An in situ IR study unraveled that adsorption of CO2 on 20 μm TEPA film at 50 °C followed a zwitterion-intermediate pathway: zwitterion → ammonium carbamate. H2O in the mixed TEPA/H2O (5:1) film decreased the rate of CO2 adsorption but increased the amine efficiency. CO2 preferentially adsorbs on primary amine sites and H2O on secondary amine sites. The presence of H2O promotes the formation of carbamic acid and produces a broad IR band centered at 2535 cm–1, which can be assigned to the ν(O–H) of hydronium carbamate, −NCOO–···H–OH2+. The broadness of this 2535 cm–1 band ranging from 2100 to 2800 cm–1 persists at 120 °C. These broad components of the band can be ascribed to ν(N–H) in hydrogen-bonded ammonium carbamate, a R–NH3+/R1R2–NH2+···–NCOO– moiety. The binding strength of adsorbed species on the TEPA film increases in the order adsorbed H2O < carbamic acid < ammonium carbamate < hydrogen-bonded water/ammonium carbamate.

CO2 diffusion limitations and readsorption of desorbed CO2 during removal from immobilized amine sorbents could significantly reduce the effectiveness of CO2 capture processes. To decouple CO2 diffusion from desorption/readsorption on silica and tetraethylenepentamine (TEPA)/silica sorbents, a new transient diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method was carried out by using benzene as a surrogate probe molecule. Comparison of the infrared intensity profiles of adsorbed CO2 and Si–OH (which adsorbs benzene) revealed that slow rates of CO2 uptake and desorption are a result of (i) CO2 diffusion through an interconnected network produced from CO2 adsorbed inside of the amine/silica sorbent pores and (ii) readsorption of CO2 on the amine sites inside of the pores and at the external surface of the sorbents. High rates of CO2 adsorption/desorption onto/from the immobilized amine sorbents could be achieved by sorbents with low amine density at the external surfaces and pore mouths.

Asymmetric hydrogenation reaction of dehydro-α-amino acid (i.e., α-amino ester) over cinchonidine (CD) modified Pd catalyst has been studied by an array of in situ infrared spectroscopic methods, including transmission, diffuse reflectance (DR), and attenuated total reflectance (ATR). Transmission FTIR spectra probed the hydrogenation reaction process, revealed OH–O and NH–N hydrogen bonding interactions between the adsorbed CD and during the reaction. DR and ATR spectra of the hydrogenation reaction under different conditions, which are consistent with but slightly different from the transmission spectra, evidenced the successful hydrogenation of the compound. The incorporation of DR and microfluidics flow-through design allowed us to investigate the adsorption of CD on the Pd surface efficiently. The results revealed that the N-bonded CD on Pd surface in a tilted configuration had increased abundance on the Pd surface with high coverage. These valuable insights provided an image of the reaction pathway to the prochiral structure (precursor state).

The nature and structure of adsorbed CO2 on immobilized amine sorbent in the presence and absence of H2O vapor have been studied by in situ infrared spectroscopy. CO2 adsorbed on the primary amine as ammonium carbamate and on the secondary amine as carbamic acid. Adsorbed H2O mainly on secondary amine enhanced CO2 capture capacity by increasing accessibility of amine sites and promoting the formation of carbamic acid. The binding strength of the adsorbed species increased in the order: carbamic acid < adsorbed H2O < paired carbamic acid; ammonium carbamate < ammonium chloride. Flowing argon over the amine sorbent at 50 °C removed weakly adsorbed H2O and carbamic acid from the secondary amine sites. Raising temperature is required to completely regenerate sorbent by removing strongly adsorbed ammonium carbamate from the primary amine sites and paired carbamic acid. The results of this study clarify the role of H2O vapor in amine-sorbents for CO2 capture and provide a molecular basis for the design of the sorbents and operation of amine-based CO2 capture processes.

