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Traditional trial-and-error experiments and theoretical simulations have difficulty optimizing catalytic processes and developing new, better-performing catalysts. Machine learning (ML) provides a promising approach for accelerating catalysis research due to its powerful learning and predictive abilities. The selection of appropriate input features (descriptors) plays a decisive role in improving the predictive accuracy of ML models and uncovering the key factors that influence catalytic activity and selectivity. This review introduces tactics for the utilization and extraction of catalytic descriptors in ML-assisted experimental and theoretical research. In addition to the effectiveness and advantages of various descriptors, their limitations are also discussed. Highlighted are both 1) newly developed spectral descriptors for catalytic performance prediction and 2) a novel research paradigm combining computational and experimental ML models through suitable intermediate descriptors. Current challenges and future perspectives on the application of descriptors and ML techniques to catalysis are also presented.

Nitrogen (N2) fixation is a challenging task for chemists. Adsorption of N2 on transition metal (TM) sites has been identified as a prerequisite for activating the very stable N≡N triple bond in both industrial and biological processes. The importance of π back-donation (filled orbitals of TM → π* orbitals of N2) between metal sites and N2 has been well elucidated while the role of another classic orbital interaction, namely σ donation (σ orbitals of N2 → empty orbitals of TM), remains ambiguous. Herein, the size-dependent reactivity of trinuclear rhodium deuteride cluster anions Rh3Dn− (n = 0–3) toward N2 adsorption in the gas phase was investigated experimentally and theoretically. A reverse relationship that higher electron-donating ability of clusters corresponds to lower N2 adsorption reactivity was experimentally observed, which is uncommon in N2 activation by gas-phase species. Theoretical analysis revealed that the σ donation rather than the π back-donation plays a predominant role in the adsorption complexes Rh3DnN2− and the enhanced reactivity upon D addition is ascribed to the lowered energy levels of active orbitals in Rh3Dn− as n increases. This study provides the first experimental evidence to declare the important role of σ donation and new clues for the design of reactive metal species in nitrogen fixation. [ABSTRACT FROM AUTHOR]

We report on cluster‐mediated C−N bond formation in the gas phase using N2 as a nitrogen source. The V3C++N2 reaction is studied by a combination of ion‐trap mass spectrometry with infrared photodissociation (IRPD) spectroscopy and complemented by electronic structure calculations. The proposed reaction mechanism is spectroscopically validated by identifying the structures of the reactant and product ions. V3C+ exhibits a pyramidal structure of C1‐symmetry. N2 activation is initiated by adsorption in an end‐on fashion at a vanadium site, followed by spontaneous cleavage of the N≡N triple bond and subsequent C−N coupling. The IRPD spectrum of the metal nitride product [NV3(C=N)]+ exhibits characteristic C=N double bond (1530 cm−1) and V−N single bond (770, 541 and 522 cm−1) stretching bands. [ABSTRACT FROM AUTHOR]

The activation and transformation of molecular nitrogen (N2) by metal hydride species has attracted widespread attention due to its critical role in nitrogen fixation. Herein, the reactions between tantalum deuteride cluster anions Ta2D2,4− and N2 were investigated experimentally and theoretically. An unprecedented reaction channel of the liberation of a single D atom was observed and much superior reactivity was identified for Ta2D4−. Theoretical investigations indicate that the releasing of D atoms benefits from the completely dissociative adsorption of N2 on the dinuclear metal centres. The extra D atoms in Ta2D4− compared to Ta2D2− are helpful to create sufficient electron density at the adsorption site and modify the symmetry of active orbitals to facilitate a further reduction of N2. This comparative study provides a molecular-level insight to understand the high structure-modulating capability of the additional hydride ligands in polyhydride species in the adsorption and activation of nitrogen molecules. [ABSTRACT FROM AUTHOR]

The increasingly stringent emission regulations make it urgent to get a fundamental understanding on the catalytic behaviors of rhodium, the most efficient component in three-way catalysts (TWCs) to convert NOx into N2. Herein, the catalytic conversion of NO and CO into N2 and CO2 by gas-phase rhodium-aluminum oxide species (RhAlO0–3 and RhAl2O1–4) was identified by mass spectrometry and quantum chemical calculations. This finding represents a significant step in the field of gas-phase catalysis, given that only elementary reactions for this conversion are commonly observed because it is extremely challenging to rupture the strong N–O bond and couple the N–N bond as well as drive the resulting product to oxidize CO. It emphasized that a polarized Rh–Al bond in gas-phase species is indispensable and the RhAlO0,1 and RhAl2O1,2 species with direct Rh–Al bonding can be involved in catalytic NO reduction by CO. [ABSTRACT FROM AUTHOR]

