Introduction

The major sources of energy currently used to satisfy the needs of human society, particularly in transportation, are non-renewable fossil fuels in the form of oil, coal and natural gas. The world’s energy demands are projected to double by 2030 as a result of the industrialization of developing countries .such India and China [1]. Consequently, the rapid growth in world population is depleting energy reserves of fossil fuels. Moreover, environmental pollution caused by burning fossil fuels is damaging the planet’s ecosystem. The Intergovernmental Panel of Climate Change (IPCC), a United Nations (UN) organization, has indicated that global warming effects are induced by the anthropogenic CO2 emissions to the atmosphere which is the product of burning fossil fuels [2]. As a result of global warming polar ice caps melt, sea levels rise and extreme weather events, such as heavy rain and tornadoes, become more frequent. Hence there has been a recent interest towards clean energy forms which can be mass produced and easily stored.

Hydrogen is considered to be an ideal energy carrier and can be used to generate clean, affordable energy. A combustion engine fuelled by pure hydrogen produces minimum pollution; its combustion product is just pure water [3]. 99% of the existing hydrogen on earth is chemically bound as H2O (water) and some more is bound as liquid or gaseous hydrocarbons, hence hydrogen cannot be used as a primary energy source [4]. Hydrogen is considered to be a clean energy carrier that can be produced from primary energy sources. There are many methods to produce hydrogen where the most common ones are steam reforming of methane and electrolysis of water. Unfortunately production from methane gas leads to increasing carbon dioxide emissions and it is not renewable. Electrolysis from water requires energy to produce the electricity for the electrolysis process and there are many different ways to produce clean electricity such as solar, wind power or hydroelectric plants. A major challenge for using hydrogen is storage, and therefore there is ongoing research to find effective and efficient ways for storing hydrogen [5].

The success of a hydrogen economy depends on finding new methods and materials to reversibly store hydrogen with high volumetric and gravimetric densities able to operate under ambient, or close to ambient, conditions. For this reason, the Department of Energy (DOE) in the U.S.A has set targets for the development of economically viable hydrogen vehicles [6].

Table 1.1 shows the 2020 ,2025 and the ultimate targets for hydrogen storage systems that include the materials, storage tank and all associated equipment. The gravimetric and volumetric capacities are both specified for their importance for vehicular use, storing of hydrogen for an on-board system needs to be both light weight and of a small volume. The storage material must also be recyclable over an operational life of 1500 cycles while taking no longer than 3.5 minutes to charge/discharge

Storage Parameter

Units 2020 2025 Ultimate

System Gravimetric Capacity: Usable, specific-energy from H2 (net useful energy/max system mass)

kWh/kg (kg H2/kg system)

1.5(0.045)

1.8(0.055)

2.2(0.065)

System Volumetric Capacity: kWh/L

1.0(0.030)

1.3(0.040)

1.7(0.050)

Usable energy density from H2 (net useful energy/max system volume)

(kg H2/L system)

Storage System Cost: $/kWh net ($/kg H2)

10333

9300

8266

Durability/Operability:Operational cycle life (1/4 tank to full)

Cycles 1500 1500 1500

Charging / Discharging Rates:System fill time • Minimum full flow rate (e.g., 1.6 g/s target for 80kW rated fuel cell power)

Min

(g/s)/kW

3.5

0.02

3.5

0.02

3.5

0.02

Hydrogen Storage

There are three main methods of hydrogen storage available that could be used for fuel cells designed for vehicular applications: pressurised gas, cryogenic liquid, and storage in a solid state material. Due to the low density of pressurised hydrogen gas, the heating value per volume in this form is significantly smaller than that of the conventional fuels. In addition, storing hydrogen using pressurized tanks is limited by the increased weight of the storage tank required for the pressures necessary as well as the potential for developing leaks [7]. In the cryogenic state, when storing as liquid hydrogen, a refrigeration system is required to cool the hydrogen to −253°C [8] . Storing hydrogen as pressurized gas or as cryogenic liquid imposes additional limitations such as weight restrictions, safety hazards and energy costs. Solid state materials such as complex hydrides (e.g. amides, borohydrides and alanates), metal organic materials, metal hydrides (including elemental metal hydrides such as MgH2, and alloys, such as LaNi5) and physisorption on high surface area materials (e.g. graphite, graphene, metal-organic frame works and aerogels) are all capable of storing hydrogen with varying volumetric and gravimetric densities.

