Dehydrogenation of isobutane over Sn/Pt/Na-ZSM-5 catalysts: The effect of SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratio,amount and distribution of Pt nanoparticles on the catalytic behavior

Sn/Pt/Na-ZSM-5 was used as catalyst for the dehydrogenation of isobutane, and the effect of SiO₂/Al₂O₃ ratio and the dispersion of Pt nanoparticles on the conversion and product selectivity were studied under atmospheric pressure at 848 K. The catalysts were characterized by various techniques such as H₂ chemisorption, TEM, SEM, EDX, XRD, FT-IR, TG/DTG, elemental analysis by XRF and ICP techniques. Higher dispersion of Pt nanoparticles in the catalyst with SiO₂/Al₂O₃ ratio of 40 resulted in higher selectivity for isobutene.     

Aqueous-Phase Reforming of Ethylene Glycol Over Supported Platinum Catalysts

Aqueous-phase reforming of 10 wt% ethylene glycol solutions was studied at temperatures of 483 and 498 K over Pt-black and Pt supported on TiO₂, Al₂O₃, carbon, SiO₂, SiO₂-Al₂O₃, ZrO₂, CeO₂, and ZnO. High activity for the production of H₂ by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO₂, carbon, and Al₂O₃ (i.e., turnover frequencies near 8-15 min^-1 at 498 K); moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO₂-Al₂O₃ and ZrO₂ (turnover frequencies near 5 min^-1); and lower catalytic activity is exhibited by Pt supported on CeO₂, ZnO, and SiO₂ (H₂ turnover frequencies lower than about 2 min^-1). Pt supported on Al₂O₃, and to a lesser extent ZrO₂, exhibits high selectivity for production of H₂ and CO₂ from aqueous-phase reforming of ethylene glycol. In contrast, Pt supported on carbon, TiO₂, SiO₂-Al₂O₃ and Pt-black produce measurable amounts of gaseous alkanes and liquid-phase compounds that would lead to alkanes at higher conversions (e.g., ethanol, acetic acid, acetaldehyde). The total rate of formation of these byproducts is about 1-3 min^-1 at 498 K. An important bifunctional route for the formation of liquid-phase alkane-precursor compounds over less selective catalysts involves dehydration reactions on the catalyst support (or in the aqueous reforming solution) followed by hydrogenation reactions on Pt.     

Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts

More than 130 Pt and Pd bimetallic catalysts were screened for hydrogen production by aqueous-phase reforming (APR) of ethylene glycol solutions using a high-throughput reactor. Promising catalysts were characterized by CO chemisorption and tested further in a fixed bed reactor. Bimetallic PtNi, PtCo, PtFe and PdFe catalysts were significantly more active per gram of catalyst and had higher turnover frequencies for hydrogen production (TOF_H2) than monometallic Pt and Pd catalysts. The PtNi/Al₂O₃ and PtCo/Al₂O₃ catalysts, with Pt to Co or Ni atomic ratios ranging from 1:1 to 1:9, had TOF_H2 values (based on CO chemisorption uptake) equal to 2.8–5.2 min⁻¹ at 483 K for APR of ethylene glycol solutions, compared to 1.9 min⁻¹ for Pt/Al₂O₃ under similar reaction conditions. A Pt₁Fe₉/Al₂O₃ catalyst showed TOF_H2 values of 0.3–4.3 min⁻¹ at 453–483 K, about three times higher than Pt/Al₂O₃ under identical reaction conditions. A Pd₁Fe₉/Al₂O₃ catalyst had values of TOF_H2 equal to 1.4 and 4.3 min⁻¹ at temperatures of 453 and 483 K, respectively, and these values are 39–46 times higher than Pd/Al₂O₃ at the same reaction conditions. Catalysts consisting of Pd supported on high surface area Fe₂O₃ (Nanocat) showed the highest turnover frequencies for H₂ production among those catalysts tested, with values of TOF_H2 equal to 14.6, 39.1 and 60.1 min⁻¹ at temperatures of 453, 483 and 498 K, respectively. These results suggest that the activity of Pt-based catalysts for APR can be increased by alloying Pt with a metal (Ni or Co) that decreases the strengths with which CO and hydrogen interact with the surface (because these species inhibit the reaction), thereby increasing the fraction of catalytic sites available for reaction with ethylene glycol. The activity of Pd-based catalysts for APR can be increased by adding a water-gas shift promoter (e.g. Fe₂O₃).     

