Presence Of Excess Hydrogen Research Articles

Selective CO production from CO2 over a metal catalyst supported on perovskite oxide in the presence of excess hydrogen

Supporting platinum on perovskite-type oxides (BZY10) increased catalytic activity while maintaining in the reverse water–gas shift reaction, which reduces carbon dioxide to carbon monoxide.

Highly stable preferential carbon monoxide oxidation by dinuclear heterogeneous catalysts

Atomically dispersed catalysts have been shown highly active for preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX). However, their stability has been less than ideal. We show here that the introduction of a structural component to minimize diffusion of the active metal center can greatly improve the stability without compromising the activity. Using an Ir dinuclear heterogeneous catalyst (DHC) as a study platform, we identify two types of oxygen species, interfacial and bridge, that work in concert to enable both activity and stability. The work sheds important light on the synergistic effect between the active metal center and the supporting substrate and may find broad applications for the use of atomically dispersed catalysts.

Synthesis of wafer-scale SWCNT forests with remarkably invariant structural properties in a bulk-diffusion-controlled kinetic regime

Robust synthesis of vertically aligned carbon nanotubes (VACNT) at large scale is required to accelerate deployment of numerous cutting-edge devices to emerging commercial applications. To address this need, this work demonstrates that the structural characteristics of single-walled CNTs (SWCNTs) produced in a growth regime dominated by bulk diffusion of the gaseous carbon precursor are remarkably invariant over a broad range of process conditions (precursor concentration increased up to 30-fold, catalyst substrate area from 1 cm2 to 180 cm2, growth pressure from 20 to 790 mbar, and gas flowrates up to 8-fold). A simple growth kinetics model is derived that accounts for both reactant bulk diffusion and competing byproduct formation in the presence of excess hydrogen, which quantitatively reproduces the experimental data in the entire isothermal parameter space. The model enables critical predictions for process scale-up optimization including a potential 6-fold increase in CNT production rate, a ∼90% carbon conversion efficiency in select conditions, and elimination of reaction rate decay observed at high pressures by using a hydrogen-free growth environment. This model-guided opportunity to substantially improve synthesis throughput and efficiency, in conjunction with the remarkably invariant CNT properties, is appealing for reliable VACNT device fabrication at both small and industrial scales.

Comparison study of preferential oxidation of CO over nanocrystalline Cu/CeO2 catalysts synthesized by different preparation methods

Preferential oxidation of carbon monoxide in presence of excess hydrogen is a promising alternative to restrict the CO deposition in the Pt-anode in the practical polymer electrolyte membrane fuel cell application. In the present work, nanocrystalline copper-ceria catalysts have been synthesized by hydrothermal method, wet impregnation method and urea nitrate combustion method. Their characterizations have been carried out by using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy . It has been found that Cu 2+ replaced Ce 4+ in cerium oxide, creating oxygen vacancy. The formation of more nano-sized CeO 2 leads to more oxygen vacancies in CeO 2 through the formation of interfacial Cu 1+ ions, which also enhances the CO oxidation activity. Among the synthesized Cu-CeO 2 catalysts, the catalyst prepared by hydrothermal method have shown both CO conversion and CO 2 selectivity as 100% towards CO oxidation at 373 K in the presence of excess H 2 making this catalyst viable for practical fuel cell application.

Optimum Hydrogen Injection in Phosphorus-Doped Polysilicon Passivating Contacts.

