Nanoporous Au Catalysts Research Articles

Predicting a Sharp Decline in Selectivity for Catalytic Esterification of Alcohols from van der Waals Interactions.

Controlling the selectivity of catalytic reactions is a critical aspect of improving energy efficiency in the chemical industry; thus, predictive models are of key importance. Herein the performance of a heterogeneous, nanoporous Au catalyst is predicted for the complex catalytic self-coupling of the series of C2 -C4 alkyl alcohols, based solely on the known kinetics of the elementary steps of the catalytic cycle for methanol coupling, using scaling methods augmented by density functional theory. Notably, a sharp decrease in selectivity for ester formation with increasing molecular weight to favor the aldehyde due to van der Waals interactions of reaction intermediates with the surface was predicted and subsequently verified quantitatively by experiment. Further, the agreement between theory and experiment clearly demonstrates the efficacy of this approach for building a predictive model of catalytic behavior for a homologous set of reactants using a small set of experimental information.

Predicting a Sharp Decline in Selectivity for Catalytic Esterification of Alcohols from van der Waals Interactions

AbstractControlling the selectivity of catalytic reactions is a critical aspect of improving energy efficiency in the chemical industry; thus, predictive models are of key importance. Herein the performance of a heterogeneous, nanoporous Au catalyst is predicted for the complex catalytic self‐coupling of the series of C2–C4 alkyl alcohols, based solely on the known kinetics of the elementary steps of the catalytic cycle for methanol coupling, using scaling methods augmented by density functional theory. Notably, a sharp decrease in selectivity for ester formation with increasing molecular weight to favor the aldehyde due to van der Waals interactions of reaction intermediates with the surface was predicted and subsequently verified quantitatively by experiment. Further, the agreement between theory and experiment clearly demonstrates the efficacy of this approach for building a predictive model of catalytic behavior for a homologous set of reactants using a small set of experimental information.

Coarsening of nanoporous Au during catalytic CO oxidation

This study focuses on the effects of catalytic activity on nanoporous Au catalysts fabricated by dealloying. Nanoporous Au structures are shown to coarsen with similar kinetics over a broad temperature range. A variation in the apparent activation energy suggests that coarsening is controlled by different rate-limiting steps at high and low temperatures.

Ag surface segregation in nanoporous Au catalysts during CO oxidation

The present study focuses on the modification of surface compositional profiles induced in nanoporous (NP) Au catalysts by the catalytic oxidation of carbon monoxide to carbon dioxide in the presence of oxygen. The phenomenon has deep implications concerning the catalytic behavior of NP Au foams in particular, and more in general for the design of more efficient catalysts. Aimed at gaining deeper insight into the mechanisms governing surface segregation, we exposed NP Au foams containing residual Ag to a mixture of gaseous carbon monoxide and oxygen at different temperature. Structural and surface composition analyses pointed out the concomitant occurrence of both NP Au coarsening and Ag surface segregation processes. Experimental findings suggest for Ag surface segregation a two-stage kinetics. During the initial, rapid coarsening of the NP Au structure, Ag surface segregation is mediated by surface rearrangements, which allow the Ag atoms to reach the surface at anomalously fast rate. As coarsening decelerates, the slower diffusion of buried Ag atoms towards the surface predominates, due to favorable chemical interactions with adsorbed oxygen. This novel mechanism’s understanding can benefit strategic areas of science and technology.

