Adsorbed Hydrogen Species Research Articles

Improved Nitrate-to-Ammonia Electrocatalysis through Hydrogen Poisoning Effects.

Electrochemical conversion from nitrate to ammonia is a key step in sustainable ammonia production. However, it suffers from low productive efficiency or high energy consumption due to a lack of desired electrocatalysts. Here we report nickel cobalt phosphide (NiCoP) catalysts for nitrate-to-ammonia electrocatalysis that display a record-high catalytic current density of -702±7 mA cm-2, ammonia production rate of 5415±26 mmol gcat-1 h-1 and Faraday efficiency of 99.7±0.2 % at -0.3 V vs. RHE, affording the estimated energy consumption as low as 22.7 kWh kgammonia-1. Theoretical and experimental results reveal that these catalysts benefit from hydrogen poisoning effects under low overpotentials, which leave behind catalytically inert adsorbed hydrogen species (HI*) at Co-hollow sites and thereupon enable ideally reactive HII* at secondary Co-P sites. The dimerization between HI* and HII* for H2 evolution is blocked due to the catalytic inertia of HI* thereby the HII* drives nitrate hydrogenation timely. With these catalysts, the continuous ammonia production is further shown in an electrolyser with a real energy consumption of 18.9 kWh kgammonia-1.

Efficient ammonia synthesis from the air using tandem non-thermal plasma and electrocatalysis at ambient conditions

While electrochemical N2 reduction presents a sustainable approach to NH3 synthesis, addressing the emission- and energy-intensive limitations of the Haber-Bosch process, it grapples with challenges in N2 activation and competing with pronounced hydrogen evolution reaction. Here we present a tandem air-NOx-NOx−-NH3 system that combines non-thermal plasma-enabled N2 oxidation with Ni(OH)x/Cu-catalyzed electrochemical NOx− reduction. It delivers a high NH3 yield rate of 3 mmol h−1 cm−2 and a corresponding Faradaic efficiency of 92% at −0.25 V versus reversible hydrogen electrode in batch experiments, outperforming previously reported ones. Furthermore, in a flow mode concurrently operating the non-thermal plasma and the NOx− electrolyzer, a stable NH3 yield rate of approximately 1.25 mmol h−1 cm−2 is sustained over 100 h using pure air as the intake. Mechanistic studies indicate that amorphous Ni(OH)x on Cu interacts with hydrated K+ in the double layer through noncovalent interactions and accelerates the activation of water, enriching adsorbed hydrogen species that can readily react with N-containing intermediates. In situ spectroscopies and density functional theory (DFT) results reveal that NOx− adsorption and their hydrogenation process are optimized over the Ni(OH)x/Cu surface. This work provides new insights into electricity-driven distributed NH3 production using natural air at ambient conditions.

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.

Enhanced hydrodechlorination of 4-chlorophenol through carboxymethylcellulose-modified Pd/Fe nanosuspension synthesized by one-step methods

Palladized iron (Pd/Fe) represents one of the most common modification strategies for nanoscale zero-valent iron (nZVI). Most studies prepared Pd/Fe by reducing iron salts and depositing Pd species on the surface of pre-synthesized nZVI, which can be called the two-step method. In this study, we proposed a one-step method to obtain Pd/Fe by the concurrent formation of Fe0 and Pd0 and investigated the effects of these two methods on 4-chlorophenol (4-CP) removal, with carboxymethylcellulose (CMC) coated as a surface modifier. Results indicated that the one-step method, not only streamlined the synthesis process, but also Pd/Fe-CMCone-step, synthesized by it, exhibited a higher 4-CP removal rate (97.9%) compared to the two-step method material Pd/Fe-CMCtwo-step (82.4%). Electrochemical analyses revealed that the enhanced activity of Pd/Fe-CMCone-step was attributed to its higher electron transfer efficiency and more available reactive species, active adsorbed hydrogen species (Hads*). Detection of intermediate products demonstrated that, under the influence of Pd/Fe-CMCone-step, the main route of 4-CP was through hydrodechlorination (HDC) to form phenol and H* was the main active specie, supported by EPR tests, quenching experiments and product analysis. Additionally, the effects of initial 4-CP concentration, initial pH, O2 concentration, anions such as Cl−, SO42−, HCO3−, and humic acid (HA) were also investigated. In conclusion, the results of this study suggest that Pd/Fe-CMCone-step, synthesized through the one-step method, is a convenient and efficient nZVI-modifying material suitable for the HDC of chlorinated organic compounds.

