The oscillating Fischer-Tropsch reaction.

The mechanistic steps that underlie the formation of higher hydrocarbons in catalytic carbon monoxide (CO) hydrogenation at atmospheric pressure over cobalt-based catalysts (Fischer-Tropsch synthesis) have remained poorly understood. We reveal nonisothermal rate-and-selectivity oscillations that are self-sustained over extended periods of time (>24 hours) for a cobalt/cerium oxide catalyst with an atomic ratio of cobalt to cerium of 2:1 (Co2Ce1) at 220°C and equal partial pressures of the reactants. A microkinetic mechanism was used to generate rate-and-selectivity oscillations through forced temperature oscillations. Experimental and theoretical oscillations were in good agreement over an extended range of reactant pressure ratios. Additionally, phase portraits for hydrocarbon production were constructed that support the thermokinetic origin of our rate-and-selectivity oscillations.

X-ray Photoelectron Spectroscopy and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Insight into the Pathways of Manganese Oxalate Thermal Decomposition to MnO and MnCO3.

X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed to study the isothermal decomposition of MnC2O4 under ultrahigh vacuum and N2 environmental conditions, respectively. High-resolution core-level XP spectra, X-ray-induced Auger spectra, and infrared spectra were obtained as a function of annealing time. In XPS studies, the time-dependent thermal decomposition characteristics were elucidated by analyzing surface composition, chemical shifts, satellites in the Mn 2p3/2 and Mn LMV bands, and Auger parameters for Mn and O. Functional groups developing during the ongoing reaction were identified by DRIFTS from characteristic vibrations. For the first time, the isothermal decomposition of manganese oxalate was shown to proceed via two pathways involving nucleation and accumulation of MnO and MnCO3. The kinetics of the decomposition in vacuum could be described by the Prout-Tompkins or/and by the Avrami-Erofeev models. The results obtained by XPS, DRIFTS, and ex situ XRD allowed concluding that the final product of oxalate decomposition was composed of MnO and MnCO3 or/and unidentate/polydentate carbonate structures populating the surface of the sample. A substantial formation of graphitic carbon was also observed and associated with interface chemical reactivities between the MnO particles and the supporting gold foil.

Directional Gateway to Metal Oxidation: 3D Chemical Mapping Unfolds Oxygen Diffusional Pathways in Rhodium Nanoparticles.

The interaction of oxygen with a reactive metal is ubiquitous, yet the precise atomic-level mechanisms and pathways leading to the formation of a surface oxide are not well-understood. We report oxygen atom distributions inside Rh single nanoparticles using atom probe microscopy (APM) and demonstrate that mainly facets of the ⟨022̅⟩ crystallographic directions act as oxygen-permeable gateways. The highly anisotropic spatial distribution of incorporated oxygen atoms is in agreement with video-field emission analyses according to which {113} facets of the ⟨022̅⟩ zones act as portals for subsurface diffusion. In addition to providing a more fundamental understanding of the precursor states to metal corrosion, in particular for the case of nanosized metal particles, our studies are also relevant for heterogeneous catalysis where catalytic activity and selectivity conform to reaction-induced structural changes of metal nanoparticles.

Rate and selectivity hysteresis during the carbon monoxide hydrogenation over promoted Co/MnOx catalysts.

While cobalt-based catalysts have been used in industrial Fischer-Tropsch synthesis for decades, little is known about how the dynamics of the Co-Co2C phase transformation drive their performance. Here we report on the occurrence of hysteresis effects in the Fischer-Tropsch reaction over potassium promoted Co/MnOx catalyst. Both the reaction rate and the selectivity to chain-lengthened paraffins and terminally functionalized products (aldehydes, alcohols, olefins) show bistability when varying the hydrogen/carbon monoxide partial pressures back and forth from overall reducing to carbidizing conditions. While the carbon monoxide conversion and the selectivity to functionalized products follow clockwise hysteresis, the selectivity to paraffins shows counter-clockwise behavior. In situ X-ray diffraction demonstrates the activity/selectivity bistability to be driven by a Co-Co2C phase transformation. The conclusions are supported by High Resolution Transmission Electron Microscopy which identifies the Co-Co2C transformation, Mn5O8 layered topologies at low H2/CO partial pressure ratios, and MnO at high such ratios.

