1,3-Butadiene is an important commodity chemical and new, selective routes of catalytic synthesis using green feedstocks, such as ethanol, is of interest.

No abstract

The reaction of CO and O 2 with submonolayer and multilayer CoO x films on Pt(111), to produce CO 2 , was investigated at room temperature in the mTorr pressure regime. Using operan do ambient pressure X-ray photoelectron spectroscopy and high pressure scanning tunneling microscopy, as well as density functional theory calculations, we found that the presence of oxygen vacancies in partially oxidized CoO x films significantly enhances the activity of CO oxidation to form CO 2 upon exposure to mTorr pressures of CO at room temperature. In contrast, CoO films without O-vacancies are much less active for CO 2 formation at RT, and CO only adsorbed in the form of carbonate species stable up to 260° C. On submonolayer CoO x islands, the carbonates form preferentially at island edges, deactivating the edge sites for CO 2 formation, even while the reaction proceeds inside the islands. These results provide a detailed understanding of CO oxidation pathways on systems where noble metals such as Pt interact with reducible oxides. ASSOCIATED CONTENTSupporting Information. XPS peak fitting procedures, further temperature dependent XPS measurements, XPS of multilayer films in CO and CO + O 2 mixtures, irreducibility of carbonate films, further discussion of CoO x films, and additional DFT calculation details and results. This material is available free of charge via the internet at http://pubs.acs.org.

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.

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 multi-pronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge which increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogenous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step towards developing structure function relationships that inform the design of new catalysts.

In this work, we thoroughly examined the structure of the Co3O4(111) surface in oxidative and reductive conditions, i.e. in equilibrium with realistic pressures of O2/H2O and H2/H2O, using density functional theory with self-interaction and dispersion corrections. We found that this surface is, in fact, hydroxylated under most reaction conditions, and that subjecting the surface to H2 increases surface Co 2+ concentration. Large structural distortions facilitate the reduction and stabilization of the Co-rich termination. At 423 K, hydroxylation readily occurs on both the Orich and Co-rich surfaces even at water pressure as low as 10 -15 bar, and non-dissociated water molecules appear on the O-rich surface when water pressure is above ~10 -11 bar. Our approach showed good agreement with hybrid functional calculations and vibrational spectroscopy experiments. Under most catalytic conditions, where water is present as a reactant, product, or impurity, we predict that the Co3O4(111) surface will be hydroxylated. Hydroxyls groups and structural distortions undoubtedly play large roles in shaping the surface's catalytic properties and interaction with supported structures. The results of the study show the necessity of the inclusion of hydroxylation and surface Co concentration in computational studies of Co3O4 and provide surface structures under various conditions to aid in future studies on structure and catalytic reactivity of Co3O4(111) used as a support or as an active phase.

Understanding the structural dynamics of a catalyst under reaction conditions is challenging but crucial for designing catalysts. By combining in situ/operando characterization and first principles modelling, here we show that supported Rh catalysts undergo restructuring at the atomic scale in response to carbon monoxide, a gaseous product formed during the steam reforming of methane. Despite the transformation of the initially prepared single Rh cation catalyst into large Rh particles during hydrogen pretreatment, the formed Rh particles re-disperse to lownuclearity CO-liganded Rh clusters, Rhm(CO)n (m=1-3, n=2-4) under catalytic conditions.Theoretical simulations under reaction conditions suggest that the pressure of CO product stabilizes Rhm(CO)n sites, while in situ/operando spectroscopy reveals a reversible restructuring between Rh3(CO)3 clusters and Rh nanoparticles driven by CO pressure. The findings demonstrate the significance of including product molecules in the atomic-scale understanding of catalytic active sites and mechanisms.

The interaction of water with metal oxides controls their activity and stability in heterogeneous catalysis and electrocatalysis. In this work, we combine density functional theory (DFT) calculations and infrared reflection absorption spectroscopy (IRAS) to identify the structural motifs formed upon interaction of water with an atomicallydefined Co3O4(111) surface. Three principal structures are observed: (i) strongly bound isolated OD, (ii) extended hydrogen-bonded OD/D2O structures, and (iii) a third structure which has not been reported to our knowledge. In this structure, surface Co2+ ions bind to three D2O molecules to form an octahedrally coordinated Co2+ with a "half hydration shell". We propose that this hydration structure represents an important intermediate in re¬organization and dissolution on oxide surfaces which expose highly unsaturated surface cations. The interaction of transition metal oxides with water heavily affects the stability and reactivity of these materials. Water can induce surface restructuring, phase transitions, sintering or dissolution, all important for heterogeneous catalysis and electrocatalysis. 1-7 Despite the pivotal importance of oxide/water interactions, it has been a long standing challenge to understand these processes at the atomic level. 2 Recently, sophisticated characterization techniques and theoretical analyses led to major breakthroughs. Researchers could identify structural motifs of molecular and dissociated water on several well-defined oxide surfaces at the atomic level. 2,3,8-20 The interaction of Co3O4 and Fe3O4 surfaces with water is of high importance in water oxidation and water-gas shift catalysis. 3,21 Water also affects the interaction between supported Au atoms and Fe3O4. 22 Water adsorption on Fe 3+ terminated Fe3O4(111) was investigated by Freund, Schauermann, Sauer, Paier, and coworkers. 16,17,19 The authors identified OD/D2O structures arranged as ordered overlayers which are weakly bound and desorb below 300 K. 16,17,19