Single Crystal Catalysts Research Articles

Sulfur adsorbed on Pt catalyst: its chemical state and effect on catalytic properties as studied by electron spectroscopy and n-hexane test reactions

Pt black samples in a fresh state, deactivated by coking and treated with H 2S were studied with electron spectroscopy. Their catalytic properties were tested in skeletal reactions of n-hexane. The Pt 4f XPS peaks show that Pt is predominantly in the metallic state after the treatments. After sulfidation, a minor amount of PtS can be detected in the difference spectrum (Pt unsulfided - Pt sulfided) only. The growth of the O 1s intensity after sulfidation can partly be attributed to the sulfate component. C 1s shows, as a rule, residual carbon in low amounts on sulfided samples. The S 2p band shows 4–8 at.-% S with respect to the total surface in two valence states: sulfate and sulfide, the latter including also minor amounts of organic sulfur which could arise from the sulfidation of hydrocarbonaceous residues. UPS demonstrated chemical Pt—S interaction even after O 2/H 2 regeneration when the Fermi-edge Pt intensity rose to a height almost equal to that of a regenerated sulfur-free Pt. ISS shows that sulfur — as opposed to carbon and oxygen — is a surface component. Sputtering effect of He + ions used in ISS can remove sulfate while a large part of sulfide is removed during O 2/H 2 regeneration and/or during n-hexane test runs. Both sulfidation and carbonization strongly decrease the overall catalytic activity. The selectivity of Pt with S present mainly as sulfate was similar to that of carbonized Pt, mainly producing hexenes. Pt mainly containing sulfide, produced initially mainly methylcyclopentane, as reported also for the sulfided single crystal catalyst. Carbonized Pt could be fully regenerated by O 2/H 2 treatment at 600 K but the same treatment — even repeated — restored the overall activity of a sulfided Pt only partially. We believe that sulfide acts mainly as a bonding modifier whereas sulfate can be a structural modifier influencing the selectivity similarly as coke or coke precursors.

Influence of acoustic wave excitation on CO oxidation over a Pt{110} single crystal

A unique ultra-high vacuum (UHV) compatible excitation system combined with an advanced ultra-high amplitude and frequency resolution acoustic spectrometer has been designed and constructed to permit accurate studies of the fundamental mechanism by which acoustic excitation influences heterogeneous catalytic reactions. A clean Pt{110} thin film single-crystal catalyst was excited with low-energy acoustic waves (Rayleigh waves) under high vacuum and UHV conditions to increase the reaction rate for carbon monoxide oxidation. A remarkable six-fold increase in the chemical activity was observed. By using a new, very accurate method to monitor the sample temperature using high-resolution acoustic wave resonance spectroscopy (HRAWRS), a non-thermal acoustic-wave-induced enhancement of the reaction rate is clearly demonstrated. The pressure and temperature dependences of the enhancement provide some insight into the mechanism by which acoustic waves enhance catalytic reactions on solid surfaces.

Direct conversion of methanol to formaldehyde in the absence of oxygen on Cu(210)

In the previous literature, people have prepared supported copper catalysts which were modestly active for the direct conversion of methanol to formaldehyde in the absence of oxygen. However, single crystal catalysts have shown much less activity. In this paper, the direct conversion of methanol to formaldehyde was attempted using a very basic copper surface, Cu(210). TPD measurements show that when one adsorbs methanol onto a clean, 100 K, Cu(210) sample, about half of the methanol desorbs intact and about half of the methanol reacts to form formaldehyde and hydrogen: CH 3OH → H 2CO + H 2. The formaldehyde desorbs in a peak centered at 390 K. Some of the hydrogen desorbs in a broad peak between 150 and 350 K, while the remainder desorbs in a peak centered at 390 K. These results show that the Cu(210) surface is special in that it is the only face of copper examined so far that is able to convert a significant amount of methanol to formaldehyde in the absence of oxygen.

