Aqueous-phase reforming of oxygenated hydrocarbons over Sn-modified Ni catalysts

Reaction kinetics measurements were conducted for aqueous-phase reforming of oxygenated hydrocarbons over Pt/Al2O3, Ni/Al2O3, NiSn/Al2O3, Raney-Ni, and Raney-NiSn catalysts at temperatures of 498 and 538 K. Raney-Ni, Raney-NiSn, and Pt/Al2O3 catalysts display good stability with time on stream during aqueous-phase reforming, whereas Al2O3-supported Ni and NiSn catalysts exhibit deactivation caused by sintering. Aqueous-phase reforming of sorbitol, glycerol, and ethylene glycol solutions produces an effluent gas stream composed of 50-70 mol% H-2, 30-40 mol% CO2, and 2-11 mol% alkanes (dry basis) at high conversion. Addition of Sn to Ni improves the selectivity for production of H-2 by ethylene glycol reforming from 35 to 51% at a Ni:Sn ratio of 270:1, while the alkane selectivity is reduced from 44 to 33%. At a Ni: Sn ratio of 14:1, the hydrogen selectivity increases to 90%, while alkane production is nearly eliminated. As the system pressure decreases to the bubble point of the feed (25.1 bar at 498 K), production of alkanes decreases and the hydrogen selectivity increases accordingly. Hydrogen selectivity is also maximized by operation at higher reactor space velocities. The addition of Sn to Ni significantly decreases the rate of methane formation from C-O bond cleavage, while maintaining sufficiently high rates of C-C bond cleavage required for hydrogen formation. Turnover frequencies for hydrogen production at 498 K over Raney-Ni-based catalysts are several times lower than that over 3 wt% Pt/Al2O3 based on CO chemisorption. However, the high CO uptakes and high densities of Raney-Ni-based catalysts lead to comparable rates of hydrogen production per unit reactor volume as 3 wt% Pt/Al2O3 at 498 K. Results from XRD, SEM, and Sn-119 Mossbauer spectroscopy suggest that Raney-NiSn catalysts comprise alumina and nickel particles surrounded by a Ni-Sn alloy. After exposure to reaction conditions, Sn is present primarily as Ni3Sn alloy with small amounts of Sn(IV) probably associated with alumina. (C) 2003 Elsevier Inc. All rights reserved.