Desorption Temperature Of Water Research Articles

Ordinary hydrophobicity of mesoporous faujasites boosting the catalytic ketalization of glycerol with acetone

A series of faujasite zeolites were used in the glycerol ketalization reaction with acetone. This zeolitic structure is widely explored in cracking reactions and has known mesoporosity and hydrophobicity, resulting from the treatments conducted to remove aluminum atoms from the framework and produce a stabilized structure containing different Si/Al ratios. Such properties, in combination with the typical acidity of zeolites, are extremely suitable for driving the ketalization reaction, a two-phase reaction that involves bulky compounds (compared to the size of the zeolitic pores) and that is reversible with the formation of water as a byproduct. The presence of mesopores in the zeolites was confirmed from the pore size distribution using nitrogen physisorption isotherms. Hydrophobicity was confirmed indirectly by thermogravimetry, measuring the amount of water adsorbed and the water desorption temperature. The contact angle of zeolites with glycerol showed only a slight increase with the increase in the Si/Al ratio due to a rise in SiOH groups, defects that naturally arose from removing aluminum atoms. Despite the high concentration of structural hydrophilic SiOH groups, the accessibility and hydrophobicity of isolated Brønsted acid sites (up to Si/Al = 250) inside the zeolitic pores were important to guarantee an improved performance in the reaction.

Accelerated Water Desorption of Oligomeric Poly(ethylene glycol) by Addition of Poly(propylene glycol) for Energy-Efficient Water Recovery Systems.

Adsorbents are used to recover water vapor from the atmosphere in desiccant air conditioning (DAC) and atmospheric water harvesting (AWH) systems. Solid adsorbents have been conventionally used in these systems, though liquid adsorbents are considered to be more effective for energy-efficient fluidic thermosystems because of their low regeneration temperatures (45-70 °C). While most previous studies have focused on improving the adsorption performance, the desorption performance of adsorbents can also be a critical factor in improving the energy efficiency of these systems. Thus, this study aimed to improve the water desorption efficiency, focusing on the liquid adsorbents. We found that mixing hydrophobic molecules into a liquid adsorbent decreases the desorption temperature and increases the water-desorption efficiency. Oligomeric poly(ethylene glycol) (PEG), a common moisture-adsorbing liquid oligomer used in detergents and cosmetics, was selected as the liquid adsorbent. Oligomeric poly(propylene glycol) (PPG), which has a structure analogous to PEG and lower hygroscopicity, was selected as the hydrophobic molecule. Water adsorption and desorption experiments showed that the mixture of PPG with PEG promoted the desorption of water molecules beyond that of PEG, while thermogravimetric differential thermal analysis revealed a decrease in the water desorption temperature with increasing PPG content. The improved desorption efficiency was ascribed to the likely water adsorption equilibrium between PEG and PPG in the blend; water molecules are preferentially desorbed from PPG, which has weaker water-adsorbate interactions. The proposed concept is expected to be incorporated into various hygroscopic liquids to develop energy-efficient liquid adsorbents for DAC and AWH.

Crystalline Porous Organic Salt for Ultrarapid Adsorption/Desorption-Based Atmospheric Water Harvesting by Dual Hydrogen Bond System.

In recent years, the porous sorbent-assisted atmospheric water harvesting (AWH) method has emerged as an effective approach for solving water crises without geographical restrictions. However, there is a limited array of porous adsorbent materials that can be used for AWH, which are inadequate to meet the needs under different climatic conditions. In light of this, herein, we synthesize a new crystalline porous organic salt (CPOS; denoted as CPOS-6) possessing a dual hydrogen bond system and verify its applicability toward AWH for the first time. Unlike other reported CPOSs, CPOS-6 displays an S-shaped water sorption isotherm owing to the presence of the dual hydrogen bond system. Under simulated drought conditions in Xinjiang Uygur Autonomous Region, CPOS-6 exhibits long-term water adsorption-desorption cycling stability, low water desorption temperature, and ultrarapid adsorption-desorption kinetics. The results confirm that CPOS-6 is an effective sorbent material for AWH.

