Theoretical surface science and catalysis—calculations and concepts

Analysis of methane-to-methanol conversion on clean and defective Rh surfaces

We investigate by density-functional theory simulations several elementary reactions associated to direct methane-to-methanol conversion on clean Rh(111) surfaces and on Rh adatoms on Rh(111). Energy barriers and reaction paths have been determined by the nudged elastic band method. The rate-limiting step in the process, C-O bond formation, has higher activation energy than the one for complete methane dehydrogenation. Our analysis enables us to understand the effect of defects on the reactivity and rules out Rh as candidate catalyst for methanol synthesis.

A density functional theory study of sulfur poisoning

Density functional theory calculations have been used to investigate the chemisorption of H, S, SH, and H(2)S as well as the hydrogenation reactions S+H and SH+H on a Rh surface with steps, Rh(211), aiming to explain sulfur poisoning effect. In the S hydrogenation from S to H(2)S, the transition state of the first step S+H-->SH is reached when the S moves to the step-bridge and H is on the off-top site. In the second step, SH+H-->H(2)S, the transition state is reached when SH moves to the top site and H is close to another top site nearby. Our results show that it is difficult to hydrogenate S and they poison defects such as steps. In order to address why S is poisoning, hydrogenation of C, N, and O on Rh(211) has also been calculated and has been found that the reverse and forward reactions possess similar barriers in contrast to the S hydrogenation. The physical origin of these differences has been analyzed and discussed.

Surface-Enhanced Raman Spectroscopic Study of 1,4-Phenylene Diisocyanide Adsorbed on Gold and Platinum-Group Transition Metal Electrodes

Self-assembled monolayers of 1, 4-phenylene diisocyanide (PDI) were formed on Au and Pt-group transition metals and examined by surface-enhanced Raman spectroscopy under controlled applied potential. On all of the metals examined, PDI adsorbs in an edge-on manner, with one NC group bound to the surface and the other pointing away from the surface. The N-C stretching frequency (nu(NC)) suggests that depending on the metal, PDI adsorbs on different binding sites: terminal sites on Au, both terminal and bridging on Rh and Pt, and predominantly 3-fold hollow sites for Pd. This binding site preference can be understood in terms of the difference in d-band center energy and d-orbital filling among the metals. The applied potential affects the N-C bonding differently as inferred from the potential dependence of nu(NC). On Au, Rh, and Pd, the nu(NC) increases linearly with the applied potential, yielding a Stark tuning slope, dnu(NC)/dE, of 25, 12, and 10 cm(-1)/V, respectively. On Pt, the nu(NC) is nearly independent of the applied potential. On all of the metals studied, the frequencies of benzene ring vibration modes are not dependent on the applied potential, consistent with the edge-on orientation in which the ring does not directly interact with the surface. Several ring vibrations are, however, sensitive to the nature of metal substrate due to different binding sites involved. The ability of the free NC group to function as an anchoring point is demonstrated by the attachment of gold nanoparticles on PDI-covered Au and Pd. The study provides useful NC-metal bonding information for isocyanide-based molecular electronic developments.

How adiabatic is activated adsorption/associative desorption?

Using density-functional theory we calculate friction coefficients describing the damping of nuclear motion into electron-hole pair excitation for the two best-known examples of activated adsorption: H2 dissociation on a Cu(111) surface and N2 dissociation on a Ru(0001) surface. In both cases, the frictions increase dramatically along the reaction path towards the transition state and can be an order of magnitude larger there than typical in the molecularly adsorbed state. In addition, the frictions for N2/Ru(0001) are typically an order of magnitude larger than for H2/Cu(111). We rationalize these trends in terms of the electron structure as the systems proceed to dissociation along the reaction paths. Combining these friction coefficients with the potential-energy surface in quasiclassical dynamics allows first-principles studies of the importance of the breakdown in the Born-Oppenheimer approximation in describing the chemistry. We find that nonadiabatic effects are minimal for the H2/Cu(111) system, but are quite important for N2/Ru(0001).

