Poisoning of Pt/C catalysts by CO and its consequences over the kinetics of hydrogen chemisorption

The initial rate of hydrogen dissociation was studied as a function of irreversible CO coverage at 353 K on 30 wt.% Pt/carbon catalysts (Pt/C) prepared according to different processes. The Pt/C catalysts exhibit similar Pt dispersion (D  0.07) and mean Pt particles size (dp  16 nm). The turnover frequency (number of hydrogen molecules dissociated per CO-free surface Pt atom) was determined as a function of CO coverage from 0.0 to 0.8. The evolution of TOF as a function of CO coverage is in agreement with the model of CO adsorbing on low coordination sites (edges, corners) and then spreading across the faces to grow islands as Brandt suggested in the past (R.K. Brandt, M.R. Hughes, L.P. Bourget, K. Truszkowska, R.G. Greenler, Surf. Sci. 286 (1993) 15–25). At high CO coverage (0.8), TOF depends on the process by which the Pt/C catalyst was prepared. In particular, a Pt/C elaborated according to a colloidal process exhibits a low sensitivity to CO poisoning with an increase of TOF by one order of magnitude.     

Oscillatory CO Oxidation Over Pt/Al2O3 Catalysts Studied by In situ XAS and DRIFTS

Fresh and mildly aged Pt/Al₂O₃ model diesel oxidation catalysts with small and large noble metal particle size have been studied during CO oxidation under lean burn reaction conditions to gain more insight into the structure and oscillatory reaction behaviour. The catalytic performance, CO adsorption characteristics using in situ DRIFTS and oxidation state using in situ XAS were correlated. Stable and pronounced oscillations only occurred over the catalyst with smaller particle sizes. Characteristic for this catalyst are low-coordinated surface Pt sites (more corner and edge atoms) which seem to become oxidized at elevated temperature as evidenced by in situ DRIFTS and in situ XAS. In situ XAS further uncovered that the oxidation of the Pt surface starts from the end of the catalyst bed and the oxidation state oscillates like the catalytic activity.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.     

Pt-Fe催化剂次表层Fe结构表面的CO氧化反应性能

利用紫外光电子能谱等表面科学方法,研究了Pt-Fe模型催化剂的次表层Fe结构[即Pt/Fe/Pt(111)结构]在不同条件下CO的吸附及其氧化反应。结果表明,Pt/Fe/Pt(111)结构在H_2气氛或者超高真空中是种稳定结构,最外层的原子与Pt(111)相同,是密排的铂原子面;但次表层的原子中有约0.5单层的铁原子,使费米边附近(0~2.0)e V的电子态密度明显低于Pt(111)表面,从而改变表面的CO和O_2吸附以及反应性能。程序升温的紫外光电子能谱结果显示,Pt/Fe/Pt(111)表面在(100~300)K,CO的吸附受温度的影响不明显,且O_2能够吸附、活化并使共吸附的CO发生氧化反应;当温度为300 K时,O_2无法在Pt/Fe/Pt(111)表面吸附、活化,所以CO氧化反应无法进行。Pt/Fe/Pt(111)结构虽然能有效地减弱CO的吸附从而避免CO毒化的问题,但O_2的吸附和活化也受到显著抑制并影响到一定条件下CO的氧化反应。     

On the activation of Pt/Al2O3 catalysts in HC-SCR by sintering: determination of redox-active sites using Multitrack

A highly dispersed Pt/Al₂O₃ catalyst was used for the selective catalytic reduction of NO_x using propene (HC-SCR). Contact with the reaction gas mixture led to a significant activation of the catalyst at temperatures above 523 K. According to CO chemisorption data and HRTEM analysis, Pt particles on the activated catalyst had sintered. The redox behavior of the fresh and sintered catalysts was investigated using Multitrack, a TAP-like pulse reactor. If Pt particles on the catalyst are highly dispersed (average size below 2 nm), only a small part (10%) of the total number of Pt surface sites as determined by CO chemisorption (Pt_surf) participates in H₂/O₂ redox cycles (Pt_surf,redox) in Multitrack conditions. For a sintered catalyst, with an average particle size of 2.7 nm, the number of Pt_surf and Pt_surf,redox sites are in good agreement. Similar results were obtained for both catalysts using NO as the oxidant. The low number of Pt_surf,redox sites on highly dispersed Pt/Al₂O₃ is explained by the presence of a kinetically more stable—probably ionic—form of Pt---O bonds on all surface sites of the smaller Pt particles, including corner, edge and terrace sites. When the average particle size shifts to 2.7 nm, the kinetic stability of all Pt---O bonds is collectively decreased, enabling the participation of all Pt surface sites in the redox cycles.

