# Determination of Differential Enthalpy and Isotherm by Adsorption Calorimetry

Authors: V. Garcia-Cuello; J. C. Moreno-Piraján; L. Giraldo-Gutiérrez; K. Sapag; G. Zgrablich
Journal: Research Letters in Physical Chemistry
 (2008)

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## Abstract

An adsorption microcalorimeter for the simultaneous determination of the differential heat of adsorption and the adsorption isotherm for gas-solid systems are designed, built, and tested. For this purpose, a Calvet heat-conducting microcalorimeter is developed and is connected to a gas volumetric unit built in stainless steel to record adsorption isotherms. The microcalorimeter is electrically calibrated to establish its sensitivity and reproducibility, obtainingK=154.34±0.23 WV−1. The adsorption microcalorimeter is used to obtain adsorption isotherms and the corresponding differential heats for the adsorption of CO2 on a reference solid, such as a NaZSM-5 type zeolite. Results for the behavior of this system are compared with those obtained with commercial equipment and with other studies in the literature.


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## Body

## 1. Introduction

It is widely accepted that the knowledge of adsorption
heats is vital in the description of gas-solid interaction. This is
particularly useful when adsorption heat measurements are combined with
simultaneous measurement of the adsorption isotherm. These measurements
obviously may provide information about the energetic of surface processes. In
some simple cases, even information on the structure of the surface itself,
like for example the energetic topography, can be retrieved from adsorption
heats and isotherms [1, 2]. Chemisorption
and catalyzed reactions, like any chemical reaction, are associated with
changes of enthalpy and can therefore be studied by means of calorimeters. Many
calorimeters, operating on different principles, have indeed been used for this
purpose [3–5]. Adsorption calorimeters are particularly
convenient for these studies [4]. They offer a number of advantages which will
be illustrated by means of selected examples.Adsorption calorimetry,
preferably in association with other physicochemical or physical techniques,
may be used to describe the surface properties of a solid.Information on the binding
energy, deduced from calorimetric data, is needed to achieve a theoretical
description of the adsorbate-adsorbent bond. It has been shown, for instance,
that, in the case of the adsorption of hydrogen on nickel-copper alloys, a
correlation between heats of adsorption and surface magnetic properties can be
found. The correlation indicates that the energy of the bond between adsorbed
hydrogen and nickel atoms is regulated by the electron density of states, near
the Fermi level, for the metal surface [6–8].In these works,
we present the design, construction, and test of an adsorption microcalorimeter
capable of measuring simultaneously adsorption isotherms and heats.


## 2. Experimental

### 2.1. Description of the New Microcalorimeter

Figure1 shows
a complete view of the adsorption calorimeter built here, which is not very
common and has not been considered in the literature.Figure 1
General scheme of station for the simultaneous measurement of isotherm and heat of adsorption.A detailed general view of the equipment calorimeter is shown in Figure1. The diagram shows microcalorimeter with the calorimetric cells made of stainless steel (sample and reference), which are embedded inside a large
block (also divided in two parts) in stainless steel, which acts as deposit of
the thermostatic liquid. Due to its thermal diffusion coefficient, this set
allows the rapid heat conduction towards the surrounding of the calorimeter. The
whole set is placed inside a nylon block to isolate it from the surroundings
and to allow the rapid stabilization of the temperature. The thermal effects
are sensed through ten thermopiles and trademark Melcor Corporation, NJ, USA, connected in series to increase the sensitivity of the microcalorimeter. The
microcalorimeter designed in this work connected to the adsorption system
constructed specially for this equipment in stainless steel to allow the
simultaneous measurement of the heat of adsorption and the isotherm. The
connection is through two pressure transducers, one in the range of high
pressure (1000 Torr), and the other in the range of low pressure (10 Torr) (see
Figure 1).


### 2.2. Electric Calibration of the Adsorption Microcalorimeter

In order
to establish the correct functioning of the microcalorimeter, which is then
connected to the volumetric adsorption unit, the sensitivity is evaluated
determining the calorimeter constant. The calibration constant reports the
voltage generated by the calorimeter when a heat flow is emitted from inside
the microcalorimetric cell. There are two methods to determine the calibration
constantK:  by application of electric
power and by the stationary method [9, 10].


### 2.3. Description
of the Unit for Simultaneous Measurement of Isotherm and Adsorption Heat

Heats of adsorption have been measured at 273 K by means of the adsorption microcalorimeter and by contacting the
solid with small successive doses of the adsorptive. This allows
the evolution of the interaction energy along with the coverage to be measured.
In the system, an ultra-high vacuum
pump (Pfeiffer Vacuum Ref. TSH 071E) is previously connected to an oil rotary pump which
initially allows to have a previous vacuum in all the system. Once the system
has a pressure of about10−3 Torr, the ultra-high vacuum pump starts
working and is kept functioning until the pressure reaches at least 10−5 Torr.
This part of the station is also composed of a joint built all in one unit, all
in stainless steel, which was previously calibrated and was specially designed to
obtain precise and accurate measurements. This is a novel contribution to the
research equipments normally used in this type of measurements where this part
consists of equipment constructed in glass with the problems
associated with it. The cell containing
the sample is also shown as well as the pirani pressure transductor which is connected to
a computer through an interface RS-232.The
differential heat of adsorption is obtained directly from the calorimetric, measuring the heat
evolved, as small
increments of adsorbate are added. This method is the one used in this work.


