SO2 Deactivation Research Articles

Mechanism for SO2 poisoning of Cu-CHA during low temperature NH3-SCR

Density Functional Theory (DFT) calculations are used to investigate low temperature SO2 deactivation of Cu-CHA during ammonia assisted selective catalytic reduction of NO (NH3-SCR). SO2 is found to adsorb on [Cu2IINH34O2]2+ forming a copper sulfate complex. NO and NH3 react over the sulfate complex forming N2, H2O and H2SO4. H2SO4 undergoes an acid-base reaction with NH3 yielding SO4(NH4)2 and HSO4(NH4), where HSO4(NH4) is thermodynamically preferred during typical reaction conditions. The SO2-derived species are bulky and have considerable barriers for inter-cage diffusion. Moreover, the presence of HSO4(NH4) species reduces the probability of having two [CuI(NH3)2]+ complexes in one cage, which is a requirement for O2 activation. The results suggest that the key mechanism for low temperature SO2 deactivation is of physical origin and that the catalyst can be regenerated by exposure to high temperatures where HSO4(NH4) decomposes. The suggested mechanism agrees with experimental observations and provides atomistic understanding of sulfur poisoning of Cu-CHA during NH3-SCR.

Promoting NH3-SCR denitration via CO oxidation over CuO promoted V2O5-WO3/TiO2 catalysts under oxygen-rich conditions

Ammonia selective catalytic reduction (NH3-SCR) technique for nitrogen oxide (NOx) elimination of low temperature flue gas (<200 °C) requires huge energy consumption and operation costs. In this work, a CuO modified V2O5-WO3/TiO2 (Cu/VWTi) catalyst are constructed to achieve the aim of enhancing the temperature of flue gas via using the heat released by CO oxidation, thereby promoting the NH3-SCR of NOx reaction. Benefiting from the synergistic effect between CuO and VWTi in the adsorption and activation of reactants, Cu/VWTi catalyst shows high NOx (>90%) and CO (>90%) removal efficiency at the temperature range of 225–270 °C and a GHSV of 40,000 h−1. Moreover, it is found that the completely oxidation of 2000 ppm CO can increase the flue gas temperature by about 20 °C. Therefore, developing catalyst that can achieve simultaneously NH3-SCR of NOx and CO oxidation is a feasible method to deduce the energy consumption and operation costs of NH3-SCR of NOx system in low temperature flue gas. Finally, the reaction process and SO2 deactivation effect over Cu/VWTi catalyst was studied via in-situ DRIFT. This work might provide some guidance for the design of catalysts to eliminate multiple pollutants.

High temperature H2S selective oxidation on a copper-substituted hexaaluminate catalyst: A facile process for treating low concentration acid gas

H2S selective catalytic oxidation technology is a prospective way for the treatment of low concentration acid gas with simple process operation and low investment. However, undesirable results such as large formation of SO2 and catalyst deactivation inevitably occur, due to the temperature rise of fixed reaction bed caused by the exothermic reaction. Catalyst with high activity in wide operating temperature window, especially in high temperature range, is urgently needed. In this paper, a series of copper-substituted hexaaluminate catalysts (LaCux, x = 0, 0.5, 1, 1.5, 2, 2.5) were prepared and investigated for the H2S selective oxidation reaction at high temperature conditions (300-550°C). The LaCu1 catalyst exhibited excellent catalytic performance and great stability, which was attributed to the best reductive properties and proper pore structure. Besides, two facile deep processing paths were proposed to eliminate the remaining H2S and SO2 in the tail gas.