Adsorption and activation of dinitrogen (N2) is an indispensable process in nitrogen fixation. Metal nitride species continue to attract attention as a promising catalyst for ammonia synthesis. However, the detailed mechanisms at a molecular level between reactive nitride species and N2 remain unclear at elevated temperature, which is important to understand the temperature effect and narrow the gap between the gas phase system and condensed phase system. Herein, the 14N/15N isotopic exchange in the reaction between tantalum nitride cluster anions Ta314N3− and 15N2 leading to the regeneration of 14N2/14N15N was observed at elevated temperature (393−593 K) using mass spectrometry. With the aid of theoretical calculations, the exchange mechanism and the effect of temperature to promote the dissociation of N2 on Ta3N3− were elucidated. A comparison experiment for Ta314N4−/15N2 couple indicated that only desorption of 15N2 from Ta314N415N2− took place at elevated temperature. The different exchange behavior can be well understood by the fact that nitrogen vacancy is a requisite for the dinitrogen activation over metal nitride species. This study may shed light on understanding the role of nitrogen vacancy in nitride species for ammonia synthesis and provide clues in designing effective catalysts for nitrogen fixation. [ABSTRACT FROM AUTHOR]

The side‐on‐end‐on coordination of N2 can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas‐phase species (a polynuclear vanadium nitride cluster anion [V5N5]−) that can capture N2 efficiently (12 %), and the quantum chemistry modelling suggests an unusual side‐on‐end‐on coordination. The cluster anions were generated by laser ablation and the reaction with N2 has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back‐donation interactions between the localized d–d bonding orbitals on the low‐coordinated dual metal (V) sites and the antibonding π* orbitals of N2 are the driving forces to adsorb N2 with a high binding energy (about 2.0 eV). [ABSTRACT FROM AUTHOR]

Dinitrogen (N2) activation by metal hydride species is of fundamental interest and practical importance while the role of hydrogen in N2 activation is not well studied. Herein, the structures of Co3Dn− (n = 0–4) clusters and their reactions with N2 have been studied by using a combined experimental and computational approach. The mass spectrometry experiments identified that the Co3Dn− (n = 2–4) clusters could adsorb N2 while the Co3Dn− (n = 0 and 1) clusters were inert. The photoelectron imaging spectroscopy indicated that the electron detachment energies of Co3D2–4− are smaller than those of Co3D0,1−, which characterized that it is easier to transfer electrons from Co3D2–4− than from Co3D0,1− to activate N2. The density functional theory calculations generally supported the experimental observations. Further analysis revealed that the H atoms in the Co3Hn− (n = 2–4) clusters generally result in higher energies of the Co 3d orbitals in comparison with the Co3Hn− (n = 0 and 1) systems. By forming chemical bonds with H atoms, the Co atoms of Co3H2–4− are less negatively charged with respect to the naked Co3− system, which leads to higher N2 binding energies of Co3H2–4N2− than that of Co3N2−. [ABSTRACT FROM AUTHOR]

The side‐on‐end‐on coordination of N2 can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas‐phase species (a polynuclear vanadium nitride cluster anion [V5N5]−) that can capture N2 efficiently (12 %), and the quantum chemistry modelling suggests an unusual side‐on‐end‐on coordination. The cluster anions were generated by laser ablation and the reaction with N2 has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back‐donation interactions between the localized d–d bonding orbitals on the low‐coordinated dual metal (V) sites and the antibonding π* orbitals of N2 are the driving forces to adsorb N2 with a high binding energy (about 2.0 eV). [ABSTRACT FROM AUTHOR]

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N 2 activation mechanism. However, the effective design and search of reactive clusters in N 2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N 2 initial adsorption energy (Δ E ads ) is an important parameter to control the N 2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N 2 activation reactivity of many previously reported metal clusters when combined with the known factor determining the N≡N cleavage process. Furthermore, based on this guideline of Δ E ads , we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV 2 S 2 - , TaV 2 C 2 - , and TaV 2 C 3 - , the high N 2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of Δ E ads revealed in this work not only reshapes our understanding of N 2 activation reactions in the gas phase but also could have implication for other N 2 activation processes in the condensed phase. The more general establishment of this new perspective on N 2 activation reactivity warrants future experimental and computational studies.

