Temperature-programmed Reaction Studies Research Articles

Direct Conversion of Ammonia to Electricity on a PCFC and an SOFC

A protonic ceramic fuel cell (PCFC) and a solid oxide fuel cell (SOFC) using NiO/BZCY and NiO/YSZ anodes, respectively, were compared in H2 and NH3 fuels at 600 °C. The effect on the cell performance by adding lanthanum strontium ruthenium titanate (LSRT) into the anode were investigated. The performance of PCFCs in NH3 was found to resemble that in H2 and remain stable; by contrast, the SOFC was subject to a rigorous fluctuation of voltages in NH3, followed by performance loss. Surface characterization evidenced that phase separation between nickel and electrolyte YSZ is a major reason to the deactivation of SOFCs. The structure degradation occurred due to repetitive nitridation of nickel and subsequent oxidation reaction. Despite containing nickel in the anode too, the PCFC experienced the little effect since the oxidation reaction occurs at the cathode. Furthermore, the addition of the LSRT helped stabilize the PCFC anode by facilitating decomposition of ammonia with exsolved ruthenium sites, which are characterized by temperature programmed reaction studies.

Sulfur-treated TiO2 shows improved alcohol dehydration activity and selectivity.

The dehydration of alcohols is an important class of reactions for the development of fossil-free fuel and chemical industries. Acid catalysts are well known to enhance the reactivity of alcohols following two main pathways of either dehydration to olefins or dehydrogenation to ketones/aldehydes. TiO2 surfaces have been well documented for primary and secondary alcohol dehydration with selectivity ranging from 1-100% towards dehydration products based on process conditions and catalyst structure. In this work we document the effects of various sulfur treatments of TiO2 surfaces which induce higher activity and, more importantly, higher selectivity for alcohol dehydration than untreated surfaces. The increase in activity and >99% dehydration selectivity is coupled with demonstrated stability for several hours on stream at high conversion. Using temperature programmed reaction studies, XPS and FT-IR spectroscopy, we identify Lewis acidic sites correlated with sulfate species on TiO2 surfaces as active sites for the reaction.

Below-Room-Temperature C-H Bond Breaking on an Inexpensive Metal Oxide: Methanol to Formaldehyde on CeO2(111).

Upgrading of primary alcohols by C-H bond breaking currently requires temperatures of >200 °C. In this work, new understanding from simulation of a temperature-programmed reaction study with methanol over a CeO2(111) surface shows C-H bond breaking and the subsequent desorption of formaldehyde, even below room temperature. This is of particular interest because CeO2 is a naturally abundant and inexpensive metal oxide. We combine density functional theory and kinetic Monte Carlo methods to show that the low-temperature C-H bond breaking occurs via disproportionation of adjacent methoxy species. We further show from calculations that the same transition state with comparable activation energy exists for other primary alcohols; with ethanol, 1-propanol, and 1-butanol explicitly calculated. These findings indicate a promising class of transition states to search for in seeking low-temperature C-H bond breaking over inexpensive oxides.

Investigation of reactions in a fluidized bed reactor during chemical looping combustion of coal/steam with copper oxide-iron oxide-alumina oxygen carrier

This paper presents data on conversion of two different coals with a chemical looping oxygen carrier, CuO-Fe2O3-alumina, and over a range of conditions including steam and various levels of reduction of the oxygen carrier.Reactions of coal/steam/CuO-Fe2O3-alumina oxygen carrier and coal/steam/partially reduced CuO-Fe2O3-alumina oxygen carrier were investigated with Wyodak coal and Illinois #6 coal in a fluidized bed reactor. Temperature programmed reaction studies indicated that the oxygen carrier enhanced the steam gasification/combustion rates of both coals. Rates of gasification/combustion were higher with Wyodak coal (sub bituminous) than that with Illinois #6 coal (bituminous). In addition to the increase in reaction rates, the total moles of carbon that were gasified and combusted from coal/steam increased in the presence of the oxygen carrier. The reduced oxygen carrier promoted the water-gas shift reaction when reacted with synthesis gas in the presence of steam, but the reverse water gas shift reaction was observed when steam was not present. The partially reduced oxygen carrier enhanced the production of H2 from coal/steam, which was different from the observations with un-reduced oxygen carrier. Water splitting reaction to produce H2 was also observed with the reduced oxygen carrier. CuO-Fe2O3-alumina reacted with coal during the temperature ramp to 850°C even in the absence of steam due to the chemical-looping oxygen uncoupling (CLOU) reaction. Fourier transform infra-red (FTIR) analysis indicated the presence of volatile aromatics during the temperature ramp and these may have also contributed to the reactions with the oxygen carrier in the absence of steam. Increasing steam concentration had a negative effect on the CLOU reaction.

