Jechan Lee

Aqueous-phase hydrogenation and hydrodeoxygenation of biomass-derived oxygenates with bimetallic catalysts

The reaction rate on a per site basis for aqueous-phase hydrogenation (APH) of propanal, xylose, and furfural was measured over various alumina-supported bimetallic catalysts (Pd–Ni, Pd–Co, Pd–Fe, Ru–Ni, Ru–Co, Ru–Fe, Pt–Ni, Pt–Co, and Pt–Fe) using a high-throughput reactor (HTR). The results in this paper demonstrate that the activity of bimetallic catalysts for hydrogenation of a carbonyl group can be 110 times higher than monometallic catalysts. The addition of Fe to a Pd catalyst increased the activity for hydrogenation of propanal, xylose, and furfural. The Pd1Fe3 catalyst had the highest reaction rate for APH of propanal among all catalysts tested in the HTR. The addition of Fe to the Pd catalyst increased the reaction rate for xylose hydrogenation by a factor of 51, compared to the monometallic Pd catalyst. However, no bimetallic catalyst tested in this study was more active than the monometallic Ru catalyst for hydrogenation of xylose. The Pd1Fe3 catalyst had the highest reaction rate for APH of furfural, which was 9 times higher than the rate of the Pd catalyst. The Pd1Fe3/Zr–P, a bimetallic bifunctional catalyst, was 14 times more active on a per site basis than a Pd/Zr–P catalyst for aqueous-phase hydrodeoxygenation (HDO) of sorbitol in a continuous flow reactor. The addition of Fe to the Pd catalyst increased the rate of C–C cleavage reactions and promoted the conversion of sorbitan and isosorbide in HDO of sorbitol. Pd1Fe3/Zr–P also had a higher yield of gasoline-range products than the Pd/Zr–P catalyst.

Enhanced stability of cobalt catalysts by atomic layer deposition for aqueous-phase reactions

A thin atomic layer deposition (ALD) TiO2 coating successfully stabilizes cobalt particles supported on TiO2 for aqueous-phase hydrogenation (APH) reactions by preventing leaching and sintering of cobalt. The uncoated conventional cobalt catalysts leach under the same conditions. Using Al2O3 coating of Co/γ-Al2O3 causes the formation of an irreducible cobalt aluminate phase which has no catalytic activity. The ALD TiO2 decorated cobalt catalyst is active for APH of a range of feedstocks including furfuryl alcohol, furfural, and xylose whereas classic non-ALD cobalt catalysts have very low activity for these reactions.

The electrocatalytic hydrogenation of furanic compounds in a continuous electrocatalytic membrane reactor

The electrocatalytic hydrogenation of biomass derived oxygenates in a continuous electrocatalytic membrane reactor presents a promising method of fuel and chemical production that minimizes usage of solvents and has the potential to be powered using renewable electricity. In this paper we demonstrate the use of a continuous-flow electrocatalytic membrane reactor for the reduction of aqueous solutions of furfural into furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA), 2-methylfuran (MF) and 2-methyltetrahydrofuran (MTHF). Protons needed for hydrogenation were obtained from the electrolysis of water at the anode of the reactor. Pd was identified as the most active monometallic catalyst of 5 different catalysts tested for the hydrogenation of aqueous furfural with hydrogen gas in a high-throughput reactor. Thus Pd/C was tested as a cathode catalyst for the electrocatalytic hydrogenation of furfural. At a power input of 0.1W, Pd/C was 4.4 times more active (per active metal site) as a cathode catalyst in the electrocatalytic hydrogenation of furfural than Pt/C. The main products for the electrocatalytic hydrogenation of furfural were FA (54–100% selectivity) and THFA (0–26% selectivity). MF and MTHF were also detected in selectivities of 8%. Varying the reactor temperature between 30 °C and 70 °C had a minimal effect on reaction rate for furfural conversion. Using hydrogen gas at the anode, in place of water electrolysis, produced slightly higher rates of product formation at a lower power input. Sparging hydrogen gas on the cathode had no effect on reaction rate or selectivity, and was used to examine the addition of recycling loops to the continuous electrocatalytic membrane reactor.

