Silicon microfabricated reactor for operando XAS/DRIFTS studies of heterogeneous catalytic reactions

Operando X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) provide complementary information on the catalyst structure, surface reaction mechanisms and activity relationships. The powerful combination of the techniques has been the driving force to design and engineer suitable spectroscopic operando reactors that can mitigate limitations inherent to conventional reaction cells and facilitate experiments under kinetic regimes. Microreactors have recently emerged as effective spectroscopic operando cells due to their plug-flow type operation with no dead volume and negligible mass and heat transfer resistances. Here we present a novel microfabricated reactor that can be used for both operando XAS and DRIFTS studies. The reactor has a glass–silicon–glass sandwich-like structure with a reaction channel (3000 μm × 600 μm; width × depth) packed with a catalyst bed (ca. 25 mg) and placed sideways to the X-ray beam, while the infrared beam illuminates the catalyst bed from the top. The outlet of the reactor is connected to MS for continuous monitoring of the reactor effluent. The feasibility of the microreactor is demonstrated by conducting two reactions: i) combustion of methane over 2 wt% Pd/Al2O3 studied by operando XAS at the Pd K-edge and ii) CO oxidation over 1 wt% Pt/Al2O3 catalyst studied by operando DRIFTS. The former shows that palladium is in an oxidised state at all studied temperatures, 250, 300, 350, 400 °C and the latter shows the presence of linearly adsorbed CO on the platinum surface. Furthermore, temperature-resolved reduction of palladium catalyst with methane and CO oxidation over platinum catalyst are also studied. Based on these results, the catalyst structure and surface reaction dynamics are discussed, which demonstrate not only the applicability and versatility of the microreactor for combined operando XAS and DRIFTS studies, but also illustrate the unique advantages of the microreactor for high space velocity and transient response experiments

Assessing the effects of partially decarbonising a diesel engine by co-fuelling with dissociated ammonia

The proven feasibility of ammonia combustion in compression-ignition engines has led to it being considered as a carbon-free replacement for diesel fuel. Due to its high auto-ignition temperature, however, a more realistic strategy would be to aim for a step-change reduction in carbon emissions by co-fuelling a diesel engine with ammonia. In assessing this strategy, ammonia gas was introduced into the air-intake manifold of a compression-ignition engine, while diesel fuel was injected directly into the cylinder to ignite the mixture. By substituting only 3% of the air intake by ammonia, the diesel consumption and the CO2 emissions decreased by 15%. The combustion and emission characteristics were then compared when the same percentage of air intake (by mass) was substituted by either dissociated ammonia (a mixture of H2, N2 with small percentages of NH3) or pure hydrogen, to mimic the other possible forms in which the co-fuel can be delivered to the engine. The addition of pure hydrogen resulted in the best engine performance, both in terms of combustion efficiency and regulated emission quality. The thermal combustion efficiency declined by only ∼0.5% when the H2 was replaced by undissociated ammonia at low load, but N2O now appeared in the emissions. Co-fuelling the engine with dissociated ammonia may provide the ideal compromise in terms of thermal combustion efficiency and emission quality, while also providing a waste-heat recovery mechanism

Rationalization of interactions in precious metal/ceria catalysts using the d-band center model

A correlation between ceria reducibility and precious metal d-band center is reported for ceria-supported precious metal catalysts. The results could provide the missing link to fully explain the occurrence of strong metal–support interaction (SMSI) and hydrogen spillover in catalysts that consist of dispersed metals in contact with reducible metal oxides

Biogas upgrade to syngas through thermochemical recovery using exhaust gas reforming

Exhaust gas fuel reforming has been examined as a process for upgrading biogas into hydrogen enriched gaseous fuel for use in transportation. The effect of different O2/CH4 mole ratios and space velocities on the reformer product has been studied. In addition, 20% volume fraction of exhaust gas recirculation (EGR) has been introduced into the engine system to investigate the effect of engine-out exhaust composition on the reforming process. Our results imply that fuel reforming process efficiencies of up to 95% (based on the energy content of the reformate produced compared to the energy content of the biogas used) can be achieved at optimum biogas and engine exhaust gas ratios for the different engine conditions. Furthermore, the total carbon dioxide content is reduced by reforming biogas to form better quality fuel

Silicon microfabricated reactor for operando XAS/DRIFTS studies of heterogeneous catalytic reactions

Operando X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) provide complementary information on the catalyst structure, surface reaction mechanisms and activity relationships. The powerful combination of the techniques has been the driving force to design and engineer suitable spectroscopic operando reactors that can mitigate limitations inherent to conventional reaction cells and facilitate experiments under kinetic regimes. Microreactors have recently emerged as effective spectroscopic operando cells due to their plug-flow type operation with no dead volume and negligible mass and heat transfer resistances. Here we present a novel microfabricated reactor that can be used for both operando XAS and DRIFTS studies. The reactor has a glass-silicon-glass sandwich-like structure with a reaction channel (3000 μm × 600 μm; width × depth) packed with a catalyst bed (ca. 25 mg) and placed sideways to the X-ray beam, while the infrared beam illuminates the catalyst bed from the top. The outlet of the reactor is connected to MS for continuous monitoring of the reactor effluent. The feasibility of the microreactor is demonstrated by conducting two reactions: i) combustion of methane over 2 wt% Pd/Al2O3 studied by operando XAS at the Pd K-edge and ii) CO oxidation over 1 wt% Pt/Al2O3 catalyst studied by operando DRIFTS. The former shows that palladium is in an oxidised state at all studied temperatures, 250, 300, 350, 400 °C and the latter shows the presence of linearly adsorbed CO on the platinum surface. Furthermore, temperature-resolved reduction of palladium catalyst with methane and CO oxidation over platinum catalyst are also studied. Based on these results, the catalyst structure and surface reaction dynamics are discussed, which demonstrate not only the applicability and versatility of the microreactor for combined operando XAS and DRIFTS studies, but also illustrate the unique advantages of the microreactor for high space velocity and transient response experiments.</p

Promotion of Ceria Catalysts by Precious Metals: Changes in Nature of the Interaction under Reducing and Oxidizing Conditions

By depositing ceria over supported precious metal (PM) catalysts and characterizing them with in situ diffuse reflectance UV (DR UV) and in situ Raman spectroscopy, we have been able to prove a direct correlation between a decrease in ceria band gap and the work function of the metal under reducing conditions. The PM–ceria interaction results in changes on the ceria side of the metal–ceria interface, such that the degree of oxygen vacancy formation on the ceria surface also correlates with the precious metal work function. Nevertheless, conclusive evidence for a purely electronic interaction could not be provided by X-ray photoelectron spectroscopy (XPS) analysis. On the contrary, the results highlight the complexity of the PM–ceria interaction by supporting a spillover mechanism resulting from the electronic interaction under reducing conditions. Under oxidizing conditions, another effect has been observed, namely, a structural modification of ceria induced by the presence of PM cations. In particular, we have been able to demonstrate by in situ Raman spectroscopy that, depending on the PM ionic radius, it is possible to create PM–ceria solid solutions. We observed that this structural modification prevails under an oxidizing atmosphere, whereas electronic and chemical interactions take place under reducing conditions