Charles Sykes

Tufts University | Tufts · Department of Chemistry

The drive to improve the safety and efficacy of radiotherapies for cancers has prompted the development of nanomaterials that can locally amplify the radiation dose at a tumor without damaging the surrounding healthy tissue. Gold nanoparticles (Au NPs), in particular, exhibit promising radiosensitizing properties under kilovolt X-ray exposure, alth...

The origin of the enantiospecific decomposition of L- and D-tartaric acid on chiral Cu surfaces is elucidated on a structure-spread domed Cu(110) crystal by spatially resolved XPS and atomic-scale STM...

Benzene, a high-volume chemical, is produced from larger molecules by inefficient and environmentally harmful processes. Recent changes in hydrocarbon feedstocks from oil to gas motivate research into small molecule upgrading. For example, the cyclotrimerization of acetylene reaction has been demonstrated on Pd, Pd alloy, and Cu surfaces and cataly...

The drive to improve the safety and efficacy of radiotherapies for cancers has prompted the development of nanomaterials that can locally amplify the radiation dose at a tumor without damaging the surrounding healthy tissue. Gold nanoparticles (Au NPs), in particular, exhibit promising radiosensitizing properties under kilovolt X-ray exposure, alth...

Single‐atom catalysts are a fast‐emerging area in which late‐transition metal atoms are supported on oxides, metals, and carbonaceous supports. They show great promise for selective chemical reactions due to their well‐defined active sites, and significantly reduce precious metal loading. However, oxide‐supported single‐atom catalysts have drawback...

Single-atom catalysts have received significant attention for their ability to enable highly selective reactions. However, many reactions require more than one adjacent site to align reactants or break specific bonds. For example, breaking a C-O or O-H bond may be facilitated by a dual site containing an oxophilic element and a carbophilic or "hydr...

Recent experiments have demonstrated an intriguing phenomenon in which adsorption of a nonracemic mixture of aspartic acid (Asp) enantiomers onto an achiral Cu(111) metal surface leads to autoamplification of surface enantiomeric excess, ees, to values well above those of the impinging gas mixtures, eeg. This is particularly interesting because it...

Single-atom alloys (SAAs) have drawn significant attention in recent years due to their excellent catalytic properties. Controlling the geometry and electronic structure of this type of localized catalytic active site is of fundamental and technological importance. Dual-atom alloys (DAAs) consisting of a heterometallic dimer embedded in the surface...

The relative stability of reactive intermediates and reactants on a surface, which dictates the rate and selectivity of catalytic reactions in both gas and liquid phases, is dependent on numerous factors. One well-established example is secondary interactions, such as van der Waals interactions between the catalyst surface and the pendant group of...

Single-atom alloy catalysts combine catalytically active metal atoms, present as dopants, with the selectivity of coinage metal hosts. Determining whether adsorbates stick at the dopant or spill over onto the host is key to understanding catalytic mechanisms on these materials. Despite a growing body of work, simple descriptors for the prediction o...

Dicarbonyl species are ubiquitous on Rh/oxide catalysts and are known to form on Rh+ centers. However, dicarbonyl species have never been directly observed on single-atom alloys (SAAs) where the active site is metallic. Herein, using surface science and theoretical modeling, we provide evidence of dicarbonyl species at isolated Rh sites on a RhCu(1...

Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multipronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ...

We report the comparison of a series of 2D molecular crystals formed from the intermediates of the dehalogenation reaction of iodoethane versus various fluorinated iodoalkanes on Cu(111). High-resolution scanning tunneling microscopy enables us to distinguish the alkyl groups from the iodine atoms, and we find that the ethyl groups and iodine atoms...

Recent heterogeneous catalysis studies have demonstrated that synergy between Ag and Cu can lead to more selective partial oxidation chemistries. We performed a series of scanning tunneling microscope experiments to gain a better understanding of the AgCu system under oxidative conditions. These experiments were carried out by exposing sub‐monolaye...

Single-Atom Alloys (SAAs) are a special class of alloy surface catalysts that offer well defined, isolated active sites in a more inert metal host. The dopant sites are generally assumed to have little or no influence on the properties of the host metal, and transport of chemical reactants and products to and from the dopant sites is generally assu...

Rhodium atoms for alkane dehydrogenation Nanoparticles of rhodium dispersed on metal oxides are generally poor catalysts for alkane dehydrogenation because the reactants bind too strongly to the metal. Hannagan et al. performed first-principle calculations indicating that single rhodium atoms in a copper surface should be stable and selective for c...

Carbon–carbon coupling is an important step in many catalytic reactions, and performing sp³–sp³ carbon–carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this...

