NH3 Temperature-programmed Desorption Research Articles

Mechanism study on direct synthesis of glycerol carbonate from CO2 and glycerol over shaped CeO2 model catalysts

Direct synthesis of glycerol carbonate (GC) from CO2 and glycerol (a byproduct of biodiesel production) is a route to obtain a high-value chemical from waste and low-cost byproducts but has not yet industrialized due to the lack of efficient catalysts. Ceria (CeO2) exhibits the highest catalytic activity and GC selectivity among the heterogeneous catalysts studied so far. However, the mechanism of this reaction over CeO2 catalysts has not been studied in detail. Herein, we synthesized CeO2 nanocrystals with different morphologies as model catalysts that can predominantly expose {111}, {110}, and {100} facets, and their surface acid-base properties were characterized using high-sensitivity temperature-programmed desorption of NH3 and CO2 with quadrupole mass spectrometry as detector (NH3-TPD-QMS and CO2-TPD-QMS). We found that the catalytic performance (GC formation rate) is strictly linearly dependent on the density of basic sites, which is relevant to the adsorption and activation of CO2. In addition, to illustrate a more microscopic reaction mechanisms underlying the formation of GC from CO2 and glycerol on all three low-index surfaces (111), (110) and (100), we also performed comprehensive first principles calculations. A three-step Langmuir–Hinshelwood (LH) mechanism was identified in which the annulation reaction is the rate-limiting step. The CeO2 (111) surface exhibits the lowest overall activation energy, which agrees well with the catalytic performance that the CeO2 nano-octahedra, predominantly exposing {111} facets, have the highest GC formation rate. This work is the first to combine experiments on shaped CeO2 model catalysts with first-principles calculations to gain insight into the mechanism of direct synthesis of GC from CO2 and glycerol, and will aid in the development of catalysts with improved performance.

Promotional Mechanisms of Activity and SO2 Tolerance of NdVOx/TiO2 Catalysts for Selective Catalytic Reduction of NOx with NH3

A series of NdV/Ti-x% catalysts were prepared by a microwave-assisted deposition-precipitation method, and their catalytic performance for the selective catalytic reduction (SCR) of NO by NH3 was investigated. Impressively, the SCR activity of the obtained catalyst was markedly influenced by the amount of vanadate added. The results show that the NdV/Ti-x% catalysts not only possess good low-temperature catalyst activities but also exhibit excellent SO2 resistance performances in the presence of 300 ppm SO2 and/or 10 vol % H2O. Furthermore, SO2 promotes the activity of the NdV/Ti-10% catalyst. These catalysts were characterized using X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance Fourier transforms (DRIFTs), etc. The in situ DRIFT results indicate that the NH3-SCR reaction over the well-dispersed vanadate catalyst follows the Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms. In particular, the SCR reaction following the E–R mechanism is rapid and significantly improves SO2 tolerance. In addition, the SO2 adsorption could be restrained on the NdV/Ti-10% catalyst, which effectively restrained the deposition of sulfate species and endowed catalysts with strong SO2 tolerance. Meanwhile, the effective suppression of SO2 adsorption allowed the NdV/Ti-10% catalyst to maintain Eley–Rideal (E–R) pathways. This work paves the way for the development of high-performance SO2-tolerant SCR catalysts.

Cu(Ⅰ) anchoring in MOF-808 as a stable catalyst in ultra-deep oxidation desulfurization

