Improving Stability of Palladium Nitrate Precursor Solutions by Fuming Nitric Acid Recrystallization Towards Efficient Bimetallic Ni-Pd/SiO2 Catalysts for Hydrogenation reactions

Abstract

Bimetallic systems in catalysts could lead to enhanced catalysis resulting in milder reaction conditions and higher turnover rate. The monometallic Nickel on Silica oxide catalysts is often used for hydrogenation reactions, but has limited activity at low temperature. Adding a noble metal in the form of Palladium could enhance the catalyst leading to a bimetallic alloy favorable to certain hydrogenation reactions such as CO2 methanation. Yet, addition of Palladium does not result directly into an improved bimetallic system over the monometallic Nickel catalyst as most of the Palladium in monometallic system is on the outside of the support caused by large sintering. Here we show that recrystallization of the Palladium nitrate source, containing Palladium oxide particles, by using fuming nitric acid as a recrystallization agent leads to enhanced stability of the precursor solution. During synthesis of Palladium catalysts different Palladium nitrate sources were used containing large differences in purity. As a result, the optimal precursor conditions leading to the most active and stable catalyst were not affected by the acidity of the solution but by the fast sintering of Palladium (oxide) particles. These results show that recrystallizing Palladium nitrate sources by fuming nitric acid and vacuum evaporation leads to a general form of Palladium nitrate consisting of little to no Palladium oxide. By repeating the recrystallization with fuming nitric acid ultimately could lead to pure Palladium nitrate sources and enhanced stability of precursor conditions showing no sign of particle formation. Most of the Palladium metal can then enter the support leading to Nickel-Palladium alloying and optimal ratio and weight percentage of metal can be characterized improving the catalyst performance making large CO2 emissions useful for reusage.