Nature of the metal-support interface in supported metal catalysts: Results from X-ray absorption spectroscopy

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

General Descriptors for CO2-Assisted Selective C-H/C-C Bond Scission in Ethane

The selective C-H/C-C bond scission in CO₂-assisted alkane activation represents an opportunity for simultaneously upgrading greenhouse gas CO₂ and light alkanes for the synthesis of value-added syngas (CO and H2), olefins, aromatics, and oxygenates. Here, Pd bimetallic (PdM_x)-derived catalysts were investigated for ethane-CO₂ reactions by combining kinetic analysis, in situ characterization, and density functional theory calculations. Two types of catalyst structures were identified under the reaction conditions, with the PdCo_x alloy surface favoring ethoxy formation, a critical precursor for further C-C bond scission, and the reaction-induced InO_x/Pd interface promoting C-H bond scission. Our results revealed a general strategy to capture the reaction-induced surface configurations and in turn control the selectivity in C-C/C-H bond scission over PdM_x-derived catalysts, featuring the interplay of two general descriptors: formation energy of PdM_x surfaces and their binding energy to oxygen. Our study provides insight into the rational design of selective catalysts for light alkane-CO₂ reactions.

Dynamic structure of active sites in ceria-supported Pt catalysts for the water gas shift reaction

Oxide-supported noble metal catalysts have been extensively studied for decades for the water gas shift (WGS) reaction, a catalytic transformation central to a host of large volume processes that variously utilize or produce hydrogen. There remains considerable uncertainty as to how the specific features of the active metal-support interfacial bonding—perhaps most importantly the temporal dynamic changes occurring therein—serve to enable high activity and selectivity. Here we report the dynamic characteristics of a Pt/CeO₂ system at the atomic level for the WGS reaction and specifically reveal the synergistic effects of metal-support bonding at the perimeter region. We find that the perimeter Pt⁰ − O vacancy−Ce³⁺ sites are formed in the active structure, transformed at working temperatures and their appearance regulates the adsorbate behaviors. We find that the dynamic nature of this site is a key mechanistic step for the WGS reaction.

Revealing the structure and dynamics of active sites is essential to understand catalytic mechanisms. Here the authors demonstrate the dynamic nature of perimeter Pt⁰−O vacancy−Ce³⁺ sites in Pt/CeO₂ and the key effects of their dynamics on the mechanism of the water gas shift reaction.

Ligand-coordinated Ir single-atom catalysts stabilized on oxide supports for ethylene hydrogenation and their evolution under a reductive atmosphere

Effective, stable, durable, and tunable Ir-ligand single-atom catalysts for ethylene hydrogenation, studied in situ for structural evolution of Ir single-atoms under a reducing atmosphere.

Supported metal and metal oxide particles with proximity effect for catalysis

External influence is essential for any change to occur in this world. Similarly, the reaction path of a chemical reaction can be changed with the addition of a catalyst from outside. Sometimes a catalyst performs better when it remains associated with an inert substance, which is called a support material (SM). Improved catalyst accomplishment arises from the 'proximity effect'. Even inert supports play a role in better product formulation or environmental remediation. In this review, it has been shown how the SM, as a nest, aids the catalyst particle synergistically to perform a good job in a chemical reaction. The structure-function relationship of SM helps in catalyst activation to some extent, and produces active centres that are difficult to fully ascertain. In the text, Langmuir-Hinshelwood (L-H), Mars-van Krevelen (MVK), and Eley-Rideal (E-R) mechanisms are highlighted for the adsorption processes as the case may be. Again, the importance of SM for both catalyst and substrates has been consolidated here in the text. Finally, the role of the initiator and the promoter is also discussed in this review.

Oxychlorination Redispersion of Pt Catalysts: Surface Species and Pt-support Interactions Characterized by X-Ray Absorption and FT-IR Spectroscopy

To help elucidate the oxychlorination redispersion reaction mechanism, the surface species formed on the surface of -Al2O3 was characterized by X-ray absorption spectroscopy (XAS). The efficacy of redispersion was assessed by the Pt–Pt coordination number (CNPt–Pt) of redispersed, and then reduced samples. A nearly fully redispersed complex (Ptrd52) was prepared by treating a sintered model Pt/-Al2O3 catalyst at 520 °C, Air/EDC (ethylene dichloride) of 30, and WHSV (Weight Hourly Space Velocity) of 0.07 h−1 for 16 h. For investigating temperature effects, samples treated at 460 (Ptrd46) and 560 C (Ptrd56) were also prepared for comparison. It was found that, while an octahedral resembling Pt(Os)3–4(O–Cl)2–3 (Os represents support oxygen or hydroxyl oxygen) complex was formed on -Al2O3 of Ptrd52, less O–Cl ligands were formed on the redispersed complexes, Ptrd46 and Ptrd56. A negative correlation of CNPt–Pt with CNPt–Cl* (Cl* represents the Cl atom in O–Cl ligand) for these three samples further suggested that the formation of Pt–O–Cl played a key role in the redispersion process. Pt–O–Cl could be formed in the reaction of reactive Cl and PtO2. At an operation temperature of lower-than-optimal temperatures of 520 C, less Cl2 dissociation and less O–Cl ligands were formed. On the other hand, higher temperatures may facilitate Cl2 dissociation, but reduce the equilibrium conversion of HCl to Cl2, leading to increased HCl reaction with Pt (PtO2) clusters to form Pt–Cl (Cl is the atom bonded directly to Pt), and decreased formation of Pt–O–Cl.

Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports

Single-site Ir(CO)2 complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)2(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.

Air Regeneration of Ethanol-Laden Pellet NaY-SiO2 and Pt/NaY-SiO2: Effects of Air Flow Rate on Pt Morphology and Regeneration Efficiency

Regeneration process and adsorbent performance were investigated by a fixed-bed adsorber at 300 °C. Surface species, zeolite structure, and Pt morphology were characterized by FT-IR, XRPD and EXAFS, respectively. Performance test results indicated that ethanol adsorption capacity of Pt/NaY-SiO2 is about 2.5 times that of NaY-SiO2. After regeneration, adsorption-capacity loss is 2.5 and 43%, respectively, for Pt/NaY-SiO2 regenerated at superficial velocity of 13.2 (PtR(HF)) and 5.3 cm/min (PtR(LF)); in contrast, it is 8 and 21%, respectively, for NaYR(HF) and NaYR(LF). The appearance of absorption bands in the CH stretching region (υCH) of the IR spectra characterizing the regenerated NaY-SiO2 suggested that the adsorption-capacity loss for NaY-SiO2 was mainly caused by the deposition of carbonaceous species formed in regeneration, which cannot be burned off readily at 300 °C. In contrast, no υCH bands have been observed for the IR spectra of PtR(HF) and PtR(LF), indicating that Pt helps to burn off carbonaceous species. However, Pt agglomeration was observed in TEM and EXAFS for Pt/NaY-SiO2(LF). The appearance of a υCO band at about 2085 cm−1 of the IR spectra characterizing PtR(LF) suggested that Pt agglomeration was induced by CO adsorption. The growth of Pt particles decreases the ethanol adsorbed on Pt together with the conversion of ethanol to ethoxides and aldehyde, leading to a decrease of adsorption capacity.

Importance of platinum particle size for complete oxidation of toluene over Pt/ZSM-5 catalysts

Pt-1.9/ZSM-5 exhibits the highest activity in the removal of toluene due to a balance of Pt dispersion and Pt⁰ proportion.

Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2

An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO(2)-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H(2) using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO(2) support was observed, and Pd(δ+) species were detected. This process parallels the high production of N(2)O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N(2) (∼80%). Concomitant with the onset of N(2) production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H(2)-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO(x) was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.

Microstructural and metal-support interactions of the Pt-CeO2/C catalysts for direct methanol fuel cell application

To understand the ceria promotion effect of Pt-CeO(2)/C catalysts on methanol oxidation, microstructural and metal-oxide interactions of Pt-CeO(2)/C catalysts with an atomic ratio of Pt/Ce between 0.14 and 1.4 were systematically examined using high-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS). With an increasing Pt content in the catalysts, Pt particles gradually invaded into the ceria supports and decoration on Pt particles was observed. Simultaneously, the morphology of the supports was dramatically modified with nanocrystalline and amorphous ceria formed between and/or around the Pt particles. It reveals that the Pt-ceria interaction could take place in the catalysts and the influence of the interaction was enhanced with an increasing Pt/Ce ratio. The EELS study demonstrated that the strong Pt-ceria interaction was related to the redox reaction between Pt and ceria. Experimental results also suggested that the strong interaction between Pt and ceria could contribute to the promotion effect of ceria on the oxidation of methanol.

Synthesis and characterization of catalytic iridium nanoparticles in imidazolium ionic liquids

The reduction of [Ir(cod)Cl](2) (cod=1,5-cyclooctadiene) dissolved in 1-n-butyl-3-methyl tetrafluoroborate, hexafluorophosphate and trifluoromethane sulphonate ionic liquids in the presence of 1-decene by molecular hydrogen produces Ir(0) nanoparticles. The formation of these nanoparticles follows the two-step [A-->B, A+B-->2B (k(1),k(2))] autocatalytic mechanism. The same mean diameter values of around 2-3 nm were estimated from in situ TEM and SAXS analyses of the Ir(0) nanoparticles dispersed in the ionic liquids and by XRD of the isolated material. XPS and EXAFS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating the formation of an ionic liquid protective layer surrounding the iridium nanoparticles. SAXS analysis indicated the formation of an ionic liquid layer surrounding the metal particles with an extended molecular length of around 2.8-4.0 nm depending on the type of the anion.

