H2 Temperature-programmed Reduction Research Articles

Does H2 Temperature‐Programmed Reduction Always Probe Solid‐State Redox Chemistry? The Case of Pt/CeO2

Redox reactions on the surface of transition metal oxides are of broad interest in thermo, photo, and electrocatalysis. H2 temperature‐programmed reduction (H2‐TPR) is commonly used to probe oxide reducibility by measuring the rate of H2 consumption during temperature ramps, assuming that this rate is controlled by oxide reduction. However, oxide reduction involves several elementary steps, such as H2 dissociation and H‐spillover, before surface reduction and H2O formation occur. In this study, we evaluated the kinetics of H2 consumption over CeO2 and Pt/CeO2 with varying Pt loadings and structures to identify the elementary steps probed by H2‐TPR. Literature often attributes changes in H2‐TPR characteristics with Pt addition to increased CeO2 reducibility. However, our analysis revealed that the H2 consumption rate is measurement of the rate of H‐spillover at Pt‐CeO2 interfaces and is determined by the concentration of Pt species on Pt nanoclusters that dissociate H2. Therefore, lower temperature H2 consumption observed with Pt addition does not indicate higher CeO2 reducibility. Measurements on samples with mixtures of Pt single‐atoms and nanoclusters demonstrated that H2‐TPR can effectively quantify dilute Pt nanocluster concentrations, suggesting caution in directly linking H2‐TPR characteristics to oxide reducibility while highlighting alternative material insights that can be gleaned.

Does H2 Temperature-Programmed Reduction Always Probe Solid-State Redox Chemistry? The Case of Pt/CeO2.

Redox reactions on the surface of transition metal oxides are of broad interest in thermo, photo, and electrocatalysis. H2 temperature-programmed reduction (H2-TPR) is commonly used to probe oxide reducibility by measuring the rate of H2 consumption during temperature ramps, assuming that this rate is controlled by oxide reduction. However, oxide reduction involves several elementary steps, such as H2 dissociation and H-spillover, before surface reduction and H2O formation occur. In this study, we evaluated the kinetics of H2 consumption over CeO2 and Pt/CeO2 with varying Pt loadings and structures to identify the elementary steps probed by H2-TPR. Literature often attributes changes in H2-TPR characteristics with Pt addition to increased CeO2 reducibility. However, our analysis revealed that the H2 consumption rate is measurement of the rate of H-spillover at Pt-CeO2 interfaces and is determined by the concentration of Pt species on Pt nanoclusters that dissociate H2. Therefore, lower temperature H2 consumption observed with Pt addition does not indicate higher CeO2 reducibility. Measurements on samples with mixtures of Pt single-atoms and nanoclusters demonstrated that H2-TPR can effectively quantify dilute Pt nanocluster concentrations, suggesting caution in directly linking H2-TPR characteristics to oxide reducibility while highlighting alternative material insights that can be gleaned.

Comparing low-temperature NH3-SCR activity, operating temperature window and kinetic properties of the Mn-Fe-Nb/TiO2 catalysts prepared by different methods

In order to overcome the shortcomings of traditional V-based catalysts, such as poor low-temperature activity, narrow temperature window, and toxicity. Therefore, the Mn-Fe-Nb/TiO2 catalysts were prepared using impregnation (IM), citric acid complexation (CA), co-precipitation (CP), hydrothermal synthesis (HY), and sol–gel (SG) methods to develop novel NH3 selective catalytic reduction (NH3-SCR) catalysts with excellent low-temperature activity, a wide temperature window, and environmental friendliness. Their NH3-SCR catalytic activity was evaluated, followed by a detailed analysis of their morphology, structure, specific surface area, chemical state, redox properties, acidity, and interactions using X-ray diffraction (XRD), Bruaner-Emmet-Teller (BET), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD), and Fourier transform infrared spectroscopy (FTIR) techniques. The 5Mn-7Fe-4Nb/TiO2-SG catalyst, prepared via sol–gel, exhibited the highest denitrification (de-NOx) activity and the widest operating temperature window, achieving over 80% Nitric oxide (NO) conversion at 180–350 °C and a peak conversion of 97% at 250 °C. The XRD, BET, SEM, and HR-TEM characterization results showed that the catalyst prepared using the sol–gel method had the smallest particles, the highest specific surface area, and the greatest dispersion. XPS and NH3-TPD results revealed that the catalyst prepared using the sol–gel method possessed the highest surface adsorbed oxygen (Oα) content and the largest number of acidic sites. H2-TPR and FTIR analyses indicated that the catalyst prepared using the sol–gel method enhanced the reduction capacity and possessed the strongest interaction forces among support-active-promoter components. Steady-state kinetic tests confirmed that in the NH3-SCR reaction, the rate-determining step was the adsorption activation of NO on the catalyst surface. The sol–gel method reduced the activation energy for de-NOx reactions, enhancing the low-temperature activity of the catalyst.

