Authors: Haijin Zhu
Categories: Perspective, catalysis, physical chemistry, electromagnetic waves
Source: iScience
Authors: Haijin Zhu
Most chemical reaction processes require a certain amount of energy input, known as activation energy, for the reaction to go forward. The activation energy can be provided to the reacting molecules in the form of heat, electricity, electromagnetic (EM) waves, or a combination of these. This article discusses the theory, practice, and technical aspects of activation of chemical reactions using electromagnetic waves (EMWs) as the energy input, instead of heat or electricity. The matching condition between the EM wavelength and the thermal activation energy is discussed, as well as the practical considerations for the application of the EMW-driven chemical reactions.
Electromagnetic (EM) energy can be transferred to matter through resonance, which occurs when the frequency of the EM wave matches the natural frequency of the particles (nuclei, electrons, atoms, bonds, functional groups, or molecules). For instance, the sunlight is an approximate black-body emission spectrum with a continuous broadband EM frequency. When the outgoing irradiation from the sun passes through the atmospheric gases, only the energy of the wavelengths that match the activation energies (from ground state to excited states) of various gas molecules will be absorbed, whereas the wavelengths that have higher or lower energy would not interact with the atmosphere, as shown in Figure 1. In well-established theories of light-matter interaction, electromagnetic-wave (EMW) absorption arises when the oscillating electric field acts as a time-dependent perturbation to the molecular or electronic Hamiltonian. Within time-dependent perturbation theory, the transition probability between an initial and a final state is governed by Fermi’s golden rule, which requires both an energy-matching condition between the photon energy and the system’s energy-level separation and a non-zero transition matrix element consistent with selection rules. From a macroscopic level, this same process is described by dielectric response theory, in which absorption occurs when the frequency of the EMW coincides with a resonant polarization mode of the material. Under such conditions, the imaginary part of the complex, frequency-dependent permittivity becomes nonzero, indicating irreversible energy transfer from the EM field to internal degrees of freedom such as electronic, vibrational, or rotational modes. These microscopic and macroscopic descriptions are complementary and mutually consistent, and together they form the physical foundation for light-matter interaction across the EM spectrum.Figure 1Electromagnetic wave absorption spectra of various gases in the atmosphereImage Wikipedia; released under a CC0 1.0 Universal (CC0 1.0) Public Domain Dedication.
A well-known scenario for the application of EMW energy transfer through resonance is the photocatalytic water splitting,^1^^,^^2^^,^^3^ where photocatalyst materials (e.g., metal oxides, metal oxysulfides, and metal oxynitrides) are irradiated with light of suitable wavelength (Figure 2). The catalyst often has a semiconductor property—if the photon energy matches the energy of the band gap, negatively charged electrons and positively charged holes are generated. Protons can then be reduced by the electrons forming H2, and hydroxides can be oxidized by the holes forming O2. For this reaction to proceed, it is required that the bottom-level potential of the conduction band must be lower than the reduction potential of H^+^/H2 (0 V vs. SHE), and the top-level potential of the valence band needs to be higher than the potential of the oxygen evolution reaction O2/H2O (1.23 V vs. SHE). Therefore, the theoretical minimum band gap for the catalyst of the water splitting reaction is 1.23 eV. Photon energy can be written (Equation 1)E=hυ=hCλ>1.23eVWhere υ is the frequency, λ the wavelength, C the speed of light, and h is Plank’s constant 6.63 × 10^34^ m^2^ kg s^−1^. Using 1 eV = 1.6 × 10^−19^ J, one can obtain the wavelength of the light λ < 1010 nm.Figure 2Photocatalysis of water splitting reaction using powdered catalyst in water
For practical considerations, the design of photocatalysts requires a match between the band gap of the catalyst and the photon energy of incident EMW. For instance, to design a visible-light-driven photocatalyst, knowing that visible light has a wavelength of 400–700 nm, the band gap of the catalyst should be engineered such that it falls within the range of 3.1–1.8 eV. Band gap larger than 3.1 eV will result in catalysts responding only to UV light.
