Iron tungsten (FeW) catalyst is a potential candidate for the selective catalytic reduction (SCR) of NOx with ammonia because of its excellent performance in a wide operating window. Sulfur poisoning effects in SCR catalysts have long been recognized as a challenge in development of efficient catalysts for applications. In this paper, the impact of sulfuration on catalyst structure, NH3-SCR reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches. The sulfuration inhibited the SCR activity at low temperatures (<300 °C), while no evident effect was observed at high temperatures (≥300 °C). After sulfuration for FeW oxides catalyst, the organic-like with covalent S═O bonds sulfate species were mainly formed over the FeW catalysts. Combining TPD with in situ DRIFTS results, it was found that the Lewis and the Brønsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S═O double bonds. The in situ DRIFTS study showed that the formation of NO2 was hindered, leading to the “fast-SCR” pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures. However, the Langmuir–Hinshelwood reaction pathway between adsorbed NH3/NH4+ species and nitrate species was facilitated and dominated at high temperatures, making the as-synthesized FeW catalysts resistant to SO2 poisoning.Keywords: iron−tungsten oxides; mechanism; SCR; sulfate species; sulfuration;

Novel Cu–Ce–Zr mixed oxides were synthesized by a citric acid sol–gel method, and they exhibited an excellent NH3-SCO activity (180 °C, TOF = 1.33 h−1). The finely dispersed CuO, Cu–Ce–Zr solid solution and monomeric Cu2+ ions in octahedral sites were the main active sites. The finely dispersed CuO species were the NH3 adsorption sites, and their adsorption capacity could be improved by their good reducibility. The Cu–Ce–Zr solid solution was an important intermediate in oxygen transfer from bulk to surface. In situ EPR analysis indicated that the isolated Cu2+ located in the octahedral sites was more active compared with that located in the square-planar pyramidal sites, and it induced the formation of more Cu–Ce–Zr solid solution. Meanwhile, the in situ O2-TPD results showed that both adsorbed oxygen and bulk lattice oxygen were the active oxygen adspecies, and the adsorbed oxygen molecules were more active than the bulk lattice oxygen in NH3 oxidation.

Novel iron–tungsten catalysts were first developed for the selective catalytic reduction of NOx by NH3 in diesel exhaust, achieving an excellent performance with a wide operating temperature window above 90% NOx conversion from 225 or 250 to 450 °C (GHSVs of 30 000 or 50 000 h–1). It also exhibited a pronounced stability and relatively high NOx conversion in the presence of H2O, SO2 and CO2. The introduction of W resulted in the formation of α-Fe2O3 and FeWO4 species obtained by HRTEM directly. The synergic effect of two species contributed to the high SCR activity, because of the increased surface acidity and electronic property. The FeWO4 with octahedral [FeO6]/[WO6] structure acted as the Brønsted acid sites to form highly active NH4+ species. Combining DFT calculations with XPS and UV–vis results, it was found that the fine electron interaction between α-Fe2O3 and FeWO4 made the electron more easily transfer from W6+ sites to Fe3+ sites, which promoted the formation of NO2. Judging by the kinetics and SCR activity studies, the Fe0.75W0.25Oδ with an appropriate W amount showed the strongest interaction, and thereby the lowest activation energy of 39 kJ•mol–1 and optimal catalytic activity. These findings would be conducive to the reasonable design of NH3–SCR catalysts by adjusting the fabrication.

Highly dispersed Fe2O3 nanoparticles supported on carbon nanotubes, prepared by a simple ethanol-assisted impregnation method, showed above 90% NO conversion and selectivity at low temperatures (200–325 °C). Moreover excellent durability and stability towards SO2/H2O was obtained.

•Al doped Ce0.4Zr0.6O2 catalyst showed better NH3 oxidation activity.•Inclusion of Al induced formation of smaller particle and better oxygen mobility.•The more oxygen vacancies were generated by Al modification.•Oxygen cycles among gaseous oxygen, oxygen vacancies and active oxygen was formed.•A new NH3 oxidation reaction was proposed in Al–Ce0.4Zr0.6O2 catalyst.A novel Al doped Ce0.4Zr0.6O2 catalyst prepared by surfactant-templated method was tested for NH3 selective oxidation, and it was found that the addition of an appropriate amount of Al to Ce0.4Zr0.6O2 catalysts improved the NH3 oxidation activity. Strikingly, the NH3 conversion obtained on 3% Al–Ce0.4Zr0.6O2 catalyst was, on average, 12.7% higher than that of undoped Ce0.4Zr0.6O2 catalyst. The XRD and H2-TPR results showed that the inclusion of Al components resulted in the formation of smaller sized CeO2 (4.4–5.8 nm), and improved the oxygen mobility and reducibility. The normalized BET and NH3-TPD analysis also confirmed that the adsorption centers for NH3 in Al–Ce0.4Zr0.6O2 catalysts was increased with the Al addition. More importantly, the oxygen vacancies, of which the amount was increased by modification of Al, were considered as the essential oxygen adsorption and activation sites. Consequently, more active oxygen species (O− and O2−) were formed, which facilitated the NH3 oxidation reaction.