Energetically Driven Reorganization of a Modified Catalytic Surface under Reaction Conditions

The compositional and structural rearrangements at the catalyst surface during chemical reactions are issues of great importance for understanding and modeling the catalytic processes. Low-energy electron microscopy and photoelectron spectromicroscopy studies of the real-space structure and composition of a Au-modified Rh(110) surface during water formation reveal reorganization processes due to Au mass transport triggered by the propagating reaction fronts. The temporal evolution of the surface reaction results in a 'patterned' surface consisting of separated Au-rich and Au-poor phases with different oxygen coverage, Rh surface structure, and reactivity. The experimental results are complemented by ab initio electronic-structure calculations of the O and Au adsorption phases, which demonstrate that the reorganization of the Au adlayer by the propagating reaction fronts is an energetically driven process. Our findings suggest that reaction-induced spatial inhomogeneity in the surface composition and structure is a common feature of metal catalysts modified with adatoms which become mobile under reaction conditions.

Platinum monolayer electrocatalysts for oxygen reduction: Effect of substrates, and long-term stability

We describe a novel concept for a Pt monolayer electrocatalyst and present the results of our electrochemical, X-ray absorption spectroscopy, and scanning tunneling microscopy studies. The electrocatalysts were prepared by a new method for depositing Pt monolayers involving the galvanic displacement by Pt of an under potentially deposited Cu monolayer on substrates of Au (111), Ir(111), Pd(111), Rh(111) and Ru(0001) single crylstals, and Pd nanoparticles. The kinetics of O2 reduction showed significant enhancement with Pt monolayers on Pd(111) and Pd nanoparticle surfaces in comparisonwith the reaction on Pt(111) and Pt nanoparticles, respectively. This increase in catalytic activity is attributed partly to the decreased formation of PtOH, as shown by in situ X-ray absorption spectroscopy. The results illustrate that placing a Pt monolayer on a suitable substrate of metal nanoparticles is an attractive way of designing better O2 reduction electrocatalysts with very low Pt contents.

Electronic structure and catalysis on metal surfaces

The powerful computational resources available to scientists today, together with recent improvements in electronic structure calculation algorithms, are providing important new tools for researchers in the fields of surface science and catalysis. In this review, we discuss first principles calculations that are now capable of providing qualitative and, in many cases, quantitative insights into surface chemistry. The calculations can aid in the establishment of chemisorption trends across the transition metals, in the characterization of reaction pathways on individual metals, and in the design of novel catalysts. First principles studies provide an excellent fundamental complement to experimental investigations of the above phenomena and can often allow the elucidation of important mechanistic details that would be difficult, if not impossible, to determine from experiments alone.

Electrocatalysis of fuel cells reaction on Pt and Pt-bimetallic anode catalysts: A selective review

In this review we selectively summarize recent progress, primarily from our laboratory, in the development of interrelationships between the kinetics of the fuel cells reactions and the structure/composition of anode catalysts. The focus is placed on two types of metallic surfaces: platinum single crystals and bimetallic surfaces based on Pt. In the first part it was illustrated that the hydcogen reaction is structure sensitive process, with Pt(110) being an order of magnitude more active than either of the atomically "flatter" (100) and (111) surfaces. The hydrogen reaction on Pt(hkl) modified by pseudomorphic Pd (sub)monolayers shows the "volcano-like" behavior, having the maximum rate on Pt(111) modified by 1 ML of Pd. The Pt(111)-Pd system is used to demonstrate how the energetics of intermediates formed in the hydrogen reaction is affected by interfacial bonding and energetic constraints produced between pseudomorphic Pd films and the Pt(111) substrate. In the second part it was shown that the oxidation of Ha in the presence of CO occurs concurrently with CO oxidation on Pt and Pt bimetallic surfaces. The Pt-Ru system is used to demonstrate that both the bifunctional effect and the ligand effect contribute to the influence of Ru on the CO oxidation rate and for Hz oxidation process in the presence of CO. The knowledge is then used to create the real-life catalyst with the catalytic activities which are, to the greatest extend possible similar to the tailor-made surface.