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Figure 2.1: Schematic diagram of TPD-TGA system

Simultaneous thermogravimetric analysis (TGA) and temperature programmed desorption (TPD) was found to be one of the most useful methods available for the initial

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TGA unit in this system has an accuracy of 0.01 mg and allows the measurement of weight changes as small as 0.01%. After exposure to the vapors of interest, the sample can be evacuated by a diffusion pump, which ensures that the pressure above the sample is low enough to minimize secondary reactions. Desorbing species can be monitored with high sensitivity by a mass

spectrometer. From the TPD-TGA results, one can quantify the desorption amount and determine whether there is a reaction on the sample during desorption.

In a typical TPD-TGA experiment, the sample is first heated to 830 K in vacuum to remove water and other impurities. After cooling to room temperature, the sample is exposed to several Torr of the probe molecule of interest. After evacuation, the temperature is then ramped at 10 K/min to 830 K, while the sample mass and desorbing products are monitored.

If readsorption is negligible, the rate of desorption from a unit surface area can be written as:

𝑁(𝑡) = −

𝑑𝑡

= 𝑣

𝜎

𝑛

_{𝑒𝑥𝑝 (}

𝑅𝑇

) 2.1)

Where: 𝑁 is the desorption rate

σ is the surface coverage (molecules/cm2) 𝑛 is the order of the reaction

𝑣𝑛is the rate constant

𝐸 is the activation energy of reaction(kJ/mol)

Because the temperature is linearly ramped to 830 K, temperature of the system can be written as a function of time 𝑇 = 𝑇0+ 𝛽𝑡. If we assume that the reaction is first order and the

activation energy 𝐸 is independent of σ, then the expression of peak temperature 𝑇𝑝 below can be

21 𝐸 𝑅𝑇_𝑝2

=

𝑒𝑥𝑝 (

) 2.2)

While this description is not appropriate for desorption from porous catalysts due to readsorption effects, the approach is useful for describing decomposition reactions where the products leave the sample once they are formed [20]. In this circumstance, the activation energy of the reaction may be obtained from the peak temperature of the products using Eqn 2.2. For example, a TPD-TGA of 1-propanol over TiO2 is shown in Fig. 2.2. The dehydration reaction of 1-propanol into propylene and water occurs during TPD and propylene cannot readsorb on titania. Using the peak temperature for propylene of 596 K, the activation energy for dehydration of 1- propanol can be calculated to be 171 kJ/mol by assuming a normal reaction pre-exponential of 1013 _s-1_.

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2.1.2 Calorimetric measurement

Calorimetry is a direct method for measuring heats of adsorption for gaseous adsorbates on acid sites. Our home-built, Calvent-type calorimeter allows the use of relatively large samples (0.5g~1g) spread into very thin beds (~1 mm thick) for rapid adsorption and heat transfer. As shown in Fig. 2.3, the calorimeter cell is surrounded on five of six sides by highly sensitive thermopiles which generate a voltage proportional to the heat flow released by adsorption. The calorimeter cell is enclosed in a large aluminum block, which in turn is kept within a Styrofoam cooler, to maintain isothermal conditions. A dosing loop is connected to a six-port valve which allows calibrated volumes of gas to be admitted into the calorimeter cell. Two pressure

transducers are separately positioned on the dosing system, which is used to fill the dosing loop, and the calorimeter cell.

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The dosing procedure involves switching the six-port valve to the dosing side, filling the dosing loop with the adsorbate gas, and then switching the six-port valve to the calorimeter side. Usually, adsorption equilibrium is achieved in approximately 20 min. The amount of gas

introduced to the dosing loop and the amount remaining at equilibrium can be determined by the volume, pressure, and the temperature.

In a typical measurement, the samples were heated in the evacuated calorimeter cell to ~600 K overnight to clean the sample before beginning to expose the sample to gaseous adsorbates. Aliquots of adsorbate were then dosed onto the sample by switching the six-port valve from dosing side to the calorimeter side. Dosing continued until saturation coverage was reached. The adsorption isotherm was obtained simultaneously with the heats by measuring the amount of gas remaining at equilibrium with the pressure above the sample.

2.1.3 FTIR

Fourier Transform Infrared Spectroscopy (FTIR) is another useful technique to study the adsorption properties on the catalyst. Comparing spectra before and after adsorption of certain probe molecules provides a convenient method to characterize solid Lewis acid catalysts. The spectra were recorded using Mattson Galaxy 2020 FTIR spectrometer which has an accuracy of 2 cm-1 _{resolution. A Spectra-Tech diffuse-reflectance accessory (Collector II), which allowed} programmed heating during measurements on powder samples, was available on the FTIR spectrometer.