Heterogeneous photocatalysis is a potentially competitive solution for the direct production of solar fuels. This research field has seen tremendous growth over the last five decades, and with such an exciting research topic, it has seenand will continue to seean increasing number of papers being published in a variety of journals. However, it is becoming increasingly difficult to compare the efficiencies of heterogeneous photocatalyst powders, because different researchers report their results in different ways. Efforts have been made to create standards for reporting data in this field, but there continues to be a discrepancy in published works. This article intends to clarify efficiency definitions, and clarify misconceptions as to why researchers should avoid reporting rates of evolution per gram, per surface area of catalyst, or as turnover frequencies (TOFs) alone, to be able to compare photocatalytic efficiency among different materials. By providing an example of a photoreactor for water splitting in the authors’ laboratory, the paper also intends to guide new researchers in the field. This article does not discuss how to improve photocatalysis but rather how to improve the reporting of photocatalysis to ensure reproducibility and effective benchmarking. Researchers should not only ensure that they have all the appropriate characterization and statistical data to support their claims but should also recognize that improperly reported data may lead to faulty benchmarking that prevents their results from being compared with those of other photocatalysts, inhibiting the progress of photocatalytic research.
for this application. [1][2][3][4][5][6] Recently, however, new classes of materials are being investigated for hydrogen evolution from water, such as nitrogen doped graphene oxide, [7] covalent organic frameworks, [8][9][10] conjugated polymer networks, [11][12][13][14] and linear conjugated polymers. [15][16][17][18][19][20] With the notable exception of graphitic carbon nitride, [21] most organic semiconductors have required an additional metal co-catalyst to produce an appreciable H 2 evolution rate, often introduced by Pt photo deposition, [22] or the addition of a molecular catalyst. [8,23,24] However, recent reports of photocatalysis employing both linear and cross-linked conjugated polymers suggest that these systems are able to produce H 2 under visible light irradiation without any added co-catalyst, at a much faster rate than commercial graphitic carbon nitride, even when the latter is subjected to Pt photodeposition. [11] Activity without the need for co-catalysts, which are often precious metals or molecular complexes with limited stability, [25] could be a very attractive feature of such polymer photocatalysts with the potential to vastly reduce the cost and complexity of current photocatalytic systems.A feature of these linear and cross-linked conjugated polymers that has not yet been systematically studied is the presence of significant quantities of residual Pd originating from their synthesis via Pd catalyzed polycondensation reactions. [11,12,15,26] It has previously been shown that the molecular Pd catalysts used in these reactions can decompose via deligation and subsequent Ostwald ripening to form metallic Pd 0 particles, which are strongly retained within the polymer matrix and are difficult to remove via classical purification techniques. [27][28][29][30][31] Similar to Pt 0 , Pd 0 is a highly active proton reduction electrocatalyst. [32][33][34] Therefore, it is possible that the residual Pd, dispersed within these polymer structures, facilitates H 2 evolution and removes the need for additional co-catalysts. Much uncertainty remains about the nature of the active site in conjugated polymer systems, and there are conflicting views in the literature on whether or not the residual amounts of Pd play a significant role. No correlation was seen between Pd concentration and hydrogen evolution reaction (HER) rate in a range of conjugated micro porous polymers, [11] and in a series of linear polymersThe effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the h...
The present work compares the oxygen evolution reaction (OER) in electrocatalysis and photocatalysis in aqueous solutions using nanostructured NiFeO x as catalysts. The impacts of pH and reaction temperature on the electrocatalytic and photocatalytic OER kinetics were investigated. For electrocatalysis, a NiFeO x catalyst was hydrothermally decorated on Ni foam. In 1 M KOH solution, the NiFeO x electrocatalyst achieved 10 mA cm −2 at an overpotential of 260 mV. The same catalyst was decorated on the surface of Ta 3 N 5 photocatalyst powder. The reaction was conducted in the presence of 0.1 M Na 2 S 2 O 8 as a strong electron scavenger, thus likely leading to the OER being kinetically relevant. When compared with the bare Ta 3 N 5 , NiFeO x /Ta 3 N 5 demonstrated a 5-fold improvement in photocatalytic activity in the OER under visible light irradiation, achieving a quantum efficiency of 24% at 480 nm. Under the conditions investigated, a strong correlation between the electrocatalytic and photocatalytic performances was identified: an improvement in electrocatalysis corresponded with an improvement in photocatalysis without altering the identity of the materials. The rate change at different pH was likely associated with electrocatalytic kinetics that accordingly influenced the photocatalytic rates. The sensitivity of the reaction rates with respect to the reaction temperature resulted in an apparent activation energy of 25 kJ mol −1 in electrocatalysis, whereas the apparent activation energy in photocatalysis was 16 kJ mol −1 . The origin of the difference in these activation energy values is likely attributed to the possible effects of temperature on the individual thermodynamic and kinetic parameters of the reaction process. The work described herein demonstrates a method of "transferring the knowledge of electrocatalysis to photocatalysis" as a strong tool to rationally and quantitatively understand the complex reaction schemes involved in photocatalytic reactions.
