CO2 formation

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

Another possibility tliat would reduce the overall process efficiency is the kinetics of CO2 formation at the anode. There is some evidence that die first step in tire sequence... 

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

The latter case (Fig. 8.40b) is more interesting. Initially both rates decrease but at steady state the rate of epoxidation has decreased, while the rate of CO2 formation has increased. Thus epoxidation exhibits electrophobic behaviour but oxidation to C02 exhibits electrophilic behaviour.45... 

Formate dehydrogenases are a diverse group of enzymes found in both prokaryotes and eukaryotes, capable of converting formate to CO2. Formate dehydrogenases from anaerobic microorganisms are, in most cases, Mo- or W- containing iron-sulfur proteins and additionally flavin or hemes. Selenium cysteine is a Mo- ligand. 

There have been some reports of herbicides disturbing the metabolism of microorganisms in soil. For example, dichlobenil was shown to increase the rate of CO2 formation from glucose in soil (Somerville and Greaves 1987). 

Ignition temperature estimated by extrapolating the steeply ascending portion of the CO2 formation curve to zero CO2 concentration Temperature which shows the maximum CO2 concentration... 

Figure 2.1S. The rate of CO2 formation in the CO + O2 reaction on Pt(l 110) oscillates synchronously with the surface reconstruction, from (1x1) to (1x2), shown in Fig. 2.14. [Adapted from M. Eiswirth, P. Moeller, K. Wetzl, R. Imbihl, and G. Ertl, /. Chem. Phys. 90 (1989) 510.1...

Here small amounts of CO and O atoms were adsorbed at relatively low temperature, after which the surface was heated linearly in time, and the CO2 formation monitored by mass spectrometry. The reaction sequence for this process is... 

What are the opportunities for using forms of energy that do not lead to CO2 formation Nuclear power from fission reactors presents problems with the handling and deposition of nuclear waste. Fusion reactors are more appealing, but may need several decades of further development. However, solar and wind energy offer realistic alternatives. 

Figure 10.7. CO2 formation rate from CO and O2 over Rh(l 11) and Rh(l 10) surfaces [Adapted from M. Bowker, Q. Guo, and R.W. Joyner, Catal. Lett. 18 (1993) 119]. Note the similarity to the simple model used to describe the rate in Fig. 2.12.

O showed a profound difference in CO2 formation rate [M.J.P. Hopstaken and J.W. Niemantsverdriet, J. Chem. Phys. 113 (2000) 5457]. Hence, care should be taken to interpret apparent structure sensitivity found under normal operating conditions of high pressure and coverage in terms of the intrinsic reactivity of sites. From the theory of chemisorption and reaction discussed in Chapter 6 it is hard to imagine how the concept of structure insensitivity can be maintained on the level of individual sites on surfaces, as atoms in different geometries always possess different bonding characteristics. 

Infrared spectroscopy has also been employed to follow the formation of acetaldehyde and acetic acid on Pt during ethanol electro-oxidation. On the basal planes, acetaldehyde could be observed starting at about 0.4 V (vs. RHE), well before the onset of CO oxidation, while the onset of acetic acid formation closely follows CO2 formation [Chang et al., 1990 Xia et al., 1997]. This is readily explained by the fact that both CO oxidation and acetic acid formation require a common adsorbed co-reactant, OHads, whereas the formation of acetaldehyde from ethanol merely involves a relatively simple proton-electron transfer. 

Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

The CO2 signal was calibrated by comparing against the well-known amount of CO2 formation produced during oxidation of a saturated CO adlayer ( CO stripping ) [Jusys et al., 2001], determining the calibration constant K between the two signals via the relation... 

Figure 13.3 Potentiodynamic electrooxidation of (a) formic acid, (b) formaldehyde, and (c) methanol on a Pt/Vulcan thin-film electrode (7 xgpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH (a), HCHO (b), or CH3OH (c). The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p-L s at room temperature). The top panels show the faradaic current (solid lines), the partial currents for Ci oxidation to CO2 (dashed lines) and for formic acid formation (dash-dotted line), calculated from the respective ion currents, and the difference between the measured faradaic current and the partial current for CO2 oxidation (formic acid oxidation (a), formaldehyde oxidation (b)), or the difference between faradaic current and the sum of the partial currents for CO2 formation and formic acid oxidation (methanol oxidation, (c)) (dotted line). The solid lines in the lower panels in...

The current efficiencies for CO2 formation and formic acid formation during poten-tiodynamic formaldehyde oxidation, calculated from the data in Fig. 13.3b as the ratio of the partial currents to the total faradaic current (in %), are plotted in Fig. 13.4a. 

Figure 13.4 Current efficiency plots for the potentiodynamic electro-oxidation of formaldehyde (a) and methanol (h positive-going scan c negative-going scan) on a Pt/Vulcan thin-fihn electrode (data from Fig. 13.3a, h) dashed lines, current efficiency for CO2 formation dash-dotted fines, current efficiency for HCOOH formation dotted fines, current efficiency for HCHO formation.

Figure 13.6 Potential-step electro-oxidation of formaldehyde on a Pt/Vulcan thin-film electrode (7 p,gpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCHO upon stepping the potential from 0.16 to 0.6 V (electrolyte flow rate 5 pL at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCHO oxidation to CO2 dotted line, difference between the net faradaic current and that for CO2 formation, (b) Solid line, m/z = 44 ion current transients gray line potential-step oxidation of pre-adsorbed CO derived upon HCHO adsorption at 0.16 V, in HCHO-free sulfuric acid solution, (c) Current efficiency transients for CO2 formation (dashed line) and formic acid formation (dotted line).

Conversion of the m/z = 44 ion current into a partial faradaic reaction current for formaldehyde oxidation to CO2 (four-electron reaction) shows that, under these experimental conditions, formaldehyde oxidation to CO2 is only a minority reaction pathway (dashed line in Fig. 13.6a). Assuming CO2 and formic acid to be the only stable reaction products, most of the oxidation current results from the incomplete oxidation to formic acid (dotted hne in Fig. 13.6a). The partial reaction current for CO2 formation on Pt/Vulcan at 0.6 V is only about 30% of that during formic acid... 

The current efficiencies for the different reaction products CO2, formaldehyde, and formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d. The CO2 current efficiency (solid line) is characterized by an initial spike of up to about 70% directly after the potential step, followed by a rapid decay to about 54%, where it remains for the rest of the measurement. The initial spike appearing in the calculated current efficiency for CO2 formation can be at least partly explained by a similar artifact as discussed for formaldehyde oxidation before, caused by the fact that oxidation of the pre-formed COacurrent efficiency. The current efficiency for formic acid oxidation steps to a value of about 10% at the initial period of the measurement, and then decreases gradually to about 5% at the end of the measurement. Finally, the current efficiency for formaldehyde formation, which was not measured directly, but calculated from the difference between total faradaic current and partial reaction currents for CO2 and formic acid formation, shows an apparently slower increase during the initial phase and then remains about constant (final value about 40%). The imitial increase is at least partly caused by the same artifact as discussed above for CO2 formation, only in the opposite sense. 

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