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Review

Advances in Catalytic Decomposition of N2O by Noble Metal Catalysts

by
Yong Zhang
,
Zhigao Tian
,
Lin Huang
,
Honghong Fan
,
Qiufei Hou
,
Ping Cui
and
Wanqiang Wang
*
Department of Chemical Engineering and Food Science, Hubei University of Arts and Science, Xiangyang 441053, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(6), 943; https://doi.org/10.3390/catal13060943
Submission received: 18 April 2023 / Revised: 24 May 2023 / Accepted: 24 May 2023 / Published: 27 May 2023
(This article belongs to the Special Issue Homogenous and Heterogenous Catalysis in Bioactive Compound Synthesis and Small Molecule Activation)
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Abstract

:
Nitrous oxide (N2O) is an environmental pollutant that has a significant greenhouse effect and contributes to the depletion of the ozone layer. To address the issues caused by N2O, direct catalytic decomposition of N2O to N2 and O2 has been demonstrated as one of the most efficient methods for its removal. Various metals, particularly noble metals, including Rh, Ru, Pd, Pt, Au, and Ir, have been widely used and investigated as catalysts to facilitate this transformation. Therefore, this review aims to provide an overview of the advances in noble metal-based catalysts studied in recent years. The comprehensive discussion includes the influence of multiple factors, such as catalyst supports, preparation methods, additives, and impurity gases (such as O2, H2O, SO2, NO, and CO2) on the performance of versatile catalysts. Furthermore, this review offers insights into the future trends of catalyst systems for the direct catalytic decomposition of N2O.

Graphical Abstract

1. Introduction

Nitrous oxide (N2O) is a greenhouse gas that contributes to ozone layer depletion [1,2,3,4] and has a long lifetime of 116 ± 9 years in the atmosphere [5,6]. The global warming potential (GWP) is a number related to the amount of global warming from a substance, and it is a ratio of the warming caused by the substance to that generated from a similar mass of CO2. In particular, the GWP of N2O is approximately 273 times higher than that of CO2. N2O emissions result from both natural sources (soil, ocean, and atmosphere) and human activities (nitric acid production, adipic acid, fossil fuel combustion, waste incineration, and automobile exhaust emission) [7,8]. According to a report by the United Nations Environment Programme (UNEP), about 5.3 million tons of N2O are emitted into the atmosphere each year [9]. Since 1750, the concentration of N2O has increased from about 270 ppb in the pre-industrial era to around 332 ppb in 2020 [9]. Despite being the largest known anthropogenic threat to the stratospheric ozone layer, N2O is regulated only under the 1997 Kyoto Protocol due to its simultaneous ability to warm the climate. As a result, the elimination of N2O has become an urgent environmental issue, attracting a lot of attention.
Various methods have been developed to reduce N2O emissions, including high-temperature decomposition, selective catalytic reduction, and low-temperature catalytic decomposition [10,11,12,13]. Although high-temperature decomposition does not require a catalyst, its energy demand is high because the decomposition temperature typically exceeds 800 °C. Selective catalytic reduction, on the other hand, has a lower decomposition temperature, but it involves adding a reducing agent, which leads to additional costs and secondary pollution. Low-temperature catalytic decomposition is one of the most promising methods, owing to its lower energy requirements and, consequently, lower cost [14]. To date, a number of novel catalytic materials, such as metal oxides [15,16], noble metal-supported catalysts [17,18], and ion-exchanged zeolites [19,20] have been successively developed for low-temperature catalytic N2O abatement.
Metal oxides are widely used for low-temperature catalytic N2O decomposition due to their low cost and thermal stability, but their activity is relatively low [21]. Although zeolite-based catalysts also exhibit high activity at low temperature, the hydrothermal stability of zeolite is poor, and irreversible inactivation can easily occur when high-temperature water vapor exists. Representative noble metal catalysts, such as Rh [21,22], Ru [23,24], Pd [25,26], Pt [27,28], Au [29,30], and Ir [31,32] are highly active towards N2O decomposition and are more resistant to impurity gases than other catalysts.
Recently, several reviews on the decomposition of N2O have been conducted. However, these reviews have been overly broad, combining discussions on noble metals, non-noble metals, molecular sieves, etc. into one topic, with an emphasis on non-noble metals. Consequently, no single topic concentrates solely on precious metals, and their significance is often disregarded. Therefore, this review is mainly to refine the field of N2O decomposition catalyzed by noble metals, allowing readers to have a comprehensive and timely understanding of different aspects of noble metals alone. Indeed, noble metals play pivotal roles in activating and promoting the decomposition of N2O. Even though single-atom noble metal catalysts are still at an early stage, they have gained considerable attention and hold promise for practical applications. Based on the above considerations, this review aims to describe the recent progress in the decomposition of N2O with a major focus on the examples applying noble metal catalysts and the effects of important parameters on catalytic performance. Moreover, the catalytic mechanisms and potential industrial applications are also illustrated. Finally, the conclusions and future perspectives are presented.

