IR spectroscopic investigation of structural hydroxyl groups in W−Si heteropolycompound supported on Al2O3

Structural hydroxyl and deuteroxyl groups within the K₄[SiW₁₂O₄₀]/Al₂O₃ and K₆[SiW₁₁PdO₃₉]/Al₂O₃ systems were studied by diffuse-reflectance FTIR spectroscopy in a spectral range of fundamental stretching vibrations, first overtones, and combination bands of stretching and bending vibrations. For hydroxyl groups, the region of combination vibrations is the most informative. The calculated frequencies of bending vibrations of hydroxyl groups (865 and 730 cm⁻¹) are characteristic of acidic OH groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2017–2020, October, 1999.     

The state of palladium in the system potassium palladium-11-tungstosilicate on aluminosilicate as studied by DRIFT spectroscopy

The influence of redox treatments on the state of palladium in the heteropoly acid K₆[SiW₁₁PdO₃₉] supported on amorphous aluminosilicate was studied by diffuse-reflectance IR spectroscopy using CO as a probe molecule. A bulk sample of the K₆[SiW₁₁PdO₃₉]·11H₂O heteropoly compound (HPC) was studied for comparison. The deposition of the bulk sample on aluminosilicate leads to the partial decomposition of Keggin structure and to the removal of palladium cations from the HPC framework. The metal introduced into the K₆[SiW₁₁PdO₃₉]/SiO₂-Al₂O₃ system within HPC displays higher thermal stability in redox cycles compared to that of the bulk HPC. A highly dispersed state of palladium persists up to 573 K.     

Investigation of the state of palladium in the potassium palladium-11-tungstosilicate/alumina system by diffuse-reflectance IR spectroscopy

The influence of redox treatments on the state of palladium in the K₆[SiW₁₁PdO₃₉]·11H₂O/γ-Al₂O₃ system was investigated by diffuse-reflectance IR (DRIFT) spectroscopy using CO as a probe molecule. The K₆[SiW₁₁PdO₃₉]·11H₂O heteropoly-compound (HPC) and starting γ-Al₂O₃ support were studied for comparison. It was shown that palladium is present in HPC mainly in the form of Pd²⁺ ions. Treatment of HPC in an H₂ flow results in complete reduction of palladium to Pd⁰. The HPC is unstable in redox cycles at temperatures above 373 K. When the HPC was supported on alumina, the Keggin units were stabilized on the support surface and Pd⁺ formed in the oxidized sample. The supported K₆[SiW₁₁PdO₃₉]·11H₂O/γ-Al₂O₃ catalyst displays higher thermal stability and does not decompose during redox treatments at temperatures up to 723 K. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 7, pp. 1271–1275. July, 1999.     

Study of the state of rhodium in the potassium rhodiumundecatungstosilicate/alumina system by diffuse-reflectance IR spectroscopy

Adsorption of CO on the catalytic system K₅[SiW₁₁RhO₃₉]/Al₂O₃ was studied by diffuse-reflectance IR spectroscopy. The electronic state of rhodium and thermal stability of the system in the redox cycles were studied. The oxidized heteropolycompound (HPC) contains the charged forms Rh⁺, Rh²⁺, and Rh³⁺. Their ratio strongly depends on the pre-treatment temperature of the sample. In the reduced HPC two main forms of rhodium exist: the ionic Rh⁺ and metallic Rh⁰. The supported HPC structure is stable in an oxidative medium at temperatures below ∼900 K. The properties of the K₅[SiW₁₁RhO₃₉]/Al₂O₃ system are retained in the redox cycles up to ∼670 K, and the highly dispersed state of rhodium is observed up to 900 K.     

The state of platinum in the potassium platinum-11-tungstosilicate system on alumina studied by diffuse-reflectance IR spectroscopy

The electronic state of platinum in the K₄[SiW₁₁PtO₃₉]/Al₂O₃ supported system and its thermal stability in redox cycles were investigated by diffuse-reflectance IR spectroscopy using CO as a probe molecule. The bulk sample of the K₄[SiW₁₁PtO₃₉]·13H₂O heteropolycompound (HPC) was studied for comparison. It was shown that platinum is present in the oxidized HPC mainly as the Pt¹⁺ and Pt⁰ species. In the supported system, the ratio of these species strongly depends on the temperature of oxidative pretreatment. The reduced HPC contains mainly metallic platinum in the form of Pt⁰ complexes with the bridged CO ligands. Supporting of the bulk sample HPC increases the thermal stability of its structure in an oxidative medium from 600 to 820 K. The supported K₄[SiW₁₁PtO₃₉]/Al₂O₃ system exhibits a higher stability in the redox cycles compared to that of the bulk sample of individual HPC and preserves the Keggin structure and highly dispersed state of platinum up to T 670 K.