γ-Al2O3 Surface Research Articles

The Role of Lewis Acid Sites in γ-Al2 O3 Oligomerization.

Olefin oligomerization by γ-Al2 O3 has recently been reported, and it was suggested that Lewis acid sites are catalytic. The goal of this study is to determine the number of active sites per gram of alumina to confirm that Lewis acid sites are indeed catalytic. Addition of an inorganic Sr oxide base resulted in a linear decrease in the propylene oligomerization conversion at loadings up to 0.3 wt %; while, there is a >95 % loss in conversion above 1 wt % Sr. Additionally, there was a linear decrease in the intensity of the Lewis acid peaks of absorbed pyridine in the IR spectra with an increase in Sr loading, which correlates with the loss in propylene conversion, suggesting that Lewis acid sites are catalytic. Characterization of the Sr structure by XAS and STEM indicates that single Sr2+ ions are bound to the γ-Al2 O3 surface and poison one catalytic site per Sr ion. The maximum loading needed to poison all catalytic sites, assuming uniform surface coverage, was ∼0.4 wt % Sr, giving an acid site density of ∼0.2 sites per nm2 of γ-Al2 O3 , or approximately 3 % of the alumina surface.

Freeze-drying optimized Ce–P/γ-Al2O3 coating on ceramic-supported Pd catalysts for CO2 methanation and CO oxidation

Ce–P/γ-Al2O3 coatings applied with the freeze-drying technique were deposited on foam ceramics before being further prepared as Pd catalysts. According to the research, phosphoric acid replaced the hydroxyl on the surface of γ-Al2O3 to produce polyhydroxy P-containing sites that readily attracted cerium oxide. The palladium oxide was then embedded into the cerium oxide vacancy. The impacts of the drying technique on the catalysts were investigated, and the results indicated that freeze drying created a special coating with a large specific surface area and uniform mesoporous structure by inhibiting nanoparticle agglomeration. H3PO4's altering action was augmented to boost the number of active sites and the percentage of weakly acidic sites. Due to better P–Ce interactions, more Ce3+ was added into CeO2, which created numerous oxygen vacancies. By adjusting the P-containing sites, the dispersion of cerium oxide and the active phase could be controlled, which aided the activation and spillover of hydrogen. Furthermore, the capacity for oxygen adsorption and desorption was enhanced, and the amount of adsorbed oxygen species increased. As a result, the freeze-drying technique improved the advantages of the Ce–P/γ-Al2O3 coating, increasing the conversion and selectivity of CO2 methanation to 81% and 94%, respectively, and lowering the complete conversion temperature of CO oxidation to 116 °C. This research has a guiding influence on the development of foam ceramic-loaded palladium catalysts in the fields of energy and the environment.

Dispersed Pd/alumina catalyst with finite iodine entry for boosted CO purification and dimethyl carbonate synthesis

Sintering caused catalyst deactivation resulting from the agglomeration of active metal sites. CH3I/CO based approaches are attractive owing to their effectiveness and mildness in rejuvenating sintered catalysts. However, few investigations have been reported for their application in oxides-supported catalysts. Herein, a landscape of reversible structural transformation of Pd/γ-Al2O3 catalyst is depicted. The scenario of Pd redispersion in γ-Al2O3 is significantly different from previously reported activated carbon-supported metal catalysts. Time-controlled CH3I/CO treatment facilitates the rapid redispersion of aggregated Pd particles on γ-Al2O3 surface. The redispersion of Pd promotes the reactivity of Pd/γ-Al2O3 catalyst. For dimethyl carbonate synthesis, the space-time yield increases from 556.9 gDMC·kgcat.−1·h−1 to 1008.9 gDMC·kgcat.−1·h−1. For CO purification, 100% conversion of H2 impurities and the rise of 27.4% in selectivity are achieved. The intrinsic oxidation rate is elevated from 607.8 mmolH2·gPd−1·h−1 to 1166.5 mmolH2·gPd−1·h−1. However, complete oxidation and atomic dispersion of Pd cannot be achieved through further treatments. Combined characterizations verify the particle reconstruction of Pd and the evolution of anchoring sites on γ-Al2O3via strong metal-iodine binding. It reveals a double-edged-sword character of CH3I for Pd redispersion, wherein the excessive treatment results in re-aggregated Pd particles and inferior reactivity.

