Carbon monoxide oxidation room temperature catalysts

C. Influence of Surface Oxygen on Catalytic Activity The catal3diic activity of a divided nickel oxide in the oxidation of carbon monoxide at room temperature increases with the runs (Figs. 18 and 21), if between the runs the catalyst is regenerated in vacuo at 200° [NiO(200°)] or 250° [NiO(250°)]. A constant activity is reached after the fourth regeneration treatment in the case of NiO(200°) and the third in the case of NiO(250°). The difference between the constant activities of NiO(200°) and NiO(250°) is less important than the... 

Low Temperature Oxidatix)n. The majority of heterogeneous catalysts used for oxidation are used at elevated temperatures. However, some of these metal oxide systems are capable of catalyzing specific oxidation reactions at ambient temperature. The most widely studied catalyst of this type is the mixed oxide CuMn204, which is active for the oxidation of carbon monoxide at room temperature. The same catalyst is also an active oxidation catalyst at increased temperatures, and this has been demonstrated in the previous section. The mixed copper manganese oxide is called hopcalite and was first discovered around 90 years ago (96). Early studies demonstrated that manganese oxides promoted with various transition metal oxides were active catalysts. 

The idea of using cataiysts to oxidize gases is not a new one. Cataiytic converters in cars oxidize caitwn monoxide and unbumed hydrocarbons to minimize pollution. Many substances are oxidized into new materials for manufacturing purposes. But both of these types of catalytic reactions oaur at very high temperatures. NASA s catalyst is special, because it s able to eliminate carbon monoxide at room temperature. 

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

The oxidation of CO at low temperatures was the first reaction discovered as an example of the highly active catalysis by gold [1]. Carbon monoxide is a very toxic gas and its concentration in indoor air is regulated to 10-50 ppm depending on the conditions [61]. An important point is that CO is the only gas that cannot be removed from indoor air by gas adsorption with activated carbon. On the other hand, metal oxides or noble metal catalysts can oxidize CO at room temperature. 

Catalysts were tested for oxidations of carbon monoxide and toluene. The tests were carried out in a differential reactor shown in Fig. 12.7-1 and analyzed by an online gas chromatograph (HP 6890) equipped with thermal conductivity and flame ionization detectors. Gases including dry air and carbon monoxide were feed to the reactor by mass flow controllers, while the liquid reactant, toluene was delivered by a syringe pump. Thermocouple was used to monitor the catalyst temperature. Catalyst screening and optimization identified the best catalyst formulation with a conversion rate for carbon monoxide and toluene at room temperature of 1 and 0.25 mmolc g min1. Carbon monoxide and water were the only products of the reactions. 

Nickel oxide, prepared by dehydration of nickel hydroxide under vacuum at 250°C. [NiO(250)]y presents a greater activity in the room-temperature oxidation of carbon monoxide than nickel oxide prepared according to the same procedure at 200° C. [NiO(200)]> although the electrical properties of both oxides are identical. The reaction mechanism was investigated by a microcalorimetric technique. On NiO(200) the slowest step of the mechanism is CO. i(ads) + CO(ads) + Ni3+ 2 C02(g) + Ni2+, whereas on NiO(250) the rate-determining step is O (0ds) + CO(ads) + Ni3+ - C02(g) + Ni2+. These reaction mechanisms on NiO(200) and NiO(250), which explain the differences in catalytic activity, are correlated with local surface defects whose nature and concentration vary with the nature of the catalyst. 