Crystalline Porous Organic Salt for Ultrarapid Adsorption/Desorption‐Based Atmospheric Water Harvesting by Dual Hydrogen Bond System

AbstractIn recent years, the porous sorbent‐assisted atmospheric water harvesting (AWH) method has emerged as an effective approach for solving water crises without geographical restrictions. However, there is a limited array of porous adsorbent materials that can be used for AWH, which are inadequate to meet the needs under different climatic conditions. In light of this, herein, we synthesize a new crystalline porous organic salt (CPOS; denoted as CPOS‐6) possessing a dual hydrogen bond system and verify its applicability toward AWH for the first time. Unlike other reported CPOSs, CPOS‐6 displays an S‐shaped water sorption isotherm owing to the presence of the dual hydrogen bond system. Under simulated drought conditions in Xinjiang Uygur Autonomous Region, CPOS‐6 exhibits long‐term water adsorption‐desorption cycling stability, low water desorption temperature, and ultrarapid adsorption‐desorption kinetics. The results confirm that CPOS‐6 is an effective sorbent material for AWH.

Structural Changes to Supported Water Nanoislands Induced by Kosmotropic Ions

We report the influence of lithium ions on binding and structure of water nanoislands on Au(111) by temperature-programmed desorption and variable-temperature scanning tunneling microscopy. Water coverages between a fraction and full bilayer and two lithium coverages (<0.15% ML) are explored. Lithium enhances selectively the binding of some of the water molecules on precovered Au(111) as compared to water on pristine Au(111), which is revealed by an increase of the water desorption temperature by approx. 10 K. Surprisingly, the effect of lithium on the structure of water is much more extended than expected from these desorption experiments. A small amount of lithium changes the structure of water nanoislands drastically compared to those on pristine Au(111). On pristine Au(111), water ice grows in the form of crystalline islands that are two or three bilayers high. On Li precovered Au(111), the islands are more corrugated, at a 5 times broader apparent height distribution and much smaller, at a 4 times smaller area distribution. These changes reflect the influence of lithium as a structure maker, or kosmotrope, on water. Our study provides unprecedented real-space information of the influence of a kosmotrope on the water structure at the nanoscale. We utilize its kosmotropic behavior to provide real-space images of desorption.

Adsorption Refrigeration Performance of Shaped MIL-101-Water Working Pair

A new metal-organic framework of MIL-101 was synthesized by hydrothermal method and the powder prepared was pressed into a desired shape. The effects of molding on specific surface area and pore structure were investigated using a nitrogen adsorption method. The water adsorption isotherms were obtained by high vacuum gravimetric method, the desorption temperature of water on shaped MIL-101 was measured by thermo gravimetric analyzer, and the adsorption refrigeration performance of shaped MIL-101-water working pair was studied on the simulation device of adsorption refrigeration cycle system. The results indicate that an apparent hysteresis loop appears in the nitrogen adsorption/desorption isotherms when the forming pressure is 10 MPa. The equilibrium adsorption capacity of water is up to 0.95 kg·kg−1 at the forming pressure of 3 MPa (MIL-101-3). The desorption peak temperature of water on MIL-101-3 is 82 °C, which is 7 °C lower than that of silica gel, and the desorption temperature is no more than 100 °C. At the evaporation temperature of 10 °C, the refrigeration capacity of MIL-101-3-water is 1059 kJ·kg−1, which is 2.24 times higher than that of silica gel-water working pair. Thus MIL-101-water working pair presents an excellent adsorption refrigeration performance.

Adsorption of water on sulfur dioxide pre-exposed Zircaloy-4 surfaces

We investigate the interaction of water (H2O) with sulfur dioxide (SO2) pre-exposed Zircaloy-4 (Zry-4) surfaces. Adsorption of SO2 shifts the Zr(MNN) Auger electron feature by 3.0 eV, whereas subsequent water adsorption attenuates the sulfur Auger signal and results in the development of a zirconium oxide, Zr(MNV)o, feature. No further shift in the Zr(MNN) transition is observed with increasing H2O exposures. Following higher H2O exposures on SO2-saturated Zry-4 surfaces, linear heating results in water desorption near 500 K. This temperature is more than 200 K lower than the desorption temperature of water from clean Zry-4 surfaces. Copyright © 2006 John Wiley & Sons, Ltd.

