Effect of a second metal (Y, K, Ca, Mn or Cu) addition on the carbon dioxide reforming of methane over nanostructured palladium catalysts

Abstract

The CO2 reforming of CH4 to synthesis gas was investigated over nanostructured Pd/Al2O3 catalysts promoted by various additives (K, Ca, Y, Mn and Cu) using a stoichiometric feedstock mixture. In order to understand the effect of the additives on the surface properties and catalytic performance of the catalysts, fresh and spent samples were characterized by a number of techniques, such as N2 adsorption-desorption isotherms, XRD, CO-chemisorption, H2-TPR, TPO, Raman and TEM. Additives strongly modified the reduction properties of Pd species and metal surface dispersion. The results showed that the addition of Y or Ca to Pd/Al2O3 slightly increased the initial activity expressed as CH4 and CO2 conversions, which is attributed to decreased reducibility of Pd species and enhanced Pd dispersion. In contrast, while the addition of K, Mn and Cu similarly reduced the reducibility of Pd species, their surface metal dispersions apparently decreased owing to decoration effect or alloy formation. This leads to lower initial activities for the modified catalysts than Pd/Al2O3. On the other hand, stability of catalysts activity during the long-term tests was associated with Pd sintering resistance as well as carbon deposition. The CO2 reforming of CH4 was ceased over Pd/Al2O3 after around 4.5h of reaction due to catalyst bed blockage caused by the large amount of carbon deposition. The carbon formation was accelerated by Pd sintering. Among all catalysts, Y-modified Pd/Al2O3 showed the best stability with almost no activity loss in 20h of reaction. This is probably because that the addition of Y to Pd/Al2O3 notably suppresses carbon formation and metal sintering and maintains Pd particle size below 10nm. This particle size is crucial, below which carbon formation can be completely avoided under the present reforming conditions. For catalysts with other additives the catalytic activities decreased to different degrees after long-term tests, due to metal sintering and carbon deposition.