TiO2–x@C/MoO2 Schottky junction: Rational design and efficient charge separation for promoted photocatalytic performance

Limited solar light harvesting, sluggish charge transfer kinetics, and inferior affinity for adsorbed hydrogen species (H*) severely restrict the photocatalytic hydrogen generation activity of TiO2 photocatalysts. Herein, we present a novel TiO2–x@C/MoO2 Schottky junction prepared via a simple one-step in situ phase-transition-regulation strategy. Crucially, the abundant oxygen vacancies in TiO2–x@C/MoO2 narrow the bandgap and introduce defects to improve the photoresponse. The strongly bonded carbon layer not only serves as a fast charge-transport channel to improve the interlayer charge transfer efficiency but also protects oxygen vacancies from oxidation. Moreover, the Schottky barrier effectively impairs the recombination of electrons and holes and promotes the utilization of photogenerated electrons. Furthermore, the MoO2 cocatalyst optimizes the Gibbs free energy for H2 evolution. As a result of the favorable synergy, the resulting TiO2–x@C/MoO2 presents a significantly enhanced photocatalytic H2 production rate of 506 μmol g−1 h−1 compared to those of TiO2–x and TiO2–x@C (125.5- and 15.8-times larger, respectively). Moreover, outstanding stability over 27 h was achieved because of the protection provided by the surface carbon layer. This ingenious design and facile synthetic strategy offer exciting avenues for the design of strongly coupled Schottky junction photocatalysts for efficient solar-to-chemical conversions.

Impacts of Surface Modification of Pt-Sensing Electrodes with Au on Hydrogen-Sensing Properties and Mechanism of Diode-Type Gas Sensors Based on Anodized Titania

The impacts of the surface modification of Pt-sensing electrodes with Au on the H2-sensing properties and mechanism of diode-type gas sensors based on anodized titania (TiO2) were discussed in this study. The sensors using Pt electrodes modified with and without Au (Au(n)/Pt/TiO2 (n: sputtering time (s)) and Pt/TiO2 sensors, respectively) were fabricated by employing an anodized TiO2 film on a Ti plate. The surface modification of the Pt electrodes with Au(20) having a thickness of ca. 10 nm was the most drastically enhanced H2 response of the Pt/TiO2 sensor especially in air. The oxidation activity of H2 over the Pt and typical Au(n)/Pt electrodes was investigated to clarify the H2-sensing mechanism, together with analyses of crystal structure and chemical state of these electrodes by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The oxidation activity of H2 over the Pt electrode decreased with an increase in the amount of the surface-modified Au. Besides, the addition of moisture into the gaseous atmosphere reduced the oxidation activity of H2 in air. The alloying of Pt with Au was confirmed after annealing of the Au(n)/Pt electrodes at 600 °C in air, and the number of oxygen adsorbates on the surface increased with an increase in the amount of the surface-modified Au. On the basis of these results, we can suggest that the large H2 response of the Au(n)/Pt/TiO2 sensors arises from both a decrease in the number of highly active oxygen adsorbates and an increase in dissociatively adsorbed hydrogen species on the surface. The water molecules and/or hydroxy groups adsorbed on the surface by the addition of moisture into the gaseous atmosphere seem to have a crucial role in increasing the dissociatively adsorbed hydrogen species on the surface, to enhance the H2 response.