CO2 adsorption/desorption onto/from tetraethylenepentamine (TEPA) films of 4, 10, and 20 μm thicknesses were studied by in situ attenuated total reflectance (ATR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques under transient conditions. Molar absorption coefficients for adsorbed CO2 were used to determine the CO2 capture capacities and amine efficiencies (CO2/N) of the films in the DRIFTS system. Adsorption of CO2 onto surface and bulk NH2 groups of the 4 μm film produced weakly adsorbed CO2, which can be desorbed at 50 °C by reducing the CO2 partial pressure. These weakly adsorbed CO2 exhibit low ammonium ion intensities and could be in the form of ammonium-carbamate ion pairs and zwitterions. Increasing the film thickness enhanced the surface amine–amine interactions, resulting in strongly adsorbed ion pairs and zwitterions associated with NH and NH2 groups of neighboring amines. These adsorbed species may form an interconnected surface network, which slowed CO2 gas diffusion into and diminished access of the bulk amine groups (or amine efficiency) of the 20 μm film by a minimum of 65%. Desorption of strongly adsorbed CO2 comprising the surface network could occur via dissociation of NH3+/NH2+···NH2/NH ionic hydrogen bonds beginning from 60 to 80 °C, followed by decomposition of NHCOO–/NCOO– at 100 °C. These results suggest that faster CO2 diffusion and adsorption/desorption kinetics could be achieved by thinner layers of liquid or immobilized amines.Keywords: amine; carbon dioxide; diffusion; film; in situ infrared spectroscopy

In situ Fourier transform infrared spectroscopy was used to determine the nature of adsorbed CO2 on class I (amine-impregnated) and class II (amine-grafted) sorbents with different amine densities. Adsorbed CO2 on amine sorbents exists in the form of carbamate–ammonium ion pairs, carbamate–ammonium zwitterions, and carbamic acid. The adsorbed CO2 on high-amine density sorbents showed that the formation of ammonium ions correlates with the suppression of CH stretching intensities. An HCl probing technique was used to resolve the characteristic infrared bands of ammonium ions, clarifying that the band observed around 1498 cm–1 is a combination of the deformation vibration of ammonium ion (NH3+) at 1508 and 1469 cm–1 and the deformation vibration of NH in carbamate (NHCOO–) at 1480 cm–1. Carbamate and carbamic acid on sorbents with low amine density desorbed at a rate faster than those on sorbents with high amine density after switching the flow from CO2 to Ar at 55 °C. Evaluation of the desorption temperature profiles showed that the temperature required to achieve the maximal desorption of CO2 (Tmax. des) increases with amine density. The adsorbed CO2 on sorbents with high amine density is stabilized via hydrogen bonding interactions with adjacent amine sites. These sorbents require higher temperature to desorb CO2 than those with low amine density.

The contribution of methanol (CH3OH) sacrificing reagent to the photocatalytic evolution of H2 from aqueous solutions has been studied by tracing the reaction of D2O over a Cu/S-TiO2 catalyst under UV illumination. Use of D2O/CH3OH produced higher formation rates of HD and D2 than that of H2. The low H2 formation rates indicate that the direct reaction of CH3OH with photogenerated holes does not proceed to an appreciable extent in the presence of high concentrations of D2O. The role of CH3OH in accelerating hydrogen formation can be attributed to its ability to produce an electron donor, injecting its electrons to the conduction band.

The evolution of gases from direct utilization of carbon in a solid oxide fuel cell (C-SOFC) was studied by potentiostatic/galvanostatic discharge of a fuel cell with coconut carbon, a carbonaceous material with low ash and sulfur content. Operation of C-SOFC at 750 °C produced less CO and more CO2 than those predicted by thermodynamic calculation using total Gibbs free energy minimization method. The addition of CO2 to the anode chamber increased CO formation and maximum power density from 0.09 W cm−2 to 0.13 W cm−2, indicating the occurrence of Boudouard reaction (CO2 + C ⇔ 2CO) coupling with CO electrochemical oxidation on the C-SOFC. Analysis of CO and CO2 concentration as a function of current and voltage revealed that electricity was mainly produced from the electrochemical oxidation of carbon at low current density and produced from the electrochemical oxidation of CO at high current density. The results suggest the electrochemical oxidation of solid carbon is more mass transfer limited than electrochemical oxidation of CO.Highlights► Direct utilization of solid carbon in a solid oxide fuel cell was demonstrated. ► Boudouard reaction can be observed from the operation of carbon fuel cell. ► Addition of CO2 and CO results in an increase in the carbon fuel cell performance. ► Electricity is mainly produced from carbon at low current density. ► Electricity is mainly produced from CO at high current density.

