PdO Surface Research Articles

Study on the Gas-Chromic Character of Pd/TiO2 for Fast Room-Temperature CO Detection.

As a widely used support, TiO2 has often been combined with Pd to form highly sensitive gas-chromic materials. Herein, we prepared a series of Pd/TiO2 catalysts with different Pd content (from 0.1 to 5 wt.%) by the impregnation method for their utilization in fast room-temperature CO detection. The detection was simply based on visible color change when the Pd/TiO2 was exposed to CO. The sample with 1 wt.% Pd/TiO2 presented an excellent CO gasochromic character, associated with a maximum chromatic aberration value of 90 before and after CO exposure. Systematic catalyst characterizations of XPS, FT-IR, CO-TPD, and N2 adsorption-desorption and density functional theory calculations for the CO adsorption and charge transfer over the Pd and PdO surfaces were further carried out. It was found that the interaction between CO and the Pd surface was strong, associated with a large adsorption energy of -1.99 eV and charge transfer of 0.196 e. The color change was caused by a reduction in Pd2+ to metallic Pd0 over the Pd/TiO2 surface after CO exposure.

Catalytic combustion of light hydrocarbons over Pd − Pt/Al2O3: The hidden Pt1 active sites

Rationalizing the roles of different active phases in Pd − Pt/Al2O3 for light hydrocarbon (methane, ethane, propane, ethylene, and propylene) catalytic combustion is of central importance in producing practical air pollution abatement and power generation systems. So far, considerable efforts have been paid to understanding the microstructural evolution of Pd − Pt catalysts during high-temperature ageing, while much less is known about the key features in these catalysts at industry-related mild temperatures (e.g., ≤ 550 °C). In this article, by starting from platinum and palladium nitrates impregnated on commercially available γ-Al2O3, the microstructural evolution of Pd − Pt/Al2O3 induced by calcinations at below 550 °C in air was distinguished and further disentangled by comparison with the cases of Pt/Al2O3. Supported by electron microscopy, in situ DRIFTS, and other characterizations, we showed that the PdO nanoparticles on Pd − Pt/Al2O3 catalysts worked not only as a reactive phase for alkane combustion, but also as a “trapper” that reduced the amount of single-site Pt1 species by anchoring and aggregating them on the PdO (101) surface during calcinations. Since these Pt single atoms were highly reactive for the combustion of alkenes, Pd − Pt/Al2O3 exhibited enhanced methane/ethane/propane oxidation activity with increasing Pd contents, but were weaker ethylene and propylene oxidizers than Pt/Al2O3. Water and sulfur tolerance tests were also performed to evaluate the true practicability of the catalysts, which demonstrated the overall higher robustness of the Pd-free recipes for light hydrocarbon combustion.

First principles study of hydrogen adsorption on the surface of PdO(101)

Liquid hydrogen storage equipment is insulated using high-vacuum multi-layer insulation. This insulation is manufactured from materials that emit hydrogen gas, which degrades the insulating properties. Palladium oxide is widely used in liquid hydrogen storage as an effective hydrogen absorber. This study investigates the adsorption principle of H2 molecules on the surface of PdO(101) crystals. The mechanism of hydrogen adsorption on the PdO(101) surface was investigated by analyzing the adsorption energy, stable adsorption configuration, and electronic structure of the H2/PdO(101) system. The dissociative chemisorption of hydrogen on the PdO (101) surface has been investigated using temperature-programmed desorption (TPD) and density functional theory (DFT) calculations. It is determined that greater than 90% of the hydrogen dissociation occurs at temperatures below ∼100K. The dissociative chemisorption of hydrogen on the PdO(101) surface was found to occur at temperatures below ∼100K. The majority of the dissociated hydrogen reacts and produces water on the surface during TPD, which desorbs above ∼300K. The results indicate that Pdcus is the most stable adsorption site for H2 on the PdO(101) surface, with an adsorption energy of −0.56 eV, resulting in weak chemisorption. Once the activation energy is reached, H2 and Pdcus form a stable structure. The movement of H atoms between Pdcus sites generates a negative heat of reaction. The adsorption energy of H atoms at the Ocus site (−3.26 eV) is higher than that at the Pdcus site (−3.21 eV). Hydroxylated PdO(101) promotes water production. The combination of H atoms and hydroxides to produce water overcomes the potential barrier of 0.69 eV, resulting in the production of Pd and H2O.

