Highly Selective Tandem Photoelectrochemical C-H Activation via Bromine Evolution Reaction in Two-Phase Electrolyte.

The development of environmentally friendly and safe chemical processes using renewable energy sources is important. In this study, a photoelectrochemical (PEC) cell was used for the tandem bromination of sp3 carbon within a unique two-phase electrolyte system. By incorporation of a RuOx cocatalyst, the Ta3N5 photoelectrode demonstrated a remarkable selectivity for Br2 close to 100%. The kinetic study for charge carriers of photoelectrodes reveals that the improved charge transfer at Ta3N5/RuOx interfaces contributed to excellent photoelectrochemical Br2 evolution activity. The photoelectrochemically produced Br2 was utilized for bromination of α-sp3 carbon in toluene, 1-methylnaphthalene, ethylbenzene, or cyclohexane by the Ta3N5/RuOx photoanode with 100% regioselectivity. The coupling of the Ta3N5 photoanode and InP photocathode generated H2 and Br2 under light illumination without external bias. This study provides systematic insights into the design of photoelectrodes for solar-driven tandem bromination systems within the unique environment of a two-phase electrolyte system.

Origin of Enhanced Photoelectrochemical Activity of the n-CdS/p-type Semiconductor Interface: Homojunction or Heterojunction?

Surface engineering of photoelectrodes is considered critical for achieving efficient photoelectrochemical (PEC) cells, and various p-type materials have been investigated for use as photoelectrodes. Among these, the p-type semiconductor/n-type CdS heterojunction is the most successful photocathode structure because of its enhanced onset potential and photocurrent. However, it is determined that the main contributor to the enhanced activity is the Cd-doped layer and not the CdS layer. In this study, a Cd-doped n+p-buried homojunction of a CuInS2 photocathode is first demonstrated without a CdS layer. The homojunction exhibited a more active and stable PEC performance than the CdS/CuInS2 heterojunction. Moreover, it is confirmed that Cd doping is effective for other p-type materials. These results strongly suggest that the effects of Cd doping on photocathodes should be carefully investigated when designing CdS/p-semiconductor heterojunction photoelectrodes. They also indicate that the Cd-doped layer has great potential to replace the CdS layer in future photoelectrode designs.

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia.

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

Monitoring Transformations of Catalytic Active States in Photocathodes Based on MoSx Layers on CuInS2 Using In Operando Raman Spectroscopy.

The electrochemical activation of CuInS2 /MoSx for photoelectrochemical (PEC) H2 production was revealed for the first time through in operando Raman spectroscopy. During the activation process, the initial metallic MoSx phase was transformed to semiconducting MoSx , which facilitates charge carrier transfer between CuInS2 and MoSx . Ex situ X-ray photoelectron spectroscopy and Raman spectroscopy suggest the existence of MoO3 after the activation process. However, apart from contradicting these results, in operando Raman spectroscopy revealed some of the intermediate steps of the activation process.

CO and CO2 Methanation Over Ni/γ-Al2O3 Prepared by Deposition-Precipitation Method.

Various Ni catalysts supported on γ-Al2O3 were prepared by a wet impregnation (WI) method and deposition-precipitation (DP) method with different precipitants and applied to CO and CO2 methanation. The prepared catalysts were characterized by various techniques including nitrogen physisorption, X-ray diffraction (XRD), temperature-programmed reduction with H2 (H2-TPR), H2 chemisorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Irrespective of kinds of precipitant, the Ni/γ-Al2O3 catalysts prepared with a DP method showed a remarkable enhanced catalytic performance in CO and CO2 methanation compared with the Ni/γ-Al2O3 catalyst prepared with a WI method owing to the higher catalytic active surface area (CASA). In the case of Ni/γ-Al2O3 catalysts prepared with a DP method, the high calcination temperatures are not favorable for the high catalytic activity due to the decreased reduction degree of Ni oxide species and CASA. The reduction degree of Ni oxide species can be increased with reduction temperature. However, the higher reduction temperature above 500 °C is not desirable to achieve the high catalytic activity because of the decreased CASA. The selective CO methanation was also accomplished at lower temperatures over the Ni/γ-Al2O3 catalyst prepared with a DP method than over the Ni/γ-Al2O3 catalyst prepared with a WI method.

