γ-Al2O3 Surface Research Articles

Promotion effect and mechanism of the addition of Mo on the enhanced low temperature SCR of NOx by NH3 over MnOx/γ-Al2O3 catalysts

A series of Mn/γ-Al2O3 and MnxMoy/γ-Al2O3 catalysts were prepared by using Incipient Wetness Impregnation (IWI) method. The catalytic performance tests showed that the Mn3Mo1.25/γ-Al2O3 demonstrated a higher SCR performance (NO conversion of around 96%) at a broad low temperature range (150–300 °C). The characterization showed that the addition of Mo to the Mn/γ-Al2O3 catalysts promoted the dispersion of MnOx on the surface of γ-Al2O3. The adsorption of NO could form two different species, i.e., nitrites and nitrates, on the surface of the catalyst. The presence of nitrites is beneficial to low temperature SCR. It is found that the existence of Mo in the catalyst favours the formation of Mn3+, which plays a critical role in the adsorption of NH3 and therefore improves NH3 adsorption capacity of the MnOx/γ-Al2O3 catalysts. The low temperature SCR of the Mn3Mo1.25/γ-Al2O3 catalyst was found to mainly follow L-H mechanism, but E-R mechanism also plays a role to some extent. Moreover, it is found that the addition of Mo not only mitigates the deactivation of the catalysts, but also broadens the effective temperature range of the SCR catalysts.

Roles of Structural Promoters for Direct CO2 Hydrogenation to Dimethyl Ether over Ordered Mesoporous Bifunctional Cu/M–Al2O3 (M = Ga or Zn)

The different behaviors of structural promoters such as gallium or zinc oxides on the acidic γ-Al2O3 surfaces were investigated for a direct CO2 hydrogenation to DME by using highly ordered mesopor...

The usage of carbon dioxide gas on pyrolysis process and its effect on biomass catalytic conversion

Several advantages of the usage of CO2 gas in pyrolysis theoretically could be the role for thermal efficiency, elimination oxygenated group, deep decomposition of biomass. The aim of this study was the use carbon dioxide as carrier gas and its effect on the catalytic product distribution from the rice husk type biomass. The Nickel/Alumina catalyst was prepared by impregnation method and calcination process of the Ni(NO3)2. 6H2O and Al2O3 mixture. The characterization was also performed by XRD analysis and BET adsorption method. The XRD analysis confirm the high crystallinity of NiO and NiAl2O4 phase which is considered as the interaction result between the NiO particle and the surface of γ-Al2O3. But the result of BET method suggested that the blockage the pore mouth of catalyst support γ-Al2O3 which is more likely due to the excessive amount of NiO particles. The comparison of pyrolysis carrier gas between CO2 and N2 and the variation of the operating temperature were performed to determine the product distribution distinctions. The catalytic conversion was done under the temperature 450, 475, 500, and 525°C. Experimental results show that the variation of carrier gas of the pyrolysis process and the variation of operating temperatures resulted in a difference product distribution. The dominant compounds formed as the temperature increases were ketone and benzene compounds.

Synergy effects of basic graphitic-C3N4 over acidic Al2O3 for a liquid-phase decarboxylation of naphthenic acids

Synergy effects of a basic graphitic-carbon nitride (g-C3N4) supported on a Lewis acidic γ-Al2O3 (Al@CN) were investigated to remove naphthenic acids (NAs) in a highly acidic model crude oil (dodecane used as model component) through a liquid-phase decarboxylation, where a novel bifunctional Al@CN catalyst effectively reduced the acidity of the model crude oil. Among the well-known elimination methods of NAs in acidic crude oils, catalytic decarboxylation of NAs has been largely attracted due to its simple process scheme by using Lewis acid catalysts. Even though γ-Al2O3 itself having with mesoporous structures was effective for NAs removal, the activity was largely enhanced by an incorporation of the basic g-C3N4 nanoparticles on the Lewis acidic γ-Al2O3 surfaces at an optimal 20 wt% g-C3N4/Al2O3 through the synergy effects of two active sites. The main roles of the highly dispersed g-C3N4 nanoparticles on the γ-Al2O3 surfaces were to reduce coke depositions and to adjust the adsorption strength of NAs on the newly formed defected carbon surfaces formed on the Al@CN during a liquid-phase decarboxylation reaction. A proper amount of the thermally stable g-C3N4 species on the acidic γ-Al2O3 surfaces not only enhanced the catalytic activity by adjusting the acidic and basic sites on the bifunctional Al@CN but also helped in preserving the pristine basic g-C3N4 structures by strongly interacting with the Lewis acidic γ-Al2O3 surfaces.

