Single-atom catalysts have recently been applied in many applications such as CO oxidation. Experimental in situ investigations into this reaction, however, are limited. Hereby, we present a suite of ...operando/in situ spectroscopic experiments for structurally well-defined atomically dispersed Rh on phosphotungstic acid during CO oxidation. The identification of several key intermediates and the steady-state catalyst structure indicate that the reactions follow an unconventional Mars-van Krevelen mechanism and that the activation of O
is rate-limiting. In situ XPS confirms the contribution of the heteropoly acid support while in situ DRIFT spectroscopy consolidates the oxidation state and CO adsorption of Rh. As such, direct observation of three key components, i.e., metal center, support and substrate, is achieved, providing a clearer picture on CO oxidation on atomically dispersed Rh sites. The obtained information are used to engineer structurally similar catalysts that exhibit T
values up to 130 °C below the previously reported Rh
/NPTA.
Although TiO2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti3+ ions on its photocatalytic activity is not fully understood. In this study, we ...investigated the effects of high-temperature calcination and H2 reduction treatment on the water oxidation activity of rutile TiO2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO2 owing to strong oxidation, but its initial activity was restored by H2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the high-temperature calcination created O•– radicals (trapped hole in oxygen lattice site), while the H2 reduction treatment created Ti3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H2 treatment temperature. Measurements of the sheet resistance and space charge layer capacitance of the thermally oxidized TiO2 films indicated that the H2 treatment improved the electrical conductivity owing to an increase in donor density (electron density). Thus, the increase in the photocatalytic and photoelectrochemical activities of the rutile TiO2 was attributed to donor doping by H2 reduction.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Artificial photosynthesis has recently drawn an increasing amount of attention due to the fact that it allows for direct solar-to-chemical energy conversion. However, one of the basic steps of this ...process, namely the reduction of CO
2
by H
2
O to afford O
2
and CO
2
reduction products (CO
2
RPs) such as HCOOH, CO, HCHO, CH
3
OH, and CH
4
, is very difficult to achieve. In contrast to the CO
2
reduction in plants and homogenous systems, the reduction of CO
2
to CO
2
RPs over heterogeneous photocatalysts was challenged by the competing reduction of H
+
to H
2
. Unfortunately, most of the research performed so far has focused only on the reduction of CO
2
, rather than the characterization of the H
2
O oxidation and H
2
production. Moreover, the fact that the heterogeneous photocatalytic reduction of CO
2
into CO
2
RPs by H
2
O should satisfy several selectivity criteria has often been ignored. Herein, we propose three such evaluation criteria, namely (1) the origin of carbon in CO
2
RPs (determined using isotopically labeled CO
2
(
13
CO
2
)), (2) the relative amount of H
2
and CO
2
RPs produced, and (3) the amount of O
2
produced by the oxidation of H
2
O. If all these criteria are satisfied,
i.e.
, the carbons of CO
2
RPs originate from CO
2
, the amount of H
2
produced is negligible, and a stoichiometric amount of O
2
is produced by the oxidation of H
2
O, then CO
2
introduced into the gas phase is believed to be reduced by H
2
O to CO
2
RPs in the aqueous phase.
Artificial photosynthesis has recently drawn an increasing amount of attention due to the fact that it allows for direct solar-to-chemical energy conversion.
Photocatalytic conversion of CO2 to reduction products, such as CO, HCOOH, HCHO, CH3OH, and CH4, is one of the most attractive propositions for producing green energy by artificial photosynthesis. ...Herein, we found that Ga2O3 photocatalysts exhibit high conversion of CO2. Doping of Zn species into Ga2O3 suppresses the H2 evolution derived from overall water splitting and, consequently, Zn‐doped, Ag‐modified Ga2O3 exhibits higher selectivity toward CO evolution than bare, Ag‐modified Ga2O3. We observed stoichiometric amounts of evolved O2 together with CO. Mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. Doping of the photocatalyst with Zn is expected to ease the adsorption of CO2 on the catalyst surface.
Doping of Zn species into Ga2O3 suppresses the undesirable H2 evolution derived from overall water splitting. Zn‐doped, Ag‐modified Ga2O3 exhibits higher selectivity towards CO evolution than bare, Ag‐modified Ga2O3 (see figure).
