Linear CO Species Research Articles

Influence of residual chlorine on Ru/TiO2 active sites during CO2 methanation

Titania-supported ruthenium (Ru/TiO2) is an established catalyst for the hydrogenation of carbon dioxide to methane (Sabatier reaction). Chlorine contamination, owed to the RuCl3 precursor, is demonstrated to have a detrimental impact on methanation activity. After calcination and reduction the catalyst contains residual chlorine, shown by XPS. An aqueous ammonia wash removes Cl without leaching Ru. The washed catalysts exhibit improvements in CH4 site-time yields. Low Ru loading catalysts encounter the greatest activity enhancements after washing (∼4.5–fold). DFT calculations indicate that chlorine and CO2 directly compete for adsorption on Ru step sites, with Cl impeding the adsorption of CO2 at under-coordinated sites and at higher Cl coverages. H2-chemisorption/TPR show that Cl removal lowers the onset of low temperature H2 dissociation on Ru. DRIFTS provide evidence that the removal of Cl facilitates low temperature dissociative binding of CO2, indicated by the formation of surface bound linear CO species.

Heats of adsorption of linear CO species on the Pt sites of a 1.2% Pt-2.7% Sn/Al2O3 catalyst before and after reconstruction and ageing processes

Heats of adsorption of linear CO species on the Pt sites of a 1.2% Pt-2.7% Sn/Al2O3 catalyst before and after reconstruction and ageing processes

Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1

In this study, the conversion of CO into CH4 was investigated utilizing a series of cobalt loaded on fibrous silica (KCC-1) catalysts (Co loading of 5–30 wt%), that were synthesized via microemulsion and impregnation techniques. FESEM-EDX and N2 physisorption demonstrated that the KCC-1 possessed a spherical structure with fibrous silica dendrimeric morphology with a superior surface area of 861 m2g-1. A significant decreased in the catalyst surface area was noticed upon the addition of Co, suggesting a possible occurrence of KCC-1 pore blockage. Inversely, the number of basic sites on KCC-1 was enhanced after the incorporation of Co, as observed by pyrrole adsorbed FTIR. At 523 K, bare KCC-1 exhibited a very low activity for CO methanation due to low basicity and the absence of surface active sites. The 20Co/KCC-1 demonstrated the best catalytic performance with 72.7% yield of CH4 and 6.8% of CO2. These results were plausibly attributed to the high intrinsic number of basic sites and high dispersion of Co on KCC-1 support. A detailed in-situ FTIR spectroscopy study revealed that both types of associative and dissociative mechanism pathways significantly contributed to the high catalytic methanation activity. In addition to the dissociative mechanism, the linear CO species adsorbed on the Co metal by associative mechanism was also further hydrogenated to obtain the final CH4 products.

Effect of Pt Particle Size and Valence State on the Performance of Pt/TiO2 Catalysts for CO Oxidation at Room Temperature

A series of Pt/TiO2 catalysts with various Pt particle sizes and valence states were prepared and tested for CO oxidation at room temperature. Field-emission transmission electron microscopy and X-ray photoelectron spectroscopy analyses confirmed that the activity of the Pt/TiO2 catalyst was influenced by the particle size and valence state of the catalyst particles. Excellent CO oxidation activity was observed at room temperature using highly dispersed, small metallic Pt particles. Increasing the Ptmetallic/Pttotal ratio resulted in an increase of turnover frequencies. According to the Fourier-transform infrared spectroscopy results, the linear CO species that was adsorbed on metallic Pt sites at 25 °C reacted with atmospheric O2 and was easily desorbed. However, linear CO species adsorbed on PtOx (x ≥ 2) sites was only desorbed at temperatures ≥100 °C, confirming the lack of CO oxidation activity at room temperature with ionic Pt catalysts.

