Platinum-group Catalysts Research Articles

Temperature-induced evolution of CuOx clusters in CuOx/TiO2 catalyst for boosting auto-exhaust oxidation

Developing non-noble metal oxides, replacing platinum group catalysts for auto-exhaust purification, where their usage amount exceeds 40 % of global demand, remains a challenge. Herein, we presented a combined approach on the synthesis and application of robust CuOx/TiO2 catalysts with ultra-fine CuOx clusters (ca. 1 nm). The strong interaction between CuOx and TiOx induces favorable interfacial charge accumulation, facilitating the extraction of the interfacial oxygen atoms in Cu2-x-O-Ti4-y chains and the surface lattice oxygen on CuOx clusters. Isotope 18O2 experiments and DFT calculations jointly confirmed that these oxygen atoms preferentially participate in the oxidation through the Mars-van Krevelen mechanism below 250 °C. The resulted oxygen vacancies facilitate the adsorption and activation of O2 molecules, which will participate the oxidation above 250 °C through Eley-Rideal mechanism. The increase in the temperature also drives the synergy of active oxygen species in the interfacial Cu2-x-O-Ti4-y bond chains and on CuOx clusters, cooperatively promoting the conversion of NO to NO2. As a result, the CuOx/TiO2 catalyst exhibits exceptional catalytic performance in soot oxidation, with a four-fold higher reaction rate (4.17 μmol g−1 min−1) than that of single-atom Cu₁-TiO2 catalyst. Such findings provide a novel strategy for the advancement of Cu-based cluster catalysts in environmental applications.

Unraveling the pH-Dependent Oxygen Reduction Performance on Single-Atom Catalysts: From Single- to Dual-Sabatier Optima.

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have emerged as a potential substitute for the costly platinum-group catalysts in oxygen reduction reaction (ORR). However, several critical aspects of M-N-C SACs in ORR remain poorly understood, including their pH-dependent activity, selectivity for 2- or 4-electron transfer pathways, and the identification of the rate-determining steps. Herein, by analyzing >100 M-N-C structures and >2000 sets of energetics, we unveil a pH-dependent evolution in ORR activity volcanos─from a single peak in alkaline media to a double peak in acids. We found that this pH-dependent behavior in M-N-C catalysts fundamentally stems from their moderate dipole moments and polarizability for O* and HOO* adsorbates, as well as unique scaling relations among ORR adsorbates. To validate our theoretical discovery, we synthesized a series of molecular M-N-C catalysts, each characterized by well-defined atomic coordination environments. Impressively, the experiments matched our theoretical predictions on kinetic current, Tafel slope, and turnover frequency in both acidic and alkaline environments. These new insights also refine the famous Sabatier principle by emphasizing the need to avoid an "acid trap" while designing M-N-C catalysts for ORR or any other pH-dependent electrochemical applications.

Effects of Flameless Catalytic Combustion of Hydrogen on the Parameters of Catalytic Sensors with Platinum-group Catalysts

Effects of Flameless Catalytic Combustion of Hydrogen on the Parameters of Catalytic Sensors with Platinum-group Catalysts

Alkaline Water Electrolysis – Investigation of the Charge Transport Limitations across the Separator Under Low KOH Concentration

