Preparation, characterization and photocatalytic activity of TiO2 impregnated with the heteropolyacid H3PW12O40: Photo-assisted degradation of 2-propanol in gas–solid regime

Both commercial and home prepared TiO₂ samples impregnated with tungstophosphoric acid (H₃PW₁₂O₄₀) were prepared and used for the photo-assisted degradation of 2-propanol in gas–solid regime. The characterization results evidenced a good coverage of the polyoxometalate (POM) onto the surface of both types of TiO₂ samples along with a marginal effect of the presence of ethanol or HCl during the POM impregnation step on the specific surface area, porosity, morphology, crystallinity and acidity of the samples. Propene was the main intermediate product found in 2-propanol photocatalytic degradation by using the samples containing POM as the photocatalyst, whereas propanone was mainly obtained when the photocatalyst was bare TiO₂. Acetaldehyde was also an intermediate product and its amount was always significantly smaller with respect to propanone and propene (when formed). Carbon dioxide and water were the ultimate oxidation products. The selectivity to propene has been attributed to the acidity of the POM supported samples.     

Enhanced Stability of HZSM-5 Supported Ga2O3 Catalyst in Propane Dehydrogenation by Dealumination

Gallium oxide catalysts supported on HZSM-5 with different Si/Al ratios were characterized by pyridine adsorption FT-IR, model reactions and XPS studies. As the Si/Al ratio of the support HZSM-5 zeolite rises, the acidity of the supported catalysts decreases accordingly, which comes from two aspects: the loss of acid sites present on HZSM-5 support and the loss of the acid sites present on gallium oxides. The latter was caused by the change in the interaction between Ga₂O₃ and support. The initial activity in the propane dehydrogenation decreases with increasing Si/Al ratio while the stability increases. The enhanced stability is thought to be caused by the decrease of the acidity of the catalysts, resulting in the suppression of the side reactions, such as cracking and oligomerization.     

Propene Adsorption on Gold Particles on TiO2(110)

The adsorption of propene on rutile TiO₂(110) and on gold islands dispersed on TiO₂(110) [Au/TiO₂(110)], both at 120 K, has been studied using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and He⁺ low energy ion scattering spectroscopy (LEIS). Propene adsorbs on both TiO₂(110) and Au/TiO₂(110), with desorption peak temperatures at low coverage of 190 and 240 K, respectively. When only 16% of the TiO₂(110) surface is covered by gold islands [16% Au/TiO₂(110)], moderate propene doses populate both the 240 and 190 K TPD peaks, in that order. The 190 K peak, seen also without Au, is due to propene bound to bare Ti sites. The 240 K peak is attributed to propene adsorbed to Ti sites at the edges of gold islands. Tiny doses of propene to the 16% Au/TiO₂(110) surface give this a 240 K TPD peak but no 190 K feature, showing that the propene is mobile on TiO₂(110). A TPD feature at 150 K, which is more prominent at higher Au coverages and higher propene doses, is due to propene bound only to metallic Au islands. Propene desorption shows additional intensity at 265-310 K when the gold islands are only one atom thick, due to propene adsorbed on 2D Au islands or at Ti sites near their edges.     

Evidence of Correlation between Electronic Density and Surface Acidity of Sulfated TiO2

Correlation between the electronic structure and surface acidity of TiO₂–SO₄ ^2– with different SO₄ ^2– amounts has been investigated by means of NH₃-TPD, NH₃-FT-IR and XPS. With the increase of sulfate loadings, the shift of binding energies of O 1s in hydroxyl and Ti 2p_2/3 increases and is proportional to total acidity. A linear relation is obtained between Ti 2p binding energy shift and Lewis acid sites, while the shift in O 1s binding energy is attributed both to the generation of NH₃ hydrogen bond and of Brønsted acid sites. Accordingly, the results obtained from XPS measurements provide evidence that the ammonia adsorption sites are attributed to the decrease of electron density of O 1s in hydroxyl (Brønsted type and H bonded) and Ti 2p_2/3 (Lewis type) by inductive effect of the neighboring sulfate ion.     

