The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

A Comparison of Similarities and Differences in the Activities of Pt/Ni(111) and Ni/Pt(111) Surfaces

Copper has traditionally been added to precipitated iron Fischer–Tropsch (FT) catalysts to facilitate reduction of Fe₂O₃ to zero valent iron during activation [M.E. Dry, in: J.R. Anderson and M. Boudart (eds), Catalysis Science and Technology, Vol. 1 (Springer-Verlag, New York, 1981) p. 179] by lowering the reduction temperature when activating with hydrogen, carbon monoxide or syngas [R.J. O'Brien et al., Catal. Today 36 (1997) 325]. This is particularly important when activating with hydrogen because metallic iron which is formed will sinter easily if the temperature is too high; however, it is not as critical when activating with carbon monoxide or syngas because iron carbides are formed and they are not as susceptible to sintering. The effect of copper on activity and selectivity has not been studied as thoroughly as its effect on catalyst activation. Kölbel reported that copper loadings less than 0.1% (weight % relative to iron) were sufficient to produce an active catalyst and that increased copper loading had no effect on FT activity [H. Kölbel and M. Ralek, Catal. Rev. Sci. Eng. 21 (1980) 225]. It has previously been shown that copper increases the activity of precipitated iron catalysts when operating at low temperature (<250°C) [2]. Bukur and Mukesh have reported that copper increases FT activity and water gas shift activity [D.B. Bukur et al., Ind. Eng. Chem. Res. 29 (1990) 194]. In addition, they reported that copper increased the average molecular weight of the product and increased hydrogenation of alkenes and isomerization of 1-alkenes. Soled et al., have reported that promotion with copper in conjunction with potassium increased FT activity but had little effect on alkene selectivity [S.L. Soled et al., Top. Catal. 2 (1995) 193]. Water gas shift activity and FT selectivity have been shown to depend on syngas conversion [A.P. Raje and B.H. Davis, Catal. Today 36 (1997) 335]. To determine the true impact of copper on FT selectivity and water gas shift activity comparisons should be done at similar conversion. No such study has been reported for the effects of copper. Herein are reported the effects of copper on FT activity and selectivity and water gas shift activity over a wide range of syngas conversions.     

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。     

The interaction of sulfur with Cu/Pt(111) and Zn/Pt(111) surfaces: copper-promoted sulfidation of platinum

The interaction of sulfur with Pt(111), Zn/Pt(111) and Cu/Pt(111) has been examined using X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and thermal desorption mass spectroscopy (TDS). At temperatures between 300 and 600 K, the exposure of Pt(111) to S₂ gas produces a chemisorbed layer of sulfurwithout the formation of bulk sulfides. Exposure of S₂ to a Zn/Pt(111) alloy, at room temperature, results in a breakdown of the alloy and formation of a zinc-sulfide film on Pt(111). Further S₂ exposure at 550 K sulfidizes the remaining metallic zincwithout affecting platinum. For the Cu/Pt(111) surface alloy, on the other hand, exposure to S₂2 at 550 K leads to sulfidation of the platinum. Platinum can effectively compete for sulfur atoms bonded to copper but not for those bonded to zinc. The reaction of S₂ gas with Cu/Pt(111) surfaces produces copper sulfides that promote the sulfidation of Pt by providing surface sites for the dissociation of S₂, and by favoring the diffusion of S into the bulk of the system.     

Surface modification and electrocatalytic properties of Pt(100), Pt(110), Pt(320) and Pt(331) electrodes with Sb towards HCOOH oxidation

