Alumina-supported Palladium Catalysts Research Articles

Pure component spectral analysis of surface adsorbed species measured under real conditions. BTEM-DRIFTS study of CO and NO reaction over a Pd/γ-Al2O3 catalyst

The adsorption of NO and CO was studied on an alumina-supported palladium catalyst by in situ diffuse reflectance infrared 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 Pd (primarily a variety of Pd-CO and Pd-NO species, in various oxidation states and coordinations) and alumina surface species (i.e. nitrates, nitrites, carbonates, bicarbonates, formates, and isocyanates) as well as gas phase species. Twenty seven previously known species were identified as well as three new and previously unreported or previously unassigned spectra. The present study indicates that BTEM can be meaningfully applied to Pd/Al2O3 DRIFTS in order to provide enhanced spectroscopic analysis. Moreover, the present results are compared in detail with the recent BTEM analysis of CO and NO adsorption on Pt/Al2O3 using DRIFTS (Phys. Chem. Chem. Phys. 2008, 10, 3535).

Influence of Pd Precursor and Method of Preparation on Hydrodechlorination Activity of Alumina Supported Palladium Catalysts

A series of alumina supported Pd catalysts were prepared by the novel deposition-precipitation method adopting the chloride precursor (DP-Cl) of Pd and varying the metal content from 0.25 to 1.0 wt%. The catalytic properties of prepared catalysts were studied by various characterization techniques such as N2 adsorption, CO chemisorption, TPR, XRD, XPS, and TEM techniques. The activity and stability of the catalysts were evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene operating at atmospheric pressure. At 1 wt% of Pd the catalyst showed higher chlorobenzene conversion with good stability when tested for a period of 25 h, whereas the other catalysts exhibited a loss in activity with time. In order to elucidate the exceptional activity and stability of this catalyst, a few more catalysts with 1 wt% Pd were prepared by impregnation technique and also using a non-chloride precursor, palladium nitrate. The 1 wt% DP-Cl catalyst again was found to be the best among the others. The activity and stability of the DP-Cl catalyst was also found to be superior to two low-dispersed catalysts, each with 10 wt% Pd, prepared by conventional impregnation method using the chloride and nitrate as the precursors. The characterization results reveal that the high activity and stability of the DP-Cl catalyst is related to the formation of electron deficient Pd species and its stabilization in the octahedral vacancies of alumina.

Gas phase hydrogenation of ortho-chloronitrobenzene (O-CNB) to ortho-chloroaniline (O-CAN) over unpromoted and alkali metal promoted-alumina supported palladium catalysts

Abstract A series of alkali metals (Li, Na, K and Cs) promoted alumina-supported palladium catalysts were prepared by a wet impregnation method and characterized by X-ray diffraction (XRD) and CO chemisorption measurements. The samples were tested for the gas phase hydrogenation of ortho -chloronitrobenzene (O-CNB) to ortho -chloroaniline (O-CAN) in a fixed-bed micro reactor at 250 °C under normal atmospheric pressure. The promoted-Pd/Al 2 O 3 catalysts show higher conversion for O-CNB and the hydrogenation activity of O-CNB per site decreases with the increasing ionic radius of the alkali metal promoter ions. However, the selectivity for O-CAN remains more or less the same in both unpromoted and promoted catalysts and also irrespective of the nature of the alkali metal promoter ions used for promotion of alumina support. Despite, similar activity and selectivity observed between Li- and Na-promoted Pd/Al 2 O 3 catalysts, the Na-promoted showed higher resistance for coke formation than a Li-promoted catalyst. The increase in the intrinsic activity of palladium site on alkali promotion has been attributed to the increase in hydrogenation activity over promoted catalysts.

