RuO2 Cocatalyst Research Articles

Hydrogen Generation over RuO2 Nanoparticle-Decorated LaNaTaO3 Perovskite Photocatalysts under UV Exposure.

The efficacy of LaNaTaO3 perovskites decoration RuO2 at diverse contents for the photocatalytic H2 generation has been explored in this study. The photocatalytic performance of RuO2 co-catalyst onto mesoporous LaNaTaO3 was evaluated for H2 under UV illumination. 3%RuO2/LaNaTaO3 perovskite photocatalyst revealed the highest photocatalytic H2 generation performance, indicating that RuO2 nanoparticles could promote the photocatalytic efficiency of LaNaTaO3 perovskite significantly. The H2 evolution rate of 3%RuO2/LaNaTaO3 perovskite is 11.6 and 1.3 times greater than that of bare LaNaTaO3 perovskite employing either 10% CH3OH or pure H2O, respectively. Interestingly, the photonic efficiency of 3%RuO2/LaNaTaO3 perovskite was enhanced 10 times than LaNaTaO3 perovskite in the presence of aqueous CH3OH solutions as a hole sacrificial agent. The high separation of charge carriers is interpreted by the efficient hole capture using CH3OH, hence leading to greater H2 generation over RuO2/LaNaTaO3 perovskites. This is attributed to an adjustment position between recombination electron–hole pairs and also the reduction of potential conduction alignment as a result of RuO2 incorporation. The suggested mechanisms of RuO2/LaNaTaO3 perovskites for H2 generation employing either CH3OH or pure H2O were discussed. The photocatalytic performances of the perovskite photocatalyst were elucidated according to the PL intensity and the photocurrent response investigations.

Regulation of Ferroelectric Polarization to Achieve Efficient Charge Separation and Transfer in Particulate RuO2 /BiFeO3 for High Photocatalytic Water Oxidation Activity.

Exploiting spontaneous polarization of ferroelectric materials to achieve high charge separation efficiency is an intriguing but challenging research topic in solar energy conversion. This work shows that loading high work function RuO2 cocatalyst on BiFeO3 (BFO) nanoparticles enhances the intrinsic ferroelectric polarization by efficient screening of charges to RuO2 via RuO2 /BFO heterojunction. This leads to enhancement of the surface photovoltage of RuO2 /BFO single nanoparticles nearly 3 times, the driving force for charge separation and transfer in photocatalytic reactions. Consequently, efficient photocatalytic water oxidation is achieved with quantum efficiency as high as 5.36 % at 560nm, the highest activity reported so far for ferroelectric materials. This work demonstrates that, unlike low photocurrent density in film-based ferroelectric devices, high photocatalytic activity could be achieved by regulating the ferroelectric spontaneous polarization using appropriate cocatalyst to enhance driving force for efficient separation and transfer of photogenerated charges in particulate ferroelectric semiconductor materials.

First-principles investigation of β-Ge3N4 loaded with RuO2 cocatalyst for photocatalytic overall water splitting

Abstract β-Ge3N4 loaded with nanoparticulate RuO2 as a cocatalyst is the first successful non-oxide photocatalyst for overall water splitting. To get an insight into the working mechanism of this particular photocatalytic system, we have calculated geometrical structures of low-index surfaces for β-Ge3N4. Analysis of surface energies indicates that the most preferentially exposed surface is (100). The band gap of surface is narrower than that of bulk due to the dangling bonds. Dissociative water adsorption on (100) surface is thermodynamically favorable. The adsorption behavior of (RuO2)n (n = 2, 3, and 4) clusters on the β-Ge3N4(100) surface has been explored. It is found that all the clusters bind to (100) surface strongly by forming interfacial bonds so that the adsorptions are exothermic processes. The calculation on density of states for β-Ge3N4(100) surface loaded with (RuO2)n clusters reveals that photo-induced electrons tend to accumulate on (RuO2)n clusters and holes tend to stay in β-Ge3N4. Based on the theoretical indication of Type-II staggered band alignment, we have proposed that in photocatalytic water splitting reaction, oxygen evolution reaction is inclined to occur on the surface of β-Ge3N4 while hydrogen evolution reaction is apt to occur on (RuO2)n clusters. In a word, loading RuO2 nanoparticles as a reduction cocatalyst benefits the charge separation in β-Ge3N4. Furthermore, attaching (RuO2)n clusters onto β-Ge3N4(100) surface results in the redshift of absorption edge and the increase of absorption intensity. Our calculations have reasonably explained the experimental observation on the decomposition of water into H2 and O2 after loading RuO2 cocatalyst in β-Ge3N4 photocatalyst.

