The Behavior of Catalytic, Low-Temperature N₂O Decomposition (LT-deN₂O) in the Presence of Sulfur-Containing Compounds on Nitric Acid Plants

Abstract

The production of nitric acid represents the primary source of nitrous oxide (N₂O) emissions. During pilot-scale studies of N₂O reduction on a low-temperature catalyst on nitric acid plants, it was observed that increasing the concentration of NH₃ resulted in a decrease in the degree of N₂O decomposition. This suggested that N₂O was formed by the oxidation of NH₃. Measurements at different temperatures, conducted after the N₂O reduction trials, resulted in the N₂O concentration at the inlet equal to the concentration at the outlet, indicating catalyst deactivation. To identify the causes of deactivation, the physicochemical properties of the catalyst were investigated. XRF analysis revealed the presence of sulfur. The results suggest the necessity of removing sulfur from the raw gas before the reduction of N₂O on the low-temperature catalyst in practical applications.

1. Introduction

Nitric acid production facilities are the primary source of nitrous oxide (N₂O) emissions, a greenhouse gas with a Global Warming Potential (GWP) 310 times higher than CO₂ [1,2,3,4]. Furthermore, N₂O contributes to ozone layer depletion [1,4,5,6,7,8,9]. Over recent decades, the atmospheric concentration of N₂O has significantly increased compared to pre-industrial levels [1,10]. The process of catalytic ammonia oxidation in nitric acid production generates N₂O as a byproduct, with typical concentrations in the off-gasses ranging from 1000 to 2000 ppmv, before the high-temperature N₂O decomposition [11,12,13].

The Industrial Emissions Directive 2010/75/EU mandates stringent limits on N₂O emissions for European Union member states. For existing nitric acid production installations, N₂O concentrations in released gasses must be reduced to below 300 ppm, while new installations must achieve even lower emissions, below 100 ppm [14,15]. To comply with these regulations, the implementation of advanced catalytic processes is essential. These processes can be carried out in the nitrous gas stream at high temperatures of 750–940 °C, as well as in the residual gas stream at lower temperatures of 200–450 °C [9,15,16].

In the framework of the “European Green Deal”, the European Commission set targets in 2019 for reducing greenhouse gas emissions. Non-ETS countries are required to cut their CO₂ equivalent emissions by at least 42%, and EU-ETS countries by at least 52% by the year 2030, compared to 2005 levels. This commitment necessitates significant reductions in N₂O emissions, even from installations that were previously considered low-emission. Furthermore, reducing N₂O emissions below the established limits provides financial benefits. The tightening regulations and potential savings drive the intensive improvement of N₂O removal technologies in nitric acid production plants [17,18,19,20].

The aim of this article is to identify and analyze the causes of LT-deN₂O catalyst deactivation, with a particular focus on the impact of sulfur contaminants. The presented research focuses on assessing changes in the chemical composition, morphology, and structure of the catalyst after operation. The article provides evidence of permanent catalyst poisoning, which prevents its regeneration, and it presents experimental results confirming the presence of sulfur compounds as the main cause of catalyst deactivation.

A review of the literature on catalysts for N₂O decomposition reveals significant challenges faced by these materials, including insufficient stability, durability, and susceptibility to deactivation, particularly in the presence of SO₂. Studies on catalysts based on noble metals and transition metal oxides often overlook critical industrial requirements for long-term durability and resistance to deactivation. Many of the analyzed catalysts have not been tested under conditions involving SO₂. The activity of most catalysts discussed in the literature is not limited to N₂O decomposition but also includes the oxidation of SO₂ to SO₃, which accelerates the deactivation process. The literature indicates that catalysts such as rhodium on zirconia are stable only in the absence of SO₂, whereas Fe-ZSM-5 catalysts demonstrate resistance to deactivation even during prolonged exposure to SO₂, making them more promising for industrial applications. The literature presents mixed findings regarding cobalt-based catalysts: CoMgAl-Ox exhibits resistance to deactivation in the presence of SO₂, while the K/Co-CeO₂ catalysts becomes completely deactivated under the same conditions, despite its excellent performance in the presence of O₂ and H₂O [21,22,23,24].