Currently, the water electrolysis is employed when high purity hydrogen is required, or excess of cheap electricity is available. With a broader future perspective, an important benefit of this technology represents fact, that it can be powered from renewable energy sources. The hydrogen produced from renewables is, thus, “green” CO2 neutral, playing an important role as energy vector in the so called “hydrogen economy”. Nowadays, the hydrogen economy represents strategy for transition of our economy towards the renewable energy sources, with water electrolysis (denoted as Power-to-X technology) being a crucial part of this scheme.Nowadays, the alkaline water electrolysis technology (AWE) operating at elevated temperatures (70 – 100 °C) with KOH electrolyte (25 – 30 wt.% KOH) dominates in the field of industrial hydrogen production by the water electrolysis. The alkaline environment is advantageous since it does not require platinum-group catalysts. On the other hand, highly concentrated caustic solution at elevated temperatures used as a liquid electrolyte limits the flexibility of the process operation. Currently several alternative technologies offering higher production intensity (proton-exchange membrane water electrolysis) and higher efficiency (high temperature solid-oxides steam-electrolysis) are developed and approach industrial application state. There are also promising innovations of the AWE process developed, e.g. application of an anion-selective membrane (AM) as the separator alternative to the diaphragm used in the conventional AWE. The AMs of interest exhibit improvement, contrary to diaphragm, of anionic conductivity with decreasing KOH concentration and operating temperature. Therefore, application of such AM allows to reduce KOH concentration in a liquid electrolyte (down to 5 – 15 wt.%) and operating temperature, thus, solve the above-mentioned problems of conventional AWE in connection with renewables. Moreover, the AM is dense, therefore might represent more effective barrier for produced gasses than diaphragm. However, one has to keep on mind increasing solubility of gasses with decreasing KOH concentration followed by potentially enhanced H2 cross-over. Another non-negligible benefit of lower KOH concentration and operating temperature is lower electrolyte conductivity in the manifolds contributing to reduced parasitic (bypass) current. However, systematic research aimed at evaluation of real application potential and searching for optimal conditions of the AM at diluted KOH concentrations is so far missing.In the presented contribution, study on a behaviour of two types of the separators in the laboratory scale AWE stack in bipolar zero-gap configuration is presented. The separators used are: a) a homogeneous AM based on chlormethylated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymer functionalized by 1,4-diazabicyclo-[2.2.2]octane (PSEBS-CM-DABCO) and b) a commercial composite diaphragm Zirfon™ Perl. The load curves were recorded to assess optimal performance of the stack with both separators at KOH concentration range of 1 – 15 wt.% and operating temperature range of 25 – 40 °C. The effect of different electrolyte flow rate through the Ni-foam electrodes was investigated as well. Additionally, 1-dimensional stationary mathematical model of single AWE (both with diaphragm and AM) cell corresponding to the studied laboratory set-up was developed to clarify the differences in the stack load curves recorded for the two studied separators. The Nernst-Planck equation is used to describe migration and diffusion transport in the cell, while convective flow of electrolyte in the porous electrode is considered in material balance. The gas evolution is neglected in the present model. A macrohomogeneous continuum theory is employed for description of the porous electrodes. The AM-solution interface is characterized by Donnan equilibrium. At all studied experimental conditions, the stack with AM outperforms the one with diaphragm. This fact is even more pronounced at KOH concentration below 5 wt.% due to occurring transport limitation, which is more significant in the case of the diaphragm, as demonstrated both by modelling and experimentally. The presented work clearly identifies interval of operating conditions, in which AM represents an important advantage over porous separator and opposite. Thus, it gives important hints for further development of this technology.Project has received funding from the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 862509).

