On the influence of water traces on the acidity measurement of amorphous aluminosilicates

• Unwanted H2O adsorption modifies acidity distribution in standard conditions.
• A procedure is proposed to suppress water readsorption during acidity measurement.
• First time report of the position of the bands of D2O adsorbed on undercoordinated Al.
• Water adsorption on Lewis sites leads to Brønsted sites stronger than the native ones.

For the measurement of their acidity, amorphous aluminosilicates are beforehand treated at temperature of 450 °C or higher in order to remove water adsorbed on their surface. We will show in this study that, during the standard course of an acidity measurement by low temperature CO adsorption, traces of water present in the setup may perturb the acidity measurement by desorbing from the setup and adsorbing on the sample. We will also demonstrate that the interaction of water occurs on mild and strong Lewis sites and leads to the detection of stronger Brønsted sites than those natively present in the amorphous aluminosilicate. We will finally propose an experimental procedure that allows avoiding the adsorption on water on the sample and will clearly identify the position of the band associated with H2O and D2O adsorbed on Lewis sites before and after addition of CO.

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