On the Capabilities of Transition Metal Carbides for Carbon Capture and Utilization Technologies.

The search for cheap and active materials for the capture and activation of CO2 has led to many efforts aimed at developing new catalysts. In this context, earth-abundant transition metal carbides (TMCs) have emerged as promising candidates, garnering increased attention in recent decades due to their exceptional refractory properties and resistance to sintering, coking, and sulfur poisoning. In this work, we assess the use of Group 5 TMCs (VC, NbC, and TaC) as potential materials for carbon capture and sequestration/utilization technologies by combining experimental characterization techniques, first-principles-based multiscale modeling, vibrational analysis, and catalytic experiments. Our findings reveal that the stoichiometric phase of VC exhibits weak interactions with CO2, displaying an inability to adsorb or dissociate it. However, VC often exhibits the presence of surface carbon vacancies, leading to significant activation of CO2 at room temperature and facilitating its catalytic hydrogenation. In contrast, stoichiometric NbC and TaC phases exhibit stronger interactions with CO2, capable of adsorbing and even breaking of CO2 at low temperatures, particularly notable in the case of TaC. Nevertheless, NbC and TaC demonstrate poor catalytic performance for CO2 hydrogenation. This work suggests Group 5 TMCs as potential materials for CO2 abatement, emphasizes the importance of surface vacancies in enhancing catalytic activity and adsorption capability, and provides a reference for using the infrared spectra as a unique identifier to detect oxy-carbide phases or surface C vacancies within Group 5 TMCs.

Transition Metal Carbides as Supports for Catalytic Metal Particles: Recent Progress and Opportunities.

Transition metal carbides (TMCs) constitute excellent alternatives to traditional oxide-based supports for small metal particles, leading to strong metal-support interactions, which drastically modify the catalytic properties of the supported metal atoms. Moreover, they possess extremely high melting points and good resistance to carbon deposition and sulfur poisoning, and the catalytic activities of some TMCs per se have been shown to be similar to those of Pt-group metals for a considerable number of reactions. Therefore, the use of TMCs as supports can give rise to bifunctional catalysts with multiple active sites. However, at present, only TiC and MoxC have been tested experimentally as supports for metal particles, and it is largely unclear which combinations may best catalyze which chemical reactions. In this Perspective, we review the most significant works on the use of TMCs as supports for catalytic applications, assess the current status of the field, and identify key advances being made and challenges, with an eye to the future.

Solvation of furfural at metal-water interfaces: Implications for aqueous phase hydrogenation reactions.

Metal-water interfaces are central to understanding aqueous-phase heterogeneous catalytic processes. However, the explicit modeling of the interface is still challenging as it necessitates extensive sampling of the interfaces' degrees of freedom. Herein, we use ab initio molecular dynamics (AIMD) simulations to study the adsorption of furfural, a platform biomass chemical on several catalytically relevant metal-water interfaces (Pt, Rh, Pd, Cu, and Au) at low coverages. We find that furfural adsorption is destabilized on all the metal-water interfaces compared to the metal-gas interfaces considered in this work. This destabilization is a result of the energetic penalty associated with the displacement of water molecules near the surface upon adsorption of furfural, further evidenced by a linear correlation between solvation energy and the change in surface water coverage. To predict solvation energies without the need for computationally expensive AIMD simulations, we demonstrate OH binding energy as a good descriptor to estimate the solvation energies of furfural. Using microkinetic modeling, we further explain the origin of the activity for furfural hydrogenation on intrinsically strong-binding metals under aqueous conditions, i.e., the endothermic solvation energies for furfural adsorption prevent surface poisoning. Our work sheds light on the development of active aqueous-phase catalytic systems via rationally tuning the solvation energies of reaction intermediates.

Stability and reactivity of metal nanoclusters supported on transition metal carbides.

