Effects of chlorinated Pd precursors and preparation methods on properties and activity of Pd/TiO2 catalysts

We investigated the effects of Pd precursors and preparation methods on the physicochemical properties and performance of Pd/TiO₂ catalysts in the photocatalytic degradation of methyl violet. To confirm the influence of the precursors, Pd/TiO₂ catalysts were prepared via chemical reduction (CR) using four different Pd precursors. Additionally, to determine the effects of preparation methods, Pd/TiO₂ catalysts were fabricated using K₂PdCl₄ precursor via three different methods: CR, deposition–precipitation (DP), and impregnation. The CO chemisorption results showed that the catalyst prepared via DP using the K₂PdCl₄ precursor, i.e., Pd/TiO₂_K_DP, displayed the highest Pd dispersion of 12.42% owing to the stable formation of Pd(OH)₂, which strongly interacted with the –OH groups on the TiO₂ support. Although the catalyst prepared via CR using the Pd(NH₃)₄Cl₂·H₂O (PA) precursor, i.e., Pd/TiO₂_PA_CR, had the lowest Pd dispersion of 0.7%, it exhibited the highest absorption of 26% after 30 min in the dark. It was found that high Pd²⁺/Pd⁰ ratio in dark conditions adversely affected the absorption of MV owing to electrostatic repulsion between the cationic dyes and metal nanoparticles. However, the Pd dispersion and the specific surface area played a key role in the photocatalytic activity under UV irradiation. Pd/TiO₂_K_CR with higher Pd dispersion showed the highest photocatalytic activity and reaction rate of 0.0212 min⁻¹.

We investigated the effects of Pd precursors and preparation methods on the physicochemical properties and performance of Pd/TiO₂ catalysts in the photocatalytic degradation of methyl violet.     

Fabrication of Pd3@Beta for catalytic combustion of VOCs by efficient Pd3 cluster and seed-directed hydrothermal syntheses

Subnanometric Pd clusters confined within zeolite crystals was fabricated using zeolitic seeds with premade [Pd₃Cl(PPh₂)₂(PPh₃)₃]⁺ clusters under hydrothermal conditions. Characterization of the Pd₃@Beta catalysts indicate that the Pd clusters confined in the channels of Beta zeolite exhibit better dispersion and stronger interaction with the zeolite support, leading to stabilized Pd species after heat treatment by high temperature. In the model reaction of toluene combustion, the Pd₃@Beta outperforms both zeolite-supported Pd nanoparticles prepared by conventional impregnation of Pd₃/Beta and Pd/Beta. Temperatures for achieving toluene conversion of 5%, 50% and 98% of Pd₃@Beta are 136, 169 and 187 °C at SV = 60 000 mL g⁻¹ h⁻¹, respectively. Pd₃@Beta could also maintain the catalytic reaction for more than 100 h at 230 °C without losing its activity, an important issue for practical applications. The metal-containing zeolitic seed directed synthesis of metal clusters inside zeolites endows the catalysts with excellent catalytic activity and high metal stability, thus providing potential avenues for the development of metal-encapsulated catalysts for VOCs removal.

Encapsulated Pd₃@Beta was fabricated through a novel Pd₃ cluster and seed-directed method, generating an excellent performance in VOCs catalytic combustion.     

Catalytically active nanosized Pd9Te4 (telluropalladinite) and PdTe (kotulskite) alloys: first precursor-architecture controlled synthesis using palladium complexes of organotellurium compounds as single source precursors

Several intermetallic binary phases of Pd–Te including Pd₃Te₂, PdTe, PdTe₂, Pd₉Te₄, Pd₃Te, Pd₂Te, Pd₂₀Te₇, Pd₈Te₃, Pd₇Te₂, Pd₇Te₃, Pd₄Te and Pd₁₇Te₄ are known, and negligible work (except few studies on PdTe) has been done on exploring applications of such phases and their fabrication at nanoscale. Hence, Pd(ii) complexes Pd(L1)Cl₂ and Pd(L2-H)Cl (L1): Ph–Te–CH₂–CH₂–NH₂ and L2: HO–2-C₆H₄–CH N–CH₂CH₂–Te–Ph were synthesized. Under similar thermolytic conditions, complex Pd(L1)Cl₂ with bidentate coordination mode of ligand provided nanostructures of Pd₉Te₄ (telluropalladinite) whereas Pd(L2-H)Cl with tridentate coordination mode of ligand yielded PdTe (kotulskite). Bimetallic alloy nanostructures possess high catalytic potential for Suzuki coupling of aryl chlorides, and reduction of 4-nitrophenol. They are also recyclable upto six reaction cycles in Suzuki coupling.

First precursor-architecture controlled synthesis of Pd₉Te₄ and PdTe nanostructures that have potential applications in Suzuki coupling of 4-chlorobenzaldehyde and catalytic reduction of 4-nitrophenol.     

Enhanced oxygen reduction activity of Pt shells on PdCu truncated octahedra with different compositions

Pd@Pt core–shell nanocrystals with ultrathin Pt layers have received great attention as active and low Pt loading catalysts for oxygen reduction reaction (ORR). However, the reduction of Pd loading without compromising the catalytic performance is also highly desired since Pd is an expensive and scarce noble-metal. Here we report the epitaxial growth of ultrathin Pt shells on Pd_xCu truncated octahedra by a seed-mediated approach. The Pd/Cu atomic ratio (x) of the truncated octahedral seeds was tuned from 2, 1 to 0.5 by varying the feeding molar ratio of Pd to Cu precursors. When used as catalysts for ORR, these three Pd_xCu@Pt core–shell truncated octahedra exhibited substantially enhanced catalytic activities compared to commercial Pt/C. Specifically, Pd₂Cu@Pt catalysts achieved the highest area-specific activity (0.46 mA cm⁻²) and mass activity (0.59 mA μg_Pt⁻¹) at 0.9 V, which were 2.7 and 4.5 times higher than those of the commercial Pt/C. In addition, these Pd_xCu@Pt core–shell catalysts showed a similar durability with the commercial Pt/C after 10 000 cycles due to the dissolution of active Cu and Pd in the cores.

Pd_xCu@Pt core–shell truncated octahedra were synthesized and exhibited substantially enhanced catalytic properties for oxygen reduction reaction relative to Pt/C.     

Efficient dehydrogenation of a formic acid–ammonium formate mixture over Au3Pd1 catalyst

A series of AuPd/C catalysts were prepared and tested for the first time for active and stable dehydrogenation of a formic acid–ammonium formate (FA–AF) mixture. The catalysts with different Au-to-Pd molar ratios were prepared using a facile simultaneous reduction method and characterized using transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. It was found that the catalytic activity and stability of the Au₃Pd₁/C catalyst was the best. The initial turnover frequency for the dehydrogenation of the FA–AF mixture over the Au₃Pd₁/C catalyst can reach 407.5 h⁻¹ at 365 K. The reaction order with respect to FA and AF is 0.25 and 0.55, respectively. The apparent activation energy of dehydrogenation is 23.3 ± 1.3 kJ mol⁻¹. The catalytic activity of the Au₃Pd₁/C catalyst remains ca. 88.0% after 4 runs, which is much better than the single Pd/C catalyst. The mechanism for the dehydrogenation is also discussed.

The Au₃Pd₁/C catalyst shows better performance in a formic acid–ammonium formate mixture and the mechanism of dehydrogenation is discussed.     

Reaction induced robust PdxBiy/SiC catalyst for the gas phase oxidation of monopolistic alcohols

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity (92% conversion of benzyl alcohol and 98% selectivity of benzyl aldehyde) and stability (time on stream of 200 h) in the gas phase oxidation of alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species. TEM indicates that the agglomeration of the 5.8 nm nanoparticles is inhibited under the reaction conditions. The transformation from inactive PdO–Bi₂O₃ to active Pd⁰–Bi₂O₃ under the reaction conditions is confirmed elaborately by XRD and XPS.

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity and stability in the gas phase oxidation of monopolistic alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species.     

