Surface Lewis Acid Sites Research Articles

Aluminum‐Doped Zirconia‐Supported Copper Nanocatalysts: Surface Synergistic Catalytic Effects in the Gas‐Phase Hydrogenation of Esters

AbstractThe efficient gas‐phase selective hydrogenation of a series of esters to the corresponding alcohols was achieved over well‐dispersed aluminum‐doped zirconia‐supported copper nanocatalysts (Cu/Al‐ZrO2), which were prepared through a homogeneous coprecipitation route in the presence of cetyl trimethyl ammonium bromide. The characterization revealed that the structure and catalytic performance of Cu/Al‐ZrO2 nanocatalysts were profoundly affected by the addition of Al. Compared with the Al‐free catalyst, Al‐doped materials had higher specific surface areas and smaller copper nanoparticles. In particular, the results confirmed that the incorporation of Al into the ZrO2 framework could form tetrahedrally coordinated Al3+ species, leading to the improvement of metal dispersion and the formation of more surface Lewis acid sites. In the gas‐phase selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG), 100 % DMO conversion, 97.1 % EG selectivity, and a high turnover frequency of 16.9 h−1 were achieved over Cu/Al‐ZrO2 catalyst with a Al/(Cu+Zr+Al) mass ratio of 0.1. The high efficiency of Cu/Al‐ZrO2 catalysts in DMO hydrogenation was attributed mainly to the surface synergistic catalytic effect between highly dispersed metallic copper species and strong Lewis acid sites, which promoted the hydrogenation reaction related to the ester groups, unlike the single case of Cu+Cu0 synergy reported previously that was found to control the extent of hydrogenation. The obtained catalysts displayed excellent catalytic performance in the gas‐phase hydrogenation of other esters including dimethyl succinate, dimethyl maleate, dimethyl adipate, and 1,4‐cycolhexane dicarboxylate.

A comparative study of different fluorine-containing compounds in the preparation of novel alumina binders with rich Brönsted acid sites

Alumina is commonly used as a catalyst binder together with aluminum sol in modern fluid catalytic cracking (FCC) catalysts. The surface acidity properties of alumina strongly affect the catalytic performance of FCC catalysts. Lewis acid sites tend to produce coke because of their dehydrogenation activity, while Brönsted ones produce less coke. Thus, it is beneficial to convert the surface Lewis acid sites into Brönsted type. Fluorine-containing modifiers have been demonstrated to be effective to generate Brönsted acid sites on alumina surface. However, different types of fluorine-containing compounds may have different modification effects. In this work, three fluorine-containing compounds, ammonium fluoroborate (NH4BF4), ammonium fluorosilicate [(NH4)2SiF6], and ammonium fluoride (NH4F), were tested and compared in the modification of alumina surface acidity. Results show that NH4BF4 and (NH4)2SiF6 perform equally well in the generation of Brönsted acid sites, while NH4BF4 is more effective in the reduction of Lewis acid sites. In comparison, NH4F is not so effective in the generation of Brönsted acid sites as the other two compounds.

Nanosized iron and nickel oxide zirconia supported catalysts for benzylation of benzene: Role of metal oxide support interaction

Nanocrystalline Fe2O3 and NiO mesoporous ZrO2 supported catalysts were prepared and thermally treated at 500°C. The efficacy of catalysts for catalyzing the benzylation of benzene using benzyl chloride was studied. The mesoporous ZrO2 alone showed only benzyl chloride conversion of 44% after a reaction time of 600min at 75°C. An increase in the conversion of benzyl chloride to 49% was observed after loading 10 wt% of NiO on ZrO2. The conversion was further increased to 58% with the increase of NiO loading to 20 wt%. However, a drastic increase of conversion was observed after loading 10 wt% Fe2O3 on ZrO2, the conversion reached to 78% under identical reaction conditions. The increase of Fe2O3 loading to 20 wt% led to a significant increase in conversion of benzyl chloride to 91%. The characterization results showed that ZrO2 support is mesoporous comprising monoclinic and tetragonal phases. The loading of the NiO or Fe2O3 led to phase transformation of ZrO2 into cubic phase. Diffraction peaks due to Fe2O3 were not observed; in contrast, the presence of segregated NiO phase was observed in case of NiO supported ZrO2 samples. The mesoporous ZrO2 support maintained the stability of catalytically active Fe2O3 species after six reaction cycles. The enhanced catalytic activity of this catalyst is attributed to the presence of a greater degree of dispersion of Fe2O3 over ZrO2, higher redox potential and a greater degree of accessible surface Lewis acid sites.