Oxide-Stabilized Nanoporous Ni-Pt for Enhanced Durability in ORR Catalysis

Improving the lifetime and durability of electrode catalysts is one of the significant barriers to the development of commercially practical hydrogen fuel cells. These issues, along with reduction in material costs associated with precious metal catalysts, are the centerpiece of a major research initiative to investigate new materials and processing techniques for these catalysts. Previous research has demonstrated that (111) Pt3Ni surfaces are superior in oxygen reduction reaction (ORR) electrocatalytic activity compared to pure Pt [1]. Nanoporous nickel-platinum (np-NiPt) formed by dealloying has been shown to possess excellent ORR mass activity compared to Pt3Ni films [2]. Other recent work has shown that incorporation of minority ternary components such as Mo into Pt-Ni alloy nanoparticles can strongly influence their ORR activity [3]. In addition to affecting catalytic activity, the addition of a ternary component to a binary alloy can lead to a more stable nanoporous metal after dealloying, as has been shown in the case of adding small fractions (< 5 at. %) of Pt to Ag-Au alloys [4]. This is because coarsening is mediated by the surface diffusion, and addition of a small amount of Pt to the initial Ag-Au alloy used as a precursor for nanoporous Au leads to a nanoporous Au catalyst with greatly increased structural stability due to the lower mobility Pt atoms acting as pinning points on the surface. Similar behavior is seen in the addition of Ir to Pt-Ni alloys [5]. In this work, we report the results of a survey of ternary components added at a dilute level (2 at. %) to a dealloyable Ni-Pt ingot. Ternary components including Ta, Si, Mo, Ir, and V were incorporated into precursor Ni77Pt23 ingots via RF melting and shaped into rotating disk electrodes (RDEs). These electrodes were dealloyed to a proscribed roughness factor (Rf = HUPD surface area/geometric surface area = 143), and then assessed for ORR activity and stability using rotating disk electrochemistry. These components were chosen because fabrication of these alloys via chemical reduction (e.g., as is used for nanoparticles) is difficult or impossible. Unlike Pt added to Ag-Au alloys, most of the elements of interest here form stable, irreducible oxides during dealloying. Correlations of the ternary component oxide structure to the lengthscale of the nanoporous materials formed during dealloying, as well as the effect of oxide structure on the electrochemically active surface area, will be discussed. [1] V. R. Stamenkovic et al. Science, 315, 493 (2007). [2] J. Snyder, T. Fujita, M. W. Chen, J. Erlebacher. Nature Materials, 9, 904-907 (2010). [3] T. Mueller et al. Science, 348, 1230-1234 (2015). [4] J. Snyder, P. Asanithi, A. B. Dalton, J. Erlebacher, Advanced Materials, 20, 4883-4886 (2008). [5] J. Snyder et al. ACS Catalysis, 7, 7995-8005 (2017). Acknowledgement: This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy.

Oxygen Adsorption and Low-Temperature CO Oxidation on a Nanoporous Au Catalyst: Reaction Mechanism and Foreign Metal Effects

To further our understanding of the role of trace impurities of the second metal in the catalytic performance of unsupported, nanoporous Au (NPG) catalysts, in particular for the activation of O2, we have prepared a NPG catalyst by electrochemical leaching of Cu from a AuCu alloy and investigated its behavior in the CO oxidation reaction. The structural and chemical properties of the as-prepared catalyst as well as that after reaction for 1000 min were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The nature of the surface oxygen species and the oxygen storage capacity were investigated and quantified by multi-pulse experiments in a temporal analysis of products (TAP) reactor. The catalytic behavior in the low-temperature CO oxidation reaction was evaluated both in a TAP reactor under dynamic vacuum conditions and in a conventional micro-reactor under atmospheric pressure. We discuss implications of these results and of similar data obtained previously on a Ag-containing NPG catalyst on the reaction mechanism and on the role of the second metal in the reaction and its impact on the reaction characteristics.

(Invited) De-Alloying of CuxAu(1-x) Alloys at Different Length Scales for the Development of Active Nanoporous Au Catalysts