Mono and dual hetero-structured M@poly-1,2 diaminoanthraquinone (M = Pt,Pd and Pt–Pd) catalysts for the electrooxidation of small organic fuels in alkaline medium

Oxidation of some small organic fuels such as methanol (MeOH), ethanol (EtOH) and ethylene glycol (EG) was carried out in an alkaline medium using palladium (Pd)–platinum (Pt) nanoparticles/poly1,2-diaminoanthraquinone/glassy carbon (p1,2-DAAQ/GC) catalyst electrodes. Pd and Pt were incorporated into the p1,2-DAAQ/GC electrode using the cyclic voltammetry (CV) technique. The obtained Pd/p1,2-DAAQ/GC, Pt/p1,2-DAAQ/GC, Pt/Pd/p1,2-DAAQ/GC and Pd/Pt/p1,2-DAAQ/GC nanocatalyst electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and CV methods. Real active surface area (A_real) achieved by carbon monoxide (CO) adsorption using differential electrochemical mass spectroscopy (DEMS) technique. The electrochemical activity was evaluated and normalized to A_real per metal loading mass. The electrocatalytic oxidation of the small organic fuels at the prepared nanocatalyst electrodes was studied in 1.0 M NaOH solutions by CV and chronoamperometric (CA) techniques. Pt/Pd/p1,2-DAAQ/GC nanocatalyst electrode exhibited enhanced catalytic activity, better durability and higher tolerance to carbon monoxide generated in the oxidation reaction when compared with the other three studied nanocatalysts. The present investigation suggests that the studied nanocatalysts can be successfully applied in direct oxidation of small organic fuels, especially MeOH.

Oxidation reaction of some small organic fuels such as methanol, ethanol and ethylene glycol was carried out in alkaline medium at palladium (Pd)–platinum (Pt) nanoparticles/poly1,2-diaminoanthraquinone/glassy carbon catalyst electrodes.     

Acetylene hydrochlorination over boron-doped Pd/HY zeolite catalysts

A novel boron-doped Pd/HY zeolite catalyst for acetylene hydrochlorination was prepared and exhibited an outstanding catalytic performance (the acetylene conversion was maintained at >95% for about 30 h). The boron species can stabilize catalytically active Pd²⁺ species and weaken carbon deposition and Pd²⁺ reduction during the reaction, thus improving the catalytic stability.

B doping partly weakens carbon deposition and Pd²⁺ reduction, thus enhancing catalytic stabilities of Pd/HY catalysts for acetylene hydrochlorination.

The coal-based acetylene hydrochlorination process is a major route for the production of vinyl chloride monomer (VCM) in China, which is essential for polyvinyl chloride (PVC) production. However, this process is restricted by the application of toxic and scarce mercury chloride, which is used as the active ingredient of catalysts in the industrial process.^1,2 Hence, it is imperative to explore environmentally friendly catalysts for acetylene hydrochlorination.Since Hutchings et al. suggested that the activities of metal catalysts were associated with the standard electrode potentials of the related metal ions,³ numerous studies have been carried out to develop non-mercury catalysts for efficient and sustainable hydrochlorination of acetylene in recent years, such as Au,^4–11 Pd,^12–19 Pt,²⁰ Ru,²¹ Bi,²² Cu,^23,24etc. Based on the detailed study by Hutchings'' team,³ metal chloride/activated carbon catalysts (including Pd²⁺, Hg²⁺, Cu²⁺, Cu⁺, Ag⁺) had superior catalytic activity and the order of their activities was Pd²⁺ > Hg²⁺ > Cu²⁺ > Cu⁺ > Ag⁺. Therefore, palladium-based catalysts were reported to be active for acetylene hydrochlorination.^12–19 Wang et al. prepared Pd-based/C catalysts via impregnation method and suggested that the Pd loss and the carbon deposition were main reasons for the catalyst deactivation, and the adding of K and La could improve the stability and regeneration of Pd/C catalysts.¹⁸ Mitchenko et al. reported that Pd(ii) chloro complexes were the active sites of catalysts¹⁹ and the (NH₄)₂PdCl₄ complex could stabilize the Pd species resulted in improving the catalytic performance over carbon-based materials.¹³ In our previous study, zeolites as new supports were prepared Pd/HY catalysts applied for acetylene hydrochlorination and the catalysts stabilities could be enhanced after modification with NH₄F or polyaniline.^14–17 While the carbon deposition and the active component reduction is the major obstacle to improve activity and stability of catalysts. It reported that heteroatom doping of support materials was effective for the improvement of the catalytic performance of metal catalysts²⁵ and few reports were studied on the B-doped zeolite-based catalysts for acetylene hydrochlorination at present. In this work, a novel boron-doped Pd/HY zeolite-based catalyst for acetylene hydrochlorination was prepared and its outstanding catalytic performance was further studied.The boron-doped HY zeolites supports were prepared using boron oxide (B2) as the boron source. The boron oxide aqueous solution was quantitatively (the weight percentage of B2 is 1.0%, 3.0%, 5.0%, 7.0%, 9.0%) added into HY zeolites (purchased by Nankai University, Si/Al = 9, 5.0 g) under stirring for 4 h, then washed with distiller water to pH about 7 and water was evaporated at 80 °C for 10 h, the B-doped HY zeolites supports were obtained. The Pd/B2-HY catalysts were prepared via ultrasonic-assisted impregnation using the B-doped HY or the original HY supports, as described in our previous work.^14–17 Based on the weight percentage of the boron oxide, the obtained B-doped Pd/HY catalysts were named as Pd/1B2-HY, Pd/3B2-HY, Pd/5B2-HY, Pd/7B2-HY and Pd/9B2-HY, respectively. The Pd loading in all catalysts was 0.9 wt%.X-ray diffraction (XRD) data were collected using a M18XHF22-SRA diffractometer with Cu–K irradiation at 50 kV and 100 mA in the scan range of 2 between 10°–80°. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were performed using a LEO1450VP detector to determine the morphology. High-resolution transmission electron microscopy (HRTEM) experiment was performed using a JEM-2100F (JEOL, Japan) working at 200 kV. BET surface areas analysis was performed by JW-BK Brunauer–Emmett–Teller (BET) equipment. Thermogravimetric (TG) tests were detected by a NETZSCH SAT 449F3 multifunctional thermal analyzer in an air atmosphere at a flow rate of 50 mL min⁻¹. The nature of carbon deposition was determined by an Agilent 7890A/5975C gas chromatography-mass spectrometry (GC-MS). Palladium contents were detected using an inductively coupled plasma (ICP-6300) instrument. X-ray photoelectron spectroscopy (XPS) data were recorded by AXIS ULTRA spectrometer (Kratos Analytical Ltd) and binding energies were referred to the C 1s line at 284.8 eV. Fourier transform infrared spectrometer (FTIR) was used EQUINOX-55 (Bruker Company, Germany) in transmittance (%) mode in the range 4000–400 cm⁻¹ and temperature programmed decomposition (TPD) was determined using a TP-5080 adsorption instrument with ammonia over a temperature ramp of 0–900 °C, rate ramp of 10 °C min⁻¹ and flow of 100 mL min⁻¹.The catalytic performances of catalysts were evaluated in a fixed bed with 10 mm-diameter a quartz tube micro reactor. The reactor temperature was calibrated by a CKW-110 temperature controller (Chaoyang automation instrument factory, Beijing, China) and the gas flow during the reaction was regulated by a D08-1F mass flow controller (Sevenstar Huachuang electronics. co. Ltd, Beijing, China). After N₂ flow (20 mL min⁻¹) purged, hydrogen chloride (12.6 mL min⁻¹) and acetylene (10.1 mL min⁻¹) were fed through a mixing vessel containing catalyst (3.0 g), giving a temperature at 160 °C, feed volume ratio V_HCl : V_C2H2 = 1.25 and the C₂H₂ gas hourly space velocity (GHSV) of 110 h⁻¹. The exit gas mixture from the reactor was passed through an absorption bottle filled with sodium hydroxide solution and then set into a gas chromatography (GC 2010, Shimadzu) to analyse the acetylene conversion and the VCM selectivity immediately.^13–17 Fig. 1 and S1^† display the catalytic performance of undoped and B-doped catalysts. The undoped Pd/HY catalyst exhibited a poor catalytic stability and its C₂H₂ conversion dropped dramatically from 93% to 13% in 3 h. After B doping, the C₂H₂ conversion was significantly enhance from 93% of the undoped Pd/HY catalyst to 96% of the B-doped Pd/B2-HY catalyst under the same conditions. It was clear that the Pd/7B2-HY catalyst displayed the optimal catalytic performance with the C₂H₂ conversion of 96% and the VCM selectivity of 98% after 10 h and its activity kept over 95% within 30 h (Fig. S2^†). It is suggested that the appropriate amount of B doping can improve the activity and stability of Pd-based catalysts for acetylene hydrochlorination.Open in a separate windowFig. 1The catalytic performances of the B-doped Pd/B2-HY catalysts.It is obvious that no other diffraction peaks of Pd species were detected, except the strongest characteristic peak of HY zeolites, which could be attributed to the low concentration or high dispersion of Pd species^26,27 (Fig. 2a). For the fresh Pd/7B2-HY catalyst, numerous Pd-based particles with the size about 2.81 nm were distributed uniformly on the HY support (Fig. 2b and d), which was slight bigger than that of the average pore width (by BET results, 2.49 nm) of the 7B2-HY zeolites, deducing that most Pd species involved in reaction might be on the support surface. In addition, the Pd species on the HY zeolite were visualized with well-defined fringes, which was ascribed to the (111) plane of face-centered cubic Pd with a d spacing of 0.23 nm (Fig. 2c).²⁸ The characterization of HRTEM verifies that XRD pattern is reasonable and suggests that the Pd species are highly dispersed in the fresh Pd/7B2-HY catalyst (Open in a separate windowFig. 2The characterization on the fresh catalysts: (a) XRD patterns of Pd-based catalysts. (b and c) HRTEM images of Pd/7B2-HY catalyst. (d) Pd particle size distribution of Pd/7B2-HY catalyst.Porous structure parameters of samples

