Subsequent Temperature-programmed Desorption Research Articles

Understanding platinum-based H2 adsorption/ desorption kinetics during catalytic hydrogenation or hydrogen storage-related reactions

Hydrogen is among the most promising energy carriers and plays an important role on the way to sustainable technologies. Platinum holds great promise for unlocking the potential of renewable hydrogen, as it is an essential component of proton exchange membrane technologies and in various hydrogenation reactions. For the variety of applications of energy harvesting, conversion, and storage, the optimization and reduction of Pt loading is crucial. In view of this, a platinum catalyst using a stable SiO2 support is synthesized to investigate the adsorption/desorption behavior of hydrogen on platinum nanoparticles of different sizes, obtained by treating the sample at different calcination temperatures. Pulsed chemisorption and subsequent temperature-programmed desorption are described mathematically to obtain kinetic parameters. It is shown that higher adsorption capacities could be obtained using smaller particles. However, for particles smaller than 2.4 nm, higher Pt2+ content decreases H2 adsorption. Adsorption inhibition due to the presence of monatomic Pt cannot be excluded. The size of the Pt nanoparticles does not significantly affect the desorption/adsorption energy, but there is evidence that the hydrogen adsorbed per Pt atom at the surface varies with size: about 1 for single crystal planes and 2 for nanoparticles <3 nm.

Exploring the framework of small pore zeolites for passive NOx adsorption

The use of a passive NOx adsorber (PNA) for NOx storage during the cold-start period is crucial for eliminating NOx emissions from the diesel exhaust. Palladium (Pd)-loaded zeolites have garnered significant attention owing to their high potential as PNA. In this study, small-pore zeolites with different frameworks were synthesized, and their structural characteristics and passive NOx adsorption abilities were investigated. Five zeolites, with CHA, AEI, AFX (low- and high- Si/Al ratio), and LEV framework structures, were synthesized by the hydrothermal conversion of faujasite-type (FAU) zeolite. Pd (1 wt%) was impregnated in combination with hydrothermal treatment. NOx adsorption and desorption behaviors were studied based on breakthrough NO adsorption and subsequent temperature-programmed desorption (TPD) experiments. Pd-loaded CHA, AEI, and LEV zeolites and high-silica AFX zeolites adsorbed approximately 93.88, 107.93, 97.57 and 106.53 μmol-NO g−1, respectively. On the other hand, the incorporation of Pd into AFX zeolite with a low Si/Al ratio led to significant hydrothermal damage and resulted in lower levels of adsorption and desorption capabilities. This demonstrates that hydrothermally stable small-pore zeolites on Pd-incorporation have great potential for passive NOx adsorption. The zeolite with different framework exhibited the different temperature range of NOx release. CHA, AEI, and high-silica AFX zeolites desorbed NOx in the low-temperature region (T < 250 °C), whereas the LEV zeolite showed a relatively high NOx desorption amount in the middle-temperature region (250 °C < T < 350 °C).

In-situ self-sacrificed fabrication of insulator-based SrTiO3/SrCO3 heterojunction interface for gaseous HCHO and NO photocatalytic degradation

In this work, novel heterostructured SrTiO3/SrCO3 (STO/SCO) interface was constructed via the one-pot g-C3N4(CN) self-sacrificing hydrothermal strategy. The as-developed STO/SCO photocatalyst shows the air cleaning potential in continuous-flow reactors with degradation rates of NO and HCHO at 44 % and 40 %, respectively. From XRD, FTIR, and XPS analysis, CN participates in the crystallise process as the source of CO32− to form the STO/SCO interface viewed by TEM and HRTEM. Subsequent temperature-programmed desorption (TPD) analysis and density functional theory (DFT) calculation results revealed the enhanced chemisorption effects of O2 on the catalyst surface. The existence of oxygen vacancies combined with the formation of heterojunction surface induces intermediate levels, which leads to the photocatalytic oxidation under simulated solar light. Charge difference distribution simulation coupled with electrochemistry and photoluminescence tests confirmed the internal-built electron fields at the heterojunction interface which would be beneficial for photocarriers separation. Based on the above-mentioned effects, enhanced reactive oxygen species (ROS) OH and O2− were detected under light irradiation by electron spin resonance (ESR). This work demonstrates the effectiveness of in-situ self-sacrificed strategy for construction of heterojunction interfaces and provides opportunities by utilising insulator-based materials for photocatalytic degradation of air pollutants.

