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In the catalytic oxidation of volatile organic compounds, Co-based oxide catalysts show excellent potential and draw more and more attention, but SO2poisoning in actual applications is one of the key problems to be solved. In this work, a series of treated halloysite (HLS)-supported Fe-modified Co oxide catalysts were prepared. In the catalytic oxidation of propane, the catalysts with 15 wt % Co loading amounts showed the best catalytic activity, while 2 wt % Fe modifications showed the best promotional effects. The introduction of Fe not only improved the catalyst activity but also enhanced the tolerance to SO2poisoning. Various characterizations of physical–chemical properties, in situ diffuse reflectance infrared transform spectroscopy, and density functional theory calculation results showed that the active components of Co/HLS catalysts reacted with SO2to form sulfate species, which could not provide sufficient catalytic activity afterward. The Fe species in Fe–Co/HLS catalysts could reduce the strong adsorption of SO2on Co oxides and inhibit the formation of stable sulfate species, thus effectively reducing the effect of SO2poisoning on the catalysts. Furthermore, the adsorption of reactants and the formation of intermediates proceeded on the surface of Fe–Co/HLS catalysts with a greater efficiency, and SO2on the surface mainly existed in the form of surface-adsorbed SO2species. Only a small proportion of SO2could form SO3species and sulfate species, thus achieving the purpose of protecting the active sites and maintaining the catalytic activity. The present study of halloysite-supported Fe-modified Co oxide catalysts would provide help for the development of Co oxide catalysts with excellent catalytic performance and tolerance to SO2poisoning.

In this work, mesoporous TiO2spheres supported MnCeW mixed oxide catalysts (MnCeW/m-TiO2) for selective catalytic reduction of NOxwith NH3were prepared by a wet impregnation method. It is interesting that the MnCeW/m-TiO2catalysts exhibited excellent SCR activity and N2selectivity in a wide temperature range, even under the high gas hourly space velocity. From in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTs) studies of desorption, it could be concluded that the addition of tungsten brought about more Brønsted acid sites and reduced the energy barrier of NOxspecies adsorbed on the surface. At high temperature range, there were still some Brønsted acid sites and NOxspecies including bidentate nitrate and nitro compounds in MnCeW/m-TiO2, therefore more intermediates could take part in the SCR reactions as well as better catalytic performance. Besides, the in situ DRIFTs of transient reactions indicated that the formed NH3species and NOxspecies of MnCeW/m-TiO2were more reactive due to the promotional effects of tungsten. A series of traditional characterizations also revealed the promotional effects of tungsten for surface active elements, catalytic redox properties, and acid sites of NH3adsorption. In a word, all the results confirmed that the introduction of W could enhance active NH3and NOxspecies as well as surface active elements, thus contributing to the catalytic performance. The present investigations may open a path for design and application of catalysts with outstanding catalytic activity and selectivity.

Improved NOxreduction in the presence of alkali metals is still challenging. In this work, we developed novel hollandite Mn–Ti oxide promoted Cu-SAPO-34 catalysts (HMT@Cu–S) for the selective catalytic reduction (SCR) of NOxwith NH3viathe isolation of active sites and alkali metal trapping sites. The HMT@Cu–S catalysts exhibited excellent SCR activity and N2selectivity. More importantly, the HMT@Cu–S catalysts had stronger resistance against alkali metal poisoning compared to Cu-SAPO-34 catalysts. It was found that these newly developed catalysts had superior alkali metal resistance compared to the reported catalysts, making them attractive for environmental application. The hollandite Mn–Ti oxides acted as a protective layer to trap alkali metal ions according to an ion exchange mechanism. From in situdiffuse reflectance infrared Fourier transform spectroscopy (in situDRIFTS) studies of desorption, it could be concluded that after alkali metal poisoning, the NH3species of the HMT@Cu–S catalysts were more unstable; therefore, they could easily participate in the SCR reactions. Additionally, the NOxspecies showed no change after introduction of alkali metal ions due to alkali metal trapping effects. Moreover, the in situDRIFTS of transient reactions indicated that the NH3species were much more easily adsorbed on K-HMT@Cu–S catalysts and that the formed NH3species that were unaffected by alkali metal ions were highly reactive. The present investigations provide an effective strategy for the design and the application of catalysts with outstanding catalytic activity and alkali metal resistance.

Anatase TiO2nanosheets (TiO2-NS) and nanospindles (TiO2-NSP) have been successfully prepared with F–and glacial acetic acid as structure-directing agents, respectively. The Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts were prepared by a wet incipient impregnation method with a monolayer amount of Fe2O3. All the catalysts were employed for the selective catalytic reduction of NO with NH3(NH3-SCR) in order to understand the morphology-dependent effects. It is interesting that the Fe2O3/TiO2-NS nanocatalyst exhibited better removal efficiency of NOxin the temperature range of 100–450 °C, which was attributed to more oxygen defects and active oxygen, acid sites, as well as adsorbed nitrate species based on Raman spectra, XPS, NH3-TPD, NO+O2-TPD, and in situDRIFTS. The density functional theory (DFT) method was used to clarify the NO and NH3adsorption abilities over the catalyst models of Fe2O3/TiO2{001} and Fe2O3/TiO2{101}. The results showed that the NH3adsorption energy over the TiO2{001} (−2.00 eV) was lower than that over TiO2{101} (−1.21 eV), and the NO adsorption energy over TiO2{001} (−1.62 eV) was also lower than that over TiO2{101} (−0.29 eV), which agreed well with the experimental results that Fe2O3/TiO2-NS achieved higher catalytic activity than Fe2O3/TiO2-NSP for NH3-SCR of NO. In addition, the rapid electron transfer and regeneration of Fe3+on the {001} facet of Fe2O3/TiO2-NS also promoted the NH3-SCR reaction efficiency. This work paves a way for understanding the facet–activity relationship of Fe2O3/TiO2nanocatalysts in the NH3-SCR reaction.