2. Utilization of Noble Metal Catalysts

2.1. Rh-Based Catalysts

Among the noble metal catalysts utilized, the Rh-based ones have been most extensively studied due to their remarkable activity for N2O decomposition. In a study by Doi et al. [33], Rh/Al2O3, Pd/Al2O3, and Pt/Al2O3 were prepared for this transformation, and the activity followed the trend: Rh/Al2O3 > Pd/Al2O3 > Pt/Al2O3. In particular, Rh/Al2O3 exhibited the highest activity, achieving nearly complete N2O conversion at 500 °C. This trend was also observed in another study by Parres-Esclapez et al. [34], who applied Al2O3- or CeO2- supported Rh, Pd, and Pt catalysts for the investigations (Figure 1). These consistent findings prompted us to further investigate the effects of supports, preparation methods, and particle size on the catalytic performance of Rh-based catalysts.

2.1.1. The Effect of Support

The catalytic performance of Rh catalysts is highly dependent on the interactions between Rh and the support material, as demonstrated by various studies. Yuzaki et al. [35] immobilized Rh on different types of supports (USY, NaY, Al2O3, ZrO2, FSM-16, CeO2, La2O3) and found that the potency of the resulting Rh catalysts decreased in the order of Rh/USY > Rh/Al2O3 > Rh/ZrO2 > Rh/CeO2 > Rh/FSM-16 > Rh/La2O3. The remarkable activity of Rh/USY was explained by the high dispersion of Rh species on the surface of USY. Similarly, Kim et al. [36] observed that Rh/CeO2 exhibited higher activity than Rh/Al2O3, which was attributed to better interaction between Rh and CeO2 as demonstrated by temperature-programmed reduction (TPR) measurements using H2 as the detection gas. Recently, Jing et al. [21] also found that RhOx supported on ZrO2 performed better than RhOx loaded on the other utilized supports (CeO2, Al2O3, SiO2, TiO2, MgO, CeO2-ZrO2, SiO2-Al2O3, MgO, Nb2O5, SnO2). The characterization results and kinetic studies showed that the N2O decomposition reaction over RhOx/ZrO2 took place via the redox cycles of RhOx. Further investigations revealed that the redox ability was critically important for the efficient progress of the reaction, during which N2O acted as an oxidizing agent to fill in the oxygen vacancies generated upon heating with concomitant release of gaseous O2.
Mesoporous materials have been reported to significantly enhance catalytic activity in N2O decomposition when they are used to support Ru [37]. As a typical example, Xu et al. [38] reported the use of mesoporous silica, denoted as SBA-15, to support Rh. Interestingly, the resultant catalyst exhibited much higher activity than the SiO2-supported Rh catalyst. Hussain et al. [39] prepared a series of mesoporous SiO2, including MCM-41, SBA-15-Conventional (SBA-15-C), SBA-15-Spherical (SBA-15-S), and KIT-6, and utilized them as the supports for Rh catalysts to perform N2O decomposition. Among these catalysts, Rh/SBA-15-S demonstrated the highest catalytic activity, with a T50 value of 372 °C (Figure 2). Later, Piumetti et al. [40] compared the activity of Rh/MCM-41, Rh/SBA-15, Rh/KIT-6, and Rh/MCF catalysts for this catalytic process. It was noted that the Rh/MCF catalyst displayed the highest catalytic potency, probably due to the unique textural properties of MCF silica. Specifically, the 3-D mesoporosity with ultra-large MCF cells led to a uniform distribution of small RhOx particles on the surface area of MCF. In another study, Liu et al. [41] investigated the catalytic activity of Rh-based catalysts embedded in mesoporous Al2O3 and MOx-Al2O3 (M = Mn, Fe, Co, Ni, Cu, Ba) composite materials in N2O decomposition. According to the T50 values, the catalytic activity of the Rh catalysts followed the sequence of Rh/Co-MA (283 °C) > Rh/Ni-MA (287 °C) > Rh/Fe-MA (290 °C) > Rh/Ba-MA (292 °C) > Rh/MA (301 °C) > Rh/Cu-MA (314 °C) > Rh/Mn-MA (321 °C). The transmission electron microscopy (TEM) results indicated a uniform dispersion of Rh2O3 particles on these supports.
Non-oxide carriers have been reported to exhibit excellent catalytic activity in N2O decomposition when Rh is loaded onto them. Some carriers, such as HAP (Ca10(PO4)6(OH)2), exhibit much higher activity than SiO2, Al2O3, and TiO2. Huang et al. [42] reported that the Rh/HAP catalyst exhibited good activity at a relatively low temperature, and N2O conversion reached 97% at 275 °C. The superiority of HAP over the other investigated supports could be ascribed to the generation of more basic sites and the stabilization of ultra-small Rh particles. Later, Huang et al. used a different synthetic method for HAP, leading to a Rh/HAP catalyst with enhanced catalytic activity. RhOx/M-P-O (M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts were also prepared and tested [43]. The activity followed the trend of RhOx/Ca-P-O > RhOx/La-P-O > RhOx/Mg-P-O > RhOx/Co-P-O > RhOx/Al-P-O > RhOx/Fe-P-O > RhOx/Zn-P-O. The temperature of RhOx/Ca-P-O regarding the complete conversion of N2O reached 300 °C (Figure 3). Other metal phosphate-supported Rh (Rh/LaPO4 [44] and Rh/CePO4 [45]) catalysts have also been studied.
From the above studies, it can be concluded that the supports of Rh-based catalysts have a significant effect on their catalytic activity. Mesoporous materials and metal phosphates could be a new research hotspot for Rh-based catalysts in N2O decomposition.