The effects of carrier gas flow rates and Boudouard reaction on the performance of Ni/YSZ anode-supported solid oxide fuel cells (SOFCs) have been studied with coconut coke fuels at 800 °C. Decreasing flow rates of carrier gas from 1000 to 50 ml min−1 increased open circuit voltages and current densities from 0.71 to 0.87 V and from 0.12 to 0.34 A cm−2, respectively. The increased cell performance was attributed to the increasing extent of electrochemical oxidation of CO, a product of Boudouard reaction. The contribution of CO oxidation to current generation was estimated to 66% in flowing inert carrier gas at 50 ml min−1. The pulse transient studies confirmed the effect of flow rates on cell performance and also revealed that CO and CO2 can displace adsorbed hydrogen on carbon fuels. Flowing CO2 over coconut coke fuel produced CO via Boudouard reaction. The presence of CO led to a highest power density of 95 mW cm−2, followed by a concurrent decline of power density and CO concentration. The declined power density along with decreasing CO concentration further verified contribution of gaseous CO to the power generation of C-SOFC; the decreasing CO concentration showed a typical kinetics behavior of Boudouard reaction, suggesting the loss of active sites on carbon surface for the reaction.Graphical abstractResearch highlights► Lower carrier gas flow rates enhanced the cell performance of carbon-based solid oxide fuel cells (C-SOFCs) by increasing the extent of electrochemical oxidation of CO due to the long residence time of CO. ► The power generation from C-SOFCs can be further increased by increasing gasification rate of solid carbon via Boudouard reaction. ► CO and CO2 can displace adsorbed hydrogen on the solid carbon fuels of C-SOFCs.

Hydrogen adsorption has been studied by static and dynamic methods on activated carbon (AC), platinum/activated carbon (Pt/AC), metal organic frameworks (MOF-5), and Pt/AC_MOF-5.The static method showed that all of adsorbents used in this study exhibited a Langmuir (type I) adsorption isotherm at 77 K and a linear function of hydrogen partial pressure at 298 K. The dynamic method produced breakthrough curves, indicating (i) slow rate of hydrogen diffusion in the densely packed activated carbon and Pt/AC beds and (ii) high rate of hydrogen diffusion in the loosely packed bed with large MOF-5 crystallites. Temperature variable adsorption resulted in the higher hydrogen uptake on Pt/AC than other adsorbents. The results suggested that temperature variable adsorption enhanced the hydrogen storage process by (i) initiating hydrogen dissociation at high temperature and (ii) facilitating spillover at low temperature on Pt/AC.Highlights► The dynamic method produced breakthrough curves of which slopes reflected H2 diffusion rate. ► The temperature variable adsorption enhanced the H2 dissociative at high temperature and facilitated the subsequent spillover during cooling.

The rate-determining step of ethanol photocatalytic oxidation was identified to be the adsorption of O2 by an infrared (IR) spectroscopy coupled with mass spectrometry method. Dosing O2 during reaction showed that adsorption of O2 controls the accumulation of photogenerated electrons and the formation of acetate (CH3COO−ad), acyl species (CH3COad), acetaldehyde (CH3CHOad), CO2, and H2O. Accumulation of CH3COO−ad on the TiO2 surface slowed down the conversion of ethanol to CO2 and H2O. Removal of CH3COO−ad from the TiO2 surface holds the key to accelerating the rate of ethanol photocatalytic oxidation. This study bridges the gap between results of nanosecond and millisecond transient absorption studies and those of minute scale photocatalytic oxidation studies.

