Alumina-supported Palladium Catalysts Research Articles

Effect of Iron Content in Alumina-Supported Palladium Catalysts and Their Reduction Conditions on Diclofenac Hydrodechlorination in an Aqueous Medium

Effect of Iron Content in Alumina-Supported Palladium Catalysts and Their Reduction Conditions on Diclofenac Hydrodechlorination in an Aqueous Medium

Selective hydrogenation of phenylacetylene over high-energy facets exposed nanotubular alumina supported palladium catalysts

High-energy facets exposed nanotubular alumina as well as conventional alumina supported Pd catalysts were synthesized and applied to the selective hydrogenation of phenylacetylene. Novel alumina supported catalyst presented higher selectivity at complete conversion of phenylacetylene. This could be related to the well dispersion of Pd and strong metal-support interaction, which are the consequences of the unique structural properties of the crystal plane regulated alumina. It helps create more active sites for hydrogenation meanwhile facilitates the desorption of alkene and thus suppresses the excess hydrogenation towards ethylbenzene.

The adsorption of nitrobenzene over an alumina-supported palladium catalyst: an infrared spectroscopic study.

As part of an on-going programme of development of an aniline synthesis catalyst suitable for operation at elevated temperatures, the geometry of the adsorption complex for nitrobenzene on a 5 wt% Pd/Al2O3 catalyst is investigated by infrared (IR) spectroscopy. Via an appreciation of the reduced site symmetry resulting from adsorption, application of the metal surface selection rule, and observation of in-plane modes only, the adsorption complex (Pd-nitrobenzene) at 28 °C is assigned as occurring vertically or tilted with respect to the metal surface, adopting Csσv(yz) symmetry. Moreover, adsorption occurs via a single Pd-O bond. Single molecule DFT calculations and simulated IR spectra assist vibrational assignments but indicate a parallel adsorption geometry to be energetically favourable. The contradiction between calculated and observed structures is attributed to the DFT calculations corresponding to an isolated molecule adsorption complex, while IR spectra relate to multi molecule adsorption that is encountered during sustained catalytic turnover. Residual hydrogen from the catalyst reduction stage leads to aniline formation on the Pd surface at low nitrobenzene coverages but, on increasing nitrobenzene exposure, the aniline is forced on to the alumina support. A reaction scheme is proposed whereby the nitrobenzene adsorption geometry is inherently linked to the high aniline selectivity observed for Pd/Al2O3 catalysts.

Hydrogenation of 1,3-pentadiene on modified Pd-containing catalysts

Activity and selectivity of alumina-supported palladium catalysts, particularly the sulfurized ones, in the hydrogenation of 1,3-pentadiene to pentenes were investigated. Preliminary sulfurizing of palladium catalysts considerably enhances the selectivity to olefins in a wide range of the H2 /diene molar ratio (from 2.5 to 10). Samples activated in H2 at elevated temperatures demonstrate a higher activity toward diene hydrogenation. Palladium catalysts are more active at low temperatures of the reaction; however, the earlier studied sulfurized Ni catalysts compare well with palladium ones in terms of their selectivity to olefins and productivity, and are more selective in an excess of hydrogen. Modification of palladium with chromium, silver or lithium increases the selectivity to olefins.

Nano-sized alumina supported palladium catalysts for methane combustion with excellent thermal stability

Pd/Al2O3 catalysts supported on Al2O3 of different particle sizes were synthesized and applied in methane combustion. These catalysts were systematically characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), high resolution-transmission electron microscopy (HR-TEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), H2-temperature-programmed reduction (H2-TPR), O2-temperature-programmed oxidation (O2-TPO), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS). The characterization results indicated that nano-sized Al2O3 enabled the uniform dispersion of palladium nanoparticles, thus contributing to the excellent catalytic performance of these nano-sized Pd/Al2O3 catalysts. Among them, Pd/Al2O3-nano-10 (Pd/Al2O3 supported by alumina with an average particle size of 10 nm) showed superior catalytic activity and stability for methane oxidation under harsh practical conditions. It maintained excellent catalytic performance for methane oxidation for 50 hr and remained stable even after harsh hydrothermal aging in 10 vol.% steam at 800°C for 16 hr. Characterization results revealed that the strong metal-support interactions and physical barriers provided by Al2O3-nano-10 suppressed the coalescence ripening of palladium species, and thus contributed to the superior sintering resistance of the Pd/Al2O3-nano-10 catalyst.