However, none of these materials simultaneously fulfil all the requirements for facilitating hydrogen storage. There are problems due to the kinetics of hydrogen uptake and release and/or the thermodynamic properties of the material [8] or reversibility and cycling stability. Metal hydride systems with high gravimetric densities such as LiH are often too stable, requiring high temperature for desorption [9]. Despite this, light metal hydrides are still promising candidates for hydrogen storage, mainly because they are capable of storing large amount of hydrogen at feasible pressures in a small volume. A suitable metal hydride system might optimally have an enthalpy of formation of ΔH=−10 to −40 KJ mol−1, corresponding to an H2 equilibrium pressure of a few bars during operational conditions where the entropy of the reaction is typically 130 J/K per mol H2 (the entropy lost when the hydrogen gas combines with the metal) [9]. The alkali and alkaline earth metal hydrides are of particular interest due to their low weight. These ionic hydrides have high hydrogen content, yet they are thermodynamically too stable to release hydrogen at practicable temperatures [10]. The introduction of nanostructured composite materials may assist in improving the metal hydride kinetics as well as possibly providing a means to modify their thermodynamic stability. The composite consisting of light metal hydride phase and catalytic additive (such as carbon nano-tubes) [11] also provides method of enhanced hydrogen interaction.

Project Overview

This project aims to investigate the catalytic effects of Mg on the hydrogen storage properties of one or more light metal hydrides. Specifically of interest is the enhancement gained through using the ball milling method for constructing nanostructured composite materials. These materials represent new approaches to utilising light metals, alkali and alkaline earth hydrides in practical hydrogen storage systems. The goals of the project are to characterise the performance of these materials as a hydrogen storage medium and to relate hydrogen storage performance to their structure and kinetics.

Experimental determination of the kinetics of hydrogen uptake and release were performed using a custom built Sieverts apparatus. The crystallographic structures of the samples were investigated using ex situ XRD (X-ray diffraction). The surface topography and composition of a number of samples was investigated using Scanning Electron Spectroscopy (SEM).

Measurement Techniques

Sieverts Method

Two of the more popular methods of determining the uptake of gas into or on to materials, which is crucial to hydrogen storage material characterisation as well as other areas of research, are the Sieverts technique and the gravimetric technique. The Sieverts technique employs measurement of the pressure and temperature of the gas together with knowledge of the volumes involved to determine the number of moles of gas before and after the gas is released into the volume containing the sample. The Sieverts method was used for this project[12] .

Fig. 1 e Schematic for a simplified Sieverts apparatus (usedwith permission from Ref. [13]).

Fig. 1 shows a schematic of a simplified Sieverts apparatus consisting of an accurately known reference volume, Vref, and a volume containing the sample, Vcell. With valve S closed, Vref is brought to a desired pressure and, after allowing a period of time for equilibration, the pressure and temperature are recorded and the number of moles of gas in Vref is

3.1.1

where

Piref = is the pressure,

Vref =is the volume,

n= is the number of moles,

R= is the gas constant,

Tiref= is the temperature and

Z= is the compressibility.

The compressibility Z, is a function of temperature and pressure and represents the variation from an ideal gas (for which 𝑍=1).

In a single step, the sample cell, which has a known pressure of hydrogen gas, is isolated by closing valve S and the required pressure of hydrogen is prepared in Vref. Sufficient time is then needed for the pressure and temperature to reach equilibrium in Vref, then valve S is opened to allow hydrogen gas to react with the sample in the cell volume Vcell . Again sufficient time must be allowed for the sample to come to equilibrium with the hydrogen pressure and the temperature. The temperature of the sample cell Tcell and the reference volume Tref are recorded as well as the pressure throughout the hydrogenator Psys before and after the valve is opened when the volumes are connected to each other the pressure throughout the apparatus is the same at equilibrium.

The number of moles of hydrogen absorbed or desorbed by the sample in the kth step ΔnHk , is

calculated from the difference between the number of moles in the system before and after valve S is opened using the following molar balance in equation[14]

3.1.2

Where,

Prefk= is the pressure in Vref before the valve S is opened, Psysk= is the pressure of the entire system after opening valve S,

Psysk−1= is the pressure of the system prior to adding the pressure of the reference volume,

Tref k and Tcell k=are the temperature of the reference and the cell respectively,

the term (Vcell−Vxk) refers to the void volume displaced by the sample and

Z= is the compressibility as a function of pressure and temperature, and the volume of the sample is 𝑉𝑥=𝑚𝑥/𝜌𝑥 (3.1.3)

where mx= is the mass of the sample and ρx= is its density.