A linear correlation between the NO_x conversion in HC-SCR, and the amount of Pt_surf,redox was found. This suggests that redox-active Pt sites are necessary for catalytic activity. In addition, the correlation could be significantly improved by assuming that Pt_surf,terrace sites of the particles larger than 2.7 nm are mainly responsible for HC-SCR activity in steady state conditions. Implications of these results for the pathway of HC-SCR over Pt catalysts are discussed.     

Kinetics study of hydrogen adsorption over Pt/MoO3

The rate controlling step and the energy barrier involved in the hydrogen adsorption over Pt/MoO₃ were studied. Rates of hydrogen adsorption on Pt/MoO₃ were measured at the adsorption temperature range of 323–573 K and at the initial hydrogen pressure of 6.7 kPa. The rate of hydrogen uptake was very high for the initial few minutes for adsorption at and above 473 K, and reached equilibrium within 2 h. At and below 423 K, the hydrogen uptake still continued and did not reach equilibrium after 10 h. The hydrogen uptake exceeded the H/Pt ratio of unity for adsorption at and above 423 K, indicating that hydrogen adsorption involves hydrogen atom spillover and surface diffusion of the spiltover hydrogen atom over the bulk surface of MoO₃ followed by formation of H_xMoO₃. The hydrogen uptake was scarcely appreciable for Pt-free MoO₃. The rate controlling step of the hydrogen adsorption on Pt/MoO₃ was the surface diffusion of the spiltover hydrogen with the activation energy of 83.1 kJ/mol. The isosteric heats of hydrogen adsorption on Pt/MoO₃ were 18.1–16.9 kJ/mol for the hydrogen uptake range 2.4–2.8 × 10¹⁹ H-atom/g-cat. Similarities and differences in hydrogen adsorption on Pt/SO₄^2?–ZrO₂, Pt/WO₃–ZrO₂ and Pt/MoO₃ catalysts are discussed.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation

We have investigated a simple approach for the deposition of platinum (Pt) nanoparticles onto surfaces of graphite oxide (GO) nanosheets with particle size in the range of 1-5 nm by ethylene glycol reduction. During Pt deposition, a majority of oxygenated functional groups on GO was removed, which resulted in a Pt/chemically converted graphene (Pt/CCG) hybrid. The electrochemically active surface areas of Pt/CCG and a comparative sample of Pt/multi-walled carbon nanotubes (Pt/MWCNT) are 36.27 and 33.43 m²/g, respectively. The Pt/CCG hybrid shows better tolerance to CO for electro-oxidation of methanol compared to the Pt/MWCNT catalyst. Our study demonstrates that CCG can be an alternative two-dimensional support for Pt in direct methanol fuel cells.     

MnOx对Pt/CeZrO2催化剂在水汽和CO2环境下CO氧化的影响

以CeZrO₂固溶体为载体,发现MnO_x的添加能促进Pt/CeZrO₂催化剂的CO氧化性能,并研究了MnO_x含量对催化剂CO氧化活性及抗H₂O和CO₂性能的影响。结果表明,随着MnO_x含量增加,催化剂活性呈现先升高后降低的趋势,在MnO_x含量为0.5%（质量分数）时活性最佳。MnO_x的添加降低了Pt颗粒尺寸并影响催化剂还原性能从而促进反应活性。水汽和CO₂对Pt/CeZrO₂催化剂的CO氧化活性有抑制作用,而MnO_x的加入能显著提高催化剂的抗水汽和CO₂的能力。反应动力学结果表明,在Pt/CeZrO₂催化剂上,反应气中引入H₂O和CO₂后,CO的反应级数有明显升高,说明H₂O和CO₂在催化剂表面与CO竞争吸附,导致CO反应活性下降;而在Pt/MnO_x/CeZrO₂催化剂上,CO的反应级数略有升高,说明MnO_x的添加能有效抑制H₂O和CO₂与CO的竞争吸附,从而改善了催化剂的抗H₂O和CO₂性能。     