Aromatization of propane over Ga/H-ZSM-5: An explanation of the synergy observed between Ga3+ and H+

The aromatization of propane is investigated for Ga₂O₃, H-ZSM-5 and Ga₂O₃/H-ZSM-5 catalysts, and the results are discussed for a series of ZSM-5 catalysts containing varying SiO₂/Al₂O₃ ratios. It is apparent that on addition of a gallium phase to H-ZSM-5, the yield of methane is significantly decreased. These results are discussed with respect to the mechanism of formation of the initial reaction product from propane. It is proposed that the synergy observed between the gallium compound and the zeolite can be explained in terms of a mechanism in which the role of the gallium phase is to induce C-H bond polarization in the propane, which leads to attack via the Bronsted acid sites of the zeolite, which leads to initial C-H bond cleavage occurring.     

Marked Effect of Mo and Fe Addition Upon Liquid Phase Methanol Reforming with Water Over Al2O3 Supported Pt Catalysts

Successively impregnated Pt–Mo/Al₂O₃ and Pt–Fe/Al₂O₃ catalysts exhibited large enhancement effect in H₂ formation rate of liquid phase methanol reforming. Added Mo oxide forms monolayer on Al₂O₃ and facilitates the higher dispersion of Pt particles. In the case of Fe, formation of some surface bimetallic clusters between Pt and Fe was confirmed by XAS analysis, which causes the enhancement effect of H₂ formation in MeOH–H₂O reaction.     

Comparison of Ethylene Glycol Steam Reforming Over Pt and NiPt Catalysts on Various Supports

Steam reforming of ethylene glycol (EG) was studied on Pt and NiPt catalysts supported on γ-Al₂O₃, TiO₂, and carbon. On all supports bimetallic NiPt catalysts show higher activity for H₂ production than the corresponding Pt catalysts as predicted from model surface science studies. The kinetic trends are similar for all catalysts (Pt and NiPt) with the H₂ production rate being zero-order and fractional order with respect to water and ethylene glycol, respectively. Slight differences in selectivity to minor products are observed depending both on active metal and support. On γ-Al₂O₃, NiPt shows higher H₂ and less alkane formation than Pt. TiO₂ supported catalysts show increased water-gas shift activity but also increased selectivity to alkane precursors. NiPt/C is identified as an active and selective catalyst for EG reforming.     

Hydrogen production for fuel cell by oxidative reforming of diesel surrogate: Influence of ceria and/or lanthana over the activity of Pt/Al2O3 catalysts

A series of Pt catalysts supported on Al₂O₃ (Pt/A), Al₂O₃-CeO₂ (Pt/A-C), Al₂O₃-La₂O₃ (Pt/A-L) and Al₂O₃-La₂O₃-CeO₂ (Pt/A-L-C) have been prepared and tested in the oxidative reforming of diesel surrogate with the aim of studying the influence of ceria and lanthana additives over the activity and stability toward hydrogen production for fuel cell application. Several characterization techniques, such as adsorption-desorption of N₂, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, H₂ chemisorption, and thermogravimetric analysis, have been used to define textural, structural, and surface properties of catalysts and to establish relationships with their behaviour in reaction. This physicochemical characterization has shown that lanthana inhibits the formation of α phase in alumina support and decreases ceria dispersion. Activity results show a better performance of ceria-loaded catalysts, being the Pt/A-C sample the system that offers higher H₂ yields after 8 h of reaction. The greater H₂ production for ceria-loaded catalysts, particularly in the case of the system Pt/A-C, is attributed to the Pt-Ce interaction that may change the electronic properties and/or the dispersion of active metal phase. Also, the Ce^III form of Ce^IV/Ce^III redox pair enhances the adsorption of oxygen and water molecules, thus increasing the catalytic activity and also decreasing coke deposition over surface active Pt phases. Stability tests showed that catalysts in which Pt crystallites are deposited on the alumina substrate covered by a lanthana monolayer, give rise to an increase in stability toward H₂ production.     

