γ‐alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large‐scale ...heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of γ‐alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopy and solid‐state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O)5Al(VI)‐OH sites on the (100) segments of massively restructured (110) facets on typical rhombus‐platelet γ‐alumina as well as the (100) segments of irrational surfaces (invariably always present in all γ‐alumina samples) responsible for its unique catalytic activity. Such (O)5Al(VI)‐OH sites are also present on the macroscopically defined (100) facets of γ‐alumina with elongated/rod‐like geometry. The mechanism by which these sites lose ‐OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta‐coordinate Al+3O5 sites is clarified. These coordinatively unsaturated penta‐coordinate Al sites produce well‐defined thermally stable Al‐carbonyl complexes. Our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ‐alumina and their role as catalytically active sites.
γ‐Alumina is one of the oldest and largest‐scale commercial catalytic materials, but the precise identification of its catalytically active sites has remained elusive. With the aid of state‐of‐the‐art FTIR, high‐field solid state NMR, and HRTEM techniques, clarification of the nature of the active sites on the surface of γ‐alumina is proposed.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Because of their heterogeneous nature, supported metal catalysts always contain metal centers in a rather broad dispersion range, and the presence of even atomically dispersed metals has been ...reported on oxide supports. The role of the atomically dispersed metal centers in the overall catalytic performances of these supported metal catalysts, however, has not been addressed to date. In this study, temperature programmed reaction and scanning transmission electron microscopy experiments were applied to show the fundamentally different reactivity patterns exhibited by Pd metal in atomically dispersed and traditional 3D clusters in the demanding reaction of CO2 reduction. The requirement for two different catalyst functionalities in the reduction of CO2 with hydrogen on Pd/Al2O3 and Pd/MWCNT catalysts was also substantiated. The results obtained clearly show that the oxide support material, even when it is considered inert like Al2O3, can function as a critical, active component of complex catalyst systems.
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IJS, KILJ, NUK, PNG, UL, UM
The catalytic performance of a series of Ru/Al2O3 catalysts with Ru content in the 0.1–5% range was examined in the reduction of CO2 with H2. At low Ru loadings (≤0.5%) where the active metal phase ...is highly dispersed (mostly atomically) on the alumina support, CO is formed with high selectivity. With increasing metal loading, the selectivity toward CH4 formation increases, while that for CO production decreases. In the 0.1% Ru/Al2O3 catalyst, Ru is mostly present in atomic dispersion, as scanning transmission electron microscopy (STEM) images obtained from the fresh sample prior to catalytic testing reveal. STEM images recorded from this same sample, following the temperature programmed reaction test, clearly show the agglomeration of small metal particles (and atoms) into 3D clusters. The clustering of the highly dispersed metal phase is responsible for the observed dramatic selectivity change during elevated temperature tests: dramatic decrease in CO and large increase in CH4 selectivity. Apparent activation energies, estimated from the slopes of Arrhenius plots, of 82 and 62 kJ/mol for CO and CH4 formation were determined, respectively, regardless of Ru loading. These results suggest that the formation of CO and CH4 follow different reaction pathways or proceed on active centers of a different nature. Reactions with CO2/H2 and CO/H2 mixtures (under otherwise identical reaction conditions) reveal that the onset temperature of CO2 reduction is about 150 °C lower than of CO reduction.
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IJS, KILJ, NUK, PNG, UL, UM
Self-assembly has proven to be a widely successful synthetic strategy for functional materials, especially for metal-organic materials (MOMs), an emerging class of porous materials consisting of ...metal-organic frameworks (MOFs) and metal-organic polyhedra (MOPs). However, there are areas in MOM synthesis in which such self-assembly has not been fully utilized, such as controlling the interior of MOM crystals. Here we demonstrate sequential self-assembly strategy for synthesizing various forms of MOM crystals, including double-shell hollow MOMs, based on single-crystal to single-crystal transformation from MOP to MOF. Moreover, this synthetic strategy also yields other forms, such as solid, core-shell, double and triple matryoshka, and single-shell hollow MOMs, thereby exhibiting form evolution in MOMs. We anticipate that this synthetic approach might open up a new direction for the development of diverse forms in MOMs, with highly advanced areas such as sequential drug delivery/release and heterogeneous cascade catalysis targeted in the foreseeable future.