Photo-catalytic reactions are extensively studied for water electrolysis,^4^ CO2 conversion,^5^ and many other chemical and electrochemical reactions.^6^^,^^7^ In such reactions, a substance known as a photocatalyst absorbs light, raises its energy level, and then transfers that energy to a substance that is reacting to cause a chemical reaction. An important feature of the photo-catalytic reactions is that the catalytic mechanisms are based on the semiconductor properties of catalysts - when the photon energy matches the bandgap of the semiconductor (catalyst), photons are absorbed, and electrons are excited, creating an electrochemical potential difference between the negatively charged electrons and positively charged holes, which drives subsequent redox reactions. Microwave-enhanced reactions have also been widely explored in the literature.^8^^,^^9^ However, in most of these cases, the EM field primarily interacts with the catalyst or solvent, leading to rapid volumetric heating or localized hot-spot formation. In other words, the role of microwaves in these systems is largely thermal or catalytic enhancement, rather than direct energy transfer to reaction coordinates. The present work proposes a different concept in which the EMW, through a carefully selected resonance frequency, directly couples with the target chemical bond of the reactant. This resonance-driven activation differs from both conventional photocatalytic and microwave-assisted processes by focusing on the direct energy coupling between the EM field and the reaction activation barrier, enabling selective and non-thermal reaction pathways.
According to the first law of photochemistry, developed by Christian von Grotthuss and John Draper in the early 1800s, only the light absorbed by a substance can produce a photochemical reaction.^10^^,^^11^ This absorption is most efficient at the resonance condition. Although this seems to be obvious, it implies the existence of a peak photocatalysis efficiency with varying wavelength, due to the most efficient energy transfer. While photons of a smaller wavelength bare more Energy, most of them may be wasted without any productive interaction with the reactant. Using Eq. 1, the resonance wavelength λ0 is reciprocally related to the activation energy (Equation 2)λ0=hCEa/NAwhere Ea is the activation energy required for the chemical reaction. NA is the Avogadro constant 6.02 × 10^23^/mole. Note that h, C, and NA are constants, Eq. 2 can be rewritten (Equation 3)λ0=0.12Ea
Table 1 summarizes the typical activation energies, their peak EM wavelengths calculated using Eq. 3, and their corresponding frequency and EM categories. Ea in Table 1 represents the thermal activation energy, which can be obtained from measurements of the reaction rate constants (k) at various temperatures, followed by fitting the experimental data to the Arrhenius equation. The validity of Eq. 3 can be verified using absorption band wavelength of nitrogen gas as an example. It is well known that the activation energy of nitrogen triple bonds at 298 K is 945 kJ/mol,^12^ one of the strongest bonds, leading to extremely slow reaction kinetics in ammonia production. Using Eq. 3, the resonance wavelength for the activation of N2 is calculated to be 127 nm. This value agrees well with the experimentally observed absorption band for nitrogen gas in the range of 107–165 nm, with maximum absorbance at around 130 nm.^13^Table 1Typical thermal activation energies and their corresponding electromagnetic activation wavelengths and frequenciesEa(kJ)Wavelength (μm)CategoryAbbreviationFrequency (THz)1012long-wave infrared (8–15 μm)IR - C20–37206mid-wave infrared (3–8 μm)IR - C37–100502.5short-wave infrared (1.4–3 μm)IR - B100–2151001.2near Infrared (0.75–1.4 μm)IR - A215–430
Equation 3 allows us to predict the most efficient EM wavelength from the activation Energy, which can be determined from theoretical calculations or experimental study of temperature-dependent reaction kinetics. Note that in practice the reaction pathways can be quite complex, involving multiple intermediate states.^14^ Each transition among these states corresponds to an activation energy and hence a resonance EM wavelength. Depending on the lifetime and energy level of these intermediate states, one or more of the transitions will be the rate-determining step(s).