Synergistic Effect of Copper and Cobalt in Cu-Co-O Composite Nanocatalyst for Catalytic Ozonation

A novel Cu-Co-O composite nanocatalyst was designed and prepared for the ozonation of phenol. A synergistic effect of copper and cobalt was observed over the Cu-Co-O composite nanocatalyst, which showed higher activity than either copper or cobalt oxide alone. In addition, the Cu-Co-O composite revealed good activity in a wide initial pH range (4.11-8.05) of water. The fine dispersion of cobalt on the surface of copper oxide boosted the interaction between catalyst and ozone, and the surface Lewis acid sites on the Cu-Co-O composite were determined as the active sites. The Raman spectroscopy also proved that the Cu-Co-O composite was quite sensitive to the ozone. The trivalent cobalt in the Cu-Co-O composite was proposed as the valid state.

A novel CuTi-containing catalyst derived from hydrotalcite-like compounds for selective catalytic reduction of NO with C3H6 under lean-burn conditions

A series of hydrotalcite-based CuxTi1 (x=2, 3, 4, 5)-mixed oxide catalysts have been synthesized by homogeneous precipitation and tested in the selective catalytic reduction (SCR) of NO by C3H6 in the presence of excess oxygen. Cu atoms can be categorized into surface and bulk species. The surface Cu species are the active sites while the bulk Cu species possess adsorption property for nitrogen species, and such bulk Cu species varies from CuxTiyOz to CuO with increasing Cu content. The Cu3Ti1 catalyst with the highest amount of surface copper and Lewis acid sites exhibits the best catalytic performance due to its improved electrophilicity of CuxTiyOz, which enhance the adsorption capability of nitrogen species. In situ infrared characterization revealed that nitrates in terms of bridging, bidentate and chelating species are reactive toward acetate and formate, and they are the key intermediates produced during the SCR of NO.

Structure–activity relation of iron oxide catalysts in soot oxidation

In this paper, a systematic series of bare iron oxide catalysts was evaluated towards their activity for the oxidation of soot under oxygen-rich conditions. The catalytic studies were performed by temperature programmed oxidation (TPO) using tight contact mixtures of catalyst and soot. A home-made carbon black originated from propene combustion was taken as model soot. The catalysts were thoroughly characterised by employing powder X-ray diffraction (PXRD), N2 physisorption, high resolution transmission electron microscopy (HRTEM), temperature programmed reduction by H2 (HTPR), temperature programmed desorption of NH3 (NH3-TPD), temperature programmed desorption of O2 (O2-TPD) and thermogravimetry coupled with difference thermal analysis (TG/DTA). Special notice was put on the NH3-TPD data, which were used for the modelling of the NH3 adsorption and desorption. This model included elementary reactions and implied the specific number of Bronsted and Lewis acid sites. The obtained kinetic parameters were found to be close to that of related oxides such as ZrO2 and H-BEA zeolite. The results of the physical–chemical characterisation as well as NH3 adsorption/desorption kinetics were coupled with the catalytic performance to identify determining properties of the iron oxide catalysts. From these correlations it was derived that both Lewis acid surface sites as well as crystallinity drive the catalytic activity for soot oxidation. The effect of these characteristics is in line with the postulated role of the catalyst implying the transport of oxygen from iron oxide to soot by surface and bulk diffusion.

