Formaldehyde Desorption Research Articles

Impact of fiber structure on the heat and mass transfer characteristics of medium density fiberboard from fractal-based microstructure reconstruction

Microwave heating can rapidly desorb residual volatile organic compounds (VOCs) such as formaldehyde in medium density fiberboard (MDF) because of its penetration and selective heating characteristics. To further understanding the desorption of formaldehyde from MDF by microwave heating, it’s essential to reveal the relationship between specific fiber structure and the heat and mass transfer characteristics of MDF. The study used scanning electron microscopy (SEM) to acquire the microstructural characteristics of MDF before and after low-temperature (350 ℃) pyrolysis treatment, including porosity and pore size distribution. A reconstruction model of MDF incorporating fractal theory and microstructural information was developed to explore the impacts of fiber structure on the heat and mass transfer parameters. The results indicate that low-temperature pyrolysis has a minimal impact on the pore structure of MDF, the porosity increased to ∼ 45 % from ∼ 40 % after slightly pyrolysis while the average pore diameter is maintained at 7 μm, thus the changes in structure can be disregarded when studying microwave-induced desorption of VOCs in MDF. Further analysis combined with fractal reconstructed structure shows that the inherent fiber organization in MDF introduces anisotropy to the mass transfer parameters, resulting in approximately ∼ 10 % higher mass transfer capability perpendicular to the thickness direction compared to that along the thickness direction. However, the heat transfer parameters of MDF are not significantly affected by the fibre structure, which indicates its isotropy. This study provides a theoretical basis for future research on microwave-induced desorption of VOCs in MDF by offering insights into heat and mass transfer theory.

Systematic Search for Thermal Decomposition Pathways of Formic Acid on Anatase TiO2 (101) Surface**

AbstractIn this study, the reaction pathways for the thermal decomposition of formic acid on the anatase TiO2 (101) surface were systematically investigated. The investigation was carried out using a single‐component artificial force induced reaction method that combines density functional theory calculations. In order to uncover the overall mechanism at low surface coverage, we explored reaction path networks for three different conditions of the anatase TiO2 (101) surfaces: clean, protonated, and oxygen‐deficient surfaces. Previous temperature‐programmed desorption (TPD) experiments had shown that H2O desorption starts at a low temperature of about 300 K, while CO and formaldehyde desorption starts at high temperatures of about 500 K. The present reaction path networks are consistent with the overall trend observed in the TPD experiments. Using the reaction pathways extracted from these networks, the overall dissociation mechanism has been discussed.

Zinc-doped titanium oxynitride as a high-performance adsorbent for formaldehyde in air

The potential utility of titanium oxynitride doped with 5% zinc (ZnTON) has been investigated as an adsorbent for the treatment of gaseous formaldehyde (FA) using a fixed-bed adsorption system. The adsorption capacity of ZnTON, when estimated at 10%/100% breakthrough (BT) levels from a dry feed gas consisting of 10 Pa FA, was far superior to two reference materials (i.e., commercial P25-TiO2 and activated carbon (AC)) by factors of 1.7/1.3 and 10/2.5, respectively. The adsorption capacity of ZnTON increased with the increase in the initial feeding concentration of FA (5–12.5 Pa), while decreasing with the rising temperature (25–100 oC). An increase in moisture level (0–100% relative humidity) also led to 5.4- and 2.5-fold reductions in adsorption capacity of ZnTON at 10% and 100% BT levels, respectively. Thermodynamically, the adsorption of FA onto ZnTON is an exothermic (ΔHo = - 9.69 kJ.mol-1) to be feasible in nature based on physisorption mechanism. Further, the adsorption of FA onto ZnTON was governed by surface interactions and monolayer surface coverage (Van der Waal's force/electrostatic attraction), as it obeyed the Langmuir isotherm and pseudo-second-order kinetic models. Regeneration tests indicated a positive effect of moisture on FA desorption and durability of ZnTON (i.e., over three adsorption-desorption cycles). This study offers valuable mechanistic insights into the synthesis of an advanced adsorbent for the efficient removal of hazardous volatile organic compounds under near-ambient conditions.

