The effect of SMSI (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts I. IR spectra of adsorbed CO prior to and during reaction conditions
1981; Elsevier BV; Volume: 71; Issue: 1 Linguagem: Inglês
10.1016/0021-9517(81)90211-6
ISSN1090-2694
Autores Tópico(s)Catalysis and Oxidation Reactions
ResumoAn infrared cell was designed and operated as a differential, plug-flow reactor so that ir spectra and kinetic data could be obtained simultaneously under well-defined, steady-state conditions. In agreement with results from other reactor systems, turnover frequencies varied over two orders of magnitude from the most active catalysts, TiO2-supported Pd in the SMSI state, to the least active, PdSiO2. Infrared spectra of CO adsorbed on PdTiO2 and PdSiO2-Al2O3 were recorded for the first time, and spectra for PdAl2O3 and PdSiO2 were also obtained. The effects of temperature and hydrogen on the spectra were studied, and spectra were obtained under reaction conditions for five different catalysts. In the absence of H2, a high-frequency band above 2080 cm−1 and a broader, dominant low-frequency band below 2000 cm−1 were present on all catalysts. Hydrogen had little influence on the CO spectra on the typical Pd catalysts, but it markedly reduced the CO band intensities on the PdTiO2 catalysts, even at 300 K. The effect was most pronounced for the PdTiO2 (SMSI) catalyst, and under reaction conditions no CO bands were detected whereas all other Pd catalysts had observable ir bands. Primarily for this reason, no obvious correlation was found between observed ir-active CO species and specific activity. However, these results strongly indicate that only a small fraction of the Pd surface atoms constitute active sites, and the large surface concentrations of CO detected by ir are not directly involved in the rate-determining step for methanation.
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