Alkaline water-splitting reactions over Pd/Co-MOF-derived carbon obtained via microwave-assisted synthesis

Cobalt-based metal–organic framework-derived carbon (MOFDC) has been studied as a new carbon-based support for a Pd catalyst for electrochemical water-splitting; i.e., the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium. The study shows a high increase in the HER activity, in terms of low onset overpotential (onset η = 35 mV vs. RHE), high exchange current density (j_o,s ≈ 0.22 mA cm⁻²), high mass activity (j_o,m ≈ 59 mA mg⁻¹), high kinetic current (j_K ≈ 5–8 mA cm⁻²) and heterogeneous rate constant (k⁰ ≈ 4 × 10⁻⁴ cm s⁻¹), which are attributed to the high porosity of MOFDC and contribution from residual Co, while the large Tafel slope (b_c = 261 mV dec⁻¹) is ascribed to the high degree of hydrogen adsorption onto polycrystalline Pd as a supplementary reaction step to the suggested Volmer–Heyrovsky mechanism. These values for the catalyst are comparable to or better than many recent reports that adopted nano-carbon materials and/or use bi- or ternary Pd-based electrocatalysts for the HER. The improved HER activity of Pd/MOFDC is associated with the positive impact of MOFDC and residual Co on the Pd catalyst (i.e., low activation energy, E_A ≈ 12 kJ mol⁻¹) which allows for easy desorption of the H_ads to generate hydrogen. Moreover, Pd/MOFDC displays better OER activity than its analogue, with lower onset η (1.29 V vs. RHE) and b_a (≈78 mV dec⁻¹), and higher current response (ca. 18 mA cm⁻²). Indeed, this study provides a new strategy of designing and synthesizing MOFDC with physico-chemical features for Pd-based electrocatalysts that will allow for efficient electrochemical water-splitting processes.

Palladium nanoparticles supported on MOF-derived carbon serve as an efficient bifunctional electrocatalyst for alkaline water-splitting reactions.     

Fabrication of polyoxometalate-modified palladium–nickel/reduced graphene oxide alloy catalysts for enhanced oxygen reduction reaction activity

Designing advanced nanocatalysts for effectively catalyzing the oxygen reduction reaction (ORR) is of great importance for practical applications of direct methanol fuel cells (DMFCs). In this work, the reduced graphene oxide (rGO)-supported palladium–nickel (Pd–Ni/rGO) alloy modified by the novel polyoxometalate (POM) with Keggin structure (Pd–Ni/rGO-POM) is efficiently fabricated via an impregnation technique. The physical characterizations such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (FESEM-EDX), and transmission electron microscopy (TEM) are utilized to confirm the structure, morphology, and chemical composition of the fabricated samples. The XRD results verify the formation of the POM-modified Pd₈Ni₂/rGO alloy electro-catalyst with the face-centered-cubic (fcc) structure and average crystallite size of 5.54 nm. The electro-catalytic activities of the nanocatalysts towards ORR in alkaline conditions are evaluated by cyclic voltammetry (CV), rotating disk electrode (RDE), and chronoamperometry (CA) analyses. The synthesized Pd₈Ni₂/rGO-POM nanomaterial shows remarkably greater ORR catalytic activity and better methanol resistance compared with the Pd₈Ni₂/rGO and Pd/rGO electro-catalysts. The promoted ORR activity of the Pd₈Ni₂/rGO-POM sample is attributed to the alloying of Pd and Ni components, the uniform scattering of Pd–Ni nanoparticles on rGO, and the alloyed catalyst being modified with POM. Moreover, these findings demonstrate that the resultant Pd₈Ni₂/rGO-POM material is attractive as a suitable and cost-effective cathodic catalyst for DMFCs, in which the decorated POMs play a vital role for the enhancement in the catalytic abilities of the nanocatalyst.

A novel nanocatalyst, polyoxometalate-modified palladium–nickel/reduced graphene oxide (Pd₈Ni₂/rGO-POM), is prepared and served as an effective ORR nanomaterial in alkaline media.     

Benzophenone assisted UV-activated synthesis of unique Pd-nanodendrite embedded reduced graphene oxide nanocomposite: a catalyst for C–C coupling reaction and fuel cell

In this work we report the use of benzophenone (BP) for the synthesis of a palladium (Pd) embedded on reduced graphene oxide (rGO) nanocomposite (Pd/rGO) using a simple aqueous solution and UV irradiation. The simple and facile evolution of thermodynamically unstable branched Pd(0) nanodendrites was achieved by BP photoactivation, circumventing the growth of more stable nanomorphologies. The synthesis of Pd(0)-embedded rGO nanosheets (PRGO-nd) was made possible by the simultaneous reduction of both the GO scaffold and PdCl₂ by introducing BP into the photoactivation reaction. The nanocomposites obtained in the absence of BP were common triangular and twinned Pd(0) structures which were also implanted on the rGO scaffold (PRGO-nt). The disparity in morphologies presumably occurs due to the difference in the kinetics of the reduction of Pd²⁺ to Pd⁰ in the presence and absence of the BP photoinitiator. It was observed that the PRGO-nd was composed of dense arrays of multiple Pd branches around nucleation site which exhibited (111) facet, whereas PRGO-nt showed a mixture of (100) and (111) facets. On comparing the catalytic efficiencies of the as-synthesized nanocatalysts, we observed a superiority in efficiency of the thermodynamically unstable PRGO-nd nanocomposite. This is due to the evolved active facets of the dendritic Pd(0) morphology with its higher surface area, as testified by Brunauer–Emmett–Teller (BET) analysis. Since both PRGO-nd and PRGO-nt contain particles of similar size, the dents and grooves in the structure are the cause of the increase in the effective surface area in the case of nanodendrites. The unique dendritic morphology of the PRGO-nd nanostructures makes them a promising material for superior catalysis, due to their high surface area, and the high density of surface atoms at their edges, corners, and stepped regions. We investigated the efficiency of the as-prepared PRGO-nd catalyst in the Suzuki–Miyaura coupling reaction and showed its proficiency in a 2 h reaction at 60 °C using 2 mol% catalyst containing 0.06 mol% active Pd. Moreover, the electrochemical efficiency for the catalytic hydrogen evolution reaction (HER) was demonstrated, in which PRGO-nd provided a decreased overpotential of 68 mV for a current density of 10 mA cm⁻², a small Tafel slope of 57 mV dec⁻¹ and commendable stability during chronoamperometric testing for 5 h.

