Effect of Acid–Base Properties on the Catalytic Activity of Pt/Al2O3 Based Catalysts for Diesel NO Oxidation

The activity of Pt catalysts supported on Al₂O₃ modified with various acid–base additives has been investigated for oxidation of NO to NO₂. Although Pt dispersion was changed by the additives, there was no clear effect of Pt dispersion on the catalytic activity. The measurement of solid acid–base properties of the modified Pt/Al₂O₃ indicated that the NO oxidation activity increased by the increase of surface density of strong acid sites and decreased by the increase of basic sites. It was suggested that platinum on the acidic supports keeps its highly active metallic state for NO oxidation, while the formation of nitrate/nitrite on the basic supports inhibits the reaction on the Pt surface.     

Simultaneous Removal of NO x and Soot Over Pt–Ba/Al2O3 and Pt–K/Al2O3 DPNR Catalysts

The effect of NO _x storage on the soot combustion activity when alkaline- and alkaline/earth-containing model DPNR catalysts are used is investigated in this work. The influence of different experimental conditions (NO concentration, temperature, and particulate loading) is addressed and discussed in relation to the NO _x storage efficiency and soot oxidation activity as well.     

Role of Pt in the Activity and Stability of PtNi/CeO2–Al2O3 Catalysts in Ethanol Steam Reforming for H2 Production

Hydrogen production from ethanol reforming was investigated on bimetallic PtNi catalysts supported on CeO₂/Al₂O₃. Pt content was varied from 0.5 to 2.5 %. Physico-chemical characterization of the as-prepared and H₂-reduced catalysts by TPR, XRD and XPS showed that Pt phase interacted with the Ni and Ce species present at the surface of the catalysts. This interaction leads to an enhancement of the reducibility of both Ni and Ce species. Loadings of Pt higher than 1.0 wt% improved the activity and stability of the Ni/CeO₂–Al₂O₃ catalyst in ethanol steam reforming, in terms of lower formation of coke, C₂ secondary products and a constant production of CO₂ and H₂. The amount and type of carbon deposited on the catalyst was analyzed by TG–TPO while the changes in crystalline phases after reaction were studied by XRD. It was found that for Pt contents higher than 1 wt% in the catalysts, a better contact between Pt and Ce species is achieved. This Pt–Ce interaction facilitates the dispersion of small particles of Pt and thereby improves the reducibility of both Ce and Ni components at low temperatures. In this type of catalysts, the cooperative effect between Pt⁰, Ni⁰ and reduced Ce phases leads to an improvement in the stability of the catalysts: Ni provides activity in C–C bond breakage, Pt particles enhance the hydrogenation of coke precursors (C_xH_y) formed in the reaction, and Ce increases the availability of oxygen at the surface and thereby further enhances the gasification of carbon precursors.     

Effect of Sulfur or Nitrogen Poisoning on the Activity and Selectivity of Y-Zeolite-Supported Pt–Pd Catalysts in the Hydrogenation of Tetralin

The partial oxidation of methane (POM) to synthesis gas over SiO₂-supported Rh and Ru catalysts was studied by in situ microprobe Raman and in situ time-resolved FTIR (TR-FTIR) spectroscopies. The results of in situ microprobe Raman spectroscopic characterization indicated that no Raman band of Rh₂O₃ was detected at 500°C over the Rh/SiO₂ catalyst under a flow of a CH₄/O₂/Ar (2/1/45, molar ratio) mixture, while the Raman bands of RuO₂ can even be detected at 600°C over the Ru/SiO₂ catalyst under the same atmosphere. The experiments of in situ TR-FTIR spectroscopic characterizations on the reactions of CH₄ over O₂ pre-treated Rh/SiO₂ and Ru/SiO₂ catalysts indicated that the products of CH₄ oxidation over Rh/SiO₂ and Ru/SiO₂ greatly depend on the concentration of O²– species over the catalysts. On the catalysts with high concentration of O²–, CH₄ will be completely oxidized to CO₂. However, if the concentration of O²– species over the catalysts is low enough, CH₄ can be selectively converted to CO without the formation of CO₂. The parallel experiments using in situ TR-FTIR spectroscopy to monitor the reaction of the CH₄/O₂/Ar (2/1/45, molar ratio) mixture over Rh/SiO₂ and Ru/SiO₂ catalysts show that the mechanisms of synthesis gas formation over the two catalysts are quite different. On the Rh/SiO₂ catalyst, synthesis gas is mainly formed by the direct oxidation of CH₄, while on the Ru/SiO₂ catalyst, the dominant pathway of synthesis gas formation is via the sequence of total oxidation of CH₄ followed by reforming of unconverted CH₄ with CO₂ and H₂O. The significant difference in the mechanisms of partial oxidation of CH₄ to synthesis gas over Rh/SiO₂ and Ru/SiO₂ catalysts can be well related to the difference in the concentration of O²– species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the two metals.     

