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Methane is the most abundant constituent of natural gas and it is difficult to fully oxidize without effective catalysts. Pd is recognized as the most efficient catalyst for the methane combustion reaction. However, few routes are available for the preparation of near-monodisperse Pd clusters for catalysis. Here we focused on the synthesis and activation of mercaptoundecanoic acid-protected Pd clusters (1.1 ± 0.3 nm) and encapsulated them in a silica matrix using sol-gel chemistry. EXAFS analysis revealed that nearly complete removal of thiolate ligands at 250 ˚C in air leads to the partial oxidation of Pd clusters and that the clusters are completely oxidized to PdO at 650 ˚C. TEM and CO chemisorption studies suggest that silica coated Pd clusters are much more resistant to sintering than the control sample of Pd clusters deposited on silica spheres. The activity studies over silica-coated and non-coated catalysts for the methane combustion reaction indicate the presence of the silica shell on the Pd clusters significantly improves the catalytic activity.

The exhaust stream from a lean-burn natural gas vehicle (NGV) typically has a maximum temperature of about 550 °C, a low concentration of methane (500–1000 ppmv) and a large amount of water vapor (around 15% by volume) which is a major cause of Pd catalyst deactivation. Such conditions pose significant challenges in the development of a small footprint catalytic converter to mitigate methane emissions. In this review, we address how the choice of support material can affect the Pd catalyst tolerance to water. Support hydrophobicity, its ability to serve as a sink for the hydroxyls, and oxygen mobility are some of the crucial factors that have contributed to improved catalyst stability and activity. In addition to the summary of the most recent advances in the field, we provide recommendations for catalyst testing and characterization techniques that are specific to the application of interest.

With the increasing demand for clean hydrogen production, both as a fuel and an indispensable reagent for chemical industries, acidic water electrolysis has attracted considerable attention in academic and industrial research. Iridium is a well-accepted active and corrosion-resistant component of catalysts for oxygen evolution reaction (OER). However, its scarcity demands breakthroughs in catalyst preparation technologies to ensure its most efficient utilisation. This minireview focusses on the wet-chemistry synthetic methods of the most active and (potentially) durable iridium catalysts for acidic OER, selected from the recent publications in the open literature. The catalysts are classified by their synthesis methods, with authors’ opinion on their practicality. The review may also guide the selection of the state-of-the-art iridium catalysts for benchmarking purposes.

The high oxygen storage capacity of exhaust gas treatment catalysts is a desirable feature for stabilizing fuel conversion at lambda variations for stoichiometric engine performance. Pd catalysts supported on Co3O4, Al2O3, CeO2 and ZrO2, as well as Pd-free Co3O4, were evaluated using methane combustion at sub-stoichiometric oxygen-to-fuel ratios at temperatures below 550 °C. The product analysis was performed with an online mass spectrometer calibrated for CH4, O2, CO2, H2O, CO and H2. Only both cobalt catalysts demonstrated the insensitivity of the methane conversion to O2/CH4 variations. The Pd/Co3O4 catalyst was the only catalyst that produced a 40 % increase in exit gas flow above the feed gas mass flow rate at ignition (light-off) between 400 °C and 550 °C, with the same decrease upon extinction. The oxygen from the catalyst participated in the combustion, even while the molecular oxygen supplies lasted. Selected catalysts were analyzed by temperature-programmed desorption, reduction in H2 and surface reaction with CH4. In the absence of O2 in the feed, Pd/Co3O4 supplied a “tithe” of its bulk oxygen at temperatures of 400−550 °C for the formation of CO2 and H2O, while Pd/Al2O3 provided only PdO-associated low oxygen for the CO and H2 formation. Co3O4 surpasses CeO2 in its oxygen-donating properties at low temperatures and at the conditions tested and, thus, is potentially capable of widening the operational lambda window of stoichiometric combustion to a larger extent than ceria.

This study addresses the hydrogenation of olefins as related to their removal from thermally cracked bitumen. The inhibition effect of a nitrogen base is evaluated on the model feed comprising a co...

Co3O4 and Pd/Co3O4 catalysts were analyzed for Pd and Co speciation during wet lean methane combustion in temperature ranges of 150–450 °C by means of in situ X-ray absorption spectroscopy. As water inhibits Pd reoxidation during methane combustion, the catalysts were reduced prior to the in situ XAS analyses to follow the evolution of the oxidized species. The contributions from metallic Pd, PdO and Pd(OH)2 and metallic Co, Co(II), and Co(III) oxides were quantified as a function of temperature. The Pd-free sample showed remarkable stability of its species regardless of temperature in the dry feed but the presence of water inhibited its reoxidation. In a partially pre-reduced Pd/CoOx catalyst in the wet feed, cobalt species progressively oxidized to the level of the Pd-free sample. Pd remained largely unoxidized below the surface, with the Pd(0) fraction remaining stable in the studied 150–450 °C region. This was ascribed to the effect of the cobalt oxide support, as Pd(0) is known to oxidize progressively for alumina and tin dioxide supports in the wet feeds. We considered bulk thermodynamics of the involved processes, as well as the reported activation energies and kinetic data from a previous study on methane combustion. In combination with the results from the in situ XAS, we suggest that cobalt oxide, being a preferential oxygen but unlikely hydroxyl acceptor, inhibits Pd oxidation, and does not provide benefits in terms of inactive Pd(OH)2 formation.

