J. Will Medlin

Control of Metal Catalyst Selectivity through Specific Noncovalent Molecular Interactions

The specificity of chemical reactions conducted over solid catalysts can potentially be improved by utilizing noncovalent interactions to direct reactant binding geometry. Here we apply thiolate self-assembled monolayers (SAMs) with an appropriate structure to Pt/Al2O3 catalysts to selectively orient the reactant molecule cinnamaldehyde in a configuration associated with hydrogenation to the desired product cinnamyl alcohol. While nonspecific effects on the surface active site were shown to generally enhance selectivity, specific aromatic stacking interactions between the phenyl ring of cinnamaldehyde and phenylated SAMs allowed tuning of reaction selectivity without compromising the rate of desired product formation. Infrared spectroscopy showed that increased selectivity was a result of favorable orientation of the reactant on the catalyst surface. In contrast, hydrogenation of an unsaturated aldehyde without a phenyl ring showed a nontunable improvement in selectivity, indicating that thiol SAMs can improve reaction selectivity through a combination of nonspecific surface effects and ligand-specific near-surface effects.

Directing reaction pathways by catalyst active-site selection using self-assembled monolayers

One key route for controlling reaction selectivity in heterogeneous catalysis is to prepare catalysts that exhibit only specific types of sites required for desired product formation. Here we show that alkanethiolate self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation and hydrodeoxygenation products during the reaction of furfural on supported palladium catalysts. Vibrational spectroscopic studies demonstrate that the selectivity improvement is achieved by controlling the availability of specific sites for the hydrogenation of furfural on supported palladium catalysts through the selection of an appropriate alkanethiolate. Increasing self-assembled monolayer density by controlling the steric bulk of the organic tail ligand restricts adsorption on terrace sites and dramatically increases selectivity to desired products furfuryl alcohol and methylfuran. This technique of active-site selection simultaneously serves both to enhance selectivity and provide insight into the reaction mechanism.

Scaling relations between adsorption energies for computational screening and design of catalysts

Adsorption energies have significant value as predictors of catalytic performance. An important method of increasing efficiency of adsorption energy calculations is to employ scaling relations, which are linear relationships between adsorption energies of similar adsorbates. They are most commonly used to unify the description of adsorbates that bind to the surface through a particular type of atom. In this work, we review the development and applications of scaling relations. Scaling relations have been observed for a variety of adsorbates bonding through C, O, H, N, and S atoms to the surfaces of transition metals, oxides, nitrides, sulfides, carbides, and nanoparticles. They can be used to increase the efficiency of predictions, simplify descriptions of surface reactivity, and sometimes to derive limits on the effectiveness of a catalyst. Because scaling relations can impose a significant limitation on catalyst design, it is also useful to explore how to design active sites that significantly deviate from them. We discuss applications to a variety of processes, including methane reforming and synthesis, alcohol decomposition and synthesis, electrochemical systems, and conversion of biomass derivatives.

Adsorption and Reaction of 1-Epoxy-3-butene on Pt(111) : Implications for Heterogeneous Catalysis of Unsaturated Oxygenates

High-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations were used to study the adsorption and reaction of 1-epoxy-3-butene (EpB) on Pt(111). These investigations were conducted to help elucidate mechanisms for improving olefin hydrogenation selectivity in reactions of unsaturated oxygenates. EpB dosed to Pt(111) at 91 K adsorbs molecularly on the surface through the vinyl group with apparent rehybridization to a di-sigma-bound state. By 233 K, however, EpB undergoes epoxide ring opening to form an aldehyde intermediate, which further decomposes upon heating to yield gas phase products CO, H2, and propylene. Comparison of the HREELS and TPD data to experiments performed with 2-butenal (crotonaldehyde) shows that EpB and 2-butenal decompose through related pathways. However, the EpB-derived aldehyde intermediate clearly has a unique structure, features of which have been elucidated by DFT calculations. In conjunction with previous surface science studies of EpB chemistry, these results can help explain selectivity trends for reactions of EpB on Pt catalysts and bimetallic PtAg catalysts, with indications that the enhanced olefin hydrogenation selectivity of PtAg catalysts likely originates from a bifunctional effect.

Benzyl Alcohol Oxidation on Pd(111): Aromatic Binding Effects on Alcohol Reactivity

To investigate how surface oxygen participates in the reaction of important aromatic oxygenates, the surface chemistry of benzyl alcohol (PhCH2OH) and benzaldehyde (PhCHO) has been studied on oxygen-precovered Pd(111) (O/Pd(111)) using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On both Pd(111) and O/Pd(111), TPD using isotopically labeled benzyl alcohol and low-temperature HREEL spectra show that the oxidation of benzyl alcohol proceeds through a benzyl alkoxide (PhCH2O-) intermediate to adsorbed benzaldehyde so that the sequence of bond scission is O-H followed by C(α)-H. In the presence of surface O, some benzaldehyde desorbs from the surface below 300 K, consistent with the presence of a weakly adsorbed η(1) aldehyde state that is bound to the surface through its oxygen lone pair. Benzaldehyde also reacts with surface oxygen to produce benzoate (PhCOO-). Shifts in the OCO stretching frequency suggest that the benzoate orientation changes as the surface becomes less crowded, consistent with a strong interaction between the phenyl group and the surface. Adsorbed benzaldehyde and benzoate undergo decomposition to CO and CO2, respectively, as well as benzene. Deoxygenation of benzyl alcohol to toluene occurs at high coverages of benzyl alcohol when the relative surface O coverage is low. Experiments conducted on (18)O/Pd(111) reveal exchange occurring between surface O and the benzaldehyde and benzoate intermediates. This exchange has not been reported for other alcohols, suggesting that aromatic binding effects strongly influence alcohol oxidation on Pd.