Electron transferring with oxygen defects on Ni-promoted Pd/Al2O3 catalysts for low-temperature lean methane combustion

Methane (CH4) is the second most consequential greenhouse gas after CO2, with a substantial global warming potential. The CH4 catalytic combustion offers an efficient method for the elimination of CH4. However, improving the catalytic performance of Pd-based materials for low-temperature CH4 combustion remains a big challenge. In this study, we synthesized an enhanced Pd/5NiAlOx catalyst that demonstrated superior catalytic activity and improved water resistance compared to the Pd/Al2O3 catalyst. Specifically, the T90 was decreased by over 100 °C under both dry and wet conditions. Introducing Ni resulted in an enormously enhanced number of oxygen defects on the obtained 5NiAlOx support. This defect-rich support facilitates the anchoring of PdO through increased electron transfer, thereby inhibiting the production of high-valence Pd(2+δ)+ and stimulating the generation of unsaturated Pd sites. Pd0 can effectively activate surface oxygen and PdO plays a significant role in activating CH4, resulting in high activity for Pd/5NiAlOx. On the other hand, the increased water resistance of Pd/5NiAlOx was mainly due to the generation of *OOH species and the lower accumulation of surface -OH species during the reaction process.

Redox-driven surface generation of highly active Pd/PdO interface boosting low-temperature methane combustion

The supported Pd catalyst has been a benchmark for methane elimination. However, the active structures have been long under debate. Here, by the aberration-corrected high angle annular dark field scanning transmission electron microscopy, operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy, we revealed the two-dimensional metallic Pd bumps on the PdO surface (litchi-like structure) generated by the redox atmosphere under the lean condition as a highly active structure of the Pd/Al2O3 catalyst. The substantially increased Pd/PdO interfaces boost the methane combustion activity higher than the similar catalysts reported previously, and remarkably enhance the reaction rate by 15.5 and 10.7 times that of pure PdO or metallic Pd counterpart under lean condition at 300 °C, respectively. Density-functional theory calculations confirm the synergistic C–C bond activation of methane on the Pd/PdO interfaces. Our work provides new insight into the traditional understanding of the chemical state and particle size effects of the industrial Pd catalysts for methane oxidation.

Pd-based eye-readable H2 sensors: Principles, developments, and perspectives

Hydrogen gas is considered as the ultimate energy carrier to solve the energy crisis, but safely handling hydrogen energy relies on the development of high-performance hydrogen sensors. Compared with widely investigated electric sensors, eye-readable hydrogen sensors (EHSs) have the advantage of higher safety coefficient and lower cost. Therefore, EHSs are more promising to provide basic sensing services for broader application scenarios in daily life, and they have attracted ever-increasing attention recently. However, comprehensive summaries of recent developments in EHSs are rarely reported. In this review paper, three types of Pd-based EHSs with the sensing mechanisms of localized surface plasmon resonance (LSPR), mechanical deformation, and PdO reduction are presented. The development, working principles, sensing performances, and relative merits of each type of EHS are summarized and discussed. This review not only attempts to expound the present strategies for fabricating EHSs, but also aims to inspire the design of more ingenious sensors to pursue higher sensing performance and promote further development in this field. Furthermore, a perspective and some unsolved challenges in EHSs are discussed in the end.