Exploiting basic principles to control the selectivity of the vapor phase catalytic oxidative cross-coupling of primary alcohols over nanoporous gold catalysts

Achieving high selectivity for high volume chemical synthesis is important for lowering energy consumption through reduction in waste. We report the selective synthesis of methyl esters—methyl acetate and methyl butyrate—through catalytic O2-assisted cross-coupling of methanol with ethanol or 1-butanol using activated, support-free nanoporous gold (npAu). Both well-controlled studies on ingots in UHV and experiments under ambient pressure catalytic conditions on both ingots and microspherical hollow shell catalysts reveal guiding principles for controlling selectivity. Under UHV conditions, the ester products of the cross-coupling of methanol with both ethanol and 1-butanol evolve near room temperature in temperature-programmed reaction studies, indicating that the reactions occur facilely. Under steady-state catalytic operation, high stable activity was observed for cross-coupling in flowing gaseous reactant mixtures at atmospheric pressure and 423K with negligible combustion. Optimum selectivity for cross-coupling is obtained in methanol-rich mixtures due to a combination of two factors: (1) the relative coverage of the respective alkoxys and (2) the relative facility of their β-H elimination. The relative coverage of the alkoxys is governed by van der Waal’s interactions between the alkyl groups and the surface; here, we demonstrate the importance of these weak interactions in a steady-state catalytic process.

The effects of Pt addition to supported Ni catalysts on dry (CO2) reforming of methane to syngas

Ni/CeZrO2 and Ni/Al2O3 catalysts promoted by Pt were explored for dry reforming of methane at temperatures ranging from 100°C to 800°C. Temperature programmed reduction (TPR) and temperature programmed reaction studies showed that Pt increases the reducibility of Ni-based monometallic catalysts. TPR studies suggested an alloy formation between Pt and Ni. The induction period with the Pt is much lower than on the Ni alone especially on the Al2O3. At lower temperatures, the Pt–Ni bimetallic catalyst showed slightly higher activity for hydrogen production during dry reforming of methane. Reaction investigations over alumina supported catalysts demonstrated a higher H2:CO ratio than ceria supported catalysts. A mechanism involving more steam reforming due to more water consumption is proposed for the reaction on alumina supported catalysts.

Mechanism of Methane Chemical Looping Combustion with Hematite Promoted with CeO2

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion that produces sequestration-ready CO2 stream, reducing the energy penalty of CO2 separation from flue gases. An effective oxygen carrier for CLC will readily react with the fuel gas and will be reoxidized upon contact with oxygen. This study investigated the development of a CeO2-promoted Fe2O3–hematite oxygen carrier suitable for the methane CLC process. Composition of CeO2 is between 5 and 25 wt % and is lower than what is generally used for supports in Fe2O3 carrier preparations. The incorporation of CeO2 to the natural ore hematite strongly modifies the reduction behavior in comparison to that of CeO2 and hematite alone. Temperature-programmed reaction studies revealed that the addition of even 5 wt % CeO2 enhances the reaction capacity of the Fe2O3 oxygen carrier by promoting the decomposition and partial oxidation of methane. Fixed-bed reactor data showed that the 5 wt % cerium oxides with 95 wt % iron oxide produ...

