He-TPD

The thermal stability of NOx adsorbed species and their reactivity in the presence of gaseous reductant molecules was addressed by thermal decomposition in He (TPD) or by heating in flowing H2/He mixtures [temperature-programmed surface reaction (TPSR)], respectively. In these cases, after NOx adsorption and He purge at the adsorption temperature (300 100oC), the samples were cooled to RT under flowing He. Then the samples were heated at 15°C/min up to 500-600°C in He (TPD) or in He + H2 (2000 ppm) (H2-TPSR). 

NO was stored on the catalyst surface under controlled conditions at 350°C (see Section 1 in Chapter 3) then the catalyst regeneration was performed at constant temperature by step addition of H2 (TRM), by thermal decomposition in He (TPD) and by heating in flowing H2 (TPSR). This allowed the analysis of the thermal stability/reactivity of the stored nitrates. 

For the TPD and FTIR experiments the samples were purged in a helium stream at 120°C for 1 h or in vacuum for 30 minutes at 100°C, respectively. Subsequently, samples were loaded with ammonia at 100°C. After second purging to remove the physisorbed NH3 (2-3 h at 100°C in He (TPD) or 0.5 h at 100°C in vacuum (FTIR)) the conventional TPD runs were performed at a heating rate of 10 K/min and a helium flow of 0.5 ml/min. The desorbed amount of ammonia was analysed continuously using a thermal conductivity cell. 

At low conversion and for catalysts with equal metal content, the TOF for RO was found to decrease with increasing acidity. Addition of acid sites to a metal/zeolite catalyst apparently not only opens an additional catalytic pathway but also modifies the metal sites (73). An alternative explanation of this observation, namely, deactivation of metal sites for RO by the benzene that is formed by RE, was rejected on the basis of experiments wherein catalysts were tested by TPD of adsorbed MCP. In this case no RE takes place and no benzene is present but he TPD products of adsorbed MCP from acidic Pd/HY and neutralized Pd/NaY were found to be entirely different (314). 

The TPR-MS and TPD-MS experiments were performed with a VG Sensorlab 200D Mass Spectrometer as analytical device. The gas flow rates of either H2(5%)/Ar (TPR) or He (TPD) were always 60 cm min-i, and the heating rate 10 K-min-i. We have also run TPO-fike experiments in a flow (60 cm min-i) of 02(5%)/He. In tins latter case, the anal3dical tool consisted erf a TCD detector the heating rate was 10 K-min. ... 

Fig. 9.3 NO2 adsorption and TPD over Fe-zeolite. Adsorption phase T = 200 °C, Q = 71 cmv min (STP), NO2 = 1,000 ppm, O2 = 2 % v/v (only for green lines), carrier gas = He TPD T-ramp = 20 °C/min, Q = 71 cm /min (STP), He flow, a NO/(N02 -NO ) as a function of time during the first 500 s of NO2 feed, b NO concentration as a function of time during the first 500 s of NO2 feed, c NO2 concentration as a function of time during the first 500 s of NO2 feed, d NO concentration as a function of catalysts temperature during TPD phase. Black lines = prereduced sample. Red lines = preoxidized sample. Green lines = preoxidized sample in the presence of oxygen during adsorption phase. Adapted from [14]...

He-TPD-MS of activated carbons treated by different methods are shown in Fig. 6.30. The types and contents of the surface groups have large changes after... 

Fig. 6.30 The He-TPD-MS spectra of the activated carbons (1) after graphitization treatment at 1,900 for C1900, (2) without treatment, (3) after by gas oxidation for C1900, (4) After treated by HNO3 for C1900.

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

The loss of sulfate during the reaction steps or during regeneration may become a critical issue when analyzing the potential of these materials as commercial catalysts. Sulfate losses during the butene TPD, made evident by the evolution of SO2 (m/e=64), started to occur at about 500°C. We have previously demonstrated the evolution of SO2 in the presence of adsorbates such as ammonia, benzene, or pyridine at temperatures much lower than those required to produce SO2 from clean sulfated zirconia [14]. For instance, A treatment in He at 600°C causes drastic losses which result in a significant drop in activity (see Fig. 3) It is... 