Reaction kinetics of methane combustion is investigated on Co3O4 and Pd/Co3O4 (0.27 wt% Pd) catalysts for a fuel-lean feed. The temperature ranges between 250 and 550 °C in the presence of 5 and 10 vol% water with CH4 concentrations that varied between 1000 and 5000 ppmv. Significant cobalt oxide contribution to the activity of the bimetallic catalyst is observed, especially at higher water concentration and lower temperature (up to 70%). Co3O4 demonstrates first order to CH4, 0 order to H2O and activation energy of 69 kJ/mol. Pd/Co3O4 catalyst shows first order to CH4, negative 0.37 order to H2O and an observed activation energy of 90.7 kJ/mol, which is corrected for water adsorption to 60.6 kJ/mol. The latter is a typical activation energy for Pd/Al2O3 catalyst at similar conditions, indicating that the Co3O4 contribution is not only in performing the methane combustion itself but also in supplying surface oxygen rather than in affecting the activation energy. The kinetic evidence shows that the observed behaviour of Pd/Co3O4 catalyst is not a summation of individual activities of Co3O4 and Pd, but rather the effect of strong metal-support interactions (SMSI).

Deterioration of intrinsic activity of Ir-based electrocatalysts during the oxygen evolution reaction (OER) has received much less attention compared to the metal dissolution. Combining chronoamperometry with operando electrochemical impedance spectroscopy and cyclic voltammetry, we show that the deactivation via active site phase transformation from hydrous Ir oxide/hydroxide into anhydrous Ir oxide, is concomitant with the dissolution-induced loss of electrochemical surface area. The relative contributions from these deactivation paths were found to be structure sensitive. Systematic evaluation of different Ir-based catalysts at identical electro-oxidative conditions showed that hydrous IrOx with structural short-range order exhibited an initial minor degradation of intrinsic activity but the most significant dissolution after the extended stability test. In contrast, newly-reported Ir superstructures with higher crystallinity and larger proportion of low-index crystal terminations exhibited enhanced resistance to dissolution but a major degradation of intrinsic activity, as the performance-relevant hydrous oxide/hydroxide species developed only on the surface of metallic Ir. The Ir/IrOx catalyst regeneration was demonstrated.

Oxidative dry reforming of methane has been performed for 100 h on stream using Ni supported on MgAl2O4 spinel at different loadings at 500–700 °C, CO2/CH4 molar ratio of 0.76, and variable O2/CH4 molar ratio (0.12–0.47). Syngas with an H2/CO ratio of 1.5–2.1 has been produced by manipulating reforming feed composition and temperature. The developed oxidative dry reforming process allowed high CH4 conversion at all conditions, while CO2 conversion decreased significantly with the lowering of the reforming temperature and increasing O2 concentration. When considering both greenhouse gas conversions and H2/CO ratio enhancement, the optimal reforming condition should be assigned to 550 °C and O2/CH4 molar ratio of 0.47, which delivered syngas with H2/CO ratio of 1.5. Coke-free operation was also achieved, due to the combustion of surface carbon species by oxygen. The 3.4 wt% Ni/MgAl2O4 catalyst with a mean Ni nanoparticle diameter of 9.8 nm showed stable performance during oxidative dry reforming for 100 h on stream without deactivation by sintering or coke deposition. The superior activity and stability of MgAl2O4 supported Ni catalyst shown during reaction trials is consistent with characterization results from XRD, TPR, STEM, HR-STEM, XPS, and CHNS analysis.

Tin dioxide was assessed as a substitute for part of platinum-group metals in a catalytic converter, and the activity of various Pd and PdPt catalysts was compared on conventional alumina support and SnO2 in wet lean methane combustion. Remarkably, Pd-only catalysts supported on SnO2 revealed higher activity compared with PdPt/Al2O3. The catalysts benefit from the strong metal-support interactions and high oxygen mobility in SnO2 with dual Sn4+/Sn2+ valency. SnO2 can thus be considered a potential replacement of Pt in a catalytic converter for a natural gas vehicle under lean burn conditions. This potentially decreases the price of the converter and eliminates the need for scarce and expensive Pt.

We present a single-step ligand-directed colloidal synthesis protocol for the selective growth of dimensionally- and shape-controlled niobium disulfide (NbS2) nanostructures. A systematic modulation of the reaction conditions resulted in the development of monolayer nanosheets, which alternatively grew as laterally confined (

Palladium-platinum bimetallic catalysts are known for high activity in methane combustion, with phase transformations and structural changes occurring in a high-temperature oxidative atmosphere. Previous research has typically dealt with reaction temperatures exceeding those of the natural gas vehicle (NGV) exhaust, especially for lean burn engines, and often in the absence of water. This study evaluates the effect of the Pd:Pt ratio (from 5:1, 4:1, … to 1:4, 1:5) on the Pd-Pt/γ-Al 2 O 3 catalyst stability during and after 40-h in situ hydrothermal ageing at 400–550 °C (5% water). Pt presence at Pd-Pt 1:1 to 4:1 ratios is found to be optimal from the viewpoint of activity, with the most stable formulation being a 1:1 Pd-Pt catalyst. Platinum excess above a 1:1 ratio suppresses deactivation at 400 °C, but at lower activity levels. The effect is not governed significantly by the particle size but by the ratio of Pd:Pt. The comparative experiments performed with Pd core–Pt shell nanoparticles, co-deposited monometallic particles and monometallic mixed supported catalysts, along with EDX mapping of the used catalysts, suggest significant structural changes when such particles progressively transform into alloyed structures, with activity and stability approaching those of the alloyed nanoparticles. The results suggest that platinum vaporization is significant in the wet feed at low surface oxygen concentrations, even at the temperature interval of 400–550 °C. It appears that the proper Pd-Pt metal ratio has a governing effect on the activity and stable behaviour in wet low-temperature methane combustion rather than the catalyst preparation method.

