H2O Desorption Research Articles

Adsorption-desorption mediated separation of low concentrated D2O from water with hydrophobic activated carbon fiber

The adsorption and desorption of D2O on hydrophobic activated carbon fiber (ACF) occurs at a smaller pressure than the adsorption and desorption of H2O. The behavior of the critical desorption pressure difference between D2O and H2O in the pressure range of 1.25–1.80kPa is applied to separate low concentrated D2O from water using the hydrophobic ACF, because the desorption branches of D2O and H2O drop almost vertically. The deuterium concentration of all desorbed water in the above pressure range is lower than that of water without adsorption-treatment on ACF. The single adsorption-desorption procedure on ACF at 1.66kPa corresponding to the maximum difference of adsorption amount between D2O and H2O reduced the deuterium concentration of desorbed water to 130.6ppm from 143.0ppm. Thus, the adsorption–desorption procedure of water on ACF is a promising separation and concentration method of low concentrated D2O from water.

Detecting the Photoactivity of Anatase TiO2(001)-(1 × 4) Surface by Formaldehyde

We report our investigation using temperature-programmed desorption to detect the activity and the photoactivity of the reduced anatase TiO2(001)-(1 × 4) surface by using CH2O as a probe. We obtain the adsorption energy of 0.55 eV for CH2O on the surface and find that the defect sites in the reduced surface are the only active sites for thermally driven reaction to produce C2H4. We also identify the pathways for photodecomposition of CH2O on the anatase TiO2(001)-(1 × 4) surface, where the C−H bond breaking to form the intermediates of HCOO− should be a key step in the reactions. After the ultraviolet light irradiation, the dissociation of HCOO– produces CO and H2O at elevated temperatures. Accompanied by desorption of H2O, we observe higher production of C2H4 than that in the non-irradiated sample. We interpret our experimental results by attributing the temporary change of the (1 × 4) ridge structure to water desorption, that is, the initially oxygen-rich five-fold coordinated Ti atoms change to four-fo...

Reduction of Hydrogenated ZrO2 Nanoparticles by Water Desorption.

Reduction of zirconia by water desorption from a hydrogenated surface is the topic of this study. The focus is on the role of nanostructuring the oxide reducibility measured by the cost of formation of oxygen vacancies by water desorption. We have performed density functional theory calculations using the Perdew–Burke–Ernzerhof + U approach and including dispersion forces on the adsorption, dissociation, diffusion of hydrogen on the ZrO2 (101) surface and on Zr16O32, Zr40O80, and Zr80O160 nanoparticles (NPs). The process involves the formation of a precursor state via diffusion of hydrogen on the surface of zirconia. The results show that O vacancy formation via H2O desorption is more convenient than via direct O2 desorption. The formation of an OsH2 surface precursor state to water desorption is the rate-determining step. This step is highly unfavorable on the ZrO2 (101) surface both thermodynamically and kinetically. On the contrary, on zirconia NPs, characterized by the presence of low coordinated ions, water desorption becomes accessible such that even at temperatures close to 450 K the reaction becomes exergonic. The study shows the role of nanostructuring on the chemical and electronic properties of an oxide.

Thermal Analysis of Hydrated Gold Cluster Cations in the Gas Phase

The binding forms and desorption processes of H2O molecules bound in hydrated gold clusters, Aun(H2O)m+ (n = 2–8, m = 1–4), were investigated by gas-phase thermal analyses under chemical equilibrium conditions and by quantum chemical calculations. The gas-phase clusters were prepared by the laser ablation of a gold target in the presence of water vapor and detected by mass spectrometry. Aun(H2O)m+ clusters were found to release H2O upon heating, eventually losing all H2O molecules to generate Aun+ at temperatures below 1000 K. In sufficiently concentrated H2O vapor, adsorption and desorption of H2O to/from a given cluster approached equilibrium during heating, and the temperature dependence of this equilibrium provided binding energies of H2O to Aun(H2O)m+. The experimental and computational results suggest two types of H2O binding forms, one in which the H2O molecules are directly bound to the Aun+ core and the other in which H2O molecules are bound to other H2O molecules through hydrogen bonding. The bi...

Development of Novel CeO2-based CO2 adsorbent and analysis on its CO2 adsorption and desorption mechanism

Affection toward CO2 adsorption-desorption processes under wet flue gas is significant. CO2 adsorption-desorption properties of CeO2 under dry and wet condition were measured. Unlike zeolite, CO2 adsorption amounts for CeO2 under dry and wet condition were almost equivalent. Compared to dry condition, CO2 temperature-programmed desorption under wet condition showed larger desorption amount at low temperature below 463 K. This could result from formation of hydrogen carbonate species by enriching surface hydroxyl groups due to H2O. CO2 and H2O desorption measurement showed that CO2 and H2O desorption occurred at close temperature, which was explained by degradation of hydrogen carbonate species.

