Recently, plasma catalysis is gaining interest as an alternative to traditional thermo-catalytic techniques. Due to the non-equilibrium physical state of the plasma, with much energy stored in a limited number of degrees of freedom, specific chemical processes can be selectively stimulated or inhibited, and the location of the chemical equilibrium can be shifted. Various physical effects at the plasma–catalyst interface—such as vibrationally excited molecules, excess charges, and electric fields—are nonexistent under purely thermal conditions, and can dramatically change the chemistry at the catalyst surface. However, very little is known about this new frontier in surface science due to lack of dedicated experiments or detailed models.
I work on several topics related to the chemistry induced by out-of-equilibrium plasma effects, theoretically as well as experimentally (although really just as the resident theory person).
Gas adsorption or storage on charged materials
A solid in contact with plasma accumulates a negative surface charge due to influx of free electrons. Excess surface electrons can reach densities in the order of 1017 m-2, and remain trapped for long times up to days. Large electric fields are also found in common plasma-catalytic setups, which can polarize the surface.
How does this affect the surface chemistry?
As it turns out, this effect has been theoretically studied in the context of gas capture and storage. Nanomaterials such as hexagonal boron nitride (h-BN) or 2D borophene typically do not bind molecules such as CO2 or H2, but they do if excess electrons or strong electric fields are present. Moreover, transition metal dichalogenides (such as MoS2 and the like) can undergo charge-induced phase transitions.
However, when trying to reproduce published results, I encountered quite a few inconsistencies in the literature. These could be traced back to an inconsiderate treatment of periodicity (you may recall that the energy of a charged infinite system diverges). As a first step towards more accurate models, I proposed an approach to eliminate these inconsistencies.
In addition, by performing calculations on several different charged materials (all derived from h-BN through doping or defect engineering) and using different theoretical approximations, I uncovered some general principles.
- Adsorption of CO2 on charged materials can be described by a universal response function. The amount of work required to charge the material is directly correlated to the adsorption strength of the chemisorbed molecule. The efficiency by which this energy is converted depends on the adsorption site.
- Materials with defects or dopants are easier to charge. Consequently, the doped materials are stabilized by excess charge. This way, charging can be a tool to selectively synthesize certain doped materials.
- The self-interaction error is a very prominent factor in theoretical calculations of adsorption thermochemistry on charged materials, as opposed to its minor impact on (neutral) traditional catalysis. Therefore, careful benchmarking with hybrid DFT functionals is a must in this field.
Surface chemistry under plasma-induced charging
While the charge response of simple 2D materials is interesting from a theoretical point of view, it is not quite clear how these insights translate to more realistic catalysts exposed to a plasma. To this end, I constructed DFT-based model systems that capture some key aspects of these systems. I developed a simple model of the charge distribution around the surface—the plasma sheath—and applied it to supported M/Al2O3 (M = Ti, Ni, Cu) single atom catalysts.
Again considering CO2 activation as a probe reaction, i found that:
- The presence of a negative surface charge dramatically improves the reductive power of the catalyst, strongly promoting the splitting of CO2 to CO and oxygen.
- The relative activity of the investigated transition metals is also changed upon charging, making "weak" catalysts much stronger.
These results strongly point to plasma-induced surface charging of the catalyst as an important factor contributing to the plasma-catalyst synergistic effects frequently reported for plasma catalysis, and a powerful additional parameter to tune catalyst activity and selectivity.