Small particles of transition metals (TM) supported on transition metal carbides (TMC) - TMn@TMC - provide a plethora of design opportunities for catalytic applications due to their highly exposed active centres, efficient atom utilisation and the physicochemical properties of the TMC support. To date, however, only a very small subset of TMn@TMC catalysts have been tested experimentally and it is unclear which combinations may best catalyse which chemical reactions. Herein, we develop a high-throughput screening approach to catalyst design for supported nanoclusters based on density functional theory, and apply it to elucidate the stability and catalytic performance of all possible combinations between 7 monometallic nanoclusters (Rh, Pd, Pt, Au, Co, Ni and Cu) and 11 stable support surfaces of TMCs with 1 : 1 stoichiometry (TiC, ZrC, HfC, VC, NbC, TaC, MoC and WC) towards CH4 and CO2 conversion technologies. We analyse the generated database to unravel trends or simple descriptors in their resistance towards metal aggregate formation and sintering, oxidation, stability in the presence of adsorbate species, and study their adsorptive and catalytic properties, to facilitate the discovery of novel materials in the future. We identify 8 TMn@TMC combinations as promising catalysts, all of them being new for experimental validation, thus expanding the chemical space for efficient conversion of CH4 and CO2.

The determination of the HOR/HER reaction mechanism from experimental kinetic data.

Hydrogen oxidation and evolution are important processes from both a fundamental and applied perspective. In interpreting experimental kinetic data, few studies have explicitly accounted for the impact of H* coverage and mass transport, which lead to discrepancies in the kinetic parameters and the resultant reaction mechanism. Here, we present how to determine the kinetic parameters accounting for both effects. We discuss the use of the kinetic parameters towards mechanistic interpretations for HOR/HER and show that, in general, knowledge of the coverage of H* or activation energies may be required to assign a reaction mechanism. We apply these ideas to activity data of several HOR and HER electrocatalysts, such as Au, Pt, MoS2, and CoP.

Is There Anything Better than Pt for HER?

The search for cheap and abundant alternatives to Pt for the hydrogen evolution reaction (HER) has led to many efforts to develop new catalysts. Although the discovery of promising catalysts is often reported, none can compete with Pt in intrinsic activity. To enable true progress, a rigorous assessment of intrinsic catalytic activity is needed, in addition to minimizing mass-transport limitations and following best practices for measurements. Herein, we underline the importance of measuring intrinsic catalytic activities, e.g., turnover frequencies (TOFs). Using mass-selected, identical Pt nanoparticles at a range of loadings, we show the pervasive impact of mass-transport limitations on the observed activity of Pt in acid. We present the highest TOF measured for Pt at room temperature. Since our measurements are still limited by mass transport, the true intrinsic HER activity for Pt in acid is still unknown. Using a numerical diffusion model, we suggest that hysteresis in cyclic voltammograms arises from H2 oversaturation, which is another indicator of mass-transport limitations.

Boosting the activity of transition metal carbides towards methane activation by nanostructuring.

The interaction of methane with pristine surfaces of bulk MoC and Mo2C is known to be weak. In contrast, a series of X-ray photoelectron spectroscopy (XPS) experiments, combined with thermal desorption mass spectroscopy (TDS), for MoCy (y = 0.5-1.3) nanoparticles supported on Au(111)-which is completely inert towards CH4-show that these systems adsorb and dissociate CH4 at room temperature and low CH4 partial pressure. This industrially-relevant finding has been further investigated with accurate density functional theory (DFT) based calculations on a variety of MoCy supported model systems. The DFT calculations reveal that the MoCy/Au(111) systems can feature low C-H bond scission energy barriers, smaller than the CH4 adsorption energy. Our theoretical results for bulk surfaces of Mo2C and MoC show that a simple Brønsted-Evans-Polanyi (BEP) relationship holds for C-H bond scission on these systems. However, this is not the case for methane activation on the MoCy nanoparticles as a consequence of their unique electronic and chemical properties. The discovery that supported molybdenum carbide nanoparticles are able to activate methane at room temperature paves the road towards the design of a new family of active carbide catalysts for methane activation and valorisation, with important implications in climate change mitigation and carbon cycle closure.

Assessing the usefulness of transition metal carbides for hydrogenation reactions.

