Mechanistic Investigation of the Heterogeneous Hydrogenation of Nitrite over Pt/Al2O3 by Attenuated Total Reflection Infrared Spectroscopy

The mechanism of the heterogeneous hydrogenation of nitrite over a Pt/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. In addition to adsorbed nitrite, hydrogenation intermediates NO(ads), “HNO”(ads), and HNO2−(ads) are formed on the platinum surface. Hydrogenation of the surface intermediates mainly results in NH4+, but also traces of N2O are observed as well, which is believed to be an intermediate in the formation of nitrogen. “HNO”(ads) is the most prominent surface species during steady state operation and is therefore involved in the rate-determining step. Some NO(ads) accumulates at steps in transient experiments, showing a very low reactivity toward N2O. The results show that although the reaction pathways of nitrite hydrogenation on platinum and palladium are rather similar; the rate-determining steps on the metals are clearly different

CO Adsorption and Oxidation at the Catalyst-Water Interface: An Investigation by Attenuated Total Reflection Infrared Spectroscopy

Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases

In situ ATR-IR study of CO adsorption and oxidation over Pt/Al2O3 in gas and aqueous phase: Promotion effects by water and pH

The adsorption and oxidation of carbon monoxide over a Pt/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated both in gas phase and water using attenuated total reflection infrared spectroscopy. A preparation method is described that results in a strongly attached layer that is stable for many days in a water flow. Both adsorption and oxidation of CO are largely affected by the presence of liquid water. It influences the metal particle potential as well as the CO molecule directly, which is reflected in large red shifts (45 cm−1) and a fourfold higher intensity when the experiments are carried out in water. Furthermore, the rate of CO oxidation changes significantly when carried out in water compared with gas phase. Finally, with increasing pH, CO stretching frequencies shift to lower wavenumbers, accompanied by a large increase in CO oxidation rate.\ud \u

The influence of water and pH on adsorption and oxidation of CO on Pd/Al2O3—an investigation by attenuated total reflection infrared spectroscopy

Adsorption and oxidation of carbon monoxide over a Pd/Al2O3catalyst layer was investigated both in gas phase and water. Both adsorption and oxidation of CO are significantly affected by the presence of liquid water. Water influences the potential of the metal particles as well as the dipole moment of the adsorbed CO molecule directly, which is reflected both in large red shifts and a higher infrared intensity when experiments are carried out in water. Furthermore, the rate of COoxidation increases significantly by both the presence of water and by increasing the pH. Enhancement of the oxidation rate is attributed to a weakening of the CO bond by increasing potential of the metal particle, similar to COoxidation over Pt/Al2O3 as recently published [S. D. Ebbesen et al., J. Catal., 2007, 246, 66]. However, on Pd/Al2O3 the oxidation of palladium\ud is clearly promoted at increasing pH, further enhancing the oxidation of CO over Pd/Al2O3

In Situ Attenuated Total Reflection Infrared (ATR-IR) Study of the Adsorption of NO2-, NH2OH, and NH4+ on Pd/Al2O3 and Pt/Al2O3

In relation to the heterogeneous hydrogenation of nitrite, adsorption of NO2-, NH4+, and NH2OH from the aqueous phase was examined on Pt/Al2O3, Pd/Al2O3, and Al2O3. None of the investigated inorganic nitrogen compounds adsorb on alumina at conditions presented in this study. NO2-(aq) and NH4+(aq) on the other hand show similar adsorption characteristics on both Pd/Al2O3 and Pt/Al2O3. The vibrational spectrum of the NO2- ion changed substantially upon adsorption, clearly indicating that NO2- chemisorbs onto the supported metal catalysts. On the contrary, adsorption of NH4+ does not lead to significant change in the vibrational spectrum of the ion, indicating that the NH4+ ion does not chemisorb on the noble metal but is stabilized via an electrostatic interaction. When comparing the adsorption of hydroxylamine (NH2OH(aq)) on Pd/Al2O3 and Pt/Al2O3, significant differences were observed. On Pd/Al2O3, hydroxylamine is converted into a stable NH2(ads) fragment, whereas on Pt/Al2O3 hydroxylamine is converted into NO, possibly via HNO(ads) as an intermediate

