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CeO2-based catalysts are widely investigated for selective catalytic reduction (SCR) of NO with NH3. Interaction of NO/O2with CeO2generally produces two surface species, i.e., nitrates and nitrites. However, the explicit quantification of these two species is still unresolved. Herein, we reported that spectrophotometry characterization was effective in determining surface adsorbed NOxspecies on CeO2by measuring the corresponding ions (NO2–and NO3–) dissolved in aqueous solution. Experimental results show that both nitrate (–NO3) and nitrite (–NO2) species can be quantitatively evaluated and the accuracy is verified by calibrating with NOx-TPD result. Exclusive transfer of adsorbed NOxfrom catalyst surface to aqueous solution is confirmed and the dissolution process can be accelerated by ultrasonic treatment. Moreover, useful information related to evolution of surface NOxspecies under various conditions (O2treatment, different adsorption temperature and duration) and over different catalysts (Fe2O3, MnO2and MnOx–CeO2) are provided. The result of present study demonstrates the potential of spectrophotometry for quantitative discrimination of surface NOxspecies on CeO2and other oxide-based materials, which is conducive to mechanism analysis of SCR reaction.

Mn-based catalysts preferred in low-temperature selective catalytic reduction (SCR) are susceptible to SO2poisoning. The stubborn sulfates make insufficient O2activation and result in deficient reactive oxygen species (ROS) for activating reaction molecules. H2O has long been regarded as an accomplice to SO2, hastening catalyst deactivation. However, such a negative impression of the SCR reaction was reversed by our recent research. Here, we reported a H2O contribution over Mn-based SCR catalysts to counteract SO2poisoning through accessible O2activation, in which O2was synergistically activated with H2O to generate ROS for less deactivation and more expected regeneration. The resulting ROS benefited from the energetically favorable route supported by water-induced Eareduction and was actively involved in the NH3activation and NO oxidation process. Besides, ROS maintained high stability over the SO2+ H2O-deactivated γ-MnO2catalyst throughout the mild thermal treatment, achieving complete regeneration of its own NO disposal ability. This strategy was proven to be universally applicable to other Mn-based catalysts.

In the work, supported catalysts of FeOxand MnOxco-supported on aluminum-modified CeO2was synthesized for low-temperature NH3-selective catalytic reduction (NH3-SCR) of NO. Impressively, the SCR activity of the obtained catalyst is markedly influenced by the adding amount of Al and the appropriate Ce/Al molar ratio is 1/2. The activity tests demonstrate that Fe–Mn/Ce1Al2 catalyst shows over 90% NO conversion at 75–250 °C and exhibits better SO2resistance compared to Fe–Mn/CeO2. Fe–Mn/Ce1Al2 shows the expected physicochemical characters of the ideal catalyst including the larger surface and increased active reaction active sites by controlling the amount of Al doping. Also, the better catalytic activity is well correlated with the present advantaged surface adsorption oxygen species, Mn4+species, Ce3+species and the enhanced reducibility of Fe–Mn/Ce1Al2, which is superior to the Fe–Mn/CeO2catalyst. More importantly, we further demonstrate that the amount and strength of surface acid sites are improved by Al-doping and more active intermediates (monodentate nitrate) is generated during NH3-SCR reaction. This work provides certain insight into the rational creation of simple and practical denitration catalyst environmental purification.

CeO2/TiO2(denoted as CeTi) catalysts obtained by solid-phase impregnation behaved better in low-temperature selective catalytic reduction of NOxwith NH3(NH3-SCR) than that by conventional wet impregnation. To explore the main factors for activity distinction, the texture property, CeO2dispersion and structure changes of TiO2were comprehensively analyzed. It was found that surface changes of TiO2had a significant impact on the improved activity. From results of inductively coupled plasma atomic emission spectrometer (ICP-AES), diffuse reflectance UV–vis spectroscopy (UV–vis-DRS) and Raman, it was inferred that Ce ions were partially incorporated into TiO2lattice, accompanied with the formation of defects and vacancies during solid-phase impregnation. Accordingly, CeTi catalysts from solid-phase impregnation exhibited superiority in adsorption and activation of reactants. Further result from monitoring the preparation process indicated that the evolved NO played an important role in promoting Ce doping through depriving oxygen atoms on TiO2surface. The interaction between Ce and Ti was enhanced. The catalyst performed better in NH3-SCR, especially at low temperature, which testified the solid-phase impregnation could be an effective method to modulate interface structure for designing efficient catalyst.

