Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites.

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

Synthesis and Characterization of Stable Cu-Pt Nanoparticles under Reductive and Oxidative Conditions.

We report a synthesis method for highly monodisperse Cu-Pt alloy nanoparticles. Small and large Cu-Pt particles with a Cu/Pt ratio of 1:1 can be obtained through colloidal synthesis at 300 °C. The fresh particles have a Pt-rich surface and a Cu-rich core and can be converted into an intermetallic phase after annealing at 800 °C under H2. First, we demonstrated the stability of fresh particles under redox conditions at 400 °C, as the Pt-rich surface prevents substantial oxidation of Cu. Then, a combination of in situ scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, and CO oxidation measurements of the intermetallic CuPt phase before and after redox treatments at 800 °C showed promising activity and stability for CO oxidation. Full oxidation of Cu was prevented after exposure to O2 at 800 °C. The activity and structure of the particles were only slightly changed after exposure to O2 at 800 °C and were recovered after re-reduction at 800 °C. Additionally, the intermetallic CuPt phase showed enhanced catalytic properties compared to the fresh particles with a Pt-rich surface or pure Pt particles of the same size. Thus, the incorporation of Pt with Cu does not lead to a rapid deactivation and degradation of the material, as seen with other bimetallic systems. This work provides a synthesis route to control the design of Cu-Pt nanostructures and underlines the promising properties of these alloys (intermetallic and non-intermetallic) for heterogeneous catalysis.

Holmium (Ho) oxide, carbide, and dioxide cation bond energies and evaluation of the Ho + O → HoO+ + e- chemi-ionization reaction enthalpy.

Guided ion beam tandem mass spectrometry (GIBMS) and quantum chemical calculations are employed to evaluate the title chemi-ionization reaction with holmium. Exchange reactions of Ho+ with O2, CO, and SO2 and HoO+ with CO, as well as collision-induced dissociation (CID) reactions of HoO+ with Xe, O2, and CO, were performed using GIBMS. Formation of HoO+ is exothermic in reactions with O2 and SO2 but endothermic for reaction with CO, as is the exchange reaction of HoO+ with CO. Quantitative analysis of these reactions and the three CID reactions provides a robust method to determine the bond dissociation energy (BDE) of Ho+-O, 6.02 ± 0.13 eV. BDEs for Ho+-C and OHo+-O are also measured as 2.27 ± 0.19 and 2.70 ± 0.27 eV, respectively. All three measurements are the first direct determinations of these BDEs. By combining the BDE of HoO+ with the well-established ionization energy of Ho, the exothermicity of Ho in the title chemi-ionization reaction can also be obtained as 0.00 ± 0.13 eV. All experimental thermochemistry was then compared to quantum chemical calculations for the purpose of establishing benchmarks and validation. BDEs determined via these calculations are in agreement with the experiment within the inherent experimental and theoretical uncertainties, with results obtained at the coupled-cluster with single, double, and perturbative triple excitations, CCSD(T), using all-electron basis sets yielding the most accurate results.

Comparative study of single-atom gold and iridium on CeO2{111}.

Oxide-supported single-atom catalysts have shown promise for a variety of heterogeneous processes. In addition to their inherent activity and selectivity, these materials come at much lower financial cost, avoiding the use of full-bodied precious-metal catalysts, but at the conceptual expense that more complex structural and electronic considerations need to be understood if we are to exploit their full potential. Here, we focus on the adsorption of single-atom iridium at both stoichiometric and defective CeO2{111} surfaces, by means of first-principles density functional theory. Reference calculations for the adsorption of single-atom gold, on the same set of substrates, provide a valuable set of benchmarks against which to interpret our iridium results.

Formation of a Ti-Cu(111) single atom alloy: Structure and CO binding.

