CO Oxidation In H2-rich Gas Research Articles

Mechanistic study of preferential CO oxidation on a Pt/NaY zeolite catalyst

Mechanistic study of the preferential CO oxidation was carried out over a Pt(1.0wt.%)/NaY zeolite catalyst from 300 to 523K by combination of kinetics and spectroscopy. The mechanism of CO oxidation at 300K was proposed on the basis of spectroscopic study, and the activity at low temperatures was assumed to originate from small Pt clusters. The analysis of XPS and TEM revealed that metallic state of Pt was rather stable after the reaction. The kinetics suggests a Langmuir–Hinshelwood mechanism in the oxygen-limited low-rate branch (O2 adsorption/dissociation is strongly depressed by adsorbed CO), being responsible for CO oxidation in H2-rich gas in the temperature range of 423–523K. In situ DRIFT spectra show that the Pt surface is dominated by linearly bound CO species, and the presence of H2 leads to a significant red-shift of the CO stretch frequency, which indicates a Had-induced modification of the PtCO bond, not just a coverage-dependent frequency shift. Consequently, CO oxidation was significantly enhanced in H2-rich gas.

Morphology effects of nanocrystalline CeO2 on the preferential CO oxidation in H2-rich gas over Au/CeO2 catalyst

Gold supported on CeO2 nanocrystals of rod, cubic and polyhedral shapes were used for CO preferential oxidation (CO-PROX) in H2-rich gas. The catalytic activity ranked by CeO2 nanocrystals as follows: rods > polyhedra > cubes. The results of X-ray photoelectron spectroscopy, temperature-programmed reduction/desorption and in situ Fourier transform infrared spectroscopy indicated that the surface gold species and also the adsorption/desorption properties for CO and oxygen species were closely related to the nature of exposed crystal planes of CeO2 nanocrystals. The gold catalyst on CeO2-rods with {1 0 0}/{1 1 0}-dominant surfaces showed the best CO-PROX performance.

Preferential CO oxidation promoted by the presence of H2over K–Pt/Al2O3

In preferential CO oxidation in H2-rich gas, K-Pt/Al2O3(K/Pt = 10) was very effective in decreasing CO concentration below 10 ppm in the 375-410 K range, and the turnover frequency of the K-Pt/Al2O3 was 20 times as high as that of Pt/Al2O3 at 353 K; furthermore, the activity of CO oxidation was promoted drastically by the presence of H2.

Kinetic study of selective CO oxidation in H2-rich gas on a Ru/γ-Al2O3 catalyst

The preferential oxidation (PROX) of CO on Ru/γ-Al2O3 in simulated reformer gas (1.0 kPa CO, 75 kPa H2, rest N2) was investigated over a wide range of CO partial pressures (0.02–1.5 kPa) and O2 excess (pO2∶pCO = 0.5–5.0). Integral flow measurements in a microreactor at λ = 2 show that ∼150 °C is the optimum temperature for the PROX of CO, combining sufficiently high rates with a high selectivity (S ≈ 50%). Ru/γ-Al2O3 shows a significantly higher activity and selectivity under these conditions than the conventionally used Pt/γ-Al2O3 catalyst. Differential flow experiments allow the quantitative determination of CO and O2 oxidation rates at 150 °C, with reaction orders of α = −0.48 for CO and α = 0.85 for O2. In the low temperature regime, 135–200 °C, the apparent activation energy, Ea*, is 95 ± 5 kJ mol−1. Our findings are consistent with a Langmuir–Hinshelwood reaction mechanism in the low-rate branch, where both CO oxidation and H2 oxidation are limited by the presence of a CO adlayer. The saturation of the reaction rate at high oxygen excess (λ 6) as well as previous data support a model where the reaction proceeds on oxygen loaded/surface oxide covered Ru particles. The minimum amount of catalyst and oxygen excess for complete removal (<50 ppm) of CO from methanol reformate in the second stage of a double stage reactor (pCO ⩽ 0.2 kPa) was calculated in reaction simulations, which were based on a plug-flow reactor model and use the kinetic parameters determined in this study. Including contributions from CO and CO2 methanation and from the reverse water gas shift reaction (RWGS) the Ru/γ-Al2O3 catalyst shows a much better performance than Pt/γ-Al2O3, both because of its much lower minimum noble metal mass, lower by a factor of 20, and the much better behavior under dynamic load conditions, where only the Ru catalyst remains below the critical CO level of 50 ppm for load variations to 10% and 1%, respectively.

Activity, Selectivity, and Long-Term Stability of Different Metal Oxide Supported Gold Catalysts for the Preferential CO Oxidation in H2-Rich Gas

A comparative study of the catalytic performance and long-term stability of various metal oxide supported gold catalysts during preferential CO oxidation at 80°C in a H2-containing atmosphere (PROX) reveals significant support effects. Compared to Au/γ-Al2O3, where the support is believed to behave neutrally in the reaction process, catalysts supported on reducible transition metal oxides, such as Fe2O3, CeO2, or TiO2, exhibit a CO oxidation activity of up to one magnitude higher at comparable gold particle sizes. The selectivity is also found to strongly depend on the employed metal oxide, amounting, e.g., up to 75% for Au/Co3O4 and down to 35% over Au/SnO2. The deactivation, which is observed for all samples with increasing time on stream, except for Au/γ-Al2O3, is related to the build-up of surface carbonate species. The long-term stability of the investigated catalysts in simulated methanol reformate depends crucially on the ability to form such by-products, with magnesia and Co3O4 supported catalysts being most negatively affected. Overall, Au/CeO2 and, in particular, Au/α-Fe2O3 represent the best compromise under the applied reaction conditions, especially due to the superior activity and the easily reversible deactivation of the latter catalyst.

Kinetics of the Selective CO Oxidation in H2-Rich Gas on Pt/Al2O3

The selective CO oxidation reaction on Pt/γ-Al2O3insimulated reformer gas (75% H2; the rest is N2) was investigated over a wide range of CO concentrations (0.02–1.5%)at low stoichiometric O2excess (pO2/pCO=0.5–1.5). Integral flow measurements in a microreactor showed that the optimumtemperature for the PROX process (preferential oxidation) was∼200°C; rates of CO methanation in this range wereinsignificant. At higher temperatures a significant loss inselectivity was observed. The quantitative determination of COoxidation rates and selectivities as a function of CO and O2concentration between 150 and 250°C by differential flow measurements led to reaction orders of −0.4 forpCOand +0.8 forpO2at an apparentactivation energy of 71 kJ/mol. These kinetics are consistentwith the selective CO oxidation reaction occurring in thelow-rate branch, on a surface predominantly covered withadsorbed CO. Its ramifications on the observed dependence of theselectivity on both CO concentration and temperature arediscussed. A H2-induced increase in the CO oxidation rate by afactor of ∼2 was observed. Comparison of the kinetic rateexpression for the selective CO oxidation with the performanceof a plug-flow reactor (variable flow rates) led to quantitativeagreement.