Abatement of photochemical smog precursors through complete hydrocarbon oxidation over commercial Pd catalysts under fuel-lean conditions with NO promoting effect.

Currently, severe environmental issues have led to a great transition in the automotive industry from internal combustion engine vehicles to electric vehicles, but this transition will take time more than 10 years, which still requires the use of internal combustion engine vehicles. However, these vehicles emit a significant amount of hydrocarbons, in addition to nitrogen oxides (NOx), due to incomplete fuel combustion. They contribute to the formation of photochemical smog when they react with NOx in the presence of sunlight. To effectively remove these hydrocarbons from the exhaust gas of turbo-gasoline engines or diesel engines, we investigated the abatement of propane and iso-pentane, two typical hydrocarbons. In particular, we studied commercial Pd catalysts and revealed how the Pd loading and aging process simulating 4k and 100k mileage affected hydrocarbon abatement abilities, and their phases were identified using characterization technique, including CO chemisorption, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM). We also suggested the reaction pathway for the complete oxidation of propane over Pd catalyst based on the reaction orders of propane and oxygen: Propane adsorbs on O atoms of PdO, and the kinetically relevant C-H bond cleavage step occurs by the interaction with abundant neighboring O atoms of PdO. Finally, the propane and iso-pentane abatement ability of the Pd catalyst aged for 100k mileage were evaluated under realistic exhaust gas conditions, and the effect of each gas component in the realistic exhaust gas was identified; water inhibits the catalytic reaction of hydrocarbons by occupying the active sites, whereas NO catalyzes the hydrocarbon oxidation reaction by either changing the reaction pathway or active sites under fuel-lean conditions. These findings enable us to effectively reduce environmental pollution and facilitate a smoother transition from internal combustion engine vehicles to electric vehicles.

Transducer-Aware Hydroxy-Rich-Surface Indium Oxide Gas Sensor for Low-Power and High-Sensitivity NO2 Gas Sensing.

Low-power metal oxide (MOX)-based gas sensors are widely applied in edge devices. To reduce power consumption, nanostructured MOX-based sensors that detect gas at low temperatures have been reported. However, the fabrication process of these sensors is difficult for mass production, and these sensors are lack uniformity and reliability. On the other hand, MOX film-based gas sensors have been commercialized but operate at high temperatures and exhibit low sensitivity. Herein, commercially advantageous highly sensitive, film-based indium oxide sensors operating at low temperatures are reported. Ar and O2 gases are simultaneously injected during the sputtering process to form a hydroxy-rich-surface In2O3 film. Conventional indium oxide (In2O3) films (A0) and hydroxy-rich indium oxide films (A1) are compared using several analytical techniques. A1 exhibits a work function of 4.92 eV, larger than that of A0 (4.42 eV). A1 exhibits a Debye length 3.7 times longer than that of A0. A1 is advantageous for gas sensing when using field effect transistors (FETs) and resistors as transducers. Because of the hydroxy groups present on the surface of A1, A1 can react with NO2 gas at a lower temperature (∼100 °C) than A0 (180 °C). Operando diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) shows that NO2 gas is adsorbed to A1 as nitrite (NO2-) at 100 °C and nitrite and nitrate (NO3-) at 200 °C. After NO2 is adsorbed as nitrate, the sensitivity of the A1 sensor decreases and its low-temperature operability is compromised. On the other hand, when NO2 is adsorbed only as nitrite, the performance of the sensor is maintained. The reliable hydroxy-rich FET-type gas sensor shows the best performance compared to that of the existing film-based NO2 gas sensors, with a 2460% response to 500 ppb NO2 gas at a power consumption of 1.03 mW.

Excellent activity and selectivity of Pd/ZSM-5 catalyst in the selective catalytic reduction of NOx by H2.

Superior de-NOx activity and N2 selectivity of the Pd/ZSM-5 catalyst was observed at low temperature (<200 °C) for the selective catalytic reduction of NOx by H2 (H2-SCR). Various Pd/ZSM-5 catalysts were prepared by calcinating at different temperatures (e.g., 500 °C, 650 °C, 750 °C, and 850 °C) and treated at reductive conditions before the H2-SCR reaction was performed. Among the prepared catalysts, the one prepared at the calcination temperature at 750 °C resulted in 96.7% NOx conversion and 96.8% N2 selectivity at 150 °C. Based on the H2-O2 reaction, the higher activity of the Pd/ZSM-5 catalyst calcined at 750 °C was attributed to its superior H2 activation ability for the H2-SCR reaction. The combined X-ray diffraction (XRD), temperature-programmed hydride decomposition (TPHD), and transmission electron microscopy (TEM) results revealed that highly dispersed Pd particles were generated on the catalyst calcined at 750 °C, while large Pd agglomerates were formed on the one calcined at 500 °C. It can be concluded that the catalytic activity of Pd/ZSM-5 improves by optimizing the calcination temperature, resulting in high Pd dispersion. Moreover, the Pd catalyst calcined at 750 °C showed high resistance to CO, maintaining >94% NOx conversion at 175 °C under 1000 ppm CO in the feed gas. Therefore, the catalyst calcined at 750 °C can be potentially used for industrial applications because of its simple preparation method and high resistance to CO.

