H2O Desorption Research Articles

Understanding A-site tuning effect on formaldehyde catalytic oxidation over La-Mn perovskite catalysts

A combination study of density functional theory (DFT) calculation and microkinetic analysis was carried out to investigate A-site tuning effect on formaldehyde (HCHO) oxidation over La-Mn perovskite catalysts (A = Sr, Ag, and Sn). The oxygen mobility of A-doped LaMnO3 catalysts and reaction mechanism of HCHO oxidation on catalyst surfaces were investigated. The microkinetic simulation was performed to quantitatively determine the activity of catalysts toward the HCHO catalytic oxidation. The results indicated that A-site tuning weakens the binding energy of Mn–O bond of LaMnO3 surface and facilitates the formation of surface oxygen vacancy. The presence of dopants can significantly reduce the activation energy of O2 dissociation, which ascribes to the facilitation of electron transfer between oxygen species and catalyst surfaces. The reaction cycle of HCHO oxidation contains seven steps: HCHO adsorption, HCHO* dehydrogenation, CHO* dehydrogenation, CO2 desorption, H2O desorption, O2 adsorption and oxygen vacancy recovery. The dopants promote HCHO adsorption and reduce the activation energy of HCHO oxidation. Two elementary steps control the overall reaction rate of HCHO oxidation. CHO* dehydrogenation step has the largest degree of rate control value at low temperature and O2 adsorption step controls the whole reaction at high temperature

Carbon-Supported Mg–Al Oxide Hybrid Catalysts for Aqueous Ethanol Conversion into 1-Butanol in a Flow Reactor

Carbon-supported Mg–Al oxide (hydrotalcite-derived) hybrid catalysts for aqueous ethanol conversion into 1-butanol were prepared and characterized by X-ray diffraction, scanning electron microscopy, low-temperature nitrogen ad(de)sorption, Fourier transform infrared and X-ray photoelectron spectroscopies, and temperature-programmed desorption of NH3, CO2, and H2O. Enhanced catalytic activity and stability of operation over time on stream of the hybrid catalysts versus Mg–Al oxides were achieved using both rectified and aqueous ethanol as feed. Redistribution of the active sites of the Mg–Al oxide phase over a highly dispersed support and increasing their availability for the reagents were suggested to be the causes of the enhanced activity of the hybrid catalysts.

1,3-Butadiene production from aqueous ethanol over ZnO/MgO-SiO2 catalysts: Insight into H2O effect on catalytic performance

A series of ZnO/MgO-SiO2 systems have been prepared and characterized by one-pass temperature-programmed desorption of CO2, NH3 and H2O, FTIR spectroscopy with adsorbed pyridine and CO2, temperature programmed surface reaction of ethanol and isopropanol, and reaction of isopropanol conversion. Effect of MgO:SiO2 ratio in the catalysts on catalytic performance in 1,3-butadiene production from aqueous ethanol have been investigated. The results indicate that hydrophilicity of ZnO/MgO-SiO2 are determined by MgO:SiO2 ratio, and chemisorption of water occurs preferably on Mg-containing sites. The presence of water in the initial ethanol leads to a decrease in formation of the CC coupling products. The hydrated sites formed in the interface of magnesia and silica remain active in the CC coupling reaction even in the presence of water. 1,3-Butadiene selectivity >60 % in the conversion of aqueous ethanol (80 vol%) is achieved in the presence of ZnO/MgO-SiO2 catalysts with the ratio of MgO:SiO2 = (1:1) and (3:1).

