Thermodynamics of reduction and oxidation reactions on oxidized or reduced Pt supported on γ-Al2O3

The integral enthalpies of H₂ or O₂ reactions with oxidized or reduced Pt supported on Al₂O₃ have been measured in a dual calorimeter at 60 ° C. These enthalpies were calculated assuming the formation of Pt _x ^s O and Pt^sH surface groups and using accepted values of heat of H₂ and O₂ chemisorption on bare polycrystalline Pt. The calculated and measured reaction enthalpies agree under the following conditions: 1) Pt²O and Pt ₂ ^s O surface stoichiometries are acceptable (1 x 2) whereas Pt²O₂ must be rejected (x = 0.5); 2) the water formed in the reduction or oxidation process must be dissociatively chemisorbed on the Al₂O₃ surface; 3) the spiltover hydrogen is atomic and its enthalpy of combination with surface electron deficient site is about -10 kcal/g atom.The significance of dispersion measurements is discussed in relation with hydrogen spillover. Neglecting spillover in reduction at 60 ° C leads to unrealistic values of dispersion.     

Kinetics of acetone hydrogenation over Pt/Al2O3 catalysts

The hydrogenation of acetone to isopropanol has been studied in the vapour phase over Pt/Al₂O₃ catalysts. The rate law obtained at a total pressure of 1 atm and temperatures between 303 and 363 K is of the form V=kP⁰_aP^1/2_H exp (-44 kJ mol^?1 RT^?1). The kinetic results are consistent with a Langmuir-Hinshelwood hydrogenation mechanism involving a half hydrogenated species and a non-competitive chemisorption of acetone and hydrogen on the platinum surface. The specific activity (calculated per platinum surface atom) has been found to be scarcely dependent on the platinum particle size. It is suggested that the chemisorption sites are made of a very small ensemble of platinum atoms.     

In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.     

NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al₂O₃ powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H₂. NO pulsing and NO–H₂ pump-probe experiments demonstrate the effect of catalyst temperature, NO–H₂ pulse delay time and H₂/NO ratio on N₂, N₂O and NH₃ selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H₂, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H₂ at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N₂O or recombine to form N₂. In excess H₂, higher temperatures and longer delay times favor the production of N₂. The longer delay enables NO decomposition on reduced Pt with the role of H₂ being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N₂ formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.     

Correlation of Surface Characteristics with Catalytic Performance of Potassium Promoted Pd/Al2O3 Catalysts: The Case of N2O Reduction by Alkanes or Alkenes

The present study aims at investigating the role of potassium (K) promoter on the surface and catalytic behavior of Pd/Al₂O₃ catalysts during N₂O reduction by alkanes (CH₄, C₃H₈) or alkenes (C₃H₆). In this context, the surface properties of un-promoted and K-promoted catalysts were evaluated by means of X ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption and FTIR-pyridine adsorption. The results reveal that both the electronic and structural properties of Pd entities can be substantially modified by potassium ions, which are in close proximity with catalyst surface without exhibiting a tendency to accumulate. This in turn results in significant modifications on reactants/intermediates chemisorption bonds, affecting the catalytic performance of K-promoted catalysts. Indeed, it was found that potassium strongly promotes the N₂O reduction by propane or propene yielding notably lower N₂O light off temperatures (ca. 100 °C) as compared to those obtained over un-promoted catalysts. On the contrary, a slight inhibition upon K-promotion was observed, when using CH₄ as reducing agent, implying that the action of the K-promoter is strongly related with the reducing agent used and its?? relative interaction with the catalyst??s surface. The results are discussed in terms of the correlation between the surface chemistry modifications induced by electropositive promoters and the de-N₂O performance of K-promoted catalysts.     

