herein, we focus on an in-depth understanding of the dynamics of atoms with in-situ X-ray absorption spectroscopy (XAS) 19 diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using a series of 4d (Ru, Rh, and Pd) and 5d (Ir and Pt) elements under CO and CO + O 2 reaction conditions. Ceria (CeO 2 ) has been widely studied for its exceptional properties such as its oxygen storage capacity and ability to stabilize isolated atoms, clusters, and nanoparticles, making it ideal for many catalytic applications.…”

“…(b) Is it a metal single site that is catalytically active or whether the dynamic structural changes of SACs play an essential role under operating conditions as indicated by many researchers? − (c) Is there any trend in CO binding behavior that directly or indirectly depends on catalyst loading, method of preparation, and surface area of the resulting catalysts with metals from different rows in the periodic table? Hence, herein, we focus on an in-depth understanding of the dynamics of atoms with in-situ X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using a series of 4d (Ru, Rh, and Pd) and 5d (Ir and Pt) elements under CO and CO + O 2 reaction conditions.…”

MATEO: InterMolecular α-Amidoalkylation Theoretical Enantioselectivity Optimization. Online Tool for Selection and Design of Chiral Catalysts and Products

“…Unfortunately, these techniques are less useful when it is necessary a fast scanning/optimization of new CPA catalysts for large libraries of reactions with diverse substrates, nucleophiles, products, and conditions (temperature, time, catalyst load, etc.). Cheminformatics methods relying upon Artificial Intelligence/Machine Learning (AI/ML) algorithms could help to speed up the discovery of new molecules [7][8][9] and also in the design new chiral catalysts and products without engaging in a long term, empirical or quantum investigation [10][11][12][13]. Therefore, there is a need to develop fast-track computational tools able to predict the enantiomeric excess saving time and experimental resources.…”