Surface Hydrogen Species Research Articles

Promoting Water Dissociation and Weakening Active Hydrogen Adsorption to Boost the Hydrogen Transfer Reaction over a Cu−Ag Superlattice Electrocatalyst

AbstractThe prerequisite for electrocatalytic hydrogenation reactions (EHRs) is H2O splitting to form surface hydrogen species (*H), which occupy catalytic sites and lead to mismatched coverage of *H and reactants, resulting in unsatisfactory activity and selectivity. Thus, modulating the splitting pathway of H2O is significant for optimizing the EHR process. Herein, a Cu−Ag alloy with a superlattice structure of staggered‐ordered Cu and Ag is theoretically predicted and experimentally proven to undergo a pathway for H2O splitting called the hydrogen transfer reaction (HTR) in the water layer, which involves the formation of *H, the capture of *H by a water cluster to form H*(H2O)x and subsequent hydrogenation reactions by H*(H2O)x. Taking acetylene hydrogenation as a model case, the as‐proposed HTR pathway could lead to a relaxation hydrogenation process to modulate the matching degree of C2H2 and *H, thus enabling a 91.2 % C2H4 Faradaic efficiency at a partial current density of 0.38 A cm−2, greatly outperforming its counterpart without a superlattice structure.

Promoting Water Dissociation and Weakening Active Hydrogen Adsorption to Boost the Hydrogen Transfer Reaction over a Cu-Ag Superlattice Electrocatalyst.

The prerequisite for electrocatalytic hydrogenation reactions (EHRs) is H2O splitting to form surface hydrogen species (*H), which occupy catalytic sites and lead to mismatched coverage of *H and reactants, resulting in unsatisfactory activity and selectivity. Thus, modulating the splitting pathway of H2O is significant for optimizing the EHR process. Herein, a Cu-Ag alloy with a superlattice structure of staggered-ordered Cu and Ag is theoretically predicted and experimentally proven to undergo a pathway for H2O splitting called the hydrogen transfer reaction (HTR) in the water layer, which involves the formation of *H, the capture of *H by a water cluster to form H*(H2O)x and subsequent hydrogenation reactions by H*(H2O)x. Taking acetylene hydrogenation as a model case, the as-proposed HTR pathway could lead to a relaxation hydrogenation process to modulate the matching degree of C2H2 and *H, thus enabling a 91.2% C2H4 Faradaic efficiency at a partial current density of 0.38 A cm- 2, greatly outperforming its counterpart without a superlattice structure.

Unlocking the Potential of Cu/Ti3C2Tx MXene Catalyst in Plasma Catalytic CO2 Hydrogenation

Plasma catalytic CO2 hydrogenation has emerged as a promising approach for converting CO2 into valuable chemicals such as CO, offering a potential solution to mitigate greenhouse gas emissions. In this study, we investigate the synergistic effect between plasma and a Cu/Ti3C2Tx MXene catalyst for CO2 hydrogenation via the reverse water gas shift (RWGS) reaction in a dielectric barrier discharge (DBD) reactor. Our findings reveal a significant enhancement in CO2 conversion (33.1%) and CO yield (31.9%) when using the Cu/Ti3C2Tx catalyst compared to plasma alone or with a Ti3C2Tx foam. Plasma treatment creates unsaturated Ti sites on the Ti3C2Tx surface, dramatically improving CO2 adsorption and activation, while Cu nanoparticles significantly enhances the activation and dissociation of H2 molecules on the catalyst surface. These surface adsorbed hydrogen species readily hydrogenate adsorbed CO2 molecules through the Langmuir-Hinshelwood mechanism. Furthermore, these surface hydrogen species can also participate in the conversion of gas-phase CO2 molecules through the Eley-Rideal pathway. This work sheds light on the synergistic plasma catalytic mechanism and provide insights for optimizing CO2 hydrogenation processes using MXene catalysts.

Synergistic Mechanism of Platinum-GaOx Catalysts for Propane Dehydrogenation.