Catalytic Conversion of N2O to N2 over Metal-Based Catalysts in the Presence of Hydrocarbons and Oxygen

The catalytic conversion of N₂O to N₂ in the presence or the absence of propene and oxygen was studied. The catalysts examined in this work were synthesized impregnating metals (Rh, Ru, Pd, Co, Cu, Fe, In) on different supports (Al₂O₃, SiO₂, TiO₂, ZrO₂ and calcined hydrotalcite MgAl₂(OH)₈·H₂O). The experimental results varied both with the type of the active site and with the type of the support. Rh and Ru impregnated on -alumina exhibited the highest activity. The performance of the above most promising catalysts was studied using various hydrocarbons (CH₄, C₃H₆, C₃H₈) as reducing agents. These experimental results showed that the type of reducing agent does not affect the reaction yield. The temperature where complete conversion of N₂O to N₂ was measured was independent of the reductant type. The activity of the most active catalysts was also measured in the presence of SO₂ and H₂O in the feed. A shift of the N₂O conversion versus temperature curve to higher temperatures was observed when SO₂ and H₂O were added, separately or simultaneously, to the feed. The inhibition caused by SO₂ was attributed to the formation of sulfates and that caused by water to the competitive chemisorption of H₂O and N₂O on the same active sites.     

NOx storage properties of Pt/Ba/Al model catalysts prepared by different methods: Beneficial effects of a N2 pre-treatment before hydrothermal aging

The influence of a pre-treatment at 700 °C, either under a O₂/N₂ mixture or only under N₂, and followed by a hydrothermal aging at 700 °C under wet air, was studied for Pt/Ba/Al NSR model catalysts prepared by different methods: (i) successive impregnation of Ba and Pt, (ii) co-addition of Pt and Ba and (iii) barium precipitation followed by Pt impregnation. The catalysts were evaluated by NOx storage capacity measurements and were characterized by N₂ adsorption, XRD, CO₂-TPD, H₂ chemisorption and H₂-TPR. The pre-treatment under N₂ largely improves the NOx storage performance in the whole studied temperature range (200–400 °C), with or without H₂O and CO₂ in the inlet gas. The better NOx storage properties of the catalysts treated under N₂ before aging are due to: (i) a higher NO oxidation activity (mainly linked to a higher platinum dispersion), (ii) a higher number of NOx storage sites resulting from a higher barium dispersion, and consequently to (iii) a higher Pt-Ba proximity.     

Enantioselective hydrogenation of ethyl pyruvate on tin promoted Pt/Al2O3 catalysts

The enantioselective hydrogenation of ethyl pyruvate has been studied on a Pt/Al₂O₃-dihydrocinchonidine catalyst promoted with different amount of tin. The surface reaction between hydrogen adsorbed on Pt and tin tetraalkyls is used for the tin introduction. This reaction leads to the formation of surface organometallic complexes (I), with SnR_(4-x) moieties anchored to the platinum surface. The enantioselectivity of the Pt/Al₂O₃-dihydrocinchonidine catalyst is found to change only slightly upon promotion with tin, while the rate of ethyl pyruvate hydrogenation depends strongly on the amount and the form of tin introduced. The hydrogenation activity is suppressed completely at relatively low tin coverage (Sn/Pt_s < 0.06). The highest hydrogenation rate is measured over catalysts containing complex (I) (Sn/Pt_s = 0.025) on the platinum surface. On Sn-Pt alloy type active sites, which are formed after decomposition of (I) in hydrogen, the rate of hydrogation is considerably lower than on the unpromoted reference Pt/Al₂O₃ catalyst.On leave from the Central Research Institute for Chemistry of the Hungarian Academy of Sciences.     