Coadsorption of D2O and CO on Pt(111) Studied by in Situ High-Resolution X-ray Photoelectron Spectroscopy

The adsorption of D 2 O on Pt(111) and the coadsorption with CO have been revisited by quantitative and in situ high-resolution X-ray photoelectron spectroscopy (XPS). During D 2 O adsorption at 110 K, O is spectra clearly show the known formation of a bilayer and multilayers. As indicated by a binding energy change of the multilayer peak in the XP spectra recorded during heating, a structural change in the D 2 O multilayer at the onset of multilayer desorption (∼145 K) is observed. Coadsorption of CO and D 2 O leads to occupation of bridge and on-top sites by CO, with the order in site occupation reversed as compared to the clean surface, as previously observed. In addition, a new CO species is clearly resolved in C 1s spectra, which may be identified with CO on hollow sites. Upon D 2 O desorption, this new species disappears and the CO site occupation changes to the value observed on the clean Pt(111). Induced by CO coadsorption, partial replacement of D 2 O from the surface into higher layers is observed, indicated by a reduced desorption temperature of water and an O is binding energy shift toward the value of the muitilayer signal. Our results suggest an intermixed first layer of CO and D 2 O molecules; D 2 O on top of this layer also influences the CO site distribution. On a surface precovered with 0.5 monolayer (ML) of CO, D 2 O molecules can only adsorb on top of the CO layer and no mixed layer is formed; hence, no site change is observed. The preadsorbed D 2 O bilayer lowers the initial CO sticking coefficient by a factor of ∼13 as compared to the clean surface; this reduction is larger than previously detected.

Kinetics of Abstraction of D and O on Cu(110) Surfaces by Gaseous H Atoms

The kinetics of HD and water formation during admission of H atoms to D- and O-covered Cu(110) surfaces was measured and compared to the kinetics of the analogous reactions on Cu(100), Cu(111), and other metal surfaces. The phenomenology of the HD abstraction kinetics on Cu(100) is according to an Eley-Rideal (ER) mechanism with a cross-section of 2.4 Å2 and only a small contribution of D2 products provides evidence for the operation of a hot-atom mechanism. Similar features were observed on the other Cu surfaces. As origin of the ER- phenomenology on Cu surfaces the inefficiency of electron−hole excitation for hot-atom sticking due to the small density of states at the Fermi level of Cu is proposed. Unlike at Cu(100), the H adsorption-induced reconstruction of the surface does not affect the abstraction kinetics. The interaction of gaseous H with oxygen adsorbed on Cu(100) leads to adsorbed and gaseous water products at 100 K. The occurrence of a considerable fraction of gaseous water at reaction temperatures 60 K below the water desorption temperature indicates that the substantial exothermicity of the reaction leads to energetically ejected product upon their formation. In accordance with other water formation reactions between gaseous H and adsorbed oxygen the mechanism is identified as two sequential hydrogenation reactions, O to adsorbed OH and OH to gaseous and adsorbed water, of which the latter is rate-determining with a cross-section of 0.6 Å2. At reaction temperatures well above the water desorption temperature, isothermal desorption is absent in the water kinetics and only gaseous water was monitored. The coverage of oxygen has no effect on the water kinetics.

Deactivation of PdO–Al 2O 3 oxidation catalyst in lean-burn natural gas engine exhaust: aged catalyst characterization and studies of poisoning by H 2O and SO 2

A palladium oxide on alumina oxidation catalyst was employed to remove combustible pollutants from the exhaust of a spark-ignited, lean-burn natural gas engine. Rapid deactivation was seen for the oxidation of methane and ethane. Characterization results are consistent with sulfur as the primary source of catalyst activity loss. In microreactor studies, deactivation of the engine aged catalysts was only apparent if water was present in the feed stream. In dry feed gas, the activity of fresh and engine aged samples was the same. SO 2 in dry gas was shown to cause both inhibition and deactivation for methane oxidation. This deactivation is partly reversible at 733 K and completely reversible at 793 K. Water inhibits the rate of methane oxidation and causes some permanent activity loss. Activity studies at 733 and 793 K indicate that activity loss is greater when both water and SO 2 are present. Sulfur oxide groups on the surface increase both the amount of water sorbed and the water desorption temperature in TPD experiments. It is proposed that water and SO 2 compete for adsorption sites on the alumina surface. Enhanced activity loss in the presence of both poisons is attributed to enhanced water inhibition and spillover of sorbed SO 2 and SO 3 species from alumina to the PdO surface.