The direct electrochemical oxidation of coal gas was studied by pyrolyzing a sample of Ohio #5 coal in flowing Ar at 700, 800, 900, and 950 °C and transporting the resulting gaseous products to a Cu (Copper) anode solid oxide fuel cell (SOFC) operated at 950 °C. Pyrolysis of coal at 700 °C produced a H2-rich coal gas containing 89% H2, 4% CO, 6% CH4, 1% CO2, and sulfur compounds (i.e., 1% COS and 1% SO2), which yielded a maximum current density of 320 mA/cm2 at 0.5 V. Raising the pyrolysis temperature from 700 to 950 °C increased the CO concentration in the coal gas (i.e., 42 vol % CO), which in turn reduced the fuel cell maximum current density. No sulfur compounds were present in the coal gas produced at temperatures higher than 700 °C. Coal gas fuel operation did not degrade the fuel cell performance. A fuel composition of 25 vol % CH4 in He generated a current density of 340 mA/cm2 at 0.5 V. These results demonstrated that the Cu anode is effective for the electrochemical oxidation of sulfur-containing coal gas at 950 °C. The addition of CO2 and D2O to the pyrolysis reactor led to the formation of CO and HD, indicating the occurrence of reforming reactions. Diffuse reflectance infrared Fourier transformation (DRIFT) spectra showed that coal pyrolysis proceeded by dehydrogenation of hydroaromatics, dealkylation of aromatics, and oxidation reactions, leading to the formation of coke with a surface containing C−O, C−S, and S−O bonds. The results of the fuel cell performance strongly support the feasibility of direct power generation from coal gas in a Cu-anode SOFC.

Catalytic decomposition of nitric oxide (NO) over Pd/Al2O3 and Ag−Pd/Al2O3 has been studied using the pulse transient response technique coupled with in situ infrared (IR) and mass spectrometry (MS) at 723−823 K. In the absence of H2, pulsing NO over the Pd/Al2O3 catalyst produces adsorbed NO species (i.e., Pd+−NO, Pd0−NO, and Pd−NO−) as well as gaseous N2, O2, and N2O products. Transient responses of the N2 and O2 profiles show that the addition of Ag onto Pd/Al2O3 catalyst shifts the O2 profile forward, increases oxygen formation and the oxidation resistance of Pd, but did not decrease the amount of retained oxygen (Oret) and did not improve the catalytic cycle for NO decomposition. Oret on the Pd surface is not able to desorb in the temperature range of this study; however, Oret on Ag−Pd/Al2O3 can be desorbed at higher temperatures than its formation and adsorption temperature. The presence of H2 during the NO pulse allowed NO reduction to occur, producing N2, N2O, O2, NH3, and H2O. Pd/Al2O3 is a more active catalyst for the formation of NH3 and H2O than Ag−Pd/Al2O3. Comparison of the transient gaseous product responses over Pd/Al2O3 and Ag−Pd/Al2O3 catalysts show that Ag (i) promotes the formation of N2, shifting its profile forward, and (ii) suppresses the formation of NH3 and H2O, delaying their formation. The lack of the initial activity of Ag−Pd/Al2O3 for NH3/H2O formation can be attributed to the alloy state of Ag−Pd on Al2O3. As the NO reduction proceeds in the presence of H2, adsorbed oxygen produced from N−O dissociation could cause the dealloying of Ag−Pd, producing Pd sites, which exhibited high selectivity for NH3/H2O formation.

This paper provides an overview of the use of various transient infrared methods to determine the role of infrared observable species in the mechanisms of the NO–CO reaction, heterogeneous ethylene hydroformylation, and photocatalytic oxidation of ethanol. The transient infrared methods with a judicious choice of ways in changing the concentration of reactants and their isotope counterparts produce responses, allowing (i) identification of the spectators, (ii) determination of active adsorbed species, and (iii) verification of kinetic models and their parameters. The method has also been recently extended to monitor infrared absorbance of photogenerated electrons during photocatalysis, correlating variation in the concentration of photogenerated electrons and adsorbed species. The specific discussion focuses on limitations of the approaches and the type of mechanistic information that can be obtained.