Oxidative desulfurization (ODS) is a promising technique for obtaining clean fuel. Designing and researching zirconium-based metal–organic frameworks (Zr-MOFs) with bimetallic catalytic sites are encouraging for its further development. In this study, Cu(Ⅰ)-MOF-808 was successfully synthesized through anchoring Cu+ in MOF-808. Pyridine infrared spectrum (Py-IR) and NH3 temperature programmed desorption (NH3-TPD) confirmed that Lewis (L) acid sites existed in the synthesized material. Cu(Ⅰ)-MOF-808 was applied into the ODS reaction, during the reaction process, L acid sites adsorbed sulfide, Cu+ sustainably provided electron to H2O2 to formed into •OH, with the synergistic effect of L acid sites and Cu+, 99.9 % of the dibenzothiophene was oxidated to dibenzothiophene sulfone in 120 min. In addition, the cyclic experiment indicated that the ODS performance of Cu(Ⅰ)-MOF-808 was maintained at 99.2 % after 12 times recycling. Powder X-ray diffraction (PXRD) spectra indicated the framework structure of the reused catalyst was stable, inductively coupled plasma optical emission spectrometer (ICP-OES) result displayed no metal ions (Zr, Cu) leaching after the ODS reaction, which also proved the catalytic stability of the Cu(Ⅰ)-MOF-808.

Efficient transformation of hemicellulosic biomass into sugar alcohols with non-precious and stable bimetallic support catalyst

The conversion of hemicellulosic biomass to sugar alcohols commonly applies homogeneous acids and noble metals as catalysts, which is neither economical nor environmentally friendly. In order to overcome the deficiencies of the reports, a series of non-precious catalysts were synthesized through the co-precipitation method and incipient wetness impregnation (IWI) method in converting hemicellulosic biomass, i.e., hemicellulose and xylose into sugar alcohols (i.e., xylitol and arabitol). Reaction experiments showed that the Ni8.98Fe1 @ 1.54SiO2 catalyst prepared by the IWI method has excellent catalytic performance for converting xylose to sugar alcohols in the neutral aqueous solution, and can be applied to the direct conversion of hemicellulose as well. Specifically, the optimum yield of xylitol and arabitol could obtain 99.48 mol% and 26.01 mol% for xylose as the substrate and 88.16 mol% and 20.55 mol% for hemicellulose as the substrate, respectively. Characterization techniques such as X-ray diffraction (XRD), N2 adsorption-desorption isotherm, transmission electron microscopy (TEM), scanning electron microscope (SEM), H2 temperature-programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy of pyridine (Py-FTIR), and SQUID vibrating sample magnetometry (SQUID -VSM) were performed to study physicochemical properties of the catalyst. N2 adsorption-desorption isotherm and NH3-TPD characterizations indicated that the catalyst with mesoporous properties and proper acidic sites is vital for hydrolyzing hemicellulose to pentose (i.e., xylose and arabinose). TEM, H2-TPR, and XPS techniques revealed that the (111) plane of metallic Ni is the main active phase for the hydrogenation of pentose into sugar alcohols, and Fe2+ and Fe3+ species act as effective promoters to enhance the pentose hydrogenation. Py-FTIR characterization and reaction conditions indicated that the appropriate ratio of Brønsted to Lewis acidic sites could facilitate the conversion of xylose to arabitol. The reaction mechanisms for converting hemicellulosic biomass to sugar alcohols were proposed based on the systematic study of reaction conditions and characterizations. N2 adsorption-desorption, H2-TPR, and XPS indicated that the catalyst with strong metal-support interaction, large specific surface area and pore volume could be beneficial for catalytic stability, which is verified in leaching and reusability tests. The catalyst has superparamagnetic, which can be quickly separated from the reaction system under the external magnetic field, facilitating the recovery and utilization of the catalyst.

