Lignocellulosic biomasses have a tremendous potential to cover the future demand of bio-based chemicals and materials, breaking down our historical dependence on petroleum resources. The development ...of green chemical technologies, together with the appropriate eco-politics, can make a decisive contribution to a cheap and effective conversion of lignocellulosic feedstocks into sustainable and renewable chemical building blocks. In this regard, the use of an indirect H-source for reducing the oxygen content in lignocellulosic biomasses and in their derived platform molecules is receiving increasing attention. In this contribution we highlight recent advances in the transfer hydrogenolysis of cellulose, hemicellulose, lignin, and of their derived model molecules promoted by heterogeneous catalysts for the sustainable production of biofuels and biochemicals.
Carbon (Vulcan XC-72)-supported bimetallic Pd–Ir catalysts with different Pd/Ir proportions (5–50 mol% Ir, 2 wt% Pd) were prepared by “water-in-oil” microemulsion method (w/o) using solutions of low ...(0.02 M, L series) and high concentration (0.2 M, H series) of the metals precursors (PdCl
2
and IrCl
3
). The bimetallic particles were examined in terms of nanoscale phase properties (extent of Pd–Ir alloying, phase separation), surface composition (Pd and Ir fractions) and electrocatalytic performance for the formic acid oxidation reaction. Structural characterization was performed using XRD, SEM and HRTEM techniques. Electrochemical characterization allowed estimating the PdH formation ability and the surface composition of Pd–Ir particles what was confirmed by XPS data. The Pd–Ir nanoparticles of similar average size (ca. 4 nm), close to that of Ir (3.8 nm) and below that of Pd (6.2 nm) were formed regardless of the Pd/Ir proportion and the concentration of the metals precursors in the w/o. In contrast to the largely alloyed PdIr nanoparticles with the Pd-rich surface formed at low concentration of the metals precursors (0.02 M), the particles of almost closed surface and bulk Pd/Ir ratios composed mostly of randomly distributed single-phase domains were formed at high concentration (0.2 M). At the lowest bulk Ir content, 5 mol%, the particles have Ir-rich surface regardless of the preparation method. The catalytic studies involving formic acid electrooxidation reaction showed the activity enhancement for the L series catalysts with respect to monometallic Pd/C (twofold TOF increase) and H series counterparts. The Pd
85
Ir
15
/C catalyst of the Pd–Ir alloyed and the surface composition expressed by the Pd/Ir atomic ratio near to 6 displayed the highest activity which was 2.9-times higher relative to that of Pd.
Graphic abstract
Heating PdAu nanoparticles deposited on silica in hydrogen results in palladium silicide(s) formation. The degree of palladium extraction by silicon species (ex-silica) is conversely correlated with ...the gold content in the PdAu alloys. In addition, the phase changes are less extensive than analogous transformations in a previously investigated PdCu-silica composite. Very high temperatures (>550 °C) are required to initiate the transformation of PdAu-SiO2 into respective bulk metal silicide(s), whereas for the PdCu-SiO2 (and Pd-SiO2) similar changes have already been observed at ≤450 °C. Based on existing literature data, it is considered that the reason for this difference is a relative availability of palladium-rich material at the Pd-IB alloy-silica phase boundary. In the case of PdAu, the segregation of gold at the surface is very pronounced, making this alloy more resistant against Pd extraction required for the silicide formation.
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•H2 pretreatment of Pd-Au/SiO2 at ≤550 °C does not result in palladium siliciding.•Inverse correlation between ability of PdSix formation and Au content in Pd-Au/SiO2.•Pd content in shell layer explains difference between siliciding of Pd-Au and Pd-Cu.
We present an efficient strategy for synthesising the PdAu catalysts with a homogeneous PdAu alloy phase for environmentally important hydrodechlorination of tetrachloromethane in the gas phase. The ...synthesis of carbon-supported catalysts involved two major steps: (i) incorporation of palladium and gold nanoparticles into carbon support and (ii) activation of the catalysts. The critical part of this work was to find the optimal conditions for both steps. Thus, the incorporation of the nanoparticles was carried out in two ways, by impregnation and direct redox reaction method using acetone solutions of metal precursor salts. The activation was performed either by a conventional thermal reduction in hydrogen or flash irradiation in a microwave oven. The homogeneity and structure of the PdAu alloy were found to depend on the catalyst activation method critically. In all cases, we observed better homogeneity for catalysts that were subject to microwave irradiation. Moreover, the flash microwave irradiation of prepared catalysts provided catalysts of better stability and selectivity towards the desired products (hydrocarbons) in the hydrodechlorination of tetrachloromethane as compared to the catalyst obtained by conventional thermal activation in hydrogen.
Gas flow-through microcalorimetry has been applied to study the Pd/Al2O3 type catalysts in the exothermic hydrogen recombination process: H2 + O2 → H2O, in view of the potential application in the ...passive autocatalytic recombination (PAR) technology. The flow mode experiments revealed thermokinetic oscillations, i.e., the oscillatory rate of heat evolution accompanying the process and the corresponding oscillations in the differential heat of process, in sync with oscillatory conversion of hydrogen. Mathematical evidence has been found for the deterministic character of the aperiodic oscillations. In the outburst of quasiperiodic oscillations of large amplitude, the instances of differential heats as high as 700 kJ/mol H2 have been detected, exceeding the heat of the gaseous water formation from elements (242 kJ/mol H2) by a factor of nearly 3. Another occurrence of anomalously high thermal effects has been measured in calorimetric oxygen titration using 0.09 μmol pulses of O2 injected onto hydrogen- or deuterium-saturated catalysts, including 2%Pd/Al2O3, 5%Pd/Al2O3 and 2%PdAu/Al2O3. Repeatedly, the saturation/oxidation cycles showed the heat evolutions in certain individual O2 pulses as high as 1100 kJ/mol O2, i.e., 550 kJ/mol H2, again 2 times as much as the heat of water formation. It has been pointed out that it seems prudent for the PAR technologists to assume a much larger rate of heat evolution than those calculated on the basis of a standard thermodynamic value of the heat of water formation, in order to account for the possibility of large thermokinetic oscillation occasionally appearing in the recombination process of hydrogen. A possible relation of the anomalous heat evolution to an inadvertent occurrence of low energy nuclear reaction (LENR) phenomena is also briefly considered.
Supported Pd/Ir bimetallic catalysts were synthesized by the “water-in-oil” microemulsion method at different precursor concentrations and characterized by XRD, XPS, SEM, TEM, and cyclic voltammetry. ...Depending on the preparation conditions, formation of bimetallic catalysts with different metal segregation and surface composition can be easily obtained, thus tuning the bimetallic structure of catalysts as well as their relative catalytic properties. Bimetallic Pd/Ir systems were efficiently tested in the hydrogenation of cinnamaldehyde showing a better performance than analogous monometallic catalysts.
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