Thermal-catalytic gas processing is integral to many current industrial processes. Ever-increasing demands on conversion and energy efficiencies are a strong driving force for the development of alternative approaches. Similarly, synthesis of several functional materials (such as nanowires and nanotubes) demands special processing conditions. Plasma catalysis provides such an alternative, where the catalytic process is complemented by the use of plasmas that activate the source gas. This combination is often observed to result in a synergy between plasma and catalyst. This Review introduces the current state-of-the-art in plasma catalysis, including numerous examples where plasma catalysis has demonstrated its benefits or shows future potential, including CO2 conversion, hydrocarbon reforming, synthesis of nanomaterials, ammonia production, and abatement of toxic waste gases. The underlying mechanisms governing these applications, as resulting from the interaction between the plasma and the catalyst, rend...

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Plasma Catalysis: Synergistic Effects at the Nanoscale

Plasma N2-fixation : 1900-2014

We study catalyst−support and catalyst−carbon interactions during the chemical vapor deposition of single-walled carbon nanotubes by combining environmental transmission microscopy and in situ, time-resolved X-ray photoelectron spectroscopy. We present direct evidence of what constitutes catalyst functionality by comparing the behavior of Ni, Fe, Pd, and Au model catalyst films on SiO2 during preannealing in O2 and NH3 and during C2H2 decomposition. The catalyst metal surface supplies sites to dissociate the hydrocarbon precursor and then guides the formation of a carbon lattice and the liftoff of a carbon cap. The catalysts are sharply distinguished by their reactivity toward activation of the hydrocarbon precursor, following trends known from heterogeneous catalysis. For Fe and Ni, the active state of the catalyst is a crystalline metallic nanoparticle. Graphitic networks do not form on oxidized Fe. Pd forms a silicide on SiO2 under our reducing conditions. Pd (silicides) and Au nanocrystals are catalyt...

State of Transition Metal Catalysts During Carbon Nanotube Growth

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.

Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: A Review

The application of plasma techniques in the preparation of catalysts and in catalytic reactions published during the last fifteen years is briefly reviewed. Two different types of plasma are used in this field: low temperature plasma (low pressure plasma) and high temperature plasma. The first type of plasma technique is widely used for the modification of surface oxides, and for the preparation and regeneration of catalysts as well as for catalytic synthesis and decomposition. The second type is applied for the preparation of ceramic-based supports, recovery of precious metals from used catalysts, and catalytic reactions. Both oxides and supported metal catalysts have been prepared by this method. The catalytic reactions involving the plasma technique essentially involve NO, methane or ammonia as reactants. An interesting potential for this technique is its application in the recovery of the precious metals from car exhaust catalysts.

A review of the use of plasma techniques in catalyst preparation and catalytic reactions

The catalytic effect of iron wires on plasma syntheses of ammonia and hydrazine has been studied in the nitrogen-hydrogen plasma prepared using rf discharge at a pressure of 650 Pa (5 Torr). The product was mainly ammonia including a small amount of hydrazine. When iron wires were placed in the plasma downstream of the gas flow, the yields of both products increased, about two times in ammonia and two orders of magnitude in hydrazine. The yields increased with increasing number of wires (the surface area of the catalyst). The dissociative adsorption of nitrogen molecules and/or molecular ions on the iron surface and the formation of NHx by the reaction with hydrogen in the plasma followed by the formation of NH3 or N2H4 are considered as a reaction scheme. This is supported by the identification of NH3 with XPS of the surface of iron wires.

Catalytic effect of iron wires on the syntheses of ammonia and hydrazine in a radio-frequency discharge

The synergistic effects o1 driving frequency of the discharge and catalysis of iron and molybdenum wires when then are placed in nitrogen-h ydrogen radio-frequency and microwave plasmas mere investigated. The ammonia Yield increased in the plasmas prepared using both driving frequencies. but the hydrazine yield increased only in fire radio-frequency discharge with the catalysts. The direct adsorption of NHx formed in the plasma on the catalyst surface followed by the formation of NH3 and N2H4 are considered as a reaction scheme in the radio-frequency discharge. On the other hand, the adsorption of N atoms and/or formation of the metal- N bond favors the formation of ammonia but does not affect the hydrazine formation in the microwave discharge.

Shigeyuki Tanaka

Papers

Catalytic effect of iron wires on the syntheses of ammonia and hydrazine in a radio-frequency discharge

Synergistic effects of catalysts and plasmas on the synthesis of ammonia and hydrazine