A single atom Ti-Cu(111) surface alloy can be generated by depositing small amounts of Ti onto Cu(111) at slightly elevated surface temperatures (∼500 to 600 K). Scanning tunneling microscopy shows that small Ti-rich islands covered by a Cu single layer form preferentially on ascending step edges of Cu(111) during Ti deposition below about 400 K but that a Ti-Cu(111) alloy replaces these small islands during deposition between 500 and 600 K, producing an alloy in the brims of the steps. Larger partially Cu-covered Ti-containing islands also form on the Cu(111) terraces at temperatures between 300 and 700 K. After surface exposure to CO at low temperatures, reflection absorption infrared spectroscopy (RAIRS) reveals distinct C-O stretch bands at 2102 and 2050 cm-1 attributed to CO adsorbed on Cu-covered Ti-containing domains vs sites in the Ti-Cu(111) surface alloy. Calculations using density functional theory (DFT) suggest that the lower frequency C-O stretch band originates specifically from CO adsorbed on isolated Ti atoms in the Ti-Cu(111) surface alloy and predicts a higher C-O stretch frequency for CO adsorbed on Cu above subsurface Ti ensembles. DFT further predicts that CO preferentially adsorbs in flat-lying configurations on contiguous Ti surface structures with more than one Ti atom and thus that CO adsorbed on such structures should not be observed with RAIRS. The ability to generate a single atom Ti-Cu(111) alloy will provide future opportunities to investigate the surface chemistry promoted by a representative early transition metal dopant on a Cu(111) host surface.

Methane Adducts of Gold Dimer Cations: Thermochemistry and Structure from Collision-Induced Dissociation and Association Kinetics.

The bond dissociation energies at 0 K (BDE) of Au2+-CH4 and Au2CH4+-CH4 have been determined using two separate experimental methods. Analyses of collision-induced dissociation cross sections for Au2CH4+ + Xe and Au2(CH4)2+ + Xe measured using a guided ion beam tandem mass spectrometer (GIBMS) yield BDEs of 0.71 ± 0.05 and 0.57 ± 0.07 eV, respectively. Statistical modeling of association kinetics of Au2(CH4)0-2+ + CH4 + He measured from 200 to 400 K and at 0.3-0.9 Torr using a selected-ion flow tube (SIFT) apparatus yields slightly higher values of 0.81 ± 0.21 and 0.75 ± 0.25 eV. The SIFT data also place a lower limit on the BDE of Au2C2H8+-CH4 of 0.35 eV, likely an activated isomer, not Au2(CH4)2+-CH4. Particular emphasis is placed on determining the uncertainty in the derivation from association kinetics measurements, including uncertainties in collisional energy transfer, calculated harmonic frequencies, and possible contribution of isomerization of the association complexes. This evaluation indicates that an uncertainty of ±0.2 eV should be expected and that an uncertainty of better than ±0.1 eV is unlikely to be reasonable.

Bond dissociation energy of Au2 +: A guided ion beam and theoretical investigation.

Guided ion beam tandem mass spectrometry was employed to measure the kinetic energy-dependent product ion cross sections for the collision induced dissociation of Au2 + with Xe. Gold dimer cations were formed in a glow discharge flow tube source that should create ions in their ground electronic state with thermal internal energies. Analysis of the endothermic kinetic energy dependent cross section accounts for multiple collisions, lifetime effects, and the internal energy of the reactant ion. The value obtained for the bond dissociation energy (BDE) of Au2 + is the first direct measurement and is reported here as 2.20 ± 0.21 eV. For comparison with experimental results, theoretical calculations were also completed at the B3LYP, M06-2X, and coupled cluster singles, doubles, and perturbative triples [CCSD(T,full)] levels of theory using the def2-TZVPPD basis set and at the CCSD(T)-F12/correlation-consistent polarized valence triple zeta basis with pseudopotential level. These results predict a 2Σg + electronic ground state for Au2 + with BDEs calculated at the B3LYP and both CCSD(T) levels of theory in agreement with the experiment within the uncertainty. Several electronically excited states are also evaluated theoretically.

Experimental and theoretical investigations of infrared multiple photon dissociation spectra of lysine complexes with Zn2+ and Cd2.