The development of highly active, durable, and cost-effective catalysts is of paramount importance for the fuel cell industry. Most catalysts for relevant applications contain Pt that is known as one of the most expensive metals.1 To date, the best way to reduce the total catalyst cost is associated with the use of an inert support coated by a small amount of Pt-based catalyst. A good candidate for the support is nanoporous Au (NPG).1-3 As highly conductive material, NPG has an open, 3D interconnected porous framework with surface area that even in ultrathin NPG layer configuration could be tens to hundreds of times larger than the area of planar Au. Once developed on a C-based support, the NPG can be terminated with Pt or Pt-containing alloys by surface-limited redox replacement (SLRR), and then applied as the catalyst in the key catalytic reactions.4 In our previous work, we established an all-electrochemical synthetic approach for a NPG-based catalyst.4 The fabrication procedure entails AgxAu(1−x) alloy electrodeposition followed by selective dissolution of Ag (de-alloying) ultimately generating the desired NPG structure with a thickness of less than 20 nm. To further study the NPG applicability in catalysis, in this report we switch our focus to a CuxAu(1−x) precursor alloy synthesized for this work in different size and shape through various routes. Generally, the precursor is subjected to de-alloying and the resulting NPG is structurally and morphologically characterized and assessed for performance and durability in formic acid oxidation tests after platinization with ultra thin catalytically active layer. More specifically, CuxAu(1−x) bulk alloys, electrodeposited alloys, and chemically synthesized Cu3Au nanorods (intermetallic and random alloy)5-7 are subjected to a comparative de-alloying study with emphasis on their parting behavior and resulting structures. The de-alloying curves exhibit quite unique behavior owing to size and structural differences. Overall, the de-alloying curves for the system of interest illustrate multiple peaks that indirectly suggest phase coexistence dependent upon the synthetic route of the precursor alloy (melting, electrodeposition nanoparticle synthesis). The NPG resulting from the de-alloying process has been studied for surface area development by Pb underpotential deposition (Pb UPD). XRD is used to study the potential presence of different phases. SEM and TEM images display the morphology before and after de-alloying. Finally, basic activity and durability tests of the catalytic properties of various Pt-coated de-alloyed structures during formic acid oxidation are presented and discussed as well. In these tests the assessment of accordingly prepared catalysts in standard HCOOH oxidation experiments attests to their high catalytic activity which in some aspects exceeds the performance of recently studied nanoparticle based catalysts.

Catalytic production of methyl acrylates by gold-mediated cross coupling of unsaturated aldehydes with methanol

Modern methods of esterification, one of the most important reactions in organic synthesis, are reaching their limits, as far as waste and expense are concerned. Novel chemical approaches to ester formation are therefore of importance. Here we report a simple procedure free of caustic reagents or byproducts for the facile direct oxidative methyl esterification of aldehydes over nanoporous Au catalysts. Complementary model studies on single crystal gold surfaces establish the fundamental reactions involved. We find that methanol more readily reacts with adsorbed active oxygen than do the aldehydes, but that once the aldehydes do react, they form strongly-bound acrylates that block reactive sites and decrease the yields of acrylic esters under steady flow conditions at 420K. Significant improvements in yield can be achieved by operating at higher temperatures, which render the site-blocking acrylates unstable.

Designing for selectivity: weak interactions and the competition for reactive sites on gold catalysts.

A major challenge in heterogeneous catalysis is controlling reaction selectivity, especially in complex environments. When more than one species is present in the gas mixture, the competition for binding sites on the surface of a catalyst is an important factor in determining reaction selectivity and activity. We establish an experimental hierarchy for the binding of a series of reaction intermediates on Au(111) and demonstrate that this hierarchy accounts for reaction selectivity on both the single crystal surface and under operating catalytic conditions at atmospheric pressure using a nanoporous Au catalyst. A partial set of measurements of relative binding has been measured by others on other catalyst materials, including Ag, Pd and metal oxide surfaces; a comparison demonstrates the generality of this concept and identifies differences in the trends. Theoretical calculations for a subset of reactants on Au(111) show that weak van der Waals interactions are key to predicting the hierarchy of binding strengths for alkoxides bound to Au(111). This hierarchy is key to the control of the selectivity for partial oxidation of alcohols to esters on both Au surfaces and under working catalytic conditions using nanoporous gold. The selectivity depends on the competition for active sites among key intermediates. New results probing the effect of fluorine substitution are also presented to extend the relation of reaction selectivity to the hierarchy of binding. Motivated by an interest in synthetic manipulation of fluorinated organics, we specifically investigated the influence of the -CF3 group on alcohol reactivity and selectivity. 2,2,2-Trifluoroethanol couples on O-covered Au(111) to yield CF3CH2O-C([double bond, length as m-dash]O)(CF3), but in the presence of methanol or ethanol it preferentially forms the respective 2,2,2-trifluoroethoxy-esters. The ester is not the dominant product in any of these cases, though, indicating that the rate of β-H elimination from adsorbed trifluoroethoxy is slower than that for either adsorbed methoxy or ethoxy, consistent with their relative estimated β-C-H bond strengths. The measured equilibrium constants for the competition for binding to the surface are 2.9 and 0.38 for ethanol and methanol, respectively, vs. 2,2,2-trifluoroethanol, indicating that the binding strength of 2,2,2-trifluoroethoxy is weaker than ethoxy, but stronger than methoxy. These results are consistent with weakening of the interactions between the surface and the alkyl group due to Pauli repulsion of the electron-rich CF3 group from the surface, which offsets the van der Waals attraction. These experiments provide guiding principles for understanding the effect of fluorination on heterogeneous synthesis and further demonstrate the key role of molecular structure in determining reaction selectivity.