Emergence of Chemical Oscillations from Nanosized Target Patterns.

This work investigates experimentally the mechanism by which chemical oscillations emerge in a nanometric system. We monitor the spatiotemporal dynamics of an oscillating reaction on the surface of a nanosized three-dimensional Pt model catalyst. Using high-resolution field emission techniques, we are able to show that the oscillations are generated by nanoscale chemical target patterns of much shorter characteristic time than the period with which the oscillations occur. Our observations are made for a specific reaction system-NO_{2} reduction with hydrogen-and represent the first experimental evidence for the presence of target patterns at the nanoscale. They can be seen as an experimental demonstration of reaction-diffusion mechanisms to hold at the nanoscale as they do at the macroscale. These results shed new light on the emergence of complexity through different time and length scales.

Tuning the catalytic CO hydrogenation to straight- and long-chain aldehydes/alcohols and olefins/paraffins.

The catalytic CO hydrogenation is one of the most versatile large-scale chemical syntheses leading to variable chemical feedstock. While traditionally mainly methanol and long-chain hydrocarbons are produced by CO hydrogenation, here we show that the same reaction can be tuned to produce long-chain n-aldehydes, 1-alcohols and olefins, as well as n-paraffins over potassium-promoted CoMn catalysts. The sum selectivity of aldehydes and alcohols is usually >50 wt% whereof up to ∼97% can be n-aldehydes. While the product slate contains ∼60% n-aldehydes at /pCO=0.5, a 65/35% slate of paraffins/alcohols is obtained at a ratio of 9. A linear Anderson-Schulz-Flory behaviour, independent of the /pCO ratio, is found for the sum of C4+ products. We advocate a synergistic interaction between a Mn5O8 oxide and a bulk Co2C phase, promoted by the presence of potassium, to be responsible for the unique product spectra in our studies.

Au 4f spin-orbit coupling effects in supported gold nanoparticles.

Using X-ray photoelectron spectroscopy we examine the Au 4f spin-orbit components in Au 4f spectra of nanosized Au particles on a TiO2 support. In general, the peak ratios of the Au 4f7/2 and 4f5/2 excitations are found to deviate from the statistical ratio of 4 : 3 and their linewidths (FWHM) are not equal. We reveal that both the FWHM and the Au 4f7/2-to-4f5/2 peak ratios increase appreciably as the Au atomic concentration on the surface of the TiO2 support and the size of Au nanoparticles decrease. On the contrary, the Au 4f spin-orbit splitting remains essentially unchanged. Our findings are discussed in terms of alterations in the electronic band structure.

Combining in situ NEXAFS spectroscopy and CO2 methanation kinetics to study Pt and Co nanoparticle catalysts reveals key insights into the role of platinum in promoted cobalt catalysis.

The mechanistic role of platinum and precious metals in promoting cobalt hydrogenation catalysts of the type used in reactions such as Fischer-Tropsch synthesis is highly debated. Here we use well-defined monometallic Pt and Co nanoparticles (NPs) and CO2 methanation as a probe reaction to show that Pt NPs deposited near Co NPs can enhance the CO2 methanation rate by up to a factor of 6 per Co surface atom. In situ NEXAFS spectroscopy of these same Pt NP plus Co NP systems in hydrogen shows that the presence of nearby Pt NPs is able to significantly enhance reduction of the Co at temperatures relevant to Fischer-Tropsch synthesis and CO2 methanation. The mechanistic role of Pt in these reactions is discussed in light of these findings.

A nanoscale demonstration of hydrogen atom spillover and surface diffusion across silica using the kinetics of CO2 methanation catalyzed on spatially separate Pt and Co nanoparticles.