(Invited) New Mixed-Anion Photocatalysts for Water Splitting and CO2 Fixation

Water splitting and CO2 fixation using semiconductor photocatalysts are important reactions from the viewpoint of solar-to-fuel energy conversion toward artificial photosynthesis. Mixed-anion compounds containing more than one anionic species in a single-phase have attracted attention as visible-light-driven photocatalysts, since as compared to oxygen 2p orbital, p orbitals of less electronegative anion can form a valence band that possesses more negative potential. In this talk, recent progress on the development of new mixed-anion photocatalysts that are applicable to water splitting and CO2 fixation will be given. We developed Ta and N-codoped rutile TiO2 for visible-light-driven water oxidation. TiO2:Ta,N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation to produce O2 in the presence of reversible electron acceptors (IO3 – or Fe3+), while TiO2:N exhibited negligible activity. Results of time-resolved infrared absorption spectroscopy revealed that the codoping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Overall water splitting into H2 and O2 was achieved using TiO2:Ta,N, in combination with a H2-evolution photocatalyst of SrTiO3:Rh and an Fe3+/Fe2+ or IO3 –/I– redox couple even under simulated sunlight (AM1.5G). A similar result was obtained with rutile TiO2 codoped with N and F. TiO2:Ta,N also served as a very stable photoanode to oxidize water under visible light, and could be coupled to not only a H2 evolution cathode but also a molecular photocathode for CO2 reduction. A photoelectrochemical cell constructed with RuO2/TiO2:Ta,N photoanode and Ru(II)–Re(I) supramolecular photocathode was capable of reducing CO2 into CO using water as the electron source upon visible light, with stoichiometric production of CO and O2. Another interesting example of mixed-anion photocatalyst is an oxyfluoride pyrochlore Pb2Ti2O5.4F1.2. It had been believed that oxyfluorides are unsuitable as visible-light-responsive photocatalysts because of the highest electronegativity of fluorine. Surprisingly, Pb2Ti2O5.4F1.2 had an unprecedented small band gap of ca. 2.4 eV, and worked as a stable photocatalyst for visible-light H2 evolution and CO2 reduction when modified with suitable promoters. Density functional theory calculations indicated that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 originates from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb–O bond in the pyrochlore lattice due to the fluorine substitution.

TiO2 hollow spheres with separated Au and RuO2 co-catalysts for efficient photocatalytic water splitting

Abstract Hollow mesoporous TiO2 photocatalysts with dual co-catalysts, located at specific positions, were prepared using Polystyrene (PS) as sacrificial templates. Au nanoparticles (NPs) were in situ loaded on the surface of PS spheres and the resulting nanocomposites were coated with TiO2 shell using sol-gel reaction. The outer surface of core-shell spheres was impregnated with Ru and the subsequent calcination produced hollow anatase spheres with Au and RuO2 dual co-catalysts. The hollow mesoporous spheres of Au@TiO2@RuO2 were proved by various techniques such as TEM, EDX, and SEM images. Photocatalysts were applied for hydrogen generation from water splitting and that with dual co-catalysts showed efficient catalytic activity under simulated solar light. The catalytic activity of photocatalysts with both oxidation and reduction co-catalysts (Au@TiO2@RuO2) showed hydrogen evolution (3165 μmol g−1) almost two times more than that Au@TiO2 and TiO2@RuO2 with single co-catalysts. And the hydrogen evolved is more than three times as compared to TiO2 (935 μmol g−1) without any co-catalyst. Hollow mesoporous morphology with different co-catalysts on inner and outer surfaces is believed to enhance photocatalytic activity which is due to better separation of photo-generated charges.