•Direct synthesis calcination method was developed to synthesize Ag nanoparticles.•Highly dispersed uniformly sized Ag nanoparticles were supported on silica.•DDA was a key factor in obtaining ordered support and dispersed silver particles.•High activity in CO oxidation (T98%=20 °C) was obtained.A novel and simple one pot synthesis approach using dodecylamine (DDA) as capping agent and structure director has been employed to synthesize the silica-supported Ag nanoparticles. The catalysts were characterized by XRD, N2 adsorption–desorption isotherms, TEM and TG-DTA. It was found that the loading and particle size of Ag were strongly related to the surfactant DDA. In addition, the formed local reductive environmental due to the decomposition of DDA during thermal treatment procedure could prevent the aggregation of Ag nanoparticles and be benefit for the formation of uniformly dispersed Ag nanoparticles on the HMS support (1.5–4.5 nm). The excellent catalytic activity for CO oxidation (T98(CO) = 20 °C) demonstrated that the method was effective for synthesizing the highly dispersed nano-silver catalyst.Graphical abstract

•A small amount doping of Cu strongly improved the activity of HCHO oxidation for HAP.•Different synthesis methods were used to adjust Cu locations and species on HAP.•Five possible sites of Cu on HAP were proposed based on the experimental results.•Relationship of Cu species & location with HCHO oxidation activity was obtained.•Highly-dispersed Cu(II) clusters was mainly responsible for HCHO oxidation.Different methods including ion exchange, co-precipitation and impregnation were used for the preparation of copper doped hydroxyapatite catalysts (CuHAP) to adjust the location of Cu on HAP and resulted in the difference of catalytic performance. Adjustments of synthesis conditions (ion exchange) were adopted for the further identification of active Cu sites for formaldehyde oxidation over CuHAP catalysts. Methods of characterization including XRD, H2-TPR, EPR and XPS were used for the identification of Cu species and its environment on the HAP. A hypothesis of five possible sites for Cu location was proposed based on the features of HAP structure and the experimental results. It has also been concluded that the highly-dispersed Cu(II) clusters was mainly responsible for HCHO oxidation. The best activity was achieved over CuHAP (1.4 wt.%) prepared by the ion exchange method with the complete conversion temperature of 180 °C.

•Addition of template in synthesized Al2O3 resulted in the difference in pore textures.•All studied Al2O3 behaved as philic adsorbents for formaldehyde.•HCHO adsorption capacity was directly with the surface area and pore volume.•The smaller pore size resulted in the higher desorption active energy (Ed).•The surface hydroxyl was favorable to the chemical adsorption of HCHO.In this work, Al2O3 materials with different pore properties were synthesized using some kinds of surfactant templates including cationic (CTAB), anionic (SDS), nonionic (P123) and blank templates. The gaseous formaldehyde adsorption–desorption behavior was studied using adsorption breakthrough curves and temperature programmed desorption (TPD) methods. The addition of ionic surfactant template (containing anionic and cationic) in synthesized solution resulted in the decrease of the crystalline size of Al2O3 particles compared with Al2O3-Blank. The adsorption capacity for formaldehyde on the Al2O3 samples was as follows: Al2O3-P123 > Al2O3-Blank > Al2O3-SDS > Al2O3-CTAB, which was in direct proportion to the surface area and pore volume. A technique based on TPD was used for estimating the desorption activation energy (Ed) of HCHO from different porosity Al2O3 samples. Al2O3-CTAB exhibited the highest deception activation energy for HCHO, which was thought to be related to its narrowest pore size distribution. The presence of abundant surface hydroxyl groups for Al2O3-P123 would be in favor of the chemical adsorption with formaldehyde molecular, and then resulted in higher desorption activation energy even though its broadest pore size distribution.