HER activity of nickel molybdenum sulfide electrocatalyst as function of the ionomer in the ink formulation

Transition metal sulfides (NiSx, MoSx and Ni0·5Mo0.5Sx) are synthesized and have their HER performance evaluated in three-electrode cell setup by applying LSV analyzes in 1 M KOH. XRD, Raman, TEM and XPS characterizations are applied aiming to correlate physicochemical properties/electrocatalytic activity. It is evidenced a synergistic effect in which nickel oxide, present in the surface of Ni0·5Mo0.5Sx, accelerates the water dissociation, while molybdenum sulfide catalyzes the reduction of adsorbed hydrogen species. When physically mixed with Carbon Vulcan, Ni0·5Mo0.5Sx only needs 60 mV higher overpotential to achieve −200 mA cm−2 than 40% Pt/C. Furthermore, Ni0·5Mo0.5Sx activity is assessed in interaction with different ionomers (Nafion® or Sustainion® XB-7). It is shown by an exploratory study that, when Nafion® is replaced by XB-7, the amount of ionomer and water to isopropanol volume ratio must be diminished in the ink formulation to, at least, maintain a comparable activity of the electrode material.

Enhancement of ammonia synthesis activity on La2O3-supported Ru catalyst by addition of ceria

The addition of Ce species in La2O3 can enhance the number of defect sites and generate the Ce1-xLaxO2-δ solid solution, thereby increasing the amount of the exposed ruthenium species and proportion of metallic ruthenium species. The presence of Ce species in Ru/La2O3 promotes the desorption of hydrogen, and hydrogen species prefers to desorb in the H2 molecule formation pathway. On the other hand, the difference in the Ce addition method strongly affects the exchange between the adsorbed hydrogen species on Ru catalyst with the gaseous hydrogen species. Owing to improvement of the proportion of oxygen vacancy, Ru0 and number of the exposure of ruthenium species, H2 species adsorbed on Ru/LaCe–C prepared by coprecipitation method preferentially desorbs in the formation of H2 pathway, and a large proportion of the adsorbed H species would exchange with the hydrogen species from the gaseous phase, which leads to improvement of ammonia synthesis rate by 75% in comparison with Ru/La2O3 catalyst.

Cover Feature: Electrochemical Proton Reduction over Nickel Foam for Z‐Stereoselective Semihydrogenation/deuteration of Functionalized Alkynes (ChemSusChem 1/2022)

The Cover Feature shows an alternative approach to the use of noble metals and flammable hydrogen gas for the semireduction of alkynes. Water electrolysis on the surface of commercially available nickel foam can deliver adsorbed hydrogen species that are chemo- and stereoselectively transferred to alkynes under ambient conditions. More information can be found in the Research Article by A. Valiente et al.

Observation of Adsorbed Hydrogen Species on Supported Metal Catalysts by Inelastic Neutron Scattering

Observation of Adsorbed Hydrogen Species on Supported Metal Catalysts by Inelastic Neutron Scattering

Highly Selective and Efficient Electrocatalytic Semihydrogenation of Diphenylacetylene to Z-Stilbene Using Pt-Pd Alloy Cathode Catalyst

