The 2020 plasma catalysis roadmap Bogaerts, Annemie; Tu, Xin; Whitehead, J Christopher ...
Journal of physics. D, Applied physics,
10/2020, Letnik:
53, Številka:
44
Journal Article
Recenzirano
Odprti dostop
Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons ...or oxygenates, and NH3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NOx removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over 'conventional' catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.
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.
Renewable energy sources such as lignocellulosic biomass provide an attractive source of renewable carbon that can be sustainably converted into fuels and chemicals to allow for a reduction in the ...need for fossil fuels and a net reduction in the amount of CO2 emitted. This perspective highlights recent advances in catalyst development for the production of biofuels from lignin and lignin-derived model compounds. Recent reports have focused on the direct cleavage of C–O bonds through various catalytic processes including (i) zeolite upgrading, (ii) hydrodeoxygenation, (iii) multiple-step upgrading, and (iv) unsupported, organometallic catalysts for slurry processing. Results show progress toward the potential commercialization of biofuels technologies from lignocellulosic biomass resources.
Bimetallic phosphides are promising materials for biomass valorization, yet many metal combinations are understudied as catalysts and require further analysis to realize their superior properties. ...Herein, we provide the synthesis, characterization, and catalytic performance of a variety of period 4 and 5 solid solutions of molybdenum-based bimetallic phosphides (MMoP, M = Fe, Co, Ni, Ru). From the results, the charge sharing between the metals and phosphorus control the relative oxidation of Mo and reduction of P in the lattice, which were both indirectly observed in binding energy shifts in X-ray photoelectron spectroscopy (XPS) and absorption energy shifts in X-ray absorption near-edge spectroscopy (XANES). For MMoP (M = Fe, Co, Ni), the more oxidized the Mo in the bimetallic phosphide, the higher the selectivity to benzene from phenol via direct deoxygenation at 400 °C and 750 psig. This phenomenon was observed in the bimetallic materials synthesized across period 4, where aromatic selectivity and degree of Mo oxidation both decreased in the following order FeMoP ≫ CoMoP > NiMoP. Alternatively, in the case of MMoP (M = Fe, Ru), the P in RuMoP is more oxidized compared to that in FeMoP, and the selectivity toward the hydrogenation pathway increased due to the interaction between the aromatic rings and the P species on the surface. For RuMoP and NiMoP, cyclohexanol was selectively produced from phenol with >99% selectivity when the reaction temperature was lowered to 125 °C at 750 psig, whereas FeMoP and CoMoP were not active under these conditions. Last, complete deoxygenation of phenol to benzene, cyclohexane, and cyclohexene was accomplished using mixtures of RuMoP and FeMoP in flow and batch experiments. These results highlight the versatility and wide applicability of transition metal phosphides for biomass conversions.
Nonthermal plasma-driven catalysis is an emerging subfield of heterogeneous catalysis that is particularly promising for the chemical transformation of hard-to-activate molecules (e.g., N2, CO2, ...CH4). In this Review, we illustrate this promise of plasma-enhanced catalysis, focusing on the ammonia synthesis and methane dry reforming reactions, two reactions that have received wide attention and that illustrate the potential for plasma excitations to mitigate kinetic and thermodynamic obstacles to chemical conversions. We highlight how plasma activation of reactants can provide access to overall reaction rates, conversions, product yields, and/or product distributions unattainable by thermal catalysis at similar temperatures and pressures. Particular emphasis is given to efforts aimed at discerning the underlying mechanisms at play in these systems. We discuss opportunities for and challenges to the advancement of the field.
Transition-metal carbides (TMCs) are important materials for a variety of applications and industrial processes, in part because of their variable crystal structures and surfaces. However, the ...synthesis of TMCs often proceeds through metastable phases during particle growth, the appearance of which cannot be described by traditional phase diagrams. Here, we use density functional theory calculations and thermodynamic analyses to construct particle size-dependent phase diagrams for Mo and W carbides and reveal the relationships between phase stability and TMC nanoparticle size. We compute size-dependent phase diagrams for a wide range of Mo carbide and W carbide phases, determine predicted crystallization pathways during synthesis, and compare model results with experimental data. We provide insights for the influence of nanoparticle size on TMC nucleation and growth during synthesis and provide a computationally guided road map for navigating the synthesis of target TMC surfaces and phases.