Behaviors of irreversibly adsorbed Sb adatoms on Pt(100), Pt(110), Pt(320) and Pt(331) single crystal surfaces and electrocatalytic properties of the modified electrodes towards formic acid oxidation were investigated. It was determined that Sb adatoms are stable at potentials below 0.45 V (SCE) on Pt(100) and Pt(110), below 0.40 V on Pt(320), and below 0.35 V on Pt(331). Different coverage of Sb_ad was obtained conveniently by partially stripping Sb_ad from saturation coverage of Sb_ad. It has demonstrated that the redox behaviors of Sb adatoms and the coadsorption properties of Sb_ad with H_ad depend strongly on the orientations of the Pt single crystal electrode. Significant catalytic effects towards HCOOH oxidation were observed on Pt single crystal electrodes modified with Sb adatoms, which consist of (1) the inhibition of dissociative adsorption of HCOOH, (2) the enhancement of oxidation current, and (3) the negative shift of oxidation potential that was measured about 220 mV on Pt(110)/Sb, 110 mV on (110) sites of Pt(320)/Sb, and 100 mV on Pt(331)/Sb electrode. Neither enhancement of oxidation current nor negative shift of oxidation potential can be observed on Pt(100)/Sb electrode. The results suggested that electronic effect is the main effect presented on Pt(110), Pt(320) and Pt(331) surface upon Sb modification, while geometric effect is considered to the major effect on Pt(100) electrode.     

UV/HREELS spectroscopy of benzene on Pt(110)

Some years ago, LEED studies of Ogletree et al. and Wander et al. found that benzene undergoes a structural rearrangement when the benzene adsorbs on platinum. In contrast, Horsley et al. proposed that the benzene adsorbs without undergoing significant distortion. In this paper UV/HREELS spectroscopy is used to examine benzene adsorption on Pt(110). We find that the benzene can adsorb into two different states: a weakly bound form of benzene with UV/HREELS spectrum similar to gas-phase benzene, and a strongly bound form of benzene with UV/HREELS and IR/HREELS spectra which are consistent with formation of a diene (e.g., 1,4-cyclohexadiene). A comparison of our data to the results of Wander et al. and Horsley et al. shows that the weakly bound form of benzene is consistent with Horsley’s assignments, while the strongly bound species looks as expected from the work of Wander et al. The conclusion from our study then is that Wander et al. and Horsley et al. are both correct. They are just studying different forms of benzene on the platinum surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conversion of methylcyclopentane (MCP) on Pt/MoO2, Ir/MoO2 and Pt–Ir/MoO2 catalysts

The effect of the metal and reaction temperature was investigated in the conversion of MCP with hydrogen at atmospheric pressure. The highly dispersed 0.5 wt.% Pt/MoO₂, 0.5 wt.% Ir/MoO₂ and 0.25 wt.%–0.25 wt.% Pt–It/MoO₂ metal catalysts were prepared by incipient wetness impregnation or co-impregnation methods. The most active catalyst in the conversion of MCP was Pt/MoO₂ and the most selective to MCP ring opening was Ir/MoO₂. At low temperature, Ir/MoO₂ opened the MCP ring at the secondary–secondary position. High temperature promoted ring opening at the secondary–tertiary positions, which was attributed to the adlineation sites. At low temperatures, Pt/MoO₂ and Pt–Ir/MoO₂ promoted only the ring enlargement reaction while Ir/MoO₂ promoted both ring opening and ring enlargement. Ring enlargement of MCP to cyclohexane and benzene was catalysed by electron deficient adduct sites, while ring opening to 2-meythylpentane (2-MP), 3-methylpentane (3-MP) and n-hexane (n-H) was catalysed by metallic sites. At high temperatures, MCP broken into C₁–C₅ fragments and deactivation of the catalysts was observed. The Ir/MoO₂ showed the highest selectivity for cracking. The differences in selectivity were attributed to the presence of adsorbed agostic species, where the electronic environment of Ir and Pt are different.     

