Effect of Pt and Pd additives on the state of the surface layer and catalytic activity of niobium oxides in the oxidation of hydrogen

Abstract

Catalysts prepared by the hydrogen reduction of Nb2O5 in the presence of Pt or Pd have specific surface much greater than for the starting oxide and their catalytic activity in the oxidation of hydrogen is much greater than the activity of Pt/Al2O3 or Pd/Al2O3. X-ray phase analysis and X-ray photoelectron spectroscopy were used to establish the existence of Nb2O5–x nonstoichiometric oxides in the catalyst, which enhances the catalytic activity of the surface. The kinetic behavior of the oxidation of hydrogen on these catalysts is explained in the framework of the Eley–Riedel mechanism. In previous work [1], we have shown that molybdenum oxide and tungsten oxide reduced by hydrogen in the presence of small amounts of platinum or palladium display high catalytic activity in the oxidation of small molecules such as H 2 , CO, and CH 4 due to the formation of H x MO 3–x and MO x phases (M = Mo or W). The formation of hydrogen-reduced bronze oxides H x MO 3–x and partially reduced oxides MO x proceeds due to hydrogen spillover from the platinum or palladium metal to the corresponding oxide [2]. Increases in the specific surface (Ssp) and catalytic activity in the oxidation of H2 were also observed in our previous work [3] for hydrogen-reduced Ta 2 O 5 in the presence of platinum or palladium. In the present work, we studied the effect of the addition of platinum and palladium on the state of the surface of niobium oxides and their catalytic activity in the oxidation of hydrogen. Samples of the catalysts were prepared by impregnation of Nb 2 O 5 (99.995 mass %) by solutions of H 2 PtCl 6 or PdCl 2 leading to 0.1 or 0.5 mass % supported platinum metal in the composite composition, drying at 393 K for 1 h, and reduction with hydrogen (10 vol.% H 2 in argon) ove r1ha t 673 K. Forcomparison of the activity of the samples obtained with the activity of Pt or Pd catalysts deposited on an inert support, samples of 0.5 mass % Pt/Al 2 O 3 and Pd/Al 2 O 3 were prepared by the same method. Treatment of these catalysts with hydrogen was carried out in the same manner as for the Pt/Nb 2 O 5–x and Pd/Nb 2 O 5–x samples. The phase composition of the catalysts was determined by X-ray phase analysis (XPA). The diffraction spectra of samples of Pt/Nb 2 O 5–x and Pd/Nb 2 O 5–x were taken on a Stoe STAPIDI diffractometer using MoK a radiation. The crystal phases in the catalysts studied were identified in a framework of the algorithms in a standard program [4] using the experimental spectra and a standard diffraction data base [5].