Comment on Meunier, F.C.; Kaddouri, A. Microwave-Assisted Oxidation of N₂ into NO_x over a La-Ce-Mn-O Perovskite Yielding Plasmas in a Quartz Flow Reactor at Atmospheric Pressure. Catalysts 2024, 14, 635

1. Background and Significance

These “Comments” stem from a recent publication by Meunier and Kaddouri [1]. The microwave-induced plasma generation by certain perovskites and spinel oxide catalysts, or, indeed, their combination, is very important in the electrification of chemical processes, especially in sustainable green fertilizer, ammonia, nitric acid, and fuel production. Furthermore, these catalysts are fundamentally a step change from the classical chemical catalysts because under plasma and microwave, they become more active and their activity can be understood in terms of quantum effects based on their chemical and morphological heterogenic structures, as studied recently by Akay [2,3,4]. The simultaneous microwave irradiation in air and plasma generation can be used to prepare catalysts with oxygen vacancies, nitrogen substitution, and localized non-stoichiometric clusters, which are reflected in their morphology. This method is known as High Intensity Power (HIP) processing [4,5,6]. The use of HIP (i.e., microwave and plasma) in processing emanates not only in a very short processing time but also in a non-equilibrium state during solid-state chemical reactions, which create the chemical and morphological heterogeneity formed by nano-scale structures and particles in the catalyst [4,5]. In plasma reactions, the resulting catalysts function through surface catalysis facilitated by several quantum phenomena (Havnes and Casimir effects while lattice vacancies, such as oxygen vacancies and vacancy clusters, act as quantum wells and wires [4,5]). They also act as a Plasma Catalysis Promoter (PCP) through modulation. Therefore, these catalysts are called Quantum Catalysts (QCs) [4,5,6]. At an industrial scale, QCs can be utilized in packed-bed or fluidized-bed configurations using microwave reactors with self-generated plasma over the catalyst particles. The PCP approach represents an important advancement in plasma catalysis [4,5,6].

2. Discovery of Microwave-Induced Plasma Generation in Perovskites and Spinel Oxides

Meunier and Kaddouri [1] indicate that the microwave-induced plasma generation in La-Ce-Mn-O perovskite can be used for nitrogen fixation from air. The authors state in the paper [1] as well as in their communications with the referees that the reason for the lack of analysis in their paper is “it is important to quickly report that it is possible to fix nitrogen” and that the article is “a short communication aimed at reporting exciting new results”.

In their reply to Referee-2 (Round-2), the authors also state that “there is no previous report in the literature that present such a simple reactor design to produce up to 2% of NOx from air in a flow system and an inexpensive commercial microwave oven. We would yet be happy to complete the reference section if important previous studies are missing.”

Therefore, the authors do not acknowledge the existence of pioneering publications on this subject. In fact, in 2020/2021/2023, Akay [2,3,4] published the first-ever papers on this subject of microwave radiation-induced plasma generation using perovskites, in which BaTiO₃ was chosen as the model catalyst. The choice of BaTiO₃ was based on the fact that it is the most important and most researched perovskite with well-documented characteristics. These studies [2,3,4] were also extended to the silane-coated SiO₂-supported spinel oxide quantum catalysts and the combination of BaTiO₃ and spinel oxides in order to enhance the plasma generation and catalytic activity [2,3,4]. In fact, the first publications on the subject (a paper [2] and a US patent application [3]) had the title of “Plasma-generating chemical-looping catalysts” and they included a section on NO_x evolution with steady or pulsatile microwave power, generating continuous NO_x, induced by plasma generation over BaTiO₃ in air.

These papers were follow-on studies of another publication by Akay [7], which provided a detailed background for the subsequent papers cited above. These catalysts were used in numerous studies by the author’s research group at Newcastle University including NH₃ synthesis, dry reforming of CH₄, CO₂ conversion to CO, direct syngas conversion to NH₃-fertilizers, Fischer–Tropsch synthesis (see references [8,9,10] for example) as well their use in agriculture for the delivery of micronutrients at the end of the QC’s service life [11]. Their end-of-catalyst life usage in agriculture thus provides sustainability and a circular economy for plasma catalysts.

Meunier and Kaddouri [1] cite one of the authors’ papers [2] (Reference 27 in [1]) towards the end of the “Introduction” in a manner which is erroneous: It is stated the following [1]:

“NOx formation has been reported upon the MW-irradiation of SiO₂-supported BaTiO₃ perovskites in air, although NOx concentrations higher than 500 ppm could not be measured [27].”

The catalysts used in NO_x formation during continuous or pulsed plasma were not “SiO₂—supported BaTiO₃ perovskites”. They were piezoelectric BaTiO₃, which generated plasma upon microwave irradiation in air [2]. Due to the limitations of the analytical equipment, it was not possible to measure the full NO_x concentrations; instead, a nitric oxide (NO) concentration up to 500 ppm was measured. Usually, [NO₂] ≈ 2 [NO] when air is used as the source of N₂ and O_2. Therefore, the total NO_x concentration of [NO_x] > 1500 ppm was achieved, which could be enhanced through the use of higher O₂/N₂ input concentration and higher plasma power. However, the aim was not to investigate NO_x generation; hence, no further investigation was carried out until recently, when plasma-generating spinel QCs (provided by the author) were used in fertigation as reported by Zhuang et al. [12]. It was shown that these catalysts enhanced N₂ conversion as much as sixfold, while the well-known NH₃ plasma catalysts had no effect or a negative effect on NO_x generation.