Hydrogen spillover is of great importance to understanding many phenomena in heterogeneous catalysis and has long been controversial. Here we exploit well-defined nanoparticles to demonstrate its occurrence through evaluation of CO2 methanation kinetics. Combining platinum and cobalt nanoparticles causes a substantial increase in reaction rate, but increasing the spatial separation between discrete cobalt and platinum entities results in a dramatic ∼ 50% drop in apparent activation energy, symptomatic of H atom surface diffusion limiting the reaction rate.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO.

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Exploring surface science and restructuring in reactive atmospheres of colloidally prepared bimetallic CuNi and CuCo nanoparticles on SiO2 in situ using ambient pressure X-ray photoelectron spectroscopy.

Bimetallic nanoparticles (to approximately 11 nm diameter) of CuNi and CuCo were prepared by a new synthetic route and the 1:1 atomic ratio of their constituent elements confirmed using STEM-EDS at a single particle level. These nanoparticles, supported on the native oxide layer of a silicon wafer, were studied in situ in a series of reactive gas atmospheres (H2 --> CO or CO/H2 --> O2 --> H2) using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Despite the deliberate similarity of nickel and cobalt with respect to copper, their restructuring behaviour is different. CuNi nanoparticles were found to surface segregate nickel in H2, but copper in O2 reversibly, while CuCo nanoparticles were found to surface segregate copper irreversibly when exposed to O2, such that the surface remains copper rich when re-exposed to H2. Both systems also restructure in opposition to the behaviour predicted by the surface free energies and enthalpies of oxide formation of the elements from which they are comprised--factors that are seen to control restructuring and surface segregation in many similar systems.

Long-chain terminal alcohols through catalytic CO hydrogenation.

We show that long-chain 1-alcohols can be produced with high selectivities using heterogeneous CO hydrogenation catalysis. This breakthrough is achieved through the targeted design of "CoCuMn" nanosized core-shell particles using co-precipitation of metal salts into oxalate precursors and subsequent thermal decomposition. Using stoichiometric CO/H2 feeds, the selectivities to 1-alcohols or combined 1-alcohols/1-alkenes are usually higher than 60% and occasionally up to 95%. The Anderson-Schulz-Flory chain-lengthening probabilities for these products are higher than 0.6, but usually below 0.9 so as to optimize the C8-C14 slate as feedstock for plasticizers, lubricants, or detergents.

Adsorption-induced restructuring and early stages of carbon-nanotube growth on Ni nanoparticles.

Carbon adsorption on various Ni surfaces is investigated as a function of coverage via a combination of first-principles simulations and field emission microscope experiments. It is found that carbon can be efficiently stored as subsurface carbides, but with different energetics on differently oriented surfaces depending on their compactness and density of adsorption sites. In the resulting morphological reshaping, {113} facets are predicted to grow at the expense of {111} and {100} facets, in excellent agreement with experimental observations. Moreover, at high coverage on the {113} surface the carbon adsorption energy passes through a maximum after which a structural crossover is realized such that carbon atoms tend to ascend to the surface to form one-dimensional chains (which are the precursors of graphitic nanostructures). This rationalizes the experimental observation of an incubation time between carbon storage and the beginning of catalytic growth, and provides insight into the early stages (nucleation mechanism) of carbon nanotubes on Ni nanoparticles.

Hydrocarbon chain lengthening in catalytic CO hydrogenation: evidence for a CO-insertion mechanism.

We studied CO hydrogenation over Co/MgO (10/1) model catalysts using chemical transient kinetics. Quantification of the time-dependent response during fast changes of the gas flow composition enabled the counting of surface amounts of carbon, oxygen, and hydrogen from the onset of adsorption to the steady state of the reaction and vice versa. Under the atmospheric pressure conditions of the reaction, the total amount of adsorbed species exceeded the monolayer limit on Co metal. The time response in transients and back-transients of gaseous reactants and products is in accordance with a CO insertion mechanism. Furthermore, the Anderson-Schulz-Flory chain lengthening probability is directly proportional to the CO pressure, whereas no such dependence is measured for the amounts of accumulating or fading surface carbon.

Size-controlled model Co nanoparticle catalysts for CO2 hydrogenation: synthesis, characterization, and catalytic reactions.

