Active and stable methane oxidation nano-catalyst with highly-ionized palladium species prepared by solution combustion synthesis

We report on the synthesis and testing of active and stable nano-catalysts for methane oxidation. The nano-catalyst was palladium/ceria supported on alumina prepared via a one-step solution-combustion synthesis (SCS) method. As confirmed by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HTEM), SCS preparative methodology resulted in segregating both Pd and Ce on the surface of the Al 2 O 3 support. Furthermore, HTEM showed that bigger Pd particles (5 nm and more) were surrounded by CeO 2 , resembling a core shell structure, while smaller Pd particles (1 nm and less) were not associated with CeO 2 . The intimate Pd-CeO 2 attachment resulted in insertion of Pd ions into the ceria lattice, and associated with the reduction of Ce 4+ into Ce 3+ ions; consequently, the formation of oxygen vacancies. XPS showed also that Pd had three oxidation states corresponding to Pd0, Pd 2+ due to PdO, and highly ionized Pd ions (Pd (2+x)+ ) which might originate from the insertion of Pd ions into the ceria lattice. The formation of intrinsic Ce 3+ ions, highly ionized (Pd2+ species inserted into the lattice of CeO 2 ) Pd ions (Pd (2+x)+ ) and oxygen vacancies is suggested to play a major role in the unique catalytic activity. The results indicated that the Pd-SCS nano-catalysts were exceptionally more active and stable than conventional catalysts. Under similar reaction conditions, the methane combustion rate over the SCS catalyst was ~18 times greater than that of conventional catalysts. Full methane conversions over the SCS catalysts occurred at around 400 C but were not shown at all with conventional catalysts. In addition, contrary to the conventional catalysts, the SCS catalysts exhibited superior activity with no sign of deactivation in the temperature range between ~400 and 800 C. 2018 by the authors. Licensee MDPI, Basel, Switzerland.Acknowledgments: This paper was made possible by an NPRP Grant #6-290-1-059 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.Scopu

Galvanic exchange as a novel method for carbon nitride supported coag catalyst synthesis for oxygen reduction and carbon dioxide conversion

A bimetallic alloy of CoAg nanoparticles (NPs) on a carbon nitride (CN) surface was synthesized using a galvanic exchange process for the oxygen reduction reaction (ORR) and carbon dioxide electrocatalytic conversion. The reduction potential of cobalt is ([Co2+(aq) + 2e? ? Co(s)], ?0.28 eV) is smaller than that of Ag ([Ag+(aq) + e? ? Ag(s)], 0.80 eV), which makes Co(0) to be easily replaceable by Ag+ ions. Initially, Co NPs (nanoparticles) were synthesized on a CN surface via adsorbing the Co2+ precursor on the surface of CN and subsequently reducing them with NaBH4 to obtain Co/CN NP. The Co NPs on the surface of CN were then subjected to galvanic exchange, where the sacrificial Co atoms were replaced by Ag atoms. As the process takes place on a solid surface, only the partial replacement of Co by Ag was possible generating CoAg/CN NPs. Synthesized CoAg/CN bimetallic alloy were characterized using different techniques such as powder x-ray diffraction (PXRD), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron diffraction spectroscopy (EDS) to confirm the product. Both the catalysts, Co/CN and CoAg/CN, were evaluated for oxygen reduction reaction in 1M KOH solution and carbon dioxide conversion in 0.5 M KHCO3. In the case of ORR, the CoAg/CN was found to be an efficient electrocatalyst with the onset potential of 0.93 V, which is comparable to commercially available Pt/C having Eonset at 0.91 V. In the electrocatalytic conversion of CO2, the CoAg/CN showed better performance than Co/CN. The cathodic current decreased dramatically below ?0.9V versus Ag/AgCl indicating the high conversion of CO2. - 2019 by the authors. Licensee MDPI, Basel, Switzerland.Acknowledgements: The authors would like to gratefully acknowledge financial support from Total Research & Technology Feluy (Grant Number: QUEX-CENG-TRT-17/18) in conducting this research. The statements made herein are solely the responsibility of the authors. The authors also wish to gratefully acknowledge the Gas Processing Centre (GPC) at Qatar University for carrying out XRD and XPS analysis, and the Central Laboratory Unit (CLU) at Qatar University for services related to electron microscopy.Scopu

Electrocoalescence of water drop trains in oil under constant and pulsatile electric fields

This study addresses the effectiveness of constant and pulsed DC fields in promoting coalescence of dispersed water drops in an oil-continuous phase. For this purpose, a train of drops of relatively uniform size is injected into a stream of flowing sunflower oil. This stream is then admitted to a coalescing section, where an electric field is applied between a pair of ladder-shape bare electrodes. The capability of this device to enhance coalescence of droplets in a chain is investigated at different field intensities, frequencies and waveforms. The effect of the initial inter-droplet separation distance on the process performance is also addressed under constant DC fields. The dominant coalescence mechanism is found to be due to dipole–dipole interaction at low field strength, whereas electrophoresis becomes predominant at higher field strength. Experiments reveal the existence of an optimal frequency, where the average droplet size enlargement is maximized, especially at low field strengths. The droplet size at the outlet of the coalescer is also found to be dependent on the field waveform

