The curious relationship of sintering to activity in supported gold catalysts for the hydrochlorination of acetylene

Abstract

Gold catalysts for the hydrochlorination of acetylene are currently being studied as an environmentally benign replacement for industrial mercuric chloride catalysts, which undergo reduction and subsequent sublimation into the atmosphere. In this work the method of strong electrostatic adsorption was used with a cationic gold precursor to controllably deposit the gold precursors over a variety of activated carbon and metal oxide supports. The catalysts were characterized by XRD, STEM, and XPS before and after reaction or aging at temperature (180°C) in HCl.The synthesis procedure resulted in highly dispersed Au nanoparticles (usually below the 1.5nm limit of detection and some from 2 to 3nm) over all supports. The series of catalysts exhibited an unusual relationship of sintering to activity; the catalysts which best anchored the Au crystallites were the least active; titania and silica catalysts showed almost no sintering and were virtually inactive, and even a graphitic carbon catalyst exhibited good anchoring but very poor activity. The sintering was caused by the HCl atmosphere and not the temperature; catalysts submitted to the reaction temperature in He were stable.It would appear that the metal nanoparticles, and Au at the edges of metal nanoparticles, are inactive. The current results support recent reports in the literature that the active sites are derived from isolated Au ions, but we can conclude that oxidized amorphous carbon also plays a key role in generating the active site. In this environment the active species is highly mobile and susceptible to sintering.