Carbon Monoliths in Adsorption-based Post-combustion CO2 Capture

The development of adsorption-based technologies for post-combustion CO2 capture requires finding an adsorbent with adequate equilibrium and transport properties. Structured adsorbents are appealing for fixed-bed TSA processes, because they present lower pressure drop and higher thermal conductivity than conventional adsorbent beds, which facilitates the use of higher flowrates and shorter cycle times, maximizing throughput. In this work, the equilibrium of adsorption of the main flue gas components, CO2, N2, O2 and H2O over two carbon honeycomb monoliths with different textural development has been measured in a pressure and temperature range of interest for post-combustion CO2 capture: between 0°C and 70°C and up to 120 kPa for CO2, N2, and O2, and between 30°C and 70°C up to the corresponding saturation pressure for H2O. The maximum adsorption capacity and isosteric heat of adsorption follows the order: H2O > CO2 > N2 ≈ O2. The carbon monoliths present equilibrium selectivity towards CO2 and H2O over N2 and O2 at typical flue gas conditions. Moderate activation is preferred to maximize the CO2 adsorption capacity and selectivity in these conditions. The Toth model was employed to fit the equilibrium data for the adsorption of CO2, N2, and O2 with highly satisfactory results. The adsorption isotherms of H2O presents the characteristic “s” shape of hydrophobic adsorbents, with low uptakes at low relative pressures, which will facilitate H2O desorption during cyclic operation. These were fitted using the extended CMMS model, which describes satisfactorily the experimental data in the full relative humidity range. The adsorption kinetics were preliminary evaluated by measuring the rate of mass uptake from a mixture with 10% CO2 (balance N2) at 30°C, 50°C and 70°C, and the data were fitted to the linear driving force model to obtain the kinetic rate constants.

H2O Adsorption on WO3 and WO3-x (001) Surfaces.

The nature of the interaction of water with the WO3 surface is of crucial importance for the use of this semiconductor oxide in photocatalysis. In this work, we investigate water adsorption and dissociation on both clean and O-deficient (001) WO3 surfaces by means of an accurate DFT approach. The O vacancy formation energy (computed with respect to O2) has been evaluated for all possible surface configurations, and the removal of the terminal O atom along the c axis is found to be preferred, costing about half the corresponding energy in the bulk. The presence of oxygen vacancies leads to a semiconductor to metal transition, confirming the experimental evidence of n-type conductivity in defective WO3 films. H2O preferably adsorbs on WO3 in a molecular undissociated form, due to the presence of W ions at the surface that act as Lewis acid sites. This interaction, about -1 eV per H2O molecule, is not very strong. Contrary to what is usually expected, the presence of oxygen vacancies does not significantly affect H2O adsorption. Finally, we investigated the H2O desorption from a hydroxylated surface. This suggests that the exposure of WO3 to H2 directly results in a hydroxylated surface and the corresponding H2O desorption turns out to be a very efficient mechanism to generate a reduced oxide surface, with important consequences on the electronic structure of this oxide.

Isotope effect on water adsorption on hydrophobic carbons of different nanoporosities

The adsorption isotherms of H2O and D2O on pitch-based activated carbon fibers (ACFs) of different pore widths from 0.6 to 1.5 nm were measured at 298 K. Briefly speaking, the D2O adsorption isotherms are close to the H2O adsorption isotherms. However, we observed an evident difference in the rising and desorption relative pressures for the H2O and D2O adsorption isotherms. The D2O adsorption rises at a higher relative pressure than the H2O adsorption. Correspondingly the D2O desorption occurs at a higher relative pressure than the H2O desorption. Detailed comparison of the H2O and D2O adsorption isotherms confirmed a reproducible difference over the whole relative pressure range. The different adsorption behavior of H2O and D2O in the hydrophobic micropores is discussed with the relevance to the difference in the hydrogen bonding of H2O and D2O.

