Alumina-supported Platinum Catalysts Research Articles

Ceria promoted γ-Al2O3 supported platinum catalyst for catalytic wet air oxidation of oxalic acid: kinetics and catalyst deactivation

Alumina-supported platinum catalysts were prepared for catalytic wet air oxidation of oxalic acid.

Sulfur Dioxide Exposure: A Way To Improve the Oxidation Catalyst Performance

The oxidation of nitric oxide and propylene was studied over a model alumina-supported platinum catalyst. Two treatments (22 h at 250 degrees C and 2 h at 800 degrees C) involving sulfur dioxide were performed in order to understand the effect of SO2 aging. The role and stability of sulfur species stored during aging were investigated by the reduction of sulfated samples at 500 degrees C and temperature-programmed reduction (TPR) up to 800 degrees C. The low activity obtained after aging without reduction revealed the poisoning effect of surface sulfur species. The reduction at 500 degrees C released half of the surface species, which increased the catalytic activity for both NO and C3H6 oxidation. The TPR removed the most stable sulfur species and improved the activity during cooling. The catalyst aged in SO2 at high temperature showed the greatest activity for both reactions because of SO2 exposure and low sulfur storage.

N-butanol removal over alumina supported platinum catalysts

Pure alumina and alumina supported platinum catalysts were used in n-butanol removal reaction. Platinum catalysts were prepared by two different methods and characterized by N2 adsorption isotherms, ICP, H2 chemisorption and TEM. Acid–base properties of catalysts were studied by pyridine and carbon dioxide adsorption–desorption followed by FTIR. Experiments were performed by changing conditions (with or without oxygen and/or water) to determine some correlation between the reactivity and the acid–base properties of the catalysts and to study the effect of water on the reactivity. In the absence of oxygen, dehydration of n-butanol occurs whereas in the presence of oxygen the n-butanol is oxidized. In presence of oxygen, butanal is the only by-product observed whereas in absence of oxygen butene isomerization products are also observed. N-butanol dehydration involves the catalyst acidity whereas the reaction of dehydrogenation involves the basic character of the catalyst. Platinum addition changes the properties of alumina and increases the activity of catalyst for all these reactions. In large amount, water appears as an inhibitor for n-butanol oxidation.

Hydrodechlorination of Tetrachloromethane Over Supported Platinum Catalysts. Effects of Hydrogen Partial Pressure and Catalyst’s Screening Protocol on the Catalytic Performance

Two series of silica- and alumina-supported platinum catalysts were investigated in the hydrodechlorination (HdCl) of tetrachloromethane. During an initial period of reaction carried out at a lower H2/CCl4 ratio the catalysts, especially those characterized by high metal dispersion, deactivated with time-on-stream. Two catalyst screening protocols were used. The first one concerned a gradually increased hydrogen partial pressure, whereas during the second one the H2 pressure was decreased. Although, in general, the hydrogen-rich reaction conditions resulted in improved catalyst performance (higher overall activity and selectivity to CHCl3), the second protocol led to even better results. Reasons for such a behaviour are suggested. Because of very high activity of a few tested samples, changes in CCl4 conversion with the hydrogen partial pressure do not reflect real reaction orders in hydrogen. The same reason may lead to falsification of apparent activation energies. In certain cases the relation between conversion and hydrogen pressure showed a maximum, suggesting that HdCl undergoes via a Langmuir–Hinshelwood mechanism, when hydrogen and CCl4 compete for metal surface sites. Both carbon- and chlorine-containing deposits were found in the post-reaction catalyst samples.