Mesoporous nanonickel oxide: Anode with good initial discharge capacity and efficiency in lithium ion batteries at 1C rate

Abstract Transition metal oxides are extensively investigated as alternative anode materials for lithium ion batteries owing to their high theoretical capacity. In this work, mesoporous nickel oxide nanostructure is synthesized by a simple solution method without high calcination temperature. The sample is characterized using X-ray diffraction, Fourier Transform Infra Red (FT-IR), Scanning Electron Microscopy, Transmission Electron Microscopy, Nitrogen adsorption–desorption isotherm and surface pyridine adsorption. Large surface area, nanosphere particles, mesoporous structure and surface lewis acidic and Bronsted sites are confirmed. The electrochemical properties of mesoporous nanonickel oxide is investigated including the cycling performance as anode material for lithium ion batteries. The results show that nanonickel oxide has a high discharge capacity at 1 C, especially the first 5 cycles, compared to previous studies at low rates (0.035 and 0.14 C) with the same material. Also, nanonickel oxide shows good efficiency through charge–discharge cycles. All results and data are correlated and discussed.

Role of the Surface Lewis Acid and Base Sites in the Adsorption of CO2 on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study

An understanding of the adsorption of CO2, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO2 on oxidized, reduced, and platinized TiO2 nanotubes (Ti-NTs) was studied using infrared spectroscopy. The Ti-NTs were characterized with TEM and XRD, and XPS was used to determine the oxidation state as a function of oxidation, reduction, and platinization. The XPS data demonstrate that upon oxidation, surface O atoms become more electronegative, producing sites that can be characterized as strong Lewis bases, and the corresponding Ti becomes more electropositive producing sites that can be characterized as strong Lewis acids. Reduction of the Ti-NTs produces Ti3+ species, a very weak Lewis acid, along with a splitting of the Ti4+ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti3+ is not observed on reduction of the platinized Ti-NTs, presumably because Pt ac...

Novel magnetically separable nanomaterials for heterogeneous catalytic ozonation of phenol pollutant: NiFe2O4 and their performances

Magnetic NiFe2O4 nanomaterials were proposed as novel ozonation catalysts for the first time. The samples which were synthesized by the hydrothermal method and the calcined method were dominated as NiFe2O4–H and NiFe2O4–C, respectively. Compared with ozonation alone, the presence of NiFe2O4–H significantly enhanced the degradation of phenol, while NiFe2O4–C was noneffective. The interaction between NiFe2O4–H and ozone was obviously stronger than that in ozonation/NiFe2O4–C. This interaction between catalyst and ozone was proved to be a surface process and played a vital role in the heterogeneous catalytic ozonation. Phosphate revealed significantly inhibitive influence on the decomposition of ozone and the degradation of phenol in ozonation/NiFe2O4. Lewis acid sites were proved as reactive centers for catalytic ozonation in aqueous solution. The results indicated that not only the more surface Lewis acid sites, causing the strong interaction between catalyst and ozone, but also the enhanced interfacial electron transfer lead to the higher catalytic activity of NiFe2O4–H than NiFe2O4–C. Furthermore, the NiFe2O4–H nanocatalyst could be easily and efficiently separated from the reaction mixture with an external magnet, which made it an attractive nanomaterial with prospective application in catalytic ozonation of organic pollutants in water treatment.

Influence of cerium modification methods on catalytic performance of Au/mordenite catalysts in CO oxidation

This work investigated influence of cerium modification methods (impregnation and ion-exchange) on the gold dispersion, surface chemical states, synergistic effect between gold and cerium and CO oxidation activity of Au catalysts supported on Ce modified mordenites. The supports and catalysts were characterized by XRF, ICP, XRD, N2 adsorption–desorption, UV–vis spectroscopy, HRTEM, in situ FT-IR and XPS. In general, Ce-containing Au catalysts were found to be more active in CO oxidation than the Ce-free counterpart, with the impregnation method of cerium modification being more effective than the ion exchange method. The different catalytic performance of Ce-containing catalysts was related to multiple factors: (1) the impregnation method of cerium modification was more favorable to form Au particles with smaller mean size than the ion-exchange method, due to the higher ratio of surface Lewis acid sites; (2) the interface between Au and CeO2 particles produced by impregnation method was more beneficial for the redox equilibrium of Au0+Ce4+↔Au3++Ce3+ to shift to right to form more active Au3+ species than the interface between Au particles and Ce cations produced by ion-exchange method, which enhanced the synergistic effect between gold and cerium and the catalytic performance of CO oxidation.