Reaction mechanism of dichloromethane oxidation on LaMnO3 perovskite

The reaction mechanism of dichloromethane (CH2Cl2) oxidation on LaMnO3 catalyst was investigated using density functional theory calculations. The results showed that CH2Cl2 dechlorination proceeds via CH2Cl2 → CH2ClO → HCHO. The adsorbed Cl∗ and formaldehyde (HCHO) are identified as the important intermediates of CH2Cl2 dechlorination process. The dissociated Cl atoms prefer to adsorb on the surface Mn sites. Surface hydroxyl groups are not directly involved in the CH2Cl2 dechlorination process, but react with the adsorbed Cl∗ to form HCl. The energy barrier of HCl formation is lower than that of Cl2 formation, indicating that hydroxyl groups facilitate the removal of adsorbed Cl∗ species. Three possible pathways of HCHO oxidation with the assist of lattice oxygen, active oxygen atom and hydroxyl groups were investigated. HCHO catalytic oxidation contains four steps: HCHO → CHO → CO → H2O desorption → CO/CO2 desorption. Compared with the HCHO oxidation by lattice oxygen and hydroxyl groups, HCHO oxidation assisted with activated oxygen atom is more thermodynamically favorable. A complete catalytic cycle was proposed to understand the preferable reaction pathway for CH2Cl2 oxidation on LaMnO3 catalyst. The catalytic cycle includes CH2Cl2 dechlorination, HCl formation and HCHO oxidation. The microkinetic analysis indicates that there are four steps controlling the reaction cycle: CH2Cl2∗ + ∗ → CH2Cl∗ + Cl∗, CH2OCl∗ + Cl∗ → CH2O∗ + Cl∗, O2∗ + ∗ → 2O∗, and CHO2∗ + OH∗ → CO2 + H2O∗.

Water Adsorption Behavior on a Highly Dense Single-Walled Carbon Nanotube Film with an Enhanced Interstitial Space.

In this study, we describe the adsorption behavior of water (H2O) in the interstitial space of single-walled carbon nanotubes (SWCNTs). A highly dense SWCNT (HD-SWCNT) film with a remarkably enhanced interstitial space was fabricated through mild HNO3/H2SO4 treatment. The N2, CO2, and H2 adsorption isotherm results indicated remarkably developed micropore volumes (from 0.10 to 0.40 mL g–1) and narrower micropore widths (from 1.5 to 0.9 nm) following mild HNO3/H2SO4 treatment, suggesting that the interstitial space was increased from the initial densely-packed network assembly structure of the SWCNTs. The H2O adsorption isotherm of the HD-SWCNT film at 303 K showed an increase in H2O adsorption (i.e., by ∼170%), which increased rapidly from the critical value of relative pressure (i.e., 0.3). Despite the remarkably enhanced adsorption capacity of H2O, the rates of H2O adsorption and desorption in the interstitial space did not change. This result shows an adsorption behavior different from that of the fast transport of H2O molecules in the internal space of the SWCNTs. In addition, the adsorption capacities of N2, CO2, H2, and H2O molecules in the interstitial space of the HD-SWCNT film showed a linear relationship with the kinetic diameter, indicating an adsorption behavior that is highly dependent on the kinetic diameter.

Laboratory-based sticking coefficients for ices on a variety of small-grain analogues

Abundances and partitioning of ices and gases produced by gas-grain chemistry are governed by adsorption and desorption on grains. Understanding astrophysical observations rely on laboratory measurements of adsorption and desorption rates on dust grains analogs. On flat surfaces, gas adsorption probabilities (or sticking coefficients) have been found close to unity for most gases. Here we report a strong decrease of the sticking coefficients of H2O and CO2 on substrates more akin to cosmic dust, such as submicrometer-sized particles of carbon and olivine, bare or covered with ice. This effect results from the local curvature of the grains, and then extends to larger grains made of aggregated small particles, such as fluffy or porous dust in more evolved media (e.g. circumstellar disks). The main astrophysical implication is that accretion rates of gases are reduced accordingly, slowing the growth of cosmic ices. Furthermore, volatile species that are not adsorbed on a grain at their freeze-out temperature will pertain in the gas phase, which will impact gas-ice partitions. We also found that thermal desorption of H2O is not modified by grains size, and thus the snowlines temperature should be independent on the dust size distribution.

Wavelength-Dependent Water Oxidation on Rutile TiO2(110).

Understanding the microscopic mechanism of water photocatalysis on TiO2 is of great value in energy chemistry and catalysis. To date, it is still unclear how water photocatalysis occurs after the initial light absorption. Here we report the investigation of the photoinduced water dissociation and desorption on a R-TiO2(110) surface, at different wavelengths (from 250 to 330 nm), using temperature-programmed desorption and time-of-flight techniques. Primary photooxidation products, gas phase OH radicals and surface H atoms, were clearly observed at wavelengths of ≤290 nm. As the laser wavelength decreases from 290 to 250 nm, the relative yield of H2O oxidation increases significantly. Likewise, photoinduced H2O desorption was also observed in the range of 320-250 nm, and the relative yield of H2O desorption also increases with a decrease in wavelength. The strong wavelength-dependent H2O photooxidation and photodesorption suggest that the energy of charge carriers is important in these two processes. More importantly, the result raises doubt about the widely accepted photocatalysis model of TiO2 in which the excess energy of charge carriers is useless for photocatalysis. In addition, the H2O photooxidation is more likely initiated by nonthermalized holes and is accomplished on the ground state potential energy surface via a non-adiabatic decay process.

