Hydrogen Affinity Research Articles

FEM simulation of palladium thin film coated surface acoustic wave hydrogen sensor for high frequency applications

A higher operating frequency is desirable for Surface Acoustic Wave (SAW) based sensors as they become more sensitive at high frequencies. The acoustic wave gets more confined near the surface at high frequencies and become more sensitive to the external stimulations. This makes SAW devices a suitable device for sensing gaseous state chemicals. SAW devices have become the basic building block of wireless sensor networks with its advantages enabling remote sensing. In this paper, a SAW based Hydrogen sensor is realized through the Finite Element Analysis tool ANSYS. Hydrogen even though has a significant role in many industries, its explosive nature demands constant monitoring. SAW delay line made up of XY-LiNbO3 as substrate with a thin layer of Palladium coated along the delay length as the sensing element is modeled. Palladium with its high affinity for Hydrogen absorbs the same and undergoes changes in properties like density and stiffness. This disturbs the surface wave propagation and in turn, affects the operating frequency which is the sensor response parameter. The frequency shift of 1.91 MHz for Hydrogen concentration of 0.3 a.f. as compared to 0.49 MHz with YZ- LiNbO3. The operating frequency also shifts to a higher range as the acoustic velocity of the substrate increases.

Synthesis and hydrogen storage behavior of Mg–V–Al–Cr–Ni high entropy alloys

The ternary MgVAl, MgVCr, MgVNi, quaternary MgVAlCr, MgVAlNi, MgVCrNi and quinary MgVAlCrNi alloys were produced by high energy ball milling (HEBM) under hydrogen pressure (3.0 MPa) as a strategy to find lightweight alloys for hydrogen storage applications. Most of the ternary and quaternary alloys presented multiphase structure, composed mainly of body-centered cubic (BCC) solid solutions and Mg-based hydrides. Only the quinary MgVAlCrNi high entropy alloy (HEA) formed a single-phase structure (BCC solid solution), which is a novel lightweight (ρ = 5.48 g/cm3) single-phase HEA. The hydrogen storage capacity of this alloy was found to be very low (approximately 0.3 wt% of H). Two non-equiatomic alloys with higher fraction of Mg and V (strong hydride former elements), namely Mg28V28Al19Cr19Ni6 and Mg26V31Al31Cr6Ni6, were then designed, aiming at higher storage capacity. Both alloys were produced by HEBM. The results show that the non-stoichiometric alloys also presented low hydrogen storage capacity. The low affinity of these alloys with hydrogen was discussed in terms of enthalpy of hydrogen solution and enthalpy of hydride formation of the single components. This study brought to light the importance of considering both enthalpy of hydrogen solution and enthalpy of hydride formation of the alloying elements for designing Mg-containing HEA for hydrogen storage. Once Mg has a positive enthalpy of hydrogen solution, the alloys composition must be balanced with alloying elements with higher hydrogen affinity, i.e., negative values of enthalpy of solution and hydride formation.

Biomass hydrodeoxygenation catalysts innovation from atomistic activity predictors.

Circular economy emphasizes the idea of transforming products involving economic growth and improving the ecological system to reduce the negative consequences caused by the excessive use of raw materials. This can be achieved with the use of second-generation biomass that converts industrial and agricultural wastes into bulk chemicals. The use of catalytic processes is essential to achieve a viable upgrade of biofuels from the lignocellulosic biomass. We carried out density functional theory calculations to explore the relationship between 13 transition metals (TMs) properties, as catalysts, and their affinity for hydrogen and oxygen, as key species in the valourization of biomass. The relation of these parameters will define the trends of the hydrodeoxygenation (HDO) process on biomass-derived compounds. We found the hydrogen and oxygen adsorption energies in the most stable site have a linear relation with electronic properties of these metals that will rationalize the surface's ability to bind the biomass-derived compounds and break the C-O bonds. This will accelerate the catalyst innovation for low temperature and efficient HDO processes on biomass derivates, e.g. guaiacol and anisole, among others. Among the monometallic catalysts explored, the scaling relationship pointed out that Ni has a promising balance between hydrogen and oxygen affinities according to the d-band centre and d-band width models. The comparison of the calculated descriptors to the adsorption strength of guaiacol on the investigated surfaces indicates that the d-band properties alone are not best suited to describe the trend. Instead, we found that a linear combination of work function and d-band properties gives significantly better correlation. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Thermal perturbation of NMR properties in small polar and non-polar molecules

