Low-temperature catalytic oxidation of carbon monoxide over palladium metal, hydrous palladium oxides, and anhydrous palladium oxides

The chemical state of Pd at the surfaces of two sizes of Pd powders and ceria-supported Pd during low-temperature CO oxidation has been studied using X-ray photoelectron spectroscopy (XPS). During oxidation in O2 metallic Pd is converted into PdO, and the thickness of the PdO layer increases with increasing reaction temperature. A similar Pd oxidation process occurs while running the catalytic CO oxidation reaction, but the extent of the Pd oxidation is less due to the presence of CO which is a reducing agent. Catalytic CO oxidation data obtained from 10 wt% Pd supported on nanocrystalline ceria powders indicate that there is a strong chemical interaction between the ceria and the supported Pd. The Pd is present as PdO on the fresh catalyst. During reaction, small amounts of Pd metal and PdO2 are formed at 50 °C while Pd metal and only a small amount of PdO2 are present after running the reaction at 110 °C. Stoichiometic oxygen concentration plays an important role during CO oxidation over palladium metal. For 1.0 % CO at 200 °C, Pd metal exhibits 100 % CO conversion with 0.51 % O2 in the feedstream. However, the same Pd powder exhibits only 45 % CO conversion with 0.47 % O2. CO oxidation catalytic enhancements from 5 to 100 % are found for a short time period over the temperature range of 185 to 200 °C. During feedstream bypass, the bicarbonate species dissociate into a hydroxyl group and a carbon dioxide molecule. XPS data indicate that a carbon C 1s peak due to bicarbonate is present during the bypass step.; Hydrous palladium oxide (PdO·H2O) exhibits 100 % CO conversion even at room temperature. Existence of lattice water and its reaction with CO/O2 are believed to be the reason for the excellent CO oxidation activity. Hydrous PdO pretreated in He at 400 °C does not exhibit any CO oxidation activity even at 100 °C while hydrous PdO exhibits CO oxidation activity with only 1.0 % CO in the feed (no O2 feed). Pd 3d, Pd 3p and O 1s XPS features indicate that the chemical state of palladium is shifted from oxide to metal and that the intensity of the metallic peaks become more predominant at longer reaction times and higher reaction temperatures. For 100 mg of hydrous PdO tested at near-stoichiometric conditions (slightly O2 rich) and various reaction temperatures ranging from 40 to 300 °C, the length of the period of initial 100 % CO conversion is proportional to the temperature. At 40 °C 100 % CO conversion lasts 15 min compared to 380 min at 200 °C. The optimum temperature for CO oxidation over hydrous PdO is at 220 °C at which complete conversion is maintained for more than 440 min.