The present invention relates to a programmed thermal desorption
procedure as well as to a simple and easy to assemble piece of apparatus for carrying
out said procedure.
Thermo-programmed desorption is a widely used analysis technique
in which the gas moleculeg absorbed by a solid surface are extracted by thermal
heating. Since its beginnings in the 1940's it has only been applied to the desorption
of gases. In 1990 IUPAC (International Union of Pure and Applied Chemistry) described
it as an experimental technique for characterizing surfaces (Pure and Applied
Chemistry vol 62, N° 12 pp 2297-2322, 1990) and they too make reference only to
the desorption of gases. The present invention can be considered as the first piece
of equipment described for carrying out TPD analysis in solution, opening up the
possibility of extending the technique to a great many areas of research.
BACKGROUND OF THE INVENTION
In a typical thermo-programmed desorption experiment a small amount
of solid containing an absorbed gas is introduced into a reactor arranged inside
an oven. The reactor is heated, generally following a linear increase in temperature
with time. As the temperature rises the absorbed gas desorbs. An inert gas, generally
helium, flows through the reactor and carries the desorbed gas molecules towards
a detector. Alternatively the molecules are drawn by a vacuum.
A small thermocouple inserted inside the reactor measures the temperature
while the detector in contact with the current of carrier gas analyzes the concentration
of the gas desorbed. The response of the detector is proportional to the rate
or desorption. This rate increases with temperature, reaches a maximum value and
returns to zero when the surface is completely empty.
The desorption spectrum (thermogram) is a recording of the concentration
of the gas desorbed as a function of temperature. Normally the spectrum can exhibit
more than one maximum (peak).
The number, shape and position of the peaks, as well as the area
contained within the thermogram, hold a great deal of information about the gas,
the surface and the interaction between the two.
The thermo-desorption technique has its origins in the 1930's when
URBACH, in is experiments on luminescence, observed the escape velocity of electrons
from a continuously heated material. However, the application of this idea to the
study of the interaction between gases and solids took place somewhat later.
The first work which refers to desorption itself was carried out
by APKER and is described in his studies published in 1948 about the existing methods
of measuring low pressures. These studies describe the difficulty in using ionization
manometers as a result of the surface contamination of the filament by the abssorption
of gases, but show, nevertheless, that when subjected to abrupt heating, flash,
there was a sudden increase in pressure due to the desorption of said gases.
In 1953 in the Bell Telephone laboratories (Murray, New Jersey) HAGSTRUM
designed and built several pieces of apparatus for studying the extraction of electrons
from metal surfaces by bombardement with positive ions. These experiments show
the inportance of working with surfaces which are atomically clean. One indication
of this contamination was the increase in pressure which took place when said surfaces
were heated quickly to high temperature 1750 K(Mo) or 2200 K (W). Furthermore
the observed that this increase was not uniform with temperature but could have
In the same laboratories it was shown that the rate of gas desorption
is dependant on temperature. The experiment was carried out in a vacuum system
where an auxiliary filament of W or Mo was heated using a continuous current. An
electronic circuit was designed to display the increase in pressure against the
temperature on an oscilloscope screen. In this way the first desorption thermogram
was obtained, i.e. the first representation of a variable related to the amount
desorbed against temperature.
From this date on flash desorption began to develop widely, the heating
process varying between 10 and 1200 K/S. In general, the equipment and procedures
used were very similar. The solid under investigation was immersed in a gas connected
to a vacuum system in which was located a device able to produce rapid heating.
The amount of gas desorbed from the sample during the heating process could be
determined by the increase in pressure inside the system, generally by means of
an ionization manometer&sup4;&supmin;&sup7;. By passing a current of gas to be
absorbed by the surface after the flash, the equipment was once again ready for
carrying out another desorption experiment.
Many studios have been carried out using the flash desorption equipment
of the type described above. The first experiments concentrated on studying the
absorption states of diatomic gases by W, while at the same time the theory required
for the quantitative analysis of the experiments was developed. Later on said experiments
dealt with the phenomena of interaction and interchange between gases absorbed
by a surface. By 1963, the flash desorption technique had been more or less perfected
. Among the many studies examined, it is worthmentioning the one carried out by
AMENOMIYA and CVETANOVIC regarding the interaction of ethylene with a surface
of aluminium oxide. The apparatus was fitted with a controller, which enabled various
linear rates of heating to be set, and a thermal conductivity thermistor for detecting
the ethylene desorbed and carried along by a current of helium. Since the surface
was non-metallic the rates of heating were much lower, between 0.5 and 40 K/min.
