Tie Yu

NH3-SCR over Cu/SAPO-34 catalysts with various acid contents and low Cu loading

Four Cu/SAPO-34 catalysts are used to research the effect of their acid content on their NH3-SCR activity, by changing the proportion of Si in the catalyst. Though the catalysts exhibit similar Cu loading performed by an ion-exchange method, they lead to different NO conversions in the temperature range of 120–600 °C. The NH3-TPD results show that varying the proportion of Si can adjust the acid content of SAPO-34. In the low temperature range, kinetic tests prove that the various acid contents did not affect the apparent activation energy (Ea) of the Selective Catalytic Reduction by NH3 (NH3-SCR) over Cu/SAPO-34. But it is found that the acid densities are related to the NO conversion at low temperature. In the high temperature range, increasing the acid content of the Cu/SAPO-34 catalysts inhibits the observed NH3 conversions in the NH3 oxidation results. As NH3 oxidation and NH3-SCR are competing reactions and the former can decrease NO conversion, this inhibition is therefore beneficial to NH3-SCR. Finally, the influence of the acid content of the catalysts on the NH3 oxidation is also investigated by kinetic tests.

The effect of synthesis methods on Cu species and active sites over Cu/SAPO-34 for NH3-SCR reaction

The changed status of copper species and active Cu sites of NH3-SCR over the Cu/SAPO-34 zeolites synthesized by three different methods has been investigated in this work. The combination of the EPR, H2-TPR and SEM results demonstrate that different synthesis methods do not affect the types of Cu species, but affect the distribution. The dominant Cu species is CuO-clusters for the precipitation sample, and it is isolated Cu2+ for the ion-exchange sample. While, for the one-pot catalyst, both isolated Cu2+ and surface CuO species are present in comparable amount. Additionally, hydrothermal treatment can also change the distribution of Cu species, it prompts the migration of Cu species from the external surface CuO into the ion exchange locations and forms more isolated Cu2+. The kinetic results show all the Cu/SAPO-34 catalysts display the same apparent activation energy and TOF values, indicating that the three synthetic methods do not change the reaction pathway of NH3-SCR and the active Cu sites (isolated Cu2+ in the vicinity of six-membered rings).

Deactivation mechanism of SO2 on Cu/SAPO-34 NH3-SCR catalysts: structure and active Cu2+

The deactivation mechanism of Cu/SAPO-34 ammonia selective catalytic reduction catalysts (NH3-SCR) by SO2 poisoning has been systematically investigated using a range of analytical techniques in order to study the influence on both the zeolitic framework and the active Cu2+ ions. The different sulfate samples were obtained by SO2 poisoning over Cu/SAPO-34 NH3-SCR catalysts as a function of time and concentration in the feed. The obtained results reveal that the SO2 poisoning could seriously decrease NO conversion during the whole temperature range (100–500 °C). The XRF results shows that there is almost no sulfur existing on the SAPO-34 support. The ex situ DRIFTS and BET results reveal that SO2 poisoning has a less-pronounced effect on its framework structure. The TPR and EPR results demonstrate that SO2 poisoning has a significant influence on the coordination environment and the content of the active isolated Cu2+ species. The kinetic results demonstrate that the SO2 poisoning does not influence the apparent activation energy (Ea) of the NH3-SCR reaction over Cu/SAPO-34 catalysts. The decline of the NH3-SCR activity is due to the reduction of the number of isolated Cu2+ ions.

The migration of Cu species over Cu–SAPO-34 and its effect on NH3 oxidation at high temperature

The influence of Cu species migration, through hydrothermal treatment, on the NH3 oxidation mechanism and its inhibition on NH3-SCR activity over Cu–SAPO-34 catalysts were studied. XRD, SEM, EPR, H2-TPR, CO-DRIFTS were conducted to estimate the Cu species distribution. The results revealed that the CuO species initially existed on the external surface of the fresh impregnated Cu/SAPO-34 catalyst, and converted to isolated Cu2+ and nanosized CuO in the cavity of the SAPO-34 support during hydrothermal treatment. The NH3-SCR activity over the hydrothermally treated Cu/SAPO-34 catalysts improved due to the increase of the isolated Cu2+ amount, while the NH3 oxidation activity declined. Furthermore, the NO amount generated in the NH3 oxidation reaction over the hydrothermally treated samples was much less than the fresh one, which was also caused by the migration of Cu species. In addition, the kinetic NH3 oxidation was performed at temperatures from 340 °C to 440 °C, and the results indicated that CuO species were the active sites for the NH3 oxidation. In conclusion, the Cu species migration caused the decrease of the NH3 oxidation activity and the great variation in the generated NO amount. Furthermore, it was proposed that the NH3 oxidation mechanism over the Cu/SAPO-34 catalysts contained two steps: firstly, molecular NH3 reacted with O2 at CuO sites; secondly, the generated NO was reduced by NH3 to N2 at isolated Cu2+ sites.

