Chen Wang
Introduction:
In the recent years most effective post-processing technology for incline NOxcontrol in dieselis the specific catalytic reduction of NOxwith ammonia (NH3-SCR). Cu/SSZ-13 catalysts, especially in the last five years, have gained much concentration because they exhibit good NH3-SCR results at low-temperature regions and outstanding N2 selectivity. As a result, Cu/SSZ-13catalysts could be a great SCR catalyst to meet with the harsh emission regulations in diesel vehicles. Actually, a number of necessaryconclusions and contributions have been made to properly to understand the nature of copper ions and their impact on NH3-SCR mechanisms over Cu/SSZ-13 catalysts. The authors found that copper sulfate content decreased when increasing at 750 °C owing to coppermigration and the contents of copper sulfate enhanced at 850 °C aging as CuOx/CuAlOx easy to react with SO2. Without any doubt, the above studies have provided important results to understand SO2 poisoning. However, the impact of SO3 should not be ignored becausediesel oxidation catalysts (DOCs) will be applied upstream to SCR catalysts. As of now, few studieshave reported on the influence of SO3 on Cu-loaded Chabazite (Cu/CHA) catalysts.
Discussion:
The Variation of the CHA Structure under SO3 Poisoning:
It is normally accepted that SO2 has no influence on the structure of zeolites as both SO2 and
zeolites show acidity . Hence, no modifications in the CHA framework on S-Cu-0 could be without difficulty understood. Most importantly, in this study, the data of SOxsulfated samples showed the identicalresults with S-Cu-0, indicating that the presence of SO3 also had no authority on the CHA structureof SSZ-13. This is supported by the minor decline in the specific surface area, as well as the samerelative crystallinity, the intensity of Si-O(H)-Al bonds and the chemical environment of Si and Al on the sulfated catalysts.It should be recalled from our preceding study on SO3 poisoning in Cu/SAPO-34 catalysts thatthe CHA structure of Cu/SAPO-34 was spoiled during high contents of SO3 poisoning (20% SO3 inSOx) as the method of dealuminationoccured. Considering the unbroken topological structure of Soxsulfated Cu/SSZ-13, Cu/SSZ-13 catalysts show better acid resistance than Cu/SAPO-34.
In fact, as SSZ- 13 have a good peak of NH3 at higher temperature compared with that of SAPO-34 (~420 °C forSSZ-13 vs. ~350 °C for SAPO-34) [15,39], SSZ-13 shows great acidity than SAPO-34.
Materials and Methods
Methods:
The Cu/SSZ-13s needed to completely dehydrate (300 °C for 24 h under vacuum), and then, the exact surface area of Cu/SSZ-13s was measured using Brunauer–Emmett–Teller measurements
(Micromeritics, Norcross, GA, USA).The relative crystallinity of catalysts was calculated when the normalizedwhole areas of six peaks relative to that for a fresh one. A Nicolet 6700 FTIR (Thermo Fisher Scientific, Waltham, MA, USA) was used to probe thechange of -OH vibration on Cu/SSZ-13s. KBr was selected as the standard substance to make thebackground for ex-situ tests.27Al and 29Si NMR measurements were analysed on a Varian Infinity plus 300 WB spectrometer (Palo Alto, CA, USA) at resonance frequencies of 59.57 and 78.13 MHz, respectively, with samples pinningrates of 8 kHz for 27Al and 4 kHz for 29Si. A Mettler Toledo thermal gravimetric analyzer (Zurich, Switzerland) was used to recognize sulfur and give their amounts, and hence, 15 mg samples were examined within thetemperature ranges from 40 to 850 °C at a ramping rate of 10 °C/min in a gas flow containing 10%O2/N2.The total copper contents on Cu/SSZ-13s were probed by H2-TPR methods. Allsamples were considered in a flow of 5% H2/N2 (10 sccm) from 40 °C to 800 °C at a ramping rate of 10°C/min.The contents of active sites were quantitative calculated by EPR at −150 °C when hydratedCu/SSZ-13s and dilute CuSO4·5H2O aqueous solution with known concentration were used as areference to standardize the copper amounts over Cu/SSZ-13.NH3-SCR activity at ordinary and kinetic conditions was performed on all Cu/SSZ-13 catalysts. The comprehensive description of the experimental process could be found in our previous studies.
SO2 poisoning of NH3-SCR over Cu-SAPO-34 was studied, specifically to assess the forms/states of stored S and the effect of such species on low-temperature NOx lessening activity. Two primary sulfur species types were experimental and were found to be exchangeable depending on whether NH3 was offered or not. Disperse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO adsorption was used to examine the nature and accessibility of the Cu species before and after sulfate formation, without the intrusion of ammonium sulfate; these data revealed that the Cu2+ inside the six-membered rings was entirely blocked by sulfur and that the nature of the [CuOH]+ close to the eight-membered ring altered.
Conclusions:
The SO3 effect on a Cu/SSZ-13 catalyst has been investigated as a function of Sox contents.
The main conclusions of this study are listed below:
As a best acid-resistant ability, even at a high ratio of SO3/SOxflux, the CHA structure of
Cu/SSZ-13 keeps intact. Copper sulfate form during sulfation with SO2 alone or SOxat 250oC and the contents of sulfate show the linear relationship with the SO3 contents in SOx. Cu(OH)+ show the stronger response to sulfate, compared with Cu2+ ions, and no trace of Cu(OH)+ has been found during sulfation. For sulfated catalysts, the loss of Cu2+ contents contributed to the inferior SCR activity at low temperatures.
This work is partly presented at 9th World Congress on Green Chemistry and Technology September 17-19, 2018 Amsterdam, Netherlands