The performance of monodispersed Ag/SBA-15 catalyst prepared by post-grafting method was studied. Compared with impregnation method, the catalyst prepared by post-grafting method exhibited higher metal dispersion, smaller silver particle size and better catalytic activity for formaldehyde oxidation. The HCHO can be completely oxidized into CO2 and H2O over the silver catalyst at about 100 °C. Temperature programmed surface reaction (HCHO-TPSR) and temperature programmed desorption (HCHO-TPD) experiments demonstrated that the catalyst prepared by post-grafting method exhibited better HCHO adsorption and activated performance at low temperatures. In situ FT-IR spectra revealed that DOM and formate species can be formed on all catalysts, but CO and CO2 species can only be found on silver supported catalysts and the post-grafting sample trended to form more intermediates. Dynamic test showed that the oxidation of HCHO with gas O2 can be occurred at lower temperatures on post-grafting catalyst than on other catalysts, which was consistent with the activity test.Graphical abstractHighlights► Highly active Ag/SBA-15 catalyst using post-grafting method for HCHO oxidation. ► HCHO can be completely oxidized into CO2 and H2O at 100 °C on the novel catalyst. ► Appropriate adsorption intensity between HCHO and silver sites was necessary. ► Silver particles highly dispersed on SBA-15 were active for HCHO oxidation. ► The formed intermediates can be easily activated on smaller silver particles.

CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co3O4 catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co3O4 catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co3O4 and Ag/Co3O4 catalysts for the both reactions was different. And the addition of silver on mesoporous Co3O4 did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co3O4 should be indispensable for the high activity of HCHO oxidation at low temperature.Addition of silver on Co3O4 did not always enhance the activity of final catalyst for CO oxidation at room temperature, but could improve the low-temperature activity of HCHO oxidation.Download full-size image

We report an Ag/SBA-15 catalyst with low loading (1.42 wt%) that shows excellently high activity in CO oxidation at room temperature after oxygen pretreatment at 900 °C. A 98% conversion of CO is achieved at 20 °C (T98 = 20 °C). An evaporation–deposition–diffusion mechanism for Ag/SBA-15 catalyst is proposed. The oxygen adsorbate-induced decrease in the surface-free energy at 900 °C induces the evaporated silver atoms to be redeposited on the support, and meanwhile diffuse into the channels of SBA-15, forming more highly dispersed small silver particles inside the channels. The favorable structure of silver catalyst for CO oxidation is believed to arise from the synergistic effect of oxygen atmosphere, high temperature (900 °C), and the channel of SBA-15. In addition, the formation of Ag+ on the surface of support after oxygen pretreatment at 900 °C is also suggested to be essential for the following reduction and high catalytic activity for CO oxidation.Graphical abstractHigh-temperature oxygen pretreatment at 900 °C induces lots of silver atoms to diffuse into channel of SBA-15, forming more highly dispersed and stable small silver particles inside the channels. The formed stable Ag+ on the surface of the support after oxygen treatment at 900 °C is also suggested to be in favor for the following reduction and the enhanced activity for CO oxidation. A 98% conversion of CO is achieved at 20 °C on Ag/SBA-15 after further treating the catalyst with H2 at 300 °C.Download high-res image (206KB)Download full-size imageHighlights► O2 treatment at 900 °C induced more Ag atoms to diffuse into the channels of SBA-15. ► A 98% conversion of CO was achieved at 20 °C over 1.42 wt% Ag/SBA-15. ► Catalytic activity was related to the restructuring and distribution of Ag particles. ► The channel of SBA-15 was effective for the stability of Ag particles. ► Ag+ formed on the surface at 900 °C was favorable for the following H2 reduction.

The dynamic adsorption/desorption behavior of VOCs (C7H8) was evaluated for mesoporous SBA-15 silicas with four kinds of morphologies and pore sizes on a fixed bed unit. The SBA-15 silica with interconnected rodlike morphology exhibited exceptionally good breakthrough behavior, a higher adsorption capacity, and better desorption performance for toluene. The large dynamic VOC capacity of the interconnected rodlike silica was attributed to the pore system of the micropores and mesopore size, group-togethering rods which can aggregate to enhance the ability of adsorption, together with the smoother surface.

Novel Cu–Ce–Zr mixed oxides were synthesized by a citric acid sol–gel method, and they exhibited an excellent NH3-SCO activity (180 °C, TOF = 1.33 h−1). The finely dispersed CuO, Cu–Ce–Zr solid solution and monomeric Cu2+ ions in octahedral sites were the main active sites. The finely dispersed CuO species were the NH3 adsorption sites, and their adsorption capacity could be improved by their good reducibility. The Cu–Ce–Zr solid solution was an important intermediate in oxygen transfer from bulk to surface. In situ EPR analysis indicated that the isolated Cu2+ located in the octahedral sites was more active compared with that located in the square-planar pyramidal sites, and it induced the formation of more Cu–Ce–Zr solid solution. Meanwhile, the in situ O2-TPD results showed that both adsorbed oxygen and bulk lattice oxygen were the active oxygen adspecies, and the adsorbed oxygen molecules were more active than the bulk lattice oxygen in NH3 oxidation.

Highly dispersed Fe2O3 nanoparticles supported on carbon nanotubes, prepared by a simple ethanol-assisted impregnation method, showed above 90% NO conversion and selectivity at low temperatures (200–325 °C). Moreover excellent durability and stability towards SO2/H2O was obtained.