Selective Catalytic Reduction of No Byc3h6 over a Commercial Automotive Iridium Based Catalyst

Ir/TiO2 coated on cordierite monolith was investigated in the selective catalytic reduction (SCR) of NO with propene using a broad temperature range and simulated diesel exhaust gas conditions under the standard mixture (1000 vpm NO, 2000 vpm C3H6, 500 vpm CO, 10 vol.% O2). The physicochemical characterization of the commercial catalyst showed that the BET surface area of the fresh and aged samples are very low ( 5m/g). XRD patterns revealed the presence of rutile TiO2 and Ir metallic phases. Irreversible H2 adsorption performed on the fresh and used catalysts indicated a very low accessibility of H2to metallic surface. The catalyst exhibited a significant DeNOx performance (>80% reduction of NO into N2with a selectivity approaching 90% at the end ofheating-up cycle (620°C)). During the cooling-down step, the NO reduction into N2 activity was further weakly decreased (20% loss in DeNOx activity). The commercial automotive catalyst was not stable after hydrothermal treatment (10% H2O/N2, 720°C). The NO-N2 and the NO-NO2 conversions start at a temperature close to that at which the oxidation of CO and C3H6 by oxygen has been already complete. N2 forms under conditions where reductants are fully converted to CO2. INTRODUCTION There have been numerous reports concerning lean NOx selective catalytic reduction (SCR) technology categorized by reducing agent such as urea (or ammonia) [1–3], hydrocarbons(HC) [4], oxygenated hydrocarbons (OHCs) [5, 6], and hydrogen [7]. These reports show highly practical potential of lean NOx SCR technology for application to the exhaust line of real diesel engine vehicle fleet with more than 50% NOx conversion at relatively low temperatures similar to diesel exhaust temperatures under lean conditions. Selective catalytic reduction (SCR) of NOx by hydrocarbons is an attractive means of limiting the NOx emissions of diesel and lean-burn gasoline engines. Noble metal-based catalysts are effective for this reaction and can tolerate water and sulphur but their DeNOx activity is significant within a narrow range of temperatures. Platinum was demonstrated to exhibit the highest activity of the metal based catalysts series but its operating temperature range would be 200–350°C and it suffers the production of large amounts of nitrous oxide in addition of nitrogen. Iridium-loaded solids have been proposed as active and durable catalysts [8–10]. The SCR of NO by propene over Ir/Al2O3 under lean-burn conditions has been previously studied in our laboratory [11, 12]. The dispersion of supported Ir catalysts was found to be an important factor influencing the activity in propene-SCR of NO. A dispersion in the 0.08–0.15 range was found to be required to obtain the best activity in NO reduction into N2. Such a dispersion could be achieved after heating cycles in the reaction mixture up to 620°C. Highly dispersed Ir particles obtained after preparation of the fresh catalysts were found to sinter under reaction conditions, regardless of whether the catalyst was oxidised in O2 or in situ reduced in H2 [11] before reaction. Furthermore, it has been shown that, besides the size of Ir particles, the composition of the reaction mixture is another important factor influencing the activation of the Ir catalyst toward reduction of NO to N2. The presence of both CO and O2 was found to be necessary for activating Ir/Al2O3 while NO would not be [12]. In situ FTIR results revealed that initially fully oxidized Ir particles partially reduced in the reaction feed to form reduced Ir surface sites adsorbing CO at temperatures as high as 350–400°C[12]. In a direct connection with our previous reported studies [11, 12], the present work has been undertaken to investigate the catalytic behaviour of a coated monolith Ir supported catalysts for NOx reduction by propene (C3H6) under simulated conditions of automotive engine operating taking into account the flexibility towards temperature variation, presence or absence of O2 and the feed gas composition. The SCR runs have been conducted under the various treatments to examine their effects on the total conversion of NO and the product selectivity using catalysts [Nawdali., 3(1): January, 2016] ISSN 2349-4506 Impact Factor: 2.265 Global Journal of Engineering Science and Research Management http: // www.gjesrm.com © Global Journal of Engineering Science and Research Management [27] in the fresh and wet aged states. The ultimate objective is to better understand the operating mode of such catalysts in a practical DeNOx activity operation for an improved converter design. MATERIALS AND METHODS Catalyst preparation A commercial available Ir/TiO2 powder was used as a model substance. The catalytic material was coated on a cordierite honeycomb. For the catalytic investigation, the cordierite monolith was cut into pieces of the size 3.8 cm x 1.7 cm x 1.2 cm fitting to the sample holder of the tube reactor. Catalyst characterization The elemental composition was determined by inductively coupled plasma (ICP) method (λ = 212.861 nm). In order to avoid the possible loss of iridium under reaction conditions [13–15], special care was devoted to the chemical attack of the samples followed by subsequent reduction in H2 at 400°C prior to elemental analysis, as described in a previous work [11]. The catalysts were characterized also using X-ray diffraction (XRD, Model: INEL 120) using Cu K1 radiation in the 2θ range of 20 – 