Electrocatalyst adsorption

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

In this work we present results obtained both with batch and continuous flow operation of the gas-recycle reactor-separator utilizing Ag and Ag-Sm203 electrocatalysts and Sr(lwt%) La203 catalysts, in conjunction with Linde molecular sieve 5A as the trapping material, and discuss the synergy between the catalytic and adsorption units in view of the OCM reaction network. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

The electrocatalysts for oxygen reduction were prepared as follows. These complex compounds were inoculated onto the carbon (AG-3, BET area near 800 m2/g) by means of adsorption from dimethylformamide solutions. The portion of complex compound weighed so as to achieve 3% of Co content was mixed with the carbon, then 5 ml of dimethylformamide per 1 g of the carbon were added and the mixture was cured at room temperature for 24 hours. Series of samples obtained were thermally treated (pyrolyzed), and the resulting grafted carbons were tested as electrode materials in the reaction of molecular oxygen reduction. 

Electrocatalyst see also specific catalysts adsorbate-support interactions, 30 273-279 adsorption, 30 240-264 isotherms, 30 241-243 bimetallic activity, 30 275... 

The adsorptions of H, O, and S04 on Pt/C electrocatalyst electrodes have been further investigated by O Grady and Ramaker by comparing the XANES data at the Pt L2 and L3 absorption edges. In their analysis, the difference spectrum, which they term AS for antibonding state, is obtained as follows ... 

More recently, Stamenkovic et al. [95,107] reported on the formation of Pt skins on Pt alloy electrocatalysts after high-temperature annealing. Pt skins were reported to exhibit strongly enhanced ORR activity. It was argued that the electronic properties of the thin Pt layer on top of the alloy alter its adsorption properties in such a way as to reduce the adsorption of OH from water and therefore to provide more surface sites for the ORR process (see Section 5.2 in Chapter 4 for a detailed discussion of skin catalysts, compare also Section 4.1.5 in the present Chapter). 

The solid curve in Figure 6.19 displays the cyclic voltammetric response of a carbon-supported high surface area Pt nanoparticle electrocatalyst in perchloric acid electrolyte under de-aerated conditions. The hydrogen adsorption range between... 

Figure 6.27. Schematic of the CO electro-oxidation on a Pt or Pt alloy electrocatalyst in the presence of hydrogen. CO adsorption blocks the dissociative adsorption and oxidation of hydrogen.

Electrocatalysis has just been described. One important feature of an electrocata -lyst is that it goes through the electrodic reaction unchanged. Its sole function is to act as an electron source or sink and as a surface for the adsorption of any intermediates involved in the reaction. Or, if one prefers to think in terms of the crystalline lattice that constitutes the solid electrocatalyst, it is clear that the lattice neither disintegr ates by its constituent particles walking off into solution nor grows by particles from the solution adding onto the lattice permanently. The surface of the electrocatalyst is a stable frontier it neither advances nor recedes. 

According to UHV investigations (156), the cracking reaction of methanol at Pt surfaces yields strongly adsorbed carbon monoxide. There had been much dispute as to whether the species that blocks the electrocatalyst platinum in methanol-containing electrolytes is really carbon monoxide (157), but today the different schools seem to converge with the opinion that it is CO or CO, some species resembling adsorbed carbon monoxide very closely in its adsorptive, optical, and electrochemical properties. 

One such reaction that has been studied is the electrocatalytic reduction of oxygen directly to water.25,27 The electrocatalysts for this process are often based on metal porphyrins and phthalo-cyanins. Thus a graphite electrode whose surface was modified by the irreversible adsorption of a cofacial dicobalt porphyrin dimer was able to reduce oxygen under conditions where the reduction did not occur on the bare electrode itself. Similarly, a catalytic chemically modified electrode for the oxidation of chloride to chlorine has been prepared28 where the active catalyst was reported to be a ruthenium dimer, [(bipy)2(OH)RulvORuvO(bipy)2]4+, which was reduced to the corresponding [Rum-RuIV] dimer during the reaction. 

In conclusion, the computational study of ternary Pt-Ru-X alloys suggests that future strategies toward more active electrocatalysts for the oxidation of methanol should be based on a modification of the CO adsorption energy of Pt (ligand effect), rather than on the enhancement of the oxophilic properties of alloy components (enhanced bifunctional effect). 

The use of MPc complexes as electrocatalysts for the detection of analytes relies on the formation of modified electrodes. Thiol, alkylthio, or arylthio substituted MPc complexes readily form self-assembled monolayers (SAMs) on gold. Amino substituted MPc complexes can easily be electropolymerized onto electrodes. Direct adsorption of the monomer onto electrodes (especially carbon electrodes) also occurs. The modified electrodes are then employed for detection of analytes ranging from neurotransmitters, thiols, phenols, and other biologically and environmentally important molecules. 

---