2.1.2. The Effect of Particle Size and Valence States of Rh

Except for the utilized supports, the particle size of Rh-based catalysts also has a profound impact on the catalytic activity. This size-dependent catalytic activity is associated with either low coordination surface sites due to the low energy charge fluctuations in their d-bands, or quantum size effects caused by electron confinement within a small volume. To investigate the influence of Rh particle size on N2O decomposition, Parres-Esclapez et al. [46] conducted experiments on the influence of Rh particle sizes on N2O decomposition and prepared four catalysts with particle sizes ranging from 1.20 to 2.00 nm. The results showed that a smaller Rh particle size resulted in higher catalytic activity, except for Rh/α-Al2O3. This trend was also observed by Piumett et al. [40] in Rh/mesoporous silica catalysts, where smaller RhOx particles exhibited better performance. The relationship between N2O conversion rates and mean Rh particle sizes in the 1–2.5 nm range is shown in Figure 4, where a significant dependence of conversion rates on particle size is observed in this size domain [40].
In contrast, Beyer et al. [47] synthesized Rh particles with diameters ranging from 1.0 to 4.0 nm on different supports, including MgO, SiO2, CeO2, Al2O3, and TiO2. Surprisingly, the highest catalytic activity was observed with Rh/MgO and Rh/SiO2 at low temperatures, which was attributed to the presence of Rh particles with a mean diameter of 2.1–2.4 nm. Rh/CeO2, Rh/Al2O3 and Rh/TiO2 predominantly had smaller Rh particles with a mean diameter of 1.0–1.4 nm, and these materials exhibited significantly lower activities (Figure 5). In addition, even smaller single-atom Rh particles were prepared by Xie et al. on the Rh/CeO2 catalyst [48]. Systematic characterizations, activity tests, and kinetics studies implied that the high activity of this catalyst was attributed to the potential atomic dispersion states of Rh species on the CeO2 support. These results indicated that the abundant oxygen vacancies near the Rh single atoms could benefit the adsorption and activation of N2O, while the more reactive Rh single atom sites with a slightly higher coordination number of Rh-O bond and higher reducibility could better facilitate the O2 desorption, altogether accounting for the superior activity achieved on the Rh/CeO2 catalyst over the other catalysts. Additionally, Wang et al. [49] prepared a series of Rh catalysts encapsulated within silicalite-1 of different sizes, from single atoms to nanoclusters and nanoparticles, with an average particle size of 0.3–2.0 nm. The results clearly demonstrated that the Rh catalysts in the form of nanoclusters and nanoparticles showed high activity and good stability toward N2O decomposition.
The catalytic performance of Rh species is significantly influenced by their valence states. Although Rh3+ species are generally considered the primary active center for N2O decomposition, there is evidence suggesting that partially reduced Rh3+ can also enhance catalytic activity. Lin et al. [43] reported an increase in activity upon reduction of the catalysts by H2, which indicated the potential of these partially reduced Rh3+ species to improve catalytic performance. Additionally, Hussain et al. [39] observed that the mesoporous silica-supported Rh catalyst possessed a higher Rh+ percentage than Rh0 or Rh3+, which reflected the pivotal role of Rh+ in this catalysis.

2.1.3. The Effect of Catalyst Preparation Methods

The choice of preparation methods is critical for the development of highly effective Rh-based catalysts with the aim of achieving low metal loadings and high dispersion. Xu et al. [38] evaluated the impact of different preparation methods on the activity of Rh/Al-SBA-15 catalysts. Specifically, Rh-Al-SBA-im, Rh-Al-SBA-pa, and Rh-Al-SBA-pc were prepared through impregnation, ammonia precipitation, and carbonate precipitation, respectively. The study found that the activity sequence of the catalysts followed Rh-Al-SBA-pa > Rh-Al-SBA-im > Rh-Al-SBA-pc. Ammonia precipitation was found to be a more effective preparation method than carbonate precipitation. In another study, Kim et al. [36] investigated the N2O decomposition activity of Rh/CeO2-Al2O3 catalysts prepared through three different synthetic approaches: impregnation, coprecipitation, and ball milling. The coprecipitation method yielded the highest catalytic activity, while the impregnation method resulted in the lowest activity. This finding was attributed to the higher surface area and better reducibility induced by the coprecipitation method compared to the other two methods. Recently, Li et al. [13] prepared Rh-containing zeolite catalysts using three methods: (1) Rh solid-state exchange into H-zeolite, (2) Rh solid-state exchange, followed by Ce impregnation, and (3) Rh-Ce co-impregnation on H-zeolite. Rh-Ce/BEA (Ce/Rh co-impregnated) was found to be much more active than Ce/Rh-BEA (Ce impregnated on Rh-BEA). Moreover, Ho et al. used an in situ synthesis method to prepare Rh/Mg/Al HT precursors with electrodeposition followed by calcination, and the resulting catalysts were still stable after a 24 h test at 475 °C in the presence of O2 and NO [50].