Sulfur-resistant methane combustion invoked by surface property regulation on palladium-based catalysts

Improving sulfur resistance of palladium-based catalysts is crucial for the sustained and efficient methane combustion. Herein, the surface property of catalysts was rationally tailored to tune the dynamic behavior of sulfur species on active sites and supports, which was qualitatively and quantitatively studied through a combination of in-situ and on-line characterizations. Results reveal that the content of accumulated sulfur species in catalysts increased linearly (0.20–0.59 mmol SO2·g−1) with the rise of the surface basic sites, while it was restrained in catalysts with high surface acidity. The zirconium doped alumina supported palladium catalyst exhibited a moderate surface acidity/basicity, meanwhile it showed an excellent catalytic activity in the presence of SO2 and after regeneration. Its enhanced sulfur resistance derived from the proper interaction between supports and sulfur species, the facile decomposition of zirconium sulfates, the suppressed formation of surface/bulk aluminum sulfates, as well as the effective regeneration of PdO phase. The established quantitative correlation between accumulated sulfur species, surface acidity/basicity and catalytic performance paves the way to develop sulfur-resistant palladium-based catalysts for oxidation reactions.

Toward High Selectivity Aniline Synthesis Catalysis at Elevated Temperatures.

In connection with an initiative to enhance heat recovery from the large-scale operation of a heterogeneously catalyzed nitrobenzene hydrogenation process to produce aniline, it is necessary to operate the process at elevated temperatures (>100 °C), a condition that can compromise aniline selectivity. Alumina-supported palladium catalysts are selected as candidate materials that can provide sustained aniline yields at elevated temperatures. Two Pd/Al2O3 catalysts are examined that possess comparable mean Pd particle sizes (∼5 nm) for different Pd loading: 5 wt % Pd/Al2O3 and 0.3 wt % Pd/Al2O3. The higher Pd loading sample represents a reference catalyst for which the Pd crystallite morphology has previously been established. The lower Pd loading technical catalyst more closely corresponds to industrial specifications. The morphology of the Pd crystallites of the 0.3 wt % Pd/Al2O3 sample is explored by means of temperature-programmed infrared spectroscopy of chemisorbed CO. Reaction testing over the range of 60–180 °C shows effectively complete nitrobenzene conversion for both catalysts but with distinction in their selectivity profiles. The low loading catalyst is favored as it maximizes aniline selectivity and avoids the formation of overhydrogenated products. A plot of aniline yield as a function of WHSV for the 0.3 wt % Pd/Al2O3 catalyst at 100 °C yields a “volcano” like curve, indicating aniline selectivity to be sensitive to residence time. These observations are brought together to provide an indication of an aniline synthesis catalyst specification suited to a unit operation equipped for enhanced heat transfer.

A Comparison of Experimental Procedures for the Application of Infrared Spectroscopy to Probe the Surface Morphology of an Alumina-Supported Palladium Catalyst

Structure/function relationships in heterogeneous catalysis play an important role in catalyst design strategies. The combination of chemisorption of suitable probe molecules alongside application of infrared spectroscopy is an established technique for providing information on the metal crystallite morphology of supported metal catalysts. Following a review of key literature on this topic, a variety of experimental arrangements that may be adopted for this task are examined. Specifically, the adsorption of CO over a 5wt% Pd/Al2O3 catalyst is investigated using transmission and diffuse reflectance sampling options and two research grade spectrometers. Although comparable spectra are obtained on all the platforms examined, differences are noted. In particular, temperature-programmed IR spectroscopy on one platform enables resolution of two features assigned to linear CO bound to the Pd particles. The relevance of this sub-division of terminal sites with respect to selective hydrogenation reactions is briefly considered.

