Temperature-programmed desorption vs. N2 desorption in determining pore-size distribution of mesoporous silica molecular sieves

Two mesoporous silica molecular sieves, one synthesized by cationic template method and another by neutral template route, were characterized for their pore-size distribution by a novel Temperature-Programmed Desorption (TPD) method and the widely used N₂ desorption method. The pore-size distributions determined by the two methods agree quite well and are within experimental errors. For example, TPD method gave a pore size distribution (radius) centered at 14 Å while N₂ desorption method showed a peak centered at 13.3 Å. for the mesoporous silica prepared by cationic template route. The new TPD method based on thermogravimetric analysis is a viable option for mesopore characterization of silica-based materials.     

The temperature-programmed desorption of N2 from a Ru/MgO catalyst used for ammonia synthesis

The temperature-programmed desorption (TPD) of N₂ from a Ru/MgO catalyst used for ammonia synthesis was studied in a microreactor flow system operating at atmospheric pressure. Saturation with chemisorbed atomic nitrogen (N-*) was achieved by exposure to N₂ at 573 K for 14 h and subsequent cooling in N₂ to room temperature. With a heating rate of 5 K/min in He, a narrow and fairly symmetric N₂ TPD peak at about 640 K results. From experiments with varying heating rates a preexponential factor A_des = 1.5×10¹⁰ molecules/(site s) and an activation energy E_des = 158 kJ/mol was derived assuming secondorder desorption. This rate constant of desorption is in good agreement with results obtained with a Ru(0001) single crystal surface in ultra-high vacuum (UHV). The rate of dissociative chemisorption was determined by varying the N₂ exposure conditions. Determination of the coverage of N-^* was based on the integration of the subsequently recorded N₂ TPD traces yielding A_ads = 2×10^–6 (Pa s)^–1 and E_ads = 27 kJ/mol. The corresponding sticking coefficient of about 10^–14 at 300 K is in agreement with the inertness of Ru(0001) in UHV towards dissociative chemisorption of N₂. However, if the whole catalytic surface were in this state, then the resulting rate of N₂ dissociation would be several orders of magnitude lower than the observed rate of NH₃ formation. Hence only a small fraction of the total Rumetal surface area of Ru/MgO seems to be highly active dominating the rate of ammonia formation.     

N2Desorption in the Isothermal Decomposition of N2O on Rh(110) at Low Temperatures

Transient N₂desorption was analyzed in an angle-resolved form while a constant N₂O flux was introduced to clean Rh(110) or an oxygen-modified one. Even at 60 K, the decomposition proceeded and the product N₂desorption collimated at either 65 ± 3° or 30 ± 1° off normal towards the [001] direction.     

Highly active and stable bimetallic Ir/Fe-USY catalysts for direct and NO-assisted N2O decomposition

The current research investigated N₂O decompositions over the catalysts Ir/Fe-USY, Fe-USY and Ir-USY under various conditions, and found that a trace amount of iridium (0.1 wt%) incorporated into Fe-USY significantly enhanced N₂O decomposition activity. The decomposition of N₂O over this catalyst (Ir/Fe-USY-0.1%) was also partly assisted by NO present in the gas mixture, in contrast to the negative effect of NO over noble metal catalysts. Moreover, Ir/Fe-USY-0.1% can decompose more than 90% at 400 °C (i.e. the normal exhaust temperature) under simulated conditions of a typical nitric acid plant, e.g. 5000 ppm N₂O, 5% O₂, 700 ppm NO and 2% H₂O in balance He, and such an activity can be kept for over 110 h under these strict conditions. The excellent properties of bimetallic Ir/Fe-USY-0.1% catalyst are presumably related to the good dispersion of Fe and Ir on the zeolite framework, the formation of framework Al–O–Fe species and the electronic synergy between the Ir and Fe sites. The reaction mechanism for N₂O decomposition has been further discussed on the temperature-programmed desorption profiles of O₂, N₂ and NO₂.     