Effect of SO2 on the catalytic activity of Ga2O3–Al2O3 for the selective reduction of NO with propene in the presence of oxygen

The effect of coexisting SO₂ on the catalytic activity of Ga₂O₃–Al₂O₃ prepared by impregnation, coprecipitation and sol–gel method for NO reduction by propene in the presence of oxygen was studied. Although the activity of Al₂O₃ and Ga₂O₃–Al₂O₃ prepared by impregnation (Ga₂O₃/Al₂O₃(I)) and coprecipitation (Ga₂O₃–Al₂O₃(CP)) was depressed considerably by the presence of SO₂, NO conversion on Ga₂O₃–Al₂O₃ prepared by sol–gel method (Ga₂O₃–Al₂O₃(S)) was not decreased but increased slightly by SO₂ at temperatures below 723 K. From catalyst characterization, SO₂ treatment was found to cause two important effects on the surface properties: one is the creation of Brønsted acid sites on which propene activation is promoted (positive effect), and the other is the poisoning of NO_x adsorption sites on which NO reduction proceeds (negative effect). It was presumed that the influence of SO₂ treatment on the catalytic activity is strongly related to the balance between the negative and positive. The activity enhancement of Ga₂O₃–Al₂O₃(S) by SO₂ was accounted for by the following consideration: (1) increase of the propene activation ability by SO₂, (2) incomplete inhibition of NO_x adsorption sites by SO₂.     

Conversion of 2-propanol over chromium aluminum orthophosphates

The catalytic conversion (dehydration/dehydrogenation) of 2-propanol on a series of CrPO₄-AlPO₄ (CrAlP) catalysts, which were differently prepared and thermally treated at 773–1073 K, has been studied by microcatalytic pulse reactor technique at different temperatures (473–573 K). Kinetic parameters for conversion of 2-propanol to propene have been obtained by analysis of the data through the Bassett-Habgood equation for first-order reaction processes. The influence of the reaction temperature upon alcohol conversion and product selectivities was also investigated. Catalytic performance was affected by the precipitation agent. Catalysts obtained in propylene oxide-aqueous ammonia showed the highest activity towards propene compared to other catalysts. Calcination at increasing temperatures caused a decrease in the activity due to the decrease in surface acid character. The results of dehydration to propene can be well interpreted through the differences in the number and strength of acid sites, which were gas-chromatographically measured using pyridine and 2,6-dimethylpyridine chemisorbed at different temperatures (573 and 673 K). Dehydrogenation to 2-propanone occurred to a small extent at all reaction temperatures and, besides, its conversion changed slightly with reaction temperature. Propene selectivity strongly increased with increasing reaction temperature.     

Fe/Beta@SBA‐15 core‐shell catalyst: Interface stable effect and propene poisoning resistance for no abatement

A series of Fe/Beta@SBA‐15 core‐shell structural catalysts were designed and controllablly constructed by an ultradilute liquid‐phase coating method. Their catalytic activities were examined for ammonia selective catalytic reduction of NO_x. It was found that their acidity and redox property were strongly dependent on SBA‐15 shells, Fe/Beta@SBA‐15 catalysts exhibit good propene and SO₂ poisoning resistance, excellent hydrothermal stability, and wide NO_x conversion temperature range (325–600°C) under a high gas hour space velocity. The kinetics results indicate that the mesoporous structure of SBA‐15 shell lowers the diffusion limitation and promotes the reactants accessing to active sites. Moreover, TGA and in situ DRIFT results reveal that SBA‐15 shell can not only prevent the generation and deposition of coke and nitrate species from blocking active sites but also serve as an effective “obstacle” to inhibit active FeO_x nanoparticles from agglomerating at high temperature, leading to the higher propene resistance than Fe/Beta. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3967–3978, 2018     

Photocatalytic activity of TiO2-based materials for the oxidation of propene and benzene at low concentration in presence of humidity

This paper complements previous studies devoted to the photocatalytic oxidation of a low-concentration propene gaseous stream in absence of humidity using agglomerated TiO₂-based materials. These prepared agglomerated materials have shown very good oxidation activities and complete selectivity towards CO₂. The present paper analyses the role of humidity on propene oxidation, which has not been studied before, and extends the use of the prepared agglomerated photocatalysts to low concentration benzene oxidation, both in absence and presence of humidity. The obtained results have shown that humidity must be totally avoided, or kept as low as possible, to achieve high propene conversions. In the case of benzene, some insight to controversial published results is given; very low conversions together with benzene cracking on the surface of the photocatalyst occur in absence of humidity. However, the introduction of humidity leads to high conversions and avoids benzene cracking. The performance of the agglomerated photocatalyst containing a high surface area activated carbon (TiO₂/C1) must be underlined in terms of activity.     