Surface reactions during temperature-programmed desorption and reduction experiments with oxygen-functionalized carbon blacks

Carbon black was functionalized by gas-phase oxidation using nitric acid vapor at 150 °C, and temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR) experiments were performed in a plug-flow reactor to analyze the decomposition mechanisms of oxygen-containing surface groups by monitoring evolved H2O, CO2, and CO quantitatively. Subsequent TPD measurements detected an enrichment of acidic surface groups with increasing duration of the HNO3 functionalization from 2 h to 24 h. A significant amount of H2O was released during the TPD experiments, yielding H2O evolution profiles which were deconvoluted into two Gaussian peaks at 162 °C and 228 °C. The combined analysis of the CO2 and H2O profiles indicates that desorbed H2O originates from chemisorbed water bound to carboxylic acid groups and from condensation reactions of carboxylic acids and phenols. Phenols and carbonyls were found to be reduced selectively by H2 during TPR, generating a pronounced H2O peak at 650 °C. A new peak in the CO2 evolution profile appeared at 575 °C in reducing atmosphere, which is assigned to the hydrolysis of anhydrides and lactones with subsequent decomposition. Thus, taking H2O into account is mandatory for a complete quantitative analysis of the decomposition mechanisms occurring during TPD and TPR experiments.

Coexistence of first-order and second-order desorption processes during temperature-programmed desorption of Bi on Ni(100) analyzed by kinetic Monte Carlo techniques

The application of temperature-programmed desorption (TPD) techniques in heterogeneous catalysis to probe, among other things, the nature of the reactions on the surface solid catalyst, and ultimately, the kinetics of desorbed species, is hampered by the inability to make direct observations of adsorbates when they are most catalytically active. Thus, it is almost impossible to make a direct association between a given elementary surface process and a key feature on the TPD spectra, such as the nature and number of peaks. What kinetic Monte Carlo simulations have shown, on the other hand, is that the complex surface evolution during TPD is controlled effectively by just a few parameters that relate to the surface kinetics and energetics. In this study, we will use kinetic Monte Carlo approach to show that double-peaked TPD spectra obtained from adsorption of Bi adsorbates on Ni(100) at an initial temperature of 800 K and subsequent TPD runs for high preadsorbed coverages can indeed be explained satisfactorily by assuming predominant first-order desorption kinetics coupled with adsorbate-adsorbate lateral interactions, in agreement with prior studies. While not totally discounting the presence of Bi dimers, and thus second-order desorption, our study is sensitive enough to reveal the extent of their presence. In other words, we propose that dimers can coexist with Bi adatoms in small amounts while retaining the key features of the TPD spectra, provided the kinetic parameters associated with dimer formation (and dissolution) are well within a certain range. On the other hand, any model in which dimers are present to a degree in which they are not totally dominated by adatoms cannot produce TPD spectra that are consistent with the experiment.

Formation and thermal stability of subsurface deuterium in Ni(110)

A unique feature of plasma-enhanced catalysis compared to thermal catalysis is the presence of reactive hydrogen radicals and ions in the gas phase above the catalyst surface. The uptake and subsequent thermal desorption of deuterium on a Ni(110) surface have been measured using incident gaseous D2 molecules, D atoms, and D2+ ions. Molecular D2 exposures on Ni(110) at 90 K form adsorbed D adatoms at the surface, but do not populate subsurface deuterium binding states under UHV conditions. In contrast, such subsurface states on Ni(110) are readily populated at 90 K by incident D atoms and D2+ ions. Subsurface D atoms recombine to desorb as D2 gas in temperature programmed desorption (TPD) measurements to create new characteristic subsurface-derived D2 thermal desorption peaks near 175 K for incident D atoms and 190–260 K, depending on the energy of the incident D2+ ions used, along with a new peak at 435 K for incident ions. While there are small differences for adsorbed D adatoms on Ni(110) and Ni(111) surfaces regarding their thermal stability and subsequent D2 TPD curves, the thermal stability and D2 TPD peaks from subsurface D atoms are nearly the same for these two substrates. This information will be helpful for a fuller understanding of the role of subsurface hydrogen and its reactivity in hydrogenation for plasma-enhanced catalysis over Ni-based catalysts.

