Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3-SCR of NOx
Graphical abstract
Introduction
Motor vehicles are the main emission source of nitrogen oxides (NOx) into the atmosphere, which reached dangerous levels and projected emission values above 1.6 Mt/year only in European light duty diesel vehicles [1]. NOx are known to cause the formation of acid rain, photochemical smog, and ground-level ozone [2,3]. Ground-level ozone is a very potent greenhouse gas and it has a direct warming effect on climate [3,4]. Several researches are addressed in decreased NOx vehicular emissions varying the engine type, operating conditions, and fuel used [5]. Notwithstanding, these efforts for reducing NOx emissions do not solve completely the problem, and even increase them [5,6]. In the treatment of NOx from exhausts, the three major technologies are lean NOx traps (LNT), ammonia (or urea) Selective Catalytic Reduction (NH3-SCR) and Hydrocarbon Selective Catalytic Reduction (HC-SCR) [7]. The leading technology is the NH3-SCR using zeolites with Fe and Cu ions, which have better hydrothermal stability compared with commercial V2O5 catalysts [8,9], and operates at a wider range of temperatures [[8], [9], [10]]. The iron-based catalysts have attracted attention owing to high NH3-SCR activity, low cost and lack of toxicity relative to vanadium [10]. Iron oxides catalysts have been used in the NH3-SCR, but they show low-temperature activity (between 150 and 300 °C) due to the low surface area and acidity [9,10]. On the other hand, iron-based zeolites have high activity at temperatures above 300 °C and in comparison with copper-based zeolites, they produce fewer amounts of side products such as N2O and have better hydrothermal stability [11].
Volcanic rocks containing natural zeolites have been mined worldwide for more than 1000 years for use as cements and building stone. Since the 1950’s, natural zeolites have found a variety of applications in adsorption, catalysis, building industry, agriculture, soil remediation, and energy [12,13]. The use of natural zeolites for environmental applications is gaining new research interests mainly due to their properties, and significant worldwide occurrence. A conversion of NOx above 80% in NH3-SCR was obtained over natural chabazite with iron, however low stability of the material was observed under hydrothermal conditions [14]. The presence of an iron-rich natural zeolite (around 12 wt% of Fe2O3) with heulandite (HEU) framework mixed with small amounts of the zeolites chabazite (CHA), phillipsite (PHI) and mordenite (MOR) was recently reported in the andesitic basalts from the Combia geological formation in Colombia (South America) [15]. This natural zeolite showed activity for the simultaneous NOx and o-dichlorobenzene removal (conversions of: NO around 5% and o-DCB around 25% at 550 °C) using methane as reducing agent under wet and lean conditions [16].
The present study aim is to report the characterization and potential of an iron Colombian natural zeolite [15] as geo-catalyst for NH3-SCR of NOx under lean and hydrothermal conditions. The natural zeolite was tested in the reaction without any treatment and after the proton incorporation for obtaining an acid material. Because Fe3+ and Fe2+ species may have a different effect on NH3-SCR of NO reaction [[18], [19], [20]], this effect was also evaluated on the natural zeolite used as catalyst. The fresh catalysts were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Mössbauer spectroscopy, scanning electron microscopy with energy dispersive X-Ray spectroscopy (SEM/EDX), X-ray fluorescence spectroscopy (XRF), BET surface area, temperature-programmed reduction under hydrogen (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD) and nitrogen oxides (NOx-TPD). The catalyst with the best performance in the NH3-SCR of NO reaction was tested during 30 h on-stream for evaluation of its durability; furthermore, the material was characterized after reaction by XRD, FTIR, BET, and NH3-TPD.
Section snippets
Extraction of natural zeolite
Natural zeolite used in this work as Geo-catalyst was collected from the riverbed of the creek “La Sucia”, located north of the La Pintada municipality in the state of Antioquia-Colombia (South America) that is part of the volcanic member of the Combia geological formation [15]. The separation process of the zeolite from the rock was performed with dense liquids method [21] using LST heavy liquid from Central Chemical Consulting − Australia (density of 2.25 g cm−3 operating at room temperature)
XRD
X-ray diffraction (XRD) patterns of Nat-zeo, H-zeo-Fe3+, and H-zeo-Fe2+ catalysts are shown in Fig. 1. According with XRD analysis is possible to establish the presence of three crystalline phases in the Nat-zeo material. Heulandite/clinoptilolite and plagioclase feldspar (anortita and labradorite) are found as major phases, while celadonite appears as minority phase. By means of Rietveld refinement it can be established that the heulandite (heulandite + clinoptilolite) phase represents about
Conclusions
An iron Colombian natural zeolite composed mainly by heulandite and celadonite phases was successfully evaluated as a potential geo-catalyst for NH3-SCR of NOx. After the treatments for obtaining the acid material and the Fe3+ and Fe2+ species, the NO conversion increased with operating temperature window of 350–500 °C. Maximum conversion increased from 21% in the natural zeolite until 76% in the presence of Fe2+ species and to 86% with the Fe3+ species. The presence of Fe3+ favored the NO
Acknowledgments
JFG, LD and MAM acknowledge financial support to Universidad Nacional de Colombia sede Medellín; ACA, LMG and ALV thank to Universidad de Antioquia (UdeA) and Colciencias for financial support through the project 1115-569-33782, and UdeA for project PURDUE 14-2-05.
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