The synergy between metals and metal oxides can effectively improve the heterogeneous catalytic process. This paper describes the intrinsic effect of Pt modification over GaOx (Pt-GaOx ) on propane dehydrogenation. The presence of Pt promotes H2 dissociation and surface coverage of hydrogen species, which is beneficial for the activation of C-H in propane. With excessive Pt, Gaδ+ can be further reduced to form Pt-Ga alloy with less surface hydrogen species. Consequently, the relative propylene formation rate between Pt-GaOx and the summed contribution of individual Pt and GaOx increases linearly with the content of hydrogen species. Optimally, the relative propylene formation rate of Pt-GaOx with 0.03 wt % Pt exceeds 25 % of the summed contribution of individual components.

Synergistic Mechanism of Platinum‐GaOx Catalysts for Propane Dehydrogenation

AbstractThe synergy between metals and metal oxides can effectively improve the heterogeneous catalytic process. This paper describes the intrinsic effect of Pt modification over GaOx (Pt−GaOx) on propane dehydrogenation. The presence of Pt promotes H2 dissociation and surface coverage of hydrogen species, which is beneficial for the activation of C−H in propane. With excessive Pt, Gaδ+ can be further reduced to form Pt−Ga alloy with less surface hydrogen species. Consequently, the relative propylene formation rate between Pt−GaOx and the summed contribution of individual Pt and GaOx increases linearly with the content of hydrogen species. Optimally, the relative propylene formation rate of Pt−GaOx with 0.03 wt % Pt exceeds 25 % of the summed contribution of individual components.

Modulating the Formation and Evolution of Surface Hydrogen Species on ZnO through Cr Addition

Modulating the Formation and Evolution of Surface Hydrogen Species on ZnO through Cr Addition

A simulated-TPD study of H2 desorption on metal surfaces

The desorption process of small molecules on the surface/interface is of great importance in understanding novel interaction mechanisms between the adsorbate and the active sites of metal catalysts. Temperature programmed desorption(TPD) is a common technology for measuring molecule desorption characters at different heating rates. We herein present a simulated-TPD study of H2 desorption on different metal nanoparticles which are constructed with the (100), (110) and (111) crystal planes based on density functional theory (DFT) calculations. Via a recombinative desorption model, the simulated-TPD curves of H2 molecules desorption on different (100), (110) and (111) surfaces of Ni, Pd, Pt and Cu are consistent with the measured experiments. We present the further simulated-TPD results of H2 molecules desorption on Ni, Pd and Pt nanoparticles by using the weighting approach in a desorption temperature region. These important results will contribute to an in-depth understanding of the interactions between surface hydrogen species and metal catalysts in heterogeneous catalysis.

A Quasi-Metal–Organic Framework Based on Cobalt for Improved Catalytic Conversion of Aquatic Pollutant 4-Nitrophenol

To generate purposely defects that can increase the catalytic activity, cobalt-based metal–organic framework (MOF) TMU-10 has been subjected to thermal treatment under an air atmosphere at temperatures between 100 and 700 °C. This process causes partial ligand removal, generating structural defects and additional hierarchical porosity in a convenient way. The resulting materials, denoted as quasi-MOFs, were subsequently employed as catalysts for the room-temperature borohydride reduction of 4-nitrophenol (4-NP). The quasi TMU-10 framework obtained at 300 °C (QT-300) exhibits excellent catalytic performance with an apparent rate constant, activity factor, and half-life time of 2.8 × 10–2 s– 1, 282 s–1 g–1, and 24.8 s, respectively, much better values than those of parent TMU-10. Coexistence of micro and mesopores, coordinatively unsaturated cobalt nodes, tetrahedral Co(II) ions, and Co(III) in QT-300 are responsible for this enhanced activity. Kinetic studies in the range of 25–40 °C varying the 4-NP and BH4– concentrations agree with the Langmuir–Hinshelwood model in which both reactants are adsorbed on the catalyst surface. Reduction of 4-NP by the surface-hydrogen species is the rate-determining step.

Visible-light-driven semihydrogenation of alkynes via proton reduction over carbon nitride supported nickel

Semihydrogenation of alkynes represents one of the most viable route to produce functional alkene products. Herein we describe the visible-light-driven alcohol or water donating semihydrogenation catalyzed by nickel supported on carbon nitride scaffold (Ni/C3N4) under ambient condition, exhibiting excellent alkene selectivity and broad substrate scope. The catalyst design takes advantage of C3N4 to harvest visible irradiation and to tune the interaction of Ni with hydrogenation intermediates, which is essential for the excellent selectivity toward alkene products. The hydrogen atom incorporated in alkene products originates from hydroxyl group of methanol or water, via a Ni catalyzed proton reduction by photogenerated electrons to give the active surface hydrogen species (H*). Such hydrogenation pathway not only avoids harsh reaction condition but also enables facile synthesis of valuable deuterated alkenes using deuterated alcohols or D2O, promising enormous application potential for well-designed catalyst architectures in the light-driven selective transfer hydrogenation (deuteration) of alkynes and other organic substrates.