The hydrogenation of CO2 was investigated over a wide range of reaction conditions, using two Pd/γ-Al2O3 catalysts with different Pd loadings (5% and 0.5%) and dispersions (∼11% and ∼100%, ...respectively). Turnover rates for CO and CH4 formation were both higher over 5% Pd/Al2O3 with a larger average Pd particle size than those over 0.5% Pd/Al2O3 with a smaller average particle size. The selectivity to methane (22–40%) on 5% Pd/Al2O3 was higher by a factor of 2–3 than that on 0.5% Pd/Al2O3. The drastically different rate expressions and apparent energies of activation for CO and CH4 formation led us to conclude that reverse water gas shift and CO2 methanation do not share the same rate-limiting step on Pd and that the two pathways are probably catalyzed at different surface sites. Measured reaction orders in CO2 and H2 pressures were similar over the two catalysts, suggesting that the reaction mechanism for each pathway does not change with particle size. In accordance, the DRIFTS results reveal that the prevalent surface species and their evolution patterns are comparable on the two catalysts during transient and steady-state experiments, switching feed gases among CO2, H2, and CO2 + H2. The DRIFTS and MS results also demonstrate that no direct dissociation of CO2 takes place over the two catalysts and that CO2 has to first react with surface hydroxyls on the oxide support. The thus-formed bicarbonates react with dissociatively adsorbed hydrogen on Pd particles to produce adsorbed formate species (bifunctional catalyst: CO2 activation on the oxide support and H2 dissociation on the metal particles). Formates near the Pd particles (most likely at the metal/oxide interface) can react rapidly with adsorbed H to produce CO, which then adsorbs on the metallic Pd particles. Two types of Pd sites are identified: one has a weak interaction with CO, which easily desorbs into gas phase at reaction temperatures, whereas the other interacts more strongly with CO, which is mainly in multibound forms and remains stable in He flow at high temperatures, but is reactive toward adsorbed H atoms on Pd leading eventually to CH4 formation. 5% Pd/Al2O3 contains a larger fraction of terrace sites favorable for forming these more multibound and stable CO species than 0.5% Pd/Al2O3. Consequently, we propose that the difference in the formation rate and selectivity to CH4 on different Pd particle sizes stems from the different concentrations of the reactive intermediate for the methanation pathway on the Pd surface.
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•Strong Lewis acid sites on alumina inhibit metal sintering during PDH.•Excess Lewis acid sites increased coke deposition during PDH.•Strong but less residual Lewis acid sites are ...important for control the deactivation.
The surface properties of catalyst supports are important in regulating the catalytic properties of heterogeneous catalysts. Herein, we studied the effect of acid-base properties of alumina on metal-support interaction and coke deposition, and investigated the stability of catalysts in propane dehydrogenation (PDH) using PtSn/Al2O3. We prepared γ-Al2O3 (A750) from ammonium aluminum carbonate hydroxide (AACH) and compared it with a commercial sample (Sasol Puralox SBA-200; P200). We loaded 0.5 wt% Pt and 0.9 wt% Sn on alumina then conducted propane dehydrogenation at 590 °C (WHSV = 5.2 h−1). PtSn/A750 and PtSn/P200 showed compatible initial activity (conversion = ˜50%) and selectivity (> 95%). After 20 h of reaction, PtSn/A750 showed a slight decrease in activity (39.9%) while the activity of PtSn/P200 dropped significantly (28.4%). Spent catalysts showed different metal sintering behavior and coke deposition which are well known causes for catalyst deactivation. A high strength of Lewis acid sites in A750 (higher Td in ethanol TPD) prevented the sintering of metal by strong metal-support interaction. Also, the lower number of Lewis acid sites in A750 than that of P200 reduced deposited coke on the catalysts (PtSn/A750: 1.8 wt% and PtSn/P200: 8.6 wt%). Furthermore, diffuse reflectance infrared Fourier-transform spectroscopy after CO adsorption at -150 °C clearly demonstrated that coke deposition was initiated from Lewis acid sites on the alumina surface, but then aromatization occurs at these sites. These results suggested that strong metal-support interactions to hold metal particles and less residual Lewis acid sites after metal loading to reduce coke deposition are important factors for designing stable and coke-resistant PtSn on alumina catalysts. Furthermore, precise characterization and understanding of the acid-base properties of alumina will contribute in developing catalysts with high stability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
After hydrothermal aging at 800°C for 16h, the NOx reduction performance of Cu-ZSM-5, Cu-beta and Cu-Y were significantly reduced, while that of Cu-SSZ-13 was not affected. Display omitted
► ...Hydrothermal aging does not affect the NH3-SCR performance of Cu-SSZ-13. ► Low temperature SCR activity of Cu-ZSM-5 and Cu-beta significantly reduced by hydrothermal aging. ► CuO and Cu-aluminate species formed in Cu-ZSM-5 and Cu-beta during hydrothermal treatment. ► No structural changes were observed for Cu-SSZ-13.