Oxidation of CO on PdCu(110) single crystal alloy catalyst: steady state, hysteresis and related surface phenomena

CO oxidation was studied on Pd-rich surfaces of a PdCu (110) single crystal alloy. The top layer contained initially ~100% Pd atoms. The techniques used were Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. The experiments were performed at constant CO and O 2 pressures while varying the temperature of the sample up to 575 K. Measurement of the rate of CO 2 production versus the temperature enabled the detection of regions of bistability and hystereses to be recorded. AES, with surface region analysis of ~ 4 layers, enabled the recording of an increase in the Cu Pd ratio from ⩽ 0.1 to ~ 0.7 after CO-oxidation experiments. This occurred due to the segregation of Cu from the bulk mostly into the three subsurface layers beneath the top layer. For this reason the type of lattice changed from fcc to bcc in the surface region. The (1 × 1) clean surface structure changed to c(2 × 2) after being exposed to intensive CO-oxidation treatment. Recording the CO- TDS after each measurement provided valuable information about the mechanism which controls the adsorption and desorption on the surface. The relative CO coverage versus the doses of CO demonstrated that after each CO-oxidation experiment less CO was able to adsorb on the surface. The results together fulfil one of the requirements of a system that could display surface kinetic oscillations.

Pattern formation during the electrodeposition of a silver-antimony alloy

It is described how during the electrodeposition of a silver-antimony alloy patterns can develop in the deposit. The patterns occur as moving bands, target patterns, and spiral waves, and the characterized using various types of microscopy. Forthe patterns to develop, the hydrodynamic conditions close to the electrode appear to be of prime importance. The wave characteristics are discussed in relation with recent studies on patterns on (single crystal) catalyst surface and convection-induced chemical instabilities.

Catalysis: from single crystals to the “real world”

The “pressure and material gaps” separating UHV and technical catalytic investigations have been bridged in recent years by combining in a single apparatus the ability to measure kinetics at elevated pressures on single-crystal catalysts with the capabilities to carry out surface analytical measurements. In these high-pressure/surface analytical studies a well-defined, single-crystal plane is used to model a site or set of sites expected to exist on practical high-surface-area catalysts. This “surface science” approach has been used to study structure sensitivity, the effects of promoters/inhibitors on catalytic activity, and mixed-metal catalysis. In this article, some of the progress made during the last twenty years to integrate modern surface science into fundamental catalytic research will be reviewed, including recent efforts to simulate oxide-supported metal systems using thin oxide films.

Methane coupling at low temperatures on Ru(0001) and Ru(11�20) catalysts

The conversion of methane to higher hydrocarbons on single crystal Ru catalysts has been investigated using combined elevated-pressure kinetic measurements/surface science studies. The reaction consists of activation of methane on Ru(0001) and Ru(11¯20) surfaces to produce carbonaceous intermediates at temperatures $$\left( {T_{CH_4 } } \right)$$ between 350 and 700 K and rehydrogenation of these species to ethane and propane at $$T_{H_2 } $$ ≈ 370 K. It is found that under the reaction conditions employed, the maximum yield in ethane/propane production occurs at $$T_{CH_4 } $$ ≈ 500 K on both surfaces. Influence of the hydrogenation temperature on the production of ethane and propane is also examined. On Ru(0001), the yields of ethane and propane maximize at $$T_{H_2 } $$ = 400 K, whereas no maximum yield was observed on Ru(11 $$\bar 2$$ 0) in the 300–500 K temperature range. Under optimum reaction conditions, hydrocarbon products consist of 16% ethane and 2% propane. High-resolution electron energy-loss spectroscopy (HREELS) has been used to identify various forms of hydrocarbonaceous intermediates following methane decomposition. An effort is made to relate the hydrocarbon intermediates identified by HREELS to the gas phase products observed in the elevated pressure experiments.

Adsorbed HNO 2/NO redox couple at Pt(111) single-crystal electrode

Nitrogen oxide (NOx), produced mainly in the exhaust gases of automobile engines and factory turbines, is known as an injurious substance to the human respiratory organs and nervous system. It is also an important source of acid rain. Nitric oxide has been the subject of intense attention in the last 5 years because of its important biological role in the body which had not been recognized previously [1]. The adsorption and decomposition mechanisms of NO x on a well-defined single-crystal catalyst surface have been studied extensively in the gas phase using a variety of methods, such as low energy electron diffraction (LEED), Auger electron spectroscopy, IR absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) etc., and these results have been well reviewed [2-4]. Electrochemical investigations of NOx have been carried out by many groups, mainly on polycrystalline Pt electrodes. The electrochemical behaviour appears to be more complicated than that observed in gas + solid catalysis. Schmid and Neumann [5] investigated the equilibrium potential of the HNO2/NO redox system in solution and obtained E ° = 0.983 V. Many groups have concluded from the kinetic analysis of cyclic voltammograms obtained in acid solution that the formation of NO is the first reduction step of HNO2 [6-9]. Clavilier and Chauvineau [10] have reported that a (poly)platinum surface was covered by NO during cleaning with an argon glow discharge. The NO was removed irreversibly at 0.45V/SCE in a positive potential scan. Gadde and Bruckenstein [11] carried out bulk electrolysis of nitrite in 0.1 M HCIO4 at a Pt electrode. Their product analysis showed only NO above 0.5V/RHE. Nishimura et al. [12] examined the electroreduction product of nitrite and nitrate ions at a porous Pt electrode in H2SO 4 using the differential electrochemical mass spectroscopy (DEMS) method. NO has been detected in the potential region 0.6-1.0 V/RHE.