The electrical energy can be provided from renewable energy source such as solar and wind, thus making the electrocatalytic hydrogenation process is expected one of important technology to reduce carbon dioxide emission in organic synthesis industry. The selective hydrogenation of alkynes to Z-alkenes is highly important for the construction of numerous valuable compounds, such as bioactive molecules, natural products, flavors, and industrial materials. Although a number of methods for the stereoselective synthesis of Z-alkenes have been developed so far, the Lindlar catalytic semihydrogeneation of alkynes is a long-standing synthetic transformation and still remains the favored system. HoweveTo develop more environmentally benign and efficient semihydrogenation systems, we have demonstrated electrocatalytic hydrogenation of diphenylacetylene (DPA) in a proton exchange membrane (PEM) reactor. 1 In our system, the oxidation of water was employed as an anodic process (Figure 1 (a)).Especially, in this work, we have investigated systematically influences of catalyst materials such as Pt, Pd, and PtPd alloy for a PEM reactor on the product selectivity for Z-stilbene and partial current density for Z-stilbene formation in the hydrogenation of diphenylacetylene. As a result, PtPd alloy with a composition of Pt1Pd99 was found to be a most suitable catalyst material in order to achieve both high partial current density for Z-stilbene formation and high Z-stilbene selectivity (Figures 1 (b) and (c)). As shown in Figure 1 (b), it was also found that the partial current density for Z-stilbene is much improved by the addition of a small amount of Pt which has the high hydrogenation activity. Moreover, the synergy of Pt electrocatalysis for the formation of adsorbed hydrogen species (Had) (H+ + e− → Had) and Pd catalysis for hydrogenation of DPA (2Had + DPA → Z-stilbene) might be required in order to achieve the high stereoselective formation of Z-stilbene. Acknowledgement This work was support by JST CREST Grant No. JPMJCR18R1, Japan. The authors gratefully acknowledge for ISHIFUKU Metal Industry corporation the generous gift for cathode catalysts. References 1) A. Fukazawa, J. Minoshima, K. Tanaka, Y. Hashimoto, Y. Kobori, Y. Sato, M. Atobe, ACS Sustain. Chem. Eng. 2019, 7, 11050–11055. Figure 1

Disposition of Iridium on Ruthenium Nanoparticle Supported on Ketjenblack: Enhancement in Electrocatalytic Activity toward the Electrohydrogenation of Toluene to Methylcyclohexane.

Electrohydrogenation of toluene (TL) to methylcyclohexane (MCH) has been recognized as a promising technology for the hydrogenation process in the organic hydride hydrogen storage system. Recently, we found that the Ketjenblack (KB)-supported Ru–Ir alloy electrocatalyst showed a high electrocatalytic activity for the electrohydrogenation of TL to MCH, and there was the synergy of Ir electrocatalysis for the formation of adsorbed hydrogen species (Had) (H+ + e– → Had) and Ru catalysis for hydrogenation of TL (6Had + TL → MCH). In this paper, the Ir-modified Ru nanoparticle supported on KB (Ir/Ru/KB) electrocatalyst was synthesized by using a modified spontaneous deposition method. The method enables to control the surface structure of Ru–Ir nanoparticles. The Ir/Ru/KB cathode showed higher electrohydrogenation activity than the Ru−Ir alloy/KB cathodes even at lower loadings of precious Ir. Characterization studies using a scanning–transmission electron microscope with an energy-dispersive X-ray spectrometer and X-ray absorption fine structure proved selective and uniform modification of Ru nanoparticle with Ir. Cyclic voltammetry measurements in H2SO4 aqueous solution indicated higher electrochemical active surface areas of Ir at the Ir/Ru/KB electrocatalysts than that at the Ru–Ir alloy/KB electrocatalysts, which is the reason for the strong synergy of Ru and Ir for the electrohydrogenation of TL to MCH.

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Spectrometric Evidence for Pt-Catalysed Decarboxylation at Anode-Relevant Potentials.

The carbon oxidation reaction (COR) is a critical issue in proton-exchange membrane fuel cells (PEMFCs), as carbon in various forms is the most used electrocatalyst support material. The COR is thermodynamically possible above the C/CO2 standard potential, but its rate becomes significantly important only at high overpotential (e. g. PEMFC cathode potential). Herein, using on-line differential electrochemical mass spectrometry, we show that oxygen-containing carbon surface groups present on high-surface aera carbon, Vulcan XC72 or reinforced graphite are oxidized at PEMFC anode-relevant potential (E=0.1 V vs. the reversible hydrogen electrode, RHE), but not at E=0.4 V vs. RHE. We rationalized our findings by considering a Pt-catalysed decarboxylation mechanism in which Pt nanoparticles provide adsorbed hydrogen species to the oxygen-containing carbon surface groups, eventually leading to evolution of carbon dioxide and carbon monoxide. These results shed fundamental light on an unexpected degradation mechanism and facilitate the understanding of the long-term stability of PEMFC anode nanocatalysts.