Catalytic hydrogenation of CO2 to aromatics over indium-zirconium solid solution and sheet HZSM-5 tandem catalysts

The presence of acids and bases, coupled with its exceptional CO2 adsorption capacity and thermal stability, has established zirconia as a highly esteemed promoter and carrier. Researchers have found that the In2O3 supported ZrO2 exhibits high activity and remarkable stability. Therefore, xIn-yZr(T) solid solutions were prepared with different calcination temperatures and In/Zr molar ratios. The solid solutions were then combined with sheet HZSM-5 zeolite from tandem catalysts, which were investigated for their catalytic performance in converting CO2 to aromatics. The X-ray diffraction, scanning electron microscopy, N2 adsorption-desorption, NH3 temperature-programmed desorption, pyridine infrared radiation, X-ray photoelectron spectroscopy, electron paramagnetic resonance, CO2 temperature-programmed desorption and H2 temperature-programmed reduction characterization methods were used to investigate the physicochemical properties of catalysts. Density Functional Theory calculations were used to investigate the energy of thermal desorption and H2 reduction to generate oxygen vacancies on the surface of In2O3(111) and Zr/In2O3(111). Additionally, the influence of oxygen vacancies on CO2 adsorption energies was simulated. It was found that the incorporation of a moderate amount of zirconium carriers promoted the generation of oxygen vacancies and provided better metal-carrier interactions. The 4In-1Zr(500 °C)/HZSM-5 tandem catalyst exhibits excellent catalytic stability, achieving a CO2 conversion of 24.3 % and aromatics selectivity of 37.3 %.

Steam reforming of dodecane using Ni-red mud catalyst: A sustainable approach for hydrogen production

Conventional energy sources emit huge amounts of carbon dioxide (CO2) into the atmosphere causing global warming. Hydrogen (H2) is an alternative clean energy carrier that produces only heat and water vapor upon combustion. In this work, we utilized industrial waste material (Red Mud, RM) to develop a cost-effective and efficient catalyst for steam reforming of diesel (n-dodecane as a model compound) for hydrogen production. We impregnated nickel (Ni), a prudent and effective metal, in red mud (RM) in varied proportions to employ as a catalyst for hydrogen production. The catalysts' structure was characterized by powdered X-ray Diffraction (PXRD), Fourier Transform Infrared spectroscopy (FTIR), and X-ray Photoelectron Spectroscopy (XPS). Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Electron Dispersive Spectroscopy (EDS) were used to measure the elemental composition while Scanning Electron Microscopy (SEM) was used to investigate the catalysts’ morphology. Surface area and porosity were analyzed using the N2 adsorption/desorption isotherms, and H2-Temperature Programmed Reduction (H2-TPR) to study the reducibility and interaction of Ni species to the RM. The catalysts were then evaluated for hydrogen production by the steam reforming process of n-dodecane (SRD). It was found that the catalyst with 20% Ni loading (20% Ni@RM) can produce hydrogen with selectivity as high as 75%, and stability for 20 h with a negligible decline in the catalyst performance.

