Hydroisomerization of n-hexadecane over Brønsted acid site tailored Pt/ZSM-12

Hydroisomerization of n-hexadecane is performed over ZSM-12 framework having tailored Brønsted acidity to investigate the effect in terms of product selectivity and yield. For this purpose, pure phase of ZSM-12 (bulk molar ratio Si/Al ~ 60) has been synthesized using TEABr as a structure directing agent. The framework Brønsted acidity is tailored with group II elements (M) viz. Ca, Ba and Mg, by means of ion-exchange method. The samples so prepared have been characterized for phase purity, textural parameters, morphology by employing powder X-ray diffraction, nitrogen adsorption–desorption isotherm measurement at 77 K, and scanning electron microscopy technique, respectively. Similarly, % metal exchange is estimated using inductively coupled plasma technique. The quantification of Brønsted acidity for H⁺–M⁺⁺–ZSM-12 samples has been estimated by means of ammonia temperature programmed desorption (NH₃-TPD) and Fourier transform infrared spectroscopy of ammonia (NH₃-FTIR). The well characterized H⁺–M⁺⁺–ZSM-12 samples were loaded with Platinum (Pt, 0.5 wt%) and subjected to hydroisomerization of n-hexadecane using an up-flow fixed bed reactor to verify the effect of process parameters like temperature and WHSV. Pt/H⁺–Ba²⁺–ZSM-12 with tailored Brønsted acidity in the range of about 25 % demonstrated the optimum performance among all the catalysts with an increased isomer selectivity and yield (89.2 and 80.3 %, respectively) by about 4 wt% at a conversion level of about 90 % compared to Pt/H⁺–ZSM-12 framework at 568 K. Such enhancement in isomer selectivity and yield is found to be significant from commercial application point of view. Based on the obtained trend, the potential benefits of implementation of Pt/H⁺–Ba²⁺–ZSM-12 (bulk molar ratio Si/Al ~ 60) framework for cold flow property improvement of ‘bio-ATF’ have been envisaged.     

Iron, and Manganese Doped SO4 2−/ZrO2–TiO2 Mixed Oxide Catalysts: Studies on Acidity and Benzene Isopropylation Activity

Sulfated zirconia–titania and its transition metal doped compositions with 1.5 wt% iron, and 0.5 wt% manganese were prepared and characterized for surface acidity, and activity towards benzene alkylation. Acidity measurement of the 600 °C calcined samples showed that upon doping with Mn, the protonic acid strength of the material increased, whereas iron doping decreased the strength of surface protons and followed the order Mn-SO₄ ^2?/ZrO₂–TiO₂ > SO₄ ^2?/ZrO₂–TiO₂ > Fe-SO₄ ^2?/ZrO₂–TiO₂. Samples calcined at 110 and 600 °C were XRD amorphous. However, on calcination at 800 °C these samples showed crystalline phases characteristic of ZrTiO₄. The Mn doped catalyst also showed highest activity and improved catalyst stability towards isopropylation of benzene. A good correlation between surface acidity and isopropylation activity was obtained. Increase in acidity in case of Mn doped sample was attributed to the increased stability of the surface sulfate groups.     

Comparison of Alkali Metal Promoted MgO Catalysts for Their Surface Acidity/Basicity and Catalytic Activity/Selectivity in the Oxidative Coupling of Methane

Alkali metal (viz. Li, Na, K, Rb and Cs) promoted MgO catalysts (with an alkali metal/Mg ratio of 0·1) calcined at 750°C have been compared for their surface properties (viz. surface area, morphology, acidity and acid strength distribution, basicity and base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) to C₂-hydrocarbons at different temperatures (700–750°C), CH₄/O₂ ratios (4·0 and 8·0) in feed, and space velocities (10320 cm³ g⁻¹ h⁻¹). The surface and catalytic properties of alkali metal promoted MgO catalysts are found to be strongly influenced by the alkali metal promoter and the calcination temperature of the catalysts. A close relationship between the surface density of strong basic sites and the rate of C₂-hydrocarbons formation per unit surface area of the catalysts has been observed. Among the catalysts calcined at 750°C, the best performance in the OCM is shown by Li–MgO (at 750°C). © 1997 SCI.     

