Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour

The relationships between morphology and Lewis acid and base character of surface sites of two types of titania powders (TiO₂ P25 and TiO₂ Merck) were studied by HRTEM and FTIR spectroscopy of adsorbed molecules. Electron micrographs revealed that TiO₂ P25 microcrystals have a prismatic shape, mainly exposing (0 0 1) and (0 1 0) surface planes. TiO₂ Merck powder, which exhibits a significantly lower specific surface area, appeared constituted by large roundish microcrystals. FTIR spectra of adsorbed CO indicated that Ti⁴⁺ ions exposed on (0 0 1) and (0 1 0) faces of TiO₂ P25 particles are Lewis acid centres significantly stronger than those present on the surface of TiO₂ Merck microcrystals. As in both cases the exposed cations are coordinated to five oxygen anions, the observed differences in Lewis acidity are ascribed to some difference in the geometric arrangement of the O²⁻ ligands. Such difference in structure affects the basicity of these centres also. In fact, a fraction of O²⁻ ions on the surface of TiO₂ P25 behave as basic centres toward CO₂ linearly adsorbed on neighbour Ti⁴⁺ centres, and then Lewis acid–base pairs can be recognised. By contrast, no basic activity towards CO₂ was detected for the TiO₂ Merck sample.

The two titania powders exhibited different chemical behaviour in condition of high surface hydration also. Hydroxyl groups on the surface of hydrated TiO₂ P25 are able to transform benzaldehyde molecules in hemiacetalic-like species, whereas C₆H₅CHO molecules are only weakly perturbed by interaction with the OH groups on TiO₂ Merck particles. This feature could be related to the different photocatalytic behaviour in the oxidation of toluene in gas phase, where benzaldehyde was found as a relevant intermediate species.     

Conversion of N2O to N2 on TiO2(1 1 0)

In this study, we examine the interaction of N₂O with TiO₂(1 1 0) in an effort to better understand the conversion of NO_x species to N₂ over TiO₂-based catalysts. The TiO₂(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO₂(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti³⁺ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N₂O molecules adsorbed at 90 K on TiO₂(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N₂O molecules exposed to TiO₂(1 1 0) at 90 K decompose to N₂ via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N₂ ejection from the surface and vacancy oxidation. We propose that this channel involves N₂O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N₂O that decomposes at 170 K to liberate N₂ in the gas phase and deposit oxygen adatoms at non-defect Ti⁴⁺ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N₂O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti⁴⁺ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N₂O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N₂O at 90 K, and this charge transfer facilitates N₂O decomposition. Based on these results, it appears that the decomposition of N₂O to N₂ requires trapping of the molecule at vacancies and that the lifetime of the N₂O–vacancy interaction may be key to the conversion of N₂O to N₂.     

The role of acidity in the decomposition of 1,2-dichlorobenzene over TiO2-based V2O5/WO3 catalysts

The influence of Lewis and Brønsted acid sites on the performance of V₂O₅/TiO₂ and V₂O₅–WO₃/TiO₂ catalysts in the total oxidation of o-dichlorobenzene was investigated. Catalytic activity of these materials resulted strongly affected by their acidic properties. The presence of Brønsted acid sites significantly increases the o-DCB conversion but also leads to the uncompleted degradation of chlorinated compounds, promoting the formation of partial oxidation products, as dichloromaleic anhydride. On the contrary, Lewis acid sites, acting as absorbing sites, promote the further oxidation of intermediates to CO and CO₂, without any by-products desorption.

Furthermore, the presence of water in the feed-stream was proven to decrease o-DCB conversion but also to play a positive role on process selectivity, increasing COx production. Plausible reasons for this effect are the reduction of Brønsted acid sites and the hydrolysis of anhydride during wet tests.     

Hartree–Fock periodic study of the chemisorption of small molecules on TiO2 and MgO surfaces

We present ab initio periodic Hartree–Fock calculations (CRYSTAL program) of the adsorption of small molecules on TiO₂ and MgO. These may be molecular or dissociative, depending on the acidic and basic properties of the molecules in gas phase and of the nature of the surface oxide. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages of the molecule; they also depend on the surface oxide; the oxygen atom of MgO, in spite of its large charge, is poorly reactive and dissociation on MgO is not favorable.