2.2. Ru-Based Catalysts

2.2.1. The Effect of Support

Ru-based catalysts for N2O decomposition are mainly supported on ZrO2, TiO2, Al2O3, MCM-41, and ZSM-5. The selection of support materials significantly influenced the activity of the Ru-based catalysts. For instance, Hinokuma et al. [10] investigated the catalytic activity of Ru supported on various oxides. The T50 values increased in the order of SnO2  <  ZrO2  <  Al2O3  <  CeO2  <  Ta2O5  <  TiO2  ≈  WO3  ≈  Nb2O5. According to H2-TPR results, the redox properties of RuO2 at low reaction temperatures were closely associated with the N2O decomposition reaction. Ru/SnO2 showed high catalytic performance because the SnO2 support induced high reducibility (redox property) for the catalysts. Zheng et al. [51] examined the effects of different oxides (MgO, SiO2, CeO2, Al2O3, TiO2) as supports on the activity of Ru catalysts. They observed that Ru particles supported on SiO2, Al2O3, or TiO2 demonstrated higher activity than those supported by MgO or CeO2. Lin et al. [52] reported that the use of rutile-TiO2 as a support to load RuO2 for N2O decomposition resulted in higher activity than those using TiO2, Al2O3, SnO2 or SiO2 as supports (Figure 6). The outstanding performance of RuO2/rutile-TiO2 was attributed to the formation of a uniformly coated RuO2 thin film on rutile-TiO2, which effectively maximized the dispersion of the active phase.
The use of mesoporous materials, such as MCM-41, KIT-6, and SBA-15, to load Ru has resulted in the development of composite catalysts [38,53], among which Ru/MCM-41 has been found to be more active for N2O decomposition than Ru/ZSM-5 [54]. This observation could be attributed to the higher specific surface area of Ru/MCM-41 compared to Ru/ZSM-5, which led to an increased dispersion of active species. The higher dispersion of active species on Ru/MCM-41 also enhanced its activity towards N2O decomposition. Therefore, the use of mesoporous materials significantly improves the catalytic activity of Ru-based composite catalysts for N2O decomposition.
Cui et al. [55] studied the impact of various M-P-O (M = Mg, Al, Ca, Fe, Co, Zn, La) supports on the performance of RuOx/M-P-O catalysts for N2O decomposition (Figure 7). The order of decreasing activity was observed as RuOx/Ca-P-O > RuOx/Mg-P-O > RuOx/La-P-O > RuOx/Co-P-O > RuOx/Al-P-O > RuOx/Zn-P-O ~ RhOx/Fe-P-O, highlighting the significant influence of different M-P-O support materials. The highest activity was exhibited by RuOx/Ca-P-O, with a determined temperature of 400 °C for the full conversion of N2O. However, its activity was still lower than that of RhOx/Ca-P-O prepared by Lin et al. Therefore, the choice of support material significantly affects the activity of a catalyst, and Ca-P-O is the optimal support for the Ru catalysts to achieve high performance in N2O decomposition.

2.2.2. The Effect of Particle Size and the Chemical Valences of Ru

Ru-supported catalysts are known to be enhanced by the presence of small Ru particles [51]. However, the chemical valences of the active components in Ru-supported catalysts have yielded different findings. Christoforou et al. [56] developed a Ru/Al2O3 catalyst for N2O decomposition, discovering that partially oxidized and fully oxidized catalysts exhibited higher activity than non-oxidized ones. Ru species bearing higher oxidation states were identified as the likely cause. Similarly, Chang et al. [57] also found that the main Ru species in Ru-NaZSM-5 and Ru-HNaUSY-5 were present as Ru3+, with these electron-rich Ru species (Ru3+) donating electrons to the N2O molecules to weaken the N–O bond and thus result in higher reaction rates of N2O decomposition. In situ IR spectroscopy conducted by Pinna et al. [23] revealed that the Ru species in a Ru/ZrO2 catalyst existed as RuO2 and RuO3, which were probably involved in the catalytic reaction. Kumvokis et al. [17] found that different pretreatment methods (He, O2, H2) had obvious effects on Ru/γ-Al2O3, with H2-prepared catalysts showing higher activity than He- or O2-prepared ones. Metallic Ru was determined to be more conducive to the N2O decomposition reaction. Sui et al. [58] observed that Ru/Al2O3 (Ru/Al2O3-H2, Ru/Al2O3-NaBH4, Ru/Al2O3-air) catalysts with different amounts of metallic Ru yielded different levels of N2O conversion. The linear relationship between the amount of metallic Ru and N2O conversion suggests that metallic Ru is the active site in N2O decomposition (Figure 8).

2.2.3. The Effect of Preparation Methods

Preparation methods also have an important influence on the catalytic performance of Ru-based catalysts. Komvokis et al. [59] used an in situ reduction method to prepare Ru/γ-Al2O3 catalysts, resulting in Ru nanoparticles with a particle size of 1–3 nm and a dispersion of up to 70%. This method exhibited significantly better catalytic activity compared to Ru/γ-Al2O3 catalysts prepared by impregnation-calcination, which possessed a particle size of 10–80 nm and a dispersion of only 10%. Using the former as a catalyst, N2O completely decomposed at the temperature of 475 °C, while with the latter as a catalyst, N2O completely decomposed at the temperature of 550 °C. On the other hand, Reddy et al. [60] found that deposition-precipitation of Ru on Al2O3 created more electron-deficient Ru sites during preparation, leading to greater catalytic activity than the catalyst fabricated by an impregnation-calcination method.
The supports and their interaction with noble metals have a crucial role in the dispersion of active sites and thus impact the catalytic potency in N2O decomposition. Mesoporous materials and metal phosphates hold potential as support systems to enhance the dispersion of active sites due to their high surface area and basicity. Therefore, the four main factors, including supports, particle size, chemical valences, and preparation methods, should be considered when designing an efficient catalytic system.