Revisiting the influence of Ni particle size on the hydrogenation of CO2 to CH4 over Ni/CeO2

Nickel nanoparticles were supported on CeO2 and evaluated in the hydrogenation of CO2 to CH4 at 538 K and 1 atm. Chemisorption of H2 was used to estimate the average Ni particle size, which ranged from 1.3 to 17 nm. The apparent activation energy for CH4 formation and turnover frequency for CO2 conversion was independent of Ni particle size. The smallest Ni particles (1.3 nm) were less selective to CH4. Comparisons to alumina-supported Ni reveal the same performance as Ni/CeO2 when normalized to active Ni atoms counted by H2 chemisorption. A beneficial effect of ceria relative to alumina is the enhanced reducibility of Ni as evaluated by H2 temperature-programmed reduction. The constancy of the turnover frequency on Ni particles suggests the acid-base and/or redox properties of the support do not play a significant role in the CO2 methanation reaction under the conditions of study, which contrasts many earlier works.

Bimetal (CuNi and CuCo) substituted CeO2: An approach for low temperature dry reforming of methane

The solution combustion synthesis method was used to make CuNi substituted CeO2 (7.5 at.% of Cu+7.5 at.% of Ni), and CuCo substituted CeO2 (7.5 at.% of Cu+7.5 at.% of Co) catalysts. The catalysts were characterized by x-ray diffraction (XRD),XPS, BET surface area measurements, scanning electron microscopy (SEM), and H2-temperature-programmed reduction(H2-TPR). Further, these catalysts were tested for endothermic dry reforming of methane (DRM) reaction. The CuNi substituted CeO2 catalyst started dry reforming of methane (DRM) at around 350 °C, while the CuCo substituted CeO2 catalyst started DRM at around 450 °C. This indicates that the CuNi catalyst has a lower activation temperature for the DRM reaction compared to the CuCo catalyst. The CuNi substituted CeO2 catalyst converted CH4 better than the CuCo substituted CeO2 at all temperatures reaching 98 % conversion at 800 °C The CuNi substituted CeO2 showed higher H2/CO ratio in comparison to the CuCo substituted CeO2 but the long-term stability followed the opposite order. Thermal gravimetric analysis (TGA), SEM and O2-TPO were used to quantify as well as to understand the nature of carbon that had built up on spent catalysts and it is mostly amorphous. Transient studies have shown that along with the lattice oxygen participation, controlled methane decomposition step is necessary for stability of catalysts. Transient studies also recommended additional reaction steps during DRM where an exothermic methane partial oxidation step reduces the endothermicity of DRM. Also, CO2 activation also goes via the defect dissociation route, an additional step to the conventional carbon oxidation (CO2+C→CO) step. So, novelty of this work is in elucidating the exact role of lattice oxygen in imparting the activity and stability to the DRM catalyst.

Salt assistant bimetallic sludge-derived porous carbon for efficient peroxymonosulfate activation

The development of cost-effective, environmentally benign, and high-performance peroxymonosulfate (PMS) activation towards wastewater treatment is still considered a challenge. In this work, the sludge-derived porous carbon loaded with bimetal oxides (DSC-FeMn) was fabricated by molten salt assistant strategy using the combination of zinc chloride, metal ions and Fe containing sludge. The obtained DSC-FeMn exhibited high catalytic activity for nearly 100 % 4-chlorophenol (4-CP) removal within 30 min with the k of 0.304 min−1, which was 1.07 and 11.94 times that of the single Fe and Mn-sludge-derived carbon (DSC-Fe, DSC-Mn), respectively. Quenching experiments, electron paramagnetic resonance tests (EPR), X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR) and electrochemical experiments proved that the radical pathway, dominated by OH and SO4−, led to the excellent degradation performance of DSC-FeMn, which was attributed to the redox cycling between Fe and Mn. This study provides a new idea for sludge resource utilization, and it deepens the understanding of the mechanism of PMS activation by the synergistic interaction between bimetal.

