Characterization of nickel catalysts with transient methods

Abstract

The Ni surface area is a key factor in catalytic performance of supported Ni catalysts employed in heterogeneously catalyzed reactions. For its characterization, transient measurement techniques are powerful but need a proper experimental design and a high level of accuracy. In this study, several transient methods, namely temperature programmed desorption of H2 (H2-TPD), pulsed H2 chemisorption and N2O chemisorption, were applied and evaluated for the characterization of Ni catalysts. Results were compared with those from static H2 chemisorption. Ni powder, as well as NiAlOx and Ni/γ-Al2O3 with varying Ni loadings, synthesized via precipitation and incipient wetness impregnation, respectively, were used as model catalysts. H2-TPD within the temperature range from 84 to 753K is able to completely describe the interaction of H2 with supported Ni. However, a quantitative analysis of the specific Ni surface area based on H2-TPD is difficult whereas pulsed H2 chemisorption leads to comparable results as obtained from static measurements. Adsorption and pre-treatment conditions have a strong impact on the desorption spectra. Reversible morphologic changes of the Ni surface were revealed by H2-TPD when changing the gas atmosphere during pre‐treatment. N2O chemisorption occurs in three steps including fast oxygen uptake, growth of the oxide layer and subsequent layer thickening due to subsurface and bulk oxidation. A separation of surface and subsurface/bulk oxidation of NiAlOx and Ni/γ-Al2O3, both of which are readily oxidized even under mild conditions, is only achievable at temperatures between 190 and 195K. In this temperature range, the ratio of adsorbed oxygen atoms, O, and the number of Ni surface atoms, NiS, (O/NiS) is 0.38±0.07, which can be applied for specific Ni surface area determination. The experimental mode of N2O chemisorption (flow or titration mode) does not influence the extent of subsurface oxidation. The separation of surface and subsurface/bulk oxidation is more feasible in the case of Ni powder. Here, a suitable temperature for Ni surface area determination lies within 265 and 285K with an O/NiS ratio of 0.96±0.05.