Linear CO Band Research Articles

Electrochemical CO Oxidation at Platinum on Carbon Studied through Analysis of Anomalous in Situ IR Spectra.

The oxidation of adsorbed CO is a key reaction in electrocatalysis. It has been studied extensively on both extended model surfaces and on nanoparticles; however, correlation between the two is far from simple. Molecular insight into the reaction is often provided using in situ IR spectroscopy; however, practical challenges mean in situ studies on nanoparticles have yet to provide the same level of detail as those on model surfaces. Here we use a new approach to in situ IR spectroscopy to study the mechanism of CO adlayer oxidation on a commercial carbon-supported Pt catalyst. We observe bipolar IR absorption bands but develop a simple model to enable fitting. Quantitative analysis of band behavior during the oxidation prepeak using the model agrees well with previous analysis based on conventional absorption bands. A second linear CO band is observed during the main oxidation region and is assigned to the distinct contribution of CO on step as opposed to terrace sites. Analysis of the step and terrace CO bands during oxidation shows that oxidation begins on the terraces of the nanoparticles before CO on steps is removed. Further correlation of this behavior with the current shows that step CO is only lost in the first of the two main oxidation peaks.

Characterization of Pd/γ-Al2O3 Catalysts Prepared Using [Pd(hfac)2] in Liquid CO2

A series of Pd/γ-Al2O3 catalysts was prepared from [Pd(hfac)2] (hfac = hexafluoroacetylacetonate) in liquid carbon dioxide using the method reported by Kim et al. [Chem Mater 18:4710 (2006)]. The catalysts were characterized using CO pulse chemisorption, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron microscopy. The catalysts were reduced initially in the high-pressure CO2 reaction cell using H2 at 75 °C. Samples were removed, stored in a desiccator, and re-reduced in situ at 250 °C prior to pulse chemisorption, DRIFTS and XAFS. CO pulse chemisorption evidenced that the Pd dispersion decreased from 55% to 5% as the Pd loading increased from 0.58 to 3.94 wt.%. The as-prepared 0.58 and 1.77 wt.% Pd/γ-Al2O3 catalysts (after air exposure) contained oxidized Pd species that were converted after in situ reduction to supported Pd particles. The average Pd particle sizes of these two catalysts (16 and 23 A, respectively) estimated from the first-shell Pd–Pd coordination numbers are in good agreement with the CO chemisorption results. DRIFTS evidenced a prevalence of weakly bound linear CO (νCO = 2083 cm−1) adsorbed on the 0.58 wt.% Pd catalyst. A 2.95 wt% Pd catalyst (49 A average particle size) also exhibited a strong linear CO band (νCO = 2093 cm−1). In contrast, CO chemisorption on a commercial 1 wt.% Pd/Al2O3 catalyst (37 A average particle size) gave predominantly 2-fold bridging CO species. We infer that the supported Pd particles prepared from [Pd(hfac)2] are rougher on the atomic scale (with a higher percentage of edge and corner atoms) than equivalently sized particles in conventionally prepared Pd/γ-Al2O3 catalysts.

The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

Kinetics, operando FTIR, and controlled atmosphere EXAFS study of the effect of sulfur on Pt-supported catalysts during CO oxidation

The effect of the presence of sulfur on the state of the catalytic surface during oxidation reactions on supported Pt catalysts has been investigated by kinetic studies and operando infrared (IR) and in situ extended X-ray absorption fine structure (EXAFS) spectroscopies. The experimental results clearly show that the catalytic surface is not static, and the reaction atmosphere strongly affects the state of the surface and consequently the catalytic activity. The activity results show that the light-off temperature for CO oxidation increases with ex situ H 2S addition. In situ IR results show a shift of the linear CO band, indicating a change in the bonding of adsorbed CO due to the presence of sulfur. Continuous co-feeding of 20 ppm of SO 2 with the reactant mixture also increases the light-off temperature. Activity and IR results demonstrate that alumina acts as a sulfur storage reservoir. This delays the initial deactivation, but after extended time on stream in the presence of SO 2 the activities of the silica- and alumina-supported catalysts are similar. The results indicate that sulfur poisoning is not only due to blocking of the sites where oxygen preferentially adsorbs, but also to modification of the Pt CO bonding.

