Effect of LiO Doping on the Surface and Catalytic Properties of the CrO/AlO System

Two Cr 2 O 3 /Al 2 O 3 samples with the nominal compositions 0.06Cr 2 O 2 /Al 2 O 3 and 0.125Cr 2 O 3 /Al 2 O 3 (AlCr-I and AlCr-II, respectively) were prepared by mixing a known amount of finely powdered Al(OH) 3 with calculated amounts of CrO 3 , followed by drying at 120°C and calcination at 700°C and 800°C. Doped solid specimens were prepared by treating Al(OH) 3 samples with known amounts of LiNO 3 dissolved in the minimum amount of distilled water prior to mixing with CrO 3 . Dopant concentrations of 0.75, 1.50, 3.00 and 6.00 mol% Li 2 O were employed. The surface and catalytic properties of the pure and doped solids thus prepared were investigated using nitrogen adsorption at −196°C and studies of the catalysis of CO oxidation by O 2 over the solid specimens at 300–400°C. The results of such studies showed that Li 2 O doping followed by calcination at 700°C led to a maximum increase in the specific surface area, S BET , of 26% for AlCr-I and of 55% for AlCr-II when these samples were doped with 3.00 mol% Li 2 O. The reverse effect was found when the calcination temperature was increased to 800°C, where a decrease of 34% in the S BET value of the AlCr-II sample doped with 3.00 mol% Li 2 O was detected. The catalytic activities measured at 350°C over the pure and doped solids decreased on increasing the dopant concentration, the maximum decrease in such activity being ca. 33% and 50%, respectively, for the AlCr-I and AlCr-II samples calcined at 700°C. Doping led to noticable changes in the magnitude of the activation energy for the catalytic reaction. Such changes were accompanied by parallel changes in the value of the pre-exponential factor in the Arrhenius equation. These results may indicate that Li 2 O doping has no effect on the mechanism of the catalytic reaction but modifies (decreases) the concentration of catalytically active sites taking part in chemisorption during the catalysis of CO oxidation by O 2

Surface Characteristics of the Pure and LiO-doped MoO/AlO System

Two series of MoO 3 /Al 2 O 3 solids, having the nominal compositions 0.2MoO 3 : Al 2 O 3 and 0.5MoO 3 :A12O 3 , were prepared by impregnating finely powdered Al(OH) 3 samples with calculated amounts of ammonium molybdate solutions. The solids thus obtained were dried at 120°C and then calcined in air at temperatures varying between 400°C and 1000°C. The doped samples were prepared by treating Al(OH) 3 with LiNO 3 solutions prior to impregnation with ammonium molybdate. The dopant concentrations employed were 1.5 and 6.0 mol% Li 2 O, respectively. The surface characteristics, viz. the specific surface area (S BET ), the total pore volume (VP) and the mean pore radius (r) of the various pure and doped solids were measured from nitrogen adsorption isotherms conducted at -196°C. The S BET data measured for different adsorbents calcined at various temperatures enabled the apparent activation energy for sintering (ΔE 3 ) to be determined for all the adsorbents investigated. The results obtained reveal that the S BET value of the pure and doped solids decreased on increasing the calcination temperature in the range 400–1000°C. The decrease was, however, more pronounced when the calcination temperature increased from 500°C to 700°C due to the formation of Al 2 (MoO 4 ) 3 . Lithium oxide doping decreased the S BET value of the solid samples investigated and also decreased the activation energy for sintering to an extent proportional to the amount of dopant present. The sintering process for the pure and doped solids proceeds, mainly, via a particle adhesion mechanism

Texture Properties of Undoped and NaO-doped VO/AlO, Catalysts

Pure alumina, V 2 O 5 –Al 2 O 3 and Na 2 O-doped V 2 O 5 /Al 2 O 3 samples have been obtained and roasted in air over the temperature range 400–750°C. The textural properties of all the calcination products have been determined from nitrogen adsorption studies at 77 K. The inclusion of V 2 O 5 in alumina modifies the textural characteristics leading to an enhancement of activation at temperatures ≤450°C and to an enhancement of sintering at higher temperatures. Furthermore, AlV 2 O 4 which formed at temperatures below 750°C resulted in a tremendous change in the textural properties. Three types of sintering were detected, i.e. particle-particle adhesion, pore widening and phase change. Doping with Na 2 O retards sintering and at low calcination temperatures gave solids with higher surface areas compared to undoped samples. This has been attributed to activation brought about by the decomposition of NaNO 3 and the evolution of nitrogen oxides