Accessibility to Cu sites was determined by temperature programmed desorption of NO (NO TPD), using an experimental setup similar to that used for TPR, except the detector was a quadrupole mass spectrometer (Balzers QMS421) calibrated on standard mixtures. The samples were first activated in air at 673 K, cooled to room temperature in air, and saturated with NO (NO/He 1/99, vol/vol). They were then flushed with He until no NO could be detected in the effluent, and TPD was started up to 873 K at a heating rate of 10 K/min with an helium flow of 50 cm min. The amount of NO held on the surface was determined from the peak area of the TPD curves. 

A series of spectra taken during the TPD of NO into a stream containing 2.14% CH4 in He are shown in Figure 3. At temperatures up to 300 °C these spectra are identical to those presented in Figure 2. However, at 300 °C the nitrito peak at 1515 cm- [shifted from its position at 1528 cm-1 at room temperature] is notably absent, presumably due to the reaction of Co-ONO with CH4. 

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

Figure 6.11. The TPD in He (heating rate 15°C/min, total flow lOONcc/min, catalyst weight 60mg) and TPSR in H2 (2000ppm) balance He (heating rate 15°C/min, total flow lOONcc/min, catalyst weight 60 mg) after N02 adsorption at 350°C over Ba/Al203 (20/100 w/w) catalyst. NO, N02, 02, N2, NH3 and H2 are outlet concentrations.

A different picture was obtained in the case of the reference Pt—BaAy-A Oj (1/20/100 w/w) sample (Figure 6.12). In the case of the TPD experiment performed after NO, adsorption at 350°C, the decomposition of stored NO, species was observed only above 350°C. Evolution of NO and 02 was observed in this case, along with minor quantities of N02 [25,28,33,35], Complete desorption of NO, was attained already slightly below 600°C. As in the case of the binary Ba/y-Al203 sample, the data hence indicate that nitrates formed upon N0/02 adsorption at 350°C followed by He purge at the same temperature, did not appreciably decompose below the adsorption temperature during the TPD run under inert atmosphere. 

The acidic properties were studied by temperature programmed desorption of ammonia (TPD-NH3). TPD experiments were carried out in the temperature range of 20 to 780°C in a flow of dry He (30 ml/min.). The rate of heating was 8 °C/min. 

Rieck and Bell/Mitchell and Vannice—Ce addition promotes WGS during methanation. Rieck and Bell351 examined 2%Pd/Si02 catalysts promoted with various lanthanides (La, Ce, Pr, Nd, and Sm) in the range 4.4-4.7%, among them, ceria, and carried out TPR (H2) and TPO (02) and H2 and CO TPD (He flow) experiments, along with TPSR experiments (H2 used instead of He). The catalysts were ramped at a rate of 1 °C per sec in a flow of 75 ccm H2 and 25 ccm CO. The authors observed that in addition to methanation, C02 was produced, and that the... 

The MS signals of NO (m/e=30) and O2 (m/e=32) as functions of desorption temperature were recorded during NO-TPD imder He flow for perovskites (Table 6). A broad NO desorption centered at 200 °C with one minor shoulder at 90 °C and another one at 388 °C was observed in the TPD of NO profdes for LaCoOs. Upon Cu substitution the low temperature peak is entirely suppressed, the 200 °C one is substantially increased and shifted to higher temperature so that it interferes with the minor high temperature one. Similar NO desorption features were found for lanthanum manganites and ferrites after NO adsorption, representing three superimposed desorption peaks. In Section 4, TPD of NO + O2 studies over... 

