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The unexpected role of NO x during catalytic ozone abatement at low temperatures
Houcine Touati, Alexandre Guerin, Yousef Swesi, Catherine Dupeyrat, Régis Philippe, Valérie Meille, Jean-Marc Clacens
To cite this version:
Houcine Touati, Alexandre Guerin, Yousef Swesi, Catherine Dupeyrat, Régis Philippe, et al.. The unexpected role of NO x during catalytic ozone abatement at low temperatures. Catalysis Communi- cations, Elsevier, 2021, 148, pp.106163. �10.1016/j.catcom.2020.106163�. �hal-02945242�
1
The unexpected role of NO
xduring catalytic ozone abatement at low temperatures
Houcine Touati1, Alexandre Guerin2, Yousef Swesi2, Catherine Batiot Dupeyrat1, Régis Philippe2, Valérie Meille2,§, Jean-Marc Clacens1,*
1 Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285 Université de Poitiers - CNRS, 4 rue Michel Brunet, BP633, 86022 Poitiers Cedex, France.
2 Univ Lyon, CNRS, Université Claude-Bernard Lyon 1, CPE-Lyon, Laboratoire de Génie des Procédés Catalytiques, UMR 5285, F-69616 Villeurbanne, France.
§ Present address: Univ Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, F- 69626, Villeurbanne, France.
Corresponding Author: [email protected]
2 Abstract
The catalytic decomposition of ozone was carried out at 40 °C over a Pd/Al2O3 catalyst in the presence or absence of nitrogen oxides traces. NOx were produced during generation of ozone from air. Two reactor set-ups were used to examine the possible effects of NOx poisoning, namely, a fixed bed with a powder catalyst and a ceramic monolith coated with the same catalyst. It was shown that NOx significantly poisoned the catalyst, resulting in a decrease of ozone conversion over time on stream. NOx-free ozone feed can lead to the total recovery of the initial catalytic activity indicating a reversible poisoning effect.
Keywords: Ozone conversion; NOx poisoning; PdO catalyst; Air treatment.
3 1. Introduction
Ozone is one of the most powerful oxidizing and whitening agent that decomposes without leaving harmful residues. Therefore, it is widely used in water treatment due to its important biocidal properties or in air purification due to its long lifetime but also in waste gas treatment, medical disinfection, treatment of diseases and in other fields [1-4]. However, ozone is present in the form of a thermodynamically unstable gas (ΔG° at 298 K = -163 kJ mol-1 and ΔH° = -138 kJ mol-1). It is corrosive, strongly oxidizing and toxic to human health, even at low concentrations [4-6]. It is naturally formed in the stratosphere, the ozone protective layer, after a photochemical reaction involving oxygen and UV radiation from sunlight or at the ground level by lighting during storms.
Peaks in ozone concentration at ground level are increasing and constitute a major health issue of ambient air pollution [7]. The ozone concentration in urban air at ground level is usually less than 100 ppb but it can exceed this threshold in polluted seasons [8]. In addition to naturally occurring ozone or ozone intentionally produced for human needs, like water treatment or disinfection, some equipment such as laser printers or electrical devices can also produce ozone, thereby increasing its concentration in these closed environments to a level above that admitted for human health. Indeed, long-term exposure to high concentrations of ozone can cause serious damages to human health and even cause cancer [9-11].
It is therefore necessary to eliminate the ozone from the air to preserve human health.
Decomposition of ozone can be done by thermal, chemical or catalytic treatment [12-14]. The catalytic decomposition of ozone can be performed using various catalysts [4-6,15-16], and it is considered as the most widely used ozone elimination process due to the mild reaction conditions, high yield and low operational cost.
The catalytic decomposition of ozone has been mainly based on transition metal oxides alone or supported, which can be also doped with noble metals. Generally, manganese- and palladium-based catalysts are known to be very effective in the decomposition of ozone [17- 20]. Catalytic steps for ozone decomposition on Pd supported catalysts are well described in the literature [5]. Renewed interest in the catalytic ozone abatement is observed today with the emergence of future standards in aeronautics. It leads to the search of very active catalysts at low temperatures that are stable to many side-pollutants and for 10,000 h typically. Therefore, it is of particular importance to estimate the sensitivity of the catalysts towards other pollutants than ozone, such as nitrogen oxides, which are principally emitted into the
4 atmosphere by diesel engines [21]. Moreover, this pollutant can also be found in some laboratory studies when O3 is generated from air by DBD plasma technology [3, 22-24].