Ammonia production through nitrogen reduction reaction (NRR) is an extremely energy-intensive process that consumes approximately 1.8% of the global energy output, and accounts for 3% of global carbon emission.^15^ At an industry scale, ammonia is primarily synthesized through the Haber-Bosch process, where the H2 and N2 gases react at elevated temperature (450 °C) and high pressure (200 atm) using iron (Fe) based catalysts. Although the Haber-Bosch method has been developed for over a century, it still dominates the current ammonia industry, contributing 90% of annual production. Electrochemical technology has been proposed as a potential alternative to synthesize ammonia directly from nitrogen and water, without needing hydrogen gas as the feedstock. Electrochemical nitrogen reduction can save about 20% of energy as compared to the thermal chemical Haber-Bosch process,^16^ and if the electricity is produced from renewable resources, it can enable deep decarbonization through society, offering enormous social and economic benefits. However, one of the main issues in the electrochemical synthesis of ammonia is the extremely low production rate. The majority of electrochemical ammonia production rates in the literature fall within the range of 10^−12^ – 10^−9^ mol/(cm^2^·s),^17^^,^^18^^,^^19^^,^^20^^,^^21^^,^^22^ with the highest rate being 1.4 × 10^−8^ mol/(cm^2^·s),^17^ reported so far. These are still orders of magnitude lower than the Haber-Bosch production rate, which falls within the order of 10^−6^ mol/(cm^2^·s), and the DOE target of 10^−7^ mol/(cm^2^·s) for electrochemical synthesis.^21^
Many factors lead to the low production rate of electrochemical NRR - for instance, the low nitrogen solubility in the liquid electrolytes and competing hydrogen evolution side reaction. Among these factors, one of the critical issues is the sluggish activation kinetics of the nitrogen molecules, which is inherently linked to the extremely high N≡N bonding energy. The relationship between reaction rate constant k and activation energy Ea is described by the Arrhenius (Equation 4)k=k0e−Ea/RTwhere R is the gas constant 8.31 J/K, T is the temperature in Kelvin, k0 is the maximum possible reaction constant. Reaction constant k can be enhanced either by increasing T or decreasing Ea. As discussed earlier, uncatalyzed nitrogen gas has a large activation energy of 945 kJ/mol, resulting in small k at room temperature. The use of a catalyst is compulsory in industry to reduce the activation energy and hence increase the reaction constant k. Using Cs/Ru/SrZrO3 catalyst, for example, can reduce the activation energy to 121 kJ/mol.^23^
It is important to note that in the presence of a catalyst, the NRR may consist of multiple elementary steps. Each elementary step, as shown in Figure 3, has a characteristic intermediate state and a corresponding activation energy. In this case, the apparent activation energy obtained from the kinetics study (reaction constant as a function of temperature) is only a manifestation of the apparent energy barrier for the multiple reactions in total. Thus, irradiation with photons corresponds to this apparent activation energy, which would not necessarily lead to the absorption and activation of reaction kinetics. Best practice in this case is to use an in situ broadband EMW spectroscopy to determine the absorption bands during practical operation conditions (temperature, pressure, catalyst, voltage, and so forth). Ideally, the spectroscopy should cover the wavelength of 50 nm–100 μm, corresponding to activation energy of 2400–1.2 kJ/mol (Eq. 3), covering vast majorities of chemical reactions. The extreme ultraviolet absorption bands (short wavelength end) are critical since they are related to elementary reactions with higher activation Energy, hence are often the rate-determining steps. For electrochemical reactions, it is important to consider the fact that the activation energy varies with applied voltage. NRR, for example, the activation energy in the presence of Cs/Ru/SrZrO3 catalyst is approximately 121 kJ/mol, whereas applying a voltage to the catalyst resulted in a drastic decrease in the apparent activation energy to 37 kJ/mol,^23^ highlighting the importance of in situ identification of absorption band experimentally.Figure 3DFT calculation of N2 reduction mechanisms on Ru(A) Dissociative, and (B) associated mechanisms. White numbers indicate N-Ru bond lengths. Reproduced from Suryanto et al.^24^ with permission. Copy right Nature 2019.