Effect of CeO2 and La2O3 on the Activity of CeO2−La2O3/Al2O3-Supported Pd Catalysts for Steam Reforming of Methane

The effect of the addition of CeO₂ or La₂O₃ on the surface properties and catalytic behaviors of Al₂O₃-supported Pd catalysts was studied in the steam reforming of methane. The FTIR spectroscopy of adsorbed CO and the Pd dispersion suggest the partial coverage of Pd⁰ by ceria or lanthana species. This could lead to the formation of an adduct MPd _x O (M = Ce or La) at the surface of the metal crystallites. The addition of ceria or lanthana resulted in an increase of the turnover rate and specific rate for steam reforming of methane. One possible explanation if that the Pd⁰*Pd^δ+O–M interfacial species (M = Ce or La) are oxidized by H₂O or CO₂, promoting the O* transfer to the metal surface. This could facilitate the removal of C* species from the metal surface, resulting in the increase of specific reaction rate and increase of the accessibility of CH₄ to metal active sites.     

Labeled 15NO Study on N2 and N2O Formation Over Pt–Ba/Al2O3 NSR Catalysts

Mechanistic aspects involved in the formation of N₂ and of N₂O during the reduction of NO, stored nitrites and stored nitrates in the presence of NO are investigated in this work by means of isotopic labeling experiments over a model PtBa/Al₂O₃ NSR catalyst. The reduction of gaseous labeled NO with unlabelled NH₃ leads to the formation of N₂O at low temperature (below 180 °C), and of N₂ at high temperature. All N₂ possible isotopes are observed, whereas only labeled molecules have been detected in the case of N₂O. Hence the formation of nitrous oxide involves undissociated NO molecules, whereas that of N₂ can be explained on the basis of the statistical coupling of ¹⁵N- and ¹⁴N-adatoms on Pt. However, due to a slight excess of the mixed ¹⁵N¹⁴N isotope, a SCR-like pathway likely operates as well. The reduction of the stored labelled nitrates is very selective to N₂ and all isotopes are observed, confirming the occurrence of the recombination pathway. However also in this case a SCR-like pathway likely occurs and this explains the abundance of the ¹⁴N¹⁵N species. When the reduction of the stored nitrates is carried out in the presence of NO, this species is preferentially reduced pointing out the higher reactivity of gaseous NO if compared to the nitrates.     

Deep Oxidation of Benzene on Pt/V2O5–TiO2 Catalysts

The combustion of benzene was studied on Pt supported on V₂O₅–TiO₂ samples containing different amounts of V₂O₅. Vanadium was highly dispersed as V⁴⁺ for low V₂O₅ loadings, forming a vanadia monolayer on titanium dioxide for a V₂O₅ concentration of about 5%wt. V₂O₅–TiO₂ samples were more active than V₂O₅ and TiO₂ single oxides, and the activity increased with the vanadia content. The platinum dispersion in Pt/V₂O₅–TiO₂ catalysts increased with the V₂O₅ loading but the activity for the deep oxidation of benzene exhibited an opposite trend. Benzene combustion was a structure sensitivity reaction promoted on larger metallic Pt crystallites which were preferentially formed when the V₂O₅ content in the sample was decreased.     

