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

• Ag/TiO2–Al2O3 was effective in desulfurizing fuels down to PEMFC application range.
• Dispersion of anatase TiO2 and Ag oxide resulted in higher sulfur capacity.
• Surface bound Ag+ and OH groups were primarily responsible for sulfur adsorption.

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).

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