Spill-Over Effects on Bimetallic Pt/Ru(0001) Surfaces

The concept of spill-over of adsorbed species has a long tradition in Heterogeneous Catalysis and has been explored also for adsorption on bimetallic surfaces, in particular by the Goodman group. In the present paper, we report results of a comprehensive temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy study on spill-over effects in the adsorption and desorption of CO on structurally well defined bimetallic Pt/Ru(0001) surfaces, where part of the substrate is covered by monolayer Pt islands. While upon adsorption at 90 K, the mobility of CO_ad molecules on the surface is very limited, it is activated when the adlayer is annealed to 150 K or, more directly, if CO exposure is done at 150 K or higher temperatures. This enables diffusion of CO_ad molecules to the Pt free Ru(0001) areas, even at local CO_ad coverages which preclude further adsorption from the gas phase on the Ru parts of the surface. Spill-over processes are shown to have significant impact on the TPD spectra; furthermore they provide an additional adsorption channel for adsorption on the bare Ru(0001) areas, allowing uptake of CO at local coverages where adsorption from the gas phase is precluded. This indicates that the apparent CO saturation coverage of 0.68 ML determined for direct adsorption on Ru(0001) under UHV conditions is limited by kinetics rather than thermodynamics. The data are discussed in comparison with results and interpretations in earlier studies, which indicate that these effects are not limited to the Pt/Ru(0001) surface, but may be found on a wide range of bimetallic systems.     

n-butane hydrogenolysis on planar and faceted Pt/W(111) surfaces

The Relationship between surface structure and reactivity is investigated by means ofn-butane hydrogenolysis, a known structure sensitive reaction, for planar and faceted Pt/ W(111) surfaces. The W(111) surface reconstructs to form pyramidal facets with [211] orientation upon vapor deposition of Pt (>1.3 ML) and annealing above 750 K. The hydrogenolysis kinetics over the planar and the faceted surface are found to be quite different. The planar surface has a higher selectivity towards ethane formation and a higher reaction rate. The apparent activation energies are found to be 33 ± 4 kJ/mol for the planar surface and 76 ± 6 kJ/mol for a surface covered with 20 nm facets. There appears to be a correlation between the concentration of fourfold coordination (C₄) sites on the surface and the amount of ethane produced. The C₄ concentration is altered by changing the facet size (annealing temperature). The results indicate the presence of a different intermediate on the C₄ sites as evidenced by the differences in the apparent activation energy, the reaction rate and the overall selectivity.     

On the interpretation of XPS spectra of metal (Pt,Pt–Sn) nanoparticle/graphene systems

This Letter discusses the X-ray photoelectron spectroscopy (XPS) results of an article on Pt and Pt–Sn nanoparticles dispersed on graphene nanosheets. I argue that the authors’ interpretation is largely unwarranted because it neglects the spin–orbit multiplicity and the branching ratio of XPS signals arising from electron levels with l > 0, and the fact that XPS peaks of given electron levels and given chemical species possess precise full with at half maximum (FWHM) values, and binding energy (BE) values. I suggest an interpretation which offers a more accurate insight into the chemical composition and the catalytic properties of these nanoparticle systems.     

Oxidation of Pt(II) to Pt(IV) complex with hydrogen peroxide in glycols

The reaction of cis, cis-[Pt^II(9-fm)(dmpda)] (9-fm=9-fluorenylidenemalonate, dmpda=2,2-dimethyl-1,3-propanediamine) with hydrogen peroxide in ethyleneglycol produces cis,trans,cis-[Pt^IV(9-fm)(OCH₂CH₂OH)(OH)(dmpda)]. Its crystal structure shows that the local geometry around the platinum atom approximates a typical octahedral arrangement with the added OCH₂CH₂OH/OH ligands in trans coordination sites. The molecules are packed in a two-dimensional assembly via van der Waals interactions, where the hydrophobic fluorenyl groups are arranged on both faces of the assembly plate whereas the hydrophilic groups fill up the inner part of the plate. The compound bearing both “inorganic OH” and “organic OH” is a potential precursor for further various functionalizations.     

Measurement of the Metal Surface Acidity/Electronegativity of Pt(110)

In previous work, it has been found that a hydrogen-covered Pt(110) surface is acidic, but quantification of the acidity has not yet been done. In this paper a spectroscopic method is developed to measure the acidity of a metal surface for the first time. The technique involves measuring the intensity of the N–H stretch from the C₅H₄XNH⁺ that forms when hydrogen coadsorbs with pyridine, 2-fluoropyridine and 3-fluoropyridine. The Bethe approximation is then used to estimate the metal surface acidity/electronegativity (MSAEL). The proton affinity/MSAEL of Pt(110) has been determined to be 907 ± 4 kJ/mol at high coverage. This is the first time the MSAEL has been measured on a metal surface. Implications for fuel cell catalysis are discussed.     