This is effectively the H2 gas ‘lost’ by the sample and therefore assumed to be taken up by the sample [15] . This assumption is valid provided the volumes are calibrated accurately and there are no leaks. After N steps the total change in the hydrogen content is given by

(3.1.4)

For the hydrogen storage material, its hydrogen content is usually expressed in terms of H/X , the number of atoms of hydrogen per host atom, which is given as:

(3.1.5)

Where

nNH= is the number of moles of the hydrogen,

nx= is the number of moles of sampleMx= is the molar mass of the sample, mH= is the mass of hydrogen and mx is the mass of the sample.

The mass fraction of hydrogen in the sample is

(3.1.6)

For mobile applications the hydrogen storage of a material is typically expressed in terms of weight percentage (wt%), which is given by 𝑤𝑡%=𝑓𝐻×100 (3.1.6)due to the need for the hydrogen storage capacity per unit weight of the storage material to be as high as possible, thus high energy density.

Normally, to characterize hydrogen uptake, an isotherm or pressure-composition-temperature (PCT) is determined, by performing multiple steps at increasing pressures while keeping the temperature constant. For this project, the desired characteristics were the kinetics of absorption and the maximum uptake, so it was sufficient to perform a single step measurement using a pressure of 50 bar at 250 ℃ which ensures full hydrogentation of the sample.

Volume Calibration and Divided.

The reference and empty cell volumes were determined using the volume calibration procedure described by Gray [14]. The two required volumes in the Sieverts apparatus, the reference (Vref) and the cell (Vcell) volumes must be measured accurately in order to calculate reliable hydrogen adsorption isotherms. The volume calibration can be performed by connecting a known calibration volume with a high precision pressure transducer to the system. In this calibration a Paroscientific 31K-101 pressure transducer was connected to the calibration volume which has an precision of 0.01% of full scale, which was 70 bar [16]. The volume calibration procedure starts by bringing the system to a desired pressure of ultra high purity helium gas with the number of moles given by the non-ideal gas law:

where :𝑃𝑖 =is the initial pressure,

𝑉𝑐𝑎𝑙= is the calibration volume,

𝑍= is the compressibility factor and

𝑇𝑖 =is the initial temperature of the calibration volume.

The valve to the unknown volume is opened and a sufficient amount of time (approximately 15 minutes) is allowed for the helium to equilibrate into the target volume. This process assumes that there is no adsorption or leaks in the system. The number of moles of helium in the target volume plus what is currently in the calibrator must equal the initial number of moles that was contained in the calibrator before the helium entered the unknown volume. This gives:

where :

f’=is refers to the final and

T =is refers to the target volume.

Rearranging equation (3.2.2) for the preparation volume to produce the following equation

Thus, for a known calibration volume, the target volume can be determined. This is typically performed multiple times to determine the uncertainty in the measurement. Subsequent unknown volumes can be determined by repeating this process, hence Vref and Vcell.

The previous volume calibration technique is valid to determine the required volumes for operating systems at a specific constant temperature. The sample cell is typically used at a variety of temperatures, resulting in the existence of a thermal gradient along the volume that connects to the rest of the apparatus which is typically at room temperature. The used temperature sensors cannot accurately model these thermal gradients, since they can only measure the temperature at positions that are fixed, and thus the volumes need to be calibrated using a temperature calibration model. The divided volume technique was used here

Figure 3.2: Divided-volume schematic of temperature distribution in the sample cell volume

Figure 3.2 shows the sample cell volume conceptually divided into two volumes, Vman at temperature Tman and Vsam at Tsam, so that:𝑉𝑐𝑒𝑙𝑙=𝑉𝑚𝑎𝑛+𝑉𝑠𝑎𝑚Where Tman =is the manifold temperature (normally the room temperature or Tref) Tsam= is the sample working temperature as provided by furnace or other heating (or cooling) facility. The divided volume calibration procedure begins by bringing the whole system to a suitable pressure of hydrogen gas with the number of moles given by the non-ideal gas law:

where the whole system is at the RT calibration temperature. The pressure in the system then settled to a value of p(Tman,Tsam). This pressure depends on the temperature of the sample section and the manifold section. The temperature of the sample cell is then increased in steps (say 25°C ) to the highest temperature to be used (400°C) and after the temperature has equilibrated the pressure is measured. Using the following calculation, values for Vman and Vsam can be determined,