Benzophenone photoinitiator aided synthesis of Pd-nanodendrite embedded rGO nanocatalyst possessing superior potential in C–C coupling reaction and fuel cell application.     

Confinement of a Au–N-heterocyclic carbene in a Pd6L12 metal–organic cage

A Au(i)–N-heterocyclic-carbene (NHC)-edged Pd₆L₁₂ molecular metal–organic cage is assembled from a Au(i)–NHC-based bipyridyl bent ligand and Pd²⁺. The octahedral cage structure is unambiguously established by NMR, electrospray ionization-mass spectrometry and single crystal X-ray crystallography. The electrochemical behaviour was analyzed by cyclic voltammetry. The octahedral cage has a central cavity for guest binding, and is capable of encapsulating PF₆⁻ and BF₄⁻ anions within the cavity.

A Au(i)–NHC-edged Pd₆L₁₂ molecular cage is assembled from a Au(i)–NHC-based bipyridyl bent ligand and Pd²⁺.     

Preparation of a magnetic and recyclable superparamagnetic silica support with a boronic acid group for immobilizing Pd catalysts and its applications in Suzuki reactions

Palladium is one of the best metal catalysts for Suzuki cross-coupling reaction to synthesize unsymmetrical biaryl compounds. However, homogeneous palladium (Pd) is limited in an industrial scale due to the high cost, separation, removal, and recovery issues. In this paper, a novel, high activity magnetic nanoparticles (Fe₃O₄@SiO₂-APBA-Pd) catalyst was prepared by a simple, cost-effective procedure. The as-prepared functional nanoparticles (Fe₃O₄@SiO₂-APBA) with boric acid group immobilized Pd through adding Pd(OAc)₂ to Fe₃O₄@SiO₂-APBA in absolute ethanol and maintaining for a certain time under a nitrogen atmosphere. The as-prepared catalyst was characterized by FT-IR, SEM, EDX, TEM, ICP-MS, XPS, and XRD. The results showed that the Pd (0.2–0.6 nm) was successfully anchored on the magnetic silica material with boric acid group. The amount of Pd was 0.800 mmol g⁻¹. This magnetic nanostructure (8–15 nm) is especially beneficial as a nanocatalyst because each nanoparticle can catalyze a reaction in a certain time without steric restriction, which could effectively improve the reaction efficiency. The current nanoparticles with the Pd catalyst could be used as a novel, green, and efficient heterogeneous catalyst for Suzuki reactions. This catalyst showed promising catalytic activity and excellent yields toward 14 kinds of Suzuki coupling reactions under mild reaction conditions, which was similar to homogeneous Pd and many reported heterogeneous Pd catalysts. In addition, the turnover number (TON) and turnover frequency (TOF) for the Suzuki reaction were high. TOF and TON were 9048 h⁻¹ and 20 250 for the Suzuki reaction of bromobenzene and phenylboronic acid. Furthermore, the nanoparticles could be easily separated by a magnet, and could be used repeatedly seven times without any significant loss in activity.

A novel, high activity and magnetic nanoparticles (Fe₃O₄@SiO₂-APBA-Pd) catalyst was prepared. It is 8–15 nm with 0.2–0.6 nm Pd particles. It can be reused 7 runs and catalyze 14 kinds of Suzuki reactions.     

Electrochemical oxidation of As(iii) on Pd immobilized Pt surface: kinetics and sensing performance

Pd nanoparticles were electrochemically immobilized on a Pt surface in the presence of sodium dodecyl sulfate (SDS) molecules to study the electrokinetics of arsenite oxidation reactions and the corresponding sensing activities. The X-ray photoelectron spectroscopy (XPS) analysis showed that on the Pt surface, Pd atoms exist as adatoms and the contents of Pd(0) and Pd(ii) were 75.72 and 24.28 at%, respectively, and the particle sizes were in the range of 61–145 nm. The experimental results revealed that the catalytic efficiency as well as the charge transfer resistance (at the redox potential of the Fe(ii)/Fe(iii) couple) increased in the order of Pt < Pt–Pd < Pt–Pd_sds. A Pt–Pd_sds electrode exhibited an open circuit potential (OCP) of 0.65 V in acidic conditions; however, when 50.0 mM NaAsO₂ was present, the OCP value shifted to 0.42 V. It has been projected that the As(iii) oxidation proceeds using a sequential pathway: As(iii) → As(iv) → As(v). After optimization of the square wave voltammetric data, the limits of detection of As(iii) were obtained as 1.3 μg L⁻¹ and 0.2 μg L⁻¹ when the surface modification of the Pt surface was executed with Pd particles in the absence and presence of the SDS surfactant, respectively. Finally, real samples were analyzed with excellent recovery performance.

Amplification of true surface area can be improved when Pd particles are deposited on a substrate in the presence of sodium dodecyl sulfate (SDS) surfactant. In acidic medium, As(iii) undergoes a two-step oxidation process.     

Highly efficient hydrogen peroxide direct synthesis over a hierarchical TS-1 encapsulated subnano Pd/PdO hybrid

We report a hierarchical TS-1 encapsulated subnano Pd/PdO hybrid catalyst that shows unprecedented activity in H₂O₂ direct synthesis from H₂ and O₂. The macro reaction rate in 30 min is up to 35 010 mmol g_Pd⁻¹ h⁻¹ at ambient temperature. Such high catalytic activity is achieved due to the hierarchical porous structure of TS-1 and the formation of the encapsulated subnano Pd/PdO hybrid after oxidation/reduction/oxidation treatment.

A hierarchical TS-1 encapsulated subnano Pd/PdO hybrid catalyst that shows unprecedented activity in H₂O₂ direct synthesis from H₂ and O₂.