High Performance AEM Water Electrolysis with Aemion® Membranes

By 2030 up to 50% of energy is expected to be carried in the bonds of H2. Global electrolysis capacity must increase from the current 240 MW to an anticipated 300 GW in 2030 and 3500 GW in 2050 to enable this transition. Alkaline and PEM electrolyzers are commercially mature with the currently market share of new installations roughly an equal split between these technologies. However, each of these electrolyzers are associated with challenges – alkaline electrolyzers operate at low current density, and require high concentration electrolytes (30 wt% KOH) to conduct hydroxides through the porous electrode separator (I.e., Zirfon). PEM electrolyzers use a proton conductive membrane to enable high current densities, however, running the reaction in acidic electrolyte requires platinum group catalysts and component coatings that hinder scalability at 2050 targets.AEM water electrolyzers address both of these challenges by pairing anion exchange membrane with alkaline electrolyte to enable high current density operation, at high pressure, without noble metal catalysts. These attributes enable the most cost-effective green hydrogen - bringing the DOE hydrogen shot target of $1/kg within reach.Anion exchange membrane chemistries have previously hindered this type of electrolyzer – AEMs based on quaternary amines, or pendant imidazolium groups chemically degrade in concentrated alkaline electrolyte, and mechanically degrade (from swelling) in low concentration alkaline media. Ionomr’s Aemion+ membranes are based on a sterically-protected polybenzimidazole chemistry and are chemically robust (stable in up to 10 M KOH), and exhibit low swelling to enable operation in low concentration electrolytes. These membranes are an enabling technology for long duration water and CO2 electrolysis.This talk highlights how Ionomr’s Aemion+ membranes enable performance in excess of 1 A/cm2 at 1.8 V with non-PGM catalysts and a variety of configurations, and >4000 hours of durability in continuous operation. Figure 1

Design of Co-NC as efficient electrocatalyst: The unique structure and active site for remarkable durability of proton exchange membrane fuel cells

Fe-N-C catalysts are promising alternatives to the platinum-group catalysts for use in oxygen reduction reactions of proton exchange membrane fuel cells. However, Fe-N-C catalysts suffer from poor durability, compared to non-precious metal catalysts, because of their accelerated demetallation by the Fenton reaction. In this study, we report the synthesis of a melamine-encapsulated Co-ZnO-C composite as a precursor and template for zeolite-imidazole-frameworks (ZIF-8). This approach allows formation of Co-N-C for constructing unique structures at meso- and macropore scales, while maintaining microporosity. Density functional theory analysis confirms the superior stability of the Co-N-C catalyst over other M-N-C catalysts (M = Fe, Ni, Cr, and Mn). Furthermore, it reveals that a closed interaction between the Co-N 4 moiety and organic adducts enhances oxophilicity, which prefers a 4-electron ORR activity. The Co-NC catalyst with a developed pore structure shows remarkable durability (6.7% performance degradation for 100 h) and full cell performance in H 2 /O 2 under 1 bar of backpressure (723 mW/cm 2 of maximum power density). Consequently, the unique structure of the synthesized catalyst successfully translates to the computationally-established ORR activity in the half-cell; superior durability is seen in the real device operation and stability analysis. This work is expected to support next-generation fuel cell development. • Melamine encapsulated Co-ZnO-C was prepared as precursor and template for atomic-cluster and porous Co-N-C catalyst. • ma-Co-NC exhibited excellent proton exchange membrane fuel cell performance and stability. • This approach allows formation of M-N-C for constructing unique structures at meso- and macropore scales, while maintaining microporosity.

Вплив швидкості потоків модельного палива та окисника на електричні властивості керамічних паливних комірок

Fuel Cells are one of the most efficient and environmentally friendly devices for electricity generation, which are developing rapidly and are already in the early stages of commercialization. Solid Oxide Fuel Cells (SOFC) areone of the most promising their types due to the highest efficiency, fuel flexibility (H2, CnHm, CO etc.) and no needs in platinum group catalysts. The performance of SOFC is affected by various polarization losses, which aredependant on selected materials, their structure and SOFC operation parameters. Over the last decade, much attention is given to the study of SOFC’s electrochemical properties at different operating regimes: temperatures, fuels, fuel and oxidantflow rates etc. The work is devoted to studying the influence of the model fuel (5% H2—Ar) and air (oxidant) flow rates on electrical properties of Solid Oxide Fuel Cellat 800 °C to determine the best combination of gas flow rates, which provide the maximum values of specific electric power. The fuel (0,35 l/min) and oxidant (1 l/min)flow rates was found as the optimal operation regime of fuel and air supply for the SOFC tested. The highest electrical densityto be ensured by the model fuel was determined as 34 mW/cm2. The amount / flow rate of oxidant and fuel gases supplied to the fuel cell does not correspond to the ratio of the reagents of the chemical reaction of oxidation of the fuel. This difference is explained by the fact that the SOFC effectiveness of fuel and oxidant utilization depends not only from to the properties structure and materials of each components: anode, cathode, electrolyte, but also from concentration of fuel and oxidant in model fuel or air, which also creates a barrier for oxidant and fuel molecules to reach the reaction zone. Keywords: Solid Oxide Fuel Cell, electrical properties, fuelgasmixtures, hydrogen, oxidant.