The reduction of NO with H2 over Ru/MgO

Ruthenium supported on magnesia was found to be a highly active and selective catalyst for the reduction of NO to N₂ with H₂. The adsorption of NO on Ru/MgO was studied at room temperature by applying frontal chromatography with a mixture of 2610 ppm NO in He. Subsequently, temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) experiments in H₂ were performed. The adsorption of NO was observed to occur partly dissociatively as indicated by the formation of molecular nitrogen. The TPD spectrum exhibited a minor NO peak at 340 K indicating additional molecular adsorption of NO during the exposure to NO at room temperature, and two N₂ peaks at 480 K and 625 K, respectively. The latter data are in good agreement with previous results with Ru(0001) single‐crystal samples, where the interaction with NH₃ was found to lead to two N₂ thermal desorption states with a maximum coverage of atomic nitrogen of about 0.38. Heating up the catalyst after saturation with NO at room temperature in a H₂ atmosphere revealed the self‐accelerated formation of NH₃ after partial desorption of N₂, whereby sites for reaction with H₂ become available. As a consequence, the observed high selectivity towards N₂ under steady‐state reduction conditions is ascribed to the presence of a saturated N+O coadsorbate layer resulting in an enhanced rate of N₂ desorption from this layer and a very low steady‐state coverage of atomic hydrogen. The formation of H₂O by reduction of adsorbed atomic oxygen is the slow step of the overall reaction which determines the minimum temperature required for full conversion of NO. This revised version was published online in July 2006 with corrections to the Cover Date.     

The dissociative adsorption of N2 on a multiply promoted iron catalyst used for ammonia synthesis: a temperature-programmed desorption study

The temperature-programmed desorption (TPD) of N₂ from a multiply promoted iron catalyst used for ammonia synthesis has been studied in a microreactor system at atmospheric pressure. From TPD experiments with various heating rates a preexponential factorA = 2 × 10⁹ molecules/site s and an activation energyE = 146 kJ/mol was derived assuming second-order desorption. The observed dependence of the TPD peak shapes on the heating rates indicated the influence of readsorption of N₂ in agreement with the results obtained for various initial coverages. Simulating the N₂ TPD curves using the model by Stoltze and Nørskov revealed that the calculated TPD curves were not influenced by the molecular precursor to desorption. However, the calculated rate of readsorption was found to be overestimated at high coverage compared with the experimental results. A coverage-dependent net activation energy for dissociative chemisorption (E*) was introduced as the simplest assumption rendering the dissociative chemisorption of N₂ activated at high coverage. The best fit of the experimental data yieldedE* = (–15+30) kJ/mol using only a single type of atomic nitrogen species. These findings are in satisfactory agreement with the parameters underlying the Stoltze-Nørskov model for the kinetics of ammonia synthesis as well as with the data reported for Fe(111) single crystal surfaces.     

Kinetic isotope effect in the reaction of NOads and COads on the Pt(100) surface

The reaction of CO with ¹⁵NO and ¹⁴NO mixtures in a co-adsorption layer on the Pt(100)-(hex) surface was studied by TPR. The kinetic isotope effect (KIE) manifests itself in the variation of the temperature of the maximum of the N₂ desorption peak depending on the isotopic composition: T_max(¹⁴N₂)<T _max(¹⁴N¹⁵N)≈ T_max(¹⁵N₂). The KIE observed is consistent with the assumption that the NO_ads dissociation is the rate-determining step of the reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of regeneration conditions on the cyclic performance of amine-grafted mesoporous silica for CO2 capture: An experimental and statistical study

This work deals with the behavior of amine-grafted mesoporous silica (referred to as TRI-PE-MCM-41) throughout adsorption–desorption cycles in the presence of 5% CO₂/N₂ using various regeneration conditions in batch experiments. The criteria proposed to determine the optimum regeneration conditions are the working adsorption capacity, the rate of desorption and the change of adsorption capacity between consecutive cycles. Using a 2³ factorial design of experiments, the impact on the performance of the adsorbent of different levels of temperature, pressure, and flow rate of purge gas during desorption was determined. It was found that all the parameters under study have a statistically significant influence on the working adsorption capacity, but only temperature is influential with respect to desorption rate. Regeneration using temperature swing was found to be attractive, as the highest CO₂ adsorption capacity (1.95 mmol g^?1) and the fastest desorption rate (9.82×10^?4 mmol g^?1 s^?1) occurred when desorption was carried out at 150 °C. However, if vacuum is applied, regeneration can be achieved at a temperature as low as 70 °C with only a 13% penalty in terms of working adsorption capacity. It was also demonstrated that under the proper regeneration conditions, TRI-PE-MCM-41 is stable over 100 adsorption–desorption cycles.     

Isotopic study of nitrous oxide decomposition on an oxidized Rh catalyst: mechanism of oxygen desorption

N₂O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O/oxidized Rh catalyst at 493 K and desorbed O₂ molecules were monitored. The ¹⁸O fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N₂O decomposition reaction at low temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Second Order Isothermal Desorption Kinetics

A new method is presented to analyze recombinative desorption from surfaces in an isothermal mode. The activation energy for desorption obtained this way is accurate as long as it is coverage independent. Second order recombinative desorption experiments of ¹⁵N₂ and D₂ from Ru(001) were used to demonstrate this method. The activation energies were E _a(N₂) = 48±2 kcal/mol and E _a(D₂) = 22±1 kcal/mol for coverages below 0.1 and 0.2 of saturation coverage, respectively. Studying N/Ru(001) provides evidence for bulk nitrogen atoms that slowly diffuse to the surface leading to isotope scrambling.     

