Low-temperature methane oxidation over oxide-supported Pd catalysts: inhibitory effect of water vapor

An Experimental and Kinetic Modelling Study for Methane Oxidation over Pd-based Catalyst: Inhibition by Water

“…The temperature was then increased up to 700 °C in Ar flow to perform a degreening of the sample in order to ensure stability during the different experiments. At this temperature, the monolith was treated twice for 60 min under lean conditions (500 ppm CH 4 , 500 ppm NO, 300 ppm CO, 8% O 2 , 5% H 2 O, balanced by Ar) with a treatment under rich conditions (2% H 2 , 5% H 2 O, Ar) for 20 min in-between lean steps. Thereafter, the sample was purged by Ar for 10 min and was pre-treated using 8% O 2 , 5% H 2 O in Ar flow for 30 min at 700 °C.…”

“…However, the catalytic activity of these systems is decreasing with time-on-stream and significant research has been addressed to understand the mechanism of Pdbased catalyst deactivation during the methane oxidation. The water inhibition effect has been discussed extensively [4][5][6][7][8][9][10][11][12][13][14][15] and the formation of surface hydroxyls is considered to be a key deactivation pathway, which results in the slow deactivation of the catalyst due to the continuous formation and accumulation of these surface species [8,16]. Moreover, it was shown that competitive adsorption between methane and water molecules plays a role in the catalytic activity loss [5,9,15,17,18].…”

“…For methane oxidation most commonly the Langmuir-Hinshelwood-Hougen-Watson (LHHW) type of the rate equations is used in literature [22][23][24][25][26][27][28]; however, Mars-van Krevelen (MVK) based equations can also be used to describe mechanism of methane oxidation [29,30]. The oxidation of CH 4 over PdO is strongly inhibited by H 2 O, with a negative reaction order (−1) [15]. Groppi et al [22,23] used a pseudo LHHW kinetic model R w = K r C CH 4 ∕ 1 + K H 2 O C H 2 O to describe methane oxidation over PdO/Al 2 O 3 .…”

“…Specchia et al [30] examined different models for steady state methane oxidation under dry conditions over Pd/ Ce x Zr 1−x O 2 , including the above mentioned mechanism, and found that MVK is the best mechanism to describe the experimental data. Inhibiting effect of water on methane oxidation under transient conditions has been also studied by Kikuchi et al [4] and recently by Alyani and Smith [31]. Kikuchi et al [4] demonstrated experimentally that the Pd/ Al 2 O 3 catalyst has been deactivated by water, and derived rate parameters for the system.…”

“…Kikuchi et al [4] demonstrated experimentally that the Pd/ Al 2 O 3 catalyst has been deactivated by water, and derived rate parameters for the system. However, these parameters were not used to model any experimental data [4]. Based on a simplified kinetic model of methane oxidation in the presence of water Alyani and Smith [31] demonstrated the Ce-promoting effect on the Pd/Al 2 O 3 catalyst.…”