Controlling Catalytic Selectivity via Adsorbate Orientation on the Surface: From Furfural Deoxygenation to Reactions of Epoxides.

Specificity to desired reaction products is the key challenge in designing solid catalysts for reactions involving addition or removal of oxygen to/from organic reactants. This challenge is especially acute for reactions involving multifunctional compounds such as biomass-derived aromatic molecules (e.g., furfural) and functional epoxides (e.g., 1-epoxy-3-butene). Recent surface-level studies have shown that there is a relationship between adsorbate surface orientation and reaction selectivity in the hydrogenation pathways of aromatic oxygenates and the ring-opening or ring-closing pathways of epoxides. Control of the orientation of reaction intermediates on catalytic surfaces by modifying the surface or near-surface environment has been shown to be a promising method of affecting catalytic selectivity for reactions of multifunctional molecules. In this Perspective, we review recent model studies aimed at understanding the surface chemistry for these reactions and studies that utilize this insight to rationally design supported catalysts.

A density functional study of C1–C4 alkyl adsorption on Cu(111)

To better understand the nature of alkyl intermediates often invoked in reactions involving hydrocarbon reactants and products, the adsorption of linear and branched C(1)-C(4) alkyls on Cu(111) at 1/4 ML and 1/9 ML coverages was studied using density functional theory. The adsorption energy and site preference are found to be coverage-dependent, and both direct alkyl-alkyl interactions and changes in the Cu electronic structure play a role in these trends. It was found that methyl strongly prefers the hollow sites, the branched alkyls strongly prefer the top site, and the linear C(2)-C(4) alkyls have weak site preferences that change with coverage. To explain these differences, rationalize alkyl adsorption trends, and predict the binding energy of other alkyls, a simple model was developed in which the binding energy is fit as a linear function of the number of C-Cu and C-H-Cu interactions as well as the C-H bond energy in the corresponding alkane. Site preference can be understood as a compromise between C-Cu interactions and C-H-Cu interactions. Density of states analysis was used to gain a molecular-orbital understanding of the bonding of alkyls to Cu(111).

Adsorption and reactivity of 2,3-dihydrofuran and 2,5-dihydrofuran on Pd(111): influence of the C=C position on the reactivity of cyclic ethers.

High-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) were used to study the adsorption and thermal chemistry of 2,3-dihydrofuran (2,3-DHF) and 2,5-dihydrofuran (2,5-DHF) on Pd(111). The results, paired with earlier computational results, indicate that 2,3-DHF and 2,5-DHF both adsorb on Pd(111) primarily via their respective olefin functional groups at low temperature (<170 K). Both molecules undergo dehydrogenation by 248 K to form species that produce furan in a reaction limited process above 300 K. The furan-producing intermediate intermediate can also undergo decomposition to form C(3)H(x) and CO. In addition, benzene resulting from C-C coupling reactions is detected on the surface and as a desorption product from both species, at about 520 K. A key difference between the two species is that 2,3-DHF can be hydrogenated to produce tetrahydrofuran at about 330 K, whereas 2,5-DHF is more likely to dehydrogenate, producing furan in an additional low-temperature channel at approximately 320 K. The results point to the importance of the position of the olefin functional group in relation to the ether function in determining the reactivity of cyclic oxygenates.

Hydrogenation of Cinnamaldehyde over Pd/Al2O3 Catalysts Modified with Thiol Monolayers

Modification of supported Pt catalysts with thiols has recently been shown to improve the hydrogenation selectivity of α,β-unsaturated aldehydes to unsaturated alcohols. Here, we apply a variety of organic thiol coatings to Pd/Al2O3 catalysts that typically have a much lower intrinsic selectivity for desired product formation. Thiol monolayers were found to increase hydrogenation selectivity to cinnamyl alcohol; however, unlike with Pt catalysts, the increase was independent of the identity of the organic tail.

Computational investigation of defect segregation at the (001) surface of BaCeO3 and BaZrO3: the role of metal–oxygen bond strength in controlling vacancy segregation

The low proton grain boundary conductivity observed in perovskites such as BaZrO3 is generally attributed to a positive grain boundary core charge that repels protonic defects away from the interfacial region. This core charge develops from the segregation of positively charged defects to the grain boundary interface. Here, the results of a density function theory investigation of defect segregation at two (001) surfaces of BaCeO3 and BaZrO3 perovskites are presented that give insight into this phenomenon. Yttrium dopant and oxygen vacancy segregation were explored independently and synergistically at these simplified perovskite–vacuum interfaces in order to establish the enthalpic driving forces that promote defect segregation at different surfaces caused by intrinsic material properties. It was found that dopant segregation strongly influences the stability of oxygen vacancies at the perovskite surface because of favorable pairing between dopants and vacancies. Furthermore, the results demonstrate that selecting materials on the basis of metal–oxygen bond strengths offers a potential means of engineering perovskites to mitigate the development of a strong positively charged grain boundary core.