Strong metal–support interactions between palladium and iron oxide and their effect on CO oxidation

Abstract

Pd/FeO x catalysts were prepared by co-precipitation and characterized before and after reduction using X-ray powder diffraction, thermal analysis, CO chemisorption, electron microscopy, and X-ray photoelectron spectroscopy. Results give evidence for the encapsulation of palladium particles by iron oxide after reduction at high temperatures (523 K). Oxidation of carbon monoxide was applied as test reaction to characterize catalyst samples in different states. Strong metal-support interactions significantly enhance catalytic activity for oxidation of carbon monoxide. However, this state is not stable under the applied reaction conditions. Catalyst deactivation occurs in two ways: 1. via changes in the oxidation state of iron species and 2. due to sintering of palladium particles.

CO oxidation as a test reaction for strong metal–support interaction in nanostructured Pd/FeO powder catalysts

“…The size of the particles was very small, indicated by the broad XRD peak. As shown previously for hydrogen reduction, the HTR peak is due to reduction of the support from hematite to magnetite phase, Fe 3 O 4 [11]. After HTR, magnetite formation was also confirmed by XRD in case of reduction with CO.…”

“…Table 1 summarizes selected parameters of the pre-catalysts after calcination at 823 K. While the specific surface area of the lower loaded samples, which have been precipitated at pH = 4, were similar at about 35 m 2 /g, the higher loaded sample, prepared at pH = 9 as in Ref. [11], showed a value of 61 m 2 /g after calcination.…”

“…These UHV-studies have been recently complemented by investigations in our group on more realistic catalysts [10,11]. Post-synthesis treatment of co-precipitated Pd/iron oxide catalysts was found to be crucial for the induction of phase-transition in the supporting iron oxide (from Fe 2 O 3 to Fe 3 O 4 polymorph).…”

“…Activity studies of CO revealed an enhanced CO-oxidation rate in the state of decoration of the noble metal particles (E a determined to 33 kJ/mol), as well as the instability of this SMSI state leading to fast deactivation during reaction (E a = 70 kJ/mol) [11]. A possible explanation for the different behavior of the co-precipitated Pd/Fe 2 O 3 catalysts, compared to the Pd/Fe 3 O 4 model system could be release of oxygen during the phase transformation of the iron oxide support, which might facilitate the palladium encapsulation, as proposed by Wang et al [8].…”

“…For this purpose, the analytical methods described previously [11] were applied and complemented by additional CO-infrared studies (DRIFTS) to probe the surface properties in different reduction states.…”