Efficient adsorptive removal of dibenzothiophene from model fuels by encapsulated of Cu+ and phosphotungstic acid (PTA) in Co-MOF

Co-containing materials have raised much concern in adsorption desulfurization due to its unsaturated metal sites. Herein, we introduced Cu+ and phosphotungstic acid (PTA) to Co-MOF to prepare Co-MOF-Cu+/PTA through post-modification method under mild condition, and then Co-MOF-Cu+/PTA were used for adsorption and removal dibenzothiophene (DBT) from model fuels. The physico-chemical property of adsorbents was characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), NH3-temperature programmed desorption (NH3-TPD), and inductively coupled plasma mass spectrometry (ICP-MS). The ICP-MS and EDX characterization results showed that Cu+ had been uniformly dispersed on the Co-MOF-Cu+/PTA. The NH3-TPD demonstrated that the PTA could enhance surface acidity of the Co-MOF-Cu+/PTA. Due to the introduction of Cu+ and PTA, the adsorption capacity of the Co-MOF-Cu+/PTA for DBT was much greater than Co-MOF. The Cu+ ions served as active sites for π-π interaction and sulfur-metal (S-M) adsorption, meanwhile the Co2+ and PTA adsorbed DBT through acid-base interaction. The adsorption capacity of Co-MOF-Cu+/PTA for DBT could reach 188.7 ​mg/g. The adsorption behavior was spontaneous and endothermic, and be well fitted by Langmuir isotherm and pseudo-second-order kinetic. The Co-MOF-Cu+/PTA had excellent selectivity and regeneration performance.

Effect of Ga substitution with Al in ZSM-5 zeolite in methanethiol-to-hydrocarbon conversion.

The catalytic properties of conventional H-[Al]-ZSM-5 and gallium-substituted H-[Ga]-ZSM-5 were evaluated in the conversion of methanethiol to ethylene (CH3SH → 1/2C2H4 + H2S). Dimethyl sulfide (DMS), aromatics, and CH4 were formed as byproducts on the H-[Al]-ZSM-5 catalyst. The introduction of Ga into the ZSM-5 structure provided a high ethylene yield with relatively high selectivity for olefins. Based on the temperature-programmed desorption of NH3 and pyridine adsorption on zeolites, the strength of acid sites was decreased by introducing Ga into the ZSM-5 structure. Undesirable reactions seemed less likely to occur at weakly acidic sites. The suppression of the formation of dimethyl sulfide (CH3SH → 1/2C2H6S + 1/2H2S) and the sequential reaction of ethylene to produce aromatics provided a high yield of ethylene over H-[Ga]-ZSM-5.

Low-temperature selective catalytic reduction of NO with NH3 over an FeO x /β-MnO2 composite.

A series of Fe-modified β-MnO2 (FeO x /β-MnO2) composite catalysts were prepared by an impregnation method with β-MnO2 and ferro nitrate as raw materials. The structures and properties of the composites were systematically characterized and analyzed by X-ray diffraction, N2 adsorption-desorption, high-resolution electron microscopy, temperature-programmed reduction of H2, temperature-programmed desorption of NH3, and FTIR infrared spectroscopy. The deNO x activity, water resistance, and sulfur resistance of the composite catalysts were evaluated in a thermally fixed catalytic reaction system. The results indicated that the FeO x /β-MnO2 composite (Fe/Mn molar ratio of 0.3 and calcination temperature of 450 °C) had higher catalytic activity and a wider reaction temperature window compared with β-MnO2. The water resistance and sulfur resistance of the catalyst were enhanced. It reached 100% NO conversion efficiency with an initial NO concentration of 500 ppm, a gas hourly space velocity of 45 000 h-1, and a reaction temperature of 175-325 °C. The appropriate Fe/Mn molar ratio sample had a synergistic effect, affecting the morphology, redox properties, and acidic sites, and helped to improve the low-temperature NH3-SCR activity of the composite catalyst.