The surface hydrosyl of MgO are more basic than the O of the lattice and water is not dissociated under adsorption. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs basic behavior, but in terms of hard and soft acidity. MgO surface is a “soft” acidic surface that reacts preferentially with the soft base, CO₂.

Another important factor is the adsorbate–adsorbate interaction: favorable cases are the sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface, so that their mutual orientation resembles the geometry of the CO₂ dimer.     

Study of the NOχ reaction with reducing gases on Fe/ZrO2 catalyst

The Fe/ZrO₂ catalyst (1% Fe by weight) shows a strong adsorption capacity toward the nitric oxide (at room temperature the ratio NOFe is ca. 0.5) as a consequence of the formation of a highly dispersed iron phase after reduction at 500–773 K. Nitric oxide is adsorbed mainly as nitrosyl species on the reduced surface where the Fe²⁺ sites are prevailing, but it is easily oxidised by oxygen forming nitrito and nitrato species adsorbed on the support. However, in the presence of a reducing gas such as hydrogen, carbon monoxide, propane and ammonia at 473–573 K the Fe-nitrosyl species react producing nitrogen, nitrous oxide, carbon dioxide and water, as detected by FTIR and mass spectrometers. The results show that nitric oxide reduction is more facile with hydrogen containing molecules than with CO, probably due the co-operation of spillover effects. Experiments carried out with the same gases in the presence of oxygen show, however, a reduced dissociative activity of the surface iron sites toward the species NO_χ formed by NO oxidation and therefore the reactivity is shifted to higher temperatures.     

Pt–Ba–Al2O3 for NOx storage and reduction: Characterization of the dispersed species

Pt–Ba–Al₂O₃ active and selective for NO_x storage and selective reduction to N₂ has been prepared and tested. Characterization of the parent Al₂O₃, Pt–Al₂O₃ and Ba–Al₂O₃ materials, as well as of Pt–Ba–Al₂O₃ catalyst in the oxidized, reduced and sulphated state has been performed by FT-IR spectroscopy of low-temperature adsorbed carbon monoxide and of adsorbed acetonitrile. XRD, TEM and XPS analyses have also been performed. Evidence for the predominance of Ba species, which are highly dispersed on the alumina support surface, and may be carbonated or sulphated, has been provided. Competitive interaction of Pt and Ba species with the surface sites of alumina has also been found.     

Catalytic activity of Pd loaded on WO3/Al2O3 for NO–CH4–O2 in the presence of water vapor

Pd loaded on various kinds of monolayer supports was applied for selective reduction of NO by methane in the presence of O₂ and water vapor. Pd/WO₃/Al₂O₃ exhibited the highest conversion of NO to N₂ among Pd loaded monolayer supports. The catalyst was relatively tolerant and reversible upon the exposure of water vapor. This is due to the enhanced amount of Brønsted acid sites under the moisture as evidenced by the IR measurement of adsorbed pyridine. The Brønsted acid sites generated on WO₃/Al₂O₃ support were required to give the dispersed Pd species, similar to on the zeolite.     

Improvement of catalytic activity and sulfur-resistance of Ag/TiO2–Al2O3 for NO reduction with propene under lean burn conditions

Ag-based catalysts supported on various metal oxides, Al₂O₃, TiO₂, and TiO₂–Al₂O₃, were prepared by the sol–gel method. The effect of SO₂ on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO₂–Al₂O₃ in comparison to Ag/Al₂O₃ and Ag/TiO₂, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO₂–Al₂O₃. NO₃⁻ rather than NO₂⁻ or NOx reacted with the carboxylate species to form CN or NCO. NO₂ was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO₂–Al₂O₃. More amount of formate (HCOO⁻) and CN were generated on the Ag/TiO₂–Al₂O₃ catalyst than the Ag/Al₂O₃ catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO₂ oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO₂–Al₂O₃ system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO₂ toleration to the Ag/Al₂O₃ catalyst.     