Hydride Doping of Chemically Modified Gold-Based Superatoms.

Atomically size-selected gold (Au) clusters protected by organic ligands or stabilized by polymers provide an ideal platform to test fundamental concepts and size-specific phenomena, such as the superatomic concept and metal-to-nonmetal transition. Recent studies revealed that these stabilized Au clusters take atomlike quantized electronic structures and can be viewed as chemically modified Au superatoms. An analogy between Au and hydrogen (H) atoms is an interesting proposal made for bare Au clusters: a Au atom at a low-coordination site of a Au cluster can be replaced with a H atom while retaining the structural motif and electronic structure. However, this proposal has not been experimentally proved in chemically modified Au superatoms while a recent theoretical study predicted the formation of [HAu25(SR)18]0 (RS = thiolate). This Account summarizes our recent studies on the interaction of hydride(s) with two types of chemically modified Au-based superatoms: (1) the Au cores of [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+ formally described as (Au9)3+ and (PdAu8)2+, respectively, and (2) Au34 cluster stabilized by poly( N-vinyl-2-pyrrolidone) (PVP). The (Au9)3+ and (PdAu8)2+ cores correspond to oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the Au34 cluster in PVP is viewed as a nearly spherical superatom having a closed electronic structure with 34 electrons and multiple uncoordinated sites on the surface. Through this study, we aimed to deepen our understanding on the role of a hydride in the formation processes of Au superatoms, the effect of adsorbed hydride(s) on the electronic structure of Au superatoms, and the activity of adsorbed hydrogen species for hydrogenation catalysis. Mass spectrometry and nuclear magnetic resonance spectroscopy demonstrated that a single hydride (H-) was selectively doped to (Au9)3+ and (PdAu8)2+ upon reactions with BH4- to form (HAu9)2+ and (HPdAu8)+, respectively. Density functional theory (DFT) calculations showed that (HAu9)2+ and (HPdAu8)+ were more spherical than the original superatoms and had a closed electronic structure with eight electrons. The hydride-doped (HAu9)2+ was selectively converted to the well-known (Au11)3+ by electrophilic addition of two Au(I) units whereas (HPdAu8)+ was converted to a new hydride-doped (HPdAu10)3+. A two-step mechanism was proposed for hydride-mediated growth of Au-based superatoms: closure of the electronic structures by adsorption of a hydride, followed by the addition of two Au(I) units. The selective formation of Au34 superatoms in PVP is also explained by assuming that hydride-doped Au clusters with 34 electrons were involved as key intermediates. The Au34 superatom exhibited the localized surface plasmon resonance (LSPR) band by reacting with BH4- due to the electron donation by multiply adsorbed hydrides. The LSPR band disappeared by exposing hydride-doped Au34 to dissolved O2, but reappeared by reaction with BH4-. Catalysis for hydrogenation of C═C bonds was generated by doping a single Pd or Rh atom to Au34. The results reported here demonstrate that the hydride doped to chemically modified Au superatoms mimics Au- in terms of electron count. The hydride-mediated growth processes observed will contribute to the development of an atomically precise, bottom-up method of synthesizing new artificial elements in a periodic table for nanoscale materials. The interaction of hydride(s) with Au superatoms will find application in hydrogenation catalysis and hydrogen sensing.

