Plasma Catalysis for Hydrogen Production: A Bright Future for Decarbonization.

Thermal approaches have played a dominant role in driving chemical reactions within the chemicals and fuels industries, benefiting from ongoing enhancements in efficiency via heat integration, catalyst development, and process intensification. Nevertheless, these traditional thermal approaches remain heavily reliant on fossil fuels, and there exists an urgent demand for the implementation of renewable energy technologies to synthesize fuels, commodity chemicals, and specialty chemicals. Nonthermal plasmas have gained considerable attention in recent years as a promising solution, and the prospects of combining plasmas with suitable catalysts have become even more appealing. Moreover, the evolution of nonthermal plasma catalysis approaches for the generation of clean hydrogen could be transformative in reducing greenhouse gas emissions. This comprehensive review highlights the influential contributions in plasma catalysis for hydrogen production, discusses recent advancements, and provides future prospects for researchers aiming to advance the production of clean hydrogen.

Interrogation of the Plasma-Catalyst Interface via In Situ/Operando Transmission Infrared Spectroscopy.

Plasma-surface coupling has emerged as a promising approach to perform chemical transformations under mild conditions that are otherwise difficult or impossible thermally. However, a few examples of inexpensive and accessible in situ/operando techniques exist for observing plasma-solid interactions, which has prevented a thorough understanding of underlying surface mechanisms. Here, we provide a simple and adaptable design for a dielectric barrier discharge (DBD) plasma cell capable of interfacing with Fourier transform infrared spectroscopy (FTIR), optical emission spectroscopy (OES), and mass spectrometry (MS) to simultaneously characterize the surface, the plasma phase, and the gas phase, respectively. The system was demonstrated using two example applications: (1) plasma oxidation of primary amine functionalized SBA-15 and (2) catalytic low temperature nitrogen oxidation. The results from application (1) provided direct evidence of a 1% O2/He plasma interacting with the aminosilica surface by selective oxidation of the amino groups to nitro groups without altering the alkyl tether. Application (2) was used to detect the evolution of NOX species bound to both platinum and silica surfaces under plasma stimulation. Together, the experimental results showcase the breadth of possible applications for this device and confirm its potential as an essential tool for conducting research on plasma-surface coupling.

Ethylene oligomerization on Ni2+ single sites within lacunary defects of Wells Dawson polyoxometalates.

Oligomerization of light olefins has become an essential step to convert gaseous olefins to liquid fuels and value-added chemicals. Here, we report the synthesis and application of nickel single sites isolated on Wells Dawson polyoxometalate defects as stable and regenerable catalysts for ethylene oligomerization.

Unconventional Catalytic Approaches to Ammonia Synthesis.

Ammonia is a critically important industrial chemical and is largely responsible for sustaining the growing global population. To provide ammonia to underdeveloped regions and/or regions far from industrial production hubs, modular systems have been targeted and often involve unconventional production methodologies. These novel approaches for ammonia production can tap renewable resources at smaller scales located at the point of use, while decreasing the CO2 footprint. Plasma-assisted catalysis and electrochemical ammonia synthesis have promise owing to their atmospheric pressure and low-temperature operation conditions and the ability to construct units at scales desired for modularization. Fundamental and applied studies are underway to assess these processes, although many unknowns remain. In this review, we discuss recent developments and opportunities for unconventional ammonia synthesis with a focus on plasma-stimulated systems.

Synergistic effects of plasma-catalyst interactions for CH4 activation.

The elucidation of catalyst surface-plasma interactions is a challenging endeavor and therefore requires thorough and rigorous assessment of the reaction dynamics on the catalyst in the plasma environment. The first step in quantifying and defining catalyst-plasma interactions is a detailed kinetic study that can be used to verify appropriate reaction conditions for comparison and to discover any unexpected behavior of plasma-assisted reactions that might prevent direct comparison. In this paper, we provide a kinetic evaluation of CH4 activation in a dielectric barrier discharge plasma in order to quantify plasma-catalyst interactions via kinetic parameters. The dry reforming of CH4 with CO2 was studied as a model reaction using Ni supported on γ-Al2O3 at temperatures of 790-890 K under atmospheric pressure, where the partial pressures of CH4 (or CO2) were varied over a range of ≤25.3 kPa. Reaction performance was monitored by varying gas hourly space velocity, plasma power, bulk gas temperature, and reactant concentration. After correcting for gas-phase plasma reactions, a linear relationship was observed in the log of the measured rate constant with respect to reciprocal power (1/power). Although thermal catalysis displays typical Arrhenius behavior for this reaction, plasma-assisted catalysis occurs from a complex mixture of sources and shows non-Arrhenius behavior. However, an energy barrier was obtained from the relationship between the reaction rate constant and input power to exhibit ≤∼20 kJ mol-1 (compared to ∼70 kJ mol-1 for thermal catalysis). Of additional importance, the energy barriers measured during plasma-assisted catalysis were relatively consistent with respect to variations in total flow rates, types of diluent, or bulk reaction temperature. These experimental results suggest that plasma-generated vibrationally-excited CH4 favorably interacts with Ni sites at elevated temperatures, which helps reduce the energy barrier required to activate CH4 and enhance CH4 reforming rates.