Unprecedented Reductive Esterification of Carboxylic Acids under Hydrogen by Reusable Heterogeneous Platinum Catalysts

Supported metal catalysts have been tested for an unprecedented reductive dimerization of carboxylic acids to esters under 8 bar hydrogen in solvent‐free conditions. Among various metal‐loaded tin oxide catalysts, platinum‐loaded tin dioxide (Pt/SnO₂) shows the highest ester yield for the reaction of dodecanoic acid. Among Pt catalysts on various supports, Lewis acidic oxides, especially SnO₂, show high activity. The most active catalyst, 5 wt% Pt/SnO₂ reduced at 100 °C, is effective for the reductive esterification of various carboxylic acids, and the catalyst is reusable for nine cycles, demonstrating the first successful example for the title reaction. Infrared (IR) studies of a model compound (formic acid) on some metal oxides indicate a strong Lewis acid‐base interaction between SnO₂ and the carbonyl oxygen. For Pt/SnO₂ catalysts with different Pt particle sizes, the activity increases with decreasing size of Pt metal. A cooperative catalysis of the Pt metal nanoparticles and the Sn⁴⁺ Lewis acid sites is proposed.

     

Influence of the nature of the platinum precursor on the surface properties and catalytic activity of alumina-supported catalysts

The influence of the nature of the metal precursor — platinum acetylacetonates and chloroplatinic acid — on the surface properties and catalytic activity of Pt/Al₂O₃ catalysts is reported. The obtained results indicate that the catalysts prepared by the organometallic route present higher metal dispersion and lower acidity compared with those prepared from H₂PtCl₆. On the other hand, XPS results showed that the state of platinum is essentially Pt⁰ in the catalysts obtained from Pt(acac)₂ while in the solids prepared by impregnation of H₂PtCl₆ there exists an important contribution of Pt⁺ species which plays a positive role in the hydrogenation of toluene. An additional hydrogen spillover due to the presence of more acidic support is also suggested as an explanation of the observed catalytic results.     

Structure-Sensitivity of Propylene Hydrogenation over Cluster-Derived Bimetallic Pt-Au Catalysts

A series of Pt and Pt-Au catalysts supported on TiO₂ has been studied using C₃H₆ hydrogenation as a probe reaction to determine the composition of the active catalytic surface. The catalysts were characterized by H₂ chemisorption and TEM analysis to determine concentrations of surface Pt sites for TOF calculations and metal particle size distributions, respectively. Similar TOF values for C₃H₈ formation (approximately 30 sec⁻¹) were observed for a monometallic Pt/TiO₂ and a bimetallic Pt–Au/TiO₂ sample prepared by impregnation from individual salt precursors. In contrast, the TOF for C₃H₈ formation over a Pt₂Au₄/TiO₂ sample prepared from an organometallic Pt₂Au₄ cluster precursor was decreased to 0.07 sec⁻¹, suggesting strong structure sensitivity for the hydrogenation reaction over this catalyst. Characterization results indicate that Pt on the surface of the Pt₂Au₄/TiO₂ catalyst is heavily diluted by Au atoms. In combination with the kinetic results, this suggests that the highly diluted surface ensembles of Pt are too small to effectively catalyze C₃H₆ hydrogenation, although electronic effects induced by the presence of Au adjacent to Pt sites can not be excluded.     