Different limits for convergent Pd-Pd lengths in Pd slabs grown over different oxides

The question about possible influence of oxide support on the structure of deposited metal slabs was discussed for a series of TiO2 (rutile), tetragonal form of ZrO2, and some forms of γ-Al2O3 oxides, all of them being frequently applied for metal deposition and СН4 oxidation. The Pd(100) or Pd(110) slab models were constructed over defective and non-defective γ-Al2O3(100), non-defective ZrO2(001), and TiO2(001) supercells within a wide range of charge transfer. The latter includes the transfer from support to Pd slab or an opposite way, i.e., the total charge of all Pd atoms of the unit cells varies from − 10.456 to 2.768 e (both charge boundaries correspond to defective γ-Al2O3 oxide models with higher Al or O surface concentrations, respectively). Quick charge relaxation to nearly zero values starting already from the second Pd layer (the next one after contacting Pd layer) was observed irrespective of the presence of any defects that can charge the metallic slab both positively or negatively. The slab geometry, i.e., Pd-Pd length, of all layers depends on the oxide type and the defect type. After partial defect passivation over defective γ-Al2O3 surface with O2 or OH groups, Pd-Pd length converges to the same length as obtained with non-defective γ-Al2O3 model. The different Pd-Pd bond lengths were obtained in the top of 4-layer Pd slabs deposited over ZrO2 and TiO2 after relaxation relative to the convergent value for the Pd over γ-Al2O3 support. It illustrates the importance of the support type for growing metal nanoparticles if the contact domain with the support is sufficiently large.

Synthesis of layered magnesium-aluminum hydroxide on the γ-Al2O3 surface for modifying the properties of supported platinum catalysts

Layered magnesium-aluminum hydroxides of hydrotalcite-like structure were synthesized on the surface of γ-Al2O3. The synthesis was performed via the interaction of aluminum and magnesium hydroxides and carbonate ions in the pores of alumina support. Novelty of the approach was that the synthesis involved the boehmite phase AlO(OH) that formed directly on the surface of γ-Al2O3 under hydrothermal conditions. This approach provides fixation of the forming layered phase on the surface of initial support and retains textural parameters of the support. The formation of a layered structure on the γ-Al2O3 surface was confirmed by XRD analysis and electron microscopy studies. Investigation of the H2[PtCl6] anchoring on γ-Al2O3 with supported magnesium-aluminum hydroxide by UV–vis DRS, 195Pt NMR and HRTEM showed that platinum was adsorbed on the surface as chloride forms of the complexes with a low degree of hydrolysis even at a low metal content. Catalytic properties of the synthesized supported platinum catalysts were examined in a model reaction of propane dehydrogenation. In comparison with the alumina-platinum catalyst, the modified sample showed a better stability at a somewhat higher yield of propylene because the formation of heavy products was suppressed.

Seeking a Structure–Function Relationship for γ-Al2O3 Surfaces

The surface of γ-Al2O3 is perhaps the most exploited surface in chemistry. It is used as a catalyst and as a catalyst support. Its porosity is often evoked as the key quality of this material. Howe...

Experimental research on selective adsorption of gaseous mercury (II) over SiO2, TiO2 and γ-Al2O3