Niobium sulfide (NbS2) has shown a promising performance in versatile applications, but its formation from Nb oxide is thermodynamically limited, which hinders its usage. We predicted, based on thermodynamic calculations, and experimentally verified that the addition of copper (Cu) to niobium promotes Nb oxide sulfidation at practical temperatures. A series of bimetallic bulk NbCu structures at varying Cu/Nb molar ratios were synthesized via a coprecipitation technique. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) results revealed that copper facilitated sulfidation and reduction of niobium oxide. The synthesized NbCu catalysts were evaluated in hydrodesulfurization (HDS) of dibenzothiophene (DBT) at 325 °C and 3 MPa. Copper promotes sulfidation but does not change the turnover frequency of surface NbS2 and behaves as a spectator. The optimal Cu/Nb molar ratio was found to be 0.3, below which there is not enough Cu to ensure maximum sulfidation and above which copper s...

Cu2+ and Zn2+ ion-exchange locations in mordenite (MOR) were evaluated using infrared spectroscopy, pore-size distribution, and temperature-programmed reduction. Isolated copper ions were the most abundant ion-exchanged species, as detected by UV-vis spectroscopy, in addition to oxide nanoparticles, with no presence of binuclear species, which was assigned to a low copper loading of 0.3 Cu/Al. The characterization revealed that only zinc could exchange in 8-membered rings. Hartree-Fock modeling confirmed copper exchange into 12-membered rings involving at least one T1 atom, and zinc exchange in T4 sites and in 8-membered structures, including T3 sites. Copper ion exchange did not offer improvement in the dimethyl ether carbonylation rate or selectivity over acidic mordenite. Zinc ion exchange led to the selectivity and stability improvement with some loss of activity. This work contributes to the understanding of acid and metal site contribution to DME carbonylation and contributes to the understanding for Cu2+ and Zn2+ ion-exchange locations in MOR with a low metal/Al loading (

A kinetic study of lean methane combustion on a silica-encapsulated bimetallic Pd–Pt (1:1 molar ratio) catalyst at varying methane concentrations and temperatures and in the absence/presence of added water is presented. With dry feed, the kinetic behavior of the bimetallic catalyst is correlated using a previously reported rate expression that is first order in methane and negative one order in water. The model does not adequately correlate the conversion of wet lean CH4 combustion in the temperature range of 550 to 750 K. For wet conditions, an alternative mechanism is suggested that is based on the previous experimental observations of the prevailing chemical state of Pd in wet feed, the ability of Pt to activate methane in oxygen-deficient atmospheres, and the inhibitory effect of water on the support-mediated oxygen exchange. The corresponding rate expression successfully predicts the activity of the silica-encapsulated Pd–Pt catalyst with wet feed (5 vol % water) in the temperature range of 550 to 75...

High-loading silica-encapsulated PdPt catalysts (PdPt@SiO2, 4 wt.% Pd, 7 wt.% Pt) are synthesized using a Stober-based method and are tested in lean methane combustion in the presence of water up to 550 °C. The as-synthesized bimetallic core particles have an average size of 7 nm, are uniform alloys of Pd and Pt, and are well-dispersed inside the oxide shells. The SiO2 shells are about 60 nm in diameter, with a specific surface area of 600 m2/g and a median pore diameter of 3.4 nm. The catalyst shows a stable methane conversion during a hydrothermal ageing (HTA) test which is two- and ten-fold higher than the conversion for the impregnated Al2O3 and SiO2-supported catalysts of the same metal loading, respectively. The surface area of the porous shell remains unaffected after HTA; however, the metal dispersion evaluated by CO chemisorption increases after the ageing and some changes in the morphology of the bimetallic PdPt nanoparticles occur.

The study focuses on the determination of metal site availability in silica-encapsulated Pd catalysts (Pd@SiO2). Existing synthetic methods are modified to achieve a high metal loading (up to 6 wt%) and porosity (surface area of 700 m2 g−1) while maintaining the original Pd nanoparticle size of 8 nm. Two synthesis schemes are used for encapsulating Pd NPs, and the resulting catalysts are assessed in lean methane combustion at up to 823 K. Application of poly(vinylpyrrolidone) (PVP) as a Pd particle stabilizer and a potential porogen alone is concluded to be inadvisable as it results in catalysts with a surface area of 70 m2 g−1 that show extremely low activity due to Pd inaccessibility. The high-surface area materials (700 m2 g−1) prepared via a separate introduction of PVP and an additional porogen (cetyltrimethylammonium bromide, CTAB) are active and exhibit the same turnover frequencies as the traditional catalysts but require smaller reactor sizes because of the high metal loading. However, 2/3 of the Pd nanoparticle surface is blocked by the shell material even in the highly porous catalysts. The silica-encapsulated catalysts, thus, offer advantages of high mass-based activity and sintering resistance of the metal cores, but their high porosity must be ensured for efficient mass transfer by the addition of a porogen (such as CTAB) during the Stober process. Above a certain limit, the increased amount of the porogen does not improve the metal accessibility and only leads to precious metal loss during synthesis.