The gas-phase structures of zinc and cadmium complexes of lysine (Lys) are investigated via a combination of infrared multiple photon dissociation action spectroscopy and ab initio quantum chemical calculations. In order to unambiguously identify the experimentally observed species, [Zn(Lys-H)]+ and CdCl+(Lys), the action spectra were compared to linear absorption spectra calculated at the B3LYP level of theory, using 6-311+G(d,p) and def2-TVZP basis sets for the zinc and cadmium systems, respectively. Single point energies were also calculated at the B3LYP, B3P86, MP2, and B3LYP-GD3BJ (accounting for empirical dispersion) levels of theory using larger basis sets. Identification of the experimentally formed isomers is possible through good agreement between infrared multiple photon dissociation action spectra and the theoretically predicted spectra. The [Zn(Lys-H)]+ complex adopts a tridentate orientation involving the amino acid backbone amine and deprotonated carboxylic acid groups as well as the side-chain amine group, [Nα,CO-,Nɛ]. The CdCl+(Lys) complex similarly adopts a tridentate chelation involving the amino acid backbone amine and carbonyl groups, as well as the side-chain amine group, [Nα,CO,Nɛ]. In both cases, the identified complexes are the lowest energy gas-phase structures at all levels of theory.

Structures of the dehydrogenation products of methane activation by 5d transition metal cations revisited: Deuterium labeling and rotational contours.

A previous infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory (DFT) study explored the structures of the [M,C,2H]+ products formed by dehydrogenation of methane by four, gas-phase 5d transition metal cations (M+ = Ta+, W+, Ir+, and Pt+). Complicating the analysis of these spectra for Ir and Pt was observation of an extra band in both spectra, not readily identified as a fundamental vibration. In an attempt to validate the assignment of these additional peaks, the present work examines the gas phase [M,C,2D]+ products of the same four metal ions formed by reaction with perdeuterated methane (CD4). As before, metal cations are formed in a laser ablation source and react with methane pulsed into a reaction channel downstream, and the resulting products are spectroscopically characterized through photofragmentation using the free-electron laser for intracavity experiments in the 350-1800 cm-1 range. Photofragmentation was monitored by the loss of D for [Ta,C,2D]+ and [W,C,2D]+ and of D2 in the case of [Pt,C,2D]+ and [Ir,C,2D]+. Comparison of the experimental spectra and DFT calculated spectra leads to structural assignments for all [M,C,2H/2D]+ systems that are consistent with previous identifications and allows a full description of the systematic spectroscopic shifts observed for deuterium labeling of these complexes, some of the smallest systems to be studied using IRMPD action spectroscopy. Further, full rotational contours are simulated for each vibrational band and explain several observations in the present spectra, such as doublet structures in several bands as well as the observed linewidths. The prominent extra bands in the [Pt,C,2D/2H]+ spectra appear to be most consistent with an overtone of the out-of-plane bending vibration of the metal carbene cation structure.

Experimental and theoretical investigations of infrared multiple photon dissociation spectra of glutamic acid complexes with Zn2+ and Cd2.

Complexes of glutamic acid (Glu) cationized with Zn2+ and Cd2+ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. Complexes of deprotonated Glu with Zn2+, [Zn(Glu-H)ACN]+ (where ACN = acetonitrile, CH3CN), and intact Glu with CdCl+, CdCl+(Glu) were formed. Notably, photodissociation induces Glu fragmentation rather than ACN loss in the Zn2+ complex. In order to identify the structures formed experimentally, the experimentally obtained spectra were compared to those calculated from optimized structures at the B3LYP/6-311+G(d,p) level for [Zn(Glu-H)ACN]+ and B3LYP/def2-TZVP level with an SDD effective core potential on cadmium for the CdCl+(Glu) system. The main binding motif observed for the heavy metal complex is a charge solvated, tridentate [N,COs,CO] structure where the metal binds to the backbone amino group and carbonyl oxygens of the side-chain and backbone carboxylic acid groups. The Zn2+ system similarly prefers a [N,CO-,COs] binding motif, where binding is observed at one oxygen of the backbone carboxylate site along with the backbone amino and side-chain carbonyl groups. In both cases, the theoretically determined lowest-energy conformers explain the experimental [Zn(Glu-H)ACN]+ and CdCl+(Glu) spectra well.

Correction: Experimental and theoretical investigations of infrared multiple photon dissociation spectra of glutamic acid complexes with Zn2+ and Cd2.

Correction for 'Experimental and theoretical investigations of infrared multiple photon dissociation spectra of glutamic acid complexes with Zn2+ and Cd2+' by Georgia C. Boles et al., Phys. Chem. Chem. Phys., 2017, 19, 12394-12406.

Low-index mesoscopic surface reconstructions of Au surfaces using Bayesian force fields.