Individual Heat of Adsorption of Adsorbed CO Species on Palladium and Pd–Sn Nanoparticles Supported on Al2O3 by Using Temperature-Programmed Adsorption Equilibrium Methods

The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 (weight %, x = 2 or 5 wt %) in the 300–713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using Fourier transform infrared (FTIR) spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted as “B”), which are the dominant species on Pd0 particles and (b) a significant increase in the Pd sites forming linear CO species (denoted as “L”). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature-programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al2O3, in parallel to the formation of two strongly adsorbed B CO species, two linear L1Pd0 and L2Pd0 CO species are formed, which exhibit different heats of adsorption. For the dominant L1Pd0 CO species, the heat of adsorption decreases linearly, from 92 kJ/mol to 54 kJ/mol, as its coverage increases at coverage 0 and 1, respectively, while that of the L2Pd0 species is >165 kJ/mol at coverage 1. On the two Pd–Sn containing particles, two linear CO species are formed denoted L12Pd–xSn and L22Pd–xSn with x = 2 or 5. The L12Pd–xSn species dominates the CO adsorption on the two solids. It is shown that its heat of adsorption (which is slightly dependent on x) linearly varies with its coverage: ∼90 kJ/mol and ∼50 kJ/mol at low and high coverage, respectively. The comparison with the heat of adsorption of L1Pd0 indicates that the electronic effect of tin is very modest, compared to its geometric effect. This conclusion is consistent with literature data dedicated to DFT calculation. Moreover, (a) XRD and TEM/EDX analysis suggest that bimetallic particles such as Pd3Sn and Pd2Sn are present and (b) the impact of tin on the H2 chemisorption on Pd0 sites are presented.

Heat of adsorption of CO on EUROPT-1 using the AEIR method: Effect of analysis parameters, water and sample mode

Fourier transform infrared spectroscopy (FTIR) in transmission and in diffuse reflectance (DRIFT) modes is used to characterize the adsorbed species formed during the adsorption of CO on EUROPT-1 (a 6.3% Pt/SiO2 catalyst) as a function of different experimental parameters such as the duration ta of the adsorption at 300K, the adsorption temperature Ta (300–673K range) and pressure Pa (1–4kPa range), the reduction temperature TR in hydrogen (423, 523, 673K) and the presence of H2O. Whatever the experimental conditions the adsorption of CO on EUROPT-1 is dominated by linear CO species (denoted L CO) characterized by IR bands in the 2090–2040cm−1 range with a small contribution of bridged CO species (denoted B CO) with IR bands in the 1880–1830cm−1 range. It is shown that the intensity, shape and positions of the IR bands of the L CO are strongly dependent on the experimental conditions. Increasing the duration of adsorption ta at 300K leads to an increase of the IR band intensity due to an activated reconstruction process. The presence of H2O leads to a shift of the L CO species IR band towards lower wavenumbers. Transmission and DRIFT modes provide similar qualitative data. However, the measurement of the heats of adsorption of the L CO species through the adsorption equilibrium infrared spectroscopy (AEIR) method, which is based on the quantification of the IR bands of adsorbed species, reveals the limitations of the DRIFT technique at high temperatures. Using transmission mode, it is shown that the heats of adsorption of the L CO species linearly vary which its coverage θL from 90kJ/mol to 250kJ/mol at θL=1 and 0 respectively whatever (a) the reduction temperature and (b) the absence or the presence of H2O. These values are consistent with previous measurements on others Pt° particles supported on different metal oxides. However the value at θL=0 is higher by about 30kJ/mol, possibly due to the high Pt dispersion (D≈0.9) of the reduced-reconstructed EUROPT-1. This study emphasizes that the use of CO to characterize supported Pt° particles is not straightforward considering the different experimental parameters that may affect the Pt structure and IR band features of the adsorbed species.

Time-resolved, in situ DRIFTS/EDE/MS studies on alumina-supported rhodium catalysts: effects of ceriation and zirconiation on rhodium-CO interactions.

