Temperature-programmed Desorption Profiles Research Articles

Experimental study of the binding energy of NH3 on different types of ice and its impact on the snow line of NH3 and H2O

Context. Nitrogen-bearing molecules (such as N2H+ and NH3) are excellent tracers of high-density and low-temperature regions, such as dense cloud cores. Notably, they could help advance the understanding of snow lines in protoplanetary discs and the chemical evolution of comets. However, much remains unknown about the chemistry of N-bearing molecules on grain surfaces, which could play an important role in their formation and evolution. Aims. In this work, we experimentally study the behaviour of NH3 on surfaces that mimic grain surfaces under interstellar conditions in the presence of some other major components of interstellar ices (i.e. H2O, CO2, CO). We measure the binding energy distributions of NH3 from different H2O ice substrates and also investigate how it could affect the NH3 snow line in protoplanetary discs. Methods. We performed laboratory experiments using the ultra-high vacuum (UHV) set-up VENUS (VErs des NoUvelles Syntheses). We co-deposited NH3 along with other adsorbates (H2O, 13CO, and CO2) and performed temperature programmed desorption (TPD) and temperature programmed-during exposure desorption (TP-DED) experiments. The experiments were monitored using a quadrupole mass spectrometer and a Fourier transform reflection absorption infrared spectrometer (FT-RAIRS). We obtained the binding energy distribution of NH3 on crystalline ice (CI) and compact amorphous solid water ice by analysing the TPD profiles of NH3 obtained after depositions on these substrates. Results. In the co-deposition experiments, we observed a significant delay in the desorption and a decrease of the desorption rate of NH3 when H2O is introduced into the co-deposited mixture of NH3–13CO or NH3–CO2, which is not the case in the absence of H2O. Secondly, we noticed that H2O traps roughly 5–9% of the co-deposited NH3, which is released during the phase change of water from amorphous to crystalline. Thirdly, we obtained a distribution of binding energy values of NH3 on both ice substrates instead of an individual value, as assumed in previous works. For CI, we obtained an energy distribution between 3780 K and 4080 K, and in the case of amorphous ice, the binding energy values were distributed between 3630 K and 5280 K; in both cases we used a pre-exponential factor of A = 1.94 × 1015 s−1. Conclusions. From our experiments, we conclude that the behaviour of NH3 is significantly influenced by the presence of water, owing to the formation of hydrogen bonds with water, in line with quantum calculations. This interaction, in turn, preserves NH3 on the grain surfaces longer and up to higher temperatures, making it available closer to the central protostar in protoplanetary discs than previously thought. It explains well why the NH3 freeze-out in pre-stellar cores is efficient. When present along with H2O, CO2 also appears to impact the behaviour of NH3, retaining it at temperatures similar to those of water. This may impact the overall composition of comets, particularly the desorption of molecules from their surface as they approach the Sun.

Saccharide formation by sustainable formose reaction using heterogeneous zeolite catalysts.

The formose reaction is a unique chemical reaction for the preparation of saccharides from formaldehyde, a single carbon compound. We applied zeolite materials as heterogeneous catalysts to the formose reaction. The simple addition of Linde type A zeolite containing calcium ions (Ca-LTA) to an aqueous solution of formaldehyde and glycolaldehyde produced saccharides at room temperature. A quantitative analysis performed by high-performance liquid chromatography revealed that triose, tetrose, pentose, and hexose saccharides were produced with few byproducts. Ca-LTA was recovered from the reaction mixture by filtration, and the retrieved zeolite was found to be reusable under the same conditions. The catalytic activity of Ca-LTA was higher than those of conventional calcium catalysts and other solid materials such as silica, alumina, and hydroxyapatite. Several other types of zeolites with different crystal structures and alkali/alkali-earth metal ions also showed catalytic activity for saccharide formation. Based on the analytical results obtained by infrared spectroscopy, temperature-programmed desorption profiles and NMR measurements, we propose a reaction mechanism in which C-C bond formation is promoted by the mild basicity of the oxygen atoms and acidity on the metal ions of the aluminosilicate on the zeolite surfaces with low SiO2/Al2O3 ratios.

