@article{thompson_lamb_2024, title={Alloying and Segregation in PdRe/Al<sub>2</sub>O<sub>3</sub> Bimetallic Catalysts for Selective Hydrogenation of Furfural}, volume={14}, ISSN={["2073-4344"]}, DOI={10.3390/catal14090604}, abstractNote={X-ray absorption fine structure (XAFS) spectroscopy, temperature-programmed reduction (TPR), and temperature-programmed hydride decomposition (TPHD) were employed to elucidate the structures of a series of PdRe/Al2O3 bimetallic catalysts for the selective hydrogenation of furfural. TPR evidenced low-temperature Re reduction in the bimetallic catalysts consistent of the migration of [ReO4]− (perrhenate) species to hydrogen-covered Pd nanoparticles on highly hydroxylated γ-Al2O3. TPHD revealed a strong suppression of β-PdHx formation in the reduced catalysts prepared by (i) co-impregnation and (ii) [HReO4] impregnation of the reduced Pd/Al2O3, indicating the formation of Pd-rich alloy nanoparticles; however, reduced catalysts prepared by (iii) [Pd(NH3)4]2+ impregnation of calcined Re/Al2O3 and subsequent re-calcination did not. Re LIII X-ray absorption edge shifts were used to determine the average Re oxidation states after reduction at 400 °C. XAFS spectroscopy and high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) revealed that a reduced 5 wt.% Re/Al2O3 catalyst contained small Re clusters and nanoparticles comprising Re atoms in low positive oxidation states (~1.5+) and incompletely reduced Re species (primarily Re4+). XAFS spectroscopy of the bimetallic catalysts evidenced Pd-Re bonding consistent with Pd-rich alloy formation. The Pd and Re total first-shell coordination numbers suggest that either Re is segregated to the surface (and Pd to the core) of alloy nanoparticles and/or segregated Pd nanoparticles are larger than Re nanoparticles (or clusters). The Cowley short-range order parameters are strongly positive indicating a high degree of heterogeneity (clustering or segregation of metal atoms) in these bimetallic catalysts. Catalysts prepared using the Pd(NH3)4[ReO4]2 double complex salt (DCS) exhibit greater Pd-Re intermixing but remain heterogeneous on the atomic scale.}, number={9}, journal={CATALYSTS}, author={Thompson, Simon T. and Lamb, H. Henry}, year={2024}, month={Sep} }
 @article{thompson_lamb_2023, title={Palladium-Rhenium Catalysts for Selective Hydrogenation of Furfural: Influence of Catalyst Preparation on Structure and Performance}, volume={13}, ISSN={["2073-4344"]}, DOI={10.3390/catal13091239}, abstractNote={PdRe/Al2O3 catalysts are highly selective for hydrogenation of furfural to furfuryl alcohol (FAL). Moreover, the synergy between the metals can result in greater specific activity (higher turnover frequency, TOF) than exhibited by either metal alone. Bimetallic catalyst structure depends strongly on the metal precursors employed and their addition sequence to the support. In this work, PdRe/Al2O3 catalysts were prepared by: (i) co-impregnation (CI) and sequential impregnation (SI) of γ-Al2O3 using HReO4 and Pd(NO3)2, (ii) SI using NH4ReO4 and [Pd(NH3)4(NO3)2], (iii) HReO4 addition to a reduced and passivated Pd/Al2O3 catalyst, and (iv) impregnation with the double complex salt (DCS), [Pd(NH3)4(ReO4)2]. Raman spectroscopy and temperature-programmed reduction (TPR) evidence larger supported PdO crystallites in catalysts prepared using Pd(NO3)2 than [Pd(NH3)4(NO3)2]. Surface [ReO4]− species detected by Raman exhibit TPR peak temperatures from ranging 85 to 260 °C (versus 375 °C for Re/Al2O3). After H2 reduction at 400 °C, the catalysts were characterized by chemisorption, temperature-programmed hydride decomposition (TPHD), CO diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and scanning transmission electron microscopy (STEM) with energy-dispersive x-ray (EDX) spectroscopy. The CI catalyst containing supported Pd–Re alloy crystallites had a TOF similar to Pd/Al2O3 but higher (61%) FAL selectivity. In contrast, catalysts prepared by methods (ii–iv) containing supported Pd-Re nanoparticles exhibit higher TOFs and up to 78% FAL selectivity.}, number={9}, journal={CATALYSTS}, author={Thompson, Simon T. and Lamb, H. Henry}, year={2023}, month={Sep} }