2.3. Pd-Based Catalysts

In general, Rh-based catalysts are more effective for N2O decomposition than Pd-based catalysts, while the latter show higher activity than Pt-based catalysts [33]. Specifically, Parres-Esclapez et al. [34] found that the trend in activity for N2O decomposition was Rh/CeO2 > Pd/CeO2 > Pt/CeO2, concluding that the catalytic activity was correlated with the reduction of noble metals. The researchers found that the activity of the catalysts was related to the reduction of noble metals: the easier the reduction, the higher the catalytic activity. In a similar vein, Cheng et al. [61] found that the 0.7Pd/FeAlPO-5 catalyst showed high activity for N2O decomposition, with 50% conversion of N2O at 360 °C and 90% conversion at 373 °C. Dacquin et al. [62] observed that Pd/LaCoO3 exhibited higher activity than Pd/Al2O3 due to the anionic vacancies formed by the interaction between Pd and LaCoO3 (Figure 9). Recently, Richards et al. [63] found that the preparation method for Pd/γ-Al2O3 catalysts greatly affected their activity, with increased concentrations of Pd–Cl species leading to improved control of Pd particle size and consequently higher activity. Finally, density functional theory (DFT) calculations indicated that the easier molecular adsorption of N2O onto the metal surface by Pd atoms is responsible for its catalytic activity in N2O decomposition [26].

2.4. Pt-Based Catalysts

Compared to Rh-, Ru-, and Pd-based catalysts, Pt-based catalysts typically exhibit lower activity in N2O decomposition due to the hindering effect of strongly adsorbed oxygen atoms produced during N2O decomposition. For instance, Pt/Al2O3 catalysts demonstrated poor activity in N2O decomposition, with an active temperature range above 400 °C [34]. To improve catalytic activity, Konsolakis et al. [64] employed CeO2-La2O3 to modify the Pt/Al2O3 catalyst. The modified catalyst, Pt/Al2O3-(CeO2-La2O3), showed significant improvement in catalytic activity (Figure 10), with complete N2O conversion being achieved at 500 °C, while the N2O conversion of 20% at 600 °C was offered by the Pt/Al2O3 catalyst. The superior catalytic performance of Pt/Al2O3-(CeO2-La2O3) was attributed to the formation of electron-enriched Pt sites (Ptδ−) at the metal-support interfacial area, which are highly active towards N2O decomposition. Furthermore, Wang et al. [65] prepared the Pt-Ni-Co catalyst, which exhibited good activity and stability, sustaining the reaction for 300 h without obvious activity decline.

2.5. Au-Based Catalysts

Supported Au catalysts have gained significant attention since Haruta et al. discovered their highly efficient activity for CO oxidation at relatively low temperatures [66,67]. Despite this, research on Au-based catalysts for N2O decomposition remains minimal. However, Yan et al. [68] demonstrated the superior performance of nano-Au supported on Co3O4 for N2O decomposition, with an onset temperature as low as 180 °C. Xu et al. [29] subsequently prepared an Au catalyst using Co-Al hydrotalcite as the support through the ion exchange method. Figure 11 illustrates the N2O conversion at a fixed temperature of 425 °C, applying various catalysts with varying Au loadings. The results implied that 1.1% Au/Co-Al exhibited the best catalytic performance. In addition, a T50 value of 380 °C and a T100 value (T100 represents the required temperature to achieve 100% N2O conversion) of 450 °C were identified as the best catalysts. Other Au-based catalysts, such as Au/ZSM-5 [69], have also been investigated for N2O decomposition. These studies collectively demonstrate that Au has the potential to act as an active species for N2O decomposition and exhibits excellent catalytic performance. Wu et al. [70] calculated the energy barrier of Au3 +/0/− clusters with N2O by DFT calculations and found that the Au3 neutral cluster exhibited the highest catalytic activity on the decomposition of N2O, with a calculated energy barrier of only 11.60 kcal/mol.

2.6. Ir-Based Catalysts

Ir-based catalysts have been found to be promising for the decomposition of N2O, owing to their unique physicochemical properties and lower cost when compared to conventional Pt-, Pd-, and Rh-based catalysts [71]. Several studies have reported high performance for Ir catalysts in N2O decomposition. Ohnishi et al. [31] reported that Ir supported on γ-alumina had high initial activity, while Zhu et al. [32] found that Ir-substituted hexaaluminate catalysts demonstrated remarkable performance. Shen et al. showed that Ir/Fe-USY-0.1% had high stability for N2O decomposition due to the electronic synergy between Ir and Fe sites, as well as the good dispersion of these metals within the zeolite framework [72]. Recently, Hinokuma et al. [73] studied the effects of various supports on catalytic N2O decomposition reactions. The T50 catalytic activity increased in the order of Ir/ZrO2 < Ir/SnO2 < Ir/Al2O3 < Ir/CeO2 < Ir/Nb2O5 < Ir/TiO2 < Ir/MgO < Ir/SiO2. Ir/ZrO2 exhibited particularly high performance since the oxidation states of Ir and ZrO2 during the reaction were almost preserved (Figure 12). Yentekakis et al. [74] found that a smaller Ir particle size correlated with higher activity in catalytic N2O decomposition. Additionally, Pachatouridou et al. [75] discovered that Ir/Al2O3 performed substantially better than Pt/Al2O3 and Pd/Al2O3 (Figure 13). Subsequent research conducted by the same group [76] revealed that the activity of the Ir/Al2O3 catalysts could be further enhanced through modification with CeO2. Despite concerns surrounding the volatility of Ir at elevated temperatures, Ir-based catalysts have demonstrated satisfactory stability and performance at moderate temperatures, positioning them as potential alternatives for N2O abatement in practical applications.
Although research on Pd-, Pt-, Au-, and Ir-based catalysts remains limited and unsystematic, these noble metals are present as cheaper alternatives to Rh-based catalysts and exhibit high catalytic activity, with Ir-based catalysts in particular emerging as a viable research direction for N2O decomposition.