The recorded desorption rate increased with temperature and later decreased as
the absorbed gas was used up, tracing a peak. At the same time the temperature
of the system was picked up by another recorder connected to the thermocouple.
From the experimental point of view the need to determine partial
pressures in the gas phase of a system stimulated the use of various types of mass
spectrometer, this kind of detector finding a clear application in the study of
isotopic surface interchange reactions as well as for the study of the decomposition
of substances absorbed by surfaces.
Another contribution in the field of the thermo-desorption which
is worthy of mention is that of CZANDERNA which deals with following the desorption
process by direct weight using a microbalance. In this way it is possible to obtain
a more direct measurement and work at high pressures . The studies of FARNETH
are along the same lines and deal with the mechanism of oxidation of alcohols on
MoO&sub2;, where the desorption process was studied simultaneously by means of
a balance and a mass spectrometer.
More recently the technique of programmed temperature desorption
found an important application in the study of catalytic process. It was of course
necessary to modify somewhat the previously described equipment as well as the
process, due principally to the porous structure of the catalytic materials as
opposed to the relatively uniform surface of the metallic materials which had previously
Of the first work carried out it is worth mentioning that of CVETANOVIC
Their first study involves the modifications which have to be made
to the flash desorption equipment. An oven was used to increase the temperature
of the catalyst and an inert gas, helium, was used to carry the sample desorbed
which was then analyzed by a chromatograph. The rates or heating were much lower,
between 10 and 30 K/min much that the system remained close to a position of equilibrium
between absorption and desorption.
Once the equipment had been modified the authors in their subsequent
work moved on to the study of different catalytic systems: butene/aluminium³²,
Later on slight modifications were introduced, relating principally
to the means of detecting the species desorbed. This is the case for the equipment
designed and perfected by MENON which uses a chromatograph as a detector in the
study of n-pentane on Pt-Al&sub2;O&sub3;, the same as ANDERSON in his work on
the desorption of hydrogen from the catalysts Pt and Au. Another means of detection
is described in the work by TOPSOE which was to study the desorption of ammonium
and pyridine from zeolites. In both cases infrared spectroscopy was used as the
identification technique. The rate of heating varied between 5 and 40 K/min.
A more sophisticated modification was made by the investigators LATZEL
and KAES who built a piece of apparatus in which the sample desorbed was drawn
along by vacuum and which could function automatically. Both the oven and the
type of heating were regulated by a computer which also controlled the mass spectrometer
usedas a detector and at the same time collected and stored all the data such as
m/e, intensities, time, temperature, etc.
By the beginning of the 1980 's the experimental equipment had already
been more or less perfected. There have therefore been very few modifications since
that time and work on thermo-desorption basically describes the results obtained
or the theoretical considerations concerning the technique. A typical diagram of
the apparatus from this period is included in the study by FALCONER.
Before concluding, there are two further issues worth mentioning:
one is the changes in the rate of heating, and the other is the increase in complexity
of the surfaces to be studied.
At first, the heating processes involved in flash desorption were
very abrupt and poorly controlled, varying between 10 and 1200 K/s. As the equipment
was perfected, this rate was reduced accordingly. For example, RIGBY worked with
rates of heating between 5 and 32 K/S and years later AMENOMIYA and CVETANOVIC
managed to work with rates of between 0.5 and 40 K/min. This reduction lead to
the modification of the temperature detection system, the sensitivity of the thermocouples
being insufficient, and enabled the problem of temperature gradients set up in
the absorbent to be solved.
It is also worth mentioning the introduction of nonlinear heating
programmes such as those in which temperature and time vary reciprocally (hyperbolic
heating). Hyperbolic heating implies greater complexity from the experimental
point of view, but at the same time can improve the resolution of the thermogram
and simplify the processing of the equations.
In 1962 REDHEAD published his work concerning the theoretical aspects
of determining the activation energy, using the rates and orders of reaction for
both types of heating, linear and hyperbolic, to make a comparative study.
In more recent years, studies on the thermo-programmed desorption
of ammmonium absorbed by zeolites using hyperbolic heating have shown that the
kinetic parameters obtained with this procedure are more accurate than those obtained
with linear heating and avoid the fairly frequent drawback of the sometimes observed
dependence of these parameters on the rate of heating.
With regard to the surfaces studied the technique has undergone a
long evolution. Initially, as has already been mentioned, the aim was to eliminate
the absorbed contaminants absorbed by the filaments of ionization manoneters. However,
within a short time interest was centred instead on the absorption of these gases
by metal surfaces and there are a great many studies concerning the absorption
of nitrogen, hydrogen and carbon monoxide by metals, in most cases W. The reason
for this continued interest is the direct relation to catalysis.