Water is an important constituent in an abundant number of chemical systems; however, its presence complicates the analysis of in situ1H MAS NMR investigations due to water’s ease of solidification and vaporization, the large changes in mobility, affinity for hydrogen bonding interactions, etc., that are reflected by dramatic changes in temperature-dependent chemical shielding. To understand the evolution of the signatures of water and other small molecules in complex environments, this work explores the thermally-perturbed NMR properties of water in detail by in situ MAS NMR over a wide temperature range. Our results substantially extend the previously published temperature-dependent 1H and 17O chemical shifts, linewidths, and spin-lattice relaxation times over a much wider range of temperatures and with significantly enhanced thermal resolution. The following major results are obtained: Hydrogen bonding is clearly shown to weaken at elevated temperatures in both 1H and 17O spectra, reflected by an increase in chemical shielding. At low temperatures, transient tetrahedral domains of H-bonding networks are evidenced and the observation of the transition between solid ice and liquid is made with quantitative considerations to the phase change. The 1H chemical shift properties in other small polar and non-polar molecules have also been described over a range of temperatures, showing the dramatic effect hydrogen bonding perturbation on polar species. Gas phase species are observed and chemical exchange between gas and liquid phases is shown to play an important role on the observed NMR shifts. The results disclosed herein lay the foundation for a clear interpretation of complex systems during the increasingly popular in situ NMR characterization at elevated temperatures and pressures for studying chemical systems.

Surface microstructural controls on electrochemical hydrogen absorption at polycrystalline palladium

The ease by which hydrogen is absorbed into a metal can be either advantageous or deleterious, depending on the material and application in question. For instance, in metals such as palladium (Pd), rapid absorption kinetics are seen as a beneficial property for hydrogen purification and storage applications, whereas the contrary is true for structural metals such as steel, which are susceptible to mechanical degradation in a process known as hydrogen embrittlement. It follows that understanding how the microstructure of metals (i.e., grains and grain boundaries) influences adsorption and absorption kinetics would be extremely powerful to rationally design materials (e.g., alloys) with either a high affinity for hydrogen or resistance to hydrogen embrittlement. To this end, scanning electrochemical cell microscopy (SECCM) is deployed herein to study surface structure-dependent electrochemical hydrogen absorption across the surface of flame annealed polycrystalline Pd in aqueous sulfuric acid (considered to be a model system for the study of hydrogen absorption). Correlating spatially-resolved cyclic voltammetric data from SECCM with co-located structural information from electron backscatter diffraction (EBSD) reveals a clear relationship between the crystal orientation and the rate of hydrogen adsorption-absorption. Grains that are closest to the low-index orientations [i.e., the {100}, {101}, and {111} facets, face-centered cubic (fcc) system] facilitate the lowest rates of hydrogen absorption, whereas grains of high-index orientation (e.g., {411}) promote higher rates. Apparently enhanced kinetics are also seen at grain boundaries, which are thought to arise from physical deformation of the Pd surface adjacent to the boundary, resulting from the flame annealing and quenching process. As voltammetric measurements are made across a wide potential range, these studies also reveal palladium oxide formation and stripping to be surface structure-dependent processes, and further highlight the power of combined SECCM-EBSD for structure-activity measurements in electrochemical science.

