Calorimetric cell

In this microcalorimeter, the heat sink is not a massive metal block but is divided into several parts which are mobile with respect to each other. Each thermoelectric element (E) and a cell guide (D) are affixed to a fluxmeter holder (C). The holder (C) is mobile with respect to a massive arm (B) which, in turn, rotates around a vertical axle (A). All parts of the heat sink are made of brass. Surfaces in contact are lubricated by silicone grease. Four thermoelectric elements (E) are mounted in this fashion. They enclose two parallelepipedic calorimetric cells, which can be made of glass (cells for the spectrography of liquids are particularly convenient) or of metal (in this case, the electrical insulation is provided by a very thin sheet of mica). The thermoelectric elements surrounding both cells are connected differentially, the Petit microcalorimeter being thus a twin differential calorimeter. 

The purpose of the particular arrangement of the heat sink in the Petit microcalorimeter is to ensure an excellent and reproducible contact, at any temperature, between the surface of the thermoelectric elements and the outside walls of the calorimetric cells (31) and, moreover, to avoid... 

The adsorption cell, connected to the volumetric line out of the calorimeter, constitutes a path for thermal leakage and, for instance, heat is transferred continuously from the calorimetric cell to the outside via the... 

Heat-flow microcalorimetry may be used, therefore, not only to detect, by means of adsorption sequences, the different surface interactions between reactants which constitute, in favorable cases, the steps of probable reaction mechanisms, but also to determine the rates of these surface processes. The comparison of the adsorption or interaction rates, deduced from the thermograms recorded during an adsorption sequence, is particularly reliable, because the arrangement of the calorimetric cells remains unchanged during all the steps of the sequence. Moreover, it should be remembered that the curves on Fig. 28 represent the adsorption or interaction rates on a very small fraction of the catalyst surface which is, very probably, active during the catalytic reaction (Table VI). It is for these... 

One of the conclusions deduced from the thermochemical cycle 2 in Table V, for instance, is that in the course of the catalytic combustion of carbon monoxide at 30°C, the most reactive surface sites of gallium-doped nickel oxide are inhibited by the reaction product, carbon dioxide. This conclusion ought to be verified directly by the calorimetric study of the reaction. Small doses of the stoichiometric reaction mixture (CO + IO2) were therefore introduced successively in the calorimetric cell of a Calvet microcalorimeter containing a freshly prepared sample of gallium-doped... 

For a well-designed set-up, and if the main and reference experiments are performed under the same conditions, it is fair to assume that the reflected energies will be small and that E[ Er. With regard to the transmittance and the luminescence energies, we have to consider two possibilities. If the calorimetric cell is opaque, then these terms will all be zero, that is,... 

On the other hand, if the calorimetric cell is transparent, then Et, E[, E, and E[ have to be determined for example by using data for the transmittance of the media and fluorescence and phosphorescence quantum yields (see following discussion). 

The initiation system consists of a nitrogen laser and the necessary optics to lead the beam to the sample cell. The laser emits pulses at 337.1 nm with 800 ps duration, with a typical repetition rate of less than 5 Hz. The optical components, aligned between the laser and the calorimetric cell, consist of an iris (I), a support for neutral density filters (F), and a collimating lens (L). The iris is used to cut out most of the laser output and allow only a thin cylinder of light to pass through its aperture, set to 2 mm. The laser energy that reaches the cell is further... 

Thermal Analyzer equipped with a differential calorimetric cell. TGA thermograms were obtained on a DuPont 1090 Thermal Analyzer. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. 

The duration of each dosing experiment is about 15-50 minutes (depending on the sample and of the time constant of the calorimeter), which was long enough to yield well-resolved heat-flow peaks and a stable horizontal baseline of the microcalorimeter. For all catalysts presented here, adsorption always reached thermodynamic equilibrium. Prior to adsorption measurements, the samples were pretreated in the calorimetric cell by heating overnight under vacuum. 

Let us now consider how these quantities are related to experimentally determined heats of adsorption. An essential factor is the condition under which the calorimetric experiment is carried out. Under constant volume conditions, AadU 1 is equal to the total heat of adsorption. In such an experiment a gas reservoir of constant volume is connected to a constant volume adsorbent reservoir (Fig. 9.3). Both are immersed in the same calorimetric cell. The total volume remains constant and there is no volume work. The heat exchanged equals the integral molar energy times the amount of gas adsorbed ... 

