Abstract
The state of palladium and copper on the surface of the PdCl2–CuCl2/γ-Al2O3 nanocatalyst for the low-temperature oxidation of CO by molecular oxygen was studied by various spectroscopic techniques. Using X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), freshly prepared samples of the catalyst were studied. The same samples were also evaluated after interaction with CO, O2, and H2O vapor in various combinations. It was shown that copper exists in the form of Cu2Cl(OH)3 (paratacamite) nanophase on the surface of the catalyst. No palladium-containing crystalline phases were identified. Palladium coordination initially is comprised of four chlorine atoms. It was shown by XAS that this catalyst is not capable of oxidizing CO at room temperature in the absence of H2O and O2 over 12 h. Copper(II) and palladium(II) are reduced to Cu(I) and Pd(I,0) species, respectively, in the presence of CO and H2O vapor (without O2). It was found by DRIFTS that both linear (2114 cm−1, 1990 cm−1) and bridging (1928 cm−1) forms of coordinated CO were formed upon adsorption onto the catalyst surface. Moreover, the formation of CO2 was detected upon the interaction of the coordinated CO with oxygen. The kinetics of CO oxidation was studied at 18–38 °C at an atmospheric pressure for CO, O2, N2, and H2O (gas) mixtures in a flow reactor (steady state conditions).
Highlights
Carbon monoxide (CO) oxidation is among the simplest and most widely used oxidation reactions
The Cu K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicated a partial reduction of Cu(II) to Cu(I) with a general retention of the paratacamite structure subject to some disordering (Figure 5). These results suggest that the stoichiometric carbon monoxide oxidation occurs, and this process is associated with palladium(II) and copper(II) reduction to Pd(0) and Cu(I), respectively, in the presence of CO and H2O but in the absence of oxygen
The introduction of a 100-fold excess of carbon dioxide did not affect the carbon monoxide conversion, which remained the same as in the experiment conducted under the identical conditions but in the absence of carbon dioxide
Summary
Carbon monoxide (CO) oxidation is among the simplest and most widely used oxidation reactions. The oxidation of CO to CO2 by molecular oxygen is catalyzed by heterogeneous and homogeneous catalysts in the gas and liquid phases, respectively, and this reaction has been investigated in sufficient detail [1,2,3,4,5,6,7,8,9,10]. The mechanism of low-temperature carbon monoxide oxidation over supported metal complex catalysts has been investigated to a lesser extent. Low-temperature CO oxidation involving a homogeneous or heterogeneous catalytic system is characterized by an induction period attributed by most researchers to the generation of catalytically active sites of palladium in an oxidation state intermediate between 2+ and 0 [23]. Depending on the reaction conditions and partial pressures of carbon monoxide, oxygen, and water, the formal kinetic orders of the reaction with respect to CO, O2, and H2O vary over a wide range [2,10,11]
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