Selectivity roadmap for electrochemical CO2 reduction on copper-based alloy catalysts

Due to the complex reaction network of the electrochemical CO2 reduction reaction (CRR), developing highly selective electrocatalysts for desired products remains a major challenge. In this study, a series of Cu-based single atom alloys (M@Cu) with multiple active sites are modelled to investigate their CRR selectivity trends by evaluating various adsorption configurations and energetics. The hydrogen (H) and oxygen (O) affinity of the secondary metals in the M@Cu model catalysts are found to be effective descriptors in determining CRR selectivity. The observed product grouping offers valid theoretical elucidation for available reports of CRR selectivity trends for Cu-based alloy catalysts. It also provides further mechanistic insight into the CRR product selectivity for an extensive range of Cu-based bimetallic materials. The selectivity trend based on the intrinsic catalyst properties provides a rational design strategy for highly selective CRR electrocatalysts.

C-H activation of light alkanes on MXenes predicted by hydrogen affinity.

C-H activation of light alkanes is one of the most important reactions for a plethora of applications but requires catalysts to operate at feasible conditions. MXenes, a new group of two-dimensional materials, have shown great promise as heterogeneous catalysts for several applications. However, the catalytic activity of MXenes depends on the type and distribution of termination groups. Theoretically, it is desired to search for a relation between the catalytic activity and the termination configuration by employing a simple descriptor in order to avoid tedious activation energy calculations. Here, we show that MXenes are promising for splitting C-H bonds of light alkanes. Furthermore, we present how a quantitative descriptor - the hydrogen affinity - can be used to characterize the termination configuration of Ti2CTz (T = O, OH) MXenes, as well as the catalytic activity towards dehydrogenation reactions, using propane as model system. First-principles calculations reveal that the hydrogen affinity can be considered as an intrinsic property of O and OH terminated Ti2C MXenes, in which the mean hydrogen affinity for the terminated Ti2C MXenes is linearly correlated to the statistical average of their OH fraction. In addition, the C-H activation energies exhibit a strong scaling relationship to the hydrogen affinity. This quantity can therefore yield quick predictions of catalytic activity of terminated Ti2C MXenes towards C-H activations, and even predict their chemical selectivity toward scissoring different C-H bonds. We believe that the hydrogen affinity will accelerate the discovery of further applications of the broad family of MXenes in heterogeneous catalysis.

Effects of various functionalisation layers on ammonia gas sensing using AlGaN/GaN high electron mobility transistors

Low concentration (<50 ppm) sensing of NH3 over a wide temperature range of 30-275°C using AlGaN/GaN high electron mobility transistor (HEMT) based gas sensors with Pt, Pd and Ag functionalisation layers has been studied. Pt/AlGaN/GaN HEMT exhibited the sensitivity of 0.51% and 0.83% for 50 ppm NH3 at 30°C and 275°C, respectively. The current decreased when the sensor was exposed to different concentrations of NH3 at 30°C while it increased above 200°C. The sensing mechanisms corresponding to the temperature dependent response of NH3 have been explained. The sensor with the Pd functionalisation layer exhibited higher sensitivity at 275°C as compared to Pt and Ag sensors due to higher hydrogen affinity of Pd. The sensor with the Ag functionalisation layer demonstrated faster response and recovery times (<3 min) as compared to other sensors using Pt and Pd.

Structure and Composition Modification of Ultrasmall Palladium Nanoparticles upon Hydrogenation from First Principles

Palladium-based materials exhibit a high affinity for hydrogen, making them ideal candidates for hydrogen storage or hydrogen sensing applications owing to the existence of a hydride phase. From the theoretical point of view, a bulk or slab is often used to model such systems but lacks the flexibility to adapt its lattice parameter to an increased load of hydrogen. Using density functional theory, we follow the change in the structure and composition of small palladium nanoparticles upon hydrogenation. We show that a cuboctahedral nanoparticle with hydrogen in the core and on the surface is a relevant model under experimental conditions. CO and NO are then adsorbed on this nanoparticle to highlight the crucial importance of hydrogen loading when considering palladium nanomaterials for-wider sensing applications.