Effect of Si-Doping on Thermal Stability and Diesel Oxidation Activity of Pt Supported Porous γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Monolithic Catalyst

Abstract  

A series of different contents of Si-stabilized aluminas with high thermal stability were synthesized by the coprecipitation method and were used as the support of Pt diesel oxidation catalysts. The physicochemical properties of SiO₂–Al₂O₃ (SA) and the catalytic performance of Pt/SiO₂–Al₂O₃ (Pt/SA) were characterized in detail by TG–DTA, XRD, infrared spectroscopy, N₂ adsorption, NMR, CO-TPD, and the catalytic activity evaluation of CO and C₃H₆ oxidations as well as NO reduction in simulating diesel exhaust. The results indicate that the presence of Si can remarkably enhance the thermal stability and phase transition temperature of alumina. It was also found that the catalytic activity is virtually independent of surface area, and only appropriate amount of Si doping can improve the diesel oxidation activity, as compared to pure Pt/Al₂O₃ under the same conditions as a result of the better dispersion of Pt on SA–W supports.     

On the Production of Hydrogen from Bio-Oil: A Representative Study Using Propylene Glycol

In a bio-refinery focused on fast pyrolysis, hydrogen (H₂) producible from reforming of the aqueous fraction of bio-oil with steam can be utilized for upgrading pyrolytic lignin into fuels by hydrotreatment. In this work, propylene glycol (PG) was chosen as a typical compound symbolizing higher polyols in the bio-oil aqueous fraction. Catalytic processing of PG into H₂ at low temperature (T = 500°C) was investigated using several commercial catalysts such as Ni/Al₂O₃, Ru/Al₂O₃, Ru/C, Pt/C, and Pd/C in a laboratory-scale fixed-bed reactor. The efficiencies of the catalysts were presented as selectivity to CO, CO₂, CH₄ and H₂, and PG conversion into gaseous phase. Wide ranges of temperature (300–500°C), W/F_O (18.6–92.9 g h/mol), and S/C ratio (5.6–12.7 mol/mol) were examined using Ni/Al₂O₃. At T = 500°C, H₂ selectivity (73.7%) and PG conversion (66.2%) were maximized using ratios of catalyst mass to molar flow rate of PG (W/F_O) = 18.6 g h/mol and steam to carbon (S/C) = 12.7 (10 wt% PG solution). It was found that Ni/Al₂O₃ demonstrates stable operation for at least 6 h of time-on-stream. Finally, a plausible reaction pathway for PG reforming was proposed.     

Catalytic cracking of isobutane over HZSM-5, FeHZSM-5 and CrHZSM-5 catalysts with different SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratios

Several systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with different ratios of SiO₂/Al₂O₃ (25,38,50,80, and 150) were prepared and they were characterized by means of X-ray diffraction (XRD), UV–Vis, NH₃-TPD and BET techniques. The results indicated that, compared with uncalcined HZSM-5 zeolites, the total acid amounts, acidic site density and acidic strength of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts obviously decreased, while those of weak acid amounts obviously enhanced with the decrease of SiO₂/Al₂O₃ molar ratio. When the ratio of SiO₂/Al₂O₃ is less than 50, the three systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with same ratio of SiO₂/Al₂O₃ gave similar and high isobutane conversions. However, when the ratio of SiO₂/Al₂O₃ was equal to or greater than 80, these three systems of catalysts possessed different altering tendencies of isobutane conversions, thus their isobutene conversions were different. High yields of light olefins were obtained over the FeHZSM-5 and CrHZSM-5 zeolite catalysts with high ratio of SiO₂/Al₂O₃ (≥80). The ratio of SiO₂/Al₂O₃ has large effects on the surface area, and acidic characteristics of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolites catalysts, and thus further affect their catalytic performances for isobutane cracking. That is the nature of SiO₂/Al₂O₃ ratio effect on the catalytic performances.     

The Synergy of the Support Acid Function and the Metal Function in the Catalytic Hydrodeoxygenation of m-Cresol

The hydrodeoxygenation (HDO) of m-cresol is investigated as a model for the HDO of phenolic compounds from lignin pyrolysis. Pt catalysts supported on ??-Al₂O₃ and SiO₂ are effective for the conversion of m-cresol to toluene and methylcyclohexane at 533?K and 0.5?atm H₂. Experiments using Pt/??-Al₂O₃ show that the reaction proceeds by a combination of Pt-catalyzed hydrogenation and acid-catalyzed dehydration reactions. Dehydration of a partially hydrogenated oxygenate intermediate is most likely the dominant reaction pathway to toluene. The acidity of the ??-Al₂O₃ support was modified by base (K₂CO₃) and acid (NH₄F) treatments, and increasing the number and strength of acid sites was found to increase the rate of HDO. Pt/SiO₂ was more active for m-cresol HDO than Pt/Al₂O₃. The reaction rate on Pt/Al₂O₃ and Pt/SiO₂ decreased after 5?h on stream, but Pt/Al₂O₃ regained initial reactivity after reductive treatment in H₂.     