Engineering binary CrmNb1-mOx (m = 0.1, 0.3, 0.5) oxides catalysts to enhance its NH3-SCR denitration activity and SO2 resistance: Study on the influence of redox property and acidity

It is still a challenge to create novel simply environmentally friendly NH3-SCR denitration catalysts in presence of SO2 and H2O, and to further understand the role of redox property and acidity of catalyst. In this work, a series of binary CrmNb1-mOx (m = 0.1, 0.3 and 0.5) oxides with different Cr/Nb ratio were engineered for NH3-SCR denitration by using the citric acid complex method. Among these catalysts, Cr0.3Nb0.7Ox revealed best catalytic performance with over 90 % NO conversion in the range of 210–390 ℃ and great SO2 and H2O resistance. The effects of different Cr/Nb ratio on catalytic activity was investigated though following characterization such as Raman spectroscopy, Transmission electron microscopy (TEM), X-ray powder diffraction (XRD), N2 physical adsorption, NO-Temperature programmed desorption (NO-TPD), H2-Temperature program reduction (H2-TPR), X-ray photoelectron spectra (XPS), NH3-Temperature programmed desorption (NH3-TPD) and In situ Diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTS). The impact of surface redox property and acidity on catalytic performance was further investigated by changing the Cr/Nb ratio. The results suggest that different Cr/Nb ratio result in catalysts having different Brunauer-Emmett-Teller (BET) surface area, and variation of crystal structure, generating the improvement of the surface acid amount and the enhancement of the oxidizing ability due to existence of more surface active oxygen species. Besides, the results of in situ DRIFTS verify that more acid sites are exposed on the Cr0.3Nb0.7Ox surface and react with absorbed NOx even in the existence of SO2, which followed Langmuir-Hinshelwood (L-H) mechanism.

Catalytic upgrading of Naomaohu coal pyrolysis volatiles over NiAl and NiLiAl oxides

Catalytic upgrading of volatiles is an essential technology to improve the product distribution of pyrolysis for the clean and efficient utilization of low-rank coals. In this work, novel acid-base bifunctional catalysts composed of Ni/Li/Al oxides were designed and prepared using LDH precursors with tunable chemical composition and structural properties. The catalysts were systematically characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), physisorption, NH3-temperature programmed desorption (NH3-TPD), CO2-temperature programmed desorption (CO2-TPD), H2-temperature programmed reduction (H2-TPR), and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). A two-stage fixed bed reactor was employed to evaluate the performance of the catalysts. Characterization results suggest that the crystal size of NiO and the surface area of the catalyst decreased with increasing Li content and decreasing Ni content in the catalyst, while the pore volume and pore diameter were positively correlated with Li content. Moreover, the addition of Li reduced the amount of weak acid sites and raised the amount of total basic sites. Consequently, the bifunctional catalysts boosted the yield of light tar and the contents of aromatics and phenols in tar. Specifically, with the highest total acid sites (569 μmol/g) promoting the cracking of heavy pitch components, Ni2.5Li0.5Al1 decreased the content of pitch in tar to 23.8 wt%, representing a 42.7 % drop from the corresponding non-catalytic pyrolysis. Accordingly, it improved the light tar yield to 7.2 wt%, which was 18 % higher than non-catalytic pyrolysis. 450 °C is identified as a suitable temperature for the catalytic upgrading of volatiles from Naomaohu coal; higher catalysis temperatures at 500 °C and 550 °C led to lower tar yield and lower phenols content, along with higher gas yield and more carbon deposition.

Magnesium borate whiskers/La2-XCeXZr2O7 catalysts preparation on cordierite honeycomb ceramic surfaces for soot catalytic oxidation͏

