In Situ Dispersion of Palladium on TiO2 During Reverse Water-Gas Shift Reaction: Formation of Atomically Dispersed Palladium

Cu/CeO2 catalysts for reverse water gas shift reactions: the effect of the preparation method

The reverse water gas shift reaction is one of the most prospective CO2 utilization approaches. Cu has excellent selectivity for CO and CeO2 is rich in surface oxygen vacancies for CO2 activation. These unique properties are often used to develop efficient Cu/CeO2 catalysts in RWGS. In this paper, Cu/CeO2 is prepared by plasma-induced micro-combustion. The effect of the subsequent calcination after micro-combustion on the structure and catalytic property is systemically studied. Because of the mild temperature of micro-combustion, highly dispersed Cu species load on the surface of CeO2 for the catalyst without calcination (Cu/CeO2-mc). During calcination, the highly dispersed Cu species form two kinds of species, Cu-Ce solid solution structure and small CuO clusters (Cu/CeO2-mcc). The Cu-Ce solid solution effectively enhances the generation of oxygen vacancies, which improves the adsorption and activation of CO2. The catalytic performance of Cu/CeO2-mcc thereby is superior to Cu/CeO2-mc in RWGS. In situ diffuse reflectance infrared fourier transform spectroscopy analysis demonstrates that the formate pathway is the main mechanism of RWGS. CO2 adsorbed on the surface of Cu/CeO2-mcc mainly forms bidentate species. While monodentate generates on the surface of Cu/CeO2-mc. And decomposes to CO easier than , thus Cu/CeO2-mcc exhibits excellent catalytic properties. This work provides a new approach for structural modulation of catalysts with excellent catalytic performance in RWGS.

A Ni-O-Ag photothermal catalyst enables 103-m2 artificial photosynthesis with >17% solar-to-chemical energy conversion efficiency

The scalable artificial photosynthesis composed of photovoltaic electrolysis and photothermal catalysis is limited by inefficient photothermal CO2 hydrogenation under weak sunlight irradiation. Herein, NiO nanosheets supported with Ag single atoms [two-dimensional (2D) Ni1Ag0.02O1] are synthesized for photothermal CO2 hydrogenation to achieve 1065 mmol g-1 hour-1 of CO production rate under 1-sun irradiation. This performance is attributed to the coupling effect of Ag-O-Ni sites to enhance the hydrogenation of CO2 and weaken the CO adsorption, resulting in 1434 mmol g-1 hour-1 of CO yield at 300°C. Furthermore, we integrate the 2D Ni1Ag0.02O1-supported photothermal reverse water-gas shift reaction with commercial photovoltaic electrolytic water splitting to construct a 103-m2 scale artificial photosynthesis system (CO2 + H2O → CO + H2 + O2), which achieves more than 22 m3/day of green syngas with an adjustable H2/CO ratio (0.4-3) and a photochemical energy conversion efficiency of >17%. This research charts a promising course for designing practical, natural sunlight-driven artificial photosynthesis systems.

Fundamental Structural and Electronic Understanding of Palladium Catalysts on Nitride and Oxide Supports

The nature of the support can fundamentally affect the function of a heterogeneous catalyst. For the novel type of isolated metal atom catalysts, sometimes referred to as single-atom catalysts, systematic correlations are still rare. Here, we report a general finding that Pd on nitride supports (non-metal and metal nitride) features a higher oxidation state compared to that on oxide supports (non-metal and metal oxide). Through thorough oxidation state investigations by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), CO-DRIFTS, and density functional theory (DFT) coupled with Bader charge analysis, it is found that Pd atoms prefer to interact with surface hydroxyl group to form a Pd(OH)x species on oxide supports, while on nitride supports, Pd atoms incorporate into the surface structure in the form of Pd-N bonds. Moreover, a correlation was built between the formal oxidation state and computational Bader charge, based on the periodic trend in electronegativity.

