Catalytic activity of nanosized Au/CeO2 catalyst towards H2O2 decomposition and the role of cationic/metallic ratio in its activity

The catalytic decomposition of H2O2 on differently pre-treated Au/CeO2 catalyst was studied by kinetic measurements at 20-50 °C. The prepared catalyst was subjected to pre-treatment by heating either in oxidative (10% O2/N2) or inert (pure N2)atmosphere at 400 °C. The different oxidation states of gold were determined by X-ray photoelectron spectroscopy measurements. The Au/CeO2 catalyst exhibited an excellent catalytic activity towards H2O2 decomposition. The catalytic activity of oxygen pre-treated sample was about twice higher than that measured for nitrogen pre-treated sample. This finding ran parallel to the amount of Aun+ as determined by XPS, indicating the role played by Aun+ species as the most active catalyst’s constituent. However, one cannot overlook the role of metallic gold in catalyzing the H2O2, decomposition showing small activity compared to that of cationic gold. The average crystallites size of metallic gold particles was found to be 7±0.5 nm independent of the pre-treatment conditions. The apparent activation energy of the catalyzed reaction was found to be 46.5 and 47.8 kJ/mol for oxygen and nitrogen pre-treatment, respectively

The Effects of LiO Doping on the Surface and Catalytic Properties of CuO/AlO Solids

The effects of doping CuO/Al 2 O 3 solids with Li 2 O on their surface and catalytic properties were investigated using nitrogen adsorption at −196°C, the decomposition of H 2 O 2 at 20–40°C and the oxidation of CO by O 2 at 175°C. The pure solids were prepared by wet impregnation of finely powdered solid Al(OH) 3 which had been precalcined at 400°C; the resulting material was then dried and calcined at 500°C with copper nitrate dissolved in the least amount of distilled water. The amount of copper oxide in such solids was fixed at 13.5 wt% while the amounts of Li 2 O added varied between 0.19 wt% and 3.80 wt%. The results obtained showed that such Li 2 O doping enhanced the crystallization of the CuO phase to an extent proportional to the amount of dopant added and increased the concentration of surface OH groups. This treatment led to a progressive small increase in the BET surface areas (S BET ) of the treated solids, which attained a maximum limit at 0.76 wt% Li 2 O but decreased upon increasing the dopant concentration above this limit. The addition of 0.76 wt% Li 2 O effected an increase of 14.6% in the S BET values of the treated solids while the addition of 3.80 wt% Li 2 O led to a corresponding decrease of 38.5% in this value. Doping with Li 2 O resulted in a progressive decrease in the catalytic activity of the solids towards CO oxidation by O 2 while the presence of 3.80 wt% Li 2 O effected a decrease of 72.5% in the value of the reaction rate constant measured at 175°C. In contrast, such treatment of CuO/Al 2 O 3 solids with Li 2 O brought about a progressive increase in their catalytic activity towards H 2 O 2 decomposition, which reached a maximum limit in the presence of 1.90 wt% Li 2 O and then decreased when the amount of Li 2 O added was increased above this limit, falling to values which were smaller than those measured for the pure catalyst samples. The doping process did not modify the activation energy of the catalyzed H 2 O 2 reaction but modified the concentration of the catalytically active constituent present in the system

Surface and Catalytic Properties of the CuO/AlO System as Influenced by Doping with CeO or ZrO and by γ-Irradiation

The effects of doping the CuO/Al 2 O 3 system with CeO 2 or ZrO 2 , or alternatively treatment with γ-irradiation, on its surface and catalytic properties were investigated using nitrogen adsorption at −196°C, the decomposition of H 2 O 2 at 20–40°C and the oxidation of CO by O 2 at 175°C. The pure solids were prepared by wet impregnation with copper nitrate dissolved in the least amount of distilled water of finely powdered solid Al(OH) 3 precalcined at 400°C, followed by drying the resulting product and subjecting the same to calcination at 500°C. The doped solids were prepared by treating Al(OH) 3 precalcined at 400°C with a known amount of dopant, i.e. cerium or zirconyl nitrate dissolved in the least amount of distilled water, prior to impregnation with the copper nitrate solution. The amount of copper oxide thus introduced was fixed at 13.5 wt% while the amounts of dopants were varied between 1 wt% and 10 wt% CeO 2 or ZrO 2 . The results obtained indicated that ZrO 2 doping increased the degree of dispersion of the CuO phase, while CeO 2 treatment had the reverse effect. Doping the CuO/Al 2 O 3 system with CeO 2 or ZrO 2 led to an increase of 15.4% or 8.1%, respectively, in its BET surface area. The catalytic activity of the system towards the decomposition of H 2 O 2 decreased on doping with ZrO 2 but increased when CeO 2 was used as a dopant. γ-Irradiation (at 20–160 Mrad) of CuO/Al 2 O 3 solids resulted in a measurable and progressive decrease in their catalytic activity towards H 2 O 2 decomposition. In CO oxidation with O 2 , ZrO 2 treatment of the CuO/Al 2 O 3 solids brought about a progressive increase in their catalytic activity with the maximum value (a 31% increase) being observed in the presence of 3 wt% ZrO 2 but then decreasing with further increases in the amount of dopant present until the final value attained with 10 wt% ZrO 2 was smaller than that measured for the pure CuO/Al 2 O 3 catalyst sample. In contrast, the addition of the smallest amount of CeO 2 (1 wt%) led to an effective increase of 69% in the catalytic activity of the CuO/Al 2 O 3 system towards the O 2 oxidation of CO, which then decreased when further amounts of CeO 2 were added to the system although still exhibiting a catalytic activity greater than that of the undoped catalyst sample. Doping or γ-irradiation of the CuO/Al 2 O 3 system had no influence on the activation energy for the decomposition of H 2 O 2 in the presence of the resulting solid catalysts although the concentrations of catalytically active sites present on the surfaces of the solids investigated were modified by such treatment

Geometric and electronic structure of Au on Au/CeO2 catalysts during the CO oxidation: Deactivation by reaction induced particle growth

Abstract Changes of the geometric and electronic structure of gold on Au/CeO2 catalysts induced by different pre-treatments (oxidative and reductive) and by the CO oxidation reaction at 80°C were followed by operando XANES / EXAFS measurements. The results showed that i) oxidative pre-treatment (O2) leads to larger Au nanoparticles than reductive pre-treatment (CO), that ii) Au is predominantly metallic during CO oxidation, irrespective of the preceding pre-treatment, and that iii) there is a reaction induced Au particle growth. Correlations with the activity of the respective catalysts and its temporal evolution give insights into the origin of deactivation of these catalysts under reaction conditions, in particular on reaction induced changes in the Au particle size