Selective reduction of NOx by propylene over silver catalyst under oxidative conditions

ARTICLE

Selective reduction of NOx by propylene over silver catalyst under oxidative conditions

Julia María D. Cónsul; Daniel Thiele; Ione M. Baibich^*; Renato C. Veses

Instituto de Química, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500, 91501-970 Porto Alegre - RS, Brazil

ABSTRACT

In this study, a Ag/Al₂O₃ and Ag/Al₂O₃/Cordierite catalyst was prepared by a conventional impregnation method and NOx reduction investigated using propylene as a reducing agent. The catalysts were characterized by Temperature Programmed Reduction (TPR), oxygen chemisorption and surface area, using the standard BET method. The catalytic activities were measured under the same reaction conditions, i.e., 100 ppm NO, 250 ppm C₃H₆, 2% of oxygen and 100000 h^-1 space velocity. The reaction products were analyzed by FTIR. The results of the catalytic activity obtained for the 2.22%Ag/Al₂O₃ catalyst showed 56% NO conversion at 723 K. The catalysts based on Cordierite presented an analogous behavior, but the temperature of maximum NO conversion was lower (620 K).

Keywords: NOx reduction, silver/alumina catalyst, propylene, Cordierite

RESUMO

Neste trabalho foram estudados catalisadores Ag/Al₂O₃ e Ag/Al₂O₃/Corderita, preparados por um método de impregnação convencional, na reação de redução de NO utilizando propeno como agente redutor. Os catalisadores foram caracterizados por redução a temperatura programada (RTP), quimissorção de oxigênio e área superficial, pelo método BET. A atividade catalítica foi determinada utilizando uma mistura reacional contendo 100 ppm de NO, 250 ppm de C₃H₆ e 2% de oxigênio e uma velocidade espacial de 100000 h^-1. Os produtos da reação foram estudados por FTIR. Os resultados dos testes catalíticos mostraram 56% de conversão de NO para o catalisador 2,22%Ag/Al₂O₃ a 723 K. Os catalisadores de cordierita apresentaram um comportamento semelhante, mas a temperatura de máxima conversão de NO deslocou-se para temperaturas menores.

Introduction

The generation of NOx effluents from stationary and mobile sources poses an important environmental problem.^1-4 Gas effluent control in internal combustion engines, whose air/gasoline ratio is stoichiometric, can be achieved by using the so-called "three way catalysts" (TWC) typically containing Pt and Rh as the active metals. On the other hand, lean-burn engines, such as diesel engines and stationary sources for industrial use, operate with excess oxygen to ensure complete combustion. In these cases, TWCs are ineffective for NO elimination, for which reason selective catalytic reduction (SCR) is used. A typical commercial SCR catalyst uses V₂O₅-WO₃-TiO₂ as the metal system and NH₃ as the reducing agent. This catalyst involves problems relating to the transport and storage of NH₃ and to vanadium toxicity.⁵ In their studies of several catalysts prepared from zeolites substituted with H⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Cr³⁺, Fe³⁺, Co³⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Ag, Iwamoto et al.^6,7 found that Cu-ZSM5 provided better NO reduction activity when hydrocarbons were used as reductants. However, this catalyst has a low thermal stability⁶ and deactivates in the presence of water vapor and SO₂. Moreover, large amounts of water vapor inhibit the catalyst's action, promoting the migration of Al⁺³ out of the zeolite structure and leading to a loss of the activity of the Cu ions attached to the Al sites.⁸

Platinum SCR catalysts are very active at low temperatures, and are thermally stable and resistant to SO₂. Obuchi and co-workers⁹ studied the SCR reaction in the presence of propylene with Pt, Ir, Pd, Rh and Ru metals based on g-alumina, obtaining the best results with the metals Pt, Rh and Ru, with NO conversions of over 50%. The selectivity for N₂ was better with Rh (more than 90% of consumed NO was converted into N₂). In contrast, platinum was not as selective, i.e., only 2/3 of the NO consumed was transformed into N₂O. Silver catalysts supported on alumina, with oxidized hydrocarbons as reductants, have recently been used for NO reduction, presenting conversions that might justify their commercial use.^10-13

Ceramic materials are often used as a "primary" support, especially Cordierite (2MgO.2Al₂O₃.5SiO₂ ). High temperatures are tolerable with the use of Cordierite. In addition, ceramic materials have a relatively high melting point and excellent thermal shock resistance. Since Cordierite possesses a low surface area and the catalytically active component is supported on the high surface area material, g-alumina is the most commonly used.¹⁴ In this way, alumina-coated Cordierite is an interesting support for environmental catalysts.

This paper reports on the results found for catalysts prepared from Ag on alumina and alumina-coated Cordierite in the NO reduction reaction, using propylene as the reducing agent. The effect of particle size on the activity is also discussed.

