Plasma meets catalysis, equipping catalysts with a "lightning engine"

When a sufficiently high electric field energy is applied to a gas, electrons are "stripped" from atoms, forming an ionized gas coexisting with high-energy electrons (1~10 eV), ions, radicals, excited molecules, and photons – plasma, the fourth state of matter.

The core characteristic of low-temperature plasma is that the electron temperature is much higher than the temperature of heavy particles (ions and neutral atoms), and it is in a thermodynamic non-equilibrium state. The macroscopic system exhibits a low temperature state, close to room temperature, and it will not cause thermal damage to the catalyst at all. Therefore, low-temperature plasma is often used in catalytic reactions and the synthesis of catalysts.

Low-temperature plasma can be divided into dielectric barrier discharge plasma (DBD), microwave plasma (MW), radio frequency plasma (RF), gliding arc plasma (GA), atmospheric pressure glow discharge plasma, etc., based on different plasma generation methods^[1,2].

Figure 1. Schematic diagram of various types of low-temperature plasma^[2]

In traditional thermal catalysis, the entire system needs to be heated to provide the reactant molecules with sufficient kinetic energy to overcome the reaction energy barrier. However, in plasma, high-energy electrons directly collide with the reactant molecules, either exciting them to a higher energy state or dissociating them into free radicals^[3]. Plasma not only activates the reactants but also fundamentally alters the properties of the catalyst^[2].

The high-energy active species abundant in low-temperature plasma can activate inert gas molecules (such as CO₂, N₂, CH₄, H₂O). This enables certain thermodynamically difficult reactions to proceed under mild conditions through low-temperature plasma, thereby enhancing energy conversion efficiency. Utilizing catalysts in conjunction with cold plasma for synergistic catalytic reactions, directly converting certain inert gaseous molecules into high-value chemicals has emerged as a novel and efficient catalytic mode ^[4~8]. Currently, the application of low-temperature plasma technology in the field of catalytic reactions mainly encompasses the following three aspects: (1) CO₂ conversion ^[9,10]; (2) methane reforming reaction ^[11,12]; (3) synthetic ammonia reaction ^[13,14].

Figure 2. CO₂ hydrogenation to methanol using plasma^[8]

High-energy active species in an excited state within low-temperature plasma can play a positive role in the preparation or post-treatment of catalysts^[2]. Typically, the macroscopic temperature of low-temperature plasma is close to room temperature, which is crucial for the synthesis of catalysts with poor thermal stability. When materials are bombarded by high-energy particles, existing chemical bonds break and new ones form, making reactions with higher energy barriers easier to occur under mild conditions. Furthermore, the synthesis or treatment of catalysts using low-temperature plasma does not require the addition of external chemicals, making it a green and environmentally friendly method for catalyst preparation and modification^[15]. Low-temperature plasma technology has been widely applied in the reduction and oxidation of catalysts^[16-19], the removal of organic ligands and pollutants^[20,21], surface treatment^[22,23], and PECVD^[24].

Figure 3. Generation of cold plasma and surface functionalization process^[25]

In addition to its high chemical activity, low-temperature plasma also exhibits collateral thermal and photonic effects, making it a new tool in the field of multi-energy field synergistic catalysis research. Low-temperature plasma breaks the thermodynamic constraints on chemical reactions, enabling transformations that were previously unachievable under mild conditions; it provides a novel approach to addressing energy crises and environmental issues.