Methanol oxidation over rutile Au1@TiO2 catalyst: Importance of facets and oxygen vacancy

Single-atom catalysts have attracted exceptional interest of researchers. However, the arguing whether tuning the interactions between the dispersed single metals and the support can enhance the activity, selectivity and stability of single atom catalysts is still open. Herein, single atom Au catalyst (rutile Au1@TiO2) is presented via density functional theory calculations, which exhibits high efficiency for methanol oxidation to formate, especially the exposed (110) facet. Rutile Au1@TiO2 (110) facet prefers to localize at 6-fold Ti site under rich-oxygen conditions; methanol goes through adsorption, OH dissociation, CH dissociation, O2 molecule adsorption, and withdraws H2O and formate to gas phase separately. In particular, a significant step on the way to methanol oxidation is the oxygen vacancy formation at the reduced rutile Au1@TiO2 (110) surface after CH scission and H2O desorption. The subsequent adsorbed O2 molecule does only provide oxygen species for methanol oxidation, but not fill surface oxygen vacancy, which promotes formate formation and desorption to develop very well.

Strain dependence of dissociative adsorption of H[formula omitted]O on epitaxially strained, out-of-plane polarized, BaTiO[formula omitted](001) thin films

The strain dependence of the apparent activation energy of desorption of H2O on epitaxial, TiO2-terminated, out-of-plane polarized, BaTiO3(100) thin films has been investigated using X-ray diffraction, X-ray photoelectron spectroscopy and temperature-programmed desorption. The apparent activation energy of desorption increases significantly with the in-plane strain, suggesting that the strain-induced ferroelectric distortion determines the strength of the chemical bond between the dissociatively adsorbed water and the surface.

New insights in MgAl and MgFe-LDH affinity towards carbon dioxide – role of the hydrophilic character on CO2 retention strength

The surface basicity and hydrophilic character of Mg and Al or Fe-based layered double hydroxides were assessed through thermal programmed desorption (TPD) of CO2 and H2O in terms of CO2 and water retention capacity (CRC and WRC), respectively. LDH calcination up to 450 °C produced slightly irreversible and detrimental effects on both properties, much less pronounced for Fe-based samples. These properties turned out to be completely revived after calcination below 200 °C. Iron-based LDH showed higher hydrophilic character but lower surface basicity than MgAl-2. Both CRC and WRC increased simultaneously with increasing PNLCO beyond a certain level. Water and CO2 adsorb via competitive interactions with weak to medium adsorption sites, but favor mutually their retention on strong basic sites. These results allow predicting LDH behavior in the adsorption of acidic gases and potential large-scale CO2 capture in humid conditions without previous dehydration.

Activity differences of rutile and anatase TiO2 polymorphs in catalytic HBr oxidation

This article investigates the activity of TiO2-rutile and TiO2-anatase polymorphs in the catalytic HBr oxidation, which is an enabling process to close the halogen loop in bromine-mediated transformation of natural gas to high-value chemicals and liquid fuels. The evaluation of rutile-, anatase-, and rutile-anatase TiO2 catalysts, exhibiting the variable specific surface areas, revealed that anatase phase is also active in this reaction. Nonetheless, in contrast to photocatalytic processes in which anatase is typically more active than rutile, rutile exhibits ca. 2–5 times higher intrinsic rates of HBr oxidation than anatase. Thereby, the apparent activation energies and reaction orders with respect to HBr, O2, and H2O display similar values for the two polymorphs. The activity differences were rationalized by density functional theory analysis, which showed that HBr oxidation follows a similar defect-driven mechanism over the most stable rutile (110) and anatase (101) surfaces. Herein, HBr activates the catalyst through a self-doping mechanism that involves the substitution of surface oxygen by bromine with the concomitant reduction of Ti4+ to Ti3+ centers. This forms a defect level that is placed in the band gap and allows for the O2 activation on the catalyst surface. While the HBr adsorption and H2O desorption display a similar energy profiles on both polymorphs, the O2 activation and Br2 evolution are more facile over rutile compared to anatase surface due to shorter distances between the coordinatively unsaturated Ticus sites and easier reduction of Ti4+ centers upon product desorption, respectively.