Alloying and oxidation of PdAu thin films

The relation between catalytic activity and the presence of oxides on the catalyst’s surface has proven very complex, especially in the case of methane oxidation over Pd. While the metallic Pd surface and a thin, but at least two atomic layers thick, oxide film has been found catalytically active, a single-layer surface oxide or a too thick oxide film are both low-active. Unfortunately, under reaction conditions, the oxide tends to grow thick and deactivate. The reason for this deactivation is believed to be exposure of the PdO(100) surface, which does not have any suitable active sites for the methane adsorption and activation, in contrast to PdO(101), which is exposed for thin films. In an attempt to limit the thickness of the oxide film, and hence stabilise the active PdO orientation, we have investigated the oxidation of thin PdAu films. The effects of different mixtures (25%, 50%, and 75% Au, respectively) and treatments on the oxidation and orientation of the alloy and oxide films were investigated. As intended, PdAu turned out to be significantly more difficult to oxidise compared to pure Pd. This effect was even stronger than expected. Depending on the sample temperature and the amount of Au present in the alloy, the orientation of the oxide is affected. At lower temperatures and Au concentration, the desired [301] oxide orientation (corresponding to the (101) planes being parallel to the sample surface) is favoured, while higher Au concentration favours the low-active [100] oxide orientation. Thus, PdAu might be a good candidate for methane oxidation if the Au concentration is low, probably below 25%. The larger lattice constant of Au compared to Pd might also affect the oxide orientation, so another choice of alloying material, e.g. PdPt, may also work to stabilise PdO[301]. Independent of the alloy composition, the sample temperature during oxide growth should be optimised in order to achieve an active oxide film.

From AuPd Nanoparticle Alloys towards Core‐Shell Motifs with Enhanced Alcohol Oxidation Activity

AbstractThe best activity of bimetallic AuPd nanoparticles for alcohol oxidation reaction is usually achieved by adjusting the Au/Pd molar ratio. Herein, alloy AuPd nanoparticles of ∼2.5–3.5 nm, ranging from Au‐rich to Pd‐rich compositions, were synthesized through a sol‐immobilization method. A calcination step under air conditions caused an increase in size up to 5.5 nm, but also driven the segregation of Pd to the surface, which generated an Au rich core with Pd on the shell. At high Pd content, the shell structure is formed by PdO. The catalytic activity of Pd‐rich samples (1 : 1, 1 : 2 and 1 : 4 Au : Pd) for benzyl alcohol oxidation increased ∼4‐fold after calcination. Upon reduction with H2, surface PdO motifs are reduced to Pd that did not dissolve back into the AuPd alloy core, maintaining the metallic Pd shell. The catalytic activity decreased. Therefore, restructuring alloyed AuPd nanoparticles into core‐shell like structures with PdO in the shell is beneficial to the catalytic activity, but reduction of PdO layer was detrimental. These results provide new insights on the optimization of AuPd catalysts, considering not only the Au/Pd molar ratio, but also the importance of PdO, which was not anticipated in the previous literature.

Creating and Stabilizing an Oxidized Pd Surface under Reductive Conditions for Photocatalytic Hydrogenation of Aromatic Carbonyls.

Photocatalysis provides an eco-friendly route for the hydrogenation of aromatic carbonyls to O-free aromatics, which is an important refining process in the chemical industry that is generally carried out under high pressure of hydrogen at elevated temperatures. However, aromatic carbonyls are often only partially hydrogenated to alcohols, which readily desorbs and are hardly further deoxygenated under ambient conditions. Here, we show that by constructing an oxide surface over the Pd cocatalyst supported on graphitic carbon nitride, an alternative hydrogenation path of aromatic carbonyls becomes available via a step-wise acetalization and hydrogenation, thus allowing efficient and selective production of O-free aromatics. The PdO surface allows for optimum adsorption of reactants and intermediates and rapid abstraction of hydrogen from the alcohol donor, favoring fast acetalization of the carbonyls and their consecutive hydrogenation to O-free hydrocarbons. The photocatalytic hydrogenation of benzaldehyde into toluene shows a high selectivity of >90% and a quantum efficiency of ∼10.2% under 410 nm irradiation. By adding trace amounts of HCl to the reaction solution, the PdO surface remains stable and active for long-term operation at high concentrations, offering perspective for practical applications.

Understanding of Active Sites and Interconversion of Pd and PdO during CH4 Oxidation.

