Article pubs.acs.org/JPCC
Recent NH3‑SCR Mechanism Research over Cu/SAPO-34 Catalyst Tie Yu,† Teng Hao,† Dequan Fan,† Jun Wang,† Meiqing Shen,*,†,‡ and Wei Li§ †
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China ‡ State Key Laboratory of Engines, Tianjin University, Tianjin 300072, P. R. China § General Motors Global Research and Development, Chemical Sciences and Materials Systems Lab, 3500 Mound Road, Warren, Michigan 48090, United States S Supporting Information *
ABSTRACT: The preliminary mechanism research of NH3SCR and NH3 oxidation over Cu/SAPO-34 catalyst is explored. The XRD, SEM, in situ EPR, NH3-SCR, NH3 oxidation test, DRIFTs, and the kinetic tests were performed for the bulk characterization, catalytic activity measurement, and the mechanism estimation. The NH3-SCR result showed that Cu/SAPO-34 revealed excellent activity during 120−600 °C, while the NH3 oxidation appeared above 300 °C and caused the decline of NO conversion. The order of various reactants (NO, NH3, and O2 for NH3-SCR and NH3, O2, for NH3 oxidation) was estimated by the kinetic tests to explain their behaviors during the NH3-SCR process. The NH3-SCR presents strong dependence on adsorbed NH3 species on Cu/SAPO-34, but the NH3 oxidation does not. Furthermore, the EPR experiment proved that the isolated Cu2+ species are the active sites and the ammonia nitrites species are the intermediate for NH3-SCR over Cu/SAPO-34 catalysts. The different behavior of adsorbed NH3 species in NH3-SCR and NH3 oxidation was studied through DRIFTs to explain the competition between the two reactions for NH3 consumption at high temperature. Meanwhile, combining the results of previous research, the primary reaction mechanism over Cu/SAPO-34 during the NH3-SCR process was conducted.
1. INTRODUCTION
In this research the variation of NO conversion during the NH3-SCR process and the influence of NH3 oxidation on the NH3-SCR activity are studied. First, the SAPO-34 was synthesized, and the Cu/SAPO-34 was prepared using the ion-exchange method. Then the orders of NH3, NO, and O2 for NH3-SCR and NH3 and O2 for NH3 oxidation were examined by the kinetic experiments. The in situ EPR and DRIFTs experiments were also conducted to evaluate the active sites of NH3−SCR above 200 °C and explain the reactive behavior of reactants. Furthermore, the NH3 oxidation at high temperature was evaluated by DRIFTs to study its influence on NH3 consumption of NH3-SCR. Finally, the conclusion about the different reaction behaviors about NH 3 -SCR and NH 3 oxidation is conducted over Cu/SAPO-34.
Selective catalytic reduction by NH3 (NH3-SCR) is considered to be one of the most efficient ways to abate NOx from the exhaust of vehicles.1,2 Since the report about NH3-SCR over Cu/SAPO-34 in 2010, this catalyst has become a potential candidate due to its excellent NH3-SCR activity and hydrothermal stability.3 Our previous works4 also reported that the Cu/SAPO-34 showed 80% NO conversion from 200 to 550 °C, and the aged sample treated at 750 °C by 10% H2O/air for 12 h kept the superior SCR activity. In addition, Xue5 pointed out the isolated Cu2+ species are the active sites of NH3-SCR due to the same turnover frequency (TOF) over different Cu/ SAPO-34 catalysts during 100−200 °C. And the active sites of Cu2+ were also inferred by Korhonen,6 because only the isolated Cu2+ species existed in Cu/CHA catalysts through EXAFS and UV−vis spectra. Moreover, our group7 studied the adsorption properties of NO/NH3 over Cu/SAPO-34 catalyst by DRIFTs. And it was found that the NH3 is the adsorbed reactant and the formation of nitrate/nitrite over Cu sites is the key step during the NH3-SCR process, which illustrates the similar conclusions with Wang.8 Though there are many researches about the NH3-SCR process recently, no clear SCR mechanism has been proposed, and less research about the effect of NH3 oxidation on the NH3-SCR at high temperature has been concluded. © 2014 American Chemical Society
2. EXPERIMENTAL METHODS 2.1. Preparation of Catalysts. The SAPO-34 was synthesized with the mole composition of 0.2 morpholine (MA):0.1 Al2O3:0.1 P2O5:0.1 SiO2:6.5 H2O by the hydrothermal method. The sources of Si, P, and Al were silica sol, 85% phosphoric acid, and pseudoboehmite, respectively. First, the phosphoric acid and the pseudoboehmite were mixed with Received: November 20, 2013 Revised: March 17, 2014 Published: March 18, 2014 6565
dx.doi.org/10.1021/jp4114199 | J. Phys. Chem. C 2014, 118, 6565−6575
The Journal of Physical Chemistry C
Article
