Reactivity of Hydrogen on and in Nanostructured Molybdenum Nitride: Crotonaldehyde Hydrogenation

Transition metal nitride catalysts for selective conversion of oxygen-containing molecules

Earth abundant transition metal nitrides (TMNs) are a promising group of catalysts for a wide range of thermocatalytic, electrocatalytic and photocatalytic reactions, with potential to achieve high activity and selectivity while reducing reliance on the use of Pt-group metals. However, current fundamental understanding of the active sites of these materials and the mechanisms by which selective transformations occur is somewhat lacking. Recent investigations of these materials from our group and others have utilized probe molecules, model surfaces, and in situ techniques to elucidate the origin of their activity, strong metal-support interactions, and unique d-band electronic structures. This Perspective discusses three classes of reactions for which TMNs have been used as case studies to highlight how these properties, along with synergistic interactions with metal overlayers, can be exploited to design active, selective and stable TMN catalysts. First, studies of the reactions of C1 molecules will be discussed, specifically highlighting the ability of TMNs to activate CO2. Second, the upgrading of biomass and biomass-derived oxygenates over TMN catalysts will be reviewed. Third, the use of TMNs for H2 production via water electrolysis will be discussed. Finally, we will discuss the challenges and future directions in the study of TMN catalysts, in particular expanding on opportunities to enhance fundamental mechanistic understanding using model surfaces, the elucidation of active centers via in situ techniques, and the development of efficient synthesis methods and design principles.

Nickel Carbide Nanoparticle Catalyst for Selective Hydrogenation of Nitriles to Primary Amines

Despite its unique physicochemical properties, the catalytic application of nickel carbide (Ni3 C) in organic synthesis is rare. In this study, we report well-defined nanocrystalline Ni3 C (nano-Ni3 C) as a highly active catalyst for the selective hydrogenation of nitriles to primary amines. The activity of the aluminum-oxide-supported nano-Ni3 C (nano-Ni3 C/Al2 O3 ) catalyst surpasses that of Ni nanoparticles. Various aromatic and aliphatic nitriles and dinitriles were successfully converted to the corresponding primary amines under mild conditions (1 bar H2 pressure). Furthermore, the nano-Ni3 C/Al2 O3 catalyst was reusable and applicable to gram-scale experiments. Density functional theory calculations suggest the formation of polar hydrogen species on the nano-Ni3 C surface, which were attributed to the high activity of nano-Ni3 C towards nitrile hydrogenation. This study demonstrates the utility of metal carbides as a new class of catalysts for liquid-phase organic reactions.

Neutron Scattering Studies of Heterogeneous Catalysis

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

Molybdenum Oxide Precursors that Promote the Low-Temperature Formation of High-Surface-Area Cubic Molybdenum (Oxy)nitride

Molybdenum nitrides and oxynitrides have been increasingly realized as (electro)catalysts for a variety of reactions. In this context, the cubic "γ-Mo₂N", also known to contain oxygen in the bulk, is of particular interest. The γ phase is typically derived from ammonolysis of MoO₃, and a high temperature is needed to fully react the stable MoO₂ intermediate that often forms along the reaction pathway. In this study, ammonolysis of atypical bronze (H_xMoO₃) and peroxo (H₂MoO₅) precursors was undertaken to avoid the formation of this undesired intermediate with the aim of synthesizing "γ-Mo₂N" at reduced temperatures and thus with a high surface area. It was found, using in situ powder diffraction, that, when the phase I bronze (x ≈ 0.3) served as the precursor, MoO₂ formed as an intermediate and was retained in the reaction product until 700 °C. In contrast, ammonolysis of the phase III bronze (x ≈ 1.7) and of H₂MoO₅ circumvented the MoO₂ intermediate. From these latter two precursors, "γ-Mo₂N" was formed at the lowest maximum reaction temperatures reported in the literature, namely, 480 °C in the case of H_xMoO₃-III and 380 °C for H₂MoO₅. The resulting products displayed extremely high surface areas of 206 and 152 m²/g, respectively, presumably as a consequence of the low synthesis temperatures. While the H_xMoO₃-III precursor showed evidence of a topotactic transformation pathway, with morphological similarity between precursor and product phases, H₂MoO₅ transformed via amorphization. Electrochemical characterization showed moderate activity for the hydrogen evolution reaction (HER), which increased after exposure to reducing potentials and loosely scaled with the catalyst-specific surface area. This work points toward new low-temperature synthesis pathways for accessing molybdenum (oxy)nitrides with high surface areas.

Fundamental understanding of the synthesis of well-defined supported non-noble metal intermetallic compound nanoparticles

Fundamental insights into the synthesis of model-like, supported, non-noble metal intermetallic compound nanoparticle catalysts with phase pure bulk and bulk-like 1st-atomic-layer particle surface composition.

