Spectroscopic investigation of a Co(0001) model catalyst during exposure to H2 and CO at near-ambient pressures.

Cobalt catalysts, although already used industrially for Fischer-Tropsch synthesis, are prone to a number of deactivation mechanisms such as oxidation of the active phase, and the deposition of carbon and reaction products. We have performed near-ambient-pressure X-ray photoelectron spectroscopy on Co(0001) model catalysts during exposure to gases relevant to Fischer-Tropsch synthesis, i.e., CO and H2, at 0.25 mbar total pressure. At this pressure, CO seems to be more efficient at keeping the Co(0001) surface metallic than H2, which is the opposite behavior as reported in the literature for other pressure ranges. We offer an interpretation of these differences based on the preferred adsorption and dissociation sites of CO and H2 compared to the oxidizing agent water (present as impurity in the gas feed and one of the products of the reaction). Additionally, detailed carbon spectra measured at the HIPPIE beamline of MAX IV allow for the distinction of different adsorbed species: CO and COx species are present in correlation to the presence of oxygen on the surface. Carbidic carbon and graphitic carbon can both be removed by hydrogen, whereas adsorbed hydrocarbons possibly poison the surface.

Steps on Pt stereodynamically filter sticking of O2.

Low coordinated sites on catalytic surfaces often enhance reactivity, but the underlying dynamical processes are poorly understood. Using two independent approaches, we investigate the reactivity of O2 impinging onto platinum and resolve how step edges on (111) terraces enhance sticking. At low incident energy, the linear dependence on step density, independence of step type, and insensitivity to O2's molecular alignment show that trapping into a physisorbed state precedes molecular chemisorption and dissociation. At higher impact energies, direct molecular chemisorption occurs in parallel on steps and terraces. While terraces are insensitive to alignment of the molecule within the (111) plane, steps favor molecules impacting with their internuclear axis parallel to the edge. Stereodynamical filtering thus controls sticking and dissociation of O2 on Pt with a twofold role of steps.

Scaling Platinum-Catalyzed Hydrogen Dissociation on Corrugated Surfaces.

We determine absolute reactivities for dissociation at low coordinated Pt sites. Two curved Pt(111) single-crystal surfaces allow us to probe either straight or highly kinked step edges with molecules impinging at a low impact energy. A model extracts the average reactivity of inner and outer kink atoms, which is compared to the reactivity of straight A- and B-type steps. Local surface coordination numbers do not adequately capture reactivity trends for H2 dissociation. We utilize the increase of reactivity with step density to determine the area over which a step causes increased dissociation. This step-type specific reactive area extends beyond the step edge onto the (111) terrace. It defines the reaction cross-section for H2 dissociation at the step, bypassing assumptions about contributions of individual types of surface atoms. Our results stress the non-local nature of H2 interaction with a surface and provide insight into reactivity differences for nearly identical step sites.

It's not just the defects - a curved crystal study of H2O desorption from Ag.

We investigate water desorption from hydrophobic surfaces using two curved Ag single crystals centered at (111) and (001) apices. On these types of crystals the step density gradually increases along the curvature, allowing us to probe large ranges of surface structures in between the (001), (111) and (110) planes. Subtle differences in desorption of submonolayer water coverages point toward structure dependencies in water cluster nucleation. The B-type step on hydrophobic Ag binds water structures more strongly than adjacent (111) planes, leading to preferred desorption from steps. This driving force is smaller for A-type steps on (111) terraces. The A'-type step flanked by (001) terraces shows no indication of preferred desorption from steps. Extrapolation to the (311) surface, not contained within either curved surface, demonstrates that both A- and A'-type steps can be regarded chemically identical for water desorption. The different trends in desorption temperature on the two crystals can thus be attributed to stronger water adsorption at (001) planes than at (111) planes and identical to adsorption at the step. These results show that our approach to studying the structure dependence of water desorption is sensitive to variations in desorption energy smaller than 'chemical accuracy', i.e. 1 kcal mol-1.

Heterogeneous Catalytic Oxidation of Ammonia by Various Transition Metals.

A well-known demonstration is adapted to simplify the illustration of heterogeneous catalytic oxidation of ammonia. Various metal catalyst wires are placed above the liquid level in a flask containing concentrated ammonia. After brief preheating, some metal wires continue to glow, providing visual evidence of an overall exothermic reaction taking place at the catalyst surface. Thermal heating by a butane flame prior to insertion and in situ resistive heating using a power supply yield identical results. Active catalysts are the group 9 and 10 elements Rh, Ir, Pd, and Pt. Besides the illustration of the Sabatier principle, the effect of the ammonia-to-oxygen ratio can also be visualized, and active metals vary in the production of a grayish smoke. These observations provide a starting point to discuss catalytic selectivity, a topic of great relevance to industrial catalytic oxidation of ammonia.

A state-selected continuous wave laser excitation method for determining CO2's rotational state distribution in a supersonic molecular beam.

State-resolved experiments can provide fundamental insight into the mechanisms behind chemical reactions. Here, we describe our methods for characterizing state-resolved experiments probing the outcome of the collision between CO2 molecules and surfaces. We create a molecular beam from a supersonic expansion that passes through an ultra-high vacuum system. The CO2 is vibrationally excited by a continuous wave infrared (IR) laser using rapid adiabatic passage. We attenuate the fractional excitation using a CO2 absorption cell in the IR beam path. We combine Monte Carlo simulations and molecular beam energy measurements to find the initial rotational state distribution of the molecular beam. We find that our pure CO2 beam from a 300 K source has a rotational temperature of ∼26 K.

Site-specific reactivity of molecules with surface defects-the case of H2 dissociation on Pt.

The classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.