H2O, and the mixture was fiercely stirred for 1 h. Then the silica sol and the MA were added and blended fiercely. Third, the mixture was sealed in an autoclave and heated at 200 °C for 24 h. After crystallization, the as-synthesized sample was obtained through centrifuging, washing, and drying at 100 °C for 6 h. Finally, the samples were calcined in air from 30 to 650 °C and kept at 650 °C for 6 h. The composition of SAPO-34 was tested by an X-ray fluorescence spectrometer (XRF), and its molecular formula can be denoted as Si0.14Al0.44P0.42O2. The preparation of Cu/SAPO-34 contains two steps by the ion-exchange method. First, the NH4−SAPO-34 was obtained by exchanging H-SAPO-34 in ammonium nitrate solution at 80 °C for 3 h. Then the NH4−SAPO-34 was stirred with copper sulfate solution to obtain Cu/SAPO-34. The exchange time for the support was precisely controlled to get proper Cu loading according to the previous work.4 After each ion-exchange procedure, the slurry was filtered and washed, and the solid was dried at 90−100 °C for 16 h. And then the dried Cu/SAPO-34 was calcined at 550 °C for 4 h in a muffle furnace. In order to eliminate the degreened phenomenon, the fresh Cu/SAPO-34 catalyst was hydrothermally treated in 10% H2O/air at 750 °C for 4 h. The Cu loading is 1.6 (wt %) by ICP. 2.2. Characterization of Catalysts. The XRD patterns were performed using an X’Pert Pro diffractometer operating at 40 kV and 40 mA with nickel-filtered Cu Kα radiation (λ = 1.5418 Å) in the range 5°< 2θ < 50° with a step size of 0.02°. The scanning electron microscopy (SEM) image of the sample was measured on a HITACHI S4800 field emission microscope. The sample was pasted on a sample holder using a carbon tape and then covered with Pt film to become conductive. The SEM image was taken at magnifications of 1000 with the 3 kV electron beam. The electron paramagnetic resonance (EPR) spectra were recorded on a Bruker ESP320 spectrometer. The Bruker ESP320E software and the special Bruker program were used for data analysis. In order to explore the status of Cu2+ species during the NH3-SCR process, the in situ EPR spectra of Cu/ SAPO-34 under the following treatment were carried out at 270 °C. The sample (45 mg) was sealed in the tube with specific quartz wool. First, the sample was pretreated in O2 at 500 °C for 1 h. After the temperature kept stable at 270 °C, the sample was exposed to NH3 for 2 h. Then, only the NO was cut in and kept for 2 h. Finally, the sample was treated by O2 for 2 h. After each step, the sample was sealed and the EPR spectrum was recorded immediately at the same temperature. 2.3. Activity Tests and the Kinetic Tests. The NH3-SCR activity was tested in a quartz reactor using 0.1 g of sample (60−80 mesh) mixed with 0.9 g of quartz (60−80 mesh) at atmospheric pressure. The catalyst was sealed in the tube with quartz wool. The temperature was controlled by a type K thermocouple inserted into the center of the catalyst. The Fourier transform infrared (FTIR) spectrometer (MKS-2030) equipped with a 5.11 m gas cell was used to measure the concentration of NO, NO2, N2O, and NH3. The gas flow rate and the volume hourly space velocity in all experiments were controlled at 500 mL/min and 300 000 h−1, respectively. Prior to the experiment, the catalysts were pretreated at 500 °C for 30 min under 5% O2/N2. The activity tests were performed for all catalysts using a feed gas composition of 500 ppm NO, 500 ppm NH3, and 5% O2. In addition, in the presence of H2O and CO2, their concentrations were 3% and 6%, respectively. The test temperature was from 120 to 600 °C. The NO conversion was calculated from the equation
NO conversion [%] =
NOinlet − NOoutlet × 100 [%] NOinlet
(1)
The NH3 oxidation was performed in the equivalent conditions with the NH3-SCR test. The inlets contained 500 ppm NH3 and 5% O2/N2. The NH3 oxidation was performed from 300 to 500 °C. The NH3 conversion was calculated as eq 2: NH3 conversion [%] =
NH3,inlet − NH3,outlet NH3,inlet
× 100 [%] (2)
The NH3-SCR kinetic experiments were performed in a thin quartz tube using 25 mg of catalyst and 125 mg of quartz sand. A relative small particles size (80−100 mesh) and the volume hourly space velocity (3 600 000 h−1) ensured the elimination of internal and external diffusion, respectively. Prior to the kinetic experiments, the samples were pretreated in 5% O2/N2 at 500°C. In order to determine the reaction order of gases, the typical reactant gas compositions were as follows: 250−800 ppm NO, 250−800 ppm NH3, 2−8% O2 with N2 as the balance for NH3-SCR at 180, 200, and 220 °C; 250−800 ppm NH3, 2− 8% O2 with N2 as the balance for NH3 oxidation at 450 °C. In the differential reactor, NO conversions were controlled at
