Catalytic Decomposition of N2O and NO

As generally known, nitrogen oxides NOx (NO, NO2) and nitrous oxide (N2O) are significant pollutants. The emissions of NOx produced by fossil fuel combustion in power plants, by transport and chemical industry, represent a serious environmental problem, since they contribute to the formation of acid rains and photochemical smog. More than 90% of emitted NOx from stationary sources is NO. Various techniques have been developed for NO elimination, such as commercially commonly used selective catalytic reduction of NOx (SCR) and selective non-catalytic reduction of NOx (SNCR). In particular, less efficient SNCR technology will no longer be appropriate due to the tightening of emission limits. Conversely, SCR NOx technology is very effective, but its disadvantage, like that of SNCR, is the need to add a reducing agent (ammonia, urea), which increases costs, causes undesirable ammonia slip and requires increased safety precautions. From this perspective, direct catalytic decomposition of NO to oxygen and nitrogen, without a reducing agent, is a big challenge. Since the beginning of 20th century, many catalysts have been studied, including noble metals, simple metal oxides, rare-earth oxides, complex metal oxides and zeolites [1,2], with the aim to find catalysts which are suitable for practical applications. In spite of the great scientific effort, direct catalytic NO decomposition remains in the stage of basic laboratory research. Mixed oxides with alkali metal promoters appear to be active for this reaction, but there are a number of issues that need to be addressed. These are the stability of catalysts, sufficient activity at industrially suitable temperatures and inhibition of the reaction by oxygen and other components present in waste gases. While the promotional effect of potassium on Co3O4 catalytic performance has already been established in the literature [3,4], it remains unknown if K is also a promoter of NO decomposition over similar simple first-row transition metal spinels like Mn3O4 and Fe3O4. The answer to this question can be found find in an interesting paper written by researchers at the Toyota Research Institute of North America [5]. Potassium also has a beneficial effect on Co4MnAlOx mixed oxide for NO catalytic decomposition [6,7]. However, the disadvantage of K/Co4MnAlOx catalysts and generally of K doped catalysts, is that potassium is not stable at the reaction temperature of NO decomposition (above 650 ◦C) and desorbs from the catalyst surface [6]. Therefore, other promoters such as cerium, known for its redox properties and oxygen storage capacity, was tested for modification of Co4MnAlOx. In the study of a Czech research group [8] presented here, the following question has been answered: does the presence of cerium in K-promoted Co4MnAlOx catalysts substantially affect the physical-chemical properties, activity and stability in direct NO decomposition? N2O, a well-known greenhouse gas, is emitted from some processes together with NOx. In this case, direct catalytic decomposition of N2O is also an elegant method for reducing its emissions. This technology is now at the stage of its first commercial applications, mainly in nitric acid plants, which belong to the biggest industrial source of N2O emission. Different variants for abatement of N2O emitted from nitric acid plants were analyzed in the work of M. Inger group from Poland [9]. A two-stage catalytic abatement of N2O from nitric acid plants consisting of high-temperature decomposition in the nitrous gases stream

