Comparison between hydrogen production via H2S and H2O splitting on transition metal-doped TiO2 (101) surfaces as potential photoelectrodes

Doping is an effective way to engineer the electronic band structure of semiconductor materials and consequently their photocatalytic activity for hydrogen generation. In this work, periodic Density Functional Theory (DFT) was employed to compare the adsorption of H₂S and H₂O molecules on TiO₂(101) anatase surfaces compared to four transition metal-doped TiO₂(101) anatase surfaces; Cr⁴⁺-TiO₂, V⁴⁺-TiO₂, Mn⁴⁺-TiO₂, and Nb⁴⁺-TiO₂. The defect formation energy, molecular adsorption energy, hydrogen splitting energies, geometrical changes, electronic structure and charge transfer characteristics were investigated to determine and compare the changes in adsorption of H₂S and H₂O on the pristine vs. doped surfaces. The defect formation energy calculations revealed the Nb⁴⁺-TiO₂ surface resulted in the highest stability, smallest change in neighboring bond lengths and the highest dopant to surface charge transfer. However, upon H₂S and H₂O adsorption, the calculations concluded that the V⁴⁺-TiO₂ surface resulted in the most stable structure for adsorbed H₂S and lowest hydrogen splitting energy requiment compared to the other dopant metals and the lowest for H₂S vs H₂O, indicating its potential catalytic activity for facile dehydrogenation for industrial applications.     

Interaction of H2S with perfect and S-covered Ni(110) surface: A first-principles study

First-Principles study based on Density functional theory (DFT) calculations are employed to investigate the dissociative mechanism of H₂S adsorption and its dissociation on perfect, and sulfur covered Ni(110) surface. On both surfaces, we probe the site preference for H₂S, HS, H, and S adsorption mechanisms. The results indicate that H₂S is energetically adsorbed on their high symmetry adsorption sites with the preferred short-bridge (SB) site on both surfaces. Furthermore, we found that chemisorption of HS is stronger in contrast to H₂S at favorable short-bridge (SB) with a binding energy of −3.59 eV on perfect Ni(110) surface, and on S-covered Ni(110) surface at the favorable hollow site having a binding energy of −3.57 eV. In the first H₂S dehydrogenation, energy barriers for S–H bond breaking over the clean surface are 0.08–0.46 eV and a little bit higher on the S-covered surface are 0.1–0.78 eV, while in second dehydrogenation the energy barrier on a clean surface is 0.19 eV. For further detail, electronic densities of states and d-band center model are used to characterize the interaction of adsorbed H₂S with both surfaces. Hence, our results show that decomposition of H₂S over perfect and S-covered Ni(110) surface is exothermic and also an easy process. However, kinetically and thermodynamically, the subsistence of surface sulfur avoids the H–S bond breaking process.     

Theoretical study on the effect of an O vacancy on the hydrogen storage properties of the LaFeO3 (010) surface

Based on first-principles calculations, we investigated the hydrogen adsorption dissociation on the LaFeO₃ (010) surface with an O vacancy. It was confirmed that H₂ molecules have four kinds of adsorption modes on LaFeO₃ (010) surfaces with an O vacancy. First, H atoms are adsorbed on O atoms to form an OH group. Second, H atoms are adsorbed on Fe atoms to form FeH bonds. Third, two H atoms are adsorbed on the same O atom to form H₂O. Fourth, two H atoms are adsorbed on the same Fe atom and it is a new type of adsorption, which does not exist in the ideal surface. The main channel of dissociative adsorption is the fourth adsorption mode of OH and FeH, where the H atoms adsorbed on the surface of Fe can be easily diffused into O atoms. Charge population analysis showed that increasing the O vacancy enhanced the interaction between FeH. In the system containing O vacancies adsorbed H atoms in the top of Fe to diffuse to the top of O need to overcome the energy barrier decreased from 0.968 eV to 0.794 eV. So the existence of an O vacancy enhances the hydrogen absorption properties of Fe atoms in LaFeO₃.     

