TPD Apparatus

Partial pressures were measured using a Dycor M200M quadru-pole mass spectrometer. In its PRESSURE-TIME mode the mass spectrometer displayed and recorded the partial pressures of 

TPD data acquisition for both temperature and pressure was accomplished using an IBM-XT PC. The acquisition software was written in ASYST by W. Clark and modified by G. Chottiner in the Physics Department of Case Western Reserve University. Pressure data was digitized by the mass spectrometer, but the continuous voltage signal from the thermocouple was converted into discrete numbers and fed to the computer by a Keithley 570 A/D converter. The pressure data could be plotted as a function of time or 

To minimize confusion between the desorption features from the sample surface and those from the sample holder and/or mounting materials, a collimator for the mass spectrometer was used. The circular opening of the collimator was about the size of the sample (1.1 cm diameter). During TPD experiments, the sample surface faced the opening of the collimator and was 3-5 mm away from it. 

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Temperature programmed desorption (TPD) of NH3 adsorbed on the samples was carried out on an Altamira TPD apparatus. NH3 adsorption was performed at 50°C on the sample that had been heat-treated at 120°C in a helium flow. After flushing with helium, the sample was subjected to TPD from 50 to 600°C (AT = 10°C min 1). The evolved NH3, H20 and N2 were monitored by mass spectroscopy by recording the mass signals of m/e = 16, 18 and 28, respectively using a VG Trio-1 mass spectrometer. 

The TPD apparatus consisted of a stainless steel flow system connected to a thermal conductivity cell. Catalyst samples of 0.1 g were placed in one arm of an L-shaped, 6 mm Vycor tube. A dual adsorption bed containing alumina and Oxy-Trap (Alltech) was placed in the other arm to prevent contamination by water and respectively. Frequent regeneration in and He was required. This in-situ adsorption bed was found necessary despite purification traps on all gas lines coming into the flow system. Pulses of 0.25 cc of a 10% mixture of CO in He were injected into the He carrier gas and passed over the pretreated catalyst at room temperature. All runs were programmed heated at a rate of 20 K min . The Pt catalysts, either commercial or laboratory produced, were prepared by the impregnation of chloroplatinic acid on Cyanamid s Aero 1000 alumina, except for two catalysts which were prepared by platinum diamino dinitrite impregnation. 

To measure the solid acidity of supports, NHs-temperature-programmed-desorption (TPD) experiments of zeolite supports were carried out on a special NH3-TPD apparatus (Ohkura Riken, Model ATD700A) interfaced to a personal computer. TPD profiles were obtained under vacuum conditions with the temperature varying from 100 to 600°C at 10°C/min in the TPD process. Samples were degassed at 500°C for 1 h under vacuum condition before measurement. The surface area and micropore volume of the zeolite... 

Fig. 4.3 Experimental setups for temperature-programmed desorption, reduction and oxidation, (a) The reactor is placed inside the furnace which is connected with temperature programmer. Detection of evolved gas(es) is performed by monitoring the variations in thermal conductivity of gas mixture, (b) The TPD apparatus equipped with mass spectrometer as a detector [5]...

TPD spectra of ammonia were measured with a conventional TPD apparatus, and desorption was detected by a thermal conductivity detector. Zeolite sample (0.1 g) was evacuated in a quartz cell at 673 K for Ih, exposed to ammonia used as probe base at room temperature for 15 min, then evacuated at room temperature. TPD measurements were made from room temperature to 823 K with a heating rate of 30 K min in a helium flow of 150 ml mln. ... 

N2-BET analysis and porosity measurements were done on a Micrometries ASAP 2000 apparatus at liquid nitrogen temperature. Temperature-programmed desorption of ammonia (NH3-TPD) and temperature-programmed reduction by H2 (H2-TPR) were performed with a Micromeritics AutoChem 2910 apparatus. 

Fig. 3.8 (a) Desorbed hydrogen (wt%) from the NaAlH milled for 5 h as a function of heating time while heating to 225, 300, 400 and 425°C. Quasi-Temperature Programmed Desorption (TPD) carried out in a Sieverts-type apparatus, (b) Temperature profile change as a function of time while heating to 225, 300 and 400°C in a Sieverts apparatus... 

NHj-TPD measurements were performed in the same apparatus. Prior to NH3 saturation, the samples (0.05 g) were treated at 600°C in 02/He or CH4/02/He flowing mixtures for half an hour. After saturation at 150°C, the samples were purged at the same temperature in the He carrier flow (30 Ncm -min" ) for Ih. 

