Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow

The overall goal of the following experiment is to characterize the nature of stable radicals on the surface of solid carbon substrates and the effect of oxidation on those radicals. This is achieved by preparing dried carbon samples at 95 degrees Celsius in a vacuum oven. As a second step, a Diamagnetic gas flow is flashed through the samples and the nature of the carbon radicals is detected by EPR spectroscopy, which characterizes the nature and rate of formation of carbon radicals in an inert environment.

Next air or oxygen is flowed through the carbon sample in order to follow the disappearance of carbon radicals in an oxidizing environment. The results show that carbon substrates are affected differently by oxidation treatment, depending on their composites and poor density based on B-E-T-F-T-I-R and EPR measurements. The implications of this technique extend toward solid energy resources such as coal.

Because these oxidation processes have important implications to proper storage and utilization. Though this method can provide insight into surface functional groups on solid surfaces, it can also be applied to other systems such as zeolite, meso, post materials, and monolayers. We first had this idea when we found out that various skull samples are characterized by different radical natures.

Prior to starting this procedure, prepare carbon samples for EPR measurements by heating the samples in open glass vials under an inert environment inside a vacuum oven. Then fill each vial with a coal sample and stopper each sample vial with a rubber septum and an aluminum cap to remove all traces of oxygen. Connect the vial to a vacuum system and seal all of the valves.

Turn on the vacuum pump. After opening the vacuum valve, wait until the monitors show a vacuum of approximately 0.1 millibar. Make sure leakage is minimal by closing the vacuum valve and counting to 30.

Open the sample valve to remove the atmosphere in the vial. Once the pressure returns to the initial pressure value determined in the previous step, repeat the leakage test. After achieving a vacuum and effectively purging the vials of the remaining atmosphere, close the vacuum valve.

Then immediately open the inert gas valve and allowed the pressure to reach 0.5 atmospheres. Following this, close the inert gas valve and open the vacuum valve to remove the gas. After the system has been purged, close the vacuum valve and immediately open the gas valve, allowing the pressure to reach 0.5 atmospheres.

Once the system has been purged a second time, open the gas valve and allow the pressure to reach one atmosphere. After closing the gas valve and the sample valve, remove the vial by gently pulling downward and removing the needle. Next, open the vacuum valve to purge the gas from the vacuum system.

Open the sample valve to allow air into the system and simultaneously turn off the vacuum pump to prevent a backflow of the oil. At this point, gently turn the open end of a freshly rinsed EPR tube into the vial containing the carbon sample. Depress and turn the EPR tube.

Then gently tap until the sample has evenly dispersed at the bottom. Once the tube contains a sufficient amount of sample, seal the tip using about 0.5 to one centimeter of rubber Teflon putty. To set up the flow system, insert the quartz tube into the EPR resonator, making sure that the section of the EPR tube filled with the coal fills the entire resonator cavity.

Set up a tank with the desired flow gas, making sure there are two operation valves in order to control the flow. Following this connect rubber tubing to the tank and ensure that the tubing reaches the tip of the EPR quartz tube with enough pull so as not to put strain on the quartz tube. Connect a flow controller to the rubber tubing to monitor the gas flow.

After attaching a small gauge needle to the rubber tubing, insert the needle through the rubber Teflon putty until it is about three to four centimeters above the sample, so as not to affect the magnetic field. Then poke a hole in the rubber putty to release the outflow gas. After turning on the EPR spectrometer, open the microwave tuning panel.

Locate the dip at 33 decibel power and use Auto-Tune for obtaining the best tuning conditions. Once the microwave power has been set to two milliwatts, open the 2D experiment as a function of magnetic field and time. Then set the appropriate parameters of the experiment.

Next, start the measurement cycle and turn on the gas flow. After the sample has reached equilibrium, and there is no further change in the EPR line shape, stop the gas flow, expose the sample to air and continue with the measurements. Until 50 spectra are obtained or until equilibrium is reached.One.

When performing the EPR experiments on various coal samples, a second species at a G value of around 2.0028 was observed. This G value is close to the value of a free electron and consistent with unsu substituted atic carbon centered radicals. However, the total spin concentration for each sample remained constant within plus or minus 10%Experimental error shown here are zero seconds and 1900 seconds scans after the coal sample was exposed to carbon dioxide.

The EPR spectrum at 1900 seconds is characterized by two species. The first at a G value of 2.004 with a line width of five 5G, and a second species, which is much narrower at a G value of 2.0028 with a line width of 2.0 G.It was found that the rate of formation of this second species is approximately 500 seconds. Interestingly enough, the extent of formation of this second species after stabilization is similar for all coal samples and was evaluated as four to 5%relative to the initial spin concentration.

The rest of the spins correspond to either carbon centered radicals or a carbon centered radical with an oxygen atom. The different G values of the dominant radical species in each sample are dependent on the nature of the coal. Since the kinetics of formation of this second species for each coal sample is different, it should be related to the sample's pore area and or the surface functional groups.

In order to better characterize these functional groups, other techniques such as BET and NMR are required to supplement the EPR data. While attempting this procedure, it's important to remember to dry the cold samples effectively. This step is crucial in order to enable the creation of a baseline where no water molecules are reabsorbed on the call substrates.

Following these procedures, other methods like elemental analysis NMR BT and FT IR can be performed in order to answer additional questions like, what is the nature of the functional group, the porosity of the samples and the compositions. After watching this video, you should have a good understanding of how to conduct the simple characterization of solid surface with radical under oxygenation treatment.