Cerium-based catalysts are very attractive for the catalytic abatement of nitrogen oxides (NOx) emitted from stationary sources. However, the main challenge is still achieving satisfactory catalytic activity in the low-temperature range and tolerance to SO2poisoning. In the present work, two series of Mo-modified CeO2catalysts were respectively obtained through a wet impregnation method (Mo-CeO2) and a co-precipitation method (MoCe-cp), and the roles of the Mo species were systematically investigated. Activity tests showed that the Mo-CeO2catalyst displayed much higher NO conversion at low temperature and anti-SO2ability than MoCe-cp. The optimal Mo-CeO2catalyst displayed over 80% NO elimination efficiency even at 150 °C and remarkable SO2resistance at 250 °C (nearly no activity loss after 40 h test). The characterization results indicated that the introduced Mo species were highly dispersed on the Mo-CeO2catalyst surface, thereby providing more Brønsted acid sites and inhibiting the formation of stable adsorbed NOxspecies. These factors synergistically promote the selective catalytic reduction (SCR) reaction in accordance with the Eley-Rideal (E-R) reaction path on the Mo-CeO2catalyst. Additionally, the molybdenum surface could protect CeO2from SO2poisoning; thus, the reducibility of the Mo-CeO2catalyst declined slightly to an adequate level after sulfation. The results in this work indicate that surface modification with Mo species may be a simple method of developing highly efficient cerium-based SCR catalysts with superior SO2durability.

•Manganese oxide (MnOx) with attractive feature of rich oxygen vacancy was constructed by a surfactant-assisted method.•MnOx-OV owned amorphous structure and large specific surface area (326 m2·g-1).•The NO conversion of MnOx-OV reached 100% at the low temperature window of 100–200 °C under a high space velocity of 120,000 mL·g-1·h-1.•Detailed experimental results proved that MnOx-OV obeyed the mechanism of Langmuir-Hinshelwood (L-H).

Understanding the influence of sulfates over catalysts for selective catalytic reduction of NO with NH3(NH3-SCR) is crucial due to the universal presence of SO2in exhaust gas. Depending on the degree of sulfation, there mainly exist surface and bulk sulfates and NH3-SCR activity is generally considered to suffer more from bulk sulfates. Herein, the unique function of bulk sulfates over CeO2in promoting high-temperature SCR reaction is revealed. Notably, compared with CeO2dominated with surface sulfates (S-CeO2–4h) and commercial V2O5-WO3/TiO2, CeO2with bulk sulfates (S-CeO2–72h) exhibits admirable NO conversion at the temperature range of 400–550 °C. Bulk sulfates provide more Brønsted acid sites with stronger strength for NH3adsorption. Moreover, the oxidation ability of CeO2is significantly inhibited due to electron-withdrawing effect from bulk sulfates, which alleviates NH3oxidation at high temperatures. More NH3adsorption with high stability and limited NH3oxidation capacity ensure the excellent catalytic performance for S-CeO2–72h in high-temperature denitration. This work provides new insight of bulk sulfates in promoting SCR activity and open a new avenue to design deNOxcatalysts employed at high temperatures.

The employment of CeTi-based catalysts in the denitration of exhaust from biomass combustion is significantly limited by the poor tolerance to alkali metals. Herein, a CuO modification strategy was proposed to enhance the resistance of CeTi catalysts to K2O. The NO conversion of pristine Ti/Ce decreased from 95% to less than 10% at 250 °C after 1 wt % K loading. As a sharp comparison, CuO-modified Ti/Ce reserved more than 70% in NO conversion under the same test conditions. Combined with diverse characterizations, it was found that the addition of CuO promoted the formation of Cu–O–Ce and Cu–O–Ti species, which showed enhanced redox and acid properties. CuO modification also suppressed the adsorption of inert nitrates resulting from K loading. Moreover, Brønsted acid sites introduced by CuO served as capture sites of alkali metals, thus alleviating the active sites from being poisoned. More retained acidic and redox capacities ensured the excellent catalytic performance of TiCu/Ce under K poisoning.