Complete oxidation of short chain alkanes using a nanocrystalline cobalt oxide catalyst

Exceptionally high activity for the complete oxidation of propane is reported for the first time using a nanocrystalline cobalt oxide catalyst. The catalyst shows stable activity for prolonged time on stream and when the reaction temperature is cycled. Furthermore, the nanocrystalline catalyst demonstrates considerably higher activity than an alumina supported palladium catalyst, which is recognised as one of the most active reported. The high activity of the cobalt oxide catalyst is associated with the nanocrystalline nature of the material, which gives rise to new catalytically active surface sites. A broader comparison with other reported high activity catalysts emphasises the high efficacy of nanocrystalline cobalt oxide for total oxidation and demonstrates that it has significant potential for important applications, such as control of emissions from liquid petroleum gas powered vehicles and alkane volatile organic emissions from stationary sources.

The application of temperature-programmed desorption, adsorption isotherms and temperature-programmed oxidation to investigate the interaction of CO with alumina-supported palladium catalysts

The interaction of CO with two alumina-supported palladium catalysts has been investigated by a combination of temperature-programmed desorption (TPD), adsorption isotherms and temperature-programmed oxidation (TPO). In agreement with numerous other studies on this adsorption system, both CO and substantial quantities of CO 2 are seen in the desorption spectra. Replacement of 12C 16O with 13C 18O as the dosing gas showed the CO 2 signal not to originate from either the Boudouard reaction or the oxidation of alumina hydroxyl groups, as has been previously reported. Rather, the CO 2 signal is attributed to the decomposition of carboxy species associated with the alumina support material. TPO experiments demonstrate that the support can make a substantial contribution to the TPO profile, complicating the use of this technique to evaluate carbon retention by finely divided metal particles. Further ‘blank’ TPD experiments on plain alumina indicate the formation of CO 2 in the TPD spectrum to be metal-mediated.

Kinetics of the Hydrogenation of Sunflower Oil over Alumina Supported Palladium Catalyst

The present work studies the sunflower oil hydrogenation on supported palladium catalysts, by analyzing the surface kinetics and the mass transfer limitations of products and reactants. Initially, a simplified model was studied. This model took into account only the consecutive hydrogenation of linoleic acid (diene), to reach the production of oleic (monoene) and stearic (saturated) acids. Using the adjusted values of the kinetic constants and the activation energies of the hydrogenation obtained with this model, a new scheme was investigated considering the geometric isomerization reactions (cis-trans). The diene hydrogenation constant was larger than that of the monoene. This fact confirms the higher reaction rate of the diene hydrogenation in comparison with that of the monoene. With respect to the isomerization rates, these have an activation energy superior to that of the monoene hydrogenation, and slightly superior to the diene hydrogenation activation energy. This fact verifies the influence of temperature on the formation of trans-isomers.

Nitrate catalytic reduction in water using niobia supported palladium–copper catalysts

Niobia and alumina supported palladium catalyst promoted by copper were investigated in the reaction of nitrate catalytic reduction in water and characterized by temperature programmed reduction, physisorption, H2 chemisorption and X-ray diffraction. Niobia supported Pd–Cu catalysts were as active and selective as an alumina supported catalyst. All catalysts had similar turnover frequencies independent of the support. The control of pH and the interaction between Pd and Cu were critical to improving the selectivity and activity of Pd–Cu/Nb2O5 catalysts.

Synthesis of carbon nanotubes and nanofibers by decomposition of acetylene over a SMAD palladium catalyst

AbstractFilament‐like Carbon Nanostructures were synthesized by decomposition of acetylene over an alumina supported palladium catalyst prepared by the SMAD method. The structure of carbon products was analyzed by transmission electron microscopy. Between 500 and 700 °C the predominant structures were carbon nanofibers. Between 750 and 800 °C an important mass and volume increase of carbon products was observed. For this temperature range most of the Carbon was in the form of high quality multiwall carbon nanotubes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Continuous catalytic oxidation of solid alcohols in supercritical CO 2: A parametric and spectroscopic study of the transformation of cinnamyl alcohol over Pd/Al 2O 3