Oxygen adsorption capability and electrochemical properties induced by oxygen vacancies in cerium-doped LaFeO3 perovskite oxide

The effect of Ce doping on the electrochemical properties of LaFeO3 perovskite oxides was investigated in terms of the oxygen adsorption capability and polarization resistance. The Ce-doped LaFeO3 perovskite oxide had a single perovskite phase with an orthorhombic structure. The presence of oxygen vacancies was confirmed by EPR spectra and O 1 s core-level spectra. An O2 temperature-programmed desorption profile demonstrated an increase in the quantity of surface-active oxygen species, with LCFO5 exhibiting the highest oxygen adsorption capability. The area-specific resistance gradually decreased with the cerium doping from 11.36 Ωcm2 (LFO) to 4.65 Ω cm2 (LCFO5). A notable decrease of 68 % was confirmed in the polarization resistance related to the mass transfer, from 7.66 Ωcm2 (LFO) to 2.44 Ωcm2 (LCFO5). This significant decrease indicates an enhanced oxygen reduction reaction activity, which is attributed to the accelerated oxygen adsorption reaction.

Conformer-Specific Desorption in Propanol Ices Probed by Chirped-Pulse Millimeter-Wave Rotational Spectroscopy.

We present a new technique for the detection of molecules desorbed from an ice surface using broad-band millimeter-wave rotational spectroscopy. The approach permits interrogation of molecules that have undergone the slow warmup process of temperature-programmed desorption (TPD), analogous to the warmup phase of icy grains in the interstellar medium as they approach the central protostar. The detection is conformer- and isomer-specific and quantitative, as afforded by chirped-pulse rotational spectroscopy. To achieve this, we combine ice TPD with buffer gas cooling, followed by detection in the millimeter-wave regime. In this report we examine the TPD profiles of n- and i-propanol, the former of which may be in five different conformational isomeric forms, and which display distinct desorption profiles. The limited conformational isomerization and temperature-dependent relative yields of n-propanol conformers observed show that the desorption is highly conformer-specific.

Homogeneous-like Alkyne Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation, which is currently accomplished by modified palladium catalysts. Herein, we rep...

Effect of coking and propylene adsorption on enhanced stability for Co2+-catalyzed propane dehydrogenation

The effects of Si-additive modification on nonoxidative propane dehydrogenation (PDH) over γ-Al2O3 sheet-supported isolate Co2+ catalysts have been investigated. Si addition cannot change the nanosheet structure and tetrahedral Co2+ coordination of the Co catalysts; nonetheless, it introduces a number of moderate-strength acid sites by the formation of composite SiAl oxides. Promoted catalytic stability is achieved on Si-modified Si–Co–Al2O3 in comparison with the Si-free Co–Al2O3 catalyst. In spite of similar total content and H/C ratios of deposited coke species on these two spent catalysts, a relatively lower graphitization degree and a higher ratio of highly reactive coke species are observed on the Si–Co–Al2O3 catalyst. Furthermore, the co-feeding of C3H6 significantly changes the coking reaction rates but not the coke distributions on Co-based catalysts. The C3H6 temperature-programmed desorption profiles and in situ diffuse reflectance infrared Fourier transform spectra suggest that the interaction of C3H6 with Co catalysts can be affected by Si modification tuning the coking property and the catalytic stability of Co2+–Al2O3-based catalysts for the PDH reactions.