Kinetics of ethylene oxidation on plane Pt/SiO2 catalysts in the viscous pressure regime: evidence of support activity

C₂H₄ oxidation on plane Pt/SiO₂ model catalysts with various Pt loadings was studied atT = 373–473 K and in the pressure ranges 10^–6–10² Torr C₂H₄ and 0.3–1500 Torr 02 (1 Torr = 133.3 Pa). Mass spectrometry combined with spatially resolved gas sampling enabled kinetic data to be collected far into the viscous pressure regime. Reaction orders and activation energies were similar to those of a macroscopic Pt surface. However, under fuel-lean conditions the global reaction rate decreases faster than the decrease in metal area. On the other hand, the global rate wasindependent of Pt loading and metal surface area in fuel-rich gas mixtures. This is interpreted in terms of a spillover effect.     

Characterization of Pt?CAu and Ni?CAu Clusters on TiO2(110)

The surface composition and properties of Pt?CAu and Ni?CAu clusters on TiO₂(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS) and soft X-ray photoelectron spectroscopy (sXPS). STM studies show that bimetallic clusters are formed during sequential deposition of the two metals, regardless of the order of deposition. At the 2?ML of Au/2?ML of Pt or Ni coverages studied here, the second metal contributes to the growth of existing clusters rather than forming new pure metal clusters. LEIS experiments demonstrate that the surfaces of the bimetallic clusters are almost 100% Au when 2?ML of Au is deposited on top of 2?ML of Pt or Ni. However, a much larger fraction of Pt or Ni (50 and 20%, respectively) remains at the surface when 2?ML of Pt or Ni is deposited on 2?ML of Au, most likely due to limited diffusion of atoms within the clusters at room temperature. According to sXPS investigations, the binding energies of the metals in the bimetallic clusters are shifted from those observed for pure metal clusters; the Pt(4f_7/2) and Ni(3p_3/2) peaks are shifted to lower binding energies while the position of the Au(4f_7/2) peak is dominated by surface core level shifts. Pure Pt clusters as well as 0.4?ML of Au on 2 ML of Pt clusters reduce the titania support upon encapsulation after annealing to 800?K, whereas 2?ML of Au on 2?ML of Pt clusters do not reduce titania, presumably because there is no Pt at the surface of the clusters. Pure Ni clusters are also known to become encapsulated upon heating, but the reduction of titania is much less extensive compared to that of pure Pt clusters.     

Low-temperature catalytic combustion on Pt/SO 4 2− /ZrO2 and Pd/SO 4 2− /ZrO2 catalysts

Low-temperature combustion of various organic compounds on Pt/SO ₄ ^–2 /ZrO₂ and Pd/SO ₄ ^–2 /ZrO₂ was studied. For these organic compounds, especially saturated hydrocarbons, the combustion activities of Pt/SO ₄ ^–2 /ZrO₂ are higher than those of Pt/ Al₂O₃. Pt/SO ₄ ^–2 /ZrO₂ combustion catalysts can be used in a wide range of space velocity and oxygen content. The catalytic activity is enhanced with an increase of Pt loading from 0.1 to 1.0 wt%. The superacidity of the support material is responsible for the improvement in activity rather than an increase in catalyst surface area or metal dispersion. Pd/SO ₄ ^–2 /ZrO₂ are less active than Pt/SO ₄ ^–2 /ZrO₂.     

Hydrodeoxygenation of dibenzofuran over noble metal supported on mesoporous zeolite

Hydrodeoxygenation has been considered to be one of the most promising methods for bio-oil upgrading. In this paper, the catalytic performance of noble metal supported on mesoporous zeolite in model bio-oil compound hydrodeoxygenation was examined. Dibenzofuran was chosen here because of its refractory nature and large molecular size. Our results indicate that Pt supported on mesoporous ZSM-5 show better performance in dibenzofuran hydrodeoxygenation than Pt/ZSM-5 and Pt/Al₂O₃. The excellent catalytic performance is attributed to the combination of strong acidity and mesopore structure in mesoporous zeolite.     

