Progress on metal-support interactions in Pd-based catalysts for automobile emission control.

The interactions between metals and oxide supports, so-called metal-support interactions (MSI), are of great importance in heterogeneous catalysis. Pd-based automotive exhaust control catalysts, especially Pd-based three-way catalysts (TWCs), have received considerable research attention owing to its prominent oxidation activity of HCs/CO, as well as excellent thermal stability. For Pd-based TWCs, the dispersion, chemical state and thermal stability of Pd species, which are crucial to the catalytic performance, are closely associated with interactions between metal nanoparticles and their supporting matrix. Progress on the research about MSI and utilization of MSI in advanced Pd-based three-way catalysts are reviewed here. Along with the development of advanced synthesis approaches and engine control technology, the study on MSI would play a notable role in further development of catalysts for automobile exhaust control.

Destructive and Protective Effects of NH3 on the Low-Temperature Hydrothermal Stability of SAPO-34 and Cu-SAPO-34.

The influences of gaseous, weakly adsorbed, and strongly adsorbed NH3 on the low-temperature (<100 °C) hydrothermal stability of SAPO-34 and Cu-SAPO-34 were investigated. NH3 temperature-programmed desorption (NH3-TPD), 1H magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted to characterize the adsorption states of NH3 and H2O in SAPO-34, and the destruction of the SAPO-34 framework was revealed by direct and cross-polarization 29Si, 27Al, and 31P MAS NMR. Gaseous NH3 coadsorbed with H2O inside SAPO-34 micropores and induced the hydrolysis of framework P-O-Al and Si-O(H)-Al bonds. Weakly adsorbed NH3 was released during aging and played a similar negative role to gaseous NH3. When being combined with hydrolyzed Al species from the framework, active Cu ions transformed to inactive CuAl2O4-like species, leading to deactivation in low-temperature SCR of Cu-SAPO-34. Strongly adsorbed NH4+ via 200 °C preadsorption protected the framework integrity of SAPO-34 and the SCR activity of Cu-SAPO-34.

A strategy to construct a highly active CoxP/SrTiO3(Al) catalyst to boost the photocatalytic overall water splitting reactions.

Hydrogen production from overall water splitting using SrTiO3(Al)-based semiconductors is one of the most promising routes to address energy and environmental concerns. Noble metals are needed to accelerate water splitting by promoting the charge transfer and providing active sites. However, noble metal-based catalysts have high prices and rare resources. Herein, we demonstrate a strategy to construct highly active CoxP/SrTiO3(Al) for overall water splitting. Hydrothermal method followed by an ultrasonic process was applied to prepare CoxP dots, which were loaded on the whole surface of SrTiO3(Al) as bifunctional cocatalysts. Interestingly, the CoxP dots on the (110) planes of SrTiO3(Al) were partially oxidized for the OER reaction. However, CoxP dots on the (100) planes of SrTiO3(Al) for HER kept it as it was. The as-prepared CoxP/SrTiO3(Al) photocatalyst shows a stable HER rate of 1.36 mmol-1 h-1 and OER rate of 0.635 mmol-1 h-1. The strong interaction between CoxP and SrTiO3(Al) not only facilitates rapid charge separation but also provides a highly active site for overall water splitting. Our study provides a valuable method for constructing noble-metal-free SrTiO3(Al)-based photocatalysts.

Quasi-operando quantification of Cu(II) ions in Cu-SSZ-13 catalyst by an NH3 temperature-programmed reduction method.

A quasi-operando NH3 temperature-programmed reduction method (NH3-TPR), with N2:Cu = 1:1, is developed to quantify total Cu(ii) ions in Cu-SSZ-13 quenched from SCR-relevant reactions, and its accuracy is confirmed by in situ EPR. [Cu(OH)]+-Z and Cu2+-2Z can be further distinguished by NH3 reduction temperatures, and their different reducibility in SCR is revealed.

Size effect of Pt nanoparticles in acid-assisted soot oxidation in the presence of NO.