Metal surfaces have long been known to reconstruct, significantly influencing their structural and catalytic properties. Many key mechanistic aspects of these subtle transformations remain poorly understood due to limitations of previous simulation approaches. Using active learning of Bayesian machine-learned force fields trained from ab initio calculations, we enable large-scale molecular dynamics simulations to describe the thermodynamics and time evolution of the low-index mesoscopic surface reconstructions of Au (e.g., the Au(111)-'Herringbone,' Au(110)-(1 × 2)-'Missing-Row,' and Au(100)-'Quasi-Hexagonal' reconstructions). This capability yields direct atomistic understanding of the dynamic emergence of these surface states from their initial facets, providing previously inaccessible information such as nucleation kinetics and a complete mechanistic interpretation of reconstruction under the effects of strain and local deviations from the original stoichiometry. We successfully reproduce previous experimental observations of reconstructions on pristine surfaces and provide quantitative predictions of the emergence of spinodal decomposition and localized reconstruction in response to strain at non-ideal stoichiometries. A unified mechanistic explanation is presented of the kinetic and thermodynamic factors driving surface reconstruction. Furthermore, we study surface reconstructions on Au nanoparticles, where characteristic (111) and (100) reconstructions spontaneously appear on a variety of high-symmetry particle morphologies.

Learning local equivariant representations for large-scale atomistic dynamics.

A simultaneously accurate and computationally efficient parametrization of the potential energy surface of molecules and materials is a long-standing goal in the natural sciences. While atom-centered message passing neural networks (MPNNs) have shown remarkable accuracy, their information propagation has limited the accessible length-scales. Local methods, conversely, scale to large simulations but have suffered from inferior accuracy. This work introduces Allegro, a strictly local equivariant deep neural network interatomic potential architecture that simultaneously exhibits excellent accuracy and scalability. Allegro represents a many-body potential using iterated tensor products of learned equivariant representations without atom-centered message passing. Allegro obtains improvements over state-of-the-art methods on QM9 and revMD17. A single tensor product layer outperforms existing deep MPNNs and transformers on QM9. Furthermore, Allegro displays remarkable generalization to out-of-distribution data. Molecular simulations using Allegro recover structural and kinetic properties of an amorphous electrolyte in excellent agreement with ab-initio simulations. Finally, we demonstrate parallelization with a simulation of 100 million atoms.

Active learning of reactive Bayesian force fields applied to heterogeneous catalysis dynamics of H/Pt.

Atomistic modeling of chemically reactive systems has so far relied on either expensive ab initio methods or bond-order force fields requiring arduous parametrization. Here, we describe a Bayesian active learning framework for autonomous "on-the-fly" training of fast and accurate reactive many-body force fields during molecular dynamics simulations. At each time-step, predictive uncertainties of a sparse Gaussian process are evaluated to automatically determine whether additional ab initio training data are needed. We introduce a general method for mapping trained kernel models onto equivalent polynomial models whose prediction cost is much lower and independent of the training set size. As a demonstration, we perform direct two-phase simulations of heterogeneous H2 turnover on the Pt(111) catalyst surface at chemical accuracy. The model trains itself in three days and performs at twice the speed of a ReaxFF model, while maintaining much higher fidelity to DFT and excellent agreement with experiment.

Decoding reactive structures in dilute alloy catalysts.

Rational catalyst design is crucial toward achieving more energy-efficient and sustainable catalytic processes. Understanding and modeling catalytic reaction pathways and kinetics require atomic level knowledge of the active sites. These structures often change dynamically during reactions and are difficult to decipher. A prototypical example is the hydrogen-deuterium exchange reaction catalyzed by dilute Pd-in-Au alloy nanoparticles. From a combination of catalytic activity measurements, machine learning-enabled spectroscopic analysis, and first-principles based kinetic modeling, we demonstrate that the active species are surface Pd ensembles containing only a few (from 1 to 3) Pd atoms. These species simultaneously explain the observed X-ray spectra and equate the experimental and theoretical values of the apparent activation energy. Remarkably, we find that the catalytic activity can be tuned on demand by controlling the size of the Pd ensembles through catalyst pretreatment. Our data-driven multimodal approach enables decoding of reactive structures in complex and dynamic alloy catalysts.