The mechanism of the water dissociation and dehydrogenation of glycerol on Au (111) and PdAu alloy catalyst surfaces

Hydrogen is an energy carrier found from renewable sources such as biomass, geothermal, solar, or wind. Water splitting and dehydrogenation of glycerol is a sustainable process of H₂ production from renewables because water is abundant, and the glycerol is formed from the biomass-derived compounds. However, finding a suitable and best catalyst for these processes is challenging. Thus, this paper proposed a theoretical study to find the mechanism of the dissociation of water and dehydrogenation of glycerol using Au metal and PdAu alloy catalysts using the density functional theory (DFT) method. Four PdAu alloys have been constructed with different atomic compositions ranging from 1 to 3 of Pd metal to Au metal. The result showed strong adsorption on the Pd₁Au₃ catalyst surface, and the water splitting is best on the Pd₃Au₁ catalyst surface. Simultaneously, the glycerol adsorption on catalyst surfaces is tested before proceeding for the complete dehydrogenation mechanism of glycerol. Strong adsorption was found at the Pd₁Au₃ catalyst compared to other catalyst surfaces on the glycerol adsorption. The dehydrogenation mechanism was found toward a downhill reaction and removed eight hydrogens from the glycerol compared to Au metal, referring to easy dehydrogenation of glycerol using the alloy PdAu. The final species that adsorbed on the Pd₁Au₃ surface is the carbon monoxide will be turned later into carbon dioxide.     

Strain facilitated small gas molecules sensing properties on Au doped SnSe2 monolayer

By the first-principles calculations, the sensitivity of CO, H₂O and NO adsorption on Au doped SnSe₂ monolayer surface is investigated. The results show that CO and H₂O molecules are physically adsorbed on Au doped SnSe₂ monolayer and act as donors to transfer 0.012 e and 0.044 e to the substrate, respectively. However, the NO molecule is chemically adsorbed on substrate and acts as an acceptor to obtain 0.116 e from the substrate. In addition, our results also show that the biaxial strain can effectively improve the adsorption energy and charge transfer of gas molecules adsorbed on the substrate surface. Also, the recovery time of desorbed gas molecules on the substrate surface is calculated, and the results indicate that the Au doped SnSe₂ is a perfect sensing material for detection and recovery of CO and NO under ?8% strain.     

New insights into the interfacial interactions of O2 and H2O molecules with PuH2 (110) and (111) surfaces from first-principles calculations

The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O₂ adsorption on (110) and (111) surfaces of PuH₂ by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm^?1. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm^?1. It shows that non-dissociative oxygen could be considered as superoxide (O₂^?) or peroxide (O₂^2?) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H₂O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H₂O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.     

A comparative experimental and density functional study of glucose adsorption and electrooxidation on the Au-graphene and Pt-graphene electrodes

At present, the graphene is covered on Cu foil with the 5 sccm hexane (C₆H₁₄) flow rate, 50 sccm hydrogen (H₂) flow rate, and 20 min deposition time parameters by the CVD method. The graphene on the Cu foil is then covered onto few-layer ITO electrode. Furthermore, the Pt and Au metals are electrodeposited on graphene/ITO electrode with electrochemical method. These electrodes are characterized by Raman spectroscopy and Scanning Electron Microscopy-Energy Dispersive X-Ray analysis (SEM-EDX). The graphene structure is approved via Raman analysis. Au, Pt, and graphene network are openly visible from SEM results. In addition, glucose (C₆H₁₂O₆) electrooxidation is investigated with cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) measurements. As a result, Pt-graphene/ITO indicates the best C₆H₁₂O₆ electrooxidation activity with 9.21 mA cm⁻² specific activity (highly above the values reported in the literature). In all electrochemical measurements, Pt-graphene/ITO exhibits best electrocatalytic activity, stability, and resistance compared to the other electrodes. The adsorption of the C₆H₁₂O₆ molecule is examined theoretically over metal atom (gold and platinum)-doped graphene surfaces using the density functional theory (DFT) method. The interaction between C₆H₁₂O₆ molecule and OH adsorbed Pt-doped surface is stronger than that of OH adsorbed Au-doped graphene surface thermodynamically according to the reaction energy values.     