Rearranging equation (3.1.5) yields The equation for the ratio for the sample volume to the manifold volume:

Based on the ratio of equation (3.1.6) and the total value for the cell volume (Vcell) found in the ambient temperature calibration, the volume for the sample stick and the manifold can be determined. This procedure is usually repeated multiple times to ensure a reliable result and determine any uncertainty.

Temperature Programmed Desorption

Typically, the desired measurement is a determination of the hydrogen uptake at the maximum pressure available. Another characterization of a hydrogen storage material is a measure of the kinetics, how fast the material can absorb and desorb the hydrogen. Temperature Programmed Desorption (TPD), is an important technique for the determination of the kinetics as well as establishing minimum temperature requirements [17, 18].

The basic principle of TPD is that the sample is heated with a constant linear temperature increase and the pressure of H2 released from the desorbing sample is monitored. Another method that could also be used is measuring the pressure of a fixed volume through mass flow meter [19].

Ball milling

Mechanochemical synthesis using ball milling of magnesium hydride with and without additives and alloying metals has been employed to produce defect-enhanced potential hydrogen storage materials [20]. Ball milling is a mechanical process that was used to synthesis the samples used in this project to create fresh surface with desired properties.

Figure 3.3: Schematic view of motion of the ball and powder mixture [21]

The milling vial, containing the material to be ground and the grinding balls, rotates about its own axis on a main supporting disk, which is rotating rapidly in the opposite direction. This motion causes large centrifugal forces which act upon the grinding balls and the material. As seen in Figure 3.3 the reduction of grain sizes in the powder is mainly due to the frictional effect, caused by the movement of the ball along the inner walls of the milling vial, and the effect of impact, caused by balls hitting the opposite wall of the milling vial [22]. The final average size depends on the number of balls, the amount of material, the size of the balls, time of milling and the rotational speed.

Structural Characterization Methods X-ray Diffraction X-ray diffraction (XRD) is a versatile analytical method to analyse material properties such as phase composition, structure, texture and many more properties of powder, solid and liquid samples. XRD is noncontact and non-destructive if used appropriately, which makes it a perfect tool for in situ studies. It is most sensitive to elements with high atomic number, since the diffracted intensity is proportional to the element’s electron number. Most crystalline materials have unique diffraction patterns, which enables identifying chemical compounds using a database of diffraction patterns. Based on the diffraction pattern, the purity of the sample can be determined, as well as the composition of any impurities present. Also diffraction pattern could be used to determine and refine the lattice parameters of a crystal structure as well as providing information on the crystallite size.

Bragg’s Law When X-rays strike the surface of a crystal they are partially scattered by atoms in a variety of ways. The diffraction patterns are formed when the wave length of the radiation is of the same scale as the atomic spacing in the crystal where the diffraction gives rise to a set of well-defined beams arranged with a characteristic geometry. The collected XRD data is the result of relative intensity for each reflection with a set of planes in crystal, known as the miller indices (h, k, l), accompanied by the corresponding scattering angle for that reflection.

Figure 3.4: Bragg diffraction in a 2D lattice

As shown in Figure 3.4, a diffraction peak is observed when a constructive interference occurs from X-rays scattered by the atomic planes in a crystal [23]. The intensities and positions of the diffracted beams are a function of the arrangements of the atoms in space alongside other atomic properties. Therefore, if the intensities and the positions of the diffracted beams are recorded, it is feasible to deduce the arrangement of the atoms in the crystal.The condition for constructive interference from planes is given by Bragg’s law:

nλ=2dhklsinθwhere n = an integer, for peak order, 𝜆 = wavelength of the radiation (X-ray), 𝜃 =Bragg angle or the incidence of the X-ray angle and 𝑑ℎ𝑘𝑙 =interplanar spacing of the (h k l) planes in the crystal lattice.