Pt/Al2O3 catalysts with mean Pt particle size ranged from 2.7 to 7.1 nm were synthesized by chemical reduction method, and the sulfated counterparts were prepared by impregnation of sulfuric acid. The turnover frequency of platinum for soot oxidation under loose contact conditions in a feed flow containing NO and O2 are positively correlated with the size of platinum. The sulfated Pt/Al2O3 exhibits higher catalytic activity for soot oxidation in the presence of NO despite their reduced ability for NO2 production. Such a contradiction is more significant for those catalysts with smaller platinum particles. Herein, the catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), inductive coupled plasma (ICP) emission spectrometry, CO chemisorption, thermogravimetric analysis (TGA), NH3 temperature-programmed desorption (NH3-TPD), NO temperature-programmed oxidation (TPO) and NOx temperature-programmed desorption (TPD). Possible effect of Pt particle size for the catalytic oxidation of soot in the presence of NO was presented based primarily on the promoted NO2 transfer efficiency onto the soot pushed by the acidic catalysts.

MOF-derived (MoS2, γ-Fe2O3)/graphene Z-scheme photocatalysts with excellent activity for oxygen evolution under visible light irradiation.

Constructing Z-scheme heterojunctions is considered as an effective strategy to obtain catalysts of high efficiency in electron-hole separation in photocatalysis. Unfortunately, suitable heterojunctions are difficult to fabricate because the direct interaction between two semiconductors may lead to unpredictable negative effects such as electron scattering or electron trapping due to the existence of defects which causes the formation of new substances. Furthermore, the van der Waals contact between two semiconductors also results in bad electron diffusion. In this work, a MOF-derived carbon material as a Z-scheme photocatalyst was synthesized via one-step thermal treatment of MoS2 dots @Fe-MOF (MIL-101). Under visible light irradiation, the well-constructed Z-scheme (MoS2, γ-Fe2O3)/graphene photocatalyst shows 2-fold photocatalytic oxygen evolution activity (4400 µmol g-1 h-1) compared to that of γ-Fe2O3/graphene (2053 µmol g-1 h-1). Based on ultraviolet photoelectron spectrometry (UPS), Mott-Schottky plot, photocurrent and photoluminescence spectroscopy (PL) results, the photo-induced electrons from the conduction band of γ-Fe2O3 could transport quickly to the valence band of MoS2 via highly conductive graphene as an electron transport channel, which could significantly enhance the electron-hole separation efficiency as well as photocatalytic performance.

VxMn(4-x)Mo3Ce3/Ti catalysts for selective catalytic reduction of NO by NH3.

A series of vanadium based catalysts (VxMn(4-x)Mo3Ce3/Ti) with different vanadium (x wt.%) and manganese ((4-x) wt.%) contents have been prepared by the wet impregnation method and investigated for selective catalytic reduction (SCR) of NOx by NH3 in the presence of 8 vol.% H2O and 500 ppmV SO2. The physicochemical characteristics of the catalysts were thoroughly characterized. The SCR of NOx by NH3 (NH3-SCR) activity, especially the low-temperature activity, significantly increased with increasing V2O5 content in the catalyst until the V2O5 content reached 1.5 wt.%, which corresponds well with the redox properties of the catalyst. All of the metal oxides were well dispersed and strongly interacted with each other on the catalyst surface. V mainly exists in the V5+ state in the catalysts. The strong synergistic effect between the vanadium and cerium species led to formation of more Ce3+ species, and that between the vanadium and manganese species contributed to formation of more manganese species with low valences. All of the catalysts exhibited strong acidity, while the redox properties determined the NH3-SCR activity, especially the low-temperature activity. H2O and SO2 had severe inhibiting effects on the activity of V1.5Mn2.5Mo3Ce3/Ti. However, good H2O and SO2 resistance and high NOx conversion by V1.5Mn2.5Mo3Ce3/Ti could be achieved in the presence of SO2 and almost no decline was observed in a long-term test at 275°C for 168 hr in the presence of SO2 and H2O, which can be attributed to the sulfate species formed on the catalyst surface.

Deactivation of Cu-SAPO-34 by urea-related deposits at low temperatures and the regeneration.