Chelating agent-free, vapor-assisted crystallization method to synthesize hierarchical microporous/mesoporous MIL-125 (Ti).

Titanium-based microporous heterogeneous catalysts are widely studied but are often limited by the accessibility of reactants to active sites. Metal-organic frameworks (MOFs), such as MIL-125 (Ti), exhibit enhanced surface areas due to their high intrinsic microporosity, but the pore diameters of most microporous MOFs are often too small to allow for the diffusion of larger reactants (>7 Å) relevant to petroleum and biomass upgrading. In this work, hierarchical microporous MIL-125 exhibiting significantly enhanced interparticle mesoporosity was synthesized using a chelating-free, vapor-assisted crystallization method. The resulting hierarchical MOF was examined as an active catalyst for the oxidation of dibenzothiophene (DBT) with tert-butyl hydroperoxide and outperformed the solely microporous analogue. This was attributed to greater access of the substrate to surface active sites, as the pores in the microporous analogues were of inadequate size to accommodate DBT. Moreover, thiophene adsorption studies suggested the mesoporous MOF contained larger amounts of unsaturated metal sites that could enhance the observed catalytic activity.

CO2 capture and conversion with a multifunctional polyethyleneimine-tethered iminophosphine iridium catalyst/adsorbent.

Tunable, multifunctional materials able to capture CO2 and subsequently catalyze its conversion to formic acid were synthesized by the modification of branched polyethyleneimine (PEI) with an iminophosphine ligand coordinated to an Ir precatalyst. The molecular weight of the PEI backbone was an important component for material stability and catalytic activity, which were inversely related. The amine functionalities on PEI served three roles: 1) primary amines were used to tether the ligand and precatalyst, 2) amines were used to capture CO2 , and 3) amines served as a base for formate stabilization during catalysis. Ligand studies on imine and phosphine based ligands showed that a bidentate iminophosphine ligand resulted in the highest catalytic activity. X-ray photoelectron spectroscopy revealed that an increase in Ir 4f binding energy led to an increase in catalytic activity, which suggests that the electronics of the metal center play a significant role in catalysis. Catalyst loading studies revealed that there is a critical balance between free amines and ligand-metal sites that must be reached to optimize catalytic activity. Thus, it was found that the CO2 capture and conversion abilities of these materials could be optimized for reaction conditions by tuning the structure of the PEI-tethered materials.

Carbothermal reduction of Ti-modified IRMOF-3: an adaptable synthetic method to support catalytic nanoparticles on carbon.

This work describes a novel method for the preparation of titanium oxide nanoparticles supported on amorphous carbon with nanoporosity (Ti/NC) via the post-synthetic modification of a Zn-based MOF with an amine functionality, IRMOF-3, with titanium isopropoxide followed by its carbothermal pyrolysis. This material exhibited high purity, high surface area (>1000 m(2)/g), and a high dispersion of metal oxide nanoparticles while maintaining a small particle size (~4 nm). The material was shown to be a promising catalyst for oxidative desulfurization of diesel using dibenzothiophene as a model compound as it exhibited enhanced catalytic activity as compared with titanium oxide supported on activated carbon via the conventional incipient wetness impregnation method. The formation mechanism of Ti/NC was also proposed based on results obtained when the carbothermal reduction temperature was varied.

Controlling the density of amine sites on silica surfaces using benzyl spacers.

The synthesis and characterization of benzyl-spaced aminopropyl-functionalized mesoporous silica is reported by a method designed to reduce silanol-amine and amine-amine interactions. The new material is characterized by N(2) physisorption, thermogravimetric analysis, potentiometric titration, X-ray diffraction, FT-Raman spectroscopy, and (13)C and (29)Si solid-state NMR spectroscopy. The degree of clustering of the amines is studied by solid-state fluorescence spectroscopy of 1-pyrenecarboxylic acid bound to the deprotected benzyl-spaced aminosilica. The results obtained provide evidence of an amine loading comparable to traditional dense amine grafting approaches on SBA-15. Thermogravimetric analysis, FT-Raman spectroscopy, and (13)C solid-state NMR spectroscopy results show evidence that the protected imine can be quantitatively cleaved to yield the corresponding amine. As evidenced by fluorescence spectroscopy, the benzyl-spaced material is isolated enough to prevent excimer formation of the probe molecule, unlike aminopropyl-functionalized silica materials prepared by traditional grafting approaches.