Pd Oxide Research Articles

Changes of Pd Oxidation State in Pd/Al2O3 Catalysts Using Modulated Excitation DRIFTS

Infrared spectroscopy is typically not used to establish the oxidation state of metal-based catalysts. In this work, we show that the baseline of spectra collected in diffuse reflectance mode of a series of Pd/Al2O3 samples of increasing Pd content varies significantly and reversibly under alternate pulses of CO or H2 and O2. Moreover, these baseline changes are proportional to the Pd content in Pd/Al2O3 samples exhibiting comparable Pd particle size. Similar measurements by X-ray absorption spectroscopy on a different 2 wt.% Pd/Al2O3 confirm that the baseline changes reflect the reversible reduction-oxidation of Pd. Hence, we demonstrate that changes in oxidation state of metal-based catalysts can be determined using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and that this behavior is part of the spectral changes that are returned by experiments under operando conditions.

Oxidation of Pd(II) with disilane in a palladium-catalyzed disilylation of aryl halides: a theoretical view.

Density functional theory (DFT) calculation has been used to reveal the mechanism of the Pd-catalyzed disilylation reaction of aryl halides. The DFT calculations indicate that the reaction starts with the oxidative addition of the C-I bond to the Pd(0) catalyst. Concerted metalation-deprotonation (CMD) can then generate a five-membered palladacycle. Insertion of Pd(ii) into the Si-Si bond in disilane followed by two sequential steps of reductive eliminations yields the disilylation product and regenerates the Pd(0) catalyst. According to the NPA charge analysis along the reaction coordinates, the formal oxidative addition of the Si-Si bond to palladium could be considered as the insertion of palladium into the Si-Si bond. However, the conventional oxidative addition of the C-I bond to palladium is exactly an oxidation process with the electron transfer from the palladium atom to the C-I bond. Therefore, electron rich Pd(0) is beneficial for the oxidation process, and Pd(ii) prone to acquire electrons is beneficial for the insertion process.

An unusual Pd(III) oxidation state in the Pd-Cl chain complex with high thermal stability and electrical conductivity.

The Pd(iii) oxidation state is unusual and unstable since it strongly tends to disproportionate. We synthesized the quasi-one-dimensional (1D) halogen-bridged Pd(iii)-Cl complex [Pd(dabdOH)2Cl]Cl2 (1-Cl; dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol) with multiple hydrogen bonds. From single-crystal X-ray diffraction, the bridging Cl- ions were located at the midpoint of the Pd-Cl-Pd moieties in the 1D chains, indicating that the Pd ions are in a Pd(iii) average valence (AV) state. Moreover, bright spots for the Pd(iii) dz2 orbitals in the upper Hubbard band above the Fermi level were observed every ∼5 Å using scanning tunnelling microscopy. These results clearly indicate that the Pd ions are in a Pd(iii) AV state in 1-Cl. In addition, 1-Cl has the highest thermal stability (470 K) among the Pd(iii) complexes reported and the highest electrical conductivity (0.6 S cm-1 at 300 K) among the 1D Pd-Cl chains reported so far.

Modulation of the superficial electronic structure via metal-support interaction for H2 evolution over Pd catalysts.

Electronic interactions can radically enhance the performance of supported metal catalysts and are critical for fundamentally understanding the nature of catalysts. However, at the microscopic level, the details of such interactions tuning the electronic properties of the sites on the metal particle's surface and metal–support interface remain obscure. Herein, we found polarized electronic metal–support interaction (pEMSI) in oxide-supported Pd nanoparticles (NPs) describing the enhanced accumulation of electrons at the surface of NPs (superficial Pdδ−) with positive Pd atoms distributed on the interface (interfacial Pdδ+). More superficial Pdδ− species mean stronger pEMSI resulting from the synergistic effect of moderate Pd–oxide interaction, high structural fluxionality and electron transport activity of Pd NPs. The surface Pdδ− species are responsible for improved catalytic performance for H2 evolution from metal hydrides and formates. These extensive insights into the nature of supported-metal NPs may open new avenues for regulating a metal particle's electronic structure precisely and exploiting high-performance catalysts.

Characterization of flame synthesized Pd–TiO2 nanocomposite catalysts for oxygen removal from CO2-rich streams in oxy combustion exhausts

Metallic Pd and/or reduced Pd oxide on Pd–TiO2 is found to be the intrinsic active site for O2 removal.