The Application of Attenuated Total Reflection Infrared Spectroscopy to Investigate the Liquid Phase Hydrogenation of Benzaldehyde Over an Alumina-Supported Palladium Catalyst

The hydrogenation of benzaldehyde in cyclohexane over a 5 wt% Pd/Al2O3 catalyst at 313 K is firstly investigated at ambient pressure in a stirred batch reactor. The formation of benzyl alcohol is a facile process and a small mass imbalance is indirectly attributed to the formation of benzene as a by-product. No hydrogenolysis reaction to form toluene is observed. Secondly, examination of this reaction system by attenuated total reflection infrared (ATR-IR) spectroscopy enables the chemistry at the liquid/solid interface to be probed. Specifically, the ν(C=O) modes of solvated and adsorbed benzaldehyde are evident at 1712 and 1691 cm−1 respectively, providing information on how the reagent is partitioning within the reaction medium. Spectral acquisition on initiation of hydrogenation then enables the benzaldehyde → benzyl alcohol transition to be tracked. The additional presence of a broad CO stretching band of chemisorbed carbon monoxide (1852–1929 cm−1) is attributed to the hydrogen-assisted decarbonylation pathway that forms the benzene by-product.

Sequential growth reveals multi-spinel interface promotion for methane combustion over alumina supported palladium catalyst

Tailoring of active components/metal oxides interface is an effective strategy to improve the performance of supported noble metal catalysts. However, the trade-off between activity and stability accompanies with the modification process. Herein, we propose a facile multi-spinel interface promotion way for simultaneous improvement in activity and stability of Pd/Al2O3 for methane oxidation. Multi-spinel NiCo2O4/NiAl2O4 interface is introduced over γ-Al2O3 support through sequential growth, and then acts as a structural template for the growth and dispersion of PdO nanoparticles to obtain Pd/NiCo2O4/NiAl2O4/Al2O3. The reducible property of NiCo2O4, and structural matching between NiCo2O4/NiAl2O4 and PdO lead to the enhanced performance during catalytic methane oxidation with respect to a low CH4 activation energy, high turnover frequency value and Pd utilization efficiency as well as long-term stability. Such a multi-spinel interface promotion strategy provides a new way to design efficient supported noble metal catalysts.

Supported palladium nanoclusters: morphological modification towards enhancement of catalytic performance using surfactant-assisted metal deposition

In the present study, alumina-supported palladium catalysts were prepared by modified electroless deposition (ED) method in presence of surfactants and their performance was significantly improved compared to that prepared in absence of surfactants. Depending on the type and concentration of surfactant, loading and morphology of deposited palladium varied. Both anionic (sodium dodecyl sulphate) and non-ionic (Tween 20) surfactants were observed to be the most effective in dispersing the metals in the precursor solution. Average metal cluster sizes obtained for catalysts prepared by wetness impregnation, modified electroless deposition and surfactant-assisted deposition method were 11.89, 4.6 and 1.18 nm, respectively. The conversion of butane and selectivity to butene was observed to be function of size of deposited Pd cluster. The conversion and selectivity towards butenes increased with the decreasing particle size. The SDS surfactant-assisted prepared catalyst, having the lowest metal cluster size (1.18 nm), showed the highest activity (33% conversion at 600 °C) and over 99% selectivity for butenes.

Effect of an Alumina Supported Palladium Catalyst on the Magnesium Sulfate Decomposition Kinetics

The sulfur related thermochemical water-splitting cycles are an important class of chemical processes considered for hydrogen production. Recently, the magnesium sulfate thermal decomposition has been reported as a potential unit operation in one of these cycles. Therefore, some interest has been observed in the use of catalysts to lower the activation energy of such reactions. In this context, the present manuscript reports the thermodynamics and kinetics modeling results associated with this reaction system in the presence of a Pd catalyst supported over γ-Al2O3. The presence of such species is responsible for shifting the decomposition temperature to lower values in at least 100 °C. It was observed that the magnesium content is still oriented towards MgO formation. The obtained results indicate that the Pd/Al2O3 catalyst could be a good alternative in reducing the thermal decomposition temperature as its presence was responsible for diminishing the process activation energy from 368.2 to 258.8 kJ.mol−1.

