2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

Since the 1970s, splitting water using solar energy has been a focus of great attention as a possible means for converting solar energy to chemical energy in the form of clean and renewable hydrogen fuel. Approaches to solar water splitting include photocatalytic water splitting with homogeneous or heterogeneous photocatalysts, photoelectrochemical or photoelectrocatalytic (PEC) water splitting with a PEC cell, and electrolysis of water with photovoltaic cells coupled to electrocatalysts. Though many materials are capable of photocatalytically producing hydrogen and/or oxygen, the overall energy conversion efficiency is still low and far from practical application. This is mainly due to the fact that the three crucial steps for the water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic reduction and oxidation reactions, are not efficient enough or simultaneously. Water splitting is a thermodynamically uphill reaction, requiring transfer of multiple electrons, making it one of the most challenging reactions in chemistry. This Account describes the important roles of cocatalysts in photocatalytic and PEC water splitting reactions. For semiconductor-based photocatalytic and PEC systems, we show that loading proper cocatalysts, especially dual cocatalysts for reduction and oxidation, on semiconductors (as light harvesters) can significantly enhance the activities of photocatalytic and PEC water splitting reactions. Loading oxidation and/or reduction cocatalysts on semiconductors can facilitate oxidation and reduction reactions by providing the active sites/reaction sites while suppressing the charge recombination and reverse reactions. In a PEC water splitting system, the water oxidation and reduction reactions occur at opposite electrodes, so cocatalysts loaded on the electrode materials mainly act as active sites/reaction sites spatially separated as natural photosynthesis does. In both cases, the nature of the loaded cocatalysts and their interaction with the semiconductor through the interface/junction are important. The cocatalyst can provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; the cocatalysts could improve the photostability of the catalysts by timely consuming of the photogenerated charges, particularly the holes; most importantly, the cocatalysts catalyze the reactions by lowering the activation energy. Our research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible. All of these findings suggest that dual cocatalysts are necessary for developing highly efficient photocatalysts for water splitting reactions.

https://faculty.missouri.edu/~glaserr/3700s15/Cocats_ar300227e.pdf

Roles of cocatalysts in photocatalysis and photoelectrocatalysis.

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

In this review, the progress made in the last ten years concerning the synthesis of porous carbon materials is summarized. Porous carbon materials with various pore sizes and pore structures have been synthesized using several different routes. Microporous activated carbons have been synthesized through the activation process. Ordered microporous carbon materials have been synthesized using zeolites as templates. Mesoporous carbons with a disordered pore structure have been synthesized using various methods, including catalytic activation using metal species, carbonization of polymer/polymer blends, carbonization of organic aerogels, and template synthesis using silica nanoparticles. Ordered mesoporous carbons with various pore structures have been synthesized using mesoporous silica materials such as MCM-48, HMS, SBA-15, MCF, and MSU-X as templates. Ordered mesoporous carbons with graphitic pore walls have been synthesized using soft-carbon sources that can be converted to highly ordered graphite at high temperature. Hierarchically ordered mesoporous carbon materials have been synthesized using various designed silica templates. Some of these mesoporous carbon materials have successfully been used as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for enzyme immobilization. Ordered macroporous carbon materials have been synthesized using colloidal crystals as templates. One-dimensional carbon nanostructured materials have been fabricated using anodic aluminum oxide (AAO) as a template.

Recent Progress in the Synthesis of Porous Carbon Materials

A surface science perspective on TiO2 photocatalysis

Surface phases of TiO2 nanoparticles (30 ∼ 200 nm) were studied by UV Raman spectroscopy and FT-IR spectroscopy with CO and CO2 as probe molecules. UV Raman spectroscopy can differentiate the surface phase structure of TiO2 calcined at different temperatures. IR spectra of adsorbed CO and CO2 on TiO2 calcined at different temperatures are in good agreement with the results from UV Raman spectra. IR results evidently confirm that UV Raman spectroscopy is a surface-sensitive technique for TiO2. Both UV Raman and IR spectra indicate that the crystalline phase of TiO2 in the surface region is usually different from that in the bulk which is characterized by XRD. CO is weakly adsorbed on Ti4+ ions of anatase phase but is hardly adsorbed on those of rutile phase at room temperature. Adsorbed CO2 on anatase phase produces mainly bidentate carbonate, while on rutile phase produces mainly bicarbonate species. These results suggest that the surface Lewis acidity of anatase phase is stronger than that of rutile phas...