Molecular adsorption studies of formaldehyde and methanol on novel twisted bilayer beta phosphorene sheets – a first-principles investigation

The success of graphene and other two-dimensional layered materials opens the way for potential applications. In this report, we investigated the structural stability of twisted bilayer beta phosphorene (TB-PNS) and found stable from the formation energy. Besides, TB-PNS falls in the semiconductor classification with an energy band gap of 0.663 eV. The TB-PNS is deployed as a base substrate to adsorb formaldehyde and methanol vapours. Moreover, the chemi-resistive nature of TB-PNS is observed owing to the adsorption and desorption of formaldehyde and methanol molecules. Also, the electronic properties such as band structure, density of states, and electron density difference show a significant variation due to the adsorption of formaldehyde and methanol. Hence, the results suggest that TB-PNS is a suitable sensing base substrate for formaldehyde and methanol.

Prediction of Ti3C2O2 MXene as an effective capturer of formaldehyde

Overexposure to formaldehyde can pose significant health risk, and much recent research effort has been focusing on the reduction of indoor formaldehyde pollution. Low-dimensional materials with the advantage of having high surface area to volume ratio and abundant active sites have been intensively investigated for their potential use as adsorbents for HCHO. Using density functional theory (DFT) simulations, we predict that nanoflakes of O-terminal titanium carbide MXene, i.e., Ti3C2O2 nanosheets, are able to effectively adsorb formaldehyde. Our results reveal an intermediate adsorption energy of about 0.3 eV per molecule for single molecule adsorption and 0.45 eV for monolayer coverage. These moderate adsorption energies suggest that Ti3C2O2 nanosheets may provide an ideal recyclable material for indoor formaldehyde removal, and, at the same time, facilitate the formaldehyde desorption at higher temperatures. Indeed, our first-principles molecular dynamics (MD) simulation confirms the stability of the adsorption up to 450 K, and desorption of HCHO molecules is observed when the simulation temperature exceeds 500 K. Moreover, we predict an impressively high adsorption capacity exceeding 6 mmol per gram of Ti3C2O2 adsorbent.

First-Principles and Microkinetic Simulation Studies of the Structure Sensitivity of Cu Catalyst for Methanol Steam Reforming

High CO2 and H2 selectivity are important issues for methanol steam reforming (MSR) to provide H2 as a clean energy carrier. The structure sensitivity and the factors governing the activity and selectivity for MSR on Cu catalysts are systematically investigated using density functional theory calculations and microkinetic simulation. Potential energy surfaces including water dissociation, methanol dehydrogenation, formaldehyde desorption and coupling with oxygen-containing intermediates, and formation of hydrogen and carbon dioxide are calculated over Cu(111), (100), (221), (211), and (110) surfaces, respectively. It is found that Cu(110) facet is the most active and selective toward carbon dioxide; Cu(221) is also active but highly selective toward formaldehyde, whereas Cu(111) is nearly inactive. Degree of rate control analysis shows that the activity is controlled mainly by methanol dehydrogenation to formaldehyde, whereas the degree of selectivity control shows that the selectivity toward formaldehyde...

Proof of Equivalent Catalytic Functionality upon Photon‐Induced and Thermal Activation of Supported Isolated Vanadia Species in Methanol Oxidation

AbstractIn this study, evidence is provided that isolated surface vanadia (VO4) species on SiO2 can similarly act as a thermal heterogeneous catalyst and as a heterogeneous photocatalyst. Structurally identical surface VO4 species catalyze the selective oxidation of methanol both by thermal activation and by UV‐light induction. Selectivity to formaldehyde appears to be unity. For the photocatalytic reaction at room temperature, formaldehyde desorption is rate limiting. With larger agglomerates or V2O5 nanoparticles, on the contrary, only the thermal reaction is feasible. This is tentatively attributed to the different positions of electronic states (HOMO/LUMO, valence/conduction band) on the electrochemical energy scale owing to the quantum size effect. Besides providing new fundamental insight into the mode of action of nanosized photocatalysts, our results demonstrate that tuning the photocatalytic reactivity of supported transition‐metal oxides by adjusting the degree of agglomeration is feasible.