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this paper, NiRu, NiRh, and NiPd catalysts were synthesized and evaluated in the hydrogenolysis of lignin C–O bonds, which is proved to be superior over single-component catalysts. The optimized ...NiRu catalyst contains 85% Ni and 15% Ru, composed of Ni surface-enriched, Ru–Ni atomically mixed, ultrasmall nanoparticles. The Ni85Ru15 catalyst showed high activity under low temperature (100 °C), low H2 pressure (1 bar) in β-O-4 type C–O bond hydrogenolysis. It also exhibited significantly higher activity over Ni and Ru catalysts in the direct conversion of lignin into monomeric aromatic chemicals. Mechanistic investigation indicates that the synergistic effect of NiRu can be attributed to three factors: (1) increased fraction of surface atoms (compared with Ni), (2) enhanced H2 and substrate activation (compared with Ni), and (3) inhibited benzene ring hydrogenation (compared with Ru). Similarly, NiRh and NiPd catalysts were more active and selective than their single-component counterparts in the hydrogenolysis of lignin model compounds and real lignin.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Abstract
Single-atom metal catalysts offer a promising way to utilize precious noble metal elements more effectively, provided that they are catalytically active and sufficiently stable. Herein, we ...report a synthetic strategy for Pt single-atom catalysts with outstanding stability in several reactions under demanding conditions. The Pt atoms are firmly anchored in the internal surface of mesoporous Al
2
O
3
, likely stabilized by coordinatively unsaturated pentahedral Al
3+
centres. The catalyst keeps its structural integrity and excellent performance for the selective hydrogenation of 1,3-butadiene after exposure to a reductive atmosphere at 200 °C for 24 h. Compared to commercial Pt nanoparticle catalyst on Al
2
O
3
and control samples, this system exhibits significantly enhanced stability and performance for
n
-hexane hydro-reforming at 550 °C for 48 h, although agglomeration of Pt single-atoms into clusters is observed after reaction. In CO oxidation, the Pt single-atom identity was fully maintained after 60 cycles between 100 and 400 °C over a one-month period.
The dehydrogenation of lower alkanes was investigated on Pt–Sn/SiO2 catalysts prepared by impregnation with different thermal treatments. The treatment atmosphere played an important role in both the ...catalytic performance and the catalyst structure. An ensemble containing several adjacent Pt and Sn atoms from Pt–SnO2 or Pt–Sn alloy nanoparticles acts as the active site for dehydrogenation of lower alkanes, which can be respectively induced by treatment in an inert atmosphere (N2) or reductive atmosphere (H2) at high temperature.
Display omitted
•Pt-SnO2 SMSI in Pt–Sn/SiO2 can be constructed via thermal treatments.•An ensemble containing Pt and Sn atoms acts as the active site for dehydrogenation.•The geometric effect of Sn on Pt contributes to the superior catalytic performance.
The addition of tin (Sn) is commonly used as a design strategy for catalyst optimization of platinum-based catalysts. The mechanistic understanding of this class of systems is, however, obscured by the structural complexity. Herein, a series of catalyst characterization techniques including X-ray absorption fine structure (XAFS) and in-situ CO diffuse reflectance infrared fourier transform spectroscopy (CO-DRIFTS) were utilized to study the catalyst structure. It was found that the structure and catalytic properties are closely related with the interaction between Pt and SnO2 (specifically the Pt-SnO2 strong metal-support interaction (SMSI)), which can be continuously tuned by thermal treating the Pt-Sn/SiO2 precursor at different atmospheres and temperatures. The treatment in an oxidative atmosphere (O2) also can generate Pt-SnO2 SMSI, which became weak at higher temperatures and led to the growth of Pt nanoparticles (NPs). Pt-SnO2 SMSI became stronger when the oxygen atmosphere was changed to an inert (N2) atmosphere. Small metallic Pt NPs were formed and their dispersion was increased with increasing treatment temperature with inert gas. The catalyst presented a moderate activity in the dehydrogenation of lower alkanes. The treatment in a reductive atmosphere (H2) produced the strongest Pt-SnO2 SMSI and most active catalyst. Highly dispersed Sn surface-enriched Pt–Sn alloy NPs were formed on SiO2, in which Pt was most electron rich. The apparent activation energy in n-butane dehydrogenation is higher on Pt–Sn/SiO2_1073 K H2 than the corresponding one on Pt–Sn/SiO2_1073 K N2. The kinetic studies revealed that the extreme isolation of Pt on Pt–Sn/SiO2_1073 K H2 (geometrical effects) dominantly contributed to its superior catalytic performance. The present work highlights the effects of thermal treatment-induced Pt-SnO2 SMSI, providing a new insight into the structure of Pt–Sn bimetallic catalysts and the promotional role of Sn in the dehydrogenation of lower alkanes to olefins on Pt surfaces.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
PDF