Pd-based catalysts are widely used in the oxidation of CH4 and have a significant impact on global warming. However, understanding their active sites remains controversial, because interconversion between Pd and PdO occurs consecutively during the reaction. Understanding the intrinsic active sites under reaction conditions is critical for developing highly active and selective catalysts. In this study, we demonstrated that partially oxidized palladium (PdOx) on the surface plays an important role for CH4 oxidation. Regardless of whether the initial state of Pd corresponds to oxides or metallic clusters, the topmost surface is PdOx, which is formed during CH4 oxidation. A quantitative analysis using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy demonstrated that a surface PdO layer was formed on top of the metallic Pd clusters during the CH4 oxidation reaction. Furthermore, the time-on-stream test of CH4 oxidation revealed that the presence of the PdO layer on top of the metallic Pd clusters improves the catalytic activity. Our periodic density functional theory (DFT) calculations with a PdOx slab and nanoparticle models aided the elucidation of the structure of the experimental PdO particles, as well as the experimental C-O bands. The DFT results also revealed the formation of a PdO layer on the metallic Pd clusters. This study helps achieve a fundamental understanding of the active sites of Pd and PdO for CH4 oxidation and provides insights into the development of active and durable Pd-based catalysts through molecular-level design.

Improving time-resolution and sensitivity of in situ X-ray photoelectron spectroscopy of a powder catalyst by modulated excitation.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful tool to characterize the surface structure of heterogeneous catalysts in situ. In order to improve the time resolution and the signal-to-noise (S/N) ratio of photoemission spectra, we collected consecutive APXP spectra during the periodic perturbation of a powder Pd/Al2O3 catalyst away from its equilibrium state according to the modulated excitation approach (ME). Averaging of the spectra along the alternate pulses of O2 and CO improved the S/N ratio demonstrating that the time resolution of the measurement can be limited solely to the acquisition time of one spectrum. Through phase sensitive analysis of the averaged time-resolved spectra, the formation/consumption dynamics of three oxidic species, two metal species, adsorbed CO on Pd0 as well as Pdn+ (n > 2) was followed along the gas switches. Pdn+ and 2-fold surface PdO species were recognised as most reactive to the gas switches. Our approach demonstrates that phase sensitive detection of time-resolved XPS data allows following the dynamics of reactive species at the solid-gas interface under different reaction environments with unprecedented precision.

Effect of Pd precursors on the catalytic properties of Pd/CeO2 catalysts for CH4 and CO oxidation

Herein, Pd/CeO2 catalysts were prepared using different Pd precursors and applied to CH4 and CO oxidation. Significantly, the Pd/CeO2 catalyst prepared using palladium nitrate (Pd(2)/CeO2 (N)) was highly active toward CH4 oxidation, whereas the catalyst prepared using palladium acetate (Pd(2)/CeO2 (A)) exhibited high CO oxidation activity. The effects of the precursor on the catalytic activity and other properties were studied using various characterization techniques. The Pd(2)/CeO2 (N) catalyst contained surface PdO nanoparticles, which were active sites for CH4 oxidation. On the contrary, the Pd(2)/CeO2 (A) catalyst possessed Pd ions stabilized at CeO2 defect sites, which were not very reactive. However, the high Pd dispersion was beneficial for the CO oxidation reaction. Therefore, the Pd precursor clearly altered the Pd dispersion and state in the Pd/CeO2, which are critical for catalytic activity.

Adsorption of ammonia ([formula omitted]) on palladium oxide (PdO) surface (001): Ab initio study

Dacapo code with the GGA-PW91 approximation has been used to study the adsorption of ammonia molecular (NH3) on PdO(001) termination surfaces, via N atom. We have determined the geometrical parameters of NH3 in the free phase, the PdO mesh parameters, at different sites of PdO(001) surface, the adsorption energies, the geometrical parameters of NH3 in adsorbed phase, the PDOS of the system, as well as the decomposition. The hollow (H) site of the PdO-O(001) termination is found to be most energetically stable with an adsorption energy of −0.445 eV, while the bridge-oy (B2) site of the PdO-Pd(001) termination is the most energetically stable with an adsorption energy of −1.105 eV. The isosurface calculations of NH3/PdO(001) show an accumulation of charges around the nitrogen, oxygen and palladium atoms, and charge depletion for hydrogen atoms. The thermochemistry of NH3 is very favourable on the PdO-O(001) surface, however for the PdO-Pd(001) surface the last two reaction paths are unfavourable. Finally, all the obtained results are in agreement with those found in the literature.