Below-Room-Temperature C-H Bond Breaking on an Inexpensive Metal Oxide: Methanol to Formaldehyde on CeO2(111).

Upgrading of primary alcohols by C-H bond breaking currently requires temperatures of >200 °C. In this work, new understanding from simulation of a temperature-programmed reaction study with methanol over a CeO2(111) surface shows C-H bond breaking and the subsequent desorption of formaldehyde, even below room temperature. This is of particular interest because CeO2 is a naturally abundant and inexpensive metal oxide. We combine density functional theory and kinetic Monte Carlo methods to show that the low-temperature C-H bond breaking occurs via disproportionation of adjacent methoxy species. We further show from calculations that the same transition state with comparable activation energy exists for other primary alcohols; with ethanol, 1-propanol, and 1-butanol explicitly calculated. These findings indicate a promising class of transition states to search for in seeking low-temperature C-H bond breaking over inexpensive oxides.

Regulation of sorption processes in natural nanoporous aluminosilicates. 3. Impact of electromagnetic fields on adsorption and desorption of formaldehyde by clinoptilolite

The effect of electromagnetic fields on the adsorption–desorption properties of natural zeolites is a new direction in the theory of physical and chemical processes associated with the surface characteristics of aluminosilicates. The influence of two types of fields, an ultrahigh-frequency electromagnetic field (UHF EMF) and a weak pulsed magnetic field (WPMF) is considered. Modes for treatment of natural zeolite (clinoptilolite) and its activated forms for the most effective sorption of formaldehyde, water, and a mixture thereof were found. It was shown that the preadsorption activation of zeolite in a UHF EMF enhances the selectivity of water vapor sorption, whereas treatment of the sorbent in a WPMF largely affects the selectivity of the absorption of formaldehyde vapors. The decreased desorption of formaldehyde as a result of activation of the mineral in a UHF EMF, as well as complete absence of formaldehyde desorption when a WPMF was used as an activator, was detected.

Descriptor Analysis in Methanol Conversion on Doped CeO2(111): Guidelines for Selectivity Tuning

Descriptors are crucial to systematize and optimize the activity, selectivity, and stability of catalysts. Adsorption energies have usually been taken as the main representative parameters that can summarize reaction energies and activation barriers for simple reactions on relatively simple reaction sites. However, more chemically sound terms, which can be directly mapped to experiments, would be more desirable. In addition, larger molecules with more than one potentially active position and complex sites, such as the acid–base pairs present in oxides, are typically beyond the scope provided by common linear-scaling methods. In the present work, we have analyzed the selectivity of the conversion of a polyfunctional molecule on a complex oxide that presents both acid–base and redox chemistry. The conversion of methanol to formaldehyde or CO on isovalently doped ceria(111) has been taken as an example. The selectivity toward CO is triggered by the competition between formaldehyde desorption and C–H cleavage. Our results show that, by introduction of dopant cations, the activation energy of the first H stripping of formaldehyde can be decreased so that its conversion becomes favorable over desorption for Zr- and Hf-doped systems and expanded lattice ceria. More importantly, desorption is controlled by geometric and acid–base factors, whereas C–H cleavage is exclusively electronically governed through acid–base and redox factors. Thus, both geometric and electronic structure parameters are needed to optimize the performance of ceria to attain the desired selectivity. Selectivity is then estimated by a collective descriptor of the surface that incorporates the ensemble size, acid–base, and redox contributions that can be directly compared to experimental values. In addition, this scaling relationship reduces the error associated with more traditional energy-based descriptors. We anticipate that the present scheme can be extended to metal oxides and other polyfunctionalized catalysts.

