Atomic Cluster Expansion without Self-Interaction

The Atomic Cluster Expansion (ACE) (Drautz, Phys. Rev. B 99, 2019) has been widely applied in high energy physics, quantum mechanics and atomistic modeling to construct many-body interaction models respecting physical symmetries. Computational efficiency is achieved by allowing non-physical self-interaction terms in the model. We propose and analyze an efficient method to evaluate and parameterize an orthogonal, or, non-self-interacting cluster expansion model. We present numerical experiments demonstrating improved conditioning and more robust approximation properties than the original expansion in regression tasks both in simplified toy problems and in applications in the machine learning of interatomic potentials.Comment: Typo fix and minor changes in wording in v

Intercalation of Co-complex into the layered structure of VOPO4.2H2O for the preparation of vanadyl pyrophosphate, (VO)2P2O7 catalyst

Layered vanadyl phosphate dihydrate, VOPO4·2H2O is one of the precursor to vanadyl pyrophosphate (VPO) catalyst which is the sole catalyst used industrially for the partial oxidation of n-butane to maleic anhydride. With a basal spacing of 0.74 nm, layered VOPO4·2H2O was used as the host and Co-complex (Co(acac)2) as a guest The obtained precursor, VOHPO4·0.5H2O was confirmed by XRD and were activated in a reaction flow of n-butane/air mixture (0.75% n-butane/air) to form vanadyl pyrophosphate catalyst ((VO)2P2O7) at 460oC for 18 h. Both catalysts were characterised by using several methods i.e. X-ray Diffraction (XRD), Braunner Emmer Teller (BET) surface area and Temperature Programmed Reduction (TPR), Redox titration and Scanning Electron Microscopy (SEM). Co-complex was succesfully intercalated into the layer and as proven by XRD with a presence of a new peak appeared at 2θ = 6.8º and another new peak was also observed at 2θ = 13.5 º. TPR studies of Co intercalated VPO shows a sharp peak come with larger area (compared to unintercalated catalyst) which correspond to the removal of oxygen species associated to V4+ phase. Another peak at lower temperature which corresponds to the oxygen species released from V5+ phase. An improved of n-butane conversion is expected due to the increment of the active oxygen species (O-) which responsible to the activation of n-butane. Higher amount of oxygen linked to V5+ also will contribute to the activity of the Co-intercalated catalyst

Supercritical water gasification of empty fruit bunches from oil palm for hydrogen production

Empty fruit bunches (EFBs) from the palm plantation sector are abundant agricultural waste products in Malaysia. Supercritical water gasification (SCWG) is a prominent way to convert high-moisture-content biomass such as EFBs into valuable end products. This investigation is focused on EFB conversion into hydrogen-rich products using SCWG (temperature = 380 °C and pressure ≈ 240 bar). Lignocellulosic model compounds (xylan, cellulose, and lignin) were used to study the degradation patterns and gas compositions under similar reaction conditions. The effect of the EFB/water ratio and the SCWG reaction time on the composition of the product gas was examined. Carbon gasification does not improve with increasing EFB/water ratio as well as with increasing reaction time caused by the thermally stable tar formation during reaction. The hydrogen concentration was found to be increased with reaction time along with raising the EFB/water ratio to 0.3 g (3.75 wt%). In addition, the possibility of using palm oil mill effluent as a reaction medium in comparison to deionized water was analyzed

The effect of Bi promoter on vanadium phosphate catalysts synthesized via sesquihydrate route

A series of 1%, 3% and 5% Bi-doped vanadyl pyrophosphate catalysts were prepared via sesquihydrate route (VPOs method). These catalysts were denoted as VPOs-Bi1%, VPOs-Bi3% and VPOs-Bi5%. Bulk and Bi-promoted vanadyl pyrophosphate catalysts prepared via sesquihydrate route exhibited a well-crystallized (VO)2P2O7 phase. Two V5+ phases, i.e. β-VOPO4 and αII-VOPO4 were observed in all Bi-promoted VPO catalysts, which led to an increase in the specific surface area and average oxidation state of vanadium. Bi-promoted VPO catalysts showed six to nine times higher amounts of oxygen evolved than the bulk VPO catalyst in oxygen TPD and a significant shift in the reduction peaks to lower temperatures. Catalytic tests revealed that both activity and selectivity to maleic anhydride increased with the presence of bismuth promoter