The hydrogen evolution reaction on single crystal gold electrode

As one of the important candidate of power sources for the future, the research and production of hydrogen gas has a significant importance. In this article, the emphasis is on the influence of impurities on hydrogen evolution reaction, i.e., the influence of an addition of decacyclene, C₁₂H₃₅C₆H₄SO₄Na, CH₃CH₂OH, chromanone, H₂SO₄, HNO₃, 4,4′-biphenediol and 1,2,3,4-tetraphenyl-1,3-cyclopentadiene was studied by electrochemical impedance technique. The adsorption structure for some organics was measured by scanning tunneling spectroscopy techniques. Superstructure of adsorbed decacyclene on Au(111) surface was captured. The ordered adsorption structure of 4,4′-biphenyldiol on Au(111) and (100) was also observed. The addition of decacyclene has shown an opposite effects on hydrogen evolution for Au(111) and (100) surface, i.e., it inhibits the reaction at Au(100) but enhances the one at Au(111). The results show that the addition of C₁₂H₃₅C₆H₄SO₄Na and HNO₃, especially the latter, can improve the hydrogen evolution. In the article the adsorption structure and hydrogen evolution reaction have been studied in order to give some useful information about the relation between the adsorption structure and the properties. The purpose of this article is to attempt to find the relation between electrochemical performance and the adsorption structure, and to explore the effect of some additives.     

Atomistic study of LaNbO4; surface properties and hydrogen adsorption

We have calculated fundamental properties of pure and hydrogen-covered (010), (101), (100) and (001) surfaces of the low temperature monoclinic phase of LaNbO₄ (LN). The (010) surface was the most stable one, exhibiting electronic structure and local geometric configurations similar to bulk. As the first stage of proton migration into the electrolyte, the ability of LN surfaces to split H₂ molecules was probed indirectly by calculating the adsorption energy of H atoms on two of the LN surfaces. H adsorption on the (010) surface was found to be strongly endothermic, and thus cannot contribute much in splitting H₂. The adsorption energy on the relatively unstable (101) surface was on the other hand approximately −0.6 eV, in the right range for surface H₂ to be catalyzed beneficially. H adsorption on this surface was induced by surface states in the band gap of the clean surface. Since the unstable (101) surface is not abundant, the rate of dissociative adsorption of H₂ on the LN surface can be anticipated to be very low. Application of the energies to simple adsorption isotherm calculations for typical proton conducting fuel cells (PCFCs) operating temperatures correspondingly showed very low H coverage, and it is not expected that LaNbO₄ surfaces can contribute much to the H₂ activation reaction of a PCFC anode.     

The adsorption and dissociation of H2O on TiO2(110) and M/TiO2(110) (M = Pt,Au) surfaces–A computational investigation

The adsorption and dissociation of H₂O on clean TiO₂(110) and metal-deposited M/TiO₂(110) (M = Pt and Au) surfaces were studied by performing calculations of periodic density-functional theory. M/TiO₂(110) surfaces catalytically decompose H₂O with barriers (decreased by ca. 15–19 kcal/mol) much smaller than for their clean TiO₂(110) counterparts. The Au-deposited TiO₂ surface has the least energy barrier (ca. 3.5 kcal/mol less than the Pt analogue), explicable with a Bader charge analysis.     

Alkali and transition metal atom-functionalized germanene for hydrogen storage: A DFT investigation

In this work, we have performed density functional theory-based calculations to study the adsorption of H₂ molecules on germanene decorated with alkali atoms (AM) and transition metal atoms (TM). The cohesive energy indicates that interaction between AM (TM) atoms and germanene is strong. The values of the adsorption energies of H₂ molecules on the AM or TM atoms are in the range physisorption. The K-decorated germanene has the largest storage capacity, being able to bind up to six H₂ molecules, whereas the Au and Na atoms adsorbed five and four H₂ molecules, respectively. Li and Ag atoms can bind a maximum of three H₂ molecules, while Cu-decorated germanene only adsorbed one H₂ molecule. Formation energies show that all the studied cases of H₂ molecules adsorbed on AM and TM atom-decorated germanene are energetically favorable. These results indicate that decorated germanene can serve as a hydrogen storage system.     