Macroporous Monolithic Pt/γ-Al2O3 and K–Pt/γ-Al2O3 Catalysts Used for Preferential Oxidation of CO

Macro-porous monolithic γ-Al₂O₃ was prepared by using macro-porous polystyrene monolith foam as the template and alumina sol as the precursor. Platinum and potassium were loaded on the support by impregnation method. TG, XRD, N₂ adsorption–desorption, SEM, TEM, and TPR techniques were used for catalysts characterization, and the catalytic performance of macro-porous monolithic Pt/γ-Al₂O₃ and K–Pt/γ-Al₂O₃ catalysts were tested in hydrogen-rich stream for CO preferential oxidation (CO-PROX). SEM images show that the macropores in the macro-porous monolithic γ-Al₂O₃ are interconnected with the pore size in the range of 10 to 50 μm, and the monoliths possess hierarchical macro-meso(micro)-porous structure. The macro-porous monolithic catalysts, although they are less active intrinsically than the particle ones, exhibit higher CO conversion and higher O₂ to CO oxidation selectivity than particle catalysts at high reaction temperatures, which is proposed to be owing to its hierarchical macro-meso(micro) -porous structure. Adding potassium lead to marked improvement of the catalytic performance, owing to intrinsic activity and platinum dispersion increase resulted from K-doping. CO in hydrogen-rich gases can be removed to 10 ppm over monolithic K–Pt/γ-Al₂O₃ by CO-PROX.     

CO Methanation Over Ru�CAl2O3 Catalysts: Effects of Chloride Doping on Reaction Activity and Selectivity

The selective CO methanation (CO-SMET) process via Ru?CAl₂O₃ catalysts was investigated as a tool for complete CO removal in fuel processors, when the H₂-rich gas so produced is employed for PEM-FCs applications to vehicles, boats, yachts and residential co-generators. CO-SMET seems, in fact, to be a good alternative to the most widely used CO preferential oxidation (CO-PROX) process. The performance of Ru-based catalysts on alumina carrier for efficient CO removal through CO-SMET was studied, exploring the role of two different Ru precursors (chloride and nitrate), and the doping effect of chloride and of Ru load (1%, 3% and 5%). First, two catalytic families (Ru?CAl₂O₃_Cl and Ru?CAl₂O₃_NO₃) were prepared by incipient wetness impregnation of alumina powder synthesized via solution combustion synthesis, by varying the Ru load. Then, based on the best obtained results, a third catalytic family was prepared adding chloride to Ru?CAl₂O₃_NO₃ catalysts by impregnation. The CO removal performance was determined at catalyst powder level in a fixed bed micro reactor. Better performances were exhibited when Ru was deposited from chloride precursor, but the post-addition of chlorine to fresh Ru?CAl₂O₃ catalysts prepared with nitrate precursor tremendously improved their selectivity toward CO methanation. In particular, with both 1% and 3% Ru?CAl_NO₃ catalyst chlorine doped, complete CO conversion was reached in a proper temperature range where the CO₂ methanation was suitably kept at a low acceptable level.     

Influence of Indium Content on the Properties of Pt–Re/Al2O3 Naphtha Reforming Catalysts

The influence of indium on the properties of Pt–Re/Al₂O₃ catalysts used in naphtha reforming is studied. The addition of indium to the Pt–Re/Al₂O₃ catalyst produces a big decrease of acidity. It also produces an inhibition of the metal function, i.e., dehydrogenation and hydrogenolysis activity. The reaction of n-C₅ isomerization shows that indium addition decreases the total activity of the Pt–Re catalyst but increases the selectivity to the i-C₅ isomers. The selectivity to low cost light gases (C₁–C₃) is particularly decreased. The reaction of n-C₇ reforming showed that addition of indium increases the stability of the catalyst and the selectivity to aromatics, and decreases the production of light gases.     