Experimental

The silver catalysts were prepared by incipient wetness impregnation of AgNO₃ (Merck 99.8%) on alumina (Condea, 210 m²/g) or alumina-coated Cordierite prepared as follows. Alumina (Condea) was used in the form of particles with diameters ranging from 0.25 to 0.42 mm. The Alumina/Cordierite support was obtained by impregnating the 0.25-0.42 mm diameter Cordierite particles with a 4% weight Al(OH)₃ solution (pH 4.5 with HNO₃). After impregnation, the support was dried at 383 K for 4h and calcined at 623 K for 2h. The process was repeated ten times, resulting in a support with 7% weight Al₂O₃on the Cordierite surface. Metal loads were determined by AAS.

The catalytic tests were conducted in a fixed-bed quartz flow reactor with a space velocity of 100000 h^-1 and a total gas flow (100 ppm NO, 250 ppm propylene and 2% oxygen in He) of 155 cm³ min^-1. The reactor's effluent gases were analyzed by FTIR (BOMEM, MB100 model) equipped with a 2 L gas cell (optical length = 2.4 m). The NO, N₂O and C₃H₆ concentrations were measured by integrating the peaks in selected regions of its absorbance spectrum (1955 - 1879.5, 2262.3 - 2242.7 and 928.84 - 897.9 cm^-1, respectively) and comparing these to a calibration curve.

Catalysts characterization were performed by temperature-programmed reduction (TPR), raising the temperature to 793 K at a rate of 10 K min^-1. During the measurements, 5%H₂/Ar was introduced into the reactor at a flow rate of 15 cm³min^-1 . Oxygen chemisorption measurements were taken by the pulse technique, using a Thermal Conductivity Detector (TCD) at 443 K, the optimum temperature for oxygen adsorption on silver particles. The dispersions were calculated assuming a stoichiometry of unity for the O/Ag ratio.^15,16 The catalysts specific surface areas were measured using the BET technique in a volumetric apparatus. The catalysts were dried at 423 K for 1 h before their characterization, which was based on the physical adsorption of nitrogen (N₂) at the liquid nitrogen temperature (77 K). Data are the average of three measurements.

Results and Discussion

Catalysts characterization

Table 1 summarizes the results of dispersion (g), particle diameter (d) and metallic surface area (Am) obtained by oxygen chemisorption measurements.

The increase of metal loads was found to cause agglomeration of Ag particles and a consequent decrease in dispersion and exposed metal area as expected.

A comparison of the Ag/alumina catalysts in Figure 1 shows that the maximum Ag surface area was obtained for the 2%Ag catalyst and decreased with higher metal contents, as a result of particle agglomeration, causing the dispersion values to decrease and the particle diameters to increase. The 0.8%Ag/Al₂O₃ catalyst showed a different behavior. The low silver concentration supported on the alumina's high surface area probably generated a stable phase with alumina, that did not adsorb oxygen at 443 K (chemisorption measurements temperature). Based on their XRD analysis of Cu, Ni and Co/Al₂O₃, Shimizu et al.¹⁷ confirmed the formation of an aluminate phase and concluded that this phase was responsible for the activity and selectivity of the catalysts studied. Miyadera¹³ also mentioned that the interaction between Ag and alumina support is important to inhibit the deactivation caused by the reduction of the oxidized Ag species during the reaction. Nakatsuji et al¹⁸ showed that a low loading catalyst displayed oxidic species of silver, possibly as an aluminate.

The catalyst prepared on alumina-coated Cordierite with a 2.5% silver load presented 2% dispersion, a metallic surface area of 0.3 m² g^-1 and large particles. The values found were analogous to the 7.6%Ag/Al₂O₃ catalyst. Assuming that Cordierite contains 7% wt. of alumina and that all the Cordierite is perfectly coated with alumina, the Ag content would be greater than 30%, leading to very large particles. Therefore, the 0.12%Ag/Al₂O₃/Cord catalyst was prepared to contain the same ratio of metal atoms on the alumina covering the Cordierite as the 2.2%Ag/Al₂O₃ catalyst. However, it did not present the same characteristics; instead, the chemisorption parameters decreased (dispersion and metallic surface area) or increased (particle diameter) twofold. In this case, the Cordierite was probably not perfectly coated with the alumina and part of silver particles were deposited on the surface cavities. SEM (scanning electron microscopy) analysis (Figure 2) revealed that the surface Al₂O₃/Cord support presented cavities. These are several parameters that have to be controlled to obtain a homogeneous alumina coated film. Even so, cavities are often found.^{14, 19- 21}