Templating fabrication of platinum nanoparticles and nanowires using the confined mesoporous channels of FSM-16—their structural characterization and catalytic performances in water gas shift reaction

The robust trigonal prismatic Pt 18 and Pt 15 cluster anions are selectively synthesized via [Pt(CO)Cl 3] − in FSM-16 (4.7 nm) and cis-Pt(CO) 2Cl 2 in FSM-16 (2.8 nm) by the reductive carbonylation of H 2PtCl 6 in the confined mesoporous channels of FSM-16, respectively. The Pt cluster anions extracted from the resulting samples in the THF solution by cation metathesis with [PPN]Cl are identified as [Pt 3(CO) 6] 5 2− in FSM-16 (2.8 nm) and [Pt 3(CO) 6] 6 2− in FSM-16 (4.7 nm) by the FTIR and UV–Vis spectroscopic data. The EXAFS and FTIR studies demonstrated that [Pt 3(CO) 6] 5 2−/FSM-16 (2.8 nm) ( ν CO=2078, 1880 cm −1) and [Pt 3(CO) 6] 6 2−/FSM-16 (4.7 nm) ( ν CO=2056, 1879 cm −1) were transformed by heating at 300–343 K into the partially decarbonylated Pt clusters (Pt–Pt; C.N.=7.6, R=2.73 Å) characteristic of a linear CO band ( ν CO=2015 cm −1). According to the EXAFS and TEM observation, Pt carbonyl clusters are eventually converted above 473 K to naked Pt particles of 11 Å (Pt–Pt C.N.=6.7, R=2.72 Å) in FSM-16 (2.8 nm) and of 15 Å (C.N.=7.9, R=2.74 Å) in FSM-16 (4.7 nm). On the other hand, the platinum nanowires [2 and 4 nm (diameter)×50–500 nm (long)] are prepared using the cylindrical mesoporous channels of FSM-16 (2.8 and 4.7 nm) in the templating reduction of H 2PtCl 6 by the exposure to γ-ray or UV-light ( λ max>254 nm) under the atmosphere of 2-propanol and water. It was demonstrated by the TEM observation and EXAFS characterization that the diameters of Pt nanowires formed in FSM-16 are consistent with those of FSM-16 used (2.8 and 4.7 nm), whereas the average lengths of Pt wires are varied by the function of the Pt loading mass and time exposure to γ-ray and UV-light. The observed electron beam diffraction patterns indicate that the Pt wires consist of a single crystal phase of Pt(110) having a lattice fringe of 2.27 Å. It is of interest to find that the Pt nanowires in FSM-16 (2.8 nm and 4.7 nm) exhibited an unique IR spectrum in CO chemisorption and the remarkable activities per surface Pt atom (TOF) for water gas-shift reaction at 323–393 K by 60–90 times larger than those of Pt nanoparticles in FSM-16 and even conventional metal catalysts.

Hydrogen as a Modifier of the Structure and Electronic Properties of Platinum in Acidic Zeolite LTL: A Combined Infrared and X-ray Absorption Spectroscopy Study

The structure and electronic properties of platinum in WH-LTL after reduction at 300 C and heating in helium to 500 or 690 C were determined using X-ray absorption and infrared spectroscopy. After reduction at 300 C, the platinum particles were metallic, consisted of 4 or 5 atoms, and were located at 2.64 A from the oxygen atoms in the zeolite framework. The particles remained metallic but increased in size to e13 atoms during hydrogen desorption by heating in a helium flow up to 690 C. Simultaneously, the distance between metal particle and oxygen atoms of the zeolite framework was shortened to 2.05 A. After reduction at 300 C and in the presence of chemisorbed hydrogen, the platinum atoms in the PT/H-LTL catalyst had more holes in the d-band than bulk platinum. Hydrogen desorption decreased the number of holes of the platinum atoms in the WH-LTL catalyst to levels lower than bulk metal values. The linear CO band shifted from 2071 to 2084 cm-' upon hydrogen desorption, due to the increased particle size andor the change in the structure of the metal-support interface. The apparent contradiction between the shift to higher wavenumbers of the linear CO band and the decreased number of holes in the d-band was attributed to the interaction of CO with filled d-orbitals and the effect of chemisorbed hydrogen on the distribution of the local density of states.