Activity versus Selectivity of the Methanol Oxidation at Ceria Surfaces: A Comparative First-Principles Study

Ceria nanoparticles may expose (100) and (111) facets depending on the preparation method. Motivated by that, we study the reactions of methanol with the CeO2(100) surface using dispersion-corrected PBE+U subject to periodic boundary conditions and compare with results for the CeO2(111) surface. At (100) facets, the oxidative dehydrogenation of methanol to formaldehyde occurs with an intrinsic barrier of only 0.94 eV (91 kJ/mol), which is significantly lower than the corresponding barrier of 1.23 eV (119 kJ/mol) at the (111) surface. The lower barrier maps to lower formaldehyde desorption temperatures as observed in temperature-programmed desorption spectroscopy. The higher activity is in accordance with predicted lower oxygen defect formation energy in the (100) surface. Thus, defect formation energies are valid reactivity descriptors for oxidation reactions following a Mars-van Krevelen mechanism. At both surfaces, formaldehyde either desorbs or forms a bridging dioxymethylene intermediate, which can be...

Enhanced Photo-Oxidation of Formaldehyde on Highly Reduced o-TiO2(110)

Photo-oxidation of formaldehyde is important because it is an atmospheric pollutant and a possible intermediate along the pathway for CO2 reduction to methanol. By using temperature-programmed reaction spectroscopy as the main tool, we demonstrate that the efficiency for photo-oxidation of formaldehyde to adsorbed formate on rutile TiO2(110) is strongly dependent on the degree of reduction of the surface and near-surface region. The most efficient photoreaction occurs when O adatoms are present on titania that is highly bulk reduced. These data suggest that electron–hole pairs created deep in the bulk do not play a significant role in the photo-oxidation of formaldehyde. Exposure of the material to O2 heals most bridging oxygen vacancies, creates O adatoms in the Ti five-coordinate rows, and quenches charge at the surface due to the presence of interstitial defects in the near-surface region. Under these conditions, the surface is oxidized but the bulk remains reduced. The major product of formaldehyde photo-oxidation is adsorbed formate; the highest yield is for a highly reduced sample containing O adatoms. The efficiency of formate production on a highly reduced surface is about four times smaller than on one with a similar degree of bulk reduction but containing O adatoms on the surface. On reduced surfaces (not containing O adatoms), the source of the additional O to make formate is derived from a less efficient and more complex photodecomposition pathway of formaldehyde. With increasing bulk reduction of the crystal, the photoefficiency on the reduced surface is severely diminished, suggesting that bridging O vacancies quench photo-oxidation. All photoproducts remain on the surface; only formaldehyde desorption is detected during exposure to UV light. A combination of scanning tunneling microscopy and density functional theory calculations were used to establish that formaldehyde forms a surface dioxyalkylene complex (H2CO–Oad) with oxygen adatoms. This intermediate loses hydrogen to a hole at a nearby bridging O to yield the formate photoproduct. These results are discussed in the broader context of photo-oxidation.

Support effect in oxide catalysis: methanol oxidation on vanadia/ceria.

Density functional theory is used for periodic models of monomeric vanadia species deposited on the CeO2(111) surface to study dissociative adsorption of methanol and its subsequent dehydrogenation to formaldehyde. Dispersion-corrected PBE+U calculations are performed and compared with HSE and B3LYP results. Dissociative adsorption of methanol at different sites on VO2·CeO2(111) is highly exothermic with adsorption energies of 1.8 to 1.9 eV (HSE+D). Two relevant pathways for desorption of formaldehyde are found with intrinsic barriers for the redox step of 1.0 and 1.4 eV (HSE+D). The calculated desorption temperatures (370 and 495 K) explain the peaks observed in temperature-programmed desorption experiments. Different sites of the supported catalyst system are involved in the two pathways: (i) methanol can chemisorb on the CeO2 surface filling a so-called pseudovacancy and the H atom is transferred to an V-O-Ce interphase bond or (ii) CH3OH may chemisorb at the V-O-Ce interphase bond and form a V-OCH3 species from which H is transferred to the ceria surface, providing evidence for true cooperativity. In both cases, ceria is directly involved in the redox process, as two electrons are accommodated in Ce f states forming two Ce(3+) ions whereas vanadium remains fully oxidized (V(5+)).