3. Microwave and Plasma-Induced Phase Transition in Perovskites and Spinel Oxides

Following the analysis of the XRD-patterns (Figure S5), on page 6, Meunier and Kaddouri [1] state that “No evidence of spinel phase formation following plasma occurrence was evidenced, in contrast to an earlier report [27].”

This may refer to the phase transformation when ferroelectric/paraelectric BaTiO₃ particles are microwaved, generating plasma while acquiring a piezoelectric state as well as porogenesis. Phase transition happens because of impurities in BaTiO₃, which act as susceptors, or due to the presence of a small number of oxygen vacancies (as defects), which act as nucleation sites for microwave-induced plasma generation in air with NO_x formation. As a result, BaTiO₃ undergoes a rapid phase transition from a paraelectric-to-piezoelectric state, which can be detected by XRD (intensity reversal at 2θ = 45°), and it can be visually observed due to the colour change from white/cream (colour of the stoichiometric barium titanate) to black [2,3,4]. Therefore, the resulting catalyst is also known as black-BaTiO₃. Akay [2] has provided a method based on Energy-Dispersive X-ray Spectroscopy (EDS) analysis of the perovskites and spinel catalysts to quantify the local variations in composition and the levels of oxygen vacancies and nitrogen substitution. Hence, the piezoelectric barium titanate with local chemical heterogeneity is represented as BaTi_1−vO_3−x{#}_yN_z, where {#} represents surface oxygen vacancy. However, the values of v, x, y, and z in the above representation change from site to site, and v and x can be negative or positive, reflecting the chemical heterogeneity of these QCs.

These observations are valid for perovskites and supported spinel oxides (unary, binary, or high-entropy-supported catalysts). The piezoelectric, highly porous SiO₂-supported spinel oxide QCs prepared by microwave-induced plasma processing also generate plasma during microwave irradiation [2,3,4,5]. Their chemical heterogeneity is represented as M⁽¹⁾_3−jM⁽²⁾_kO_4−m{#}_nN_r/SiO₂ (k = 0 unary and k = 1 binary spinel) where M⁽¹⁾ and M⁽²⁾ are metal catalysts. Their nano-structures allow the manifestation of the Havnes and Casimir quantum effects during plasma reactions [2,3,4,5,11].

In single-phase catalysts of the type M_3−jO_4−m{#}_nN_r/SiO₂ at high metal concentrations, when [M]/[Si] ≥ 1, the microwave-induced plasma generation results in the appearance of new phases as the spinel phase is transformed to silicate perovskite (MSiO₃) and olivines (M₂SiO₄) as observed by XRD studies [2,13]. The microwave-induced plasma generates a phase transition in stoichiometric barium titanate to piezoelectric BaTi_1−vO_3−x{#}_yN_z [2] with porogenesis. This process results in the formation of nano-sized layered structures with lattice exudates, which separate the nano-planes and cause Casimir forces [2,4]. It is not clear if this process is similar to spinel phase formation observed in layered structures, including perovskites (requiring high temperatures), at the molecular level, yet it leads to the densification of perovskites [14,15].

Finally, chemical and morphological structure modification in HIP-processed perovskite (LaCoO₃) catalysts with Sr doping at the A-site (La) have been confirmed recently by Luan et al. [16]. This catalyst has spatial variation in Co/(La+Sr), similar to that reported by Akay for piezoelectric BaTiO₃ [2,4,5,7]. Luan et al. [16] also concluded that the resulting compound is denoted as La_1−sSr_sCo_1−λO_3−δ, where s is the level of Sr substitution at the A-site and λ and δ represent Co and oxygen vacancies, respectively. Further, silica support itself can also have oxygen vacancies (SiO_2-β) and the interface between silica support and catalyst act as quantum wires.

4. Technology Transfer

Meunier and Kaddouri [1] also cite three patent applications, which, in fact, do not consider any catalysts in the plasma field, and yet, they do not cite the author’s granted US patent [13] or currently pending US patent application [3] (which was first published in 2021 by the World Intellectual Property Organization, WIPO, and was cited in [4]). This patent application [3] was based on the original publication of the microwave-induced plasma generation and product removal to prevent product dissociation. An AI-based patent search would reveal that Akay’s patent [13] and patent application [3] represent the first ever intellectual property on microwave-induced plasma generation and its use in plasma synthesis.

As indicated previously [3,4,5,6] and in this “Comment”, microwave-induced plasma generation over the catalyst particles is important in increasing the energy efficiency of plasma catalytic reactions, as these types of catalyst beds (fixed or fluidized) enhance reactor efficiency, promote plasma activity on the catalyst surface, and reduce product decomposition through the Havnes effect [3,4]. It (microwave-induced plasma generation) also eliminates the necessity of high electric voltages and the use of Dielectric Barrier Discharge (DBD) reactors; instead, it utilizes the experience gained in microwave reactor technology.