It has previously been shown that ex situ phosphorus-doped polycrystalline silicon on silicon oxide (poly-Si/SiOx) passivating contacts can suffer a pronounced surface passivation degradation when subjected to a firing treatment at 800 °C or above. The degradation behavior depends strongly on the processing conditions, such as the dielectric coating layers and the firing temperature. The current work further studies the firing stability of poly-Si contacts and proposes a mechanism for the observed behavior based on the role of hydrogen. Secondary ion mass spectrometry is applied to measure the hydrogen concentration in the poly-Si/SiOx structures after firing at different temperatures and after removing hydrogen by an anneal in nitrogen. While it is known that a certain amount of hydrogen around the interfacial SiOx can be beneficial for passivation, surprisingly, we found that the excess amount of hydrogen can deteriorate the poly-Si passivation and increase the recombination current density parameter J0. The presence of excess hydrogen is evident in selected poly-Si samples fired with silicon nitride (SiNx), where the injection of additional hydrogen to the SiOx interlayer leads to further degradation in the J0, while removing hydrogen fully recovers the surface passivation. In addition, the proposed model explains the dependence of firing stability on the crystallite properties and the doping profile, which determine the effective diffusivity of hydrogen upon firing and hence the amount of hydrogen around the interfacial SiOx after firing.

In-situ experimental and computational approach to investigate the nature of active site in low-temperature CO-PROX over CuOx-CeO2 catalyst

Preferential oxidation (PROX) of carbon monoxide (CO) in presence of excess hydrogen is a necessity to prevent poisoning of the Pt-anode by CO in PEMFCs (Polymer Electrolyte Membrane Fuel Cell). A highly active catalyst, 5 wt% Cu-CeO2 showed 100 % CO conversion at 65 °C without any undesirable oxidation of H2. The catalyst showed no deactivation even after 100 h on stream, making it viable for practical fuel cell application. Operando EPR in the PROX reaction condition revealed highly dispersed mixed valent cations with oxygen vacancies, responsible for the low-temperature PROX activity. At the active site, both Cu and Ce ions were shown to reversibly change their valence states to facilitate the abstraction of bridging lattice oxygen for CO oxidation to CO2. This surface oxygen mediated CO oxidation was found to be much faster than that of H2 to H2O, which could explain the complete selectivity of oxygen for CO2 formation.

Influence of cobalt on performance of Cu–CeO2 catalysts for preferential oxidation of CO

Copper and cobalt oxides supported on CeO2 were investigated for preferential oxidation of carbon monoxide (CO-PROX) in the presence of excess hydrogen and CO2. (CuO)1−x(Co3O4)x/3−(CeO2)2.5 (x = 0, 0.25, 0.50, 0.75, 0.85 and 1) catalysts were prepared by coprecipitation method. These mixed oxide catalysts were characterized by several physicochemical techniques, such as BET surface area (SBET), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). XRD studies show the peaks related to CuO and Co3O4 phases in copper and cobalt containing CeO2 catalysts. The average particle size of the CeO2 crystallites is in the range of 8–10 nm as evaluated from HRTEM studies. XPS studies demonstrate that Cu, Co and Ce in (CuO)1−x(Co3O4)x/3−(CeO2)2.5 catalysts are presented in +2 and +1, +3 and +2 and +4 and +3 oxidation states, respectively. The catalyst with x = 0.75 shows better activity and selectivity towards CO-PROX. Though the catalyst with only copper (CuO–CeO2, x = 0) shows good activity but reverse water gas shift (RWGS) reaction is noticed at high temperature. On the other hand, RWGS reaction is suppressed on the cobalt containing CuO–CeO2 catalyst. Cobalt on CeO2 with x = 1 shows hardly any activity for PROX reaction at low temperatures. No methanation activity is observed on CuO–CeO2 or Co3O4–CeO2 catalysts. In contrast, combination of copper and cobalt on CeO2 shows methanation of CO where enhanced activity is observed with increasing in cobalt content.

A High-Performance Nanoreactor for Carbon–Oxygen Bond Hydrogenation Reactions Achieved by the Morphology of Nanotube-Assembled Hollow Spheres

Hydrogenation of carbon–oxygen bonds is extensively used in organic synthesis. However, a high partial pressure of hydrogen or the presence of excess hydrogen is usually essential to achieve favorable conversions. In addition, because most hydrogenations are consecutive reactions, the selectivity is difficult to manipulate, leading to an unsatisfactory distribution of products. Herein, a copper silicate nanoreactor with a nanotube-assembled hollow sphere (NAHS) hierarchical structure is proposed as a solution to these problems. In the case of dimethyl oxalate (DMO) hydrogenation, the NAHS nanoreactor achieves remarkable catalytic activity (the yield of ethylene glycol is 95%) and stability (>300 h) when the H2/DMO molar ratio is as low as 20 (in comparison to typical values of 80–200). For further investigation, nanotubes and lamellar-shaped Cu/SiO2 catalysts with similar surface areas of active sites of NAHSs were investigated as contrasts. By a combination of high-pressure hydrogen adsorption and Monte ...