Selective catalytic reduction (SCR) with urea catalyzed by Cu-SAPO-34 is an effective method to eliminate NOx from diesel exhaust. However, urea-related deposits may form during cold-start and urban driving due to low exhaust temperatures. The activity of Cu-SAPO-34 at 175°C is significantly degraded by urea exposure, and 300°C is required for regeneration. Through in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed hydrolysis studies, the dominant stable deposit at 175°C is identified as biuret, which can be eliminated at 300°C. The urea-derived deactivation and regeneration mechanisms of Cu-SAPO-34 were compared with those of anatase-supported catalysts.

Roles of cobalt and cerium species in three-dimensionally ordered macroporous CoxCe1-xOδ catalysts for the catalytic oxidation of diesel soot.

Three-dimensionally ordered macroporous (3DOM) CoxCe1-xOδ oxides with different Co/Ce atomic ratios were synthesized by a colloidal crystal template (CCT) method. They show higher activity for soot combustion in O2 than single CeO2 and Co3O4 under loose contact conditions. XRD, Raman, XPS, H2-TPR and soot-TPR characterizations were carried out to study the surface and bulk oxygen vacancies and to correlate them to the activity. There exists electron transfer from Ce to Co in the matrix. Both Ce3+ and Co2+ species contribute importantly to the creation of surface and bulk oxygen vacancies, which determine the ignition and burnout temperatures of the catalysts, respectively.

High selectivity of CO2 hydrogenation to CO by controlling the valence state of nickel using perovskite.

The selectivity of CO2 hydrogenation can be significantly tuned by controlling the valence state of nickel using lanthanum-iron-nickel perovskites. Nickel with higher valence states weakens the binding of CO and increases the activation barrier for further CO hydrogenation, leading to a higher CO selectivity than the metallic nickel.

Effect of barium sulfate modification on the SO2 tolerance of V2O5/TiO2 catalyst for NH3-SCR reaction.

Sulfur poisoning of V2O5/BaSO4-TiO2 (VBT), V2O5/WO3-TiO2 (VWT) and V2O5/BaSO4-WO3-TiO2 (VBWT) catalysts was performed in wet air at 350°C for 3hr, and activities for the selective catalytic reduction of NOx with NH3 were evaluated for 200-500°C. The VBT catalyst showed higher NOx conversions after sulfur poisoning than the other two catalysts. The introduction of barium sulfate contributed to strong acid sites for the as-received catalyst, and eliminated the redox cycle of active vanadium oxide to some extent, which resulted in a certain loss of activity. Readily decomposable sulfate species formed on VBT-S instead of inactive sulfates on VWT-S. These decomposable sulfates increased the number of strong acid sites significantly. Some sulfate species escaped during catalyst preparation and barium sulfate was reproduced during sulfur poisoning, which protects vanadia from sulfur oxide attachment to a great extent. Consequently, the VBT catalyst exhibited the best resistance to sulfur poisoning.

Improved activity and durability of Rh-based three-way catalyst under diverse aging atmospheres by ZrO2 support.

The catalytic activity and durability of Rh/ZrO2 catalyst were investigated compared with Rh/Al2O3 catalyst under diverse aging atmospheres, including lean, rich and lean-rich cyclic aging atmospheres, to simulate the real working conditions of three-way catalyst. Oxidation states and microstructures of rhodium species were investigated to correlate with the catalytic performance of the catalysts. The catalytic performance and durability of the Rh catalyst under diverse aging atmospheres were drastically enhanced by ZrO2 support. ZrO2 support was confirmed to be able to effectively inhibit rhodium sintering even under diverse aging conditions. It can also successfully keep Rh species in an active low-valence state on the surface of the catalyst. The superiority of ZrO2 support compared to Al2O3 was verified by the Rh-based monolith catalyst.

Optimizing the crystallinity and acidity of H-SAPO-34 by fluoride for synthesizing Cu/SAPO-34 NH3-SCR catalyst.