Progress in the development of heteroatom-doped nickel phosphates for electrocatalytic water splitting

Hydrogen energy is expected to replace fossil fuels as a mainstream energy source in the future. Currently, hydrogen production via water electrolysis yields high hydrogen purity with easy operation and without producing polluting side products. Presently, platinum group metals and their oxides are the most effective catalysts for water splitting; however, their low abundance and high cost hinder large-scale hydrogen production, especially in alkaline and neutral media. Therefore, the development of high-efficiency, durable, and low-cost electrocatalysts is crucial to improving the overpotential and lowering the electrical energy consumption. As a solution, Ni2P has attracted particular attention, owing to its desirable electrical conductivity, high corrosion resistance, and remarkable catalytic activity for overall water splitting, and thus, is a promising substitute for platinum-group catalysts. However, the catalytic performance and durability of raw Ni2P are still inferior to those of noble metal-based catalysts. Heteroatom doping is a universal strategy for enhancing the performance of Ni2P for water electrolysis over a wide pH range, because the electronic structure and crystal structure of the catalyst can be modulated, and the adsorption energy of the reaction intermediates can be adjusted via doping, thus optimizing the reaction performance. In this review, first, the reaction mechanisms of water electrolysis, including the cathodic hydrogen evolution reaction and anodic oxygen evolution reaction, are briefly introduced. Then, progress into heteroatom-doped nickel phosphide research in recent years is assessed, and a discussion of each representative work is given. Finally, the opportunities and challenges for developing advanced Ni2P based electrocatalysts are proposed and discussed.

РАЗРАБОТКА ЭФФЕКТИВНЫХ СЕНСОРОВ ОБНАРУЖЕНИЯ ДОВЗРЫВООПАСНЫХ КОНЦЕНТРАЦИЙ H2

We studied the response of catalytic sensors to hydrogen with various types of platinum-group catalysts (Pt, Pd, Ir, Rh, Pt+Pd) in the pre-explosive concentration range. Temperature dependences of sensory response are analysed. Dependences of the sensory response on the applied voltage demonstrates hysteresis behavior that can be explained by the partial transition of the oxides of the platinum group metals into the metallic phase at temperatures above 500 °С and the reverse oxidation of metals if temperature is below 400 °С. Catalytic sensors with Ir and Rh catalysts are more preferable for practical use in the detection of hydrogen.

Investigation of catalytic hydrogen sensors with platinum group catalysts

Environmental deterioration and limited resources of hydrocarbons push the development of alternative power sources. One of the most promising energy carriers is hydrogen. However, handling hydrogen is more hazardous than the use of hydrocarbons because it has a significantly wider flammable range. Thus the development of new sensors for preventing hydrogen leakage is the actual task of modern materials science and chemical engineering. In this work, the response of catalytic sensors to hydrogen with different catalysts of platinum group (Pt, Pd, Ir, Rh, Pt + Pd, Pt + Pd + Rh, Pt + Pd + Ir) in the pre-explosion concentration range is studied. Temperature dependencies of sensitivity are discussed. A hysteresis in sensor response is observed during the cycling of the supply voltage. This phenomenon can be explained by partial transformation of platinum group metal oxides into metallic phase at a temperature of more than 500 °C and reverse metal oxidation at temperatures less than 400 °C. It has been shown that the sensors with catalysts containing Ir and Rh demonstrate more preferable characteristics for practical applications.

