Stability of Pd-Pt catalysts in low-temperature wet methane combustion: Metal ratio and particle reconstruction

Article
H. Nassiri, R.E. Hayes, N. Semagina
Chemical Engineering Science, 186, 44-51
Publication year: 2018

Highlights

  • Pd-Pt catalysts with 1:5 to 5:1 ratio were studied in wet lean methane combustion
  • Hydrothermal ageing tests were performed for 40 h at 400–550 °C
  • The optimal combination of activity and stability is a Pd:Pt 1:1 (atomic) ratio
  • Significant structural changes occur in situ, even at such low temperatures.

Abstract

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.

Promotion of niobium oxide sulfidation by copper and its effects on hydrodesulfurization catalysis

Article
A. Mansouri, N. Semagina
ACS Catalysis, 8, 7621-7632
Publication year: 2018

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.

Palladium islands on iron oxide nanoparticles for hydrodesulfurization catalysis

Article
A. Mansouri, N. Semagina
Catalysis Science and Technology, 8, 2323-2332
Publication year: 2018

Deposition of thin Pd islands on iron oxide nanoparticles results in a 4-fold activity enhancement in HDS and suppresses cracking:

Abstract

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.

Kinetics of low-temperature methane oxidation over SiO2-encapsulated bimetallic Pd-Pt nanoparticles

Article
A.H. Habibi, N. Semagina, R.E. Hayes
Industrial & Engineering Chemistry Research, 57, 8160-8171
Publication year: 2018

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.

Evaluation of hydrothermal stability of encapsulated PdPt@SiO2 catalyst for lean CH4 combustion

Article
A.H. Habibi, R.E. Hayes, N. Semagina
Applied Catalysis A: General 556, 129-136
Publication year: 2018

Highlights

  • PdPt nanoparticles are encapsulated in silica for wet methane combustion catalysis
  • The encapsulated catalyst shows higher conversion than impregnated Al2O3 and SiO2
  • The shell remains intact but PdPt morphology and dispersion change after ageing.

Abstract

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.

Effect of Cu and Zn ion-exchange locations on mordenite performance in dimethyl ether carbonylation

Article
A.A.C. Reule, V. Prasad, N. Semagina
Microporous and Mesoporous Materials, 263, 220-230
Publication year: 2018

 

Highlights

  • Cu(II) and Zn(II) ion exchange locations on MOR are studied
  • Cu exchanges into 12-membered rings (12-MR), Zn into 8-MR of MOR
  • Spectral and porosity analysis is supported by Hartree-Fock modeling
  • Cu/MOR and Zn/MOR showed different selectivity in dimethyl ether carbonylation.

Abstract

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.

Copper affects the location of zinc in bimetallic ion‐exchanged mordenite

Article
A.A.C. Reule, J. Shen, N. Semagina
ChemPhysChem, 19, 1500-1506
Publication year: 2018

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.

Bringing attention to metal (un)availability in encapsulated catalysts

Article
A.H. Habibi, R.E. Hayes, N. Semagina
Catalysis Science and Technology, 8, 798-805
Publication year: 2018

Encapsulation method significantly affects shell porosity, availability of active sites and catalytic behavior of Pd@SiO2 materials in methane combustion:

Abstract

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.

Water shifts PdO-catalyzed lean methane combustion to Pt-catalyzed rich combustion in Pd-Pt catalysts: In situ X-ray absorption spectroscopy

Article
H. Nassiri, K.-E. Lee, Y. Hu, R.E. Hayes, R.W.J. Scott, N. Semagina
Journal of Catalysis, 352, 649-656
Publication year: 2017

Highlights

  • Pd-Pt catalysts are relatively insensitive to water poisoning in methane combustion
  • In situ spectroscopy shows a water-promoted increase of Pd(0)-to-oxidized Pd ratio
  • In situ study and thermodynamic analysis indicate a water-induced lack of oxygen
  • In the lack of oxygen, Pt(0) allows for CH4 activation in Pd-Pt catalysts
  • PdO activates methane in dry combustion, and Pt(0) does so in the wet feed.

Abstract

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 an 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 the 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. © 2017 Elsevier, reprinted with permission.

Thermal kinetics analysis in microwave‐assisted oil sands separation

Article
K. Rambabu, N. Semagina, C.F. Lange
The Canadian Journal of Chemical Engineering 95 (1), 127-135
Publication year: 2017

This computational study addresses oil sands separation under microwave irradiation. Electric field distribution and microwave energy dissipation were analyzed at the level of sand particles and bitumen. The simplified model does not include water or gas present in the oil sands. The microwave power dissipated in bitumen was found to be about 32 times that of the sand particle. In order to investigate the feasibility of heating the bitumen phase preferentially and sustaining a thermal gradient between the bitumen phase and the solid phase, thereby reducing the thermal losses to the solid phase, time scales were estimated using several different scenarios. The small size of the particles in the solid phase caused the time needed to reach thermal equilibrium between phases to be extremely short, in the order of 10 to 100 ms. To achieve separation times shorter than these, pressures up to 100 GPa would have to be sustained across the oil sands layer. Alternatively, buoyancy-driven separation by settling would require accelerations in the order of 10^(5) g. Therefore, the heating and separation of bitumen within the thermal conduction time scales seem to be theoretically possible but associated with high technological challenges in their implementation. © 2016 Canadian Society for Chemical Engineering.