To the best of our knowledge, only few studies [25-28] were published on the influence of NOx in the catalytic conversion of O3 at low temperatures. These studies showed under different operating conditions (e.g., T = 20 to 85°C; O3 concentration = 5 to 1600 ppm) and over various catalysts (e.g., activated carbon, NiMo oxides, Cu and Mn / Al2O3 or α-Fe2O3) an inhibitory effect of NOx on the decomposition of ozone.
To improve the understanding of the ozone catalytic decomposition in the presence of NOx at low temperatures on noble metal catalysts, a commercial Pd/Al2O3 under various experimental conditions (time on stream (TOS), ozone concentration, mass of the catalyst, nature of feed gas to generate ozone) was investigated. The catalyst was either in powdered form or as monolithic washcoated catalyst.
2. Experimental
2.1. Synthesis and characterization of catalyst
The ozone conversion was studied over a commercial 5 wt% Pd/Al2O3 catalyst (purchased in reduced form, Alfa Aesar (ref. 11713, batch No. X12E021). It was used either in the powder form or coated on a ceramic monolith. A 600 cpsi ceramic monolith in the form of 1”
(diam) x 1” (L) cartridge was provided by ALSYS group. Additional characterization data of the monolith are available in the Electronic Supporting Information (ESI) (see Figs. S1-S2 and Tables S1-S3). The coating of the ceramic monolith by the commercial Pd-based catalyst was carried out according to a previously established procedure [29-30], which resulted in the deposition of 195 mg of catalyst. In both cases (powder or washcoating), the catalytic phase was oxidized in air at 450°C for 4 h. This thermal treatment consisted in a heating under the flow (5 L min-1) of air up to 450 °C with a ramp of 2 °C min-1 followed by a stay at 450 °C for 4 h. Finally, cooling of the sample to room temperature was conducted under the same air flow rate. Powder catalyst characterization procedures are described in the ESI.
2.2. Catalytic performance tests
Two reactor set-ups were used. The first one was dedicated to catalytic tests on powders at lab scale, while the second one was designed to mimic current ozone converters and used a catalytic washcoated monolith. A wide range of operating conditions (ozone concentration,
5 gas flow-rate and catalyst shape) was thus studied. The catalytic activity tests on powders (Fig. S3, ESI) were carried out in a conventional flow reactor at atmospheric pressure at 40
°C. The composition of the reacting gas flow (100 NmL min-1) was 13 ppmv of O3 diluted in air. A 10-mg of powder catalyst sample was used. Ozone was generated by flowing pure oxygen or air through a non-thermal plasma reactor, where the exiting ozone-oxygen or ozone-air mixture was then diluted by air with residence time (τ) of 1.2 ms. The space velocity (expressed in molO3 gcat-1
h-1) was 3.10-4 in this reactor. Ozone concentration was then analysed online by an ozone analyser (Environment S.A. type O3 42 M) based on the UV photometric method at 254 nm.
The second reactor set-up (washcoated monolith – Fig. S4) allows to test structured catalysts. It consists of a 32 mm i.d. tube filled with one 1” x 1” catalyst-coated monolith.
Upstream, a compressed air delivering system was able to deliver up to 20 Nm3 h-1 of pure air (dew point of -60 °C). A fraction of the air (or Ar/O2 80:20) flow was used to generate ozone by corona discharge (Ozone service, Mimaud equipments, France). The ozone generator was able to produce nominally 3 g h-1 and the excess of ozone was catalytically destroyed in an auxiliary guard bed. The air and ozone gas mixture (nominally 4.9 Nm3 h-1 with an ozone content of 1.9 ppmv) was homogenized through a static mixer and then entered the catalytic zone. The typical space velocity was 2.10-3 molO3 gcat-1 h-1 at 40 °C, one order of magnitude higher than in the powder catalyst reactor system. The temperature was varied from 40 to 250
°C. The ozone content was measured upstream and downstream of the catalytic zone thanks to a BMT932 analyser (BMT-Berlin, Germany) based on the UV photometric method at 254 nm.
3. Results and discussion 3.1. Catalyst characterization
All the catalyst characterizations were performed after calcination at 450 °C. A basic catalyst characterization can be found in ESI (Table S4). Briefly, a type IVa N2 adsorption- desorption isotherm [31] and a BET surface area of 144 m2 g-1 were found (Fig. S5). The actual Pd concentration is 4.47 wt% Pd. Powder XRD shows the presence of PdO and Al2O3
(Fig. S6).