Nanosize Effect in Heterogeneous Catalytic Processes Over Copper, Iron, and Zirconium Oxides

A summary is given of the results of studies of the catalytic properties of nanophase systems derived from copper, iron, and zirconium oxides in the ethanol steam reforming, the oxidation of carbon monoxide, the preferential oxidation of CO in the presence of excess hydrogen, the oxidation of benzyl alcohol, and the cracking of triglycerides. A relationship was found between the catalytic activity of copper, iron, and zirconium oxides and the size of their nanoparticles. The reasons for the nanophase effect in these processes were elucidated for a number of systems.

Effect of support carbon materials on Ag catalysts used for CO oxidation in the presence and absence of H2

Effects of the support carbon materials on Ag-catalyzed CO oxidation at low temperature and CO preferential oxidation (PROX) in the presence of excess hydrogen were studied for the catalyst systems of Ag/AC (activated carbon), Ag/CB (carbon black) and Ag/CNT (carbon nanotube). By comparison, Ag/CNT exhibited a higher CO conversion with high CO2 selectivity, especially pretreated in H2 for 2h at 300°C. On the basis of XPS and N2-sorption analysis, the CNT surface gave rise to high extent of the surface defectiveness and thermal-stable oxygen state, which is necessary for the high dispersion of active species. XRD results revealed that the active species Ag partly transfers from surface into CNTs, increasing the interface between the Ag particles and CNTs. Thus, Ag particles can be protected by CNT coating against sintering and supply more adsorption sites.

Gold catalysts supported on Y-modified ceria for CO-free hydrogen production via PROX

The preferential CO oxidation in the presence of excess hydrogen (PROX) was studied over gold catalysts on yttrium-modified ceria supports synthesized by impregnation (IM) and co-precipitation (CP) methods. In order to avoid oxygen vacancies ordering by heavily yttrium doping, a low extent Y-modification of ceria (up to 7.5wt.% Y2O3, i.e., 11at.% Y) was investigated. The samples were characterized by XRD, HRTEM/HAADF, Raman and XP spectroscopy and TPR measurements. At the operating temperature of the fuel cells (80–120°C) the selectivity of the Y-doped gold catalysts was larger than that of Au/CeO2. However, at these temperatures all studied catalysts exhibited quite similar PROX activity and selectivity. This was explained by the similarity in relatively small average size of gold particles (estimated by HRTEM in the 2.1–3.5nm range) and the reducibility of ceria surface layers. Raman spectroscopy showed a higher extent of bulk defects, O vacancies and interstitials with increasing Y-doping in the supports made by the CP method and-based on sequential spectra obtained under oxidizing and reducing conditions- enabled the assignment of a band at ∼605cm−1 to a vibrational mode within the anionic (oxygen) sub-lattice that involve mobile (detachable) oxygen atoms that can be delivered by the support under suitable conditions. The absence of correlation between CO conversion and yttrium loading could suggest a preferential role played by the surface modification and surface oxygen mobility in PROX. Although under ideal conditions there was not substantial dependence of the behavior in PROX on the preparation method of doped supports, the IM method was more appropriate than the CP method for PROX with CO2 and water addition to the gas feed. The improved tolerance to CO2 could be explained by the lowering of ceria surface basicity due to the nanosized Y2O3, covering ceria grains.

In situ study of the initiation of hydrogen bubbles at the aluminium metal/oxide interface.