Preparation of Supported Noble Metal Catalyst for Methanol Catalytic Combustion at Room Temperature

An alumina-supported platinum catalyst has been prepared by incipient wetness impregnation–organic compounds reduction (IWI-OCR) and incipient wetness impregnation–hydrogen reduction (IWI-HR) methods. The effect of the reducing reagents on the catalytic activity of the catalyst has been studied. It was proved that some organic compounds such as methanol and formaldehyde are more suitable reducing reagents than hydrogen. The catalytic combustion activity of the catalyst prepared by the methanol reduction method (IWI-MR) is the highest. When the feed gas consisted of 16% methanol vapor in air (volume) and GHSV was 30,000 mL · h−1 · g−1, the methanol conversion on this kind of catalyst can reach up to ∼100% at room temperature, which is 20% higher than that on the catalyst reduced by hydrogen.

NOx-Assisted Soot Oxidation on Pt–Mg/Al2O3 Catalysts: Magnesium Precursor, Pt Particle Size, and Pt–Mg Interaction

Pt–Mg/Al2O3 soot oxidation catalysts were prepared by impregnating either magnesium acetate or magnesium nitrate on alumina-supported platinum catalyst. The influence of Mg addition on the structure and catalytic behaviors of Pt/Al2O3 catalysts were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, transmission electron microscopy (TEM), H2 chemisorption, thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), NOx temperature-programmed desorption (NOx-TPD), NO temperature-programmed oxidation (NO-TPO), and soot temperature-programmed oxidation (soot-TPO). In spite of the coverage of surface Pt by magnesium species and the weakened oxidation resistance of Pt, the Pt–Mg/Al2O3 catalyst derived from magnesium acetate exhibits a higher soot oxidation activity than that prepared with magnesium nitrate, which is mainly determined by the larger Pt particle size on this catalyst. Additionally, the synergistic effect between Pt and Mg enhances the NO oxidation activity and NOx storage capacity of Pt–Mg/Al2O3 catalyst. More NO2 is produced in the temperature range of soot oxidation on this catalyst than on the Mg-free Pt/Al2O3 catalyst with a similar Pt particle size, which efficiently promotes the ignition of soot.

Chromium Addition to Alumina-Supported Platinum Catalysts and Effect on Catalytic Activity in Methane Combustion

Chromium Addition to Alumina-Supported Platinum Catalysts and Effect on Catalytic Activity in Methane Combustion

Portable Thermoelectric Power Generator Based on a Microfabricated Silicon Combustor with Low Resistance to Flow

A portable-scale thermoelectric power generator is designed, fabricated, and tested. The basis of the system is a mesoscale silicon reactor for the combustion of butane over an alumina-supported platinum catalyst. The system is integrated with commercial bismuth telluride thermoelectric modules to produce 5.8 W of electrical power with a chemical-to-electrical conversion efficiency of 2.5% (based on LHV). The energy and power densities of the demonstrated system are 321 W h kg–1 and 17 W kg–1, respectively. The pressure drop through the system is 130 Pa for the highest flow rate used, resulting in a parasitic power requirement for air-pressurization of ∼0.1 W. The demonstration represents an order-of-magnitude improvement in portable-scale electrical power from thermoelectrics and hydrocarbon fuels, and a notable increase in the conversion efficiency compared with previous studies.

Steam reforming of volatile fatty acids (VFAs) over supported Pt/Al 2O 3 catalysts