Solid selective adsorption to partition mercury speciation (Hg0, Hg2+) in flue gas is known as the innovative technology in developing solid agents application in online measurement of mercury speciation by continuous emission monitoring system (Hg-CEMS). Porous materials SiO2, TiO2 and γ-Al2O3 were chosen as the selective adsorbents and the systematical experiments were conducted in a fixed-bed reactor. Adsorption tests indicated that Hg0 was scarcely adsorbed by these three porous adsorbents while Hg2+ exhibited an apparent adsorption behavior. γ-Al2O3 was verified to be efficient in Hg2+/Hg0 separation because of its high HgCl2 adsorption capacity and SO2 anti-interference ability. The intrinsic mechanisms of HgCl2 selective adsorption and SO2 competition over γ-Al2O3 surface sites were further investigated. Temperature programmed desorption (TPD) analyses demonstrated that the weak interaction between Hg0 and γ-Al2O3 surface was ascribed to a physisorption manner while the adsorption of HgCl2 was characterized chemical bonding. Hydroxyl (Al-OH), Lewis Aln+-Bronsted On− pairs and the coordinatively unsaturated oxygen (O2− ions) were deduced to be effective basic sites for the chemisorption of HgCl2 on γ-Al2O3. Simultaneously, the competitive adsorption between SO2 and HgCl2 in O2− ions was confirmed to be responsible for the increased breakthrough of HgCl2 over γ-Al2O3 layer with the existence of SO2.

Structure and Vibrational Properties of Potassium-Promoted Tungsten Oxide Catalyst Monomeric Sites Supported on Alumina (K2O/WO3/Al2O3) Characterized Using Periodic Density Functional Theory

Al2O3-supported tungsten oxide catalysts have been instrumental in many industrially relevant reactions and their reactivity is controlled by their molecular structure. In turn, their molecular structure has primarily been derived via Raman measurements with assignments made using model compounds of known local (molecular) coordination. In this work, the structure and simulated Raman spectra of unpromoted and K+-promoted tungsten oxide catalyst monomeric sites supported on γ-Al2O3(110) (K2O/WOx/γ-Al2O3(110)) were studied using periodic DFT methods. Two different WOx-grafted monomers on the γ-Al2O3(110) surface were identified with a total energy difference of 0.17 eV between both structures. Importantly, both structures showed the presence of W═O and W–OH moieties, thus providing additional insights into experimental Raman data, which typically describe only W═O moieties. The grafted WO3 species were stabilized when the present W–OH groups were oriented toward the alumina surface leading to the formation ...

Layered Double Hydroxide Method for Preparing Ni–Mo/γ-Al2O3 Ultradeep Hydrodesulfurization Catalysts

This article reports a novel strategy for preparing NiMo/γ-Al2O3 ultradeep hydrodesulfurization (HDS) catalysts via thermal decomposition of layered double hydroxides (LDHs). Ni–Al–[C6H4(COO)2]2–-LDHs/γ-Al2O3 (terephthalate-pillared Ni–Al-LDHs/γ-Al2O3) composites were first obtained by in situ crystallization of Ni–Al–[C6H4(COO)2]2–-LDHs on the surface of γ-Al2O3. Then Ni–Al–Mo7O246–-LDHs/γ-Al2O3 composites were synthesized from the above composites by the anion exchange of [C6H4(COO)2]2– with Mo7O246–. Finally, bimetallic Ni–Mo/γ-Al2O3 catalysts were obtained by the subsequent decomposition of Ni–Al–Mo7O246–-LDHs/γ-Al2O3. The LDHs strategy imposes significant effects on improving the Ni/Mo dispersion with weak interaction of active metal and support and thereby greatly improves the HDS properties of the resulting catalysts. The HDS assessment results revealed that the catalyst obtained by LDHs procedure exhibits outstanding HDS activities for 4,6-dimethyldibenzothiophene and Dagang FCC diesel. The novel ...

Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment.

Inorganic oxides play a crucial role in the activation of atomically dispersed metal oxides for catalytic olefin transformations, but the inefficient activation processes remain poorly understood. Activation of methyltrioxorhenium (MTO) for propene metathesis via its deposition on the surface of γ-Al2O3 typically results in <5% active sites, and these sites deactivate rapidly. Simple substitution of the support by a less crystalline (largely amorphous) alumina ( a-Al2O3) results in ca. 4× more activity and at least 10× more productivity. On both types of alumina, metathesis is initiated only at specific sites, whose availability limits the catalytic activity. While the two aluminas have similar total numbers of Lewis acid sites, the less crystalline support activates twice as many grafted MTO sites. Interestingly, a-Al2O3 has nearly double the number of strong Lewis acid sites. However, the number of active sites is ca. 10× lower than the total number of strong Lewis acid sites, and metathesis proceeds even when most are occupied by pyridine. DQSQ and D-HMQC 1H and 27Al solid-state NMR reveal that many Lewis acid sites are co-located with surface hydroxyl groups, which prevent activation and/or cause rapid deactivation. Undercoordinated Al sites ondominant (110) facets, which retain hydroxyl groups under catalyst preparation conditions, are therefore unlikely to lead to stable active sites. In contrast, the minor (100) facets of γ-Al2O3, which are completely dehydroxylated, contain strongly Lewis-acidic five-coordinate Al sites that are necessarily remote from surface hydroxyl groups. Such sites, which are relatively more abundant on less well-crystallized aluminas, are inferred to be responsible for generatingstable metathesis sites.

