Selective CO oxidation

Selective CO Oxidation in Excess of H2 (PrOX) over the Nanostructured CuxCe1 x02 y Catalyst... 

By carrying out selective CO oxidation with some addition of C02 and H20 to the feed gas over a similar nanostructured CTy.Cej x02 catalyst prepared by a coprecipitation method [56], 15 vol% of C02 in the feed gas decreases the activity of the catalyst. Under these conditions the same values of activity and selectivity were obtained at temperatures 15 to 35°C higher. The addition of 10 vol% of H20 shifted the activity and selectivity curves to temperatures 20 to 40°C higher with respect to the curves where only CO, 02, H2, and He were used in the feed. 

The kinetics of selective CO oxidation over the Cu Cej r02, nanostructured catalysts can be well described by employing Mars and van Krevelen type of kinetic equation derived on the basis of a redox mechanism ... 

Figure 7.13 represents the calculated vs. experimental values of reaction rates for the Mars and van Krevelen model of the selective CO oxidation in excess of hydrogen over the catalyst used. From the figure one can see that most scatter of data represents the use of eight different catalyst samples the data obtained over one catalyst sample lie almost on a straight line, within 95% confidence limits. 

Figure 7.13 Calculated vs. experimental values of reaction rates for selective CO oxidation in excess hydrogen. (Reprinted from [49], With permission from Elsevier.)...

In our studies we have demonstrated that the redox mechanism that was used to model dynamic behavior of CO oxidation is consistent with a kinetic model of the selective CO oxidation obtained under steady-state mode of operation [62], We propose the following tentative scheme (Figure 7.15) for the selective CO oxidation over the CuolCe(J902 v catalyst CO and H2 adsorb on the... 

The CiijCe, r02 v nanostructuied catalyst prepared by the sol-gel method is a very efficient and selective CO oxidation catalyst even under the highly reducing conditions which are present in a PrOX reactor. It is energy efficient toward the PEM fuel cell technology, because it oxidizes CO... 

In catalytic processes, such as CO conversion (CO + H20 —> C02 + H2), selective methanisation (CO + 3 H2 —> CH4 + H20) or selective CO oxidation (CO + Vi 02 —> C02), the achievable efficiencies depend on reaction parameters, such as temperature, pressure, volume flow, raw gas concentration and catalyst material. These are capable of achieving contamination levels from 1 % down to a few ppm. The selection of different reaction paths is based on the use of different types of catalyst. 

G. K. Bethke, and H. H. Kung, Selective CO oxidation in a hydrogen-rich stream over Au/gamma-AI2O3 catalysts, Appl. Catal. A-Gen. 194 43-53 (2000). 

To study the promotion mechanism of Pt wire/FSM-16 in the PROX reaction, the Pt nanowires were extracted by HF/EtOH treatment from FSM-16, and the wires were again deposited on the external surface of FSM-16 from the ethanol solution. We found that the resulting external Pt wire/FSM-16 catalyst gave low TOFs (>35) and lower CO selectivity (>30%) in the PROX reaction [32]. This implies that the encapsulation of Pt wires in the silica channels of FSM-16 is a key to promote the selective CO oxidation in the PROX reaction. Furthermore, from the structural characterization by XANES, XPS and IR in CO chemisorption... 

Figure 15.26 Pictorial representation of proposed mechanism for selective CO oxidation in PROX reaction through the carboxyl intermediates (COOH) on Pt nanowires and particles supported on FSM-16 and HMM-1 with active OH groups.

We have observed similar IR bands (1520, 1352 and 1295 cm ) on the Pt wire/ FSM-16 sample in an in situ IR study of the PROX reaction. From these results, we propose that the selective CO oxidation in the PROX on Pt wire/FSM-16 proceeds through the reaction of a carboxyl intermediate (COOH) on Pt nanowires (and particles) supported on FSM-16 with active OH groups (Figure 15.26). CO reacts with an active silica surface OH of FSM-16 to convert the HCOO intermediate on Pt wires and particles into CO2, thereby leading to selective CO oxidation. The subsequent H2/O2 chemisorption generates active surface OH groups near the Pt wires and particles on FSM-16. Smaller HCOO intermediates due to the smaller OH interaction on Pt particle/HMM-1 and Pt necklace wire/HMM-1 may reflect in their lower TOFs and lower CO selectivity in the PROX reaction (Figure 15.25a and b). 

For successful operation a selective CO oxidation catalyst in a reformer-PEFC system must be operated at ca. 353-373 Kin a complex feed consisting of CO, 02, H2, C02, H20 and N2, and be capable of reducing CO concentrations from about 1% to below 50 ppm - this is equivalent to a CO conversion of at least 99.5% [4, 54, 60], In addition, this conversion must be achieved with the addition of equimolar 02 (twice the stoichiometric amount) and the competitive oxidation of H2 must be minimized. This is expressed as selectivity, which is defined as the percentage of the oxygen fed consumed in the oxidation of CO for commercial operation a selectivity of 50% is acceptable, since at this selectivity minimal H2 is oxidized to water. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

The practical application of CO oxidation is growing, especially in relation to the development of polymer electrolyte fuel cells. An ongoing attempt is focused on the selective CO oxidation in H2 stream. The key challenging question related to the development of direct methanol fuel cells is whether CO oxidation can proceed at low temperatures, even under a strongly acidic environment. 