Size effects have been recognized to promote the catalytic activity and selectivity of metal oxide particles. So far, limited works and studies are conducted to investigate the size effect of metal oxide with the tailored shape in the selective catalytic reduction of NOxwith NH3(NH3–SCR). Herein, the MnOx-FeOynanocage catalysts with varied scales (0.25, 0.5, 1, and 2 μm) were synthesized via a Prussian blue analogue (PBA)-derived method and used for NH3–SCR of NO. By preforming a series of the activity tests over the nanocages with different scales, the NH3–SCR activity of 0.5 μm MnOx-FeOynanocage catalysts exhibits the highest deNOxactivity in the temperature range of 80–200 °C owing to more preferable physical and chemical properties. It has been demonstrated that there is a strong interaction among Mn and Fe cations in the 0.5 μm MnOx-FeOynanocages. Moreover, the H2-TPR and XPS analysis prove 0.5 μm nanocages exhibit excellent redox properties, which contribute to the higher conservation of NOx. Through the DFT studies, it is also demonstrated that the 0.5 μm MnOx-FeOynanocage catalysts could provide more preferable electronic charge, which gives rise to the varied adsorption behavior of the NH3species and NOxspecies compared to the nanocages with other scales. The in situ DRIFTs were also employed to evaluate the adsorption status of NH3with NOxspecies over MnOx-FeOynanocage catalysts with varied scales. Finally, the scale-activity relationship of the MnOx-FeOynanocage catalysts and their corresponding activities are also established. The deep insight into the scale-activity relationship of the PBA-derived MnOx-FeOynanocage catalyst paves the way for developing and designing highly efficient Mn-based catalyst at lower temperature.

Nb modified SnO2–CeO2catalysts were synthesized viaa co-precipitation method and used for selective catalytic reduction of NO with NH3(NH3-SCR). After modifying the niobium oxides, the NH3-SCR activity of the SnO2–CeO2catalyst is greatly promoted in the temperature range of 120–210 °C and the H2O tolerance of the catalyst is also improved in the long term operation. It is demonstrated that there is a strong interaction among Nb, Sn and Ce cations in the Nb modified SnO2–CeO2catalyst. Moreover, H2-TPR, O2-TPD, XPS analysis and DFT calculations prove that Nb modified SnO2–CeO2catalysts could provide more positive electronic charge and active electron orbits, which give rise to the varied adsorption behavior of the NH3species and NOxspecies. Consequently, the reaction mechanism of SnO2–CeO2catalysts might change after Nb modification. In situDRIFTS was also employed to evaluate the reactivity of NH3with NOxspecies over SnO2–CeO2catalysts and Nb modified SnO2–CeO2catalysts, respectively. NH3-SCR reaction over the SnO2–CeO2catalyst follows both the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism. In view of the Nb modified SnO2–CeO2catalyst, its reaction mainly follows the changed E–R mechanism, where abundant active coordinated NH3species are preferable to be activated to amide species (NH2) and thereafter quickly react with gaseous NO to generate N2and H2O. Additionally, Nb contributed to the generation of oxygen vacancies to capture the active O2species, leading to the enrichment of the catalytic cycle. Accordingly, the catalytic pathway of the SnO2–CeO2catalyst is greatly optimized after the modification of Nb. Finally, the structure–activity relationship of the SnO2–CeO2catalyst and Nb modified SnO2–CeO2catalyst is also established. Deep insight into the structure–activity relationship of the Nb modified SnO2–CeO2mixed oxide catalysts paves the way for a better understanding of the mechanistic aspect of Ce-based catalysts, which is helpful for developing highly efficient Ce-based catalysts for practical applications.

Surface engineering of TiO2is significant in the SCR reaction and worth studying. In a previous study, we found that TiO2nanosheets (TiO2-NS) with the (001) plane exhibited a much better SCR performance than TiO2nanoparticles (TiO2-NP) with the (101) plane. In this work, in situDRIFTS and DFT calculations were applied to address the promotional effect of the (001) plane of TiO2-NS in the SCR reaction. The behavior of adsorption and desorption of NOxand NH3on the two surfaces was studied. It was found that NH3adsorbed on both TiO2-NS and TiO2-NP was in the form of NH3(g) on the Lewis acid sites. NOxon TiO2-NP were mainly trans-(NO)2and N2O4which were non-reactive. Differently, NO on TiO2-NS was mainly in the form of NO2which was probably due to the high energy and abundant active oxygen species of the (001) facet. The formed NO2could trigger the subsequent ‘fast SCR’ reaction thereafter promoting the activity. Therefore, TiO2-NS with more (001) facets had better SCR activity than TiO2-NP with the exposed (101) facet.