| Cobalt based catalysts supported on TiO2 and ZrO 2 were studied for the oxidation of NO to NO2 in excess oxygen. NO oxidation was studied as the first step in a two-step catalytic scheme where NO is oxidized to NO2 and, in turn, NO2 is reduced with CH4 to N2 under lean conditions. Catalysts were prepared by sol-gel (SG) and incipient-wetness impregnation (IWI) techniques. It was found that increasing the calcination temperature had an adverse effect on the activity of the IWI catalysts. Catalysts were characterized by temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), laser Raman spectroscopy (LRS), and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). 10%Co/ZrO2 IWI catalysts were found to be more active than the 10%Co/TiO2 IWI catalysts. The most active Co/ZrO2 catalyst reached equilibrium at 250°C, giving a 94% NO2 yield. Higher activity for NO oxidation was observed to correlate with the formation of the Co3O4 phase. Characterization by several techniques demonstrated that this metal oxide phase is more dominant on the ZrO2 support than on the TiO2 support.; For the second stage, reduction of NO2 with methane, a series of Pd-based catalysts on a sulfated zirconia (SZ) support were prepared and tested. The Pd/SZ catalysts showed much higher N2 yields during the reduction of NO2 as opposed to NO. When the Pd/SZ catalysts were combined with the cobalt-based NO oxidation catalysts for the reduction of NO with CH4 in excess O2, the N2 yields were similar to those obtained when only the reduction catalyst was used to reduce NO2. Studies in which the reactor feed contained species at concentrations that more closely resembled actual operating conditions (NOx, O2, CO, CO2, methane, ethane, and propane) showed promising results that did not significantly deviate from the N2 yields that were obtained in the earlier experiments. Experiments on the mixed catalyst bed in which water vapor was introduced to the feed led to a decrease in the N2 yield. The loss of activity in the presence of water has been attributed to competitive adsorption.; The studies for the reduction in NOx emissions also revealed that the cobalt-based NO oxidation catalysts had potential to be used as a catalyst for the oxidation of carbon monoxide. The Co/ZrO2 catalyst had high activity for the oxidation of CO to CO2, and under certain conditions was able to completely oxidize CO at room temperature. The catalyst was also studied for the preferential oxidation of carbon monoxide in excess hydrogen. While the oxidation of carbon monoxide in lean, excess oxygen, conditions has applications for air quality improvement, the preferential oxidation of carbon monoxide in the presence of hydrogen has application for the purification of hydrogen streams for use in PEM fuel cells. A range of test conditions with various gas-hourly space velocities and concentrations of CO, O 2, H2, CO2, and H2O were examined. It was found that an increase in the ratio of CO-to-O2 led to higher O2 selectivity to CO2 and lower conversion of CO for a given temperature. At a given temperature, the presence of CO2 or H2O led to an inhibition of the CO oxidation reaction and a decrease in the CO conversion, as compared to the reactions in which CO 2 or H2O were not present, was observed. The effect of GHSV showed that higher GHSV led to lower CO conversion but higher O2 selectivity to CO2. Stability studies showed that operation the catalyst in its oxide form was fairly stable while operating around 100°C, but operation at higher temperatures (175--250°C) could lead to a loss of activity, likely caused by the partial reduction of cobalt oxide. At higher temperatures, the reduction of cobalt oxide also led to the formation of methane through the CO + 2H2 methanation reaction. Cursory investigations of cobalt supported on various metal oxide supports showed that in the test conditions, Co/ZrO2 had the higher act... |
PDF Full Text Request