Pore Surface Engineering of Covalent Triazine Frameworks@MoS2 Electrocatalyst for the Hydrogen Evolution Reaction.

Electrochemical water splitting is an important strategy for the mass production of hydrogen. Development of synthesizable catalysts has always been one of the biggest obstacles to replace platinum-group catalysts. In this work, a high quality crystal polymer covalent triazine framework [CTF; Brunauer-Emmett-Teller (BET) surface area of 1562.6 m2 g-1 ] is synthesized and MoS2 nanoparticles are grown in situ into/onto the 1 D channel arrays or the external surface for electrocatalysis [hydrogen evolution reaction (HER)] . The state-of-the-art CTFs@MoS2 structure exhibits superior catalytic kinetics with an overpotential of 93 mV and Tafel slope of 43 mV dec-1 , which is improved over most other reported analogous catalysts. The inherent π-conjugated crystal channels in CTFs provides a multifunctional support for electron transmission and mass diffusion during the hydrogen evolution process. Catalytic kinetics analysis shows that the HER performance is closely correlated to the hierarchical pore parameters and aggregated thickness of MoS2 nanoparticles. This work provides an attractive and durable alternative to synthesize high activity and stable catalysts for HER.

Supported Ag Catalysts on Mg–Al Oxides toward Oxidant-Free Dehydrogenation Reaction of Benzyl Alcohol

Oxidant-free dehydrogenation of alcohols has attracted considerable attention, owing to the largely improved utility value of alcohols. Herein, mixed metal oxides (MMOs) via topological transformation of Mg–Al hydrotalcite precursors were prepared and used as support for the immobilization of metallic Ag nanoparticles. The resulting Ag/MgAl-MMO shows an excellent catalytic performance toward oxidant-free dehydrogenation of benzyl alcohol (conversion of benzyl alcohol: 95.9%; selectivity of benzaldehyde: 93.3%), which is close to platinum-group catalysts commonly used in this reaction. A combination study based on a series of characterization methods reveals a synergistic catalysis between Agδ+ and medium-strong acid/basic sites (Mgδ+–Oδ−) from support: Agδ+ facilitates the breakage of α-C–H bond while Mgδ+–Oδ− accelerates the cleavage of O–H bond. This work provides an effective approach to enhance catalytic behavior toward dehydrogenation reactions by virtue of metal–support interactions.

Investigating Novel Anion-Conducting Polymers Via X-Ray Scattering

Hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) is a hydroxide-resistant, high-performing anion-conducting polymer. Such materials are very interesting, as they may allow the use of hydrogen fuel cells without platinum-group catalysts [1]. We have investigated the structure of this material using small-angle x-ray scattering (SAXS,) small-angle neutron scattering (SANS,) and computational methods. Our SAXS results revealed three distinct length scales, which we assigned to ion-polymer, interchain, and monomer length scales. A molecular dynamics (MD) simulation reproduced these features of the scattering curve and corroborated our interpretations of them. No structure at larger length scales was observed; the lack of large watery regions, common in high-performance ion-exchange membranes, may contribute to the material’s remarkable mechanical stability. [1] A.G. Wright and S. Holdcroft, ACS Macro Lett. 3, 444 (2014). Figure 1

Engineering Bimetallic Ag–Cu Nanoalloys for Highly Efficient Oxygen Reduction Catalysts: A Guideline for Designing Ag‐Based Electrocatalysts with Activity Comparable to Pt/C‐20%