Bottom-Up Construction of Mesoporous Cerium-Doped Titania with Stably Dispersed Pt Nanocluster for Efficient Hydrogen Evolution

Hydrogen generation is one of the crucial technologies to realize sustainable energy development, and the design of advanced catalysts with efficient interfacial sites and fast mass transfer is significant for hydrogen evolution. Herein, an in situ coassembly strategy was proposed to engineer a cerium-doped ordered mesoporous titanium oxide (mpCe/TiO2), of which the abundant oxygen vacancies (Ov) and highly exposed active pore walls contribute to good stability of ultrasmall Pt nanoclusters (NCs, ∼ 1.0 nm in diameter) anchored in the uniform mesopores (ca. 20 nm). Consequently, the tailored mpCe/TiO2 with 0.5 mol % Ce-doping-supported Pt NCs (Pt-mpCe/TiO2-0.5) exhibits superior H2 evolution performance toward the water-gas shift reaction with a 0.73 molH2·s-1·molPt-1 H2 evolution rate at 200 °C, which is almost 6-fold higher than the Pt-mpTiO2 (0.13 molH2·s-1·molPt-1 H2). Density functional theory calculations confirm that the structure of Ce-doped TiO2 with Ce coordinated to six O atoms by substituting Ti atoms is thermodynamically favorable without the deformation of Ti-O bonds. The Ov generated by the six O atom-coordinated Ce doping is highly active for H2O dissociation with an energy barrier of 2.18 eV, which is obviously lower than the 2.37 eV for the control TiO2. In comparison with TiO2, the resultant Ce/TiO2 support acts as a superior electron acceptor for Pt NCs and causes electron deficiency at the Pt/support interface with a 0.17 eV downshift of the Pt d-band center, showing extremely obvious electronic metal-support interaction (EMSI). As a result, abundant and hyperactive Ti3+-Ov(-Ce3+)-Ptδ+ interfacial sites are formed to significantly promote the generation of CO2 and H2 evolution. In addition, the stronger EMSI between Pt NCs and mpCe/TiO2-0.5 than that between Pt and mpTiO2 contributes to the superior self-enhanced catalytic performance during the cyclic test, where the CO conversion at 200 °C increases from 72% for the fresh catalyst to 99% for the used one. These findings reveal the subtle relationship between the mesoporous metal oxide-metal composite catalysts with unique chemical microenvironments and their catalytic performance, which is expected to inspire the design of efficient heterogeneous catalysts.

Selective Electroreduction of 5-Hydroxymethylfurfural to Dimethylfuran in Neutral Electrolytes via Hydrogen Spillover and Adsorption Configuration Adjustment

5-Hydroxymethylfurfural (HMF), one of the essential C6 biomass derivatives, has been deeply investigated in electrocatalytic reduction upgrading. Nevertheless, the high product selectivity and rational design strategy of electrocatalysts for electrocatalytic HMF reduction is still a challenge. Here, a high selective electro-reduction of HMF to dimethylfuran (DMF) on palladium (Pd) single atom loaded on titanium dioxide (Pd SA/TiO2 ) via hydrogen spillover and adsorption configuration adjustment in neutral electrolytes is achieved. Combining density functional theory calculations and in situ characterization, it is revealed that Pd single atom could weaken the interaction between Pd atoms and adsorbed hydrogen (*H) to promote the *H spillover for increasing *H coverage on the surface and maintain the tilted adsorption configuration to activate C═O bond; thus the selectivity of DMF on Pd SA/TiO2 increases to 90.33%. Besides, it is elaborated that low *H coverage on TiO2 favors the formation of bis(hydroxymethyl)hydro-furoin (BHH), and the flat adsorption configuration of HMF on Pd nanoparticles benefits to form 2,5-dihydroxymethylfuran (DHMF). This work provides a promising approach for modifying electrocatalysts to realize the selective electroreduction of HMF to value-added products.