XPS investigation of surface oxidation layers on a platinum electrode in alkaline solution

A thick oxidation layer on a platinum electrode has been grown in 1 N NaOH at 3 V vs Ag/AgCl reference electrode. After transferring the Pt electrode into an ultrahigh vacuum chamber the surface layer was analysed by X-ray photoelectron spectroscopy. Pt4f_5/2 amd O1s electron binding energies of 74.3, 77.6 and 530.9 eV respectively, as well as the broad peak shape of the O1s signal and the oxygen-to-platinum intensity ratio of 3.08 point towards a platinum—oxyhydroxide PtO(OH)₂. This formula is in good agreement with cyclic voltammetry curves, measured for the same electrode, that revealed two cathodic reduction peaks for oxygen surface coverages equivalent to more than two hydrogen monolayers. These two peaks were assigned to PtOH and PtO. XPS analysis at elevated temperatures showed that the thick (5 nm) oxidation layer decomposes at 400 K to a mixture of several oxides and hydroxides of Pt⁴⁺ and Pt²⁺ and Pt metal with a ratio of O-to-Pt of 1. This mixture further gradually decomposes to only a monolayer of oxygen at 770–870 K. Sodium cations were found to be present in trace amounts in the adlayer and to strongly shift the O1s binding energy to lower values.     

Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

Low temperature water–gas shift: Differences in oxidation states observed with partially reduced Pt/MnOX and Pt/CeOX catalysts yield differences in OH group reactivity

The Pt–ceria synergy may be described as the dehydrogenation of formate formed on the surface of the partially reducible oxide (PRO), ceria, by Pt across the interface, with H₂O participating in the transition state. However, due to the rising costs of rare earth oxides like ceria, replacement by a less expensive partially reducible oxide, like manganese oxide, is desirable. In this contribution, a comparison between Pt/ceria and Pt/manganese oxide catalysts possessing comparable Pt dispersions reveals that there are significant differences and certain similarities in the nature of the two Pt/PRO catalysts. With ceria, partial reduction involves reduction of the oxide surface shell, with Ce³⁺ at the surface and Ce⁴⁺ in the bulk. In the case of manganese oxide, partial reduction results in a mixture of Mn³⁺ and Mn²⁺, with Mn²⁺ located at the surface. With Pt/CeO_X, a high density of defect-associated bridging OH groups react with CO to yield a high density of the formate intermediate. With Pt/MnO_X, the fraction of reactive OH groups is low and much lower formate band intensities result upon CO adsorption; moreover, there is a greater fraction of OH groups that are essentially unreactive. Thus, much lower CO conversion rates are observed with Pt/MnO_X during low temperature water–gas shift. As with ceria, increasing the Pt loading facilitates partial reduction of MnO_X to lower temperature, indicating metal–oxide interactions should be taken into account.     

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 Divalent Metal Component (MeII) on the Catalytic Performance of MeIIFe2O4 Catalysts in the Oxidative Dehydrogenation of n-Butene to 1,3-Butadiene

A series of metal ferrite (Me^IIFe₂O₄) catalysts were prepared by a co-precipitation method with a variation of divalent metal component (Me^II = Zn, Mg, Mn, Ni, Co, and Cu) for use in the oxidative dehydrogenation of n-butene to 1,3-butadiene. Successful formation of metal ferrite catalysts with a random spinel structure was confirmed by XRD, ICP-AES, and XPS analyses. The catalytic performance of metal ferrite catalysts in the oxidative dehydrogenation of n-butene strongly depended on the identity of divalent metal component. Acid properties of metal ferrite catalysts were measured by NH₃-TPD experiments, with an aim of correlating the catalytic performance with the acid property of the catalysts. It was revealed that the yield for 1,3-butadiene increased with increasing surface acidity of the catalyst. Among the catalysts tested, ZnFe₂O₄ catalyst with the largest surface acidity showed the best catalytic performance in the oxidative dehydrogenation of n-butene.     