Hydrogen peroxide as a clean and strong oxidant is one of the commonly used chemicals in various fields of chemical industry, such as the pulp and paper industry, the textile industry, wastewater treatment, green chemical synthesis metallurgy, electronics manufacture, propulsion and the food industry.¹ Compared to the traditional anthraquinone process (sequential hydrogenation and oxidation of alkyl anthraquinone), the direct synthesis of hydrogen peroxide (DSHP) from hydrogen and oxygen was recognized as an efficient and environmental alternative process owing to its remarkable adherence to green chemistry perspectives, such as low energy consumption, minimized toxicity and infrastructure investment.^2–5Pd supported catalysts were the most extensively and earliest studied catalysts for the DSHP since 1914.⁶ Both DFT and experimental results indicated that subnano Pd particles were most effective for the selective oxygen hydrogenation to hydrogen peroxide,⁷ and the activity and selectivity are also highly dependent upon the oxidation state of the Pd particles.⁸ However, there were limitations in applying Pd nanoparticles catalyst to the reaction due to the thermal vulnerability in a calcination and reduction activation process.⁹ To solve this problem, many preparation methods have been adopted to stabilise Pd nanoparticles and control the particle size and morphology, such as yolk–shell structure,¹⁰ core–shell structure¹¹ and other encapsulation structure supports. But there were still problems that the size of metal particles is larger than 2.5 nm. Encapsulation of Pd species by mercaptosilane-assisted dry gel conversion (DGC) synthesis method can provide a precise control over the nanoparticle size as well as limitating the aggregation under high temperature during activation.¹² However, active sites deep inside the encapsulated nanoparticles were often hardly accessible since the internal configuration diffusion limitations of reactants and products in micropores, leading to low H₂ conversion and decomposition of the long residence time of synthetic H₂O₂.¹³ So, the role of the porous structured catalyst was essential for encapsulated metal nanoparticles.Titanium silicalite-1 (TS-1) has already been used as an excellent catalyst for a variety of selective oxidation reactions employing hydrogen peroxide as oxidant.^14,15 Moreover, in situ H₂O₂ generation coupled with these selective oxidation reactions leading to the desired products such as propylene,^16,17 benzyl alcohol,¹⁸ cyclohexene¹⁹ was a desirable, green and lower cost route. More importantly, the Ti–OOH species formed on the TS-1 during selective oxidation might improve the stability of OOH, which is a key reaction intermediate during the DSHP.²⁰ Hutchings et al. reported that hierarchical titanium silicalite supported Au–Pd catalysts showed high peroxide production rate and benzaldehyde production rate for oxidation of benzyl alcohol by in situ generated H₂O₂.²¹ In this report, the encapsulation of subnano-sized Pd metal particles within conventional (Pd@TS-1) and hierarchical titanium silicalite-1 (Pd@HTS-1) has been achieved (see Scheme 1). The Pd@HTS-1 catalyst after oxidation–reduction–oxidation pre-treatment showed unprecedented activity in direct synthesis of hydrogen peroxide from hydrogen and oxygen under ambient temperature without any promoter.Open in a separate windowScheme 1Schematic diagram of the preparation method for Pd@HTS-1.The TS-1 and HTS-1 encapsulated Pd sub-nanoparticles were first synthesized via solvent evaporation-assisted dry gel conversion method, where the Pd was encapsulated in situ through hydrothermal crystallization in assistance of 3-mercaptopropyl-trimethoxysilane (Scheme 1). The results of ICP analysis confirmed that total Pd contents in Pd@TS-1 and Pd@HTS-1 were 0.094 and 0.106 wt%, respectively. The characteristic diffraction “finger peak” on the X-ray diffraction in Fig. S1^† proved that the TS-1, Pd@TS-1 and Pd@HTS-1 had a well-crystallized MFI structure,²² which was further confirmed by the asymmetric stretching of Si–O–Ti in the spectra of Fourier Transform Infrared Spectroscopy (FT-IR, see Fig. S2^†). For all of the samples, the diffraction peak at 2θ of 25.4° was not observed. Meanwhile, the diffraction peak of crystalline Pd was also not detected for Pd@HTS-1 and Pd@TS-1, indicating that the Pd particles were well dispersed in the zeolite.⁷ Besides, the diffuse reflectance UV-vis spectra of the TS-1, Pd@TS-1 and Pd@HTS-1 were shown in Fig. S3.^† The band at 210 nm in three samples confirmed the tetrahedral structural geometry of Ti in these silicates, and the weak band at 280 nm was assigned to small amounts of penta/hexacoordinated Ti species.²³ Moreover, the absorption band around 300 nm indicated that the three samples contain anatase TiO₂.²⁴The textural properties of the synthesized Pd@TS-1 and Pd@HTS-1 were characterized by N₂ adsorption/desorption and the results were shown in Fig. 1 and Table S1.^† Notably, typical irreversible type IV adsorption isotherms with an H1 hysteresis loop were observed over the Pd@HTS-1 sample (Fig. 1b), indicating the presence of a mesoporous structure. The mesopore size of Pd@HTS-1, obtained through the BJH method, and the obtained graph peaked at about 7.0 nm. Volume of the micropores was around 0.14 cm³ g⁻¹ for both Pd@TS-1 and Pd@HTS-1, but the surface area of Pd@HTS-1 (509.9 m² g⁻¹) was 48.9 m² g⁻¹ larger than that of Pd@TS-1 (461.0 m² g⁻¹) due to its mesoporous structure, which is beneficial for the diffusion of reactants and products through the catalysts.²⁵Open in a separate windowFig. 1Nitrogen adsorption–desorption isotherms of the synthesized TS-1: (a) Pd@TS-1 and (b) Pd@HTS-1.Comparison between the experimentally obtained results from ammonia temperature-programmed desorption (NH₃-TPD) analysis (Fig. S4^†) and the previously reported data showed that the peaks observed were corresponding to weak acid sites, medium acid sites, and strong acid sites of the catalysts.²⁶ Furthermore, pyridine adsorption peak on the FT-IR spectra of these samples (Fig. S5^†) revealed that titanium silicate (TS-1) was an acidic support with a large number of Lewis acid segments and few Brønsted acid segments. As shown in scanning electron microscopy (SEM) image (Fig. 2), Pd@TS-1 particles were crystallites with a morphology close to cuboids and a mean particle size of about 3–5 μm, while the Pd@HTS-1 has spherical morphology with a particle size of about 1.3 μm.Open in a separate windowFig. 2SEM images of the synthesized Pd-modified TS-1: (a) Pd@TS-1 and (b) Pd@HTS-1.The synthesized TS-1 and HTS-1 encapsulated Pd sub-nanoparticles were then subjected to oxidation/reduction/oxidation treatment to adjust the valence states of Pd.²⁷ Such heat treatment cycle can switch off the sequential hydrogenation and decomposition reactions in the DSHP. However, Ostwald ripening, thus the migration and coalescence of metal clusters, will occur at a higher temperature. Therefore, high temperature treatments was used to emulate the conditions used in the literature mentioned before,^28,29 and the thermal stability of the encapsulated Pd@TS-1 catalysts before and after the treatments were also evaluated and compared to investigate the effect of high temperature and the thermal treatments on the catalysts. The Pd@TS-1 and Pd@HTS-1 samples after an air/H₂/air thermal treatments at 500/400/500 °C for 4/2/6 h were denoted as Pd@TS-1-O, Pd@TS-1-OR, Pd@TS-1-ORO, Pd@HTS-1-O, Pd@HTS-1-OR, Pd@HTS-1-ORO respectively with O denoting oxidation and R denoting reduction. The Pd particle size distribution after such treatments was first released by the high-resolution transmission electron microscopy (HRTEM) image in Fig. 3 and S6.^† The Pd particles encapsulated within microporous TS-1 zeolites were well dispersed and uniformly distributed throughout the zeolite crystals. The average sizes of Pd particles encapsulated in the TS-1 and HTS-1 were in the range of 1–2 nm, which, however, was bigger than those of the MFI topology channels (0.53 × 0.56 nm) and intersectional channels (∼0.9 nm). Nevertheless, the successful encapsulation of the Pd particles in the TS-1 zeolites was verified by comparing the hydrogenation rates of a mixture of nitrobenzene and 1-nitronaphthalene. As shown in Fig. S7,^† the reaction rate for the hydrogenation of nitrobenzene and 1-nitronaphthalene was much higher over the Pd@HTS-1-OR compared to the Pd@TS-1-OR. We anticipated that the slightly larger Pd size than the zeolite channels might reflect the local disruption of the crystal structures near the location of the particles during the in situ synthesis. More detailed size distributions of Pd particles encapsulated in the TS-1 and HTS-1 zeolites after air, Ar/H₂ and air treatments were shown in Fig. 3d–f and j–l, respectively. The particle sizes of most of the Pd species still remain below 2 nm on average, which indicated the absence of metal clusters migration and coalescence by Ostwald ripening even after such higher temperature treatments. The high thermal stability of the Pd subnano particles resulted from the embedding confinement.³⁰Open in a separate windowFig. 3HRTEM images and metal particle size distributions of the Pd@TS-1 and Pd@HTS-1 before and after high-temperature oxidation–reduction–oxidation treatments. (a, d Pd@TS-1-O. b, e Pd@TS-1-OR. c, f Pd@TS-1-ORO. g, j Pd@HTS-1-O. h, k Pd@HTS-1-OR. i, l Pd@HTS-1-ORO.)The Pd dispersion and average Pd nanoparticle size for Pd@TS-1 and Pd@HTS-1 after the air/H₂ treatment were further determined by CO chemisorption measurements (see Table S2^†). The dispersions of Pd in Pd@TS-1 and Pd@HTS-1 are 85% and 81%, respectively. The average Pd particle sizes for Pd@TS-1 and Pd@HTS-1 calculated by CO adsorption measurements are 1.06 nm and 1.17 nm, respectively, which was smaller than that estimated from the TEM analysis. This was probably due to the presence of Pd nanocluster or single atoms, which cannot be directly observed by HRTEM.We now turn to the Pd valence states of the catalysts after the oxidation/reduction/oxidation treatment by the XPS (see Fig. S8^†). The Pd3d spectra signals were hardly observed when the concentration of Pd atoms was low, the binding energy peaks for different oxidation states of Pd atoms were collected after peak fitting by prolonging the scanning time.³¹ The XPS results demonstrated the presence of both metallic Pd and PdO. The binding energy of peaks for Pd⁰3d_5/2 and Pd⁰3d_3/2 correspond to 335.5 and 340.6 eV, respectively, while the binding energy for Pd²⁺3d_5/2 and Pd²⁺3d_3/2 were at 337.8 and 341.9 eV, respectively.³¹ The transformation of valence state could be observed in Fig. S8a–c,^† which was derived from XPS measurements. Moreover, the ratios for Pd⁰ and Pd²⁺ atoms in Pd@TS-1 and Pd@HTS-1 were approximately 2 and 1, respectively. On the basis of these results, we proposed a reaction mechanism for the synthetic process of the catalysts, subnano-sized Pd particles might be oxidated from Pd⁰ to Pd²⁺ to form PdO on the surface of the catalysts during reoxidation.The catalytic performance of the TS-1 and HTS-1 encapsulated subnano-sized Pd/PdO hybrid in the direct synthesis of hydrogen peroxide from H₂ and O₂ were tested at ambient temperature without any promoters. Compared to the Pd supported by the active carbon, the selectivity of hydrogen peroxide was higher, the reason might be the formation of Ti–OOH³² and the confinement effect of the Pd encapsulated in the channel of the zeolite (Scheme 2). Both HTS-1 zeolite and Pd@zeolites showed significant amount of O₂ adsorption according to the O₂-TPD (Fig. S9^†), which might be the reason for high activity/selectivity. The selectivity for hydrogen peroxide on Pd@TS-1-OR is lower than that on Pd@TS-1-O, while the degradation rate of hydrogen peroxide on Pd@TS-1-OR are higher than that on Pd@TS-1-O (Fig. 4 and Table S3^†), which was attributed to the change in oxidation state from Pd²⁺ to Pd⁰ after reductive treatment, in agreement with previous reports.^27,33 The selectivity of hydrogen peroxide over Pd@TS-1 increased after an oxidation/reduction/oxidation cycle, the reason might be the weaker adsorption of O₂ and H₂, the intermediate OOH and the production H₂O₂ and the suppression of H₂O₂ decomposition.²⁰Open in a separate windowScheme 2Schematic of the mechanism for DSHP by Pd@TS-1.Open in a separate windowFig. 4H₂O₂ selectivity of DSHP over Pd@TS-1 with different oxidation states for 5 min reaction. Reaction conditions (same as Fig. 5 and ​and6):6): H₂/Ar (2.9 MPa) and air (1.35 MPa), 8.5 g solvent (2.9 g water, 5.6 g MeOH), 0.02 g catalyst, RT, 1200 rpm.The productivity of DSHP over Pd@TS-1 increased with oxidation, reduction and reoxidation treatment in 30 minutes (Fig. 5 and Table S3^†), demonstrated that PdO layer on monometallic Pd catalysts could suppress oxygen dissociation and H₂O₂ degradation,¹² the appropriate PdO formed on the surface of the catalysts after reoxidation can optimize the H₂O₂ production. The hierarchical Pd@TS-1 (35 010 mmol g_Pd⁻¹ h⁻¹) is remarkably higher than those of conventional Pd@TS-1 (3210 mmol g_Pd⁻¹ h⁻¹), the superior hydrogen peroxide production rate of Pd@HTS-1-ORO indicating that the Pd encapsulated by uniformed topology structure of TS-1 highly limited by the effect of pore-diffusion resistance.¹¹ Compared to Pd@TS-1, it was noteworthy that Pd@HTS-1 with only 0.1 wt% Pd content and subnano size after oxidative treatments showed famous reaction activity without any promoters under mild condition, which could be mainly ascribed to the presence of internal diffusion limitation within encapsulated micropore zeolites. The micropore structure limited the use of Pd metal because a part of the Pd crystal surface was blocked by zeolite supports, the hydrogen and oxygen were restricted by the configurational diffusion of zeolite to the Pd surface. Moreover, the formed and desorption H₂O₂ was also constrained by the micropore and thereby resulted in prolonged residence time of the product leading to degradation of H₂O₂. The intracrystal diffusion no longer limited the mass transport process of the hierarchical zeolite due to the presence of additional porosity. Although the physical and structural properties (including the primary particle size, the properties of the external surface and so on) were different between Pd@HTS-1 and Pd@TS-1, we may still draw a conclusion that the excellent catalytic activity is mainly attributed to the presence of mesopore favours diffusion of both reactants and products to and off the active sites in micropores.Open in a separate windowFig. 5Macro reaction rate for H₂O₂ production over Pd@TS-1 and Pd@HTS-1. ^aPd/C#C&Pd/C#Ex from Young-Min Chung;^34bPd–Sn/TiO₂ from Hutchings.²⁹The TON of H₂O₂ production at different reaction time over the six different Pd@TS-1 and Pd@HTS-1 catalysts were shown in Fig. 6. The TON increases with increasing reaction time, however, the slop of the TON–time curves (d_TON/d_t) seems decreased with increasing time, which revealed that the net productivity rate of hydrogen peroxide synthesis declined slightly with increasing time, especially for the Pd@HTS-1-OR at the reaction period of 30–60 min. The accumulative productivity of hydrogen peroxide slowed down, the reason might be the rapid decrease of hydrogen partial pressure in the medium and the ongoing H₂O₂ degradation.Open in a separate windowFig. 6The TON of H₂O₂ production with different reaction time over Pd@TS-1 and Pd@HTS-1 catalysts. TON (turnover number) = mol (H₂O₂)/mol (surface Pd).In summary, successful encapsulation of subnano-size Pd metal particles within titanium silicate (TS-1) voids was achieved via the mercaptosilane-assisted DGC synthesis method. The subnano-size Pd nanoparticles encapsulated in HTS-1 zeolites exhibited superior thermal stability after the oxidation/reduction/oxidation heat treatment process adjusting Pd/PdO hybrid owing to the embedding confinement. The synthesized high-efficiency Pd@HTS-1-ORO showed the famous hydrogen peroxide synthesis productivity, a hydrogen peroxide production rate as high as about 35 010 mmol H₂O₂ g_Pd⁻¹ h⁻¹. Our strategy brings about a finely tailored method to control particle size down to the subnano level and eliminate the diffusion inside metal encapsulated microporous zeolites, which is advantageous for catalytic activity and selectivity in direct synthesis of hydrogen peroxide. Thus, our approach opens up the possibility that the titanium-containing zeolites encapsulated noble metal catalyst can be extended further to selective oxidation reactions with H₂O₂ generated in situ from H₂ and O₂.     