Observation of anomalous reactivities of Ni/Pt(111) bimetallic surfaces

We have investigated the surface reactivities of Ni/Pt(111) bimetallic model catalysts using ethylene and cyclohexene as probing molecules. The bimetallic surfaces were generated by evaporating Ni onto a Pt(111) single- crystal surface held at 600 K. The surface chemistry was investigated using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). The reactivities of the bimetallic surfaces were compared with those of the clean Pt(111) surface and a thick Ni(111) film on the Pt(111) substrate. Formation of the bimetallic surface led to a significantly reduced reactivity towards the decomposition of ethylene when compared to either Pt(111) or Ni(111)/Pt(111) surfaces. Furthermore, although the surface reactivity towards cyclohexene was retained for the bimetallic surface, the decomposition mechanism was distinctly altered from that of either Pt(111) or Ni(111)/Pt(111) surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ethylene dimerization over a model Sn/Pt(111) catalyst

The dimerization reaction of ethylene was studied over Pt(111) and (3×3)R30°-Sn/ Pt(111) model catalysts at moderate pressures (20–100 Torr). The catalyst surfaces were prepared and characterized in a UHV surface analysis system and moderate pressure catalytic reactions were conducted with an attached batch reactor. The overall catalytic activity of the (3×3)R30°-Sn/Pt(111) surface alloy for C₄ products was slightly higher than that at Pt(111). In addition to the dimerization reaction, hydrogenolysis of ethylene to propane and methane was also observed, with the (3×3)R30°-Sn/Pt(111) surface alloy less active than Pt(111). Among the C₄ products, butenes andn-butane were the major components. Carbon buildup was observed to be significant above 500 K with the (3×3)R30°-Sn/Pt(111) surface alloy much more resistant than Pt(111). The dimerization of ethylene was not eliminated by the presence of surface carbonaceous deposits and even at significant surface coverages of carbon the model catalysts exhibited significant activities. The results are discussed in terms of the surface chemistry of ethylene and the previously reported catalytic reactions of acetylene trimerization andn-butane hydrogenolysis at these surfaces.     

催化剂Pt／Mg（Al）O的制备与特性

用共沉淀法制备得一种类似于水滑石的载体Ｍｇ（Ａｌ）Ｏ，再用浸渍法制得载于这种载体上的铂催化剂Ｐｔ／Ｍｇ（Ａｌ）Ｏ。该载体的比表面积为２４４ｍ２ｇ－１，该催化剂铂分散度为（Ｈ／Ｐｔ）ｉｒｒ＝０．９，平均直径ｄ＝１．３８ｎｍ，且有均匀的颗粒度。Ｐｔ－Ｍｇ（Ａｌ）Ｏ间的相互作用比Ｐｔ－Ａｌ２Ｏ３间的强。由于电子从载体Ｍｇ（Ａｌ）Ｏ中的碱性（氧）离子大量转移至细小的Ｐｔ颗粒上，故该催化剂中的Ｐｔ颗粒具有很强的负电性，表现了优异的催化性质     

Selective Ring-Opening of Methylcyclopentane Over Titania-Supported Monometallic (Pt, Ir) and Bimetallic (Pt–Ir) Catalysts

