Effect of Co Loading on the Activity and Selectivity of PtCo Aqueous Phase Reforming Catalysts

The reaction site time yields (STYs, normalized to CO chemisorption sites) and product selectivity were measured for a series of bimetallic, multiwalled carbon nanotube supported PtCo catalysts with varying Pt/Co ratios for aqueous phase glycerol reforming. The STYs for all products increased by factors of around 2 for PtCo 1:0.5 and 1:1, and a factor of 4 for PtCo 1:5 relative to a monometallic Pt catalyst. The PtCo catalysts had similar hydrogen selectivity (>85%) at glycerol conversions up to 60%. X-ray absorption spectroscopy and scanning transmission electron microscopy characterization revealed that PtCo catalysts adopt monometallic Pt, mixed PtCo alloy, and Pt shell/Co core particle configurations. A linear correlation between the fraction of mixed PtCo alloy particles and the STY was found, indicating that higher Co loading resulted in a higher fraction of mixed PtCo alloy particles (the promoted phase) that provided the STY increase

PtMo Bimetallic Catalysts Synthesized by Controlled Surface Reactions for Water Gas Shift

Supported PtMo bimetallic catalysts were prepared by controlled surface reactions (CSR) and studied for water gas shift (WGS) at 543 K. Carbon and silica supports were used for the preparation of monometallic Pt catalysts, and Mo was deposited onto these catalysts by reaction with cycloheptatriene molybdenum tricarbonyl ((C₇H₈)­Mo­(CO)₃). Catalysts were characterized by CO chemisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), STEM/EDS, and XAS analysis. We report that carbon-supported Pt nanoparticles are saturated with Mo species at a Mo:Pt atomic ratio of 0.32. Molybdenum has a strong promotional effect in these catalysts, increasing the TOF by up to a factor of more than 4000. Silica-supported catalysts were found to be more active, but the TOF promotional effect of Mo was smaller than for the carbon-supported catalysts at 15. EDS analyses and activity studies showed that the formation of bimetallic catalysts was therefore more efficient using the carbon support. The active sites for WGS are suggested to be at the interface between Pt atoms and Mo moieties that are possibly in an oxidized form

Determination of CO, H2O and H2 coverage by XANES and EXAFS on Pt and Au during water gas shift reaction

The turn-over-rate (TOR) for the water gas shift (WGS) reaction at 200 1C, 7% CO, 9% CO2, 22% H2O, 37% H2 and balance Ar, of 1.4 nm Au/Al2O3 is approximately 20 times higher than that of 1.6 nm Pt/Al2O3. Operando EXAFS experiments at both the Au and Pt L3 edges reveal that under reaction conditions, the catalysts are fully metallic. In the absence of adsorbates, the metal–metal bond distances of Pt and Au catalysts are 0.07 A ˚ and 0.13 A ˚ smaller than those of bulk Pt and Au foils, respectively. Adsorption of H2 or CO on the Pt catalysts leads to significantly longer Pt–Pt bond distances; while there is little change in Au–Au bond distance with adsorbates. Adsorption of CO, H2 and H2O leads to changes in the XANES spectra that can be used to determine the surface coverage of each adsorbate under reaction conditions. During WGS, the coverage of CO, H2O, and H2 are obtained by the linear combination fitting of the difference XANES, or DXANES, spectra. Pt catalysts adsorb CO, H2, and H2O more strongly than the Au, in agreement with the lower CO reaction order and higher reaction temperatures

Palladium Nanoparticle Formation on TiO₂(110) by Thermal Decomposition of Palladium(II) Hexafluoroacetylacetonate

Palladium nanoparticles were synthesized by thermal decomposition of palladium­(II) hexafluoroacetylacetonate (Pd­(hfac)₂), an atomic layer deposition (ALD) precursor, on a TiO₂(110) surface. According to X-ray photoelectron spectroscopy (XPS), Pd­(hfac)₂ adsorbs on TiO₂(110) dissociatively yielding Pd­(hfac)_ads, hfac_ads, and adsorbed fragments of the hfac ligand at 300 K. A (2 × 1) surface overlayer was observed by scanning tunneling microscopy (STM), indicating that hfac adsorbs in a bidentate bridging fashion across two Ti 5-fold atoms and Pd­(hfac) adsorbs between two bridging oxygen atoms on the surface. Annealing of the Pd­(hfac)_ads and hfac_ads species at 525 K decomposed the adsorbed hfac ligands, leaving PdO-like species and/or Pd atoms or clusters. Above 575 K, the XPS Pd 3d peaks shift toward lower binding energies and Pd nanoparticles are observed by STM. These observations point to the sintering of Pd atoms and clusters to Pd nanoparticles. The average height of the Pd nanoparticles was 1.2 ± 0.6 nm at 575 K and increased to 1.7 ± 0.5 nm following annealing at 875 K. The Pd coverage was estimated from XPS and STM data to be 0.05 and 0.03 monolayers (ML), respectively, after the first adsorption/decomposition cycle. The amount of palladium deposited on the TiO₂(110) surface increased linearly with the number of adsorption/decomposition cycles with a growth rate of 0.05 ML or 0.6 Å per cycle. We suggest that the removal of the hfac ligand and fragments eliminates the nucleation inhibition of Pd nanoparticles previously observed for the Pd­(hfac)₂ precursor on TiO₂

Surface Chemistry of Trimethylaluminum on Pd(111) and Pt(111)