The Cosmic Ray Staircase: the Outcome of the Cosmic Ray Acoustic Instability

Recently, cosmic rays (CRs) have emerged as a leading candidate for driving galactic winds. Small-scale processes can dramatically affect global wind properties. We run two-moment simulations of CR streaming to study how sound waves are driven unstable by phase-shifted CR forces and CR heating. We verify linear theory growth rates. As the sound waves grow non-linear, they steepen into a quasi-periodic series of propagating shocks; the density jumps at shocks create CR bottlenecks. The depth of a propagating bottleneck depends on both the density jump and its velocity; {\Delta}P_c is smaller for rapidly moving bottlenecks. A series of bottlenecks creates a CR staircase structure, which can be understood from a convex hull construction. The system reaches a steady state between growth of new perturbations, and stair mergers. CRs are decoupled at plateaus, but exert intense forces and heating at stair jumps. The absence of CR heating at plateaus leads to cooling, strong gas pressure gradients and further shocks. If bottlenecks are stationary, they can drastically modify global flows; if their propagation times are comparable to dynamical times, their effects on global momentum and energy transfer are modest. The CR acoustic instability is likely relevant in thermal interfaces between cold and hot gas, as well as galactic winds. Similar to increased opacity in radiative flows, the build-up of CR pressure due to bottlenecks can significantly increase mass outflow rates, by up to an order of magnitude. It seeds unusual forms of thermal instability, and the shocks could have distinct observational signatures.Comment: 29 pages, 29 figure

Near-bottom flow characteristics of currents at arbitrary angle to 2D ripples

Experimental results for near-bottom current velocity profiles for flows over artificial, definitely 2D ripples made of 1.5 cm high aluminum angle -profile spaced at 10 cm intervals are obtained for the following cases: (i)current alone perpendicular to ripples; (ii) current alone parallel to ripples; (iii) combined orthogonal wave-current flows for current parallel to ripples; and (iv) current alone at an angle of 30° to the ripple axis. The velocity profiles are analyzed by the log-profile method, and show the roughness experienced by the current to increase as the angle between ripple and current direction increases, i.e. demonstrating convincingly the reality of the concept of a direction-dependent roughness for flows over a 2D rippled bottom. Roughness experienced by the velocity component perpendicular to the ripples is, however, found to be independent of the direction of the mainstream flow relative to that of the ripples, and the different roughness experienced by the perpendicular and parallel velocity components gives rise to a turning of the current velocity vector to become increasingly aligned with the ripple crests as the bottom is approached from above. Implications of this feature, in terms of net sediment transport direction in combined wave-current flows in inner-shelf coastal waters, is discussed.Singapore–MIT Alliance for Research and Technology ((SMART) Center for Environmental Sensing and Modeling)Singapore. National Research Foundatio

Synthesis and characterization of Fe2O3/CaO derived from Anadara Granosa for methyl ester production

Taufiq Yap Yun Hin/ Mohd Izham Saiman/ / Ibrahim M. Lokman In this study, the iron (III) oxide (Fe2O3) doped on natural CaO catalyst (Fe2O3/CaO) was prepared and utilized in biodiesel production from used frying oil by a single-step reaction process. The heterogeneous Fe2O3/CaO catalyst was synthesized using impregnation method; followed by calcination at 500 °C. The catalyst was characterized in detail by both qualitative and quantitative methods such as X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscope (SEM), thermal gravimetric analysis (TGA), ammonia and carbon dioxide-temperature programmed desorption (NH3-TPD and CO2-TPD), and Brunauer-Emmett-Teller (BET) analyses. The operating parameters such as molar ratio of methanol, catalyst amount and reaction time were investigated in order to optimize the reaction condition for the biodiesel production. As a result, the optimum reaction parameters found were 15:1 methanol-to-oil molar ratio, 65 °C reaction temperature, 3 h of reaction time and 1 wt.% of the Fe2O3/CaO. The reported results revealed suggestively high potential of the heterogeneous Fe2O3/CaO catalyst for direct conversion of used frying oil to biodiesel-with the possibility to reuse at least 5 reaction cycles without any reactivation process