Magnesium borate whiskers were used to prepare a hierarchical microstructure on cordierite ceramic substrate through the precipitation-molten salt method. The La2-XCeXZr2O7 catalyst was then loaded onto the magnesium borate whiskers to form the hierarchical microstructure. The Ce-doped catalysts exhibited a significant improvement in the catalytic activity of soot combustion compared to the La2Zr2O7 catalyst. The study employed various techniques to characterize the structural morphology and phase composition, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The growth of Magnesium borate whiskers (Mg2B2O5) with diameters of 100–200 nm and lengths of 2–5 μm were grown and tightly bonded to the cordierite ceramic substrate to mimic the structure of human respiratory system cilia. This could improve the filtration efficiency of soot in car exhaust. BET analysis showed that the specific surface area of cordierite increased from 0.684 m2/g to 1.363 m2/g after whisker growth. The study confirmed that the whiskers were firmly attached to the cordierite ceramic substrate, as demonstrated by the binding strength experiments. The addition of Ce metal has been found to significantly enhance soot combustion due to the presence of more active oxygen species and superior mobility of lattice oxygen species with more oxygen vacancies. The catalyst's intrinsic redox property was characterized using X-ray photoelectron spectroscopy (XPS) and H2 temperature-programmed reduction (H2-TPR). The semi-quantitative thermogravimetric analysis (TG-DTG) and the temperature-programmed oxidation (TPO) of catalysts showed that Z-La1.9Ce0·1Zr2O7 exhibited the highest catalytic activity for soot combustion, with an ignition temperature of 268 °C, a peak temperature of 418 °C, and a temperature of 90 % soot conversion of 464 °C. Furthermore, cyclic stability experiments demonstrate that the Z-La1.9Ce0·1Zr2O7 catalyst exhibits excellent structural stability and consistent catalytic performance. This distinctive microstructure holds great potential for applications in the field of DPF.

Revealing the effect of synthetic method on integrated CO2 capture and conversion performance of Ni-Na2ZrO3 bifunctional materials

Integrated CO2 capture and utilization by reverse water gas shift (ICCU-RWGS) reaction for syngas production for C1 chemical industry has gained increasing attention. Designing highly active bifunctional materials that enables CO2 adsorption and in-situ conversion is essential. In this work, Ni-Na2ZrO3 bifunctional materials are prepared by different synthetic methods for ICCU-RWGS. N2 physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), CO2 temperature-programmed desorption (CO2-TPD) and H2 temperature-programmed reduction (H2-TPR) analysis reveals that Ni-Na2ZrO3-CP prepared by the co-precipitation (CP) method shows small Ni crystalline size, moderate surface basicity, and excellent reducibility. ICCU-RWGS performance is investigated under isothermal conditions of 650 °C, 10 %CO2 and 35 %H2. The results indicate that Ni-Na2ZrO3-CP demonstrates excellent performance in ICCU-RWGS, with a notable CO2 capture capacity of 3.76 mmol CO2/g and a CO production rate of 3.10 mmol CO/g. Additionally, it exhibits relatively good operational stability. Detailed reaction paths of the ICCU-RWGS procedure for the Ni-Na2ZrO3 bifunctional materials are proposed. The findings will shed new perspectives on the design of synthetic bifunctional catalysts for ICCU-RWGS.

Interpretation of H2-TPR from Cu-CHA Using First-Principles Calculations.

Temperature-programmed reduction and oxidation are used to obtain information on the presence and abundance of different species in complex catalytic materials. The interpretation of the temperature-programmed reaction profiles is, however, often challenging. One example is H2 temperature-programmed reduction (H2-TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NOx (NH3-SCR). The TPR profiles of Cu-CHA consist generally of three main peaks. A peak at 220 °C is commonly assigned to ZCuOH, whereas peaks at 360 and 500 °C generally are assigned to Z2Cu, where Z represents an Al site. Here, we analyze H2-TPR over Cu-CHA by density functional theory calculations, microkinetic modeling, and TPR measurements of samples pretreated to have a dominant Cu species. We find that H2 can react with Cu ions in oxidation state +2, whereas adsorption on Cu ions in +1 is endothermic. Kinetic modeling of the TPR profiles suggests that the 220 °C peak can be assigned to Z2CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z2Cu and Z2CuHOOHCu species (360 °C) or paired Z2Cu and Z2CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.