3. The Effect of Additives and Impurity Gases

3.1. The Effect of Additives

Related studies have investigated the potential of additives to enhance the activity of noble metal oxide-based catalysts for N2O decomposition. Specifically, Kim and colleagues [36] examined the impact of Pr and La on the catalytic performance of a Rh/CeO2 catalyst. These findings revealed that the addition of Pr and La led to an improvement in the redox properties of the catalyst, with Rh/CeO2(Pr) demonstrating the highest activity, followed by Rh/CeO2(La) and Rh/CeO2. In another study, Parres-Esclapez et al. [46] observed that Sr had a positive influence on the activity of Rh/Al2O3, likely due to the improved dispersion and reducibility of Rh species. Additionally, Konsolakis et al. reported that CeO2 and La2O3 acted as promoters to enhance the catalytic activity of Pt/Al2O3.
On the other hand, alkali metal cations have also been used as promoters to improve the dispersion of noble metal oxides, consequently increasing catalyst activity. Haber et al. [77] investigated the impact of Li+, Na+, K+, and Cs+ on the activity of a Rh/Al2O3 catalyst and discovered that these ions influenced the dispersibility of Rh. The extent of the promotion effect depended on the type of cation, with K+ and Cs+ showing obvious increases in activity as the promoter amount increased, while Li+ and Na+ demonstrated slower increases in activity with increasing promoter concentration. Very recently, Bozorgi et al. [78] examined the effect of alkali metal promoters (Na+, K+, and Cs+) on Pt-based catalysts, and found that Na+ and Cs+ led to a drop in N2O conversion from 65% to 25% and 35%, respectively. In contrast, K+ triggered an increase of 18% in N2O decomposition (Figure 14).

3.2. The Effect of Impurity Gases

The direct catalytic decomposition of N2O represents a highly effective method for the elimination of N2O from tail gas. However, in reality, exhaust gas tends to contain a high concentration of impurities, including gases, such as NO, O2, SO2, CO2, and H2O [55,76,79,80]. These impurities have been found to interfere with the ability of the catalysts to decompose N2O, thus decreasing its activity and stability. As such, it is essential to establish the extent to which different impurity gases impact the effectiveness of catalytic N2O decomposition, thereby underscoring the need for a more comprehensive understanding of their role in this process.

3.2.1. The Effect of O2

The inhibiting influence of O2 on N2O decomposition has been demonstrated in various studies exploring the impact of O2 on Rh-, Ru-, and Pt-based catalysts, resulting in significant reductions in catalytic activity. The degree of inhibition can vary depending on the support materials utilized. Specifically, Beyer et al. [47] found that the basicity of the support material could affect the inhibition effect of O2. The catalytic activities of Rh(N)/MgO and Rh(N)/CeO2 reduced notably in the presence of O2, whereas the effect was less pronounced for Rh(N)/Al2O3 and Rh(N)/TiO2, and minimal for Rh(N)/SiO2. Similarly, Cui et al. [55] observed a significant reduction in N2O conversion for the RuOx/HAP catalyst upon the introduction of O2 (Figure 15), primarily due to competitive adsorption between O2 and N2O. However, the inhibiting effect of O2 on RuOx/HAP was shown to be reversible, as the N2O conversion returned to the initial activity upon removal of O2. These findings indicate that the efficiency of N2O decomposition when applying Rh-, Ru-, and Pt-based catalysts can be hampered by the presence of O2. Besides, Pachatouridou et al. [75] observed that the presence of O2 largely decreased the catalytic activity of both Ir/Al and Ir/Ce catalysts. Nevertheless, it was noted that the CeO2-promoted Ir/AlCe catalyst exhibited only minor susceptibility to O2 in the feed stream [76]. These findings imply that the efficiency of N2O decomposition by Rh-, Ru-, and Pt-based catalysts may be impaired in the presence of O2, and the addition of certain additives may enhance tolerance to O2-induced inhibition.

3.2.2. The Effect of H2O

The inhibiting effect of H2O is more pronounced than that of O2 as reported in the literature. Specifically, for a Rh/LaPO4 catalyst, it was found that the N2O conversion was significantly lower in the presence of H2O than in the presence of O2 at the same temperature [44]. Similar findings have been reported for RuOx/HAP [55] and Rh/CePO4 [45]. This inhibiting effect could be attributed to the competitive adsorption of N2O and H2O on the catalyst surface. The influencing effect of H2O on catalytic activity is generally reversible, except for triggering structural change and compromising N2O decomposition due to changes in the active center of the catalyst. Previous research by Liu et al. indicated that the influence of H2O was comparable to that of O2 since the activity could be quickly restored once H2O was removed. However, the semi-reversible character of H2O-poisoning was noticed over a Pt(K)/Al2O3-(CeO2-La2O3) catalyst [79]. Apart from the competitive adsorption of H2O, the changes in the active center of the catalyst (e.g., oxidation of Pt sites by hydroxyl groups) hindered N2O adsorption/decomposition and were thus considered a significant contributing factor.