TEM images (
Figure 1) reveal a fairly good distribution of Pd particles on the alumina surface, with a mean Pd particle size 5 nm. Table 1 shows results of the nitrogen elemental analysis of the 5
6 wt% Pd/Al2O3 catalyst before and after catalytic tests, in the presence of O3 generated from air or pure O2. It is to be noted that no elemental nitrogen is present on the catalyst before catalytic activity testing.
3.2. Catalytic activity performance studies
Fig. 2 shows the decomposition of ozone behaviour in terms of conversion vs time at a low temperature, ca. 40 °C on the calcined 5 wt% Pd/Al2O3 powdered catalyst. Firstly, the catalytic conversion of ozone is very high when ozone is generated from pure O2, with only a slight reduction of the conversion during the first 3 h of reaction. On the other hand, if ozone is generated from air, after one hour of reaction, the conversion of ozone dropped considerably. This conversion decrease with time can be explained by the formation of nitrogen oxides (NOx) in the plasma discharge (DBD) in the presence of nitrogen (N2) in air as it was shown in the literature [25-27].
After a longer reaction time (8 h), the 5 wt% Pd/A2O3 was effective for the elimination of ozone and is stable during 8 h of reaction in the presence of ozone formed from pure O2, while when ozone is formed from air, a 25 % decrease in ozone conversion was observed after 8 h on reaction stream (Fig. 2).
A similar comparison was carried out in the 1” x 1” monolithic cartridge, coated with 0.195 g of Pd/Al2O3 catalyst and which was exposed to the air-ozone flow (4.9 Nm3 h-1) during ca. 3 h in the standard configuration of the pilot unit (i.e. ozone generated from air).
The experiment was repeated by replacing air at the ozone generator inlet with an argon/oxygen (80:20) gas mixture. In the first case (Figure 3, left), whereas the ozone concentration at the reactor inlet is very stable (ca. 1.9 ppmv), the ozone concentration at the reactor outlet increases continuously during the first 3 h of operation, leading to a conversion drop from 83 to 70 %. In both cases, a deactivation rate of 4 %/h is observed (this deactivation rate is reached after a few hours in the case of the powder catalyst because the conversion reached 100 % initially). On the contrary, when ozone is generated from an argon- oxygen gas mixture (Figure 3, right), the activity of the catalytic cartridge remains very stable and a conversion of 83 % is maintained. It should be pointed out that in both series of experiments (powder and structured reactor), only the ozone source was changed. Therefore, the formation of nitrogen oxides (NOx) from air is clearly responsible for the activity decrease.
7 Further experiments performed at 120 °C in the monolithic reactor did not show any activity decrease over 2 h, regardless of the ozone precursor (Fig. S7). This shows that the detrimental role of nitrogen oxides is limited to the low temperatures. Another experiment consisted in heating the monolith at a higher temperature (ca. 250 °C) without introducing any ozone in the air feed. An ozone release was observed, showing that adsorption without reaction occurred (Figure 4). A competitive adsorption at low temperature between ozone and NOx is thus expected to be the reasonable cause of the activity decrease at 40 °C.
It is known [28] that when ozone is formed from air, nitrogen oxides are present in the gas phase and can block ozone decomposition. The presence of NOx has an inhibitory effect on the catalyst active sites as shown by Mehandjiev et al. [26]. The presence of adsorbed NOx on the catalyst surface after ozone decomposition tests was proven by several techniques on powder catalysts. Using chemical analysis (Table 1), we observed the presence of nitrogen on the catalyst exposed to ozone generated from air, while no nitrogen was measured on the freshly calcined catalyst or on the one exposed to ozone generated from pure oxygen. The amount of chemisorbed nitrogen on the solids also depends on the time on stream, with an increase from 0.10 % after 3 h to 0.66 % after 8 h. In addition, FTIR spectra (Fig. S8) of the fresh and used catalysts also showed the NOx poisoning effect of the catalyst surface via the formation of the nitrate groups (IR band at 1380 cm-1) [32,33].
In the present case, the used catalyst "after test O3 (air)" shows an intense IR absorption band at 1380 cm-1, while this band is not observed on the used catalyst "after test O3 (O2)".
We were not able to discriminate if this band corresponds to aluminium or palladium nitrate.