Oxidative carbonylation of amines with alcohols provides an environmentally benign pathway to isocyanates and carbamates. Currently, the most active catalysts for the oxidative carbonylation of aniline with methanol are Pd-based catalysts. To further improve the economic feasibility of the carbamate synthesis process, we have investigated the activity of CuCl2–NaI, CuCl2–NaCl, and CuCl–NaI at 438 K and 0.41 MPa by in situ infrared spectroscopy. The activity of the catalysts for carbamate synthesis increased in the order: CuCl2–NaCl < CuCl–NaI < CuCl2–NaI. The presence of promoter (i.e., NaI or NaCl) in the reactant–catalyst mixture is essential to promote carbamate synthesis. The formation of by-product, CO2, can be suppressed by the sequential addition of NaI to CO/O2/methanol/aniline/CuCl2. Transient profiles of reactants/products obtained from in situ infrared spectroscopic studies revealed that CO2 and carbamate were formed via two independent pathways. The infrared observation of a Cu0(CO)2 species and O2 participation in carbamate synthesis suggest that the carbamate synthesis reaction involves a redox cycle of Cu0/CuII.

The thermal stability and reactivity of adsorbed pyridine on sulfated zirconia and Pt/sulfated zirconia were studied by temperature-programmed desorption (TPD) coupled with infrared spectroscopic and mass spectrometric analyses. Adsorption of pyridine on sulfated zirconia produced two types of adsorbed species: a pyridinium ion on the Brønsted acid site (Pyr-B) giving rise to infrared bands at 1638, 1611, 1486, and 1540 cm−1 and a covalently bound species on Lewis acid sites (Pyr-L), giving characteristic bands at 1486 and 1445 cm−1 at the same rate, accompanied by the desorption of sulfates to produce gaseous SO3. TPD studies showed that Pyr-L nearly completely decomposed to CO2 at 773–823 K, whereas more than 60% Pyr-B remained on the surface in the same temperature range, indicating that Pyr-B possesses higher thermal stability than Pyr-L. The addition of Pt to sulfated zirconia lowers the decomposition temperature of sulfate as SO2. The study shows that that temperature-programmed desorption/decomposition with simultaneous infrared spectroscopic and mass spectrometric analyses is an effective approach for identifying the structure of the adsorbed species leading to decomposed products. TPD of adsorbed pyridine is not a reliable approach for measuring acid strength of the acid/base catalysts.

The reaction pathways for the photocatalytic oxidation of ethanol on the TiO2 surface at 30 °C were studied by in situ infrared (IR) spectroscopy. The coverage of ethanol and water was found to play a key role in how the reaction is initiated. The low ethanol coverage on the H2Oad-containing TiO2 surface produced adsorbed formate (HCOO−ad) as a primary intermediate; the high ethanol coverage on the H2Oad-deficient TiO2 surface produced adsorbed acetate (CH3COO−ad) as a major intermediate during the initial period (i.e., 2 min) of the photocatalytic oxidation. The adsorbed species and reaction products observed during in situ IR studies suggest the low-coverage ethanol reaction is initiated by •OH, whereas the high-coverage ethanol reaction is initiated by hole. The hole-initiating ethanol oxidation on the H2Oad-deficient TiO2 surface produced adsorbed acetic acid (CH3COOHad)/CH3COO−ad, and built up photogenerated electrons, giving a parallel increase in the IR intensity of CH3COO−ad and the IR background at 2000 cm−1 (i.e., a measure of photogenerated electrons). As the high-coverage ethanol reaction proceeded toward producing CO2/H2O, adsorbed H2O accumulated and the coverage of CH3CH2OHad/CH3CH2Oad decreased on the TiO2 surface, shifting the reaction from hole-initiating to •OH-initiating.