Ammonia decomposition to produce hydrogen can be promoted by the combination of plasma and catalyst. In this study, molybdenum nitride (Mo
2
N) catalyst prepared by low pressure nitriding method was ...applied for the DBD plasma-driven ammonia decomposition for hydrogen production. At the NH
3
feed flow rate of 40 sccm, the ammonia decomposition efficiency of more than 95% was achieved using plasma-catalytic method at an input power of 40 W without extra heating, which is more than twice that achieved by plasma-only or thermal-catalyst mode. Confirmed by various characterization results, through the low-pressure nitriding method, a composite structure of Mo/Mo
2
N was introduced on the surface of the catalyst, which is considered to play an important role in improving the catalytic performance. Owing to the pre-generated Mo/Mo
2
N active species, the plasma catalytic ammonia decomposition proceeded straightly without evident induction period.
Although it has been widely accepted that the crystal phase, morphology, and facet significantly influence the catalytic and photocatalytic activity of TiO2, establishing the correlation between ...structure and activity of heterogeneous reactions is very difficult because of the complexity of the structure. Utilizing ultrahigh vacuum (UHV) based temperature-programmed desorption (TPD) and density functional theory (DFT) calculations, we have successfully assessed the photoreactivity of two well characterized rutile surfaces ((011)-(2×1) and (110)-(1×1)) through examining the photocatalyzed oxidation of methanol. The photocatalytic products, such as formaldehyde and methyl formate, are the same on both surfaces under UV illumination. However, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which contradicts previous reports in aqueous environments where characterization of TiO2 structure is difficult. The discrepancy probably comes from the differences of the TiO2 structure in these studies. Our DFT calculations reveal that the rate-determining step of methanol dissociation on both surfaces is C–H scission,; however, the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher than that on (110)-(1×1) because of their distinct surface atomic configurations. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO2, but also provides an example for building the correlation between structure and activity using surface science techniques and DFT calculations.
Many physical and chemical processes on TiO2 surface are linked to the excess electrons originated from band gap states. However, the sources (surface and/or subsurface defects) of these states are ...controversial. We present quantitative ultraviolet photoelectron spectroscopy (UPS) measurements on the band gap states of TiO2(110) with constant subsurface defect density and varied surface bridging hydroxyls (ObrH) prepared through photocatalyzed splitting of methanol, in combination with density functional theory (DFT) calculations. Our results clearly suggest both surface and subsurface defects contribute to the band gap states, whereas the contribution of subsurface defects corresponds to that of only 1.9% monolayer ObrH at the current bulk reduction level. As the surface defect concentration is usually much larger than 1.9% monolayer in real studies and applications, our work demonstrates the importance of surface defects in changing the electronic structure of TiO2, which dictates the surface chemistry.
Ethanol on TiO2(110) has been studied using the temperature-programmed desorption (TPD), femtosecond two-photon photoemission spectroscopy (2PPE), and density functional theory (DFT) calculations. ...The first layer of ethanol (binds to Ti5c) whose molecular state has been predicted to be more stable by DFT desorbs at 295 K. A photoinduced excited state that is associated with bridging hydroxyls has been detected at ∼2.4 eV above the Fermi level on ethanol/TiO2(110) interface using 2PPE. Detailed TPD studies show that ethanol on Ti5c can be photocatalytically converted to acetaldehyde by near-band-gap excitation with the hydrogen atoms transfer to bridging-bonded oxygen sites, which is consistent with the 2PPE results. TPD results also show a low-temperature water TPD peak that seems to bind to the Ti5c sites in addition to the ethylene TPD product. These results suggest that the Ti5c sites on TiO2(110) are the primary active sites for photocatalysis of ethanol on TiO2(110), while bridging-bonded oxygen sites also play an important role, as in the case of methanol. The kinetics of photocatalyzed ethanol dissociation on TiO2(110) has also been measured using the 2PPE technique, which is of heterogeneous nature.
Exploring photocatalysts to foster CO2 photoreduction into high value‐added chemicals is of great significance. Lead halide perovskites (LHPs) have recently been extensively investigated as ...photocatalysts, owing to their facile fabrication and prominent optoelectronic properties. However, the toxicity of lead and instability will hinder their future large‐scale applications. To address these challenges, a series of lead‐free Sb‐based all‐inorganic mixed halide perovskite Cs3Sb2(BrxI1−x)9 (0 ≤ x ≤ 1) nanoplatelets (NPLs) is synthesized. The perovskite NPLs are prepared using a ligand‐assisted re‐precipitation approach at 50 °C. The authors observe the tunability of their optical band gaps from 2.1 to 2.5 eV, and they can maintain the excellent stability over 120 h under heating at 100 °C or UV light irradiation. The resultant materials are employed as efficient photocatalysts for visible‐light driven CO2 reduction at the gas–solid interface. The Cs3Sb2(Br0.7I0.3)9 perovskite NPLs afford an impressive overall yield of 27.7 µmol g−1 for the selective photocatalytic conversion of CO2 into CO. This study represents a significant demonstration for practical artificial photosynthesis by using LHP materials as photocatalysts.
A series of stable lead‐free Sb‐based inorganic mixed halide perovskite Cs3Sb2(BrxI1−x)9 (0 ≤ x ≤ 1) nanoplatelets (NPLs) are synthesized. It is demonstrated that the direct tuning of the halide compositions of the Cs3Sb2(BrxI1−x)9 (0 ≤ x ≤ 1) perovskite NPLs is feasible, highlighting an alternative method for controlling the NPLs properties. The as‐synthesized Cs3Sb2(BrxI1−x)9 (0 ≤ x ≤ 1) perovskite NPLs are efficient photocatalysts for CO2 reduction under visible light illumination.