Computational Modeling Predicts the Stability of Both Pd+ and Pd2+ Ion-Exchanged into H-CHA.

Passive NOx adsorbers (PNA) using Pd/zeolites have emerged as a promising solution for the reduction of cold-start emissions from vehicle exhaust. However, the nature of the active sites and the mechanisms underlying NOx adsorption in Pd/zeolites remain a subject of ongoing investigation. In this study, we employ quantum chemical simulations to investigate the structure of Pd species in cation-exchange sites at isolated Al and Al pairs in the 6-ring and 8-ring of the CHA framework, before the introduction of NOx. Our calculations show that the speciation of Pd in these exchange sites strongly depends on the precise Al arrangement within the framework, as well as the operating conditions. Ionically dispersed Pd is found to be the most favorable species over a wide range of oxidizing and reducing conditions. Small oligomers of PdO and metallic Pd do not appear to be competitive at either isolated Al or Al pairs. Notably, our calculations show that ion exchange sites other than next-next-nearest neighbor Al pairs in the 6-ring will be preferentially occupied by Pd+ instead of Pd2+. The stability of Pd+ in the zeolite environment is an interesting contrast with its rareness in molecular Pd compounds. Nonetheless, a detailed analysis of the electronic structure shows that predicted Pd oxidation states are consistent with chemical intuition for all complexes investigated in this study. We also discuss the potential ambiguity in Pd characterization provided by typical experimental techniques such as XANES, EXAFS and UV-VIS, and highlight the need for additional EPR spectroscopy studies to further elucidate the initial Pd speciation in zeolites for PNA applications.

Deactivation of Industrial Pd/Al2O3 Catalysts by Ethanol: A Spectroscopic Study

AbstractThe chemical processes involved in the decomposition of ethanol over two industrial Pd/Al2O3 catalysts characterized by different Pd dispersions, particles morphology and reducibility of the active metal phase was thoroughly investigated in gas‐phase and in liquid‐phase by transmission FT‐IR and ATR‐IR spectroscopies, respectively. In both cases, two main products were detected, arising from two competitive paths: acetates from ethanol oxidation and carbonyls from acetaldehyde decarbonylation. The regularity of the Pd particles greatly affects the relative proportion of the two products, while the initial Pd oxidation state influences the kinetics of the two processes. Ethanol oxidation to acetates is the kinetically favored reaction path both in gas‐phase and in liquid‐phase. In contrast, the presence of a solvent dampens the decarbonylation process to very low levels, because the solvent competes with ethanol in occupying the available surface sites. Since adsorbed carbonyls are considered as surface poisons of industrial Pd/Al2O3 catalysts in processes involving aliphatic as well as aromatic alcohols, the rationalization of the experimental variables affecting alcohols decomposition can help in establishing working protocols and in designing preparation procedures that minimize the unwanted poisoning effects.

Mechanistic insight in NO trapping on Pd/Chabazite systems for the low-temperature NOx removal from Diesel exhausts

We have applied transient response methods to study Pd-CHA systems as Passive NOx Adsorbers (PNA) for the low-temperature NOx removal from Diesel exhausts. NO adsorption/TPD experiments were carried out at different temperatures in gaseous mixtures containing the main exhausts components (O2, CO, H2O) with the aim to clarify the mechanism determining the capability of Pd-CHA to trap and desorb NO. PdI was identified as the main NO storage site, both in mixtures containing a strong reductant like CO and in the presence of O2 only. When the oxidizing capability of the environment was enhanced, the Pd utilization, i.e. NO stored/Pdtot, dropped significantly: this loss is attributed to the competition between the NO storage on the reduced Pd sites and their re-oxidation. Pd oxidation played an important role also in the desorption phase: TPD experiments performed in gaseous mixtures with greater oxidizing strength enabled NO desorption at lower temperatures. The present data emphasize therefore the impact of the Pd redox state on the NO storage/release performances of Pd-CHA based PNAs.

Metal-Organic Framework (MOF)-Derived Electron-Transfer Enhanced Homogeneous PdO-Rich Co3 O4 as a Highly Efficient Bifunctional Catalyst for Sodium Borohydride Hydrolysis and 4-Nitrophenol Reduction.