Perovskite-Derivative Ni-Based Catalysts for Hydrogen Production via Steam Reforming of Long-Chain Hydrocarbon Fuel

Large-scale hydrogen production by the steam reforming of long-chain hydrocarbon fuel is highly desirable for fuel-cell application. In this work, LaNiO3 perovskite materials doped with different rare earth elements (Ce, Pr, Tb and Sm) were prepared by a sol-gel method, and the derivatives supported Ni-based catalysts which were successfully synthesized by hydrogen reduction. The physicochemical properties of the as-prepared catalysts were characterized by powder X-ray diffraction, high-resolution transmission electron microscopy, N2 adsorption–desorption isotherms, H2 temperature-programmed reduction, and X-ray photoelectron spectroscopy. The catalytic performance of the as-prepared catalysts for hydrogen production was investigated via the steam reforming of n-dodecane. The results showed that the catalyst forms perovskite oxides after calcination with abundant mesopores and macropores. After reduction, Ni particles were uniformly distributed on perovskite derivatives, and can effectively reduce the particles’ sizes by doping with rare earth elements (Ce, Pr, Tb and Sm). Compared with the un-doped catalyst, the activity and hydrogen-production rate of the catalysts are greatly improved with rare earth element (Ce, Pr, Tb and Sm)-doped catalysts, as well as the anti-carbon deposition performance. This is due to the strong interaction between the uniformly distributed Ni particles and the support, as well as the abundant oxygen defects on the catalyst surface.

Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions

A series of Ag-modified manganese-mullite (SmMn2O5) catalysts with different Ag contents (1, 3, and 6 wt.%) were prepared via a citric acid sol–gel method for catalytic soot oxidation. The catalysts were characterized by powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy analysis (HRTEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). The soot oxidation activity of the mullite was significantly promoted by the addition of silver and affected by the loading amount of the metal. Herein, the influences of silver loading on the metal size distribution and its interactions with the mullite were studied. Based on these characterizations, a possible soot oxidation reaction mechanism was proposed for silver-modified SmMn2O5.

The pivotal role of oxygen vacancy and surface hydroxyl in the adsorption and activation of CO2 on ceria-zirconia mixed oxide

Ceria-zirconia mixed metal oxide along with pure ceria and zirconia were prepared by the co-precipitation method and characterized by N2-Physisorption, X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), CO-TPR with mass spectroscopy (CO-TPR/MS), oxygen storage capacity (OSC), CO and CO2-temperature programmed desorption with MS (TPD/MS), Raman spectroscopy, XPS, and diffuse reflectance infrared Fourier transform Spectroscopy (DRIFTS) techniques. Furthermore, Density Functional Theory (DFT) calculations were performed to acquire molecular insights into the role of oxygen vacancies and surface hydroxyl species in CO2 adsorption and subsequent activation. Our experimental and theoretical results corroborated the fact that the incorporation of zirconium ions into the ceria matrix enhanced the overall CO2 adsorption and activation by forming thermally stable surface species, which were eventually detected by DRIFTS. Additionally, DFT calculations revealed the oxygen vacancy mediated change in the electronic structure of ceria-zirconia mixed oxide, which had been argued to improve its catalytic activity. Furthermore, the oxygen vacancy formation energy (OVFE), oxygen storage capacity (OSC) and CO2 adsorption energies were correlated to augment the importance of these properties.

Selective hydrodeoxygenation of guaiacol to phenol over CoFe/reduced graphene oxide

A reduced graphene oxide (rGO)-supported CoFe bimetallic catalyst (CoFe/rGO) was synthesized and used for hydrodeoxygenation of guaiacol to phenol (Ph). The CoFe/rGO catalyst without reduction pretreatment showed excellent catalytic performance, achieving 94.0 mol% conversion and 70.8 mol% selectivity to Ph at mild reaction temperature and low H2 pressure. The microstructure and chemical composition of the CoFe/rGO catalyst were characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and H2 temperature-programmed reduction (H2-TPR) characterization analysis. Co and Fe are supported on rGO in forms of the isolated metal atoms embedded in the rGO matrix and the CoFe oxide nanoparticles attached on rGO surface. Fe as a promoter weakens the hydrogenation activity of Co to benzene ring and dramatically improves the selectivity to Ph. The unique of rGO on constructing catalytic active sites and the synergistic effect of Co and Fe bimetals make CoFe/rGO an excellent catalyst.