Scherrer Formula The crystallite size of the powder can be determined using the Scherrer formula that relates the peak width to the particle size. The crystallite thickness (t) is

𝑡=0.9𝜆/√𝐵𝑀2−𝐵𝑆2cos𝜃with 𝜆: is the x-ray wavelength, 𝐵𝑀: is the observed peak width, 𝐵𝑆: is the peak width of a crystalline standard (natural width plus instrument broadning) : is the angle of diffraction.

Figure xx: Schematic diagram of lab X-Ray Diffractometry.

A schematic of a laboratory X-ray Diffractometer (XRD) is shown in Figure xx, which uses an electron gun as a source of electrons which are fired at a target ‘sample holder’. The high energy electrons knock core electrons out of the atoms in the target. Electrons from higher energy orbital drop back into the low lying orbitals in the target, which results in emission of X-ray photons. The emitted radiation is the characteristic wavelength for the element [24]. The XRD apparatus used in the project is fitted with a silver tube to produce X-rays with an average energy of 22keV.

These X-rays are collimated and directed onto the sample. As the detector and sample are rotated, the reflected X-ray intensity is recorded. Constrictive interference and a peak in the intensity occur when the geometry of the incident X-rays hitting the sample satisfies the Bragg equation. A detector records and processes the data and converts the signals to a count rate which is then accumulated on to a computer. In a XRD, the sample rotates in the path of the collimated X-ray beam at an angle θ whereas, the detector is mounted on an arm to collect the diffracted X-rays and rotates at an angle 2θ. A goniometer is the instrument used to maintain the angle and rotate the sample and or detector.

XRD was the main method used for the analysis of the samples before and after synthesis as well as after absorption/ desorption tests. The XRD patterns were obtained using the PANalytical diffractometer system the (Empyrean). Samples were loaded into glass capillaries of 0.5 or 0.8 mm internal diameter in the glove box and sealed with super glue gel in order to avoid oxidation during the XRD measurements..

Scanning Electron Microscopy (SEM) Scanning electron microscopes (SEM) is used to gain both quantitative and qualitative information about the composition and the morphology of the sample features to enhance the correlation of microscopic with chromatographic data. This method yields a reproducible characterization of all image features such as inclusions, particle size and internal pores of a solid structure. In SEM a source of electrons is focused into a fine probe, in vacuum, that is rastered over the surface of the sample. During the electron’s penetration of the surface, a number of interactions take place that can result in the emission of electrons or photons from or through the surface. The majority of the emitted electrons are collected by a detector and counted, so that each point struck by the beam on the sample is mapped onto a corresponding point on a screen [23].

Experimental Apparatus and Process

In this chapter, the experimental methods used to prepare the materials investigated are described. The handling of the prepared materials and the techniques used for materials characterisation and hydrogen storage studies are discussed.

4.1 300 bar Sieverts Apparatus An existing custom manometric Sieverts apparatus constructed from commercial Sitec and VCR components was used to perform the hydrogen desorption and absorption kinetics measurements. The Sieverts apparatus two different types of pressure transducer were used.

Figure 4.1:Instrument schematic showing the main components for the preparation, reference and sample sections[25].

A Presens Precise 400 pressure gauge rated to 400 bar and an accuracy of 0.01% FSR (full-scale range) was used to measure the pressure of the sample. The other pressure gauge was used to measure the reference pressure, which was a Quartzdyne model DSB301-05-C85 rated to 340 bar and an accuracy of 0.01% FSR. In addition, a vacuum guage, 925 MicroPirani Transduser, was used for the TPD experiment. To maintain the temperature regulation, all the valves, including the sample stick valve (Vstick) were mounted on a water cooled backplane. In addition the reference volume was encased in a water cooled copper jacket. Furthermore, the preparation section was mounted on the adjacent side to the rest of the system to minimise temperature changes in the reference volume due to heating and cooling of the reservoir in the preparation section.The vacuum pump used was a Pfeiffer turbo molecular pump. A needle valve (VN) was designed to be used to regulate the flow of the gas. In this project it was left permanently open four turns. The hydrogen reservoir was a 150 cc Swage lock stainless steel volume with a rated pressure of 340 bar incorporating approximately 400 g of MnNi4:2Co0:8 (Japan Metals and Chemicals Co.). This is the storage medium where the hydrogen is absorbed into the reservoir alloy by cooling the reservoir and is released from the reservoir by heating. The cooling was done using liquid nitrogen in a hand-held dewar and the heating by using a hot air gun. Silver plated stainless steel gaskets were used to connect the VCR component of bottom part of the sample cell due to their ability to withstand high temperatures (>700K) and it was replaced with each sample change to avoid any residue from previous sample to contaminate the new sample. For the top part of the sample cell a silver plated stainless steel filter gasket was used to connect the VCR component to contain the sample in the sample cell during pressure changes and hence, avoid contaminating the system.

