Production of green hydrogen from sewage sludge / algae in agriculture diesel engine: Performance Evaluation

Alternative fuel opportunities can satisfy energy security and reduce carbon emissions. In this regard, the hydrogen fuel is derived from the source of environmental pollutants like sewage and algae wastewater through hydrothermal gasification technique using a KOH catalyst with varied gasification process parameters of duration and temperature of 6-30 min and 500-800 °C. The novelty of the work is to identify the optimum gasification process parameter for obtaining the maximum hydrogen yield using a KOH catalyst as an alternative fuel for agricultural engine applications. Influences of gasification processing time and temperature on H2 selectivity, Carbon gasification efficiency (CE), Lower heating value (LHV), Hydrogen yield potential (HYP), and gasification efficiency (GE) were studied. Its results showed that the gasifier operated at 800 °C for 30 min, offering maximum hydrogen yield (26 mol/kg) and gasification efficiency (58 %). The synthesized H2 was an alternative fuel blended with diesel fuel/TiO2 nanoparticles. It was experimentally studied using an internal combustion engine. Influences of H2 on engine performance, like brake-specific fuel consumption, brake thermal efficiency and emission performances, were measured and compared with diesel fuel. The results showed that DH20T has the least (420g/kWh) brake-specific fuel consumption (BSFC) and superior brake thermal efficiency of about 25.2 %. The emission results revealed that the DH20T blend showed the NOX value increased by almost 10.97 % compared to diesel fuel, whereas the CO, UHC, and smoke values reduced by roughly 31.25, 28.34, and 42.35 %. The optimum fuel blend (DH20T) result is recommended for agricultural engine applications.

Hybrid MWCNT and TiO2 Nanoparticle-Suspended Waste Tyre Oil Biodiesel for CI Engines

Nowadays, scarcity arises in almost all our basic needs, including water, fuel, and food. Recycling used and scrapped things for a valuable commodity is highly appreciable for compensating for the globally fast-growing demand. This paper aims to investigate waste tyre oil for preparing biodiesel for CI engines by enhancing their performance with hybrid nanoparticles for preparing nanofuel and hybrid nanofuel. The nanoparticles (30-40 nm) of MWCNT and TiO2 were utilized to prepare nanofuels with nanoparticle concentrations of MWCNT (300 ppm) and TiO2 (300 ppm), respectively. In the case of hybrid nanofuel, the nanoparticle concentration of MWCNT (150 ppm) and TiO2 (150 ppm) was preferred. The performance of the proposed nanofuel and hybrid nanofuel with pure diesel was evaluated. The proposed fuel performance outperforms the combustion performance, has higher engine efficiency, and has fewer emissions. The best performances were noticed in hybrid nanofuel that has 32% higher brake thermal efficiency than diesel and 24% and 4% lower BSFC and peak pressure than diesel, respectively. The emission performance is also 29%, 50%, and 13% lower in CO, HC, and CO2 emissions than that in pure diesel.

First principles study on thickness dependent structural and electronic properties unveiling the growth and stability of 2D layered II-VI semiconducting compounds

By employing first principles density functional theory calculations on thickness dependent structural and electronic properties of (0001) surface slabs of wurtzite MX compounds, our study demonstrated the possibility of the existence of 2D layered materials from II-VI group traditional semiconducting compounds that are widely used in various fields. Our calculations revealed that (0001) surface slabs of wurtzite ZnO and CdO compounds prefer to stabilize as sp² hybridized - atomically thin graphitic layers as observed in earlier work, which are separated by van der Waals distances, when compared to the respective wurtzite slabs. On the other hand, for surface slabs of other ZnX and CdX (X = S, Se, Te) compounds, sp³ hybridized bilayers, which comprise an X-Zn(Cd)-Zn(Cd)-X structural arrangement, are energetically stable until certain thicknesses of the slabs. Both 2D layered MO and MX systems are electronically insulating in nature. When increasing the number of layers in these systems, the band gap decreases due to the widening of the energy bands. Our calculations further confirmed that all of these 2D systems possess structural, elastic, and lattice dynamical stabilities, depicting their compatibility in optoelectronics and photovoltaic applications, as confirmed by their effective mass and mobility.

