In situ K-edge X-ray absorption spectroscopy of the ligand environment of single-site Au/C catalysts during acetylene hydrochlorination

Reaction-Driven Dynamic and Reversible Transformations of Au Single Atoms and Au-Zr Alloys on Zirconia for Efficient Acetylene Hydrochlorination

Catalysis involving gold supported on metal oxides has undergone extensive examination. However, the nature of the catalytic site under actual reaction conditions and the role of the support continue to be vigorously debated. This study addresses these issues through experimental investigations and theoretical simulations. We explore a novel catalytic mechanism that employs dynamic single-atom catalysis for the hydrochlorination of acetylene. This catalytic mechanism occurs in defective ZrO2-supported Au-Zr single-atom alloys. Specifically, the dynamic single-atom catalysis is a result of the mobility of the gold cation, which is accelerated by Cl radicals and strongly couples with the abundant unsaturated surface sites of ZrO2 in a synergistic manner. As a result, the Au electronic structure dynamically evolves, leading to a decrease in the addition reaction energy barrier. Notably, the Au cation can detach from the Au-Zr alloy structure to catalyze the hydrochlorination of acetylene near the Zr-Ov-Zr sites and then reintegrate back into the Au-Zr alloy structure upon completion of the reaction. This study underscores the significance of dynamic active sites under reaction conditions and their pivotal role in catalysis.

Metal-Organic Frameworks Encaged Ru Single Atoms for Rapid Acetylene Harvest and Activation in Hydrochlorination

Ruthenium (Ru)-based catalysts have been candidates in hydrochlorination for vinyl chloride monomer (VCM) production, yet they are limited by efficient acetylene (C₂H₂) utilization. The strong adsorption performance of HCl can deactivate Ru active sites which resulted in weak C₂H₂ adsorption and slow activation kinetics. Herein, we designed a channel that employed metal-organic framework (MOF)-encaged Ru single atoms to achieve rapid adsorption and activation of C₂H₂. Low-Ru (∼0.5 wt %) single-atom catalysts (named Ru-NC@MIL) were assembled by hydrogen-bonding nanotraps (the H-C≡C-H^δ+···O^δ- interactions between C₂H₂ and carboxylate groups/furan rings). Results confirmed that C₂H₂ could easily enter the encapsulation channels in an optimal mode perpendicular to the channel with a potential energy of 42.3 kJ/mol. The harvested C₂H₂ molecules can be quickly passed to Ru-N₄ active sites for activation by stretching the length of carbon-carbon triple bonds (C≡C) to 1.212 Å. Such a strategy guaranteed >99% C₂H₂ conversion efficiency and >99% VCM selectivity. Moreover, a stable long-term (>150 h) catalysis with high efficiency (∼0.85 kg_vcm/h/kg_cat.) and a low deactivation constant (0.001 h^-1) was also achieved. This work provides an innovative strategy for precise C₂H₂ adsorption and activation and guidance for designing multi-functional Ru-based catalysts.

Migration: A Neglected Potential Contribution of HCl-Oxidized Au(0)

In this study, the typical oxidation process of Au/C catalysts exposed to HCl is presented. Although the process violates the standard electrode potentials, the "oxidized" tendency of Au(0) species is analyzed. This oxidation behavior can only be triggered over the Au/C sample within residual cationic Au species, and terminated over the completely metallic Au(0)/C sample. This study demonstrates that the presence of surface chlorination species cannot facilitate the oxidation of Au(0) and Au(I) when the sample is treated with HCl alone, which excludes the oxidation paths of: Au(0) → Au(III) and Au(I) → Au(III). The reported "HCl-oxidized Au(0)" behavior is partially caused by the migration of Au(III) species in the carbon bulk-phase, which occurs outside the XPS detection limit region and into the detection limit rather than the "HCl-oxidized Au(0)" itself. The mechanism of driving the bulk-phase Au(III) migrated from the steady destabilized state to the carbon surface is then studied. This study demonstrates that the migration of Au cannot be neglected behind the curious oxidation phenomenon by HCl, which provides a new perspective for the oxidation of other noble metals by HCl.