The effects of rhenium and sulfur on the reactivity and selectivity of Pt single-crystal catalysts

Platinum-rhenium bimetallic surfaces have been prepared by condensing Re on Pt(111) crystal surfaces in ultrahigh vacuum (UHV) using a pulsed metal vapor vacuum arc plasma gun. Conversion reactions of n-hexane and cyclohexane have been studied. The same reactions were also performed over sulfided surfaces made by depositing atomic sulfur on the metal in UHV. The presence of Re on Pt(111) greatly increases the hydrogenolysis of n-hexane. Sulfiding of the catalysts suppresses hydrogenolysis and enhances the cyclization activity of n-hexane toward methylcyclopentane. The maximum reactivity for cyclization to form five-membered rings over the sulfided Pt-Re surfaces is obtained when the Pt surface is covered by 1 monolayer (ML) of Re. When the Pt(111) surface is covered by 1 ML of Re, the sulfided catalyst is twice as active as the unsulfided catalyst for cyclization of n-hexane. Presulfiding also enhances the dehydrogenation activity of cyclohexane over the Pt-Re bimetallic surfaces. In that case, maximum reactivity of sulfided catalysts is observed when the Pt surface is covered by 0.5 ML of Re. We conclude that the Pt-Re bimetallic catalyst shows its unique properties when it is combined with sulfur.

An investigation of ethylene hydrogenation catalyzed by metallic molybdenum using an isolatable high-pressure reactor: Identification of the reaction site and the role of carbonaceous deposits

Ethylene hydrogenation catalyzed by a model Mo(100) single crystal catalyst is investigated using an isolatable high-pressure reactor. The observed reaction kinetics are in agreement with those measured for supported transition metal hydrogenation catalysts showing a zero-order dependence in ethylene and a first-order dependence in hydrogen partial pressures. The reaction activation energy is 8.1 ± 0.5 kcal/mol for a Mo(100) single-crystal model catalyst and for a foil and mass spectral analysis of the products formed following substitution of deuterium for hydrogen shows that a maximum of two deuterium atoms are incorporated into the ethane that is produced. Auger analysis of the surface following reaction reveals the presence of large amounts of carbon (up to ∼7 monolayers). This carbonaceous deposit decomposes on heating the surface to above 1000 K to desorb C 1 species. The experimental data suggest that either hydrogen or ethylene can permeate this layer so that hydrogen transfer takes place either at the metal surface or on top of the layer. Blocking the fourfold sites on Mo(100) by oxygen or carbon has only a marginal effect on the overall reaction kinetics, and does not substantially alter the amount of carbon deposited onto the surface during reaction. This result implies that the enhanced catalytic activity observed on metal carbide catalysts is not only due to site blocking, but also arises because of chemical modification of the catalyst by compound formation.

Molecular mechanism of alkene epoxidation: A model study with 3,3-dimethyl-1-butene on Ag(111)

On chlorine-promoted Ag(111) oxygen adatoms selectively oxidise 3,3-dimethyl-1-butene (DMB) to the corresponding epoxide with very high selectivity providing direct verification of the mechanism advanced earlier for alkene epoxidation over single crystal silver and silver catalysts.

Monte Carlo simulation of catalytic CO oxidation

A simple Monte Carlo model of the CO oxidation on a single-crystal catalyst surface is presented. The simulation model considers the following elementary reaction steps: 1. (1) chemisorption of a CO molecule, its surface migration and possible desorption 2. (2) physisorption of an O 2 molecule to a precursor state and its subsequent dissociative chemisorption 3. (3) activated reaction of adsorbed O and CO (the Langmuir - Hinshelwood reaction mechanism), formation of CO 2 and its rapid desorption. The changes in the activation energy of reaction and in the adsorption energy of CO resulting from the interactions between adsorbed species are also considered. The model makes possible to monitor temperature programmed reaction spectra or reaction spectra obtained during changes of the ratio of the partial pressures of CO and O 2. The results of simulations for a Pd(111) single-crystal plane are compared with experiment.