The effects of ceria and zirconia on the structure–function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ-Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ-Al2O3. The structure–function correlations of ceria and/or zirconia-doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive extended X-ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ-Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.

Experimental Microkinetic Approach of the Surface Reconstruction of Cobalt Particles in Relationship with the CO/H2 Reaction on a Reduced 10% Co/Al2O3 Catalyst

The adsorption of x% CO/He (x = 1 and 2) on a reduced 10 wt % Co/Al2O3 catalyst at an adsorption temperature of 560 K > Ta > 420 K leads to the formation of a linear CO species on Co° sites (denoted LCo° species) with an infrared (IR) band at ≈2030 cm–1 at full coverage. This adsorption is associated with the formation of carbon that is implicated in the reconstruction of the surface of the cobalt particles, leading to the progressive transformation of the LCo° species into a new linear CO species on Co°C sites of the reconstructed surface with an IR band at ≈2060 cm–1. An experimental microkinetic approach of the reconstruction process via the LCo° → LCo°C transformation reveals the impact of different kinetic parameters such as the reaction temperature, the CO partial pressure, and the presence of hydrogen. During the reconstruction the coverages of the LCo° and LCo°C species remain very high (>0.95) due to their high heats of adsorption determined by the Adsorption Equilibrium Infra Red spectroscopy method. It is shown that the rate of the reconstruction process is controlled by that of the disappearance of the LCo° species via its dissociation into adsorbed elemental carbon and oxygen species according to a pseudo first kinetic order elementary step involving a small amount of free cobalt sites (≈4 × 1012 sites/cm2 of cobalt particles). The impact of the reaction temperature on the LCo° → LCo°C transformation indicates that (a) the activation energy of the LCo° dissociation which controls the reconstruction process is ≈125 kJ/mol and (b) the formation of elemental carbon via the disproportionation of the LCo° species seems unlikely. These two conclusions are consistent with literature data on density functional theory (DFT) calculations. For adsorption temperature >560 K, the formation of superficial graphitic like species overlaps the reconstruction process. In the presence of hydrogen, the reconstruction is observed for H2/CO ratios <3 but not for higher values (i.e., H2/CO = 10).

Heats of adsorption of linearly adsorbed CO species on Co2+ and Co° sites of reduced Co/Al2O3 catalysts in relationship with the CO/H2 reaction

IR spectra in transmission mode are used to measure the heats of adsorption at different coverages θ:E(θ), of two linearly adsorbed CO species formed on Co2+ and Co° sites (noted LCo2+ and LCo°) of reduced x% Co/Al2O3 (wt%, x≤10) catalysts according to the adsorption equilibrium infrared spectroscopy procedure developed previously. For LCo2+ species characterized by an IR band at 2151cm−1, ELCo2+(θ) varies linearly with θ from ELCo2+(1)=45 kJ/mol to ELCo2+(0)=52 kJ/mol. These values are modified by the presence of neither Co° particles nor carbon deposition (from CO disproportionation reaction) nor H2 using CO/H2 gas mixtures. During the CO adsorption at high temperatures (i.e. 538K), C deposition on the surface and in the bulk of the cobalt particles modifies the Co° adsorption sites. This leads to a transformation LCo° (IR band at 2020cm−1)→LCo°C (IR band at 2060cm−1) where LCo°C denotes a linear CO species formed on Co° sites modified by the C deposition. The heat of adsorption of the LCo°C species varies linearly with its coverage from ELCo°C(1)=93 kJ/mol to ELCo°C(0)=165 kJ/mol. In the presence of H2 with a ratio H2/CO=10, the C deposition is strongly decreased and the LCo° species dominates the surface of the cobalt particles. However, in these conditions its hydrogenation into CH4 disturbs its adsorption equilibrium in a large coverage range and only ELCo°(1)=108 kJ/mol has been determined. For a ratio H2/CO=3, the carbon deposition cannot be prevented leading to the formation of the LCo°C species. It is shown that the ELCo°C(θ) values are not modified by the presence of adsorbed hydrogen. The heats of adsorption of the different adsorbed CO species on Co° sites are consistent with some literature data on DFT calculations and experimental values obtained on model cobalt surfaces (i.e. single crystals). In particular, they confirm DFT calculations which indicate that the C deposition on reduced cobalt particles decreases slightly the heats of adsorption of the linear CO species adsorbed on Co° sites.