Surface oxygen generated upon N2O activation on iron containing ZSM-5 type zeolites with different elemental composition

Various ZSM-5 zeolites with iron contents ranging from of 0.86 to 4.98 and Si/Al ratios of 11.5–140 were prepared by solid-state ion exchange with FeCl2. The catalysts were characterised by XRD, H2-TPR and NH3-TPD. The formation and stability of surface oxygen over these zeolites were investigated with a transient multipulse technique combined with subsequent temperature-programmed desorption (TPD). Higher iron contents enhance the formation of surface oxygen. However, when a critical iron content is reached, larger iron oxide clusters and particles are formed and these larger clusters do not contribute significantly to surface oxygen formation. Moreover, the amount of desorbed oxygen is more than that formed from N2O due to the additional desorption of oxygen present in the lattice of such oxide clusters. TPD studies indicate the presence of different surface oxygen species depends on both the zeolite iron content and Si/Al ratio.

Influence of graphite surface properties on the first electrochemical lithium intercalation

The electrochemical insertion of lithium ions into graphite materials having different surface chemistry and defect concentration was studied during the first cycle in half-cell containing 1 M LiPF 6 in an electrolytic solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The graphite surface properties were varied by thermal treatments in either hydrogen, oxygen, or nitrogen oxide or chemical treatment in boiling nitric acid. The influence of the surface modifications on the course of the first electrolyte reduction was investigated. The surface group chemistry was analyzed by temperature-programmed desorption coupled with mass spectrometry. The surface defect concentration was determined in terms of the active surface area (ASA) measured by oxygen chemisorption and a subsequent temperature-programmed desorption. The experimental results showed that the ASA parameter governs the exfoliation tendency of the graphite negative electrode material with the existence of a critical value below which the graphite systematically exfoliates. The specific charge loss during the first electrochemical insertion of lithium and the exfoliation behavior of the graphite negative electrode material are not influenced by the type and amount of oxygen surface groups. But hydrogen present on the graphite surface increased the graphite exfoliation tendency even for graphite materials with an ASA above the critical value.

Oxygen plasma activation of Cr(CO) 6 on α-Fe 2O 3(0001)

The chemistry of Cr(CO) 6 on the Fe 3O 4(111) surface termination of α-Fe 2O 3(0001) was explored using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED) both with and without activation from an oxygen plasma source. No thermal decomposition of Cr(CO) 6 was detected on the surface in the absence of O 2 plasma treatment, with first layer molecules desorbing in TPD at 215 K from a close-packed overlayer. The interaction of first layer Cr(CO) 6 with the Fe 3O 4(111)-termination was weak, desorbing only ∼ 30 K above the leading edge of the multilayer state. Activation of multilayer coverages of Cr(CO) 6 with the O 2 plasma source at 100 K resulted in complete conversion of the outer Cr(CO) 6 layers, presumably to a disordered Cr oxide film, with Cr(CO) 6 molecules near the surface left unaffected. Absence of CO or CO 2 desorption states suggests that all carbonyl ligands are liberated for each Cr(CO) 6 molecule activated by the plasma. AES and SSIMS both show that O 2 plasma activation of Cr(CO) 6 results in a carbon-free surface (after desorption of unreacted Cr(CO) 6). LEED, however, shows that the Cr oxide film was disordered at 600 K and likely O-terminated based on subsequent water TPD. Attempts to order the film at temperatures above 650 K resulted in dissolution of Cr into the α-Fe 2O 3(0001) crystal based on SSIMS, an observation linked to the Fe 3O 4(111) termination of the surface and not to the properties of α-Cr 2O 3/α-Fe 2O 3 corundum interface. Nevertheless, this study shows that O 2 plasma activation of Cr(CO) 6 is an effective means of depositing Cr oxide films on surfaces without accompanying carbon contamination.

Influence of O 2-induced surface roughening on the chemistry of water on TiO 2(1 1 0)

The impact of oxygen induced regrowth of TiO 2 on the reduced rutile TiO 2(1 1 0) surface has been studied using temperature programmed desorption (TPD) of adsorbed water multilayers. Pre-exposure of UHV annealed TiO 2(1 1 0) surfaces to O 2 at temperatures from 300 to 850 K induced changes in subsequent water TPDs that were interpreted in terms of the rougher surface morphologies resulting from the regrowth process. Water TPD from TiO 2(1 1 0) previously oxidized at 300 K exhibited a new peak at ∼312 K due to reaction of water with O adatoms. These O adatoms were produced by dissociative adsorption of O 2 at O-vacancy sites. Additionally, oxygen reacted (slowly) with surface Ti 2O 3 strands at RT. Water TPD from surfaces pre-oxidized at higher temperatures (⩾500 K) exhibited features reflective of desorption from rough surfaces, namely loss of peak resolution and eventual merger of the second layer and ice peaks, formation of a high temperature tail on the second layer peak, and broadening of the first layer TPD peak. The multiplicity of kinetically different adsorption sites on the roughened TiO 2(1 1 0) surfaces contributed to the widening of the desorption features.