Adsorption and interaction of methanol with zinc oxide: Single crystal faces and zinc oxide-copper catalyst surfaces studied by photoelectron spectroscopY (XPS and UPS)

Adsorption of methanoi on (101̄0) and (0001̄) faces of ZnO and on ZnO(Cu) catalyst surfaces was studied by XPS and UPS in the temperature range from 100 to 320 K. At 100 K not only molecular adsorption and condensation occurred, but also formation of methoxy species (CH 3O-) was observed on all surfaces. Only on (101̄0) dissociation yielding a small amount of CH n groups took place at 115 K. On the catalyst surface this type of species was formed above 150 K, while on (0001̄) faces (CH 2O) n (para-formaldehyde) was observed at 185 K. The temperature dependent behaviour is analogous and the electron binding energies coincide for the three surfaces. On the catalyst surface formation of methoxy groups is enhanced at room temperature, while no hydroxyl groups are formed on this surface in contrast to the behaviour of the pure ZnO surfaces. A tentative explanation for this behaviour is given.

Adsorption and interaction of methanol with zinc oxide: Single crystal faces and zinc oxide-copper catalyst surfaces studied by photoelectron spectroscopy (XPS and UPS)

Abstract Adsorption of methanoi on (1010) and (0001) faces of ZnO and on ZnO(Cu) catalyst surfaces was studied by XPS and UPS in the temperature range from 100 to 320 K. At 100 K not only molecular adsorption and condensation occurred, but also formation of methoxy species (CH 3 O-) was observed on all surfaces. Only on (1010) dissociation yielding a small amount of CH n groups took place at 115 K. On the catalyst surface this type of species was formed above 150 K, while on (0001) faces (CH 2 O) n (para-formaldehyde) was observed at 185 K. The temperature dependent behaviour is analogous and the electron binding energies coincide for the three surfaces. On the catalyst surface formation of methoxy groups is enhanced at room temperature, while no hydroxyl groups are formed on this surface in contrast to the behaviour of the pure ZnO surfaces. A tentative explanation for this behaviour is given.

Kinetics of carbon monoxide oxidation on single-crystal palladium, platinum, and iridium

The activity of Pt(100), Pd(110), Ir(111), and Ir(110) single-crystal catalysts for CO oxidation has been studied as a function of temperature and partial pressure of O/sub 2/ and CO in a high-pressure reactor-ultra-high-vacuum surface analysis apparatus over the temperature range 425-725 K and pressure range 0.1-600 Torr. The specific rates and the partial pressure dependencies determined for the single crystals are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. The single-crystal catalysts exhibit simple Arrhenius behavior over most of the temperature range studied, and the observed activation energies lie between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results are consistent with the generally accepted model in which the surface is primarily covered with CO and the reaction rate is controlled by the desorption of CO. Deviation from Arrhenius behavior below 500 K for Pt is interpreted as a change in the reaction mechanism. Under highly oxidizing conditions surfaces of both Pd and Ir show negative-order dependence on O/sub 2/ partial pressure, indicating the presence of a strongly bound oxygen species. The oxygen species was similar to surface oxide formed bymore » deliberate oxidation and could be detected as CO/sub 2/ desorbing at high temperatures in postreaction temperature-programmed desorption. Oxide formed by oxidation of the Pd and Ir samples prior to high-pressure reaction was only stable at 475 K on Pd(110) in an 11:1 O/sub 2/:CO mixture and to 500 K on Ir(111) in an 80:1 O/sub 2/CO:CO mixture. Deliberate oxidation resulted in a rate decrease but did not affect the activation energy significantly, indicating that the oxide served merely as a simple site blocker.« less

The role of adsorbate overlayers in thiophene hydrodesulfurization over molybdenum and rhenium single crystals

Thiophene hydrodesulfurization (HDS) has been investigated over model single crystal catalysts of molybdenum and rhenium. Thiophene HDS is a structure sensitive reaction over rhenium and a structure insensitive reaction over molybdenum. Adsorbed sulfur decreases the activity of both Re(0001) and Mo(100) surfaces while adsorbed carbon has distinctly different effects on the catalytic properties of the two metals. Carbon overlayers deactivate the Re(0001) surface but have no effect on the HDS activity of the Mo(100) surface. Radiotracer35S and14C studies indicate that HDS occurs on an adsorbate overlayer on molybdenum comprised primarily of carbon. HDS over rhenium, on the other hand, occurs on the bare metal surface. This appears to be responsible for the observed differences in the influence of surface structure on HDS activity.