Surface Species Formed During CO and CO 2 Hydrogenation over Rh/TiO 2 (W6+) Catalysts Investigated by FTIR and Mass-Spectroscopy

Surface species formed over Rh/TiO 2 (W 6+) catalysts during CO and CO 2 hydrogenation were investigated by in situ FTIR spectroscopy and transient techniques. It was found that at least four carbon-containing species, namely, linear CO, bridged CO, active carbon (C α) and less-active carbon (C β), exist on Rh crystallites, while formate and/or carbonate and/or hydrocarbonate species are on the TiO 2 support. A reduction of CO coverage by 25 ∼ 40% and a shift of the linear CO band to higher wavenumber values by 10 ∼ 14 cm −1 were observed under CO and CO 2 hydrogenation upon doping the TiO 2 carrier with small amounts of W 6+ cations (< 1 at.%). This implies that the Rh-CO bond is significantly weakened by doping, presumably due to an alteration of the electronic structure of the TiO 2 carrier, which modifies the electronic state of surface Rh via electronic interactions at the metal-support interface. The concentrations of the C α and C β species on Rh crystallites and carbon-containing species on the support were also found to be influenced by doping. While the concentration of the C α species (including CH x species) was reduced and that of the C β species was enhanced with increasing W 6+ dopant content, the concentration of the carbonate and/or hydrocarbonate species on the carrier exhibited a maximum at a W 6+ content of 0.11 ∼ 0.22 at.%. It is reasoned that hydrogen chemisorption is favored on the doped catalysts, as the CO coverage and the strength of the Rh-CO bond are significantly reduced. This has twofold consequences which result in a large increase of the CO and CO 2 hydrogenation activity, as previously observed over the Rh/TiO 2 (W 6+) catalysts: there is an increase in the concentration of surface hydrogen participating in the rate-determining step and an enhancement of the formation of Rh carbonyl hydride species, through which the reaction proceeds via a route of lower activation energy (H-assisted CO dissociation).

Platinum-Mordenite Catalysts for n-Hexane Isomerization: Characterization by X-Ray Absorption Spectroscopy and Chemical Probes

Abstract Platinum-mordenite (Pt-MOR) catalysts were prepared from NH 4 -MOR by ion exchange with [Pt II (NH 3 ) 4 ][OH] 2 , calcination in O 2 at 350°C, and reduction in H 2 at 350°C. The resultant Pt-H-MOR was active for n -hexane isomerization and hydrocracking via bifunctional catalysis at 240-300°C and 1 atm. The observed activation energies for C 6 branched-isomer formation are unusually low, suggesting that the isomerization rates were controlled by pore diffusion. A Pt-KH-MOR catalyst was prepared by ion exchange with aqueous KNO 3 and re-reduction at 350°C; elemental analysis evidenced 90% exchange of protons for K + ions. The product distribution and observed activation energies for C 6 branched-isomer formation over Pt-KH-MOR are consistent with n -hexane isomerization via bifunctional catalysis. Hydrocracking was strongly suppressed, and light hydrocarbons were formed primarily by Pt-catalyzed hydrogenolysis. From in situ extended X-ray absorption fine structure spectroscopy and H 2 temperature-programmed desorption, we conclude that the Pt-MOR catalysts consist of small Pt clusters hosted within the mordenite crystals. The Pt L III X-ray absorption near-edge structure (XANES) spectra of Pt-H-MOR and Pt-KH-MOR are closely similar, suggesting that the electronic structure of the Pt clusters is unaffected by mordenite acid-base chemistry. For the freshly reduced catalysts, a XANES feature at 10 eV relative energy is assigned to Pt-H antibonding states. The infrared spectrum of CO adsorbed on Pt-H-MOR contains an intense band at 2084 cm −1 , which is assigned to linear CO moieties on Pt clusters. A small peak at 2124 cm −1 is assigned to isolated Pt I -CO species, which we infer are formed by oxidative fragmentation of Pt clusters. The infrared spectrum of CO adsorbed on Pt-KH-MOR evidences a red shift of the linear CO band, which we suggest is due to electrostatic interactions between carbonyl O atoms and nearby K + ions.

