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Origin of low temperature deactivation of Ni5Ga3 nanoparticles as catalyst for methanol synthesis

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Abstract

In an effort to find alternative energy sources capable to compete with fossil fuels, methanol synthesis could represent a realistic solution to store “green” hydrogen produced from electrolysis or photo-induced water splitting. Recently, density functional theory (DFT) calculations [1] proposed Ni-Ga alloys as active catalysts for methanol production from syngas mixtures and Ni-Ga nanoparticles supported on highly porous silica have been prepared using an incipient wetness impregnation technique from a solution of nickel and gallium nitrates [2]. Tests conducted in a fixed-bed reactor showed that the highest methanol yield is obtained with a Ni5Ga3 alloy exposed to a 25% CO2 – 75% H2 reaction mixture at 210 °C [2]. Under these experimental conditions, the catalyst is found to lose 35% of its activity after 20 hours of continuous testing at both 1 and 5 Bars. Although in situ XRD and EXAFS studies [3] confirm dealloying as responsible for high temperature (T > 300 °C) deactivation, this does not explain the activity loss in the low temperature regime (T < 300 °C). This work presents an extensive study on the low temperature (T = 200, 210, 250 °C) deactivation of silica supported Ni5Ga3 nanoparticles as catalyst for methanol production. Synthesis, followed by deactivation and a series of regeneration steps at increasing temperature in pure H2 has been carried out in a fixed-bed reactor connected to a gas chromatography system. In each regeneration step, CH4 is generated and the activity of the catalyst is subsequently increased, suggesting the presence of carbon containing species blocking the active sites of the alloy nanoparticles (Figure 1). Carbon deposition has furthermore been investigated by temperature programmed oxidation (TPO) of a deactivated catalyst in a fixed-bed reactor connected to a mass spectrometer. CO2 and H2O evolution at T > 200 °C confirms the presence of carbon containing species on the catalyst. Finally, an electron microscopy study aiming at direct carbon probing has been performed, requiring the synthesis of a new support consisting of 200 nm Stöber silica spheres [4]. Unlike porous silica, on this new support Ni5Ga3 nanoparticles becomes directly exposed to the electron beam (Figure 2) allowing us to obtain high resolution TEM images and perform more accurate electron energy loss spectroscopy (EELS) measurements.