An advantage of EMW activation compared to the traditional thermal activation is that it allows the reaction to proceed at relatively low temperatures. This is extremely beneficial for exothermic reactions with a high activation energy, such as NRR. In such reactions, high temperature is required to overcome the energy barrier and achieve a reasonable reaction kinetics, whereas the exothermic nature means the thermochemical equilibrium is favored at a lower temperature. As a result, the production rate would first increase with temperature due to kinetics, reaching a maximum, then decrease due to unfavored equilibrium. Using EMW activation would allow the reaction to proceed at a lower temperature with both a high production rate and a high conversion rate.
It is expected that EMW irradiation can enhance the chemical reaction kinetics significantly at the matching condition between the EMW and the reaction activation energy. However, the predicted kinetic enhancement may not necessarily translate directly into a proportional increase in production rate. Several practical factors must be considered.1.Efficiency of energy transfer η. While the energy transfer is most efficient at the resonance condition, there is always a certain amount of EW energy being wasted. Increasing incident beam power can increase the amount of energy intake by the reactant, but this may be at a cost of energy transfer efficiency η. A compromise should be made between the input EWW energy and the output production rate.2.Reaction pathways and lifetime of the excited states. While the EMW irradiation can excite the reactant molecules from ground state to excited state thus increasing the excited state concentration, the final production rate is also critically dependent on the possibility of the excited state turning into the final product. This possibility, being reflected by the reaction constant k0 in Eq. 4, signifies the maximum possible reaction constant (100% reactant molecules being at the excited state).3.Concentration of the reactant(s). In a first-order reaction, the reaction rate is directly proportional to the concentration of the reactant. The low solubility of nitrogen in water has been recognized as one of the main reasons for the extremely low production rate of electrochemical nitrogen reduction in water.^22^
In summary, this article discusses the theories, practices, and technical aspects of activating chemical reactions with EMW irradiation. A concise expression of λ0=0.12Ea is proposed as the matching condition for EMW driven activation. This relationship is verified by the EMW absorbance of nitrogen molecules, which is known to have an activation energy of 945 kJ/mol. The matching EM wavelength calculated using the above equation is 127 nm, consistent with the experimentally observed absorption spectrum peaked around 130 nm. However, the catalytic efficiency of the UV light at 127 nm has yet to be tested experimentally. Considering the complexity of chemical reaction pathways, in practice, it is recommended to conduct an in situ broadband continuous EMW scan to determine the actual absorption bands of a chemical reaction in practical operation conditions. The shorter-wavelength absorption bands are particularly relevant, as they correspond to elementary steps with higher activation Energy, hence are often the rate-determining. The concept proposed in this perspective offers a potential pathway for designing EMW generators that can selectively activate specific chemical reactions - such as N2 splitting, without relying on conventional catalysts. The EM-driven chemical reactions will demonstrate not only improved reaction kinetics, but also higher energy efficiency compared to the traditional thermal-activated reactions.
The author H.Z. gratefully acknowledges the financial support from the Guangdong Basic and Applied Basic Research fund no. 2024A1515011865, the Guangdong Major Project of Basic and Applied Basic Research no. 2021B0301030005, and the GTIIT Changzhou Innovation Institute 2025 Seed Project no. GCII-Seed-202503.
H.Z. conceptualized the work, wrote the original draft, edited the draft, and answered the reviewer’s questions.
The author declares no competing interests.
During the preparation of this work, the author used Gemini in order to improve language, correct grammatical errors, and ensure technical clarity. After using this tool, the author reviewed and edited the content as needed and takes full responsibility for the content of the published article.