Volatile fatty acids (VFAs), easily produced using acid fermentation of biomass, were used to generate hydrogen via steam reforming. Three short-chain carboxylic acids (C2–C4) – acetic, propionic and butyric acids – were used as model compounds in addition to VFAs produced in a typical anaerobic batch reactor. Catalytic steam reforming of VFAs using alumina-supported platinum catalysts was studied in a fixed-bed quartz reactor at various temperatures between 300 and 600 °C. The influence of reaction conditions such as temperature, oxygen to carbon ratio (O/C) and gas hourly space velocity (GHSV) was investigated. VFAs were successfully converted to CO x and hydrogen. A hydrogen yield of up to 70% was achieved, based on typical stoichiometry at 600 °C and a GHSV of 25,000 h −1. Temperature-programmed oxidation (TPO), X-ray diffraction (XRD) and pore size distribution (PSD) were used to characterize coke deposition. Graphitic carbon on catalysts was not identified by XRD, which implies that amorphous coke had formed in the small pores. The catalysts could be reactivated by oxidation and reduction. A detrimental effect on hydrogen yield was observed by adding a small amount of O 2 to the VFA feed, due to the high concentration of oxygen in the feed composition. Steam reforming of real VFAs (S/C = 9) in the acid fermentation of food waste was performed with different GHSVs at a reaction temperature of 600 °C. Conversion of VFAs decreased significantly with increasing GHSV, but the hydrogen selectivity was still above 60%. The conversion pathways of the VFAs to CO x and hydrogen are most likely complex, particularly due to the variety of the chemical compounds present in the real VFAs. The steam reforming of VFAs was investigated over various noble metal (Ruthenium, Palladium, Rodium, Nickel) catalysts supported on alumina, the specific activity based on the active surface area decreased in the order of Ru > Pd∼Rh > Pt > Ni.

CoAl2O4 spinel catalyst for soot combustion with NO /O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NOx + O2 stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N2 adsorption–desorption measurements. The CoAl2O4 spinel was able to oxidise soot as fast as the reference Pt/Al2O3 catalyst. Its catalytic activity can be attributed to a high NOx chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO2.

Effects of tungsten oxide on soot oxidation activity and sulfur poisoning resistance of Pt/Al2O3 catalyst

Tungsten oxide was impregnated into the alumina-supported platinum catalyst in order to improve the soot oxidation activity and the resistance to sulfur dioxide. The catalysts were characterized by X-ray diffraction (XRD), Raman, UV-vis spectroscopy, Brunauer–Emmett–Teller (BET), Fourier transform infrared (FT-IR) spectroscopy, temperature-programmed desulfation, NOx temperature-programmed desorption (NOx-TPD), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). The deposition of WOx was found to reduce the availability of Pt active sites on the fresh catalyst. This, as well as the acidic property of tungsten oxide, decreases the NO oxidation and the NOx adsorption abilities. However, a higher soot oxidation activity was achieved on the WOx-modified catalyst in the presence of NO and O2, which is associated with the presence of more platinum in the metallic state by interacting with the electronegative tungsten oxide. The acidity of tungsten oxide and the improved oxidation-resistance of platinum may be critical to the NO ↔ NO2 recycling efficiency and decomposition of surface oxygen complexes. After the sulfur poisoning treatment, fewer sulfates are formed on the WOx-modified catalyst and decompose at lower temperatures. The IR spectra of CO adsorption indicate that less platinum active sites on this catalyst are affected by sulfates and a higher level of Pt dispersion is obtained, which is responsible for the high NO oxidation and soot oxidation activities.

Recovery of spent alumina-supported platinum catalyst and reduction of platinum oxide via plasma sintering technique

A thermal plasma process for the recovery of the spent alumina-supported platinum catalyst (Pt/Al 2O 3) and reduction of platinum oxide (PtO 2) was developed. The spent Pt/Al 2O 3 and PtO 2 catalysts were sintered at >1500 K under plasma condition and were reformed or reduced to metallic platinum, which were proven by XRD and EDX. Results from SEM and BET specific surface area analyses indicated that the organic tar with the intermolecular and intramolecular water on the surface of the spent catalysts was decomposed and converted to syngas (CO, CO 2, and H 2), which might be the reducing agents in the process. Time-programmed hydrogenation study of urocanic acid was carried out to compare the catalytic efficacy of recovery and fresh Pt/Al 2O 3 catalysts. The experimental result indicated that the plasma sintered Pt/Al 2O 3 catalyst showed the similar catalytic efficacy with respect to the fresh catalyst.

ChemInform Abstract: Preparation, Activity, and Durability of Promoted Platinum Catalysts for Automotive Exhaust Control.