Effect of reduction temperature on selective hydrogenation of crotonaldehyde over Ir/TiO2 catalysts

Vapor-phase selective hydrogenation of crotonaldehyde was conducted over Ir/TiO2 catalysts to investigate the effect of reduction temperature on the catalytic behaviors. The catalyst reduced at 100°C contained large amount of strong Lewis acid sites, which was detrimental to the activity because of the strong adsorption of crotonaldehyde molecules on the active sites. Also, the selectivity to crotyl alcohol was suppressed due to the interaction between CC bond and the charge-deficient Ir species, which favored the formation of butanal. When the catalyst was reduced at high temperature, it was found that 300°C was appropriate for high activity and selectivity, due to the moderate interaction between the CO bond and the Ir species, as well as the weakened surface Lewis acid sites. However, higher reduction temperature (500°C) led to a strong bonding of carbonyl carbon and the charge-rich Ir species and suppresses the activity, which may be related to a strong metal–support interaction (SMSI).

Niobia-modified aluminas prepared by impregnation with niobium peroxo complexes for dimethyl ether production

Use of a water-soluble niobium peroxo complex allowed the preparation of niobium-modified aluminas containing up to 90% of the theoretical niobia monolayer in one impregnation step. There was a maximum in the density of surface Lewis acid sites at 45% of the theoretical monolayer. FTIR of adsorbed pyridine and adsorbed CO2 suggest the vertical growth of the Nb2O5 layer for the largest niobium contents. The addition of 22.5% of the theoretical monolayer eliminated about 80% of the basic surface hydroxyls, inhibiting the adsorption of gas phase CO2 by the samples. The niobia/alumina catalysts suffered less inhibition by CO2 than the pure alumina in the methanol dehydration reaction, confirming that Nb2O5 is mainly deposited on sites where the CO2 adsorption is stronger, leaving free sites that are active in catalytic dehydration and less inhibited by CO2, however none of the niobia/aluminas was more active than the pure alumina. Nevertheless, in the direct syngas to DME conversion using a mixed catalyst system comprised of a CuZnAl methanol synthesis catalyst and a methanol dehydration component, the activity was significantly larger with a niobia/alumina as a dehydrating component than with the pure alumina.

Dechlorinating transformation of propachlor through nucleophilic substitution by dithionite on the surface of alumina

The aim of this study was to evaluate the roles of aluminum-based minerals, especially their surface Lewis acid sites (LASs), on the transformation and fate of chloroacetanilide herbicide contaminants when nucleophilic reagents are present. Batch experiments were used to study propachlor transformation processes under different reaction conditions. The surface bonding of aluminas before and after interacting with propachlor and dithionite were characterized by Fourier transform infrared (FTIR) spectra. The LASs on different aluminas, acting as the key role for accelerating the propachlor transformation, were analyzed by pyridine adsorption/FTIR spectroscopic technique. Rates of dithionite-initiated propachlor dechlorination were increased in the presence of aluminas. Transformation efficiencies of propachlor on different aluminas were found to be as γ-Al2O3 > γ-AlOOH > α-Al2O3. A higher reaction temperature, higher pH, and higher alumina dosage can further increase the propachlor dechlorination rate. The addition of citric acid may block the active sites on alumina and reduce propachlor transformation by dithionite. FTIR and pyridine adsorption/diffuse reflectance Fourier transform infrared spectroscopy indicated that Al–S and Al–O bonds on the LASs of alumna play a key role in accelerating propachlor transformation. LASs on alumina surfaces can effectively accelerate the propachlor transformation by dithionite. The strong electron accepting ability of LASs gives dithionite a favorable affinity to form surface sulfur compounds, which are stronger reductants and nucleophilic reagents for propachlor dechlorination. The intensities of LASs on alumina surfaces can be used to control the rate of propachlor transformation by dithionite.

Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] as a molecular model of ZnS surfaces: an experimental and theoretical study

Experimental and theoretical results pertaining to [Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3], a possible molecular model of ZnS S-terminated polar surfaces, as well as a potential source of strictly monodispersed ZnS quantum dots, are presented and discussed. The results of density functional theory (DFT) calculations provided a rationale for the peculiar arrangement of [Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] clusters in the solid state, contemporarily indicating the unsuitability of the isolated species to mimic whatever (polar or non-polar) ZnS surface. Despite the fact that such a failure is further confirmed by time-dependent DFT and UV–Vis diffuse reflectance spectroscopy, the combined use of theoretical outcomes, DRIFT measurements, and literature data pertaining to the surface chemical properties of ZnS (Hertl in Langmuir 4:594, 1988) ultimately testifies that [Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] is perfectly suited to model the interaction of pyridine molecules with ZnS surface Lewis acid sites. The herein reported theoretical results are expected to be a useful reference for the interpretation of chemisorption experiments of Py-based Lewis bases on single crystal ZnS surfaces.

Infrared studies of CO oxidation by oxygen and by water over Pt/Al2O3 and Pd/Al2O3 catalysts

CO adsorption has been studied over Pd/Al2O3 and Pt/Al2O3 catalysts using IR spectroscopy in different conditions and after different pretreatments. The oxidation of CO by oxygen and water (water gas shift) has also been studied at 130–673K. The spectra show that the most active species as oxidant for CO are dispersed Ptn+ and Pdn+ cations and PdOx species mainly “decorating” the defects of alumina crystals where they modify completely the behaviour of surface OH's and Lewis acid sites. Some of these species oxidize CO to CO2 already below ∼200K. These highly oxidized species escape detection by IR during reaction at 500–673K, when only reduced sites are apparent due to their very strong interaction with CO. The data suggest that redox mechanisms could occur for both WGS and CO oxidation occurring on highly dispersed cationic species non detectable under reaction conditions.

Activity, selectivity and stability of Zn-exchanged NaY and ZSM5 zeolites for the synthesis of o-hydroxyacetophenone by phenol acylation

The gas-phase synthesis of o-hydroxyacetophenone (o-HAP) by phenol acylation with acetic acid was studied on Zn-exchanged NaY and ZSM5 zeolites. The density, nature and strength of surface acid sites were determined by temperature programmed desorption of NH 3 coupled with infrared spectra of adsorbed pyridine. NaY zeolite contained only surface Lewis acid sites. The exchange of Na + with Zn 2+ increased both the density and strength of Lewis acid sites of parent NaY zeolite. The Lewis/Brønsted acid sites ratio on ZSM5 zeolite was about 1. The addition of Zn increased the density of Lewis acid sites but partially eliminated the protonic sites of parent ZSM5 zeolite. Thus, the Lewis/Brønsted acid sites ratio was 3.2 on a ZnZSM5 containing 1.19% Zn. The incorporation of Zn 2+ to zeolite NaY increased the initial selectivity to o-HAP because o-HAP was preferentially formed through the attack of the acylium ion to the phenol molecule adsorbed in vertical orientation on Lewis acid sites. Nevertheless, the activity decay rate on ZnNaY samples increased with the Zn content. ZnZSM5 zeolites were more active but less selective to the o-HAP formation than ZnNaY zeolites. However, ZnZSM5 zeolites formed only small quantities of coke and exhibited a remarkable stability on stream.

Adsorbate–substrate interaction between chlorodifluoromethane and titanium dioxide: Infrared spectroscopy and density functional theory studies

Infrared spectra of chlorodifluoromethane (CHClF 2) adsorbed on titanium dioxide (TiO 2) at room temperature have been investigated for the first time. From the comparison between the adsorption characteristics and the gas-phase spectra it can be deduced that the molecule interacts with the surface Lewis acid site (Ti 4+) mainly through the Cl atom even if also the adsorption with one F atom is also observed. Moreover, the spectra show the presence of H-bonds between the CH group and the surface Lewis basic site (OH − or O 2−). In order to obtain more information on the molecule orientation and the variation of the structural parameters, a DFT-B3LYP study has been carried out considering the anatase (1 0 1) surface and evaluating the adsorption energetics in terms of interaction, distortion and binding energies. The obtained geometries confirm that both the acid–base interactions through Cl or F atoms are possible and suggest the formation of one H-bond between the CH group of the molecule and the Lewis basic site of the surface. The calculated vibrational frequencies of the adsorbed molecule have been found to be in reasonable agreement with the experimental data.

