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Kinetic parameters have been estimated for the H2– D2 exchange reaction on a thin film Pd catalyst by fitting reaction data from T = 333 to 593 K over a range of inlet partial pressures, Pin H2 and Pin D2 . A rigorous approach to estimating the 95% confidence regions of the kinetic parameters reveals some of the issues and complexities that are not routinely considered in the estimation of kinetic parameter uncertainty from catalytic data. Three different mechanistic models were used to assess the influence of subsurface hydrogen, H′: the traditional Langmuir–Hinshelwood (LH) mechanism, the Single Subsurface Hydrogen (1H′) mechanism, and the Dual Subsurface Hydrogen (2H′) mechanism. The fitting was performed by fixing the preexponential factors for all Arrhenius rate constants and equilibrium constants to their transition state theory values. The diffusion of H and D atoms from the surface into the subsurface was constrained to be endothermic (i.e. ΔE ss > 0) and represented as an equilibrium process. Performance of the fitting routine was evaluated on a noiseless simulated dataset (created using ΔE‡ ads = 0, ΔE‡ des = 43, and ΔE ss = 25 kJ/mol) and the same simulated dataset with the inclusion of 3% Gaussian noise. In both cases, the solver was able to return the chosen values of ΔE‡ ads , ΔE‡ des , and ΔE ss . Mapping of the behavior of the residual sum of squared errors, 2 , about its global minimum within 3D ( ads , des , ss ) parameter space allowed quantification and visualization of the 95% confidence regions using 2D error ellipses for each pair of fitting parameters. For the experimental dataset on the Pd catalyst, fitting to the LH model predicted that H2– D2 exchange is adsorption rate limited, with ΔE‡ ads = 51.1 ± 0.6 kJ/mol with 95% confidence. On the other hand, fitting to both the 1H′ and 2H′ models led to predictions of ΔE‡ ads = 0, consistent with the current understanding that the barrier to H2 dissociation on Pd is low. Thus, the results detailed herein provide supporting evidence for a non-LH mechanism for H2– D2 exchange on Pd while also illustrating the issues associated with quantification of uncertainty in kinetic parameter estimation. 

Abstract Sensing of clinically relevant biomolecules such as neurotransmitters at low concentrations can enable an early detection and treatment of a range of diseases. Several nanostructures are being explored by researchers to detect biomolecules at sensitivities beyond the picomolar range. It is recognized, however, that nanostructuring of surfaces alone is not sufficient to enhance sensor sensitivities down to the femtomolar level. In this paper, we break this barrier/limit by introducing a sensing platform that uses a multi-length-scale electrode architecture consisting of 3D printed silver micropillars decorated with graphene nanoflakes and use it to demonstrate the detection of dopamine at a limit-of-detection of 500 attomoles. The graphene provides a high surface area at nanoscale, while micropillar array accelerates the interaction of diffusing analyte molecules with the electrode at low concentrations. The hierarchical electrode architecture introduced in this work opens the possibility of detecting biomolecules at ultralow concentrations. 

Surface segregation is a phenomenon common to all multicomponent materials and one that plays a critical role in determining their surface properties. Comprehensive studies of surface segregation versus bulk composition in ternary alloys have been prohibitive because of the need to study many different compositions. In this work, high-throughput low-energy He+ ionscattering spectra and energy-dispersive X-ray spectra were collected from a CuxAuyPd1−x−y composition spread alloy film under ultrahigh vacuum conditions. These have been used to quantify surface segregation across the entire CuxAuyPd1−x−y composition space (x = 0 → 1 and y = 0 → 1 − x). Surface compositions at 164 different bulk compositions were measured at 500 and 600 K. At both temperatures, Au shows the greatest tendency for segregation to the top-most surface while Pd is always depleted from the surface. Higher temperatures enhance the Au segregation. Segregation at most of the binary alloy bulk compositions matches with observations previously reported in the literature. However, surface compositions in the CuPd B2 composition region reveal segregation profiles that are nonmonotonic in bulk alloy composition. These were not observable in prior studies because of their limited resolution of composition space. An extended Langmuir−MacLean model, which describes ternary alloy segregation, has been used to analyze experimental data from the ternary alloys and to estimate pair-wise segregation free energies and segregation equilibrium constants. The ability to study surface segregation across the ternary alloy composition space with high-throughput methods has been validated, and the impact of bulk alloy phase on surface segregation is demonstrated and discussed. 

To identify superior thermal contacts to graphene, we implement a high-throughput methodology that systematically explores the Ni−Pd alloy composition spectrum and the effect of Cr adhesion layer thickness on thermal interface conductance with monolayer graphene. Frequency domain thermoreflectance measurements of two independently prepared Ni−Pd/Cr/graphene/ SiO2 samples identify a maximum metal/graphene/SiO2 junction thermal interface conductance of 114 ± (39, 25) MW/m2 K and 113 ± (33, 22) MW/m2 K at ∼10 at. % Pd in Ninearly double the highest reported value for pure metals and 3 times that of pure Ni or Pd. The presence of Cr, at any thickness, suppresses this maximum. Although the origin of the peak is unresolved, we find that it correlates with a region of the Ni−Pd phase diagram that exhibits a miscibility gap. Cross-sectional imaging by high-resolution transmission electron microscopy identifies striations in the alloy at this particular composition, consistent with separation into multiple phases. Through this work, we draw attention to alloys in the search for better contacts to two-dimensional materials for next-generation devices. 

To identify superior thermal contacts to graphene we implement a high throughput methodology that systematically explores the Ni-Pd alloy composition spectrum and the effect of Cr adhesion layer thickness on the thermal interface conductance with monolayer CVD graphene. Frequency domain thermoreflectance measurements of two independently prepared Ni- Pd/Cr/graphene/SiO2 samples both identify a maximum in the metal/graphene/SiO2 junction thermal interface conductance of 114± (39, 25) MW/m2K and 113± (33, 22) MW/m2K at ~10 atomic percent Pd in Ni—nearly double the highest reported value for pure metals and three times that of pure Ni or Pd. The presence of Cr, at any thickness, suppresses this maximum. Although the origin of the peak is unresolved, we find that it correlates to a region of the Ni-Pd phase diagram that exhibits a miscibility gap. Cross sectional imaging by high resolution transmission electron microscopy identifies striations in the alloy at this particular composition, consistent with separation into multiple phases. Through this work, we draw attention to alloys in the search for better contacts to 2D materials for next generation devices.