In situ spectroscopic studies of the effect of water on the redox cycle of Cu ions in Cu-SSZ-13 during selective catalytic reduction of NOx.

The effect of water on the NH3-assisted selective catalytic reduction of NOx (NH3-SCR) has been largely neglected, despite the inevitable presence of water vapor in real emissions produced by fuel combustion. In this work, we investigated the role of water in the behavior of active Cu2+ ions in Cu-SSZ-13 in the NH3-SCR reaction. The addition of water to the reactant feed leads to significantly increased NOx reduction over the catalyst. By combining in situ DRIFTS and XANES analyses during the NH3-SCR reaction, we found that the redox cycle of Cu ions is promoted by the presence of water.

Upgrading bio-oil model compound over bifunctional Ru/HZSM-5 catalysts in biphasic system: Complete hydrodeoxygenation of vanillin.

A complete hydrodeoxygenation(HDO) of vanillin to yield cycloalkanes was performed using bifunctional Ru loaded HZSM-5 catalysts with different metal loadings (0.1, 0.5, 1, 3, and 5 wt%) and Si/Al2 ratios (Si/Al2 = 23,300) in n-octane/water biphasic system. Both the reaction pathway and product distribution were influenced by the metal/acid balance of the catalysts. Higher metal/acid ratio promoted Caryl-C cleavage reaction, resulting in the increased yield of cyclohexane. Synergetic effect of metal and acid sites was observed in the bifunctional catalyst, attaining as high as 40-fold increase of metal efficiency in the ring hydrogenation reaction, compared to lone metal site catalyst. The effect of solvent composition was evaluated, revealing that the presence of water promoted the overall HDO reaction. By balancing metal/acid and introducing appropriate solvent system, efficient catalytic system that minimized carbon loss and improved metal efficiency for vanillin HDO was obtained.

Mobility of Cu Ions in Cu-SSZ-13 Determines the Reactivity of Selective Catalytic Reduction of NOx with NH3.

Selective catalytic reduction of NOx with NH3 (NH3-SCR) in Cu-SSZ-13 has been proposed to have a unique homogeneous-like mechanism governed by the spatial proximity of mobile Cu ions. Among factors that determine the proximity, the effect of ion density on the SCR reaction is well established; however, it has not been verified how the different mobility of the Cu ion influences the SCR reaction. Herein, we try to reveal the mobility-dependent SCR reaction by controlling the Cu species with different ion mobilities in Cu-SSZ-13. Since the reaction kinetics is governed by the diffusion of Cu ions, the Cu ion mobility determines the reactivity of the Cu-SSZ-13. In terms of this correlation, enhanced ion mobility leads to improved NH3-SCR activity. These findings help understand the behavior of Cu ions in Cu-SSZ-13 under a catalytic reaction and provide insights to design rational catalysts by tuning the ion mobility.

Enhanced activity of vanadia supported on microporous titania for the selective catalytic reduction of NO with NH3: Effect of promoters.

Vanadium oxide-based catalysts are considered a promising catalyst for selective catalytic reduction (SCR) of NO with NH3, which is an effective NOx removal technology. As environmental issues have garnered more attention, however, improvements to vanadium-based SCR catalysts are strongly required. In a previous study, we found that vanadium oxide on microporous titania as a support (V/MPTiO2) has certain advantages, such as improved thermal stability and more suppressed N2O formation, over the use of conventional nanoparticle titania (DT-51) as a support. In this study, widely used promoters, such as W, Sb, and Mo, were added to V/MPTiO2 to investigate whether they have promoting effects on V/MPTiO2 as well. Among these promoters added catalysts, the W and Mo were found to have significant promoting effects on the enhancement of deNOx activities at low temperatures, while the addition of Sb to V/MPTiO2 tended to have a negative effect on the SCR activity. Based on the characterizations, including laser Raman, H2-temperature programmed reduction (H2-TPR), and in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) analysis, we found that the addition of W and Mo increased the degree of polymerization in V/MPTiO2, which generated more reactive vanadia species. Hence, such changes, resulting from the addition of W and Mo promoters to V/MPTiO2, yielded enhanced catalytic activity at low temperatures.