NH3 Oxidation Mechanism over Cu/SAPO-34 Catalysts Prepared by Different Methods

NH3 oxidation mechanism and its inhibition effect on the selective catalytic reduction of NOx by NH3 (NH3–SCR) reaction over two series of Cu/SAPO‐34 catalysts were studied. It was found that the different NO conversions were induced by the diverse NH3 oxidation degree. X‐ray diffraction, scanning electron microscopy, H2 temperature‐programmed reduction (H2–TPR), and CO diffuse reflectance infrared Fourier transform spectroscopy (CO–DRIFTS) analyses were conducted to estimate the Cu species distribution. The TPR results showed that the CuO species was dominant in impregnated Cu/SAPO‐34 samples, and the ion‐exchanged Cu/SAPO‐34 samples contained more isolated Cu2+ species. Furthermore, H2–TPR, electron paramagnetic resonance, and CO–DRIFTS analyses were utilized to quantify the CuO, Cu2+, and Cu+ species. The results together with the kinetic results revealed the CuO is the active site for NH3 oxidation, and the NH3 oxidation over Cu/SAPO‐34 sample consists two steps: (a) NO production; (b) further NO consumption by adsorbed NH3 species on Cu2+ sites. Finally, the inhibition of NH3 oxidation on NH3–SCR activity was explained and the NO conversion at high temperature was related to the Cu2+/CuO ratio in Cu/SAPO‐34 sample.

The role of various iron species in Fe-β catalysts with low iron loadings for NH3-SCR

A series of Fe-β catalysts, containing 0.17–0.52 wt% Fe, were prepared by a liquid ion-exchange method to study the influence of various iron species on NH3-SCR activity. A combination of UV-vis and EPR techniques was applied to identify and quantify the iron species. The spectroscopic studies showed that the iron was almost all isolated Fe3+ when the Fe content was less than or equal to 0.17 wt%. At higher Fe contents (0.27–0.52 wt% Fe), small oligomers coexisted with isolated Fe3+ species. Furthermore, the quantitative analysis indicated that the percentage of tetrahedral Fe3+ isolates decreased, while the percentage of octahedral Fe3+ isolates increased with incremental iron loading. In situ EPR results suggested that isolated Fe3+ showed excellent activity for NH3-SCR; moreover, isolated Fe3+ sites in distorted tetrahedral (g ≈ 6) and octahedral (g ≈ 8.8) environments showed better redox abilities than tetrahedral Fe3+ (g ≈ 4.3). The SCR TOF values proved that isolated Fe3+ sites both in tetrahedral and octahedral coordinations were the active sites for the NH3-SCR reaction. In addition, the NH3 oxidation TOF results indicated that oligomers were the active sites for NH3 oxidation over Fe-β catalysts and the contribution of diverse clustered oligomers was unequal.

The role and activity of various adsorbed ammonia species on Cu/SAPO-34 catalyst during passive-SCR process

In this work, the adsorption and reaction performance of various adsorbed ammonia species during the passive-SCR process were investigated by temperature-programmed desorption (TPD), temperature-fixed surface reaction (TFSR), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) and kinetic tests. The NH3-TPD and DRIFTS-NH3 adsorption results showed that the number of weak and strong acid sites in the Cu/SAPO-34 sample increased due to Cu species incorporation, which formed new Lewis acid sites. Further, a decrease in the number of moderate acid sites resulted from the replacement of hydroxyls by Cu2+. The TFSR results revealed that the adsorbed NH3 species presented different SCR activity, which could be divided into active NH3 species and inactive NH3 species. In the low temperature range, a large amount inactive NH3 resulted in lower activity and resulted in a long equilibrium process. However, the performance of inactive NH3 was distinctly different at high temperatures. Furthermore, the in situ DRIFTS results illustrated the activity difference and migration between the two adsorbed NH3 species. It was proved that the NH3 migration rate from the Bronsted acid sites to Lewis acid sites was slower than the SCR rate at low temperatures, which might determine the SCR reaction rates. However, at high temperatures, the NH3 migration rate was faster than the SCR rate of active NH3. In other words, NH3 migration from the Bronsted acid sites to Lewis acid sites may be the rate determining step of passive-SCR lean period at low temperatures, while NH3 adsorption possibly was the rate determining step of the passive-SCR rich period at high temperatures. On the basis of the above results, the present work gives an insight into the potential of Cu/SAPO-34 for passive-SCR applications in the future.