Narrow band gap (1 eV) InGaAsSbN solar cells grown by metalorganic vapor phase epitaxy

Heterojunction solar cell structures employing InGaAsSbN (Eg ∼ 1u2009eV) base regions are grown lattice-matched to GaAs substrates using metalorganic vapor phase epitaxy. Room temperature (RT) photoluminescence (PL) measurements indicate a peak spectral emission at 1.04u2009eV and carrier lifetimes of 471–576 ps are measured at RT from these structures using time-resolved PL techniques. Fabricated devices without anti-reflection coating demonstrate a peak efficiency of 4.58% under AM1.5 direct illumination. Solar cells with a 250u2009nm-thick InGaAsSbN base layer exhibit a 17% improvement in open circuit voltage (Voc), 14% improvement in fill factor, and 12% improvement in efficiency over the cells with a thicker (500u2009nm-thick) base layer.

Selective Glycerol Oxidation by Electrocatalytic Dehydrogenation

This study demonstrates that an electrochemical dehydrogenation process can be used to oxidize glycerol to glyceraldehyde and glyceric acid even without using stoichiometric chemical oxidants. A glyceric acid selectivity of 87.0 % at 91.8 % glycerol conversion was obtained in an electrocatalytic batch reactor. A continuous-flow electrocatalytic reactor had over an 80 % high glyceric acid selectivity at 10 % glycerol conversion, as well as greater reaction rates than either an electrocatalytic or a conventional catalytic batch reactor.

Modeling aqueous-phase hydrodeoxygenation of sorbitol over Pt/SiO2–Al2O3

In this paper, we investigated the effects of temperature, hydrogen partial pressure, and sorbitol concentration on the aqueous-phase hydrodeoxygenation (APHDO) of sorbitol over a bifunctional 4 wt% Pt/SiO2–Al2O3 catalyst in a trickle bed reactor. APHDO involves four fundamental reactions: (1) hydrogenation; (2) dehydration; (3) C–C bond cleavage by dehydrogenation and decarbonylation; and (4) C–C bond cleavage by dehydrogenation and retro-aldol condensation. The main deoxygenation routes are decarbonylation and alcohol dehydration. Retro-aldol condensation plays a critical role in reducing the carbon number of the products. The key products in this system are C1–C6 n-alkanes, primary and secondary alcohols, and carbon dioxide. As shown in this paper, the reaction conditions can dramatically change the product selectivity for APHDO of biomass-derived feedstocks (e.g., sorbitol). A sorbitol hydrodeoxygenation reaction network was generated that predicts all of the 43 experimentally measured species. The reaction network consists of 4804 reactions and produces a total of 1178 distinct chemical species. The associated material balance equations were solved numerically to model the experimentally observed species as a function of temperature, concentration, and pressure. The model concentrations fit well the experimentally measured values, demonstrating that the model was accurately able to model the reaction families and capture the salient features of the experimental observations. The trend observed in this paper can be used for the optimization of reactors and new catalysts to selectively make targeted products by hydrodeoxygenation of biomass-derived feedstocks.

Impact of thermal annealing on bulk InGaAsSbN materials grown by metalorganic vapor phase epitaxy

Two different thermal annealing techniques (rapid thermal annealing (RTA) and in-situ post-growth annealing in the metalorganic vapor phase epitaxy (MOVPE) chamber) were employed to investigate their impact on the optical characteristics of double-heterostructures (DH) of InGaAsSbN/GaAs and on the performance of single-junction solar cell structures, all grown by MOVPE. We find that an optimized RTA procedure leads to a similar improvement in the photoluminescence (PL) intensity compared with material employing a multi-step optimized anneal within the MOVPE reactor. Time-resolved photoluminescence techniques at low temperature (LT) and room temperature (RT) were performed to characterize the carrier dynamics in bulk InGaAsSbN layers. Room temperature carrier lifetimes were found to be similar for both annealing methods, although the LT-PL (16 K) measurements of the MOVPE-annealed sample found longer lifetimes than the RTA-annealed sample (680 ps vs. 260 ps) for the PL measurement energy of 1.24 eV. InGaAsSbN-based single junction solar cells processed with the optimized RTA procedure exhibited an enhancement of the electrical performance, such as improvements in open circuit voltage, short circuit current, fill factor, and efficiency over solar cells subjected to the in-situ MOVPE annealing technique.