Low-temperature NH3-SCR performance of a novel Chlorella@Mn composite denitrification catalyst

The synthesis process of conventional Mn-based denitrification catalysts is relatively complex and expensive. In this paper, a resource application of chlorella was proposed, and a Chlorella@Mn composite denitrification catalyst was innovatively synthesized by electrostatic interaction. The Chlorella@Mn composite denitrification catalyst prepared under the optimal conditions (0.54 g/L Mn2+ concentration, 20 million chlorellas/mL concentration, 450°C calcination temperature) exhibited a well-developed pore structure and large specific surface area (122 m2/g). Compared with MnOx alone, the Chlorella@Mn composite catalyst achieved superior performance, with ∼100% NH3 selective catalytic reduction (NH3-SCR) denitrification activity at 100-225°C. The results of NH3 temperature-programmed desorption (NH3-TPD) and H2 temperature-programmed reduction (H2-TPR) showed that the catalyst had strong acid sites and good redox properties. Zeta potential testing showed that the electronegativity of the chlorella cell surface could be used to enrich with Mn2+. X-ray photoelectron spectroscopy (XPS) confirmed that Chlorella@Mn had a high content of Mn3+ and surface chemisorbed oxygen. In-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) experimental results showed that both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms play a role in the denitrification process on the surface of the Chlorella@Mn catalyst, where the main intermediate nitrate species is monodentate nitrite. The presence of SO2 promoted the generation and strengthening of Brønsted acid sites, but also generated more sulfate species on the surface, thereby reducing the denitrification activity of the Chlorella@Mn catalyst. The Chlorella@Mn composite catalyst had the characteristics of short preparation time, simple process and low cost, making it promising for industrial application.

Efficient and Stable Rice Husk Bioderived Silica Supported Cu2S-FeS for One Pot Esterification and Transesterification of a Malaysian Palm Fatty Acid Distillate

A novel heterogeneous catalyst composite (CuS-FeS/SiO2) derived from rice husk silica was engineered following pyrolysis, chemical precipitation, and chemical redox technique. The resulting catalyst was applied to the conversion of palm fatty acid distillate to biodiesel. The presence of CuS and FeS on the catalyst was verified using X-ray diffraction and Fourier transform infrared spectroscopy, nitrogen physisorption, scanning electron microscopy (FESEM) with energy dispersive X-ray (EDS) spectroscopy, and temperature-programmed desorption of NH3 (TPD-NH3), inductively coupled plasma-atomic emission spectrometry (ICP-AES), and TGA; a specific surface area of approximately 40 m2·g−1 was identified. The impact of independent variables, i.e., reaction temperature, reaction duration, methanol:oil ratio and catalyst concentration were evaluated with respect to the efficacy of the esterification reaction. The greatest efficiency of 98% with a high productivity rate of 2639.92 µmol·g−1·min−1 with k of 4.03 × 10−6 mole·S−1 was achieved with the following parameters: temperature, 70 °C; duration, 180 min; catalyst loading, 2 wt.%; and methanol to oil ratio, 15:1. The CuS-FeS/SiO2 catalyst showed relatively high stability indicated by its ability to be reused up to five times.

Study on the Performance of the Zr-Modified Cu-SSZ-13 Catalyst for Low-Temperature NH3-SCR.

Cu-SSZ-13 and Zr-modified Cu-SSZ-13 catalysts with different Zr/Cu mass ratios were prepared by ion-exchange and impregnation methods, respectively. The NH3-SCR performance tests were performed using the catalyst performance evaluation device to investigate the effects of different Zr/Cu mass ratios on the catalyst ammonia-selective catalytic reduction (NH3-SCR) performance. X-ray diffraction, ICP-OES, BET, NH3 temperature-programed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectrometry, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were used to characterize the catalysts. The results show that the prepared Cu-SSZ-13 catalyst had good catalytic activity. Zr introduction was carried out on this basis. The results showed that proper Zr doping improved the catalytic activity at low temperatures and widened the high-temperature stage, with an optimal activity stage at a Zr/Cu mass ratio of 0.2. The NO x conversion efficiency was close to 100% at 200 °C and over 80% at 450 °C. The active species were well dispersed on the catalyst surface, and the metal modification did not change the crystal structure of the zeolite. The NH3-TPD results showed that the Zr-modified catalyst had more abundant acid sites, and the H2-TPR results indicated that the Cu species on the catalyst had excellent reducibility at low temperatures. The interaction between Cu and Zr could regulate the Cu+ and Cu2+ proportion on the catalyst surface, which facilitated the increase in the Cu+ for fast SCR reaction at low temperatures. With abundant acid sites and both SCR reactions following the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism on the catalyst surface at a low temperature of 150 °C, more abundant acid sites and reaction paths created favorable conditions for NH3-SCR reactions at low temperatures.