Figure 3 depicts the TPR profiles for the 0.8%, 2.2% and 3.8% silver catalysts on alumina, showing two peaks for H₂ uptake. As the metal content increased, the two signals shifted to low temperatures, suggesting that, with the increase in particle size and resulting decrease in dispersion, the interaction between the particles and the support became weaker, leading to lower temperatures for metal reduction. Furthermore, the first peak intensity rose with the increase in Ag₂O particles, which became easily reducible, in agreement with Bethke and Kung,¹⁰ who observed two peaks for 2 and 6%Ag/Al₂O₃ catalysts. The low temperature peaks were attributed to the reduction of large Ag₂O clusters interacting weakly with the support, which typically are easier to reduce than small metal oxide particles. The second peak of the 0.8%Ag/Al₂O₃ catalyst with low metal contents was located at a high temperature and its intensity was low, confirming our assumption that, in this case, there was strong interaction with the support, which led to a very stable Ag phase with low reductibility of the species.

The TPR profiles for the 7.6%Ag/Al₂O₃, 0.12%Al₂O₃/Cord and 2.5%Ag/Al₂O₃/Cord catalysts are shown together in Figure 4 for the sake of comparison, since their chemisorption data are similar. The two temperature peaks of the 2.5%Ag/Al₂O₃/Cord catalyst were very close (around 383 K and 493 K), suggesting that the metal species on the support had similar characteristics. The first peak at 383 K may be attributed to Ag₂O particles interacting weakly with the support, although this interaction was stronger than that of the same particles of the 7.6%Ag/Al₂O₃ catalyst, as a result of different particle diameters, according to the oxygen chemisorption results. On the other hand, the second peak at a higher temperature (493 K) may be ascribed to similar species found in the 7.6%Ag/Al₂O₃ catalyst. The same peaks were observed in the 0.12%Ag/Al₂O₃/Cord catalyst but, in this case, they shifted to high temperatures (453 K and 564 K), suggesting a stronger interaction with the support than the other catalysts.

Table 2 presents the surface area of the catalysts. In general, alumina supports caused the surface area to decrease slightly as the silver concentration increased, suggesting that higher metallic particle sizes led to a blockage of the alumina pores, decreasing the surface area. On the other hand, the Al₂O₃/Cord support displayed a much higher surface area than the Cordierite, demonstrating that alumina deposition efficiently promoted an increase of the very small Cordierite surface area. The 2.5%Ag/Al₂O₃/Cord catalyst also showed a surface area decrease.

Conversion tests versus temperature

Figure 5 shows the conversions for the propylene oxidation and NO reduction reactions for 0.8%Ag/Al₂O₃ and 2.2%Ag/Al₂O₃ in the 373-873 K temperature range. Both catalysts can be seen to be active for NO reduction in the range of 673 K to 773 K, decreasing at higher temperatures. The propylene oxidation reaction revealed a different behavior: the conversion increased until total propylene conversion was reached. Both catalysts produced similar results for NO reduction, but the 2.2%Ag/Al₂O₃ catalyst showed higher propylene oxidation conversions at temperatures where NO reduction conversions were higher. Figure 6 presents the results of both reactions results for the catalysts with higher silver concentrations. A comparison of Figures 6 and 5 indicate that higher metal loads led to lower temperatures for maximum hydrocarbon conversions, with the 7.6%Ag/Al₂O₃ catalyst presenting the lowest temperature. This effect may be due to the presence of larger particles, which favor oxygen and propylene adsorption on the same active particle; hence, the propylene oxidation is more effective than the NO reduction. These results are in accord to other authors,^{10, 22-25} that found a relationship between particle size, oxidation state and catalytic activities.

If one considers the correlation between NO conversion at 673 K and Ag dispersion values (Figure 7), one finds that the maximum NO conversion was obtained for the catalyst with the highest dispersion, indicating that there is an optimum particle size at which the NO reduction reaction is effective in the presence of propylene and excess oxygen.

Figure 8 shows conversion tests versus temperature for the 2.5%Ag/Al₂O₃/Cord and 0.12%Ag/Al₂O₃/Cord catalysts. The NO conversion behavior was analogous to that of the alumina-based catalysts, but the maximum NO conversions occurred at lower temperatures than with the latter catalysts. In the case of the 2.5%Ag/Al₂O₃/Cord, this may be evidence that there is around 30% of metal on the alumina, as explained earlier, leading to lower temperatures than for the highest metal concentration alumina-based catalyst (7.6%Ag/Al₂O₃), which confirms the effect of the metal concentration on the reaction temperature. The maximum effective conversion for this catalyst is the highest one, considering that the exposed metal surface area of this catalyst (0.29 m² g^-1 cat) is much lower than the 2.2%Ag/Al₂O₃ one (1.76 m² g^-1 cat) even with similar Ag loads. Plummer et al. ²⁶ investigated a similar catalyst (0.3-0.8% Ag/Al₂O₃/Cord) for the oxidation of methanol, obtaining conversions of up to 90%.