N2O decomposition over K-promoted Co-Al catalysts prepared from hydrotalcite-like precursors

N₂O decomposition was investigated over a series of K-promoted Co-Al catalysts. The activity tests showed that doping with K greatly enhanced the catalytic activity of the Co-Al catalyst, and the enhancement was critically dependent on the amount of K and the calcination temperature. When the catalyst had a K/Co atomic ratio of 0.04 and was calcined at 700–800 °C, a full N₂O conversion could be reached at a reaction temperature of 300 °C. Moreover, even under the simultaneous presence of 4% O₂ and 2.6% water vapor, such high-temperature treated K/Co-Al catalyst exhibited high reactivity and stability, with the N₂O conversion remaining at a constant value of 92% over 40 h run at 360 °C. In contrast, non-doped Co-Al catalyst showed a severe activity loss under such reaction conditions. A combination of characterization techniques was employed to reveal the promoting role of K and the effect of calcination temperature. The results suggest that doping with K increases the electron density of Co and weakens the Co–O bond, thus promoting the activation of N₂O on the Co sites and facilitating the desorption of oxygen from the catalyst surface. High-temperature calcinations made the desorption of O₂ proceed more readily.     

Effect of reductants in N2O reduction over Fe-MFI catalysts

We investigated the effect of reductants over ion-exchanged Fe-MFI catalysts (Fe-MFI) based on the catalytic performance in N₂O reduction in the presence and absence of an oxygen atmosphere. In the case of N₂O reduction with hydrocarbons (CH₄, C₂H₆, and C₃H₆) in the presence of excess oxygen, the order of N₂O contribution was as follows: CH₄ > C₂H₆ > C₃H₆. This indicates that CH₄ is a more efficient reductant than C₂H₆ and C₃H₆. The TOFs of N₂O decomposition and the N₂O reduction by various reductants (H₂, CO, CH₄) in the absence of oxygen increased with increasing Fe/Al ratio (Fe/Al⩾0.15), wheras the TOFs were lower and constant in the range of Fe/Al⩽0.10. Temperature-programmed reduction with hydrogen (H₂-TPR) showed that the catalysts with a higher Fe/Al ratio were reduced more easily than those with a lower Fe/Al ratio. Temperature-programmed desorption of O₂ (O₂-TPD) showed that oxygen was desorbed at lower temperatures over the catalysts with a higher Fe/Al ratio. As the result of extended X-ray absorption fine structure (EXAFS) analysis, only mononuclear Fe species were observed over Fe(0.10)-MFI after treatment with N₂O or O₂. On the other hand, binuclear Fe species and mononuclear Fe species were observed over Fe(0.40)-MFI after treatment with N₂O or H₂. More reducible Fe species, which gave lower-temperature O₂ desorption, can be due to Fe binuclear species. Since the N₂O reduction with reductants proceeds via a redox mechanism, the reducible binuclear Fe species can exhibit higher activity. Furthermore, CH₄ can be oxidized by N₂O more easily than can H₂ and CO, although it is generally known that the reactivity of methane is very low.     

Adsorbed Atoms and Molecules Destined for a Reaction

The idea of an activation complex is popular for explaining reaction rates, but the characteristics of reactions and catalysis may not be explained in this way. A predestined state for each reaction composed of surface atoms and adsorbed species is responsible for these features. Two single Sn atoms trapped in adjacent half-unit cells of an Si(111) 7 × 7 surface is an example of a predestined state. An isolated Sn atom in a half-unit cell does not migrate to other half-unit cells at room temperature, but when two single Sn atoms are in adjacent half-unit cells they undergo rapid combination to form an Sn₂ dimer. In addition, these two single Sn atoms replace the center Si adatoms and an Si₄ cluster is formed. The spatial distribution of molecules desorbing from surfaces may reflect the predestined states for the desorption processes. The spatial distribution in the temperature-programmed desorption (TPD) of NO on Pd(110) and Pd(211) surfaces and that in the temperature-programmed reaction (TPR) of NO + H₂ were studied. N₂ desorbing from Pd(110) by the recombination of N atoms obeys cos⁶ – cos⁷ but the N₂ produced by a catalytic reaction of NO with H₂ obeys cos. In contrast, the N₂ desorbing with NO at 490 K in the TPD of Pd(110) shows a sharp off-normal distribution expressed by cos⁴⁶( – 38). The adsorption of NO on Pd(211) predominantly occurs on the (111) terrace but the spatial distribution suggests that the predestined states for the reaction and desorption are formed on both the (111) terrace and (100) step surfaces.     

