Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

• Published in: Journal of catalysis 2012-01, Vol.285 (1), p.48-60
• Main Authors: Hävecker, Michael, Wrabetz, Sabine, Kröhnert, Jutta, Csepei, Lenard-Istvan, Naumann d’Alnoncourt, Raoul, Kolen’ko, Yury V., Girgsdies, Frank, Schlögl, Robert, Trunschke, Annette
• Format: Article
• Language: English
• Subjects: Acrylic acid; Acrylics; Active sites; Adsorption; Catalysis; Catalysts; Catalytic oxidation; Chemistry; Exact sciences and technology; General and physical chemistry; In situ XPS; M1 phase; Microcalorimetry; MoVTeNb oxide; Operando; Propane; Propane oxidation; Surface chemistry; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2011.09.012

The surface of a highly crystalline MoVTeNb oxide catalyst composed exclusively of the M1 phase has been studied before catalysis and during operation in oxidation of propane at 693K. Increasing formation of acrylic acid in general goes along with surface depletion of Mo and accordingly the enrichment in Te, V, and Nb. A close correlation exists between the acrylic acid production and the increasing presence of surface V5+ species. [Display omitted] ► The surface of MoVTeNb oxide is characterized by a high number of energetically equivalent sites for propane adsorption. ► Restructuring of the catalyst surface during propane oxidation has been monitored under working conditions. ► Acrylic acid formation correlates with surface depletion in Mo6+ and enrichment in V5+ sites. ► The active ensemble contains V5+ oxo-species in close vicinity to Te4+ sites. The surface of a highly crystalline MoVTeNb oxide catalyst for selective oxidation of propane to acrylic acid composed of the M1 phase has been studied by infrared spectroscopy, microcalorimetry, and in situ photoelectron spectroscopy. The acid–base properties of the catalyst have been probed by NH3 adsorption showing mainly Brønsted acidity that is weak with respect to concentration and strength of sites. Adsorption of propane on the activated catalyst reveals the presence of a high number of energetically homogeneous propane adsorption sites, which is evidenced by constant differential heat of propane adsorption qdiff,initial=57kJmol−1 until the monolayer coverage is reached that corresponds to a surface density of approximately 3 propane molecules per nm2 at 313K. The decrease of the heat to qdiff,initial=40kJmol−1 after catalysis implies that the surface is restructured under reaction conditions. The changes have been analyzed with high-pressure in situ XPS while the catalyst was working applying reaction temperatures between 323 and 693K, different feed compositions containing 0mol.% and 40mol.% steam and prolonged reaction times. The catalytic performance during the XPS experiments measured by mass spectrometry is in good agreement with studies in fixed-bed reactors at atmospheric pressure demonstrating that the XPS results taken under operation show the relevant active surface state. The experiments confirm that the surface composition of the M1 phase differs significantly from the bulk implying that the catalytically active sites are no part of the M1 crystal structure and occur on all terminating plan