Fine-Tuning Pyridinic Nitrogen in Nitrogen-Doped Porous Carbon Nanostructures for Boosted Peroxidase-Like Activity and Sensitive Biosensing.

Carbon materials have been widely used as nanozymes in bioapplications, attributing to their intrinsic enzyme-like activities. Nitrogen (N)-doping has been explored as a promising way to improve the activity of carbon material-based nanozymes (CMNs). However, hindered by the intricate N dopants, the real active site of N-doped CMNs (N-CMNs) has been rarely investigated, which subsequently retards the further progress of high-performance N-CMNs. Here, a series of porous N-CMNs with well-controlled N dopants were synthesized, of which the intrinsic peroxidase (POD)like activity has a positive correlation with the pyridinic N content. Density functional theory calculations also reveal that pyridinic N boosts the intrinsic POD-like activity of N-CMNs. Pyridinic-N dopant can effectively promote the first H2O desorption process in comparison with the graphitic and pyrrolic N, which is the key endothermic reaction during the catalytic process. Then, utilizing the optimized nanozymes with high pyridinic N content (NP-CMNs) and superior POD-like activity, a facile total antioxidant capacity (TAC) assay was developed, holding great promise in the quality assessment of medicine tablets and antioxidant food for healthcare and healthy diet.

Two mechanisms for acetic acid synthesis from ethanol and water

Two catalysts, Cu/ZnO/Al2O3 and Cu/ZrO2/Al2O3, were employed for acetic acid synthesis from ethanol and water. They were characterized by X-ray fluorescence, N2 physisorption, N2O titration, in situ X-ray diffraction refined by the Rietveld method, in situ X-ray photoelectron spectroscopy, temperature-programmed reduction followed by X-ray absorption near-edge structure, and temperature-programmed desorption of ethanol, H2O, CO2, and NH3. The Zn-based catalyst is composed of ZnO nanoparticles, Cu0, CuZn alloy, and Al2O3, whereas the Cu/ZrO2/Al2O3 is composed of Cu0, ZrO2, and Al2O3. Both catalysts show the same Cu0 metallic surface area. On one hand, the spectra of the H2O temperature-programmed desorption show that the ZnO nanoparticles of Cu/ZnO/Al2O3 promote water dissociation and, consequently, the redox properties of this catalyst. On the other hand, the Zr-based catalyst shows low activity for H2O dissociation and higher acidity and basicity than for the Zn-based catalyst. These different properties lead to different mechanisms for acetic acid synthesis from ethanol. Taking the redox mechanism of the WGS reaction into account, the steps of the acetic acid synthesis promoted by the Zn-based catalyst can be described as follows: first, ethanol is dehydrogenated alloy to acetaldehyde on Cu0 and CuZn; then, this aldehyde is oxidized on the ZnO–Cu0 interface to acetate, which desorbs, forming acetic acid; finally, H2O dissociates on the O vacancies of the ZnO–Cu0 interface and reoxidizes the oxide. The ZnO–CuZn alloy interface should also be considered in this mechanism. The second mechanism, which occurs mainly on the Zr-based catalyst, is related to the acetaldehyde and ethanol condensation forming ethyl acetate. This ester is hydrolyzed, synthesizing acetic acid and ethanol.

Oxygen reduction reaction on TiO2 rutile (1 1 0) surface in the presence of bridging hydroxyl groups

The goal of this study is to provide insight into the mechanism of the oxygen reduction reaction on the TiO2 rutile (1 1 0) surface in the presence of bridging hydroxyl groups. Considering the Langmuir–Hinshelwood and Eley–Rideal mechanisms, each possible intermediate was identified using density functional theory and a cluster model along with the energy barriers of the reduction steps and the OO bond breaking. Our results show that the initial step, the O2 adsorption on the surface, is favored compared to the pure surface. At higher potentials, the oxygen reduction reaction was found to go through the formation of HO2, which can easily convert to two terminal hydroxyl groups. The rate-limiting step is the desorption of the first H2O with 0.58 eV energy requirement at zero applied potential, while at 1.23 V the reduction of the adsorbed OH to form H2O is the bottleneck with a barrier height of 1.71 eV.