Oxide- and Zeolite-Supported Molecular Metal Complexes and Clusters: Physical Characterization and Determination of Structure, Bonding, and Metal Oxidation State

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.

Pt3Ru6Clusters Supported on γ-Al2O3: Synthesis from Pt3Ru6(CO)21(μ3-H)(μ-H)3, Structural Characterization, and Catalysis of Ethylene Hydrogenation andn-Butane Hydrogenolysis

The supported clusters Pt-Ru/gamma-Al2O3 were prepared by adsorption of the bimetallic precursor Pt3Ru6(CO)21(mu3-H)(mu-H)3 from CH2Cl2 solution onto gamma-Al2O3 followed by decarbonylation in He at 300 degrees C. The resultant supported clusters were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and as catalysts for ethylene hydrogenation and n-butane hydrogenolysis. After adsorption, the nu(CO) peaks characterizing the precursor shifted to lower wavenumbers, and some of the hydroxyl bands of the support disappeared or changed, indicating that the CO ligands of the precursor interacted with support hydroxyl groups. The EXAFS results show that the metal core of the precursor remained essentially unchanged upon adsorption, but there were distortions of the metal core indicated by changes in the metal-metal distances. After decarbonylation of the supported clusters, the EXAFS data indicated that Pt and Ru atoms interacted with support oxygen atoms and that about half of the Pt-Ru bonds were maintained, with the composition of the metal frame remaining almost unchanged. The decarbonylated supported bimetallic clusters reported here are the first having essentially the same metal core composition as that of a precursor metal carbonyl, and they appear to be the best-defined supported bimetallic clusters. The material was found to be an active catalyst for ethylene hydrogenation and n-butane hydrogenolysis under conditions mild enough to prevent substantial cluster disruption.

A Site-Isolated Rhodium−Diethylene Complex Supported on Highly Dealuminated Y Zeolite: Synthesis and Characterization

The reaction of Rh(C2H4)2(acac) with the partially dehydroxylated surface of dealuminated zeolite Y (calcined at 773 K) and treatments of the resultant surface species in various atmospheres (He, CO, H2, and D2) were investigated with infrared (IR), extended X-ray absorption fine structure (EXAFS), and 13C NMR spectroscopies. The IR spectra show that Rh(C2H4)2(acac) reacted readily with surface OH groups of the zeolite, leading to loss of acac ligands from the Rh(C2H4)2(acac) and formation of supported mononuclear rhodium complexes, confirmed by the lack of Rh-Rh contributions in the EXAFS spectra; each Rh atom was bonded on average to two oxygen atoms of the zeolite surface with a Rh-O distance of 2.19 A. IR, EXAFS, and 13C NMR spectra show that the ethylene ligands remained bonded to the Rh center in the supported complex. Treatment of the sample in CO led to the formation of site-isolated Rh(CO)2 complexes bonded to the zeolite. The sharpness of the nu(CO) bands in the IR spectrum gives evidence of a nearly uniform supported Rh(CO)2 complex and, by inference, the near uniformity of the mononuclear rhodium complex with ethylene ligands from which it was formed. The supported complex with ethylene ligands reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at 294 K.

Synthesis and reactivity of dimethyl gold complexes supported on MgO: characterization by infrared and X-ray absorption spectroscopies

Dimethyl gold complexes bonded to partially dehydroxylated MgO powder calcined at 673 K were synthesized by adsorption of Au(CH3)2(acac) (acac is C5H7O2) from n-pentane solution. The synthesis and subsequent decomposition of the complexes by treatment in He or H2 were characterized with diffuse reflectance Fourier transform infrared (DRIFT), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The XANES results identify Au(III) in the supported complexes, and the EXAFS and DRIFTS data indicate mononuclear dimethyl gold complexes as the predominant surface gold species, consistent with the lack of Au-Au contributions in the EXAFS spectrum and the presence of nu(as)(CH3) and nu(s)(CH3) bands in the IR spectrum. EXAFS data show that each complex is bonded to two oxygen atoms of the MgO surface at an Au-O distance of 2.16 angstroms. The DRIFT spectra show that reaction of Au(CH3)2(acac) with MgO at room temperature also formed Mg(acac)2 and H(acac) species on the support. Treatment of the dimethyl gold complexes in He or H2 at increasing temperatures varying from 373 to 573 K removed CH3 ligands and caused aggregation forming zerovalent gold nanoclusters of increasing size, ultimately with an average diameter of about 30 angstroms. Analysis of the gas-phase products during the genesis of the gold clusters indicated formation of CH4 (consistent with removal of CH3 groups) and CO2 at 473-573 K, associated with decomposition of the organic ligands derived from acac species. O2 and CO2 were also formed in the decomposition of ubiquitous carbonates present on the surface of the MgO support.