A new La‐Doped CuBi2O4 Catalysts for the Reduction of Nitroaromatic Compounds and Toxic Organic Dyes

AbstractThe new La‐doped CuBi2O4 catalysts were prepared by one‐pot hydrothermal method. The prepared catalysts were investigated by high‐resolution transmission electron microscopy (HRTEM), powder X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), Hydrogen temperature‐programmed desorption (H2‐TPD), and Electrochemical impedance spectroscopy (EIS). The synthetic catalysts have been investigated for the reduction of 4‐nitrophenol and toxic organic dyes. La‐doping improve the surface electron transport and H2‐adsorption ability. The La−CuBi2O4(C) catalyst has higher electron transfer efficiency and stronger H absorption ability. It was proposed that the doped La3+ and surface hydrogen species play key roles in reduction reaction. The electron transfer can be achieved by La3+ capture electron to be reduced La2+, then La2+ return to original state. Meanwhile, the improvement of H absorption ability due to La‐doping is also benefit to reduce the 4‐NP. This study is helpful in acquiring an in‐depth insight into the catalytic role of doping.

Hot-Atom Mechanism in Syngas Methanation on Precovered Pd(100) Surfaces.

The excess energy of subsurface hydrogen species may facilitate overcoming reaction barriers and remarkably alters the reaction pathways. We present an in-depth study on the different reactivity of surface and subsurface hydrogen species in syngas methanation on the O/C-covered Pd(100) by using density functional theory calculations and microkinetic simulations. It is shown that the apparent energy barriers to form H2O and CH4 are reduced by 0.87 and 0.61 eV for the case in which the hot subsurface hydrogen species are involved in the whole hydrogenation process. The activity of O-covered Pd(100) is better than that of the C-covered surface, and the reactivity of subsurface hydrogen species is much higher than that of surface hydrogen species under ambient conditions. Increasing CO partial pressure strongly enhances the reactivity of subsurface hydrogen species in syngas methanation on the O-covered Pd(100). These important results are helpful for understanding the hot-atom mechanism through subsurface heterogeneous catalysis.

Effect of pore-size distribution on Ru/ZSM-5 catalyst for enhanced N2 activation to ammonia via dissociative mechanism

In the present study, a series of Ru/ZSM-5 catalysts with different pore-size distributions were prepared and investigated for NH3 synthesis. Our studies indicate that Ru/ZSM-5-Mic with micropore structure exhibits superior NH3 synthesis rate, which is much higher than those of Ru/ZSM-5-Mac (with macroporous structure) and Ru/ZSM-5-Mes (with mesoporous structure) catalysts. A series of TPD experiments demonstrate that pore-size distributions play an important role in N2 adsorption and activation over Ru/ZSM-5. Moreover, the addition of La significantly promotes the NH3 synthesis performance over Ru/ZSM-5-Mic. Additionally, in situ DRIFTS studies indicate that the main intermediate species over Ru/ZSM-5-Mic are –NH2, and most of the surface hydrogen species desorb following the H2O-formation pathway.

Discriminating the Role of Surface Hydride and Hydroxyl for Acetylene Semihydrogenation over Ceria through In Situ Neutron and Infrared Spectroscopy

Ceria has been used as a hydrogenation catalyst especially in selective alkyne hydrogenation, but the reaction mechanism regarding the role of different surface hydrogen species remains unclear. In...