NO reduction by CH4over La2O3: temperature‐programmed reaction and in situ DRIFTS studies

The chemistry between NO _x species adsorbed on La₂O₃ and CH₄ was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH₄ in the presence of O₂, NO ₃ ^- does not appear to activate CH₄, thus either an adsorbed O species or an NO ₂ ^- species is more likely to activate CH₄. In the absence of O₂, a different reaction pathway occurs and NO^- or (N₂O₂)^2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH₄ unidentate NO ₃ ^- , which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO₂ or H₂O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N₂ and N₂O from NO was observed at much lower temperature during the TPR experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantification of metallic area of high dispersed copper on ZSM-5 catalyst by TPD of H2

Metallic surface of Cu/ZSM-5 catalyst was determined quantitatively with H₂ TPD and N₂O oxidation methods. Usually well established N₂O techniques are the dissociative N₂O chemisorption performed as pulse chemisorption or in frontal technique with highly diluted N₂O to avoid high heat evolution and bulk oxidation. Here TPR/H₂ of the oxygen layer formed under relatively high N₂O partial pressure is compared with TPD/H₂. The dispersion of Cu⁰ determined by TPD is very high. The calculated metallic surface copper area was very close to the total exchanged Cu in the sample, which confirms that copper atoms are well dispersed on ZSM-5 sample. These measurements are supported by XPS data, suggesting also a high dispersion of copper ions in the zeolite. The surface metallic copper determined by H₂ TPD technique showed that this method avoids sintering, compared to the N₂O method, where sintering occurred due to heat evolution during the N₂O reaction, causing migration and agglomeration of copper externally the surface of the zeolite.     

Synergistic mechanism of isolated Fe3+ and highly dispersed Ptδ+ over zeolite for boosting propane dehydrogenation

Pt-based catalysts have been widely investigated in propane dehydrogenation (PDH) owing to their high activity in C H activation, while it suffers from Pt sintering and coke deposition. We develop a transition metal Fe and zeolite support synergistic-modified method to realize the highly dispersed and stable Pt species inside zeolite over Pt/Fe-silicate-1. And it shows the excellent PDH performance with propylene generation rate of 51.6 mol C₃H₆ g_Pt⁻¹ h⁻¹ and low deactivation rate constant k_d of 0.017 h⁻¹ as well as a high TOF_Pt of 37.6 s⁻¹ at 550°C. The systematic characterizations reveal the isolated Fe³⁺ species could significantly improve Pt dispersity and regulate Pt electronic density to realize a more positive Pt^δ+ species inside Silicalite-1 pore. And the further in situ DRIFTS experiments demonstrate that the synergistic effect between the appropriate acidic Fe sites and the highly dispersed Pt^δ+ species around Fe species are responsible for the superior PDH performance.     

Synergism Between Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Au/TiO<Subscript>2</Subscript> in the Low Temperature Oxidation of Propene

CO impedes the low temperature (<170 °C) oxidation of C₃H₆ on supported Pt. Supported Au catalysts are very effective in the removal of CO by oxidation, although it has little propene oxidation activity under these conditions. Addition of Au/TiO₂ to Pt/Al₂O₃ either as a physical mixture or as a pre-catalyst removes the CO and lowers the light-off temperature (T ₅₀) for C₃H₆ oxidation compared with Pt catalyst alone by ~54 °C in a feed of 1% CO, 400 ppm C₃H₆, 14% O₂, 2% H₂O.     

The NO-H2 reaction over Pt(100) oscillatory behaviour of activity and selectivity

The NO-H₂ reaction has been studied over a Pt(100) single crystal surface as a function of temperature and partial pressures of the reactants. The activity as well as the selectivity, shows oscillatory behaviour under isothermal conditions from 420 K to 520 K. The oscillations observed for the formation rates of N₂ and NH₃ are out of phase with those found for the formation rate of N₂O. These observations are in line with recently proposed mechanisms for the formation of N₂, NH₃ and N₂O.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. This revised version was published online in July 2006 with corrections to the Cover Date.     