3.2.3. The Effect of SO2 and CO2

When SO2 is present in the feed, it causes an irreversible inhibiting effect on the catalytic activity of the catalysts due to the formation of stable sulfates on the surface [80], which differs from the reversible inhibiting effects of O2 and H2O. Marnellos et al. [81] found that the presence of SO2 caused a dramatic decline of the N2O decomposition over the Ru/Al2O3 catalyst, and similar inactivation was also observed on the Pd/Al2O3 catalyst [80]. These sulfate species were reported to be stable on the catalytic surface at the reaction temperature and could only be decomposed at elevated temperatures. However, the partially deactivated catalysts could be regenerated by either reducing or calcining them at high temperatures.
Compared to O2, H2O and SO2, CO2 exhibits no obvious effect on catalytic activity. Huang et al. [82] found that CO2 had only slight inhibiting effects, as shown by an increase in the T50 value from 223 °C to 267 °C. According to Liu et al. [45], CO2 had no effect on the activity of the Rh/CePO4 catalyst.

3.2.4. The Effect of NO

In most cases, the addition of NO substantially decreased the decomposition of N2O. As a typical example, Pieterse et al. [83] studied the effect of NO on Rh/MOR and Ru/FER and found that the presence of NO had a strong negative effect on the performance of N2O decomposition. Sobalik et al. [84] observed that NO on the decomposition of N2O was negative over Rh-ferrierite and even inhibited decomposition over Ru-ferrierite. In some cases, the presence of NO could increase activity and decrease the apparent activation energy of N2O decomposition. For instance, NO enhanced the N2O decomposition over Pt/Fe-ferrierite and Ru/Fe-ferrierite, while the effect tended to be negative over Rh/Fe-ferrierite. Kim et al. [85] found that the influence of NO was more severe than that of O2 and H2O. In the presence of NO, the catalytic activity drastically decreased. The N2O conversions using Rh/Al2O3 catalysts in the presence of NO were 24% at 375 °C. The inhibitory effect of NO may be attributed to the formation of nitrite/nitrate species on the catalytic surface.

4. Catalytic Mechanisms

There are two typical mechanisms involved in the action of noble metal catalysts on N2O decomposition: the Kondratenko mechanism and the Hinshelwood mechanism [86]. Under the Kondratenko mechanism, N2O is first decomposed into N2 and oxygen adsorbed on the surface. Subsequently, the adsorbed oxygen reacts with N2O to form N2 and O2, which is an irreversible process. In this way, the mechanism assumes that the O2 source is half that of adsorbed oxygen and half from N2O. In contrast, the Hinshelwood mechanism suggests that O2 stems from adsorbed oxygen. Under this mechanism, N2O is transformed into adsorbed N2O on the noble metal surface. Next, adsorbed N2O is decomposed into N2 and adsorbed oxygen, which is combined to form O2.
Although noble metal catalysts exhibit outstanding performance, their catalytic activity could be inhibited by the presence of impurity gases (NO, H2O, O2, CO2, SO2). This is mainly attributed to the competitive adsorption of these gases with N2O on the same active sites [9,87], or to the hydroxylation-induced oxidation of Pt sites [88].

5. Industrial Applications of Noble Metal Catalysts

For industrial applications, it is important to assess the suitability of each catalyst in relation to the unique characteristics of the N2O source, including adipic acid and nitric acid production, fossil fuel and biomass combustion, and vehicle emissions [9,82]. Noble metal catalysts exhibit high catalytic activity at low temperatures and are suitable for low-temperature catalytic N2O decomposition in nitric acid plants, waste anesthetic gas elimination, automotive exhaust treatment, etc., where the main requirement is resistance to tail gas inhibitors such as O2 and H2O. However, these noble metal catalysts are not stable enough for use at high temperatures, such as N2O abatement directly in ammonia burners of nitric acid plants, where the catalyst is exposed to 850 °C. In addition, noble metal catalysts have been identified as promising choices for the removal of N2O in ammonia combustion processes, which generate various NOx species, including NO, NO2, and N2O [89]. Despite their satisfactory catalytic performance in N2O decomposition, noble metal catalysts are constrained by their high cost and sensitivity to various impurity gases. For example, the recent surge in the international spot price of Au to a historical record of 2079.67 USD per ounce imposes financial limitations on the practical application of these catalysts. To overcome these limitations, future research should focus on designing and fabricating highly active noble metal catalysts with ultralow metal loadings and a good tolerance to impurity gases.