WO3-based porous MCM-48 catalysts for renewable acrolein synthesis by the dehydration of glycerol

The gas-phase selective dehydration of glycerol to acrolein was studied over x%W (x: 5, 15, 25, and 35 %, w/w) supported on MCM-48. Microporous silica MCM-48 was synthesized to produce three-dimensional cubic crystals with a high surface area and resistance to hydrolytic pressure. Tungsten oxide (WO3) based solid acid catalysts supported on MCM-48 were prepared by two distinct synthesis methods, namely, oxalic acid added sol–gel (HSG) and nonhydrolytic sol–gel (NHSG) methods. The morphological and textural properties of catalysts were characterized by XRD, BET, SEM, Raman, XPS, and FT-IR techniques. The acidic properties of catalysts were elucidated by FTIR pyridine-adsorption and NH3-Temperature Programmed Desorption (NH3-TPD). The total surface acid sites’ concentration, Brønsted (B) and Lewis (L) acid concentrations, and B/(B+L) ratios were calculated from NH3-TPD and pyridine-FTIR. The steady acrolein yields were obtained with the optimal catalysts (15 %W and 25 %W loaded MCM-48 for HSG and NHSG, respectively). This study demonstrated that the medium-strength surface acidity and B/(B+L) ratios for glycerol dehydration to acrolein were correlated with both the activity and the selectivity of acrolein.

MESOPOROUS SILICOALUMINATE MATERIALS (MCM-41, SBA-15 AND MCF) BY ATRANE ROUTE FOR COBALT CATALYST

In this work, the synthesis of mesoporous silicoaluminum supports synthesized by the atrane route was performed. The obtained supports presented high homogeneity of the aluminum dispersion, high surface area and narrow pore size distribution. The synthesized mesoporous supports were: MCM-41, SBA-15 and MCF with 10% of Al2 O3 in the matrix of SiO2 . Twelve percent of cobalt was added to these supports by the incipient wetness impregnation method. These materials were characterized by adsorption of Nitrogen (BET-BJH), Scanning Microscopy Electron (SEM), X-Ray Diffraction (XDR), H2 Temperature-Programmed Reduction (TPR) and NH3 Temperature-Programmed Desorption (TPD). According to their structural properties of these catalysts, a promising application in the Fischer- Tropsch syntheses (FTS) is identified.

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.

One-pot synthesis of CNT-SAPO-34 composite supported copper and cerium catalysts with excellent surface resistance to SO2 and H2O in NH3-SCR

CuCe/CNTx-SAPO-34 (x = 0, 0.5, 1, 2) with various CNT doping quantities were synthesized through the one-pot hydrothermal synthesis method using CNT and SAPO-34 as composite supporters. The selected CuCe/CNT1-SAPO-34 catalyst exhibits remarkable SCR activity and high H2O/SO2 resistance in a wide temperature range of 200–450 °C. X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR), and NH3-temperature programmed desorption (NH3-TPD) were used to characterize the physiochemical structure, surface state of active sites reducibility, and surface acidity of the catalysts. The characterization results reveal that CuCe/CNT1-SAPO-34 possesses a hierarchical structure, small particle size, high levels of surface Cu2+, Ce3+ and chemisorbed oxygen, enhanced reducibility, and suitable surface acidity. For comparison, CuCe/CNT1-SAPO-34 (P) mixed mechanically with CuCe/CNT0-SAPO-34 and CNT was investigated through the NH3-SCR with and without H2O/SO2, it also displays lower resistance of H2O and SO2 than CuCe/CNT1-SAPO-34. The catalytic performance made CuCe/CNT1-SAPO-34 a promising catalyst for the NOx emission control from diesel vehicles.