Novel Mo/Ga/H-ZSM-5 catalyst in environmentally important reductive transformation of N2O in presence of CH4

Direct decomposition of N₂O and the reduction of N₂O with CH₄ over Ga/H-ZSM-5 and Mo/Ga/H-ZSM-5 (Si/Al = 40) catalysts in a plug flow reactor under steady-state conditions as well as by temperature programmed surface reaction (TPSR) have been investigated. Ga ions were ion-exchanged from liquid phase while Mo was deposited onto the Ga/H-ZSM-5 sample using incipient wetness technique. The catalysts were characterized by means of XRF, XPS, TPR, CO chemisorption, TEM and EDS. The N₂O forms redox centers in the Mo/Ga/H-ZSM-5 catalysts at elevated temperatures, which are extremely active in the reaction with CH₄ already at around 373 K. Addition of Mo to the Ga/H-ZSM-5 decreased the T₅₀ temperature in the N₂O decomposition and reduction of N₂O with CH₄ from 819 to 787 K and from 755 to 646 K, respectively. The oxidation/reduction of the Mo/Ga/H-ZSM-5 sample is more favoured in the interaction with N₂O/CH₄ as compared to that using O₂/H₂ and the mechanism of the redox reactions might also be different. The reduction of N₂O with CH₄ cannot be described with the Mars–van Krevelen redox mechanism, but by the participation of CH₄ via MoGa–OCH₃ species in a complex oxygen transfer mechanism is proposed at which N₂O does not directly reoxidise the reduced active centers.     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

Active species of copper chromite catalyst in C–O hydrogenolysis of 5-methylfurfuryl alcohol

The active sites of copper chromite catalyst, CuCr₂O₄·CuO, were investigated for the condensed-phase hydrogenolysis of 5-methylfurfuryl alcohol to 2,5-dimethylfuran at 220 °C. The bulk and surface features of the catalyst were characterized by XRD, H₂-TPR, N₂ adsorption, CO chemisorption, N₂O titration, NH₃-TPD, XPS, and AES. Maxima of both of the potential active species, Cu⁰ and Cu⁺, occurred after reduction in H₂ at 300 °C compared to 240 and 360 °C. These Cu⁰ and Cu⁺ maxima also coincided with the highest specific rate of reaction based on the surface area of the reduced catalyst. The trends of Cu⁰ and Cu⁺ observed by N₂O titration and CO chemisorption were also observed qualitatively by AES. Correlations between activity and the possible active species suggested that Cu⁰ was primarily responsible for the activity of the catalysts.     

Effect of pretreatment and regeneration conditions of Ru/γ-Al2O3 catalysts for N2O decomposition and/or reduction in O2-rich atmospheres and in the presence of NOX, SO2 and H2O

The effect of the pretreatment (inert, oxidative, and reducing) of Ru/γ-Al₂O₃ catalyst on its activity and stability in the decomposition of N₂O in the absence or presence of O₂, SO₂, H₂O and NO_X was studied in the present work. Decomposition of pure N₂O was slightly enhanced by the H₂-pretreated catalyst (metallic Ru) compared to the O₂- or He-pretreated ones, owing to a cyclic oxidation–reduction pathway of metallic Ru. The observed decrease of activity by O₂ or H₂O addition was reversible compared to SO₂ which caused a strong, irreversible deactivation of the catalyst, irrespective of the type of pretreatment. This was attributed to the formation of stable sulphates, mainly those on RuO₂ surface, which could only be removed by regeneration under reducing (H₂ in He) atmosphere at temperatures of ca. 500 °C. Oxidative or inert regeneration required very high temperatures (i.e. >700 °C) in order to decompose these sulphates. A method of retaining N₂O conversion activity very high (≥98%) for long reaction times is suggested and is based on frequent and short-time (ca. 10 min) regenerations of the catalyst under reducing atmosphere (ca. 5% H₂ in He). The effect of co-feeding various reducing agents, such as CO or C₃H₆, on the N₂O conversion activity in the presence of O₂, SO₂, H₂O and NO_X is negligible, mainly because they are oxidized at relatively low temperatures in the O₂-rich feeds used in this study.