110 .Specific surface area was determined by BET nitrogen adsorption using a Micromeritics ASAP2000.Adsorption measurements were conducted in a conventional volumetric apparatus. The catalysts were reduced overnight in situ at 400°C under flowing H2, and then evacuated at the same temperature for 2 h. The irreversible chemisorption uptake of H2 was measured at 25°C. The dispersion (ratio of the number of superficial Ir atoms to the total number of Ir atoms) was thus deduced from this uptake assuming a H/Irs = 1 stoichiometry [16, 17]. For TEM and EDX analysis, samples of reduced catalysts were exposed to air and suspended in ethanol, a drop of this suspension was deposited on a carboncoated grid, and the specimens were examined with a high resolution Jeol 2010 electron microscope. An ageing procedure (denoted as wet aged) was applied to the commercial automotive Ir-based catalyst coated on cordierite using a hydrothermal treatment at 720°C in the presence of 10% water in nitrogen for 20 h. Catalyst tests The selective catalytic reduction activity measurements were carried out in a down flow fixed-bed quartz reactor at atmospheric pressure, as already described [18, 19]. The sample mass was introduced into the reactor under helium flowing and the catalyst was exposed to the reactant gas mixture containing 1000 vpm NO, 2000 vpm C3H6, 500 vpm CO, 10 vol.% O2 and He as a carrier gas (total flow rate 10 L.h ) . The activity was measured as a function of temperature at increasing and decreasing temperatures following a heating cycle under the reaction mixture. Each experiment was conducted as follows: introduction of the mixture at 25°C, heating from 25 to 620°C at a rate of 2°C.min, plateau at 620°C for 20 min, cooling from 620 to 25°C (2°C min). Several successive temperature cycles could also be performed. The gas hourly space velocity (GHSV = volumetric gas flow/coated monolith volume) was constantly kept at 100000 h, which represents the flow conditions in SCR converters on board of diesel vehicles. Reactants and products (CO2, N2O, O2, N2, CO) were analysed by gas chromatography, with He as carrier gas, using a dual CTR1 column from Alltech (Porapak and molecular sieve) and a TCD detector. A Porapak column and a flame ionization detector were employed for hydrocarbons. In addition, the concentrations of NO, NO2, N2O and CO2 were continuously measured on-line by means of Rosemount IR and UV analyzers. The conversions of NO into N2, N2O and NO2 (in %) were 100x2[N2]/[NOx]i, 100x2[N2O]/[NOx]i and 100x[NO2]/[NOx]i, respectively ([NOx]i is the inlet NOx concentration). CO was never detected. The nitrogen and carbon balances were equal to or greater than 95 %.Several reactions are considered: NO + 1/9 C3H6 1⁄2 N2 + 1/3 CO2 + 1/3 H2O NO + 1⁄2 O2  NO2 C3H6 + 9/2 O2 3 CO2 + 3 H2O CO + 1⁄2 O2 CO2 NO + CO  1⁄2 N2 + CO2 Very low amounts of N2O are formed on Ir-based catalysts and the corresponding reactions are not taken into account. RESULTS AND DISCUSSION Physicochemical characterization Irrespective of the treatments described above, all our samples contained 0.470.48 wt.% Ir. The chlorine content is less than 300 ppm. Remaining elemental analysis data can be found in Table 1. [Nawdali., 3(1): January, 2016] ISSN 2349-4506 Impact Factor: 2.265 Global Journal of Engineering Science and Research Management http: // www.gjesrm.com © Global Journal of Engineering Science and Research Management [28] Table 1: Chemical analysis of the commercial automotive catalyst. Preciousmetal Additives 0.470.48 % of Ir Fe : 0.579% K : 0.152% Ti : 4.99% Zr : 0.04% The BET surface area of the fresh and wet aged samples, measured after desorption under vacuum at 400°C for 2 hours, are respectively of 4.5 and 3.9 m/g. These values are low, which indicate a low level of wash coat or the presence of phases of low BET surface area. Figure 1 shows the XRD patterns of the industrial sample. The experiments were performed on the wash coat from the fresh sample, peeled off by the N2 liquid method. The main diffraction peaks can be directly indexed to the rutile TiO2 and Ir metallic phases. No crystal phase of components of Ir, Fe, Zr, K emerged which indicated that all the active components were highly dispersed on the catalyst surface. Also, the cordierite phase has not been detected. Similar diffraction peaks have been observed with the fresh and the wet aged catalysts. Figure 1. XRD patterns of the commercial automotive catalyst. For EDX analysis, the results are similar on the two fresh and wet aged samples. In general, there is enough homogeneous coherence between the channels of the monolith and the holes corresponding typically to the cordierite containing iron. We have noted an excess of Al and Si compared to the cordierite with the presence of K and Na. There is no preferential localization for Ir and chlorine has not been detected. Hydrogen chemisorption measurements were carried out on the two samples. For this purpose, they were reduced in hydrogen before measuring hydrogen uptakes. We assumed that the reduction at 400°C does not drastically modify the size of iridium oxide agglomerates. It is well known that these agglomerates are very difficult to redisperse and that iridium sintering can proceed under oxidising atmosphere but not in reducing media [20]. The amounts of irreversibly adsorbed H2 on the initial reduced state of the fresh and wet aged catalysts are in the limit of detection of the device and consequently, the metal surfaces accessible to H2 are very low.