Alumina-supported Ni catalysts modified with silver for the steam reforming of methane: Effect of Ag on the control of coke formation

The effect of Ag loading (0.1-0.6 wt.%) on the physicochemical properties and catalytic behavior of Al2O3-supported Ni catalysts on steam reforming of methane was studied. The surface properties of catalysts were evaluated by applying FTIR of adsorbed CO. Additional information was obtained by S-BET, XRD, TPR, XANES, XPS and TEM characterizations. Ag caused a strong modification at the surface properties of the samples related to the CO adsorption and the stability to graphitic carbon deposition. The FTIR spectra of adsorbed CO on Ni/Al2O3 catalyst showed bands in two regions: (i) high frequency region (HF) with bands at 2075 cm(-1) (HF1) and 2037 cm(-1) (HF2), and (ii) low frequency region (LF) with bands at 1946 cm(-1) (LF1) and 1902 cm(-1) (LF2). The spectra of TPD-CO revealed that the HF2 species were favored with increasing the Ag, loading, while the LF species were strongly suppressed. The kinetics parameters E-a(app) versus ln A showed a compensation effect. The catalysts with Ag loading >= 0.3,wt.% submitted to stability tests showed high resistance to coke deposition. The high stability of the Ni catalysts promoted with Ag was attributed to changes in the Superficial structure of Ni sites as result of the combination of different effects: (i) the decrease of Ni ensemble by geometric effect of Ag, (ii) changes of Ni sites involved in the nucleation of the graphite structure and (iii) equilibration of different steps of reaction rate decreasing the C* formation rate. (C) 2007 Elsevier B.V. All rights reserved.