We report on cluster-mediated C-N bond formation in the gas phase using N 2 as a nitrogen source. The V 3 C + +N 2 reaction is studied by a combination of ion-trap mass spectrometry with infrared photodissociation (IRPD) spectroscopy and complemented by electronic structure calculations. The proposed reaction mechanism is spectroscopically validated by identifying the structures of the reactant and product ions. V 3 C + exhibits a pyramidal structure of C 1 -symmetry. N 2 activation is initiated by adsorption in an end-on fashion at a vanadium site, followed by spontaneous cleavage of the N≡N triple bond and subsequent C-N coupling. The IRPD spectrum of the metal nitride product [NV 3 (C=N)] + exhibits characteristic C=N double bond (1530 cm -1 ) and V-N single bond (770, 541 and 522 cm -1 ) stretching bands., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The activation and transformation of molecular nitrogen (N 2 ) by metal hydride species has attracted widespread attention due to its critical role in nitrogen fixation. Herein, the reactions between tantalum deuteride cluster anions Ta 2 D 2,4 - and N 2 were investigated experimentally and theoretically. An unprecedented reaction channel of the liberation of a single D atom was observed and much superior reactivity was identified for Ta 2 D 4 - . Theoretical investigations indicate that the releasing of D atoms benefits from the completely dissociative adsorption of N 2 on the dinuclear metal centres. The extra D atoms in Ta 2 D 4 - compared to Ta 2 D 2 - are helpful to create sufficient electron density at the adsorption site and modify the symmetry of active orbitals to facilitate a further reduction of N 2 . This comparative study provides a molecular-level insight to understand the high structure-modulating capability of the additional hydride ligands in polyhydride species in the adsorption and activation of nitrogen molecules.

Nitrogen (N 2 ) fixation is a challenging task for chemists. Adsorption of N 2 on transition metal (TM) sites has been identified as a prerequisite for activating the very stable N≡N triple bond in both industrial and biological processes. The importance of π back-donation (filled orbitals of TM → π* orbitals of N 2 ) between metal sites and N 2 has been well elucidated while the role of another classic orbital interaction, namely σ donation (σ orbitals of N 2 → empty orbitals of TM), remains ambiguous. Herein, the size-dependent reactivity of trinuclear rhodium deuteride cluster anions Rh 3 D n - (n = 0-3) toward N 2 adsorption in the gas phase was investigated experimentally and theoretically. A reverse relationship that higher electron-donating ability of clusters corresponds to lower N 2 adsorption reactivity was experimentally observed, which is uncommon in N 2 activation by gas-phase species. Theoretical analysis revealed that the σ donation rather than the π back-donation plays a predominant role in the adsorption complexes Rh 3 D n N 2 - and the enhanced reactivity upon D addition is ascribed to the lowered energy levels of active orbitals in Rh 3 D n - as n increases. This study provides the first experimental evidence to declare the important role of σ donation and new clues for the design of reactive metal species in nitrogen fixation.

Cleavage of the strong N≡N bond has long been a great challenge for energy-efficient dinitrogen (N 2 ) fixation; thus a reasonable design of reactive species to activate N 2 under mild conditions is highly desirable and meaningful. Herein a novel N 2 activation strategy of combining 5d early (E) and 3d late (L) transition metals (TMs) is proposed, which is verified by the facile and complete N≡N cleavage via the polarized Fe-Ta bond in gas-phase cluster FeTaC 2 - . The efficient N≡N cleavage benefits from an electronic-level design of highly strengthened donor-acceptor interactions, in which the 5d-ETM (Ta) mainly pushes electrons from occupied 5d-orbitals to N 2 π*-orbitals while the 3d-LTM (Fe) simultaneously pulls electrons from N 2 σ/π-orbitals to its unoccupied 3d-orbitals. Through employing 5d-ETM and 3d-LTM to play their respective roles, this work provides a new and versatile idea for activating the inert N≡N bond and inspires relevant design of TM-based catalysts.

Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N 2 has received considerable attention to gain fundamental insights into the activation of the N≡N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions Fe x S y - ( x = 1-8, y = 0- x ) were prepared and their reactivities toward N 2 were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe 5 S 2 - and Fe 5 S 3 - showed superior reactivity toward N 2 . Theoretical results revealed that N 2 binds molecularly to the iron sites of Fe 5 S 2,3 - in a common end-on coordination mode with an unprecedented back-donation interaction from the localized d - d bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N 2 . This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N 2 adsorption in the prevalent end-on binding mode.