Understanding active sites and mechanism of oxygen reduction reaction on FeN4–doped graphene from DFT study

First-principle density functional theory (DFT) calculations are performed to study the active sites in FeN₄G electrocatalysts, as well as ORR activity and mechanism. The possible intermediates and transition states existing in the possible reaction paths from Langmuir-Hinschelwood (LH) mechanism are investigated. The results show that the associative pathways of OOH1 formation is prior to that of O₂1 dissociation. The condition of proton adsorbed on top N sites (T2) is more beneficial to the reduction of O-contained species adsorbed on top Fe site (T1) compared to the conditions of proton adsorbed on top C sites (T3). However, the dissociation of O₂1, OOH1 and H₂O₂1 is more likely to occur on the paired T1-T3 sites, since their lower energy barriers compared to other paired sites. The most favorable four-electron reduction pathway follows the mechanisms: O₂1→ OOH1→ O1+H₂O→ OH1+H₂O→ 2H₂O. The rate determining step for ORR on FeN₄G is the reduction of O1 into OH1 (barrier, 0.47 eV). The most feasible pathway for ORR is downhill at a high electrode potential (0.76 V vs. SHE at pH = 0) according to the free energy diagram. Compared to the ideal catalyst, the adsorption energy of OOH1 on FeN₄G is much lower in free energy, while those of OH1 and O1 are slightly higher. Additionally, the elementary reaction rate for OOH1→O1+H₂O is much larger than that of OOH1→H₂O₂ based on the parameter of activation barrier. Therefore, the formation of H₂O₂ (l) is unfavorable on FeN₄G catalysts.     

Theoretical insights into electronic structures and durability of single-atom Pd/TiN catalysts

Density functional theory (DFT) calculations have been performed to evaluate the metal-support interactions between palladium atom and titanium nitride surface (Pd–TiN). N vacancy sites on defective TiN surface can stabilize Pd single atom under strongly oxidizing conditions, and surface defect-mediated stabilization is accompanied by obvious charge redistributions between Pd and substrate. The adsorption of several gas species on stable Pd–TiN surfaces is also explored to understand catalytic reactions. It is found that O, H, OH, O₂, and CO favorably adsorb on Ti atop site, while CO₂ and H₂ prefer the hollow and Pd atop site, respectively. Moreover, co-adsorption with either H or OH weakens the CO-surface interactions, indicating the CO poisoning effect would be alleviated under both acidic and alkaline conditions. This study provides a perspective to understand the support effect of Pd–TiN, which sheds light on the design of high-performance Pd-based electrodes for proton exchange membrane fuel cells (PEMFCs).     

Boosting visible light driven hydrogen production: Bifunctional interface of Ni(OH)2/Pt cocatalyst on TiO2

Sunlight driven hydrogen production by water splitting represents a sustainable approach for hydrogen energy utilization, however, solo semiconductor could not meet the current demand. Herein, as proof of concept, we put forward a bifunctional interface of Ni(OH)₂/Pt cocatalyst loaded on anatase TiO₂ through a selective deposition process. Ni(OH)₂/Pt interface facilitates H₂O adsorption by providing two adsorption sites: O is selectively adsorbed on Ni(OH)₂ and H adsorbed on metallic Pt. Beyond that, Ni(OH)₂/Pt interface also extends light absorption range of TiO₂ to visible light region and accelerates the separation of photo-generated electrons/holes for semiconductor TiO₂. Benefiting from this interfacial engineering in semiconductor, the visible-light driven hydrogen production performance of the as-obtained Ni(OH)₂/Pt/TiO₂ sample is greatly boosted with a rate of 5875 μmol g⁻¹ h⁻¹, 2.6 times higher than that of Pt/TiO₂.     