Heats of adsorption of linear CO species adsorbed on reduced Fe/Al 2O 3 catalysts using the AEIR method in diffuse reflectance mode

IR spectra in diffuse reflectance mode are quantitatively exploited to determine the heats of adsorption of two linearly adsorbed CO species formed on Fe 2+ and Fe° sites (denoted L Fe 2 + and L Fe° CO species) of reduced x% Fe/Al 2O 3 (wt%, x = 1 and 5) catalysts according to the AEIR procedure developed previously using the IR transmission mode. The IR transmission properties of iron containing catalysts are limited particularly for high iron loadings favoring the use of the diffuse reflectance mode. The heats of adsorption of the L Fe 2 + CO species linearly vary with the coverage of the sites from E L Fe 2 + ( 1 ) = 45 kJ / mol to E L Fe 2 + ( 0 ) = 66 kJ / mol at coverage 1 and 0, respectively. These values are modified by the presence of neither Fe° sites nor carbonaceous adsorbed species. The heats of adsorption of the L Fe° CO species on a C-free iron surface (for adsorption temperature T a < 456 K), linearly vary with its coverage from E L Fe ° ( 1 ) = 79 kJ / mol to E L Fe ° ( 0 ) = 105 kJ / mol at high and low coverages. For T a > 456 K, the CO dissociation overlaps the CO adsorption equilibrium leading to a C-containing iron surface. The presence of carbonaceous species has no significant impact on the heat of adsorption of the L Fe° species at high coverages whereas it increases significantly that at low coverages: E L Fe ° ( 0 ) = 120 kJ / mol. Considering reduced iron supported catalysts, the AEIR method is particularly useful because it allows the determination of the individual heats of adsorption of two adsorbed CO species that can be simultaneously present on the surface for high iron loadings. The study confirms that quantitative exploitations of DRIFT spectra are available for well designed experimental conditions offering an alternative for solid catalysts with low IR transmission properties.

Combining Time-Resolved Hard X-ray Diffraction and Diffuse Reflectance Infrared Spectroscopy To Illuminate CO Dissociation and Transient Carbon Storage by Supported Pd Nanoparticles during CO/NO Cycling

A novel combination of time-resolved hard X-ray diffraction, diffuse reflectance infrared spectroscopy, and mass spectrometry reveals how Pd nanoparticles dissociate CO and store atomic carbon and how this carbon storage dynamically changes the population of linear and bridging CO species at the surface of the Pd nanoparticles.

Experimental Microkinetic Approach of the Surface Reconstruction of Gold Particles during the Adsorption of CO at 300 K on 1% Au/Al2O3

The adsorption of CO at 300 K on a 1% Au/Al2O3 solid leads to the formation of a linear CO species adsorbed on the Au° particles (denoted L1) characterized by an IR band at 2095 cm−1 at high coverage. For an adsorption pressure PCO< 1 kPa the position and the intensity of this IR band is independent of the duration of the adsorption ta (ta < 30 min). For PCO ≥ 5 kPa, it is shown that there is a progressive decrease in the intensity of the IR band of the L1 CO species with the increase in ta (without modification of its position) in parallel with the progressive increase in an IR band at 2070 cm−1 ascribed to a new linear CO species (denoted L2). According to literature data, these modifications are ascribed to the reconstruction of the gold particles leading to the transformation of the Au° sites adsorbing CO. The rate of the L1 → L2 transformation allows studying the elementary steps involved in this reconstruction process according to an experimental microkinetic approach that considers the impacts of different experimental parameters in particular ta, PCO and the dispersion of the gold particles. This work shows that (a) the rate constant at 300 K of the elementary step controlling the reconstruction is dependent on the coverage θL1 of the L1 CO species that is a function of PCO and (b) the higher heat of adsorption of the L2 CO species as compared to the L1 CO species constitutes one of the driving force of the reconstruction of the gold surface and explains the stabilization of the reconstructed surface during out degassing at 300 K. It is shown that the conclusions of the present study are consistent with literature data on DFT calculations.