Effect of Gold Particle Size on Steam Reforming of Methanol Over Au/CeO2-ZrO2 Catalysts.

We examined the effect of the particle size of gold on steam reforming of methanol over Au/CeO2-ZrO2 catalysts. Gold was loaded onto CeO2-ZrO2 through deposition-precipitation. The average particle size (2-12 nm) of the gold was controlled by thermal reduction under H2 at various temperatures and by chemical reduction with various reducing agents. The catalytic activity decreased significantly with increasing particle size of the gold. The turnover frequency at the interface between gold and a support appeared to be independent of particle size in the range 2-5 nm, which implies that the perimeter of the particle may be the active site for this reaction. Methanol adsorption and conversion over these catalysts were also investigated with in-situ diffuse reflectance infrared Fourier transform spectroscopy. Analytical results for various adsorbed intermediate species during methanol conversion suggests that transformation of methoxy species is facilitated by use of smaller gold particles.

Recent Advances in CuInS2-Based Photocathodes for Photoelectrochemical H2 Evolution.

Photoelectrochemical (PEC) H2 production from water using solar energy is an ideal and environmentally friendly process. CuInS2 is a p-type semiconductor that offers many advantages for PEC H2 production. Therefore, this review summarizes studies on CuInS2-based PEC cells designed for H2 production. The theoretical background of PEC H2 evolution and properties of the CuInS2 semiconductor are initially explored. Subsequently, certain important strategies that have been executed to improve the activity and charge-separation characteristics of CuInS2 photoelectrodes are examined; these include CuInS2 synthesis methods, nanostructure development, heterojunction construction, and cocatalyst design. This review helps enhance the understanding of state-of-the-art CuInS2-based photocathodes to enable the development of superior equivalents for efficient PEC H2 production.

CuInS2 Photocathodes with Atomic Gradation-Controlled (Ta,Mo)x(O,S)y Passivation Layers for Efficient Photoelectrochemical H2 Production.

An atomic gradient passivation layer, (Ta,Mo)x(O,S)y, is designed to improve the charge transportation and photoelectrochemical activity of CuInS2-based photoelectrodes. We found that Mo spontaneously diffused to the a-TaOx layer during e-beam evaporation. This result indicates that the gradient profile of MoOx/TaOx is formed in the sublayer of (Ta,Mo)x(O,S)y. To understand the atomic-gradation effects of the (Ta,Mo)x(O,S)y passive layer, the composition and (photo)electrochemical properties have been characterized in detail. When this atomic gradient-passive layer is applied to CuInS2-based photocathodes, promising photocurrent and onset potential are seen without using Pt cocatalysts. This is one of the highest activities among reported CuInS2 photocathodes, which are not combined with noble metal cocatalysts. Excellent photoelectrochemical activity of the photoelectrode can be mainly achieved by (1) the electron transient time improved due to the conductive Mo-incorporated TaOx layer and (2) the boosted electrocatalytic activity by Mox(O,S)y formation.

Catalytic hydrogenolysis of alkali lignin in supercritical ethanol over copper monometallic catalyst supported on a chromium-based metal-organic framework for the efficient production of aromatic monomers.

The catalytic hydrogenolysis of lignin has been reported as an effective approach for lignin depolymerization owing to its high efficiency for aromatic monomer production. In this study, a series of copper monometallic catalysts over an MIL-101(Cr) support were synthesized and used for the catalytic hydrogenolysis of alkali lignin using supercritical ethanol. First, the optimal copper catalyst for lignin hydrogenolysis was selected. Subsequently, the reaction conditions for catalytic hydrogenolysis were systematically optimized to maximize the total monomer yield. The optimal conditions were determined to be 6 h of reaction time, 20 min of sonication pretreatment, 50% catalyst loading, and 5% lignin loading. Under these conditions, an aromatic monomer yield of 38.5% was obtained; this depolymerized lignin stream, which is mainly composed of G-type monomers, can serve as a promising aromatic feedstock and carbon source for further microbial upgrading and bioconversion to produce various value-added products.