Oriented growth of layered-MnO2 nanosheets over α-MnO2 nanotubes for enhanced room-temperature HCHO oxidation

Construction of heterostructures with well-defined size, dimension, surface and interface is an effective approach to develop enhanced and unprecedented functionality. Herein, oriented growth of layered-MnO2 nanosheets (L-MnO2) over α-MnO2 nanotubes backbones is demonstrated. The epitaxial relationship in the resulting α-MnO2@L-MnO2 heteroepitaxy is rationalized as (110)(α-MnO2)||(001)(layered-MnO2). With loading of 1wt% Pt nanoparticles over the MnO2 samples, the resulting Pt/MnO2 samples show catalytic activity toward room-temperature HCHO oxidation via “HCHO+O2=CO2+H2O”. Upon 1h treatment, 92.1% HCHO becomes mineralized over the Pt/α-MnO2@L-MnO2, higher than that of 81.3% and 75.9% for the Pt/α-MnO2 and Pt/L-MnO2. The oxidation of HCHO was well fitted with the second-order kinetic model, with the rate constant of the Pt/α-MnO2@L-MnO2 exceeding that of the Pt/α-MnO2 and the Pt/L-MnO2 by 2.27 and 5.92 times. Density-Functional-Theory (DFT) simulations show that α-MnO2 {100} surface facilitates adsorption/activation of O2, and layered-MnO2 {001} surface is beneficial to desorption of resultant H2O. The α-MnO2@L-MnO2 heteroepitaxy simultaneously integrates exposed facets of α-MnO2 {100} surface and layered-MnO2 {001} surface, in which the synergetic effect of the two surfaces leads to significantly enhanced room-temperature HCHO oxidation activity. The present study provides a rational design of manganese oxide-based catalysts for advanced environmental and energy applications.

A complete reaction mechanism for standard and fast selective catalytic reduction of nitrogen oxides on low coverage VOx/TiO2(0 0 1) catalysts

We present a complete catalytic mechanism describing both the Standard and the Fast selective catalytic reduction (SCR) reactions in their correct stoichiometric form on a vanadia titania (anatase 001 facet) based catalyst model. It consists of two cycles, a NO-activation cycle and a Fast SCR cycle that share the same reduction part but use NO+O2 and NO2 respectively for the reoxidation. The stoichiometry of the Standard SCR reaction is obtained by coupling the two cycles and the stoichiometry of the Fast SCR reaction is represented by the Fast SCR cycle. We establish structures and energetics for each elementary reaction allowing us to calculate the rate for the two reactions by microkinetic modeling. We find at low temperatures the rate for the Standard SCR reaction is determined by H2O formation and desorption as neither NO nor O2 reacts exothermically with the reduced site prior to H2O desorption. On the contrary NO2 reacts directly with the reduced site resulting in higher rate for the Fast SCR reaction at low temperatures. The rate for the two reactions is the same at higher temperatures as the rate determining step is in the reduction part which is common to both reactions.

The mechanism of H2 and H2O desorption from bridging hydroxyls of a TiO2(110) surface

With an increase in BBO vacancies (created by H2O desorption), the H2 desorption barrier decreases, while the H2O desorption barrier increases.

Production of H2-free CO by decomposition of formic acid over ZrO2 catalysts

The decomposition of formic acid (HCOOH) over ZrO2 catalysts synthesized by precipitation method was investigated and their catalytic activity based on unit mass of catalyst was compared by varying the calcination temperature in the 500–1000°C range. The calcined ZrO2 samples were characterized using temperature-programed desorption (TPD) of iso-propanol (IPA), H2O, and HCOOH as well as X-ray diffraction (XRD), pyridine-adsorbed infrared spectroscopy (Py-IR) and N2 sorption. The catalytic activity as a function of the calcination temperature of ZrO2 showed a volcano-type curve with a maximum at 900°C. The catalytic activity of ZrO2 in the decomposition of HCOOH well correlated with Brönsted acid site density, and it was influenced by the crystalline phase of ZrO2. Furthermore, the evolution of the Brönsted acid site density measured by Py-IR as a function of the calcination temperature was in good accordance with the desorption temperature determined by IPA- and HCOOH-TPD experiments. When H2O vapor was added, the conversion of HCOOH slightly decreased, while the CO selectivity remained almost constant. However, when CO was supplied, the conversion of HCOOH decreased continuously, and the selectivity to H2 gradually increased, indicating that the formation of zirconium suboxides with characteristics approaching those of metals was successful using CO as a reductant.

Hydrogen Oxidation-Mediated Current Discharge in Mesoporous Pt/TiO2 Nanocomposite.

Here we report on direct evidence of a correlation between hydrogen-to-water oxidation on mesoporous Pt/TiO2 nanocomposites at room temperature and the conversion of surface-released chemical energy into a stationary electrical current. The Pt phase of this heterojunction device is an electrically continuous 15 nm thick mesh deposited onto a mesoporous TiO2 substrate fabricated with a plasma electrolytic oxidation process. The H2O turnover frequency approaches an asymptotic value associated with the saturation of the Pt/TiO2 interface as the concentration of hydrogen gas is increased. In situ measurements of the reaction-induced current concurrently with mass spectrometry measurements illuminate the polarity switch of the reaction current (from thermionic emission to a reverse steady-state flow) simultaneously with the production of water. Furthermore, a concentration-dependent value of 5 min is measured as the time constant for the adsorption of the initial addition of H2 and H2O formation and desorption.