 @article{thompson_lamb_2017, title={Catalysts for selective hydrogenation of furfural derived from the double complex salt [Pd(NH3)(4)](ReO4)(2) on gamma-Al2O3}, volume={350}, ISSN={["1090-2694"]}, DOI={10.1016/j.jcat.2017.03.019}, abstractNote={The double complex salt [Pd(NH3)4](ReO4)2 was employed as precursor of supported bimetallic catalysts for selective hydrogenation of furfural. Direct reduction of [Pd(NH3)4](ReO4)2 on γ-Al2O3 in flowing H2 at 400 °C yields bimetallic nanoparticles 1–2 nm in size that exhibit significant interaction between the metals, as evidenced by temperature-programmed hydride decomposition (complete suppression of β-PdHx formation), extended X-ray absorption fine structure spectroscopy at the Pd K and Re LIII edges (PdRe distance = 2.72 Å), and scanning transmission electron microscopy with energy dispersive X-ray analysis. In contrast, calcination of [Pd(NH3)4](ReO4)2 on γ-Al2O3 at 350 °C in air and subsequent reduction in H2 at 400 °C results in metal segregation and formation of large (>50 nm) supported Pd particles; Re species cover the Pd particles and γ-Al2O3 support. A PdRe 1:2 catalyst prepared by sequential impregnation and calcination using HReO4 and [Pd(NH3)4](NO3)2 has a similar morphology. The catalyst derived by direct reduction of [Pd(NH3)4](ReO4)2 on γ-Al2O3 exhibits remarkably high activity for selective hydrogenation of furfural to furfuryl alcohol (FAL) at 150 °C and 1 atm. Suppression of H2 chemisorption via elimination of Pd threefold sites, as evidenced by CO diffuse-reflectance infrared Fourier transform spectroscopy, correlates with increased FAL selectivity.}, journal={JOURNAL OF CATALYSIS}, author={Thompson, Simon T. and Lamb, H. Henry}, year={2017}, month={Jun}, pages={111–121} }
 @article{thompson_lamb_delley_franzen_2017, title={Vibrational spectroscopy of the double complex salt Pd(NH3)4(ReO4)2, a bimetallic catalyst precursor}, volume={173}, ISSN={1386-1425}, url={http://dx.doi.org/10.1016/J.SAA.2016.10.011}, DOI={10.1016/J.SAA.2016.10.011}, abstractNote={Tetraamminepalladium(II) perrhenate, a double complex salt, has significant utility in PdRe catalyst preparation; however, the vibrational spectra of this readily prepared compound have not been described in the literature. Herein, we present the infrared (IR) and Raman spectra of tetraamminepalladium(II) perrhenate and several related compounds. The experimental spectra are complemented by an analysis of normal vibrational modes that compares the experimentally obtained spectra with spectra calculated using DFT (DMol3). The spectra are dominated by features due to the ammine groups and the ReO stretch in Td ReO4-; lattice vibrations due to the D4h Pd(NH3)42+ are also observed in the Raman spectrum. Generally, we observe good agreement between ab initio calculations and experimental spectra. The calculated IR spectrum closely matches experimental results for peak positions and their relative intensities. The methods for calculating resonance Raman intensities are implemented using the time correlator formalism using two methods to obtain the excited state displacements and electron-vibration coupling constants, which are the needed inputs in addition to the normal mode wave numbers. Calculated excited state energy surfaces of Raman-active modes correctly predict relative intensities of the peaks and Franck-Condon activity; however, the position of Raman bands are predicted at lower frequencies than observed. Factor group splitting of Raman peaks observed in spectra of pure compounds is not predicted by DFT.}, journal={Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy}, publisher={Elsevier BV}, author={Thompson, Simon T. and Lamb, H. Henry and Delley, Bernard and Franzen, Stefan}, year={2017}, month={Feb}, pages={618–624} }
 @article{sun_schulz_thompson_lamb_2016, title={Catalytic deoxygenation of octanoic acid over silica- and carbon-supported palladium: Support effects and reaction pathways}, volume={269}, ISSN={["1873-4308"]}, DOI={10.1016/j.cattod.2015.12.021}, abstractNote={Octanoic acid (OA) deoxygenation was investigated over silica- and carbon-supported palladium catalysts (each containing 5 wt.% Pd) at 235–300 °C and 1 atm in a continuous flow reactor. A commercial Pd/SiO2 (A) catalyst was active for OA decarbonylation (DCN) and hydrodeoxygenation (HDO) at 260 °C under 10% H2; subsequent hydrogenation (HY) and DCN of the primary products, 1-heptene and octanal, respectively, produced n-heptane. Under equivalent conditions, a Pd/SiO2 (B) catalyst prepared using Pd(NO3)2 and Aerosil 300 produced n-heptane with very high selectivity (>99%) via DCN/HY. In contrast, a commercial Pd/C (A) catalyst was highly active and selective to n-heptane (>98%) and CO2 (65%) under these conditions. Moreover, CO2 selectivity and n-heptane yield increased with reaction temperature consistent with direct decarboxylation (DCX). Increasing H2 partial pressure resulted in markedly lower activity and CO2 selectivity; however, Pd/C (A) had negligible activity under He. Pd/C (A) exhibited greater water–gas shift (WGS) activity than Pd/SiO2 (A); however, differences in WGS activity alone cannot explain the observed support effect. A more highly dispersed Pd/C (B) catalyst was more active at 260 °C under H2 than Pd/C (A); however, under 10% H2, it had lower activity, CO2 selectivity (55%), and stability. Pd/C (A) and Pd/C (B) have very similar textural properties, but Pd/C (A) has a much higher Na content. By comparison, Pd supported on high-purity acetylene carbon black exhibited only DCN activity. These results indicate that carbon surface properties (e.g., polar functional groups, alkali metal content) influence the fatty acid deoxygenation performance of Pd/C catalysts.}, journal={CATALYSIS TODAY}, author={Sun, Keyi and Schulz, Taylor C. and Thompson, Simon T. and Lamb, H. Henry}, year={2016}, month={Jul}, pages={93–102} }
