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In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework
Chapters
Summary February 1st, 2020
The use of FTIR spectroscopy for investigation of the surface properties of polycrystalline solids is described. Preparation of sample pellets, activation procedures, characterization with probe molecules and model studies of CO2 adsorption are discussed.
Transcript
Adsorption heterogeneous catalysis sensing too many important processes occurring on solid surfaces. To successfully design new, effective materials, it is necessary to understand in detail the gas/solid interaction. In situ infrared spectroscopy is one of the most useful techniques for this purpose.
In this video, we show the protocol we use for infrared characterization of the surface of polycrystalline solids in studies of gas/solid interaction. Enjoy video. Spread uniformly using a grit about 20 milligrams of the sample powder on the polished surface of a pressing die.
If the powder sticks to the metal surface, use mica or clear packing tape glued to the die. Place on top another die with the polished side facing the powder. Ensure even distribution of the specimen with several gentle rotating movements.
Then, put the two cylinders in a hydraulic press and apply 0.2 tons of pressure. After about two minutes, reduce slowly the pressure and remove the cylinders from the press. If the pellet is not formed, repeat the procedure, applying a higher pressure.
Using a scalpel or a blade, cut a piece of the pellet with dimensions about 10 by 10 millimeters. Measure the geometric surface and the weight of the pellet. Place the pellet into the sample holder.
Put the sample holder into the IR cell, and move the sample to the middle of the oven zone. Connect the cell to the vacuum/adsorption apparatus, placing between them a reservoir with known volume, in this case, about 0.5 milliliters. Evacuate the system.
Adjust the activation temperature to 573 Kelvins, recommended heating rate between two and five Kelvins per minute. Then evacuate the sample with this temperature for one hour. Using a magnet, move the pellet outside the oven, and wait for 10 minutes in order to reach room or ambient temperature.
During that time, register a background spectrum. Then, move the pellet to the IR beam path, and register the sample spectrum. The infrared spectrum of the sample gives rather poor information about its surface.
That is why the adsorption of the so-called probe molecules is used for obtaining detailed information. The probe molecules are substances which are specifically absorbed. Based on their IR spectra, or on the changes they caused in the spectra of the soil, one can make conclusions about the type and properties of the adsorption centers.
Ensure the sample is situated on the IR beam path. Introduce a small dose, namely 0.5 micromoles of the adsorbate to the cell, in this case deuterated acetonitrile. Record an IR spectrum.
Then, introduce a second dose of the adsorbate and repeat the procedure. Do this until no more changes in the spectrum occur. Evacuate the sample recording spectra until no more changes occur.
Then, move the sample to the oven with a preset temperature of 323 Kelvins. After 15 minutes evacuation at this temperature, place the pellet outside the oven and wait for 10 minutes in order to reach ambient temperature. During that time register a fresh background spectrum.
Move the pellet to the IR beam path and register the sample spectrum. Repeat the procedure increasing the oven temperature with steps of 50 Kelvins, until obtaining a spectrum coinciding with the initial sample spectrum. To prevent deep cooling of the cell windows during the low-temperature experiments, first turn on the water circulation system.
Then ensure the sample is situated on the IR beam path. Fill in the cell reservoir with liquid nitrogen, and keep it full during the whole experiment. After cooling the sample, record a spectrum.
Then introduce adsorbate, in this particular case carbon monoxide, on successive small doses, 0.5 micromoles each. Record a spectrum after each dose. Finish these set of experiments with zero equilibrium pressure of two millibars.
Then start to decrease the equilibrium pressure, first by dilution and then by evacuation at low temperature, again recording spectrum. Mark the pressure in each spectrum. When no more changes occur, stop filling the reservoir with liquid nitrogen and record spectra under dynamic vacuum at increasing temperature.
problem that could be solved by processes involving adsorption. Here we present the result of the characterization of the UiO-66, as well as the proper use and enhancement of its adsorption capacity towards carbon dioxide. The IR spectrum of UiO-66 registered after evacuation at ambient temperature contains bands due to the linker, residual dimethylformamide, terephthalic acid and esters, isolated and H-bonded structural OH groups.
Evacuation at 573 Kelvins leads to almost full disappearance of the residuals and of the structural hydroxyls. That is, the sample is practically clean and dehydroxylated. Adsorption of acetonitrile, a probe molecule for assessing acidity, on the just evacuated sample reveals existence of Bronsted acid sites, hydroxyl groups, through C-N stretching bands at 2276 and 2270 reciprocal centimeters.
At the same time, the OH band is red shifted by 170 and 250 reciprocal centimeters, indicating a weak Bronsted acidity. With sample activated at 573 Kelvins, the bands indicating Bronsted acidity are practically absent, which is consistent with the observed sample dehydroxylation. However, a band at 2299 reciprocal centimeters, due to acetonitrile on Zirconium 4+Lewis acid sites, is well seen.
Low temperature CO adsorption on a sample evacuated at ambient temperature revealed CO polarized by OH groups through a band at 2153 reciprocal centimeters. Simultaneously, the original OH band is red-shifted by 77 reciprocal centimeters, confirming the weak acidity of the hydroxyls. With a sample evacuated at 573 Kelvins, a very weak band due to CO polarized by hydroxyl groups was detected at 2154 reciprocal centimeters, confirming again the low hydroxyl concentration in the sample.
Importantly, no CO coordinated to Zirconium 4+sites was detected. This observation shows that the Lewis acid sites can be monitored only by relatively strong bases, as acetonitrile, probably via structural rearrangement in the Zirconium 4+environment. Carbon dioxide was put in contact with a sample evacuated at 573 Kelvins.
The adsorbed CO2 is monitored by the antisymmetric stretching modes at 2336 reciprocal centimeters. Then, water was introduced into the system, which led to gradual development of a high-frequency shoulder at 2340 reciprocal centimeters, which finally dominated the spectrum in the region. In concert, bands due to isolated and H-bonded structural hydroxyls developed.
The results show that water vapor hydroxylates the sample, creating structural hydroxyl groups that act as CO2 adsorption sites. This observation is important because evidences that CO2 adsorption could be enhanced in humid atmosphere and reveals the mechanism of this phenomenon.
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