Partial Oxidation of Propane: Single-Site Pd/γ-Al2O3 Catalysts

The paper proposes a method for synthesizing a heterogeneous catalyst that contains single Pd sites on γ-Al2O3 surface. This method involves preliminary heterogenization of a Pd complex with a hydroxyquinone (e.g., alizarin) on the γ-Al2O3 surface, followed by hydrogenolysis of the Pd–alizarin bond, reduction of Pd(II) into Pd(0), and removal of alizarin. The catalyst was examined by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and tested in propane oxidation into key petrochemicals. During the test, the catalyst exhibited high activity (242 mol of the product per g-at Pd per hour).

Preparation of single phase nano-sized AlN powder by carbothermal reduction and nitridation of hydrothermal synthesized carbon coated γ-Al2O3 from glucose via fast heating

By twice pre-milling γ-Al2O3 in absolute ethyl alcohol plus in glucose aqueous solution, γ-Al2O3 was first coated by carbon via hydrothermal treatment technique. Then, at a fast heating speed of 50 °C/min, nano-sized pure AlN powders were synthesized through carbothermal reduction and nitridation (CRN) at 1550–1600 °C for 60–120 min in N2. The pre-milling in absolute ethyl alcohol effectively broke the initial aggregation of γ-Al2O3, and the subsequent milling in glucose aqueous solution led to well contact between γ-Al2O3 and glucose, which contributed to the formation of intact carbon coating on the surface of γ-Al2O3 during hydrothermal treatment. The coated carbon served as one reactant and inhibitor of particle aggregation for CRN, which also led to retention of γ-Al2O3 and its morphology during heating with the help of high heating speed. As a result, fine AlN of ∼100 nm was directly formed by nitridation of γ-Al2O3. The synthesis mechanism of nano-sized AlN powder by the carbon coated Al2O3/C precursor was discussed.

Degradation of Perfluorooctanoic Acid on Aluminum Oxide Surfaces: New Mechanisms from Ab Initio Molecular Dynamics Simulations.

Perfluorooctanoic acid (PFOA) is a part of a large group of anthropogenic, persistent, and bioaccumulative contaminants known as per- and polyfluoroalkyl substances (PFAS) that can be harmful to human health. In this work, we present the first ab initio molecular dynamics (AIMD) study of temperature-dependent degradation dynamics of PFOA on (100) and (110) surfaces of γ-Al2O3. Our results show that PFOA degradation does not occur on the pristine (100) surface, even when carried out at high temperatures. However, introducing an oxygen vacancy on the (100) surface facilitates an ultrafast (<100 fs) defluorination of C-F bonds in PFOA. We also examined degradation dynamics on the (110) surface and found that PFOA interacts strongly with Al(III) centers on the surface of γ-Al2O3, resulting in a stepwise breaking of C-F, C-C, and C-COO bonds. Most importantly, at the end of the degradation process, strong Al-F bonds are formed on the mineralized γ-Al2O3 surface, which prevents further dissociation of fluorine into the surrounding environment. Taken together, our AIMD simulations provide critical reaction mechanisms at a quantum level of detail and highlight the importance of temperature effects, defects, and surface facets for PFOA degradation on reactive surfaces, which have not been systematically explored or analyzed.

Abrading-Induced Breakdown of Ag Nanoparticles into Atomically Dispersed Ag for Enhancing Antimicrobial Performance

Silver is among the most essential antimicrobial agents. Increasing the efficacy of silver-based antimicrobial materials will reduce operating costs. Herein, we show that mechanical abrading causes atomization of Ag nanoparticles (AgNPs) into atomically dispersed Ag (AgSAs) on the surfaces of an oxide-mineral support, which eventually boosts the antibacterial efficacy considerably. This approach is straightforward, scalable, and applicable to a wide range of oxide-mineral supports; additionally, it does not require any chemical additives and operates under ambient conditions. The obtained AgSAs-loaded γ-Al2O3 inactivated Escherichia coli (E. coli) five times as fast as the original AgNPs-loaded γ-Al2O3. It can be utilized over 10 runs with minimal efficiency loss. The structural characterizations indicate that AgSAs exhibit a nominal charge of 0 and are anchored at the doubly bridging OH on the γ-Al2O3 surfaces. Mechanism studies demonstrate that AgSAs, like AgNPs, damage bacterial cell wall integrity, but they release Ag+ and superoxide substantially faster. This work not only provides a simple method for manufacturing AgSAs-based materials but also shows that AgSAs have better antibacterial properties than the AgNPs counterpart.