An egg-shell bifunctional CeO2-modified NiPd/Al2O3 catalyst for petrochemical processes involving selective hydrogenation and hydroisomerization

The catalytic performance during the 1-butyne hydrogenation using two reduced Al2O3-supported Pd-based catalysts was carried out in a total recirculation system with an external fixed-bed reactor. The lab-prepared egg-shell NiPd/CeO2-Al2O3 catalyst (NiPdCe) with Pd loading = 0.5 wt%, Ni/Pd atomic ratio = 1 and CeO2 loading = 3 wt% was synthesized and characterized, and it was compared with an egg-shell Al2O3-supported Pd based commercial catalyst (PdCC). The reduced catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The textural characteristics and ammonia temperature-programmed desorption profiles of the fresh (unreduced) catalysts were also obtained. Both catalysts show high 1-butyne conversion and selectivity to 1-butene, but the catalysts also present important differences between hydroisomerizing and hydrogenating capabilities. NiPdCe catalyst shows higher capability for hydroisomerization reactions, while the PdCC catalyst exhibits higher hydrogenating capability. The observed catalytic performances can be interesting for some industrial processes and can provide a guideline for the development of a Pd-based catalyst with specific catalytic properties.

Unusual morphology transformation and basicity of magnesium oxide controlled by ageing conditions and its carbon dioxide adsorption

The physicochemical changes of magnesium oxide (MgO) by different preparation methods have been extensively explored for various applications; however, few studies have been conducted on the morphology and physicochemical properties of MgO controlled by various ageing conditions even though an ageing step is one of the important steps in the precipitation. In this study, morphological transformation of MgO was achieved by controlling the ageing time at different temperatures and the unusual transformation was observed; Microsphere-structured MgO, known to be unstable at low temperatures, could be formed even at 25 °C, owing to the abundant amount of OH−. In addition, the surface basicity was significantly affected by the aging conditions. Ageing at 60 °C furnished stronger basic sites on MgO, which resulted from the abundant oxygen vacancies. Notably, an unusual peak in the sample prepared at 80 °C of aging temperature was observed at 656 °C in the CO2 temperature-programmed desorption profile, and it was attributed to the desorption of strongly adsorbed hydroxyl groups. The surface area was maximized (350 m2 g−1) by employing the ageing temperature of 80 °C, and the CO2 sorption capacities of as-prepared samples were 6–7 wt.% during adsorption for 120 min at 25 °C. Understanding the physicochemical properties and basicity of MgO can facilitate the design of adsorbents or catalysts as a platform material.

An Aging Model of NH3 Storage Sites for Predicting Kinetics of NH3 Adsorption, Desorption and Oxidation over Hydrothermally Aged Cu-Chabazite

A unified transient kinetic model which can predict the adsorption, desorption and oxidation kinetics of NH3 over hydrothermally aged Cu-chabazite was developed. The model takes into account the variation of fractional coverages of NH3 storage sites due to hydrothermal aging. In order to determine the fractional coverage of these sites, the catalyst was aged for various times at a certain temperature followed by NH3 adsorption, desorption and temperature-programmed desorption (TPD) experiments. TPD profiles were deconvoluted mainly into three peaks with centres at 317, 456 and 526 °C, respectively. Hydrothermal aging resulted in the progressive increase in the intensity of the peak at 317 °C and decrease in the intensity of the peaks at 456 and 526 °C, along with decreased NH3 oxidation at high temperatures. A model for hydrothermal aging kinetics of the fractional coverage of storage sites was developed using three reactions with appropriate rate expressions with parameters regressed from experimental data. The model was then incorporated into a multi-site kinetic model for the degreened Cu-Chabazite by the addition of aging reactions on each storage site. The effects of both aging time and temperature on the kinetics NH3 adsorption, desorption and oxidation were successfully predicted in the 155-540 °C range. This study is the first step towards the development of a hydrothermal aging-unified kinetic model of NH3-Selective Catalytic Reduction over Cu-chabazite.