Promoted methane activation on doped ceria via occupation of Pr(4f) states

Steam reforming of the methane stream obtained from biomass gasification is a potential route for sustainable syngas (CO + H2) and hydrogen production. The rate-limiting step for the reactions involved in methane steam reforming (MSR) is thought to be CH4 activation. Praseodymium-doped ceria-based catalysts, with easily reducible Pr cations and high oxygen storage capacity, could be attractive for MSR. In this work, we performed density functional theory (DFT) calculations to examine the activity of a low Pr-doped CeO2(1 1 1) surface for methane reforming. The results show a significant lowering (0.48 eV) of the energy barrier for methane activation via homolytic H3CH bond cleavage. This can be mainly explained by the easy occupation of Pr(4f) gap states by electrons transferred from adsorbed hydrogen species. The promotion of methane activation upon Pr doping of ceria was corroborated with methane temperature-programed reduction (CH4-TPR) experiments conducted in both Ce0.95Pr0.05O2 and CeO2 samples.

CO-Sensing Properties of Diode-Type Gas Sensors Employing Anodized Titania and Noble-Metal Electrodes under Hydrogen Atmosphere

CO-sensing properties of diode-type sensors employing an anodized TiO2 film and noble-metal (M) electrodes (M/TiO2 sensor, M: Pd, Pt, and Pd-nPt, n: the amount of Pt (wt %) in the Pd-nPt electrode) were investigated at 50–250 °C in dry or wet H2. All the M/TiO2 sensors showed nonlinear I–V characteristics as a diode device in air and N2, but the I–V characteristics of the sensors were actually linear in H2 because of the negligible small height of Schottky barrier at their M/TiO2 interface. The Pd/TiO2 sensor showed no CO response in H2, but the Pt/TiO2 and Pd-nPt/TiO2 sensors responded to CO in H2. Among them, the Pd-64Pt/TiO2 sensor showed the largest CO response at 100 °C in H2. The reason why the mixing of Pd with Pt was effective in improving the CO response is probably because of a decrease in the amount of dissolved hydrogen species, an increase in the amount of dissociatively adsorbed hydrogen species, and an increase in the amount of adsorbed CO species in CO balanced with H2 by the mixing of Pt into Pd. The interference from moisture in the target gas on the CO response should be largely improved from a practical application perspective.

Nobel metal free, oxidant free, solvent free catalytic transformation of alcohol to aldehyde over ZnO-CeO2 mixed oxide catalyst

The catalytic transformation of alcohols to aldehydes under oxidant-free condition has drawn significant attention from the perspective of green chemistry. In this work, we designed noble metal free ZnO-CeO2 mixed oxide catalyst in four different ratios and tested for vapor phase benzyl alcohol dehydrogenation reaction as a model reaction under oxidant free condition. The ZnO-CeO2 mixed oxide catalyst having ratio Zn/Ce=30/70 composition showed highest selectivity towards formation of benzaldehyde. Interestingly in addition to benzaldehyde, toluene was formed in the reaction due to hydrogenolysis of benzyl alcohol. The lowest Ce3+/Ce4+ ratio was observed from the XPS analysis of Ce(3d) core electron for the catalyst having Zn/Ce=30/70 composition compared to others. CO2-TPD results proved that mostly the medium strength basic sites were responsible for hydrogen abstraction from benzyl alcohol producing benzaldehyde. H2-TPR results showed that ZnO-CeO2 catalyst (Zn/Ce=30:70) had lowest reduction temperature which is in the 673K to 573K temperature range. The amount of toluene was higher for the ZnO-CeO2 catalyst having Zn/Ce=40:60 ratio which had less basic sites and higher fraction of Ce3+ ion. The ZnO-CeO2 catalyst (Zn/Ce=30:70) did not deactivate for a reaction time up to 2h. While successive regenerations of the catalyst, toluene selectivity were increased. This may be due to the reduction of Ce4+ to Ce3+ by adsorbed hydrogen species. Also the ZnO-CeO2 catalyst (Zn/Ce=30/70) showed activity for the formation of ethanal, propanal, butanal and octanal along with corresponding alkanes from ethanol, 1-propanol, 1-butanol and 1-octanol in oxidant free condition demonstrating the in-situ generation of hydrogen. The micro kinetic analysis showed that there is no external and internal mass transfer limitation in the present case.