The role of defects presenting in graphitic SiC sheets and their consequences in the exfoliation of layers - a first principles approach

Recently, there has been a growing interest in exploring new 2D nanostructures, due to their unique electronic and optical properties. An atomically thin SiC sheet, which has a honeycomb structure similar to BN, as well as being a direct band gap semiconductor, is one such candidate. Despite several theoretical reports predicting the structural and dynamical stability of 2D SiC nanostructures, few experimental reports have been reported so far. In the present work, we demonstrated by employing first principles density functional theory calculations that the role of self defects on the exfoliation of SiC layers can be understood by studying monolayer, bilayer and trilayer 2D SiC systems. From our work, it can be seen that the dangled C atom on the removal of a Si atom in the SiC layer prefers to interact with an adjacent layer, owing to the compensation of the charges, whereas, a dangled Si atom (in the carbon vacancy case) in the SiC layer compensates its additional charge within the layer by forming a Si-Si bond. We concluded that the exfoliation process of SiC is significantly affected by Si vacancies, rather than the presence of carbon vacancies. This work also provides an intuitive idea to synthesise 2D SiC nanostructures as it has interesting structural and electronic properties.

The extraction and process optimization of Cu (II) and Cd (II) using Pickering emulsion liquid membrane

In the present study, the extraction of divalent heavy metals like copper [Cu (II)] and cadmium [Cd (II)] using a Pickering Emulsion Liquid Membrane (PELM) has been investigated by using three different surfactants such as Amphiphilic silica nanowires (ASNWs), Aluminum oxide nanoparticles (Alumina) and Sorbitan monooleate (SPAN 80). The influence of the process parameters such as pH, the stripping phase concentration, the agitation speed, and the carrier concentration on the extraction efficiency have been examined to find the optimum conditions at which the maximum recovery of Cu (II) and Cd (II) could take place. At optimum conditions, the extraction efficiency of 89.77% and 91.19% for Cu (II) and Cd (II) ions were achieved. Non-edible oils were used as diluent in this present study to reduce the need for toxic organic solvents in preparing PELM. The impact of each process factor on the extraction efficiency of Cu (II) and Cd (II) ions has been verified using analysis of variance (ANOVA). The higher values of F and lower values of P (less than 0.05) indicate pH is the most significant parameter on the percentage extraction of Cu (II) and Cd (II) using the Taguchi design approach.

Rational Design of Highly Efficient Perovskite Hydroxide for Electrocatalytic Water Oxidation

The production of hydrogen from ecofriendly renewable technologies like water electrolysis and fuel cells involves oxygen evolution reaction (OER), which plays a major role, but the slow kinetics of OER is a bottleneck of commercialization of such technologies. Herein, we have reported the formation of an efficient OER catalyst from SnCo(OH)₆ (SCH) by leaching of Sn atoms during electrochemical OER studies. According to density functional theory calculations, adsorption of OH* species on Sn atoms is energetically more favorable than that of Co atoms, and as a result, highly active CoOOH is generated by leaching of Sn atoms from surface layers. We observed enhanced OER performance with superior mass activity by blending SCH with activated charcoal, which displays a low overpotential of 293 mV and higher mass activity than that of pristine SCH. More importantly, it outperforms Co(OH)₂ and RuO₂ having the same carbon composition because of the formation of thermodynamically stable and amorphous CoOOH on the surface of single-crystalline SCH and strong tethering ability of activated charcoal.

Immune stimulatory and anti-HIV-1 potential of extracts derived from marine brown algae Padina tetrastromatica

Background Marine brown algae are biologically diverse and their medicinal value has been explored limited. We assessed whether Padina tetrastromatica Hauck will possess the immune stimulatory and human immunodeficiency virus (HIV-1) inhibitory activity. Materials and Methods Aqueous and methanolic extracts were tested for the Th1/Th2 cytokines using PBMC. Subsequently, leukotriene B4 (LTB4), nitric oxide (NO) and anti-oxidant effect were analyzed using RAW264.7 cells. In addition, Padina extracts were tested for the HIV-1 clade C & A by measuring the levels of viral p24 antigen in infected peripheral blood mononuclear cells (PBMCs) and against reverse transcriptase (RT). Results At 100 μg/mL, aqueous and methanolic extracts produced a significant amount of IL-10 and IFN-γ at 24 h and 72 h post-stimulation by PBMCs. It also produced a significant amount of LTB4, NO and had an antioxidant effect on RAW264.7 cell, suggesting the immune stimulating potential of P. tetrastromatica. Upon infection of PBMCs with 100 TCID50, aqueous and methanolic extracts of P. tetrastromatica inhibited HIV-1 C (>90%) and HIV-1 A (>50%) showed a significant reduction in HIV-1 p24 levels and HIV-1 RT inhibition (>50%). GC-MS study revealed a relative abundance of tetradecanoic and oleic acid in the methanolic extract of P. tetrastromatica, which might be responsible for immune stimulation and anti-HIV-1 activity. Conclusion At lower concentrations (100 mg/mL), the aqueous and methanolic extracts of P. tetrastromatica showed the strong immune stimulation and greatest anti-HIV-1 potential in vitro. This study demonstrates the therapeutic potential of these brown algae P. tetrastromatica for the benefit of mankind.