Model cobalt catalysts for CO(2) hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200-300 °C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3-10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.

Field emission techniques for studying surface reactions: applying them to NO-H2 interaction with Pd tips.

The adsorption of NO and its reaction with H(2) over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10(-3)Pa pressure range and at sample temperatures between 400 and 600K. By varying the H(2) partial pressure while keeping the other control parameters constant, the NO+H(2) reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H(2) pressures delimiting this hysteresis decrease as well. Abrupt transformations of the micrographs are observed by FIM from bright to dark patterns and vice versa. These transformations define the hysteresis region. The collected data allow establishing a novel kinetic phase diagram of the NO+H(2)/Pd system within the range of temperatures and pressures indicated. The observed features are correlated with a local chemical analysis by means of field pulses. NO(+) seems to be the dominating imaging species under all conditions. At high relative H(2) pressures (the "hydrogen-side"), H atoms seem to diffuse subsurface. This process is blocked at lower H(2) pressure (the "NO-side") due to NO(ad) and O(ad) accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO(2)(+) species in the mass spectra.

Combined sulfating and non-sulfating support to prevent water and sulfur poisoning of Pd catalysts for methane combustion.

The appropriate combination of titania and silica, sulfating and non-sulfating support, respectively, results in Pd catalysts with improved water and sulfur tolerance in methane combustion. For the first time the catalyst recovers the initial activity after one cycle under lean-burn conditions without additional regenerating treatments.

Nanometric chemical clocks.

Field ion microscopy combined with video techniques and chemical probing reveals the existence of catalytic oscillatory patterns at the nanoscale. This is the case when a rhodium nanosized crystal--conditioned as a field emitter tip--is exposed to hydrogen and oxygen. Here, we show that these nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the nanofacets that are simultaneously exposed at the tip's surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction-diffusion mechanism, plays a major role in determining the self-organizational behavior of multifaceted nanostructured surfaces. Surprisingly, this nanoreactor, composed of the tip crystal and a constant molecular flow of reactants, is large enough for the emergence of regular oscillations from the molecular fluctuations.

Early stages in the nucleation process of carbon nanotubes.

The early stages of carbon nanotube nucleation are investigated using field ion/electron microscopy along with in situ local chemical probing of a single nanosized nickel crystal. To go beyond experiments, tight-binding Monte Carlo simulations are performed on oriented Ni slabs. Real-time field electron imaging demonstrates a carbon-induced increase of the number density of steps in the truncated vertices of a polyhedral Ni nanoparticle. The necessary diffusion and step-site trapping of adsorbed carbon atoms are observed in the simulations and precede the nucleation of graphene-based sheets in these steps. Chemical probing of selected nanofacets of the Ni crystal reveals the occurrence of Cn (n=1-4) surface species. Kinetic studies prove C2+ species are formed from C1 with a delay time of several milliseconds at 623 K. Carbon dimers, C2, must not necessarily be formed on the Ni surface. Tight-binding Monte Carlo simulations reveal the high stability of such dimers in the first layer beneath the surface.

Surface elastic properties of Ti alloys modified for medical implants: a force spectroscopy study.

We report here the first nanoscale surface elasticity measurements on surface-modified titanium alloys using the force spectroscopy mode in scanning force microscopy. Samples of three vanadium-free titanium alloys, Ti-7Nb-6Al, Ti-13Nb-13Zr and Ti-15Zr-4Nb, were investigated. Surface modification of the three alloys was produced by thermal oxidation in air at 750 degrees C for different times, which resulted in the formation of protective oxide layers with different surface composition and morphology. The elastic properties of the surface layers were studied comparatively in the as-received Ti alloys and after the oxidation process using cantilevers with different stiffness to evaluate the influence of the indentation depth. In all cases, Young's modulus of the sample surfaces was found to be lower than 65GPa, and as low as 20GPa for some of the oxidized samples. Variations observed for the three oxidized Ti alloys can be related to the different chemical composition of the outer layers generated for the different oxidation times.