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•Hydrogen photocatalytic production from propan-2-ol and glycerol was studied on M/TiO2 systems (M=Pt, Pd, Au).•Glycerol underwent photo-reforming whereas propan-2-ol was ...dehydrogenated to acetone and hydrogen.•TOF values on Pt-based solids (0.5% by weight) increased with the metal particle size.•Results are discussed in terms of adsorption of both alcohols on the solids and metal work functions.•The highest hydrogen production rates were obtained with the use of propan-2-ol.
Two different C-3 alcohols (glycerol and propan-2-ol) were tested as sacrificial agents for H2 photocatalytic production on M/TiO2 (M=Pt, Pd and Au) solids. In all cases, synthetic methods led to average metal particle sizes in the 2–5nm range (for nominal metal loadings of 0.5–3% by weight). Glycerol underwent photo-reforming whereas propan-2-ol was selectively oxidized to acetone. Under the present experimental conditions, both processes were structure-sensitive with higher TOF values being obtained for bigger metal particle sizes. As for the influence of metal loading, the highest TOF values corresponded to the lowest Pt and Au contents (0.5% by weight). In the case of propan-2-ol, dehydrogenation activity followed the order expected considering the work function of the metals (Pt>Pd>Au). On the contrary, for more complex glycerol photoreforming TOF values decreased in the order Pt≈Au>Pd. For both substrates, a dissociative adsorption Langmuir-Hinshelwood mechanism was found. Results were interpreted in terms of the different adsorption strength of glycerol and propan-2-ol as evidenced by a competitive adsorption experiment. All in all, the highest hydrogen production rate values corresponded to the use of propan-2-ol as the sacrificial agent, its TOF value being ca. 5.5s−1 on Pt-0.5% synthesized by photo-deposition method.
The physicochemical properties and catalytic performance in the preferential CO oxidation (PROX) reaction of nanosized gold supported on doped-ceria were investigated. Zn- and Sm-doped Au/ceria ...catalysts were found to be more active than undoped Au/ceria, whereas the addition of lanthanum oxide had the opposite effect. A reductive pretreatment at 373 K for 1 h promoted catalytic activity. The ability of Au/doped ceria catalysts to tolerate the presence of CO
2 and H
2O in the feed was also studied. Adding CO
2 in the reactant feed provoked a decrease in catalyst activity; however, catalyst doping improved the resistance toward deactivation by CO
2. On the other hand, co-addition of CO
2 and H
2O counteracted the negative effect of CO
2, especially in the case of doped samples. IR studies of CO adsorbed at 90 K on the catalysts after different pretreatments gave information on the type of gold species present on the catalyst. The dispersion of gold depended on the nature of the dopant. Au/Zn–CeO
2 catalyst demonstrated the greatest dispersion as revealed by HRTEM measurements and comparison of FTIR intensity of the CO adsorption bands on the reduced samples. AuCe
x
clusters were formed on this catalyst by increasing the prereduction temperature. Large amounts of CO
2 were produced during the CO–O
2 interaction in the presence of a high concentration of zero-valent gold sites on the surface of the modified Au catalysts, confirming their important role in the CO oxidation reaction. IR spectra were collected after exposure to CO
+
O
2
+
H
2 and also after addition of water in the PROX reaction mixture over Au/Zn–CeO
2 at 400 K. The evolution of the FTIR spectra run at 90 K after admission of O
2 on preadsorbed CO on the most active catalyst (i.e., Au/Zn–CeO
2) demonstrates the roles of the highly dispersed gold and the reduced support in activating oxygen.
A strong influence of the amount of the Co3O4 promoter on the catalytic performance in methanol oxidation of different gold catalysts supported on ceria was observed. The activity followed the order: ...Au/10 wt% Co3O4-doped CeO2 > Au/5 wt% Co3O4-doped CeO2 > Au/15 wt% Co3O4-doped CeO2 > Au/CeO2 >> Au/Co3O4. FTIR measurements of adsorbed CO indicate that oxidized gold sites are initially present on the activated samples and that such species are involved in the methanol reaction. Methanol oxidation performed under static conditions gave rise at 75 degree C to mainly formate species on Au/CeO2 and to a large variety of different carbonate species on Au/10 wt% Co3O4-doped CeO2. FTIR and EXAFS analyses revealed that the active sites present on the best performing Au/CeO2 catalyst added with 10 wt% Co3O4 are oxidized gold species, close to Co sites, at the interface with the support, which are reduced under reaction conditions. These species are able to activate and to react with oxygen giving rise to formate and carbonate species.
