Solving the Water Hypersensitive Challenge of Sulfated Solid Superacid in Acid-Catalyzed Reactions

Converting Poisonous Sulfate Species to an Active Promoter on TiO2 Predecorated MnOx Catalysts for the NH3-SCR Reaction

MnO_x-based catalysts possess excellent low-temperature NH₃ selective catalytic reduction (NH₃-SCR) activity, but the poor SO₂/sulfate poisoning resistance and the narrow active-temperature window limit their application for NO_x removal. Herein, TiO₂ nanoparticles and sulfate were successively introduced into MnO_x-based catalysts to modulate the NH₃-SCR activity, and the active-temperature window (NO conversion above 80%, T₈₀) was significantly broadened to 100-350 °C (SO₄^2--TiO₂@MnO_x) compared to that of the pristine MnO_x catalyst (ca. T₈₀: 100-268 °C). Combined with advanced characterizations and control experiments, it was clearly shown that the poisonous effects of sulfate on the MnO_x catalyst could be efficiently inhibited in the presence of TiO₂ species due to the interaction between sulfate and TiO₂ to form a solid superacid (SO₄^2--TiO₂) species as NH₃ adsorption sites for the low-temperature process. Furthermore, such solid superacid (SO₄^2--TiO₂) species could weaken the redox ability to inhibit the excessive oxidation of NH₃ and thus enhance the high-temperature activity significantly. This work not only puts forward the TiO₂ predecoration strategy that converts sulfate to a promoter to broaden the active temperature window but also experimentally proves that the requirement of redox ability and acidity in the MnO_x-based NH₃-SCR catalyst was dependent on the reaction temperature range.

Mild Preoxidation Treatment of Pt/TiO2 Catalyst and Its Enhanced Low Temperature Formaldehyde Decomposition

The typical platinum nanoparticles loaded on titania (Pt/TiO2) were pretreated with mild oxidation (<300 °C) in pure oxygen to enhance the low-temperature formaldehyde (HCHO) decomposition performance. The structural properties of support and platinum nanoparticles were characterized by X-ray diffraction (XRD), physical adsorption/desorption, high-resolution transmission electron microscopy (HRTEM), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFITS), and temperature-programmed reduction and oxidation (TPR and TPO). The catalytic results showed that the low temperature HCHO decomposition activity of mild pre-oxidized Pt/TiO2 was around three times that of the pristine one. According to the characterization results, the structure of the Pt/TiO2 support and their Pt particle sizes had negligible change after pre-oxidation treatment. The cationic Pt content of Pt/TiO2 and surface roughness of Pt nanoparticles gradually increased with the increasing temperature of the pre-oxidation treatment. Mild pre-oxidation treatment was beneficial to the oxygen activation and water dissociation of Pt/TiO2. In situ HCHO-DFIRTS results showed that the mild pre-oxidation treatment could enhance the dehydrogenation of formate.

Complete Hydrodesulfurization of Dibenzothiophene via Direct Desulfurization Pathway over Mesoporous TiO2-Supported NiMo Catalyst Incorporated with Potassium

Mesoporous TiO2 containing different potassium content was prepared from potassium titanate by mediating the pH value of the ion exchange, which was used as catalytic support to load NiMo for hydrodesulfurization of dibenzothiophene. The as-prepared samples were characterized by X-ray diffraction, N2 physical adsorption/desorption, temperature-programmed reduction, scanning electron microscope/energy dispersive X-ray mapping analysis, high resolution transmission electron microscopy, and pyridine-adsorbed Fourier transform infrared spectroscopy. The characterization results showed that NiO and MoO3 were well dispersed on mesoporous TiO2 with varying potassium content. A crystal NiMoO4 phase was formed on the TiO2 with relatively high potassium content, which could decrease the reduction temperature of oxidized active species. The evaluation results from the hydrodesulfurization displayed that as the potassium content of the catalyst increased, the dibenzothiophene conversion firstly increased and then slightly decreased when potassium content exceeded 6.41 wt %. By contrast, the direct desulfurization selectivity could continuously increase along with the potassium content of catalyst. Furthermore, the change in direct desulfurization selectivity of a TiO2-supported NiMo catalyst was independent of the reaction condition. The mesoporous TiO2-supported NiMo catalyst incorporated with potassium could have near both 100% of dibenzothiophene and 100% of direct desulfurization selectivity. According to the structure–performance relationship discussion, the incorporation of potassium species could benefit the formation of more sulfided active species on mesoporous TiO2. Moreover, excessive free potassium species may poison the active sites of the hydrogenation pathway. Both factors determined the characteristics of complete hydrodesulfurization of dibenzothiophene via a direct desulfurization pathway for potassium-incorporated mesoporous TiO2 supported NiMo catalysts.