Analysis of Molecular Hydrogen Formation on Low-Temperature Surfaces in Temperature Programmed Desorption Experiments

The study of the formation of molecular hydrogen on low-temperature surfaces is of interest both because it enables the exploration of elementary steps in the heterogeneous catalysis of a simple molecule and because of its applications in astrochemistry. Here, we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments, beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations, and the energy barriers of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results are used in order to evaluate the formation rate of H2 on dust grains under the actual conditions present in interstellar clouds. It is found that, under typical conditions in diffuse interstellar clouds, amorphous silicate grains are efficient catalysts of H2 formation when the grain temperatures are between 9 and 14 K. This temperature window is within the typical range of grain temperatures in diffuse clouds. It is thus concluded that amorphous silicates are good candidates to be efficient catalysts of H2 formation in diffuse clouds.

The interaction of N 2O with ZSM-5-type zeolites: A transient, multipulse investigation

The interaction of N 2O with MFI zeolites was investigated by a multipulse transient response method at 250 °C with subsequent temperature-programmed desorption. It was shown that the active sites for the formation of a surface oxygen species can be determined quantitatively using this method under the conditions of catalytic applications. Iron content and several pretreatment procedures influenced the formation of surface oxygen and molecular N 2O sorption. For example, fresh zeolites with high and low iron content formed significantly more surface oxygen than zeolites that had been pretreated in N 2O. The calcination at higher temperatures had an effect only on the activity of the zeolites with high iron content, which was not as pronounced as has been reported in the literature. The adsorption of molecular N 2O as an elementary step of N 2O activation in iron-containing zeolites cannot be neglected, because it was found to be thermally quite stable. All results indicate that in these zeolites not only one, but several oxygen species were formed that differ in thermal stability and thus in reactivity.

Mechanism of Ozone‐Promoted Uptake of NO2 by a Si‐Rich H‐Type Zeolite.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anomalous Phenomena in Oxygen Desorption from Tungsten and Their Mathematical Modeling

A model is developed that describes unusual phenomena in oxygen desorption from tungsten surfaces: (1) in temperature-programmed desorption (TPD) in the transfer from the linear regime of sample heating to the isothermal regime, a maximum of the desorption rate is observed; (2) in the stepped partial desorption of oxygen, the beginning of each subsequent TPD peak shifted toward higher temperatures; (3) the apparent activation energy of desorption increased in the latter case by a factor of ∼3 (from 280 to 920 kJ/mol). Monte Carlo modeling that takes into account surface migration of adsorbed oxygen atoms and their lateral interactions gave semiquantitative description of experimental data.

Mechanism of Ozone-Promoted Uptake of NO2 by a Si-Rich H-Type Zeolite

We have observed significant enhancements in the rate and capacity of NO2 uptake caused by the injection of ozone over the zeolite H-ZSM-5, which has a high SiO2/Al2O3 ratio. We performed these experiments using a gas-flow system in which the profiles of NO2 adsorption and subsequent temperature-programmed desorption were measured by Fourier transform infrared spectroscopy. We interpret these results in two ways: either by assuming that fast surface reactions lead to N2O5 formation or by considering that ozone suppresses the blockage of the reactive Bronsted-acidic protons.

Thermal Desorption of Tris(8-hydroxyquinoline)aluminum (III) (Alq3) on Cu(111)

The thermal properties of tris(8-hydroxyquinoline)aluminum (III) (Alq3), widely used in organic light-emitting diodes, have been examined on Cu(111) using temperature-programmed desorption (TPD). Two molecular desorption peaks are evident in TPD. Strikingly, the lower temperature (α) peak at ∼450 K develops first, but is not saturating, whereas the higher temperature (β) peak at ∼500 K appears later and grows more rapidly than the (α) peak. For the model proposed, as-deposited Alq3 forms an amorphous phase that converts to a crystalline phase in competition with desorption during TPD and a possible crystalline formation from the deposition. Consistent with this model, annealing at 400 K slowly shifts the subsequent TPD intensity from the (α) to the (β) peak. TPD after heating to 470 K and rapidly cooling to <350 K exhibits a strong (β), but no (α), peak.