Pd–Pd/PdO as active sites on intercalated graphene oxide modified by diaminobenzene: fabrication,catalysis properties,synergistic effects,and catalytic mechanism

Pd–Pd/PdO nanoclusters well dispersed on intercalated graphene oxide (GO) (denoted as GO@PPD–Pd) were prepared and characterized. GO@PPD–Pd exhibited high catalytic activity (a TOF value of 60 705 h⁻¹) during the Suzuki coupling reaction, and it could be reused at least 6 times. The real active centre was Pd(200)–Pd(200)/PdO(110, 102). A change in the Pd facets on the surface of PdO was a key factor leading to deactivation, and the aggregation and loss of active centres was also another important reason. The catalytic mechanism involved heterogeneous catalysis, showing that the catalytic processes occurred at the interface, including substrate adsorption, intermediate formation, and product desorption. The real active centres showed enhanced negative charge due to the transfer of electrons from the carrier and ligands, which could effectively promote the oxidative addition reaction, and Pd(200) and the heteroconjugated Pd/PdO interface generated in situ also participated in the coupling process, synergistically boosting activity. Developed GO@PPD–Pd was a viable heterogeneous catalyst that may have practical applications owing to its easy synthesis and stability, and this synergistic approach can be utilized to develop other transition-metal catalysts.

Pd(200) and the Pd(200)/PdO(102, 110) interface generated in situ participated in coupling reactions via a synergistic effect, boosting the catalytic activity to a high level.     