The effects of hydrothermal treatment on model Cu/zeolite catalysts were investigated to better understand the nature of Cu species for the selective catalytic reduction of NOx by NH3. After hydrothermal aging at 800°C for 16h, the NOx reduction performance of Cu-ZSM-5 and Cu-beta was significantly reduced at low temperatures, while that of Cu-SSZ-13 was not affected. When the zeolite framework aluminum species were probed using solid state 27Al MASNMR, significant reduction in the intensities of the tetrahedral aluminum peak intensity was observed for Cu-ZSM-5 and Cu-beta, although no increase in the intensities of the octahedral aluminum peak was detected. When the redox behavior of Cu species was examined using H2-TPR, it was found that Cu2+ could be reduced to Cu+ and to Cu0 for Cu-ZSM-5 and Cu-beta catalysts, while Cu2+ could be reduced only to Cu+ in Cu-SSZ-13. After hydrothermal aging, CuO and Cu-aluminate species were found to form in Cu-ZSM-5 and Cu-beta, while little changes were observed for Cu-SSZ-13.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
H
2
-TPR and FTIR were used to characterize the nature of the Cu ions present in the Cu-SSZ-13 zeolite at different ion exchange levels. The results obtained are consistent with the presence of Cu ...ions at two distinct cationic positions in the SSZ-13 framework.
H
2
-TPR and FTIR were used to characterize the nature of the Cu ions present in the Cu-SSZ-13 zeolite at different ion exchange levels. The results obtained are consistent with the presence of Cu ions at two distinct cationic positions in the SSZ-13 framework.
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•The number and property of sites on alumina are important for Pt-Al2O3 interaction.•Pt dispersion increases with increasing number of sites on alumina.•Pt dispersion increases with ...ethanol desorption temperature (Td) increase on alumina.•Pt dispersion changes under bimodal distribution on Pt/Al2O3.
In this work, how the number and properties of specific sites on alumina surfaces affect the specific interaction between Pt and alumina was investigated by using X-ray diffraction, ethanol temperature programmed desorption, diffuse reflectance infrared Fourier transform spectroscopy, H2 chemisorption, scanning transmission electron microscopy and benzene hydrogenation reaction. Here, we chose two sets of model aluminas having different number of sites with the identical properties and different properties of sites with the same number based on ethanol TPD. The H2 chemisorption results for the model aluminas show that H/Pt are all similar for low Pt loadings, but significantly different for high Pt loadings. For 1 wt% Pt/Al2O3, the number of specific sites on all the aluminas was sufficient to disperse all the Pt, leading to only highly dispersed Pt clusters (∼1 nm). However, at 10 wt% Pt/Al2O3, the number of Pt atoms is greater than that of the specific sites on the alumina surface, resulting in a bimodal distribution of large agglomerated Pt (>10 nm) and highly dispersed Pt clusters (<3 nm) revealed by XRD and TEM. Overall, the results clearly demonstrated that Pt shows higher dispersion with increasing number of sites and interaction strength, because the Pt atoms can interact with specific sites on alumina in greater numbers and more strongly. However, these Pt dispersion changes do not represent the gradual change in Pt cluster sizes, but the relative population change of small (<3 nm) and large agglomerated Pt clusters (>10 nm) under bimodal distribution. The number of large agglomerated Pt clusters decreased with increasing number of sites and interaction strength. This fundamental understanding provides an important perspective for designing Al2O3-based supported catalysts.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Reduction of oxidized Pt (3D PtO2, 2D PtO2 raft, and atomically dispersed Pt) on Al2O3.
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•3D PtO2 particles are easily reduced than 2D PtO2 rafts and atomically dispersed ...Pt.•Metal-support interaction decides reduction and auto-reduction of supported Pt oxides.•Reduction behavior of oxidized Pt affects the CO oxidation rates.
The catalytic performance of supported heterogeneous catalysts is significantly affected by the particle size and morphologies of the active phase. In this work, we studied how metal-support interaction affected the morphology and size of Pt clusters on Al2O3. Under oxidizing atmosphere, when the specific interaction between oxidized Pt and Al2O3 is dominant, highly dispersed 2D PtO2 rafts (1 nm) and atomically dispersed Pt atoms formed. At high Pt loadings, when only a fraction of the metal atoms can interact with the alumina support, or on weakly interacting supports (e.g., SiO2), 3D PtO2 particles (>2 nm) and large Pt crystallites (>10 nm) formed. Due to the weak interaction with the support, these 3D PtO2 particles can be reduced at a lower temperature (−40 °C) than 2D PtO2 rafts (~110 °C) and atomically dispersed Pt (>300 °C). The catalytic behavior for CO oxidation is also affected by the reduction properties of supported Pt catalysts. The activity of Pt/Al2O3 stepwise increases with the reduction temperature, which is closely related with the reduction of these three types of oxidized Pt species. In contrast, Pt/SiO2 didn’t show drastic activity change with the subsequent reduction treatments up to 500 °C because it contains mainly 3D PtO2 particles. All these results demonstrate that specific metal-support interactions guide the geometric and chemical properties and the corresponding catalytic performances of oxidized Pt in Pt/Al2O3 catalysts, underlining the importance of careful catalyst activation for the efficient utilization of metallic Pt in real-world catalytic applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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