Electrostatic phase separation: a review

The current understanding and developments in the electrostatic phase separation are reviewed. The literature covers predominantly two immiscible and inter-dispersed liquids following the last review on the topic some 15 years. Electrocoalescence kinetics and governing parameters, such as the applied field, liquid properties, drop shape and flow, are considered. The unfavorable effects, such as chain formation and partial coalescence, are discussed in detail. Moreover, the prospects of microfluidics platforms, non-uniform fields, coalescence on the dielectric surfaces to enhance the electrocoalescence rate are also considered. In addition to the electrocoalescence in water-in-oil emulsions the research in oil-in-oil coalescence is also discussed. Finally the studies in electrocoalescer development and commercial devices are also surveyed. The analysis of the literature reveals that the use of pulsed DC and AC electric fields is preferred over constant DC fields for efficient coalescence; but the selection of the optimum field frequency a priori is still not possible and requires further research. Some recent studies have helped to clarify important aspects of the process such as partial coalescence and drop–drop non-coalescence. On the other hand, some key phenomena such as thin film breakup and chain formation are still unclear. Some designs of inline electrocoalescers have recently been proposed; however with limited success: the inadequate knowledge of the underlying physics still prevents this technology from leaving the realm of empiricism and fully developing in one based on rigorous scientific methodology

Experimental and theoretical studies on the mechanical and structural changes imposed by the variation of clay loading on poly(vinyl alcohol)/cloisite 93A nanocomposites

Polymer nanocomposites (PNC) structures are promising materials due to their novel properties. However, many of their characteristic physical, mechanical, and chemical behavior have not been quantified. Depending on the interface interactions between polymeric chains and clay sheets, various classes of polymer/clay nanocomposite exist: Intercalated, flocculated and exfoliated nanocomposites. In this work, we present an experimental study of poly(vinyl alcohol) (PVA)-cloisite 93A followed by a classical molecular dynamic (MD) simulations. The structural properties of the system were studied using X-ray diffraction (XRD), nanoscanning electron microscopy (NSEM). Both revealed intercalation between PVA chains and cloisite 93A nanoclay. Another evidence of the intercalation between PVA and cloisite 93A nanoclay was realized from the differential scanning calorimetry (DSC) which confirmed as surge in crystallinity upon intercalation. A main focus for the intercalated structure was to investigate the impact of nanofillers content on the mechanical performance. Intercalation significantly influenced both Young's modulus and the % of elongation of PVA/cloisite 93A blends. Young's modulus and tensile stress were raised with the content of the filler content up to 3 wt%. For higher content, opposite observations are addressed owing to the formation of aggregates of nanofillers and as consequence construction of microvoids. From the MD simulations, the intercalation has been shown by the increase of the d-spacing with the clay loading. By calculating the density profile, it has been demonstrated that in case of low clay loading, the cloisite 93A has its nitrogen groups on the clay surface and the long tails form layers. For the high loading of clay, nitrogen and carbon groups are together on the surface. Young modulus calculated theoretically follows the same experimental trend where an increase of the values has been observed with a clay loading up to 3 wt%, followed by a decrease of the values for higher clay loading. J. VINYL ADDIT. TECHNOL., 25:172181, 2019.This paper was made possible by an NPRP 09-260-1-048 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. We are also grateful to University College London for using their high performance computing facilities.Scopu

Ab initio molecular dynamics investigation of CH4/CO2 adsorption on calcite: Improving the enhanced gas recovery process

Ab initio molecular dynamics simulations of CH4 and CO2 on the calcite (104) surface have been carried out for the molecular level analysis of CO2-enhanced gas recovery process (EGR). This process takes advantage of the stronger interaction of CO2 with the reservoir walls compared to CH4, therefore can improve the extraction of the latter, while at the same time sequestering the former underground. Pure and mixed gases were considered and the temperature effect on the systems behavior was analyzed. For pure gases, carbon dioxide shows great stability on the surface in the studied temperature range, while methane molecules start leaving the surface at 298 K. For gas mixtures, the reported results confirm that for low to medium concentrations, a temperature of 373 K could determine the best methane extraction efficiency, as CH4 interaction with the surface is quite weak and carbon dioxide binds strongly on the surface. On the other hand, when full coverage is achieved, the best efficiency is reached for the highest temperature. Finally, when considered a 2:2 gas layer, carbon dioxide tends to adsorb preferentially to the surface while methane keeps floating above it, thereby reducing its chance to be adsorbed back. These results reveal nanoscopic details for the design of suitable EGR processes.The authors would like to acknowledge the support of Qatar National Research Fund (a member of Qatar Foundation) through Grant # NPRP10-0125-170235. The findings achieved herein are solely the responsibility of the authors. The authors would like to thank Texas A&M University in Qatar for the use of their computational resources.Scopu