Effects of Endogenous Formaldehyde in Nasal Tissues on Inhaled Formaldehyde Dosimetry Predictions in the Rat, Monkey, and Human Nasal Passages

Formaldehyde is a nasal carcinogen in rodents at high doses and is an endogenous compound that is present in all living cells. Due to its high solubility and reactivity, quantitative risk estimates for inhaled formaldehyde have relied on internal dose estimates in the upper respiratory tract. Dosimetry calculations are complicated by the presence of endogenous formaldehyde concentrations in the respiratory mucosa. Anatomically accurate computational fluid dynamics (CFD) models of the rat, monkey, and human nasal passages were used to simulate uptake of inhaled formaldehyde. An epithelial structure was implemented in the nasal CFD models to estimate formaldehyde absorption from air:tissue partitioning, species-specific metabolism, first-order clearance, DNA binding, and endogenous formaldehyde production. At an exposure concentration of 1 ppm, predicted formaldehyde nasal uptake was 99.4, 86.5, and 85.3% in the rat, monkey, and human, respectively. Endogenous formaldehyde in nasal tissues did not significantly affect wall mass flux or nasal uptake predictions at exposure concentrations > 500 ppb; however, reduced nasal uptake was predicted at lower exposure concentrations. At an exposure concentration of 1 ppb, predicted nasal uptake was 17.5 and 42.8% in the rat and monkey; net desorption of formaldehyde was predicted in the human model. The nonlinear behavior of formaldehyde nasal absorption will affect the dose-response analysis and subsequent risk estimates at low exposure concentrations. Updated surface area partitioning of nonsquamous epithelium and average flux values in regions where DNA-protein cross-links and cell proliferation rates were measured in rats and monkeys are reported for use in formaldehyde risk models of carcinogenesis.

Study on activated carbon derived from sewage sludge for adsorption of gaseous formaldehyde

The aim of this work is to evaluate the adsorption performances of activated carbon derived from sewage sludge (ACSS) for gaseous formaldehyde removal compared with three commercial activated carbons (CACs) using self-designing adsorption and distillation system. Formaldehyde desorption of the activated carbons for regeneration was also studied using thermogravimetric (TG) analysis. The porous structure and surface characteristics were studied using N2 adsorption and desorption isotherms, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results show that ACSS has excellent adsorption performance, which is overall superior to the CACs. Adsorption theory indicates that the ACSS outperforms the CACs due to its appropriate porous structure and surface chemistry characteristics for formaldehyde adsorption. The TG analysis of desorption shows that the optimum temperature to regenerate ACSS is 75°C, which is affordable and economical for recycling.

A density functional theory study of formaldehyde adsorption and oxidation on CeO 2(1 1 1) surface

The effects of different oxygen species and vacancies on the adsorption and oxidation of formaldehyde over CeO 2(1 1 1) surface were systematically investigated by using density functional theory (DFT) method. On the stoichiometric CeO 2(1 1 1) surface, the C–H bond rupture barriers of chemisorbed formaldehyde are much higher than that of formaldehyde desorption. On the reduced CeO 2(1 1 1) surface, the energy barriers of C–H bond ruptures are less than those on the stoichiometric CeO 2(1 1 1) surface. If the C–H bond rupture occurs, CO and H 2 form quickly with low energy barriers. When O 2 adsorbs on the reduced (1 1 1) surface (O 2/O v species), the C–H bond rupture barriers of formaldehyde are greatly reduced in comparison with those on the stoichiometric CeO 2(1 1 1) surface. If O 2 adsorbs on oxygen vacancy at sub-layer surface, its oxidative roles on formaldehyde are much similar to that of O 2/O v species.