Kusakabe et al.83 proposed selective CO oxidation membrane concept to facilitate SMR reaction. Yttria-stabilized zirconia (YSZ) membrane was deposited on the surface of a porous alumina support tube by sol-gel procedure. This again was impregnated with Pt and Rh aqueous solution to produce a Pt- or Rh-loaded YSZ membrane. With addition of 02 in the feed, oxidation of CO can bring CO concentration to the level appropriate for PEMFC (<30ppm). 

Recent literature data suggest that the use of promoters may be beneficial and lead to a significant improvement in CO conversion and selectivity for preferential selective CO oxidation (under an H2-rich environment).17 Lanthanum oxide (La203) is known to be an effective catalytic textural and structural promoter, increasing the... 

Avgouropoulos, G. and Ioannides, T. Selective CO oxidation over CuO-Ce02 catalysts prepared via the urea-nitrate combustion method. Applied Catalysis. A, General, 2003, 244, 155. 

Fig. 1(a) shows N2 adsorption and desorption isotherm of Pt/C. At a relative N2 pressure of 0.4-0.7, an increase in the amount of adsorbed N2 with a hysteresis loop corresponds to the filling of mesopores. This result suggests that not only micropores less than 1 nm but also mesopores were generated under pyrolysis. The BET surface area was calculated to be 623 mVg. The pore size distribution of mesopores was calculated using the BJH model for the desorption branch and is shown in Fig. 1(b). The average pore size was 3.5 nm. The neutral surfactants molecule must play an important role to generate micropores and mesopores during the carbonization. We expect that the existence of mesopores would improve the diffusion of reactants and products in selective CO oxidation. 

Au/y-Al203 deactivates in CO oxidation, but it exhibits stable activity in selective CO oxidation (SCO) in the presence of H2. The activity for CO oxidation could be suppressed significantly by thermal treatment at 100°C, and the lost activity could be recovered by exposing the catalyst to water vapor at room temperature. A catalyst deactivated in CO oxidation could be regenerated by exposing it to H2 at room temperature or by running the SCO reaction over it. The results can be explained with a reaction mechanism for CO oxidation involving an active site that consists of an ensemble of metallic Au atoms and Au-hydroxyl. Deactivation in CO oxidation is due to the formation of an inactive carbonate, and deactivation by thermal treatment is due to dehydroxylation of the Au-hydroxyl. 

CO oxidation was conducted at room temperature with a feed of 1% CO, 21% O2, and balance He in a glass reactor of about 25 mL. The products were analyzed by gas chromatography using two columns simultaneously a molecular sieve column for CO and O2, and a HayeSep Q column for CO2. Approximately O.lg of catalyst was used such that the conversion at 5 minutes time-on-stream (TOS) was above 50% at a space velocity of 150 L/g-h. Unless specified, the first product analysis was performed after 1 min of reaction. The selective CO oxidation (SCO) reaction was conducted using a reactant mixture consisting of 2% O2, 1% CO, 49% H2, and balance He at a flow rate of 200 mL/min. 

In conclusion, we have found that AU/Y-AI2O3 catalysts can be deactivated both thermally and by CO oxidation. Successful regeneration of a thermally deactivated catalyst is accomplished by exposure to H2O, whereas a reaction-deactivated catalyst can be regenerated by exposure to H2 at room temperature. The latter can be regenerated also by the selective CO oxidation reaction, which is conducted in the presence of H2. The results can be explained with a reaction mechanism in which the CO oxidation reaction proceeds via the formation and decomposition of a surface formate and bicarbonate and an active site consisting of an ensemble of Au-hydroxyl group and metallic Au atoms. Deactivation is due to dehydroxylation of the Au-hydroxyl group or the formation of a rather inactive carbonate. 

Fig. 2. Typical experimentally obtained isotopic transients for the selective CO oxidation on Pt/7-Al20s. A switch from to CO was made during the reaction run.

The normalised form of the experimentally obtained data for the selective CO oxidation on Pt/7-Al203 is shown in Fig. 5 where argon was the... 

Since SSITKA can decouple the apparent rate of reaction into the contribution from the intrinsic activity ( the reciprocal of surface residence time of intermediates) and the nrnnber of active sites ( surface concentration of intermediates), the cause of deactivation of a catalyst during reaction can often be revealed. SSITKA has been used in a number of studies for this purpose. Catalyst deactivation during n-butane isomerization and selective CO oxidation are good examples. Deactivation studies are conducted by collecting isotopic transient data at particular times-on-stream as deactivation occurs. 

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