Molecular Modeling Study of the SO2 Deactivation of an Amine Resin and a Procedure To Avoid SO2 Deactivation Using a Polyethylene Glycol/Tertiary Amine System

Since 2012, the polymeric resin Lewatit R VP OC 1065 has been investigated for removal of CO2 from various process streams and air. The present article focuses on the deactivation mechanism of the resin with SO2 and a work around. This is important for CO2 capture from flue gas of coal-fired power plants and fuel oil. The deactivation of the resin was already experimentally observed in 2013 but thus far not described computationally. Molecular modeling shows that Lewatit R VP OC 1065 is deactivated by irreversible formation of dimeric amine-SO2 charge-transfer complexes which are very stable and resist thermal and chemical desorption. Additional support for this view was found in the work on aminosilica adsorbents for CO2 capture, which are subject to SO2 deactivation also. Therefore, attention was paid to a procedure to avoid SO2 deactivation. Polyethylene glycol (PEG)/tertiary amine systems seem to be very promising. Their reported high SO2/CO2 selectivities, SO2 capacities, and ease of regeneration were computationally confirmed by identifying their mode of action and favorable thermodynamics. As a result, a combination of a PEG/tertiary amine system with Lewatit R VP OC 1065 might be a very attractive candidate two-step process to capture both SO2 and CO2 from flue gas originating from coal-fired power plants and large ships.

Unraveling the Unexpected Offset Effects of Cd and SO2 Deactivation over CeO2-WO3/TiO2 Catalysts for NOx Reduction.

It is challenging for selective catalytic reduction (SCR) of NOx by NH3 due to the coexistence of heavy metal and SO2 in the flue gas. A thorough probe into deactivation mechanisms is imperative but still lacking. This study unravels unexpected offset effects of Cd and SO2 deactivation over CeO2-WO3/TiO2 catalysts, potential candidates for commercial SCR catalysts. Cd- and SO2-copoisoned catalysts demonstrated higher activity for NOx reduction than a Cd-poisoned catalyst but lower than that for an SO2-poisoned catalyst. In comparison to SO2, Cd had more severe effects on acidic and redox properties, distinctly decreasing the SCR activity. After sulfation of Cd-poisoned catalysts, SO42- preferentially bonded with the surface CdO and released CeO2 active sites poisoned by CdO, thus reserving the highly active CeO2-WO3 sites and maintaining a high activity. The sulfation of Cd-poisoned catalysts also provided more strong acidic sites, and the synergistic effects between the formed cerium sulfate and CeO2 contributed to the high-temperature SCR performance. This work sheds light on the deactivation mechanism of heavy metals and SO2 over CeO2-WO3/TiO2 catalysts and provides an innovative pathway for inventing high-performance SCR catalysts, which have great resistance to heavy metals and SO2 simultaneously. This will be favorable to academic and practical applications in the future.

Oxide Catalysts with Ultrastrong Resistance to SO2 Deactivation for Removing Nitric Oxide at Low Temperature.

Nitrogen oxides are one of the major sources of air pollution. To remove these pollutants originating from combustion of fossil fuels remains challenging in steel, cement, and glass industries as the catalysts are severely deactivated by SO2 during the low-temperature selective catalytic reduction (SCR) process. Here, a MnOX /CeO2 nanorod catalyst with outstanding resistance to SO2 deactivation is reported, which is designed based on critical information obtained from in situ transmission electron microscopy (TEM) experiments under reaction conditions and theoretical calculations. The catalysts show almost no activity loss (apparent NOX reaction rate kept unchanged at 1800 µmol g-1 h-1 ) for 1000 h test at 523 K in the presence of 200 ppm SO2 . This unprecedented performance is achieved by establishing a dynamic equilibrium between sulfates formation and decomposition over the CeO2 surface during the reactions and preventing the MnOX cluster from the steric hindrance induced by SO2 , which minimized the deactivation of the active sites of MnOX /CeO2 . This work presents the ultralong lifetime of catalysts in the presence of SO2 , along with decent activity, marking a milestone in practical applications in low-temperature selective catalytic reduction (SCR) of NOX .