TPD of Cu-Al-MCM-41 (after NO adsorption under 0.8% NO in He) was eondueted (Table 14). NO and NO2 are the species detected coming off the surface as the temperature of the catalyst is increased. Two features were observed in the NO desorption profile a principal peak at 149 °C and a second NO desorption feature at higher temperature (440 °C). This indicates that there are at least two types of NO adsorption sites available. The presence of two types of adsorbed NO species over Cu catalysts has been reported earlier in the literature[45]. These have been proposed to be the desorption of NO from Cu ions and nitrate (NO3 ), nitrite (NO2 ) or N02 adsorbed species, respectively. Assuming the sensitivity factors of the peaks at low and high temperature are equivalent, the areas can be used to estimate the normalized desorption of NO. As listed in Table 14, the amount of NO desorbed at low temperature is close to the total amoimt of desorbed NO. This feature indicates that copper is mainly as isolated Cu in the catalyst. During NO desorption, a small amoimt of NO2 (8.2 pmol/g) desorbed at 80 °C. 

The adsorption of NO, under lean conditions was studied by imposing a step change of NO and NO2 feed concentrations in the presence and absence of excess oxygen over the reference catalysts in a fixed-bed flow microreactor operated at 350 ° C and analyzing the transient response in the outlet concentrations of reactants and products [transient response method (TRM)[. The adsorption/desorption sequence was repeated several times in order to condition the catalytic systems fully due to the regeneration procedure adopted (either reduction with 2000 ppm H2 + He or TPD in flowing He), BaO was the most Ba-abundant species present on the catalyst surface. FT-IR spectroscopy was used as a complementary technique to investigate the nature of the stored NO species. 

Figure 13.16 TPD in He and TPSR in H2 (2000ppm) + He after NO-O2 adsorption at 35O C over Pt-Ba/Al2O3 (1 20 100 w/w) catalyst. Adapted from ref. [119].

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. 

In order to elucidate the results of the CO TPD experiment, the detailed structure of the oxygen-modified Mo(l 12) surfaces and the adsorption sites of CO on these surfaces have been considered. Zaera et al. (14) investigated the CO adsorption on the Mo(l 10) surface by high-resolution electron-energy-loss spectroscopy (HREELS) and found vatop sites. Francy et al. (75) also found a 2100 cm loss for CO on W(IOO) and assigned it to atop CO. Recently, He et al. (16) indicated by infrared reflection-absorption spectroscopy that at low exposures CO is likely bound to the substrate with the C-0 axis tilted with respect to the surface normal. They, however, have also shown that CO molecules adsorbed on O-modified Mo(l 10) exhibi Vc-o 2062 and 1983 cm L characteristic to CO adsorbed on atop sites. Thus it is supposed that CO adsorbs on top of the first layer Mo atoms. 

XRD patterns were obtained with a Rigaku D/MAX-IIA diffractometer system equipped with Ni-filtered Cu-Ka radiation. N2 adsorption/desorption isotherms were measured with a Micromeritics ASAP 2010 system. NH3-TPD was conducted under He flow of 30 ml/min and a heating rate of 20 °C/min. 

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,... 

TPR of the samples in flowing He or H2 were performed in a Pyrex flow system which was also used for catalytic reactions. Acid properties of the samples were probed by TPD of NH3 preadsorbed at RT. The analysis of gaseous products was made by an on-line mass spectrometer or a thermal conductivity detector. Reactions of n-hexane in the presence of excess H2 were carried out at 623 K and atmospheric pressure. A saturator immersed in a constant temperature bath at 273 K was used to produce a reacting mixture of 6% n-hexane in H2. Reaction products were analyzed by an online gas chromatograph (HP-5890A) equipped with a flame ionization detector and an AT-1 (Alltech) capillary column. 

The nitrided catalyst (0.2 g) was heated in situ to 373 K in flowing He and held at RT for 1 h after the preparation. The catalyst was then heated at 0.0167 Ks-1 to 1263 K at a flow of 11.1 pmols-1 H2 (or He) for the TPR (or TPD) studies. The gases desorbed from the sample were monitored on-line using an ULVAC MSQ-150A quadrupole mass spectrometer equipped with a variable-leak valve heated by heating tape. 

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

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