6. Conclusions

In this review, advancements in the field of N2O decomposition on noble metal catalysts have revealed the crucial role of supports, preparation methods, particle size, additives, and impurity gases in catalytic activity. For Rh- and Ru-based catalysts, properties such as structural type, specific surface area, the amount of alkali, and interaction between active species and the support impact the dispersion of the active catalytic species, ultimately influencing its activity. However, studies on Pd-, Pt-, Au-, and Ir-based catalysts have mostly concentrated on catalytic activity alone, neglecting the necessity of studying active species, reaction conditions, and reaction mechanisms. In particular, the high activity of Ir-based catalysts is gaining attention and holds promise for actual production applications.
Although noble metals exhibit good performance in decomposing N2O at low temperatures, their high costs limit their widespread industrial applications. On the other hand, single atom catalysts have recently received considerable attention due to their higher utilization efficiency of noble metals, making them a promising research direction for noble-metal-catalyzed N2O decomposition. Additionally, the utilization of different supports can improve the metal-support interaction and practical application of noble metal catalysts by enhancing the dispersion of precious metals, increasing oxygen vacancies, and regulating the acid-base properties of the catalysts.
It has been well documented that N2O is typically present in an exhaust mixture that includes O2, H2O, SO2, NO and other gases. Nevertheless, most current catalysts have only been tested without these impurity gases, which can greatly impact their performance in N2O decomposition. The addition of O2 or H2O to N2O moderately impairs N2O decomposition, while SO2 results in severe inhibition of N2O decomposition due to the formation of stable sulfates on the surface of the catalysts. In terms of NO, it could have either positive or negative effects on catalytic activity. Therefore, further efforts are needed to enhance the activity and durability of noble metal-supported catalysts in the presence of these impurity gases.

Author Contributions

Y.Z. wrote the majority of the paper, Z.T., L.H. and Q.H. wrote some of the paper. H.F. provided financial support. W.W. and P.C. reviewed and modified the draft of the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Natural Science Foundation of Hubei Province (No. 2021CFB160).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Temperature of 50% N2O conversion (T50) on Rh, Pd, and Pt catalysts.
Figure 1. Temperature of 50% N2O conversion (T50) on Rh, Pd, and Pt catalysts.
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Figure 2. Comparison of the temperature of 50% N2O conversion abatement by different mesoporous silica and their Rh (1 wt%) supported catalysts.
Figure 2. Comparison of the temperature of 50% N2O conversion abatement by different mesoporous silica and their Rh (1 wt%) supported catalysts.
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Figure 3. N2O conversion on RhOx/M-P-O (M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts at 350 °C.
Figure 3. N2O conversion on RhOx/M-P-O (M = Mg, Al, Ca, Fe, Co, Zn, La) catalysts at 350 °C.
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Figure 4. N2O conversion rates at 350 °C, normalized per mmol of Rh, as a function of the RhOx particle size.
Figure 4. N2O conversion rates at 350 °C, normalized per mmol of Rh, as a function of the RhOx particle size.
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Figure 5. N2O conversion on Rh/MgO, Rh/SiO2, Rh/CeO2, Rh/Al2O3 and Rh/TiO2 catalysts at 350 °C.
Figure 5. N2O conversion on Rh/MgO, Rh/SiO2, Rh/CeO2, Rh/Al2O3 and Rh/TiO2 catalysts at 350 °C.
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Figure 6. N2O conversion for various RuO2 catalysts at 280 °C.
Figure 6. N2O conversion for various RuO2 catalysts at 280 °C.
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Figure 7. N2O conversions over RuOx/M-P-O at 400 °C.
Figure 7. N2O conversions over RuOx/M-P-O at 400 °C.
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Figure 8. Relationship between N2O conversion and the amount of surface metallic Ru at 350 °C.
Figure 8. Relationship between N2O conversion and the amount of surface metallic Ru at 350 °C.
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Figure 9. N2O conversions over Rd/LaCoO3 and Pd/Al2O3 at 500 °C.
Figure 9. N2O conversions over Rd/LaCoO3 and Pd/Al2O3 at 500 °C.
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Figure 10. N2O conversion in the absence of O2 over catalysts at 500 °C.
Figure 10. N2O conversion in the absence of O2 over catalysts at 500 °C.
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Figure 11. N2O conversion over Au/Co-Al catalysts with different Au loadings at 425 °C.
Figure 11. N2O conversion over Au/Co-Al catalysts with different Au loadings at 425 °C.
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Figure 12. Catalytic activity of 5.0 wt% Ir supported on various metal oxides for N2O decomposition.
Figure 12. Catalytic activity of 5.0 wt% Ir supported on various metal oxides for N2O decomposition.
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Figure 13. Effect of metal entity and loading on N2O decomposition at 600 °C.
Figure 13. Effect of metal entity and loading on N2O decomposition at 600 °C.
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Figure 14. Effect of alkali metal type on catalytic decomposition N2O.
Figure 14. Effect of alkali metal type on catalytic decomposition N2O.
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Figure 15. Effect of 5% O2 and/or 2% H2O on N2O conversion over RuOx/HAP at 400 °C.
Figure 15. Effect of 5% O2 and/or 2% H2O on N2O conversion over RuOx/HAP at 400 °C.
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Zhang, Y.; Tian, Z.; Huang, L.; Fan, H.; Hou, Q.; Cui, P.; Wang, W. Advances in Catalytic Decomposition of N2O by Noble Metal Catalysts. Catalysts 2023, 13, 943. https://doi.org/10.3390/catal13060943

AMA Style

Zhang Y, Tian Z, Huang L, Fan H, Hou Q, Cui P, Wang W. Advances in Catalytic Decomposition of N2O by Noble Metal Catalysts. Catalysts. 2023; 13(6):943. https://doi.org/10.3390/catal13060943

Chicago/Turabian Style

Zhang, Yong, Zhigao Tian, Lin Huang, Honghong Fan, Qiufei Hou, Ping Cui, and Wanqiang Wang. 2023. "Advances in Catalytic Decomposition of N2O by Noble Metal Catalysts" Catalysts 13, no. 6: 943. https://doi.org/10.3390/catal13060943

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