Simple physical mixing of zeolite prevents sulfur deactivation of vanadia catalysts for NOx removal.

NOx abatement has been an indispensable part of environmental catalysis for decades. Selective catalytic reduction with ammonia using V2O5/TiO2 is an important technology for removing NOx emitted from industrial facilities. However, it has been a huge challenge for the catalyst to operate at low temperatures, because ammonium bisulfate (ABS) forms and causes deactivation by blocking the pores of the catalyst. Here, we report that physically mixed H-Y zeolite effectively protects vanadium active sites by trapping ABS in micropores. The mixed catalysts operate stably at a low temperature of 220 °C, which is below the dew point of ABS. The sulfur resistance of this system is fully maintained during repeated aging/regeneration cycles because the trapped ABS easily decomposes at 350 °C. Further investigations reveal that the pore structure and the amount of framework Al determined the trapping ability of various zeolites.

Time-resolved observation of V2O5/TiO2 in NH3-SCR reveals the equivalence of Brønsted and Lewis acid sites.

The involvement of Lewis and Brønsted acid sites on V2O5/TiO2 catalyst in the selective catalytic reduction of NO with NH3 (NH3-SCR) is under debate. Here, a Li doping strategy is applied to selectively block Brønsted sites, which aims to prepare model catalysts with the same V loading but different ratios of the two acid sites. Time-resolved in situ DRIFTS observation demonstrates that the surface ammonia species pre-adsorbed on Lewis and Brønsted sites can participate equally in the reaction. Consideration of site redistribution in the early stages of the transient reaction is key to accurate measurement of the ammonia consumption rate.

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.

One-pot conversion of alginic acid into furfural using Amberlyst-15 as a solid acid catalyst in γ-butyrolactone/water co-solvent system.

One-pot conversion of alginic acid, which was derived from brown algae, to furfural was investigated using various solid acid catalysts. Among the solid acid catalysts tested, Amberlyst-15 showed the highest activity in furfural production in aqueous media. When the effect of reaction media was examined by applying various organic solvent mixtures, it was found that γ-butyrolactone/water co-solvent system was selected as the most appropriate system for the reaction. Maximum furfural yield of 32.2% was obtained using Amberlyst-15 in the γ-butyrolactone/H2O at 210 °C for 20 min. Catalyst showed gradual deactivation behavior as the reaction proceeded, although the catalyst recovered its activity upon the simple treatment with sulfuric acid. N2 adsorption-desorption experiments, Fourier-transform infrared spectroscopy (FT-IR), back titration, and CHNS analysis were applied to investigate the physicochemical property of post-reaction samples, confirming that the leaching of the active sulfonic acid group and decrease in acid density was the major cause of deactivation.

Oxidative Methane Conversion to Ethane on Highly Oxidized Pd/CeO2 Catalysts Below 400 °C.

Methane upgrading into more valuable chemicals has received much attention. Herein, we report oxidative methane conversion to ethane using gaseous O2 at low temperatures (<400 °C) and atmospheric pressure in a continuous reactor. A highly oxidized Pd deposited on ceria could produce ethane with a productivity as high as 0.84 mmol gcat -1 h-1 . The Pd-O-Pd sites, not Pd-O-Ce, were the active sites for the selective ethane production at low temperatures. Density functional theory calculations confirmed that the Pd-O-Pd site is energetically more advantageous for C-C coupling, whereas Pd-O-Ce promotes CH4 dehydrogenation. The ceria helped Pd maintain a highly oxidic state despite reductive CH4 flow. This work can provide new insight for methane upgrading into C2 species.

Effect of Reactant Ratios on Methane Oxychlorination Over CeO2 Catalyst.

The oxidative chlorination of methane to produce chloromethane (CH3Cl, CM) over CeO2 catalyst was investigated as a function of various reactant ratios to obtain the optimum condition with the highest yield of the target product (CH3Cl). The reactions were carried out at 480 °C by changing O2 and HCl concentrations at fixed CH4 concentration. It was found that the ratio of O2:HCl (1:2) is the most important parameter to obtain high CH4 conversion and CH3Cl selectivity. Note that excessive HCl gives rise to the formation of the increased amount of more chlorinated products, such as CH2Cl2 or CHCl3, due to the subsequent chain reaction with HCl and excessive O2 promoting the oxidation of methane to CO and CO2. The optimum reactant ratio of CH4:O2:HCl was 4:1.5:3, with the highest CH3Cl yield (14.3%). Various characterization results indicated that CeO2 catalyst was slightly sintered, and had no carbon and chlorine deposition on the surface after the reaction.

