With the aid of DFT calculations, we investigate the elementary reaction steps of dry reforming of methane (DRM) on Ni(111) and NiO(100) as simple models of metallic and oxidized Ni catalysts. The reaction-path calculations reveal that DRM is feasible on metallic Ni at elevated temperatures. However, a notable problem with metallic Ni catalysts is the coke formation because the activation barrier for the C* formation is not considerably higher than those of the competing reactions that lead to the DRM products. In contrast, NiO does not encounter issues with coke formation, but it is not an effective catalyst due to too high activation energies and slow surface diffusion of H*. We also explain the experimentally observed difference between the DRM catalysts consisting of Ni particles supported on undoped and MnO ������-doped CeO2 supports (designated as Ni/CeO2 and Ni/Mn ������CeO2, respectively). Specifically, we explain the absence of the 2020 cm-1 vibrational peak on the Ni/Mn ������CeO2 catalyst. Calculations univocally attribute the experimentally observed 2020 cm-1 peak to CO adsorbed on a top site of metallic Ni because all other sites and involved species display considerably different frequencies. The CO stretching frequency increases as Ni oxidizes, and on NiO(100), it is similar to the vibration of CO on the CeO2(111) support, about 2100 cm-1. Current results thus provide a sound explanation of why Ni/Mn ������CeO2 is a superior DRM catalyst to Ni/CeO2. In particular, the presence of the 2020 cm-1 peak on the Ni/CeO2 catalyst signals that Ni particles are sufficiently metallic and thus susceptible to carbon poisoning. In contrast, the absence of the 2020 cm-1 peak on the Ni/Mn ������CeO2 catalyst indicates that Ni particles are oxidized, i.e., the Ni oxidation is low enough to allow the DRM reaction but high enough to reduce the catalyst's carbon poisoning.

Renewable and green energy transition and meeting ever-increasing energy demands are among modern society's most pressing challenges. Currently, the world's energy systems rely on fossil fuels, leading to significant environmental pollution, including anthropogenic CO2 emissions and subsequent climate changes. The concept of hydrogen economy was first proposed half a century ago as a way to overcome these challenges by using hydrogen as the primary energy carrier instead of fossils.1 However, hydrogen production remains one of the significant obstacles to realizing the hydrogen economy, as steam reforming of methane derived from natural gas is still the dominant method of industrial-scale production, contributing significantly to the undesired emissions. Water electrolysis powered by renewables such as wind and sun energy is a sustainable way to produce high-purity hydrogen. In electrolyzers, hydrogen is obtained at the cathode side through the electrochemical hydrogen evolution reaction (HER), while the oxygen evolution reaction (OER) occurs at the anode. The HER is one of the most intensely studied electrochemical processes as it is of paramount importance for both fundamental and applicable aspects of electrocatalysis and physical chemistry in general. Traditionally, Pt has been known as the best monometallic HER catalyst due to its close-to-optimal interaction with adsorbed hydrogen atoms, which are reaction intermediates.2 However, due to its scarcity and high price, significant research efforts are being invested in designing alternative Pt-free electrocatalysts for (not only) HER. While promising results have been reported with different abundant materials such as sulfides, nitrides, carbides, and phosphides, their applicability is still challenged by rather inferior activity with respect to Pt and rapid deactivation, especially in corrosive acidic media. Currently, state-of-the-art HER catalysts are based on Pt nanoparticles dispersed over high-surface-area carbon supports (Pt/C). The main downside of such composites is the weak interaction between carbon and Pt particles, which means that support cannot affect the intrinsic activity of Pt sites. Moreover, this weak interaction results in the degradation of Pt/C due to particle detachment, agglomeration, or coalescence during HER. Upgrading the overall performance of Pt/C may be achieved by employing alternative supports able to trigger strong metal-support interaction (SMSI). SMSI can alter the electronic structure of the metallic active sites and increase their activity, while also it can lead to the stronger anchoring of nanoparticles with the support, improving stability. Different materials, such as pre-functionalized carbons, carbides, and metal oxides, have been reported to be able to provide SMSI. In this work, we investigated HER on a carbon-ceramic catalyst composed of Pt nanoparticles supported on titanium oxynitride (TiONx) embedded on reduced graphene oxide nanoribbons (Pt/TiONx).3 TiONx was obtained by incorporating nitrogen into TiO2 and provides the main merits of carbon supports, such as electrical conductivity and high surface area, combined with the ability to trigger SMSI. The screening of the electrocatalytic performance of the Pt/TiONx composite for HER revealed enhancement with respect to the Pt/C benchmark, which we ascribe to the effect of the TiONx substrate via SMSI. To confirm this, XPS was performed to compare the electronic states of Pt in Pt/TiONx and Pt/C catalysts, which revealed the ability of TiONx to trigger SMSI. DFT calculation provided additional confirmation of SMSI between the TiONx support and Pt nanoparticles and its impact on both HER activity and stability enhancement. The results presented in this work open up a perspective of using TiONx as an alternative support able to improve the catalytic performance of various active sites for different electrochemical reactions. References (1) Bockris, J. O. M. The Hydrogen Economy: Its History. Int J Hydrogen Energy 2013, 38 (6), 2579–2588. https://doi.org/10.1016/j.ijhydene.2012.12.026. (2) Hansen, J. N.; Prats, H.; Toudahl, K. K.; Mørch Secher, N.; Chan, K.; Kibsgaard, J.; Chorkendorff, I. Is There Anything Better than Pt for HER? ACS Energy Lett 2021, 1175–1180. https://doi.org/10.1021/acsenergylett.1c00246. (3) Smiljanić, M.; Panić, S.; Bele, M.; Ruiz-Zepeda, F.; Pavko, L.; Gašparič, L.; Kokalj, A.; Gaberšček, M.; Hodnik, N. Improving the HER Activity and Stability of Pt Nanoparticles by Titanium Oxynitride Support. ACS Catal 2022, 12 (20), 13021–13033. https://doi.org/10.1021/acscatal.2c03214. Figure 1