A four-fold increase in palladium (Pd) mass-based hydrodesulfurization (HDS) activity was achieved by depositing Pd species as nanosized islands on 12 nm colloidal iron oxide (FeOx) nanoparticles via the galvanic exchange reaction. The highest palladium dispersion was obtained at an optimal Pd/Fe molar ratio of 0.2, which decreased when the ratio increased. The improved dispersion was responsible for the enhanced catalytic activity per the total Pd amount in the HDS of 4,6-dimethyldibenzothiophene at 623 K and 3 MPa as compared to the iron-free Pd/Al2O3 catalyst. The lattice strain and modified electronic properties of the Pd islands suppressed deep hydrogenation to dimethylbicyclohexyl and changed the hydrocracking product distribution. Pd nanoparticles deposited on commercial Fe2O3 did not provide such an activity enhancement and catalyzed significant cracking. This study demonstrates that FeOx@Pd structures are a possible alternative to monometallic Pd catalysts with enhanced noble metal atom efficiency for ultra-deep HDS catalysis and points to their great potential to reduce the catalyst cost and move towards more earth-abundant catalytic materials.

Bimetallic Pd-Y nanoparticles were synthesized by yttrium precursor reduction on the surface of colloidal Pd seeds catalyzed by palladium hydride. The addition of yttrium improved the thermal stability of the Pd nanoparticles to agglomeration. The nanoparticles before and/or after deposition on alumina were characterized by TEM, XPS, STEM with elemental mapping, temperature-programmed reduction, CO chemisorption and CO-DRIFT spectroscopy. The supported catalysts were evaluated in hydrodesulfurization (HDS) of 4,6-dimethydibenzothiophene at 350 °C and 1 MPa. Yttrium addition did not alter the overall HDS rate but increased the direct desulfurization selectivity from 71% to 84% and suppressed cracking twice as much as monometallic Pd catalyst did. The study demonstrates that PdY structures could be a promising candidate for low-pressure HDS of refractory sulfur compounds.

The addition of platinum to palladium is known to provide bimetallic catalysts that are relatively active in lean methane combustion in the presence of water at temperatures below 773 K, which is of practical interest for exhaust treatment from natural gas vehicles. The study provides insight into the wet lean methane combustion mechanism on the Pd–Pt/Al2O3 catalyst via in situ X-ray absorption spectroscopy studies at temperatures 473–873 K. The presence of water leads to an increased fraction of metallic Pd due to the lack of surface oxygen. The fraction of metallic Pd drops as the temperature increases. Oxygen deficit results in Pt atoms available for methane dissociation, which does not occur in the dry methane-lean feed, in which oxygen poisons Pt.

Dimethyl ether carbonylation to methyl acetate was studied on a series of dealuminated mordenites with Si/Al ratios of 6.5 through 15.4. Dealumination was achieved via leaching with nitric acid. Selective coupled removal of T3 and T4 Al sites located in 4-membered rings in 8- and 12-membered channels, respectively, was suggested by a combination of DRIFTS, TPD of probe molecules, 27 Al MAS NMR, surface area, pore distribution, and XRD analyses. The methyl acetate peak productivity was found to be equal when calculated per residual T3 sites in 8-membered rings, providing experimental evidence of the reaction site specificity. While dealumination led to the decrease in product peak rate because of active and selective T3 site removal, it doubled the catalyst lifetime and increased selectivity upon deactivation from 50% to 80%. The study decoupled the contribution of 4-membered (T3 and T4) and 5-membered (T1 and T2) acid sites to the products’ formation and catalyst deactivation.

Palladium-platinum bimetallic catalysts supported on alumina with Pd/Pt molar ratios ranging from 0.25 to 4 were studied in dry lean methane combustion in the temperature range of 200 to 500 degrees Celsius. Pt addition decreases the catalyst activity, which could not be explained by the decrease in dispersion or the reaction structure-sensitivity. In situ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were conducted for monometallic Pd, Pt and Pd-Pt 2:1 catalysts. Monometallic Pd is fully oxidized in the full temperature range, whereas Pt addition promotes Pd reduction even in a reactive oxidizing environment. The Pd/PdO weight ratio in bimetallic Pd-Pt 2:1 catalysts decreases from 98/2 to 10/90 in the 200-500 degrees Celsius temperature range under reaction conditions. Thus, Pt promotes the formation of the reduced Pd phase with a significantly lower activity as compared to the oxidized Pd. The study sheds light on the effect of Pt on the state of the active Pd surface under low-temperature dry lean methane combustion conditions, which are important for methane-emission control devices.

38% to 93% selectivity enhancement toward sulfur-free products was observed upon iridium addition to a palladium-only catalyst in the hydrodesulfurization of a refractory sulfur compound 4,6-dimethyldibenzothiophene (4,6-DMDBT) at the same 40% conversion from a 300 ppm, S-containing feed at 5 MPa and 300 °C. Pd promoted hydrogenation to sulfurous intermediates, while Ir catalyzed C S hydrogenolysis and also improved Pd resistance to sintering. The selectivity in the direct desulfurization path for the Ir-containing catalysts increased up to 26% versus 5% for the Pd-only catalyst. The bimetallic catalyst allowed for a decrease in S from 300 ppm to 11 ppm. It can be used at reduced pressure (3 MPa) with only a 15% decrease in hydrodesulfurization conversion as compared to the operation at 5 MPa.

A plethora of previously unimaginable low-temperature C1 and C2 valorization reactions have become possible after the discovery of metal-exchanged solid-acid catalysts, with the most promising candidate being copper-mordenite. We show the dramatic effect of zinc addition to copper-exchanged mordenite in maintaining high Cu dispersion and reducing the catalyst poisoning as it relates to carbonylation of dimethyl ether to methyl acetate. Zinc maintains 90%+ selectivity even during deactivation versus 60% for the Cu-mordenite, which leads to 6-fold higher product yield. The concept of Zn addition is recommended for further exploration in the conversion of methane to methanol or acetic acid.

