Mechanism of hydrocarbon fuel desulfurization using Ag/TiO2–Al2O3 adsorbent

Silver supported on mixed oxides shows promises as an advanced organosulfur adsorbent with deep desulfurizing ability and scalability to be fitted into on-board fuel cell systems. However, its application into a workable technology requires thorough understanding of the organosulfur adsorption mechanism. This paper presents the performance, characterization, and desulfurization mechanism of Ag/TiO2–Al2O3 (10 wt.% Ag, Ti/Al = 1:4.4) adsorbent for applications in pre-reformate cleaning of Proton Exchange Membrane (PEM) fuel cells. The adsorbent demonstrated effective organosulfur adsorption capacities at ambient conditions (13.06 mg S/g adsorbent for model hydrocarbon fuel containing benzothiophene). This was achieved through greater TiO2 (< 4 nm, from X-ray diffraction) and Ag dispersions (~ 23% for 10 wt.% Ag, from O2 chemisorption). Anatase-TiO2 dispersion on Al2O3 provided increased adsorbent activity (3.27 eV band gap, from UV–vis spectroscopy), higher surface acidity (~ 14 cm3/g NH3 uptake at P = 800 mm Hg, from NH3 adsorption), and exerted in more defect sites (α-Lewis acid sites) for Ag incorporation. TiO2–Al2O3 provided both strong and weak Brønsted sites (from infrared studies using ammonia, 2,6-lutidine, trimethyl chlorosilane, and thiophene as probe molecules). Organosulfur adsorption on Ag/TiO2–Al2O3 adsorbent was primarily attributed to surface hydroxyls (via hydrogen/σ bonding) and surface bound silver oxides (via π bonding).