A series of H-SAPO-34 zeolites were synthesized by a hydrothermal method in fluoride media. The as-synthesized H-SAPO-34 zeolites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 physisorption, temperature-programmed desorption of NH3 (NH3-TPD) and nuclear magnetic resonance (NMR) measurements. The results showed that a certain concentration of F(-) anions promoted the nucleation and crystallization of H-SAPO-34. The H-SAPO-34 synthesized in the fluoride media showed high crystallinity, uniform particle size distribution, large specific surface area and pore volume, and enhanced acidity. Therefore, Cu/SAPO-34 based on the fluoride-assisted zeolite showed a broadened temperature window for the selective catalytic reduction of NO by NH3 (NH3-SCR) reaction due to the enhanced acidity of the zeolite and the improved dispersion of copper species.

Effect of water vapor on NH3-NO/NO2 SCR performance of fresh and aged MnOx-NbOx-CeO2 catalysts.

A MnOx-NbOx-CeO2 catalyst for low temperature selective catalytic reduction (SCR) of NOx with NH3 was prepared by a sol-gel method, and characterized by NH3-NO/NO2 SCR catalytic activity, NO/NH3 oxidation activity, NOx/NH3 TPD, XRD, BET, H2-TPR and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that the MnOx-NbOx-CeO2 catalyst shows excellent low temperature NH3-SCR activity in the temperature range of 150-300°C. Water vapor inhibits the low temperature activity of the catalyst in standard SCR due to the inhibition of NOx adsorption. As the NO2 content increases in the feed, water vapor does not affect the activity in NO2 SCR. Meanwhile, water vapor significantly enhances the N2 selectivity of the fresh and the aged catalysts due to its inhibition of the decomposition of NH4NO3 into N2O.

DRIFT study of CuO-CeO2-TiO2 mixed oxides for NOx reduction with NH3 at low temperatures.

A CuO-CeO2-TiO2 catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperatures was prepared by a sol-gel method and characterized by X-ray diffraction, Brunner-Emmett-Teller surface area, ultraviolet-visible spectroscopy, H2 temperature-programmed reduction, scanning electron microscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The CuO-CeO2-TiO2 ternary oxide catalyst shows excellent NH3-SCR activity in a low-temperature range of 150-250 °C. Lewis acid sites generated from Cu(2+) are the main active sites for ammonia activation at low temperature, which is crucial for low temperature NH3-SCR activity. The introduction of ceria results in increased reducibility of CuO species and strong interactions between CuO particles with the matrix. The interactions between copper, cerium and titanium oxides lead to high dispersion of metal oxides with increased active oxygen and enhanced catalyst acidity. Homogeneously mixed metal oxides facilitate the "fast SCR" reaction among Cu(2+)-NO, nitrate (coordinated on cerium sites) and ammonia (on titanium sites) on the CuO-CeO2-TiO2 catalyst at low temperatures.

Facile synthesis of hierarchical porous γ-Al2O3 hollow microspheres for water treatment.

Hierarchical porous γ-Al2O3 hollow microspheres were synthesized by a modified spray drying method. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for the synthesis of γ-Al2O3 hollow microspheres. The mechanism of the formation of hierarchical porous γ-Al2O3 hollow microsphere was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing and a following self-assembling process with NH4Cl as the template during spray drying. The meso-/macro-pores in γ-Al2O3 mainly arise from the stacking of the laminar boehmites which are obtained by ageing the precipitated precursors at 90°C. NH4Cl, which was the byproduct from the reaction between AlCl3·6H2O and NH3·H2O, was demonstrated to be an excellent template to act as the core and the barrier for separation of laminar boehmites. No extra NH4Cl was added. The as-synthesized hierarchical porous γ-Al2O3 hollow microsphere presented remarkably higher adsorption capacity, which is thirty times higher adsorption rate for Congo Red than the solid microsphere containing only small mesopores.

NO reduction by CO over Rh/Al2O3 and Rh/AlPO4 catalysts: Metal-support interaction and thermal aging.