Later on the technique was applied to the study of more complex surface
phenomena such as the desorption of the decomposed species from the surface, or
those formed by catalytic effects. This is the case of the desorption of some
organic compounds (ethane, methane, benzene) absorbed by metal surfaces such as
W, Tr or Pt.
Once the necessary modifications in the equipment had been achieved
and the application of the technique had been extended to the study of catalytic
effects, work broadened further to cover porous catalysts. It is therefore worth
mentioning the study of the absorption/desorption of hydrocarbons and alcohols
by catalysts such as aluminium, carbon, silica gel, magnesium oxide, etc.
In recent years programmed desorption has also been used to characterize
supported metals. The technique is now widely used for both porous and metal catalysts,
or for metal oxide catalysts, and constitutes a valuable tool for the study of
absorption/desorption surface phenomena as well as catalysis. Finally and as has
already been shown, the application of this technique has only been carried out
in the gas phase but never in the condensed phase. This could be due perhaps to
the difficulty in reaching the required temperatures for desorption under these
conditions, or to the scarcity of work and little development in the research into
desorption in solution. The equipment designed therefore widens the field of thermo-programmed
The object of the present invention is a procedure for programmed
thermal desorption based on the use of a liquid which can dissolve the substance
desorbed and carry it along in this state for analysis.
A further object of the invention is a piece of apparatus for carrying
out said procedure of programmed thermal desorption in solution, said apparatus
being of simple construction, relatively reduced in cost and easy to handle compared
with the equipment used for the thermo-programmed desorption of gases.
As in the case of the traditional processes, the procedure of the
invention comprises the heating of the sample to be desorbed inside a chamber and
the carrying of the substance desorbed to a region where it is analyzed, said
process being characterized in that it uses a liquid which can dissolve the substance
desorbed. The liquid is made to circulate around the desorption chamber during
the heating phase. The liquid circulates around the heating chamber at high pressure,
high enough to keep it in the liquid state during the heating period, such that
desorption takes place in the liquid phase. The substance desorbed is carried along
in the dissolved state to a region where it is analyzed.
A it leaves the desorption chamber the carrier liquid in which the
desorbed substancte is dissolved is subjected to a partial cooling process before
reaching the region where it is to be analyzed.
The apparatus required to carry out this procedure comprises a desorption
chamber, means of heating said chamber and a detector in which the analysis of
the desorbed substance takes place. According to the invention, the apparatus
further includes means of supplying the desorption chamber with a liquid under
pressure for carrying along the substance desorbed, means of heating the carrier
liquid before it enters the desorption chamber, means of maintaining the carrier
liquid at high pressure, at least while it circulates around the desorption chamber,
and means of partially cooling the carrier liquid, said cooling means being situated
between the desorption chamber and the detector for analyzing the desorbed substance.
All characteristics of the invention, as described in the claims,
are disclosed below in greater detail with the help of an accompanying drawing
which represents schematically a non-limiting example of the apparatus required
for carrying out the procedure of the invention.
The accompanying drawing shows an example of the apparatus required
for programmed thermal desorption in solution, said apparatus comprising a tank
1 for storing the carrier liquid, a high pressure pump 2 which forces the carrier
liquid over a pre-heater 3 and the desorption chamber 4. The pre-heater 3 consists
of a coil which, together with the desorptlon chamber 4, is located inside an oven
5 provided with a temperature programmer 6. Following the desorption chambers,
and outside the oven 5, is a cooler 7 which may take the form of a coil and which
is submerged in a thermostatic bath 9. A protective filter 8 is provided after
the cooler 7, and outside the bath 9 is a run 10 which included a reduction in
cross-sectional area and which may consist, for example, of a capillary tube or
an adjustable valve. After this run 10 is the detector 11 where the analysis of
the desorbed substance takes place. The data provided may be collected and displayed
graphically on a recorder 12 and processed by a computer 13. The novel part of
the apparatus is the region which lies between the pump 2 and the run 10 which
comprises the reduction in cross-sectional area, said region constituting the region
of high pressure, the fundamental idea of the procedure being to maintain the
desorption chamber 4 at high pressure during the entire heating process in order
that desorption takes place in solution and not in the gas phase. These conditions
of pressure may be achieved by increasing the loss of load of the carrier liquid
after it has passed through the desorption chamber, or by using a capillary tube
10 or alternatively by means of an adjustable valve, as has already been indicated.
Nevertheless, the pressure is also controlled and depends on the rate of flow
which is chosen for the carrier liquid .