Bimetallic ion exchange on a zeolite often impacts its catalytic properties compared to its monometallic counterparts. Here, we address the synergistic effect of simultaneous copper and zinc ion exchange on mordenite (MOR), as found earlier for dimethyl ether (DME) carbonylation. Samples with various Cu/Zn ratios were characterized by diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) in the 3600 and 720 cm-1 regions, pore distribution analysis through Ar physisorption, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and transmission electron microscopy (TEM). When ion-exchanged alone, copper preferentially occupies 12-membered rings, whereas zinc occupies 8-membered rings. In bimetallic combinations, the zinc addition was found to prevent the copper from sintering into nanoparticles and to increase its coordination strength to the zeolite. At a Cu/Zn ratio of 0.25 (for MOR with Si/Al=6.5), copper promotes zinc ion exchange into 12-membered rings, more specifically, into T4 sites that are known for the formation of the coke precursor in DME carbonylation on a MOR. The sites became blocked during the bimetallic ion exchange, leading to suppressed catalyst deactivation. The study contributes to the understanding of mutual ion effects in bimetallic exchanged zeolites and highlights the major role of copper as a governing factor in determining the location of co-exchanged zinc on a MOR.

Two types of bimetallic Pd-Ru catalysts with a 2:1 Ru:Pd molar ratio were prepared using a poly-(vinylpyrrolidone) stabilizer: one alloy structure with mixed-surface atoms and one core–shell structure with a Pd core and Ru shell, which were confirmed by a surface-probe reaction at mild conditions. In indan hydrogenolysis at 350 °C, inversion of the core–shell structure began with Pd atoms appearing on the surface of the particles. Both catalysts displayed distinctively different catalytic behavior and indicated the importance of structure control for this particular application within a studied time frame. For methane combustion over the 200–550 °C temperature range, both structures demonstrated identical activity, which was due to their structural evolution to one nanoparticle type with Pd-enriched shells, as evidenced by extended X-ray absorption fine structure.

The crucial role of Ni mode addition to Pd catalysts for low-temperature wet methane combustion is addressed, resulting in excellent performance of ultralow-Ni-containing catalysts versus inactive nickel–alumina spinel. Traditional impregnation–calcination and colloidal techniques of bimetallic catalyst preparation yield monometallic Pd particles on a binary NiAl2O4 support and Pd and Ni nanoparticles on the parent Al2O3 support, respectively. The catalyst is potentially valuable for natural gas catalytic combustion technologies because it decreases the required temperature for complete methane combustion in 5% water presence in the feed by 100 degrees versus monometallic Pd.

An increase in stirring speed is generally considered to be an a priori means of reducing external mass-transfer limitations in fast three-phase hydrogenations that are performed in a stirred tank. We provide experimental evidence for a 300-mL stirred reactor that, above a certain impeller speed, the efficiency of gas–liquid mass-transfer decreases, resulting in the decreased reaction rate. The phenomenon is attributed to the high degree of gas recirculation with large cavities behind the blades. The recirculation may decrease hydrogen concentration in the remainder of the tank, thus decreasing the concentration gradient that controls mass transfer. The model reaction in this work was 2-methyl-3-butyn-2-ol semihydrogenation with Lindlar catalyst Pd–Pb/CaCO3. The test impellers were a Rushton turbine, a down-pumping pitched blade turbine, and up-pumping A340 impellers. The kinetic experiments were combined with the measurement of volumetric gas–liquid mass-transfer coefficient, flow pattern analysis and im...

The answer is no. It depends on the catalyzed reaction and the active metal. For Ru and Ir nanoparticles stabilized by polyvinylpyrrolidone (PVP), their activities in an indan ring opening are not affected by the residual polymer, while CO chemisorption requires complete polymer removal. The in situ cleaning effect of the reaction atmosphere was found negligible. The findings are supported by TEM, XPS, CHN analysis, indan- and CO-TPD, CO chemisorption analyses and catalytic measurement. The necessity for complete polymer removal must be evaluated on a case-by-case basis; in some cases mild catalyst pretreatment is enough.

Alloy and core–shell bimetallic Pd–Ir nanoparticles stabilized by polyvinylpyrrolidone (PVP) and deposited on alumina were subjected to PVP removal via thermal treatments. Less than 25% of the PVP was removed at 200 °C calcination/375 °C reduction, while the 400 °C calcination eliminated over 95% of PVP. The treated samples were tested in the gas-phase ring opening of indane at 350 °C. The cleaning was paramount for the bimetallic catalysts with Pd-rich surfaces that only exhibited activity after 400 °C calcination, while the catalysts with Ir-rich surfaces were similarly active irrespective of the presence or absence of the PVP residuals. For the CO chemisorption, complete cleaning was required for all catalysts. The study is supported by CO-DRIFTS, CO-TPD, indane-TPD, carbon analysis, TEM, CO chemisorption and catalytic measurements, and shows that the necessity to remove PVP depends on the nature of metals in bimetallic catalysts and their surface arrangements.

A variety of bimetallic Ni–Ir catalysts were synthesised by preforming nanoparticles in the presence of polyvinylpyrrolidone, followed by deposition on γ-alumina and high-temperature polymer removal. The Ni–Ir (1:1 molar ratio) nanoparticles prepared by the hydrogen-sacrificial technique (Ir reduction on the preformed Ni nanoparticles with surface Ni hydride) allowed increasing indane ring opening activity per total amount of Ir as compared to monometallic Ir. The simultaneous reduction of Ni and Ir precursors was not as efficient. The catalysts were characterised with UV/Vis spectroscopy, TEM, temperature-programmed reduction, CO2 temperature-programmed desorption, CO diffuse reflectance Fourier transform spectroscopy, X-ray photoelectron spectroscopy and CHN analysis. The study only explored the catalyst’s metal function and allows saving rare and expensive iridium without loss of its outstanding performance as a ring-opening catalyst.