Cinnamyl alcohol was oxidized to cinnamaldehyde in a continuous fixed-bed reactor with molecular oxygen over an alumina-supported palladium catalyst in supercritical carbon dioxide modified with toluene. A strong dependence of the reaction performance on pressure and oxygen concentration in the feed was found. Optimization of the reaction conditions resulted in a higher catalytic activity than in the liquid phase. At 120 bar, 80 °C, and double stoichiometric oxygen concentration, a turnover frequency of 400 h −1 at a selectivity of 60% to cinnamaldehyde was achieved. Spectroscopic investigations and the knowledge of the selectivity pattern turned out to be crucial for a deeper understanding of the reaction allowing a rational optimization. Under almost all experimental conditions (even at high oxygen concentration) hydrogenated byproducts, stemming from internal hydrogen transfer reactions, were detected in the effluent. This indicated that alcohol dehydrogenation was the first reaction step; this finding was further confirmed by spectroscopic investigations. In situ XANES and EXAFS revealed that in the entire experimental range investigated, the palladium constituent was mainly in a reduced state, and its surface could be oxidized only in the absence of cinnamyl alcohol in the feed. Bulk-phase behavior studies and investigations at the catalyst–fluid interface, performed by visual inspection and combined transmission and ATR-IR spectroscopy, demonstrated that the reaction performed best in the biphasic region. Moreover, cinnamaldehyde and carbon dioxide, but hardly any toluene and cinnamyl alcohol, were detected inside the porous catalyst, indicating a significantly different product composition inside the porous catalyst compared with the bulk phase.

Reductive Hydrodechlorination of Trichloroethylene by Palladium-on-Alumina Catalyst: 13C Solid-State NMR Study of Surface Reaction Precursors

Adsorption of trichloroethylene (TCE) on alumina-supported palladium catalysts (Pd/Al2O3) was studied in the presence and absence of hydrogen using 13C-solid state NMR. Carbon-13 NMR spectra indicate that at low coverage strongly adsorbed species are formed while at high coverage additional physisorbed species are present. Carbon-13 spin-echo amplitude data measured as a function of pulse separation, tau, was used to determine the 13C-13C intramolecular dipolar coupling and the carbon-carbon bond length of adsorbed species. Results indicate that a substantial fraction of the chemisorbed carbon species had undergone carbon-carbon bond scission forming single-carbon fragments, suggesting that the activation energy for carbon-carbon bond scission is comparable to the heat of adsorption. For the remaining surface species, the double bond is elongated to 1.46 +/- 0.03 A and is suspected to be chemically bonded ethynyl. At room temperature, adding an excess of hydrogen to catalyst that is covered to saturation with TCE precursors produces only in a small amount of ethane, indicating the fraction of surface species that are hydrodehalogenation precursors is small.

Erratum: “The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts” [J. Chem. Phys. 123, 174706 (2005)

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The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

Characterization of palladium supported on γ-Al 2O 3 catalysts in hydrodechlorination of CCl 2F 2

Alumina supported palladium catalysts are prepared by wet impregnation technique with varying Pd loading. The reduced catalysts are tested for their activity and selectivity in the hydrogenolysis of CCl 2F 2 to CH 2F 2. Modified TPR set-up is used to distinguish between halide reduction and carbon reduction and, based on these results, the reduction mechanism of Pd/Al 2O 3 catalysts is elucidated for both fresh and spent forms of catalysts. TEM revealed that there is a strong re-dispersion of palladium taking place during the reaction. The palladium loading for maximum conversion of CCl 2F 2 and yielding for CH 2F 2 is 8 wt.% Pd on γ-Al 2O 3. XRD, TEM and TPR results show that Pd particle size has influence on the CCl 2F 2 hydrogenolysis activity and selectivity to CH 2F 2 and CH 4.

Solvent-Free Aerobic Oxidation of Alcohols Catalyzed by an Efficient and Recyclable Palladium Heterogeneous Catalyst

A simple alumina-supported palladium catalyst prepared by an adsorption method is highly efficient and recyclable in the solvent-free oxidation of alcohols with molecular oxygen. The adsorption method results in high dispersion of palladium probably as mononuclear or oligonuclear species on alumina surface. These palladium species are transformed to small Pd nanoparticles (ca. 5 nm), which are probably the true active species, during the course of alcohol oxidation.