Atomic and electronic structure of solids of Ge2Br2PN, Ge2I2PN, Sn2Cl2PN, Sn2Br2PN and Sn2I2PN inorganic double helices: a first principles study

We report the results of density functional theory calculations on the atomic and electronic structure of solids formed by assembling A₂B₂PN (A = Ge and Sn, B = Cl, Br, and I) inorganic double helices.

Fluorine-enriched mesoporous carbon as efficient oxygen reduction catalyst: understanding the defects in porous matrix and fuel cell applications

Herein, fluorine enrichment in mesoporous carbon (F-MC) was explored to introduce maximum charge polarization in the porous matrix, which is beneficial for the preferential orientation of O2 molecules and their subsequent reduction. Ex situ doping of F to porous carbon derived from phloroglucinol-formaldehyde resin using Pluronic F-127 as a structure-directing agent is standardized. The optimized F-MC catalyst exhibited excellent electrocatalytic activity towards the oxygen reduction reaction (ORR) in alkaline media (0.1 M KOH) with an onset potential of -0.10 V vs. SCE and diffusion-limiting current of 4.87 mA cm-2, while displaying only about 50 mV overpotential in the half-wave region compared to Pt-C (40 wt%). In the stability test, the catalyst showed only 10 mV negative shift in its half-wave potential after 10 000 potential cycles. The rotating ring disk electrode (RRDE) experiments revealed that F-MC follows the most preferable 4e - pathway (n = 3.61) with a moderate peroxide (HO2 -) yield. This was further supported by density functional theory calculations and also deeply explains the existence of defects being beneficial for the ORR. The F-MC catalyst owing to its promising ORR activity and long-term electrochemical stability can be viewed as a potential alternative ORR catalyst for anion exchange membrane fuel cell applications.

Computational Approach To Reveal the Structural Stability and Electronic Properties of Lithiated M/CNT (M = Si, Ge) Nanocomposites as Anodes for Lithium-Ion Batteries

This work is motivated to explore the structural stability and electronic and electrochemical properties of nanocomposites of M₄Li _n (M = Si and Ge)-carbon nanotube (CNT) by employing first-principles density functional theory calculations. By analyzing the structural stability of various M₄Li _n (n = 0-10) clusters, it is revealed that a tetrahedron-shaped M₄Li₄ Zintl cluster is found to be highly stable. Our study on the interaction between the lithiated clusters and CNT illustrates that the charge transfer from the former to latter plays a pivotal role in stabilizing these nanocomposites. The structural stability of those nanocomposites arises as a consequence of bonding between lithiated clusters and CNT, which is mediated through the cation-π interaction. The strength of the interaction between them is well reflected in electronic structure calculations by shifting the energy levels with respect to the Fermi energy. Further, the electrochemical properties of these nanocomposites are explored by forming an assembly of the cluster-inserted CNT. The calculated average intercalation voltage of the systems is found to be low (maximum ∼1.0 V for M = Si and 1.05 V for M = Ge), which demonstrates their anodic behavior.

dz2 orbital-mediated bound magnetic polarons in ferromagnetic Ce-doped BaTiO3 nanoparticles and their enriched two-photon absorption cross-section