The available literature on the deactivation of deN₂O catalysts by sulfur compounds is limited, indicating a significant research gap in this area. Although some studies describe general mechanisms of sulfur poisoning of catalysts, which partially helps in understanding the deN₂O catalyst deactivation process, there is a need for more detailed investigations to fully explain this mechanism. According to the literature, the deactivation of the CuO/Al₂O₃ sorbent at temperatures of 200–300 °C by SO₂ is attributed to the formation of ammonium sulfate and copper sulfate. Ammonium sulfate deactivates the SCR process by blocking or clogging the catalyst pores, while copper sulfate further contributes to this process by reducing catalyst activity and blocking pores, leading to lower activity compared to CuO. In addition, studies on the ZnCo₂O₄ catalyst indicate that SO₂ significantly reduces its catalytic activity during soot oxidation. This reduction in activity may result from the formation of sulfate or sulfide on the catalyst surface, which limits the number of active sites and decreases the specific surface area of the catalyst [25,26].

The deactivation of the Ru/γ-Al₂O₃ catalyst caused by SO₂ is reported to be irreversible and attributed to the formation of stable sulfates on the catalyst surface. This phenomenon was confirmed through TPD-MS and TPR-H₂ experiments on the deactivated catalysts. The formed stable sulfates are difficult to remove, resulting in a permanent loss of catalyst activity. This mechanism suggests that the interaction of SO₂ with the catalyst surface leads to the formation of persistent sulfate complexes that block active sites, effectively eliminating the catalyst’s ability to catalyze further reactions. Such deactivation is particularly significant in industrial applications, where catalyst regeneration can be costly or technically challenging [27,28].

The Co-ZSM-5 catalyst shows inhibited activity in the presence of SO₂, while Cu-ZSM-5 is completely deactivated. SO₂ causes deactivation of the ex-Co-Rh, Al-HTlc catalyst, indicating its susceptibility to sulfur poisoning. However, surprisingly, SO₂ promotes N₂O decomposition on the ex-FeZSM-5 catalyst, which may be related to the oxidation of SO₂ to SO₃, affecting the specific reaction mechanism on the catalyst surface. Differences in the reactions of the catalysts indicate a specific relationship between the nature of the catalyst material and its resistance to sulfur poisoning. This diversity highlights the need for further research on the mechanism of sulfur poisoning and identification of factors that can influence catalytic activity in the presence of sulfur compounds [29].

Pilot-scale studies on the deN₂O catalyst presented in this article are crucial for understanding that it is extremely important not only to remove sulfur from the residual gas but also to minimize the presence of sulfur in the industrial environment, as this catalyst is highly sensitive even to trace amounts of sulfur. In our case, the periodic operation of boilers in another department, which serves as a source of sulfur in the air, leads to significant issues related to catalyst deactivation. The effectiveness of deN₂O technology depends not only on the type of catalyst but also on the conditions of its installation, including the presence of contaminants. Given the limited research on the stability of deN₂O catalysts under industrial conditions in the presence of SO₂, it is essential to study the rate of deactivation of these catalysts to better understand their long-term stability and to develop effective strategies for extending their lifetime in real industrial applications. Our research aims to fill this gap by providing new data on the impact of even small amounts of sulfur on the stability of deN₂O catalysts, which is crucial for optimizing emission reduction technologies in industrial facilities [17,29].

The decomposition of N₂O is an exothermic, irreversible, first-order reaction (ΔH₂₉₈ = −163 kJ/mol), represented by the following equation: 2N₂O → 2N₂ + O₂. In the presence of a catalyst, the activation energy is reduced, allowing the reaction to proceed at a lower temperature. The first step in the mechanism of N₂O decomposition on the catalyst surface involves the breaking of the N–O–N bond, resulting in the formation of an N₂ molecule and the adsorption of oxygen on the active site of the catalyst (S, Equation (1)):

N₂O + S → N₂ + S…O₍surf.₎

(1)

The desorption of oxygen, leading to the regeneration of the active site, can proceed in two ways. Firstly, through the direct recombination of two oxygen molecules on the catalyst surface (Equation (2)):

2S…O₍surf.₎ ↔ 2S + O₂

(2)

Alternatively, through the reaction of a N₂O molecule with the oxygen adsorbed on the catalyst surface (Equation (3)):

S…O₍surf.₎ + N₂O → S + N₂ + O₂

(3)

At temperatures between 300 and 600 °C, the key step controlling the rate of the overall reaction is oxygen desorption, which means that the catalyst’s ability to regenerate its active site is crucial for its efficiency [12,30,31,32].