Micro-Kinetic Modelling of CO-TPD from Fe(100)—Incorporating Lateral Interactions

The experimentally determined temperature programmed desorption profile of CO from Fe(100) is characterized by four maxima, i.e., α1-CO, α2-CO, α3-CO, and β-CO (see e.g., Moon et al., Surf. Sci. 1985, 163, 215). The CO-TPD profile is modeled using mean-field techniques and kinetic Monte Carlo to show the importance of lateral interactions in the appearance of the CO-TPD-profile. The inclusion of lateral interactions results in the appearance of a new maximum in the simulated CO-TPD profile if modeled using the mean-field, quasi-chemical approach or kinetic Monte Carlo. It is argued that α2-CO may thus originate from lateral interactions rather than a differently bound CO on Fe(100). A detailed sensitivity analysis of the effect of the strength of the lateral interactions between the species involved (CO, C, and O), and the choice of the transition state, which affects the activation energy for CO dissociation, and the energy barrier for diffusion on the CO-TPD profile is presented.

The Counterion Effect of Imidazolium-Type Poly(ionic liquid) Brushes on Carbon Dioxide Adsorption.

Imidazolium-based poly(ionic liquid) brushes were attached to spherical silica nanoparticles bearing various functionalities by using a surface-initiated atom transfer radical polymerization ("grafting from" technique). A temperature-programmed desorption process was applied to evaluate and analyze the carbon dioxide adsorption performance of the synthesized polymer brushes. The confined structure of the surface-attached polymer chains facilitates gas transport and adsorption, leading to an enhanced adsorption capacity of carbon dioxide molecules compared with pure polymer powders. Temperature-programmed desorption profiles of the synthesized polymer brushes after carbon dioxide adsorption reveal that the substituent groups on the nitrogen atom at the 3-position of the imidazole ring, as well as the associated anions significantly affect the adsorption capacity of functionalized poly(ionic liquid) brushes. Of the tested samples, amine-functionalized poly(ionic liquid) brushes associated with hexafluorophosphate ions exhibit the highest carbon dioxide adsorption capacity of 2.56 mmol g-1 (112.64 mg g-1 ) at 25 °C under a carbon dioxide partial pressure of 0.2 bar.

Systematic study of the role played by ZnCl2 during the carbonization of a chemically activated carbon by TG–MS and DSC

In this study, thermal and calorimetric techniques are used in order to get a better insight into the progress of the carbonization process of a lignocellulosic material for activated carbon preparation. Therefore, the pyrolysis process of the precursor (olive waste) alone and in the presence of the activating agent (ZnCl2) at different temperatures is followed via thermogravimetry–mass spectroscopy analysis. The mass loss and the temperature programmed desorption profiles of the signals m/e = 16, 18, 28, 30, and 44 allow the determination of the role of the activating agent during the synthesis of the activated carbons at different temperatures. In order to follow the changes in energy during the pyrolysis process, differential scanning calorimetry is used. The obtained results are linked to the properties of the activated carbons obtained at the different temperatures and characterized in a previous paper. In this manner, it is possible not only to determine the thermal events taking place during the pyrolysis process but also to unravel their impact on the textural, structural, and surface properties of the final materials.

Changes in physicochemical properties of activated carbon during treatment with supercritical water

AbstractWe investigated the changes in activated carbon (AC) physicochemical properties during treatment with supercritical water (SCW). Experimental results show that SCW treatment can increase the specific surface area and pore volume of AC. Scanning electron microscopy proves that the AC surface undergoes severe corrosion, resulting in the exposure of highly porous structures. In addition, the point‐of‐zero charge of AC increases after SCW treatment without the addition of HCl, H2SO4, and H2O2. This result is confirmed by the presence of C≡C and −OH functional moieties, as established through Fourier transform infrared spectroscopy. Temperature programmed desorption profiles suggest that the decomposition of AC in the SCW treatment produces pyrone and anhydride functional groups. According to the above results, SCW treatment increases the adsorption capacities of AC. In SCW catalytic processes, therefore, active components removed from the surface of AC‐supported catalysts are directly involved in reactions that enhance the catalytic activity.