Enhanced diffusion and permeation of hydrogen species on the partially carbon covered iron surfaces

Understanding hydrogen diffusion on and permeation into the species-covered metal surfaces is of great importance in the exploration of novel hydrogenation processes. By combining of density functional theory, ab initio thermodynamics and microkinetic simulations, we present the profound coverage effects on hydrogen diffusion on and permeation into the H/O/C-covered Fe(100) surfaces at different atomic coverages. Through comparing the calculated barriers of mostly 72 possible configurations, we found that the effect of hydrogen species adsorption is very weak, and the carbon species adsorption has very favorable effect in contrast to the oxygen species adsorption. Importantly, the strongly enhanced effects are respectively found within three partially carbon-covered configurations, i.e., H1C1-covered, O1C1-covered and C1C1-covered Fe(100) surfaces. Ab initio thermodynamics calculations provide the further evidences on the stabilities of these partially carbon-covered surfaces under different hydrogen pressures and temperatures. The phase of 0.5 ML surface hydrogen and 0.25 ML subsurface hydrogen is stably formed within the conditions of low surface temperatures and high hydrogen pressures. In addition, our microkinetic simulations accurately give the atomic coverages of surface and subsurface hydrogen species on these partially carbon-covered Fe(100) surfaces. The H1C1-covered Fe(100) surface is the most favourable due to the lowest dissociation barrier of molecular hydrogen.

Enhanced photocatalytic alkane production from fatty acid decarboxylation via inhibition of radical oligomerization

The decarboxylation of bio-derived fatty acids provides a sustainable pathway for the production of alkane products under mild conditions; however, products are generally obtained in low selectivity due to the uncontrollable reactivity of radical intermediates. Here we demonstrate that photogenerated radicals can be rapidly terminated by surface hydrogen species during photocatalytic decarboxylation of fatty acids on a hydrogen-rich surface that is constructed by the interactions between H2 and Pt/TiO2 catalyst, thereby greatly inhibiting oligomerization; Cn–1 alkanes can therefore be obtained from bio-derived C12–C18 fatty acids in high yields (≥90%) under mild conditions (30 °C, H2 pressure ≤0.2 MPa) and 365 nm light-emitting dode irradiation. Industrial low-value fatty acid mixtures (namely, soybean and tall oil fatty acids) can be transformed into alkane products in high yields (up to 95%). Our research introduces an efficient biomass-upgrading approach that is enabled by subtle control of the radical intermediate conversion on a heterogeneous surface. The photocatalytic decarboxylation of fatty acids affords alkanes under mild conditions, albeit with limited selectivity due to radical-mediated side reactions. Now, a hydrogenated Pt/TiO2 catalyst is introduced for the selective conversion of C12–C18 fatty acids into Cn–1 alkanes in quantitative yields.

WOx domain size, acid properties and mechanistic aspects of glycerol hydrogenolysis over Pt/WOx/ZrO2

Supported WOx catalysts are widely investigated in glycerol hydrogenolysis for their high selectivity to 1,3-propanediol (1,3-PDO). The high performance is often related to surface Brönsted acid site. However, the intrinsic structure of Brönsted acid is unclear and its controllable preparation has not been investigated in detail. In addition, many reaction mechanisms have been proposed up to now, but with few direct evidences in spectroscopy studies. In this work, Pt/WOx/ZrO2 catalysts containing various amounts of WOx were studied in glycerol hydrogenolysis. The reaction is found to be structurally sensitive to WOx domain size, with medium polymerized WOx shown to benefit the formation of 1,3-PDO. By doping a suitable amount of Mn into monolayer covered WOx/ZrO2, large amounts of WOx with medium polymerization degree were created. Thus, the turnover frequency of 1,3-PDO (TOF1,3-PDO/W) increased 2.6 times in comparison to the best result of none Mn-doping Pt/WOx/ZrO2 catalysts. Characterizations of WOx structure and acid properties indicate that super strong Brönsted acid site is created by the interaction between medium polymerized WOx and Pt particle. This type of acid is linearly correlated with the formation rate of 1,3-PDO. The adsorption state of glycerol was studied using infrared spectroscopy, and the secondary −OH is found to be strongly adsorbed to Brönsted acid site on WOx containing catalyst, while its interaction with Pt/ZrO2 is much weaker. The natural structure of the active site is proposed to be Pt-(WOx)n-H, integrating super strong Brönsted acid site and metallic Pt site together. Combined with FTIR investigations of different surface hydrogen species and in situ 2-propanol conversion, the reaction mechanism was also determined.

Bonding-Based Wafer-Level Vacuum Packaging Using Atomic Hydrogen Pre-Treated Cu Bonding Frames.

