Sulfur deactivation of nickel methanation catalysts

Abstract

Sulfur deactivation of supported Ni in CO hydrogenation was studied in an all-quartz internalrecycle reactor with a feed containing 4% CO in H 2. Thirteen ppb H 2S reduced the steady-state methanation activity of Ni γ- Al 2 O 3 about 200-fold at 661 K; 100 ppb H 2S reduced the activity 5000-fold. A dual site Langmuir-Hinshelwood rate expression predicts both the CO partial pressure dependence and the S poisoning. Poisoning and chemisorption data indicate formation of a stable two-dimensional, surface sulfide with a S: Ni surface atom ratio of 1: 2 for 13 ppb H 2S in H 2 at 661 K. The surface sulfide has a free energy of formation of at least −26 kcal/mole which is 15 kcal/mole more stable than bulk Ni 2S 3. Sulfur poisoning is due to geometric effects, i.e., site blockage, rather than electronic effects since the activation energy for methanation over S-poisoned Ni was the same as that over unpoisoned Ni, 24 kcal/mole.