Vapor phase hydroformylation of ethene over Co/SiO 2 promoted by noble metals: dynamic in situ diffuse reflectance FT-IR study of surface species

An in situ diffuse reflectance FT-IR technique was employed to investigate the active surface species and the reaction mechanism of the oxygenate formation in the vapor phase hydroformylation of ethene on Co/SiO 2 promoted with various noble metals such as Ir, Rh, Pt, Re, Ru, and Pd. Co(A)/SiO 2 and Ir(CO)/SiO 2 which were derived from cobalt(II) acetate and Ir 4(CO) 12, respectively, were quite inactive in the reaction, and showed only quite small peaks of adsorbed CO under the conditions of 1.1 MPa of C 2H 4/CO/H 2 at 298 K. In contrast, Co(A)-Ir(CO)/SiO 2, which were very active in the reaction, exhibited strong absorption bands of linear and bridged CO species. At 423–463 K, propanal adsorbed on the catalyst and acyl species which is suggested as the intermediate for the formation of propanal were also observed on this catalyst. By exposing CO preadsorbed on this catalyst to C 2H 4/H 2 at 289 K and 0.1 MPa, the intensity of the linear CO band decreased, and the bands of propanal and acyl species emerged simultaneously, whereas that of the bridged CO band remained constant after the initial drop. These results suggested that the oxygenates are formed via the CO insertion into adsorbed ethyl species, and linear CO species plays a major role in the CO insertion on these noble metal-promoted cobalt catalysts.

Spectroscopic Study of the Effects of Carrier Doping on the Surface Structure of Rh/TiO2 Catalysts and On Their Interaction with CO and H2

FTIR and laser-Raman spectroscopies are employed to investigate the structure of Rh catalysts supported on W[sup 6+]-doped TiO[sub 2] carriers and their interaction with CO and H[sub 2]. The results of Raman study show that the surface of the TiO[sub 2] carrier (sintered at 1173 K) contains both the rutile and anatase structures, in contrast to the bulk, which is 100% rutile. Doping of TiO[sub 2] with W[sup 6+] cations increases the proportion of the anatase structure on the surface. The concentration of CO adsorbed on the Rh surface (i.e., the sum of the gemdicarbonyl Rh[sup +](CO)[sub 2], linear Rh[sup 0]CO, and bridged Rh[sup 0][sub 2]CO), following exposure to 76 Torr of CO at 300 K, is found to exhibit a maximum with respect to dopant content at a level of 0.11-0.22 at. %. Blue shifts by 9 cm[sup [minus]1] of the linear CO band on the Rh crystallites and by 11 cm[sup [minus]1] of the gem-dicarbonyl band on the isolated Rh[sup +] sites are observed when Rh is supported on the doped carriers. The alterations in the CO adsorption capacity and in the frequency of the C-O bond, upon carrier doping, are attributed to electronic interactions between the Rhmore » crystallites and the doped TiO[sub 2] carrier, which result in alterations in the electronic configuration of the rhodium sites. Preadsorbed hydrogen on the doped catalyst is found to greatly retard the rate of subsequent CO adsorption. 51 refs., 8 figs., 2 tabs.« less

An in-Situ Infrared Study of the Formation of n- and iso-Butyraldehyde from Propylene Hydroformylation on Rh/SiO 2 and Sulfided Rh/SiO 2