Elucidation of the effects of competitive adsorption of Cland Br ions on the initial stages of Pt surface oxidation by means of electrochemical nanogravimetry

At anodically polarized Pt electrodes in aqueous H₂SO₄ solution, Cl⁻ and Br⁻ ions are adsorbed competitively with the oxygen species that is deposited between 0.8 and 1.1 V, RHE, forming the initial stage of anodic oxide-film generation at Pt anodes. The Cl⁻ adsorption leads to selective and progressive blocking of the first 50% of coverage by O species at the Pt surface with increasing Cl⁻ concentration (commencing at 10⁻⁷ M) while relatively little effect is seen on the place-exchanged Pt/O→O/Pt lattice that is formed beyond 1.10 V, except at Cl⁻ concentrations greater than 10⁻¹ M, where evolution of Cl₂ commences and hence interferes. The states of co-adsorbed Cl and O-containing species, and their relative surface coverages, determine the nature of the electrocatalytic surface on which anodic Cl₂ evolution takes place. How this selective blocking behavior arises has not previously been well understood. In the present paper, complementary applications of the electrochemical quartz crystal nanobalance (EQCN) technique with cyclic voltammetry are brought to bear on this matter and provide new results at high levels of sensitivity and reproducibility which help to elucidate the competitive and selective chemisorption effects of Cl⁻ at Pt anodes. The results obtained with Cl⁻ ions exhibit some peculiarities which comparative experiments on Br⁻ adsorption help to rationalize.     

Electrocatalytic properties of Pt/carbon composite nanofibers

Pt/carbon composite nanofibers were prepared by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt nanoparticles were controlled by the electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions. The electrocatalytic activities of Pt/carbon composite nanofibers were measured by the oxidation of methanol. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm⁻² has the highest activity.     

Bimetallic catalysts as dissipative structures: stationary concentration patterns in the O2 + H2 reaction on a composite Rh(110)/Pt surface

The O₂ + H₂ reaction has been studied under low pressure conditions (10^-5 mbar) employing a microstructured Rh(110)/Pt surface as catalyst. Photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM) were used as spatially resolving in situ methods. Under reaction conditions stationary concentration patterns (Turing‐like structures) of the adsorbates develop inside the Pt domains which are associated with a compositional change of the metallic substrate. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of TiO<Subscript>2</Subscript> layers deposited on YSZ on the electrochemical promotion of C<Subscript>2</Subscript>H<Subscript>4</Subscript> oxidation on Pt

The electrochemical promotion of Pt/YSZ and Pt/TiO₂/YSZ catalyst-electrodes has been investigated for the model reaction of C₂H₄ oxidation in an atmospheric pressure single chamber reactor, under oxygen excess between 280 and 375 °C. It has been found that the presence of a dispersed TiO₂ thin layer between the catalyst electrode and the solid electrolyte (YSZ), results in a significant increase of the magnitude of the electrochemical promotion of catalysis (EPOC) effect. The rate enhancement ratio upon current application and the faradaic efficiency values, were found to be a factor of 2.5 and 4 respectively, higher than those in absence of TiO₂. This significantly enhanced EPOC effect via the addition of TiO₂ suggests that the presence of the porous TiO₂ layer enhances the transport of promoting O²⁻ species onto the Pt catalyst surface. This enhancement may be partly due to morphological factors, such as increased Pt dispersion and three-phase-boundary length in presence of the TiO₂ porous layer, but appears to be mainly caused by the mixed ionic-electronic conductivity of the TiO₂ layer which results to enhanced O²⁻ transport to the Pt surface via a self-driven electrochemical promotion O²⁻ transport mechanism.     

Probing powder supported catalysts with sum frequency spectroscopy

We report the first observation of sum frequency generation (SFG) from a high surface area metal containing catalyst. In this study, total internal reflection (TIR)-SFG has been used to probe the interface between a 5 wt% Pt/Al₂O₃ catalyst and a carbon monoxide (CO) containing gas phase. We also offer a comparison of this catalyst with CO adsorbed on a Pt thin film and with the attenuated total refection-IR (ATR-IR) spectrum. Vibrational features associated with linearly bound CO to the surface of the Pt are observed. Transient CO adsorption/desoption phenomena are also studied.