(Invited) Investigating the Hydrogenase Mechanism By Protein Film Electrochemistry

With turnover frequencies up to 106 s-1, hydrogenases are the fastest molecular hydrogen catalyst. They are classified according to the metal content of their active, distinguishing the NiFe and FeFe as the two major classes. The two classes display an outstanding organometallic cofactor, with cyanido- and carbonyl- groups coordinating the metals. Their mechanism includes the stabilization of a metal-hydride state as the most reduced intermediate of the catalytic cycle. Hydrogenase (Hase) have driven mechanistic studies for decades, including extensive works by protein film electrochemistry targetting aspects as diverse as intramolecular diffusion of gases, long distance electron transfer, or susceptibility to inhibitors. This communication will focus on two other questions, distinguishing steady state functioning and in-activation processes (see figure). The catlaytic bias is the ability of an enzyme to turnover faster one direction than another. Amongst hydrogenases, the catalytic bias spans full range, from extremely biased to essentially neutral enzymes. Several works have brought clues to understand the molecular basis of the catalytic bias for both Hase classes, pointing out to the intramolecular electron transfer chain as the main effector. Our recent results on the study of the hydrogen dependent CO2 reductase reveal the role of the affinity for hydrogen in the bias. The susceptibility to overpotential corresponds to in-activation processes triggered by redox conditions that are far from equilibrium, and plausibly encountered in the life cycle of bioelectrochemical devices. One particular phenomenon is the reductive inactivation of FeFe-Hases, undergoing both in solution and on the electrode, that remains largely to understand. Our works on the enzyme HydA1 from microalgae suggest that this inactivation can be reversed under maintained reductive conditions, unlike discussed in the literature. Figure 1

Reactivity descriptors of diverse copper-oxo species on ZSM-5 zeolite towards methane activation

Abstract Copper-containing ZSM-5 zeolite can facilitate the direct oxidation of methane to form methanol under mild condition with high selectivity. However, due to the limitation of the experimental characterization, the identification of the catalytic sites in Cu-exchanged zeolites still remains unclear. Herein, the reactivity of different [CuxOy] (x = 1, 2, 3; y = 1, 2, 3)- and [CuOH]+-ZSM-5 active sites towards methane partial oxidation process has been elucidated by using density functional theory (DFT) calculations. It is indicated that: i) C–H activation over five different active sites obeys the radical mechanism, and the reactivity follows the sequence: copper-oxyl [CuO]+ (S = 1) > tricopper [Cu3O3]2+ (S = 3/2) > bis-(μ-oxo)-dicopper [Cu2O2]2+ (S = 1) > mono-(μ-oxo)-dicopper [CuOCu]2+ (S = 1) > mono-copper [CuOH]+ (S = 1/2); ii) The rate constants for C–H bond cleavage of methane at the reaction temperature are faster at the higher temperature; iii) The descriptors for C–H activation of methane on [CuxOy] (x = 1˜3, y = 1˜3)- and [CuOH]+-ZSM-5 have been established, for which the active site with more spin density and less negative charge will induce the lower activation barriers for C–H activation of methane. Meanwhile, the stronger hydrogen affinity of the active site is, the lower the activation barriers of methane C–H activation will be. Taken the stability and activation barrier together, the catalytic performance of copper-oxyl site has been enhanced by the presence of water.

Atomistic Models for Highly‐Dispersed PtSn/γ‐Al2O3 Catalysts: Ductility and Dilution Affect the Affinity for Hydrogen

