Study of Ni–Ag/SiO2 catalysts prepared by reduction in aqueous hydrazine

We have studied bimetallic Ni–Ag (Ni + Ag = 1 wt%) catalysts supported on crystallized silica and prepared by aqueous chemical reduction with hydrazine at 353 K. Two protocols of reduction were used. Prepared catalysts were characterized by means of XRD, TEM, STEM, H2 chemisorption and H2-TPD. Their catalytic activity was studied in the gas-phase hydrogenation of benzene. The most important feature of the results obtained is the synergistic effect between Ni and Ag which led to improvement of dispersion and reactivity of nickel in the presence silver for precipitated catalysts. Silver is inactive in the test-reaction. Precipitated bimetallic catalysts give rise to total conversion from 373 K, a temperature at which conversion hardly reaches 30% for the impregnated catalysts. Dispersion and activity pass through a maximum of monotonically decrease with precipitated and impregnated catalysts, respectively. Deactivation was observed for bimetallic catalysts, particularly with precipitated samples. These results could be explained by the mechanism of metal reduction in the hydrazine media. As a result, various Ni–Ag species formed where Ni and Ag phases were separated clusters or interacted as heteroatomic groupings on the carrier surface. These grouping would be responsible of the high performances of the precipitated catalysts.

We studied Ni–Ag (1 wt%)/SiO2 catalysts prepared in aqueous hydrazine media. We found that silver, although inactive toward hydrogen or in gas-phase hydrogenation of benzene, remarkably improves dispersion and catalytic activity of nickel in bimetallic system. The results obtained may be correlated to mechanisms of hydrazine metal reduction.Figure optionsDownload as PowerPoint slide