Impact of the Reconstruction of Gold Particles on the Heats of Adsorption of Linear CO Species Adsorbed on the Au Sites of a 1% Au/Al2O3 Catalyst

The adsorption of CO at 300 K on a reduced (H2, 713 K) 1% Au/Al2O3 solid leads to the formation of a linear CO species adsorbed on the Au particles (denoted L1) characterized by an IR band at 2098 cm−1 that slightly shifts to higher wavenumbers with the decrease in the coverage. For adsorption pressures Pa > 5 kPa, it is shown that there is a progressive reconstruction of the Au surface with the increase in the duration of the adsorption ta leading to the disappearance of the Au sites associated with the L1 CO species and to the formation of new Au sites forming a linear CO species (denoted L2) characterized by an IR band at 2070 cm−1. The heats of adsorption of the two adsorbed species are determined as the function of their respective coverages by using the adsorption equilibrium infrared spectroscopy procedure previously developed. It is shown that they linearly decrease with the increase in their coverages from 62 to 43 kJ/mol for the L1 and from 100 to 50 kJ/mol for the L2 CO species at coverages 0 a...

Heats of Adsorption of Linear and Bridged CO Species Adsorbed on a 3% Ag/Al2O3Catalyst Using in situ FTIR Spectroscopy under Adsorption Equilibrium

The heats of adsorption of linear and bridged CO species adsorbed on Ag0 sites of a reduced 3% Ag/Al2O3 catalyst and of the linear CO species on Ag+ sites of the oxidized catalyst are determined as the function of their respective coverages by using the adsorption equilibrium infrared spectroscopy method previously developed. The evolutions of the intensities of the IR bands of each adsorbed species: 2045 cm-1 and 2000 cm-1 at 300 K, for the linear and bridged CO species, respectively, on Ag0 sites of the reduced solid and 2168 cm-1 for a linear CO species on Ag+ sites of the oxidized solid are determined as a function of the adsorption temperature Ta, at a constant CO adsorption pressure PCO. This provides the evolutions of the coverages of each adsorbed CO species with Ta in isobar conditions that give the individual heats of adsorption of the three adsorbed CO species at several coverages according to an adsorption model. The heats of adsorption of the linear and bridged adsorbed CO species on Ag0 sit...

Impact of the support on the heat of adsorption of the linear CO species on Pt-containing catalysts

It has been previously shown that the individual heats of adsorption of each adsorbed CO species (linear, bridged and three-fold coordinated) on Pt/Al 2O 3 catalysts can be obtained from the change of their characteristic IR bands with the adsorption temperature T a (in the range of 300–800 K) at a constant CO partial pressure P a according to a procedure denoted adsorption equilibrium infrared spectroscopy. In the present study, this procedure is used to reveal the impact of the nature of the support TiO 2, CeO 2 and Y 2O 3/ZrO 2 (two solids with high (104 m 2/g) and low (14 m 2/g) BET surface area) on the heat of the adsorption of the L CO species that dominates the CO adsorption. These reducible supports have been selected because of their expected interactions with Pt particles according to different SMSI effects. In the wavenumber range characteristic of the L CO species on Pt sites, the FT-IR spectra are dependent on the nature of the support: two strongly overlapped IR bands are detected with an intensity ratio depending on the support. However, it is shown that the nature of the support only has a slight impact on the average heats of adsorption of the L CO species (<20 kJ/mol as compared to Pt/Al 2O 3).

