A convenient method for the synthesis of 2 nm supported bimetallic nanoparticle Pt-Cu catalysts for propane dehydrogenation is reported here. In situ synchrotron X-ray techniques allow for the determination of the catalyst structure, which is typically unobtainable using laboratory instruments.
The overall goal of this research is to synthesize new bimetallic catalysts with small uniform particle size and test their performance for alkane dehydrogenation. The goal is to understand the fundamental principles which lead to high olefin selectivity, high rate, long life and thermal stability. This catalyst synthesis method optimizes anchoring of metal species and to support by preparing a solution with a pH that charges the support surface and checks metal ions of opposite charge.
Careful control of the calcination and reduction temperatures is needed to achieve small particle size, co-impregnation of the properly chosen metal precursors ensures strong bimetallic interaction which ultimately controls the activity and selectivity. After preparation, we measure the rates, selectivity and stability of the catalysts for alkane dehydrogenation to determine the differences in performance with catalyst composition and correlate these with the structure. First, carefully weight approximately five grams of dry silica on weigh paper and transfer it into a weighing dish.
While mixing, add water drop-wise until the silica is completely wet, but with no excess solution. Then, reweigh the wet silica to calculate the amount of water absorbed and to determine the pore volume of the silica support. To prepare the precursor solution, dissolve 0.125 grams of copper nitrate trihydrate in one milliliter of water in a small vile to obtain a sky-blue solution.
Add ammonia drop-wise to the copper-nitrate solution forming dark-blue precipitates of copper hydroxide. Continue adding ammonia until the dark-blue precipitates dissolve to form a dark-blue solution and the pH is greater than 10. Next, add 0.198 grams of tetraammineplatinum nitrate to the solution.
Then, add water so that the total volume of the solution is 3.5 milliliters which matches the pore volume of five-gram silica support. Heat the solution to 70 degrees Celsius until all the tetraammineplatinum nitrate salts are dissolved. After allowing the dissolved metal precursor solution to cool to room temperature, add a few drops at a time to five grams of silica in a ceramic evaporating dish and stir gently to break up the particles that stick together to achieve a homogeneous distribution of the solution.
Dry the impregnated silica-supported catalyst with a copper-to-platinum ratio of 0.7 in an oven at 125 degrees Celsius overnight. On the following day, calcine the cooled catalyst precursor in a furnace at 250 degrees Celsius with a five-degree Celsius permanent ramp rate in air for three hours. Next, transfer the calcined catalyst to a tube furnace for reduction.
Place a one-inch layer of quartz wool in the middle of a one-inch quartz tube reactor and load the cooled calcined catalyst into the tube through a plastic funnel. Then, place the tube in a clamshell temperature programmed furnace. After purging the tube with nitrogen for five minutes at room temperature, switch the flow to 5%hydrogen balanced in nitrogen at the same flow rate as nitrogen to reduce the catalyst.
Increase the temperature to 150 degrees Celsius with a five-degree Celsius permanent ramp rate and hold for five minutes. Now, start slow-ramping of the temperature at a rate of 2.5 degrees Celsius per minute to 250 degrees Celsius holding the temperature for 15 minutes after every 25-degree-Celsius increase. Ramp to 550 degrees Celsius at 10 degrees Celsius per minute and stay for 30 minutes to complete the reduction.
Switch the 5%hydrogen flow back to pure nitrogen to purge the system and cool to room temperature. Then, unload the catalyst and store it in a vial for future use. Place a half-inch layer of quartz wool against the dimple in the middle of a 3/8-inch quartz tube reactor.
Next, mix 40 milligrams of silica-supported catalyst precursor with a copper-to-platinum ratio of 0.7 and 960 milligrams of the silica in an empty vial to dilute the catalyst. Using a plastic funnel, load the catalyst mixture into the tube reactor. Wipe the outer wall of both tube ends with lint-free wipes to remove any dirt and to get a good seal with the O-ring.
Connect the tube fittings to both ends of the quartz tube reactor and attach them to the reactor system equipped with a clamshell furnace. Following this, turn on the nitrogen flow through the tube reactor. After one minute, close the ball valve on the reactor outlet.
After waiting for the system pressure to increase to five pounds per square inch gauge, close the ball valve on the inlet nitrogen line to stop the nitrogen flow and seal the reactor system. After one minute, record the pressure reading from the gauge. Open the ball valve on the reactor outlet to release the pressure before restarting the nitrogen flow by turning on the ball valve on the inlet nitrogen line to purge the system for one minute.
Start flowing hydrogen diluted in nitrogen for catalyst reduction before running the reaction and stop the nitrogen flow. Start heating the tube reactor to 550 degrees Celsius with a rate of 10 degrees Celsius. For propane dehydrogenation reaction testing, start the gas chromatograph, or GC, in the reactor system and select the proper method for gas component analysis.
Now, switch the reactor gas flow to a bypass line. Flow 100 cubic centimeters per minute of 5%propane and nitrogen diluted in 5%hydrogen diluted in nitrogen. After the propane flow stabilizes, inject the bypass flow into the GC as a reference sample.
Next, switch the gas flow back to the reactor tube line to start the reaction and record the time. After the reaction runs for four minutes, inject the reactor outlet gas flow into the GC to get the outlet gas component information. Finally, use the corresponding peak analysis software to analyze each peak.
The propylene selectivity versus time for platinum and platinum-copper catalysts is presented here. While the propylene selectivity of the platinum catalysts decreases at higher conversion, the silica-supported catalyst with a copper-to-platinum ratio of 7.3 retains high propylene selectivity at different propane conversions. The catalyst's selectivity increases almost linearly with a copper content in the platinum-copper catalysts.
Higher copper content also improves the turnover rates per mole of surface platinum for propane dehydrogenation. There is a close-to-linear relationship between the turnover rate and the catalyst atomic ratio of copper-to-platinum with the carbon balance close to 100%during all reaction tests. The average particle size of the monometallic platinum and platinum-copper catalysts determined by stem imaging are between two and three nanometers.
The change of the scattering pattern in the x-ray absorption fine structure spectra of the catalysts with increasing copper-to-platinum ratio suggests the formation of bimetallic nanoparticles with increasing copper content. The XRD pattern of platinum and platinum-copper catalysts showed that their composition differs from the ideal composition of ordered alloys and there is no peak from superlattice diffraction indicating that platinum and copper form a disordered solid solution structure in the catalysts. The diffraction peaks shift to higher angles with increasing copper-to-platinum ratio confirming that the solid solution becomes richer in copper.
Once mastered, the impregnation step can be done in about one hour and the consistency like small particles with uniform composition. When preparing metallic alloys by impregnation, it's important to adjust the pH value of the solution and use proper metal complexes according to the type of the support. The solution volume should be equal to the pore volume of the support.
After watching this video, you should have a good understanding of how to synthesize supported bimetallic catalysts and test their alkane dehydrogenation performance. This method is broadly applicable and can be used for different catalyst compositions and for many chemical reactions. Don't forget that mixing hydrogen with air in the presence of a metallic catalyst is extremely dangerous and may lead to explosions.
You must always purge the reactor with nitrogen before and after hydrogen addition to the catalyst.