Partial oxidation of methanol on well-ordered V2O5(001)/Au(111) thin films

The partial oxidation of methanol to formaldehyde on well-ordered thin V(2)O(5)(001) films supported on Au(111) was studied. Temperature-programmed desorption shows that bulk-terminated surfaces are not reactive, whereas reduced surfaces produce formaldehyde. Formaldehyde desorption occurs between 400 K and 550 K, without evidence for reaction products other than formaldehyde and water. Scanning tunnelling microscopy shows that methanol forms methoxy groups on vanadyl oxygen vacancies. If methanol is adsorbed at low temperature, the available adsorption sites are only partly covered with methoxy groups after warming up to room temperature, whereas prolonged methanol dosing at room temperature leads to full coverage. In order to explain these findings we present a model that essentially comprises recombination of methoxy and hydrogen to methanol in competition with the reaction of two surface hydroxyl groups to form water.

Effect of Relative Humidity on Adsorption of Formaldehyde on Modified Activated Carbons

This work mainly involves the study of effect of relative humidity on adsorption of formaldehyde on the activated carbons modified with organosilane solution. Modification of activated carbons was carried out by impregnating activated carbon with organosilane/methanol-containing solutions. The breakthrough curves of formaldehyde in the packed beds of original and modified activated carbons were measured, respectively, at relative humidity of 30%, 60%, and 80%. Temperature-programmed desorption (TPD) experiments were used to estimate the activation energy for desorption of formaldehyde from the activated carbon. Results showed that the relative humidity had strongly influence on breakthrough curves of formaldehyde in the packed beds. The higher the relative humidity of gas mixtures through the packed beds was, the smaller the breakthrough time of formaldehyde became. The use of organosilane compounds to modify surfaces of the activated carbon can enhance the interaction between formaldehyde and the surfaces, and as a result, the breakthrough times of formaldehyde in the packed beds of the modified activated carbon were longer than that in the packed bed of the unmodified activated carbon.

Gold, vanadium and niobium containing MCM-41 materials—Catalytic properties in methanol oxidation

This work describes the syntheses, characterisation and catalytic application of silicate MCM-41 materials modified with gold, vanadium and niobium by their introduction during the synthesis (co-precipitation) performed with the use of HCl or H 2SO 4 as pH adjustment agent. The samples were characterised by XRD, N 2 adsorption and test reactions (2-propanol and acetonylacetone transformations). Moreover, they were applied in the methanol oxidation, studied in the flow system and in situ FTIR method. It is shown that the basicity of AuMCM-41 is diminished by the introduction of transition metal species (V, Nb). The materials prepared by the use of H 2SO 4 (for the adjustment of pH) reveals a higher acidity, better ordering of mesopores, and a lower activity in methanol oxidation than those prepared with the use of HCl. The latter exhibit high activity in methanol oxidation to CO 2. The presence of both, V and Nb, besides Au in MCM-41 makes the desorption of formaldehyde much easier increasing the selectivity.

A Density Functional Theory Study of Formaldehyde Adsorption on CeO2(111) Surface

采用基于第一性原理的密度泛函理论和周期平板模型,研究了甲醛在以桥氧为端面的CeO2（111）稳定表面上的吸附行为.通过对不同覆盖度,不同吸附位的甲醛吸附构型、吸附能及电子态密度的分析发现,甲醛在CeO2（111）表面存在化学吸附与物理吸附两种情况.化学吸附结构中甲醛的碳、氧原子分别与表面的氧、铈原子发生相互作用,形成CH2O2物种;吸附能随着覆盖度的增加而减小.与自由甲醛分子相比,物理吸附的甲醛构型变化不大,其吸附能较小.利用CNEB（climbing nudged elastic band）方法计算了甲醛在CeO2（111）表面的初步解离反应活化能（约1.71eV）,远高于甲醛脱附能垒,这与甲醛在清洁CeO2（111）表面程序升温脱附实验中产物主要为甲醛的结果相一致.