In the context of the literature review, industrial-scale studies have shown that catalysts may be significantly more sensitive to sulfur compounds than previously assumed. The residual gas that was passed through the cobalt–zinc spinel catalyst supported on α-Al₂O₃ was analyzed for its composition and was found to be free of sulfur compounds. However, despite its initially high performance at low temperatures, the catalyst unexpectedly became deactivated. Further investigations revealed that the catalyst had been poisoned by sulfur originating from the ignition of boilers in another department, leading to its presence in the air [33].

2. Materials and Methods

2.1. Materials

The catalyst for low-temperature N₂O decomposition (LT-deN₂O) from tail gas was deposited on an α-Al₂O₃ support in the form of rings with a diameter of 5 mm and a height of 2 mm. The catalyst structure consists of a cobalt–zinc spinel and surface-modified with a potassium structure of which has been patented [33]. To verify the effectiveness of the catalyst on an industrial scale, it was evaluated in a pilot reactor with an internal diameter of 0.21 m. This reactor was supplied with tail gas from an industrial nitric acid production plant and connected to a bypass tail gas stream between the industrial SCR-deNO_x reactor and the expander turbine. All performed and presented measurements have been performed in triplicate. The process flow diagram of the installation is presented on Figure S1 in the Supplementary Material.

2.2. Activity of the Catalyst

2.2.1. Activity of the Catalyst in the Pilot-Scale Studies

In the pilot-scale study, two catalytic beds were placed in the radial basket: catalyst for reduction of NO_x (9 dm³ in the outer ring) and catalyst for reduction of N₂O deposited on the Raschig rings (9 dm³ in the inner ring). Ammonia was gradually added to the residual gas stream. Illustrative photo is displayed as Figure 1.

2.2.2. Activity of the Catalyst after Operation in Trials

For the catalyst sample after operation in the reactor, activity measurements were performed in the laboratory installation in a manner analogous to point 2.2.1.

2.3. Physico-Chemical Characterization of the Catalyst

2.3.1. X-ray Fluorescence (XRF) Spectroscopy

The chemical composition of the catalyst samples was determined using X-ray fluorescence (XRF) spectrometry with the X’UNIQUE II sequential spectrometer from Philips (Amsterdam, the Netherlands). The analysis compared the elemental content of fresh and spent catalysts, allowing for the assessment of changes in chemical composition during operation.

2.3.2. X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) was conducted using a Prevac photoelectron spectrometer equipped with a hemispherical R3000 analyzer (VG SCIENTA, Hastings, UK). This analysis allowed for the determination of the surface chemical composition of the samples by measuring the binding energy of electrons emitted from the surface under X-ray irradiation [34].

2.3.3. Porosity Measurement

The porosity of the samples was analyzed using mercury intrusion porosimetry with an AutoPore IV 9510 apparatus from Micromeritics (Norcross, GA, USA). Based on the obtained data, the pore size distribution, specific surface area (SSA), total pore volume (V_t), true porosity (P), average pore diameter (d), and bulk density (ρ), were determined.

3. Results and Discussion

3.1. Catalytic Activity

A pilot-scale study was conducted on a real residual gas stream, with its average composition and key parameters summarized in

Table 1. Average composition of residual gasses at the inlet to the N₂O reduction reactor.