Pt Acts As a Catalyst to Activate RuO2: The Active Sites of RuO2 Co-Catalyst for the CO Oxidation

RuO2 has been suggested to act as stable co-catalysts for CO oxidation on Pt. However, little is known on how the oxide functions to oxidize nearby CO. In this work, we use model rotating disk electrodes composed of RuO2 nanosheets (RuO2(ns)) deposited on the surface of polycrystalline Pt, polycrystalline Au and glassy carbon. A new co-catalytic mechanism regarding the Pt/RuO2 interface towards the electro-oxidation of CO is proposed. Two distinct synergetic effects were identified, decreasing the bulk CO electro-oxidation onset potential compared to bare Pt by 50 mV (the ‘high potential’ synergetic effect at 0.85 V vs. RHE) and 250 mV (the ‘low potential’ synergetic effect at 0.65 V vs. RHE). We discuss the later with a new mechanism, involving an activated surface of RuO2(ns) generated by a partial reduction catalyzed by Pt. The peculiar ‘low potential’ synergetic effect decays upon potential cycling in CO-saturated electrolyte where Pt is poisoned with COad, but can be reactivated by COad-free Pt in HUPD potential region. These results were resolved considering the formation of ‘Ru metal-like’ sites on RuO2(ns) which are generated from a partial reduction of the RuO2(ns) surface catalyzed by a Pt surface that is not poisoned by CO (spill-over hydrogen). Therefore, this ‘low potential’ synergetic effect interestingly differs from the customary bifunctional mechanism and shows a highly interdependent relationship where Pt-catalyzed activated surface of RuO2(ns) provides oxygenated species for the removal of CO adsorbed on Pt. Acknowledgments This work was supported in part by the ‘Polymer Electrolyte Fuel Cell Program’ from the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Design and Synthesis of TiO₂ Hollow Spheres with Spatially Separated Dual Cocatalysts for Efficient Photocatalytic Hydrogen Production.

TiO2 hollow spheres modified with spatially separated Ag species and RuO2 cocatalysts have been prepared via an alkoxide hydrolysis–precipitation method and a facile impregnation method. High-resolution transmission electron microscopy studies indicate that Ag species and RuO2 co-located on the inner and outer surface of TiO2 hollow spheres, respectively. The resultant catalysts show significantly enhanced activity in photocatalytic hydrogen production under simulated sunlight attributed to spatially separated Ag species and RuO2 cocatalysts on TiO2 hollow spheres, which results in the efficient separation and transportation of photogenerated charge carriers.

Fabrication of Oxygen‐Doped Double‐Shelled GaN Hollow Spheres toward Efficient Photoreduction of CO2

Double‐shelled GaN hollow spheres are prepared for photocatalytic reduction of CO2 through ammonia‐gas nitridation of shape‐analogous Ga2O3 hollow spheres. Small amount oxygen in situ doping narrows the bandgap of the GaN hollow sphere which significantly enhances the utilization of visible light. The large surface area of the hollow spheres provides more activity sites for the photoreduction reaction. The specific hollow structure also allows the trapping of the incident light for a longer time during photocatalysis process, which provides more opportunities for light absorption. Loading of Pt and RuO2 cocatalysts further enhances the separation of photogenerated electrons and holes to improve the CO2 reduction activity.

