Effect of SO2 and SO3 Exposure to Cu-CHA on Surface Nitrate and N2O Formation for NH3–SCR

We report effects of SO2 and SO3 exposure on ammonium nitrate (AN) and N2O formation in Cu-CHA used for NH3–SCR. First-principles calculations and several characterizations (ICP, BET, XRD, UV–vis–DRS) were applied to characterize the Cu-CHA material and speciation of sulfur species. The first-principles calculations demonstrate that the SO2 exposure results in both (bi)sulfite and (bi)sulfate whereas the SO3 exposure yields only (bi)sulfate. Furthermore, SOx adsorption on framework-bound dicopper species is shown to be favored with respect to adsorption onto framework-bound monocopper species. Temperature-programmed reduction with H2 shows two clear reduction states and larger sulfur uptake for the SO3-exposed Cu-CHA compared to the SO2-exposed counterpart. Temperature-programmed desorption of formed ammonium nitrate (AN) highlights a significant decrease in nitrate storage due to sulfur species interacting with copper sites in the form of ammonium/copper (bi)bisulfite/sulfate. Especially, highly stable sulfur species from SO3 exposure influence the NO2–SCR chemistry by decreasing the N2O selectivity during NH3–SCR whereas an increased N2O selectivity was observed for the SO2-exposed Cu-CHA sample. This study provides fundamental insights into how SO2 and SO3 affect the N2O formation during ammonium nitrate decomposition in NH3–SCR applications, which is a very important topic for practical applications.


S1. Catalyst synthesis and monolith preparation
Cu/SSZ-13 was prepared by following the reported synthesis methods in our previous study 1 .
The Na form of SSZ-13 was synthesized to acquire a Si to Al molar ratio of ≈ 15.The prepared Na/SSZ-13 powder was transferred to NH4 form via ion-exchange with a 0.1 M NH4NO3 solution.The obtained NH4/SSZ-13 powder was washed several times with milli-Q water (18.2MΩ•cm) until pH ≈ 7 was reached.Afterwards, the resulting powder was dried at 80℃ in a drying oven overnight and well ground as fine powder form to calcine the NH4/SSZ-13 powder at 500℃ for 8 h in static air with a 2 ℃ • min -1 heating rate to acquire the H form of SSZ-13.Finally, copper-exchanged SSZ-13 was prepared via an incipient wetness impregnation method with Cu(NO3)2 solution (0.074g of Cu(NO3)2•2.5H2O + 0.35g of ethanol).The copper-exchanged solution was dried at room temperature overnight and then calcined in a calcination oven at 600℃ and 750℃ for 8 h and 6 h, respectively with a 2 ℃•min -1 heating rate.
The washcoated monoliths were carefully dried with a heating gun.The washcoating was repeated to obtain the target washcoat loading (ca.300mg).Afterwards, the dried catalyst sample monoliths were calcined at 500℃ for 2h, with 2 ℃•min -1 heating rate.

S2. SO3 calibration and SOx-poisoning of degreened monoliths
Prior to SOx treatments of the catalyst sample monoliths, SO3 formation was calibrated by using an oxidation catalyst (Pt/Al2O3, 7.5 wt.% Pt).The temperature was set at 550℃ while 30 ppm SO2 + 8% O2 + Ar was fed to the oxidation catalyst.30 ppm SO2 and 200 ppm SO2 concentrations were tested to confirm SO3 formation, and the test results showed that SO3 was generated and SO2 oxidation reached its equilibrium state as shown in Figure S1.Afterwards, the SO3 generation test was repeated several times to ensure reproducibility of the SO3 formation via the oxidation catalyst for the SO2+SO3 treatment.SO2 and SO2+SO3 treatments were performed with the catalyst sample monoliths by the following test procedures illustrated in Figure S2.First, the fresh Cu/SSZ-13 monoliths were degreened under standard SCR conditions (400 ppm NH3/NO + 10% O2 + 5% H2O + Ar Bal.) at 750℃ for 5 h, following by SO2 or SO2+SO3 treatment steps referred to as SO2-and SO3-poisoning at 400℃, respectively.Gas compositions are specified with coloured region as degreening, SOx feed, NH3 feed, and base feed steps.

S3. Elemental analysis results from ICP, N2 physisorption and XRD
In terms of sulfated samples, the sulfated washcoated monoliths were well crushed to a very fine powder form.0.25 and 1.22 of S/Cu molar ratio was obtained via ICP analysis, giving 33 and 159 µmol• g !"#$%&"' () of sulfur content for the SO2-and SO3-exposed sample, respectively.a SAR indicates molar ratio of Si to Al.
b Specific surface area of the powder samples was measured with N2 sorption using BET method.

Figure S1 .
Figure S1.SO2 conversion under dry conditions over Pt/Al2O3 catalyst in SO3 generator upstream from SGB reactor.The equilibrium curve was computed according to the thermodynamic data obtained from NIST Chemistry Webbook 2 .

Figure S2 .
Figure S2.SO2 and SO2+SO3 treatments of monoliths in the SGB reactor.The SO2+SO3 mixture was produced from an oxidation catalyst in a separate reactor prior to the main reactor.The gas also consisted of 8% O2 + 5% H2O + Ar fed with a total flow of 1200 Nml•min -1 .