Carbon dioxide is then desorbed almost entirely to the gas phase, and the coverage of both surfaces by adsorbed C02 must be close to its maximum value. Since the capacity of adsorption of both catalysts with respect to carbon dioxide is the same (Table II), the difference of then-activities cannot be caused by a different coverage of their surfaces by carbon dioxide. A test carried out in a static reactor (catalyst weight 50 mg., initial pressure of the mixture CO + 02 3 torr, liquid nitrogen trap to condense C02) confirms that NiO(250) is more active than NiO(200) in the room-temperature oxidation of carbon monoxide (Figure 5). 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

The acid-catalyzed hydrocarboxylation of alkenes (the Koch reaction) can be performed in a number of ways. In one method, the alkene is treated with carbon monoxide and water at 100-350°C and 500-1000-atm pressure with a mineral acid catalyst. However, the reaction can also be performed under milder conditions. If the alkene is first treated with CO and catalyst and then water added, the reaction can be accomplished at 0-50°C and 1-100 atm. If formic acid is used as the source of both the CO and the water, the reaction can be carried out at room temperature and atmospheric pressure.The formic acid procedure is called the Koch-Haaf reaction (the Koch-Haaf reaction can also be applied to alcohols, see 10-77). Nearly all alkenes can be hydrocarboxylated by one or more of these procedures. However, conjugated dienes are polymerized instead. Hydrocarboxylation can also be accomplished under mild conditions (160°C and 50 atm) by the use of nickel carbonyl as catalyst. Acid catalysts are used along with the nickel carbonyl, but basic catalysts can also be employed. Other metallic salts and complexes can be used, sometimes with variations in the reaction procedure, including palladium, platinum, and rhodium catalysts. The Ni(CO)4-catalyzed oxidative carbonylation with CO and water as a nucleophile is often called Reppe carbonylationP The toxic nature of nickel... 

Interaction (1) which produces gaseous carbon dioxide in the case of NiO(250°) causes the inhibition of the surface of Ni0(200°) because it produces adsorbed carbon dioxide. Since it has been shown that the different behavior of NiO(200°) and NiO(250°) with respect to this interaction is related to their different surface structure (Section IV, C), it is concluded that the catalytic activity of a divided nickel oxide in the room-temperature oxidation of carbon monoxide is determined primarily by the number and the nature of the lattice defects which are formed on the catalyst surface during its preparation at a low temperature. 

It has been demonstrated in earlier sections that the catalytic activity of nickel oxide in the room-temperature oxidation of carbon monoxide is related to the number and the nature of the lattice defects on the surface of the catalyst and that any modification of the surface structure influences the activity of the solid. Changes of catalytic activity resulting from the incorporation of altervalent ions in the lattice of nickel oxide may, therefore, be associated not only with the electronic structure of the semiconductor (principle of controlled valency ) (78) but perhaps also with the presence of impurities in the oxide surface or a modification of the surface structure because of this incorporation. In order to determine the influence of dopants on the lattice defects in the surface of the solid and on its catalytic activity, doped nickel oxides were prepared under vacuum at a low temperature (250°). Bulk doping is not achieved and, thence, one of the basic assumptions of the electronic theory of catalysis (79) is not fulfilled. 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

Zinc oxide is a component of a number of catalysts, for example Cu/Zn0/Al203 which catalyses the synthesis of methanol from carbon monoxide and hydrogen [70]. INS spectroscopy was used to show the presence of Zn-H bonds in ZnO dosed with hydrogen. Hydrogen was chemisorbed on zinc oxide at room temperature the INS spectrum... 

In situ Mossbauer Measurements. In situ Mossbauer spectra were taken during the room temperature oxidation of carbon monoxide on Sn-Pt/Si02 catalyst ((0-3) type (sample II-3 in ref. 139) both at 27 °C and -196 °C. These spectra are presented in Figure 21 A and B, respectively and the corresponding data are summarized in Table 12. ... 

The oxidation of carbon monoxide at around room temperature is the most famous reaction known for gold catalysts. Haruta s group discovered in 1987 [1, 2] that gold is a unique catalyst for this reaction when gold metal particles are smaller than 5 nm and supported on oxides. Since then, extensive and intensive fundamental works have been published, and expanding new applications, from air purification (gas masks, gas sensors, indoor air quality control) to hydrogen purification for fuel cells (PROX, preferential selective oxidation of CO in the presence of Hj) have been developed. 

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