Surface Phases of TiO2 Nanoparticles Studied by UV Raman Spectroscopy and FT-IR Spectroscopy

Ti-substituted mesoporous SBA-15 (Ti−SBA-15) materials have been synthesized by using a new approach in which the hydrolysis of the silicon precursor (tetramethoxysilane, TMOS) is accelerated by fluoride. These materials were characterized by powder X-ray diffraction patterns (XRD), X-ray fluorescence spectroscopy (XRF), N2 sorption isotherms, diffuse-reflectance UV−visible (UV−vis) and UV−Raman spectroscopy, 29Si MAS NMR, and the catalytic epoxidation reaction of styrene. Experiments show that Ti−SBA-15 samples of high quality can be obtained under the following conditions:  F/Si ≥ 0.03 (molar ratio), pH ≤ 1.0, aging temperature ≤ 80 °C, and Ti/Si ≤ 0.01. It was found that the hydrolysis rate of TMOS was remarkably accelerated by fluoride, which was suggested to play the main role in the formation of Ti−SBA-15 materials of high quality. There is no stoichiometric incorporation of Ti, and the Ti contents that are obtained are quite low in the case of the approach that is proposed. The calcined Ti−SBA-15 m...

Direct synthesis and characterization of titanium-substituted mesoporous molecular sieve SBA-15

A [(C(18)H(37))(2)N(+)(CH(3))(2)](3)[PW(12)O(40)] catalyst, assembled in an emulsion in diesel, can selectively oxidize the sulfur-containing molecules present in diesel into their corresponding sulfones by using H(2)O(2) as the oxidant under mild conditions. The sulfones can be readily separated from the diesel using an extractant, and the sulfur level of the desulfurized diesel can be lowered from about 500 ppm to 0.1 ppm without changing the properties of the diesel. The catalyst demonstrates high performance (>/=96 % efficiency of H(2)O(2), is easily recycled, and approximately 100 % selectivity to sulfones). Metastable emulsion droplets (water in oil) act like a homogeneous catalyst and are formed when the catalyst (as the surfactant) and H(2)O(2) (30 %) are mixed in the diesel. However, the catalyst can be separated from the diesel after demulsification.

Ultra-deep desulfurization of diesel: oxidation with a recoverable catalyst assembled in emulsion.

Ce 1− X Ni X O 2 oxides with X varying from 0.05 to 0.5 were prepared by different methods and characterized by XRD and TPR techniques. Ce 0.7 Ni 0.3 O 2 sample prepared by sol–gel method shows the highest reducibility and the highest catalytic activity for methane combustion. Three kinds of Ni phases co-exist in the Ce 1− X Ni X O 2 catalysts prepared by sol–gel method: (i) aggregated NiO on the support CeO 2 , (ii) highly dispersed NiO with strong interaction with CeO 2 and (iii) Ni atoms incorporated into CeO 2 lattice. The distribution of different Ni species strongly depends on the preparation methods. The highly dispersed NiO shows the highest activity for methane combustion. The NiO aggregated on the support CeO 2 shows lower catalytic activity for methane combustion, while the least catalytic activity is found for the Ni species incorporated into CeO 2 . Any oxygen vacancy formed in CeO 2 lattice due to the incorporating of Ni atoms adsorbs and activates the molecular oxygen to form active oxygen species. So the highest catalytic activity for methane combustion on Ce 0.7 Ni 0.3 O 2 catalyst is attributed not only to the highly dispersed Ni species but also to the more active oxygen species formed.

Reduction property and catalytic activity of Ce1-XNiXO2 mixed oxide catalysts for CH4 oxidation

The phase transformation of zirconia from tetragonal to monoclinic is characterized by UV Raman spectroscopy, visible Raman spectroscopy, and XRD. Electronic absorption of ZrO2 in the UV region makes UV Raman spectroscopy more sensitive at the surface region than XRD or visible Raman spectroscopy. Zirconia changes from the tetragonal phase to the monoclinic phase with calcination temperatures elevated and monoclinic phase is always detected first by UV Raman spectroscopy for the samples calcined at lower temperatures than that by XRD and visible Raman spectroscopy. When the phase of zirconia changes from tetragonal to monoclinic, the slight changes of the phase at very beginning can be detected by UV Raman spectroscopy. UV Raman spectra clearly indicate that the phase transition takes place initially at the surface regions. It is found that the phase change from tetragonal to monoclinic is significantly retarded when amorphous Zr(OH)4 was agglomerated to bigger particles and the particle agglomeration of ...

Pinliang Ying

Papers

Surface Phases of TiO2 Nanoparticles Studied by UV Raman Spectroscopy and FT-IR Spectroscopy

Direct synthesis and characterization of titanium-substituted mesoporous molecular sieve SBA-15

Ultra-deep desulfurization of diesel: oxidation with a recoverable catalyst assembled in emulsion.

Reduction property and catalytic activity of Ce1-XNiXO2 mixed oxide catalysts for CH4 oxidation

Phase Transformation in the Surface Region of Zirconia Detected by UV Raman Spectroscopy