AbstractSupported platinum‐based sub‐nanometric particles play a central role in many catalytic applications. In particular, platinum‐tin active phases supported on γ‐Al2O3 are largely employed for dehydrogenation of alkanes and catalytic reforming of naphtha cuts, although geometric and electronic effects of the active phase in the presence of hydrogen still remains highly debated. Thanks to periodic density functional theory (DFT), we propose structural models of such systems containing thirteen metal atoms (PtxSn13‐x with 0≤x≤13) deposited on the (100) γ‐Al2O3 surface. We thus unravel the intricate effects of the composition (Pt/Sn ratio), of the support (γ‐Al2O3) and the hydrogen coverage on the stability of platinum‐tin sub‐nanometric clusters, in the case where tin is reduced (Sn0). A detailed investigation of the interaction of the supported Pt10Sn3 cluster with hydrogen by velocity‐scaled molecular dynamics provides a mapping of the hydrogen coverage as a function of the operating conditions (T, P(H2)). Our study highlights significant differences between Pt13 and PtxSn13‐x clusters in terms of ductility and dilution (also called ensemble) effects which may be at the origin of the different reactivities usually reported for Pt and PtSn supported catalysts.

Exploiting the Hydrogen Bond Donor/Acceptor Properties of PN-Heterocycles: Selective Anion Receptors for Hydrogen Sulfate.

We describe two novel hybrid receptors combining a phosphorus-/nitrogen-containing (PN) phosphonamidate heterocycle with urea recognition units in an arylethynyl backbone. Structural, spectroscopic and computational studies reveal that the origin of superior binding for hydrogen sulfate (HSO4- ) anion is correlated with the formation of strong hetero-complementary hydrogen bonds with the phosphonamidate motif. We further demonstrate that the hybrid host system is capable of capturing/transporting the HSO4- anion from an aqueous, biphasic system.

NiTiFe and NiTiZn LDHs with affinity for hydrogen – Role of the surface basicity

NiTi-based layered double hydroxides modified by isomorphic substitution of nickel by iron or zinc showed appreciable affinity towards hydrogen under ambient conditions. This affinity was correlated to their surface basicity, expressed in terms of CO2 retention capacity (CRC). The latter was assessed through thermal programmed desorption (TPD), and was found to vary almost linearly with the partial negative charge of lattice oxygen (PNCLO). Incorporation of iron or zinc induced a basicity enhancement, which was markedly affected by excessive amount of incorporated iron. This was explained in terms of structure compaction that reduces the amount of accessible basic sites. NiTiFe and NiTiZn LDHs showed almost similar affinity towards hydrogen, which appears to be enhanced by increasing amount of incorporated metal. However, both LDH types displayed thoroughly opposite HRC-CRC correlations presumably due to the different polarizing power of Fe and Zn cations within the LDH frameworks. These findings demonstrate that optimum basicity is an essential requirement for improved affinity towards hydrogen. This basicity can be judiciously tailored through adequate chemical composition for envisaging potential use in hydrogen storage.

Metal-enhanced hydrogenation of graphene with atomic pattern

Graphene hydrogenation is an attractive approach to functionalize graphene. However, the hydrogenating treatment could only be achieved under harsh conditions due to the weak hydrogen affinity of graphene. It has also made the direct writing of electronic circuits on graphene by hydrogenation very difficult. Here we propose a metal-enhanced approach to hydrogenate graphene with atomic pattern that enables to produce hydrogenation pattern on demand. First-principles calculations reveal that certain metals (Cu, Ag, Al) attached to graphene can significantly lower the energy barrier for hydrogen binding process, while the hydrogen binding energy itself is much enhanced. Such metal-promoted hydrogenation is spatially localized, which paves the way to precisely write pre-designed hydrogenation patterns on graphene with well-controlled metallic clusters and tips.

Density functional theory evaluation of cation-doped bismuth molybdenum oxide photocatalysts for nitrogen fixation