Polar magnetism and chemical bond inα-RuCl3

The micaceous black allotrope of ruthenium trichloride is the subject of many recent experimental and theoretical studies. Even so, its structural and magnetic properties remain undecided; monoclinic, trigonal and rhombohedral space groups for the crystal structure have been proposed on the basis of various types of experiments. The magnetic structure is often discussed in the context of the Kitaev state, but inevitably they are inconclusive discussions in the absence of structural and magnetic space groups. Johnsonet alinfer a candidate for the magnetic structure (C_c2/m) from results gathered in an extensive set of experiments on an untwined sample ofα-RuCl₃(Johnsonet al2015Phys. Rev.B92235119). The proposed zigzag antiferromagnetic ground state of Ru ions does not respond to bulk magnetic probes, with optical rotation and all forms of dichroism prohibited by symmetry. Experimental techniques exploited by Johnsonet alincluded x-ray and magnetic neutron diffraction. Properties of the candidate magnetic structure not previously explored include polar magnetism that supports Ru Dirac multipoles, e.g. a ruthenium anapole that is also known as a toroidal dipole. In a general case, Dirac dipoles are capable of generating interactions between magnetic ions, as in an electrical Dzyaloshinskii-Moryia interaction (Kaplan and Mahanti 2011Phys. Rev.B83174432; Zhaoet al2021Nat. Mater.20341). Notably, the existence of Dirac quadrupoles in the pseudo-gap phases of cuprate superconductors YBCO and Hg1201 account for observed magnetic Bragg diffraction patterns. Dirac multipoles contribute to the diffraction of both x-rays and neutrons, and a stringent test of the magnetic structure C_c2/m awaits future experiments. From symmetry-informed calculations we show that, the magnetic candidate permits Bragg spots that arise solely from Dirac multipoles. Stringent tests of C_c2/m can also be accomplished by performing resonant x-ray diffraction with signal enhancement from the chlorineK-edge. X-ray absorption spectra published forα-RuCl₃possess a significant low-energy feature (Plumbet al2014Phys. Rev.B90041112(R)). Many experimental studies of other Cl-metal compounds concluded that identical features hallmark the chemical bond. Using a monoclinic C_c2/m structure, we predict the contribution to Bragg diffraction at the ClK-edge absorption. Specifically, the variation of intensity of Bragg spots with rotation of the sample about the reflection vector. The two principal topics of our studies, polar magnetism and the chemical bond in the black allotrope of ruthenium trichloride, are brought together in a minimal model of magnetic Ru ions in C_c2/m.

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.

Ligand-metal bonding effects in resonance enhanced x-ray Bragg diffraction

Chlorine covalently bonded to an open shell metal is present in many materials with desirable or intriguing physical properties. Materials include highly luminescent nontoxic alternatives to lead halide perovskites for optoelectronic applications K₂CuCl₃and Rb₂CuCl₃, enantiomorphic CsCuCl₃that presents magneto-chiral dichroism at a low temperature, and cubic K₂RuCl₆that possesses a singlet ground state generated by antiparallel spin and orbital angular momenta. Structural chirality of CsCuCl₃has been confirmed by resonant x-ray Bragg diffraction. We explore likely benefits of the technique at the chlorine K-edge using a symmetry informed method of calculation applied to chlorine multipoles. Already, a low energy feature in corresponding x-ray absorption spectra of many compounds has been related to the chlorine-metal bond. Bragg diffraction from chlorine in cubic K₂RuCl₆is treated in detail. Diffraction patterns for rhombohedral compounds that present space-group forbidden Bragg spots are found to be relatively simple.

Ru supported on activated carbon and coated with a polydopamine layer for effective acetylene hydrochlorination

Ru/AC@PDA-T catalysts were synthesized by coating a protective layer of PDA derivatives on the Ru/AC catalyst. Significantly improved activity and stability provide directions for the development of environmentally friendly and economical catalysts.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Design of carbon supports for metal-catalyzed acetylene hydrochlorination

For decades, carbons have been the support of choice in acetylene hydrochlorination, a key industrial process for polyvinyl chloride manufacture. However, no unequivocal design criteria could be established to date, due to the complex interplay between the carbon host and the metal nanostructure. Herein, we disentangle the roles of carbon in determining activity and stability of platinum-, ruthenium-, and gold-based hydrochlorination catalysts and derive descriptors for optimal host design, by systematically varying the porous properties and surface functionalization of carbon, while preserving the active metal sites. The acetylene adsorption capacity is identified as central activity descriptor, while the density of acidic oxygen sites determines the coking tendency and thus catalyst stability. With this understanding, a platinum single-atom catalyst is developed with stable catalytic performance under two-fold accelerated deactivation conditions compared to the state-of-the-art system, marking a step ahead towards sustainable PVC production.

Carbons are indispensable as supports for metal-based catalysts in polyvinyl chloride manufacture via acetylene hydrochlorination. In this work, the acetylene interaction, tunable through adjusting microporosity and oxygen sites is identified as central activity and stability descriptor.

Sulfur Promotion in Au/C Catalyzed Acetylene Hydrochlorination

The formation of highly active and stable acetylene hydrochlorination catalysts is of great industrial importance. The successful replacement of the highly toxic mercuric chloride catalyst with gold has led to a flurry of research in this area. One key aspect, which led to the commercialization of the gold catalyst is the use of thiosulphate as a stabilizing ligand. This study investigates the use of a range of sulfur containing compounds as promoters for production of highly active Au/C catalysts. Promotion is observed across a range of metal sulfates, non-metal sulfates, and sulfuric acid treatments. This observed enhancement can be optimized by careful consideration of either pre- or post-treatments, concentration of dopants used, and modification of washing steps. Pre-treatment of the carbon support with sulfuric acid (0.76 m) resulted in the most active Au/C in this series with an acetylene conversion of ≈70% at 200 °C.