Temperature programmed desorption (TPD) of C02 (5 °/min, flow of He, 15 ml/min) was carried out on a conventional flow apparatus. In a typical experiment, 0.29 g of the catalyst were activated as above reported, then the system was cooled to 25°C and approximately 2 10 5 mol of Co2 were injected by means of a gas sampling valve. After degassing in flow of helium for 60 min the amount of the irreversibly adsorbed C02 was determined with an on-line g.l.c. equipped with a thermal conductivity detector,... 

Ammonia TPD Measurement. The acidic properties of the catalysts were characterized using temperature programmed desorption (TPD) of ammonia. The experiments were carried out on a flow-type apparatus equipped with a fixed-bed and a thermal conductivity detector. The samples were activated in a helium flow of 5 L/h at 773 K for 1 hour. 300 mg of the H+-form of each dehydrated sample were used to perform the ammonia TPD. Pure ammonia, with a flow rate of 3 L/h, was then passed through the sample at 423 K for 30 min. The sample was subsequently purged with helium at the same temperature for 1.5 hours in order to remove the physisorbed ammonia. The TPD was performed under a helium flow of 6 L/h from 423 K to 873 K with a heating rate of 10 K/min and subsequently at the final temperature for 30 min. 

The TPR experiments were performed in 5% H Ar gaseous mixture at 20 cm3/min. Samples were heated at 5 K/min within the temperature range of 298-1073 K. Prior to TPR experiments, samples were treated in situ with air at 723 K for 2 h, The TPD runs were carried out in the same apparatus. Catalysts were reduced in H2 at different temperatures for 1 h. After cooling the reactor down to room temperature, samples were heated under Ar at 5 K/min from 298 up to 873 K The amounts of H2 desorbed were calculated from the integrated peaks areas measured by a heat conductivity cell, which was calibrated by injection of known volumes of H2 into the carrier gas. 

The heat of adsorption of Ar was also measured for acidity evaluation. In the case of Ar-TPD, an effect of the probe molecule diffusion in micropores is observed with some samples, such as zeoUtes, at high temperature-programmed rates. The adsorption method is not influenced by diffusion of the adsorbed molecule because the Ar isotherm is measured at static equilibrium. It is also advantageous that the usual BET apparatus can be used to obtain the adsorption isotherm. In addition, the adsorption behavior of Ar is of the Henry type at temperatures around room temperature. 

Figure 4 shows a schematic diagram of an ultrahigh vacuum (5 x 10 ° Torr) apparatus that integrates LEED, XPS, TPD, LEISS, and electrochemistiy (EC). The base pressure of the chamber is 5 x 10" Torr. The sample is mounted on a probe, a tube fabricated out of stainless steel, at the top of the chamber. The probe allows experiments to be performed at very low temperatures for example, the probe is filled with hquid nitrogen for experiments at 77 K. The sample can also be heated resistively (up to 1500 K) via copper wires attached to the sample for still higher temperatures, an electron beam from a tungsten wire located behind the sample is employed. Temperature is monitored via a ReAV-Re thermocouple. 

Figiffe 4. Schematic diagram of an integrated LEED-TPD-XPS-LEISS-EC apparatus. 

An instrument that integrates ES with EC is shown in Fig. 3 a residual gas analyzer is also available in this apparatus for temperature-programmed desorption (TPD) but the use of such technique is beyond the scope of the research described here. 

The adsorption property was measured by a static method at 30 °C with a conventional volumetric apparatus as well as by the temperature programmed desorption (TPD) method. The details of the pretreatment and adsorption procedures were shown in Results and Discussion section. Metal-loaded zeolite samples were characterized by XRD, diffuse reflectance UV-Vis spectroscopy (DRS) and electron spin resonance (ESR). 

The TPD experiments are carried out using a differentially pumped quadrupole mass spectrometer (QMS), connected to the UHV apparatus employed for the MIES/UPS studies. The ramping time in TPD is considerably shorter (IK/s) than in MIES and UPS (IK/min). For this reason, the maximum desorption rate in MIES/UPS occurs about 15K earlier than in TPD. 

