Influence of Pretreatment on the Interaction of Oxygen with Silver and the Catalytic Activity of Ag/SiO_(2) Catalysts for CO Selective Oxidation in H_(2)

The interactions of oxygen with pre-reduced silver catalysts as well as theircatalytic properties for CO selective oxidation in H_2 after oxygen pre-treatment are studied inthis paper. It is found that the pretreatment exerts a strong influence on the activity andselectivity of the silver catalyst. A drop in activity and selectivity is observed after treating apre-reduced catalyst with oxygen at low temperatures, whereas a converse result is obtained after anoxidizing treatment at high temperatures (T ≥ 350℃). O_2-TPD results show that surface oxygenspecies adsorbs on silver surface after the oxygen treatment at low temperatures. However,penetration of oxygen into the silver is enhanced by a high temperature treatment, meanwhile thesurface oxygen species disappear. No other silver species except metallic silver are observed on allthe catalysts by XRD, and the size of silver particle is not changed after the treatment withoxygen at low temperatures. The surface oxygen species formed by oxygen treatment can also beremoved by hydrogen reduction. The strongly-adsorbed surface oxygen species prohibit the adsorptionand diffusion of oxygen species in reaction gas on the surface of silver catalyst, causing thedecrease in CO oxidation activity, in other words, it is important to obtain a clean silver surfacefor increasing the catalyst activity in CO removal from H_2-rich feed gas. The differences inactivity and selectivity due to the oxygen pretreatment at different temperatures are discussed interms of the changes in the surface/subsurface oxygen species of the silver particles.

Macrokinetics of Ethylene Epoxidation over A-type Silver Catalyst

By taking the surface chemical reactions as the rate-controlling step, a possible reaction mechanism for ethylene epoxidation to synthesize ethylene oxide over the A-type silver catalyst was developed, while it was assumed that the epoxidation reaction would take place between ethylene and the un-dissociated adsorbed oxygen O2 a on the solo active sites, while the deep oxidation would occur between ethylene and the dissociated adsorbed oxygen Oa on the adjacent multi-active sites. In order to describe the effect of 1,2-C2H4Cl2(EDC) inhibitor on the ethylene epoxidation process, the reversible reactions between EDC and vinyl chloride(VC) on the active sites of silver catalyst was introduced. According to the assumed mechanism, the hyperbolic macro-kinetic model of ethylene epoxidation over the A-type silver catalyst was established, and the macrokinetic experiments were carried out in an internal-recycle gradientless reactor operating at a pressure of 2.1 MPa and a temperature in the range of 217.8—249.0 ℃, with the gas composition(molar fraction) consisting of 15.82%—34.65% C2H4, 2.55%—7.80% O2, 0.88%—6.15% CO2, 0.15—2.61 μmol/mol of 1,2-C2H4Cl2 and 0.14—1.28 μmol/mol of C2H3 Cl. By means of the Simplex Optimal Method, the parameters of the macrokinetic models were estimated. Statistical test showed that the macrokinetic models developed for the A-type silver catalyst agree well with the experimental results.

NO reduction by CO over TiO_2-γ-Al_2O_3 supported In/Ag catalyst under lean burn conditions

TiO2/γ-Al2O3 supported In/Ag catalysts were prepared by impregnation method,and investigated for NO reduction with CO as the reducing agent under lean burn conditions.The microscopic structure and surface properties of the catalysts were studied by N2 adsorption-desorption,X-ray diffraction,transmission electron microscopy,X-ray photoelectron spectroscopy,ultraviolet-visible spectroscopy,H2 temperature-programmed reduction and Fourier transform infrared spectroscopy.TiO2/γ-Al2O3 supported In/Ag is a good catalyst for the reduction of NO to N2.It displayed high dispersion,large amounts of surface active components and high NO adsorption capacity,which gave good catalytic performance and stability for the reduction of NO with CO under lean burn conditions.The silver species stabilized and improved the dispersion of the indium species.The introduction of TiO2 into the γ-Al2O3 support promoted NO adsorption and improved the dispersion of the indium species and silver species.

Effect of pretreatment conditions on catalytic activity of Ag/SBA-15 catalyst for toluene oxidation

The catalytic oxidation of toluene over Ag/SBA‐15synthesized under different pretreatment conditions,including O2at500°C(denoted O500),H2at500°C(H500),and O2at500°C followed by H2at300°C(O500‐H300)was studied.The pretreated samples were investigated by N2physisorption,X‐ray diffraction,and ultraviolet‐visible diffuse reflectance.The pretreatment atmosphere greatly influences the status of the Ag and O species,which in turn significantly impacts the adsorption and catalytic removal of toluene.Ag2O and amorphous Ag particles,as well as a large amount of subsurface oxygen species,are formed on O500,and the subsurface oxygen enhances the interaction between Ag species and toluene,so O500shows good activity at higher temperature.However,its activity at lower temperature is not as high as expected,with a reduced presence of Ag2O and lower adsorption capacity for toluene.H2pretreatment at500°C is conducive to the formation of large Ag particles and yields the largest adsorption capacity for toluene,so H500exhibits the best activity at lower temperatures;however,because of poor interaction between Ag and toluene,its activity at higher temperature is modest.The O500‐H300sample exhibits excellent catalytic activity during the whole reaction process,which can be attributed to the small and highly dispersed Ag nanoparticles as well as the existence of subsurface oxygen.

2-Amino homopropargyl alcohols from the highly regioselective Ag_2CO_3-catalyzed nucleophilic openings of alkynyl epoxides by amines

The nucleophilic ring opening reaction of propargyl epoxides by amines based on a silver catalyst is presented, The reaction takes place under mild conditions and features a high regioselectivity to provide an effective method for the synthesis of 2-amino homopropargyl alcohols in moderate to high yields,