FTIR study of CO adsorption on coked Pt–Sn/Al2O3 catalysts

Infrared spectra of adsorbed CO have been used as a probe to monitor changes in Pt site character induced by the coking of Pt/Al₂O₃ and Pt–Sn/Al₂O₃ catalysts by heat treatment in heptane/hydrogen. Four distinguishable types of Pt site for the linear adsorption of CO on Pt/Al₂O₃ were poisoned to different extents showing the heterogeneity of the exposed Pt atoms. The lowest coordination Pt atoms (ν(CO) < 2030 cm⁻¹) were unpoisoned whereas the highest coordination sites in large ensembles of Pt atoms (2080 cm⁻¹) were highly poisoned, as were sites of intermediate coordination (2030–2060 cm⁻¹). Sites in smaller two‐dimensional ensembles of Pt atoms (2060–2065 cm⁻¹) were partially poisoned, as were sites for the adsorption of CO in a bridging configuration. The addition of Sn blocked the lowest coordination sites and destroyed large ensembles of Pt by a geometric dilution effect. The poisoning of other sites by coke was impeded by Sn, this effect being magnified for Cl‐containing catalyst. Oxidation or oxychlorination of coked catalyst at 823 K followed by reduction completely removed coke from the catalyst surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective Oxidation of Ethane Over Mo–V–Al–O Oxide Catalysts: Insight to the Factors Affecting the Selectivity of Ethylene and Acetic Acid and Structure-activity Correlation Studies

Catalysts of general formula, MoVAlO _x were prepared with the initial elemental composition of 1:0.34:0.167 (Mo:V:Al) at a pH value in the range of 1–4. The elemental analysis showed that the final composition of the catalysts is pH dependant. The performance of the catalysts was tested for selective oxidation of ethane to give ethylene and acetic acid. While all of them were active for ethane oxidation with a moderate conversion, the catalyst prepared at pH 2 showed a highest activity with 23% ethane conversion and a combined selectivity of 80.6% to ethylene and acetic acid. The catalyst prepared at pH 4 was least selective to ethylene and acetic acid. Various techniques like powder XRD, SEM, Raman, UV–Vis and EPR were used to characterize the catalysts and to identify the active phases responsible for the selective oxidation of ethane. The powder XRD data showed that the catalysts prepared at pH 1 and 2 contain mainly of MoO₃ and MoV₂O₈ along with traces of Mo₄O₁₁. The amount of MoO₃ was slightly higher in the catalyst prepared at pH 1. However, the catalyst prepared at pH 3 contains mainly of MoV₂O₈ with no trace of MoO₃. The catalyst prepared at pH 4 showed V₂O₅ as the major phase along with MoVAlO₄ phase. The Raman data corroborated the XRD results. EPR and UV–Vis studies indicated the presence of traces of V⁴⁺ in pH 1 and 2 catalysts and significant amount of Mo⁵⁺ in all the catalysts. Thus, the high activity and selectivity to ethylene and acetic acid are attributed to the presence of MoV₂O₈ phase and other reduced species like Mo₄O₁₁ phase supported on MoO₃. The presence of V and Mo ions in a partially reduced form seems to play a crucial role in the selective oxidation of ethane.     

Glycerol Partial Oxidation over Pt/Al2O3 Catalysts under Basic and Base-Free Conditions—Effect of the Particle Size

The glycerol partial oxidation reaction over Pt/Al₂O₃ catalysts was studied under basic (NaOH/GLY molar ratio 4) and base-free conditions (NaOH/GLY molar ratio 0). Catalysts with small (2.95 nm) and large particle sizes (260.83 nm) were synthesized according to the use of different reducing agents, formaldehyde or sodium borohydride, and hydrazine, respectively. These different Pt particle sizes lead to a dramatic change in terms of activity, irrespective of the applied conditions. The biggest particles (i.e., 260 nm) seem to generate overoxidation products leading to a decrease in the carbon balance (to ~80%) while the smallest particles exhibit the highest initial glycerol transformation rate (i.e., ~10,000 mol h⁻¹ mol_Pt⁻¹ under basic conditions at 60°C and ~2000 mol h⁻¹ mol_Pt⁻¹ in the absence of a base at 100°C). In terms of selectivities, the main products are different as a function of the initial reaction conditions. For base-free conditions, the two main products are glyceraldehyde and glyceric acid with a sum of selectivities always larger than 80%. Under basic conditions, the major product is glyceric acid while no trace of glyceraldehyde is detected.