An investigation of the selective ring-opening of methylcyclopentane (MCP) was conducted for the first time on Pt/TiO₂, Ir/TiO₂ and Pt?CIr/TiO₂ catalysts with low amounts of noble metals (0.5?wt%) over a temperature range of 180?C400?°C under hydrogen at atmospheric pressure. The catalysts were prepared by impregnation or co-impregnation and characterized by different physico-chemical techniques, including SEM, XRD, H₂-TPR, N₂-sorption, TEM and elemental analysis. The metallic particles were highly dispersed on the TiO₂ support at isodispersion of ~1?nm. The particles exhibited icosahedral Mackay structures limited by (111) planes. The catalytic results show that the activity in the MCP was strongly influenced by the intrinsic nature of the metal and by the temperature. The most active catalyst was Ir/TiO₂. The order of the reactivity as a function of the temperature and total conversion was Ir/TiO₂ 180?°C (???=?2?%)?>?Pt?CIr/TiO₂ 220?°C (???=?27.8?%)?>?Pt/TiO₂ 260?°C (???=?9.9?%). Under these conditions, all of the catalysts exhibited the ability to open the ring of MCP with an atom economy, without unwanted products of cracking and ring-enlargement reactions. The synergy between Pt?CIr bimetallic particles was assessed by the total conversion of MCP, whereas the ring-opening results indicated that the reaction took place on Ir sites. These results suggest that the bimetallic catalyst contained separate entities of two metals. Increased reaction temperatures led to reduced reaction selectivity with respect to ring-opening of MCP versus the cracking side reaction.     

Formic acid oxidation on Pd-modified Pt(100) and Pt(111) electrodes: A DEMS study

Formic acid oxidation on palladium submonolayers on well-defined Pt(100) and Pt(111) electrodes has been studied using voltammetry and Differential Electrochemical Mass Spectrometry (DEMS). A combination of the two techniques allows a better understanding of the reaction taking place on the electrode surface. Thus, an exact correlation between the CO₂ mass signal and the current density in the voltammogram corresponding to the formic acid oxidation has been obtained. On palladium modified Pt(100) electrodes and in the potential region below 0.3 V, the currents in the positive scan are higher than those recorded in the negative scan. This diminution on the signal in the negative scan has been associated with CO₂ reduction to CO on the palladium adlayer. In addition, the CO₂ reduction reaction seems to take place on the border of the palladium islands. Finally, the adsorption of (bi)sulfate anions has an inhibiting behavior on the formic acid oxidation reaction.     

Correlating Low-temperature Hydrogenation Activity of Co/Pt(111) Bimetallic Surfaces to Supported Co/Pt/γ-Al2O3 Catalysts

The low-temperature self-hydrogenation (disproportionation) of cyclohexene was used as a probe reaction to correlate the reactivity of Co/Pt(111) bimetallic surfaces with supported Co/Pt/γ-Al₂O₃ catalysts. Temperature-programmed desorption (TPD) experiments show that cyclohexene undergoes self-hydrogenation on the ~1 ML Co/Pt(111) surface at ~219 K, which does not occur on either pure Pt(111) or a thick Co film on Pt(111). Supported catalysts with a 1:1 atomic ratio of Co:Pt were synthesized on a high surface area γ-Al₂O₃ to verify the bimetallic effect on the self-hydrogenation of cyclohexene. EXAFS experiments confirmed the presence of Co–Pt bonds in the catalyst. Using FTIR in a batch reactor configuration, the bimetallic catalyst showed a higher activity toward the self-hydrogenation of cyclohexene at room temperature than either Pt/γ-Al₂O₃ or Co/γ-Al₂O₃ catalysts. The comparison of Co/Pt(111) and Co/Pt/γ-Al₂O₃ provided an excellent example of correlating the self-hydrogenation activity of cyclohexene on bimetallic model surfaces and supported catalysts.     

Pt/Au(111)表面合金电化学稳定性的第一性原理研究

利用密度泛函理论(DFT)对Pt/Au (111)表面合金的电化学稳定性进行了初步研究。形成能计算结果表明,Au与Pt不易在块体中形成合金,但能在Pt (111)面形成表面合金。溶解电位计算结果进一步表明,Pt/Au (111)面上Pt原子的溶解电位与其第一近邻Au原子数有很好的线性关系,而Au对第二近邻及更远近邻的Pt溶解电位的影响可忽略。这些结果意味着可建立表面配位环境与表面原子溶解电位间的标度关系,为揭示表面合金的"结构-电化学稳定性"构型关系奠定了基础。