Recent Advances in the Reverse Water-Gas Conversion Reaction

The increase in carbon dioxide emissions has significantly impacted human society and the global environment. As carbon dioxide is the most abundant and cheap C1 resource, the conversion and utilization of carbon dioxide have received extensive attention from researchers. Among the many carbon dioxide conversion and utilization methods, the reverse water-gas conversion (RWGS) reaction is considered one of the most effective. This review discusses the research progress made in RWGS with various heterogeneous metal catalyst types, covering topics such as catalyst performance, thermodynamic analysis, kinetics and reaction mechanisms, and catalyst design and preparation, and suggests future research on RWGS heterogeneous catalysts.

Tuning Redistribution of CuOx Nanoparticles on TiO2 Support

Nanoparticles exhibit unique catalytic performance, depending on their nanoscale size. However, controlling the particle size of the supported catalysts is still challenging. Here, we present a method for tunable redistribution of CuOx nanoparticles on rutile TiO2 support by physically adding pristine TiO2. The redistribution is driven by the work function difference (WFD) between the TiO2 support and the TiO2 additive, both of which exhibit distinct values, as determined through Kelvin probe force microscopy and electron binding energy analysis. Addition of TiO2 with lower work function (rutile) promotes electron transfer toward the CuOx/TiO2 composite, resulting in nanoparticle aggregation, while addition of TiO2 with higher work function (anatase) results in smaller CuOx on TiO2. The increase in particle size and electron density of CuOx, driven by the addition of rutile TiO2, promoted the complete conversion of nitrobenzene (100%) within 5 h. This is 2.7 times that of dispersed and degraded CuOx driven by mixing with anatase TiO2 (36.9%).

Thermodynamically and Kinetically Stabilized Pt Clusters Against Sintering on CeO2 Nanofibers Through Enclosing CeO2 Nanocubes

Sintering is a major concern for the deactivation of supported metals catalysts, which is driven by the force of decreasing the total surface energy of the entire catalytic system. In this work, a double-confinement strategy is demonstrated to stabilize 2.6 nm-Pt clusters against sintering on electrospun CeO2 nanofibers decorated by CeO2 nanocubes (m-CeO2 ). Thermodynamically, with the aid of CeO2 -nanocubes, the intrinsically irregular surface of polycrystalline CeO2 nanofibers becomes smooth, offering adjacent Pt clusters with decreased chemical potential differences on a relatively uniform surface. Kinetically, the Pt clusters are physically restricted on each facet of CeO2 nanocubes in a nanosized region. In situ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation reveals that the Pt clusters can be stabilized up to 800 °C even in a high density, which is far beyond their Tammann temperature, without observable size growth or migration. Such a sinter-resistant catalytic system is endowed with boosted catalytic activity toward both the hydrogenation of p-nitrophenol after being aged at 500 °C and the sinter-promoting exothermic oxidation reactions (e.g., soot oxidation) at high temperatures over 700 °C. This work offers new opportunities for exploring sinter-resistant nanocatalysts, starting from the rational design of whole catalytic system in terms of thermodynamic and kinetic aspects.

Cu-based high-entropy two-dimensional oxide as stable and active photothermal catalyst

Cu-based nanocatalysts are the cornerstone of various industrial catalytic processes. Synergistically strengthening the catalytic stability and activity of Cu-based nanocatalysts is an ongoing challenge. Herein, the high-entropy principle is applied to modify the structure of Cu-based nanocatalysts, and a PVP templated method is invented for generally synthesizing six-eleven dissimilar elements as high-entropy two-dimensional (2D) materials. Taking 2D Cu₂Zn₁Al_0.5Ce₅Zr_0.5O_x as an example, the high-entropy structure not only enhances the sintering resistance from 400 °C to 800 °C but also improves its CO₂ hydrogenation activity to a pure CO production rate of 417.2 mmol g^-1 h^-1 at 500 °C, 4 times higher than that of reported advanced catalysts. When 2D Cu₂Zn₁Al_0.5Ce₅Zr_0.5O_x are applied to the photothermal CO₂ hydrogenation, it exhibits a record photochemical energy conversion efficiency of 36.2%, with a CO generation rate of 248.5 mmol g^-1 h^-1 and 571 L of CO yield under ambient sunlight irradiation. The high-entropy 2D materials provide a new route to simultaneously achieve catalytic stability and activity, greatly expanding the application boundaries of photothermal catalysis.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Filtration of the preferred catalyst for reverse water-gas shift among Rhn- (n = 3-11) clusters by mass spectrometry under variable temperatures