The effects of potassium on ammonia synthesis over iron single-crystal surfaces

The effects of potassium on ammonia synthesis over model iron single-crystal catalysts of (111), (100), and (110) orientation have been studied under high-pressure reaction conditions (20 atm reactant pressure of nitrogen and hydrogen). Under these conditions, no more than 0.15 monolayers (ML) of potassium can be stabilized on the iron surfaces. The Fe(110) surface shows no activity for ammonia synthesis in this study with or without adsorbed potassium. The presence of potassium on the Fe(111) and Fe(100) surfaces increases the rate of ammonia synthesis markedly. At a low reaction conversion of 0.3% the rate over Fe(111) and Fe(100) is enhanced by a factor of two in the presence of potassium. The effect of potassium on Fe(111) and Fe(100) is enhanced as higher reaction conversions (i.e., increasing ammonia partial pressures) are achieved because potassium induces changes in the reaction orders for both ammonia and hydrogen. No change in the activation energy for the reaction is observed with potassium, suggesting that the reaction mechanism has not been altered. Temperature-programmed desorption shows that the adsorption energy of ammonia is significantly reduced when coadsorbed with potassium. A model is proposed in which the decrease of ammonia adsorption energy, induced by potassium, reduces the concentration of ammonia on the iron surface. This effect decreases the number of active sites blocked by the ammonia product, thereby increasing the rate of ammonia synthesis. The model also suggests that an additional effect of potassium is to increase the rate of nitrogen dissociative chemisorption by about 30% over Fe(111) and Fe(100) under ammonia synthesis conditions.

Estimating the density of carbon atoms on a Ni catalyst surface in equilibrium with a carbonaceous gas

The amount of carbon adsorbed on the surface of Ni in contact with carbonaceous gas mixtures such as CH 4/H 2 and CO/CO 2, is estimated from equilibrium segregation data. The results are displayed on “gas composition versus temperature” plots for the above two gas mixtures. These plots provide basic thermodynamic information relevant to reactions such as steam reforming of hydrocarbons on supported Ni catalysts. For example, the plot for CO/CO 2 gas mixtures represents the Boudouard equilibrium on a single crystal Ni catalyst, whilst the plot for CH 4/H 2 gas mixtures provides information relevant to the equilibrium hydrogenation of adsorbed C to CH 4.

A radiotracer ( 14C) and catalytic study of thiophene hydrodesulfurization on the clean and carbided Mo(100) single-crystal surface

The role of adsorbed carbon overlayers and possible product poisoning (H 2S and cis-2-butene) in the thiophene hydrodesulfurization (HDS) reaction over Mo(100) single-crystal surfaces was investigated. Using the β − emitting 14C isotope, it was shown that adsorbed carbon remains on the Mo(100) surface after many thiophene turnovers (TN ≈ 10 3). HDS activities measured for carbided surfaces were found to be identical to that of initially clean Mo(100) single crystals, suggesting that the active catalyst surface is carbon covered. Deactivation of the single-crystal catalysts was caused by adsorption of H 2S onto the active HDS sites. Removal of the H 2S by evacuation of the reaction mixture readily regenerated the catalyst's activity toward a fresh mixture of thiophene and hydrogen. The product cis-2-butene had no effect on the activity of the catalysts.

Surface structure sensitivity of alloy catalysis: Catalytic conversion of n-hexane over Au-Pt(111) and Au-Pt(100) alloy crystal surfaces

Alloys have become important catalysts for many chemical processes due to their superior properties over single component catalysts. These include increased reaction selectivity and greater resistance to deactivation. The reasons for their superior performance are not well understood, however. Catalytic studies, using single component metal single crystal catalysts, coupled with the use of surface science techniques, have proven useful in understanding the nature of the working catalyst. Extension of these investigations to alloy single crystal at high pressures should be useful for elucidating the role of alloying in altering catalytic behavior. In this note the authors report the surface structure sensitivity of the catalytic properties of platinum-gold alloys in the conversion reaction of n-hexane and present evidence for a mixed Au-Pt site on the (111) surface for the isomerization reaction. Experiments were carried out in an ultrahigh vacuum system equipped with Auger electron spectroscopy (AES), low energy electron diffraction (LEED) optics, a quadrupole mass spectrometer, and a sample isolation cell which was connected to the reactor loop and gas chromatograph equipped with a flame ionization detector. 11 references.