Infrared Spectroscopy and Temperature Programmed Desorption Study of CO on Rh/Al2O3 Catalysts: Probing Overlayer and Support Sites

Abstract

The adsorption of CO on reduced RI1/AI2O3 catalysts has been investigated using infrared (IR) spectroscopy and temperature programmed desorption (TPD). At ~120 K, carbon monoxide adsorbs on Al3+ sites of the -1203 support and in the form of gem-dicarbonyl, linear and bridge bound CO species on the Rh overlayer as identified by IR spectroscopy. TPD of CO from a 5 % Rh/AI2O3 catalyst reveals two desorption features with maximum rates of desorption at 170-185 K and 490-500 K. The former peak is assigned to CO desorbing from Al3+ sites while the latter is assigned to CO desorbing from the Rh overlayer. The CO adsorption capacity of the Rh overlayer has been quantified as a function of anneal temperature in ultrahigh vacuum using TPD. Sintering of the Rh overlayer has been identified as the primary mechanism by which exposed Rh°is lost from the catalyst surface. Indirect evidence is provided which suggests that loss of catalyst surface area (T > 1200 K) may also be a mechanism by which surface Rh is lost, possibly due to encapsulation of Rh particles. Structural changes in the Rh overlayer have been found to occur during CO TPD with oxidative disruption occurring at low temperatures and reductive desorption occurring at high temperatures.

Time‐Resolved, In Situ DRIFTS/EDE/MS Studies on Alumina‐Supported Rhodium Catalysts: Effects of Ceriation and Zirconiation on Rhodium–CO Interactions

“…Others have associated such shifts with an increase in the Rh particle size upon which the bridged CO species is adsorbed. [3] Furthermore, no trace of Rh I (CO) 2 species was evident at 573 K, as for all the other Rh catalysts.…”

“…The broad signal at ñ % 1725 cm À1 ,w hich was characteristico f bridged CO speciesb etween 2o r3Rh atoms and aC ea tom (Figure 4), was redshifted to ñ = 1685 cm À1 at 573 K. After increasingt he temperature to 573 K, ar edshift from ñ = 2073 to 2056 cm À1 was observed for the linear species, with ab lueshift for the bridgeds pecies from ñ = 1889 to 1900 cm À1 .O thers have associateds uch shifts with an increase in the Rh particle size upon which the bridged CO species is adsorbed. [3] Furthermore, no trace of Rh I (CO) 2 speciesw as evidenta t5 73 K, as for all the other Rh catalysts.…”

“…The interaction of CO with rhodium-containing catalysts has been intensively studied in recent decades. [1][2][3][4][5][6][7] This interest stems largelyf rom the use of rhodiumi nt hree-way automotive exhaust catalysts (TWC).I na ddition to the metal components of modernT WCs, ac omplex mixture of precious-metal particles( Rh, Pt, Pd) and variouso xide materials are included in the catalysts' formulation. Amongst these, cerium oxide has emerged as ah ighly important promoter material.…”

“…For example, upon oxidation, an ew The effects of ceria and zirconia on the structure-function properties of supported rhodiumc atalysts (1.6 and 4wt% Rh/ g-Al 2 O 3 )d uring CO exposure are described. Ceria and zirconia are introduced through two preparation methods:1 )ceria is deposited on g-Al 2 O 3 from [Ce(acac) 3 ]a nd rhodium metal is subsequently added, and 2) throught he controlled surface modification (CSM) technique, which involves the decomposition of [M(acac) x ]( M = Ce, x = 3; M= Zr, x = 4) on Rh/g-Al 2 O 3 . The structure-function correlationso fc eria and/or zirconiadoped rhodiumcatalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive ex-tendedX -ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions.…”

Hydrogen Production from Ethanol over Rh–Pd/CeO2 Catalysts

“…For example, on Rh/Al 2 O 3 at 300 K bands at 2082 and 2012 cm -1 were seen [23] while on Pd-Rh/SiO 2 and Rh/SiO 2 [24] they are at 2098 and 2035 cm -1 . Other evidence of gem dicarbonyl species on Rh metal are given in [25,26]. At 523 K, the bands at 2010 and 2025 cm -1 decreased and a band at 2078 cm -1 corresponding to a linear CO species is seen.…”