The structure-function relationship for alumina supported platinum during the formation of ammonia from nitrogen oxide and hydrogen in the presence of oxygen.

We study the structure-function relationship of alumina supported platinum during the formation of ammonia from nitrogen oxide and dihydrogen by employing in situ X-ray absorption and Fourier transform infrared spectroscopy. Particular focus has been directed towards the effect of oxygen on the reaction as a model system for emerging technologies for passive selective catalytic reduction of nitrogen oxides. The suppressed formation of ammonia observed as the feed becomes net-oxidizing is accompanied by a considerable increase in the oxidation state of platinum as well as the formation of surface nitrates and the loss of NH-containing surface species. In the presence of (excess) oxygen, the ammonia formation is proposed to be limited by weak interaction between nitrogen oxide and the oxidized platinum surface. This leads to a slow dissociation rate of nitrogen oxide and thus low abundance of the atomic nitrogen surface species that can react with the adsorbed hydrogen species. In this case the consumption of hydrogen through the competing water formation reaction and decomposition/oxidation of ammonia are of less importance for the net ammonia formation.

Effects of surface modification of noble-metal sensing electrodes with Au on the hydrogen-sensing properties of diode-type gas sensors employing an anodized titania film

Diode-type gas sensors employing an anodized TiO2 film and noble-metal (Pt, Pd and Pd–Pt) sensing electrodes modified with and without a small amount of Au (Au/M/TiO2 and M/TiO2, respectively, M=Pt, Pd, Pd–Pt (Pd:Pt=58:42 in weight)) have been fabricated, and the effects of surface modification of the electrodes with Au on their H2-sensing properties have been investigated at 250°C in both air and N2 under dry and wet atmospheres. H2 response of all the M/TiO2 sensors in N2 was much larger than that in air under both dry and wet atmospheres. The Pt/TiO2 sensor showed the smallest H2 response in air among them, but the surface modification of the Pt electrode with Au was quite effective in enhancing the H2 response of the Pt/TiO2 sensor in air. Therefore, the response of the Au/Pt/TiO2 sensor to 8000ppm H2 in air was comparable to that in N2, especially under wet atmosphere, and thus the response of the Au/Pt/TiO2 sensor to 8000ppm H2 was almost independent of oxygen concentration under wet atmosphere. In addition, the annealing of the sensor at 600°C did not affect the response of the Au/Pt/TiO2 sensor to 8000ppm H2. The homogeneous mixing of Au into the Pt electrode also improved the H2 response of the Pt/TiO2 sensor, but the effectiveness was much lower than that of the surface modification of the Pt electrode with Au. On the other hand, the response of the as-fabricated Au/Pd/TiO2 and Au/Pd-Pt/TiO2 sensors to 8000ppm H2 in air was much larger than that of the Au/Pd/TiO2 and Au/Pd–Pt/TiO2 sensors annealed at 600°C, especially under wet atmosphere. The large H2 response observed with the Au/Pt/TiO2 sensor and the as-fabricated Au/Pd/TiO2 and Au/Pd–Pt/TiO2 sensors can be explained by large suppression of H2 oxidation by Au components remained at the surface of their sensing electrodes and then increased amount of dissociatively adsorbed hydrogen species at the sensing electrode surface.

Invited: Hydrogen Oxidation and Evolution Reaction (HOR/HER) on Pt Electrodes in Acid vs. Alkaline Electrolytes: Mechanism, Activity and Particle Size Effects