The key to optimizing energy-consuming catalytic conversions lies in acquiring a fundamental understanding of the nature of the active sites and the mechanisms of elementary steps at an atomically precise level, while it is challenging to capture the crucial step that determines the overall temperature of a real-life catalytic reaction. Herein, benefiting from a newly-developed high-temperature ion trap reactor, the reverse water-gas shift (CO₂ + H₂ → CO + H₂O) reaction catalyzed by the Rh_n^- (n = 3-11) clusters was investigated under variable temperatures (298-783 K) and the critical temperature that each elementary step (Rh_n^- + CO₂ and Rh_nO^- + H₂) requires to take place was identified. The Rh₄^- cluster strikingly surpasses other Rh_n^- clusters to drive the catalysis at a mild starting temperature (∼440 K). This finding represents the first example that a specifically sized cluster catalyst that works under an optimum condition can be accurately filtered by using state-of-the-art mass spectrometric experiments and rationalized by quantum-chemical calculations.

(n, m) Distribution of Single-Walled Carbon Nanotubes Grown from a Non-Magnetic Palladium Catalyst

Non-magnetic metal nanoparticles have been previously applied for the growth of single-walled carbon nanotubes (SWNTs). However, the activation mechanisms of non-magnetic metal catalysts and chirality distribution of synthesized SWNTs remain unclear. In this work, the activation mechanisms of non-magnetic metal palladium (Pd) particles supported by the magnesia carrier and thermodynamic stabilities of nucleated SWNTs with different (n, m) are evaluated by theoretical simulations. The electronic metal-support interaction between Pd and magnesia upshifts the d-band center of Pd, which promotes the chemisorption and dissociation of carbon precursor molecules on the Pd surface, making the activation of magnesia-supported non-magnetic Pd catalysts for SWNT growth possible. To verify the theoretical results, a porous magnesia supported Pd catalyst is developed for the bulk synthesis of SWNTs by chemical vapor deposition. The chirality distribution of Pd-grown SWNTs is understood by operating both Pd-SWNT interfacial formation energy and SWNT growth kinetics. This work not only helps to gain new insights into the activation of catalysts for growing SWNTs, but also extends the use of non-magnetic metal catalysts for bulk synthesis of SWNTs.

Synergistic Interactions of Neighboring Platinum and Iron Atoms Enhance Reverse Water-Gas Shift Reaction Performance

The limitations of conventional strategies in finely controlling the composition and structure demand new promotional effects for upgrading the reverse water-gas shift (RWGS) catalysts for enhanced fuel production. We report the design and synthesis of a hetero-dual-site catalyst for boosting RWGS performance by controllably loading Fe atoms at the neighboring Pt atom on the surface of commercial CeO₂. The Fe-Pt/CeO₂ exhibits a remarkably high catalytic performance (TOF_Pt: 43,519 h^-1) for CO₂ to CO conversion with ∼100% CO selectivity at a relatively low temperature of 350 °C. Furthermore, the catalyst retains over 80% activity after 200 h of continuous operation. The experimental and computational investigations reveal a "two-way synergistic effect", where Fe atoms can not only serve as promotors to alter the charge density of Pt atoms but also be activated by the excess active hydrogen species generated by Pt atoms, enhancing catalytic activity and stability.