Factors affecting coupled degradation and time-dependent sorption processes of tebuconazole in mineral soil profiles

This laboratory degradation and adsorption study aimed to determine the tebuconazole degradation parameters for 6 profiles of Polish mineral soils and to find links between the tebuconazole degradation rate, its adsorption, soil microbial activity and other significant soil properties. The values of the adsorption distribution coefficient Kd, obtained in batch experiments after 96 h of shaking were in the range of 6.2–34.6 mL g−1. In both batch experiments and incubation experiments at 20 °C, the typical course of adsorption processes was observed, an initial rapid stage followed by a slow stage. In 3 of the 18 soils examined, adsorption was not reached within 51 days. The range of the half-life values was 201–433 days for the Ap horizon and up to 3904 days for subsoils, which were estimated using the two-site nonequilibrium adsorption model coupled with first-order degradation for dissolved and adsorbed pesticide. It was found that modeling the degradation of tebuconazole on the basis of the coefficients of microbial biomass activity for topsoil and two subsoils explained almost 96% of the variance of the estimated pore water degradation rate coefficients in examined soils. The degradation rate was also negatively correlated with the amount adsorbed in the time dependent adsorption sites. This fraction was the least available for soil microorganisms because it was strongly adsorbed in soil pores with a radius <2.5 nm, determined from the H2O desorption isotherm. The degradation rate was also affected by the ratio of the water content in soil during degradation experiments to the water content at field capacity. The results indicated that degradation occurred in the soil liquid phase only.

Moisture effect on the diffusion of Cu ions in Cu/Ta2O5/Pt and Cu/SiO2/Pt resistance switches: a first-principles study

ABSTRACT Cu/Ta2O5/Pt and Cu/SiO2/Pt are two of the most promising resistance switches. From experimental observations, it is speculated that the presence of H2O in the amorphous Ta2O5 and SiO2 (a-Ta2O5 and a-SiO2) facilitates the rate-limiting step during the switching process. This rate-limiting step is essentially the diffusion of Cu ions along the nanopores of the amorphous. To better understand this behavior and obtain a detailed examination of the atomic structures, a first-principles simulation was conducted. In addition, we investigate the diffusion behaviors of Cu ions in bare a-Ta2O5 nanopore and in the one covered with H2O–together with those in a-SiO2 nanopore. Our work reveals that Ta and Si atoms on the sidewalls of bare a-Ta2O5 and a-SiO2 nanopores are in the unsaturated (TaO5) and saturated (SiO4) forms, respectively. Consequently, H2O molecules are adsorbed on the nanopore sidewall strongly in the case of a-Ta2O5, and weakly in a-SiO2, by forming O-Ta and H∙∙∙O bonds, respectively. This can explain the experimental observation that the desorption of H2O occurs only at high temperatures for a-Ta2O5 films, while it is observed for a-SiO2 even when the temperature is low. The calculated diffusion barrier of Cu ions in a-Ta2O5 nanopores covered with H2O is about 0.43 eV, which is much lower than that without H2O (~1.40 eV). In view of the similar chemical environments of O and the adsorbed Cu ions in a-SiO2 and a-Ta2O5 nanopores, it is expected that the diffusion of Cu ions in a-SiO2 nanopore without H2O is much more difficult than with H2O. This could be attributed to the strong and weak adsorption of Cu ions on the sidewall in the absence and presence of H2O, respectively, for both, a-Ta2O5 and a-SiO2. Our investigation provides a full atomic picture to understand the moisture effect on the diffusion of Cu ions in Cu/a-Ta2O5/Pt and Cu/a-SiO2/Pt resistance switches.