The effect of preparation method on structural properties and activity of Cu-Mn spinel oxide catalysts for the low-temperature water-gas shift reaction (LT-WGSR) has been studied. Single-step ...urea-combustion and coprecipitation procedures were used for synthesis of the catalysts. Catalyst characterization was performed by N2 physisorption, XRD, HRTEM, CO-TPR, CO-TPD and FTIR spectroscopy. The WGS activity was evaluated in a conventional flow reactor in the temperature range of 140a240 degree C. The influence of reaction gas mixture, including either idealized or realistic reformate, H2O/CO ratio and contact time on activity were investigated. The catalytic activity tests carried out with both idealized and realistic reformate demonstrated the superior performance of the catalyst prepared by the single-step urea-combustion method. Moreover, comparison of the WGS activity of these catalysts with the one of a commercial CuO-ZnO-Al2O3 catalyst points out the potential application of Cu-Mn spinel oxide catalysts in LT-WGSR. The results revealed that the urea-combustion synthesis method is more appropriate than coprecipitation for the preparation of active and stable Cu-Mn spinel oxide catalysts for LT-WGSR.
Gold catalysts on Y-doped ceria dispersed on high surface area γ-Al2O3 were synthesized and tested in preferential CO oxidation in hydrogen rich stream (PROX). The effect of ceria loading (10, 20 or ...30 wt%) was studied. The gold catalyst with the lowest ceria amount exhibited the highest PROX activity. The addition of Y2O3 (1 wt%) led to improved performance. The most favorable effect was observed in the sample with 20 wt% ceria amount. This gold catalyst showed good PROX activity and stability in the presence of CO2 and water. Catalysts characterization by XRD, HRTEM/HAADF, XPS and H2-TPR was used to elucidate the relationship between the chemical composition, state of gold, support features and catalytic properties.
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•PROX over alumina supported Au/Y-doped ceria catalysts.•Effect of ceria loading (10, 20 or 30 wt%) and Y-addition.•Promising PROX activity and stability in the presence of CO2 and water.•Relationship between gold and support features and catalytic properties.
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•Fe-doped ceria, prepared by two different methods was used as support.•Complete benzene oxidation over Au, Pd and Pd–Au catalysts was studied.•Strong effect of preparation method on ...activity of Au-based catalysts was observed.•Fe-doped ceria prepared by impregnation is a beneficial support for Pd–Au catalyst.
Mono-(Au, Pd) and bimetallic Au–Pd catalysts supported on Fe-modified ceria were synthesized and characterized by XRD, TEM, XPS, and TPR techniques. The effect of support preparation (impregnation or mechanochemical mixing) on the structural, electronic, reductive and catalytic properties of the catalysts for complete benzene oxidation was investigated. The best catalytic performance and the positive role of Pd deposition on already deposited gold was observed over bimetallic catalyst supported on prepared by impregnation Fe–Ce support. Complete conversion of benzene was achieved over the most active bimetallic catalyst at 200°C. During the 24h stability test at 200°C the activity remained constant.
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•CuO/ZnO/Al2O3 hydrotalcites excellent precursors for WGS catalysts.•Impact of gold on the WGS activity: outstanding conversion levels at low temperature.•Au-Cu synergy as key ...parameter for optimized performance.•Superior long term stability and good tolerance to transient start/stop cycles.
Clean hydrogen production via WGS is a key step in the development of hydrogen fuel processors. Herein, we have designed a new family of highly effective catalysts for low-temperature WGS reaction based on gold modified copper-zinc mixed oxides. Their performance was controlled by catalysts’ composition and the Au-Cu synergy. The utilization of hydrotalcite precursors leads to an optimal microstructure that ensures excellent Au and Cu dispersion and favors their strong interaction. From the application perspective these materials succeed to overcome the major drawback of the commercial WGS catalysts: resistance towards start/stop operations, a mandatory requisite for H2-powered mobile devices.
An FTIR and quadrupole mass spectroscopic study of the water–gas shift (WGS), the reverse WGS reactions, and the adsorption of the individual molecules involved has been carried out on Au/Fe2O3 and ...Au/TiO2 catalysts. The chemisorptions and the reactions on the two catalysts have been compared with the aim of gaining a better understanding of the role played by the two phases present in these catalysts and of the synergistic interplay between them in gold catalysts tested for a low-temperature water–gas shift reaction. Evidences are reported that H2 is dissociated already at room temperature on both the catalysts on gold sites, giving rise to hydrogen atoms that can react with adsorbed oxygen atoms or spillover on the supports where they can reduce the support surface sites. It is shown that CO is adsorbed molecularly on different surface sites, on the support cations, on Au0 sites exposed at the surface of small three-dimensional particles and also on Auδ− sites exposed at the surface of negatively charged clusters. The CO formed in the reverse WGS reaction appears chemisorbed only on the Au0 sites. The support sites and the Auδ− sites, where the CO appears as more strongly bonded, are present but not accessible to the CO formed by CO2 reduction, probably because these sites are covered by water. Water and OH groups are adsorbed on the supports, on gold sites, and at the interface between them. The effects of CO coadsorption on water dissociation and of H2 dissociation on CO2 reduction have been evidenced. The close similarity of the catalytic activity of the two examined samples indicates that the active sites for hydrogen dissociation and for water–CO reactive interactions are located at the surface of the metallic gold small particles where the reaction can take place by a red–ox regenerative mechanism.
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