Insight into Key Parameters for Fabricating Stable Single-Atom Pt-Nix Alloy by Reduction Environment-Induced Anti-Ostwald Effects

Developing single-atom catalysts with superior stability under reduction conditions is essential for hydrogenation/dehydrogenation catalysis and green hydrogen generation. In this contribution, single-atom Pt catalysts were achieved via a reduction environment-induced anti-Ostwald approach in the highly confined Ni species (Pt-Ni_x ) on nonreducible Al₂ O₃ matrix. In-situ X-ray absorption spectroscopy indicated that the isolated Pt-Ni_x metallic bonds, generated at high reduction temperature, dominated the formation of single Pt atoms. A relatively large cluster of metallic Ni would benefit the stabilization of Pt single atom as observed via high-angle annular dark-field scanning transmission electron microscopy and validated by density functional theory simulation. Excellent performance on cellulose hydrogenolysis was demonstrated under harsh reductive and hydrothermal conditions, potentially expandable to other hydrogen involved reactions like CO₂ hydrogenation, green hydrogen production from different hydrogen carriers, and beyond.

Ordered Mesopore Confined Pt Nanoclusters Enable Unusual Self-Enhancing Catalysis

As an important kind of emerging heterogeneous catalyst for sustainable chemical processes, supported metal cluster (SMC) catalysts have received great attention for their outstanding activity; however, the easy aggregation of metal clusters due to their migration along the substrate's surface usually deteriorates their activity and even causes catalyst failure during cycling. Herein, stable Pt nanoclusters (NCs, ∼1.06 nm) are homogeneously confined in the uniform spherical mesopores of mesoporous titania (mpTiO2) by the interaction between Pt NCs and metal oxide pore walls made of polycrystalline anatase TiO2. The obtained Pt-mpTiO2 exhibits excellent stability with well-retained CO conversion (∼95.0%) and Pt NCs (∼1.20 nm) in the long term water-gas shift (WGS) reaction. More importantly, the Pt-mpTiO2 displays an unusual increasing activity during the cyclic catalyzing WGS reaction, which was found to stem from the in situ generation of interfacial active sites (Ti3+-Ov-Ptδ+) by the reduction effect of spillover hydrogen generated at the stably supported Pt NCs. The Pt-mpTiO2 catalysts also show superior performance toward the selective hydrogenation of furfural to 2-methylfuran. This work discloses an efficient and robust Pt-mpTiO2 catalyst and systematically elucidates the mechanism underlying its unique catalytic activity, which helps to design stable SMC catalysts with self-enhancing interfacial activity in sustainable heterogeneous catalysis.

Ptn-Ov synergistic sites on MoOx/γ-Mo2N heterostructure for low-temperature reverse water-gas shift reaction

In heterogeneous catalysis, the interface between active metal and support plays a key role in catalyzing various reactions. Specially, the synergistic effect between active metals and oxygen vacancies on support can greatly promote catalytic efficiency. However, the construction of high-density metal-vacancy synergistic sites on catalyst surface is very challenging. In this work, isolated Pt atoms are first deposited onto a very thin-layer of MoO₃ surface stabilized on γ-Mo₂N. Subsequently, the Pt-MoO_x/γ-Mo₂N catalyst, containing abundant Pt cluster-oxygen vacancy (Pt_n-O_v) sites, is in situ constructed. This catalyst exhibits an unmatched activity and excellent stability in the reverse water-gas shift (RWGS) reaction at low temperature (300 °C). Systematic in situ characterizations illustrate that the MoO₃ structure on the γ-Mo₂N surface can be easily reduced into MoO_x (2 < x < 3), followed by the creation of sufficient oxygen vacancies. The Pt atoms are bonded with oxygen atoms of MoO_x, and stable Pt clusters are formed. These high-density Pt_n-O_v active sites greatly promote the catalytic activity. This strategy of constructing metal-vacancy synergistic sites provides valuable insights for developing efficient supported catalysts.