A 3- and 2-fold increase in selectivities toward 2-ethyltoluene and n-propylbenzene, respectively, in indan ring opening (RO) was achieved by introducing palladium to the ruthenium catalyst. The product selectivities for the Ru–Pd system with the 4:1 molar ratio were the same as those for monometallic iridium, known for its outstanding single cleavage selectivity; the lights formation was suppressed as compared with the monometallic platinum catalyst. A further increase in the Pd amount did not result in the selectivity improvement and brought down the activity to the low level of Pd. The bimetallic catalysts were synthesized in the presence of poly-(vinylpyrrolidone). The bimetallic systems revealed sintering resistance up to 400 °C, as compared with their monoforms. The indan RO activity was maximized after precalcination at 200 °C. The suggested nanoparticles’ bimetallicity was consistent with the results of CO-TPD, CO–DRIFTS, thermal stability tests, and a chemical probe reaction (olefin hydrogenation...

Due to the complexity of the vacuum residue fraction of petroleum and bitumen, a model compound was used to probe cracking and addition reactions in the liquid phase. Hydrogenation reactions were conducted in a batch microreactor at 430 °C, 13.9 MPa H2 for 30 min using a solution of 1,3,6,8-tetrahexylpyrene (THP) in tetralin. Sulfided iron was prepared on α-alumina, γ-alumina, and glass beads as support materials. The hypothesis of this study was that addition reactions can be suppressed under hydrogenation conditions by using iron sulfide as a low-activity catalyst in the presence of hydrogen gas and a hydrogen donor solvent, by saturating olefin intermediates. The products were analyzed by high performance liquid chromatography, gas chromatography, matrix-assisted laser desorption ionization mass spectrometry, and proton nuclear magnetic resonance spectroscopy to investigate conversion and product distribution for different catalysts and without added catalyst. The results show that sulfided iron can gi...

Nearly monodispersed 1.6 nm Ir, 2.3 nm Pd nanoparticles, 2.7 nm Pd(core)–Ir(shell) and 2.2 nm Pd–Ir alloys with mixed surface atoms were synthesised in the presence of polyvinylpyrrolidone (PVP) and studied in the atmospheric ring opening of indan. The nanoparticles and supported catalysts were characterised by UV–vis spectroscopy, TGA, TEM, EDX, TPR, XPS, ISS and XANES. The nanoparticle size and structure control allowed insights into the bimetallic catalyst functioning. As soon as two contiguous surface Ir atoms exist on the nanoparticle surface, they display the same catalytic properties, most likely through the dicarbene ring-opening mechanism. Palladium serves only as a dispersing agent in Pd(core)–Ir(shell) structures providing 100% Ir dispersion, or as an inert surface diluting metal in Pd–Ir formulations with mixed nanoparticle surface. The Pd–Ir core–shell structure allows maximum selective ring-opening yield, which is 19% higher than that for the industrial Pt–Ir catalyst.

Selective ring opening of naphthenic molecules in oil upgrading should result in no loss in molecular weight. Benzocyclopentane (indan) ring opening was studied under hydrogen atmospheric pressure at 609 K over poly-(vinylpyrrolidone)-stabilized Ru, Ir and Pd monometallic and Ru–Ir and Ru–Pd bimetallic nanocatalysts prepared by simultaneous reductions. The particle size (mostly within 2 nm) and their monodispersity were confirmed by transmission electron microscopy, while X-ray photoelectron spectroscopy indicated the bimetallics' alloy structure. Pd catalysts displayed the lowest activity in the ring opening; Ru showed the highest formation of undesired o-xylene and lights. Monometallic Ir displayed the highest activity and selectivity toward 2-ethyltoluene and n-propylbenzene. In bimetallic structures, higher Ir content led to improved catalytic performance. Next to the monometallic Ir catalyst, the newly developed Ru1Ir4/γ-Al2O3 catalyst (with 1 : 4 molar ratio of Ru to Ir) displayed superior single cleavage selectivity as well as lower cracking activity compared to industrial Pt–Ir catalysts at a comparable indan conversion. The study can pave the way in the development of Pt-free Ru-containing catalysts with narrow size distributions for selective ring opening, especially taking into consideration a possibility of their higher S-resistance as compared to the Pt catalysts.

Tungsten carbide-based electrodes under mixed hydrogen–methane and methane fuels have been investigated as potential anode materials for solid oxide fuel cell (SOFC) application. Firstly, it was shown that hydrogen is not a suitable fuel for the carbide-based materials. A conventional WC–YSZ composite and a carbide infiltrated porous YSZ support were then studied. Ac impedance spectroscopy revealed that the ohmic resistance and the charge-transfer polarization of these cells were reasonably low. The chemical reaction polarization, however, was relatively large, particularly under methane fuel. The carbide-based electrodes were then modified by incorporation of ceria and/or ruthenium. Not only did the co-existence of CeO 2 and Ru synergically enhance the cell performance, more importantly it also greatly improved the stability of the polarized cell. Although bulk phase analysis confirmed the presence of a minor amount of tungsten oxide, surface analysis showed that the oxide phase remained superficial. It was then proposed that surface oxidation of the carbide phase was essentially a part of the fuel oxidation process and, as long as the rate of carbide oxidation and that of oxide recarburization remained comparable, the cell performance was stable.