Specification	Unit	Residual Gas
N2	% vol.	~97.2
O2	% vol.	2.5
H2O	% vol.	0.3
NOx	ppm	65
N2O	ppm	100
NH3	ppm	1–3
p	kPa	959

. The aim of the experiment was to evaluate the efficiency of N₂O conversion on a catalyst in 410, 420, 430, and 440 °C as well as under different residual gas flow rates (ranging from 10 Nm³/h to 100 Nm³/h, with 10 Nm³/h intervals). The results are presented in

Table 2. Efficiency of N₂O conversion in various temperatures and gas flow rates during the pilot-scale study.

No.	C0,N2O [ppm]	XN2O [%]	Flow [Nm3/h]	Tinlet [°C]	Toutlet [°C]	p [bar]
4.1	59.5	77.9	12	408.4	409.0	9.5
4.2	61.5	90.4		421.5	421.7	9.6
4.3	64.7	98.6		434.3	436.8	9.6
4.4	64.2	99.9		439.6	442.7	9.6
4.5	66.0	75.5	19	415.9	413.4	9.6
4.6	71.2	90.5		432.3	431.0	9.5
4.7	71.0	95.2		442.4	444.6	9.6
4.8	66.6	54.1	30	414.8	412.5	9.5
4.9	69.0	64.1		421.3	420.7	9.6
4.10	72.6	76.6		428.3	431.1	9.6
4.11	73.0	80.4		438.6	439.2	9.6
4.12	81.1	31.7	40	408.5	409.5	9.5
4.13	78.1	50.3		420.4	422.4	9.5
4.14	76.3	65.5		434.3	434.7	9.6
4.15	74.9	73.0		441.6	441.9	9.6
4.16	81.4	18.6	61	412.4	408.6	9.6
4.17	81.6	30.5		422.5	418.7	9.6
4.18	79.8	41.9		429.8	430.0	9.6
4.19	78.0	52.2		440.6	440.4	9.6
4.20	84.4	13.1	103	421.9	419.4	9.5
4.21	82.4	21.9		430.7	428.5	9.5
4.22	81.8	33.0		439.6	439.5	9.5
4.23	116.8	−0.71	120	437.2	445.7	9.5
4.24	115.4	−0.72		437.0	445.8	9.5
4.25	117.1	−0.69		436.7	446.3	9.5
4.26	116.2	−0.67	100	435.2	444.4	9.5
4.27	116.5	−0.66		435.9	446.6	9.5
4.28	117.5	−0.63		436.4	447.9	9.5

. Initial results indicated stable and reproducible catalytic activity in the decomposition of N₂O.

Based on the findings from the laboratory-scale investigations, which indicated a significant influence of NO_x concentration on the N₂O conversion efficiency, a strategic decision was made to implement a dual-catalyst configuration within the reactor. Specifically, the deNO_x catalyst was positioned in the outer ring of the reactor, while the de N₂O catalyst was situated in the inner ring. This arrangement was designed to optimize the overall process efficiency. In this configuration, the deNO_x catalyst in the outer section of the reactor effectively reduces the NO_x concentration before the gas stream reaches the deN₂O catalyst. Consequently, this allows for the optimal performance of the deN₂O catalyst in the inner section of the reactor, where the diminished NO_x concentration enhances the N₂O conversion rate.

Experimental data further revealed that the N₂O conversion efficiency increases with a rising reaction temperature. The highest conversion rate, exceeding 95%, was achieved at a temperature of 440 °C under a gas flow rate of less than 20 Nm³/h. This elevated conversion rate at 440 °C underscores the efficacy of the deN₂O catalyst under favorable thermal conditions, which is critical for industrial applications where precise temperature control can significantly enhance process efficiency.

Conversely, as the gas flow rate increases, a decrease in N₂O conversion efficiency is observed. Within the investigated temperature range, a conversion rate exceeding 60% was attained at flow rates below 30 Nm³/h. The reduction in conversion efficiency at higher flow rates is likely attributable to the reduced contact time between the gas stream and the catalyst surface, thereby limiting the extent of the chemical reaction. These findings underscore the necessity for precise regulation of key process parameters, such as temperature and flow rate, to maximize the performance of the deN₂O catalyst, which is essential for the optimization of N₂O abatement in industrial processes.