Comparative study of hydrodeoxygenation performance over Ni and Ni2P catalysts for upgrading of lignin-derived phenolic compound

To deeply understand the different hydrodeoxygenation (HDO) behavior of the monometallic Ni and Ni2P catalysts in HDO processes, the Ni/SiO2 and Ni2P/SiO2 catalysts were prepared, respectively. The as-prepared catalysts were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, X-ray photoelectron spectra (XPS), H2 temperature-programmed reduction (H2-TPR), transmission electron microscope (TEM), NH3 temperature-programmed desorption (NH3-TPD), and CO uptakes. Taking the lignin-derived phenolic compound as raw material, the HDO performance and reaction route of the monometallic Ni and Ni2P catalysts were compared. The results showed that the phosphorization contributed to highly dispersion of active sites, formation of smaller active sites and significantly enhanced acid sites, showing Ni2P/SiO2 catalyst is a bifunctional metal–acid catalyst. For both of the Ni/SiO2 and Ni2P/SiO2 catalysts, the HDO of m-cresol mainly proceeded by the hydrogenation (HYD) step. However, the acid sites on Ni2P/SiO2 catalyst can significantly promote the conversion of oxygenated 3-methylcyclohexanol (MCHnol) to deoxygenated methylcyclohexane (MCH) owing to its strong hydrogenolysis of C–OH bond originated from the bifunctional metal–acid sites. In addition, the unique electron structure of Ni2P phase involved in two different types of sites (tetrahedral Ni(I) and square pyramidal Ni(II)) and the highly dispersed smaller Ni2P particles also contributed to the outstanding HDO performance. Density functional calculation (DFT) calculations clarified that the Ni2P active phase possessed prior ability to cleavage of C–OH bond in comparison with the metallic Ni species. As a result, Ni2P/SiO2 catalyst showed a significantly higher selectivity to MCH (96.3 %) than that of Ni/SiO2 (14.1 %) catalyst at the conditions of 250 ℃ for 240 min.

An Efficient Zr-ZSM-5-st Solid Acid Catalyst for the Polyol Esterification Reaction

In this study, Zr active species were implanted into a ZSM-5 zeolite framework to form a solid acid catalyst through steam treatment and the liquid-solid isomorphous substitution process. The as-synthesized Zr-ZSM-5-st catalyst ensured excellent esterification of trimethylolpropane and fatty acids (FAs) to achieve a polyol ester production yield of 94.41%. Combined with N2 physisorption, X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, transmission electron microscopy mapping, X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption, and inductively coupled mass plasma spectroscopy were conducted. The results revealed that the excellent performance of Zr-ZSM-5-st catalyst could be attributed to the enhanced acidity and the developed surface area and pore structure.

Doping effect of rare earth metal ions Sm3+, Nd3+ and Ce4+ on denitration performance of MnOx catalyst in low temperature NH3-SCR reaction

The MnXOx catalysts (i.e., MnSmOx, MnNdOx, MnCeOx) were prepared by reverse co-precipitation method and used for NH3-SCR reaction. It is found that MnCeOx catalyst presents the best low temperature catalytic activity (higher than 90% NOx conversion in the temperature range from 125 to 225 °C) and excellent H2O + SO2 resistance. In order to explore the reason for this result, the characterization of X-ray diffraction (XRD), Raman spectroscopy, Brunauer–Emmett–Teller (BET), H2-temperature programmed reduction (H2-TPR), NH3-temperature programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflaxions infrared Fourier transformations spectroscopy (DRIFTS) were conducted. The obtained results suggest that MnCeOx catalyst shows the largest amount of acid sites and the best reducibility among these MnXOx catalysts. Besides, Ce4+ doping inhibits the crystallization of MnOx catalyst and shows the largest specific surface area. Finally, in situ DRIFTS experiments reveal that NH3-SCR reaction over MnCeOx catalyst follows both Langmuir–Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms, which is through “fast SCR” reaction.