On the Substantially Improved Photoelectrochemical Properties of Nanoporous WO3 Through Surface Decoration with RuO2

Solar fuel generation, either through photochemical or photoelectrochemical (PEC) routes, is one of the most prominent pathways to harvest sunlight in order to contribute to a diversified and sustainable energy supply. Oxygen evolution reaction (OER) from aqueous solutions is the half reaction of different energy related reaction schemes, most importantly stoichiometric water splitting (H2 generation) or CO2 reduction. In this paper, we present PEC water oxidation (OER) on nanoporous WO3 films, decorated with RuO2 nanoparticles. The morphology and the composition of these nanostructured assemblies were characterized by high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry while their photo-electrochemical behavior was evaluated by photovoltammetry and incident photon-to-current conversion efficiency (IPCE) measurements. The O2 evolution capability was directly assessed by determining the amount of evolved O2 gas during PEC oxidation of water. The RuO2 electrocatalyst substantially increased photoanodic current flow through facile transfer of photogenerated holes from WO3 to the solution, thereby improving interfacial charge transfer kinetics. Both absolute and relative enhancement of photocurrents was analyzed as a function of the applied external bias potential giving important insight into the mechanistic details. Furthermore, the decisive roles of the amount of RuO2 co-catalyst and thermal pretreatment were established by synthesizing hybrid assemblies with different RuO2 coverage, and applying subsequent thermal treatment, respectively. Through careful optimization of the composition of the hybrid material, the IPCE value doubled at lower bias potentials and was increased close to the theoretical 100 % limit at higher positive potential values. These results demonstrate the synergy gained by combining the excellent photoanodic properties of nanoporous WO3 with the robust O2 evolution capabilities of RuO2.

Visible Light Water Oxidation Using a Co-Catalyst Loaded Anatase-Structured Ti1–(5x/4)NbxO2–y–δNy Compound

The photocatalytic activity of anatase-structured Ti(1-(5x/4))Nb(x)O(2-y-δ)N(y) (x = 0.25, y = 0.02; NbN-25) was examined for water oxidation under UV and visible light irradiation. The semiconductor was prepared by sol-gel processing followed by nitridation in flowing ammonia and exhibits an indirect optical gap of 2.2 eV. Ti(1-(5x/4))Nb(x)O(2-y-δ)N(y) was loaded with RuO2 by an impregnation technique, and optimized conditions reveal that 1 wt % RuO2 generates 16 μmol O2 from water with concomitant IO3(-) reduction after 3 h of illumination under simulated solar radiation at a flux of 600 mW/cm(2) illumination, which corresponds to 6-sun AM1.5G illumination (compared to no detectible O2 without the RuO2 cocatalyst). A series of cut-on filters shows that the catalyst-loaded semiconductor evolves O2 for λ ≤ 515 nm, and a gas-phase mass spectrometry isotope labeling experiment shows that irradiating an iodate solution in H2(18)O in the presence of 1 wt % RuO2 loaded on NbN-25 gives rise to catalytic water oxidation: both (36)O2 and (34)O2 are observed. It is unclear whether (16)O arises from IO3(-) or surface reconstruction on the photocatalyst, but ICP-AES analysis of the postirradiated solution shows no dissolved metal ions.

Photocatalysis of heat treated sodium- and hydrogen-titanate nanoribbons for water splitting, H2/O2 generation and oxalic acid oxidation

The photocatalytic activity of sodium titanate (Na1.48H0.52Ti3O7), sodium hexatitanate (Na2Ti6O13), and hydrogen titanate (H2Ti3O7) nanoribbons and anatase TiO2 nanorods were compared for water splitting, oxalic acid photodegradation and H2 and O2 generation using sacrificial agents. The intrinsic properties of the materials were found to affect their performance depending on the particular reaction system. The Na2Ti6O13 nanoribbons, in the presence of RuO2 co-catalyst, outperformed the anatase nanorods, for the water splitting reaction, generating over 10 times more H2/O2. This was thought to derive from their tunnel-like structure which provided better electron/hole separation when compared with TiO2. However, the efficient holes and electrons scavenging in the presence of sacrificial agents, methanol or AgNO3, to generate H2 or O2, respectively, overwhelmed the tunnel-like structure effect. In this case photoactivity was governed by the crystal structure, with observed decreasing activity in the order TiO2>Na2Ti6O13>H2Ti3O7∼Na1.48H0.52Ti3O7, and by the band gap of the semiconductor which determined its capacity to absorb photons in producing electron/hole pairs.

