Carbon monoxide oxidation— experiments

Fig. 13.3 Laboratory flow reactor experiments on carbon monoxide oxidation as function of pressure [226]. The initial temperature is 1040 K and the initial mole fractions are 1.0% CO, 0.5% O2, and 0.65% H2O, with the balance N2.

F.H.M. Dekker, J.G. Nazloomian, A. Bliek, F. Kapteijn, J.A. Moulijn, D.R. Coulson, P.L. Mills, and J.J. Lerou, Carbon Monoxide Oxidation over Platinum powder A comparison of TAP and Step-Response Experiments, Appl. Catal. A, accepted for publication (1996). 

Generally, Cu based catalysts are more active than Cu-Cr based catalysts. This observation is in agreement with previous reports about carbon monoxide oxidation on Cu/AbOs catalysts(lO). From the results of oxygen chemisorption it can be seen that reaction sites are more dispersed on Cu catalysts as compared to Cu-Cr catalysts. Dispersity towards CO chemisorption is relatively very low for all the catalysts and therefore no definite conclusion could be arrived at from CO chemisorption data In addition, poor affinity of the catalysts for carbon monoxide in chemisorption experiments indicates low level of carbon monoxide adsorption during reaction... 

The experiments hitherto described dealt with catalytically active electrons and positive holes released by light. They allow only indirect conclusions regarding thermal catalysis. It is felt that direct observations are necessary in the present stage more than ever. Some work along these lines has been mentioned in the Introduction. Other observations on semiconductors of the ferrite type (d) have shown that the carbon monoxide oxidation, a donor reaction, is catalyzed best by inverse spinels, in which ferric ions, situated in octahedral positions, chemisorb carbon monoxide. Zinc ferrite, in which all the occupied octahedral positions carry ferric ions, showed a... 

Quite a number of years ago we carried out some experiments on films evaporated from Alloy 99 wire. We observed that films evaporated from this wire exhibited quite different properties than those evaporated from Nickel A or Hoskins 651. The films prepared from Alloy 99 showed (211) preferential orientation and were highly resistant to sintering as measured by the hydrogen adsorption. The activity of these films for carbon monoxide oxidation was 15 times greater than the activity of films prepared from the other nickel wires. For the disproportionation of carbon monoxide, 50-60% more CO2 was found than was theoretically possible. When the wire was heated in hydrogen a large decrease in hydrogen pressure with subsequent water formation was observed. 

Experiments on carbon monoxide oxidation catalyzed by single crystals of platinum suggest that Turing patterns may be seen in heterogeneous chemical reactions as well [82]. The stationary structures discussed in this chapter are only one of many spatiotemporal patterns that can arise in reaction-diffusion systems, and recent experiments [83] and calculations [84] suggest that there is much more to be seen, even in relatively simple systems. We may expect the rapid progress of the past several years to continue for some time to come. 

The changeover from ROO radicals to HOO radicals and the switch from organic peroxides to HOOH has been shown as temperature is increased in propane VPO (87,141). Tracer experiments have been used to explore product sequences in propane VPO (142—145). Propylene oxide comes exclusively from propylene. Ethylene, acetaldehyde, formaldehyde, methanol, carbon monoxide, and carbon dioxide come from both propane and propylene. Ethanol comes exclusively from propane. 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Curve 2 in Fig. 6.17 corresponds to experiment which we started as usual but reduced the surface of quartz trough treatment by carbon monoxide at 280°C. At this moment emission disappeared and furtiier increase in temperature would not bring it back. The sample was cooled after reaching the temperature of 320°C, oxidized and heated anew. Such subsequent oxidation of quartz resulted in formation and emission of singlet oxygen (see Fig. 6.17, curve 2) but only at temperatures around 320°C, i.e. above the temperature of initial heating. 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

Another way to eliminate the oxygen plant is to react a metal oxide with methane to yield the synthesis gas in a fluidized-bed reactor (83-86). Experiments have shown that copper oxide readily oxidizes methane to carbon monoxide and hydrogen with high selectivity at a temperature of about 1200 K and that the reduced CuO can be reoxidized with air. Lewis et al. (83-86)... 

The first scheme was ruled out by showing that, at room temperature, a surface formed by very brief exposure of the oxygen-saturated surface to carbon monoxide is stable after removal of the carbon monoxide from the reaction chamber. In other words, no further surface carbide formed by lateral reactions of adsorbed carbon monoxide with surface oxygen atoms. The second scheme was ruled out by showing that exposure of the surface formed in the latter experiment to oxygen had no effect. Consequently the third scheme is believed to represent the mechanism of oxidation of carbon monoxide at the step and kink sites of platinum. 

PCDD/F and other chlorinated hydrocarbons observed as micropollutants in incineration plants are products of incomplete combustion like other products such as carbon monoxide, polycyclic aromatic hydrocarbons (PAH), and soot. The thermodynamically stable oxidation products of any organic material formed by more than 99% are carbon dioxide, water, and HCl. Traces of PCDD/F are formed in the combustion of any organic material in the presence of small amounts of inorganic and organic chlorine present in the fuel municipal waste contains about 0.8% of chlorine. PCDD/F formation has been called the inherent property of fire. Many investigations have shown that PCDD/Fs are not formed in the hot zones of flames of incinerators at about 1000°C, but in the postcombustion zone in a temperature range between 300 and 400°C. Fly ash particles play an important role in that they act as catalysts for the heterogeneous formation of PCDD/Fs on the surface of this matrix. Two different theories have been deduced from laboratory experiments for the formation pathways of PCCD/F ... 

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