Designing Pd-on-Au Bimetallic Nanoparticle Catalysts for Trichloroethene Hydrodechlorination

Alumina-supported palladium (Pd) catalysts have previously been shown to hydrodechlorinate trichloroethene (TCE) and other chlorinated compounds in water, at room temperature, and in the presence of hydrogen. The feasibility of this catalytic technology to remediate groundwater of halogenated compounds can be improved by re-designing the Pd material in order to increase catalytic activity. We synthesized and characterized Pd supported on gold nanoparticles (Au NPs) of different Pd loadings. In all cases, we found that these catalysts were considerably more active than Pd NPs, alumina-supported Pd, ard Pd-black (62.0, 12.2, and 0.42 L x g(Pd)(-1) x min(-1), respectively). There is a synergistic effect of the Pd-on-Au bimetallic structure, with the material with the highest TCE hydrodechlorination activity (943 L x g(Pd)(-1) x min(-1)) comprised of Au NPs partially covered by Pd metal. The Pd-on-Au bimetallic catalyst structure provides a new synthesis approach in improving the catalytic properties of monometallic Pd materials. The resulting nanoparticle-based materials should be highly suitable as hydrodehalogenation and reduction catalysts for the remediation of various organic and inorganic groundwater contaminants.

Benzyl alcoholoxidation in supercritical carbon dioxide: spectroscopic insight into phase behaviour and reaction mechanism

Selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen over an alumina-supported palladium catalyst was performed with high rate at about 95% selectivity in supercritical carbon dioxide. The experiments in a continuous flow fixed-bed reactor showed that the pressure has a strong influence on the reaction rate. A marked increase of the rate (turnover frequency) from 900 h(-1) to 1800 h(-1) was observed when increasing the pressure from 140 to 150 bar. Video monitoring of the bulk fluid phase behavior and the simultaneous investigation by transmission and attenuated total reflection (ATR) infrared spectroscopy at two positions of the view cell showed that the sharp increase in activity is correlated to a transition from a biphasic to a monophasic reaction mixture. In the single phase region, both oxygen and benzyl alcohol are dissolved in the supercritical CO2 phase, which leads to a reduction of the mass transport resistances (both in the external fluid film and in the catalyst pores) and thus to the high reaction rate measured in the catalytic experiments. The phase transition could be effectively and easily monitored by transmission and ATR-IR spectroscopy despite the small concentration of the dense liquid like phase. Deposition of the Pd/Al2O3 catalyst on the ATR-crystal at the bottom of the view cell allowed to gain insight into the chemical changes and mass transfer processes occurring in the solid/liquid interface region during reaction. Analyzing the shift of the upsilon2 bending mode of CO2 gave information on the fluid composition in and outside the catalyst pores. Moreover, the catalytic reaction could be investigated in situ in this spectroscopic batch reactor cell by monitoring simultaneously the reaction progress, the phase behaviour and the catalytic interface.

In situ monitoring of oxide-supported platinum-group metal catalysts by energy dispersive EXAFS

Energy dispersive EXAFS (EDE), using a Laue monochromator on ID24 of the ESRF, and multiple ion monitoring mass spectrometry have been used to monitor the structure and function of alumina-supported rhodium and palladium catalysts within a microreactor. Metallic 5%-Rh/Al2O3 was preformed in situ by hydrogen reduction, and shown to react rapidly with NO at room temperature yielding an oxidised phase. In order for this material to catalyse the reduction of NO by H2 (to N2), it was shown that the metallic phase must be reformed. The “light-off” temperature at which the metallic phase was formed and catalysis commenced varied with NO : H2 stoichiometry, this temperature lowering in more reducing conditions. N2O formation was concomitant with the conversion from the oxidised to the metallic phase.The EDE configuration allowed the simultaneous monitoring of structure at both the rhodium and palladium K-edges. Incorporation of a 20% palladium mole fraction into 5%-Rh-Pd/Al2O3 protected rhodium from oxidation and also reduced the light-off temperature, and this is reflected in low coordination number values for the composite Pd to Rh/Pd shell. This favours palladium being preferentially on the surface of the nanoparticles. At higher temperatures there are changes in coordination numbers than imply a redistribution of metal sites under catalytic conditions. Future plans to include infrared spectroscopy as a third simultaneous technique will be outlined.