Electronically Engineering Water Resistance in Methane Combustion with an Atomically Dispersed Tungsten on PdO Catalyst

Improving the low-temperature water-resistance of methane combustion catalysts is of importance for industrial applications and it is challenging. A stepwise strategy is presented for the preparation of atomically dispersed tungsten species at the catalytically active site (Pd nanoparticles). After an activation process, a Pd-O-W1 -like nanocompound is formed on the PdO surface with an atomic scale interface. The resulting supported catalyst has much better water resistance than the conventional catalysts for methane combustion. The integrated characterization results confirm that catalytic combustion of methane involves water, proceeding via a hydroperoxyl-promoted reaction mechanism on the catalyst surface. The results of density functional theory calculations indicate an upshift of the d-band center of palladium caused by electron transfer from atomically dispersed tungsten, which greatly facilitates the adsorption and activation of oxygen on the catalyst.

Electronically Engineering Water Resistance in Methane Combustion with an Atomically Dispersed Tungsten on PdO Catalyst

AbstractImproving the low‐temperature water‐resistance of methane combustion catalysts is of importance for industrial applications and it is challenging. A stepwise strategy is presented for the preparation of atomically dispersed tungsten species at the catalytically active site (Pd nanoparticles). After an activation process, a Pd−O−W1‐like nanocompound is formed on the PdO surface with an atomic scale interface. The resulting supported catalyst has much better water resistance than the conventional catalysts for methane combustion. The integrated characterization results confirm that catalytic combustion of methane involves water, proceeding via a hydroperoxyl‐promoted reaction mechanism on the catalyst surface. The results of density functional theory calculations indicate an upshift of the d‐band center of palladium caused by electron transfer from atomically dispersed tungsten, which greatly facilitates the adsorption and activation of oxygen on the catalyst.

Potential- and Time-Dependent Dynamic Nature of an Oxide-Derived PdIn Nanocatalyst during Electrochemical CO2 Reduction.

Electrochemical reduction of CO2 into valuable fuels and chemicals is a promising route of replacing fossil fuels by reducing CO2 emissions and minimizing its adverse effects on the climate. Tremendous efforts have been carried out for designing efficient catalyst materials to selectively produce the desired product in high yield from CO2 by the electrochemical process. In this work, a strategy is reported to enhance the electrochemical CO2 reduction reaction (ECO2RR) by constructing an interface between a metal-based alloy (PdIn) nanoparticle and an oxide (In2O3), which was synthesized by a facile solution method. The oxide-derived PdIn surface has shown excellent eCO2RR activity and enhanced CO selectivity with a Faradaic efficiency (FE) of 92.13% at -0.9 V (vs RHE). On the other hand, surface PdO formation due to charge transfer on the bare PdIn alloy reduces the CO2RR activity. With the support of in situ (EXAFS and IR) and ex situ (XPS, Raman) spectroscopic techniques, the optimum presence of the Pd-In-O interface has been identified as a crucial parameter for enhancing eCO2RR toward CO in a reducing atmosphere. The influence of eCO2RR duration is reported to affect the overall performance by switching the product selectivity from H2 (from water reduction) to CO (from eCO2RR) on the oxide-derived alloy surface. This work also succeeded in the multifold enhancement of the current density by employing the gas diffusion electrode (GDE) and optimizing its process parameters in a flow cell configuration.