The enriched ferromagnetism and two-photon absorption (TPA) cross-section of perovskite BaTiO3 nanoparticles are indispensable for magnetic and optical data storage applications. In this work, hydrothermally synthesized Ce-doped BaTiO3 nanoparticles exhibited the maximum room temperature ferromagnetism (4.26 × 10-3 emu g-1) at 4 mol% due to the increase in oxygen vacancies, as evidenced by X-ray photoelectron and electron spin resonance spectroscopy and density functional theory (DFT) calculations. Hence, the oxygen vacancy-constituted bound magnetic polaron (BMP) model was invoked to explain the enhancement in ferromagnetism. The BMP theoretical model indicated an increase in BMP magnetization (M0, 3.0 to 4.8 × 10-3 emu g-1) and true spontaneous moment per BMP (meff, 4 to 9.88 × 10-4 emu) upon Ce doping. DFT calculations showed that BMPs mediate via the Ti dz2 orbitals, leading to ferromagnetism. Besides, it is known that the magnetic moment induced by Ce at the Ba site is higher than Ce at the Ti site in the presence of oxygen vacancies. The open aperture Z-scan technique displayed the highest TPA coefficient, β (7.08 × 10-10 m W-1), and TPA cross-section, σTPA (455 × 104 GM), at 4 mol% of Ce as a result of the robust TPA-induced excited state absorption. The large σTPA is attributed to the longer excited state lifetime, τ (7.63 ns), of the charge carriers created by oxygen vacancies and Ce ions, which encounter several electronic transitions in the excited sub-states.

Notes on Jasminum andamanicum N.P. Balakr. & N.G. Nair (Oleaceae) from Andaman & Nicobar Islands, India

Jasminum andamanicum is endemic to the Andaman group of Islands. It is assessed as Endangered as per the IUCN red list criteria using primary and secondary information on trends in EOO, AOO and numbers of collection. Taxonomy of the species, distribution, herbarium image, and causes of rarity are discussed.

Borophene layers on an Al(111) surface - the finding of a borophene layer with hexagonal double chains and B9 nonagons using ab initio calculations

We studied the stability of several borophene layers on an Al(111) surface and found a structure called 9R using ab initio calculations. This layer competes with χ3 and β12 borophene layers and is made up of boron nonagons that form a network of hexagonal boron double chains. Remarkably, it has no B6 hexagon unlike other borophene layers. All three layers lie significantly lower in energy than the honeycomb layer recently reported on the Al(111) surface [W. Li, et al., Sci. Bull., 2018, 63, 282]. We discuss the structural stability and electronic structures of different borophene layers in light of the role of the filling factor f of boron atoms in boron hexagons in a honeycomb layer as well as charge transfer from the Al substrate to the borophene layer as obtained from the Bader charge analysis. The electron localization function shows that the 9R layer has two-center bonding within the nonagon rings and three-center bonding between the rings. Calculations of the phonon spectra show that a free 9R layer is dynamically stable raising the hope of its isolation. The electronic structure shows that in all cases the borophene layer is metallic.

Atomic structure and electronic properties of A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) inorganic double helices: first principles calculations

We study the structural stability and electronic properties of new classes of DNA-like inorganic double helices of the type A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) by employing first principles density functional theory (DFT) calculations including van der Waals interactions. In these quaternary double helices PN or SiS forms the inner helix while the AB helix wraps around the inner helix and the two are interconnected. We find that the bromides and iodides of Ge, Sn, and Pb as well as Pb2Cl2PN form structurally stable double helices while Ge2I2SiS as well as bromides and iodides of Sn and Pb have stable double helices. The atomic structures of different double helices have been analyzed in detail to understand the stability of these systems as there is up to about 80% difference in the interatomic distances in the two helices which is remarkable. Also in these new classes of double helices there is polar covalent bonding in the inner helix due to heteroatoms. We have calculated the DDEC6 partial atomic charges and bond orders which suggest strong covalent bonding in the inner helix. The electronic structure reveals that these double helices are semiconducting and in many cases the band gap is direct. Furthermore, we have studied the effects of doping and found that hole doping is the most appropriate way to tuning their electronic properties.

Polymer brush on surface with tunable hydrophilicity using SAM formation of zwitterionic 4-vinylpyridine-based polymer

A zwitterionic vinylpyridine-based polymeric SAM was assembled on different surfaces to obtain tunable hydrophilicity.

Unveiling the multifunctional roles of hitherto known capping ligand oleic acid as blue emitter and sensitizer in tuning the emission colour to white in red-emitting phosphors

Multifunctional roles of oleic acid as capping ligand, blue emitter and sensitizer are revealed in obtaining white light emission from Eu³⁺-based red phosphors.