Adiabatic The temperature of the sample results from its thermal activity. This technique gives direct access to the thermal runaway curve. The results must be corrected by the adiabacity coefficient, since a part of the heat released in the sample is used to increase the temperature of the calorimetric cell. This rends the kinetic evaluation complex. 

In this equation, cw stands for the heat capacity of the calorimetric vessel. Traditionally, this correction was performed via the Water Equivalent of the calorimeter. This means that the heat capacity of the calorimetric cell is described by a thermally equivalent mass of water. 

Microcalorimeters are well suited for the determination of differential enthalpies of adsorption, as will be commented on in Sections 3.2.2 and 3.3.3. Nevertheless, one should appreciate that there is a big step between the measurement of a heat of adsorption and the determination of a meaningful energy or enthalpy of adsorption. The measured heat depends on the experimental conditions (e.g. on the extent of reversibility of the process, the dead volume of the calorimetric cell and the isothermal or adiabatic operation of the calorimeter). It is therefore essential to devise the calorimetric experiment in such a way that it is the change of state which is assessed and not the mode of operation of the calorimeter. 

The most common calorimetric technique is the discontinuous procedure where the adsorptive is introduced in successive steps. The calorimetric cell with its contents (adsorbent and adsorptive) must be considered as an open system (cf. Figure 3.15). It is only when the adsorptive is introduced reversibly and when the step is small enough (so that the amount introduced can be written d n and the pressure increase dp) that the derivation of a differential energy of adsorption (as defined by Equations (2.49) and (2.50)) is possible. Under these conditions, and taking into account the internal energy contributed by the gaseous adsorptive, we can write ... 

Rouquerol and Everett (Rouquerol et ai, 1980) have shown that the calculation of dWrev is easily accomplished if one notionally splits the volume of the whole adsorption system into two parts, VA (external to the calorimetric cell, but in contact with the thermostat) and Vc (located within the calorimetric cell, see Figure 3.15). If we assume a reversible compression of an ideal gas by reduction of volume the whole system exchanges work with the surroundings ... 

It is evident that a knowledge of f° (rate of adsorption) and 0 (heat flow) is not enough to derive a continuous curve of differential enthalpy of adsorption. One must also know the dead volume Vc of the calorimetric cell proper and the derivative of the quasi-equilibrium pressure with time. Note that when this derivative is very small (i.e. in the nearly vertical parts of an adsorption isotherm), Equation 2.82 becomes simply ... 

Introduction of sample bulb (2) into stainless steel calorimetric cell already filled with immersion liquid (7). 

Micro-calorimetric adsorption measurements require a proper in situ vacuum activation at a higher temperature than the adsorption process. A first pre-treatment under oxygen is performed in the calorimetric cell in order to eliminate the impurities present on the sample (essentially carbonates, nitrates, carbonaceous residues and water present from the preparation, calcination and exposure to atmosphere) and to avoid the partial reduction of the surface of an oxide that is easily reduced under vacuum. 

An adiabatic low-temperature c orimeter designed specifrcally for a vapor-deposited sample is depicted in Fig. 1. This calorimeter equips with a built-in cryorefrigerator which enables to keep the calorimetric cell at cryogenic temperatures for a long time. 

The last term in Eq. (48) is dependent on the experimental apparatus and can be eliminated by differential assembly of the calorimetric cells. Hence for a differential calorimeter Eq. (48) becomes... 

The flow-mix mode of operation allows a fast reaction (i.e. the reaction is rapid in comparison with the time constant of the instrument). Here, the reaction is initiated inside the calorimetric cell by the mixing of the pre-equilibrated reagents as they enter the cell. Figure 1. The effluent is not recycled but can be retained for further analysis. 

In both instances, the calorimeter records the change in heat through semi-conducting thermopiles positioned around the calorimetric cell. The data are recorded as a function of time and is displayed as J s. ... 

The basic operation of the gaseous flow calorimeters is essentially identical to that of the flow-through solution-phase calorimeters with an external gas/vapour source that is passed, through a single calorimetric cell, across the solid of interest and the resulting heat change measured. For these instruments, the detectors are thermistors in direct contact with the solid under study. The form of the returned data is volts as a function of time. The signal can be converted to J s via a calibration constant. 

Figure 3 Schematic for the flow-through calorimetric cell...

---