Co2 Separation from Ambient Air by Novel CeO2-Based Adsorbent

CO2 direct air capture is a process for CO2 negative emission to prevent global warming. To consider an application for CO2 direct air capture using temperature swing adsorption process, CeO2-based and zeolite-based adsorbent were investigated for their CO2 adsorption and desorption characteristics under atmospheric conditions. The CeO2-based adsorbent exhibited a larger CO2 adsorption amount than zeolite-based adsorbents at atmospheric CO2 concentration under dry conditions. Remarkably, under wet conditions, the CO2 adsorption amount of the CeO2-based adsorbent remained large while the zeolite-based adsorbents exhibited almost no CO2 adsorption. A CO2 adsorption isotherm study revealed that CO2 adsorption of the CeO2-based adsorbent was stable at a low CO2 partial pressure, indicating strong CO2 chemical adsorption. The CO2 desorption of the CeO2-based adsorbent was observed at relatively low temperatures below 200 °C under both dry and wet conditions. The CO2 desorption temperature under dry conditions was lower than that under wet conditions. In situ Fourier transform infrared spectroscopy indicated that in spite of the different CO2 desorption temperatures, the CO2 adsorbed on the CeO2 formed bidentate carbonate species under both dry and wet conditions. Temperature dependence analysis of the spectra revealed that CO2 desorption was accelerated under wet conditions after liquid H2O desorption occurred, indicating that CO2 and H2O had an attractive interaction, which led to a higher CO2 desorption temperature.

Cooperative desorption of H2O and CO from photo-excited cobalt oxide clusters: The evidence of photo-catalytic coupling

The cooperative desorption of H2O and CO with 100% selectivity from the co-adsorbed cobalt oxide clusters CoxOyH2OCO+ [(x,y) = (3,3), (5,7), (6,9), and (9,12)] upon photo excitation at 355 nm was identified by mass spectrometry. In contrast, the sole desorption of CO dominated the mass spectra of Co3O3H2OCO+ upon collisional excitation by Xe at a wide range of center-of-mass collisional energies. The experiments and further kinetic modeling proposed the photo-catalytic coupling of H2O and CO on the cobalt oxide clusters. Combined with theoretical studies, a mechanism for the photo-catalytic coupling of dissociatively adsorbed H2O and CO is proposed.

Role of Nanostructuring on the Properties of Oxide Materials: The Case of Zirconia Nanoparticles

In this review we discuss how the geometric structures, the stability, the reactivity, and the magnetic properties of zirconia nanoparticles (NPs) may differ substantially from those of bulk zirconia or of the (101) ZrO2 surface. Based on extensive density functional calculations, we show that the formation energies of neutral oxygen vacancies are considerably smaller in zirconia NPs than those on the zirconia surface. The presence of low‐coordinated sites on the NPs generates defective states in the band gap of the material that results in enhanced chemical activity towards adsorbed molecular or atomic species, which also affects the physical properties. In particular, the following cases have been considered comparing zirconia bulk and NPs: (1) the adsorption of a Au atom; (2) the adsorption of a H2 molecule; (3) the reduction of zirconia via H2O desorption from a hydroxylated surface; (4) the appearance of high‐spin, magnetic ground states in reduced zirconia NPs. This provides evidence that nanostructuring can turn zirconia, a non‐reducible, non‐magnetic material, into a reducible magnetic oxide.

It's not just the defects – a curved crystal study of H2O desorption from Ag

We investigate water desorption from hydrophobic surfaces using two curved Ag single crystals centered at (111) and (001) apices. On these types of crystals the step density gradually increases along the curvature, allowing us to probe large ranges of surface structures in between the (001), (111) and (110) planes. Subtle differences in desorption of submonolayer water coverages point toward structure dependencies in water cluster nucleation. The B-type step on hydrophobic Ag binds water structures more strongly than adjacent (111) planes, leading to preferred desorption from steps. This driving force is smaller for A-type steps on (111) terraces. The A'-type step flanked by (001) terraces shows no indication of preferred desorption from steps. Extrapolation to the (311) surface, not contained within either curved surface, demonstrates that both A- and A'-type steps can be regarded chemically identical for water desorption. The different trends in desorption temperature on the two crystals can thus be attributed to stronger water adsorption at (001) planes than at (111) planes and identical to adsorption at the step. These results show that our approach to studying the structure dependence of water desorption is sensitive to variations in desorption energy smaller than 'chemical accuracy', i.e. 1 kcal mol-1.