Propane dehydrogenation catalysis of group IIIB and IVB metal hydrides

Catalytic propane dehydrogenation (PDH) has mainly been studied using metal- and metal oxide-based catalysts. Studies on dehydrogenation catalysis by metal hydrides, however, have rarely been reported. In this study, PDH reactions using group IIIB and IVB metal hydride catalysts were investigated under relatively low-temperature conditions of 450 °C. Lanthanum hydride exhibited the lowest activation energy for dehydrogenation and the highest propylene yield. Based on kinetics studies, a comparison between the reported calculation results and isotope experiments, the hydrogen vacancies of metal hydrides were involved in low-temperature PDH reactions.

Ammonia Synthesis via an Associative Mechanism on Alkaline Earth Metal Sites of Ca3 CrN3 H

Typically, transition metals are considered as the centers for the activation of dinitrogen. Here we demonstrate that the nitride hydride compound Ca3 CrN3 H, with robust ammonia synthesis activity, can activate dinitrogen through active sites where calcium provides the primary coordination environment. DFT calculations also reveal that an associative mechanism is favorable, distinct from the dissociative mechanism found in traditional Ru or Fe catalysts. This work shows the potential of alkaline earth metal hydride catalysts and other related 1 D hydride/electrides for ammonia synthesis.

Exploring Metal Nanocluster Catalysts for Ammonia Synthesis Using Informatics Methods: A Concerted Effort of Bayesian Optimization, Swarm Intelligence, and First-Principles Computation

This paper details the use of computational and informatics methods to design metal nanocluster catalysts for efficient ammonia synthesis. Three main problems are tackled: defining a measure of catalytic activity, choosing the best candidate from a large number of possibilities, and identifying the thermodynamically stable cluster catalyst structure. First-principles calculations, Bayesian optimization, and particle swarm optimization are used to obtain a Ti8 nanocluster as a catalyst candidate. The N2 adsorption structure on Ti8 indicates substantial activation of the N2 molecule, while the NH3 adsorption structure suggests that NH3 is likely to undergo easy desorption. The study also reveals several cluster catalyst candidates that break the general trade-off that surfaces that strongly adsorb reactants also strongly adsorb products.

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.

Influence of Gas for Thermal Treatment on Hydrogen Permeation in V-Ni Alloy Membranes

Hydrogen separation is an important step for the utilization of hydrogen energy. Metallic alloys, such as vanadium-nickel, are potential hydrogen separation materials. Due to the strong propensity of vanadium to form oxides and hydrides, vanadium alloy has a lower hydrogen permeability, and it is difficult to maintain the permeability over time. Therefore, special preparation processes such as Pd coating have been suggested for hydrogen separation vanadium-based membranes. However, aside from the prohibitive price of palladium, the interdiffusion of palladium and vanadium makes the coated membrane inviable to be used at a high temperature. Thermal treatment with inert gas was investigated in this study to assess the applicability of the vanadium alloy without palladium coating for hydrogen separation and clarify the mechanism behind the thermal treatment. Argon is inert with vanadium and displayed permeability recovery after 43 h thermal treatment, but the permeability declined under certain conditions. In contrast, nitrogen is known to interact with vanadium and the hydrogen permeability was maintained at a level lower than the test with argon. Given that nitrogen can compete with hydrogen for the active sites on vanadium, nitrogen might hinder hydrogen adsorption and hydride formation, whereas argon reduced the partial pressure of hydrogen during the thermal treatment, enhancing the driving force of hydrogen desorption. In the X-ray diffraction spectrum, vanadium hydrides and oxides were confirmed after hydrogen permeation and thermal treatment. In the X-ray photoelectron spectroscopy data, oxygen was a dominant element due to vanadium oxides and adsorbed nitrogen was also observed. According to binding energy shifts of nitrogen, nitrogen used for thermal treatment might substitute or compete for active sites with adsorbed nitrogen and hydrogen, existing in vanadium lattice. Although thermal treatment can be used to recover hydrogen permeability, the alloy cannot be recovered as hydrogen-free. However, results demonstrate the potential of thermal treatment to complement an uncoated vanadium alloy for a hydrogen separation membrane.

Impact of Gas-Solid Reaction Thermodynamics on the Performance of a Chemical Looping Ammonia Synthesis Process

Novel ammonia catalysts seek to achieve high reaction rates under milder conditions, which translate into lower costs and energy requirements. Alkali and alkaline earth metal hydrides have been shown to possess such favorable kinetics when employed in a chemical looping process. The materials act as nitrogen carriers and form ammonia by alternating between pure nitrogen and hydrogen feeds in a two-stage chemical looping reaction. However, the thermodynamics of the novel reaction route in question are only partially available. Here, a chemical looping process was designed and simulated to evaluate the sensitivity of the energy and economic performance of the processes toward the appropriate gas-solid reaction thermodynamics. Thermodynamic parameters, such as reaction pressure and especially equilibrium ammonia yields, influenced the performance of the system. In comparison to a commercial ammonia synthesis unit with a 28% yield at 150 bar, the chemical looping process requires a yield greater than 38% to achieve similar energy consumptions and a yield greater than 26% to achieve similar costs at a given temperature and 150 bar. Entropies and enthalpies of formation of the following pairs were estimated and compared: LiH/Li₂NH, MgH₂/MgNH, CaH₂/CaNH, SrH₂/SrNH, and BaH₂/BaNH. Only the LiH/Li₂NH pair has satisfied the given criteria, and initial estimates suggest that a 62% yield is obtainable.

Catalytic Performance and Near-Surface X-ray Characterization of Titanium Hydride Electrodes for the Electrochemical Nitrate Reduction Reaction

The electrochemical nitrate reduction reaction (NO₃RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiH_x, 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH₂ enrichment with increasing NO₃RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH₂/Ti electrodes for use in NO₃RR, thereby decoupling hydride formation from NO₃RR performance. A wide range of NO₃RR activity and selectivity on TiH₂/Ti electrodes between -0.4 and -1.0 V_RHE was observed and analyzed with density functional theory (DFT) calculations on TiH₂(111). This work underscores the importance of relating NO₃RR performance with near-surface electrode structure to advance catalyst design and operation.

Mixed Anion Control of the Partial Oxidation of Methane to Methanol on the β-PtO2 Surface

Although the C-H bond of methane is very strong, it can be easily dissociated on the (110) surface of β-PtO2. This is because a very stable Pt-C bond is formed between the coordinatively unsaturated Pt atom and CH3 on the surface. Owing to the stable nature of the Pt-C bond, CH3 is strongly bound to the surface. When it comes to methanol synthesis from methane, the Pt-C bond has to be cleaved to form a C-O bond during the reaction process. However, this is unlikely to occur on the β-PtO2 surface: The activation energy of the process is calculated to be so large as 47.9 kcal/mol. If the surface can be modified in such a way that the ability for the C-H bond activation is maintained but the Pt-C bond is weakened, a catalyst combining the functions of C-H bond cleavage and C-O bond formation can be created. For this purpose, analyzing the orbital interactions on the surface is found to be very useful, resulting in a prediction that the Pt-C bond can be weakened by replacing the O atom trans to the C atom with a N atom. This would be a sort of process to make β-PtO2 a mixed anion compound. Density functional theory simulations of catalytic reactions on the β-PtO2 surface show that the activation energy of the rate-limiting step of methanol synthesis can be reduced to 27.7 kcal/mol by doping the surface with N.

Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H^- could supply. Polarized, electron-rich N_xH_y intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N₂, NH₃, and H₂, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH₃. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N₂ is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H^- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.