This study investigates the photocatalytic nitrogen fixation on a cation-doped surface (BixMy)2MoO6 where (M = Fe, La, Yb) in both the orthorhombic and monoclinic configurations using a density functional theory (DFT) approach with experimentally validated model inputs. The proceeding discussion focuses on the Heyrovsky-type reactions for both the associative and dissociative reaction pathway related to nitrogen reduction. Key fundamental insight in the reduction mechanism is discussed that relates the material properties of the substitutional ions to the nitrogen and hydrogen affinities. Physical insight is gathered through interpretation of bound electronic states at the surface. Compositional phases of higher Fe and Yb concentrations resulted in decreased MoO binding and increased affinity between Mo and the N and H species on the surface. The modulation of the MoO binding is induced by strain as Yb and Fe are implemented, this, in turn, shifts energy levels and modulates the band gap energy by approximately 0.2 eV. This modification of MoO bond as substitution occurs is a result of the orbital hybridization of MO (M = Fe, Yb) that causes a strong orbital interaction that shifts states up toward the Fermi. The optimal composition was predicted to be an orthorhombic configuration of (Bi0.75Fe0.25)2MoO6 with a predicted maximum thermodynamic energy barrier of 1.4 eV. This composition demonstrates effective nitrogen and hydrogen affinity that follows the associative or biological nitrogen fixation pathway.

Synthesis and Application of Thiosquaramides and Their Derivatives: A Review

Thiosquaramides are the thio analogues of squaramides that are widely applicable in the fields of asymmetric catalysis, pharmaceutical research, and chemical biology. Having four-membered ring system derived from squaric acid, thiosquaramides are feasible hydrogen bond donors and acceptors. A high affinity for hydrogen bonding is driven through a concomitant increase in aromaticity of the ring. In this review the structural properties, acidity, and lipophilicity of thiosquaramides and squaramides are compared. Different synthetic procedures starting from squarates, half-squaramides or squaramides are shown, and the main derivatization methods are outlined. Finally, the yet only few applications of this interesting family are reviewed. Considering their hydrogen bonding and aromatic switching, in combination with structural rigidity, they bear the possibility of becoming robust and tunable bifunctional organocatalysts for a range of synthetically useful transformations in the future.

Property–Activity Relations for Methane Activation by Dual‐Metal Cu–Oxo Trimers in ZSM‐5 Zeolite

AbstractThe identification of correlations between the experimentally or computationally measurable parameters of a catalytic system and its reactivity is one of the key steps toward the realization of a catalysis by design strategy. Here, periodic density functional theory calculations to establish such correlations for perspective catalysts for selective methane oxidation based on dual‐metal cation‐exchanged zeolites are employed. A representative trimeric metal–oxo active site is considered as a model reactive center. Computations reveal that the activation barrier for the homolytic CH bond cleavage in methane correlates well with the thermodynamic stability of the resulting CH3·⋯HO intermediate. The stability of the HO part approximated by the hydrogen affinity of the active site correlates with the activity trend, but deviations are observed due to inability of this descriptor to account for the stabilization of the CH3∙ moiety. Such fundamental characteristics as the atomic spin density and the basicity of reactive oxygen sites cannot be directly correlated with the catalyst reactivity, implying the complexity in property–reactivity relationship for methane activation. Calculations suggest that the initial screening of the potent zeolite‐based catalyst for methane activation can be established based on the analysis of both the thermodynamics and perturbation of base molecular adsorption probes such as pyrrole.

Portable label-free inverse opal photonic hydrogel particles serve as facile pesticides colorimetric monitoring

Long-range ordered macroporous inverse opal hydrogel particles with methanephosphonic acid (MPA) imprinted sites have been designed and prepared through a sacrificial template method for the in situ molecule recognition process. The hydrogel particles showed well specific selectivity to MPA and obvious reflection peak shift which can be recognized by naked eyes. Because of the hydrogen bond interaction between MPA and polymethacrylic acid (PMAA), the macropores of the hydrogel matrix gradually swell during the recognition process. The prepared MPA imprinted crystal colloidal arrays hydrogel particles can be used as the colorimetric sensors for the detection of the existence of the MPA, achieving a 120 nm large red shift of its reflection peak leading to the interaction between the MPA and the imprinting sites. In addition, the size of the hydrogel particles is systematically studied, which inspires us to obtain the appropriate range of MPA sensor by controlling the size of the hydrogel particles to fit the different standard. In combination with food safety, hydrogen affinity molecularly imprinted polymer (MIP) inverse opal suspension arrays sensors here are of potential application for the building of the pesticide detection system.

Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale

Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.