The effect of SO2 on NH3-SCO and SCR properties over Cu/SCR catalyst

The sulfur resistance in NH3-SCO and NH3-SCR reactions of 1%Cu/SCR catalyst prepared by improved wet impregnation method was tested at 350 °C. In the presence of 500 ppm SO2, 1%Cu/SCR maintained good SCR performance, but its NH3 oxidation efficiency was visibly decreased. The NH3 conversion was promoted to about 90% after adding NO since SCR reactions occurred. As observed in ionic chromatography spectra, no more SO3 were generated over the Cu doped catalyst. NH3-TPD, H2-TPR results showed that some sulfite and sulfate species on the catalyst surface significantly increased the amount of weak acid sites and decreased the reducibility of copper species. In addition, the influence mechanism of SO2 on NH3 and NO adsorption and conversion over 1%Cu/SCR catalyst were revealed by using in situ DRIFT methods. The suppression of NH2 → NOx conversion could account for the SO2 deactivation in NH3 oxidation. Gaseous NO can directly react with NH2 through iSCR mechanism.

Oxide Catalysts with Ultrastrong Resistance to SO2 Deactivation for Removing Nitric Oxide at Low Temperature.

Nitrogen oxides are one of the major sources of air pollution. To remove these pollutants originating from combustion of fossil fuels remains challenging in steel, cement, and glass industries as the catalysts are severely deactivated by SO2 during the low-temperature selective catalytic reduction (SCR) process. Here, a MnOX /CeO2 nanorod catalyst with outstanding resistance to SO2 deactivation is reported, which is designed based on critical information obtained from in situ transmission electron microscopy (TEM) experiments under reaction conditions and theoretical calculations. The catalysts show almost no activity loss (apparent NOX reaction rate kept unchanged at 1800 µmol g-1 h-1 ) for 1000 h test at 523 K in the presence of 200 ppm SO2 . This unprecedented performance is achieved by establishing a dynamic equilibrium between sulfates formation and decomposition over the CeO2 surface during the reactions and preventing the MnOX cluster from the steric hindrance induced by SO2 , which minimized the deactivation of the active sites of MnOX /CeO2 . This work presents the ultralong lifetime of catalysts in the presence of SO2 , along with decent activity, marking a milestone in practical applications in low-temperature selective catalytic reduction (SCR) of NOX .

Denitrification performance of non-pitch coal-based activated coke by the introduction of MnOx–CeOx–M (FeOx, CoOx) at low temperature

Modified activated coke was prepared by the introduction of MnOx, CeOx, and M (FeOx, CoOx) into non-pitch coal-based activated coke (NPAC) using a novel non-pitch binder. The modified non-pitch coal-based activated coke was characterized in detail by N2 physical adsorption, thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, fourier transform infrared spectroscopy, temperature-programmed reduction of H2, temperature-programmed desorption of NH3, and its catalytic activity was examined by the selective catalytic reduction of NO with NH3 at low temperature. In addition, the SO2 and H2O tolerance was investigated. The results indicated that compared to MnOx–CeOx–4.39, Mn–Ce–Co/NPAC-7 and Mn–Ce–Fe/NPAC-7 exhibit considerably higher catalytic activities while simultaneously maintaining NOx removal rates of 78.98% and 77.85%, respectively, at 150°C. The high catalytic performance of Mn–Ce–Co/NPAC-a and Mn–Ce–Fe/NPAC-a is related to several better properties compared to MnOx–CeOx–4.39, namely, considerably higher specific surface areas and percentages of Oα/(Oα+Oβ+Oγ) and Ce3+/(Ce3++Ce4+), more easily reduced of Mn species, high reducibility, and more acidic sites. Furthermore, Mn–Ce–Co/NPAC-7 and Mn–Ce–Fe/NPAC-7 exhibited specific resistance to SO2 and H2O poisoning, and the SO2 deactivation mechanism was proposed.