Developing a bifunctional catalyst with low cost and high catalytic performance in NaBH4 hydrolysis for H2 generation and selective reduction of nitroaromatics will make a significant impact in the field of sustainable energy and water purification. Herein, a low-loading homogeneously dispersed Pd oxide-rich Co3 O4 polyhedral catalyst (PdO-Co3 O4 ) with concave structure is reported by using a metal-organic framework (MOF)-templated synthesis method. The results show that the PdO-Co3 O4 catalyst has an exceptional turnover frequency (3325.6 molH2 min-1 molPd -1 ), low activation energy (43.2 kJ mol-1 ), and reasonable reusability in catalytic H2 generation from NaBH4 hydrolysis. Moreover, the optimized catalyst also shows excellent catalytic performance in the NaBH4 selective reduction of 4-nitrophenol to 4-aminiphenol with a high first-order reaction rate of approximately 1.31 min-1 . These excellent catalytic properties are mainly ascribed to the porous concave structure, monodispersed Pd oxide, as well as the unique synergy between PdO and Co3 O4 species, which result in a large specific surface area, high conductivity, and fast solute transport and gas emissions.

A Vacuum Ultraviolet-Enhanced Oxidation Mechanism for Pd: Near-Surface Oxidation for Atomic Layer Etching.

Density functional theory (DFT) is used to better understand the oxidation of Pd metal using vacuum ultraviolet (VUV) light co-exposed with O2, which is known to produce O and O3. The oxidation of Pd metal arising from O, O2, and O3 is assessed on bare Pd, Pd with a 0.25 monolayer of adsorbed atomic O, and Pd with increasing O incorporation into the substrate. DFT calculations are complemented experimentally by co-exposing 20 nm Pd films to 1 Torr of O2 and VUV photons (6.5 < hν < 11.3 eV) from a D2 lamp at temperatures ranging from 50 to 200 °C and times from 30 s to 40 min. Oxidation of Pd is characterized using in situ X-ray photoelectron spectroscopy. Co-exposures at 50 °C and 1 Torr O2 are performed with the Pd illuminated by the VUV light and shadowed from the VUV light in attempting to select for the oxidant that impinges on the Pd surface and causes oxidation. Results suggest that atomic O incident from the gas phase is responsible for oxidation of Pd, as no PdOx formation is observed for the same time period with the sample shadowed. Growth of PdOx via O diffusion is studied with the nudged elastic band method. Atomic O diffusion through Pd has an activation energy barrier of ∼2.87 eV with respect to a surface O. This decreases to ∼1.80 eV once the 0.25 monolayer of O occupies the surface. The extent of Pd oxidation is limited to the near-surface Pd region for all times and temperatures investigated. PdOx formation does not appear to exceed one to two atomic layers of Pd for conditions explored herein.

Orientation-Dependent Hindrance to the Oxidation of Pd-Au Alloy Surfaces.

Oxidation of monometallic Pd and bimetallic Pd3Au alloy surfaces are observed by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at an elevated pressure (100 mTorr O2 ambient). It is directly evidenced that the alloying with Au hinders the surface oxidation of Pd3Au surfaces compared with monometallic Pd surfaces. Remarkably, the oxidation behavior is clearly different between Pd3Au(111) and (100) surfaces. The (100) surface has a relatively Pd-rich surface composition, and the surface oxide layer is formed, whereas the (111) surface has a Au-rich composition, and the surface oxidation is quite limited. A combined approach of experimental and theoretical techniques reveals that Pd/Au surface composition and atomic arrangement are key factors determining the oxidation behavior.

Low-temperature synthesis of PdO-CeO2/C toward efficient oxygen reduction reaction

PdO-CeO2 nanocomposites have many potential (electro) catalytic applications but are routinely synthesized at a high temperature. Herein, we report low-temperature synthesis of uniform PdO nanoparticles decorated by nearly amorphous CeO2 supported on carbon (PdO-CeO2/C). Unlike previous chemical oxidation of Pd to PdO at 500°C, Pd is electrochemically oxidized to PdO paired with CeO2/CeO2-x at 250 C. To the best of our knowledge, the synthesis of PdO at such a low temperature has not been reported before this study. Compared with commercial Pt/C, PdO-CeO2/C exhibits an excellent oxygen reduction reaction (ORR) activity in alkaline media in terms of a remarkable mass activity (MA) of 1103 mA mgPd−1 (at 0.9 V vs. reversible hydrogen electrode), which is 5.9 times of that of commercial Pt/C. Density functional theory suggests that the high ORR activity may arise from an appropriate oxygen adsorption strength at the interface of PdO-CeO2. In addition, after 4000 cycles of accelerated durability tests, the MA of the PdO-CeO2/C only decreases 13.4% superior to commercial Pt/C with a loss of 55.5%. The significantly enhanced durability should be correlated to the strong interfacial interaction between PdO and CeO2, which well anchors PdO and suppresses the migration of PdO to form large aggregates.