Palladium nanospheres of 2.4 and 3.8 nm diameter and nanocubes of 18 nm rib length were used to catalyze a fluorometric Tsuji-Trost reaction for the transformation of a phenyl allyl ether to a fluorescent phenol in the presence of triphenylphosphine, which was pivotal to the catalytic activity. Turnover frequencies calculated per defect atoms were found similar for all nanoparticles, indicating that these atoms are the active sites. However, kinetic studies combined with Pd leaching and transmission electron microscopy analyses in the presence of various reaction components showed Pd leaching via oxidative addition of a reactant, followed by nanoparticle growth depending on the PPh3 concentration. The formation of largest particles was found for the fastest reaction with PPh3/Pd molar ratio of 4, in the range from 0 to 9. This study shows the validity of the atomic dissolution mechanism in the reaction of interest. © 2011 Elsevier Inc. All rights reserved.

A structured catalyst support was developed based on FeCrAl alloy sintered microfibers (SMF) via multiple stage thermal oxidation in air for 1 h at 930 °C, 1 h at 960 °C and 2 h at 990 °C. The procedure resulted in the formation of a forest of predominantly α-alumina whiskers (200 nm in height and 100 nm apart). Palladium deposition and reduction yielded 0.5 wt.% Pd/SMF with 20 nm nanoparticle size. The catalyst was tested in three-phase hydrogenation of 2-methyl-3-buten-2-ol, and due to its pore structure allowed eliminating internal mass transfer limitations. The developed support can be beneficial to catalytic reactions suffering from mass transfer limitations and catalyst deactivation via pore mouth blocking. As compared to other methods of structured catalyst preparation, the thermal oxidation procedure is simple, fast and environmentally benign, and eliminates problems associated with poor adhesion of traditional washcoated layers of powdered catalysts or supports.

The catalytic steam gasification of coke from Athabasca bitumen was investigated by thermogravimetric analysis using K 2 CO 3 and Na 2 CO 3 as catalysts, both of which reduced the activation energy of the reaction considerably to 1.2 × 10 5 J mol −1 and 1.3 × 10 5 J mol −1 , respectively, down from 2.1 × 10 5 J mol −1 for the uncatalyzed reaction. The reaction rates varied with the partial pressure of steam between 60 kPa and 85 kPa consistent with a Langmuir–Hinshelwood model, but a first order equation was also sufficient given the low partial pressures. The initial rate of gasification of the coke particles correlated linearly with the estimated external surface area of the particles, as expected from a surface reaction involving a non-porous solid. The initial reaction rate increased with increasing the catalyst loading up to 2.4 (mol potassium)/kg. A portion of the catalyst penetrated into the coke, as confirmed by secondary ion mass spectroscopy analysis, where it could not promote the reaction with steam. This result was consistent with a small increase observed in the reaction rate at low catalyst loading. The shrinking core model was successful in predicting the rates at higher conversions from the initial rate data, despite increases in BET surface area with conversion.

Monodisperse Pd nanocubes of 20 nm rib length and Pd nanospheres of 3.0 nm diameter were synthesized in the presence of cetyltrimethylammonium bromide and used to investigate the structure sensitivity of three-phase 2-methyl-3-buten-2-ol hydrogenation. Turnover frequencies per all surface atoms were found as 2.58 s−1 for the cubes and 3.86 s−1 for the spheres at 313 K, indicating that (100) atoms of the cubes comprising ∼98% of all surface atoms have lower activity than other surface atoms of the spheres, composed of atoms on (111), (100) terraces, edges, and vertices. Apparent activation energies of 23 kJ/mol for the cubes and 17 kJ/mol for the spheres in the verified kinetic regime confirmed the reaction structure sensitivity. Assuming that only (100) and (111) atoms are active on the sphere surface, a hypothetical most active Pd nanostructure was predicted as a tetrahedron allowing twice higher activity per Pd loading as compared to a spherical particle.

Recent advances in the liquid-phase synthesis of metal nanostructures of different sizes and shapes are reviewed regarding their catalytic properties. The controlled synthesis of nanostructures is based on the colloid chemistry techniques in the solution, which use organic nanoreactors and a variety of stabilizers. Their catalytic activity and selectivity depend on the particle’s shape and size, as shown for Suzuki and Heck coupling, hydrogenations, hydrogenolysis, oxidations, and electron-transfer reactions. The knowledge of a reaction’s structure-sensitivity relationship is important for the rational catalyst design in view of process intensification. Nanostructures can be used per se and in supported form to meet the requirements of an eventual process.

Palladium nanohexagons were prepared using a seed-mediated method. Their catalytic performance in 2-methyl-3-butyn-2-ol hydrogenation was compared to the one of monodispersed Pd nanospheres. Quantitative correlations between initial turnover frequencies (TOFs) and nanoparticle surface compositions showed independence of TOFs calculated per atoms on Pd(111) facets on particle size and shape.

Monodispersed Pd nanoparticles (8, 11, and 13 nm in iameter) as confirmed by high resolution transmission electron microscopy were prepared via the reverse microemulsion method and deposited on structured supports consisting of carbon nanofibers (CNF) grown on sintered metal fibers (SMF). The CNF/SMF supports were subjected to oxidative treatments to introduce O-functional groups on the CNF surface. These groups were characterized by temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy. The catalysts were used to study (a) the effect of Pd size and (b) the effect of the support nature on the selective acetylene hydrogenation. Antipathetic size dependence of TOF disappeared at particle size bigger than 11 nm. Initial selectivity to ethylene was found size-independent. The deactivation due to coke deposition was faster for smaller particles. The structure-sensitivity relations for the catalysts investigated are discussed in terms of “geometric” and “electronic nature” of the size effect and rationalized regarding Pd-Cx phase formation which is size-dependent. Supports with increased acidity diminished the formation of coke and changed the byproduct distribution toward ethane.