Selective Preparation of Mo2N and MoN with High Surface Area for Flexible SERS Sensing

γ-Mo₂N and δ-MoN are the two most important molybdenum nitrides, but controllable preparation of them with high surface area has not been achieved. Herein, we achieved selective preparation of γ-Mo₂N and δ-MoN. The key factor for the selective preparation of γ-Mo₂N and δ-MoN is to control the crystal phase of the precursor MoO₃. In H₂O and NH₃ mixed gas, the α-MoO₃ nanoribbons are nitridated to obtain γ-Mo₂N single-crystal porous nanobelts, while the h-MoO₃ prisms are nitrided to obtain δ-MoN hierarchical porous columns. The corrosion effect of H₂O plays a key role in the formation of single-crystal porous structure. The γ-Mo₂N flexible membrane composed of the single-crystal porous nanobelts exhibits strong localized surface plasmon resonance and surface enhanced Raman scattering effect, which show highly sensitive response to polychlorinated phenol.

Unveiling the Activity Origin of Iron Nitride as Catalytic Material for Efficient Hydrogenation of CO2 to C2+ Hydrocarbons

Developing efficient catalytic materials and unveiling the active species are significant for selective hydrogenation of CO₂ to C₂₊ hydrocarbons. Fe₂ N@C nanoparticles were reported to exhibit outstanding performance toward selective CO₂ hydrogenation to C₂₊ hydrocarbons (C₂₊ selectivity: 53.96 %; C₂ -C₄ ⁼ selectivity, 31.03 %), outperforming corresponding Fe@C. In situ X-ray diffraction, ex situ Mössbauer and X-ray photoelectron spectra revealed that iron nitrides were in situ converted to highly active iron carbides, which acted as the real active species. Moreover, the combined results of in situ diffuse reflectance infrared Fourier transform spectroscopy and control experiments suggested an in situ formed carbonyl iron-mediated conversion mechanism from iron nitrides to iron carbides.

Defect-mediated ab initio thermodynamics of metastable γ-MoN(001)

Refractory transition metal nitrides exhibit a plethora of polymorphic expressions and chemical stoichiometries. To afford a better understanding of how defects may play a role in the structural and thermodynamics of these nitrides, using density-functional theory calculations, we investigate the influence of point and pair defects in bulk metastable γ-MoN and its (001) surface. We report favorable formation of Schottky defect pairs of neighboring Mo and N vacancies in bulk γ-MoN and apply this as a defect-mediated energy correction term to the surface energy of γ-MoN(001) within the ab initio atomistic thermodynamics approach. We also inspect the structural distortions in both bulk and surfaces of γ-MoN by using the partial radial distribution function, g(r), of Mo-N bond lengths. Large atomic displacements are found in both cases, leading to a broad spread of Mo-N bond length values when compared to their idealized bulk values. We propose that these structural and thermodynamic analyses may provide some insight into a better understanding of metastable materials and their surfaces.

Investigations of Hydrocarbon Species on Solid Catalysts by Inelastic Neutron Scattering

The status of surface species on solid catalysts during heterogeneous catalysis is often mysterious. Investigations of these surface species are crucial to deconvolute the reaction network and design more efficient catalysts. Vibrational spectroscopy is a powerful technique to study the interactions between surface species and the catalysts and infrared (IR) and Raman spectroscopies have been widely applied to study reaction mechanisms in heterogeneous catalysis. However, IR/Raman spectra are difficult to model computationally and important vibrational modes may be IR-, Raman- (or both) inactive due to restrictions by optical selection rules. Inelastic neutron scattering (INS) is another form of vibrational spectroscopy and relies on the scattering of neutrons by the atomic nucleus. A consequence of this is that INS is not subject to any optical selection rules and all vibrations are measurable in principle. INS spectroscopy has been used to investigate surface species on catalysts in a wide range of heterogeneous catalytic reactions. In this mini-review, we focus on applications of INS in two important fields: petrochemical reactions and C1 chemistry. We introduce the basic principles of the INS technique, followed by a discussion of its application in investigating two key catalytic systems: (i) the behaviour of hydrocarbons on metal-oxide and zeolite catalysts and (ii) the formation of hydrocarbonaceous species on methane reforming and Fischer–Tropsch catalysts. The power of INS in studying these important catalytic systems is demonstrated.