Tuning of intrinsic antiferromagnetic to ferromagnetic ordering in microporous α-MnO2 by inducing tensile strain

By employing first principles density functional calculations, we investigated an α-MnO₂ compound with a tunnel framework, which provides an eminent platform to alter the intrinsic antiferromagnetic (AFM) to ferromagnetic (FM) ordering, through the introduction of chemical or mechanical tensile strain. Our calculations further showed that the strength of FM ordering increases until 10% triaxial tensile strain. Since long range FM ordering is induced, it is realized to be superior as compared to the experimentally observed short-range FM ordering in oxygen-deficient compound. The driving force behind this superior effect is understood from the unusual electron occupancy in Mn atoms as a result of tetrahedral distortion in the MnO₆ octahedra and an increase in the sp³ character of the oxygen atoms. Thus, the α-MnO₂ compound belongs to a class of materials that exhibit good potential for piezomagnetic applications.

Effect of Ablation Rate on the Microstructure and Electrochromic Properties of Pulsed-Laser-Deposited Molybdenum Oxide Thin Films

Molybdenum trioxide (MoO₃) is a well-known electrochromic material. In the present work, n-type α-MoO₃ thin films with both direct and indirect band gaps were fabricated by varying the laser repetition (ablation) rate in a pulsed laser deposition (PLD) system at a constant reactive O₂ pressure. The electrochromic properties of the films are compared and correlated to the microstructure and molecular-level coordination. Mixed amorphous and textured crystallites evolve at the microstructural level. At the molecular level, using NMR and EPR, we show that the change in the repetition rate results in a variation of the molybdenum coordination with oxygen: at low repetition rates (2 Hz), the larger the octahedral coordination, and greater the texture, whereas at 10 Hz, tetrahedral coordination is significant. The anion vacancies also introduce a large density of defect states into the band gap, as evidenced by XPS studies of the valence band and supported by DFT calculations. The electrochromic contrast improved remarkably by almost 100% at higher repetition rates whereas the switching speed decreased by almost 6-fold. Although the electrochromic contrast and coloration efficiency were better at higher repetition rates, the switching speed, reversibility, and stability were better at low repetition rates. This difference in the electrochromic properties of the two MoO₃ films is attributed to the variation in the defect and molecular coordination states of the Mo cation.

Excitation-dependent local symmetry reversal in single host lattice Ba2A(BO3)2:Eu3+ [A = Mg and Ca] phosphors with tunable emission colours

Excitation-dependent emission colour tuning in single host lattice phosphors by local symmetry changes due to reorganization of highly strained oxygen.

First principles calculations on oxygen vacant hydrated α-MnO2 for activating water oxidation and its self-healing mechanism

Understanding the mechanism behind water oxidation is the prime requirement for designing better catalysts for electrochemical energy devices. In this work, we demonstrate by employing first principles calculations that an initial step of water oxidation is observed to be associated with the dissociation of water dimers into hydronium and hydroxide ions, in the tunnel of a hydrated α-MnO2 compound with an oxygen vacancy. The former ion is intercalated within the network, while the latter ion occupies the oxygen vacant site and interacts strongly with the Mn atoms. Based on our calculations, the factor responsible for this dissociation of water molecules is observed to be the presence of mixed charge states of Mn atoms in the triangular lattice. Further, the coulombic attraction of a hydronium ion with a water molecule leads to the formation of a Zundel cation in the tunnel, while by dehydrogenating the adsorbed hydroxide ion, the self-healing property of the compound is achieved along with another hydronium ion as a reaction product. These cations can be exchanged with Li(+) ions. Thus, the protonic moieties formed in the tunnel of α-MnO2 leads to niche applications in the field of fuel cells and lithium ion batteries.

First principles modeling of Mo6S9 nanowires via condensation of Mo4S6 clusters and the effect of iodine doping on structural and electronic properties

By employing first principles DFT calculations, we propose a new stable model for Mo6S9 nanowires (NWs) obtained by condensing tetrahedral Mo4S6 clusters rather than octahedral Mo6S8 clusters, which are known as magic clusters in the Mo-S polyhedral cluster family. The pristine NW is found to be metallic and its local structure and physical properties can be tuned by doping of iodine atoms. This doping increases the number of Mo-Mo bonds in the NW, thus, Mo4 tetrahedra are initially fused to the Mo6 octahedron, and then, to the Mo8 dodecahedron. Further, a close correlation among the Mo-Mo bonding in the local structure, mechanical and electronic properties, is observed from our study. Finally, the stability of the pristine and iodine doped Mo8S12-xIx NW structures obtained from condensation of Mo4 tetrahedra are found to be quite comparable with that of already reported Mo6S9-xIx NWs with Mo6 octahedra as building blocks.