Rhodium was impregnated on γ-Al2O3 and AlPO4 followed by treating in a wet air at 1050°C for 5h. The catalytic activities of the catalysts were evaluated by temperature-programmed reactions in a stoichiometric mixture of NO and CO. The fresh Rh/Al2O3 and Rh/AlPO4 catalysts exhibit similar catalytic activities. After the hydrothermal aging, Rh/Al2O3 experiences a significant deactivation, whereas Rh/AlPO4 shows even higher activity than the fresh sample. A series of structural and surface characterizations were performed to explore different aging mechanisms of these two catalysts. The fresh Rh/AlPO4 catalyst shows superior low-temperature redox property, but its NOx adsorption is weakened due to the acidic nature of the support. Thus, two fresh catalysts present similar catalytic activities. The hydrothermal aging at high temperature leads to the formation of inert Al5Rh2 alloy. Contrarily, no such strong interaction occurs between rhodium and AlPO4. Furthermore, large Rh particles, which facilitate the dissociation of NO, are formed on the aged Rh/AlPO4 catalyst. Thus, a superior high activity for NO-CO is achieved over the aged Rh/AlPO4.

Effects of tungsten oxide on the activity and thermal stability of a sulfate-derived titania supported platinum catalyst for propane oxidation.

A Pt/WO3/TiO2 catalyst for propane oxidation was prepared by a stepwise wet impregnation method, and was aged at 800 degrees C for 5 hr. Compared to the sulfate-derived titania supported catalyst, the introduction of tungsten oxide as stable Brønsted acid sites led to the formation of more metallic platinum active sites at the Pt/WO3 interface. The dissociation of surface intermediates for propane oxidation was promoted on the WO3-modified catalyst. This, as well as the inhibition effects of tungsten oxide on the sintering of anatase and the phase transformation to rutile, resulting in a high activity and thermal stability for the Pt/WO3/TiO2 catalyst.

Total oxidation of propane on Pt/WOx/Al2O3 catalysts by formation of metastable Ptδ+ species interacted with WOx clusters.

A series of Pt/Al(2)O(3) catalysts with various tungsten oxide loadings were prepared by a stepwise wet impregnation method. The catalysts were characterized by X-ray diffraction, nitrogen physisorption, Raman, UV-vis diffuse reflectance, transmission electron microscopy and infrared spectroscopy of adsorbed probe molecules (CO, NH(3) or C(3)H(8)). The propane oxidation activity of Pt/Al(2)O(3) catalyst is significantly improved by the addition of tungsten oxide. The tungsten oxide overlayer is presented as monomeric/polymeric WO(x) clusters and WO(3) crystals depending on the loading amount. The most active catalyst occurs at an intermediate surface tungsten density corresponding to the maximum of polytungstate species. The electronic interactions between Pt and WO(x) clusters lead to the generation of more reducible Pt(δ+) species which are suggested to be active sites for propane oxidation. Basically, a simple model is proposed involving the initial CH bond activation at the platinum-tungsten oxide interface.

Effects of WO(x) modification on the activity, adsorption and redox properties of CeO2 catalyst for NO(x) reduction with ammonia.

A series of WO3/CeO2 (WO(x)/CeO2) catalysts were synthesized by wet impregnation of ammonium metatungstate on a CeO2 support. The resulting solid acid catalysts were characterized by X-ray diffraction (XRD), UV-Vis spectroscopy (UV-Vis), Raman spectroscopy (Raman), in-situ Fourier transform infrared spectroscopy (in-situ FT-IR) of ammonia adsorption, NH3-TPD, H2 temperature-programmed reduction (H2-TPR), NH3/NO oxidation and activity measurements for NO(x) reduction by NH3 (NH3-SCR). The results show that polytungstate (WO(x)) species are the main species of tungsten oxide on the surface of ceria. The addition of tungsten oxide enhances the Brönsted acidity of ceria catalysts remarkably and decreases the amount of surface oxygen on ceria, with strong interaction between CeO2 and WO(x). As a result, the N2 selectivity of NH3 oxidation and NH3-SCR at high temperatures (> 300 degrees C) is enhanced. Therefore, a wide working temperature window in which NO(x) conversion exceeds 80% (NO(x) conversion > 80%) from 200 to 450 degrees C, is achieved over 10 wt.% WO(x)/CeO2 catalyst. A tentative model of the NH3-SCR reaction route on WO(x)/CeO2 catalysts is presented.