Platinum inhibits low-temperature dry lean methane combustion via palladium reduction in Pd-Pt/Al2O3: An in situ X-ray absorption study

Article
H. Nassiri, K.-E. Lee, Y. Hu, R.E. Hayes, R.W.J. Scott, N. Semagina
ChemPhysChem, 18, 238-244
Publication year: 2017

Active site “spoiler”: Palladium–platinum bimetallic catalysts supported on alumina are studied in dry lean methane combustion at low temperature. Platinum addition decreases the catalyst activity by reducing active palladium oxide to inactive palladium(0):

Palladium–platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 °C. Platinum addition decreases the catalyst activity, which cannot be explained by the decrease in dispersion or the structure sensitivity of the reaction. In situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy measurements have been conducted for monometallic Pd, Pt, and 2:1 Pd−Pt catalysts. Monometallic palladium is fully oxidized in the full temperature range, whereas platinum addition promotes palladium 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 °C temperature range under the reaction conditions. Thus, platinum promotes the formation of the reduced palladium phase with a significantly lower activity than that of oxidized palladium. The study sheds light on the effect of platinum on the state of the active palladium surface under low-temperature dry lean methane combustion conditions, which is important for methane-emission control devices. © 2017 Wiley-VCH.

Enhancement of palladium-catalyzed direct desulfurization by yttrium addition

Article
A. Mansouri, N. Semagina
Journal of Applied Catalysis A: General, 543, 43-50
Publication year: 2017

Highlights

  • Bimetallic PdY nanoparticles were studied in HDS of 4,6-DMDBT at 350 °C and 1 MPa
  • Y improved the thermal stability of Pd nanoparticles against sintering
  • Y increased the direct desulfurization selectivity of Pd from 71% to 84%
  • Y addition to Pd suppressed cracking.

 

Abstract

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. © 2017 Elsevier, reprinted with permission.

Effect of selective 4-membered ring dealumination on mordenite-catalyzed dimethyl ether carbonylation

Article
A.A.C. Reule, J.A .Sawada, N. Semagina
Journal of Catalysis, 349, 98-109
Publication year: 2017

Highlights

  • Leaching with nitric acid produced mordenites with Si/Al ratios 6.5–15.4.
  • Al was preferentially removed from 4-membered rings at T3 and T4 sites.
  • Dealumination affected deactivation by doubling the selectivity and lifetime.
  • T1 and T2 sites are the major contributors to mordenite deactivation.
  • Lower density of 4-membered ring sites is recommended for delayed deactivation.

 

Abstract

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, 27Al 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. © 2017 Elsevier, reprinted with permission.

 

Zinc hinders deactivation of copper-mordenite: dimethyl ether carbonylation

Article
A.A.C. Reule, N. Semagina
ACS Catalysis 6 (8), 4972-4975
Publication year: 2016

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. © 2016 American Chemical Society, reprinted with permission.

Palladium nanoparticle size effect in hydrodesulfurization of 4,6‐dimethyldibenzothiophene (4,6‐DMDBT)

Article
J. Shen, N. Semagina
ChemCatChem 8 (15), 2565-2571
Publication year: 2016

Optimal size: 8 nm Pd nanoparticles provide the deepest hydrodesulfurization as a result of balanced hydrogenation, sulfur extraction, and sulfur poisoning.

Pd nanoparticle size sensitivity of 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization was investigated by using 4, 8, 13, and 87 nm particles and was compared with the sulfur-free and sulfur-inhibited hydrogenation of 3,3-dimethylbiphenyl, which is a product of direct desulfurization of 4,6-DMDBT. The smallest 4 nm particles provided unprecedented (for Pd at 5 MPa and 300 °C) direct desulfurization selectivity of 20 % at 40 % conversion because of the reduced contribution of the hydrogenation path. The 4 nm particles were poisoned by the adsorbed sulfur to the greatest extent. The optimal size, providing the highest Pd mass-based yield of the desulfurized products, was found to be 8 nm. The catalyst with 87 nm particles was based on Pd nanocubes with the lowest edge/terrace surface atom ratio and large terraces and this showed the lowest sulfur extraction from both 4,6-DMDBT and sulfurous intermediates as a result of the low availability of edge atoms for a perpendicular sigma-mode adsorption through the lone pair of the sulfur atom. © 2016 Wiley-VCH.