It is of great importance to study the mechanisms to activate dinitrogen (N 2 ), the very inert molecule, under mild conditions. Gas-phase metal clusters are being actively generated to react with N 2 to identify new reaction types and mechanisms. Herein, an unprecedented, mechanistically unique metal atom (Fe or V) ejection in the thermal reaction of FeV 2 C 2 - with N 2 has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations. Strong evidence suggests that the complete cleavage of the N≡N triple bond and subsequent functionalization of two N atoms via C-N coupling were achieved in this reaction. The complementary cooperation between V atoms with strong electron-donating ability and an Fe atom with large electron-withdrawing ability as well as the geometric flexibility of the Fe-V-V ring drives the whole reaction. The important role of C ligands in N≡N cleavage was also revealed. This study emphasizes the importance of heteronuclear metal systems for N 2 fixation.

Nitrogen (N 2 ) fixation is a challenging and vital issue in chemistry. Inspired by the fact that the active sites of nitrogenases are polynuclear metal sulfide clusters, the reactivity of gas-phase metal sulfide clusters toward N 2 has received considerable attention to gain fundamental insights into nitrogen fixation. Herein, neutral tantalum sulfide clusters have been prepared and their reactivity toward N 2 has been investigated by mass spectrometry in conjunction with density functional theory (DFT) calculations. The experimental results showed that Ta 3 S n ( n = 0-3) could adsorb N 2 , while Ta 3 S 4 was inert to N 2. The DFT calculations revealed that the complete cleavage of the N≡N bond on the trinuclear metal center in the Ta 3 S 0-3 /N 2 reaction systems was overall barrierless under thermal collision conditions. The sulfur ligands can facilitate the approaching of N 2 toward the metal center but weaken the electron-donating ability of the metal center. The inertness of Ta 3 S 4 is ascribed to the electron-deficient state of Ta 3 in Ta 3 S 4 and the least effective orbital interaction in the Ta 3 S 4 /N 2 couple. This study provides new insights into the ligand effect on the interaction of the metal clusters with N 2.

The side-on-end-on coordination of N 2 can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas-phase species (a polynuclear vanadium nitride cluster anion [V 5 N 5 ] - ) that can capture N 2 efficiently (12 %), and the quantum chemistry modelling suggests an unusual side-on-end-on coordination. The cluster anions were generated by laser ablation and the reaction with N 2 has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back-donation interactions between the localized d-d bonding orbitals on the low-coordinated dual metal (V) sites and the antibonding π* orbitals of N 2 are the driving forces to adsorb N 2 with a high binding energy (about 2.0 eV)., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The construction of C-N bonds by the direct incorporation of dinitrogen (N 2 ) instead of ammonia (NH 3 ) into active species is particularly desirable but has been rarely reported. Herein, a ditantalum carbide cluster anion (Ta 2 C 4 - ) capable of cleaving the N≡N bond and constructing a C-N bond under mild conditions has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum-chemical calculations. The photoelectron spectrum of Ta 2 C 4 N 2 - is remarkably different from that of Ta 2 C 4 - and matches the simulated spectrum of the Ta 2 C 4 N 2 - species with an end-on-bonded CN unit. The formation of the C-N bond has also been supported by the CN - fragment observed in the collision-induced dissociation of Ta 2 C 4 N 2 - . The exceptional reactivity of Ta 2 C 4 - is ascribed to the low-valent metal center serving as an electron reservoir. This study provides a non-NH 3 route to construct C-N bonds by incorporating N 2 into carbide compounds to produce nitrogenous species.

Dinitrogen activation/fixation is one of the most important and challenging subjects in synthetic as well as theoretical chemistry. In this study, the adsorption reactions of N 2 onto TaC n - ( n = 1-4) cluster anions have been investigated by means of mass spectrometry in conjunction with density functional theory calculations. Following the experimental results that only TaC 4 - was observed to adsorb N 2 , theoretical calculations predicted that the TaC 4 - reaction system (TaC 4 - + N 2 → TaC 4 N 2 - ) has a negligible barrier on the approach of N 2 molecule while insurmountable barriers are located on the reaction pathways of TaC 1-3 - /N 2 reaction systems. The natural bond orbital and molecular orbital analysis indicated that the more positive charge on the metal center of TaC 1-4 - would facilitate the initial approach of the nonpolar N 2 molecule, and the appropriate frontier orbital of TaC 1-4 - with proper symmetry (π-type 5d orbital) which can match up well with the π* antibonding orbital of the N 2 molecule with less σ repulsion and more possibility for π back-donation would be helpful for the formation of the stable encounter complexes. This study reveals the fundamental rules and key factors governing the N 2 adsorption.