Hydroxyls away! Tin oxide extracts hydroxyls from Pd and liberates it for methane combustion but renders Pd inactive for methane steam reforming, as opposed to Pd/Al2O3. Quantitative analysis of metallic Pd, PdO, and Pd(OH)2 is presented for Pd/Al2O3 and Pd/SnO2 catalysts as a function of temperature during methane‐lean combustion in the presence of water, as found by in situ X‐ray absorption spectroscopy.
Quantitative analysis of poisonous Pd hydroxide phase formation during methane combustion in the presence of water is performed via in situ X‐ray absorption spectroscopy from 200 to 500 deg C for palladium catalysts supported on tin and aluminum oxides. Water presence inhibits oxidation of the metallic Pd; for the Pd/Al2O3catalyst the major oxidation product is Pd(OH)2, with PdO being the dominant phase on the Pd/SnO2 system. Tin dioxide is reduced during methane oxidation in the presence of water, regardless of the excess oxygen in the feed. Temperature‐programmed surface reactions at anaerobic and low‐oxygen conditions revealed that the Pd/SnO2 catalyst has an inability to catalyze methane steam reforming, as opposed to Pd/Al2O3. We suggest that tin oxide extracts hydroxyls from the PdO surface, which makes the latter more active in methane combustion in the presence of water but inactive in methane steam reforming. © 2019 John Wiley and Sons.
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/Al2O3provided only PdO-associated low oxygen for the CO and H2 formation. Co3O4surpasses 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. © 2019 Elsevier, reprinted with permission.
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%). Co3O4demonstrates 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). © 2019 Springer Nature, reprinted with permission.
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. © 2019 Springer Nature, reprinted with permission.
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. © 2019 Elsevier, reprinted with permission.
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/IrOxcatalyst regeneration was demonstrated. © 2019 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license.
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 (<100 nm) 2D nanostructures using an appropriate mixture of coordinating ligands with different functional groups. The lateral size reduced to ∼50 nm by increasing the amount of a chalcogen source (carbon disulfide, CS2), which promoted planar growth to form 0D nanohexagons and 1D hexagonal nanorods. A noncoordinating solvent suppressed anisotropic growth and resulted in the formation of 0D nanospheres. Nanohexagons with the highest fraction of corner and edge active sites delivered the highest activity in dibenzothiophene hydrodesulfurization. © 2018 American Chemical Society, reprinted with permission.
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-Al2O3 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. © 2018 Elsevier, reprinted with permission.
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 NbS2and 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 segregates to the catalyst surface and blocks NbS2 active sites. The weight-based sulfur-removal activity of the optimal catalyst was doubled in the presence of copper. This study demonstrates that the bimetallic Earth-abundant NbCu catalyst could be a promising candidate for hydrotreating catalysis. Since Cu-promoted NbS2 was determined to be more active than molybdenum sulfide per mass and surface area, the copper addition may be recommended for thermodynamically limited niobium oxide sulfidation to promote NbS2 formation as a potential alternative to MoS2 for a variety of emerging applications with transition-metal sulfides. The study also demonstrates that the concept of promotion in catalysis can be extended to the assisted increase of the number of active sites, with no effect on their performance during catalysis. © 2018 American Chemical Society, reprinted with permission.
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. © 2018 Reproduced by permission of the Royal Society of Chemistry.
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 CH4combustion 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 750 K. The study also evaluates the internal mass transfer across the silica shell. It is shown that for the catalysts used here, the diffusion resistance across the shell is negligibly small. © 2018 American Chemical Society, reprinted with permission.
High-loading silica-encapsulated PdPt catalysts (PdPt@SiO2, 4 wt.% Pd, 7 wt.% Pt) are synthesized using a Stöber-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. © 2018 Elsevier, reprinted with permission.
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 (<0.2). © 2018 Elsevier, reprinted with permission.
Mutual ion effects: In bimetallic CuZn/MOR, zinc prevents copper sintering, whereas copper promotes zinc ion exchange into 12‐ instead of 8‐membered rings:
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−1regions, 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. © 2018 Wiley-VCH.
The study focuses on the determination of metal sites availability in silica-encapsulated Pd catalysts (Pd@SiO2). Existing synthetic methods are modified to achieve high metal loading (up to 6 wt.%) and porosity (surface area of 700 m2/g) 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 surface area of 70 m2/g that show extremely low activity due to Pd inaccessibility. The high-surface area materials (700 m2/g) 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 Stöber process. Above a certain limit, the increased amount of the porogen does not improve the metal accessibility and only leads to the precious metal loss during synthesis. © 2018 Reproduced by permission of the Royal Society of Chemistry.