Aqueous phase reforming of glycerol over Re-promoted Pt and Rh catalysts

The potential of Re promotion for carbon-supported Pt and Rh nanoparticles was investigated for aqueous phase reforming (APR) of glycerol as a model compound for biobased feedstock. Upon alloying with Re, the overall conversion rate of glycerol was substantially increased for both metal catalysts. Whereas Pt/C is more active than Rh/C in glycerol APR, the RhRe/C catalyst outperforms PtRe/C. The overall APR catalytic performance strongly correlates with the activity trend for the gas-phase water–gas shift (WGS) reaction. RhRe/C exhibited the highest activity in APR and WGS reactions. A very strong synergy between Rh and Re and Pt and Re is observed in the model WGS reaction. The role of Re in the bimetallic catalysts is to facilitate water dissociation, effectively increasing the WGS activity. During APR, this results in lower steady-state CO coverage and increased glycerol conversion rates. In terms of selectivity, the yield of renewable hydrogen is increased. The use of Re as a promoter also results in significant changes in the product selectivities during glycerol APR. Although gas-phase acetaldehyde decomposition measurements evidenced that alloying with Re increased C–C bond cleavage activities of Pt/C and Rh/C, the increased acidity due to acidic hydroxyl groups bound to Re resulted in a more substantial increase of dehydration reactions. Whereas Rh/C is more selective for formation of products involving C–C bond cleavage than Pt/C, the product mixtures of their alloys with Re reflect a much increased ratio of C–O vs. C–C bond cleavage reaction rates

Influence of particle size on the activity and stability in steam methane reforming of supported Rh nanoparticles

The influence of Rh nanoparticle size and type of support on the catalytic performance in steam methane reforming has been investigated to clarify the nature of the rate-controlling step. A set of Rh catalysts was prepared using ZrO2, CeO2, CeZrO2 and SiO2 supports. The nature and dispersion of the active Rh metal phase was studied by H2-chemisorption, TEM and X-ray absorption spectroscopy. The particle size was varied between 1 and 9 nm. The degree of Rh reduction depends on the particle size and the support. Very small particles cannot be fully reduced, especially when ceria is the support. The intrinsic rate per surface metal atom increases linearly with the Rh metal dispersion and does not depend on the type of support. With the support of kinetic data, it is concluded that dissociative CH4 adsorption is the single rate-controlling step at least at reaction temperatures above 325 °C. This implies that the overall rate is controlled by the density of low-coordinated edge and corner metal atoms in the nanoparticles. These particles contain sufficient step edge sites to provide an easy reaction pathway for CO recombination reactions. Catalysts with Rh nanoparticles smaller than 2.5 nm deactivate more strongly than catalysts with larger nanoparticles. Characterization of spent catalysts by X-ray absorption spectroscopy shows that deactivation is due to the oxidation of very small particles under the steam methane reforming reaction conditions

Identification of step-edge sites on Rh nanoparticles for facile CO dissociation

Understanding the dependence of the rate of catalytic reactions on metal nanoparticle size remains one of the great challenges in heterogeneous catalysis. Especially, methods to probe step-edge sites on technical supported nanoparticle catalysts are needed to put structure–activity relations on a surer footing. Herein, we demonstrate that N2 is a useful IR probe for the semi-quantitative identification of step-edge sites on supported metallic Rh nanoparticles. The intensity of the strongly perturbed band at 2205 cm− 1 correlates with the CO bond dissociation rate under conditions relevant to the Fischer–Tropsch reaction. Due to the intermediate reactivity of Rh, step-edge sites are required to dissociate the strong CO bond. DFT calculations show that N2 prefers to adsorb on top of low-coordinated surface atoms such as steps, corners and edges. The occurrence of the intensity maximum at intermediate particle size is explained by the presence of surface overlayers on terraces that give rise to step-edges. These step-edge sites are important in the dissociation of di-atomic molecules such as CO, NO and N2

The Influence and Removability of Colloidal Capping Agents on Carbon Monoxide Hydrogenation by Zirconia-Supported Rhodium Nanoparticles

By using wet-chemical methods, cubic, tetrahedral, and randomly shaped Rh nanoparticles (NPs) with different surface terminations were synthesized and subsequently deposited on ZrO2 supports. To guide the NP shape, three capping agents were used during the synthesis: polyvinylpyrrolidone (PVP), trimethyl(tetradecyl)ammonium bromide (TTAB), and oleylamine (OAm). TTAB and PVP could not be completely removed from the final catalyst, leaving a capping residue as confirmed by X-ray photoelectron spectroscopy (XPS). In contrast, OAm could be fully removed. The influence of the NP shape and the influence of the capping agents were evaluated under CO hydrogenation conditions. Both the PVP and TTAB residues blocked parts of the Rh surface and dominated catalytic activity beyond the effects from NP shape and surface termination. OAm could be successfully removed. The extent of metal surface blocking by the capping residue, probed by chemisorption, is larger than the observed reduction in CO hydrogenation activity. This suggests that a majority of less-active sites are being blocked by the capping residue and that successful removal of the residue from the most active sites is possible.\u3cbr/\u3

Gold stabilized by nanostructured ceria supports : nature of the active sites and catalytic performance

The interaction of gold atoms with CeO2 nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition–precipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2–6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O–H bond cleavage appears to be rate limiting, while this shifts to C–H bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria

Continuous synthesis of γ–valerolactone in a trickle-bed reactor over supported nickel catalysts

Various Ni-based catalysts were tested in the continuous liquid phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in a trickle-bed reactor using water as solvent with the aim to develop an economic and environmentally friendly way for the GVL synthesis. For this purpose, various synthesis methods were used to prepare Ni-based catalysts, which were first screened in batch reactors. Characterization by X-ray diffraction, temperature-programmed reduction, electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy showed that slow precipitation using urea resulted in a good Ni dispersion. The dispersion also improved at lower Ni loading, and smaller Ni particles mostly showed an enhanced catalytic performance for the synthesis of GVL. 5 wt % Ni/Al2O3 prepared by wet impregnation showed the highest specific activity for the hydrogenation of LA to GVL (90% LA conversion and 75% GVL yield) featuring an average Ni particle size of 6 nm. Some deactivation of the catalysts was observed, probably due to transformation of γ-Al2O3 to boehmite and sintering of the Ni particles. In addition, reoxidation of Ni particles may additionally lead to deactivation as concluded by comparison with screening studies in batch reactors