Theoretical insights into the support effect on the NO activation over platinum-group metal catalysts.

We systematically explored NO activation at metal/oxide interfaces by the combination of Sr3Ti2O7, Sr3Fe2O7, CeO2, anatase-TiO2, ZrO2, and γ-Al2O3 supports and the platinum-group metal cluster (Pd4, Pt4, and Rh4) using slab-model density functional theory calculations. These metal clusters can be strongly adsorbed at these metal oxide surfaces. The Pt4 and Rh4 clusters show larger adsorption energies than the Pd4 cluster, yet the γ-Al2O3(100) surface shows smaller adsorption energies than other metal oxide surfaces. One oxygen vacancy close to the metal cluster was constructed to evaluate the NO activation at those metal/oxide interfaces. The O atom of NO refills the oxygen vacancy after NO dissociation, while the N adatom is left on the metal cluster. The exothermic process was identified for the NO activation except for the Sr3Fe2O7 case, indicating the significant role of the interplay between the metal cluster and oxygen vacancy.

Corrosion Susceptibility of Passive Films on 1060, 2024, and 5083 Aluminum Alloys: Experimental Study and First-Principles Calculations

The corrosion characteristics of passive films on 1060, 2024 and 5083 aluminum alloys formed in citric acid solution are studied in Cl–-containing solutions by combining scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The passive film on 1060 pure Al exhibits the best corrosion resistance while the presence of alloying elements (Cu and Mg) in the passive films reduce the corrosion resistance of the passive films on 2024 and 5083 Al alloys. According to first-principles calculations, Cu/Mg doping γ-Al2O3 surfaces are looser and more reactive than pure γ-Al2O3 (110) surface, and Cl adsorption behavior is changed in Cu/Mg doping γ-Al2O3 surfaces. This illuminates the difference of corrosion resistances of 1060, 2024 and 5083 aluminum alloys from the aspects of the passive film structure and interaction of chlorine ions with the passive films.

Selective atomic layer deposition of NiO on MoOx/γ-Al2O3 to enhance the active catalytic phase formation for hydrodesulfurization of dibenzothiophene

It is well-established that the Ni-decorated MoS2 edge sites (NiMoS) on the γ-Al2O3 surface are the favorable active sites for efficient hydrodesulfurization (HDS). The intimate contact of Ni with MoOx on γ-Al2O3 (under moderate interaction with γ-Al2O3) enhances the reducibility and sulfidability of NiMo catalyst and their HDS activity, consequently. In this work, selective atomic layer deposition (S-ALD) was utilized as an efficient synthesis technique to selectively deposit NiO on MoOx/γ-Al2O3 to enhance the desired NiMoS active catalytic phase formation after sulfidation. The appropriate ALD temperature window for Ni S-ALD was determined between 150 and 180 °C. The prepared catalysts were characterized by NH3-TPD, H2-TPR, XRD, HR-TEM, and XPS. The results supported the selective deposition of NiO on MoOx/γ-Al2O3 but not on the γ-Al2O3 surface. The S-ALD catalysts enhanced the HDS conversions at lower reaction temperatures and pressures compared to their equivalents made by conventional impregnation. At lower pressures, the S-ALD catalysts resulted in up to 52% higher conversions than their corresponding impregnation ones. The study of catalysts surface by HR-TEM and XPS showed that the main reason for enhanced S-ALD catalysts activity is the formation of more active NiMoS desired catalytic phase with more sulfidability than their counterparts made by impregnation.

The adsorption of single Au atom and nucleation on γ-Al2O3 surfaces.