Noble Metal/CNT Based Catalysts in NH3 and EtOH Assisted SCR of NO

Platinum (Pt), palladium (Pd/PdO), and rhodium (Rh) decorated carbon nanotube (CNT) based catalysts were prepared, characterized and their activity was tested in ammonia (NH3) and ethanol (EtOH) assisted selective catalytic reduction (SCR) of nitric oxide (NO) at low temperatures (30–300 °C). In addition, the influence of sulphur on NH3-SCR activity was investigated. The catalysts were characterized by transmission and scanning electron microscopy, energy-dispersive X-ray analysis, and X-ray diffraction techniques. In addition, IR measurements were done to determine the adsorbed species on the catalyst surface. The maximum NO conversions over the catalysts were as high as 85 % for Pt/CNT (at 192 °C), 54 % for Pd/PdO/CNT (at 291 °C), and 48 % for Rh/CNT (at 292 °C) with NH3 as the reducing agent. The SO2 deactivation was the most severe in the case of Pd/PdO/CNTs. In the EtOH assisted SCR the maximum NO conversions were 100 % for Pt/CNT, 98 % for Pd/PdO/CNT and 85 % for Rh/CNT.

Low-temperature SCR activity and SO2 deactivation mechanism of Ce-modified V2O5–WO3/TiO2 catalyst

The promotion effect of ceria modification on the low-temperature activity of V2O5-WO3/TiO2 catalyst was evaluated for the selective catalytic reduction of NO with NH3 (NH3-SCR). The catalytic activity of 1wt% V2O5-WO3/TiO2 was significantly enhanced by the addition of 8wt% ceria, which exhibited a NOx conversion above 80% in a broad temperature range 190–450°C. This performance was comparable with 3wt%V2O5-WO3/TiO2, indicating that the addition of ceria contributed to reducing the usage of toxic vanadia in developing low-temperature SCR catalysts. Moreover, V1CeWTi exhibited approximately 10% decrease in NOx conversion in the presence of 60ppm SO2. The characterization results indicated that active components of V, W and Ce were well dispersed on TiO2 support. The synergetic interaction between Ce and V species by forming V–O–Ce bridges enhanced the reducibility of VCeWTi catalyst and thus improved the low-temperature activity. The sulfur poisoning mechanism was also presented on a basis of the designed TPDC (temperature-programmed decomposition) and TPSR (temperature-programmed surface reaction) experiments. The deposition of (NH4)2SO4 on V1CeWTi catalyst was much smaller compared with that on V1Ti. On the other hand, the oxidation of SO2 to SO3 was significantly promoted on the CeO2-modified catalyst, accompanied by the formation of cerium sulfates. Therefore, the deactivation of this catalyst was mainly attributed to the vanishing of the V–Ce interaction and the sulfation of active ceria.

H2O and SO2 deactivation mechanism of MnOx/MWCNTs for low-temperature SCR of NOx with NH3

The effect of H2O and SO2 on the catalytic activity of manganese oxides supported on multi-walled carbon nanotubes (MnOx/MWCNTs) for low-temperature selective catalytic reduction (SCR) of NOx with NH3 was studied. Also, N2 adsorption, transient response experiments, X-ray powder diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and in situ FT-IR spectroscopy were performed to investigate the deactivation mechanism of MnOx/MWCNTs catalyst in the presence of H2O or SO2. Experimental results showed that H2O had a reversible negative effect on the catalytic activity of the catalyst. When the temperature was higher than 270°C, the effect of H2O could be negligible. The competitive adsorption of H2O and NH3 on the Lewis acid sites contributed to the deactivation of the catalyst. The integrity increase of MWCNTs in the presence of H2O might be another reason for the deactivation of the catalyst. However, SO2 led to the irreversible deactivation of the catalyst. The higher the reaction temperature, the more dramatically the catalystic activity decreased. The sulfation of the active center atoms was the main poisoning route. Also, formation of ammonium sulfates on the catalyst surface and the competitive adsorption between SO2 and NO were responsible for the partial deactivation of the catalyst to some extent.