Rotation-Assisted Hydrothermal Synthesis of Thermally Stable Multiwalled Titanate Nanotubes and Their Application to Selective Catalytic Reduction of NO with NH3.

Titanate nanotubes are widely applied in various fields, including photocatalysts and electronic devices, but their weak thermal stability limits their application for catalyst support. Here, we found that titanate nanotubes with a thick multiwalled structure of 15 layers or more can be prepared by using rotation-assisted hydrothermal synthesis. The porous structure of conventional nanotubes synthesized without rotation collapsed easily after thermal treatment, whereas the nanotubes having a thick multiwalled structure retained their pore structure and the specific surface area (∼300 m2/g) even after calcination at 400 °C in air. Systematic variation of rotation speed suggested that rotation in the synthesis process accelerated the stacking of layered titanate nanosheets, which are known to be intermediates of nanotubes. Thus, the rapid assembly of titanate nanosheets facilitated by rotation led to the formation of nanotubes with a multiwalled structure. Overly fast rotation, however, caused excessive stacking and created a thicker structure that cannot be easily wrapped into nanotubes. Therefore, it is essential to maintain the optimum rotation speed to obtain both the nanotube morphology and the thick multiwalled structure. Vanadium-tungsten-oxide catalyst supported on the multiwalled titanate nanotubes was used in NH3-selective catalytic reduction, which showed stable NO x reduction performance with high selectivity to N2, which may originate from the suppressed sintering of VO x on multiwalled nanotubes. This study demonstrates that the morphology of nanotubes can be tuned by controlling the degree of interaction supplied by external forces.

Direct methanol synthesis from methane in a plasma-catalyst hybrid system at low temperature using metal oxide-coated glass beads.

A plasma-catalyst hybrid system was used to synthesize methanol directly from methane. A dielectric barrier discharge (DBD) plasma combined with the catalyst was introduced in order to overcome the difficulties of catalyst-only batch reactions such as high reaction pressure and separation of liquid product. Of the transition metal oxides, Mn2O3-coated glass bead showed the highest methanol yield of about 12.3% in the plasma-catalyst hybrid system. The reaction temperature was maintained below 100 °C due to the low plasma input power (from 1.3 kJ/L to 4.5 kJ/L). Furthermore, the reactivity of the catalyst was maintained for 10 hr without changing the selectivity. The results indicated that the plasma-induced OH radical might be produced on the Mn2O3 catalyst, which led to methanol synthesis.

Investigation on the enhanced catalytic activity of a Ni-promoted Pd/C catalyst for formic acid dehydrogenation: effects of preparation methods and Ni/Pd ratios.

In this present work, we studied the effects of preparation methods and Ni/Pd ratios on the catalytic activity of a Ni-promoted Pd/C catalyst for the formic acid dehydrogenation (FAD) reaction. Two catalysts prepared by co-impregnation and sequential impregnation methods showed completely different Pd states and catalytic activities. As the sequentially impregnated catalyst showed better activity than the co-impregnated catalyst, the sequentially impregnated catalyst was investigated further to optimize the ratio of Ni/Pd. The highest catalytic activity for the FAD reaction was obtained over the seq-impregnated catalyst having a 1 : 1.3 molar ratio of Pd : Ni. The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that small particle size is one factor improving the catalytic activity, while those of X-ray photoelectron spectroscopy (XPS) and X-ray adsorption near edge structure (XANES) indicate that the electronic modification of Pd to a positively charged ion is another factor. Thus, it can be concluded that the enhanced catalytic activity of the Ni-promoted Pd/C catalyst is attributed to the role of pre-impregnated Ni in facilitating the activity of Pd by constraining the particle growth and withdrawing an electron from Pd.

Catalytic Hydrogenation of Macroalgae-Derived Alginic Acid into Sugar Alcohols.

Alginic acid, a major constituent of macroalgae, iss hydrogenated into sugar alcohols over carbon-supported noble metals for the first time. Mannitol and sorbitol are the major products of the catalytic hydrogenation of alginic acid, which consists of two epimeric uronic acids. The main reaction pathway is the consecutive hydrogenations of the aldehyde and carboxyl ends of alginic acid dimers, followed by the cleavage of the C-O-C linkage into monomeric units by hydrolysis. The highest yield of C6 sugar alcohols is 61 % (sorbitol: 29 %; mannitol: 28 %; galactitol: 4 %). The low sorbitol/mannitol ratio is in contrast to that from cellulose hydrogenation, owing to the composition of alginic acid and isomerization between sugar alcohols under the catalytic system. This new green route to producing sugar alcohols from alginic acid might provide opportunities to diversify biomass resources.