Single-atom catalysts (SACs) in heterogeneous catalysts have attracted increasing attention and the adsorption and nucleation of single atom on the surface are closely related to the performance of the catalyst. The present work employed density functional theory calculations to examine the adsorption of single Au atom and nucleation on γ-Al2O3 surfaces at the atomic level. The effect of surface hydroxyls group on the adsorption and nucleation of single Au atom on γ-Al2O3 surfaces is explored. It was found that the spillover reactions of surface hydroxyls H atoms with the deposited Au- are not available on the hydroxylated surface. The interaction of Au to the clean surface is the stronger than to the hydroxylated surface. The even-odd alternations of Aux and weak binding of single Au atoms to γ-Al2O3 leads to large even-numbered Au cluster on the surface. Density of states and electron density difference analysis show that the electronic structure of Au/γ-Al2O3 is quite different from the reported Cu and Pd on Al2O3.

Mechanisms of ethanol dehydration to ethylene on γ-Al2O3（100）and (110C): A combined DFT and KMC study

Periodic density function theory (DFT) and kinetic Monte Carlo (KMC) method are carried out to investigate ethanol dehydration to ethylene on γ-Al2O3 surface. Data on the adsorption energy, adsorption sites and heat of reaction of key species for the reaction of ethanol dehydration to ethylene on γ-Al2O3(100) , (110C) and (110D) surfaces were obtained by the DFT method. Subsequently, KMC simulations are performed with DFT results employed as database. The kinetics of ethanol dehydration from γ-Al2O3(100) surface to ethylene was studied by KMC. The effects of water-alcohol ratios on the reaction process were investigated. Under the process conditions of 750 K, 1 MPa, and water-ethanol ratio ratios in the range of 1 to 3, appropriate reducing the water-alcohol ratio can effectively inhibit carbon build-up without having a significant impact on the rate of ethylene production, providing a theoretical basis for extending catalyst life and reduce the industrial production cost.

Co and Ni single sites on the (111)n surface of γ-Al2O3 – a periodic boundary DFT study

Coordination of Co and Ni single-atom on the most active tri-coordinated Al/Metal single-site (111)n surface of γ-Al2O3 leads to stable electron configuration of d7 for Co and d8 for Ni on the surface of γ-Al2O3.

Catalytic activity of γ-Al2O3(110) in the NO + H2 reaction: a DFT study.

In this work, the interaction of the surface of γ-Al2O3(110) with NO and H2 was studied using density functional theory calculations. Free γ-Al2O3(110) adsorbs NO and binds H atoms, but repels the H2 molecule. A triplet of low-coordinated OII-AlIII-OII atoms provides the catalytic activity of γ-Al2O3(110) along the path: (i) the adsorption of NO/AlIII is followed by the binding of H2 to form a hydroxylamine derivative NHOH through an intermediate NO/AlIII + 2 × H/OII complex; (ii) recombination of NHOH with the release of N2 through an intermediate NHOH/AlIII + NHOH/AlIV or adsorption of NO followed by the release of N2O through the intermediate NHOH/AlIII + NO/AlIV; the pathway ends with the regeneration of γ-Al2O3(110). The calculated adsorption heats ensure the diffusion of H atoms from the deposited Pt to the surface (110), initiating the formation of the NH2/AlIII + H/OII complex, which releases NH3 endothermically and is stable enough to inhibit stage (ii) of the above reaction pathway. An excess of O2 in the NO + H2 mixture excludes H/Pt and eliminates inhibition. The formation of oxynitrides is suppressed, but not excluded by more exothermic surface processes. The N-doped conductivity of bulk and surface oxynitrides Al32O47N and the dependence of the heat of adsorption of H atoms on the band gap width were revealed, which suggests a relationship between the band gap width and catalytic activity.