Confirmation of sulfur tolerance of bimetallic Pd–Pt supported on highly acidic USY zeolite by EXAFS

The sulfur tolerance (i.e., degree of sulfidation) of Pd and Pt in sulfided bimetallic Pd–Pt catalysts (Pd : Pt mole ratio of 4 : 1) supported on USY (ultrastable Y) zeolites (SiO₂/Al₂O₃ = 10.7, 48, and 310) was investigated using an extended X‐ray absorption fine structure (EXAFS) method. The sulfidation of the catalysts was done in a 1000 ppm H₂S–2% H₂/N₂ stream at 573 K for 0.5 h. In the Fourier transforms of Pd K‐edge and Pt L_III‐edge EXAFS spectra, both of the peaks due to metallic Pd and to metallic Pt for the Pd–Pt/USY (SiO₂/Al₂O₃ = 10.7) catalyst remained most after sulfidation. Further, the results of the Fourier transforms confirmed that the sulfur tolerance of both Pd and Pt decreased with increasing SiO₂/Al₂O₃ ratio, suggesting that Pd and Pt become sulfur‐tolerant when Pd–Pt bimetallic particles are supported on highly acidic USY zeolite. This revised version was published online in July 2006 with corrections to the Cover Date.     

EXAFS characterization of bimetallic Pt–Pd/SiO2–Al2O3 catalysts for hydrogenation of aromatics in diesel fuel

Bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts exhibited much higher activities in aromatic hydrogenation of distillates than monometallic Pt/SiO₂–Al₂O₃ and Pd/SiO₂–Al₂O₃ catalysts. The studies of extended X‐ray absorption fine structure (EXAFS) indicated that there was an interaction between Pt and Pd in the Pt–Pd/ SiO₂–Al₂O₃ catalyst. Furthermore, from the EXAFS, it was assumed that the active metal particle on the Pt–Pd/SiO₂–Al₂O₃ catalysts is composed of the “Pd dispersed on Pt particle” structure. Regarding both the activities of aromatic hydrogenation and the EXAFS results, it was concluded that the Pd species dispersed on Pt particles were responsible for the high activity of the bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of hydrochlorination and hydrofluorination of Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts for n-hexane hydroconversion

Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts were hydrochlorinated or hydrofluorinated with 3.0 wt%HCl or HF, respectively. These mono- and bimetallic catalysts were tested for n-hexane hydroconversion in a pulsed microcatalytic reactor in a flow of H₂ gas. The catalysts were characterized via XRD, metal dispersion via H₂ chemisorption and acid site strength distribution using temperature programmed desorption of ammonia (TPD). Although metallic promoters frequently cause inhibition of the catalytic activities of Pt and since iridium is less active than platinum, fortunately, Ir was found to enhance the hydroconversion activities of the current catalysts, particularly after hydrochlorination.     

The Role of Support Acidity in the Selective Catalytic Reduction of NO by C3H6 Under Lean-Burn Conditions

Mechanical mixtures consisting of a catalyst (Pt/SiO₂ or Pt/SiO₂–Al₂O₃) and supports of varying acidity (hydrotalcite, SiO₂, SiO₂–Al₂O₃, and ZSM-5 zeolite) were tested for the selective reduction of NO by C₃H₆. A certain degree of support acidity appears to favour N₂ selectivity, but if there are too many acid sites, carbon deposition becomes extensive and leads to catalyst deactivation.     

Formation of carbon nanotubes through ethylene decomposition over supported Pt catalysts and silica-coated Pt catalysts

Ethylene decomposition was performed over supported Pt catalysts to fabricate composites of Pt metal nanoparticles and carbon nanotubes (CNTs). All supported Pt catalysts (Pt/carbon black, Pt/CNT, Pt/MgO, Pt/Al₂O₃ and Pt/SiO₂) showed catalytic activity for ethylene decomposition at 973 K to form CNTs. Pt metal particles were found at tips of CNTs. These results indicate that Pt metal particles have catalytic activity for growth of CNTs through hydrocarbon decomposition. A broad range (5-50 nm) of CNT diameters were formed from the use of supported Pt metal catalysts although Pt metal particles in the catalysts before ethylene decomposition were relatively uniform in size (2-5 nm). These results imply that Pt metal particles in the catalysts aggregated during ethylene decomposition at 973 K. Aggregation of Pt metal particles in catalysts during ethylene decomposition could be suppressed by covering catalysts with silica layers that were a few nanometers thick. Silica-coated Pt catalysts showed high activity for ethylene decomposition to form CNTs with uniform diameters (8-10 nm) despite the uniform coverage of Pt metal particles with silica layers.     

Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al₂O₃, NiMo/Al₂O₃, NiW/Al₂O₃ catalysts as well as the effects of solvent, H₂ pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al₂O₃, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H₂ are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al₂O₃, NiMo/Al₂O₃ and NiW/Al₂O₃ catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al₂O₃ shows the highest activity among the tested conventional catalysts with 97?% selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al₂O₃ catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H₂ availability (i.e., low H₂ surface coverage) and toluene formation is favored. In supercritical n-heptane with high H₂ availability (i.e., high H₂ surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H₂ surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.     

Influence of preparation method on the catalytic performances of Re2O7/SiO2-Al2O3 catalysts in the metathesis of ethylene and 2-pentene

The metathesis of ethylene and 2-pentene for propylene production was investigated over Re₂O₇/SiO₂-Al₂O₃ catalysts. The SiO₂-Al₂O₃ (50 wt% Al₂O₃) were obtained by co-precipitation, physical mixing, and sol–gel coating. The presence of isolated layers/grains γ-Al₂O₃ either in bulk (physical mixing) or on the surface (sol–gel coating) resulted in isomerization-free metathesis. On the contrary, the Re₂O₇ catalysts supported on co-precipitated SiO₂-Al₂O₃ or the direct synthesis showed both metathesis and double-bond shift isomerization activities and as a consequence, higher propylene yield was obtained via the secondary metathesis reaction. The catalytic properties were also correlated with the Al-atom rearrangements and the acidic characters/strengths.     

Effect of V2O5 Loading on Propane Combustion over Pt/V2O5–Al2O3 Catalysts

Propane combustion was studied on Pt(0.4%)/V₂O₅–Al₂O₃ catalysts containing up to 20% V₂O₅. The density, strength and nature of surface acid sites were determined by TPD of NH₃ and FTIR spectra of adsorbed pyridine. The sample acidity increased with the vanadium content, essentially because the addition of vanadium oxide generated Brønsted acid sites. The Pt dispersion as determined by H₂ chemisorption increased with increasing V₂O₅ loading. The sample activity for propane combustion was evaluated through both conversion versus temperature (light-off curves) and kinetically-controlled conversion versus time catalytic tests. The propane combustion turnover rate on Pt/V₂O₅–Al₂O₃ increased with the amount of vanadium, probably because the intrinsic Pt oxidation activity increases with the sample acidity.     

Enhanced Sulfur Tolerance of Ceria-Promoted NO x Storage Reduction (NSR) Catalysts: Sulfur Uptake, Thermal Regeneration and Reduction with H2(g)

SO _x uptake, thermal regeneration and the reduction of SO _x via H₂(g) over ceria-promoted NSR catalysts were investigated. Sulfur poisoning and desulfation pathways of the complex BaO/Pt/CeO₂/Al₂O₃ NSR system was investigated using a systematic approach where the functional sub-components such as Al₂O₃, CeO₂/Al₂O₃, BaO/Al₂O₃, BaO/CeO₂/Al₂O₃, and BaO/Pt/Al₂O₃ were studied in a comparative fashion. Incorporation of ceria significantly increases the S-uptake of Al₂O₃ and BaO/Al₂O₃ under both moderate and extreme S-poisoning conditions. Under moderate S-poisoning conditions, Pt sites seem to be the critical species for SO _x oxidation and SO _x storage, where BaO/Pt/Al₂O₃ and BaO/Pt/CeO₂/Al₂O₃ catalysts reveal a comparable extent of sulfation. After extreme S-poisoning due to the deactivation of most of the Pt sites, ceria domains are the main SO _x storage sites on the BaO/Pt/CeO₂/Al₂O₃ surface. Thus, under these conditions, BaO/Pt/CeO₂/Al₂O₃ surface stores more sulfur than that of BaO/Pt/Al₂O₃. BaO/Pt/CeO₂/Al₂O₃ reveals a significantly improved thermal regeneration behavior in vacuum with respect to the conventional BaO/Pt/Al₂O₃ catalyst. Ceria promotion remarkably enhances the SO _x reduction with H₂(g).