TEMPERATURE-PROGRAMMED REDUCTION (TPR) IN CATALYSIS: A POWERFUL INVESTIGATION TOOL (IF CORRECTLY USED)

Temperature-programmed reduction (TPR) has become a powerful tool in the study of catalysts. Indeed it is possible to investigate complex phenomena like metal-support interaction, the role of different metals (as additives or in solid solution) as promoters or inhibitors of the catalyst reduction, or the influence of one or more phases on the reducibility of a specific component in multi-component catalysts [1]. However, such many valuable insights can be missed if an incorrect TPR experimental approach is followed [2]. Indeed reduction profiles are markedly affected by artefacts when TPR measurements are carried out under an inadequate combination of the experimental operating variables, i.e. initial amount of reducible species, total flow rate, initial hydrogen concentration and heating rate. It is shown that under a correct choice of the experimental operating variables the reduction profiles of CuO, Cu2O and CuO-ZnO catalysts are characterized by single and rather sharp reduction peaks. In contrast to that, when an improper combination of the experimental operating variables is imposed, artefacts like double peaks or a large broadening appear in the reduction profiles, all of this making the reliability of the TPR results completely lost. The origin of these artefacts is complex. When the fraction of hydrogen consumed is too large, a levelling-up of the output of the hydrogen detector occurs. Both such a peculiar dynamic situation and activated H2 adsorption-desorption phenomena coinciding with the reduction process are responsible of the observed perturbations. [1] A. Jones, B.D. McNicol, in Temperature-Programmed Reduction for Solid Materials Characterization, Chemical Industries, Vol. 24, Marcel Dekker, Inc. (New York, 1986). [2] G. Fierro, M. Lo Jacono, M. Inversi, P. Porta, R. Lavecchia and F. Cioci, J. Catal. 148, 709 (1994).