Impact of Pt dispersion on the elementary steps of CO oxidation by O 2 over Pt/Al 2O 3 catalysts

The impact of Pt dispersion (denoted by D) of Pt/Al 2O 3 catalysts on the turnover frequency (TOF) of the CO/O 2 reaction and on the elementary steps involved in the catalytic reaction is studied using transient experiments with either a mass or a FTIR spectrometer as a detector. Similar to literature data, it is observed that TOF decreases with an increase in D. The aim of the present study is to correlate this observation with the modifications of the surface elementary steps of two kinetic models, M1 and M2, developed previously for D ≲ 0.6 . The elementary steps considered are (1) adsorption of CO as a linear CO species (denoted by L and involved in models M1 and M2) and as a bridged CO species (denoted by B); (2) the oxidation by O 2 of the adsorbed CO species; and (3) the reduction by CO of the strongly adsorbed oxygen species (denoted by O sads) involved in model M2. It is shown that D has no significant impact on the heat of adsorption of the L CO species and on the mechanism of the reduction of O sads species by CO. For oxidation of the L CO species by O 2, it is shown that there is an induction period in the CO 2 production for D ≳ 0.60 not observed for a lower dispersion. This is explained by considering that the rate of formation of the Pt sites, which adsorb O 2 during oxidation, depends on D. These sites, which represent a small fraction of the Pt 0 sites, are associated with the B CO species. It is shown that the heat of adsorption of the B CO species increases with an increase in D. It is suggested that it is the removal of the B CO species (by desorption and oxidation) that controls the induction duration.

Heats of adsorption of linear CO species adsorbed on the Au degrees and Ti+delta sites of a 1% Au/TiO2 catalyst using in situ FTIR spectroscopy under adsorption equilibrium.

The heats of adsorption of two linear CO species adsorbed on the Au degrees particles (denoted L(Au degrees)) and on the Ti(+delta) sites (denoted L(Ti+delta)) of a 1% Au/TiO(2) catalyst are determined as the function of their respective coverage by using the AEIR procedure (adsorption equilibrium infrared spectroscopy) previously developed. Mainly, the evolutions of the IR band area of each adsorbed species (2184 cm(-1) for L(Ti+delta) and at 2110 cm(-1) for L(Au degrees)) as a function of the adsorption temperature T(a), at a constant CO adsorption pressure P(CO), provide the evolutions of the coverages theta(LTi+delta) and theta(LAu degrees ) of each adsorbed CO species with T(a) in isobar conditions that give the individual heats of adsorption. It is shown that they linearly vary from 74 to 47 kJ/mol for L(Au degrees ) and from 50 to 40 kJ/mol for L(Ti+delta) at coverages 0 and 1, respectively. These values are consistent with literature data on model Au particles and TiO(2). In particular, it is shown that the mathematical formalism supporting the AEIR procedure can be applied to literature data on Au-containing solids (single crystals and model particles).

Oxidation of CO on a Pt/Al 2O 3 catalyst: from the surface elementary steps to light-off tests : V. Experimental and kinetic model for light-off tests in excess of O 2

