Highly selective production of syngas (>99%) in the partial oxidation of methane at 480 °C over Pd/CeO2 catalyst promoted by HCl

• Pd/CeO 2 catalyst was used for partial oxidation of methane in the presence of HCl. • The introduction of HCl transforms Pd/CeO 2 into (PdCl X + Pd 0 )/CeOCl(0< X ≤ 2). • The transformed Pd/CeO 2 produces syngas with 99% CO selectivity at above 480 ℃. • No deactivation is observed for 40 h, resulting from no coke formation. The partial oxidation of methane (POM) is an attractive catalytic route for producing syngas (CO/H 2 ), although there are problems to overcome such as high reaction temperature (>650 °C) and the deactivation of the catalyst due to the coke formation. We addressed the problem by including HCl in the POM over a Pd/CeO 2 catalyst. Pd/CeO 2 produces CO, CO 2 , and coke with low CO selectivity in the absence of HCl, while it converts methane to chlorinated products at below 450 °C in the presence of HCl. At above 480 °C, however, highly selective CO (>99%) is produced over Pd/CeO 2 via the POM. Moreover, it is found that coke is not formed during the reaction, leading to the stable partial oxidation activity and selectivity up to 40 h. Both surface and bulk characterization results strongly propose that HCl plays an essential role in transforming Pd/CeO 2 into (PdCl x + Pd 0 )/CeOCl (0 < x ≤ 2), which provides the ideal environment for the catalytic route of the POM.