Catalytic property of Pt/AlSBA-15 in selective catalytic reduction of NO

Pt-supported on mesoporous SBA-15 catalysts (Pt/SBA-15) have been investigated for selective catalytic reduction of NO by propene in excess oxygen. Catalytic activity of Pt/SBA-15 can be improved by the post-synthesis modification of Al into SBA-15. The resulting material (AlSBA-15) retains a hexagonal order and physical properties of the parent SBA-15 and shows new acid sites. Increased acid sites are confirmed by NH₃-TPD. Medium acid site appeared at 200 °C and broad strong acid site appeared above 250 °C. A certain degree of support acidity of Pt/AlSBA-15 increased the NO conversion, but when there are too many acid sites, carbon deposition becomes extensive and lead to decrease the NO conversion. The influence of different Pt loadings on the catalytic properties was also investigated.     

Dehydrogenation of Propane in the Presence and Absence of CO2 Over β-Ga2O3 Supported Chromium Oxide Catalysts

The dehydrogenation of propane to propene in the presence and absence of CO₂ over β-Ga₂O₃ supported chromium oxide catalysts has been studied. The effect of chromium content and feed composition on the dehydrogenation of propane has been investigated. It has been found that deposition of chromium oxide enhances the dehydrogenation activity and resistance to coke deposition. Moreover, it has been shown that CO₂ promotes the dehydrogenation of propane above 848 K. At that temperature in the presence of CO₂ both the yield of propene and conversion of propane were higher than in the inert gas atmosphere.     

Optimizing activity of tungsten oxides for 1-butene metathesis by depositing silica on γ-alumina support

A series of catalysts made of tungsten oxide loaded on SiO₂, γ-Al₂O₃, SiO₂–Al₂O₃ and silica deposited γ-alumina are tested for 1-butene metathesis. Among these catalysts, the catalyst 6W/20SiO₂/Al₂O₃ gives the highest activity for 1-butene metathesis reaction with 1-butene conversion up to 71 mol% and the yield of propene up to 21 mol%. The excellent catalytic activity is related to the moderate dispersion of tungsten oxide and the suitable acidity of the support. The dispersion of WO_x species and the acidity of supports were studied by characterization of XRD, Raman spectra, UV–vis, H₂-TPR and NH₃-TPD in detail. The surface properties of silica modified γ-alumina leads to the moderate aggregation of supported tungsten oxide, which appears to be more effective for 1-butene metathesis at the low temperature of 453 K. Optimized activity was realized by tuning the dispersion of tungsten species on silica deposited alumina.     

Structure Characteristics and NO Selective Catalytic Reduction Property of Sn x Zr1-x O2 Solid Solution Catalysts

Novel catalysts, Sn_xZr_1-xO₂ solid solutions, for NO selective catalytic reduction:NO SCR) are reported. They have much higher activity and selectivity than SnO₂ and ZrO₂. Sn⁴⁺ is the main reductive sites as proved by TPR. The dilution of Sn sites by the coexisting Zr causes a suppression of propene combustion and consequently promoted the selective reduction of NO. The rutile structure might be beneficial to NO SCR.     

Mechanism for the reduction of NOx on Rh/sulfated titanias

Titanias of different surface areas have been sulfated and used as supports of Rh oxide for the selective catalytic reduction of nitrogen oxides. Only the sulfation of TiO₂ of large surface areas gives strong Br?nsted acid sites, retaining pyridine up to 773 K. Low surface area anatase is unable to retain sulfates. The catalytic activities measured at 523 K, increase with the number of acid sites, and then reach a plateau, showing the intervention of acidity in the SCR. This is true only for Rh but not for Pt. The investigation of the elemental steps on Rh/sulfated TiO₂ by in situ diffuse reflectance spectroscopy permits to clarify a few points: the oxidation of propene, presumably to acetaldehyde, occurs by the reaction with nitrates adsorbed on the support. The further oxidation of this intermediate by NO₂ yields an isocyanate, which can be hydrolysed to ammonia.     