Adsorption of H2S on carbonaceous materials of different dimensionality

The effect of the dimension of carbonaceous systems, from two to zero, on the adsorption strength of H₂S is investigated by density functional theory based methods. To this end, a carbon nanocone (CNC), a (3, 3) carbon nanotube ((3, 3)-CNT), and graphene (G) are chosen as models for zero-, one- and two-dimensional systems, respectively. Pristine G and CNC have low tendency to adsorb H₂S but on the (3, 3)-CNT the molecule adsorbs dissociatively and deforms the surface. The effect of doping the surface of these materials with Ti has also been investigated. The presence of Ti modifies H₂S adsorption capability to the point that it is chemically adsorbed on the three decorated surfaces although H₂S adsorption on Ti decorated graphene appears to be different from two other doped surfaces. Only in this case, the H₂S molecule dissociates and releases hydrogen atoms which form H₂ molecule. The resulting H₂ molecule is physisorbed on the Ti-decorated graphene surface and the S atom adsorbs directly on the Ti atom. The density of states of pristine, Ti decorated and H₂S adsorbed nanostructures demonstrate that the systems change their conductivity and magnetic properties.     

La(OH)3-decorated NiFe nanoparticles as efficient catalyst for hydrogen evolution from hydrous hydrazine and hydrazine borane

Ultrafine NiFe nanoparticles (NPs) decorated by La(OH)₃ have been successfully prepared via a facile one-step co-reduction method without the help of surfactant or support. It was found that after being decorated with La(OH)₃, the NiFe–La(OH)₃ NPs have a smaller particle size and lower crystallinity. The resultant NiFe–La(OH)₃ nanocatalyst exhibits an excellent catalytic activity, 100% H₂ selectivity, and satisfied recyclability for hydrogen evolution from hydrous hydrazine (N₂H₄·H₂O) at 343 K under alkaline conditions, giving a high turnover frequency (TOF) value of 100.6 h⁻¹, which is more than 35 times higher than pure NiFe NPs (2.8 h⁻¹). The kinetics study shows that with respect to the concentration of catalyst and N₂H₄, the N₂H₄ dehydrogenation reaction follows fist-order kinetics and zero-order kinetics, respectively. Furthermore, NiFe–La(OH)₃ also exhibits an outstanding performance and excellent recycle stability during high dehydrogenation of hydrazine borane (N₂H₄BH₃). The excellent performances for the dehydrogenation of N₂H₄ and N₂H₄BH₃ could be attributed to the strong interaction between La(OH)₃ and NiFe NPs, as well as ultrafine and low crystalline NiFe NPs due to the addition of La(OH)₃.     

Photocatalytic decomposition of formic acid and methyl formate on TiO2 doped with N and promoted with Au. Production of H2

The photo-induced vapor-phase decompositions of formic acid and methyl formate were investigated on pure, N-doped and Au-promoted TiO₂. Infrared (IR) spectroscopic studies revealed that illumination initiated the decomposition of adsorbed formate formed in the dissociation of formic acid and located mainly on TiO₂. The photocatalytic decompositions of formic acid and methyl formate vapor on pure TiO₂ occurred to only a limited extent. The deposition of Au on pure or doped TiO₂ markedly enhanced the extent of photocatalytic decomposition of formic acid. The main process was dehydrogenation to give H₂ and CO₂. The formation of CO occurred to only a very small extent. Addition of O₂ or H₂O to the formic acid decreased the CO level from ∼0.8% to ∼0.088%. Similar features were experienced in the photocatalytic decomposition of methyl formate, which dissociated in part to give surface formate. Experiments over Au deposited on N-doped TiO₂ revealed that the photo-induced decomposition of both compounds occurs even in visible light.     