Catalytic desorption of CO2-loaded solutions of diethylethanolamine using bentonite catalyst

The absorption of CO₂ gas into aqueous alkanolamine solutions is the most advanced CO₂ separation technology and a key challenge in this technique is the energy-intensive process of solvent regeneration. The tertiary amine N,N′-diethylethanolamine (or DEEA) is a candidate CO₂-capturing solvent with potential. To improve the energy efficiency of regeneration of DEEA, several catalysts were used for desorbing CO₂ from loaded solutions of DEEA (2.5 M) at T = 363 K. Desorption trials were conducted in batch mode. The initial CO₂ loading varied in the 0.3–0.35 mol CO₂/mol DEEA range. The performance was analyzed by calculating the rate of CO₂ desorption, cyclic capacity, and reduction in sensible energy. The amount of thermal energy needed for amine regeneration was significantly lowered by using nine transition metal oxide catalysts and the hierarchy was as follows: Al₂O₃ < MoO₃ < V₂O₅ < TiO₂ < MnO₂ < ZnO < Cr₂O₃ < SiO₂ < ZrO₂. Among the metal oxides, Al₂O₃ increased desorption efficiency compared to blank runs by 89%. A clay-based powder bentonite was also used as catalyst and its efficacy was compared with the metal oxides. This cheap and easily available bentonite catalyst was tuned through simple ion-exchange with four acids (HCl, H₃PO₄, HNO₃, and H₂SO₄). Upon treatment with H₂SO₄, bentonite remarkably increased desorption efficiency by 100%. Furthermore, bentonite catalyst treated with sulphuric acid (denoted here as Bt/H₂SO₄) was characterized by Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), Fourier transform infrared spectrometery (FTIR), X-ray diffraction (XRD), and ammonia temperature-programmed desorption (NH₃-TPD). In this way, a comprehensive study on catalytic desorption of DEEA was performed.     

H2/D2 adsorption and desorption studies on carbon molecular sieves with different pore structures

The critical effect of confinement on the interaction of hydrogen isotopes (H₂ and D₂) with carbon surfaces was investigated through a combined low temperature adsorption/thermal desorption spectroscopy (TDS) study on three carbon molecular sieves (CMS) possessing nanopores with nominal sizes between 0.3 and 0.5 nm. The porous structure and the sorption properties of all three adsorbents were characterized by N₂ (77 K) and CO₂ (273 K), as well as H₂ and D₂ (77 K) low pressure (up to 1 bar) adsorption measurements. The interaction of the carbons with hydrogen, deuterium, and an isotopic H₂/D₂ gas mixture was further studied by means of TDS measurements, extended to temperatures down to 20 K. The differences in the H₂/D₂ adsorption/desorption profiles of the three CMS samples are correlated with the respective micropore size distributions. The presence of very narrow micropores, with size close to the kinetic diameter of the hydrogen molecule, resulted in enhanced hydrogen (both for H₂ and D₂) interactions, giving rise to a TDS maximum centered on 122 K, the highest desorption temperature ever measured for the desorption of physisorbed hydrogen. Furthermore, the quantum effects on hydrogen/deuterium adsorption on CMS adsorbents have been addressed for the first time using the TDS technique.     

Thermally induced variation in redox chemical bonding structures of single-walled carbon nanotubes exposed to hydrazine vapor

The effect of hydrazine (N₂H₄) vapor on the properties of single-walled carbon nanotube (SWCNT) networks was investigated by sheet resistance measurement, scanning electron microscopy, Raman spectroscopy, ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS). Our results show that, even after an auxiliary thermal desorption treatment at 80 °C, the n-doping effect on our SWCNTs caused by N₂H₄ vapor still persistently remained. Further analysis on the XPS data suggests that a reactive chemical species, nitrene (NH), generated during thermal decomposition of N₂H₄, could react with SWCNTs by cycloaddition to form cyclic nitrogen-containing aziridine structures on SWCNTs. Our results also show that the formed nitrogen-containing bonding structures were thermally metastable and could be significantly eliminated upon further annealing at 350 °C. Moreover, it was found that the N₂H₄ vapor treatment could introduce nitroso groups and carbonyl groups, but not carboxyl groups, to our pristine SWCNTs. The mild oxidation could be attributed to the HNO₂ and H₂O₂ produced from the reactions of NH and N₂H₄ with oxygen, respectively, when a N₂H₄ treatment was performed in air.     