Preparation of an Unsupported Copper-Based Catalyst for Selective Hydrogenation of Acetylene from Cu2O Nanocubes

Designing cheap, earth-abundant, and nontoxic metal catalysts for acetylene hydrogenation is of pivotal importance, but challenging. Here, a nonprecious metal catalyst for selective hydrogenation of acetylene in excess ethylene was prepared from Cu₂O nanocubes. The preparation includes two steps: (1) thermal treatment in acetylene-containing gas at 160 °C and (2) hydrogen reduction at 180 °C. The resultant catalyst showed outstanding performance at low temperature (80-100 °C) and 0.1-0.5 MPa pressure, completely converting acetylene with a low selectivity to undesired ethane (<20%). The characterization results of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy corroborated that the formation of an interstitial copper carbide (Cu_xC) might give rise to significantly enhanced hydrogenation activity. Preliminary density functional theory calculation demonstrated that the lattice spacing of Cu₃C was nearly identical to that of the new Cu_xC crystallite measured in HRTEM and determined by XRD. The calculated dissociation energy of hydrogen on Cu₃C(0001) was considerably lower than that on Cu(111), suggesting superior hydrogenation activity of Cu₃C(0001). It is experimentally verified that copper(I) acetylide (Cu₂C₂) might be the precursor of Cu_xC. Cu₂C₂ underwent partial hydrogenation to fabricate Cu_xC crystallites and the thermal decomposition to Cu and carbon materials in parallel.

Recent Progress on Transition Metal Nitrides Nanoparticles as Heterogeneous Catalysts

This short review aims at providing an overview of the most recent literature regarding transition metal nitrides (TMN) applied in heterogeneous catalysis. These materials have received renewed attention in the last decade due to its potential to substitute noble metals mainly in biomass and energy transformations, the decomposition of ammonia being one of the most studied reactions. The reactions considered in this review are limited to thermal catalysis. However the potential of these materials spreads to other key applications as photo- and electrocatalysis in hydrogen and oxygen evolution reactions. Mono, binary and exceptionally ternary metal nitrides have been synthetized and evaluated as catalysts and, in some cases, promoters are added to the structure in an attempt to improve their catalytic performance. The objective of the latest research is finding new synthesis methods that allow to obtain smaller metal nanoparticles and increase the surface area to improve their activity, selectivity and stability under reaction conditions. After a brief introduction and description of the most employed synthetic methods, the review has been divided in the application of transition metal nitrides in the following reactions: hydrotreatment, oxidation and ammonia synthesis and decomposition.

New insights into the effect of nitrogen incorporation in Mo: catalytic hydrogenation vs. hydrogenolysis

The incorporation of nitrogen into bulk Mo has a contrasting effect on hydrogenation (nitrobenzene) and hydrogenolysis (benzaldehyde) processes.

Ultra-small Mo2N on SBA-15 as a highly efficient promoter of low-loading Pd for catalytic hydrogenation

Decreasing Pd usage whilst maintaining a superior performance is promising, but remains a challenge in the catalytic field. Herein, we have demonstrated the highly efficient promotion of Mo₂N with a reduced amount of Pd for the liquid-phase hydrogenation reaction. The Mo₂N (2-3 nm) was uniformly anchored onto mesoporous SBA-15 by using PMo₁₂ as the Mo source. The small size and good dispersion of Mo₂N is favourable for allowing their effective contact with post-loading Pd. This good contact is conducive to developing a synergistic catalyst, which was verified by studying the liquid-phase hydrogenation of p-nitrophenol (PNP) to p-aminophenol (PAP) with NaBH₄ as the H source. The conversion ability of PNP to PAP on 1 wt% Pd-Mo₂N/SBA-15 was vastly superior to 1 wt% Pd/SBA-15 and even better than 20 wt% Pd/SBA-15. The low-Pd, highly efficient catalysis is ascribed to the transfer of the electrons from Mo₂N to Pd for the easy activation of H. The synergy can be affected by the type of support used. SBA-15 is superior to SiO₂ and the other supports, which could be related to the large surface area and the plentiful number of pores on SBA-15, which is favourable to the dispersion of Pd and Mo₂N, and the transfer/diffusion of the reactants. In particular, a highly efficient catalyst can be achieved at an even more reduced Pd loading (0.05 wt%). The current method describes the design of a highly efficient catalyst for the hydrogenation reaction using low amounts of noble metals.

Bimetallic molybdenum nitride Co3Mo3N: a new promising catalyst for CO2 reforming of methane

The synergistic effect between Mo–Co in Co₃Mo₃N leads to its enhanced catalytic activity and stability in CO₂ reforming of methane.

Highly selective hydrodeoxygenation of anisole, phenol and guaiacol to benzene over nickel phosphide

Ni₂P supported catalysts have extensively been studied for various hydrodeoxygenation (HDO) reactions to benzene at a higher reaction temperature and lower H₂ pressure.