The behavior of trimethylaluminum (TMA) was investigated on the surfaces of Pt(111) and Pd(111) single crystals. TMA was found to dissociatively adsorb on both surfaces between 300–473 K. Surfaces species observed by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) after TMA adsorption at 300 K included Al-CH₃ and CH_<i>x</i>,ads (<i>x</i> = 1, 2, or 3) on Pt(111), and ethylidyne (CCH₃), CH_<i>x</i>,ads (<i>x</i> = 1, 2, or 3), and metallic Al on Pd(111). Density functional theory (DFT) calculations predicted methylaluminum (MA, Al-CH₃) to be the most kinetically favorable TMA decomposition product on (111) terraces of both surfaces, however, HREELS signatures for Al-CH₃ were detected only on Pt(111), whereas ethylidyne was observed on Pd(111). XPS demonstrated higher amounts of carbonaceous species on Pt(111) than on Pd(111). DFT calculations showed that further dissociation of MA to metallic aluminum and methyl groups to be more kinetically favorable on step sites of both metals. In our proposed reaction mechanism, MA migrates to and dissociates at Pd(111) steps at 300 K forming adsorbed methyl groups and metallic Al. Some methyl groups dehydrogenate and recombine forming ethylidyne. Metallic Al or ejected Pd atoms from steps diffuse across Pd(111) terraces until coalescing into irregularly shaped islands on terraces or steps, as observed by scanning tunneling microscopy (STM). Upon heating above 300 K, the Pd–Al alloy diffuses into the Pd bulk. On Pt(111), a high coverage of carbon-containing species following TMA adsorption at 300 K prevented MA diffusion and dissociation at steps, as evidenced by isolated clusters of MA in STM images. Heating above 300 K resulted in MA dissociation, but no Pt–Al alloy formation was observed. We conclude that the differing abilities of Pd and Pt to hydrogenate carbonaceous species plays a key role in MA dissociation and alloy formation, and therefore, the adsorption and dissociation chemistry of TMA depends on properties of the metal substrate surface and determines thin film morphology and composition

Zinc Promotion of Platinum for Catalytic Light Alkane Dehydrogenation: Insights into Geometric and Electronic Effects

Supported metal nanoparticles are vital as heterogeneous catalysts in the chemical transformation of hydrocarbon resources. The catalytic properties of these materials are governed by the surface electronic structure and valence orbitals at the active metal site and can be selectively tuned with promoters or by alloying. Through an integrated approach using density functional theory (DFT), kinetics, and <i>in situ</i> X-ray spectroscopies, we demonstrate how Zn addition to Pt/SiO₂ forms high symmetry Pt₁Zn₁ nanoparticle alloys with isolated Pt surface sites that enable near 100% C₂H₄ selectivity during ethane dehydrogenation (EDH) with a 6-fold higher turnover rate (TOR) per mole of surface Pt at 600 °C compared to monometallic Pt/SiO₂. Furthermore, we show how DFT calculations accurately reproduce the resonant inelastic X-ray spectroscopic (RIXS) signatures of Pt 5d valence orbitals in the Pt/SiO₂ and PtZn/SiO₂ catalysts that correlate with their kinetic performance during EDH. This technique reveals that Zn modifies the energy of the Pt 5d electrons in PtZn, which directly relates to TOR promotion, while ensemble effects from the incorporation of Zn into the catalyst surface lead to enhanced product selectivity

A Discovery of Strong Metal–Support Bonding in Nanoengineered Au–Fe₃O₄ Dumbbell-like Nanoparticles by in Situ Transmission Electron Microscopy

The strength of metal–support bonding in heterogeneous catalysts determines their thermal stability, therefore, a tremendous amount of effort has been expended to understand metal–support interactions. Herein, we report the discovery of an anomalous “strong metal–support bonding” between gold nanoparticles and “nano-engineered” Fe₃O₄ substrates by in situ microscopy. During in situ vacuum annealing of Au–Fe₃O₄ dumbbell-like nanoparticles, synthesized by the epitaxial growth of nano-Fe₃O₄ on Au nanoparticles, the gold nanoparticles transform into the gold thin films and wet the surface of nano-Fe₃O₄, as the surface reduction of nano-Fe₃O₄ proceeds. This phenomenon results from a unique coupling of the size-and shape-dependent high surface reducibility of nano-Fe₃O₄ and the extremely strong adhesion between Au and the reduced Fe₃O₄. This strong metal–support bonding reveals the significance of controlling the metal oxide support size and morphology for optimizing metal–support bonding and ultimately for the development of improved catalysts and functional nanostructures

A Discovery of Strong Metal–Support Bonding in Nanoengineered Au–Fe₃O₄ Dumbbell-like Nanoparticles by in Situ Transmission Electron Microscopy

The strength of metal–support bonding in heterogeneous catalysts determines their thermal stability, therefore, a tremendous amount of effort has been expended to understand metal–support interactions. Herein, we report the discovery of an anomalous “strong metal–support bonding” between gold nanoparticles and “nano-engineered” Fe₃O₄ substrates by in situ microscopy. During in situ vacuum annealing of Au–Fe₃O₄ dumbbell-like nanoparticles, synthesized by the epitaxial growth of nano-Fe₃O₄ on Au nanoparticles, the gold nanoparticles transform into the gold thin films and wet the surface of nano-Fe₃O₄, as the surface reduction of nano-Fe₃O₄ proceeds. This phenomenon results from a unique coupling of the size-and shape-dependent high surface reducibility of nano-Fe₃O₄ and the extremely strong adhesion between Au and the reduced Fe₃O₄. This strong metal–support bonding reveals the significance of controlling the metal oxide support size and morphology for optimizing metal–support bonding and ultimately for the development of improved catalysts and functional nanostructures

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃ are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein’s rule. Further, exchanged Cu^II ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu^IIOH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH₃ under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13