Surface and Texture Characterization of Alumina-Supported Copper and Mixed Copper–Manganese Oxide Catalysts and Their Formate Precursors

Abstract

Alumina-supported copper and copper–manganese oxide catalysts as well as the parent formates were characterized by means of FTIR spectroscopy in situ and nitrogen physisorption. The IR spectroscopic results are discussed on the basis of the deposition scheme proposed recently by Kapteijn et al. (J. Catal.150, 94 (1994). The infrared bands in the high-frequency region (OH stretches) indicate that the aqueous copper species formed in the solutions are deposited on the basic and neutral surface OH groups, while as our previous studies show the aqueous manganese species are deposited on the basic and acidic OH groups. However, the aqueous manganese species are deposited on the basic and neutral OH groups in the presence of copper ions (pH 4.6–4.7). The deposition of the aqueous metal species on the protonated basic OH groups occurs as a result of the “ion-pairing” process. The driving forces resulting in the deposition of the aqueous metal species on the neutral OH groups are considered to be hydrogen bonds. The deposition of the formate ions is also discussed. The higher uptake of copper as compared to manganese is discussed in terms of the metal ion properties (electronic configuration, the ability to form hydrogen bonds of different strength, metal species–support interaction). The nitrogen physisorption shows that the initial mesoporous character of γ-Al2O3 structure does not change during impregnation. The rFHH values which characterized the adsorbent–adsorbate interaction forces are calculated. The comparative analysis of the pore size distribution curves of the oxide-supported samples and their parent formates proves to be a useful tool to elucidate the metal species–support interaction strength.