On the various Cu-redox pathways and O2-mediated Bronsted-to-Lewis adsorbed-NH3 redistribution under SCR half-cycle conditions

The Standard SCR reaction on Cu-SSZ-13 is a complex redox process facilitated by a reduction half cycle (RHC) and an oxidation half cycle (OHC). It is generally accepted that RHC requires the simultaneous presence of NO and NH3, while OHC requires both NO and O2; however understanding of how these individual reactants impact the Cu redox cycle is limited. In this study, we provide experimental investigation of NO-only, NH3-only and NO+NH3 RHC, and their relative rates. Simple kinetic models were developed to understand the dependance of NO-only and NH3-only RHC on Cu speciation. Various OHC routes including NO+O2 and O2-only OHC, and a previously unreported H2O-mediated OHC pathway were also investigated. Detailed NH3 temperature-programmed desorption studies showed that RHC selectively consumes NH3 adsorbed on Lewis-acid sites, and that part of the remaining Bronsted-bound NH3 moves to the Lewis-acid sites during subsequent OHC, thereby resulting in a repartitioning of adsorbed NH3.

Effect of different doping elements on performance of Ce–Mn/TiO2 catalyst for low temperature denitration

Sm and Ho were doped in Ce–Mn/TiO2 catalyst respectively to enhance its denitration performance at low temperature. X-ray diffraction (XRD), N2 adsorption–desorption, X-ray photoelectron spectroscopy (XPS), NH3-temperature programmed desorption (NH3-TPD), H2-temperature programmed reduction (H2-TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques were used to analyze the structure and performance of catalysts. The results demonstrate that Ho doping increases the amount of acid sites and improves low temperature redox property of Ce–Mn/TiO2, which lead to excellent DeNOx performance of Ho-Ce-Mn/TiO2 in the whole reaction temperature range. Sm doping results in decline of redox property, but it is beneficial to increasing the acid sites of Ce–Mn/TiO2. The increased surface acid sites and moderate oxidative ability impart Sm-Ce-Mn/TiO2 higher denitration activity and N2 selectivity at temperature above 150 °C. Lewis acid sites and redox property are the main factors affecting the activity of catalysts. Doping of Ho and Sm both improves sulfur resistance performance of Ce–Mn/TiO2 by inhibiting the adsorption of SO2 and formation of sulfate. Ce–Mn/TiO2 modified by Ho shows better sulfur resistance than that doped with Sm because of its more surface acid sites.

Low-Temperature NH3-SCR on Cex-Mn-Tiy Mixed Oxide Catalysts: Improved Performance by the Mutual Effect between Ce and Ti

A series of Cex-Mn-Tiy catalysts were synthesized using the coprecipitation method, and sodium carbonate solution was used as a precipitant. The various catalysts were assessed by selective catalytic reduction of NOx with NH3, and characterized by X-ray diffraction, Raman spectroscopy, H2 temperature-programmed reduction, NH3 temperature-programmed desorption, and X-ray photoelectron spectroscopy to investigate the physicochemical properties, surface acidity, and redox abilities of the Cex-Mn-Tiy catalysts. The Ce0.1-Mn-Ti0.1 catalyst exhibited the best catalytic performance (more than 90% NOx conversion in the range of 75 to 225 °C), as a result of proper redox ability, abundant acid sites, high content of Mn4+ and Ce3+, and surface-adsorbed oxygen (OS). The results of in situ DRIFT spectroscopy showed that the NH3-SCR reaction followed both the E-R and L-H paths over the Ce0.1-Mn-Ti0.1 catalyst, and it occurred faster and more sharply when it mainly abided by the E-R mechanism.