Interaction of H atoms with oxygen adsorbed on Ni(100) surfaces: direct reactions towards OH and H 2O

The reactions of oxygen adlayers on Ni(100) surfaces with gaseous thermal H and D atoms were studied with AES, TDS and reaction-product monitoring methods in the temperature range 120–400 K. Independent of temperature and coverage, adsorbed O atoms are hydrogenated to adsorbed OH (OD) by impinging H (D) atoms. A cross-section of σ = 0.3 A ̊ 2 suggests that this reaction proceeds via an Eley-Rideal (ER)-type direct mechanism. Independent of temperature, in a second ER step with a cross-section of σ = 4.5 A ̊ 2 , adsorbed OH (OD) groups are hydrogenated to water. Well above the water desorption temperature (180 K) the product molecules desorb either through a thermally activated or an energetically mediated step. Below the water desorption temperature (at 120 K), 90% of the product molecules remain adsorbed on the surface and 10% desorb through the energy set free at reaction. An isotope effect (H/D) on the cross-sections was not observed. The processes were observed independent of the oxygen coverage and also at oxidized Ni surfaces, which could be reduced completely at sufficiently high H or D atom fluences.

Thermogravimetric analysis of selected condensed materials on a quartz crystal microbalance

Employing a quartz crystal microbalance and a quadrupole mass spectrometer in a vacuum chamber, thermogravimetric analyses (TGA) and mass spectrometer analyses have been performed at low temperature (90–190 K) on depositions of ammonia, acetone, methanol, toluene, isopropyl alcohol, water, para-xylene, and selected combinations of these materials. Deposition of a single species yielded a TGA curve characteristic of a pure substance. Homogeneous binary mixtures, like methanol and water, exhibited a desorption curve which contained structure in addition to that observed for the pure materials. Sequential layering of one material on top of another also altered the TGA response, suggesting substantial interlayer interactions, and the TGA curve shape was found to be a function of the top-layer thickness for the water-on-methanol system. Backfilling the chamber with nitrogen or argon at a pressure of 1 Torr increased the apparent desorption temperature of water by about 20 K.

The chemisorption of H 2O and O 2 on Cu films on ZnO(0001)-O

The interactions of water and oxygen with well-defined Cu films on the oxygen-terminated ZnO(0001)-O surface has been studied, mainly using temperature-programmed desorption (TPD) and low-energy He + ion scattering (LEIS). The Cu films, which were from submonolayer to multilayer in coverage, had been structurally characterized in previous studies using XPS, LEIS, ARXPS, LEED, work function measurements and CO adsorption. Submonolayer films, deposited at low temperatures in the form of 2D islands, adsorb water and oxygen much like the Cu(110) surface. Upon annealing to high temperatures, these films show a shift in the Cu-derived water TPD peak from 168 down to 158 K, consistent with a conversion of the Cu film from one offering Cu(110)-like sites to one offering Cu(111)-like sites. Even heavy doses of Cu show a water TPD peak due to free ZnO(0001)-O sites, consistent with a film structure where thick Cu islands only cover most, but not all of the surface, with clean ZnO patches between. The fraction of the ZnO surface which is exposed (as measured by this water TPD peak's area) increases with the pre-annealing temperature of the Cu film above 550 K, due to 3D clustering of the Cu. Between 130 K and 370 K, the influence of submonolayer Cu islands on the water desorption temperature from these free ZnO sites decreases, suggesting that small Cu 2D islands coalesce into larger 2D islands. Oxygen-dosed, submonolayer Cu films showed no clear evidence for a hydroxyl state in water TPD.

Steps in the reaction hydrogen + oxygen .dblharw. water on platinum(111): laser-induced thermal desorption at low temperature

The low-temperature reactions of coadsorbed hydrogen and oxygen to form adsorbed water on Pt(111) have been characterized using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD). Hydrogen and oxygen adatoms were reacted in the temperature range 110-145 K, which is below the water desorption temperature (180 K), for varying initial hydrogen coverages and an initial saturation p(2x2)oxygen layers. LITD was used to monitor the coverages of product water molecules and reactant hydrogen adatoms by measuring the H{sub 2} and H{sub 2} LITD yields calibrated against known H, O, and H{sub 2}O coverages, both before the onset of the reaction when only hydrogen and oxygen adatoms were present and also after reacting the two species at higher temperatures where a mixture of hydrogen, oxygen, water and perhaps hydroxyl species were present. The coverage of hydrogen adatoms was found to decrease with an activation energy of E{sub a} = 2.6 kcal/mol, attributed to the step H + O {r_arrow} OH. Approximately the same activation energy was obtained from the temperature dependence of the increase in water coverage. However, the initial increase in water coverage was not sufficient to account for the decrease in the hydrogen adatom coverage and, furthermore, the watermore » coverage at long times actually decreased, implying that significant amounts of adsorbed hydroxyl were formed from the reaction H{sub 2}O + O {r_arrow} 2OH. 40 refs., 9 figs., 1 tab.« less