Mechanism Study of NO Catalytic Oxidation over MnOx/TiO2 Catalysts

The MnOx/TiO2 catalysts have been proved very active in the catalytic oxidation of NO to NO2, but the mechanism of this heterogeneous reaction was still not clear. In this study, through a systematic in situ diffuse reflectance infrared Fourier transform spectroscopy investigation, the main pathway of NO oxidation over MnOx/TiO2 was revealed. Experimental results indicated that the NO was first coordinated to Mn sites, forming nitrosyls, which were readily oxidized to nitrates by lattice oxygen, and then the nitrates were decomposed to NO2 at high temperatures. However, when SO2 was present, the formation of nitrates was significantly hindered because the SO2 would permanently occupy the active Mn sites as sulfates. We believe that this revealed the mechanism of NO oxidation and SO2 deactivation would provide useful guidance for the development of better catalysts toward fast selective catalytic reduction, NOx storage-reduction, continuously regenerating trap, and NOx wet scrubbing.

Study on the poisoning tolerance and stability of perovskite catalysts for catalytic combustion of volatile organic compounds

La0.8Cu0.2MnO3 and La0.8Sr0.2MnO3 perovskite catalysts were prepared by the co-precipitation method. The resistance of these catalysts to sulfur poisoning was tested via catalytic combustion of toluene. The results show that the perovskite catalysts were poisoned in the presence of SO2. In the presence of dodecyl mercaptan (C12H25SH), La0.8Sr0.2MnO3 exhibits better resistance to sulfur poisoning than La0.8Cu0.2MnO3. It was determined that the SO2 deactivation is due to the formation of CuSO4 on the catalyst surface.

Adsorption and oxidation of NH3 over V2O5/AC surface

V2O5/AC has been reported to be active for selective catalytic reduction (SCR) of NO with NH3 at around 200°C and resistant to SO2 deactivation. To elucidate its SCR mechanism, adsorption and oxidation of NH3 over V2O5/AC are studied in this paper using TG, MS and DRIFTS techniques. It is found that the adsorption and oxidation of NH3 take place mainly at VO bond of V2O5. A higher V2O5 loading results in more NH3 adsorption on the catalyst. V2O5 contains both Brϕnsted and Lewis acid sites; NH4+ on Brϕnsted acid sites is less stable and easier to be oxidized than NH3 on Lewis acid sites. Gaseous O2 promotes interaction of NH3 with AC and oxidation of NH3 over V2O5/AC. NH3 is oxidized into NH2 and acylamide structures and then to isocyanate species, which is an intermediate for N2 formation.

SO2 resistant antimony promoted V2O5/TiO2 catalyst for NH3-SCR of NOx at low temperatures

To get the low temperature sulfur resistant V2O5/TiO2 catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V2O5/TiO2 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V2O5/TiO2 catalysts showed high catalytic activity for NH3 selective catalytic reduction (NH3-SCR) of NOx carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V2O5/TiO2 > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V2O5/TiO2 for maximum NOx conversion, which also showed high resistance to SO2 in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V2O5/TiO2 and compared with commercial W(10%)V2O5/TiO2 catalyst. High electrical conductivity difference (ΔG) for Sb(2%)/V2O5/TiO2 catalyst with temperature was observed. SO2 deactivation experiments were carried out for Sb(2%)/V2O5/TiO2 and W(10%)/V2O5/TiO2 at a temperature of 230 °C for 90 h, resulted Sb(2%)/V2O5/TiO2 was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V2O5/TiO2 than Sb(2%)/V2O5/TiO2 catalyst.