For overall water-splitting systems, it is essential to establish O -insensitive cathodes that allow cogeneration of H and O . An acid-tolerant electrocatalyst is described, which employs a Mo-coating on a metal surface to achieve selective H evolution in the presence of O . In operando X-ray absorption spectroscopy identified reduced Pt covered with an amorphous molybdenum oxyhydroxide hydrate with a local structural order composed of polyanionic trimeric units of molybdenum(IV). The Mo layer likely hinders O gas permeation, impeding contact with active Pt. Photocatalytic overall water splitting proceeded using MoO /Pt/SrTiO with inhibited water formation from H and O , which is the prevailing back reaction on the bare Pt/SrTiO photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membraneless electrolysis and photocatalysis.
Platinum single atoms are grafted by SOMC on morphology-controlled TiO2. Their structure is characterized by EXAFS and other techniques, and their activity and stability in HER and backwards reaction are studied and compared to Pt nanoparticles.
Successful conversion of renewable energy to useful chemicals requires efficient devices that can electrocatalyze or photocatalyze redox reactions, e.g., overall water splitting. Excellent electrocatalysts for the hydrogen evolution reaction (HER), such as Pt, can also cause other sidereactions, including the water-forming back-reaction from H 2 and O 2 products. A Cr-based amorphous layer coated on catalysts can work as a successful surface modifier that avoids the back-reaction, but its capabilities and limitations toward other species have not been studied. Herein, we investigated the Cr-based layer on Pt from perspectives of both electrocatalysis and photocatalysis using redox-active molecules/ions (O 2 , ferricyanide, IO 3, H 2 O 2 , and CO gas). Our systematic study revealed that utilization of the Cr-based layer realized an exclusive cathodic reaction only to HER, even in the presence of the aforementioned reactive species, suggesting that Cr-based layers work as membranes, as well as corrosion and poison inhibition layers. However, the Crbased layer experienced self-oxidation and dissolved into the aqueous phase when a strong oxidizing agent or low pH was present. Presented herein are fundamental and critical aspects of the Cr-based modifier, which is essential for the successful and practical development of solar fuel production systems.
Understanding photophysical and electrocatalytic processes during photocatalysis in a powder suspension system is crucial for developing efficient solar energy conversion systems. We report a substantial enhancement by a factor of 3 in photocatalytic efficiency for the oxygen evolution reaction (OER) by adding trace amounts (~0.05 wt%) of noble metals (Rh or Ru) to a 2 wt% cobalt oxide-modified Ta3N5 photocatalyst particulate. The optimized system exhibited high quantum efficiencies (QEs) of up to 28 and 8.4% at 500 and 600 nm in 0.1 M Na2S2O8 at pH 14. By isolating the electrochemical components to generate doped cobalt oxide electrodes, the electrocatalytic activity of cobalt oxide when doped with Ru or Rh was improved compared with cobalt oxide, as evidenced by the onset shift for electrochemical OER. Density functional theory (DFT) calculation shows that the effects of a second metal addition perturbs the electronic structure and redox properties in such a way that both hole transfer kinetics and electrocatalytic rates improve. Time resolved terahertz spectroscopy (TRTS) measurement provides evidence of long-lived electron populations (>1 ns; with mobilities μe ~0.1-3 cm 2 V -1 s -1 ), which are not perturbed by the addition of CoOx-related phases. Furthermore, we find that Ta3N5 phases alone suffer ultrafast hole trapping (within 10 ps); the CoOx and M-CoOx decorations most likely induce a kinetic competition between hole transfer toward the CoOx-related phases and trapping in the Ta3N5 phase, which is consistent with the improved OER rates. The present work not only provides a novel way to improve electrocatalytic and photocatalytic performance but also gives additional tools and insight to understand the characteristics of photocatalysts that can be used in a suspension system.
Sulfur Kβ non-resonant X-ray emission spectroscopy complements sulfur K-edge X-ray absorption spectroscopy in providing information on chemical speciation and electronic structure.
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