The carrier liquid, stored in the tank 1, is forced around the apparatus
by the high pressure pump via the pre-beater 3 which terminates at the desorption
chamber 4, said chamber being provided with filtration discs to prevent particles
of the support material from being carried along.
The chamber 4 and tbe pre-heater 3 are placed inside the oven 5 which
is provided with or connected to a temperature programmer 6 with which different
rates of heating can be achieved.
The cooler 7 may also take the form of a coil and is suspended inside
the thermal bath 9. The substance desorbed and swept along by the carrier liquid
is cooled and kept at a constant temperature by the cooler 7 before it arrives
at the detector 11. The purpose of the filter 8 is to prevent the passage of any
solid particles which may be accidentally swept along by the liquid, thereby protecting
the capillary tube 10.
The signal from the detector is displayed graphically along with
the temperature (thermogram) on the graphical recorder 12 and may also be processed
The volume of the system as a whole should be small to minimize the
amount of carrier liquid used up and so that the substance desorbed arrives immediately
at the detector 11 for analysis.
The pump 2 used for supplying the liquid under pressure should enable
high pressures to be obtained. The upper limit is determined by the critical pressure
of the carrier liquid. Furthermore, the pump should be able to provide a wide range
of flaw rates which must be both accurate and constant. The pump, as well as the
conduits and other components, must of course be inert to the various solvents
which are to be used as carrier liquids.
The pre-heater 3 must be inert to the carrier liquid and able to
withstand high pressures and temperatures, being built of a material with high
thermal conductivity so that while it is inside the pre-heater the carrier liquid
reaches the temperature of the oven. A simple pre-heater could consist of a coil
formed by a long tube made of stainless steel or of steel with an inner lining
The desorption chamber 4 must also be inert to the carrier liquid
and to the species desorbed, be able to withstand high temperatures and pressures
and have a high thermal conductivity. At the same time it must able to retain
the solid sample and allow the carrier liquid to circulate freely. The chamber
4 must be easy to open in order to fit the sample and its volume should be as small
as possible, just enough to contain said sample, with a small cross-sectional
area so that the carrier liquid flows at high speed, carrying along the desorbed
molecules and preventing them from being re-absorbed. One simple design for the
desorption chamber could take the form of a small, stainless steel cylinder with
an internal diameter of a few millimetres, conically closed and provided with filtration
plates at each end to prevent the passage of solid particles from the support
but allowing the passage of the carrier liquid.
The oven 5 may have any shape and size, and conveniently is provided
with a forced convection device so that no local temperature gradients are created
and to rapidly achieve a uniform temperature as set by the programmer 6, facilitating
and increasing the transmission of heat through the pre-heater 3 and the desorption
chamber 4. It is also advisable that it be provided with a cooling device, for
example a coil connected to a cryostat, so that the oven can be rapidly cooled
after each experiment and furthermore to be able to start the desorption process
a low temperatures, which is sometimes convenient.
Finally, the oven 5 should be provided with, or be easily connected
to, a simple and accurate programmer 6, since in solution small variations in the
rate of heating are enough to produce substantial changes in the rate of desorption.
Like the other components in the circuit, the cooler 7 must be inert
to the carrier liquid and to the substances desorbed. Furthermore, it must be able
to withstand high temperatures and pressures and have a high thermal conductivity
so that it can quickly cool the current of carrier liquid. This part of the equipment
could consist simply of a long, fine coil of steel, as shown in the drawing, with
an inner lining of glass or another inert material, submerged in the thermostatic
If the reduction in cross-sectional area, for maintaining the region
of high pressure, consists of a capillary tube 10, said tube consists of a material
which is inert to the carrier liquid and to the substances desorbed. The tube
must also be able to withstand high pressures and its length and cross-sectional
area should be such that they provide the necessary pressure inside the desorption
chamber 4 and inside the cooling coil 7 so that the system remains in the liquid
phase. As has already been mentioned, the capillary tube 10 could be replaced by
an adjustable valve to achieve the same purpose.
The detector 11 where the analysis of the desorbed substance takes
place, can be of any known type which can measure directly or indirectly the concentration
of the desorbed substance in the flow of the carrier liquid. Some detectors which
may be of good general use are: spectrophotometers (ultraviolet, visible, infrared,
fluorescent, etc.), mass spectrometers, conductivity detectors, clectrochemical
Both the process and the apparatus of the invention can be of great
use in any research or laboratory or industry involving work with solid surfaces
and processes of absorption and catalysis, either as research apparatus or as
a piece of equipment for controlling a particular process. Thus, for example, it
could be used for controlling the dying of fibres, for the control and recovery
of absorbents and catalysts, for controlling the elimination of contaminants by
means of absorbents, etc.