Abstract The effects of the addition of calcia, ceria and lanthana to alumina-supported platinum catalysts on the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide automobile emissions (three-way catalyst behaviour) were analyzed. The activity of the prepared samples was determined with steady-state, reducing and oxidizing, simulated feedstreams as well as with a cycled oxidizing-reducing feedstream averaged at the stoichiometric conditions which resembled the exhaust air/fuel fluctuations in a closed-loop emission control system. Activity of the catalysts was also analyzed after conducting accelerated thermal and chemical ageing in order to test their durability. Under normal operating conditions of the automobile engine, Pt/Al2O3 catalysts promoted by rare-earth oxides are able to achieve high HC, CO and NO conversions. The behaviour of the catalysts in the cold start period was determined by analysis of light-off temperatures and a comparison was made with those corresponding to some commercial samples and others reported in the literature. The catalysts prepared in this work showed lower light-off temperatures than those of commercial and reported Pt/Al2O3 catalysts but these temperatures were not so low as with PtRh/Al2O3. In all cases, the prepared catalysts resulted in a better resistance to accelerated ageing. Samples with ceria showed the best resistance to accelerated ageing.

Structure of alumina supported platinum catalysts of different particle size during CO oxidation using in situ IR and HERFD XAS

The structure of alumina supported platinum catalysts has been studied using in situ time-resolved high-energy resolution fluorescence X-ray absorption spectroscopy (HERFD XAS), in situ infrared (IR) spectroscopy, and kinetic measurements during CO oxidation at O 2 to CO ratio of one. Regardless of the particle size, the CO oxidation occurred in two distinctive regimes, a high-activity regime and a low-activity regime, which have high and low rates of reaction respectively, as observed in previous studies. The size of the particles slightly affects the rate in the low-activity regime, which was more difficult to assess in the high-activity regime. The catalyst surface is poisoned by adsorbed CO in the low-activity regime, as probed by HERFD and IR, and in the high-activity regime, the catalyst exhibits higher activity paralleled with the formation of a surface oxide.

Characterization of Enhanced CO Oxidation Activity by Alumina Supported Platinum Catalyst

A novel pretreatment technique was applied to the conventional Pt/alumina catalyst to prepare for the highly efficient catalyst for the preferential oxidation of carbon monoxide in hydrogen-rich condition. Their performance was investigated by selective CO oxidation reaction. CO conversion with the oxygen-treated Pt/Alumina catalyst increased remarkably especially at the low temperature below <TEX>$100^{\circ}C$</TEX>. This result is promising for the normal operation of the proton exchange membrane fuel cell (PEMFC) without CO poisoning of the anode catalyst. XRD analysis results showed that metallic Pt peaks were not observed for the oxygen-treated catalyst. This implies that well dispersed small Pt particles exist on the catalyst. This result was continued by high resolution transmission electron microscopy (HRTEM) analysis. Consequently, it can be concluded that highly dispersed Pt nanoparticles could be prepared by the novel pretreatment technique and thus, CO conversion could be increased considerably especially at the low temperatures below <TEX>$100^{\circ}C$</TEX>.

Fundamental Aspects of the Chiral Modification of Platinum with Peptides: Asymmetric Induction in Hydrogenation of Activated Ketones

A series of peptides, all containing the natural amino acid tryptophan (Trp), have been used as chiral surface modifiers for asymmetric hydrogenation reactions on alumina supported platinum catalysts. The surface chiral sites have been investigated using density functional theory calculations in order to elucidate the structure of the asymmetric environment produced by the adsorption of the peptides on the metal surface. As a test reaction, ketopantolactone (KPL) has been asymmetrically hydrogenated using the modified catalyst. Catalytic results were tested against a computational model to shed light on the phenomenon of chiral induction. The choice of Trp as the first member of the peptidic chain is due to its structural resemblance to cinchona alkaloids, already successfully used as modifiers for asymmetric hydrogenation reactions. The amino acidic moiety of Trp was elongated by addition of other natural amino acids via a peptidic bond, and the resulting peptides were tested as chiral modifiers. The ind...

