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

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.

The Origin of Au/Ce1-xZrxO2 Catalyst’s Active Sites in Low-Temperature CO Oxidation

Gold catalysts have found applications in many reactions of both industrial and environmental importance. Great interest has been paid to the development of new processes that reduce energy consumption and minimize pollution. Among these reactions, the catalytic oxidation of carbon monoxide (CO) is an important one, considering that a high concentration of CO in the atmosphere creates serious health and environmental problems. This paper examines the most important achievements and conclusions arising from the own authorship contributions concerning (2 wt. % Au)/Ce1−xZrxO2 catalyst’s active sites in low-temperature CO oxidation. The main findings of the present review are: (1) The effect of preparing conditions on Au crystallite size, highlighting some of the fundamental underpinnings of gold catalysis: the Au surface composition and the poisoning effect of residual chloride on the catalytic activity of (2 wt. % Au)/Ce1−xZrxO2 catalysts in CO oxidation; (2) The identification of ion clusters related to gold and their effect on catalyst’ surface composition; (3) The importance of physicochemical properties of oxide support (e.g., its particle size, oxygen mobility at low temperature and redox properties) in the creation of catalytic performance of Au catalysts; (4) The importance of oxygen vacancies, on the support surface, as the centers for oxygen molecule activation in CO reaction; (5) The role of moisture (200–1000 ppm) in the generation of enhanced CO conversion; (6) The Au-assisted Mars-van Krevelen (MvK) adsorption–reaction model was pertinent to describe CO oxidation mechanism. The principal role of Au in CO oxidation over (2 wt. % Au)/Ce1−xZrxO2 catalysts was related to the promotion in the transformation process of reversibly adsorbed or inactive surface oxygen into irreversibly adsorbed active species; (7) Combination of metallic gold (Au0) and Au-OH species was proposed as active sites for CO adsorption. These findings can help in the optimization of Au-containing catalysts.

Ru–polyoxometalate as a single-atom electrocatalyst for N2 reduction to NH3 with high selectivity at applied voltage: a perspective from DFT studies

Polyoxometalate supported Ru₁ achieves high NRR selectivity at applied voltage while maintaining high activity.

Supported Noble-Metal Single Atoms for Heterogeneous Catalysis

Single-atom catalysts (SACs), with atomically distributed active metal sites on supports, serve as a newly advanced material in catalysis, and open broad prospects for a wide variety of catalytic processes owing to their unique catalytic behaviors. To construct SACs with precise structures and high density of accessible single-atom sites, while preventing aggregation to large nanoparticles, various strategies for their chemical synthesis have been recently developed by improving the distribution and chemical bonding of active sites on supports, which results in excellent activity and selectivity in a variety of catalytic reactions. Noble-metal-based SACs are discussed, and their structural properties, chemical synthesis, and catalytic applications are highlighted. The structure-activity relationships and the underlying catalytic mechanisms are addressed, including the influences of surface species and reducibility of supports on the activity and stability, impact of the unique structural and electronic properties of single-atom centers modulated by metal/support interactions on catalytic activity and selectivity, and how the modified catalytic mechanism obtained by inhibiting the multiatoms involves catalytic pathways. Finally, the prospects and challenges for development in this field are highlighted.

Theoretical investigation of the ORR on boron-silicon nanotubes (B-SiNTs) as acceptable catalysts in fuel cells

Here, the potential of boron doped silicon nanotubes (7, 0) as ORR catalysts is examined. Acceptable paths for the ORR on studied catalysts are examined through DFT. The optimum mechanism of the ORR on the surface of B2-SiNT (7, 0) is shown. The ORR on the surface of B2-SiNTs (7, 0) can continue through LH and ER mechanisms. The calculated beginning voltage for the ORR on B2-SiNTs (7, 0) is 0.37 V and it is smaller than the beginning voltage (0.45 V) for platinum-based catalysts. In the acidic solution the beginning voltage for the oxygen reduction process can be evaluated to be 0.97 V, which corresponds to 0.37 V as a minimum overvoltage for the ORR. The B2-SiNTs (7, 0) are suggested as an ORR catalyst in acidic environments.

Investigation of performance of aluminum doped carbon nanotube (8, 0) as adequate catalyst to oxygen reduction reaction

The discovery of novel nano-catalysts to oxygen reduction reaction with high performance due to various application of fuel cells is very important. In present study the potential of aluminum doped carbon nanotube (8, 0) to oxygen reduction reaction in acidic condition was examined through theoretical models. The possible paths to oxygen reduction reaction on Al₂-CNT (8, 0) surfaces were investigated and optimal path was identified from thermodynamic standpoint. Results indicated that the Al₂-CNT (8, 0) catalyzed the oxygen reduction reaction through the Eley-Rideal and Langmuir-Hinshelwood mechanisms. The Al₂-CNT (8, 0) catalyst has much better methanol and CO tolerance than platinum-based catalysts. In this study, the overpotential value of oxygen reduction reaction on aluminum doped carbon nanotube (8, 0) surface (ca 0.38 V) is lower than corresponding values on platinum-based catalysts (ca 0.45 V). Finally, results demonstrated that the Al₂-CNT (8, 0) can be proposed as efficiency catalyst to oxygen reduction reaction.

Theoretical investigation of oxidation of NO (NO + ½ O2 → NO2) on surfaces of nickel-doped nanocages (Ni-C60 and Ni-B30N30)

In present study, the NO oxidation on Ni-carbon nanocage and Ni-boron nitride nanocage surfaces was investigated. The Ni-C₆₀ and Ni-B₃₀N₃₀ catalysts can oxidize the NO molecule by Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. In this study, the NO molecule was joined to Ni atom of the Ni-surface-O₂* and Ni-surface-O* to generate the intermediates with low barrier energies. It can be concluded that the cis-Ni-surface-ONOO* complex in the ER pathway is more stable than four-elements-ring complex in LH pathway ca 0.06 and 0.08 eV, respectively. In the LH pathway, the studied catalysts were deactivated by irreversible absorption of NO₂ molecules in Ni atoms of Ni-C₆₀ and Ni-B₃₀N₃₀. In contrast, in the ER pathway two NO₂ molecules were released in the normal temperature. In this study, the abilities of the Ni-C₆₀ and Ni-B₃₀N₃₀ to oxidation of NO molecule was demonstrated. Finally, the systematic scheme to design of metal-doped nano-catalysts to oxidation of toxic gases was proposed.

CO oxidation over the polyoxometalate-supported single-atom catalysts M1/POM (Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW12O40]3–): a computational study on the activation of surface oxygen species

Density functional theory calculations have been carried out to explore the catalytic performance of a series of the M₁/POM (M = Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW₁₂O₄₀]³⁻) single-atom catalysts for CO oxidation.