Ce-modified mesoporous γ-Al2O3 supported Pd-Pt nanoparticle catalysts and their structure-function relationship in complete benzene oxidation

In this investigation, a mesoporous γ-Al2O3 with large surface area and good thermal stability was synthesized and employed as the support for preparing a series of Pd-Pt nanocrystalline catalysts by using a high-temperature solution-phase reduction method. The PdOx (x = 0, 1) and PtOx (x = 0, 2) nanoparticles were found to have a small diameter (about 2 nm) and were highly dispersed on the surface of the mesoporous γ-Al2O3. The results show that catalysts with the appropriate ceria greatly improved the physical adsorption/chemisorption of benzene to Pd-Pt nanocrystals, which greatly enhanced the activity to achieve the complete oxidation of benzene. The high activity of the catalyst was mainly due to the nano-effects of Pd-Pt nanocrystals and the interactions among the active components. 0.2%Pd-Pt(1:1)/10%Ce/γ-Al2O3 demonstrated the best activity for the complete oxidation of benzene at 200 °C. Furthermore, this catalyst maintained 95% benzene conversion for 1000 h at 190 °C and displayed good resistance to water and Cl-poisoning, which indicated that the Pd-Pt nanocrystalline catalysts have promise for adoption by the industry.

Hydrogen Spillover to Copper Clusters on Hydroxylated γ-Al2O3

Density functional theory is used to examine the binding of Cun (n = 1–5) clusters onto γ-Al2O3(110) surfaces. Weak binding of Cu clusters onto a clean surface leads to three-dimensional growth. On...

Enhanced stability of Pd/Al2O3 during aqueous oxidation reaction via SiH4 treatment

γ-Al2O3 is widely present as a support in catalytic application. However, the transformation of γ-Al2O3 into the undesired hydrated boehmite (γ-AlOOH) under the aqueous reaction conditions usually results in an irreversible inactivation of supported Al2O3 catalysts. Toward suppressing the hydration of γ-Al2O3 in the process of catalytic reactions, herein we have devised a new strategy by exploring the SiH4 treatment for successful preparation of Si–Pd/Al2O3 catalysts. SiH4 treatment enables a significant improvement in the poor stability of Pd/Al2O3 for selective oxidation of toluene because SiH4 could effectively anchor palladium nanoparticles and inhibit the formation of boehmite by reacting with unsaturated aluminum sites to reduce the intensity of Lewis acid sites on the surface of γ-Al2O3. Pd/Al2O3 pretreated with SiH4, that is Si–Pd/Al2O3, provides high catalytic activity and stability in the aqueous oxidation of toluene even at high temperature. Moreover, Si–Pd/Al2O3 is reusable without obvious loss of the catalytic activity and selectivity, compared to Pd/Al2O3.

Interfacial Behavior of Orthoborate Ionic Liquids at Inorganic Oxide Surfaces Probed by NMR, IR, and Raman Spectroscopy

Absorption modes and the reactivity of nonhalogenated ionic liquids (ILs) at inorganic oxide surfaces of γ-Al2O3, MgO, and SiO2 particles were characterized using multinuclear (11B, 31P, and 29Si) solid-state magic-angle-spinning NMR, FTIR, and Raman spectroscopy. ILs are composed of the trihexyl(tetradecyl)phosphonium cation, [P6,6,6,14]+, and bis(mandelato)borate, [BMB]−, or bis(salicylato)borate, [BScB]−, anions. Spectroscopic measurements were performed on room-temperature (298 K) samples and samples exposed to 15 h at 373 K. The single-pulse 11B NMR data of heated [P6,6,6,14][BMB] mixed with the inorganic oxides showed a significant change in the spectra of the anion for all three oxides. In contrast, no such spectral changes were detected for heated [P6,6,6,14][BScB] mixed with the inorganic oxides. 31P MAS NMR data for the IL/metal oxide systems revealed interactions between [P6,6,6,14]+ and the surfaces of oxides. A significant intensity of 31P CP-MAS NMR signals indicated a low mobility of cation...