A novel surface activation technology for Cu-Cu bonding-based wafer-level vacuum packaging using hot-wire-generated atomic hydrogen treatment was developed. Vacuum sealing temperature at 300 °C was achieved by atomic hydrogen pre-treatment for Cu native oxide reduction, while 350 °C was needed by the conventional wet chemical oxide reduction procedure. A remote-type hot-wire tool was employed to minimize substrate overheating by thermal emission from the hot-wire. The maximum substrate temperature during the pre-treatment is lower than the temperature of Cu nano-grain re-crystallization, which enhances Cu atomic diffusion during the bonding process. Even after 24 h wafer storage in atmospheric conditions after atomic hydrogen irradiation, low-temperature vacuum sealing was achieved because surface hydrogen species grown by the atomic hydrogen treatment suppressed re-oxidation. Vacuum sealing yield, pressure in the sealed cavity and bonding shear strength by atomic hydrogen pre-treated Cu-Cu bonding are 90%, 5 kPa and 100 MPa, respectively, which are equivalent to conventional Cu-Cu bonding at higher temperature. Leak rate of the bonded device is less than 10−14 Pa m3 s−1 order, which is applicable for practical use. The developed technology can contribute to low-temperature hermetic packaging.

Structure sensitivity evaluation of catalytic CO2 hydrogenation to higher hydrocarbons by an iron catalyst

A series of size dependent kinetic models based on thermodynamic method are developed for indirect CO2 hydrogenation to higher hydrocarbons by an iron catalysts via the Fischer–Tropsch synthesis. The size dependent parameters in these kinetic models are evaluated over a series of iron catalysts with different average particle sizes. The results showed that the reaction between surface CO and hydrogen species is the key reaction step in producing of the higher hydrocarbons. The heats of CO2 and H2 adsorption are calculated about 45.5 and 52.1 kJ mol−1, respectively, and the apparent activation energy for CO2 hydrogenation is 146 kJ mol−1. The results showed that the best kinetic model is developed based on two hypotheses; the change in Gibbs free energy of adsorption due to the nanosize effect is negative and surface occupation is low. In addition, the calculated size-independent CO2 hydrogenation reaction for large particles is about 0.0086 [mol/(gcat h)].

Ni and Fe mixed phosphides catalysts for O-removal of a bio-oil model molecule from lignocellulosic biomass

Silica-supported catalysts based on mono and bimetallic nickel and iron phosphides with different Ni:Fe molar ratios (1:0, 3:1, 2:1 and 0:1) were synthesised and tested in the hydrodeoxygenation (HDO) reaction of phenol, a model molecule present in bio-oil derived from pyrolysis of biomass. Tests were performed in a fixed bed reactor under hydrogen pressure (1.5 and 3MPa) for different contact times. Freshly reduced catalysts were characterised by physisorption of N2 at −196°C, CO chemisorption, X-ray diffraction, transmission electronic microscopy, temperature-programmed desorption of ammonia and hydrogen, and X-ray photoelectron spectroscopy. Characterization results revealed that the presence of Ni increased their ability to activate hydrogen and the number of active sites, while the presence of Fe favored the spillover process. By increasing the amount of Ni, the surface became enriched with P, and this was associated with the presence of surface P-OH groups that provided Brönsted acid sites to activate O-containing compounds as well as surface hydrogen species that minimised deactivation by coke deposition. Ni-containing phases presented higher HDO capabilities than FeP ones, but generated some O-containing intermediates. In contrast, the FeP catalyst transformed all phenol molecules into O-free compounds (cyclohexane and mainly cyclohexene).

A new LHHW kinetic model for CO2 hydrogenation over an iron catalyst

A new Langmuir–Hinshelwood–Hougen–Watson (LHHW) type kinetic model is developed for carbon dioxide hydrogenation over a precipitated Fe/Cu/K catalyst in a continuous spinning basket reactor. A number of possible reaction sets originating from the combination of the formate mechanism for the water gas shift reaction and the enolic mechanism for the Fischer–Tropsch synthesis, with various rate determining steps, are considered. The results show that the CO2 hydrogenation process is limited by the reaction between surface CO and hydrogen species to form enolic (HCO) units. This surface reaction is the key reaction step in producing higher hydrocarbons by the Fischer–Tropsch synthesis.