Interaction of coadsorbed C and O atoms on W(100) surface: Implications of CO2 on the temperature-programmed desorption profile

The authors study the adsorption and interaction of C and O atoms on the W(100) surface by performing density functional theory based calculations. Previous experimental works showed that C and O atoms exist on the surface when CO and CO2 dissociate. In this regard, the authors investigated the possibilities of forming O2 and CO2 from the coadsorbed species. Our results reveal that O2 is not stable as a molecule and will dissociate on the surface. CO2 formation on the other hand is accompanied by an activation barrier and is stable in a bent configuration. Its formation competes with CO dissociation and recombination. The authors will demonstrate through kinetic Monte Carlo simulations the implication of CO2 formation on the temperature programmed desorption profile and show that if stable and present in significant numbers, it would lead to erosion of the β2 peak. The authors therefore deduce that CO2 formation is most likely not a stable process and that CO2 formation and its subsequent decomposition are not significant in CO/W(100).

A Non-Precious Metal Promoting the Synthesis of 5-Hydroxymethylfurfural

In this work, a new kind of catalyst was prepared for synthesis of 5-hydroxymethylfurfural. Copper ions were incorporated into manganese oxide octahedral molecular sieves (K-OMS-2). The catalysts Cu-K-OMS-2 were characterized by measuring FTIR spectra, scanning electron microscope images, X-ray diffraction patterns, and temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR) profiles. Thermogravimetric analysis (TGA) demonstrated that the stability of Cu-K-OMS-2 is almost the same as that of K-OMS-2. XRD patterns showed that introducing copper ions did not change the structure of K-OMS-2, but copper ions had an effect on the morphology of K-OMS-2 as illustrated by SEM images. TPD profiles demonstrated that both K-OMS-2 and Cu-K-OMS-2 possess basic and acidic sites, and Cu-K-OMS-2 has weak acidic sites. One-pot synthesis of 2,5-diformylfuran (DFF) from fructose was investigated under the catalysis of Cu-K-OMS-2 together with a commercial catalyst Amberlyst 15. The effect of reaction time and temperature on the DFF yield was investigated, and reaction temperature had an effect on the DFF yield. The effect of atomic ratio of Cu to Mn of Cu-K-OMS-2 on the DFF yield was also investigated. The DFF yield was improved 34.7% by Cu-K-OMS-2 in comparison to K-OMS-2, indicating the promotion effect of copper on the DFF yield. Consecutive use of Cu-K-OMS-2 demonstrated that after 6 cycles, the loss of DFF yield was 6.3%, indicating a good reusability of Cu-K-OMS-2.

A structure sensitivity approach to temperature programmed desorption

A novel approach for explaining the effect of the metal cluster size on Temperature Programmed Desorption (TPD) from supported catalysts is described. Based on the assumption that metal clusters have regular cuboctahedral geometry, it was possible to develop a model that describes how three dimensional aspects of the metal clusters may affect the TPD profiles. Depending on the way that the metal clusters are assumed to be arranged in the surface, two different approaches are described to account for desorption. The first one, named Monocluster Model, considers a surface with a narrow cluster size distribution in which an average cluster diameter reasonably well describes the cluster population. A more complete approach, named Multicluster Model, explains desorption based on the cluster size distribution of the metal sites. TPD curves of ammonia from different platinum supported carbon materials were numerically treated and compared with the geometrical information obtained from Transition Electron Microscopy. The results displayed a reasonable agreement, indicating that TPD curves should be interpreted taking into account the metal cluster size.

Numerical study of the diffusion of binary hydrocarbon blends in ZSM-12 zeolites

The experimental results for the simultaneous diffusion of toluene and ethylene in one dimensional channels of the ZSM-12 zeolite which have been reported in a previous publication are fitted with an original finite difference numerical simulation model. This system is mimicking the single file diffusion of hydrocarbons in the zeolite passive hydrocarbon trap installed at the exhaust of an internal combustion engine, prior to the catalytic muffler, to solve the hydrocarbon cold start emission problem. The study encompasses the effects of Na+, H+ and Ag+ counterions and temperature ramp on the temperature programmed desorption of the binary mixture. A simple lattice gas model is proposed to simulate the binary gas diffusion in a one dimensional pore lattice. Numerical simulations show that taking into account an Arrhenius dependence of the binary gas diffusion coefficient on temperature yielded a good fit of these experimental results, explaining many significant features of the temperature programmed desorption profile observed experimentally.