Transition Metal Carbides (TMCs) are proposed as replacements for and expensive late Transition Metals (TMs) as heterogeneous catalysts, often implying hydrogenation reactions or steps. Present density functional theory based calculations support using group IV TMCs and δ-MoC as viable TM alternatives, given the moderate exoergicity and affordable reaction step energy barriers.

Room Temperature Methane Capture and Activation by Ni Clusters Supported on TiC(001): Effects of Metal-Carbide Interactions on the Cleavage of the C-H Bond.

Methane is an extremely stable molecule, a major component of natural gas, and also one of the most potent greenhouse gases contributing to global warming. Consequently, the capture and activation of methane is a challenging and intensively studied topic. A major research goal is to find systems that can activate methane, even at low temperatures. Here, combining ultrahigh vacuum catalytic experiments, X-ray photoemission spectra, and accurate density functional theory (DFT) based calculations, we show that small Ni clusters dispersed on the (001) surface of TiC are able to capture and dissociate methane at room temperature. Our DFT calculations reveal that two-dimensional Ni clusters are responsible for this chemical transformation, confirming that the lability of the supported clusters appears to be a critical aspect in the strong adsorption of methane. A small energy barrier of 0.18 eV is predicted for CH4 dissociation into adsorbed methyl and atomic hydrogen species. In addition, the calculated reaction free energy profile at 300 K and 1 atm of CH4 shows no effective energy barriers in the system. Comparison with other reported systems which activate methane at room temperature, including oxide and zeolite-based materials, indicates that a different chemistry takes place on our metal/carbide system. The discovery of a carbide-based surface able to activate methane at low temperatures paves the road for the design of new types of catalysts which can efficiently convert this hydrocarbon into other added-value chemicals, with implications in climate change mitigation.

Unexpectedly large impact of van der Waals interactions on the description of heterogeneously catalyzed reactions: the water gas shift reaction on Cu(321) as a case example.

The molecular mechanisms of the water gas shift reaction on Cu(321) have been chosen to investigate the effect of dispersion terms on the description of the energy profile and reaction rates. The present study based on periodic DFT calculations shows that including dispersion terms does not change the qualitative picture of the overall reaction, maintaining the rate determining step and the predominant route. However, the effect of dispersion is different for different adsorbates - reactants, intermediates or products - with a clear net effect and with no compensation of errors. Thus, in the OH + OH → H2O + O process the dispersion effects imply up to three orders of magnitude in the calculated reaction rates; the formation of carboxyl is highly disfavoured when dispersion terms are explicitly included and finally, the reaction rate for CO2 production (at 463 K) through cis-COOH dissociation is enhanced by three orders of magnitude by including dispersion terms in the calculation of the energy barrier. Consequently, the inclusion of dispersion terms largely affects the overall potential energy profile and produces tremendous changes in the predicted reaction rates. Therefore, dispersion terms must be included when aiming at obtaining information from macroscopic simulations employing for instance microkinetic or kinetic Monte Carlo approaches, where these effects should be clearly shown.

ReaxFF molecular dynamics simulations of CO collisions on an O-preadsorbed silica surface.

A quasiclassical trajectory dynamics study was performed for carbon monoxide collisions over an oxygen preadsorbed ß-cristobalite (001) surface. A reactive molecular force field (ReaxFF) was used to model the potential energy surface. The collisions were performed fixing several initial conditions: CO rovibrational states (v = 0-5 and j = 0, 20, 35), collision energies (0.05 ≤ E(col) ≤ 2.5 eV), incident angles (θ(v) = 0°, 45°) and surface temperatures (T(surf) = 300 K, 900 K). The principal elementary processes were the molecular reflection and the non-dissociative molecular adsorption. CO2 molecules were also formed in minor extension via an Eley-Rideal reaction although some of them were finally retained on the surface. The scattered CO molecules tend to be translationally colder and internally hotter (rotationally and vibrationally) than the initial ones. The present study supports that CO + O(ad) reaction should be less important than O + O(ad) reaction over silica for similar initial conditions of reactants, in agreement with experimental data.