In situ infrared (IR) technique has been employed to study the reaction of adsorbed CO on Rh/SiO 2 and S-Rh/SiO 2 with C 3H 6 and H 2, and to investigate the effect of sulfur on the n- and isobutyraldehyde selectivities during steady-state propylene hydroformylation. CO adsorption on Rh/SiO 2 results in the formation of linear CO and bridged CO adsorbed on Rh 0 sites, and the gem-dicarbonyl on Rh + sites. CO adsorption on S-Rh/SiO 2 results in a high wavenumber Rh +(CO) species in addition to linear CO adsorbed on Rh 0 and the gem-dicarbonyl adsorbed on Rh +. Sulfur decreases the rate of CO adsorption and inhibits the formation of bridged CO on Rh/SiO 2. Pulse CO chemisorption on Rh/SiO 2 and S-Rh/SiO 2 reveals that the equilibrium constant for CO adsorption on S-Rh/SiO 2 is smaller than that on Rh/SiO 2 at 303 K. Exposure of adsorbed CO to C 3H 6 at 303 K does not result in the formation of the aldehyde on both Rh/SiO 2 and S-Rh/SiO 2. Exposure of adsorbed CO to C 3H 6 and H 2 on Rh/SiO 2 causes a decrease in the intensity of the linear CO band and a slight increase in the intensity of the aldehyde band. A prominent decrease in the Rh +(CO) band is observed on S-Rh/SiO 2 with a simultaneous increase in the aldehyde band on exposure of adsorbed CO to C 3H 6 and H 2. The linear CO on Rh + is more active for CO insertion than linear CO on Rh 0 at 303 K. The Rh +(CO) is removed from the surface of S-Rh/SiO 2 at temperatures above 363 K in the presence of C 3H 6/H 2; however, the species remains stable at 513 K in the absence of H 2. Sulfided Rh/SiO 2 exhibits a higher CO insertion selectivity, a lower hydrogenation activity, and a lower n-/iso-butyraldehyde selectivity than Rh/SiO 2 during steady-state propylene hydroformylation at 513 K and 0.1-1 MPa. The analogy between homogeneous and heterogeneous hydroformylation suggests that enhancement of the isobutyraldehyde formation by adsorbed sulfur could be due to the spacious environment of the protruding Rh + ion sites on the S-Rh/SiO 2 allowing isomerization of n-propyl groups before CO insertion.

CO adsorption on clean and C-, O- and H-covered Mo(110) surfaces: an IRAS study

The adsorption of CO on clean and O-, C- and H-precovered Mo(110) surfaces has been studied using infrared reflection absorption spectroscopy (IRAS). For a low CO exposure (⩽ 2 L) at a sample temperature of 90 K, CO on a Mo(110) surface shows no IR features in the 1800–2200 cm −1 frequency region, and is thus likely bound to the substrate with the C-O axis tilted with respect to the surface normal. By an exposure of ≈4 L, CO features at 2004 and 1888 cm −1 are evident, and interpreted to correspond to linear and bridging CO, respectively. Saturation CO on Mo(110) at 90 K shows a single peak at 2033 cm −1 which splits into two peaks at 2031 and 2014 cm −1 upon heating to 160 K due to the formation of ordered CO islands. Adsorption of O 2; induces two CO IR peaks at 2062 and 1983 cm −1 which are believed to arise from CO molecules that have an indirect (via the Mo substrate) and a direct electronic interaction with O, respectively. CO on a C-covered Mo(110) surface displays an IR peak at 2085 cm −1. The invariant frequency of this peak at various C coverages indicates that the C forms two-dimensional islands which grow via a mechanism of expansion at the island edges. Coadsorption of H with CO on a Mod 10) surface results in a splitting of the linear CO band. The peak induced at 2008 cm −1 is attributed to CO adsorbed in the vicinity of H.

Adsorption of CO on Rh(100) studied by infrared reflection–absorption spectroscopy

The interaction of CO with a Rh(100) surface at 90 and 300 K has been investigated with infrared reflection–absorption spectroscopy (IRAS). Absorption bands due to the C–O stretch are found in both the linear and bridging regions at all coverages for both adsorption temperatures. For adsorption at 300 K, an ordered CO layer is formed at ∼0.4 monolayers (ML) as evidenced by a sharp, highly symmetrical, linear-CO band. At θCO&amp;gt;0.45 ML, the reduction in intermolecular separation and increasing intermolecular repulsive forces in the CO adlayer are evidenced by the increased broadness of both the linear- and the bridged-CO bands. CO adsorption at 90 K yields a nonuniform adlayer dominated by island formation at θCO&amp;lt;0.5 as indicated by peak splitting of the linear-CO band and the presence of a broad bridged-CO band. At θCO∼0.5, a highly ordered CO adlayer is suggested by the appearance of a highly symmetrical linear-CO band as well as the formation of a sharp c(2×2) LEED pattern. The presence of multiple components in the linear-CO band along with the drastic variations in the relative absorption intensities for both CO bands at coverages between 0.5 and 0.7 ML imply that the CO adlayer undergoes a phase transition within this coverage regime. These IRAS data are in agreement with the observed transition of a sharp c(2×2) pattern at θCO=0.5 to an ordered LEED pattern with a p(4×4)R45° coincidence structure at θCO=0.7. From a series of temperature-dependent IRAS measurement, it has been determined that the difference in binding energy of linear and bridged sites at θCO=0.2–0.5 ranges from ∼0.1 to ∼0.4 kcal/mol with the bridged-bound CO being the more stable species.