Roles of Promoters in V2O5/TiO2 Catalysts for Selective Catalytic Reduction of NOx with NH3: Effect of Order of Impregnation.

Recently, various promoters for commercial selective catalytic reduction (SCR) catalysts are used to improve DeNOx activity at low temperature. We aimed at finding the optimum condition to prepare V2O5/TiO2 catalyst by changing promoters (W, Ce, Zr and Mn), not only for improving SCR reactivity, but also for reducing N2O formation at high temperature. In addition, we changed the order of impregnation between promoter and vanadium precursors on TiO2 support and observed its effect on activity and N2O selectivity. We utilized various analytical techniques, such as N2 adsorption-desorption, X-ray Diffraction (XRD), Raman spectroscopy, UV-visible Diffuse Reflectance Spectroscopy (UV-vis DRS) and Temperature Programmed Reduction with hydrogen (H2-TPR) to investigate the physicochemical properties of V2O5/TiO2 catalysts. It was found that W and Ce added V2O5/TiO2 catalysts showed the most active DeNOx properties at low temperature. Additionally, the difference in impregnation order affected the SCR activity. The superiority of low temperature activity of the vanadium firstly added catalysts (W or Ce/V/TiO2) is attributed to the formation of more polymerized V2O5 on the sample.

Sulfation and Desulfation Behavior of Pt-BaO/MgO-Al2O3 NOx Storage Reduction Catalyst.

The comparative study between Pt-BaO/Al2O3 and Pt-BaO/MgO-Al2O3 gives the information about the effect of MgO addition to Al2O3 support on the sulfation and desulfation behavior of Pt-BaO/MgO-Al2O3 NOx storage reduction catalyst. The sulfated two samples were analyzed by using element analysis (EA), X-ray diffraction (XRD), H2 temperature programmed reaction (H2 TPRX) and NOx uptake measurement. The amount of sulfur uptake on 2 wt% Pt-20 wt% BaO/Al2O3 and 2 wt% Pt-20 wt% BaO/MgO-Al2O3 are almost identical as 0.45 and 0.40 of S/Ba, respectively, which yields the drastic decrease in NOx uptake for both sulfated samples. However, after desulfa- tion with H2 at 600 degrees C, the residual sulfur amount on MgO-Al2O3 supported catalyst is three times larger than that on Al2O3 supported one, indicating that sulfur species formed on the former are more stable than those on the latter. It is also well corresponding to the H2 TPRX results where the main H2S peak from MgO-Al2O3 supported sample is observed at higher temperature than Al2O3 supported one, resulting in the lower NOx uptake activity of former sample than the latter one. Meanwhile, after desulfation of MgO-Al2O3 supported sample at 700 degrees C and 800 degrees C, the activity is recovered more significantly due to the removal of the large amount of sulfur while Al2O3 supported one decreases monotonically due to the sintering of Pt crystallite and the formation of BaAl2O4 phase. It is summarized that MgO-Al2O3 supported catalyst enhances the thermal stability of the catalyst, however, forms the stable sulfate species, which needs to be improved to develop the more sulfur resistant NSR catalyst system.

Suppressed N2O formation during NH3 selective catalytic reduction using vanadium on zeolitic microporous TiO2.

Emission of N2O from mobile and off-road engine is now being currently regulated because of its high impact compared to that of CO2, thereby implying that N2O formation from the exhaust gas after-treatment system should be suppressed. Selective catalytic reduction using vanadium supported TiO2 catalyst in mobile and off-road engine has been considered to be major source for N2O emission in the system. Here we have demonstrated that vanadium catalyst supported on zeolitic microporous TiO2 obtained from the hydrothermal reaction of bulk TiO2 at 400 K in the presence of LiOH suppresses significantly the N2O emission compared to conventional VOx/TiO2 catalyst, while maintaining the excellent NOx reduction, which was ascribed to the location of VOx domain in the micropore of TiO2, resulting in the strong metal support interaction. The use of zeolitic microporous TiO2 provides a new way of preparing SCR catalyst with a high thermal stability and superior catalytic performance. It can be also extended further to the other catalytic system employing TiO2-based substrate.