In the context of fuel cell technology, the selective oxidation of CO in mixtures high in hydrogen is an important reaction. In order to effectively remove the hydrogen-rich stream from CO, the reactions have been run in an excess oxygen gas at a temperature range of 150-200°C. Over nickel catalysts supported by rare earth elements, selective oxidation of natural gas to produce syngas was studied. In order to improve product yields and selectivity, it was discovered that rare earth oxides like Y2O3 and CeO2 were effective promoters. When molecular hydrogen (H2) and another substance or element come into contact chemically, it is known as hydrogenation. This reaction typically takes place with the aid of a catalyst like nickel, palladium, or platinum. To reduce or saturate organic compounds, the process is frequently used.