Sites for CO2 activation over amine-functionalized mesoporous Ti(Al)-SBA-15 catalysts

Activation of CO₂ and its utilization in the synthesis of chloropropene and styrene carbonates over functionalized, mesoporous SBA-15 solids, have been investigated. The surface basicity of SBA-15 was modified with nitrogen-based organic molecules of varying basicity viz., alkyl amines (–NH₂), adenine (Ade), imidazole (Im) and guanine (Gua). The surface of SBA-15 was also functionalized with Ti⁴⁺ and Al³⁺ species. The acid–base properties of these modified SBA-15 materials were investigated by temperature-programmed desorption (TPD) and diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy. NH₃ and pyridine were used as probe molecules for acid sites, while CO₂ was used to characterize the basic sites. CO₂ was activated at the basic amine sites forming surface carbamate species (IR peaks: 1609 and 1446 cm⁻¹). The latter reacted further with epoxides adsorbed on the acid sites forming cyclic carbonates. A correlation between the intensity of the IR peak at 1609 cm⁻¹ and cyclic carbonate yield has been observed. The cyclic carbonate yields were higher when both the acid and base functionalities were present on the surface. The Ti- and Al-SBA-15 functionalized with adenine exhibited the highest catalytic activity and selectivity. There is an optimal dependence (“volcanic plot”) of the yield of cyclic carbonates on the desorption temperature, T_max(CO₂) in the TPD experiments. These solid catalysts were structurally stable up to 473 K and could be recycled for repeated use. In addition to density, the strength and type of amine sites play a crucial role on CO₂ activation and utilization.     

Adsorption of NO and NO2 on ceria–zirconia of composition Ce0.69Zr0.31O2: A DRIFTS study

In this study, the nature of surface intermediates generated by adsorption of NO and NO₂ on a commercial ceria–zirconia powder of composition Ce_0.69Zr_0.31O₂ was investigated using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The conditions of occurrence of the main adsorbed species, i.e. nitrites and nitrates, are studied semi-quantitatively as a function of catalyst pre-treatment and/or type of adsorbed NO_x molecule. On the partially reduced ceria–zirconia, the primary role of NO_x is to re-oxidize the surface via adsorption/decomposition on reduced sites. By contrast, the formation of nitrites and nitrates readily occurs on oxidized surfaces, the latter kind of species being strongly promoted in the case of NO₂ adsorption only.     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying ¹⁵NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the ¹⁵NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

Transformation of methane formation sites into methanol formation ones during CO---H2 reaction over Pd/CeO2 in its SMSI state

The phenomena of induction period of methanol formation, observed in CO---H₂ reaction over Pd/CeO₂ catalysts under SMSI state, was investigated in depth. The magnitude of the induction period was dependent on the extent of SMSI, and higher temperature H₂ reduction lengthened it accompanied with the increase of the number of active sites for methane formation. On the contrary, by the pretreatment of SMSI surface with water vapor, this induction period almost disappeared with the drastic decrease of methane formation rate. These results indicate that methane formation sites would be transformed into methanol formation sites by the oxidation of water vapor formed during CO---H₂ reaction. Infrared spectroscopic investigation of adsorbed CO after various pretreatments indicated that during the induction period thin layers of reduced ceria, which preferentially covered Pd(1 1 1) plane under SMSI state, were removed from the Pd(1 1 1) plane by formed water vapor during CO---H₂ reaction. It was concluded that Pd(1 1 1) plane adjacent to ceria would be the efficient active sites for methanol formation.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

The mechanism of SO2 effect on NO reduction with propene over In2O3/Al2O3 catalyst

An In₂O₃/Al₂O₃ catalyst shows high activity for the selective catalytic reduction of NO with propene in the presence of oxygen. The presence of SO₂ in feed gas suppressed the catalytic activity dramatically at high temperatures; however it was enhanced in the low temperature range of 473–573 K. In TPD and FT-IR studies, the formation of sulfate species on the surface of the catalyst caused an inhibition of NO_X adsorption sites, and the absorbance ability of NO was suppressed by the presence of SO₂, and the amount of ad-NO₃⁻ species decreased obviously. This leads to a decrease of catalytic activity at higher temperatures. However, addition of SO₂ enhanced the formation of carboxylate and formate species, which can explain the promotional effect of SO₂ at low temperature, because active C₃H₆ (partially oxidized C₃H₆) is crucial at low temperature.     