## 2.1. Description of the New Microcalorimeter

Figure1 shows
a complete view of the adsorption calorimeter built here, which is not very
common and has not been considered in the literature.Figure 1
General scheme of station for the simultaneous measurement of isotherm and heat of adsorption.A detailed general view of the equipment calorimeter is shown in Figure1. The diagram shows microcalorimeter with the calorimetric cells made of stainless steel (sample and reference), which are embedded inside a large
block (also divided in two parts) in stainless steel, which acts as deposit of
the thermostatic liquid. Due to its thermal diffusion coefficient, this set
allows the rapid heat conduction towards the surrounding of the calorimeter. The
whole set is placed inside a nylon block to isolate it from the surroundings
and to allow the rapid stabilization of the temperature. The thermal effects
are sensed through ten thermopiles and trademark Melcor Corporation, NJ, USA, connected in series to increase the sensitivity of the microcalorimeter. The
microcalorimeter designed in this work connected to the adsorption system
constructed specially for this equipment in stainless steel to allow the
simultaneous measurement of the heat of adsorption and the isotherm. The
connection is through two pressure transducers, one in the range of high
pressure (1000 Torr), and the other in the range of low pressure (10 Torr) (see
Figure 1).


## 2.2. Electric Calibration of the Adsorption Microcalorimeter

In order
to establish the correct functioning of the microcalorimeter, which is then
connected to the volumetric adsorption unit, the sensitivity is evaluated
determining the calorimeter constant. The calibration constant reports the
voltage generated by the calorimeter when a heat flow is emitted from inside
the microcalorimetric cell. There are two methods to determine the calibration
constantK:  by application of electric
power and by the stationary method [9, 10].


## 2.3. Description
of the Unit for Simultaneous Measurement of Isotherm and Adsorption Heat

Heats of adsorption have been measured at 273 K by means of the adsorption microcalorimeter and by contacting the
solid with small successive doses of the adsorptive. This allows
the evolution of the interaction energy along with the coverage to be measured.
In the system, an ultra-high vacuum
pump (Pfeiffer Vacuum Ref. TSH 071E) is previously connected to an oil rotary pump which
initially allows to have a previous vacuum in all the system. Once the system
has a pressure of about10−3 Torr, the ultra-high vacuum pump starts
working and is kept functioning until the pressure reaches at least 10−5 Torr.
This part of the station is also composed of a joint built all in one unit, all
in stainless steel, which was previously calibrated and was specially designed to
obtain precise and accurate measurements. This is a novel contribution to the
research equipments normally used in this type of measurements where this part
consists of equipment constructed in glass with the problems
associated with it. The cell containing
the sample is also shown as well as the pirani pressure transductor which is connected to
a computer through an interface RS-232.The
differential heat of adsorption is obtained directly from the calorimetric, measuring the heat
evolved, as small
increments of adsorbate are added. This method is the one used in this work.


## 3. Results

### 3.1. Electric Calibration of the Adsorption Microcalorimeter

The
calibration constants were obtained for the operation conditions of the
microcalorimeter. Constants between range134.11±0.19WV−1 to 156.67±0.23WV−1 are determined. These values
show the sensitivity of the microcalorimeter built here, which is higher than
that of equipments reported in literature and even of those built in our
laboratory previously. This constitutes a significant contribution to the
construction of this type of instruments. Values by the method of state
stationary condition were obtained and were of the order same and magnitude.