At a flow rate of 100 Nm³/h and an inlet temperature of 421.9 °C, an N₂O conversion of 13.1% was achieved. Upon increasing the inlet temperature to 430.7 °C, the conversion rose to 21.9%, and at 439.6 °C conversion reached 33%. Subsequently, the flow rate of the residual gas was increased to 120 Nm³/h while maintaining an inlet temperature of approximately 440 °C. Based on previous studies, which indicated that conversion decreases as the flow rate increases, it was anticipated that at a flow rate of 120 Nm³/h, the conversion would be lower than that achieved at 100 Nm³/h at the same temperature. However, no conversion was observed at this flow rate.

The initial interpretation of the results suggested that a flow rate of 120 Nm³/h might be too high, preventing the conversion of N₂O. To verify this hypothesis, the flow rate was reduced to 100 Nm³/h at a temperature of 435.2 °C. Nevertheless, the previously achieved conversion under these conditions was not replicated. These results are presented in Table 2.

The relationship between N₂O conversion efficiency and residual gas flow rate at temperatures of 440 °C and 420 °C is depicted in Figure 2. This graphical representation elucidates how variations in flow rate affect the fresh deN₂O catalyst’s efficiency under differing thermal conditions, providing a detailed framework for optimizing operational parameters in the N₂O reduction process. These insights are pivotal for advancing the understanding of process dynamics and for refining reactor operating conditions to achieve the maximum conversion efficiency.

Further attempts to decrease the flow rate, based on the earlier conclusion that lower flow rates should enhance conversion, also failed to produce the desired outcome—no N₂O conversion was achieved (samples no. 4.26–4.28 in Table 2). Without any changes to the operational conditions, the N₂O concentration at the inlet equaled that at the outlet, indicating a complete lack of conversion. Consequently, the catalyst was deemed deactivated. Further experimental data are presented in

Table 3. Efficiency of N₂O conversion in further examination during pilot-scale study.

No.	C0,N2O [ppm]	XN2O [%]	Flow [Nm3/h]	Tinlet [°C]	Toutlet [°C]
4.29	120.1	−0.7	120	437.4	446.2
4.30	116.2	−0.7	100	435.2	444.4
4.31	111.5	−0.6	60	432.8	445.2
4.32	112.1	−0.6	39	432.1	444.8
4.33	118.3	−0.1	19	438.4	445.1
4.34	88.1	−5.9	11	422.9	452.1

.

The results presented in Table 3 contain negative conversion values, which result from measurement errors rather than the actual properties of the process. Conversion cannot take negative values, and these results should be interpreted as discrepancies related to inaccuracies in the measurements of N₂O concentrations before and after the reactor. These values indicate that the actual conversion was zero, meaning that the conversion did not occur, suggesting complete deactivation of the catalyst. The conversion (X) was calculated according to the formula in Equation (4):

$$X={\displaystyle \frac{C_{0,N_{2}O}-C_{N_{2}O}}{C_{0,N_{2}O}}}$$

(4)

where C_0,N2O represents the N₂O concentration at the reactor inlet, which under operating conditions was equal to the gas concentration in the installation’s stack within the margin of measurement error, due to the minor time delay resulting from the gas flow. This means that there are no significant differences in concentration between the measurement point before the reactor and the stack. Therefore, the negative conversion values result from minor measurement errors and indicate that the reaction did not occur, which in practice means complete deactivation of the catalyst.

A stability study of the same catalytic bed was reported elsewhere. The catalyst was proved to be resistant to contaminants, such as H₂O and O₂, which do not affect its performance. Only the concentration of NO_x impacted the conversion, causing a decrease of about 10% when NO_x concentration increased within the range of 500 to 2700 ppm. However, this process is reversible, as the N₂O conversion gradually returns to its initial level after the NO_x is removed [35].

Despite a thorough analysis of the process, the specific cause of the catalyst deactivation could not be identified. Upon detailed examination, it was noted that the onset of N₂O conversion failure coincided with the activation of boilers in the power department, which involved the emission of various pollutants, including sulfur compounds resulting from coal combustion. This coincidence suggests a potential impact of these pollutants on the catalytic process. To determine the causes of deactivation, the physicochemical properties of the catalyst were measured.