The formation of a two-phase surface molecular overlayer that transitions from isolated propene molecules to a highly ordered 1D chain structure on Cu(111) is elucidated through combined high-resolution STM imaging...

We report a comprehensive study combining surface science, Density Functional Theory (DFT) calculations, and catalyst synthesis, characterization, and testing to investigate the preferential oxidation of CO in the presence of H2 over single-site Pt1/CuxO catalysts. Surface science studies show that while Pt1/CuxO model surfaces enable low-temperatu...

The atomic scale structure of the active sites in heterogeneous catalysts is central to their reactivity and selectivity. Therefore, understanding active site stability and evolution under different reaction conditions is key to the design of efficient and robust catalysts. Herein we describe theoretical calculations which predict that carbon monox...

This paper describes a single-molecule study of N-methylbutylamine molecular rotors supported on a Cu(111) surface. It is first demonstrated that the chirality of the individual rotating molecules can be directly determined by scanning tunneling microscopy (STM) imaging and understood with density functional theory (DFT) simulations. Tunneling elec...

Metal alloys are ubiquitous in many branches of heterogeneous catalysis, and it is now fairly well established that the local atomic structure of an alloy can have a profound influence on its chemical reactivity. While these effects can be difficult to probe in nanoparticle catalysts, model studies using well defined single crystal surfaces alloyed...

Scanning probe microscopes are notoriously sensitive to many types of external and internal interferences including electrical, mechanical, and acoustic noise. Sometimes noise can even be misinterpreted as real features in the images. Therefore, quantification of frequency and magnitude of any noise is the key to discover the source and eliminate i...

Selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols is a challenging class of reactions, yielding valuable intermediates for the production of pharmaceuticals, perfumes, and flavorings. On monometallic heterogeneous catalysts, the formation of the unsaturated alcohols is thermodynamically disfavored over the saturated aldeh...

There are now many examples of single molecule rotors, motors and switches in the literature that, when driven by photons, electrons or chemical reactions, exhibit well defined motions. As a step towards using these single molecule devices to perform useful functions one must understand how they interact with their environment and quantify their ab...

The conversion of surface-bound alkyl groups to alkanes and alkenes are important steps in many heterogeneously catalyzed reactions. While Pt is ubiquitous in industry because of its high activity toward C-H activation, many Pt based catalysts tend to over-bind reactive intermediates, which leads to deactivation by carbon deposition and coke format...

Significance The energy efficiency of catalytic processes hinges on achieving high selectivity and activity; bimetallic catalysts have potential to increase both by exploiting different chemical properties of the components. Transport of intermediates between the two metals, for example, transport of hydrogen atoms for selective hydrogenation, is r...

Single-atom catalysts (SACs) have gained significant world-wide attention for exhibiting distinct reactivity and selectivity for important reactions, while utilizing precious metals at the ultimate limit of efficiency. SACs involving reducible oxide supports are good catalysts for oxidation reactions, and metal-acid site pairs enable CC bond format...

Recent studies have shown that the addition of Cu to Ag catalysts improves their epoxidation performance by increasing the overall selectivity of the bimetallic catalyst. We have prepared AgCu near-surface alloys and used scanning tunneling microscopy to gain an atomistic picture of O2 dissociation on the bimetallic system. These data reveal a high...

We demonstrate that significant entropic effects govern the stability of rotationally constrained molecular complexes on surfaces. Using a complementary combination of low-temperature scanning tunneling microscopy, temperature programmed desorption, and density functional theory calculations, we probe the energetics of surface adsorbed molecular ro...

Scanning probe microscopes are notoriously sensitive to many types of external and internal interference including electrical, mechanical and acoustic noise. Sometimes noise can even be misinterpreted as real features in the images. Therefore, quantification of the frequency and magnitude of any noise is key to discovering the source and eliminatin...

The preferential oxidation of CO (PROX) in hydrogen-rich fuel gas streams is an attractive option to remove CO while effectively conserving energy and H 2 . However, high CO conversion with concomitant high selectivity to CO 2 but not H 2 O is challenging. Here, we report the synthesis of high-loading single Pt atom (2.0 weight %) catalysts with ox...

Studies of radioactive isotopes at the liquid-solid or gas-solid interface are enabling a detailed mechanistic understanding the effects of radioactive decay on physical, biological, and chemical systems. In recent years, there has been a burgeoning interest in using radioactive isotopes for both imaging and therapeutic purposes by attaching them t...