Development of highly active and stable Pt-free oxygen reduction reaction catalysts from earth-abundant elements remains a grand challenge for highly demanded metal-air batteries. Ag-based alloys have many advantages over platinum group catalysts due to their low cost, high stability, and acceptable oxygen reduction reaction (ORR) performance in alkaline solutions. Nevertheless, compared to commercial Pt/C-20%, their catalytic activity still cannot meet the demand of commercialization. In this study, a kind of catalysts screening strategy on Agx Cu100-x nanoalloys is reported, containing the surface modification method, studies of activity enhancement mechanism, and applied research on zinc-air batteries. The results exhibit that the role of selective dealloying (DE) or galvanic displacement (GD) is limited by the "parting limitation", and this "parting limitation" determines the surface topography, position of d-band center, and ORR performance of Agx Cu100-x alloys. The GD-Ag55 Cu45 and DE-Ag25 Cu75 catalysts alloys present excellent ORR performance that is comparable to Pt/C-20%. The relationship between electronic perturbation and specific activity demonstrates that positive shift of the d-band center (≈0.12 eV, relative to Ag) for GD-Ag55 Cu45 is beneficial for ORR, which is contrary to Pt-based alloys (negative shift, ≈0.1 eV). Meanwhile, extensive electrochemical and electronic structure characterization indicates that the high work function of GD-Ag55 Cu45 (4.8 eV) is the reason behind their excellent durability for zinc-air batteries.

Ostwald ripening versus single atom trapping: towards understanding platinum particle sintering.

Ostwald ripening is a leading cause of the degradation of platinum group catalysts at high temperature in an oxidizing atmosphere. Recent experiments suggested that volatile species can be trapped on ceria, forming atomically dispersed active catalytic sites instead of large nanoparticles. Here we present a comparative density functional theory study of the interaction of PtO2(g), the most likely volatile species responsible for the process of Ostwald ripening, with various surfaces. Defect-free CeO2(111) and Al2O3(100) surfaces have a very small binding energy towards PtO2(g) compared to the platinum surface, indicative of particle growth. However, the stepped edge of the CeO2(111) surface effectively traps the mobile species, generating atomically dispersed catalysts. Such trapped single-atom platinum-on-ceria catalysts are predicted to have a square-planar [PtO4] structure, with the platinum atom strongly binding to the surface, preventing platinum atoms from aggregating into larger nanoparticles. These results provide an atomic insight into the single atom trapping and suggest a route for the development of sinter-resistant catalysts.

Development of Deuterium Labeling Method Based on the Heterogeneous Platinum Group Metal-Catalyzed C-H Activation

Deuterium (D) labeled compounds are utilized in various scientific fields such as mechanistic elucidation of reactions, preparation of new functional materials, tracers for microanalysis, deuterium labeled heavy drugs and so on. Although the H-D exchange reaction is a straightforward method to produce deuterated organic compounds, many precedent methods require expensive deuterium gas and/or harsh reaction conditions. A part of our leading research agendas is intended to the development of novel and functional heterogeneous platinum-group catalysts and the reclamation of unknown functionalities of existing heterogeneous platinum-group catalysts. During the course of the study, benzylic positions of substrates were site-selectively deuterated under mild and palladium-on-carbon (Pd/C)-catalyzed hydrogenation conditions in heavy water (D2O). Heat conditions promoted the H-D exchange reactivity and facilitated the H-D exchange reaction at not only the benzylic sites but also inactive C-H bonds and heterocyclic nuclei. It is noteworthy that platinum-on-carbon (Pt/C) indicated a quite high affinity toward aromatic nuclei, and the H-D exchange reaction was strongly enhanced by the use of Pt/C as a catalyst under milder conditions. The mixed use of Pd/C and Pt/C was found to be more efficient in the H-D exchange reaction compared to the independent use of Pd/C or Pt/C. Furthermore, simple alkanes could also be efficiently deuterated under rhodium-on-carbon (Rh/C)-catalyzed conditions. The use of ruthenium-on-carbon (Ru/C) enabled the regiospecific and efficient deuterium incorporation at α-positions of alcohols and results were applied as a regio- and stereoselective multi-deuteration method of sugar derivatives.