Influence of H2S on ZrO2-based gasification gas clean-up catalysts: MeOH temperature-programmed reaction study

Addition of H2S in the gasification gas stream has been found to improve naphthalene and ammonia conversion over ZrO2 and Y2O3-ZrO2 catalysts, whereas over SiO2-ZrO2 such effect has not been observed. The differences in the properties of the catalysts were studied by spectroscopic (MeOH-DRIFTS) and temperature-programmed methods (H2S-TPD and MeOH-TPSR). Methanol was also used to probe the changes after sulfidation of the catalysts. H2S adsorption sites were found to be different on the ZrO2 and Y2O3-ZrO2 catalysts compared with the ones on the SiO2-ZrO2 catalyst. Sulfur was also found to be more reactive on the ZrO2 and Y2O3-ZrO2 catalysts than on SiO2-ZrO2. It was suggested, that the positive effect of H2S on naphthalene conversion during gasification gas clean-up is connected with the reactions of adsorbed sulfur on their surface. In contrast, on SiO2-ZrO2 the lack of such an effect may be related to limited reactivity of sulfur on the surface.

Chemical Reactivity of Pd−Au Bimetallic Nanoclusters Grown via Amorphous Solid Water as Buffer Layer

Bimetallic Pd−Au nanoclusters were prepared on SiO2/Si(100) via amorphous solid water (ASW) as buffer layer. Selective growth pathways have led to segregated palladium and gold clusters or alloy bimetallic crystallites. Morphology and chemical composition were determined by AFM, SEM, and HR-TEM coupled to EDX analysis. Correlation between clusters morphology and composition to their chemical reactivity is reported for the first time. Temperature programmed reaction studies revealed that conversion of acetylene to ethylene and trimerization to benzene are significantly enhanced over crystalline alloy Pd−Au clusters when compared to clean Pd. A remarkable selectivity toward ethylene formation over benzene is unique to the alloy bimetallic clusters grown via ASW buffer layer on silica, presumably due to suppression of the trimerization pathway. The intrinsic, modified electronic properties of the bimetallic alloy clusters are believed to drive their enhanced chemical reactivity when compared with the pure Pd clusters.

Acrolein coupling on reduced TiO 2(1 1 0): The effect of surface oxidation and the role of subsurface defects

Reactions of acrolein, water, and oxygen with the vacuum-reduced surface of TiO 2(1 1 0) are reported in a temperature programmed reaction study of the interaction of an aldehydic pollutant with a reducible metal oxide. A total of 25% of the acrolein that binds to the surface is converted to products. Notably, carbon–carbon coupling occurs with 86% selectivity for formation of C 6 products: C 6H 8, identified as 1,3-cyclohexadiene, in a peak at 500 K and benzene immediately thereafter at 530 K. Acrolein is evolved from the surface in three peaks: a peak independent of coverage at 495 K, attributed to decomposition of an intermediate that is partly converted to C 6H 8; a coverage-dependent peak that shifts from 370 K (low coverage) to 260 K (high coverage), which is attributed to adsorption at 5-fold coordinated Ti sites; and a multilayer state at 160 K. Water and acrolein compete for 5-fold coordinated titanium sites when dosed sequentially. The addition of water also opens a new reaction pathway, leading to the hydrogenation of acrolein to form propanal. Water has no effect on the yield of 1,3-cyclohexadiene. Exposure of the surface to oxygen prior to acrolein dosing quenches the evolution of acrolein at 495 K and concurrently eliminates the coupling. From these results, we propose that reduced subsurface defects such as titanium ion interstitials play a role in the reactions observed here. The notion that subsurface defects may contribute to the reactivity of organic molecules over reducible oxide substrates may prove to be general.

MgO(111) Nanosheets with Unusual Surface Activity

MgO(111) nanosheets can be prepared via a facile wet chemical process. The MgO(111) nanosheets possessing the exposed (111) plane as a main surface have a thickness typically between 3 and 5 nm. Study of MgO(111) nanosheets by in situ diffuse reflectance Fourier transform (DRIFT) spectroscopy suggests that hydroxyl groups, oxygen vacancies, and surface oxygen anions exist on the surface and the (111) surface may be stabilized by hydroxyl groups. DRIFTS and temperature-programmed desorption (TPD) studies of CO2 adsorption reveal that there are large amounts of medium basic sites which can be attributed to the high concentration of surface O2- Lewis basic sites. DRIFTS and temperature-programmed reaction studies (TPRS) of methanol have demonstrated that, in contrast to commercial MgO, MgO(111) nanosheets are highly reactive toward the decomposition of methanol. Methanol can be decomposed and the surface CO species formed can be oxidized readily at low temperature by the high concentration of oxygen anions o...

