On the propagation rate of the chemical waves observed during the course of CO oxidation on a Ag/Pt(110) composite surface

We have analyzed the propagation rate of the chemical waves observed during the course of CO oxidation on a Ag/Pt(I 10) composite surface that were reported in our previous papers [Surf Interface Anal. 2001, 32, 179; J. Phys. Chem. B 2002, 106, 5645]. In all cases, the propagation rate v can be adequately fitted as v = v(0) + D-0/d, in which v(0) and D-0 are constants, and d is the distance between the reaction front of the chemical wave and the boundary from which the chemical wave originates. We propose that the surface species responsible for the formation of the chemical wave comes from two paths: the adsorption of molecules in the gas phase on the surface and the migration from the adjacent surface with different catalytic activity. v(0) corresponds to the contribution from the surface species due to the adsorption, and D-0/d to that of the surface species that migrates from the adjacent surface. The rate equation clearly suggests that the observed chemical wave results from the coupling between adjacent surfaces with different catalytic activities during the course of heterogeneous catalysis. These results, together with our previous reports, provide a good fundamental understanding of spillover, an important phenomenon in heterogeneous catalysis.

Study of CH4 and CO oxidation from electrochemical method

CH4 and CO oxidation reaction on perovskite-like oxides La2-xSrxMO4 (0.01 <= x <= 1.0; M = Cu, Ni) was investigated from cyclic voltammetry method, finding that for suprafacial CO oxidation reaction, the catalyst activity has a close correlation to the area of redox peaks measured in the cyclic voltammetry, the larger the peak area is, the higher the activity will be, while for interfacial CH4 oxidation reaction, the activity depends mainly on the difference in redox potentials (Delta E), and the smaller the difference in redox potentials is, the higher the activity will be.

Performance of Pt-TiO2 electrocatalyst for CO oxidation in the electrochemical gas sensor

It is reported for the first time that the Pt/TiO2 electrocatalyst was successfully used for the electrocatalytic oxidation of CO in the electrochemical gas sensor with a controlled potential mode. The stability of electrocatalytic activity of the Pt-TiO2 electrocatalyst for the CO oxidation is better than that of Pt.

NO reduction and CO oxidation over Cu/Ce/Mg/Al mixed oxide catalyst in FCC operation

Simultaneous NO reduction and CO oxidation in the presence of O-2,H2O and SO2 over Cu/Mg/AUO (Cu-cat), Ce/Mg/Al/O (Ce-cat) and Cu/Ce/Mg/Al/O (CuCe-cat) were studied. At low temperatures (<340 degreesC), the presence of O-2 or H2O enhanced the activity of CuCe-cat for NO and CO conversions, but significantly suppressed the activity of Cu-cat and Ce-cat, At high temperature (720 degreesC), the presence of O-2 or H2O had no adverse effect on the NO and CO conversions over these catalysts. The addition of SO2 to NO + CO + O-2 + H2O system had no effect on the, reaction of CO + O-2 over Cu-cat, but deactivated this catalyst for NO + CO and CO + H2O reactions; over Ce-cat, all of these reactions of NO + CO, CO + O-2 and CO + H2O were suppressed significantly; over CuCe-cat, NO + CO and CO + O-2 reactions were not affected while the reaction of CO + H2O was slightly inhibited. (C) 2002 Elsevier Science B.V. All rights reserved.

Unexpected promotion of Au/TiO2 by nitrate for CO oxidation

The catalytic activity for Au/TiO2 for CO oxidation can be significantly enhanced by the addition of nitrates and this may relate to the variable catalyst performance observed in many studies.

Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H-2) are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 similar to 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H-2 treatment at low temperatures (100 similar to 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H-2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (> 300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 similar to 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (> 300 degrees C) is still higher than that directly pretreated with H,. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.

CO adsorption and correlation between CO surface coverage and activity/selectivity of preferential CO oxidation over supported Ag catalyst: an in situ FTIR study

In situ IR measurements for CO adsorption and preferential CO oxidation in H-2-rich gases over Ag/SiO2 catalysts are presented in this paper. CO adsorbed on the Ag/SiO2 pretreated with oxygen shows a band centered around 2169 cm(-1), which is assigned to CO linearly bonded to Ag+ sites. The amount of adsorbed CO on the silver particles ( manifested by an IR band at 2169 cm(-1)) depends strongly on the CO partial pressure and the temperature. The steady-state coverage on the Ag surface is shown to be significantly below saturation, and the oxidation of CO with surface oxygen species is probably via a non-competitive Langmuir Hinshelwood mechanism on the silver catalyst which occurs in the high-rate branch on a surface covered with CO below saturation. A low reactant concentration on the Ag surface indicates that the reaction order with respect to Pco is positive, and the selectivity towards CO2 decreases with the decrease of Pco. On the other hand, the decrease of the selectivity with the reaction temperature also reflects the higher apparent activation energy for H-2 oxidation than that for CO oxidation.

Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts

Silver is well known to show peculiar catalytic activities in several oxidation reactions. In the present paper, we investigate the catalytic activity of silver catalysts toward CO-gelective oxidation in H-2. XRD, TEM, TPD, and in situ FTIR techniques were used to characterize the catalysts. The pretreatment of the catalysts was found to have great influence on their performance. The pretreatment in 02 improves the activity of the silver catalyst, whereas He pretreatment at 700 degreesC or direct hydrogen pretreatment shows an inverse effect. Silver catalysts undergo massive structural change during oxygen pretreatment at high temperatures (> 500 degreesC), and there is solid evidence for the formation of subsurface oxygen species. The existence of this silver-subsurface oxygen structure facilitates the formation of active sites on silver catalysts for CO oxidation, which are related to the size, morphology, and exposed crystal planes of the silver particles. Its formation requires a certain temperature, and a higher pretreatment temperature with oxygen is required for the silver catalyst with a smaller particle size. It is observed, for the first time, that adsorbed CO on the surface of silver particles can directly react with subsurface oxygen species at low temperatures (e.g., RT), and the surface oxygen can migrate into and refill these subsurface sites after the consumption of subsurface oxygen by the reaction with CO. This finding provides a new reaction pathway for CO oxidation on silver catalyst. (C) 2004 Published by Elsevier Inc.

Factors influencing the catalytic activity of SBA-15-supported copper nanoparticles in CO oxidation

Copper nanoparticles were deposited onto mesoporous SBA-15 support via two different routes: post-grafting method and incipient wet impregnation method. Both XRD and TEM reveal that the post-grafting can make Cu particles very small in size and highly dispersed into channels of SBA-15, while the impregnation method mainly forms large Cu particles on the external surface of SBA-15. TPR experiments show that CuO species formed by the post-grafting method is more reducible than that prepared by the impregnation method. The catalytic activity tests for CO oxidation manifests that the sample prepared by the post-grafting method has a much higher activity than that prepared by the impregnation method, with a lowering of 50 degrees C for T-50, showing a strong dependence of catalytic activity on the size and dispersion of Cu particles. Besides the preparation procedure, other factors including calcination temperature, reduction treatment, copper loading as well as the feed composition, have an important effect on the catalytic activity. The best performance was obtained when the catalyst was calcined at 500 degrees C and reduced at 550 degrees C. The calcination and reduction treatment at high temperature have been found to be necessary to completely remove the organic residue and to generate active metallic copper particles. (c) 2005 Elsevier B.V. All rights reserved.