Chiral surfaces are of growing interest for enantioselective adsorption and reactions. While metal surfaces can be prepared with a wide range of chiral surface orientations, chiral oxide surface preparation is more challenging. We demonstrate the chirality of a metal surface can be used to direct the homochiral growth of a thin film chiral oxide. S...

The reducibility of metal oxides, when serving as the catalyst support or the active sites themselves, plays an important role in heterogeneous catalytic reactions. For example, in the production of methanol from CO2 hydrogenation with Cu-based catalysts, the degree of surface oxidation plays a critical role in catalyst performance. Here we present...

We demonstrate that PdAu single-atom alloy model catalysts offer a heterogeneous route to selective Würtz-type C-C coupling. Specifically, when methyl iodide is exposed to an otherwise unreactive Au(111) surface, single...

Single‐atom alloys are a new class of heterogeneous catalyst in which trace amounts of metal dopants exist as individual, isolated sites in a host metal. In this study, we examine RhCu, a new single‐atom alloy with a combination of scanning tunneling microscopy, reflectance absorption infrared spectroscopy, and temperature programed desorption to u...

Silver-based heterogeneous catalysts, modified with a range of elements, have found industrial application in several reactions in which selectivity is a challenge. Alloying small amounts of Pt into Ag has the potential to greatly enhance the somewhat low reactivity of Ag while maintaining high selectivity and resilience to poisoning. This single-a...

Ni is one of the most extensively utilized metals in industrial catalysis. For example, Ni is the catalyst of choice for the steam reforming of hydrocarbons. However, pure Ni also detrimentally catalyzes the formation of graphitic carbon, which in turn leads to coking and deactivation of the catalyst. It has been shown that alloying small amounts o...

Surface-bound molecular rotors provide a useful way to study the structure and dynamics of molecular motion at the single molecule level. However, when most molecules adsorb on a metal surface their interaction with the metal changes their properties dramatically making a priori design impossible. We report a case in which gas phase predictions of...

The selective hydrogenation of alkynes to alkenes is an important industrial process. However, achieving high selectivity and reducing the usage of precious platinum group metals is still challenging for the conventional hydrogenation catalysts. With atomically dispersed active metal atoms in the surface of a host metal, single-atom alloys have sho...

Controlled Pt loading on TiO2 nanoparticles enables single-site catalysts. With this, the coordination environment and catalytic activity can be obtained, allowing extraction of structure-function information.

Molecular devices are capable of performing a number of functions ranging from mechanical motion to simple computation. Two molecular rotor types will be presented: altitudinal phenyl-ethyl and azimuthal amine based rotors. The altitudinal rotor is the product of spontaneously formed organometallic arrays via the Ullmann coupling reaction of precur...

Enantioselective interactions underpin many important phenomena from biological mechanisms to chemical catalysis. In this regard, there is great interest in understanding these effects at the molecular level. Surfaces provide a platform for these studies and aid in the long-term goal of designing heterogeneous enantiospecific interfaces. Herein we...

Pt based materials are used extensively in heterogeneous catalytic processes, but are notoriously susceptible to poisoning by CO. In contrast, highly dilute binary alloys formed of isolated Pt atoms in a Cu metal host, known as PtCu single-atom alloys (SAAs), are more resilient to CO poisoning during catalytic hydrogenation reactions. In this artic...

Identifying and characterizing the atomic-scale interaction of methanol with oxidized Cu surfaces is of fundamental relevance to industrial reactions, such as methanol steam reforming and methanol synthesis. In this work, we examine the adsorption of methanol on the well-defined “29” Cu oxide surface using a combination of experimental and theoreti...

ConspectusHeterogeneous catalysts are workhorses in the industrial production of most commodity and specialty chemicals, and have widespread energy and environmental applications, with the annual market value of the catalysts themselves reaching almost $20 billion in 2018. These catalysts are complex, comprising multicomponent materials and multipl...

We discuss a simple yet effective strategy for escaping traditional linear-scaling relations in heterogenous catalysis with highly dilute bimetallic alloys known as Single-Atom Alloys (SAAs). These systems, in which a reactive metal is atomically dispersed in a less reactive host, were first demonstrated with the techniques of surface science to be...

Oxidative methanol dehydrogenation is a major industrial reaction with global formaldehyde production exceeding 30 million tonnes per year. Unfortunately, oxidative dehydrogenation produces water-aldehyde mixtures that require subsequent distillation. Anhydrous alcohol dehydrogenation is a promising alternative that produces H2 instead of water. Pu...

Recent advances in single atom catalysis have sparked interest in their use as low-cost and high-efficiency catalysts in a wide variety of reactions. One such reaction that has been heavily studied with single atom catalysts is the water gas shift reaction. In addition, water participates in a rich variety of other industrially important catalytic...