Determination of the acidic properties of carboxylated carbocatalysts in an acid-catalyzed ring-opening reaction using kinetic profiling

The acid-catalyzed ring-opening reaction of styrene oxide was used as a probe reaction for evaluating the acidic properties of carboxylated carbocatalysts. Significant discrepancies in the initial reaction rates were normalized using the total number of carboxyl groups, and demonstrated that the average catalytic activities of the carboxyl moieties on the carbocatalysts differed. Comparisons between the apparent activation energy E a and the pre-exponential factor A, derived from Arrhenius analysis, demonstrated that A varied more significantly, and therefore had a more significant effect on the reaction rates than E a. The variation in the calculated pKa values of the carboxyl groups was attributed to the electronic effects of the nitro groups. This hypothesis was supported by the temperature programmed desorption profiles of nitrogen monoxide ions.

A temperature programmed desorption study of non-thermal plasma modified γ-Al2O3

In this paper, the effect of non-thermal RF plasma modification on the point of zero charge (PZC) of γ-Al2O3 has been explored. Samples of alumina were first modified with air, CO2, N2 or Ar plasma and then measurements of the PZC were taken. In order to determine the mechanism behind PZC shift, the surface area and crystal phase were measured for each sample using nitrogen physisorption (BET) and X-ray diffraction (XRD). Additionally, the presence of plasma gas on the surface of alumina was determined qualitatively using a modified temperature programmed desorption (TPD) technique. Air and CO2 modification of alumina powder decreased the PZC from 7.48 to 4.86 and 6.57 respectively. However, N2 and Ar modification did not result in any PZC shift. BET and XRD measurements showed no significant surface area or phase changes occurred due to non-thermal plasma modification. TPD profiles for air and CO2 modified samples showed new peaks corresponding to the adsorption of plasma ions on the alumina surface. These results demonstrate that PZC changes in a support after plasma treatment is caused by the adsorption of plasma species.

Degradation of Adenine on the Martian Surface in the Presence of Perchlorates and Ionizing Radiation: A Reflectron Time-of-flight Mass Spectrometric Study

Abstract The aim of the present work is to unravel the radiolytic decomposition of adenine (C5H5N5) under conditions relevant to the Martian surface. Being the fundamental building block of (deoxy)ribonucleic acids, the possibility of survival of this biomolecule on the Martian surface is of primary importance to the astrobiology community. Here, neat adenine and adenine–magnesium perchlorate mixtures were prepared and irradiated with energetic electrons that simulate the secondary electrons originating from the interaction of the galactic cosmic rays with the Martian surface. Perchlorates were added to the samples since they are abundant—and therefore relevant oxidizers on the surface of Mars—and they have been previously shown to facilitate the radiolysis of organics such as glycine. The degradation of the samples were monitored in situ via Fourier transformation infrared spectroscopy and the electron ionization quadruple mass spectrometric method; temperature-programmed desorption profiles were then collected by means of the state-of-the-art single photon photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS), allowing for the detection of the species subliming from the sample. The results showed that perchlorates do increase the destruction rate of adenine by opening alternative reaction channels, including the concurrent radiolysis/oxidation of the sample. This new pathway provides a plethora of different radiolysis products that were identified for the first time. These are carbon dioxide (CO2), isocyanic acid (HNCO), isocyanate (OCN−), carbon monoxide (CO), and nitrogen monoxide (NO); an oxidation product containing carbonyl groups (R1R2–C=O) with a constrained five-membered cyclic structure could also be observed. Cyanamide (H2N–C≡N) was detected in both irradiated samples as well.