A comparison of the photocatalytic activity of six tunneled titanates

The photocatalytic behavior of six tunneled titanates—Na0.7Ga4.7Ti0.3O8, Na0.8Ga4.8Ti1.2O10, Na0.8Ga4.8Ti2.2O12, K1Ga17Ti15O56, K1.5Ga1.5Ti6.5O16, and BaTi4O9—was investigated using methylene blue as an indicator and a xenon arc lamp as the radiation source. Powders prepared by solid state reaction had surface areas ranging from 0.40 to 1.58m2/g and particle sizes ranging from 1 to 25μm. Bandgaps, as measured from diffuse reflectance data, ranged from 2.84 to 4.15eV. Two of the tunneled titanates—Na0.8Ga4.8Ti1.2O10 and Na0.7Ga4.7Ti0.3O8—exhibited negligible photocatalytic activity. The activities of Na0.8Ga4.8Ti2.2O12, KGa17Ti15O56, and K1.5Ga1.5Ti6.5O16 were similar to each other, exhibiting apparent first-order reaction rates of 0.275 to 0.31h−1 using 100mg of powder in 125ml of 20μM methylene blue at room temperature. The BaTi4O9 sample exhibited the highest photocatalytic activity with an apparent first-order reaction rate of 0.53h−1 under the same conditions. The addition of a RuO2 co-catalyst improved the photocatalytic activity of Na0.7Ga4.7Ti0.3O8, decreased the activity of BaTi4O9, and had little effect on the activity of the other powders. A comparison of the structural features in the six materials shows that photocatalytic activity is strongly related to the density of TiO6 octahedra in the different crystal structures. Among the gallium-containing tunneled titanates, photocatalytic activity increased with decreasing band gap.

Photocatalytic oxidation of thiophene on BiVO4with dual co-catalysts Pt and RuO2under visible light irradiation using molecular oxygen as oxidant

Thiophene is one of the main sulfur-containing compounds in gasoline and difficult to be oxidized with the conventional oxidative processes. Herein for the first time we report that thiophene can be oxidized to SO3 on BiVO4 co-loaded with Pt and RuO2 co-catalysts (denoted as Pt-RuO2/BiVO4) under visible light irradiation with molecular oxygen as oxidant. The high activity of the catalyst can be achieved by only loading as low as 0.03 wt% of Pt and 0.01 wt% of RuO2 as dual co-catalysts on BiVO4. ESR measurements give the evidence that the active oxygen species (˙OH and O2˙−) generated by photocatalytic processes are involved in the photocatalytic oxidation of thiophene. The considerable enhancement of photocatalytic activity can be attributed to the simultaneous presence of the reduction and oxidation co-catalysts which are beneficial for the efficient separation and transfer of the photo-generated electrons and holes.

Electronic structure and water splitting under visible light irradiation of BiTa1−xCuxO4 (x=0.00–0.04) photocatalysts

A series of photocatalysts, BiTa1−xCuxO4 (x = 0.00–0.04), were synthesized by the conventional solid-state reaction method and their electronic structures and photocatalytic activities were investigated. The electron microscope observations revealed that the particle sizes of BiTaO4:Cu crystals were smaller and the surface with many characteristic steps was more obvious than that of the nondoped BiTaO4, which lead to the increase of photocatalytic activity of water splitting. The UV–vis spectra indicate that the Cu2+ ions doping not only enhanced the photocatalytic activity under ultraviolet–visible (λ > 300 nm) light irradiation but also induced the visible light (λ > 400 nm) response. The photocatalyst doped with 2 mol% Cu2+ and loaded with 0.3 wt% RuO2 co-catalyst was found to have the highest activity. New band gap in the visible light range is obtained by copper-doped BiTaO4, which is attributed to the transition from the donor level resulting from the Cu impurity to the conduction band of BiTaO4 doped with copper on the basis of the result of DFT calculation.