Structure-sensitivity of alumina supported palladium catalysts for N2O decomposition

The catalytic activity of Pd/γAl2O3 for N2O decomposition was found to be highly dependent on the preparation methodology and the number of reaction cycles. Chloride species on the surface of a 2.6 wt. % Pd-Al2O3 catalyst prepared by wet impregnation were identified as inhibiting the activity. Multiple reaction cycles were shown to remove these species, and a subsequent increase in activity was observed; 10.3 molN2O h−1 kgcat−1 in the first use, increased to 28.7 molN2O h−1 kgcat−1 on the fourth use both at 550 °C. Additionally, removal of physisorbed water from the support prior to metal deposition increased the thermal stability of Pd nanoparticles and increased the catalyst activity. Catalysts were subsequently prepared using a deposition technique with an increased concentration of Cl- ions resulted in increased Pd-Cl species in the final catalyst and the catalytic activity was consequently increased due to the improved control of Pd particle size.

Cooperative Catalysis of Methane Oxidation through Modulating the Stabilization of PdO and Electronic Properties over Ti-Doped Alumina-Supported Palladium Catalysts.

Poor low-temperature catalytic activity and durability are the main drawbacks of palladium-based catalysts for methane combustion. Herein, stable and active PdO particles are constructed by incorporating Ti into an alumina support, which makes the catalysts exhibit satisfactory methane combustion activity. The results of comprehensive characterization reveal that an appropriate amount of Ti doping induces the optimization of electron transfer and distribution, thus contributing to the construction and stabilization of active PdO lattices. The reactive oxygen mobility is improved and the optimal PdO/Pd0 combination is achieved, thanks to the amplified PdO–support interaction. In addition, the acid–base properties are regulated and Brønsted acid sites are generated by virtue of the adjustment of electronic properties, which facilitate stabilization of PdO as well. Hence, the Ti-containing catalyst exhibits superior activity for methane oxidation at low temperatures. Notably, the activity and cyclic performance of the catalyst can be further enhanced when undergoing long-term and isothermal heat treatment under the reactant stream and methane, and it demonstrates a high performance with 90% CH4 conversion at 340 °C.

Water Inhibition in Methane Oxidation over Alumina Supported Palladium Catalysts

In situ diffuse reflectance infrared Fourier transform spectroscopy has been used to distinguish surface hydroxyl groups on Al2O3 and PdO/Al2O3 model catalysts calcined at 500-900 °C. Employing the operando approach, the formation of surface hydroxyl groups has been correlated to the methane oxidation activity for PdO/Al2O3 catalysts using a PdO powder sample as reference. The results show that the alumina support stabilizes active PdO particles leading to enhanced apparent methane turnover frequency (TOF), which decreases slowly in dry conditions due to alumina hydroxylation. Wet conditions cause severe hydroxylation that is detrimental for the methane TOF. The hydroxylation follows two different routes, i.e., spillover of hydrogen-containing species to the PdO-Al2O3 boundary and/or the close proximity of the supported PdO particles and under wet conditions also dissociation of gas phase water on the entire alumina surface. Both hydroxylation routes obey varying kinetics such that near saturation is reached quickly (minutes) followed by a continuous slow growth for prolonged exposure times (hours). At low temperatures, inhibition of palladium active sites on the rim of the PdO particles close to alumina seems to be of particular importance for the observed detrimental effect of water, whereas water induced morphological changes (no sintering observed) of the PdO particles play a minor role.

Catalytic Activity and Stability over Nanorod-Like Ordered Mesoporous Phosphorus-Doped Alumina Supported Palladium Catalysts for Methane Combustion

Nanorod-like phosphorus-doped ordered mesoporous γ-alumina (OMA), which has abundant ordered pore channels in the nanorods, was rapidly synthesized through a modified sol–gel strategy without use of any mineral acids. Highly dispersed Pd-based catalysts were synthesized by taking as-obtained phosphorus-doped OMA materials as carriers for methane combustion. The crystallization temperature of γ-Al2O3 was increased by phosphorus doping. The surface acidity properties of γ-Al2O3 were modified upon phosphorus incorporation, which had a significant effect on catalyst activities, and this influence was much more conspicuous for the supports calcined at high temperature. The incorporation of phosphorus adjusted the distribution of palladium active species and the reducibility of catalysts, synergistically affecting the low-temperature catalytic performance. Pd/6P-OMA catalyst demonstrated enhanced low-temperature catalytic properties and stability in the 13-cycle stability and long-term stability tests. During t...

