In subsurface imaging and oil recovery where temperatures and salinities are high, it is challenging to design polymer-coated nanoparticles with low retention (high mobility) in porous rock. Herein, the grafting of poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylic acid) (poly(AMPS-co-AA)) on magnetic iron oxide nanoparticles was sufficiently uniform to achieve low adsorption on model colloidal silica and crushed Berea sandstone in highly concentrated API brine (8% NaCl and 2% CaCl2 by weight). The polymer shell was grafted via amide bonds to an aminosilica layer, which was grown on silica-coated magnetite nanoparticles. The particles were found to be stable against aggregation in American Petroleum Institute (API) brine at 90 °C for 24 h. For IO nanoparticles with ∼23% polymer content, Langmuir adsorption capacities on colloidal silica and crushed Berea Sandstone in batch experiments were extremely low at only 0.07 and 0.09 mg of IO/m2, respectively. Furthermore, upon injection of a 2.5 mg/mL IO suspension in API brine in a column packed with crushed Berea sandstone, the dynamic adsorption of IO nanoparticles was only 0.05 ± 0.01 mg/m2, which is consistent with the batch experiment results. The uniformity and high concentration of solvated poly(AMPS-co-AA) chains on the IO surfaces provided electrosteric stabilization of the nanoparticle dispersions and also weakened the interactions of the nanoparticles with negatively charged silica and sandstone surfaces despite the very large salinities.
The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ∼2 μm. The conductance versus axial magnetic flux Φ exhibits Aharonov–Bohm oscillations with maxima occurring alternately at half-integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ = 0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode.
The low viscosity and density of carbon dioxide (CO2) usually result in the poor sweep efficiency in CO2-flooding processes, especially in heterogeneous formations. Foam is a promising method to control the mobility and thus reduce the CO2 bypass because of the gravity override and heterogeneity of formations. A switchable surfactant, Ethomeen C12, has been reported as an effective CO2-foaming agent in a sandpack with low adsorption on pure-carbonate minerals. Here, the low mobility of Ethomeen C12/CO2 foam at high temperature (120°C), high pressure (3,400 psi), and high salinity [22 wt% of total dissolved solids (TDS)] was demonstrated in Silurian dolomite cores and in a wide range of foam qualities. The influence of various parameters, including aqueous solubility, thermal and chemical stability, flow rate, foam quality, salinity, temperature, and minimum-pressure gradient (MPG), on CO2 foam was discussed. A local-equilibrium foam model, the dry-out foam model, was used to fit the experimental data for reservoir simulation
Nanoparticle (NP)-stabilized foams can be generated at extreme water-deficient conditions (with quality as high as 95–99%) and yet with apparent viscosities >100 cP. This makes them greatly appealing for hydraulic fracturing applications, where minimal water consumption and leak-off to the reservoir are desired. Initial assessment of propensities of these novel fluids for fracturing applications requires field scale simulations. However, conventional fracturing models are difficult to employ because they do not consider true foam hydrodynamics. We have developed a mathematical model to simulate the transport of NP-stabilized foams for hydraulic fracturing. The model combines fluid transport in reservoir matrix and fracture with rock mechanics equations and thus allows for considering the effects of foam on fracture dynamics. Gas and water flow with mechanistic accounting of foam generation and coalescence are simulated using population balance models. Transport of nanoparticles through porous media was simulated using single site filtration model. The equations are discretized using finite-difference scheme. Settari’s approach is used to embed fracture’s moving boundary with the matrix to accordingly update transmissibility. Model’s capabilities are verified with examples on fracture growth and fracture clean up processes to illustrate the benefits of using the NP-stabilized high quality foams. Fracture propagation was simulated for water, a conventional viscous fracpad and NP-stabilized foams of different qualities and textures. The simulations confirmed that larger foam viscosity generated wider fractures with smaller fracture half-length. In addition, fracture cleanup simulations show that fracturing fluid cleanup for foam based fracturing fluids could take the order of 10 days as opposed to that of viscous fracpad which could take up to 1000 days; demonstrating the advantage of using dry foams
Urea electrooxidation has attracted considerable interest as an alternative anodic reaction in the electrochemical generation of hydrogen due to both the lower electrochemical potential required to drive the reaction and also the possibility of eliminating a potentially harmful substance from wastewater during hydrogen fuel production. Nickel and nickel-containing oxides have shown activities comparable to those of precious-metal catalysts for the electrooxidation of urea in alkaline conditions. Herein, we investigate the use of nanostructured LaNiO3 perovskite supported on Vulcan carbon XC-72 as an electrocatalyst. This catalyst exhibits an exceptionally high mass activity of ca. 371 mA mgox–1 and specific activity of 2.25 A mg–1 cmox–2 for the electrooxidation of urea in 1 M KOH, demonstrating the potential applications of Ni-based perovskites for direct urea fuel cells and low-energy hydrogen production. While LaNiO3 is shown to be stable at low overpotentials, through in-depth mechanistic studies the catalyst surface was observed to restructure and there was apparent CO2 poisoning of the LaNiO3 upon extended cycling, a result that may be extended to other Ni-based systems.