Effect of CeO2 on Supported Pd Catalyst in the SCR of NO: A DRIFT Study

The activity of Pd/Al₂O₃ and Pd/Al₂O₃–CeO₂ samples has been tested in the selective catalytic reduction of NO by propene. It is found that the activity of Pd/Al₂O₃ decreases with calcination temperature, while the activity of Pd/Al₂O₃–CeO₂ increases abnormally with increasing calcination temperature. Surface-area measurement shows both samples suffer a linear decrease in their surface area, so it is reasonable to attribute the activity enhancement to the effect of CeO₂. The adsorption behavior and state of surface-active sites have been characterized by diffuse reflectance FTIR spectroscopy using CO and NO as probes and the effect of CeO₂ has been revealed. The CeO₂ component increases and stabilizes the dispersion of surface Pd species to prevent it from aggregating at high temperature. CeO₂ may also act as a buffer during the redox cycle of Pd, lengthen the period of Pd redox procedure and render Pd a property of inertia in its redox process, thus increasing the activity of the Pd/Al₂O₃–CeO₂ sample. The essential feature of both effects is the strong interaction between Pd and CeO₂. The intensity of interaction increases linearly with calcination temperature and so does the sample activity.     

Propane oxydehydrogenation reaction on a VPO/TiO2 catalyst. Role of the nature of acid sites determined by dynamic in-situ IR studies

A VPO/TiO₂ catalyst tested in the oxydehydrogenation reaction (ODH) of propane between 300 and 400°C shows satisfactory performances (up to 80% of propene selectivity at 2% of propane conversion at 300°C or 56% of propene selectivity at 9% of propane conversion at 400°C). Addition of water or pyridine in the feed gas tends to decrease the propane conversion and enhances the propene selectivity. It is shown that water increases the number of Brönsted surface acid sites by dissociative adsorption which, in turn, enhances propene selectivity at the expense of the CO_x selectivity. These results are in good agreement with spectroscopic IR observations performed under catalytic conditions showing that the Lewis acid sites are linked to CO_xformation, whereas it seems that Brönsted sites would rather be linked to propene formation.     

In situ FTIR studies of propene adsorption over Ag- and Cu-exchanged Y zeolites

Ag- and Cu-exchanged forms of HY zeolite are prepared by the solid-state ion exchange (SSIE) technique. Samples with different metal loadings ranging from low to high exchange are obtained. Both compositional and structural investigations of the exchanged zeolites are performed by elemental analysis, powder X-ray diffraction (XRD), and nitrogen adsorption porosimetry. The acidity investigation is carried out by using the Fourier transform infrared spectroscopy (FTIR) of pyridine adsorption. Propene adsorption over the Ag- and Cu-exchanged zeolites is studied at room temperature by using in situ FTIR spectroscopy. XRD analysis shows good conservation of the zeolitic structure after ionic exchange even at high exchange degree. Porosity measurements indicate that the increase of Ag and Cu exchange degrees leads to a decrease of the porous volumes and the specific surface areas, which is more pronounced for Ag-exchanged zeolites. The acidity evaluation for both exchanged metals shows an important decrease of Brønsted acidity and a slight increase of Lewis acidity, which are more noticeable at high degree of exchange. Instantaneous oligomerization of propene on HY zeolite at room temperature is favored by the relatively high amount of Brønsted acid sites. The propene molecules interact more strongly with Cu²⁺ than with Ag⁺. The shape of the CC stretching band of the FTIR spectra involves at least two overlapping bands indicating that the propene CC bond may interact with several kinds of Ag⁺, Cu²⁺ and Cu⁺ sites of various propene bonding.     

Harmful by-products in selective catalytic reduction of nitrogen oxides by olefins over alumina

The selective reduction of nitrogen dioxide and nitrogen monoxide by olefins (ethene, propene) has been studied over two different -aluminium oxides in the temperature range 473–873 K. Nitrogen dioxide was reduced more effectively than nitrogen monoxide with both, ethene and propene, as a reductant. At temperatures exceeding 700 K, ammonia was formed as a by-product over one type of alumina. Concentrations in the range 30–40 ppm were determined for propene in combination with both, NO and NO₂, while no ammonia was produced with ethene as a reductant. In addition, significant formation of hydrogen cyanide up to 70 ppm was observed with propene over both aluminium oxides starting from either NO or NO₂. In contrast, hydrogen cyanide formation remained below 10 ppm with ethene as a reductant. Nitrous oxide formation did not exceed 10 ppm for all investigations. The results show that for alumina catalysts ethene is a more suitable reductant than propene due to its lower tendency to form undesired by-products.     

Effect of Modifiers on the Reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 Catalysts for the Oxidative Dehydrogenation of Propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (T_max and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.