Effects of pretreatment atmosphere and silver loading on the structure and catalytic activity of Ag/SBA-15 catalysts

Reduced graphene oxide as an effective promoter to the layered manganese oxide-supported Ag catalysts for the oxidation of ethyl acetate and carbon monoxide

The layered manganese oxide (δ-MnO₂)-supported reduced graphene oxide (rGO)-promoted silver catalysts (xAg- yrGO/δ-MnO₂; x and y are the Ag and rGO contents (wt%), respectively) were prepared via a polyvinyl alcohol-protected reduction route. Physicochemical properties of these materials were determined using the numerous techniques, and their catalytic activities were evaluated for the oxidation of CO and ethyl acetate. It is found that the loading of rGO as an electron transfer promoter could significantly strengthen the metal-support interaction (SMSI) between Ag and δ-MnO₂ and increase specific surface area of the sample, hence improving catalytic performance of the sample. Activity evaluation reveals that 1Ag- 1.0rGO/δ-MnO₂ showed the best catalytic activity and the lowest apparent activation energy (E_a), giving a T_90% of 140 °C and an E_a of 42.7 kJ/mol for CO oxidation, and a T_90% of 160 °C and an E_a of 39.8 kJ/mol for ethyl acetate oxidation at space velocity (SV) = 60,000 mL/(g h). The good performance of 1Ag- 1.0rGO/δ-MnO₂ was associated with its high Mn³⁺/Mn⁴⁺ or O_ads/O_latt molar ratio, good low-temperature reducibility, and strong SMSI between Ag and δ-MnO₂. The in situ DRIFTS characterization demonstrates that the carbonate and acetate species were the main intermediate products in CO and ethyl acetate oxidation over 1Ag- 1.0rGO/δ-MnO₂, respectively. The 1Ag- 1.0rGO/δ-MnO₂ sample was not significantly altered in physicochemical property after 55 h of stability test, but its activity decreased in the presence of water vapor, especially such an effect on ethyl acetate oxidation was more obvious, which was possibly due to the competitive adsorption of water and reactants on the catalyst surface.

Efficient synthesis of sub-5 nm Ag nanoparticles by the desorption effect of supercritical CO2 in SBA-15

Herein, the desorption effect of supercritical CO₂ (scCO₂) was utilized to obtain sub-5 nm Ag nanoparticles (NPs) with a high Ag loading in SBA-15. The size of the Ag NPs decreased from 3.54 ± 0.79 nm (Ag loading of 25.3 wt.% wt.%) to 2.38 ± 0.68 (Ag loading of 10.5 wt%) nm by changing the depressurization curve from 0.1 MPa min^-1 (20-14 MPa) to 3 MPa min^-1 (20-12 MPa). Meanwhile, the intensity of the crystalline Ag characteristic peaks was obviously higher than the latter sample from the x-ray diffraction (XRD) patterns. However, compared with the adsorption kinetics of the two precursors of AgNO₃ and Cu(NO₃)₂ on SBA-15, under the same deposition and depressurization conditions, when the two depositional systems used water as a co-solvent, the time taken to reach the adsorption equilibrium of Ag⁺ on the supports was longer than the time taken by Cu²⁺, which existed in the water as [Cu(H₂O)₄]²⁺. The surface of SBA-15 was hydrophilic, and then the interaction of Ag⁺ and the surface was weaker compared with Cu²⁺, making Ag⁺ highly dispersed on the surface under the scCO₂ desorption effect. After calcination, the size of the Ag NPs decreased, but the morphology of CuO was mainly characterized by nanorods (NRs). Moreover, by comparing the experiments of wetness impregnation, the dispersion ability of the bulk scCO₂ of the reactor was inefficient for Ag⁺ adsorbed on the channels in the depressurization process.

Immobilization of ultrafine Ag nanoparticles on well-designed hierarchically porous silica for high-performance catalysis

Catalyst immobilization is of much significance not only for maintaining the high activity of the ultrafine catalyst, but also for the separation of catalyst during the practical application. Herein, a novel support material, three-dimensional hierarchically porous silica (HPS) with interconnected micro-meso-macro pores and high specific surface area was successfully fabricated though a freeze-drying technique in the presence of poly(vinyl alcohol) (PVA) and subsequent calcination process. A series of characterizations revealed that the specific surface area of HPS can be well adjusted by changing the addition of PVA. The specific surface area of HPS was as high as 360 m² g^-1, which was 211-fold higher than HPS-0 (silica prepared without using PVA). To demonstrate the potential application of such novel support material, highly dispersed silver nanoparticles (AgNPs) were immobilized on the surfaces of HPS and HPS-0 through in-situ reduction. By contrast, the catalytic activity of AgNPs anchored on HPS (531 s^-1 g^-1) was about 42-fold higher than that of AgNPs anchored on HPS-0 (12.67 s^-1 g^-1). The significantly enhanced catalytic activity of AgNPs/HPS was believed to be related to their high specific surface area and interconnected macroporous scaffolds, which could provide numerous reactive sites and mass transfer routes for the reactants.

Biosynthesized Pd/γ-Al2O3 catalysts for low-temperature 1,3-butadiene hydrogenation: the effect of calcination atmosphere

The effect of pretreatment atmosphere on 1,3-butadiene hydrogenation over biosynthesized Pd/γ-Al₂O₃ catalysts was reported.

SiO2-Ag-SiO2 core/shell structure with a high density of Ag nanoparticles for CO oxidation catalysis

SiO₂-Ag-SiO₂, a sandwiched core/shell structure with a layer of Ag nanoparticles (∼4 nm) encapsulated between a shallow SiO₂ surface layer and a SiO₂ submicrosphere substrate (∼200 nm), has been synthesized from [Formula: see text] and SiO₂ spheres by a facile one-pot hydrothermal method. The composite is proposed to result from the dynamic balance between the [Formula: see text] reduction and the dissolution-redeposition of SiO₂ in mild basic media. The synthetic mechanism and the roles of the reaction time, temperature, and the amount of ammonia in the formation of this unique structure are investigated and discussed. The composite structure shows superior catalytic performance in CO oxidation to the control Ag/SiO₂ structure prepared by impregnation. Pre-treatment by O₂ at 600 °C significantly improves the catalytic performance of the composite structure and preserves the nanocomposite structure well.