Adsorption/oxidation of CO on highly dispersed Pt catalyst studied by combined electrochemical and ATR-FTIRAS methods: Part 1. ATR-FTIRAS spectra of CO adsorbed on highly dispersed Pt catalyst on carbon black and carbon un-supported Pt black

Elecrochemical ATR-FTIRAS measurements were conducted for the first time to investigate nature of CO adsorbed under potential control on a highly dispersed Pt catalyst with average particle size of 2.6 nm supported on carbon black (Pt/C) and carbon un-supported Pt black catalyst (Pt-B). Each catalyst was uniformly dispersed by 10 μg Pt/cm² and fixed by Nafion^® film of 0.05 μm thick on a gold film chemically deposited on a Si ATR prism window. Adsorption of CO was conducted at 0.05 V on the catalysts in 1 and 100% CO atmospheres, for which CO coverage, θ_CO, was 0.69 and 1, respectively. Two well-defined ν(CO) bands free from band anomalies assigned to atop CO (CO(L)) and symmetrically bridge bonded CO (CO(B)_sym.) were observed. It was newly found that the CO(L) band was spitted into two well-defined peaks, particularly in 1% CO, from very early stage of adsorption, which was interpreted in terms of simultaneous occupation of terrace and step-edge sites, denoted as CO(L)_terrace and CO(L)_edge, respectively. This simultaneous occupation was commonly observed in our work both on Pt/C and Pt-B. A new band was also observed around 1950 cm⁻¹ in addition to the bands of CO(L) and CO(B)_sym., which was assigned to asymmetric bridge CO, CO(B)_asym., adsorbed on (1 0 0) terraces, based on our previous ECSTM observation of CO adsorption structures on (1 0 0) facet. The CO(B)_asym. on the Pt/C, particularly in 100% CO atmosphere, results in growth of a sharp band at 3650 cm⁻¹ accompanied by a concomitant development of a band around 3500 cm⁻¹. The former and the latter are assigned to ν(OH) vibrations of non-hydrogen bonded and hydrogen bonded water molecules adsorbed on Pt, respectively, interpreted in term of results from a bond scission of the existing hydrogen bonded networks by CO(L)s and from a promotion of new hydrogen bonding among water molecules presumably by CO(B)_asym..It was found that the frequency ν(CO) of CO(L) both on Pt/C and Pt-B is lower than that on bulky polycrystalline electrode Pt(poly) or different crystal planes of Pt single-crystal electrodes by 30-40 cm⁻¹ at corresponding potentials, which implies a stronger electronic interaction between CO and Pt nano-particles and/or an increased contribution of step-edge sites on the particles. Determination of the band intensities of CO(L), CO(B)_asym. and CO(B)_sym. has led us to conclude a much higher bridged occupation of sites at Pt nano-particles than Pt(poly) electrodes.     

Ethylene adsorption on Pt/Au/SiO2 catalysts

Ethylene adsorption on a Pt/Au/SiO₂ catalyst (2 wt% Pt; Au/Pt atomic ratio of 10) was studied using adsorption microcalorimetry and FTIR spectroscopy. Ethylene adsorption at 300 K on Pt/Au/SiO₂ produced π‐bonded, di‐σ‐bonded, and ethylidyne species with an initial heat of 140 kJ/mol, compared to a heat of 157 kJ/mol for Pt/SiO₂ on which only ethylidyne species formed. At 203 and 263 K, ethylene adsorbed on Pt as well as on Au surface atoms for the Pt/Au/SiO₂ catalyst. Quantum chemical, DFT calculations indicate that Au exerts a significantly smaller electronic effect on Pt than does addition of Sn to Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Increased Metal Utilization in Carbon‐Supported Pt Catalysts by Adsorption of Preformed Pt Nanoparticles on Colloidal Silica

Pt/C electrocatalysts, aimed at maximizing the electrochemical surface area (ECSA) and consequently the specific mass activity of fuel cell reactions, are obtained by firstly depositing Pt nanoparticles on colloidal silica (Pt‐silica), followed by the adsorption of the latter onto a carbon support. This method of catalyst preparation increases Pt metal utilization and generates accessible void space in the interpenetrating particle network of carbon and silica for the facile transport of reactants and products. Both electrochemical hydrogen adsorption/desorption and CO oxidation measurements show an increase in the ECSA using this approach. Methanol electrooxidation is used as a test reaction to evaluate the catalytic activity. It is found that the silica modified catalyst is three times as active as a catalyst prepared without silica, under otherwise identical conditions.     