Conversion of N 2O to N 2 on TiO 2(1 1 0)

In this study, we examine the interaction of N 2O with TiO 2(1 1 0) in an effort to better understand the conversion of NO x species to N 2 over TiO 2-based catalysts. The TiO 2(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO 2(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti 3+ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N 2O molecules adsorbed at 90 K on TiO 2(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N 2O molecules exposed to TiO 2(1 1 0) at 90 K decompose to N 2 via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N 2 ejection from the surface and vacancy oxidation. We propose that this channel involves N 2O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N 2O that decomposes at 170 K to liberate N 2 in the gas phase and deposit oxygen adatoms at non-defect Ti 4+ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N 2O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti 4+ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N 2O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N 2O at 90 K, and this charge transfer facilitates N 2O decomposition. Based on these results, it appears that the decomposition of N 2O to N 2 requires trapping of the molecule at vacancies and that the lifetime of the N 2O–vacancy interaction may be key to the conversion of N 2O to N 2.

Nitrogen Uptake on Coal Char at Ambient Temperature and the Effect of the Uptake on Ultimate Analysis

The exposure of Taiheiyo and Blair Athol coal chars to air at ambient temperature led to the adsorption of an appreciable amount of N2 on the coal chars. Some of N2 thus adsorbed still remained on the chars even after purging the chars with He gas flow. The amount of remaining N2 was determined by the subsequent temperature-programmed desorption experiment, and it was found that the remaining N2 accounted for a substantial portion of nitrogen content determined by ultimate analysis using an instrumental analyzer for each coal char. In other words, the amount of nitrogen determined by an instrumental analyzer is overestimated. Accordingly, to obtain the correct nitrogen analysis value, it is necessary to remove such adsorbed N2. Evacuation of coal chars at 423 K for 5 h prior to the analysis is enough for this purpose.

Physico-chemical and catalytic properties of Fe 2BiMo 2O x catalyst ultrasound treated and promoted with Al 2O 3 in the oxidative dehydrogenation of ethylbenzene to styrene

It has been shown by means of X-ray diffraction (XRD) analysis that Fe 2BiMo 2O x and Fe 2BiMo 2Al 0.25O x catalysts are multiphase systems of α-Bi 2(MoO 4) 3, γ-Bi 2MoO 6, Fe 2(MoO 4) 3, Fe 2O 3·3–4MoO 3 and the ternary compound—FeBi 3Mo 2O 12. Using a pulse technique has been found that Fe 2BiMo 2Al 0.25O x catalyst exhibits the highest catalytic activity (100% of ethylbenzene (ETB) conversion) and styrene (STY) selectivity 97.7% at 653 K and a contact time of 3.6 s in the oxidative dehydrogenation (ODH) of ETB. The acidity and basicity measurements of the surface of the catalysts were performed by using a pulse chemisorption procedure with the subsequent temperature-programmed desorption (TPD) of ammonia and acetic acid, respectively. Small additives of Al 2O 3 to Fe 2BiMo 2O x catalyst considerably improve its catalytic activity and selectivity in the process that primarily is due to the increase of the amount of acid and moderate base sites on the catalyst surface. Active acid and base sites of the first form of NH 3 and CH 3COOH adsorption, respectively play an important role in STY formation. It is shown that ultrasonic treatment of F 2BiMo 2O x catalyst in its preparation by precipitation from solution improves its activity and selectivity in the ODH of ETB to STY.

Adsorption and decomposition of (methylcyclopentadienyl) (1,5-cyclooctadiene) iridium on Rh

The adsorption and decomposition on polycrystalline Rh of (methylcyclopentadienyl) (1,5-cyclooctadiene) iridium, (MeCp)Ir(COD), has been studied by temperature-programmed desorption (TPD) using time-of-flight mass spectrometry and Auger electron spectroscopy. During early stages of dosing at 100 K there is partial decomposition of (MeCp)Ir(COD) and, in subsequent TPD, only H2 desorbs. For higher doses, (MeCp)Ir(COD) and H2, but no hydrocarbons, desorb. The H2 evolves in several saturable peaks reflecting stepwise C–H bond breaking. In isothermal reaction spectroscopy involving dosing between 350 and 750 K, desorbing products include hydrocarbons with intensities varying with temperature and dosing time. Total sticking probabilities (St) are unity on clean Rh but decrease as deposition products accumulate. Above 750 K the decomposition reaction continues even when the Rh surface is completely covered with precursor decomposition products.