The selective oxidation of methane to formaldehyde on a metal surface is a challenging process due to the high stability of methane and the tendency of formaldehyde to undergo further oxidation. However, certain conditions and choice of metal catalysts can help improve the selectivity and activity of this reaction.1. Temperature: The reaction temperature plays a crucial role in the selective oxidation of methane. Lower temperatures around 300-400C are generally preferred to minimize the over-oxidation of formaldehyde to carbon dioxide and water. However, the temperature should be high enough to activate the methane molecule.2. Pressure: Lower pressures are typically favorable for the selective oxidation of methane to formaldehyde. This is because higher pressures can lead to the formation of undesired side products, such as methanol and dimethyl ether.3. Oxygen-to-methane ratio: The ratio of oxygen to methane in the reaction mixture is another important factor. A lower oxygen-to-methane ratio around 1:1 can help improve the selectivity towards formaldehyde by reducing the likelihood of over-oxidation.4. Choice of metal catalyst: The choice of metal catalyst has a significant impact on the selectivity and activity of the reaction. Some metals that have shown promising results in the selective oxidation of methane to formaldehyde include: a. Silver Ag : Silver is one of the most widely studied catalysts for this reaction. It has been found to exhibit good selectivity and activity, especially when supported on metal oxide materials such as alumina or silica. b. Molybdenum Mo : Molybdenum-based catalysts, particularly those containing Mo-V-Te-Nb-O mixed metal oxides, have shown high selectivity and activity for the selective oxidation of methane to formaldehyde. c. Iron Fe : Iron-based catalysts, such as iron molybdate Fe2 MoO4 3 , have also been reported to exhibit good selectivity and activity in the selective oxidation of methane to formaldehyde.5. Catalyst support and promoters: The choice of catalyst support and the addition of promoters can also influence the selectivity and activity of the reaction. For example, metal oxide supports like alumina, silica, and titania can enhance the dispersion of the active metal species and improve the catalyst's stability. Promoters such as alkali metals e.g., potassium can help in modifying the catalyst's surface properties and improve the selectivity towards formaldehyde.In summary, the selective oxidation of methane to formaldehyde on a metal surface can be influenced by various factors, including temperature, pressure, oxygen-to-methane ratio, choice of metal catalyst, and the presence of catalyst support and promoters. By optimizing these conditions, it is possible to improve the selectivity and activity of the reaction.