ACTIVOS Y PASIVOS POR IMPUESTOS

Because direct synthesis occurs via HOR and ORR being performed in concert on a single catalyst, it is certainly possible to split the two up into half-cells. This poses an interesting solution to possibly the biggest drawback in direct synthesis which is that H2 and O2 need to be mixed together. To avoid the danger of forming explosive mixtures, HOR and ORR could be performed in individual half-cells, similar to a fuel cell or electrodialysis experiment.10,11 _{In order to test this} idea, the experimental setup discussed in Chapter 2 of this work was utilized to separate direct synthesis into half-cells (Figure 6.6). The HOR half-cell was comprised of a Pt wire in saturated H2 solution, while the ORR half-cell was comprised of Vulcan carbon deposited onto a glassy carbon substrate. The two half-cells were electrically connected with no applied bias, and after a brief period of time, H2O2 was detected at a Pt UME present in the ORR half-cell. This observation gives credence to the idea that an effective direct synthesis catalyst can easily be fabricated by simply depositing minute amounts of a metal that can activate H2 onto a carbon support. There is a great deal of research on the mechanism of H2O2 formation on carbons and several studies optimizing H2O2 formation to occur at relatively low overpotentials.7,12 Additionally, HOR catalysis is one of the most exhaustively researched areas of electrochemistry.13-15 _{This research} can be used as a foundation in future work exploring the fabrication of catalysts to carry out direct synthesis in the most cost-effective way possible. Additionally, the results shown in Figure 6.4 and

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6.5 show H2O2 formation on unaltered carbon. In direct synthesis, the H2 activating catalyst would be supported onto the carbon, and will thermodynamically affect its ability to activate O2, as discussed above. Ultimately, this result shows the utility in simply utilizing carbon materials as direct synthesis catalysts and can potentially eliminate the rigorous catalyst design procedures used to make traditional direct synthesis catalysts.

From an engineering point of view, the results from this work have strong implications on optimal cell design for a thermal catalytic system. The work here shows that the formation of H2O2 is dependent on a catalyst mediating the electron transfer between H2 and O2. Therefore, there is no need for the two to be mixed together in a single reactor bed, and instead, they can be separated into two individual chambers similar a fuel cell. The only difference in this case would be that a typical fuel cell seeks to suppress H2O2 formation, while this direct synthesis type fuel cell would seek to form it. Naturally, there would need to be some variation in cell design from a traditional fuel cell, as H2O2 is known to degrade most ionic membranes that separate the chambers of the cell.16 _{It would be advantageous to utilize cell designs common to corrosion cells or bipolar} membrane electrodialysis cells.11,17 _{These cells typically involve separating two chambers with an} electrocatalytic membrane. In the case of forming H2O2, the membrane would be constructed to have one side coated with an HOR catalyst and the other with an ORR catalyst. An example of this reactor design is shown in Figure 6.3. In this design, the HOR and ORR were split up into half-cells, where H2 and O2 were bubbled into their respective half-cells. A Pt wire was shorted to a Au electrode, and it was observed that H2O2 was formed within seconds of introducing H2 to the HOR half-cell. The signal for H2O2 decreased when H2 and O2 were sequentially removed and the signal returned near to baseline upon removing gas flow entirely. This demonstration is a simple proof-of-concept that H2O2 can be formed in direct synthesis without the limitation of H2 or O2

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concentrations. Regardless, this innovation in cell design for direct synthesis will undoubtedly lead to large improvements in its economic efficiency for producing H2O2, thus giving it a much larger degree of industrial relevance.

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6.5. Figures

Figure 6.1 SECM images of HOR (A) and ORR (B) over a catalyst array of varying PtAu

compositions. This shows the importance of catalyst composition on catalytic reactivity towards H2 specifically.

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Figure 6.2 COMSOL simulation used to fit data collected from SECM images. The boundary

conditions of the substrate are altered until the UME response (green boundary) matches what is acquired experimentally. This yields catalyst spot kinetic parameters (k0_{, α) as well as catalyst spot} H2O2 selectivity.

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Figure 6.3 Shown here is an example of COMSOL being utilized to fit tip current collected in

ORR experiments (A) as well as substrate currents measured in HOR experiments (B). By fitting these data, kinetic parameters k0 _{and α as well as H2O2 selectivity are quantifiable (shown in} legends in the plots).

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Figure 6.4 Shown here is RRDE data collected from a PdAu catalyst mounted on Vulcan and the

bare Vulcan support. The nascent Vulcan support produces H2O2 at larger overpotentials and displays a small degree of overlap with the Vulcan support, albeit small.

0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.0 0.1 0.2 0.3 0.4 0.5 Vulcan XC-72 PdAu/Vulcan XC-72 Potential (V vs. Ag/AgCl) D isk C ur rent (m A ) -30 -25 -20 -15 -10 -5 0 R ing C ur rent ( µA )

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Figure 6.5 Shown here is RRDE data for various carbon supports to demonstrate its non-innocent

nature in ORR electrocatalysis. The Vulcan XC-72 carbon that was used in previous work in this presentation reduces O2 at fairly large overpotentials, but still displays some degree of overlap with the metal catalysts supported onto it.

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Figure 6.6 A) Shown here is the experimental setup for compartmentalizing direct synthesis into

a fuel cell type system. The half-cells used were a Pt containing cell saturated with H2 and a Au containing cell saturated with O2. The Pt and Au electrodes were connected through an external wire to allow electron flow. In (B) is the results of the Pt UME biased to collect any H2O2 produced from the Au electrode. An anodic signal arises from merely connecting the Pt and Au electrodes in the presence of H2 and O2, respectively. This signal decreases as the gas flow of either H2 or O2 is halted, which gives strong support for the formation of H2O2.

A

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