Surface properties of palladium catalysts supported on ternary ZrO2–Al2O3–WOx oxides prepared by the sol–gel method: Study of the chemical state of the support

The surface properties of Pd and Pd–Pt catalysts supported on binary ZrO2–WOx and ternary ZrO2–Al2O3–WOx oxides prepared by the sol–gel method were studied. Special attention was paid to the study of the texture of the catalysts as well as the chemical state of tungstated zirconia and tungstated zirconia promoted with alumina in the palladium catalysts. The catalysts were tested in the isomerization of n-hexane and were characterized by N2 physisorption, XRD, TPR, Raman spectroscopy, XPS and FT-IR of adsorbed pyridine. The catalysts had bimodal pore size distributions with mesopores in the range 55–70 Å and macropores of 1000 Å in diameter. The catalysts had a surface WOx coverage (4.4–6.0 W nm−2) lower than that of the theoretical monolayer (7.0 W nm−2). A lower acidity of the ternary ZrO2–Al2O3–WOx oxide as compared to the binary ZrO2–WOx oxide was found. Higher activity in the isomerisation of n-hexane was obtained in the Pd–Pt catalysts supported on ternary ZrAlW oxides prepared by sol–gel that is correlated with the coexistence on the surface of W4+ (WO2) or W0 and W6+ (Al2(WO4)3) species, ZrO2 in the tetragonal phase and a high amount of ZrOx suboxides species in a low oxidation state (Zr3+ and Zr2+).

► Pd catalysts supported on ZrO2–WOx and ZrO2–Al2O3–WOx prepared by sol–gel had bimodal pore size distributions.
► They had a surface WOx coverage (4.5–4.9 W nm−2) lower than that of the theoretical monolayer (7 W nm−2).
► WOx phases consisted mainly of monotungstate and two-dimensional poly-oxotungstate in tetrahedral coordination.
► Catalysts supported on ZrAlW had lower acidity than those on ZrW.
► Coexistence on the surface of both tetragonal zirconia, Al2(WO4)3 and reduced ZrOx, W0 and W4+ species was found.