Catalysis with magnetically retrievable and recyclable nanoparticles layered with Pd(0) for C–C/C–O coupling in water

Nanoparticles layered with palladium(0) were prepared from nano-sized magnetic Fe₃O₄ by coating it with silica and then reacting sequentially with phenylselenyl chloride under an N₂ atmosphere and palladium(ii) chloride in water. The resulting Fe₃O₄@SiO₂@SePh@Pd(0) NPs are magnetically retrievable and the first example of NPs in which the outermost layer of Pd(0) is mainly held by selenium. The weight percentage of Pd in the NPs was found to be 1.96 by ICP-AES. The NPs were authenticated via TEM, SEM-EDX, XPS, and powder XRD and found to be efficient as catalysts for the C–O and C–C (Suzuki–Miyaura) coupling reactions of ArBr/Cl in water. The oxidation state of Pd in the NPs having size distribution from ∼12 to 18 nm was inferred as zero by XPS. They can be recycled more than seven times. The main features of the proposed protocols are their mild reaction conditions, simplicity, and efficiency as the catalyst can be separated easily from the reaction mixture by an external magnet and reused for a new reaction cycle. The optimum loading (in mol% of Pd) was found to be 0.1–1.0 and 0.01–1.0 for O-arylation and Suzuki–Miyaura coupling, respectively. For ArCl, the required amount of NPs was more as compared to that needed for ArBr. The nature of catalysis is largely heterogeneous.

Fe₃O₄@SiO₂@SePh@Pd(0) (Pd, 1.96%) as the first example of NPs having a Pd(0) layer held by selenium can execute C–C/C–O coupling in 2–6 h (80 °C).     

Insights into the spontaneous formation of hybrid PdOx/PEDOT films: electroless deposition and oxygen reduction activity

Hybrid palladium oxide/poly(3,4-ethylenedioxythiophene) (PdO_x/PEDOT) films were prepared through a spontaneous reaction between aqueous PdCl₄²⁻ ions and a nanostructured film of electropolymerized PEDOT. Spectroscopic and electrochemical characterization indicate the presence of mixed-valence Pd species as-deposited (19 ± 7 at% Pd⁰, 64 ± 3 at% Pd²⁺, and 18 ± 4 at% Pd⁴⁺ by X-ray photoelectron spectroscopy) and the formation of stable, electrochemically reversible Pd⁰/α-PdO_x active species in alkaline electrolyte and furthermore in the presence of oxygen. The elucidation of the Pd speciation as-deposited and in solution provides insight into the mechanism of electroless deposition in neutral aqueous conditions and the electrocatalytically active species during oxygen reduction in alkaline electrolyte. The PdO_x/PEDOT film catalyses 4e⁻ oxygen reduction (n = 3.97) in alkaline electrolyte at low overpotential (0.98 V vs. RHE, onset potential), with mass- and surface area-based specific activities competitive with, or superior to, commercial 20% Pt/C and state-of-the-art Pd- and PEDOT-based nanostructured catalysts. The high activity of the nanostructured hybrid PdO_x/PEDOT film is attributed to effective dispersion of accessible, stable Pd active sites in the PEDOT matrix.

Hybrid PdO_x/PEDOT films efficiently catalyse the direct 4e⁻ oxygen reduction reaction in alkaline electrolyte.     

Continuous syntheses of carbon-supported Pd and Pd@Pt core–shell nanoparticles using a flow-type single-mode microwave reactor

Continuous syntheses of carbon-supported Pd@Pt core–shell nanoparticles were performed using microwave-assisted flow reaction in polyol to synthesize carbon-supported core Pd with subsequent direct coating of a Pt shell. By optimizing the amount of NaOH, almost all Pt precursors contributed to shell formation without specific chemicals.

Continuous syntheses of carbon-supported Pd@Pt core–shell nanoparticles were performed using flow processes including microwave-assisted Pd core–nanoparticle formation.