Determination of the pore size distribution of mesoporous silicas by means of quasi-equilibrated thermodesorption of n-nonane

Temperature programmed desorption profiles of n-nonane were measured for MCM-41, SBA-15 and HMS mesoporous silicas under quasi-equilibrium conditions using standard TPD setup with a chromatographic detector, utilizing He + 0.4% n-C₉H₂₀ mixture as a carrier gas. A method for determination of the pore size distributions according to a modified BJH scheme, based on the Kelvin equation for the fluid core radius and the BET-type function for the adsorbed film thickness, was proposed. The resulting pore size distributions are in good agreement with those obtained by standard BJH analysis of the N₂ desorption isotherms.     

Sorption studies of CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript>, N<Subscript>2</Subscript>, CO,O<Subscript>2</Subscript> and Ar on nanoporous aluminum terephthalate [MIL-53(Al)]

Aluminum terephthalate, MIL-53(Al), metal–organic framework synthesized hydrothermally and purified by solvent extraction method was used as an adsorbent for gas adsorption studies. The synthesized MIL-53(Al) was characterized by powder X-Ray diffraction analysis, surface area measurement using N₂ adsorption–desorption at 77 K, FTIR spectroscopy and thermo gravimetric analysis. Adsorption isotherms of CO₂, CH₄, CO, N₂, O₂ and Ar were measured at 288 and 303 K. The absolute adsorption capacity was found in the order CO₂>CH₄>CO>N₂>Ar>O₂. Henry’s constants, heat of adsorption in the low pressure region and adsorption selectivities for the adsorbate gases were calculated from their adsorption isotherms. The high selectivity and low heat of adsorption for CO₂ suggests that MIL-53(Al) is a potential adsorbent material for the separation of CO₂ from gas mixtures. The high selectivity for CH₄ over O₂ and its low heat of adsorption suggests that MIL-53(Al) could also be a compatible adsorbent for the separation of methane from methane–oxygen gas mixtures.     

Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite

Mechanical activation of boehmite (γ-AlOOH), synthesized by thermal decomposition of gibbsite, has been carried out in a planetary mill up to 240 min. After an initial decrease in particle size up to 15 min, the particle size shows an increase with further milling; the median size (d₅₀) has increased from 1.8 to 5 μm during 15 to 240 min of milling. Quite unexpectedly, the BET specific surface area of the sample (N₂ adsorption method) decreases continuously from 264 m²/g to 67 m²/g with milling. A detailed analysis of N₂ adsorption/desorption isotherms has indicated that the decrease in surface area is associated with: (a) change in narrow slit like pores with microporosity to slit shaped pores originating from loose aggregate of platelet type particles; and (b) shift of maxima in pore size distribution plot at ~ 2 nm and ~ 4 nm to dominantly ~ 23 nm size pores. Scanning electron microscopy (SEM) studies have revealed that during milling, initial breakage is followed by agglomeration/fusion of particles with consequent loss in porosity. Amorphisation, decrease in microcrystallite dimension (MCD) and increase in microstrain (ε) are indicated from a detailed analysis of X-ray powder diffraction patterns and Fourier Transform Infrared (FTIR) spectra. Reactivity of samples, expressed in terms of increase in dissolution in alkali (in 8 M NaOH at 90 °C) and decrease in boehmite to γ-Al₂O₃ transformation temperature, increases with milling time. The nature of correlations between reactivity and physico-chemical changes during milling has been analyzed and discussed.     

Catalytic Activity of Ruthenium Nanoparticles Supported on Carbon Nanotubes for Hydrogenation of Soybean Oil

Ruthenium catalysts supported on multiwalled carbon nanotubes (MCNTs) with different loadings (1% wt, 3% wt, 5% wt) were prepared by reduction with H₂ or NaBH₄ for selective hydrogenation of soybean oil at 338 K and initial pressure of 1.066 MPa. These catalysts were characterized using transmission electron microscopy (TEM), X-ray powder diffraction (XRD), N₂ adsorption–desorption, and H₂-temperature programmed desorption (TPD) techniques. Ru particles were dispersed more homogeneously on the surface of the nanotubes after being reduced with H₂ than with NaBH₄. The catalysts with 3% and 5% Ru loadings had higher hydrogenation activity. The NaBH₄-reduced catalyst had higher cis-isomer selectivity.