Photodriven CO2 Hydrogenation into Diverse Products: Recent Progress and Perspective

Converting CO2 into value-added chemicals through hydrogenation can optimize the energy structure dominated by fossil energy, effectively alleviate environmental problems, and achieve full utilization of carbon resources. However, the traditional CO2 hydrogenation reactions need to be carried out under high temperature and pressure, causing inevitable secondary pollution to the environment. A fundamental way to solve these problems is to use clean solar energy to convert CO2 into value-added chemicals and to establish an artificial carbon cycle process. In this Perspective, we highlight recent advances in photodriven CO2 conversion, including the reverse water-gas-shift reaction, methanation reaction, methanol synthesis reaction, and C2+ hydrocarbon synthesis reaction. Finally, we also discuss the challenges and future investigation opportunities for modulating the selective conversion of CO2. This Perspective offers guidance for the design of photodriven CO2 conversion or even the entire C1 catalyst chemistry for tuning product selectivity and activity.

Partially sintered copper‒ceria as excellent catalyst for the high-temperature reverse water gas shift reaction

For high-temperature catalytic reaction, it is of significant importance and challenge to construct stable active sites in catalysts. Herein, we report the construction of sufficient and stable copper clusters in the copper‒ceria catalyst with high Cu loading (15 wt.%) for the high-temperature reverse water gas shift (RWGS) reaction. Under very harsh working conditions, the ceria nanorods suffered a partial sintering, on which the 2D and 3D copper clusters were formed. This partially sintered catalyst exhibits unmatched activity and excellent durability at high temperature. The interaction between the copper and ceria ensures the copper clusters stably anchored on the surface of ceria. Abundant in situ generated and consumed surface oxygen vacancies form synergistic effect with adjacent copper clusters to promote the reaction process. This work investigates the structure-function relation of the catalyst with sintered and inhomogeneous structure and explores the potential application of the sintered catalyst in C1 chemistry.

Mechanistic and multiscale aspects of thermo-catalytic CO2 conversion to C1 products

The increasing environmental concerns due to anthropogenic CO2 emissions have called for an alternate sustainable source to fulfill rising chemical and energy demands and reduce environmental problems. The thermo-catalytic activation and conversion of abundantly available CO2, a thermodynamically stable and kinetically inert molecule, can significantly pave the way to sustainably produce chemicals and fuels and mitigate the additional CO2 load. This can be done through comprehensive knowledge and understanding of catalyst behavior, reaction kinetics, and reactor design. This review aims to catalog and summarize the advances in the experimental and theoretical approaches for CO2 activation and conversion to C1 products via heterogeneous catalytic routes. To this aim, we analyze the current literature works describing experimental analyses (e.g., catalyst characterization and kinetics measurement) as well as computational studies (e.g., microkinetic modeling and first-principles calculations). The catalytic reactions of CO2 activation and conversion reviewed in detail are: (i) reverse water-gas shift (RWGS), (ii) CO2 methanation, (iii) CO2 hydrogenation to methanol, and (iv) dry reforming of methane (DRM). This review is divided into six sections. The first section provides an overview of the energy and environmental problems of our society, in which promising strategies and possible pathways to utilize anthropogenic CO2 are highlighted. In the second section, the discussion follows with the description of materials and mechanisms of the available thermo-catalytic processes for CO2 utilization. In the third section, the process of catalyst deactivation by coking is presented, and possible solutions to the problem are recommended based on experimental and theoretical literature works. In the fourth section, kinetic models are reviewed. In the fifth section, reaction technologies associated with the conversion of CO2 are described, and, finally, in the sixth section, concluding remarks and future directions are provided.

