Steam reforming of sulfur-containing n-dodecane over Me/CeO2 (Me=Pt, Rh) catalysts

Diesel, with its existing logistic infrastructure, considerable well-to-wheel efficiency, safe handling feature, adequate world-wide fuel storage and high energy density, is an attractive source of H2 production via SR [1]. However, coke and carbon deposition still remain a maior challenge in the processing of sulfur-containing heavy hydrocarbons [2]. In this work, the activity and stability of Me/CeO2 catalysts (Me=Pt, Rh) were investigated towards the steam reforming (SR) of n-dodecane at different process variables including S/C ratio (S/C=1.0-2.5), space velocity (GHSV=16,000-40,000 h-1), time-on-stream and sulfur concentration (0-100 ppm). Pt-based catalysts (0.6-2.3 wt.% of Pt) and Rh-based catalyst (0.6 wt.% of Rh) were prepared by Solution Combustion Synthesis (SCS) method and characterized by XRD, N2-physisorption, CO-chemisorption, TPR, TPO and TEM techniques. Preliminary blank tests were carried out to determine the ignition temperature of n-dodecane cracking reaction (550°C). An optimized temperature-controlled bed configuration from ca. 500°C (TIN) to 800°C (TSET) was developed to avoid coke formation due to thermal cracking of n-dodecane. Both Pt/CeO2 and Rh/CeO2 systems showed high catalytic activity towards n-dodecane SR of at all the investigated S/C molar ratio and GHSV, in terms of total n-dodecane conversion and high H2 production (70-73%, in dry and N2-free basis). WGS reaction is favored, determining lower amount of CO and a higher amount of CO2 in the products mixture (compared to the thermodynamic equilibrium values) and, consequently, higher H2/CO molar ratio [3]. Superior chemical-physical properties of Rh/CeO2 system positively affect the SR performance, allowing stable catalytic activity of sulfur-free dodecane at S/C=1.5 and GHSV=16,000 h-1 (Figure 1a). The regeneration ability of the catalyst was evaluated at lower steam-to-carbon molar ratio (S/C=1.0) and higher space velocity (GHSV=40,000 h-1) under sequential start-up and shut-down cycles. After the regeneration step with air, the activity was restored, indicating that the full regeneration of the catalyst is possible. Catalyst deactivation was accelerated by the addition of increased amount of sulfur from 30 to 100 ppm (Figure 1b). The formation of increased amounts graphitic carbonaceous particles on the catalyst surface was observed, changing the surface morphology of the coke to a more refractory nature. Higher S/C ratios positively favor catalyst stability of the SR reaction of sulfur-containing n-dodecane.