Adsorption and dissociation mechanism of toluene on Pd (111) and PdO (101) surface: First principle calculation

Metal Palladium species are efficiently used to degrade volatile organic compounds (VOCs). In this paper, the adsorption properties of toluene on Pd(111) and PdO(101) surfaces were theoretically studied using the first principle calculation. Adsorption energy, charge density, density of states and electron density difference were chosen as characteristics and were calculated to evaluate the adsorption capacity of toluene on Pd(111) and PdO(101) surfaces. The results showed that the interaction of toluene on the Pd(111) surface was stronger than on the PdO(101) surface. The dissociation pathway of toluene after adsorption was also determined; the results showed that the formation of benzyl radical through the reaction with a hydroxyl group on the Pd(111) and PdO(101) surfaces was investigated using density functional theory calculations. Compared with PdO(101) (0.63 eV), the dissociation of toluene on Pd(111) (0.19 eV) has a lower energy barrier, which indicated that the reaction between toluene and Pd(111) was easier to occur. Therefore, it provides the basis for the preparation of high-performance catalysts and the calculation of catalytic oxidation reaction paths in the future.

Promoter role of tungsten in W-Pd/Al2O3 catalyst for direct synthesis of H2O2: Modification of Pd/PdO ratio

The addition of non-precious-metal is a promising approach to develop Pd-based catalysts for the direct synthesis of H2O2. Herein, we prepare W modified Pd/Al2O3 for enhanced synthesis towards H2O2. With fixed Pd loading, the H2O2 productivity show a volcano relationship with increasing W addition, and reach up to peak value for slice W content. It is found that W species in the form of WO3 existing on the surface of Pd particles. The small addition of W optimizes the ratio between surface PdO and Pd, which simultaneously augments the productivity of H2O2 and hinders hydrogenation/decomposition of H2O2. Nevertheless, excess W addition not only leads to overmuch metallic state of Pd species, but also impedes the O2 adsorption on Pd particles surface, resulting in a decrease in H2O2 synthesis. This work demonstrates the promoter mechanism of W and offers a promising strategy developing Pd-based catalysts for the direct synthesis of H2O2.

Oxidative calcination of PdCo with unexpected electrocatalytic performance for ethylene glycol oxidation

Pd-based catalysts have been widely used in alkaline fuel cells. However, up to now, the effects of oxidation treatment of Pd-based catalysts on their application in alkaline fuel cells have been rarely reported. In this paper, PdCo nano-metal catalyst was prepared by calcination-oxidation treatment. It was found that the mass specific activity and area specific activity of the resulting PdO-Co3O4 nano-composite for electrocatalytic oxidation of ethylene glycol in alkaline solution were 3.8 and 2.4 times that of commercial Pt/C, respectively. Compared with PdCo nanometals, the mass specific activity and area specific activity of the PdO-Co3O4 nanocomposite for the electrocatalytic oxidation of ethylene glycol in the alkaline solution increased by 1.6 and 1.2 times, respectively. The experimental and calculated results showed that the surface morphology and active center of the catalyst changed after calcination and oxidation treatment. The adsorption energies of O2 and OH on the Co doped PdO (101) surface decreased, which was beneficial to stabilize the intermediate C2H4OHO*. As a result, the energy barrier for the O−H dissociation on the Co-doped surface was reduced. The strong binding of Co doped PdO (101) with ethylene glycol and its intermediate species led to different electrochemical kinetics and reaction path to produce excellent electric catalytic activity. The synergistic effect of PdO and Co3O4 significantly enhanced the interaction between active oxygen and catalyst surface, which not only facilitates the formation of superoxide species on the catalyst surface, but also improves the redox properties of the catalyst and promotes the electrocatalytic oxidation activity of ethylene glycol. The strategy of bi/multi-metallic oxidation proposed in this paper provides a general methodology for the construction of other catalysts.

An enhanced electrochemical CO2 reduction reaction on the SnOx–PdO surface of SnPd nanoparticles decorated on N-doped carbon fibers

Electrochemical reduction of CO2 to formate on SnPd–NCF. The adsorbed bicarbonate ion promotes the protonation of CO2˙− to HCO2−.