HCOOH oxidation

I,eiva E, Iwasita T, Hertero E, FeUu JM. 1997. Effect of adatoms in the electrocatalysis of HCOOH oxidation. A theoretical model. Langmuir 13 6287 6293. 

Figure 13.5 Potential-step electro-oxidation of formic acid on a Pt/Vulcan thin-film electrode (7 p,gptcm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH upon stepping the potential from 0.16 to 0.6 V (electrol)Te flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCOOH oxidation to CO2. (b) Solid line, m/z = 44 ion current transients gray line, potential-step oxidation of pre-adsorbed CO derived upon HCOOH adsorption at 0.16 V, in HCOOH-ftee H2SO4 solution.

Other model reactants are simple organic molecules, for example, formic acid [381, 382]. Pt(lll) exerts lower catalytic influence on HCOOH oxidation than do Pt(lOO) and Pt(llO) faces. However, in the presence of Pb adatoms on Pt(lll) a strong catalytic influence has been observed [383]. The poisonous species production in HCOOH oxidation is then inhibited. Electrochemical reduction of CO2 to glycolate/glyoxylate and oxalic acid has been studied [384]. Other products such as formic acid accompanied by CO and methane have also been detected [385]. In the latter case, the efficiency of the competing process of hydrogen evolution has been suppressed to less than 3.5%. 

OHad (or surface bonded water) block surface sites for HCOOH oxidation. 

In electrocatalysis, notable cases of formation of strongly bound species that are not, however, the kinetically involved intermediates in the main reaction pathway arise in the electrochemical oxidations of HCOOH, HCHO, and CH3OH at Pt anodes for those reagents, a self-poisoning intermediate, variably identified as chemisorbed CO, in bridged or linear double bonding to the electrode, or the species- C—OH, is involved (43) this species is not a principal kinetically involved intermediate in, for example, HCOOH oxidation, which proceeds at unpoisoned sites by the mechanism discussed in Section V,B,3. 

Similar steps have been proposed for CHjOH oxidation in alkaline electrolytes 200 or for HCOOH oxidation (201). 

Recent work involved stepped Pt surfaces with controlled adsorption of Bi on step edges [73]. For the stepped singlecrystal surfaces Pt(554), Pt(332), and Pt(221), with nine-, five-, and three-atom wide terraces, respectively, the enhancement factor for HCOOH oxidation increases as the terrace width decreases. Bi appears to block adsorption of poison on reactive (110) oriented step sites and decreases the reaction ensemble size, which increases the rate on narrow (111) terraces. 

Pt(lOO) is observed at 0sb = 0.35. Sb causes significant up-shifting of the CO band formed from HCOOH oxidation, which suggests that dissociative chemisorption is triggered at adjacent sites, electronically modified by Sb. Figure 8 shows IR spectra for Pt(lOO) and Pt(lOO) with an Sb adlayer (0st = 0.25) and the corresponding oxidation current versus and Oqo curves. The... 

Prediction from DPT calculation shows that HCOOH oxidation under a water-covered surface behaves substantially differently than in the gas phase or using a solvation model involving only a few water molecules [106]. 

An astonishing aspect of formic acid oxidation on palladium is the formation of CO2 at potentials of hydrogen oxidation and oxygen evolution. On-line mass spectroscopy analysis of volatile products reveals the production of CO2 not only in the double-layer region but also near 0.25 and 1.75 V [101]. The fact that the current and the CO mass signal during a potential scan do not follow the same pattern indicates that HCOOH oxidation proceeds through parallel mechanisms. 

Lei H-W, Hattori H, Kita H (1996) Electrocatalysis by Pb adatoms of HCOOH oxidation at Pt (111) in acidic solution. Electrochim Acta41 1619-1628... 

Yang Y-Y, ZhouZ-Y, Sun S-G (2001) In simFTlRS studies of kinetics of HCOOH oxidation on Pt(l 10) electrode modified with antimony adatoms. J Electroanal Chem 500 233-240... 

The film thickness using a PTh matrix is much smaller than that for the other two studied matrices. In addition, only two platinization cycles are required to disperse die optimum amount of metal in this matrix. In die case of PAM and PPy a much high number of potentio namic sweeps are required to obtain die most favourable film thickness and a hi er number of platinization cycles are necessary to obtain an adequate amount of dispersed platinum. This effect can be ascribed to the different degree of porosity for these polymers and, particularly, to a hindrance of die ion exchange proems responsible for the Pt(lV) inclusion in the matrix. The time of immersion of these electrodes in a Pb(n) solution has only a slight influence on their behavior, with being all efficient catalysts of die HCOOH oxidation Electrodes form by die PTh polymeric matrix, on the other hand, show higher current densities, and die electrocatalytic activity is more stable towards successive potentiodynamic cycles. 