TGA-MS experiments were also performed under argon on the used solid that was exposed to O3 generated from air. Fig. S9 shows a weight loss between 200 and 400 °C that can be correlated to the desorption of NO (m/z = 30), while no NO2 (m/z = 46) desorption was observed. The same experiment performed on the used solid that was exposed to O3 generated from O2 did not show any desorption of NO or NO2.
Finally, successive ozone decomposition tests were performed on the powder catalyst, changing the O3 gas feed generation from pure O2 (step 1) to air (step 2), to finally pure O2
(step 3). We observed (Figure 5), after an initial small O3 conversion decrease in the presence of O3 generated from pure oxygen, a stabilisation of the O3 conversion. Changing the ozone generation feed from pure O2 to air (step 2), a larger O3 conversion drop is obtained. Then, going back to ozone generated from pure O2 (step 3), the ozone conversion of step 1 was recovered. After this third step, no nitrogen was detected on the catalyst by chemical analysis.
Therefore, it is concluded that the NOx poisoning effect is reversible and that ozone allows to
8 remove chemisorbed NOx from the catalyst surface when NOx are not present in the feed gas stream. The catalyst can thus recover its initial activity. These results show that one of the main reasons for catalyst (5 wt% Pd/Al2O3) deactivation during the decomposition of O3 (from air) is the chemisorption of NOx on the PdOx surface. The adsorption of NOx at the surface of PdOx at low temperature has been shown [34].
This chemisorption of NOx in the presence of ozone can lead to the formation of nitrate ions (NO3-
) on the surface of the palladium oxide, blocking catalytic active sites that are required to decompose ozone [28]. This deactivation mechanism can be represented by step 2 in the following Scheme 1. As soon as the introduction of NOx is stopped, desorption of NOx
proceeds (step 3) and adsorption sites for O3 are recovered (step 4). At higher temperatures (ca. 120°C), adsorption of NOx is less favourable [35], consequently competition of adsorption with ozone is limited, and the catalytic activity is thus maintained. These observations are valid in both the catalytic test at lab-scale and also at a pilot experiment with more realistic and industrial conditions (washcoated catalyst).
4. Conclusions
The low-temperature catalytic conversion of ozone was studied using a 5wt%Pd/Al2O3 catalyst. Traces of NOx whose presence is linked to the quality of the gas used in the ozone generator were found to drastically reduce the catalytic activity. Although a simple lab-scale solution is to use pure oxygen in the ozone generator, the phenomenon highlights the possible impact of ambient pollution on the efficiency and lifetime of actual ozone converters, especially at low temperatures.
Acknowledgements
Régis Philippe, Valérie Meille, Jean-Marc Clacens and Catherine Batiot-Dupeyrat acknowledge financial support from BPI France. Jean-Marc Clacens and Catherine Batiot- Dupeyrat acknowledge financial support from the European Union (ERDF) and “Région Nouvelle Aquitaine”.
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12 Figure captions
Figure 1: TEM images of 5 wt% Pd/Al2O3 catalyst.
Figure 2: Decomposition performance of O3 generated by air or O2 at 40 °C with time on stream.
Figure 3: O3 concentration generated with air (left), and with Ar/O2 (right) at 40 °C.
Figure 4: O3 release (ppmv) during heating (air flowrate = 4.9 Nm3 h-1, no O3 in the air).
Figure 5: Consecutive ozone decomposition tests; O3 alternatively generated from pure O2, air and pure O2 gas treatments of the catalyst.
13
Scheme 1: Proposed reaction mechanism for the formation of nitrate ions at the catalyst surface.
14 Table 1: Nitrogen elemental analysis of 5 wt% Pd/Al2O3 before and after O3 decomposition catalytic tests.
5 wt% Pd/Al2O3 Test time (h) N (wt%)
Before O3 decomposition test - 0.00
After O3 decomposition test (O3 from O2) 3 0.00 After O3 decomposition test (O3 from Air) 3 0.10 After O3 decomposition test (O3 from Air) 8 0.66
15 Figure 1: TEM images of 5 wt% Pd/Al2O3 catalyst.
16 Figure 2: Decomposition performance of O3 generated by air or O2 at 40 °C with time on stream.
17 Figure 3: O3 concentration generated with air (left), and with Ar/O2 (right) at 40 °C.
18 Figure 4: O3 release (ppmv) during heating (air flowrate = 4.9 Nm3 h-1, no O3 in the air).
19 Figure 5: Consecutive ozone decomposition tests at 40 °C; O3 alternatively generated from pure O2, air and pure O2 gas treatments of the catalyst.