In situ ATR-IR study of nitrite hydrogenation over Pd/Al2O3

The mechanism of nitrite hydrogenation over a Pd/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. Nitrite hydrogenates to NO(ads), NH2(ads), and NH+4 on the palladium surface. Hydrogenation of adsorbed NO on palladium results in the formation of a reaction product that is not infrared-active (most likely nitrogen), whereas no NH+4 is formed from NO(ads). NH+4 is formed solely from hydrogenation of the NH2(ads) intermediate. The present study clearly shows that formation of nitrogen and NH+4 proceeds via two separate pathways, based on which a revised reaction scheme is proposed.\ud \u

Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy

To improve the sustainability of transportation, a major goal is the replacement of conventional petroleum-based fuels with more sustainable fuels that can be used in the existing infrastructure (fuel distribution and vehicles). While fossil-derived synthetic fuels (e.g. coal derived liquid fuels) and biofuels have received the most attention, similar hydrocarbons can be produced without using fossil fuels or biomass. Using renewable and/or nuclear energy, carbon dioxide and water can be recycled into liquid hydrocarbon fuels in non-biological processes which remove oxygen from CO2 and H2O (the reverse of fuel combustion). Capture of CO2 from the atmosphere would enable a closed-loop carbon-neutral fuel cycle. This article critically reviews the many possible technological pathways for recycling CO2 into fuels using renewable or nuclear energy, considering three stages--CO2 capture, H2O and CO2 dissociation, and fuel synthesis. Dissociation methods include thermolysis, thermochemical cycles, electrolysis, and photoelectrolysis of CO2 and/or H2O. High temperature co-electrolysis of H2O and CO2 makes very efficient use of electricity and heat (near-100% electricity-to-syngas efficiency), provides high reaction rates, and directly produces syngas (CO/H2 mixture) for use in conventional catalytic fuel synthesis reactors. Capturing CO2 from the atmosphere using a solid sorbent, electrolyzing H2O and CO2 in solid oxide electrolysis cells to yield syngas, and converting the syngas to gasoline or diesel by Fischer-Tropsch synthesis is identified as one of the most promising, feasible routes. An analysis of the energy balance and economics of this CO2 recycling process is presented. We estimate that the full system can feasibly operate at 70% electricity-to-liquid fuel efficiency (higher heating value basis) and the price of electricity needed to produce synthetic gasoline at U.S.D$2/gal ($ 0.53/L) is 2-3 U.S. cents/kWh. For $3/gal ($ 0.78/L) gasoline, electricity at 4-5 cents/kWh is needed. In some regions that have inexpensive renewable electricity, such as Iceland, fuel production may already be economical. The dominant costs of the process are the electricity cost and the capital cost of the electrolyzer, and this capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind). The potential of this CO2 recycling process is assessed, in terms of what technological progress is needed to achieve large-scale, economically competitive production of sustainable fuels by this method.Sustainable fuel Hydrocarbon fuel Carbon dioxide recycling Electrolysis Energy balance Economics

Effect of pH on the Nitrite Hydrogenation Mechanism over Pd/Al2O3 and Pt/Al2O3: Details Obtained with ATR-IR Spectroscopy

It is well-known that activity and selectivity to N2 during nitrite hydrogenation over noble metal catalysts in water depend on the pH of the solution, but mechanistic understanding is lacking. Attenuated total reflection infrared (ATR-IR) spectroscopy is an ideal tool to perform detailed studies on catalytic surfaces in water. In this paper, the influence of pH was studied on adsorption and subsequent hydrogenation of nitrite in water between pH 5 and 9 over Pd/Al2O3 and Pt/Al2O3, using ATR-IR spectroscopy. On both catalysts, pH clearly influenced the surface coverage and reaction rates of intermediates. For Pt/Al2O3, lowering the pH induced the increasing surface coverage of key reaction intermediates like NOsteps1620 cm−1 and “HNO”(ads)1540 cm−1, as well as increased hydrogenation rates, explaining the higher TOF at lower pH as reported in the literature. For Pd/Al2O3, the effect of pH on selectivity is controlled by the rate constants of the formation and hydrogenation of the most stable reaction intermediates to N2 (NO(ads)1720 cm−1) and NH4+ (NH2(ads)1510 cm−1)