Temperature-Dependent Halogen-Exchange Activity Studies of Zeolite-Derived Aluminum Trifluoride

A high-surface-area (190 m2/g) amorphous aluminum trifluoride material (“plasma-AlF3”) was synthesized by plasma decomposition of zeolite, and its structural and reactivity properties were investigated. High-resolution transmission electron microscopy of plasma-AlF3 indicates morphological features on the nanometer-scale, whereas temperature-programmed X-ray diffraction is used to determine the phase-transition temperatures of plasma-AlF3 to β- and α-AlF3. Halogen-exchange reactivity is studied by temperature-programmed reaction (TPR) techniques using the dismutation of CCl2F2 as a model reaction. Plasma-AlF3 is found to possess an unexpected low-temperature (>315 °C) activity not observed with the well-known halogen-exchange catalyst β-AlF3. Supporting TPR studies on aluminum trifluoride hydrates are performed to correlate this new activity with an amorphous AlF3 structure, and a simple Lewis acid model is presented to explain the reactivity data.

Hafnium diboride thin films by chemical vapor deposition from a single source precursor

High quality, stoichiometric thin films of hafnium diboride are deposited by chemical vapor deposition from the precursor Hf[BH4]4 at deposition temperatures as low as 200°C. An activation energy of 0.43eV(41kJ∕mol) is obtained for the overall process as monitored by temperature programmed reaction studies. Films deposited at low temperatures (<500°C) are structurally amorphous to x-ray diffraction; a 12nm thick film is sufficient to prevent copper diffusion into silicon during a 600°C anneal for 30min. Films deposited above 500°C are crystalline, but have a columnar microstructure with low density. All the films are metallic, but the low temperature amorphous films have the lowest resistivity ∼440μΩcm. The process is also highly conformal, e.g., a 65nm wide trench with a 19:1 depth-width aspect ratio was coated uniformly.

The detailed kinetics and mechanism of ethyl ethanoate synthesis over a Cu/Cr 2O 3 catalyst

Ethyl ethanoate is produced industrially in a process that came on-stream in March 2003 using liquid ethanol as the feed. The catalyst was Cu/Cr 2O 3, and the reactor temperature was 473 K. The selectivity to ethyl ethanoate was 95%. Detailed kinetic and mechanistic studies of this process were conducted in a multipurpose microreactor system, using an on-line mass spectrometer controlled and interrogated by computer. Isothermal frontal gas adsorption chromatography experiments using ethanol as the adsorbate in the temperature range 300–473 K found that a H 2 pulse was detected immediately after the ethanol contacts the catalyst. The fact that H 2 evolved from the Cu/Cr 2O 3 catalyst at 300 K indicates that the dehydrogenative adsorption of ethanol to form an adsorbed ethoxy species [(CH 3CH 2O) (a)] occurred on the Cu component of the catalyst. A plot of the logarithm of the amount of H 2 evolved on adsorption against the reciprocal of the adsorption temperature gives an activation energy to adsorption of 31 kJ mol −1. Temperature-programmed studies on the adsorbed ethoxy species showed that some of these ethoxy species dehydrogenated at 425 K with a surface activation energy of 94 kJ mol −1 to form the adsorbed acetyl species (CH 3CO) (a). The adsorbed ethoxy and acetyl species combined on the surface to form adsorbed ethyl ethanoate. The ethyl ethanoate desorbed at a peak maximum temperature of 680 K. Ammonia temperature-programmed desorption experiments produced a desorption peak for NH 3 at 680 K, revealing the existence of a strong Brönsted acid site on the surface of the Cr 2O 3. Thus the ethyl ethanoate desorbing at 680 K was bonded to the Cu/Cr 2O 3. Temperature-programmed reaction (TPR) studies, in which ethanol was passed continuously over the Cu/Cr 2O 3 catalyst while raising the temperature linearly with time and monitoring the products of reaction continuously on the mass spectrometer, gave an overall activation energy of 92 kJ mol −1 for ethyl ethanoate formation, the same as that for the dehydrogenation of the adsorbed ethoxy species to form an adsorbed acetyl, suggesting that this might be the rate-determining step for the reaction. TPR studies of ethanol over unsupported polycrystalline Cu gave identical results to those of the Cu/Cr 2O 3 catalyst, confirming that Cu is the active component of the catalyst.

