Effect of surface manganese oxide species on soot catalytic combustion of Ce–Mn–O catalyst

MnCe-based catalysts for removal of organic pollutants in urban wastewater by advanced oxidation processes - A critical review

With Advanced oxidation processes (AOPs) widely promoted, MnCe-based catalysts have received extensive attention under the advantages of high efficiency, stability and economy for refractory organic pollutants present in urban wastewater. Driven by multiple factors such as environmental pollution, technological development, and policy promotion, a systematic review of MnCe-based catalysts is urgently needed in the current research situation. This research provides a critical review of MnCe-based catalysts for removal of organic pollutants in urban wastewater by AOPs. It is found that co-precipitation and sol-gel methods are more appropriate methods for catalyst preparation. Among a host of influence factors, catalyst composition and pH are crucial in the catalytic oxidation processes. The synergistic effect of the free radical pathway and surface catalysis results in better pollutants degradation. It is more valuable to utilize multiple systems for oxidation (e.g., photo-Fenton technology) to improve the catalytic efficiency. This review provides theoretical guidance for MnCe-based catalysts and offers a reference direction for future research in the AOPs of organic pollutants removal from urban wastewater.

Insight into modified CeMn based catalysts for efficient degradation of toluene by in situ infrared

Trace activated carbon (AC) and diatomaceous earth (DE) were used as structural promoters to be incorporated into Ce-Mn-based solid-solution catalysts by the redox precipitation method. The modified catalysts exhibit superior reducibility, with abundant Ce3+, Mn3+and reactive oxygen species, which are facilitated to the migration of oxygen and the generation of oxygen vacancies. In particular, the catalytic combustion temperatures of 90 % toluene (3000 ppm) on Ce1Mn3Ox-AC/DE were 84 °C (dry) and 123 °C (10 vol% H2O), respectively. The role of lattice oxygen and adsorbed oxygen was revealed by in situ DRIFTS. Additionally, in situ DRIFTS was employed to verify that the degradation of toluene by Ce1Mn3Ox-AC/DE satisfied the Langmuir-Hinshelwood (L-H) mechanism and the Mars-Van Krevelen (MvK) mechanism. The possible reaction pathway was elucidated (toluene → benzyl alcohol → benzoic acid → maleic anhydride → CO2 + H2O). Furthermore, final products attributed to toluene oxidation were detected by in situ DRIFTS at 50 °C in the absence of oxygen, confirming that the catalyst possessed outstanding performance at low temperatures beyond mere adsorption.

Unveiling the Surface Chemical Reactions during Multi-Phase Catalytic Oxidation of Soot on Nanoengineering/Interfacing/Doping-Prepared Mn-CeO2 Catalysts Using TG-MS and Operando DRIFTS-MS

The aerosol pyrolysis method from nitrate precursors was used to prepare the Mn-CeO2 catalyst containing Mn2O3, CeO2, and Mn-doped CeO2 nanoparticles for catalyzing carbonous soot oxidation. The prepared Mn-CeO2 catalysts have high specific surface areas, Ce3+ ratio, and oxygen vacancy defects; these are a benefit for soot oxidation. The T50 for soot oxidation on the 0.57Mn-CeO2 catalyst is as low as 355 °C, which is 329 °C lower than that for soot oxidation without a catalyst. The catalysts were characterized using XRD, SEM-EDS, HRTEM, XPS, Raman spectroscopy, H2-TPR-MS, O2-TPD-MS, soot-TPR-MS, and operando DRIFTS-MS. The functions of Mn2O3, CeO2, and Mn-doped CeO2 in the 0.57Mn-CeO2 catalyst are unveiled. Mn-doped CeO2 plays a key role and CeO2 participates in soot oxidation, while Mn2O3 is used to enhance higher ratios of Ce3+, via the reaction of Mn3+ + Ce4+ = Mn4+ + Ce3+. The mechanism of soot oxidation on Mn-CeO2 was proposed.

Preparation of Ce-MnOx Composite Oxides via Coprecipitation and Their Catalytic Performance for CO Oxidation

Ce-MnOx composite oxide catalysts with different proportions were prepared using the coprecipitation method, and the CO-removal ability of the catalysts with the tested temperature range of 60-140 °C was investigated systematically. The effect of Ce and Mn ratios on the catalytic oxidation performance of CO was investigated using X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), H2 temperature programmed reduction (H2-TPR), CO-temperature programmed desorption (CO-TPD), and in situ infrared spectra. The experimental results reveal that under the same test conditions, the CO conversion rate of pure Mn3O4 reaches 95.4% at 170 °C. Additionally, at 140 °C, the Ce-MnOx series composite oxide catalyst converts CO at a rate of over 96%, outperforming single-phase Mn3O4 in terms of catalytic performance. With the decrement in Ce content, the performance of Ce-MnOx series composite oxide catalysts first increase and then decrease. The Ce MnOx catalyst behaves best when Ce:Mn = 1:1, with a CO conversion rate of 99.96% at 140 °C and 91.98% at 100 °C.

Effect of oxygen vacancy and highly dispersed MnOx on soot combustion in cerium manganese catalyst

Cerium manganese bimetallic catalysts have become the focus of current research because of their excellent catalytic performance for soot combustion. Two series of cerium manganese catalysts (Na-free catalysts and Na-containing catalysts) were prepared by coprecipitation method and characterized using XRD, N₂ adsorption-desorption, SEM, Raman, XPS, H₂-TPR, O₂-TPD, Soot-TPR-MS and in situ IR. The effects of abundant oxygen vacancies and surface highly dispersed MnO_x on soot catalytic combustion of cerium manganese catalysts prepared by different precipitants were analyzed. The activity test results show that the active oxygen species released by a large number of oxygen vacancies in the cerium manganese catalyst are more favorable to the soot catalytic combustion than MnO_x which is highly dispersed on the surface of the catalyst and has good redox performance at low temperature. Because the catalytic effect of MnO_x on the surface of Na-free catalysts is more dependent on the contact condition between the catalyst and the soot, this phenomenon can be observed more easily under the loose contact condition than under the tight contact condition. The activity cycle test results show that these two series of catalysts show good stability and repeated use will hardly cause any deactivation of the catalysts.

Experimental investigation on the effects of DPF Cs-V-based non-precious metal catalysts and their coating forms on non-road diesel engine emission characteristics

Non-precious metal catalysts with good soot catalytic properties and a low cost have great potential for application in diesel particulate filters (DPF). In this study, we compared the effects of DPF supported by Cs₂V₄O₁₁ (Cs-V-based) non-precious metal catalysts and conventional Pt-Pd-based precious metal catalysts on the performance of a non-road diesel engine. Furthermore, the effects of on-wall coating and in-wall coating of Cs-V-based catalysts on DPF performance were also investigated. The results indicated that the particulate emissions from DPF with Cs-V-based catalysts were reduced slightly less than that with Pt-Pd-based catalysts; however, the particle number (PN) and particulate matter (PM) emissions were still reduced by 94.4% and 91.7%, respectively, meeting the non-road China IV limits under the non-road steady cycle (NRSC). In addition, CO, HC, and NO can also be slightly oxidized by the non-precious metal catalysts. On the other hand, the DPF with in-wall coating induced comparatively higher gaseous substances and particulate emissions and caused a higher exhaust back pressure (EBP), which was 9.6% higher than the on-wall coating under NRSC, negatively affecting engine performance. Additionally, the geometric mean diameter (GMD) for the DPF with in-wall coating was only 33.3 nm because of the large emission proportion of nuclear mode particles.