Alkylation for removing trace olefins from reforming aromatics over novel Al2O3@SO42-/ZrO2 catalysts derived from the ZrO2-coated γ-Al2O3 strategy

Sulfated zirconia, as a promising catalyst in various acid-catalytic reactions, still suffer from unsatisfactory catalytic stability and regeneration performance due to severe leaching of sulfur species, while aluminum modification can effectively overcome these drawbacks. Unfortunately, the limited acceptability of aluminum content by co-precipitation and milling methods remains a critical issue. Herein, an effective strategy of coating sulfated zirconia on the surface of γ-Al2O3 was developed to synthesize the novel Al2O3@SO42-/ZrO2 catalyst with high aluminum content. The synthesis variables were investigated and systematically optimized. The structural and acidic properties of Al2O3@SO42-/ZrO2 were determined by XRD, HR-TEM equipped with EDX, N2 adsorption-desorption isotherms, EA, FT-IR, NH3-TPD and Py-FTIR. Results indicated that the unique structure constructed by sulfated zirconia coated on γ-Al2O3 not only realized the stabilization of favorable tetragonal zirconia phase even at exceedingly high aluminum content but also promoted Brønsted acid sites. Accordingly, the optimal Al2O3@SO42-/ZrO2-50% performed outstanding catalytic activity and stability, given the olefins conversion of 98%-75% during the continuous reaction experiment. Owing to the alleviation of sulfur loss by Al-promoting effects, excellent regeneration performance, with olefins conversion only slightly declining after 4 consecutive reaction-regeneration cycles, was also obtained. The mechanistic pathway for the alkylation of olefins with aromatics over Al2O3@SO42-/ZrO2 was also investigated and proposed by GC–MS analysis.

Adsorption and decomposition of CO2 on γ-Al2O3(1 0 0): First-principles investigation

The adsorption and dissociation of CO2 on perfect and oxygen-deficient γ-Al2O3(100) is comparatively studied using first-principles calculations. CO2 molecules are found to physically absorb at perfect γ-Al2O3(100) with a linear configuration without the tendency to decompose. By contrast, the presence of oxygen vacancies at the γ-Al2O3(100) surface results in direct CO2 decomposition at the oxygen vacancy sites or the CO2 adsorption at the Al site adjacent to the oxygen vacancy, followed by the CO2 molecular rotation to the oxygen vacancy site and subsequent decomposition. The charge transfer between adsorbed CO2 and the γ-Al2O3(100) surface is further analyzed to understand the oxygen deficiency effect on CO2 adsorption and decomposition. These results demonstrate the promoting effect of oxygen vacancies of the oxide catalysts on CO2 activation, which have practical importance in catalytic CO2 conversion to value-added chemicals and controlling the oxidation of alumina-forming alloys in CO2 containing atmospheres.

Cu(II) complexes as catalyst precursors in the process of selective hydrogenation of diene hydrocarbons

The reaction of copper(II) acetate with 3-furancarboxylic acid (Hfur) and 5-nitro-2-furancarboxylic acid (Hnfur) with participation of 4-phenylpyridine (phpy) in acetonitrile resulted in mononuclear complexes [Cu(L)2(phpy)2(H2O)]·solv (L = fur (1), nfur (2); solv = phpy (1)) whose structures were determined by direct single crystal X-ray analysis. According to X-ray data, the complexing component in 1 and 2 is in a distorted square-pyramidal environment (CuN2O3); the pyramid base is formed by monodentate-bound oxygen atoms of fur-/nfur- anions and a pair of nitrogen atoms of the phpy moieties, while the water molecule occupies an axial position. It was found for the first time that copper nanoparticles immobilized on the surface of γ-Al2O3 obtained by chemical reduction of 1 and 2 (pre-deposited on the surface from solution) with sodium tetrahydroborate show high selectivity of monohydrogenation of tricyclo[5.2.1.02,6]deca-3,8-diene (dicyclopentadiene, DCPD) to give tricyclo[5.2.1.02,6]dec-4-ene (5,6-dihydrocyclopentadiene, DHDCPD). The catalytic activity tests show high selectivity (up to 100 %) of the catalysts studied in the reaction of partial hydrogenation to DHDCPD under continuous process conditions even at high conversion values (up to 96 %) and with hydrogen present in excess. Thermal behavior of 1 and 2 was studied by simultaneous thermal analysis (STA).

Promoted Activity of Surface Hydroxyls on γ-Al2O3 Mineral Dust with the Coexistence of SO2 and NH3.