A study of the mechanism of selective conversion of ammonia to nitrogen on Ni/γ-Al2O3 under strongly oxidising conditions

The activity and excellent selectivity (>90%) of γ-Al₂O₃-supported Ni for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N₂H₄ and NO have been used to model the intermediates of the SCO mechanism (direct formation of N₂ via the recombination of two NH_x species) and of the in situ SCR mechanism (two-step formation of N₂ via the reduction of an in situ produced NO species by a NH_x species), respectively. Two IR absorption bands appear during the TPO of NH₃ in the temperature range of N₂ formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N₂ occurs via the in situ SCR mechanism. This also explains the sudden “NO jump” observed on various systems once the temperature is high enough to activate 50% of the NH₃ molecules fed to the catalyst. The fact that NO and NH₃ are able to react to give N₂ at low temperature (from 100°C) confirms that activation of NH₃ is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism.     

Selective photo-oxidation of various hydrocarbons in the liquid phase over V2O5/Al2O3

More than 0.22 mmol of isolated VO₄ species of V₂O₅/Al₂O₃ exhibited the highest evolution of the partial oxidation products (alcohol and ketone) in the oxidation of cyclohexane and cyclopentane. The conversion of cyclohexane and the selectivity of the partial oxidation products were achieved to be 0.49% and 85% over 0.8 g of 3.5 wt.% V₂O₅/Al₂O₃, respectively, where the K/A ratio was 6.2. In addition, V₂O₅/Al₂O₃ can selectively oxidize various hydrocarbons in the liquid phase by the one-step oxygen atom insertion to CH bond. The order of priority was tertiary carbon > secondary carbon > primary carbon > benzene ring.     

Storage of NO2 on BaO/TiO2 and the influence of NO

The influence of NO on the adsorption and desorption of NO₂ on BaO/TiO₂ has been studied under lean conditions. The adsorption of NO₂ involves the disproportionation of NO₂ into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio,

BaO+3NO₂→NO+Ba(NO₃)₂.

Three different nitrate species form on the catalyst: surface nitrates on the TiO₂ support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD).

The reverse reaction of the NO₂ disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO₂. Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NO_x storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO₂ as an intermediate product. This NO₂ is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period.     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

Study of the NOχ reaction with reducing gases on Fe/ZrO2 catalyst

The Fe/ZrO₂ catalyst (1% Fe by weight) shows a strong adsorption capacity toward the nitric oxide (at room temperature the ratio NOFe is ca. 0.5) as a consequence of the formation of a highly dispersed iron phase after reduction at 500–773 K. Nitric oxide is adsorbed mainly as nitrosyl species on the reduced surface where the Fe²⁺ sites are prevailing, but it is easily oxidised by oxygen forming nitrito and nitrato species adsorbed on the support. However, in the presence of a reducing gas such as hydrogen, carbon monoxide, propane and ammonia at 473–573 K the Fe-nitrosyl species react producing nitrogen, nitrous oxide, carbon dioxide and water, as detected by FTIR and mass spectrometers. The results show that nitric oxide reduction is more facile with hydrogen containing molecules than with CO, probably due the co-operation of spillover effects. Experiments carried out with the same gases in the presence of oxygen show, however, a reduced dissociative activity of the surface iron sites toward the species NO_χ formed by NO oxidation and therefore the reactivity is shifted to higher temperatures.     

Influence of NH3 and NO oxidation on the SCR reaction mechanism on copper/nickel and vanadium oxide catalysts supported on alumina and titania

The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N₂O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H₂-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO₂ and N₂O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO₂ intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO₂ species could be related to the N₂O formation.