Chemisorption and FTIR study of bimetallic PtAu/SiO 2 catalysts

Pt/SiO 2, Au/SiO 2, and bimetallic PtAu/SiO 2 catalysts were prepared by incipient wetness impregnation of nonporous SiO 2. The catalysts were characterized after reduction in H 2 by static volumetric chemisorption and infrared spectroscopy. For the monometallic and the bimetallic catalysts, H 2, O 2, and CO were used as adsorbates at room temperature. Additionally, for the monometallic Au/SiO 2 catalyst, OZ adsorption at 473 K was also used. Infrared spectra of adsorbed CO were obtained on all catalysts. Addition of gold decreased the uptake of all three adsorbates at room temperature, without significantly influencing the relative amounts of weakly held adsorbed species which could be removed upon pumping for 30 min. Agreeing with CO chemisorption data, the total integrated intensity of the linear CO band decreased with increasing Au content. The IR results indicated a predominantly geometric effect of Au causing a shift of the linear CO band to lower wavenumbers. The shift could be rationalized in terms of decreased dipole-dipole coupling of adsorbed CO species. The results are discussed within the context of a previous investigation using electron microscopy, TPR, and reactivity data. Portions of the bimetallic catalysts were subjected to three high-temperature oxidation/reduction cycles and characterized by static chemisorption of H 2 to investigate the effect of this thermal treatment on Pt dispersion.

Infrared spectroscopic study of methanol and formic acid absorbates on a platinum electrode: Part II. Role of the linear CO(a) derived from methanol and formic acid in the electrocatalytic oxidation of CH 3OH and HCOOH

The formation and oxidation of the linear CO(a) from CH 3OH and HCOOH on a platinum electrode in 0.5 M H 2SO 4 has been studied by using in-situ polarization modulated infrared reflection-absorption spectroscopy as a function of the electrode potential. The formation from CH 3OH is greatly reduced in the hydrogen region, while from HCOOH it occurs more readily in the hydrogen region than at 0.4 V (vs. RHE). Linear CO(a) is oxidized off the Pt surface in a narrow potential range around 0.5 V, after which the rate of the electrooxidation of CH 3OH and HCOOH increases sharply. The poisoning effect of linear CO(a) was also confirmed by the rapid development of the infrared absorption intensity of the linear CO(a) band during continuous decrease of the electrocatalytic activity with time while the electrode was held at 0.4 and 0.2 V for methanol and formic acid, respectively. Reaction schemes to produce CO(a) from HCOOH and CH 3OH in both the hydrogen region and the double-layer region are proposed.

Infrared spectroscopic study of CO and CO2 adsorption on Rh–ZrO2, Rh–Al2O3 and Rh–MgO

The adsorption states of CO and CO2 on Rh catalysts supported on ZrO2, Al2O3 and MgO were studied using i.r. spectroscopy. Upon adsorption of CO on reduced Rh–ZrO2, the carbonate bands due to the reaction 2CO →C + CO2, together with the bands of twin, linear and bridge CO species were observed at moderate temperatures, whereas Rh–MgO gave no appreciable formation of CO2 even at higher temperatures (> 100 °C) and Rh–Al2O3 showed intermediate behaviour. On the adsorption of CO2, the linear CO band was formed at lower frequency than that on CO adsorption. The linear CO species formed from CO2 showed higher reactivity toward hydrogen compared with that from CO adsorption.