Synthesis of Reduced Graphene Oxide as a Support for Nano Copper and Palladium/Copper Catalysts for Selective NO Reduction by CO.

Copper and palladium/copper nanoparticles supported on reduced graphene oxide catalysts were synthesized and evaluated for the selective NO reduction by CO. The catalysts were characterized by XRD, nitrogen adsorption–desorption, TGA, XPS, TPR, in situ XRD, STEM, and HRTEM. The STEM and HRTEM results showed high metal oxide dispersions on the rGO. XPS results showed the presence of Cu and Pd oxide species. The reduction of copper supported on the rGO occurred in two steps for CuOx/rGOc, while that for CuOx-PdOy/rGOc occurred in one step for temperatures lower than 350 °C. Noteworthy is that the in situ XRD results showed that the rGO structure was not affected after reduction at 350 °C. The in situ XRD of reduction revealed the appearance of new phases for copper during the reduction. The catalysts were evaluated in NO reduction by CO. The tests showed that the reduced catalysts presented high performance with NO conversions and N2 selectivity above 85% at 350 °C.

Catalytic influence of light element incorporation in the lattice of palladium

The alloying of metals with foreign elements is a common approach for manipulating catalytic properties of active sites. For palladium-based catalysts, incorporation of light elements receives far less attention than Pd-metal alloys even though incorporated light element are catalytically significant. Due to their small size, light elements (H, B, C, and O) can occupy octahedral sites in Pd to form an interstitial phase, while palladium adjusts its lattice to accommodate the incorporation of larger light elements (S and P) to form distinct phases. The incorporated light element modifies the electronic structure of palladium, influencing the binding energy of substrates and reactive intermediates. Pd hydride and oxide often form in situ during hydrogenation or oxidation reactions, respectively, representing the (more, less) active phase of the catalyst. The reactive interstitial hydrogen dissolved in Pd tends to hydrogenate reactants completely and unselectively. Pd oxide can be the active phase for oxidation reactions, and the incorporated oxygen or oxygen vacancies may participate in the reaction directly. Incorporated light elements change geometric aspects of Pd ensembles, serving the role of site blocking, ensemble isolation, or serve as an additional active site, endowing Pd with bifunctional character. We review the synthetic approaches for incorporating light elements into the palladium lattice. We summarize the catalytic influence of light element incorporation into the palladium lattice with respect to electronic and geometric modification, in situ incorporated hydrogen and oxygen, and discuss the influence of light element incorporation into Pd on specific reactions (i.e., selective hydrogenation, oxidation, electrocatalysis, and CC cross coupling).

A Hydrothermally Stable Irreducible Oxide-Modified Pd/MgAl2 O4 Catalyst for Methane Combustion.

Catalytic combustion is promising in removing trace amounts of CH4 to address serious environmental concerns. Supported Pd-based catalysts are most effective but often suffer from low stability in applications owing to the water-vapor-induced sintering. Herein, we develop a universal strategy to prepare irreducible-oxide-modified Pd/MgAl2 O4 catalysts which show high activity and excellent stability against both hydrothemal aging at elevated temperatures and deactivation in long-term reaction under wet conditions. The addition of irreducible oxides inhibited the deep oxidation of Pd in the oxygen-rich conditions, which preserved not only the epitaxial structure but also a suitable active phase of Pd-PdOx on MgAl2 O4 , thus promoting both activity and stability. This work provides new insights into the effect of metal-oxide interaction on CH4 combustion and offers an avenue to design hydrothermally stable and active combustion catalysts for industrial applications.