Interactions between tetrahydrothiophene (THT) and silver species in AgNa-Y.

We studied the interaction between tetrahydrothiophene (THT), which is one of sulfur odorants in a pipeline natural gas, and AgNa-Y zeolites, which can be obtained via Ag+ exchange with Na+ of Na-Y and used for the adsorptive removal of THT at ambient temperature and atmospheric pressure, with a temperature-programmed desorption (TPD), a temperature-programmed reduction with H2 (H2-TPR), a transmission electron microscopy (TEM) with an energy dispersive X-ray spectroscopy (EDX) and an X-ray absorption fine structure (XAFS). The presence of the metallic Ag and the Ag+ in the fresh AgNa-Y can be supported with a TEM and an XAFS analysis. The fraction of metallic Ag in silver species increased with increasing the pretreatment temperature. The formation of Ag-S bond with concomitant decreasing the interaction between Ag+ and oxygen in the lattice as well as the interaction of Ag-Ag in the metallic Ag can be observed during the adsorption of THT at ambient temperature. This can explain why there is no noticeable difference in the adsorption capacity in between Ag+ -dominant AgNa-Y and Ag0-rich AgNa-Y. This Ag-S bond was transformed into the Ag-Ag bond during the heat treatment in an inert gas above 673 K. However, the fresh chemical and electronic state of Ag can be recovered after the heat treatment in air above 673 K.

Recent advances in catalytic co-pyrolysis of biomass and plastic waste for the production of petroleum-like hydrocarbons.

The global economy is threatened by the depletion of fossil resources and fluctuations in fossil fuel prices, and thus it is necessary to exploit sustainable energy sources. Carbon-neutral fuels including bio-oil obtained from biomass pyrolysis can act as alternatives to fossil fuels. Co-pyrolysis of lignocellulosic biomass and plastic is efficient to upgrade the quality of bio-oil because plastic facilitates deoxygenation. However, catalysts are required to produce bio-oil that is suitable for potential use as transportation fuel. This review presents an overview of recent advances in catalytic co-pyrolysis of biomass and plastic from the perspective of chemistry, catalyst, and feedstock pretreatment. Additionally, this review introduces not only recent research results of acid catalysts for catalytic co-pyrolysis, but also recent approaches that utilize base catalysts. Future research directions are suggested for commercially feasible co-pyrolysis process.

Enhancement of aromatics from catalytic pyrolysis of yellow poplar: Role of hydrogen and methane decomposition.

The present study examined the effects of the pyrolysis environment on BTEX (benzene, toluene, ethylbenzene, and xylenes) production in the catalytic upgrading of yellow poplar pyrolysis vapors. Three different gas environments, N2, CH4, and pre-decomposed CH4 stream (10 wt%-Ni/5 wt%-La2O3-5 wt% CeO2-Al2O3), which is a mixture of H2 (55.62%) and CH4, were studied using two types of zeolite catalysts, HZSM-5, and 1 wt% Ga/HZSM-5. The BTEX yields were enhanced linearly in the order N2 < CH4 < CH4 ex-situ decomposition. The highest BTEX yield of 9.58 wt% was obtained under the CH4 ex-situ decomposition environment over 1 wt% Ga/HZSM-5. The methane and hydrocarbons derived from biomass were activated on highly dispersed (GaO)+ sites and transformed smoothly to BTEX by aromatization on the BrØnsted acid sites of Ga/HZSM-5. The hydrogen produced from methane decomposition also assisted in aromatics production through the hydrodeoxygenation of methoxyphenols, guaiacols and catechols.

Recent Progress in Direct Conversion of Methane to Methanol Over Copper-Exchanged Zeolites.