Self-Assembled Monolayer of l-Cysteine on Au(111): Hydrogen Exchange between Zwitterionic l-Cysteine and Physisorbed Water

A self-assembled monolayer (SAM) of l-cysteine [HSCH2CH(NH2)COOH] was prepared on a Au(111) surface by vapor deposition in ultrahigh vacuum and was characterized by techniques of temperature-programmed desorption (TPD), Cs+ reactive ion scattering (Cs+ RIS), and low-energy secondary ion mass spectrometry (LESIMS). Analysis of the amino acid functional groups of SAM indicated that l-cysteine molecules exist in the zwitterionic form. Upon physisorption of the D2O overlayer on the SAM, the −NH3+ functional group of cysteine readily exchanges their H atoms with D2O in the temperature range 125−230 K. The H/D exchange of the −NH3+ group sequentially occurs with D2O molecules that are directly hydrogen-bonded to the SAM, and the long-range proton transfer to the upper layer water molecules does not occur. Temperature-programmed reaction study and kinetic analysis yielded an activation energy of 13 ± 1 kJ mol-1 for the H/D exchange reaction, which suggests proton tunneling as a mechanism.

Unique properties of Ir/ZSM-5 catalyst for NO reduction with CO in the presence of excess oxygen

The reduction of NO with CO in the presence of excess oxygen was investigated over different noble metal catalysts for probing the relationship between catalytic properties and adsorption behaviors. Among the four precious metal catalysts investigated, Ir/ZSM-5 was found to be the only active one for NO reduction with CO under lean conditions. With the decreasing of the Ir content, higher NO conversion and CO selectivity was obtained. Temperature-programmed reaction (TPR) studies of NO/H2/O2 and NO/CO/O2 showed that the Pt/ZSM-5 was active when H2 was used as the reductant, whereas, the Ir/ZSM-5 was active when CO was the reducing agent. This difference is due to the different mechanisms of the two reactions. Temperature-programmed desorption (TPD) of NO, CO and O2 showed that NO could dissociate more easily over the Ir/ZSM-5 than on the Pt/ZSM-5, while the oxidation of CO by O2 proceeded more rapidly on the Pt/ZSM-5 than on the Ir/ZSM-5. The presence of excess O2 inhibited drastically the dissociation of NO, which is considered as the key step for the NO–CO reaction. The high dissociation rate of NO over the Ir/ZSM-5 is visualized as the key factor for its superior high activity in NO reduction with CO under lean conditions.

Push–Pull Mechanism of Hydrodenitrogenation over Silica-Supported MoP, WP, and MoS 2 Hydroprocessing Catalysts

The mechanism of liquid-phase catalytic hydrodenitrogenation at 3.1 MPa on silica-supported molybdenum phosphide, MoP/SiO 2, and tungsten phosphide, WP/SiO 2, was studied using a series of pentylamines of different structures. The reactivity pattern suggested that removal of nitrogen occurred primarily by an E2 elimination mechanism involving acidic and base sites on the catalyst surfaces in a push–pull process. Infrared spectroscopy and temperature-programmed reaction studies of ethylamine indicated that alkyl ammonium species formed on Brønsted acid sites were intermediates in the reaction. Similar results were obtained with a reference MoS 2/SiO 2 sample tested at the same conditions. This suggested that sulfur was probably present on the active surface and assisted in the removal of sulfur.

Synthesis of yttria-doped strontium-zirconium oxide powders via ammonium glycolate combustion methods as precursors for dense ceramic membranes

Ammonium glycolate combustion was used to synthesize SrZr0.95Y0.05O3 − x powders. Soluble metal-glycolate complexes, detected by infrared spectroscopy, were formed with Y+3 and Zr+4, which maintain atomic level mixing of metal cations with glycolic acid. Sr+2/glycolic acid mixtures did not form glycolate complexes. Temperature-programmed reaction studies showed that only precursor solutions with metal-glycolate complexes combust, indicating that the combustion is initiated by metal-glycolate complexes. Varying the ammonium hydroxide content and glycolic acid levels above that required to form glycolate complexes did not affect surface area and crystallinity. Decreasing the glycolic acid/nitrate ratio increases the temperatures reached during combustion. Thick disks prepared from co-precipitated and ammonium glycolate powders demonstrated that combustion synthesized powders reach higher densities. Surface area and SEM indicated that the ammonium glycolate powders have smaller particle sizes, which favor densification, than co-precipitated powders.