Catalyst deactivation in the direct synthesis of dimethyl ether from syngas over CuO/ZnO/Al2O3 and γ-Al2O3 mechanical mixtures

Direct synthesis of dimethyl ether from syngas over mixed catalysts constitutes a novel route aimed to replace the traditional two-step process. Many previous studies about this one-step process showed that catalyst deactivation is unavoidable. The present study wants to characterize the deactivation of CuO/ZnO/Al2O3 and γ-Al2O3 mechanical mixtures, and develop a deactivation model for predicting catalyst performance in presence of deactivation.It was demonstrated that water adsorbs over the γ-Al2O3 surface, blocking its active sites and causing a sharp conversion drop (mainly observed during the first hours on stream). This effect was reversible and could be avoided by increasing temperature (270 °C or above). The other deactivation mechanism was the deposition of carbonaceous species over the catalyst surface. A deactivation model was proposed and fitted to the experimental data.

Theoretical Insights into the Interaction of Oxygenated Organic Molecules and Cobalt(II) Precursor with γ-Al2O3Surfaces

The interaction of two common oxygenates, ethylene glycol (EG) and acetic acid (AA), with the main exposed (100) and (110) surfaces of γ-Al2O3 are investigated by quantifying the thermochemistry of the drying process. Using density functional theory (DFT) calculations, we determine adsorption free energies and identify the adsorption modes and sites for both molecules interacting with the (100) and (110) surfaces of γ-Al2O3 at various coverages. While the affinity of both molecules is stronger for the (100) surface after drying, EG is more strongly adsorbed on alumina surfaces than AA. EG preferentially interacts through hydrogen bonds on the hydroxyl nests of the (100) and (110) surfaces, while it may also interact through Al–O bonds on the (110) surface at low coverage. AA is predominantly deprotonated and interacts as acetate anions through hydrogen bonds on the (110) and (100) surfaces and through Al–O bonds on the (100) surface at low coverage. We then propose a quantitative comparison of the interaction of Co2+ hexaaquo precursor with the same γ-Al2O3 surfaces. The adsorption configurations of the two organic molecules which may hinder the epitaxial growth of cobalt oligomers on alumina surfaces are identified. However, the thermodynamic analysis shows that EG and AA are less stable than the Co precursors after drying. The strong stabilization of Co precursors is attributed to its epitaxial relationship with the dried alumina surfaces as found in [Angew. Chem. Int. Ed. 2015, 54, 6824–6827] and also to a favorable entropy change during the drying process (vapor release and nitrate counterion decomposition in the gas phase). We finally discuss how these effects can be circumvented by changing the conditions of preparation and drying.

Interfacial stability, electronic property, and surface reactivity of α-MoO3/γ-Al2O3 composites: DFT and DFT + U calculations