The presence of excess hydrogen at the interface between a metal substrate and a protective oxide can cause blistering and spallation of the scale. However, it remains unclear how nanoscale bubbles manage to reach the critical size in the first place. Here, we perform in situ environmental transmission electron microscopy experiments of the aluminium metal/oxide interface under hydrogen exposure. It is found that once the interface is weakened by hydrogen segregation, surface diffusion of Al atoms initiates the formation of faceted cavities on the metal side, driven by Wulff reconstruction. The morphology and growth rate of these cavities are highly sensitive to the crystallographic orientation of the aluminium substrate. Once the cavities grow to a critical size, the internal gas pressure can become great enough to blister the oxide layer. Our findings have implications for understanding hydrogen damage of interfaces.

Influence of the heating rate and the potassium concentration of the feed solution on the hydrothermal liquefaction of used yeast and apple pomace under reducing conditions

In the work reported here, baker’s yeast (Saccharomyces cerevisiae), used yeast, and apple pomace were used as feed for the production of liquid biofuels in a continuous one-step process under hydrothermal conditions in the presence of excess hydrogen and K2CO3. The biomass conversion experiments were performed in an up-flow reactor under near-critical water conditions (T 330–450 °C, p 20–32 MPa). The products consisted of three phases: an oil-like organic phase, a gaseous phase, and an aqueous phase. Higher concentrations of organic carbon in the process resulted in a higher product yield. The heating value of the organic phase was up to 37.6 MJ kg−1. Liquefaction of yeast without any addition of K2CO3 also resulted in liquid oil, but the quality and the yield of the oil product were lower. The use of K2CO3 catalyst provides significant effect on the apple pomace conversion process. The carbon mass yield ranged from 270 to 400 g kg−1. Our older statement that the reaction of temperature of 400 °C is optimal for the oil yield and quality has been confirmed with the present new experimental results.

Effect of Cu Loading to Catalytic Selective CO Oxidation of CuO/CeO<sub>2</sub> –Co<sub>3</sub>O<sub>4</sub>

This work studied CuO/CeO2-Co3O4 with wt% Ce:Co ratio 95:5 for selective CO oxidation with effect of wt% Cu loading. The catalysts were prepared by co-precipitation. Characterizations of catalysts were carried out by XRD and BET techniques. The results showed a good dispersion of CuO for 5 wt% Cu loading catalysts and showed high specific surface area of catalyst. For selective CO oxidation, both 5CuO and 30CuO catalysts could remove completely CO in the presence of excess hydrogen at 423 K and 20CuO could eliminate CO completely at 443 K. Moreover, considering the selectivity to CO oxidation, the 5CuO catalyst has shown the highest selectivity of 85% while the 30CuO catalyst obtains the selectivity of 65% at the reaction temperature of 423 K.

Pd-catalyzed hydrodehalogenation of chlorinated olefins: Theoretical insights to the reaction mechanism

Density functional theory calculations are applied to study energetics of trichloroethene (TCE) hydrodechlorination over pure Pd(111), chlorine-covered Pd(111), and Pd island supported by Au(111). Our results show that in all cases CCl bond breakings take place more readily than CH bond formations and that TCE dechlorinates fully producing CCH precursors for the hydrogenations. The reaction pathway through radical-like species provides a possible explanation to the experimental product distributions that show a nominal amount of lesser chlorinated species in the presence of excess hydrogen. The surface chlorine resulted from the TCE decomposition weakens the binding of the adsorbates and reduces the gap between the dechlorination and hydrogenation barriers but is not expected to affect the overall reaction route. The underlying gold induces stronger adsorption and decreases the activation barriers considerably compared to the pure Pd(111). The change in Gibbs free energy between the gas-phase molecule and the activated surface species explains the experimentally observed reactivity ordering among chlorinated ethenes. Although vinyl chlorine has a stronger CCl bond than TCE, it dechlorinates faster when introduced from the gas phase to the catalyst surface.