Mechanisms for oxygen reduction reactions on Pt-rich and Pt-poor PtFe alloys as the fuel cell cathode

The mechanism of Pt-M (3d-metal)-enhanced performance compared to that of pure platinum as the cathode catalyst in fuel cells is not entirely clear. In this paper, a DFT calculation is applied to study the electronic structures of Pt-Fe alloys and ORR (Oxygen Reduced Reaction) on Pt(111), Pt-skinned Pt3Fe(111) and Pt-skinned PtFe3(111) surfaces. With the charge transfer between Pt and Fe, the electronic structure thoroughly differs from that of pure Pt. Consequently, a further change in interaction between the surface and adsorbates, which are produced during the ORR process, primarily causes the improvement in ORR performance. When used as a catalyst, the Pt-skinned Pt-Fe alloy surface is superior to the Pt surface and has three main advantages: (i) a more appropriate potential for each electron step (closer to the stand reversible potential), (ii) a smaller O2 adsorption energy, and (iii) a smaller H2O desorption energy. The theoretical results also show that the Pt-skinned PtFe3(111) surface has obviously higher activity than the Pt-skinned Pt3Fe(111), which implies that activity is improved with the increase in Fe content. This work provides a new approach to understand the excellent ORR performance of Pt-M alloys and design other non-noble metal catalysts.

Kinetic Analysis of the Inhibition of CH4 Oxidation by H2O on PdO/Al2O3 and CeO2/PdO/Al2O3 Catalysts

Addition of CeO2 to a PdO/Al2O3 catalyst, assessed for the oxidation of CH4 in the presence of 5% H2O at various temperatures, reduces the inhibition effects of H2O. A simplified kinetic analysis that assumes Langmuir adsorption of H2O on the catalyst active sites is used to model both the dynamic response and the steady-state CH4 conversion following H2O addition to the feed gas and to quantify the effect of CeO2 addition. The analysis shows that less H2O is adsorbed on the 2.9Ce/6.5Pd/Al2O3 than the 6.5Pd/Al2O3 catalyst and that the rate of H2O desorption is higher on the Ce-promoted catalyst. Both factors result in reduced inhibition of CH4 conversion by H2O on the 2.9Ce/6.5Pd/Al2O3 catalyst compared to the 6.5Pd/Al2O3 catalyst. The apparent activation energy for CH4 oxidation over the 2.9Ce/6.5Pd/Al2O3 catalyst (61 ± 11 kJ mol–1) is shown to be statistically the same over the 6.5Pd/Al2O3 (56 ± 9 kJ mol–1). Catalyst characterization data show minimal changes in catalyst properties after reaction, and r...

Desorption behavior of zinc atoms from zinc-sulfate solution irradiated with pulsed DC plasma

A DC pulsed plasma ignited between a metal needle and zinc sulfate (ZnSO4) solution electrode was used to investigate Zn metal desorption from an electrolyte solution. Using an ICCD camera and optical band-pass filter, 2D atomic absorption spectroscopy was carried out during irradiation of pulsed plasma to the surface of the solution. The time-resolved measurement of Zn atoms released to the gas phase revealed that the Zn desorption rate monotonically increased with increasing number of discharge repetitions. The surface temperature of the electrolyte solution was observed with a thermographic camera, and correlations between the H2O and Zn desorption rate were inspected. The correlation between the H2O and Zn desorption rate suggested that Zn desorption is assisted not only by the electric field of the discharge but also by H2O evaporating from the solution.

Density Functional Theory Study of the Mechanism of Formaldehyde Oxidation on Mn-Doped Ceria

Formaldehyde is an indoor pollutant, whose removal under mild conditions is of growing importance. Mn-doped CeO2 is a promising catalyst for the oxidation of formaldehyde to water and carbon dioxide. We have theoretically investigated the origin of the high activity of Mn-doped ceria as compared with ceria. DFT+U calculations were used to identify adsorption modes and compare different reaction mechanisms. The reaction mechanism involves HCHO adsorption, two C–H bond cleavage steps involving reactive O atoms (either structural O atoms of the support or adsorbed O2), H2O formation, and H2O and CO2 desorption. On the stoichiometric surface, a Mars–Van Krevelen mechanism occurs, which involves ceria surface O atoms. The lower coordination number of these O atoms in the stoichiometric Mn-doped ceria results in decreased barriers for C–H bond cleavage. In the presence of defects which will be ubiquitous in the Mn-doped surface, a Langmuir–Hinshelwood mechanism becomes feasible, as O2 can strongly adsorb on the...