3.2. Characterization of the Catalyst

XRF analysis revealed the following composition of the catalyst: Co₃O₄—13.7 wt%; ZnO—2.65 wt%; K₂O—0.22 wt%; SO₃—0.70 wt%; and Al₂O₃—82.73 wt%. The analysis revealed the presence of sulfur trioxide on the catalyst. The composition of the catalyst after operation in the reactor did not change and is consistent with the original specifications. The only observed disparity is the presence of sulfates in the catalyst sample.

Surface composition analysis by XPS revealed the presence of sulfur (2.5%) in the sample after operation in the reactor (Figure 3B). The position of the maximum for S at a Binding Energy (BE) around 170 eV indicates the presence of sulfur in the form of sulfates [36]. Analysis of the catalyst sample that was not placed in reactor (Figure 3A) did not show the presence of sulfates.

Porograms of the carrier (α-Al₂O₃), the fresh catalyst (K-CoZn/α-Al₂O₃_fresh), and the catalyst after trials (K-CoZn/α-Al₂O₃_used) are presented on Figure 4. After operation, the morphology of the catalyst shifts to bimodal distribution with the majority of pores in the same range as the carrier and fresh catalyst and the second peak shifted towards greater pore diameters (~900 nm). The smaller pore volume is a result of long-term exposure (sintering) at high temperature, but it does not have a negative impact on the catalyst’s activity [15].

Table 4. Measurements on the catalyst activity conducted during the third phase of the pilot-scale study.

Sample	SSA [m2/g]	Vt [cm3/g]	P [%]	d [nm]	ρ [g/cm3]
α-Al2O3	7.4	0.56	68	300	1.2
K-CoZn/α-Al2O3_fresh	7.6	0.44	65	235	1.4
K-CoZn/α-Al2O3_used	6.5	0.35	59	210	1.6

summarizes the textural properties of the α-Al₂O₃, the fresh catalyst K-CoZn/α-Al₂O₃_fresh, and K-CoZn/α-Al₂O₃_used.

4. Conclusions

Due to legislative conditions and the rapidly evolving carbon emissions trading market, characterized by rising ERU unit prices, it is crucial to focus on reducing N₂O emissions. In light of this, research on catalysts and secondary reactors in the chemical industry, particularly in nitric acid production plants, is becoming increasingly important. This paper presents research on a low-temperature N₂O decomposition catalyst, conducted on a pilot unit operating within a functioning nitric acid production installation. The use of this installation allowed for a thorough assessment of the catalyst’s effectiveness and the proposed solutions under real process conditions, which is essential for the further development of this technology. During the research, all process parameters were consistent with the actual operating parameters of the nitric acid production installation, ensuring the reliability of the results obtained. The primary objective of the initial research was to determine the durability and effectiveness of both the technological solution and the catalyst under industrial conditions. However, during stable operation of the installation, catalyst deactivation was observed. Since the tests were conducted entirely in an industrial environment, it was not possible to analyze the issue immediately when it occurred. Only after the installation was shut down was it possible to conduct detailed investigations and identify the causes of the catalyst deactivation. As a result of the conducted research, the presence of sulfur in the catalyst structure was detected, which could be one of the key factors contributing to its deactivation. Further analyses revealed that the sulfur may have originated from the air (close proximity of a coal-fired power plant) used in the oxidation ammonia or/and from contaminants within the installation.

Before using the catalyst for N₂O decomposition in an industrial installation, the source of sulfur contamination in the gas mixture stream must be identified and eliminated. The catalyst after operation does not show significant changes in composition, morphology, or structure. The presence of sulfur compounds on the surface of the catalyst was detected. Therefore, the presence of sulfur compounds is indicated as the source of the catalyst’s deactivation. The catalyst after operation in the reactor has been permanently poisoned, and its regeneration through heating at elevated temperatures is not possible. Sulfur does not desorb from the surface of the catalyst.