4-2 Preparation of Mg and NaH

Hydrides are very sensitive to air and moisture in the atmosphere, therefore exposure must be avoided to prevent sample degradation. All preparation of samples for ball milling and loading the sample cell were carried out in an argon filled glove box (MBRAUN Unilab), where both the water and oxygen levels were maintained below 1 ppm. Subsequent storage and handling of all prepared materials was carried out in the glovebox.

4-3 Experimental ProceduresThe experimental procedure consisted of two parts: synthesis the sample materials and the measuring the hydrogen uptakes for these materials by temperature programmed desorption and single step absorption. All of the sample were investigated using X-ray diffractionIn addition, selected samples were characterised by SEM.

Figure 4.3: The overall procedure of the experiments

The materials and chemicals used in this project are listed in Table

Materials/ chemicals

Formula Purity (%) Supplier

Magnesium powder

Mg powder 98% Sigma-Aldrich

Sodium hydride NaH 99.99% ALDRICHHydrogen H2 99.999% BOC

Helium He 99.999% BOCArgon Ar 99.997% BOCTitanium isopropoxide

TTIP 99.999% ALDRICH

4-3-1Material Synthesis

The optimum conditions for the preparation of MgH2 by the reaction of hydrogen on magnesium.metal have been investigated on three samples.1- Samples 1Mg-based metal hydrides can be used as solid-state hydrogen storage materials for fuel cell cars, as a hydrogen source for fuel cell auxiliary power units, for the storage of high-temperature heat in industrial processes and in power plants, or for the smoothing of irregular supply of heat and

electricity production for fuel cells in domestic energy sectors. Metal hydrides are produced from different pure metals or metal alloys under the influence of hydrogen[26]. Thisprocess of hydrogenation is accompanied by the release of heat (exothermic reaction). The same amount of heat must be supplied to the metal hydride for the decomposition and the release of hydrogen .These processes are infinitely repeatable, and reversible Mg-based metal hydrides can therefore be used for the storage of heat and hydrogen [27]. Depending on the ther-modynamics of the reaction, different temperatures applyto different metal hydrides . heat release

Mg +H2 MgH2 heat storage

Starting Materials

SynthesisStructural

Characterization

Hydrogen Storage Measurement

Absorption/Desorption

XRD/SEM

Ball Milling

Mg+H2 = MgH2Magnesium hydride prepared from magnesium powder of high purity, MgH2 was prepared from Mg powder heated 400 oC under 300 bar H2 for two days. after that the sample milled under 10 bar of hydrogen pressure for 0.5 h rotation 6 min pause, The pre-milled MgH2 was further formed then again heated.2- Samples 2 Mg+NaH = MgH + Na (1; 1)The Mg and NaH was further milled in tool steal for 24 h under 80 bar of H2 for (2h, rotation 6 min pause) where the sample was milled using six zirconia balls. The magnesium powder and Sodium hydride with ratio 1:1g.3- Samples 3 Mg+NaH +(TIIP 2mol %)The MgH2 was prepared by milled Magnesium powder, Sodium hydride and Titanium iso-propoxide for 20 h using a 10 zirconia balls. The amounts of TTIP 2 mol%.

The composite materials investigated in this project were synthesised by ball milling, which is known to produce small grain sizes, dislocations and other defects which enhance absorption/desorption kinetics by providing pathways for hydrogen diffusion and shorten bulk diffusion lengths. The ball milling was conducted in a PQ-N04 planetary mill (Across International), in which the milling vial and balls were made out of zirconia. The milling vial was loaded with the sample and sealed inside the glove box under argon.

4-3-2 Temperature programed desorption (TPD)Temperature programed desorption (TPD) studies were carried out on all the samples used for this project with the same temperature program. The internal temperature of the sample cell was recorded using a Platinum Resistance Thermometer (PRT). The TPD studies were initiated when the sample cell was at room temperature, and then heated by a furnace controlled by a Eurotherm 2404 temperature controller.

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