Decreasing iridium loading in the electrocatalyst presents a crucial challenge in the implementation of proton exchange membrane (PEM) electrolyzers. In this respect, fine dispersion of Ir on electrically conductive ceramic supports is a promising strategy. However, the supporting material needs to meet the demanding requirements such as structural stability and electrical conductivity under harsh oxygen evolution reaction (OER) conditions. Herein, nanotubular titanium oxynitride (TiON) is studied as a support for iridium nanoparticles. Atomically resolved structural and compositional transformations of TiON during OER were followed using a task-specific advanced characterization platform. This combined the electrochemical treatment under floating electrode configuration and identical location transmission electron microscopy (IL-TEM) analysis of an in-house-prepared Ir-TiON TEM grid. Exhaustive characterization, supported by density functional theory (DFT) calculations, demonstrates and confirms that both the Ir nanoparticles and single atoms induce a stabilizing effect on the ceramic support via marked suppression of the oxidation tendency of TiON under OER conditions.

Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON x ) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON x support. The Pt/TiON x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal-support interaction (SMSI) triggered between the TiON x support and grafted Pt nanoparticles. SMSI between TiON x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.

Hydrogen bonds between either a water molecule or metal hydroxides and small organic molecules with functional groups that contain N, O, S, or P heteroatoms were analyzed using DFT calculations to shed some light on the question of whether hydroxylated nanoparticles and surfaces can be stabilized with organic molecules via hydrogen bonding interactions. Two different models of metal hydroxides were used, that is, small discrete clusters and periodic slab models of surfaces, where Al(OH)3 and Cu(OH)2 served as model systems. For small discrete cluster models, formula units of Al(OH)3 and Cu(OH)2 were taken, whereas for extended surface models, boehmite-AlOOH(010) and Cu(OH)2(001) surfaces were used. According to our results, the Cu(OH)2 cluster is usually a better H-bond acceptor and donor than the water molecule, whereas the Al(OH)3 cluster prefers to either act as an H-bond donor or to form two H-bonds, one as an H-bond donor and the other as an H-bond acceptor. Among the considered organic molecules with functional groups containing N, O, S, or P heteroatoms, imidazole and (CH3)2POOH form the strongest H-bonds; the two molecules are very good H-bond acceptors as well as H-bond donors. These two molecules were also used to analyze hydrogen bonding with the boehmite-AlOOH(010) and Cu(OH)2(001) surfaces. The comparison between the surface and small-cluster calculations reveals that although cluster calculations can give reasonable estimates of adsorption energy provided that all formed H-bonds are properly accounted for (which is not always trivial), there are nevertheless structural intricacies—such as additional H-bonds with second-neighbor OH groups that may form on surfaces—that cannot be captured with small clusters. The more realistic aqueous conditions were also analyzed using the continuum solvation model. They not only influence the properties of H-bonds that are usually shorter than in vacuum but also induce deprotonation of adsorbed molecules, as observed for (CH3)2POOH on a Cu(OH)2 surface.