A model high surface area alumina-supported palladium catalyst

A catalyst preparative procedure is described that produces a high surface area alumina-supported palladium catalyst that yields an atypical chemisorbed carbon monoxide infrared spectrum. This inherently residue-free substrate provides a useful reference for evaluation of catalyst crystallite morphology and its effect on reactivity profiles.

Synthesis of carbon nanotubes and carbon nanofilaments over palladium supported catalysts

The synthesis of carbon nanotubes (CNTs) and carbon nanofilaments (CNFs) was investigated using the catalytic decomposition of acetylene over alumina-supported palladium catalysts prepared with the impregnation of palladium acetylacetonate (Pd/Al2O3) and deposition of dodecanethiol-protected palladium nanoparticles (DT-Pd/Al2O3). The synthesized carbon products consisted of multi-walled carbon nanotubes together with a significant number of CNFs. At 700 °C over Pd/Al2O3, the carbon products were mainly made up of CNFs in the range of 9–26 nm in diameter, while CNTs with well-defined layer structures were synthesized at 800 °C. When Pd lo ding was 0.5 wt.%, carbon tubules showed better crystallinity than those of 2.5 wt.% at 700°C. DT-Pd/Al2O3, showing better dispersion of DT-Pd nanoparticles, showed narrower size-distribution of CNTs than that of Pd/Al2O3. Over several supports (Al2O3, SiO2–Al2O3, SiO2, and Y) with DT-Pd nanoparticles, we obtained amorphous carbon-free CNTs showing a narrower diameter range of 6–21 nm at 700 °C.

Study on the combustion behavior of methane over Ce–Zr-modified Pd/Al 2O 3 catalysts

The effects of ceria and zirconium additions to alumina-supported palladium catalysts on methane combustion behavior have been investigated. A series of Ce–Zr-modified Al 2O 3 supports and their supported Pd catalysts are prepared using impregnation method and then characterized by BET, XRD, LR, TEM, XPS techniques. The results show that the addition of Ce–Zr improves the thermal stability of alumina. The thermal stability of alumina increases with increasing the loading of Zr. The methane oxidation activity and thermal stability is found to be dependent on the Ce:Zr ratio over the Pd/Ce x Zr 1− x /Al 2O 3 catalysts. The Pd/Ce 0.2Zr 0.8/Al 2O 3 exhibits the highest activity and thermal stability for methane oxidation. The temperatures for the 90% methane conversion are 448 and 455 °C for the Pd/Ce 0.2Zr 0.8/Al 2O 3 catalysts calcined at 500 and 1100 °C, respectively. TEM results show that the particle size of Pd is not the key factor influencing the activity of Pd/Ce 0.2Zr 0.8/Al 2O 3 catalyst. An induction period is present during methane combustion reaction for the Pd/Al 2O 3 and Pd/Ce x Zr 1− x /Al 2O 3 catalysts (calcined at 1100 °C), but the lengths of induction period are obviously different and the Pd/Ce 0.2Zr 0.8/Al 2O 3 catalyst exhibits the shortest induction period. XPS results show that comparing to Pd/Al 2O 3 catalyst the addition of Ce–Zr stabilizes Pd in a high oxidation state and enhances the reoxidation of metallic Pd to the active PdO under reaction conditions. Therefore the Pd/Ce x Zr 1− x /Al 2O 3 catalysts compared to Pd/Al 2O 3 catalysts have higher thermal stability and shorter induction period.