NO and deuterium co-adsorption on the reconstructed Pt(100)- hex surface: a temperature programmed reaction study

The deuterium adsorption at 270 K on a reconstructed Pt(100)- hex surface covered by NO ads was studied by means of temperature programmed reaction (TPR). In the case of adsorption on a clean Pt(100)- hex surface the saturated D ads coverage is 0.06 ML at P H 2 =6×10 −8 mbar, whereas the D ads uptake is enhanced considerably by NO pre-adsorption. First the D ads uptake increases with increasing NO ads coverage, θ NO, reaching a maximum at θ NO≈0.25 ML, and then decreases to zero. This phenomenon is explained as follows. The NO adsorption on the hex surface leads to the formation of 1×1 islands saturated by NO ads and surrounded by the hex phase. The NO ads/1×1 islands are assumed to modify the hex phase adjacent to the island boundaries, adapting this area for deuterium adsorption. TPR in the co-adsorption layer of NO ads and D ads is initiated by D 2 desorption and shows an ‘explosive’ behaviour, manifesting itself in the narrow TPR peaks of N 2 and D 2O at ∼370 K. The NO ads pre-coverage affects the reaction temperature as well. Thus, at NO ads coverage of 0.35–0.40 ML the temperature of the surface explosion increases abruptly by ∼15–20 K. At this coverage the NO ads/1×1 islands are supposed to modify the rest of the hex phase so that, after further D 2 adsorption, the surface becomes completely saturated by D ads and NO ads species. A possible mechanism for this is discussed.

The thermal chemistry of model organosulfur compounds on gallium arsenide (110)

Organothiols and related organosulfur compounds hold promise for use as self-assembled ultra-thin electron beam resists, II–VI material growth precursors, and monolayer passivation layers. We report the results of temperature-programmed reaction and desorption (TPR/D) studies of the surface chemistry of three model organosulfur compounds, CH 3SH, (CH 3S) 2 and (CH 3) 2S, on the (110) face of gallium arsenide. Our measurements indicate that each of these species interacts strongly with the GaAs(110) surface. The first monolayer of CH 3SH desorbs at ∼300 K, with a desorption wave peak temperature (extrapolated to the zero coverage limit) corresponding to a desorption activation energy of 0.81 eV. This value is high compared to ∼110 K for the second layer, suggesting a chemical interaction between the thiol and the surface, despite the fact that the monolayer feature in the TPR/D spectra exhibits first-order kinetics, and the molecule is observed to desorb intact. The behavior of (CH 3S) 2 is markedly different; the disulfide decomposes upon adsorption (or during the TPR/D temperature ramp) and desorbs predominantly as (CH 3) 2S at ∼500 K, perhaps as the result of a concerted associative/dissociative desorption process that leaves sulfur at the surface. This result is also evidence that adsorbed CH 3SH and (CH 3) 2S do not assume equivalent surface-bound forms and supports the idea that the hydrogen atom of the thiol remains in close association with the adsorbed parent molecule, while the disulfide undergoes dissociative adsorption. Finally, our results indicate that (CH 3) 2S, like CH 3SH, molecularly adsorbs/desorbs. The desorption activation energy, extrapolated to zero coverage, is 0.79 eV, consistent with a strong adsorbate–substrate interaction. The similarity of this value to that for CH 3SH suggests that a specific sulfur–surface interaction dominates the desorption kinetics at low coverage. In the monolayer regime, with increasing exposure, the monolayer desorption feature in the TPR/D spectra broadens and exhibits strong asymmetry, and ultimately develops a lower temperature feature, indicating complex kinetics consistent with coverage-dependent phase behavior. Finally, by comparison of the measured desorption energies of the organosulfur compounds with those of the alkyl halides, for which the adsorption on GaAs(110) has been extensively investigated, an estimation of the chemical and physical contributions to the molecule–surface interaction is obtained. Our analysis suggests that both types of interactions play important roles in the adsorption energetics.