The oxidation of gold is an important step in a number of catalytic reactions and the oxidation of Au(111) with ozone has been well-studied using surface science techniques. We report that the addition of 1% Pd in the surface in the form of a Pd/Au(111) single-atom alloy dramatically alters the desorption temperature of molecular oxygen after oxida...

The delicate balance between hydrogen bonding and van der Waals interactions determines the stability, structure, and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces. Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging. In this quest, small a...

The delicate balance between H-bonding and van der Waals interactions determine the stability,structure and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces.Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging.Small alcohols adsorbed on metal s...

Molecular devices are capable of performing a number of functions from mechanical motion to simple computation. Two molecular rotors will be presented: altitudinal phenyl-ethyl and azimuthal amine rotors (see Fig 1 for an example). The altitudinal rotor, where the rotor is the ethyl tail and its axis of rotation is parallel to the metal surface, is...

The non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen is an industrially relevant chemical conversion. Although Cu-based catalysts show high reactivity toward oxidative ethanol dehydrogenation, the flat Cu(111) surface is rather inactive for ethanol dehydrogenation in the absence of water, surface oxygen or defects. Herein we sh...

Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poiso...

Doping isolated single atoms of a platinum group metal into the surface of a noble metal host is sufficient to dramatically improve the activity of the unreactive host yet also facilitates the retention of the host’s high reaction selectivity in numerous catalytic reactions. The atomically dispersed highly active sites in these Single Atom Alloy (S...

The recent availability of shale gas has led to a renewed interest in C–H bond activation as the first step towards the synthesis of fuels and fine chemicals. Heterogeneous catalysts based on Ni and Pt can perform this chemistry, but deactivate easily due to coke formation. Cu-based catalysts are not practical due to high C–H activation barriers, b...

Single-atom catalysts have attracted great attention in recent years due to their high efficiencies and cost savings. However, there is debate concerning the nature of the active site, interaction with the support, and mechanism by which single-atom catalysts operate. Here, using a combined surface science and theory approach, we designed a model s...

The geometric and electronic structural characterization of thin film metal oxides is of fundamental importance to many fields such as catalysis, photovoltaics, and electrochemistry. Surface defects are also well known to impact a material’s performance in any such applications. Here, we focus on the “29” oxide Cu2O/Cu(111) surface and we observe t...

Silica supported and unsupported PdAu single atom alloys (SAAs) were investigated for the selective hydrogenation of 1-hexyne to hexenes under mild conditions. The catalysts were prepared by adding a trace amount of Pd (0.4 at. %) into the surface of pre-formed Au nanoparticles through a sequential reduction method. TEM and XRD analyses indicate th...

Molecular devices are capable of performing a number of functions from mechanical motion to simple computation. Their utility is somewhat limited, however, by difficulties associated with coupling them with either each other or with interfaces such as electrodes. Self-assembly of coupled molecular devices provides an option for the construction of...

Supplementary Figures and Notes.

80 frames of a 575 frame movie showing the correlated rotational switching events occurring in the active rows of the major crystal structure formed from the reaction of 4-bromo- 1-ethyl-2-fluorobenzene with Cu(111). Frames were acquired at -10 mV and 30 pA and rendered at a frame rate of 2 frames per second.

Rotational switching events in different molecular crystal packing structures. Frames were acquired at -10 mV and 30 pA (major), -10 mV and 10 pA (minor), -30 mV and 30 pA, and +30 mV and 100 pA for surface reaction intermediates of 4-bromo-1-ethyl-2- fluorobenzene, 1-bromo-4-ethylbenzene, and 1-bromo-3-ethylbenzene, respectively. The frame rate of...

Using a surface science approach, the selectivity in the Ullmann cross-coupling of aryl halides on Cu(111) has been understood and controlled. The binding strength of the reactants and repulsion between them dictates which organometallic intermediates form, and hence the product distribution. Cross coupling can be maximized at low reactant concentr...

Control of the translational motion of individual molecules on surfaces is necessary for the development of novel methods for mass transport, separations, enantiopurifications and reactions, as well as in new applications including pumps, sensors, and assembly. Herein we discuss a concept whereby a flashing temperature ratchet-like mechanism compri...

The non-oxidative dehydrogenation of alcohols is considered as an important method to produce aldehydes for the chemical industry and hydrogen gas. However, current industrial processes are oxidative, meaning that the aldehydes are formed along with water, which in addition to being less energy efficient, poses separation problems. Herein the produ...