As generally known, nitrogen oxides NO x (NO, NO 2 ) and nitrous oxide (N 2 O) are significant pollutants. The emissions of NO x produced by fossil fuel combustion in power plants, by transport and chemical industry, represent a serious environmental problem, since they contribute to the formation of acid rains and photochemical smog. More than 90% of emitted NO x from stationary sources is NO. Various techniques have been developed for NO elimination, such as commercially commonly used selective catalytic reduction of NO x (SCR) and selective non-catalytic reduction of NO x (SNCR). In particular, less efficient SNCR technology will no longer be appropriate due to the tightening of emission limits. Conversely, SCR NO x technology is very effective, but its disadvantage, like that of SNCR, is the need to add a reducing agent (ammonia, urea), which increases costs, causes undesirable ammonia slip and requires increased safety precautions. From this perspective, direct catalytic decomposition of NO to oxygen and nitrogen, without a reducing agent, is a big challenge.
Since the beginning of 20th century, many catalysts have been studied, including noble metals, simple metal oxides, rare-earth oxides, complex metal oxides and zeolites [1,2], with the aim to find catalysts which are suitable for practical applications. In spite of the great scientific effort, direct catalytic NO decomposition remains in the stage of basic laboratory research. Mixed oxides with alkali metal promoters appear to be active for this reaction, but there are a number of issues that need to be addressed. These are the stability of catalysts, sufficient activity at industrially suitable temperatures and inhibition of the reaction by oxygen and other components present in waste gases.
While the promotional effect of potassium on Co 3 O 4 catalytic performance has already been established in the literature [3,4], it remains unknown if K is also a promoter of NO decomposition over similar simple first-row transition metal spinels like Mn 3 O 4 and Fe 3 O 4 . The answer to this question can be found find in an interesting paper written by researchers at the Toyota Research Institute of North America [5].
Potassium also has a beneficial effect on Co 4 MnAlO x mixed oxide for NO catalytic decomposition [6,7]. However, the disadvantage of K/Co 4 MnAlO x catalysts and generally of K doped catalysts, is that potassium is not stable at the reaction temperature of NO decomposition (above 650 • C) and desorbs from the catalyst surface [6]. Therefore, other promoters such as cerium, known for its redox properties and oxygen storage capacity, was tested for modification of Co 4 MnAlO x . In the study of a Czech research group [8] presented here, the following question has been answered: does the presence of cerium in K-promoted Co 4 MnAlO x catalysts substantially affect the physical-chemical properties, activity and stability in direct NO decomposition? N 2 O, a well-known greenhouse gas, is emitted from some processes together with NO x . In this case, direct catalytic decomposition of N 2 O is also an elegant method for reducing its emissions. This technology is now at the stage of its first commercial applications, mainly in nitric acid plants, which belong to the biggest industrial source of N 2 O emission. Different variants for abatement of N 2 O emitted from nitric acid plants were analyzed in the work of M. Inger group from Poland [9]. A two-stage catalytic abatement of N 2 O from nitric acid plants consisting of high-temperature decomposition in the nitrous gases stream and low-temperature decomposition in the tail gas stream was evaluated as economically advantageous.
The effort of researchers in the area of N 2 O catalytic decomposition has been focused on increasing catalysts efficiency in conditions simulating real waste gases. Among studied catalysts, nanocrystalline cobalt spinel has been recognized as a very active catalytic material for N 2 O decomposition. Bulk, surface and interface promotion of Co 3 O 4 for low-temperature N 2 O decomposition was studied by the Kotarba group [10,11], while the question of optimal calcination temperature of Co 4 MnAlO x was solved in [12]. The advantages of active Co-spinel phase deposition on zeolite foam support are shown in work [13]. In addition, Co, Fe species were also considered as active sites for N 2 O catalytic decomposition, which is discussed in the work of M. Rutkowska et al., [14], dealing with the optimization of iron form in layered 2D zeolite MCM-22.
Another issue is the decomposition of N 2 O in the presence of suitable semiconductor materials and light with appropriate wavelength and intensity in indoor and outdoor environments. Research findings focusing on the fundamental exploration of the synthesis, characterization and application of nanostructured graphitic carbon nitride/zinc oxide for N 2 O photocatalytic decomposition are explored in the work of the Kočí group [15].
In conclusion, this collection of publications together with references herein well represent the state-of-the art in the area of direct catalytic decomposition of NO or N 2 O; methods suitable for the reduction of their emissions in waste gases and the abatement of N 2 O in the environment via photocatalytic decomposition.
I would like to express my sincerest thanks to all authors for their valuable contributions and I hope that this Special Issue will be an inspiration for many scholars active in this field. I would also like to thank the Editorial team of Catalysts for their kind support and prompt responses. Without them, this Special Issue would not have been possible.

Conflicts of Interest:
The author declares no conflict of interest.