Visible light assisted highly efficient hydrogen production from H2S decomposition by CuGaO2 and CuGa1−In O2 delafossite oxides bearing nanostructured co-catalysts

The present communication is the report of our research work on synthesis of new delafossite oxides containing Ga/Ga and In p-block elements, and their visible light driven catalytic activity in solar H2 production from H2S decomposition. CuGaO2 and its indium doped analogue CuGa1−xInxO2 (x = 0.065) delafossite oxides without/with NiO and RuO2 co-catalysts loading in nanostructures were prepared by solid state reaction method. These materials possess hexagonal rhombohedral structure (XRD); morphologically CuGaO2 has irregularly-shaped plate like particles while all others have ordered hexagonal rod-like arrangements (FESEM). Co-catalyst deposits of few nm sizes are observable as white spots/patches on the surface of naked oxide catalysts. Acquiring p-type conductivity from the mixed valence of Cu (+1 and +2) and oxygen deficiency, these catalysts strongly absorb visible light (Eg = 1.85 eV) in a wide wavelength range. They decompose H2S in aqueous 0.5 M KOH solution under visible light (λ ⩾ 420 nm) irradiation and generate H2 to the tune of 4300 μmol/h, giving rise to a high quantum efficiency of 13.6% at 550 nm. The exceedingly higher rate of H2 production appears to result from a combined contribution of chemical nature (p-block elements Ga and In), p-type conductivity and an efficient e−–h+ separation.

Structural, photocatalytic, and photophysical properties of perovskite MSnO3 (M = Ca, Sr, and Ba) photocatalysts

The photophysical properties of MSnO3 (M = Ca, Sr, and Ba) including optical absorption, photoluminescence, and energy band structure including band edge positions were investigated experimentally and theoretically in association with their photocatalytic properties. Photocatalytic reactions for H2 and O2 evolution in the case of sacrificial reagents were performed under ultraviolet (UV) light irradiation. The order of the activities of H2 evolution was CaSnO3 > SrSnO3 > BaSnO3, agreeing not only with that of the conduction-band edges (or band gaps) but also with that of the transferred excitation energy, while that of O2 evolution was CaSnO3 < SrSnO3 < BaSnO3, consistent with that of the angle of the Sn–O–Sn bonds as well as the delocalization of excited energy. When loaded with RuO2 cocatalyst, both CaSnO3 and SrSnO3 can efficiently split pure water into hydrogen and oxygen in a stoichiometric ratio under UV light irradiation. In addition, RuO2-loaded SrSnO3 showed higher water splitting activity than RuO2-loaded CaSnO3 did. This is attributed to the suitable conduction and valence band edges and to high mobility of the photogenerated charge carriers caused by the proper distortion of SnO6 connection in SrSnO3. The RuO2-loaded BaSnO3 photocatalyst cannot split pure water, which might be because of a high concentration of defect centers such as Sn2+ ions and the probability of radiative recombination in BaSnO3.

Photophysical and Photocatalytic Activities of a Novel Photocatalyst BaZn1/3Nb2/3O3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Photophysical and Photocatalytic Activities of a Novel Photocatalyst BaZn1/3Nb2/3O3

A new ABO3 perovskite photocatalyst BaZn1/3Nb2/3O3 has been synthesized by a solid-state reaction process. Nine Raman active vibration modes observed from its Raman scattering spectrum suggest that BaZn1/3Nb2/3O3 possesses a distorted perovskite structure. The formation rate of H2 evolution from CH3OH/H2O solution with 5.0 wt % Pt cocatalyst under the irradiation of a 400 W Hg lamp is about 2685.7 μmol/(h·g catal). for the photocatalyst BaZn1/3Nb2/3O3. It also splits pure water into H2 and O2 in a stoichiometric ratio with NiOx and RuO2 cocatalysts under UV light irradiation. The formation rates of the H2 and O2 evolution from pure water in the case of 1.0 wt % NiOx cocatalyst under UV light irradiation are 291.22 and 145.61 μmol/(h·g catal), respectively. The photoelectrochemical properties of BaZn1/3Nb2/3O3 film as deposited by using a pulsed laser deposition (PLD) technique suggested that the lower conduction band should be composed of the Zn2+ 4s state, which is in good agreement with its UV−vis diffu...