Valentín Esteves, Keren

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    Novel synthesis and characterization of supported bimetallic films for hydrogen detection
    (2014) Valentín Esteves, Keren; Martínez Iñesta, María M.; College of Engineering; Suleiman Rosado, David; Hernández Maldonado, Arturo J.; Department of Chemical Engineering; Meléndez, Enrique
    Continuous increase in energy demand and high levels of pollution generated by the existing energy sources have led researchers to explore other clean and renewable alternatives. Hydrogen gas (H2) is an option that has been implemented in some parts of the country but there are still some challenges in its worldwide implementation. Hydrogen has a low explosion limit of 4%v/v (H2/air) at atmospheric pressure, high diffusivity and low liquefaction point. These characteristics demand small, sensitive, selective, economic and reliable gas sensors that can detect accurately and precisely H2 gas at different concentrations. Palladium (Pd) has a high sensitivity and selectivity towards hydrogen that is used in resistance based sensors. However, pure Pd undergoes large internal stresses when it goes through α to β phase transition that significantly affect its response deeming necessary the design of more reliable materials as hydrogen sensors. The approach of this work was to synthesize Pd90/Ag10, Pd90/Pt10, Pd90/Ni10 and Pd90/Cr10 bimetallic materials using a modified Solid State Reduction method (SSR) method developed in our laboratory to delay the α to β phase transition to higher concentrations. The crystalline phase, the films’ morphology, metal distribution, and the metals’ oxidation state were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Photoelectron Spectroscopy (XPS), respectively. Results of the Pd90/Ag10 films suggest an alloy type interaction between the two metals. Results also show that the Pd90/Ni10 films were not formed presumably because the hydrated salts used as precursors can’t be reduced by our synthesis method and because Ni has a negative standard reduction potential. Pd90/Cr10 films were presumably not formed for the same reasons as Pd90/Ni10. Sensing results show that Pd90/Ag10 and Pd90/Pt10 bimetallic materials can detect H2 with precision. Pd90/Pt10 delayed the α to β phase transition to 1.5%v/v and could detect at least 10%v/v H2/N2 before failure. Pd90/Ag10 did not delay the phase transition but could detect up to 4% v/v H2/N2. Results suggest that the bimetallic materials have an alloy interaction which constricts the structure expansion thus reducing the mechanical stresses during the phase transition. The results of both samples also showed that the sensitivity normalized by the response time (S/tr) have two linear relationships with [H2] 1/2, before and after the phase transition. The mentioned relation allows the implementation of our bimetallic films as sensing materials for H2 detection. In conclusion, the modified SSR yielded bimetallic films that can sense hydrogen, but it is limited to the use of anhydrous salts as precursors and the use of metals with a positive standard reduction potential such as Pd, Pt and Ag.