Although we expected the NO conversion of the 0.12%Ag/Al₂O₃/Cord to be similar to that of the 0.8%Ag/Al₂O₃ catalyst, it was lower than the latter. In this case, the 0.12%Ag/Al₂O₃/Cord catalyst had a lower silver load than 0.8%Ag/Al₂O₃ catalyst and, as explained earlier, the alumina/Cordierite support have cavities; therefore, possibly part of the silver particles were deposited inside these cavities, preventing them from participating in the SCR reaction.

Conversion tests versus time

Table 3 shows the NO and C₃H₆ conversions after a 150 min test period at the maximum NO conversion temperature for the Ag/alumina catalyst. During the C₃H₆- SCR of NO, the NO and propylene conversions remained at approximately the values reported in Table 3 in each time interval.

Higher NO conversions were obtained for the 2.2% silver loaded catalyst. In addition, propylene conversions exceeded 90% for the higher loaded catalysts. This result is ascribed to the fact that SCR reactions compete with the combustion of the reducing agent. Meunier et al.¹¹ found that the catalytic activity actually increased slightly after 50h at 815 K with a lower loaded catalyst.

The different tendency of the NO conversion with temperature in comparison to the TPR values for the 0.8% catalyst is yet not clear. This catalyst presents a lower maximum conversion temperature than the one with 2.2% silver, in contradiction to the reduction temperatures in the TPR profiles (Figure 3). Moreover, the ethylene conversions follow the TPR values, as expected. It seems here that, for the catalyst with the lowest silver load (smaller particles) the NO reduction reaction occurs before the C₂H₄ oxidation begins as shown in Figure 5. As explained above, larger particles favor oxygen and propylene adsorption on the same particle. Probably it is more difficult for these two larger molecules to be adsorbed in the 0.8%Ag catalyst.

N₂O production

The reactions selectivities were estimated by the N₂O production (Figures 9 and 10). The N₂O concentration was low (< 6%) in all the cases studied, indicating high selectivity to N₂, which is in accord to the proposed mechanism for selective reduction, where the inorganic ad-NOx species, formed from the interaction of the oxigen with NO, can subsequently produce N₂O + N₂ and/or further react with the reductant or a derived species to form organo-NOx compounds as well. In particular, organo-nitro and organo-nitroso compounds and/or their derivatives are suggested to react with NO or the organo-nitrite to yield N₂.²⁴ In general, N₂O begins to be detected after the NO conversion starts and increases concentration when the NO conversion declines. This behaviour agrees with Meunier and co-workers'²⁵ results for silver and cobalt catalysts. The highest N₂O production was presented by the 7.6%Ag/Al₂O₃ catalyst, that had the highest N₂O production at lower temperature than the one for maximum NO conversion.

The catalysts based on Cordierite presented the lowest N₂O production, indicating high selectivity to N₂.

The NO₂ formed could not be quantified with accuracy, since its IR band is overlapped by the ethylene band, as pointed out by others.²⁴ Even so, it was possible to estimate that there was NO₂ production, especially at the temperature where the NO conversion was higher. In the catalyst with a silver load of 2.2% (2.2%Ag/Al₂O₃) there was a NO₂ production of approximately 6 ppm, while for the catalyst with a silver load of 7.6% (7.6%Ag/Al₂O₃) it reaches 20 ppm, following the same pattern as for N₂O production.

Conclusions

The maximum NO conversion temperature was found to be around 673 K for all the catalysts studied.

The catalysts with silver loads of more than 4% promoted propylene oxidation reaction in detriment to the NO reduction, indicating that the silver particles were in a reduced state. The higher silver load produced a reduction of the Ag/Al₂O₃ interface owing to the greater diameter of the silver particles, thereby accelerating the propylene oxidation. In addition, the metallic surface area was an important parameter, since a larger area facilitates oxygen and propylene adsorption on the same active site, causing competition with NO adsorption.

The Ag/Al₂O₃/Cordierite catalysts promoted hydrocarbon oxidation owing to the larger particle diameters.

The maximum NO conversion temperature was approximately 673 K and close to the maximum nitrate decompositions temperature, indicating that selective NO reduction involved the formation of surface nitrates.

The two catalysts based on Cordierite showed the lowest N₂O production and consequently high selectivity to N₂.

Acknowledgements

The authors acknowledge Carlos Gigola (PLAPIQUI, Argentina) for the grateful discussions and the financial support provided by CNPq (Brazil), FAPERGS (Brazil), COPESUL (Brazil) .

Received: July 17, 2003

Published on the web: July 14, 2004

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Publication Dates

• Publication in this collection
  16 Sept 2004
• Date of issue
  Aug 2004

History

• Received
  17 July 2003