Molecular nature of support effects in single-site heterogeneous catalysts: silica vs. alumina

Well-defined silica- and alumina-supported catalysts can display very different reactivity, which is often attributed to support effects. Advanced spectroscopic studies in combination with computational chemistry show that the origin of these differences is mainly molecular in nature. Silica partially dehydroxylated at 700 °C features mainly isolated silanols (SiOH) as surface functional groups, which can be exploited to generate well-defined and site isolated siloxy metal species. In contrast, alumina, a more complex support, exhibits both hydroxyl groups and highly reactive Al,O Lewis acid and base surface sites. Therefore, it is better understood as a multifunctional and adaptable ligand, which can lead to a variety of surface species, often more reactive (cationic) and/or stable than their known silica counterparts.

Effects of NaCl on Pt/ZrO 2 catalysts for selective hydrogenation of crotonaldehyde

A series of NaCl-modified Pt/ZrO 2 catalysts were prepared and tested for vapor phase selective hydrogenation of crotonaldehyde. It was found that the reactivities of the catalysts increased with NaCl contents, with the highest selectivity to crotyl alcohol (60%) at a NaCl content of 1%. Ammonia temperature programmed desorption and Fourier transmission infrared spectra of pyridine adsorption results indicated that the strengths of surface Lewis acids of the catalysts were weakened with increasing NaCl contents, such weakening was responsible for the enhancement of the reactivity. Diffuse reflectance infrared transform spectra of CO adsorption and X-ray photoelectron spectrometer results suggested that increased electron density on Pt species with the NaCl modification was responsible for the enhanced selectivity. However, high NaCl content (>3%) in the catalyst resulted in the growth of Pt particles and fewer interfacial Pt sites contacting with the surface Lewis acid sites, such changes were unfavorable for the selectivity.

ChemInform Abstract: Surface Modified Niobium Oxide Catalyst: Synthesis, Characterization, and Catalysis.

Abstract The molecular structures and reactivity of surface modified niobium oxide catalysts were determined by Raman spectroscopy, pyridine adsorption as well as the methanol oxidation reaction. Metal oxides (Re 2 O 7 CrO 3 , WO 3 , MoO 3 , and V 2 O 5 ) and acids (phosphate and sulfate) form a surface overlayer on the niobia support, and possess a structure similar to the metal oxide ionic species present in acidic solutions (ReO 4 − , Cr 3 O 2− 10 , W 12 O −8 40 , Mo 8 O 4− 26 , V 10 O 6− 28 , H 2 PO − 4 , and HSO − 4 ) when the surface is hydrated. Upon dehydration, only one dehydrated surface ReO 4 species is present on niobia. For the CrO 3 , WO 3 , MoO 3 , V 2 O 5 , P 2 O 5 , and sulfate supported on niobia two dehydrated surface metal oxide species, highly distorted and slightly distorted structures, are present. Acidity and BET studies reveal that the presence of the surface metal oxide species stabilizes the surface acidic properties and surface area of niobia during high calcination temperatures (500°C). The 1% V 2 O 5 and 1% P 2 O 5 on Nb 2 O 5 samples possess a high concentration of surface acid sites as well as surface area, and the strength of surface Lewis acid sites on 1% P 2 O 5 /Nb 2 O 5 is stronger than the other systems. In addition, the surface V 2 O 5 , MoO 3 , CrO 3 , and Re 2 O 7 sites on niobia behave as redox sites and the surface WO 3 , P 2 O 5 , and sulfate sites on niobia behave as acid sites during methanol oxidation. The weak interaction between the surface rhenia species and the niobia support results in volatilization of surface rhenia during methanol oxidation and a corresponding low activity. Thus, the surface properties of niobia catalysts could be altered by the addition of surface metal oxide overlayer.