Controlled decoration of the surface with macromolecules: polymerization on a self-assembled monolayer (SAM)

Polymer functionalized surfaces are important components of various sensors, solar cells and molecular electronic devices. In this context, the use of self-assembled monolayer (SAM) formation and subsequent reactions on the surface have attracted a lot of interest due to its stability, reliability and excellent control over orientation of functional groups. The chemical reactions to be employed on a SAM must ensure an effective functional group conversion while the reaction conditions must be mild enough to retain the structural integrity. This synthetic constraint has no universal solution; specific strategies such as "graft from", "graft to", "graft through" or "direct" immobilization approaches are employed depending on the nature of the substrate, polymer and its area of applications. We have reviewed current developments in the methodology of immobilization of a polymer in the first part of the article. Special emphasis has been given to the merits and demerits of certain methods. Another issue concerns the utility - demonstrated or perceived - of conjugated or non-conjugated macromolecules anchored on a functionally decorated SAM in the areas of material science and biotechnology. In the last part of the review article, we looked at the collective research efforts towards SAM-based polymer devices and identified major pointers of progress (236 references).

Structural stability and bonding nature of Li–Sn–carbon nanocomposites as Li-ion battery anodes: first principles approach

The atomic structural stability and electronic properties of Li_nSn₄–carbon nanotube (CNT) and Li_nSn₄–graphene nanocomposites were studied by first principles calculations.

Nanoscale functionalization of surfaces by graft-through Sonogashira polymerization

“Graft-through” Sonogashira polymerization has been performed on functionalized self-assembled monolayer.

Atomic structure and edge magnetism in MoS(2+x) parallelogram shaped platelets

The structural, electronic, and magnetic properties of MoS(2+x) parallelogram shaped platelets having m and n Mo atoms on the adjoining edges have been studied using first principles calculations and by varying m and n from 1 to 6. These platelets have 100% S coverage on two adjoining edges while 50% S coverage on the other two edges. The structural stability of the platelets increases with size but the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap in general decreases. There is a triangular metallic corner at the intersection of 100% S covered edges of the platelets when m = n. On the other hand magnetism is observed on the 50% S covered edges of the platelets for the sizes greater than that of the (3,4) platelet. The magnetic moments mainly arise from the undercoordinated S(2c) atoms at the 50% S covered edges rather than from Mo atoms. The criteria for the existence of the magnetic moments on S(2c) atoms are suggested and the electronic structure of the platelets on the edges as well as inside is discussed.

Assembling nanowires from Mo-S clusters and effects of iodine doping on electronic structure

Using ab initio calculations, we find high stability of octahedral Mo6S8 clusters, which can further be condensed to form Mo3nS3n+2 (n, an integer) nanowires. These linear structures are energetically more favorable compared with other closed-packed polyhedral isomers of Mo-S clusters. The octahedral units in nanowires are stabilized by strong Mo-Mo interactions and p-d hybridization between Mo 4d and S 2p orbitals. There is a free electron-like band that crosses the Fermi energy in infinite nanowires and leads to their metallic character. Iodine doping acts as electron donor and can be used to tailor the electronic conductivity. For Mo12S8I4 nanowires, both electrons and holes are found to contribute to conduction. These nanowires are energetically more favorable than the experimentally obtained Mo12S6I12 nanowires.

Ab initio Study of Structural Stability of Mo−S Clusters and Size Specific Stoichiometries of Magic Clusters

Using first-principles calculations with ultrasoft pseudopotential formalism and the generalized gradient approximation for the exchange-correlation functional, we study the stability of MonSm (n =1-6 and m ranging from n to 3n) clusters and obtain the optimal stoichiometry for each n corresponding to the magic cluster. It is found that in this size range, the lowest-energy structures favor a core of metal atoms, which is covered by sulfur. In particular, we observe that for Mo6S14 isolated clusters, a 3D structure is significantly lower in energy as compared to platelet structures found recently on Au (111) surface. The composition ratio between S and Mo in the magic clusters is less than 2 for n=3 and greater than 2 for n<3. The structural stability of the magic clusters arises from the optimization of the Mo-Mo and Mo- S bonding as well as the symmetry of the cluster. Addition of a terminal sulfur in a magic cluster generally lowers its binding energy. The presence of partially occupied d-orbitals in Mo atoms contributes to Mo-Mo bonding and for higher S concentration it leads to sulfur-sulfur bond formation. The variation in energy due to a change in the sulfur composition suggests that sulfurization of the magic clusters is generally more favorable than desulfurization.