Isotopic substitution evidence against the formation of (H 3O +) ad from coadsorption of hydrogen and water on Cu(110)

We present both high resolution electron energy loss spectra (HREELS) and thermal desorption spectra (TDS) for hydrogen plus water coadsorption on Cu(110). Hydrogen adsorbed at 100 K. increases the desorption temperature of water by ∽ 10 K., indicating a fairly weak interaction. This stabilised water is associated with dramatic changes in the HREELS spectra: the scissor mode at 1600 cm −1 is attenuated and a new vibration is observed at 1150 cm −1. The assignment of this mode to the symmetric bend of an (H 3O +) ad species as recently reported for Pt(111), therefore seemed plausible also for Cu(110). However, TDS showed no isotopic exchange which indicates that a reaction between (H) ad and (H 2O) ad does not occur on Cu(110). The different origin of the 1150 cm −1 mode on Cu(110) is briefly discussed.

Hydrophilic versus hydrophobic coadsorption: Carbon monoxide and water on Rh(111) versus Pt(111)

Carbon monoxide is a major poison to anodic reactions in aqueous fuel cells. To ascertain the effects of a controlled aqueous environment on the adsorption of CO on electrocatalytically active metals, the coadsorption of CO and water on Rh(111) and Pt(111) surfaces at 100 K was studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). On Rh(111) low coverages of CO shift monolayer water desorption from 182 to 207 K, indicating net attractive COH 2O interactions. Water shifts the CO stretching frequency from 2020 to 1620 cm −1, suggesting a displacement of CO from atop to three-fold hollow sites. A site shift is corroborated by XPS data. Concurrent changes in water vibrational features suggest formation of a mixed phase in which CO and water occupy adjacent sites. It has been hypothesized that such an adsorption geometry would facilitate the normally slow electrooxidation of CO and of fuels such as methanol. On Pt(111), in contrast, all coverages of CO decrease the desorption temperature of water, indicating net repulsive COH 2O interactions. Water splits the single 2110 cm −1 CO stretch of low coverages of CO into two peaks corresponding to the atop and bridging species also seen for higher coverages of CO on the water-free surface. This result and the lack of a change in water vibrational features suggest that on Pt(111) the coadsorbates separate into incompressible islands containing only water and compressible, internally repulsive, patches containing only CO. If the disparate behavior of Rh(111) and Pt(111) also occurs in room temperature aqueous solutions, comparison of their electrooxidation activity in the presence of CO could provide a conclusive test of whether or not rational provision of adjacent CO and water binding sites is likely to be a productive strategy for the development of improved electrooxidation catalysts.

Coadsorption of oxygen and water at Ni(110) surfaces

The interaction of water with oxygen covered Ni(110) surfaces has been studied using high resolution electron energy loss spectroscopy (HREELS). As suggested by a recent TDS/ESDIAD study, it is found that hydroxyl groups are formed at the surface which are stable even above the water-desorption temperature.

Coadsorption of oxygen and water at Ni(110) surfaces

The interaction of water with oxygen covered Ni(110) surfaces has been studied using high resolution electron energy loss spectroscopy (HREELS). As suggested by a recent TDS/ESDIAD study, it is found that hydroxyl groups are formed at the surface which are stable even above the water-desorption temperature.

The interaction of water with the Pt(111) surface

The interaction of water with a platinum (111) surface has been examined by thermal desorption (TDS), ultraviolet photoemission (UPS), and X-ray photoemission (XPS) spectroscopies. UPS and XPS results indicate that water adsorbs molecularly at 100K. TDS studies show that water desorbs with approximately zero order kinetics and that a monolayer of water has a maximum desorption rate at 180 K. Heavier coverages exhibit additional desorption from multilayer water (ice) which peaks at temperatures near 165 K. The coadsorption of atomic oxygen with water leads to an increase in the desorption temperature of water of about 30 K. XPS studies of this surface indicate that adsorbed OH species are involved in the reaction mechanism which increases the water desorption temperature.