In situ XAS with high-energy resolution: The changing structure of platinum during the oxidation of carbon monoxide

The catalytically active species during the oxidation of carbon monoxide over a real alumina-supported platinum catalyst under atmospheric pressure are determined by combining in situ high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS) at the Pt L 3 edge and kinetic measurements. Catalysts were prepared by incipient wetness impregnation. The oxidation of carbon monoxide occurred in two distinctive regimes, a high-activity regime and a low-activity regime, which have high and low rates of reaction respectively. In the low-activity region, the catalyst is poisoned by carbon monoxide, limiting the dissociative adsorption of oxygen. In the high-activity regime, all CO is desorbed from the surface, increasing the rate of reaction. The low carbon monoxide concentration enables oxygen to react to the surface in this reaction regime generating a more reactive surface. HERFD showed that different phases are active depending on the reaction conditions in nano-sized catalyst particles.

Spectral reconstruction of surface adsorbed species using band-target entropy minimization. Application to CO and NO reaction over a Pt/γ-Al2O3 catalyst using in situ DRIFT spectroscopy

The adsorption of NO and CO was studied on an alumina-supported platinum catalyst by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The temperature range was 50-160 degrees C and a wide variety of partial pressures was used. The band-target entropy minimization (BTEM) algorithm was applied to the DRIFTS data sets resulting in the pure component spectra of numerous species adsorbed on both the Pt (primarily a variety of Pt0-CO and Pt2+ -CO species) and alumina surface (i.e. nitrates, nitrites, bicarbonates, isocyanates) as well as gas-phase species. Thirty-one previously known species were identified as well as 6 new and previously unreported or previously unassigned spectra. The present results indicate that BTEM analysis of DRIFTS data is a very promising tool for the study of heterogeneous catalytic in situ spectroscopic data.

Pilot-scale investigation of Pt/alumina catalysts deactivation by organosilicon in the total oxidation of hydrocarbons

The deactivation of alumina-supported platinum catalyst for VOC oxidation by long-term exposure for hexamethyldisiloxane (HMDS) has been investigated in a pilot scale reactor for up to 1000 h of exposure. Catalyst samples were characterized by ICP-AES, BET and silicon deposition by SEM. Catalyst activity was investigated using oxidation of ethyl acetate showing deactivation of the catalyst samples as exposed to HMDS increasing with exposure time. Silicon was shown to deposit on the catalyst as well as on the blank alumina support. SEM investigation of an individual pellet revealed a radial eggshell silicon distribution. CO chemisorption showed strong decrease of Pt active sites after deactivation, while still maintaining reasonable oxidation levels of ethyl acetate.

Catalytic Activities of Platinum Supported on Silica-modified Alumina for Hydrodesulfurization of Thiophene

Hydrodesulfurization (HDS) of thiophene over silica-modified alumina-supported platinum (Pt/SiO2-Al2O3) catalysts was examined. The HDS activity of a Pt/Al2O3 catalyst was enhanced by silica modification. However, the optimal silica loading for HDS activity could not be identified. The dispersion of platinum on alumina, as measured by the hydrogen adsorption method, was enhanced by silica modification and the optimal silica loading for platinum dispersion was 10 to 40 wt%. The acidity of SiO2-Al2O3 was evaluated by 2-propanol dehydration (200°C) and cumene cracking (400°C). The optimal silica loading for the acidity of SiO2-Al2O3 was 90 wt%. Furthermore, the presence of Brönsted acid sites on SiO2-Al2O3 was confirmed by a pyridine adsorption method with FT-IR spectroscopy. The high HDS activity of the Pt/SiO2-Al2O3 catalyst is caused by both high Pt dispersion and formation of Brönsted acid sites.