The Role of Active Sites of CoH-ZSM-5 Catalysts for the C2H4-SCR of NO

Catal Lett (2010) 135:182–189 DOI 10.1007/s10562-010-0270-y The Role of Active Sites of CoH-ZSM-5 Catalysts for the C2H4-SCR of NO Xiaomei Chen • Aimin Zhu • C. T. Au Xuefeng Yang • Chuan Shi • Received: 11 November 2009 / Accepted: 11 January 2010 / Published online: 11 February 2010 Ó Springer Science+Business Media, LLC 2010 Abstract The selective catalytic reduction of NO with ethylene was investigated and compared over fresh CoHZSM-5, H2-reduced (450 °C) CoH-ZSM-5 and H-ZSM-5. The dispersion of cobalt species for fresh and H2-reduced CoH-ZSM-5 was studied by UV–vis and FTIR spectra. Combined with DRIFTS results of ad-species and reaction experiments, the active sites of CoH-ZSM-5 for the C2H4SCR were identified and the catalytic functions of active cobalt species and support were also clarified. Notably, the formation of –CN or –NCO species was heavily dependent on the type of nitrates. Co3O4 particles contributed to the oxidation of NO to bidentate nitrates. The latter species were crucial for the formation of –NCO. Whereas exchanged Co2? ions facilitated the oxidation of NO to monodentate and/or bridged nitrates which participated in the formation of –CN species. Keywords NO selective catalytic reduction
Ethylene
CoH-ZSM-5
DRIFTS
Active sites X. Chen
A. Zhu
X. Yang
C. Shi Laboratory of Plasma Physical Chemistry, Dalian University of Technology, 116024 Dalian, People’s Republic of China C. T. Au Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, People’s Republic of China C. Shi (&) State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116012 Dalian, People’s Republic of China e-mail: 123 1 Introduction Selective catalytic reduction of NOx with hydrocarbons under oxygen-rich condition has been widely studied for its potential to clean up exhausts from lean burn and diesel engines. Since the early work of Held and Li [1, 2], Co-ZSM-5 is reported to be an active catalyst for the selective catalytic reduction of nitrogen oxide with hydrocarbons (HC-SCR) and extensive research has been devoted to the investigation of reaction mechanism [3–6]. According to the literatures and our previous study [3–7], a mechanism for the HC-SCR of NO over Co-ZSM-5 was proposed. The reaction begins with the formation of nitrates and carboxylate species. Subsequently, the reduction of nitrates occurs by carboxylate species, resulting in the formation of key intermediates such as –CN, –NCO, or R-NOx. Finally, –CN and –NCO become active towards nitrates, leading to the production of N2, H2O and COx. Although many studies have focused on the mechanism of HC-SCR reaction, the nature of active sites of Co-ZSM-5 during the HC-SCR reaction is still a matter of debate. The structure of Co-ZSM-5 catalyst has been investigated by many research groups [8–11]. These materials may contain at least two different kinds of cobalt species on cationic ion-exchanged positions in ZSM-5 and cobalt oxide particles in zeolite channels or over the external surface of zeolite microcrystal when the samples are heated under oxidant atmosphere [8–10]. It is widely believed that substitutional Co2? ions are effective for SCR reactions [5, 11]. Some research groups proposed that the exchanged Co2? ions were active in the HC-SCR of NO, while dispersed Co3O4 particles were undesired because they promoted the oxidation of hydrocarbon with oxygen rather than with NO2 [5, 11, 12]. Nevertheless, some recent research [13, 14] on HC-SCR revealed that Co oxide Active Sites of CoH-ZSM-5 in C2H4-SCR particles were found in active catalysts and could function catalytically. Cobalt-oxygen microaggregates or small clusters of cobalt oxide located inside the channels have been found to be the active sites in the CH4-SCR of NO over Co-ZSM-5 [13]. In our previous work [14], Co-ZSM5 was characterized by TPR and UV–vis techniques. It was found that three types Co species co-existed over Co-ZSM5: Co3O4 particles, dispersed CoOx and ion exchanged Co2? ions. However, the structure of active Co species and their catalytic roles during the C2H4-SCR of NO still remain unclear. In this work, we investigated the catalytic functions of different cobalt species on the formation of surface species in each reaction step using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The catalytic activity and DRIFT spectra recorded in different gas mixtures were compared over fresh CoH-ZSM-5, H2-reduced CoH-ZSM-5 and H-ZSM-5. These results were correlated with the active Co sites required for C2H4-SCR reaction through UV–vis and FTIR analyses. It has been discovered that the cobalt oxidation state has a significant influence on the behaviors of surface intermediates formed in the reaction. 183 5%H2/He at 450 °C for 1 h. After cooling to 150 °C in He, the reaction gas mixture was fed to the reactor. The conversions of NO and C2H4 were measured as a function of temperature and the reaction temperature was increased from 150 to 500 °C in steps of 10 °C/min. The effluent gases of N2 and C2H4 were monitored by using online gas chromatography with both TCD and FID detectors (equipped with 5A molecular sieve and GDX 502 columns, respectively). 2.3 In Situ DRIFT Measurements The catalyst was placed in a DRIFT cell with CaF2 windows and treated in a He flow at 500 °C or a 5%H2/He flow at 450 °C for 1 h. In each experiment, the pretreated catalyst was exposed to reaction gas mixtures for 30 min at the desired temperature, and then DRIFT spectra were recorded (128-scan accumulation and 4 cm-1 resolution) using a Bruker Tensor 27 spectrometer equipped with a DLATGS detector. The gas composition was the same as that in the activity tests. All gas mixtures were fed at a flow rate of 100 mL min-1. The reactivity of surface adsorbed species was evaluated by the transient response of the IR spectra. 2 Experimental 3 Results and Discussion 2.1 Catalyst Preparation and Characterization 3.1 Catalyst Characterization The CoH-ZSM-5 catalyst was prepared as described in our precious work by the thrice-repeated ion-exchange procedure at 80 °C using H-ZSM-5 powders (SiO2/Al2O3 = 25, Nankai University, China) and aqueous cobalt acetate solutions [14]. The content of Co in the Co-ZSM-5 catalyst was determined to be 1.7 wt. % using an inductively coupled plasma emission spectrometry method. The calcined sample is denoted as ‘‘fresh CoH-ZSM-5’’ hereinafter. The catalyst treated in 5 vol% H2/He (50 mL min-1) for 1 h at 450 °C is denoted as ‘‘H2-reduced CoH-ZSM-5’’ hereinafter. Diffuse reflectance UV–visible spectra were recorded at room temperature on a JASCO V-550 spectrometer. 2.2 Catalytic Activity The activity measurements of catalysts were carried out in a quartz tube reactor (i.d. 6 mm) using 0.2 g catalyst of 20– 40 mesh. The feed gas mixture contained 1,000 ppm NO, 1,000 ppm C2H4 and 10% O2, with He as balance gas. The total flow rate of feed gas mixture was 100 mL min-1, (...truncated)