Systematic studies of mass transfer interactions with intrinsic reaction kinetics were performed for the threephase selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) over a modified Pd/CaC${O}_{3}$ catalyst under solvent free conditions. Hydrogen concentration in the liquid phase (${C}_{H2,b}$) was monitored in situ during the catalytic reaction by means of the Fugatron” analyzer. Reactions were carried out in an autoclave at different stirring rates at two concentrations of hydrogen (5 and 13 mol ·${m}^{-3}$ For stirring speeds higher than 1500 rpm no influence of gas-liquid mass transfer was observed. Hydrogen liquid-solid (L-S) mass transfer was found to be negligible, whereas the MBY mass L-S transfer becomes important at high MBY conversions at high hydrogen concentration. Low stirrer speed caused the reaction rate and MBE selectivity to decrease. No internal mass transfer limitations were observed, and conditions for the kinetic regime were found. The kinetics modeled ollowed the Langmuir-Hinshelwood mechanism and was consistent with the experimental data.

An innovative structured catalyst based on sintered metal fibers (SMFs) coated by a grain-structured ZnO layer was developed for the selective 3-phase hydrogenation of functionalized alkynes. The catalyst was synthesized by the deposition of Pd0 -sol with 7-nm nanoparticles stabilized by molybdate anions on ZnO/SMF after reduction in hydrogen at 773 K. The crystallites of PdZn alloy (24 nm) were formed by high-temperature treatment in hydrogen and contained partly Pd(0) nanoparticles. Pd/ZnO/SMF (0.2 wt% Pd, 6 wt% ZnO) allowed > 95 % yield of 2-methyl-3-buten-2-ol (MBE) during 2-methyl-3-butyn-2-ol hydrogenation at 308 K and 5 bar pressure using water as the solvent and adding a small amount of quinoline . The consecutive reaction of MBE hydrogenation was suppressed due to the presence of PdZn in the active phase. The catalyst activity of the catalyst was an order of magnitude higher compared with that of commercial Lindlar catalyst (Pd, Pb/CaCO 3), which quickly deactivated in aqueous media due to irreversible chemical transformations of the CaCO3 support. The structured catalyst developed in this study could be reused after ultrasonic regeneration, retaining the original selectivity and most of its initial activity. The catalyst showed high mechanical stability, precluding leaching of its components during the reaction. Due to easy shaping, high permeability, and low pressure drop during the fluid passage, the structured Pd/ZnO/SMF catalyst is suitable for a continuously operated staged bubble column reactor.

Monodispersed Pd-nanoparticles ( ∼ 8nm ) prepared via a modified microemulsion method were deposited on active carbon fibers (ACF)fabrics and studied in a semi-batch 1-hexyne liquid-phase hydrogenation. The catalyst Pd(0.45wt%)/ACF demonstrated > 96% selectivity to1-hexene up to 90% of conversion. Initial activity at 303K, 1.3MPa pressure, in n-heptane as a solvent was 0 . 14kmol H 2 kg −1Pd s −1 . Assuminga Langmuir–Hinshelwood kinetics with a weak hydrogen adsorption a kinetic model was developed. It was shown to be consistent with theexperimental data and allowed determining the main kinetic parameters. 2006 Elsevier Ltd. All rights reserved. Keywords: Palladium nanoparticles; Microemulsion; Hydrogenation; Structured catalyst; Kinetics modeling 1. Introduction Catalytic hydrogenation is one of the key processes in finechemicals production. The performance of the hydrogenationprocess and product distribution are strongly influenced by thecatalyst activity and selectivity, the interaction of chemical ki-netics and mass transfer, as well as the reactor design. To avoidinternal and external mass transfer limitations and to attain highproduct selectivity catalyst particles of small diameter are re-quired. In technical applications the minimal size, however, islimited due to the catalyst handling.One approach to overcome these problems is the use of fi-lamentous catalytic materials (Kiwi-Minsker, 2002). The diam-eters of the filaments are in the order of several micrometersand correspond to the diameter of the traditional suspensioncatalysts. Therefore, internal mass transfer limitations could beminimized. Platinum-group metals were deposited on filamen-tous woven cloths and demonstrated high efficiency in variousliquid-phase hydrogenations (Holler et al., 2000; Macias Pereset al., 1997).

The structure sensitivity of a liquid-phase 1-hexyne hydrogenation was studied using monodispersed nonsupported Pd nanoparticles of 6, 8, 11, 13, and 14 nm diameter. The particles were prepared via a reverse microemulsion water/AOT/isooctane. The size was varied by the water-to-surfactant ratio. A 15-fold TOF increase was observed with increase of the particle diameter from 11 to 14 nm. For particles above 14 nm, TOF approached the value of the Pd black catalyst. It was observed that the selectivity toward 1-hexene did not vary with the particle size and was 96.5% at the conversion of 1-hexyne of 85%. The byproduct distribution was characterized by the ratio of selectivities to 2-hexenes/n-hexane and decreased 5-fold with particle diameter from 6 to 11 nm but remained constant for bigger particles. The experimental results indicate a predominance of a “geometric” nature of the size effect observed during selective 1-hexyne hydrogenation, suggesting that an ensemble of neighboring Pd surface atoms constitutes the active center responsible for hydrogenation.