Rheological characterization of NA2CO3 activated calcium bentonite for oil and gas drilling applications

Calcium bentonite was activated with soda ash (Na2CO3) at various ratios and due to the ion exchange; the original Ca-bentonite was converted to Na-bentonite. It was found that the Na2CO3 activation improves the swelling and rheological properties of the bentonite suspension. The results showed that the magnitude of the viscosity, η, elastic modulus, G′, and viscous modulus, G″, depend on the strength of the bentonite gel structure and interactions. The activated bentonite showed higher values of η, G′, and G″ compare to inactivated bentonite. A strong correlation between the bentonite type, soda ash concentration and rheological behaviour of bentonite is established.Scopu

Mechanism of GaAs surface sulfidation

The mechanism of GaAs sulfidation under illumination and potentiodynamic polarization was investigated in acidified thiourea (TU) electrolytes. Sulfidation generated smooth surfaces, as revealed by scanning electron microscopy and atomic force microscopy images; but analysis by inductively coupled plasma - mass spectroscopy (ICP-MS) of spent electrolytes showed that this was in part due to GaAs dissolution. The initial step in sulfidation occurred through formation of elemental arsenic which then reacted with TU and forms As(III) sulfide, which was subsequently oxidized into As(V) sulfide and finally to arsenic sulfate. X- Ray photoelectron spectroscopy (XPS) demonstrated the initial formation of elemental As (XPS peak at 42 eV of As - As bond). XPS also showed three S 2p doublets at 162.4 eV which were assigned to (As(III) - S), 164.4 eV for (As(V) - S) and 169.1 eV for (S - O). The intensity of the XPS peak due to As(V) - S (S 2p at 164.4 eV) increased with prolonged exposure to sulfidation by the electrolyte, indicating the oxidation of As(III) into As(V) sulfide. Furthermore, the intensity of the S - O bond (S 2p at 169.1 eV) decreased with time, presumably due to arsenic sulfate dissolution. Both XPS and ICP-MS studies revealed that arsenic species were preferentially segregated on the surface but Ga ions diffused towards the electrode bulk.Scopu

Synthesis, characterization and performance of Pd-based core-shell methane oxidation nano-catalysts

In this paper, a comparative investigation of the catalytic performances of Pd@TiO2 and Pd@CeO2, core-shells nanocatalysts supported over functionalized alumina, for application to methane oxidation is presented. The results indicated that the Pd@CeO2/SiO2.Al2O3 core-shell nanocatalyst exhibited higher activity and stability than the Pd@TiO2/SiO2.Al2O3 nanocatalyst. Complete combustion of methane over the Pd@CeO2/SiO2.Al2O3 nanocatalyst was achieved at about 400 ?C. By contrast, the maximum combustion of methane over the Pd@TiO2/SiO2.Al2O3 nanocatalyst was only attained at ?550 ?C. The Pd@TiO2/SiO2.Al2O3 nanocatalyst experienced deactivation, and a transient dip in methane conversion in the temperature region between 580 ?C and 750 ?C was also observed. The exceptional activity of the Pd@CeO2/SiO2.Al2O3 nanocatalyst was attributed to the intimate interaction between palladium (Pd) and ceria (CeO2) and efficient oxygen back-spillover at Pd and CeO2 interface resulting from the core-shell structure. ? 2018 Elsevier B.V.This paper was made possible by an NPRP Grant # 6-290-1-059 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors

Optimization of selection of chain amine scrubbers for CO2 capture

In order to optimize the selection of a suitable amine molecule for CO2 scrubbers, a series of ab initio calculations were performed at the B3LYP/6-31+G(d,p) level of theory. Diethylenetriamine was used as a simple chain amine. Methyl and hydroxyl groups served as examples of electron donors, and electron withdrawing groups like trifluoromethyl and nitro substituents were also evaluated. Interaction distances and binding energies were employed as comparison operators. Moreover, natural bond orbital (NBO) analysis, namely the second order perturbation approach, was applied to determine whether the amine-CO2 interaction is chemical or physical. Different sizes of substituents affect the capture ability of diethylenetriamine. For instance, trifluoromethyl shields the nitrogen atom to which it attaches from the interaction with CO2. The results presented here provide a means of optimizing the choice of amine molecules for developing new amine scrubbers.NPRP Grant # 5-1437- 1-243 from the Qatar National Research Fund (a member of Qatar Foundation)�Scopu