The conversion of methane into an easily transportable liquid fuel or chemicals has become a highly sought-after goal spurred by the increasing availability of cheap and abundant natural gas. While utilization of methane for the production of syngas and its subsequent conversion via an indirect route is typical, it is cost-intensive, and alternative direct conversion routes have been investigated actively. One of the most promising directions among these is the low-temperature partial oxidation of methane to methanol over a metal-loaded zeolite, which mimics facile enzymatic chemistry of methane oxidation. Thus mono-, bi-, and trinuclear oxide compounds of iron and copper stabilized on ZSM-5 or mordenite, which are structurally analogous to those found in methane monooxygenases, have demonstrated promising catalytic performances. The two major problems of theses metal-loaded zeolites are low yield to methanol and batch-like non-catalytic reaction systems challenging to extend to an industrial scale. In this mini-review, attention was given to the direct methane oxidation to methanol over copper-loaded zeolite systems. A brief introduction on the catalytic methane direct oxidation routes and current status of the applied metal-containing zeolites including the ones with copper ions are given. Next, by analyzing the extensive experimental and theoretical data available, the consensus among the researchers to achieve the target of high methanol yield is discussed in terms of zeolite topology, active species, and reaction parameters. Finally, the recent efforts on continuous methanol production from the direct methane oxidation aiming for an industrial process are summarized.

Enhanced Selectivity for CO2 Adsorption on Mesoporous Silica with Alkali Metal Halide Due to Electrostatic Field: A Molecular Simulation Approach.

Since adsorption performances are dominantly determined by adsorbate-adsorbent interactions, accurate theoretical prediction of the thermodynamic characteristics of gas adsorption is critical for designing new sorbent materials as well as understanding the adsorption mechanisms. Here, through our molecular modeling approach using a newly developed quantum-mechanics-based force field, it is demonstrated that the CO2 adsorption selectivity of SBA-15 can be enhanced by incorporating crystalline potassium chloride particles. It is noted that the induced intensive electrostatic fields around potassium chloride clusters create gas-trapping sites with high selectivity for CO2 adsorption. The newly developed force field can provide a reliable theoretical tool for accurately evaluating the gas adsorption on given adsorbents, which can be utilized to identify good gas adsorbents.

Nanosized Pt-Co catalysts for the preferential CO oxidation.

The preferential CO oxidation in the presence of excess hydrogen was studied over Pt-Co/gamma-Al2O3. CO chemisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX) and temperature programmed reduction (TPR) were conducted to characterize active catalysts. The catalytic activity for CO oxidation and methanation at low temperatures increased with the amounts of cobalt in Pt-Co/gamma-Al2O3. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Co and Pt was determined to be 10. The co-impregnated Pt-Co/gamma-Al2O3 appeared to be superior to Pt/Co/gamma-Al2O3 and Co/Pt/gamma-Al2O3. The reductive pretreatment at high temperature such as 773 K increased the CO2 selectivity over a wide reaction temperature. The bimetallic phase of Pt-Co seems to give rise to high catalytic activity in selective oxidation of CO in H2-rich stream.

Direct conversion of cellulose into polyols over Ni/W/SiO2-Al2O3.

The direct conversion of cellulose into polyols over Ni/W/SiO(2)-Al(2)O(3) catalysts with different Al molar fractions was examined. For comparison, Cu/W/SiO(2)-Al(2)O(3), Fe/W/SiO(2)-Al(2)O(3), and Co/W/SiO(2)-Al(2)O(3) were also evaluated. The bulk crystalline structure was determined using X-ray diffraction (XRD). The surface acidity was probed via temperature-programmed desorption of ammonia (NH(3)-TPD). The textural properties were investigated using N(2) physisorption. The metal contents were confirmed via inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Among the various metal catalysts, Ni/W/SiO(2)-Al(2)O(3) was confirmed to be the most favorable for hydrogenolysis of cellulose into polyols. The effect of the Al molar fraction in SiO(2)-Al(2)O(3) on this reaction over Ni/W/SiO(2)-Al(2)O(3) was also investigated. It was found that the polyol yield was closely related to the total acidity of the support. Compared to Ni/W/SBA-15, Ni/W/SiO(2)-Al(2)O(3) (Al/(Al+Si)=0.6) showed better stability during the recycling test. The catalyst deactivation was confirmed to be caused by metal leaching.