Hydrogen release mechanism and performance of ammonia borane catalyzed by transition metal catalysts Cu‐Co/MgO(100)

The catalytic dehydrogenation of ammonia borane (NH₃BH₃, AB) molecule is most frequently employed by metal catalysts, but a reliable dehydrogenation mechanism in molecular level has yet to be fully illuminated. Herein, adopting the density functional theory (DFT) method, the dehydrogenation mechanism and performance of NH₃BH₃ under the transition metal catalysts (Cu/MgO, Co/MgO, CuCo/MgO) were studied. The calculated results show that the dehydrogenation mechanism of AB refers to stepwise dehydrogenation mechanism: AB is adsorbed in the transition metal catalysts firstly, then one H(N) atom transferred to H(B) of ―BH₃ and to form H₂ molecule via the broken of B―H and N―H bond, finally, H₂ molecule desorption from the catalyst complexes. Among the transition metal catalysts, CuCo/MgO have the perfect catalytic activity in dehydrogenation reaction of NH₃BH₃, its barrier energy of the feasible pathway (path A) is 22.26 kcal/mol, which is lower than the barrier energy of AB‐Cu/MgO(28.13 kcal/mol), AB‐Co/MgO(27.46 kcal/mol), and the results of thermogravimetric analysis further verified the reasonability of DFT calculational results. Besides, partial density of states calculational results show the electron orbital hybridization of Cu, Co atom may account for the excellent catalytic performance of CuCo/MgO(100) compared with the Cu/MgO(100) and Co/MgO(100) in dehydrogenation process of AB.     

The mechanism characterizations of methane steam reforming under coupling condition of temperature and ratio of steam to carbon

To elucidate the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming process, the characterizations of methane steam reforming at different temperature and ratio of steam to carbon in term of distribution of H₂ and CO, and the elementary reaction rate were investigated. Meanwhile, the formation mechanisms of H₂ and CO via sensitivity analysis and reaction path analysis were obtained. The results showed that the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming were higher than that of individual factor. The effects of temperature on the methane steam reforming were higher than that of the ratio of steam to carbon. The adsorption and desorption reaction of CH₄ on the surface of Ni-based catalyst had the most obvious effect on the sensitivity of H₂, CH₄ and CO. Besides, the effects of adsorption and desorption reaction of H₂O on the sensitivity of H₂ were higher than that of CH₄ and CO. Hydrogen was generated by the desorption reaction of H(s) in the adsorbed state and from three generating paths: a) CH₄(s) dissociated directly or reacted with O(s) to form H(s); b) The dissociation reaction of H₂O(s) produced H(s); c) OH(s) dissociated directly or reacted with C(s) to form H(s). Carbon monoxide was generated from single path: CH₄(s)→CH₃(s)→CH₂(s)→CH(s)→C(s)→CO(s)→CO(g).     

Two-dimensional modelling for catalytically stabilized combustion of a lean methane-air mixture with elementary homogeneous and heterogeneous chemical reactions

The catalytically stabilized combustion (CST) of a lean (equivalence ratio Φ = 0.4) methane-air mixture was investigated numerically in a laminar channel flow configuration established between two platinum-coated parallel plates 50 mm long and 2 mm apart. A two-dimensional elliptic fluid mechanical model was used, which included elementary reactions for both gaseous and surface chemistry. Heat conduction in the solid plates and radiative heat transfer from the hot catalytic surfaces were accounted for in the model. Heterogeneous ignition occurs just downstream of the channel entrance, at a streamwise distance (x) of 4 mm. Sensitivity analysis shows that key surface reactions influencing heterogeneous ignition are the adsorption of CH₄ and O₂ and the recombinative desorption of surface-bound O radicals; the adsorption or desorption of radicals other than O has no effect on the heterogeneous ignition location and the concentrations of major species around it. Homogeneous ignition takes place at x = 41 mm. Sensitivity analysis shows that key surface reactions controlling homogeneous ignition are the adsorption/desorption of the OH radical and the adsorption/desorption of H₂O, the latter due to its direct influence on the OH production path. In addition, the slope of the OH lateral wall gradient changes from negative (net-desorptive) to positive (net-adsorptive) well before homogeneous ignition (x = 30 mm), thus exemplifying the importance of a detailed surface chemistry scheme in accurately predicting the homogeneous ignition location. The effect of product formation on homogeneous ignition was studied by varying the third body efficiency of H₂O. Product formation promotes homogeneous ignition due to a shift in the relative importance of the reactions H + O₂ + M → HO₂ + M and HCO + M → CO + H + M.