The experimental techniques and apparatus have been described in detail elsewhere. (8, 9) Titania and niobia were deposited onto the clean metal foils by vaporizing either a Ti-Ta alloy wire or a niobium wire in 10-7 Torr H2 at 800K. Auger electron spectroscopy (AES) was used to determine the oxide coverages, and temperature programmed desorption (TPD) was used to determine the effect of the oxide layers on the adsorption of CO and H2. Methanation rates were measured in a side chamber which allowed the sample to be characterized by AES before and after rates were measured. All rates were measured with 100 Torr CO and 400 Torr H2, and conversion to methane was always kept less than 1%. 

Unlike the techniques described above, more details will be given of TPD. The reason is not only that TPD is a less common technique than LEED, AES and XPS, but also that it has been used very heavily in this work and the apparatus has been developed in this laboratory. 

TPD The experiments were carried out using a standard apparatus. Helium was used as a carrier gas. The thermal conductivity detector was employed to detect the changes of desorption. A dry ice trap was used to remove the water in the carrier gas. The weight of sample was lOmg. The sample was pretreated in He flow at room temperature for Ih before the experiment. The heating rate was 10°C min . 

FT-IR spectra were obtained with a Nicolet Magna 750 apparatus. NH3 TPD measurements were carried out in a flow apparatus with a TCD detector as described before [4]. A KOHwater trap was placed before the detector in order to avoid water interference on TCD signal. Some TPD measurements were carried out by treating the sample with a 0.6% water vapour/He mixture after NH3 adsorption. 

The catalysts were characterized by X-ray diffraction (Philips PW 1710 diffractometer, (CuKa radiation) and FT-IR (Perkin Elmer 1750 instrument) analyses. N2 adsorp-tion/desorption isotherms were recorded at 77°K using a Catasorb apparatus. NH3-TPD (thermodesorption) tests were made in a flow system (0.3 g sample) after adsorption of ammonia (5% NH3 in He) at 1(X)°C up to complete saturation, flushing with He for 10 min and then heating up to 550°C at a rate of 10°C/min. The catalysts were pretreated in a pure He flow at 4(X)°C up to complete removal of adsorbed water. The NH3 concentration was followed with an on-line mass quadrupole detector. 

For TGA, a Setaram TGA-DTA 92 apparatus was used. Alternatively, a home-built apparatus was employed for TPD-MS. GC analysis was on a Chrompack CP-Sil-5 column, eventually coupled to a Fisons mass spectrometer. ESR spectra were recorded with a Bruker ESP-300 and a TE104 cavity at temperatures between 130 and 300 K. N2 sorption experiments were performed with an Omnisorp-100 instrument. The t-plot method was applied for the analysis of the pore volume. Solid state NMR spectra were recorded using a Bruker MSL 400 spectrometer at a resonance frequency of 100.61 MHz. Cross polarization was optimized with glycine as a reference. For the measurement of liquid samples, a Bruker AMX 300 system was used, operating at 300.13 and 75.47 MHz for iH and C, respectively. 

Figure 1. (a) Schematic of the apparatus which may be employed in a surface photochemistry experiment. The Sample (S) is mounted on a temperature controlled block in UHV. It may be cleaned by argon ion bombardment. The adsorbate is dosed onto the surface (D). Its dark state is determined by TPD (as measured by mass spectrometry, MS), EELS, UPS etc. Irradiation is by an arc lamp via a monochromator or filters. Products arc detected by the same analysis tools. 

The apparatus used for these experiments has been described previously (10). In a typical TPD experiment, 25 mg of sample were placed in a quartz microreactor which was mounted inside a furnace. Following evacuation for 1 h at room temperature, helium was flowed over the sample at a rate of 100 cc/min (STP) and the temperature was raised at 0.5 K/s. During heating, the desorption products were swept from the reactor by the helium stream and monitored downstream with a UTI Model 100 C quadrupole mass spectrometer. Upon completion of each TPD experiment, the mass spectrometer was calibrated for oxygen as described below, and then a TPR experiment was performed using a hydrogen flow rate of 200 cc/min (STP). After each TPR experiment, the mass spectrometer calibration was repeated. 

The equipment and procedures have been described in detail elsewhere [2-4]. The apparatus consists of a microbalance which is mounted inside a high vacuum chamber that can be evacuated to a background pressure of lO" torr. A quadrupole mass spectrometer, interfaced to a microcomputer, was used to sample the desorbing products. Approximately 15 mg of sample were spread in a thin layer over a flat sample pan in order to minimize bed effects in desorption [7,8]. The TPD-TGA experiments were performed with a heating rate of 10K/min. Isopropylamine was the molecule which was studied in most detail, and all of it was found to desorb completely during the experiment. 

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