Rapid reduction of aqueous ClO4– to Cl– by H2 has been realized by a heterogeneous Re(hoz)2–Pd/C catalyst integrating Re(O)(hoz)2Cl complex (hoz = oxazolinyl-phenolato bidentate ligand) and Pd nanoparticles on carbon support, but ClOx– intermediates formed during reactions with concentrated ClO4– promote irreversible Re complex decomposition and catalyst deactivation. The original catalyst design mimics the microbial ClO4– reductase, which integrates Mo(MGD)2 complex (MGD = molybdopterin guanine dinucleotide) for oxygen atom transfer (OAT). Perchlorate-reducing microorganisms employ a separate enzyme, chlorite dismutase, to prevent accumulation of the destructive ClO2– intermediate. The structural intricacy of MGD ligand and the two-enzyme mechanism for microbial ClO4– reduction inspired us to improve catalyst stability by rationally tuning Re ligand structure and adding a ClOx– scavenger. Two new Re complexes, Re(O)(htz)2Cl and Re(O)(hoz)(htz)Cl (htz = thiazolinyl-phenolato bidentate ligand), significantly mitigate Re complex decomposition by slightly lowering the OAT activity when immobilized in Pd/C. Further stability enhancement is then obtained by switching the nanoparticles from Pd to Rh, which exhibits high reactivity with ClOx– intermediates and thus prevents their deactivating reaction with the Re complex. Compared to Re(hoz)2–Pd/C, the new Re(hoz)(htz)–Rh/C catalyst exhibits similar ClO4– reduction activity but superior stability, evidenced by a decrease of Re leaching from 37% to 0.25% and stability of surface Re speciation following the treatment of a concentrated “challenge” solution containing 1000 ppm of ClO4–. This work demonstrates the pivotal roles of coordination chemistry control and tuning of individual catalyst components for achieving both high activity and stability in environmental catalyst applications.
Graphene has great potential for fabrication of ultrafast opto-electronics, in which relaxation and transport of photoexcited carriers determine device performance. Even though ultrafast carrier relaxation in graphene has been studied vigorously, transport properties of photoexcited carriers in graphene are largely unknown. In this work, we utilize an ultrafast grating imaging technique to measure lifetime (τr), diffusion coefficient (D), diffusion length (L) and mobility (μ) of photoexcited carriers in mono- and multi-layer graphene non-invasively. In monolayer graphene, D∼10,000 cm2/s and μ∼120,000 cm2/V have been observed, both of which decrease drastically in multilayer graphene, indicating that the remarkable transport properties in monolayer graphene originate from its unique Dirac-Cone energy structure. Mobilities of photoexcited carriers measured here are several times larger than the Hall and Field-Effect mobilities reported in literature (<15,000 cm2/V), due to the high energy of photoexcited carriers. Our results indicate the importance of obtaining monolayer graphene to realize high-performance graphene devices, as well as the necessity to use transport properties of photoexcited carriers for predicting the performance of graphene-based opto-electronics.