In situ hydrogen generation from cycloalkanes using a Pt/CNF catalyst

Pt catalyst supported on carbon nanofibers (CNFs) has been prepared via ion-exchange and it was characterized by XRD, TEM, N₂ physisorption and CO chemisorption. The Pt/CNF catalyst has a small Pt crystallite size in the range of 2–3 nm. This catalyst has been tested in the dehydrogenation of decalin, which is a cycloalkane proposed in the literature as H₂ storage media for vehicles and portable devices. The objective is finding a Pt catalyst suitable for in situ generation of H₂ from chemical storage in decalin. The results revealed that Pt supported on CNF outperforms a Pt catalyst supported on micro–mesoporous activated carbon. Finally, we propose a reactor configuration aiming at the intensification of H₂ production in continuous.     

Effect of sintering on the catalytic activity of a Pt based catalyst for CO oxidation: Experiments and modeling

The goal of this paper was to make the link between sintering of a 1.6% Pt/Al₂O₃ catalyst and its activity for CO oxidation reaction. Thermal aging of this catalyst for different durations ranging from 15 min to 16 h, at 600 and 700 °C, under 7% O₂, led to a shift of the platinum particle size distributions towards larger diameters, due to sintering. These distributions were studied by transmission electron microscopy. The number and the surface average diameters of platinum particles increase from 1.3 to 8.9 nm and 2.1 to 12.8 nm, respectively, after 16 h aging at 600 °C. The catalytic activity for CO oxidation under different CO and O₂ inlet concentrations decreases after aging the catalyst. The light-off temperature increased by 48 °C when the catalyst was aged for 16 h at 600 °C. The CO oxidation reaction is structure sensitive with a catalytic activity increasing with the platinum particle size. To account for this size effect, two intrinsic kinetic constants, related either to platinum atoms on planar faces or atoms on edges and corners were defined. A platinum site located on a planar face was found to be 2.5 more active than a platinum site on edges or corners, whatever the temperature. The global kinetic law {r (mol m⁻² s⁻¹) = 103 × exp(−64,500/RT)[O₂]^0.74[CO]^−0.5)} related to a reaction occurring on a platinum atom located on planar faces allows a simulation of the CO conversion curves during a temperature ramp. Modeling of the catalytic CO conversion during a temperature ramp, using the different aged catalysts, allows prediction of the CO conversion curves over a wide range of experimental conditions.     

Pt Skin Versus Pt Skeleton Structures of Pt3Sc as Electrocatalysts for Oxygen Reduction

In order for low temperature polymer electrolyte membrane fuel cells to become economically viable Pt catalyst loading must be significantly reduced. The cathode of the polymer electrolyte membrane fuel cell, where oxygen reduction takes place, is responsible for the main activity loss. The development of new materials for this reaction is essential in order to increase the overall effeciency of the fuel cell. Herein, we study the effect of ultra high vacuum annealing on the structure and activity of polycrystalline Pt₃Sc. Upon annealing in ultra high vacuum a Pt overlayer is formed on the polycrystalline Pt₃Sc. The reactivity of the overlayer is probed by temperature programmed desorption of CO. The onset of CO desorption is around 130 K lower on UHV annealed Pt₃Sc than on Pt(111) and the temperature of the desorption peak maximum at saturation was ～50 K lower on UHV annealed Pt₃Sc, relative to Pt(111), consistent with the CO adsorption energies calculated using density functional theory calculations. Exposing the annealed Pt₃Sc sample to 200 mbar O₂ at room temperature results in ～14 % Sc oxide as measured by X-ray photoelectron spectroscopy. Electrochemical testing of the oxygen reduction reaction shows the same activity as sputter cleaned Pt₃Sc.     