Preparation and Characterization of SnO2-Based Composite Metal Oxides: Active and Thermally Stable Catalysts for CH4 Oxidation

A series of SnO2-based catalysts modified by Fe, Cr and Mn were prepared by the combination of redox reaction and co-precipitation methods, and applied to catalytic CH4 oxidation. The modified catalysts show generally higher activity than the unmodified SnO2. XRD analysis indicates that Fe, Cr and Mn cations could be incorporated into the lattice of rutile SnO2 (cassiterite) to form solid solution structure. As a result, more reducible and active oxygen species was formed in the samples, as substantiated by the H2-TPR results. Moreover, the specific surface areas of the modified catalysts are much higher than that of pure SnO2 and their crystallite sizes are smaller, indicating they are more resistant to thermal sintering. Indeed, the high specific surface areas and the formation of more active oxygen species in the modified samples are believed to be the predominant reasons leading to their enhanced CH4 oxidation activity. Eventually, it is noted that SnCrO displays not only remarkable CH4 oxidation activity, but also potent resistance to SO2 and water deactivation, which makes it a promising catalyst with the potential to be applied in some real CH4 oxidation processes.

The Effect of MoO3 Addition to V2O5/Al2O3 Catalysts for the Selective Catalytic Reduction of NO by NH3

The effect of MoO3 addition to alumina supported vanadia catalysts on the catalytic activity for the selective catlaytic reduction of NO is investigated. Upon the addition of MoO3, catalytic activity is enhanced and the particle size of V2O5 which is shown by the results of XRD and Raman spectroscopy is decreased. The MoO3-V2O5/Al2O3 catalyst also exhibits more resistance to SO2 deactivation than V2O5/Al2O3 does.

Deactivation of highly excited CS2 and SO2 by rare gases

The time dependent thermal lensing (TDTL) technique has been used to study collisional energy transfer from highly excited CS2 in baths of Xe, Kr, and Ar, and from highly excited SO2 in Kr and Ar. Bath gas pressures ranged from about 50 to about 600 Torr. The data were analyzed by simulating the observed TDTL signals with a unified hydrodynamic TDTL theory. The results are expressed in terms of 〈ΔE〉, the bulk average energy transferred per collision as a function of 〈E〉, the mean energy content. The results show that 〈ΔE〉 increases dramatically at 〈E〉≈17 500–23 500 cm−1 for CS2 deactivation, and at 〈E〉≈18 000–22 500 cm−1 for SO2 deactivation. This enhancement of energy transfer, which was observed previously in NO2 and CS2 deactivation, has been linked to the presence of nearby excited electronic states. Furthermore, at lower energy, our results reveal an unusual systematic dependence of 〈ΔE〉 on bath pressure; energy transfer per collision is significantly more efficient at lower collision frequency. These results and data from the literature can be explained with a phenomenological model which includes collisional vibrational relaxation within each of two sets of vibronic levels, and collision-induced intersystem crossing (CIISC), which exhibits mixed order kinetics.

Role of Sulfur in Reducing PCDD and PCDF Formation

Past research has suggested that the presence of sulfur (S) in municipal waste combustors (MWCs) can decrease the downstream formation of chlorinated organic compounds, particularly polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Thus, co-firing a MWC with coal, because of the S species from coal, may reduce PCDD and PCDF emissions. Experiments were carried out to test this hypothesis and to determine the role of S. A field-sampled MWC fly ash was injected into the EPA's pilot-scale reactor, doped with hydrogen chloride (HCl). The tests involved either natural gas or coal combustion. Besides the combustion environment, MWC fly ash injection temperature and sulfur-to-chlorine ratio (S/Cl) were varied. Flue gas was sampled and analyzed for PCDD and PCDF to determine in-flight formation. In the natural-gas-fired reactor, when S was added (as sulfur dioxide, SO2), the PCDD and PCDF formation decreased dramatically at S/Cl ratios as low as 0.64, and with varying furnace conditions, the inhibitory effect was consistent for S/Cl ratios of about 1. In tests with the coal-fired furnace, the S inhibitory effect was again observed at S/Cl values of 0.8 and 1.2, respectively, for the two coals tested. S inhibition mechanisms were studied in a bench-scale reactor. Results show that the depletion of molecular chlorine (Cl2), an active chlorinating agent, by SO2 through a gas-phase reaction appears to be a significant inhibition mechanism in addition to previously reported SO2 deactivation of copper catalysts.