Evidence for dissolved hydrogen in the mixed ionic-electronic conducting perovskites La0.6Sr0.4FeO3-δ and SrTi0.7Fe0.3O3-δ.

Two mixed ionic-electronic conducting, Fe-containing perovskites were investigated regarding their reducibility in dry H2, namely lanthanum strontium ferrite (LSF4, La0.6Sr0.4FeO3-δ) and strontium titanium ferrite (STF3, SrTi0.7Fe0.3O3-δ). Upon treatment under comparable reduction conditions, LSF4 is by far more affected by reduction and is reduced more deeply than STF3. Thermal treatments of fully oxidized or slightly reduced LSF4/STF3 at decreased O2 partial pressure lead to spontaneous desorption of O2. Temperature-programmed desorption (TPD) spectra of H2 reveal distinct differences in H2 and H2O desorption. A simple mass balance of H2 reveals that oxygen vacancies formed on STF3 are more resilient towards O2 re-oxidation compared to those on LSF4. The results also imply that substantial amounts of hydrogen are dissolved in the bulk of LSF4 or STF3. 4.9 × 10-2 mol H2 per mol LSF4 and 1.6 × 10-2 mol H2 per mol STF3 are incorporated if the specimens are heated in flowing/dry H2 up to 550 °C and 612 °C, respectively. For LSF4 this equals about 13 hypothetical ML of H2 and for STF3 about 20 hypothetical ML of H2. This conclusion is also supported by Fourier-transform infrared spectroscopy (FT-IR). FT-IR reveals water formation during static H2 treatment of LSF4/STF3, which indicates perovskite reduction. Furthermore, both samples behave extraordinarily hydrophobic and no chemistry involving surface hydroxy groups was observed.

Thermal release of hydrogen retained in multilayer graphene films prepared by mist-chemical vapor deposition

In this study, we investigated the absorption and thermal desorption processes of H and H2O and the thickness of multilayer graphene films deposited on Cu foils using a mist-chemical vapor deposition method. Ion beam analysis techniques such as nuclear reaction analysis (NRA), elastic recoil detection (ERD), and Rutherford backscattering spectrometry (RBS) were employed. The RBS measurements revealed that the thickness of the multilayer graphene films was approximately 8±3nm (24±9 layers). The depth distribution of H was analyzed using NRA and ERD. Based on these measurements, the residual H/C ratio for multilayer graphene was estimated to be approximately 0.03 in the bulk and 0.88 on the top-most surface. Additionally, the thermal desorption temperature for H from the multilayer graphene film was less than 373K, which was much lower than that from isotropic graphite bulk (approximately 673K). These results suggest that the thermal release of H did not occur because of desorption from sp2- and sp3-hybridized C atoms, such as intercalation and defect sites. Instead, it occurred owing to the desorption of H2O adsorbed near the surface.

Atomic Mechanism of Electrocatalytically Active Co-N Complexes in Graphene Basal Plane for Oxygen Reduction Reaction.

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the oxygen reduction reaction (ORR) are crucial to the large-scale practical application of fuel cells. The nonprecious metal/N modified graphene electrocatalysts are regarded as one of potential candidates, and the further enhancement of their catalytic activity depends on improving active reaction sites at not only graphene edges but also its basal plane. Herein, the ORR mechanism and reaction pathways of Co-N co-doping onto the graphene basal plane have been studied by using first-principles calculations and ab initio molecular dynamics simulations. Compared to singly N-doped and Co-doped graphenes, the Co-N co-doped graphene surface exhibits superior ORR activity and the selectivity toward a four-electron reduction pathway. The result originates from catalytic sites of the graphene surface being modified by the hybridization between Co 3d states and N 2p states, resulting in the catalyst with a moderate binding ability to oxygenated intermediates. Hence, introducing the Co-N4 complex onto the graphene basal plane facilitates the activation of O2 dissociation and the desorption of H2O during the ORR, which is responsible for the electrocatalyst with a smaller ORR overpotential (∼1.0 eV) that is lower than that of Co-doped graphene by 0.93 eV. Our results suggest that the Co-N co-doped graphene is able to compete against platinum-based electrocatalysts, and the greater efficient electrocatalysts can be realized by carefully optimizing the coupling between transition metal and nonmetallic dopants in the graphene basal plane.