Preparation and characterization of Pt/HMFI–SBA-15 hybrid catalyst for tetralin transformation

A Pt catalyst supported on a hybrid material, HMFI–SBA-15, was prepared. Both, support and catalyst (Pt/HMFI–SBA-15) were characterized by nitrogen physisorption, small and wide (2θ) angle XRD patterns, FT-IR, SEM and HRTEM. The acidic properties of the hybrid material were studied by cumene dealkylation and those of the catalyst were studied by FT-IR of adsorbed pyridine. The catalyst, Pt/HMFI–SBA-15, was tested for tetralin transformation at various reaction temperatures 498, 523, 548, 573, 585 and 598 K. Wide-angle XRD and FT-IR in the skeletal region indicate the presence of MFI zeolite fragments incorporated onto SBA-15. The characterization of the acid sites on the support by cumene dealkylation and FT-IR pyridine adsorption revealed the presence of Brönsted acid sites related to the HMFI zeolite fragments in the hybrid materials. For the catalyst, a homogeneous distribution of Pt clusters was found by HRTEM. In the transformation of tetralin, at all the reaction temperatures, the main products were trans + cis-decalins. However, at high reaction temperature ring contraction to spirodecane and dehydrogenation to naphthalene were observed. At 598 K, a maximum of 8% of ring contraction products was obtained.     

Sintering Behavior of Pt Metal Particles Supported on Silica-Coated Alumina Surface

The sintering behavior of Pt metal particles was studied by supporting them on silica-coated alumina. Silica coating was found to be effective for the retention of a large surface area of alumina even after calcination at elevated temperatures. Before sintering, the size of Pt metal particles on all the silica-coated aluminas, including the uncoated alumina, was identical, while the particle size was larger on silica than on alumina. After sintering the Pt catalyst at 1073 K, the particle size increased on uncoated alumina as well as on alumina coated with thicker silica layers, especially on the supports previously calcined at >1473 K. On the other hand, the size of Pt metal particles did not increase much on alumina coated with monolayer silica. The observed suppression of sintering of Pt metal particles resulted from the retention of a large surface area of alumina with a thinner silica layer. In the case of a thicker silica layer, although a large surface area was maintained after calcination at elevated temperatures, the existence of a bulk silica-like property of the support did not favor the suppression of sintering of Pt metal particles.     

Subsurface Oxygen on Pt(111) and Its Reactivity for CO Oxidation

Abstract  

Temperature has an important effect on the dissociative adsorption of molecular oxygen on platinum surfaces. Here, we show that if the substrate temperature is increased to 400–600 K, the total amount of oxygen loaded onto Pt(111) can be more than twice the well-established maximum coverage of 0.25 ML. While low energy electron diffraction and STM reveal a conventional p(2 × 2) structure of the topmost layer, temperature programmed desorption measurements indicate that additional oxygen is stored under the surface of platinum. Reactivity measurements show that this sub-surface oxygen layer does not lower the activity of such platinum surface towards CO oxidation. Therefore, while sub-surface oxygen layer does form under catalytically relevant temperatures on Pt(111), it has no great influence on the oxidizing ability of such surface. This sheds new light on the initial stages of platinum oxide formation, and may help bridge the understanding of catalytic oxidation of CO on Pt in ultra high vacuum and in high-pressure catalysis studies.     

Characterization of Pt/L-zeolite catalysts by chemisorption, EXAFS and reaction of neopentane with H2

We have prepared four series of Pt/L-zeolite Catalysts that were calcined at six different temperatures and subsequently reduced at the same temperature (773K). Two series were prepared by ion exchange and two by impregnation. In all cases the dispersion (H/Pt or CO/Pt) was a maximum after calcination at 543–643 K. The Catalysts were further characterized by EXAFS and reaction of neopentane with H₂. There is a good correlation between the particle size determined by chemisorption and the Pt first shell coordination number determined by EXAFS. The neopentane isomerization selectivity ratio was observed to be a linear function of particle size measured by chemisorption or EXAFS coordination number. In addition, enhanced isomerization and hydrogenolysis activities are found to correlate with an increase in the support acidity.     

Infrared Study of Benzene Hydrogenation on Pt/SiO2 Catalyst by Co-adsorption of CO and Benzene

FT-IR spectra of the co-adsorption of benzene and CO have been performed to identify the preferred adsorption sites of hydrogen and benzene on a Pt/SiO₂ catalyst for hydrogenation of benzene. Results of CO adsorbed on atop sites on Pt/SiO₂ includes: an α peak at 2091 cm⁻¹, a β peak at 2080 cm⁻¹ and a γ peak at 2067 cm⁻¹ indicating three kinds of adsorption sites for dissociative hydrogen on Pt/SiO₂. The site of lowest CO stretching frequency offers stronger adsorbates–metal interaction for benzene and hydrogen. Hydrogen binding on the site of lowest CO stretching frequency before benzene adsorption significantly enhances the reaction rate of benzene hydrogenation.