The design of α-MoO3/γ-Al2O3 composite is of high interest because such composites are with extraordinary catalytic selectivity for petroleum refining. However, the role of each component in heterogeneous catalyst as well as the essential relationship between interfacial structure and surface reactivity is still ambiguous. To deeply understand these details, we investigated the structure, stability, electronic property, and surface reactivity of α-MoO3/γ-Al2O3 composites by density functional theory DFT and DFT + U. The models of α-MoO3/γ-Al2O3 composites were constructed by combining the α-MoO3 (0 1 0) surface with non-spinel and spinel γ-Al2O3 (1 0 0) and (1 1 0) facets. We found that both the nature of γ-Al2O3 support and the surface coverage of α-MoO3 significantly influenced the interfacial stability and surface catalytic activity. For all composites, the interfaces were stabilized via the formation of different AlOMo bonds between the γ-Al2O3 slab and the α-MoO3 slab. The interaction energy and adhesion work of interface indicated that the spinel γ-Al2O3 (1 1 0) surface is most favorable for the stabilization of α-MoO3 surface. Fermi softness (SF), a readily obtainable electronic property was used to evaluate the surface reactivity of the composites. The results indicated that the surface reactivity of all composites is clearly higher than pure α-MoO3, and the monolayer coverage composites with exposed (1 1 0) surface of γ-Al2O3 exhibited the highest surface reactivity. By analysis of the charge density difference and density of states, we found that the electrons on the interface are largely redistributed and charges transfer from γ-Al2O3 to α-MoO3, which promoted the delocalization of both interfacial and surface electronic states near the Fermi level, resulting in strengthened the interfacial interaction and surface reactivity. In addition, the adsorption and dissociation of H2S on the surfaces of all ML composites was investigated. The reaction pathway and kinetic barrier were determined. The results shown that ML- Mo/nspAl(1 1 0) with the highest Femi softness (1.39) showed the lowest energy barrier (0.35 eV), which further confirmed the effectiveness of the Femi softness.

Effects of Myo-inositol Hexakisphosphate on Zn(II) Sorption on γ-Alumina: A Mechanistic Study

Myo-inositol hexakisphosphate (IHP), a most common organic phosphorus in many soils, can strongly interact with aluminum (Al) oxides and influence the fate of metal ions. In this study, the effects of presorbed IHP on γ-Al2O3 (γ-alumina) surfaces on Zn(II) sorption were investigated in batch experiments using a combination of powder X-ray diffraction (XRD), in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), 31P and 27Al solid-state nuclear magnetic resonance spectroscopies (NMR), and Zn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). The results of the batch experiments show that the presorption of IHP increases the sorption density of Zn(II) on γ-Al2O3 surfaces. The XRD data indicate that the presorption of IHP hinders the formation of Zn–Al layered double hydroxide (LDH) precipitates by raising the critical concentration of Zn(II) required to precipitate the complex. Solid-state NMR spectra further suggest that the chemical environment and speciation of IHP presorbed change, i.e., from inner-sphere surface complexes to ternary surface complexes and to zinc phytate precipitates (Zn-IHP) with the increase in Zn(II) concentration or pH. Linear combination fittings (LCFs) of the EXAFS spectra indicate that the proportion of Zn(II) in binary or ternary surface complexes decreases and that in Zn–Al LDH increases with increasing concentration of Zn(II) at pH 7. Furthermore, the order at which IHP and Zn are added in the reaction can influence the cosorption mechanism. At pH 7, more binary or ternary Zn surface complexes and Zn-IHP form, and less Zn–Al LDH precipitates form if Zn is added first. These results demonstrate that both the timing and concentration of IHP and divalent metals have sweeping influences on their solubility and speciation and these intricacies need to be taken into consideration toward predicting their fates in the environment.

Insight into the formation mechanism of C[sbnd]C chain in ethanol synthesis at the interface of partially hydroxylated γ-Al2O3 (110D) surface and polyethylene glycol solvent

A detailed reaction network has been constructed for ethanol synthesis at the interface of partially hydroxylated γ-Al2O3 (110D) surface and polyethylene glycol (PEG) solvent using periodic density functional theory calculations, which provides a first glimpse into the plausible reaction mechanism for ethanol synthesis. The formation of key intermediate CH3 is the rate determining step of the overall process, accompanied with a reaction barrier of 1.29 eV. Based on thermodynamic, kinetic data and Bader charge analyses, we confirm that the methyl carbonylation is responsible for the initial CC bond formation, which needs to overcome a 0.96 eV barrier and is exothermic by1.67 eV. The hydroxyl coverage conditions formed by dissociative adsorbed H2O govern the key active sites on the γ-Al2O3 (110D) surface which in turn has a significant role in determining the dominant reaction mechanism. Meanwhile, the absence of PEG solvent also affects the optimal reaction route of ethanol synthesis and the essential reason is the existence of fairly strong hydrogen bonds. Our findings manifest the interface located between partially hydroxylated γ-Al2O3 (110D) surface and PEG solvent possesses decent catalytic performance for ethanol synthesis and facilitates providing new strategy to tune and design promising CuZnAl catalysts for ethanol synthesis.