HCOOH Observed current densities for HCOOH oxidation are ... 

Pb alloyed with Pt also enhanced the HCOOH oxidation rate [151-153]. Intermetallic PtPb nanoparticles (diameter 20-32 nm) synthesized in THF and diglyme from organometallic precursors showed mass-specific activities toward HCOOH oxidation up to almost ten times higher compared to commercial PtRu (Figure 4.28) [152]. Fuel cell experiments are awaited to further validate the... 

Figure 4.28. Mass specific activity with respect to HCOOH oxidation for PtPb nanoparticles (prepared from organometallic precursors in diglyme) compared to commercial PtRu/C. 0. 5 M HCOOH - 0.1 M H2SO4, 10 mV s 298 K [152]. (Reproduced with permission from Chem Mater 2006 18 5591-6. Copyright 2006 American Chemical Society.)...

PtCo bimetallic nanoparticles supported on highly oriented pyrolitic graphite showed good activity toward HCOOH oxidation compared to pure Pt, particularly in the composition range Pt Co between 1 1.1 and 1 3.5 (atomic ratio) [154]. 

Bismuth as an adatom or alloying element with Pt has been proposed as an extremely active co-catalyst for HCOOH oxidation on Pt. The unusual activity of Bi has been attributed to a combination of geometric (third-body) and electronic effects [155-157]. However, the role of Bi is quite complex and in some cases contradictory results were obtained. Tripkovic et al. clearly showed flie activity of PtBi is dependent on flie redox behavior of Bi and on its surface arrangement [158]. Oxidized Bi species such as Bi203 and BiO(OH) were identified by XPS on the surface at both open circuit and anodic potentials. Furthermore, the possibility of Bi leaching was proposed, with subsequent underpotential deposition of the resulting Bi on flie surface forming in-situ an adlayer, which could also contribute to the enhanced catalytic activity, as shown by a shift of the HCOOH oxidation onset potential by -0.25 V compared to pure Pt [158]. 

Complex spatio-temporal oscillatory patterns of the HCOOH oxidation rate on Pt were observed in the presence of 10 M Bi in the solution (i.e., in effect a Bi underpotential deposited layer on the Pt surface) [160]. The instabilities are exemplified in the cyclic voltammogram shown by Figure 4.29. It was concluded that there is a continuous oscillatory deactivation-reactivation of the surface with major impact from surface defects, as well. 

Figure 4.29. Cyclic voltammetry of HCOOH oxidation on Pt, a) in the presenee of 10 M Bi and b) in its absenee. 0.5 M H2SO4 - 1 M HCOOH. Sean rate 10 mV s [160]. (Reprinted with permission from Lee J, Christoph J, Strasser P, Eiswirth M, Ertl G. Spatio-temporal interfacial potential patterns during electrocatatyzed oxidation of formie acid on Bi-modified Pt. J Chem Phys 2001 115 1485-92. Copyright 2001 American Institute of Physics.)...

The co-catalytic role of Sb is somewhat similar to that of Bi. The redox behavior of Sb in conjunction with Had on Sb determines the electrocatalytic activity in a Pt structure-sensitive fashion [161, 162]. Sb modification had a beneficial effect on the HCOOH electrooxidation activation energies on Pt(lll) and Pt(331), while exercising an inhibitory role on Pt(100), Pt(llO), and Pt(320) [161]. But the same group presented cyclic voltammetry data showing increased HCOOH oxidation currents on Sb-modified Pt(llO) and Pt(320) [162]. The effect of Sbad was very dependent on its surface coverage and there was an interaction... 

Petty and co-workers published a study on the effect of oxotungstate adlayers on the electrocatalytic activity of Pt toward HCOOH oxidation [163]. In the presence of 1 mM Na3PWi2O40 in 0.5 M H2SO4 - 0.1 M HCOOH, at E < 0.55 V vs. RHE the oxidation current on a Pt electrode was higher by about 2.5-3 times. The direction of scanning also had an effect on the relative behavior with respect to phosphotungstate, since conditions had to be created on the surface for its effective adsorption [163]. 

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