A Hydrothermally Stable Irreducible Oxide‐Modified Pd/MgAl2O4 Catalyst for Methane Combustion

AbstractCatalytic combustion is promising in removing trace amounts of CH4 to address serious environmental concerns. Supported Pd‐based catalysts are most effective but often suffer from low stability in applications owing to the water‐vapor‐induced sintering. Herein, we develop a universal strategy to prepare irreducible‐oxide‐modified Pd/MgAl2O4 catalysts which show high activity and excellent stability against both hydrothemal aging at elevated temperatures and deactivation in long‐term reaction under wet conditions. The addition of irreducible oxides inhibited the deep oxidation of Pd in the oxygen‐rich conditions, which preserved not only the epitaxial structure but also a suitable active phase of Pd‐PdOx on MgAl2O4, thus promoting both activity and stability. This work provides new insights into the effect of metal–oxide interaction on CH4 combustion and offers an avenue to design hydrothermally stable and active combustion catalysts for industrial applications.

Reduction of Oxidized Pd/Ag(111) Surfaces by H2: Sensitivity to PdO Island Size and Dispersion

Understanding the migration of species across interfaces in bimetallic systems is key to exploiting their bifunctionality for chemical reactivity and heterogeneous catalysis. The present study demonstrates that the sizes and dispersion of oxidized Pd islands present on oxidized Ag(111) in addition to the concentration of active Pd sites have a significant influence on the rate of surface reduction by H2. Two distinct types of Pd oxide islands were generated for this investigation and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy. Small, uniformly dispersed PdOx islands (1–5 nm diameter) were created by depositing Pd onto AgOx surfaces, while larger, non-uniformly dispersed PdOx agglomerates (30–50 nm) were produced by depositing Pd on Ag(111) prior to oxidizing. Based on XPS, the small PdOx islands have a higher concentration of undercoordinated Pd atoms than the large agglomerates. Both types of PdOx are found to dramatically enhance the reduction of AgOx by H2 at 300 K due to the ability of PdOx to dissociate H2; the pure AgOx surfaces are unreactive toward H2. The rate of reduction at 300 K is found to be 2–4 times larger for the AgOx surface covered by small, uniformly dispersed PdOx islands. The higher reactivity of this surface is attributed to enhanced migration of oxygen and hydrogen atoms between the PdOx and AgOx phases due to the sizes and high dispersion of the small PdO islands as well as the higher concentration of active Pd sites on the PdOx domains. This study demonstrates that reactant migration between co-existing surface phases is highly sensitive to both the intrinsic chemical activity and morphological properties of the active phase (PdOx) and reveals that these properties can be significantly influenced by the method of synthesizing the oxidized bimetallic surfaces.

Engineering Electronic Structure of Single-Atom Pd Site on Ti 0.87 O 2 Nanosheet via Charge Transfer Enables C–Br Cleavage for Room-Temperature Suzuki Coupling

The palladium (Pd)-catalyzed Suzuki reaction is widely applied in the pharmaceutical industry, where constructing highly active and low-cost Pd sites are impendent. Here, we report the fabrication ...

Change in the Pd Oxidation State during Light-Off of Three-Way Catalysis Analyzed by in situ Diffuse Reflectance Spectroscopy

The changes in the oxidation state of supported Pd catalysts during light-off of NO–CO–C3H6–O2 reactions were evaluated during temperature ramp up and down cycles (10 °C min–1) under fuel-rich (air...

In-Situ diffuse reflectance spectroscopy analysis of Pd/CeO2–ZrO2 model three-way catalysts under Lean-Rich cycling condition

The redox behavior of Pd/CeO2–ZrO2 (Pd/CZ) model three-way catalysts with different particle sizes was studied by in situ diffuse reflectance spectroscopy under the lean-rich perturbation condition. To estimate the contribution of three-phase boundary (TPB) between Pd, CZ, and gas phase, a kinetic analysis of the Pd redox was performed using time-resolved reflectance data. Because the Pd oxidation upon rich-to-lean switching was strongly retarded by oxygen storage in CZ, the ratio of apparent initial rate constants of the Pd redox, kR→L/kL→R, was significantly correlated with the presence of TPB. When both Pd and CZ particles are so small that most Pd is close to TPB, the retardation effect is most obvious (kR→L/kL→R ∼ 0.3). Increasing particle sizes of not only Pd but also CZ reduces the abundance of TPB and thus the retardation effect is less pronounced (kR→L/kL→R > 1). The rate constant ratio provides a useful index of the degradation of TPB to estimate the extent of deactivation of the three-way catalyst via thermal sintering.