Mechanisms and site requirements for NO and NH3 oxidation on Cu/SSZ-13

Two series of Cu/SSZ-13 catalysts were synthesized via aqueous solution and solid-state ion exchange using SSZ-13 supports of varying Si/Al ratios. The isolated and multinuclear Cu content of these catalysts were determined by H2 temperature programmed reduction (H2-TPR). Multinuclear Cu in these catalysts, including in situ Cu-dimers formed from ZCuIIOH coupling and permanent CuO clusters, are active species for dry NO oxidation. NH3 oxidation on these catalysts follows an internal SCR (i-SCR) mechanism, i.e., a portion of NH3 is first oxidized to NO, then NO is selectively reduced by the remaining NH3 to N2. NH3 oxidation displays distinct kinetic behavior below ∼300 °C and above ∼400 °C. At low temperature the results indicate that NH3-solvated mobile Cu-ions are the active centers. CuO clusters, when present, also contribute to the low temperature activity by catalyzing NH3 oxidation to NO. At high temperature, in situ Cu-dimers and CuO clusters catalyze NH3 oxidation to NO, and isolated Cu-ions catalyze SCR to realize the cascade turnovers. For both NO and NH3 oxidation, Cu-dimers balanced by framework charges of close proximity appear to be more active than Cu-dimers balanced by distant framework charges. However, the former Cu-dimers are less stable than the latter and tend to split into monomers in the presence of vicinal Brønsted acid sites. Via density functional theory (DFT) calculations, the i-SCR mechanism for low temperature NH3 oxidation, i.e., the energetic favorability for the involvement of the NO intermediate, is justified. The DFT results also agree with experimental data that the formation of Cu-dimers from ZCuIIOH dimerization is essential for NH3 oxidation at high temperature.

Cu modified VOx/Silicalite-1 catalysts for propane dehydrogenation in CO2 atmosphere

Propane dehydrogenation in CO2 atmosphere (PDH-CO2) is of interest with a potential to raise propane conversion. Here the catalytic performance of Cu modified 10VOx/Silicalite-1(S-1) for PDH-CO2 is presented for the first time. The initial propane conversion of 68.8% on 0.5Cu-10VOx/S-1 was reached from 59.3% on 10VOx/S-1, along with the CO2 conversion of 44.5% from 32.4%. The role of S-1 support and Cu modification were elucidated by means of X-ray diffraction (XRD), ultraviolet–visible diffuse reflection (UV–Vis), NH3/CO2 temperature-programmed desorption (NH3/CO2-TPD), H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) characterization. While dispersing VOx, non-acidic S-1 minimized the side reactions usually occurring on the strong acidic sites. Cu modification promoted the dispersion of VOx and enabled 10VOx/S-1 exposing more weak acidic sites and basic sites, boosting respectively the adsorption of propane and CO2, for concurrently accelerating the reverse water gas shift (RWGS). Moreover, the highly dispersed V5+ was formed on the surface whereas the percentage of inactive V3+ decreased.

Double-stable Ce-based oxide: Capturing atomic precious metals via Ostwald ripening for multicomponent three-way catalysts construction

Precious metal catalysts have been widely used to catalyze various reactions, particularly three-way catalysts for automobile exhaust purification. Pt, Pd, and Rh, which are dispersed on Ce-based oxides that store and release oxygen, serve as the active center of a catalyst. However, methods to construct multifunctional three-way catalysts to maximize their efficiency and stabilize them are lacking. In this study, Ce-based oxides with a double-stable mode were synthesized to capture and stabilize the precious metals Pt and Rh migrated by Ostwald ripening (OR), and a multicomponent three-way catalyst was accurately synthesized, which showed good catalytic activity and thermal stability. The origin of Ce-based oxides with a dual-stable mode was studied by TEM, XRD, oxygen storage/release capacity and rate test, and XPS. AC-STEM, EXAFS, in situ diffuse reflectance Fourier transform infrared, and H2 temperature-programmed reduction studied the OR migration of precious metals and the composition change after aging. Results showed that in the lean/rich oxygen alternating shift atmosphere containing CO, the proportion of OR migration of Pt and Rh precious metals whose initial state is single atoms can reach 100 %. This discovery provides a basic understanding of the sintering mechanism of precious metals, which is of great importance for the development and industrial application of three-way catalysts.