Fuel cells and electrolyzers based on proton-exchange membranes (PEMs), operating at low pH (pH≈0), offer high power densities, but require large amounts of noble metal for the oxygen reduction reaction (ORR) in fuel cells and the oxygen evolution reaction (OER) in electrolyzers. For the hydrogen oxidation/evolution reaction (HOR/HER) only very small amounts of Pt is required due to its high activity for the HOR/HER.1 In alkaline electrolyte, non-noble metal catalysts are very active for the ORR and for the OER. Therefore, a replacement of the noble-metal intensive PEM technology by alkaline membrane technology seems promising. Unfortunately, for yet unclear reasons, the HOR/HER kinetics on Pt are much slower in alkaline than in acid electrolytes, and more active catalysts are needed to catalyze the HOR/HER in alkaline environment.2 It is therefore critical to elucidate the reasons for the poor HOR/HER activity of Pt in alkaline electrolytes, and this will be possible only by having a clear idea about the reaction mechanism and the rate determining step.Historically, the HOR/HER equilibrium has been described by invoking over potential deposited hydrogen, H-OPD, as the reactive intermediate species. 3 In contrast with adsorbed H intermediate present on noble metal surfaces at potentials positive to the reversible H+/H2 potentials in the process of the so-called underpotential-deposition (UPD), the believed reacting H-OPD species would form at potentials negative to the reversible H+/H2 potentials. So far, no clear evidence of the difference in the physical nature of H-OPD and H-UPD species has ever been reported. Recently, computational studies have addressed the HOR/HER rates by considering one of the three microscopic steps (Tafel, Volmer, Heyrovsky) as rate determining and only one state of adsorbed hydrogen species as reaction intermediate. 4 However, the link between this reacting adsorbed hydrogen intermediate and the H-UPD has never been explicitly stated.By comparing HOR/HER exchange current densities of carbon supported nanoparticles (Fig. 1) with the H-UPD charge transfer resistance (Fig. 2) which was determined by electrochemical impedance spectroscopy on polycrystalline surfaces (following refs.5 , 6) and, for the first time, on carbon supported nanoparticles, we will show that the H-UPD charge transfer rates closely match the HOR/HER exchange current densities in acid and alkaline electrolytes. Therefore, no H-OPD species has to be invoked to describe the two decades HOR/HER activity decrease from acid to base. This fundamental result gives credence to a HOR/HER rate controlled by the Volmer reaction.Moving away from the platinum system, iridium being an oxophilic surface is an interesting electrocatalyst system as it probes the hypothesis advanced by Strmcnik et al. that a more oxophilic surface might be effective in catalyzing the interaction with H2O/OH-, which they proposed to be the governing reaction in base. 7 However, our findings demonstrate explicitly that Ir does not have a higher activity than Pt in base (nor in acid). This leads to the conclusion that water dissociation is not the rate determining step in base, and, therefore, points toward identical microscopic HOR/HER reaction steps in acid and base.In conclusion, these novel insights confirm the H-binding energy as a relevant HOR/HER activity descriptor.8 Future studies will have now to address the pH effect on this binding energy and the synthesis of electrocatalysts with tuned H-binding energies.1. K. C. Neyerlin, W. Gu, J. Jorne and H. A. Gasteiger, J. Electrochem. Soc., 2007, 154, B631-B635.2. W. Sheng, H. A. Gasteiger and Y. Shao-Horn, J. Electrochem. Soc., 2010, 157, B1529-B1536.3. B. Conway and B. Tilak, Electrochim. Acta, 2002, 47, 3571-3594.4. E. Skúlason, V. Tripkovic, M. E. Björketun, S. Gudmundsdottir, G. Karlberg, J. Rossmeisl, T. Bligaard, H. Jónsson and J. K. Nørskov, J. Phys. Chem. C, 2010, 114, 18182-18197.5. K. Schouten, M. van der Niet and M. Koper, Phys. Chem. Chem. Phys., 2010, 12, 15217-15224.6. B. Łosiewicz, R. Jurczakowski and A. Lasia, Electrochim. Acta, 2012, 80, 292-301.7. D. Strmcnik, M. Uchimura, C. Wang, R. Subbaraman, N. Danilovic, D. Van der Vilet, A. P. Paulikas, V. Stamenkovic and N. Markovic, Nat. Chem., 2013, 5, 300-306.8. W. Sheng, M. Myint, J. G. Chen and Y. Yan, Energy Environ. Sci., 2013, 6, 1509-1512.