Self-regeneration of supported transition metals by a high entropy-driven principle

The sintering of Supported Transition Metal Catalysts (STMCs) is a core issue during high temperature catalysis. Perovskite oxides as host matrix for STMCs are proven to be sintering-resistance, leading to a family of self-regenerative materials. However, none other design principles for self-regenerative catalysts were put forward since 2002, which cannot satisfy diverse catalytic processes. Herein, inspired by the principle of high entropy-stabilized structure, a concept whether entropy driving force could promote the self-regeneration process is proposed. To verify it, a high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox is constructed as a host model, and interestingly in situ reversible exsolution-dissolution of supported metallic species are observed in multi redox cycles. Notably, in situ exsolved transition metals from high entropy Zr0.5(NiFeCuMnCo)0.5Ox support, whose entropic contribution (TΔSconfig = T⋆12.7 J mol-1 K-1) is predominant in ∆G, affording ultrahigh thermal stability in long-term CO2 hydrogenation (400 °C, >500 h). Current theory may inspire more STWCs with excellent sintering-resistance performance.

Unlocking the Catalytic Potential of TiO2-Supported Pt Single Atoms for the Reverse Water-Gas Shift Reaction by Altering Their Chemical Environment

Single-atom catalysts (SACs) often exhibit dynamic responses to the reaction and pretreatment environment that affect their activity. The lack of understanding of these behaviors hinders the development of effective, stable SACs, and makes their investigations rather difficult. Here we report a reduction-oxidation cycle that induces nearly 5-fold activity enhancement on Pt/TiO2 SACs for the reverse water-gas shift (rWGS) reaction. We combine microscopy (STEM) and spectroscopy (XAS and IR) studies with kinetic measurements, to convincingly show that the low activity on the fresh SAC is a result of limited accessibility of Pt single atoms (Pt1) due to high Pt-O coordination. The reduction step mobilizes Pt1, forming small, amorphous, and unstable Pt aggregates. The reoxidation step redisperses Pt into Pt1, but in a new, less O-coordinated chemical environment that makes the single metal atoms more accessible and, consequently, more active. After the cycle, the SAC exhibits superior rWGS activity to nonatomically dispersed Pt/TiO2. During the rWGS, the activated Pt1 experience slow deactivation, but can be reactivated by mild oxidation. This work demonstrates a clear picture of how the structural evolution of Pt/TiO2 SACs leads to ultimate catalytic efficiency, offering desired understanding on the rarely explored dynamic chemical environment of supported single metal atoms and its catalytic consequences.

Tuning the shape and crystal phase of TiO2 nanoparticles for catalysis

Synthesis of TiO₂ nanoparticles with tunable shape and crystal phase has attracted considerable attention for the design of highly efficient heterogeneous catalysts. Tailoring the shape of TiO₂, in the crystal phases of anatase, rutile, brookite and TiO₂(B), allows tuning of the atomic configurations on the dominantly exposed facets for maximizing the active sites and regulating the reaction route towards a specific channel for achieving high selectivity. Moreover, the shape and crystal phase of TiO₂ nanoparticles alter their interactions with metal species, which are commonly termed as strong metal-support interactions involving interfacial strain and charge transfer. On the other hand, metal particles, clusters and single atoms interact differently with TiO₂, because of the variation of the electronic structure, while the surface of TiO₂ determines the interfacial bonding via a geometric effect. The dynamic behavior of the metal-titania interfaces, driven by the chemisorption of the reactive molecules at elevated temperatures, also plays a decisive role in elaborating the structure-reactivity relationship.

Probing Single-Atom Catalysts and Catalytic Reaction Processes by Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Developing advanced characterization techniques for single-atom catalysts (SACs) is of great significance to identify their structural and catalytic properties. Raman spectroscopy can provide molecular structure information, and thus, the technique is a promising tool for catalysis. However, its application in SACs remains a great challenge because of its low sensitivity. We develop a highly sensitive strategy that achieves the characterization of the structure of SACs and in situ monitoring of the catalytic reaction processes on them by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) for the first time. Using the strategy, Pd SACs on different supports were identified by Raman spectroscopy and the nucleation process of Pd species from single atoms to nanoparticles was revealed. Moreover, the catalytic reaction processes of the hydrogenation of nitro compounds on Pd SACs were monitored in situ, and molecular insights were obtained to uncover the unique catalytic properties of SACs. This work provides a new spectroscopic tool for the in situ study of SACs, especially at solid-liquid interfaces.