Nanofiber-like mesoporous alumina supported palladium nanoparticles as a highly active catalyst for base-free oxidation of benzyl alcohol

A nanofiber-like mesoporous alumina supported palladium catalyst, Pd/γ-Al2O3-fibr, was successfully prepared by a hydrothermal method and used in the aerobic oxidation of benzyl alcohol to benzaldehyde with molecular oxygen under base-free conditions. The material was characterized by N2 physisorption, FT-IR, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and X-ray photo-electron spectroscopy (XPS). XRD and TEM results revealed that the palladium nanoparticles were uniformly dispersed in the framework of nanofiber-like mesoporous γ-alumina. The higher activity of Pd/γ-Al2O3-fibr compared to Pd/γ-Al2O3 and Pd/SBA-15 in the aerobic oxidation of benzyl alcohol reaction suggested that the unique fibrous architecture of our material and synergic effect between Pd species and γ-Al2O3 are beneficial to enhance the reactivity of palladium NPs. Furthermore, the unique architecture of Pd/γ-Al2O3-fibr catalyst also offers it good mass-transfer property and accessibility, which make it an effective catalyst for aerobic oxidation of steric bulk alcohol substrate. The Pd NPs supported on nano-fibrous mesoporous alumina in this paper is demonstrated to be a recoverable noble metal-based nanocatalyst with easy accessibility and may be great potential as a promising candidate for numerous catalytic applications.

Low Reynolds number isotope transient kinetic modeling in isothermal differential tubular catalytic reactors

A novel method is presented for modeling steady state isotope transient kinetics of heterogeneous catalytic reactions when the flow regime is laminar and conversion is differential. It is based on a factorization theorem which is deduced from the observation that transport functions fluctuate radially in porous beds. Factorization separates the radial from axial and temporal coordinates of the flow rate and concentration functions. It is shown that in transient tracing with a differential conversion, the radial components of the transport functions in the material conservation equations can be integrated into constant parameters to be determined from experimental data. The method is in particular useful as the knowledge of the radial profile of velocity and other transport functions and parameters are not prerequisites for data correlation. The methodology was successfully applied to the adsorption of carbon monoxide in Boudouard reaction on an alumina supported palladium catalyst. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1317–1329, 2018

Structure/activity relationships applied to the hydrogenation of α,β-unsaturated carbonyls: The hydrogenation of 3-butyne-2-one over alumina-supported palladium catalysts

The gas phase hydrogenation of 3-butyne-2-one, an alkynic ketone, over two alumina-supported palladium catalysts is investigated using infrared spectroscopy in a batch reactor at 373K. The mean particle size of the palladium crystallites of the two catalysts are comparable (2.4±0.1nm). One catalyst (Pd(NO3)2/Al2O3) is prepared from a palladium(II) nitrate precursor, whereas the other catalyst (PdCl2/Al2O3) is prepared using palladium(II) chloride as the Pd precursor compound. A three-stage sequential process is observed with the Pd(NO3)2/Al2O3 catalyst facilitating complete reduction all the way through to 2-butanol. However, hydrogenation stops at 2-butanone with the PdCl2/Al2O3 catalyst. The inability of the PdCl2/Al2O3 catalyst to reduce 2-butanone is attributed to the inaccessibility of edge sites on this catalyst, which are blocked by chlorine retention originating from the catalyst’s preparative process. The reaction profiles observed for the hydrogenation of this alkynic ketone are consistent with the site-selective chemistry recently reported for the hydrogenation of crotonaldehyde, an alkenic aldehyde, over the same two catalysts. Thus, it is suggested that a previously postulated structure/activity relationship may be generic for the hydrogenation of α,β-unsaturated carbonyl compounds over supported Pd catalysts.

Voltammetry of MicroParticles in Thin Layer

We describe a new experimental device for the characterization of electroactive microparticles and of the soluble species released during their electrochemical transformation. By using only a few microparticles, it provides, in a faster way, similar information as modified carbon paste electrodes with electrolytic binders (CPEEBs) and the sensitivity and resolution are must better. The device geometry was modelled with ferrocyanide ions and the voltammograms for ferrocene, pyrolusite and hematite were compared with the literature ones concerning CPEEBs. Finally, an alumina-supported palladium catalyst and pyrite-containing argillite were successfully characterized.