### 3.2. Isotherms and Differential Heats of Adsorption

Table1
reports the characterization results obtained with the equipment built here for
the probe sample, type NaZSM-5 zeolite, previously characterized in an Autosorb
Quantachrome 3B equipment. The superficial characteristics and microporosities
obtained with the two equipments are compared.Table 1
Comparison of principal superficial characteristics of NaZSM-5 zeolite.
NaZSM-5 ZEOLITEQuantachrome 3BTMMicrocalorimeter built in this workSμp-DRK-method (m2/g)285296Vμp-DRK-method (cm2/g)0.240.26These values
are evaluated from the adsorption ofCO2 at 298 K. The results show
a very good agreement between the commercial equipment and the microcalorimeter
built here, reinforcing the excellent functioning of this equipment. Figure 2
shows the adsorption isotherm of CO2 at 273 K obtained for the zeolite
analyzed in this investigation. This isotherm was reproduced also on the commercial equipment
with a good concordance, reinforcing again the satisfactory behavior of our
apparatus.Figure 2
Adsorption
isotherm for zeolite in CO2 at 273 K.It is interesting
to analyze jointly the data obtained from the adsorption isotherm (see Figure2) and those for the
differential heat of adsorption (see Figure 3). In Na exchanged ZSM-5 zeolites,
Na cations neutralize the acidity of the zeolite and develop the basicity for
adsorbing acidic CO2. Thus, NaZSM-5 provides two kinds of adsorption
sites for CO2: stronger sites around a Na cation (which saturates
rapidly) and weaker sites on the pore walls [11]. The steep increase of the
adsorption isotherm and the high value of qd at low pressure (p/po<0.01) in Figures 2 and 3 reveal in a clear way the presence of these
strong sites, which become rapidly saturated. Note also that the steep decrease
in qd at very low pressure from 48 to 46 kJ/mol is indicating that the
adsorption strength of these sites is not uniform (indication of energetic
heterogeneity). After the strong sites become saturated (p/po>0.01), qd steps down to a lower and almost constant value, corresponding to adsorption on
the zeolite walls, and simultaneously the amount adsorbed increases more slowly
in the isotherm. This is in [11] and shows the potentiality of the
microcalorimetric station.Figure 3
Differential heats of
adsorption for CO2 on NAZSM-5 zeolite.


## 3.1. Electric Calibration of the Adsorption Microcalorimeter

The
calibration constants were obtained for the operation conditions of the
microcalorimeter. Constants between range134.11±0.19WV−1 to 156.67±0.23WV−1 are determined. These values
show the sensitivity of the microcalorimeter built here, which is higher than
that of equipments reported in literature and even of those built in our
laboratory previously. This constitutes a significant contribution to the
construction of this type of instruments. Values by the method of state
stationary condition were obtained and were of the order same and magnitude.


## 3.2. Isotherms and Differential Heats of Adsorption

Table1
reports the characterization results obtained with the equipment built here for
the probe sample, type NaZSM-5 zeolite, previously characterized in an Autosorb
Quantachrome 3B equipment. The superficial characteristics and microporosities
obtained with the two equipments are compared.Table 1
Comparison of principal superficial characteristics of NaZSM-5 zeolite.
NaZSM-5 ZEOLITEQuantachrome 3BTMMicrocalorimeter built in this workSμp-DRK-method (m2/g)285296Vμp-DRK-method (cm2/g)0.240.26These values
are evaluated from the adsorption ofCO2 at 298 K. The results show
a very good agreement between the commercial equipment and the microcalorimeter
built here, reinforcing the excellent functioning of this equipment. Figure 2
shows the adsorption isotherm of CO2 at 273 K obtained for the zeolite
analyzed in this investigation. This isotherm was reproduced also on the commercial equipment
with a good concordance, reinforcing again the satisfactory behavior of our
apparatus.Figure 2
Adsorption
isotherm for zeolite in CO2 at 273 K.It is interesting
to analyze jointly the data obtained from the adsorption isotherm (see Figure2) and those for the
differential heat of adsorption (see Figure 3). In Na exchanged ZSM-5 zeolites,
Na cations neutralize the acidity of the zeolite and develop the basicity for
adsorbing acidic CO2. Thus, NaZSM-5 provides two kinds of adsorption
sites for CO2: stronger sites around a Na cation (which saturates
rapidly) and weaker sites on the pore walls [11]. The steep increase of the
adsorption isotherm and the high value of qd at low pressure (p/po<0.01) in Figures 2 and 3 reveal in a clear way the presence of these
strong sites, which become rapidly saturated. Note also that the steep decrease
in qd at very low pressure from 48 to 46 kJ/mol is indicating that the
adsorption strength of these sites is not uniform (indication of energetic
heterogeneity). After the strong sites become saturated (p/po>0.01), qd steps down to a lower and almost constant value, corresponding to adsorption on
the zeolite walls, and simultaneously the amount adsorbed increases more slowly
in the isotherm. This is in [11] and shows the potentiality of the
microcalorimetric station.Figure 3
Differential heats of
adsorption for CO2 on NAZSM-5 zeolite.


## 4. Conclusions

A modern adsorption microcalorimeter was built for the simultaneous measurement
of isotherms and adsorption heats, establishing its correct
functioning through adequate calibration of both the calorimeter part and the
volumetric equipment of the adsorption part. For this purpose, the microcalorimeter
calibration constant was found with values that go
from134.11±0.19WV−1 to 156.67±0.23WV−1. The
adsorption isotherm was determined for a type NAZSM-5 zeolite as a reference
solid to establish the correct functioning of the equipment. Micropore volume and
superficial area were determined to be 0.20cm3/g and 296m2/g,
respectively. These results agree very well with those obtained with commercial
equipment. Finally, the differential heats of adsorption, for the same solid,
were measured. The analysis of results gives valuable information about the
studied CO2/NaZSM-5 system, which is in concordance with other
studies in the literature.



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*Source: 127328-2008-07-15.xml*