Deactivation Mechanism Study for Sulfur-Tolerance Enhanced NiO Nanocatalysts of Lean Methane Oxidation

The treatment of lean methane from sulfur-containing exhausts of natural gas vehicles is important, but challenging, because of the strong and irreversible catalyst poisoning by sulfur species. Herein, sulfur-tolerance enhanced NiO nanocatalysts (NiO-SPP) are prepared via surface polymeric phosphate (SPP) modification of NiO NPs, and a self-catalyzed sulfation (SCS) mechanism is proposed to reveal the insights on the boosted sulfur-tolerance capacity. In the SCS mechanism, the sulfation process is divided into two steps, namely, (1) the initial sulfation process and (2) the self-catalyzed sulfation process accelerated by previously formed sulfates. Stability and durability tests of NiO NPs and NiO-SPP reveal that the improved sulfur-tolerance capacity via SPP modification should be attributed to two reasons, namely, the external reason as the protection on the surface structure of NiO-SPP, and the internal reason as the deceleration of the initial sulfation rate. Specifically, characterizations and analyses of the catalysts before and after stability tests indicate that the SPP modification could efficiently cut down the interaction possibility of active sites with sulfur species. On the basis of the SCS mechanism, the initial sulfation rate of NiO NPs under long-term deactivation tests is calculated to be 12.8 times that of NiO-SPP, and the self-catalyzed sulfation rate constant for NiO NPs is twice that of NiO-SPP. The SPP modification of catalysts and the SCS mechanism provide a reference for the improvement of catalyst durability on the perspective of deactivation.