Sulfate and ammonium formed on mineral dust can be mutually accelerated through the heterogeneous reactions of coexisting SO2 and NH3. However, little is known about the underlying mechanism, especially the pivotal reactive sites. Using combined Born-Oppenheimer molecular dynamics simulations and density functional theory calculations, the results show that, compared to that of SO2 or NH3 alone on the γ-Al2O3 surface, the increased level of formation of sulfate and ammonium can be attributed to the promoted activity of the surface-bridged hydroxyl with the coexistence of SO2 and NH3. In the specific mechanism, the O and H of the surface-bridged hydroxyl group are attacked by the adjacent SO2 and NH3, respectively, which directly enhances the formation of absorbed sulfite and ammonium, and indirectly facilitates the production of sulfate by oxidation of atmospheric O2. The proposed mechanisms can be broadly applied to other aluminum-based suspended dust particles, such as kaolinite, montmorillonite, and clay dust.

Removal of As2O3 in coal-fired flue gas by metal oxides: Effects of adsorption temperature and flue gas components

Arsenic emitted from coal-fired power plants has attracted widespread attention. Metal oxides are promising adsorbents for arsenic removal, but the influence laws and mechanisms of temperature and flue gas components on their arsenic removal performance are unclear. In this work, systematic research on the effects of adsorption temperature and flue gas components on As2O3 removal by Fe2O3, CaO, and γ-Al2O3 was conducted. The results show that As2O3 removal ability satisfies Fe2O3 > CaO > γ-Al2O3 at 300–900 °C, which is mainly related to their basic site strength. At 500 °C, Fe2O3 has the largest As2O3 removal capacity (adsorption capacity: 158.4 μg/g; removal efficiency: 84.68%). Increasing adsorption temperature is beneficial to the transformation of arsenic to high-valent arsenic (As(V)) on the surface of Fe2O3, CaO, and γ-Al2O3. Low-concentration SO2 will compete with As2O3 for adsorption on the three metal oxides surface, which thereby inhibits their As2O3 removal ability. The presence of NO has an obvious inhibitory effect on the As2O3 removal of Fe2O3, CaO, and γ-Al2O3, which is little affected by the change of NO concentration. Fe2O3 is a metal oxide adsorbent with strong arsenic removal potential. Compared with basic simulated flue gas (BSFG) components, the presence of SO2, NO, and SO2+NO in the simulated flue gas all inhibits the adsorption of As2O3 by Fe2O3. Finally, future work on arsenic removal from coal-fired flue gas adsorbents is presented. This work has important engineering applications and theoretical research guiding significance for the development of high-performance arsenic removal adsorbents.

Coupling Molecular-Scale Spectroscopy with Stable Isotope Analyses to Investigate the Effect of Si on the Mechanisms of Zn-Al LDH Formation on Al Oxide.

While silicate has been known to affect metal sorption on mineral surfaces, the mechanisms remain poorly understood. We investigated the effects of silicate on Zn sorption onto Al oxide at pH 7.5 and elucidated the mechanisms using a combination of X-ray absorption fine structure (XAFS) spectroscopy, Zn stable isotope analysis, and scanning transmission electron microscopy (STEM). XAFS analysis revealed that Zn-Al layered double hydroxide (LDH) precipitates were formed in the absence of silicate or at low Si concentrations (≤0.4 mM), whereas the formation of Zn-Al LDH was inhibited at high silicate concentrations (≥0.64 mM) due to surface-induced Si oligomerization. Significant Zn isotope fractionation (Δ66Znsorbed-aqueous = 0.63 ± 0.03‰) was determined at silicate concentrations ≥0.64 mM, larger than that induced by sorption of Zn on Al oxide (0.47 ± 0.03‰) but closer to that caused by Zn bonding to the surface of Si oxides (0.60-0.94‰), suggesting a presence of Zn-Si bonding environment. STEM showed that the sorbed silicates had a close spatial coupling with γ-Al2O3, indicating that >Si-Zn inner-sphere complexes (">" denotes surface) likely bond to the γ-Al2O3 surface to form >Al-Si-Zn ternary inner-sphere complexes. This study not only demonstrates that dissolved silicate in the natural environment plays an important role in the fate and bioavailability of Zn but also highlights the potential of coupled spectroscopic and isotopic methods in probing complex environmental processes.