Aerobic Oxidation of Alcohols with a Nanoporous Au Catalyst

Key words aerobic oxidation - alcohols - nanoporous gold - flow reaction

On the Role of Residual Ag in Nanoporous Au Catalysts for CO Oxidation: A Combined Microreactor and TAP Reactor Study

AbstractThe influence of residual Ag on the formation of stable adsorbed active oxygen species (oxygen storage capacity, OSC) and its correlation with the activity and stability in the CO oxidation reaction on nanoporous gold (NPG) was investigated by a combination of kinetic and temporal analyses of products (TAP) reactor measurements, comparing four different NPG samples with different Ag contents. The data demonstrated that oxygen can be activated and stored on the NPG catalysts at room temperature. Clearly the surface Ag content influenced both the OSC and activity, with a more qualitative correlation for the fresh catalysts and an approximately linear relationship for the stable catalysts after 1000 min on stream. A strictly linear correlation between catalytic activity and OSC, both before and after the reaction, indicated that the active oxygen detected by the TAP reactor measurements also represents the active oxygen species for the reaction at atmospheric pressure. Consequences for the mechanistic understanding of the CO oxidation reaction on NPG catalysts are discussed.

Complete Electrochemical Fabrication of a Platinized Nanoporous Au Catalyst for Formic Acid Oxidation

not Available.

The mystery of gold's chemical activity: local bonding, morphology and reactivity of atomic oxygen

Recently, gold has been intensely studied as a catalyst for key synthetic reactions. Gold is an attractive catalyst because, surprisingly, it is highly active and very selective for partial oxidation processes suggesting promise for energy-efficient "green" chemistry. The underlying origin of the high activity of Au is a controversial subject since metallic gold is commonly thought to be inert. Herein, we establish that one origin of the high activity for gold catalysis is the extremely reactive nature of atomic oxygen bound in 3-fold coordination sites on metallic gold. This is the predominant form of O at low concentrations on the surface, which is a strong indication that it is most relevant to catalytic conditions. Atomic oxygen bound to metallic Au in 3-fold sites has high activity for CO oxidation, oxidation of olefins, and oxidative transformations of alcohols and amines. Among the factors identified as important in Au-O interaction are the morphology of the surface, the local binding site of oxygen, and the degree of order of the oxygen overlayer. In this Perspective, we present an overview of both theory and experiments that identify the reactive forms of O and their associated charge density distributions and bond strengths. We also analyze and model the release of Au atoms induced by O binding to the surface. This rough surface also has the potential for O(2) dissociation, which is a critical step if Au is to be activated catalytically. We further show the strong parallels between product distributions and reactivity for O-covered Au at low pressure (ultrahigh vacuum) and for nanoporous Au catalysts operating at atmospheric pressure as evidence that atomic O is the active species under working catalytic conditions when metallic Au is present. We briefly discuss the possible contributions of oxidants that may contain intact O-O bonds and of the Au-metal oxide support interface in Au catalysis. Finally, the challenges and future directions for fully understanding the activity of gold are considered.