 @article{thompson_lamb_2016, title={Palladium-Rhenium Catalysts for Selective Hydrogenation of Furfural: Evidence for an Optimum Surface Composition}, volume={6}, ISSN={["2155-5435"]}, DOI={10.1021/acscatal.6b01398}, abstractNote={Bimetallic catalysts comprising a platinum-group metal (e.g., Pt, Pd, Ru) and rhenium (Re) have important applications in petroleum refining, industrial chemicals production, and biomass conversion. In this work, a series of PdRe/Al2O3 catalysts was investigated for selective hydrogenation of furfural to furfuryl alcohol (FAL) at 150 °C and 1 atm in a differential reactor. The results demonstrate that PdRe/Al2O3 catalysts have greater FAL selectivity and activity than Pd/Al2O3 catalysts. Over the bimetallic catalysts, decreased furan production is accompanied by a marked increase in hydrogenation activity. PdRe 1:1 catalysts prepared using [Pd(NH3)4](NO3)2 are significantly more active than catalysts prepared using Pd(NO3)2. PdRe 1:2 catalysts are more selective to FAL but less active than 1:1 catalysts prepared using the same precursors. The superior activity of PdRe/Al2O3 catalysts for selective hydrogenation of furfural is inferred to result from Re surface modification of Pd nanoparticles—disrupting Pd ensembles and creating new highly active Pd–Re sites. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicates an enhanced linear to bridging ratio for CO chemisorbed on surface Pd atoms, supporting this hypothesis. The H/CO chemisorption ratio at 35 °C varies inversely with Re surface coverage and correlates with furfural turnover frequency (TOF) and FAL selectivity. Thus, the observed TOF maximum at H/CO ≈ 0.25 suggests an optimum surface composition of approximately 75% Re and 25% Pd. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) with energy-dispersive X-ray (EDX) analysis shows intimate contact of Re clusters with Pd nanoparticles in the most active catalysts.}, number={11}, journal={ACS CATALYSIS}, author={Thompson, Simon T. and Lamb, H. Henry}, year={2016}, month={Nov}, pages={7438–7447} }
 @article{sun_wilson_thompson_lamb_2015, title={Catalytic Deoxygenation of Octanoic Acid over Supported Palladium: Effects of Particle Size and Alloying with Gold}, volume={5}, ISSN={["2155-5435"]}, DOI={10.1021/cs501865n}, abstractNote={Catalytic deoxygenation of octanoic acid (OA) to n-heptane was investigated over silica-supported Pd and PdAu catalysts at 260 °C and 1 atm in a fixed-bed microreactor. Pd/SiO2 catalysts were prepared by incipient wetness (IW) and ion exchange (IE). Bimetallic catalysts were prepared using an IE procedure that is known to produce supported PdAu nanoparticles. The Pd nanoparticles (7.5 nm average size) in the Pd/SiO2 (IW) catalyst exhibit well-defined (100) and (111) facets, as evidenced by high-resolution electron microscopy (HREM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. As expected, the smaller nanoparticles (1.5 nm average size) in the Pd/SiO2 (IE) catalyst display strong linear and bridging CO DRIFTS bands. The PdAu/SiO2 (1/1 atomic ratio) catalyst contains 5 nm alloy nanoparticles with Pd-rich surfaces, as evidenced by HREM with energy-dispersive X-ray (EDX) analysis and in situ EXAFS spectroscopy. DRIFTS thermal desorption experiments demonstrated that alloying with Au reduced the CO adsorption energy on surface Pd sites. The Pd/SiO2 (IE) catalyst initially exhibited OA decarboxylation and decarbonylation activity but lost decarboxylation activity rapidly with time on stream (TOS). In contrast, the Pd/SiO2 (IW) catalyst had only decarbonylation activity, deactivated less rapidly with TOS, and could be regenerated by heating in H2 to remove OA residues. Alloying Pd with Au was found to improve catalyst stability without significantly affecting decarbonylation activity, as evidenced by the equivalent OA turnover frequencies of the Pd/SiO2 (IW) and PdAu/SiO2 (2/3) catalysts. The geometric and electronic effects of alloying reduce the CO adsorption energy and mitigate self-poisoning by OA and related species.}, number={3}, journal={ACS CATALYSIS}, author={Sun, Keyi and Wilson, Adria R. and Thompson, Simon T. and Lamb, H. Henry}, year={2015}, month={Mar}, pages={1939–1948} }