This article is the final part of a study on the CO/O 2 reaction over a 2.9% Pt/Al 2O 3 in line with the microkinetic approach of the heterogeneous gas–solid catalysis. Mainly, the kinetic parameters of each elementary step of two kinetic models (Models M1 and M2) determined previously are used to explain the evolution of the coverage of the adsorbed CO intermediate species (a strongly adsorbed linear CO species on Pt 0, denoted by L) as well as the turnover frequency (TOF in s −1) during light-off tests (increase in the reaction temperature T r) using 1% CO/ x% O 2/He gas mixtures with x⩽50. Model M1 involves a L-H elementary step between L CO species and a weakly adsorbed oxygen species (O wads). It is operative (a) whatever T r in excess CO and (b) only at low T r values in excess O 2. Model M2 involves a L-H elementary step between a L CO and a strongly adsorbed oxygen species: O sads is operative at high T r values in excess O 2. It is shown that the switch for M1 to M2, at the ignition process, during the heating stage occurs for a high CO conversion (>60%) at a specific T r value (denoted by T i) depending on the oxygen partial pressure. Similar to the observations on Pt single crystals, it is shown that the ignition process is associated with a surface-phase transformation from a Pt surface mainly covered by L CO species (denoted by Pt–CO) to a Pt surface mainly covered by O sads (denoted by Pt–O). The Pt–CO → Pt–O transformation is due to an oxidative removal of the adsorbed L CO species into CO 2 and not to a competitive chemisorption. The high CO conversion associated with the Pt–CO → Pt–O transformation indicates that mass-transfer processes contribute to the ignition process. During a cooling stage from T r> T i, the switch from M2 to M1 (extinction process) is associated with the surface-phase transformation Pt–O → Pt–CO at a reaction temperature T e< T i.

Oxidation of CO on a Pt/Al 2O 3 catalyst: from the surface elementary steps to light-off tests : IV. Kinetic study of the reduction by CO of strongly adsorbed oxygen species

Transient experiments using mass and FTIR spectroscopy as detectors are performed at 300 K with a reduced 2.9% Pt/Al 2O 3 catalyst to study the reduction of strongly adsorbed oxygen species (denoted O sads) formed by O 2 chemisorption using several y% CO/ z% Ar/He mixtures ( y and z in the range 0.5–10). During the first seconds of the reaction C mass balances reveal that the CO consumption is mainly due to the formation of a strongly adsorbed CO species identified as a linear CO species (denoted L) interacting with the O sads species (IR band at 2084 cm −1). The evolution of the CO 2 production rate with time on stream presents different profiles according to the reaction temperature: decreasing exponential at T r<273 K and peak profiles for T r⩾300 K. The CO 2 production at T r<273 K is in agreement with a kinetic model considering two elementary steps: the adsorption of the L CO species without competition with O sads followed by a L–H elementary step (denoted S3b): O sads+L→CO 2ads, with a rate constant k 3b= ν 3bexp(− E 3b/ RT) and E 3b=65 kJ/mol at θ Osads≈1. For T r>300 K, mass transfer processes contribute to the apparent CO 2 production rate. At high θ Osads values, they compete with the surface reactions for 273 K< T r<360 K and finally dominate the CO 2 production at T r>360 K. However, kinetic studies can be performed at T r>300 K after a significant decrease in θ Osads due to the increase in E 3b: E 3b=110 kJ/mol at θ Osads=0.4. Several conclusions of the present study are in very good agreement with the reduction of O sads species on Pt single crystals using a CO molecular beam under UHV conditions.

Impact of the Residual Chlorine on the Heat of Adsorption of the Linear CO Species on Cu/Al2O3 Catalysts

The adsorption of CO at several adsorption temperatures (T a ) in the range of 300-550 K on several reduced 5 wt % Cu/Al 2 O 3 solids, with and without residual chlorine, is studied by FTIR spectroscopy. Whatever the solid,the main adsorbed species is a linear (denoted by L) CO species adsorbed on Cu° sites. On the Cl-free solids, the L CO species is detected at 2095 cm - 1 , while the presence of Cl leads to a shift to higher wavenumbers. The evolution of the coverage θ L of the L CO species with the adsorption temperature for a constant CO partial pressure (P C O = 1 kPa) allows us to determine the heat of adsorption of the L CO species as a function of the coverage on the various Cu-containing solids by using the AEIR method. It is shown that the presence of residual Cl has no impact on the heat of adsorption at low coverages (82 kJ/mol), while it decreases slightly the heat of adsorption at high coverages from 57 to 45 kJ/mol.