Continuous flow syntheses have attracted attention as a powerful method for organic, nanomaterial, and pharmaceutical syntheses because of various features that produce benefits in terms of efficiency, safety, and reduction of environmental burdens.^1–7 Advances of homogeneous heating and mixing techniques in continuous flow reactors have engendered further developments for precise reaction control, which is expected to create innovative materials through combination with multiple-step flow syntheses.Microwave (MW) dielectric heating has been recognized as a promising methodology for continuous flow syntheses because rapid or selective heating raises the reaction rate and product yield.^8–18 For the last two decades, most MW apparatus has been batch-type equipped with a stirring mechanism in a multi-mode cavity. Therefore, conventionally used MW-assisted flow reactors have been mainly of the modified batch-type. Results show that the electromagnetic field distribution can be spatially disordered, causing inhomogeneous heating of the reactor.^19–25 Improvements of reactors suitable for flow-type work have been studied actively in recent years to improve their energy efficiency and to make irradiation of MW more homogeneous.^26–37We originally designed a MW flow reactor system that forms a homogeneous heating zone through generation of a uniform electromagnetic field in a cylindrical single-mode MW cavity.^26,30 The temperatures of flowing liquids in the reactor were controlled precisely via the resonance frequency auto-tracking function. Continuous flow syntheses of metal nanoparticle, metal-oxide, and binary metal core–shell systems with uniform particle size have been achieved using our MW reactor system.^26,38,39 Furthermore, large-scale production necessary for industrial applications can be achieved through integration of multiple MW reactors.³⁰Carbon-supported metal catalysts are widely used in various chemical transformations and fine organic syntheses. Particularly, binary metal systems such as Pd@Pt core–shell nanoparticles have attracted considerable interest for electro-catalysis in polymer electrolyte membrane fuel cells (PEMFC) because of their enhanced oxygen reduction activity compared to a single-use Pt catalyst. Binary metal systems also contribute to minimization of the usage of valuable Pt.^40–51 Earlier studies of carbon-supported Pd@Pt syntheses involved multiple steps of batch procedures such as separation, washing and pre-treatment of core metal nanoparticles, coating procedures of metal shells, and dispersion onto carbon supports. Flow-through processes generally present advantages over batch processes in terms of simplicity and high efficiency in continuous material production.We present here a continuous synthesis of carbon-supported Pd and Pd@Pt core–shell nanoparticles as a synthesis example of a carbon-supported metal catalyst using our MW flow reactor. This system incorporates the direct transfer of a core metal dispersion into a shell formation reaction without isolation. Nanoparticle desorption is prevented by nanoparticle synthesis directly on a carbon support. The presence of protective agents that are commonly used in nanoparticle syntheses, such as poly(N-vinylpyrrolidone), can limit the chemical activity of the catalyst. Nevertheless, this system requires no protective agent. Moreover, this system is a simple polyol synthesis that uses no strong reducing agent. It therefore imposes little or no environmental burden. For this study, the particle size and distribution of metals in Pd and Pd@Pt core–shell nanoparticles were characterized using TEM, HAADF-STEM observations, and EDS elemental mapping. From electrochemical measurements, the catalytic performance of Pd@Pt core–shell nanoparticles was evaluated.A schematic view of the process for the continuous synthesis of carbon-supported Pd@Pt core–shell nanoparticles is presented in Fig. 1. Details of single-mode MW flow reactor are described in ESI.^† We attempted to conduct a series of reactions coherently in a flow reaction system, i.e., MW-assisted flow reaction for the synthesis of carbon supported core Pd nanoparticles with subsequent deposition of the Pt shell. Typically, a mixture containing Na₂[PdCl₄] (1–4 mM) in ethylene glycol (EG), carbon support (Vulcan XC72, 0.1 wt%), and an aqueous NaOH solution were prepared. This mixture was introduced continuously into the PTFE tube reactor placed in the center of the MW cavity. Here, EG works as the reaction solvent as well as the reducing agent that converts Pd(ii) into Pd(0) nanoparticles. The MW heating temperature was set to 100 °C with the flow rate of 80 ml h^−l, which corresponds to residence time of 4 s. The carbon-supported Pd nanoparticles were transferred directly to the Pt shell formation process without particle isolation. The dispersed solution was introduced into a T-type mixer and was mixed with a EG solution of H₂[PtCl₆]·6H₂O (10 mM). The molar ratio of Pd : Pt was fixed to 1 : 1. Subsequently, after additional aqueous NaOH solution was mixed at the second T-mixer, the reaction mixture was taken out of the mixer and was let to stand at room temperature (1–72 h) for Pt shell growth.Open in a separate windowFig. 1Schematic showing continuous synthesis of carbon-supported Pd and Pd@Pt core–shell nanoparticles. The Pd nanoparticles were dispersed on the carbon support by MW heating of the EG solution. The solution was then transferred directly to Pt shell formation.Rapid formation of Pd nanoparticles with average size of 3.0 nm took place at the carbon-support surface during MW heating in the tubular reactor (Fig. 2a). Most of the Pd(ii) precursor was converted instantaneously to Pd(0) nanoparticles and was well dispersed over the carbon surface. Fig. 2b shows the time profile of the outlet temperature and applied MW power during continuous synthesis of carbon-supported Pd nanoparticles. The solution temperature rose instantaneously, reaching the setting temperature in a few seconds. This temperature was maintained with high precision (±2 °C) by the continuous supply of ca. 18 W microwave power. No appreciable deposition of metal was observed inside of the PTFE tube. It is noteworthy that Pd of 98% or more was supported on carbon by heating for 4 s at 100 °C from ICP-OES measurement. Our earlier report described continuous polyol (EG) synthesis of Pd nanoparticles as nearly completed with 6 s at 200 °C.³⁹ The reaction temperature in polyol synthesis containing the carbon was considerably low, suggesting that selective reduction reaction occurs on the carbon surface, which is a high electron donating property.Open in a separate windowFig. 2(a) TEM image of carbon-supported Pd nanoparticles synthesized using the MW flow reactor. The average particle size was 3.0 nm. (b) The time profile of the temperature at the reactor outlet and applied microwave power during continuous synthesis of carbon-supported Pd nanoparticles. Na₂[PdCl₄] = 2 mM, NaOH = 10 mM.The concentrations of Na₂[PdCl₄] precursor and NaOH affect the Pd nanoparticle size. Results show that the Pd particle size increased as the initial concentration of Na₂[PdCl₄] increased (Fig. S1a and b^†). Change of NaOH concentration exerted a stronger influence on the particle size. Nanoparticles of 12.3 nm were observed without addition of NaOH, whereas 2.6 nm size particles were deposited at the concentration of 20 mM (Fig. S1c and d^†). The higher NaOH concentration led to instantaneous nucleation and rapid completion of reduction. The Pd nanoparticle surface is equilibrated with Pd–O⁻ and Pd–OH depending on the NaOH concentration. The surface is more negative at high concentrations of NaOH because of the increase of the number of Pd–O⁻, which inhibits the mutual aggregation and further particle growth. Furthermore, to control the Pd nanoparticle morphology, we conducted synthesis by adding NaBr, which has been reported as effective for cubic Pd nanoparticle synthesis.⁵² However, because reduction of the Pd precursor derives from electron donation from both the polyol and the carbon support, morphological control was not achieved (Fig. S2^†). That finding suggests that morphological control is difficult to achieve by adding surfactant agents to the polyol.For Pt shell formation, carbon supported Pd nanoparticles (3.0 nm average particle size) were mixed with H₂[PtCl₆]·6H₂O solution with the molar ratio of Pd : Pt = 1 : 1. Then additional NaOH solution was mixed. As described in earlier reports,³⁹ alkaline conditions under which base hydrolysis and reduction of [PtCl₆]²⁻ to [Pt(OH)₄]²⁻ takes place are necessary for effective Pt shell formation. It is noteworthy that the added Pt precursor was almost entirely supported on carbon within 24 h in cases where an appropriate amount of additional NaOH (5 mM) was mixed by the second T-mixer (Fig. 3a). However, for 10 mM, nucleation and growth of single Pt nanoparticles were enhanced in place of core–shell formation. Consequently, a mixture of Pd@Pt and single Pt nanoparticles was formed on the carbon support (Fig. 3b). Very fine Pt nanoparticles were observed in the supernatant solution.Open in a separate windowFig. 3(a) Time profiles of residual ratio of Pt in the mixed solutions. Horizontal axis was left standing time. Carbon-support in the mixed solution after added the Pt precursor was precipitated by centrifugation. The supernatant solution was measured by ICP-OES. Concentrations of additional NaOH were 0, 5, and 10 mM. (b) TEM image of carbon-supported Pd@Pt core–shell nanoparticles. The synthesis conditions of Pd nanoparticles were Na₂[PdCl₄] (2 mM) and NaOH (10 mM). The molar ratio of Na₂[PdCl₄] : H₂[PtCl₆]·6H₂O was 1 : 1, and additional NaOH concentration was 10 mM. After left standing for 72 h, the mixture of Pd@Pt and single Pt nanoparticles (1–2 nm) was formed on carbon-support. Fig. 4a portrays a TEM image of carbon supported Pd@Pt core–shell nanoparticles. The average particle size of Pd@Pt core–shell nanoparticles was 3.6 nm after being left to stand for 24 h: larger than the initial Pd nanoparticles (3.0 nm). Fig. 4b shows the HAADF-STEM image of Pd@Pt core–shell nanoparticles supported on carbon. The core–shell structure of the particles can be ascertained from the contrast of the image. The Z-contrast image shows the presence of brighter shells over darker cores. Actually, the contrast is strongly dependent on the atomic number (Z) of the element.⁵³ The Z values of Pt (Z = 78) and Pd (Z = 46) differ considerably. Therefore, the image shows the formation of Pd@Pt core–shell structure with the uniform elemental distribution. Elemental mapping images by STEM-EDS show that both Pd and Pt metals were present in all the observed nanoparticles (Fig. 4c). Based on the atomic ratio (Pd : Pt = 49 : 51), they show good agreement with the designed values. The Pt shell thickness was estimated as about 0.6 nm, which corresponds to 2–3 atomic layer thickness of Pt encapsulating the Pd core metal, indicating good agreement with Fig. 4b image. For an earlier study, uniform Pt shells were formed by dropwise injection of the Pt precursor solution because the Pt shell growth rate differs depending on the crystal plane of the Pd nanoparticle.⁴⁶ For more precise control of shell thickness in our system, the Pt precursor solution should be mixed in multiple steps.Open in a separate windowFig. 4(a) TEM image and (b) HAADF-STEM image of carbon-supported Pd@Pt core–shell nanoparticles and the line profile of contrast. (c) Elemental mapping image of carbon-supported Pd@Pt core–shell nanoparticles, where Pd and Pt elements are displayed respectively as red and green. The EDS atomic ratio of Pd : Pt was 49 : 51. The synthesis conditions of Pd nanoparticles were Na₂[PdCl₄] (2 mM) and NaOH (10 mM). The molar ratio of Na₂[PdCl₄] : H₂[PtCl₆]·6H₂O was 1 : 1. The concentration of additional NaOH were 5 mM. It was left standing for 24 h.A comparison of the catalytic performance of the carbon-supported Pd@Pt core–shell and Pt nanoparticles is shown in Fig. S3.^† For this experiment, carbon-supported Pt nanoparticles with Pt 2 mM were prepared as a reference catalyst using a similar synthetic method. The initial Pt mass activities of the carbon-supported Pd@Pt and Pt nanoparticles were, respectively, 0.39 and 0.24 A mg_Pt⁻¹, improving by the core–shell structure. In addition, durability tests for carbon-supported Pd@Pt nanoparticles show that the reduction rate of Pt mass activity after 5000 cycles was only 2%. The catalytic activities of carbon-supported Pd@Pt nanoparticles were superior in terms of durability, suggesting that the Pt shell was firmly formed.     

Thin robust Pd membranes for low-temperature application

It is known that hydrogen embrittlement could result in warping and destruction of pure Pd membranes, which limits the working temperatures to be above 293 °C. This study attempted to investigate the relationship between hydrogen embrittlement resistance and membrane geometry of ultrathin pure Pd membranes of 2.7–6.3 μm thickness. Thin tubular Pd membranes with an o.d. of 4 mm, 6 mm and 12 mm immediately suffered from structural destruction when exposed to H₂ at room temperature. In contrast, thin hollow fiber membranes (outer diameter, 2 mm, thickness < 4 μm) exhibit strong resistance against hydrogen embrittlement at temperatures below 100 °C during repeated heating/cooling cycles at a rate up to 10 °C min⁻¹ under H₂ atmosphere. This is ascribed to reduced lattice strain gradients during α–β phase transition in cylindrical structures and lower residual stresses according to in situ XRD analysis, which shows a great prospect in low temperature applications.

Thin tubular membranes (outer diameter, 2 mm, thickness < 4 mm) exhibits strong resistance against hydrogen embrittlement at temperatures below 100 °C due to reduced lattice strain gradients in cylindrical structures and lower residual stresses.