Stable platinum nanoparticles on specific MgAl2O4 spinel facets at high temperatures in oxidizing atmospheres

The development of thermally stable, nanometer-sized precious metal-based catalysts remains a daunting challenge. Such materials, especially those based on the use of costly platinum metal, are essential and, to date, non-replaceable for a large number of industrially important catalytic processes. Here we report a well-defined cuboctahedral MgAl2O4 spinel support material that is capable of stabilizing platinum particles in the range of 1-3 nm on its relatively abundant {111} facets during extremely severe aging at 800 °C in air for 1 week. The aged catalysts retain platinum dispersions of 15.9% with catalytic activities for methanol oxidation being ~80% of that of fresh ones, whereas a conventional Pt/γ-Al2O3 catalyst is severely sintered and nearly inactive. We reveal the origin of the markedly superior ability of spinel {111} facets, resulting from strong interactions between spinel surface oxygens and epitaxial platinum {111} facets, inspiring the rational design of anti-sintering supported platinum group catalysts.

Characterization of Water/Sucrose Laurate/n‐Propanol/Allylbenzene Microemulsions

AbstractWater/n‐propanol/sucrose laurate/allylbenzene micellar systems were formulated and applied in the isomerization of allylbenzene in the presence of heterogenized derivatives of some platinum group catalysts. The ratio (w/w) of n‐propanol/surfactant studied herewith was 2/1. Temperature insensitive microemulsions were found. The microemulsions were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity, and isentropic compressibility. The densities increase with increases in the water volume fraction. Excess volumes of the microemulsions decrease for water volume fractions below 0.2, level off for water volume fractions between 0.2 and 0.6 then increase for water volume fractions above 0.6. Excess volumes of the studied micellar systems increase with temperature. Isentropic compressibilities increase with temperature for water volume fractions below 0.8 and decrease for water volume fractions above 0.8. Structural transitions from water‐in‐oil to bicontinuous to oil‐in‐water occur along the microemulsion phase. The particle hydrodynamic diameter of the oil‐in‐water microemulsions at the 0.95 water volume fraction was found to decrease with temperature.

Solid-phase catalytic reactions of tritium with carbohydrates: 4. Mechanism of isomerization of D-glucose in the course of solid-phase catalytic hydrogenation with tritium

The influence exerted on the solid-phase catalytic hydrogenation of lactulose with tritium by temperature in the range 130–160°C, platinum group catalysts, solid phase composition, and support surface area was studied. Lactose was identified in the reaction products along with labeled lactulose. The mechanism of isomerization of sugars in the solid phase under the action of hydrogen spillover was suggested. Isomerization of sugars occurs by a complex mechanism similar to acid-catalyzed keto-enol tautomerization of epimeric sugars in solution, and the active species in SCH of sugars with tritium is spillover hydrogen in the form of proton.

Controlled selectivity for palladium catalysts using self-assembled monolayers

The selective reaction of one part of a bifunctional molecule is a fundamental challenge in heterogeneous catalysis and for many processes including the conversion of biomass-derived intermediates. Selective hydrogenation of unsaturated epoxides to saturated epoxides is particularly difficult given the reactivity of the strained epoxide ring, and traditional platinum group catalysts show low selectivities. We describe the preparation of highly selective Pd catalysts involving the deposition of n-alkanethiol self-assembled monolayer (SAM) coatings. These coatings improve the selectivity of 1-epoxybutane formation from 1-epoxy-3-butene on palladium catalysts from 11 to 94% at equivalent reaction conditions and conversions. Although sulphur species are generally considered to be indiscriminate catalyst poisons, the reaction rate to the desired product on a catalyst coated with a thiol was 40% of the rate on an uncoated catalyst. Interestingly the activity decreased for less-ordered SAMs with shorter chains. The behaviour of SAM-coated catalysts was compared with catalysts where surface sites were modified by carbon monoxide, hydrocarbons or sulphur atoms. The results suggest that the SAMs restrict sulphur coverage to enhance selectivity without significantly poisoning the activity of the desired reaction.