In reduced TiO2, electronic transitions originating from the Ti3+-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material’s blue ...color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti3+ species is identified at 2.5 ± 0.2 eV above the Fermi level (E F) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below E F, as they both result from the Jahn–Teller induced splitting of the 3d orbitals of Ti3+ ions in reduced TiO2. Localized excitation of Ti3+ ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti3+ self-doping. Our work reveals the physical origin of the Ti3+ related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.
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•Partially oxygen-covered TiN surface is highly efficient for H2O activation by oxygen-assisted dissociation.•Water gas shift (WGS) on Pt/TiN proceeds by the association of CO ...adsorbed on Pt with the adjacent hydroxyl on Ti sites.•The superior catalytic performance of WGS on Pt/TiN arises from the improved H2O dissociation at Pt-TiN interface.
The reactive interfacial sites upon supported catalysts provide an alternative pathway to enable the reactants activation in an energetically favorable mode, thus decreasing the energy barrier and promoting the catalytic efficiency. By virtue of structural characterization, in-situ spectroscopic analysis, kinetic assessments and density functional theory (DFT) calculations, supported Pt/TiN catalysts are systematically studied to interrogate H2O activation on the Pt-TiN interfacial sites and its catalytic function during the low-temperature water gas shift (WGS). It is found that the partially oxygen-covered TiN surface is highly efficient for H2O activation by oxygen-assisted dissociation with an energy barrier of 0.41 eV, much less than that on metallic Pt surface. The formed hydroxyls binding on Ti sites, observed by in-situ X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD), could readily react with the adjacent CO adsorbed on Pt clusters with the reaction energy barrier of 0.43 eV to attain the individual catalytic cycle. Benefiting from the enhanced H2O activation, the catalytic turnover rate of CO-H2O reaction on Pt/TiN is 3.8 and 20.3 times higher than that on traditional Pt/TiO2 and Pt/SiO2 at 403 K, respectively. The interfacial reactive sites are the origin of low-temperature WGS on Pt/TiN catalysts.
Infrared spectra have been widely used to study CO adsorption and reaction on Au surfaces to reveal the underlying reaction mechanism. However, the results and interpretation of IR spectra of CO ...adsorption on Au surfaces remain highly controversial, which prevents reliable conclusions from earlier studies. Herein, we studied CO adsorption on Au thin films and nanoparticles by in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) with highly purified CO. Only a single CO adsorption feature above 2100 cm–1 is observed on Au nanoparticles and thin films with CO adsorption pressure varying from 1 × 10–6 to 100 Torr. This observation contradicts the general findings that CO adsorption on Au surfaces at elevated CO pressures shows strong adsorption bands at 2090–1900 cm–1 mostly assigned to CO bound to negatively charged Au. We rule out the contradiction is caused by the pressure gap between vacuum and elevated pressure conditions employed for CO adsorption. We provide clear evidence that the CO adsorption bands in the region of 2090–1900 cm–1, widely observed in earlier studies of high-pressure CO adsorption on Au surfaces, are actually caused by Ni-carbonyl (Ni(CO)4) contamination from incompletely purified CO gas rather than CO adsorption on negatively charged Au. Ni-carbonyl can be adsorbed and decomposed on Au surfaces, resulting in the formation of NiAu bimetallic alloys. We further investigate CO adsorption behaviors on bimetallic NiAu thin films and nanoparticles by in situ PM-IRRAS. We find that Au is enriched on the NiAu surface under vacuum conditions, while Ni is segregated on the surface under elevated CO pressure conditions. The strong CO adsorption on Ni sites on NiAu bimetallic surfaces leads to an atop CO adsorption band at 2090–2040 cm–1, while CO bound on Au sites yields an adsorption band above 2100 cm–1. Our study clearly clarifies the origin of the widely reported CO adsorption bands at 2090–1900 cm–1 and resolves the puzzle of IR spectra of CO on Au surfaces, thus providing an important paradigm that complete purification of CO gas is a prerequisite for any in situ studies associated with CO at elevated pressures.
Abnormal permeation behavior of hydrogen through niobium has been investigated in this paper, i.e. the permeation flux saturated with long-term decrease after reaching a maximum. The diffusivity and ...permeability have been deduced from the decay edge of permeation transient. Three kinds of polycrystalline niobium foils with different annealing temperature have been compared, to verify the effect of defects and grain properties on the permeability and diffusivity. In the temperature range of (773–1023) K, the heat treatment along with the permeation cycles could either reduce or increase the permeability and diffusivity depending sensitively on temperature and showing a temperature threshold around 950 K. The permeation flux is proportional to square root of pressure, revealing that the abnormal permeation was still bulk diffusion-limited. The diffusivity gradually decreased with permeation cycles, and became more and more sensitive to pressure. The niobium foil expanded macroscopically along the gradient of hydrogen concentration, which reveals the strong and unrecoverable lattice distortion in this temperature and pressure range. The X-ray diffraction studies showed that splitting of all the Nb peaks and shifting of Nb-D peaks along with hydrogen loadings. The phase transition was expected to eliminate the lattice strain during hydrogen loading and which in turn acted as a diffusion barrier.
•The abnormal permeation transient of hydrogen in Nb is systematically investigated.•The abnormal permeation is bulk-diffusion controlled and pressure dependent.•The temperature of annealing strongly affects the behavior of abnormal permeation.•The reason for abnormal permeation is the formation of Nb-D due to phase transition.