We explore the consequences of nonthermal plasma-activation on product yields in catalytic ammonia synthesis, a reaction that is equilibrium-limited at elevated temperatures. We employ a minimal ...microkinetic model that incorporates the influence of plasma-activation on N2 dissociation rates to predict NH3 yields into and across the equilibrium-limited regime. NH3 yields are predicted to exceed bulk thermodynamic equilibrium limits on materials that are thermal-rate-limited by N2 dissociation. In all cases, yields revert to bulk equilibrium at temperatures at which thermal reaction rates exceed plasma-activated ones. Beyond-equilibrium NH3 yields are observed in a packed bed dielectric barrier discharge reactor and exhibit sensitivity to catalytic material choice in a way consistent with model predictions. The approach and results highlight the opportunity to exploit synergies between nonthermal plasmas and catalysts to affect transformations at conditions inaccessible through thermal routes.
Carbon dioxide adsorption from a simulated flue gas stream was successfully performed with a hyperbranched aminosilica (HAS) material. The HAS was synthesized by a one-step reaction, spontaneous ...aziridine ring-opening polymerization off of surface silanols, to form a 32 wt % organic/inorganic hybrid material. The adsorption measurements were performed in a fixed-bed flow reactor using humidified CO2. The advantage of this adsorbent over previously reported adsorbents is the stability of the organic groups covalently bound to the silica support compared to those made by physisorbed methods. Furthermore, a large CO2 capacity (∼3 mmol CO2/g adsorbent) associated with the high loading of amines was observed.
Bimetallic FeMo phosphide is shown to be a highly selective catalyst for selectively breaking C-O bonds in a lignin model compound as well as fully deoxygenating the resulting phenolic compounds. The ...combination of depolymerization and deoxygenation allows a single catalyst to potentially upgrade biomass into transportation fuels while conserving hydrogen. Display omitted
•Sulfur-free, Fe-based bimetallic hydrodeoxygenation catalyst.•Highest reported selectivity for phenol deoxygenation to benzene.•FeMoP catalyst is selective to cleaving C–O bonds in lignin.•New synthesis method for FeMo bimetallic phosphide.•Detailed crystallographic analysis of FeMoP crystal.
We report the synthesis of an industrially applicable, non-sulfided bimetallic catalyst with remarkable selectivity to the hydrodeoxygenation of aryl ethers and phenol. Bimetallic FeMo phosphide catalysts have selectivities as high as ∼90% benzene and 10% cyclohexane at hydroprocessing temperatures (400°C) and industrially low pressures (2.1MPa H2) at near complete conversion (>99%). Similarly, the selectivities for the hydrodeoxygenation of anisole were 90% benzene, 4% toluene, and 6% cyclohexane at 92% conversion. Furthermore, this catalyst has the ability to cleave aryl ether bonds and specifically β-O-4 linkages with higher selectivities to aromatics compared to hydrocarbons, thus minimizing the use of expensive H2. The selectivity to aromatics from phenol was highly dependent on H2 pressure, where doubling the pressure to 4.2MPa reduced the selectivity to benzene, while less substantial decreases were observed when anisole or a β-O-4 model compound was reacted. These high selectivities were attributed to the unique FeMo phosphide material, as catalytic studies with FeP, MoP, and the combination of FeP and MoP resulted in selectivities much lower than the FeMo phosphide material.
Plasma-assisted catalysis is the process of electrically activating gases in the plasma-phase at low temperatures and ambient pressure to drive reactions on catalyst surfaces. Plasma-assisted ...catalytic processes combine conventional heterogeneous surface reactions, homogeneous plasma phase reactions, and coupling between plasma-generated species and the catalyst surface. Herein, we perform kinetically controlled ammonia synthesis measurements in a dielectric barrier discharge (DBD) plasma-assisted catalytic reactor. We decouple contributions due to plasma phase reactions from the overall plasma-assisted catalytic kinetics by performing plasma-only experiments. By varying the gas composition, temperature, and discharge power, we probe how macroscopic reaction conditions affect plasma-assisted ammonia synthesis on three different γ-alumina-supported transition metal catalysts (Ru, Co, and Ni). Our experiments indicate that the overall reaction and plasma-phase reaction are first-order in both N2 and H2. In contrast, the rate contributions due to plasma-catalyst interactions are first-order in N2 but zeroth order in H2. Furthermore, we find that the tuning of the plasma discharge power is more effective in controlling catalytic performance than the increasing of bulk gas temperature in plasma-assisted ammonia synthesis. Finally, we show that adding a catalyst to the DBD reaction alters the way that productivity scales with the specific energy input (SEI).
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