Novel PtCuO/CeO2/α-Al2O3 sponge catalysts for the preferential oxidation of CO (PROX) prepared by means of supercritical fluid reactive deposition (SFRD)

Ceramic sponges (α-alumina) supporting ceria wash-coats and PtCuO layers of varying compositions were tested in the preferential oxidation (PROX) of CO in the presence of excess hydrogen. Copper and platinum were loaded simultaneously on the ceria-coated sponge by means of supercritical fluid reactive deposition (SFRD) which includes adsorption and reduction of metal-organic complexes dissolved in supercritical CO2. Products were characterized by means of SEM, STEM HAADF, EDX, H2-TPR, CO-TPD, CO2-TPD and N2-adsorption. Catalysts prepared after the SFRD-method exhibit high selectivity. The CO oxidation activity roughly correlates with the copper content and is superior as compared to the activity of PROX-catalysts reported in the literature so far.

The Beneficial Effect of Hydrogen on CO Oxidation over Au Catalysts. A Computational Study

Density functional theory calculations have been carried out to explore the effect of hydrogen on the oxidation of CO in relation to the preferential oxidation of CO in the presence of excess hydrogen (PROX). A range of gold surfaces have been selected including the (100), stepped (310) surfaces and diatomic rows on the (100) surface. These diatomic rows on Au(100) are very efficient in H-H bond scission. O2 hydrogenation strongly enhances the surface-oxygen interaction and assists in scission of the O–O bond. The activation energy required to make the reaction intermediate hydroperoxy (OOH) from O2 and H is small. However, we postulate its presence on our Au models as the result of diffusion from oxide supports to the gold surfaces. The OOH on Au in turn opens many low energy cost channels to produce H2O and CO2. CO is selectively oxidized in a H2 atmosphere due to the more favorable reaction barriers while the formation of adsorbed hydroperoxy enhances the reaction rate.

Chemical speciation of adsorbed glycine on metal surfaces

Experimental studies have reported that glycine is adsorbed on the Cu(110) and Cu(100) surfaces in its deprotonated form at room temperature, but in its zwitterionic form on Pd(111) and Pt(111). In contrast, recent density functional theory (DFT) calculations indicated that the deprotonated molecules are thermodynamically favored on Cu(110), Cu(100), and Pd(111). To explore the source of this disagreement, we have tested three possible hypotheses. Using DFT calculations, we first show that the kinetic barrier for the deprotonation reaction of glycine on Pd(111) is larger than on Cu(110) or Cu(100). We then report that the presence of excess hydrogen would have little influence on the experimentally observed results, especially for Pd(111). Lastly, we perform Monte Carlo simulations to demonstrate that the aggregates of zwitterionic species on Pt(111) are energetically preferred to those of neutral species. Our results strongly suggest that the formation of aggregates with relatively large numbers of adsorbed molecules is favored under experimentally relevant conditions and that the adsorbate-adsorbate interactions in these aggregates stabilize the zwitterionic species.

Conversion of yeast by hydrothermal treatment under reducing conditions

In the work reported here, baker’s yeast (Saccharomyces cerevisiae) was used as feed for the production of liquid biofuels in a continuous one-step process under hydrothermal conditions in the presence of excess hydrogen and K2CO3. The yeast conversion experiments were performed in an up-flow reactor under near-critical water conditions (T 330–450°C, p 20–32MPa). The products consisted of three phases, an oil-like organic phase, a gaseous phase, and an aqueous phase. Higher concentrations of organic carbon in the process resulted in a higher product yield. The heating value of the organic phase was up to 38.6MJ/kg. Liquefaction of yeast without any addition of K2CO3 also resulted in liquid oil, but the quality and the yield of the oil product were lower. A reaction temperature of 400°C was found to be optimal for the oil yield and quality.

Ce0.78Sn0.2Pt0.02O2−δ: A new non-deactivating catalyst for hydrogen production via water–gas shift reaction

We demonstrate the activity of Ce0.78Sn0.2Pt0.02O2−δ, a new catalyst, towards water–gas shift (WGS) reaction. Over 99.5% CO conversion to H2 is observed at 300±25°C. Based on different characterization techniques we found that the present catalyst is resistant to deactivation due to carbonate formation and sintering of Pt on the surface when subjected to longer duration of reaction conditions. The catalyst does not require any pre-treatment or activation between start-up/shut-down reaction operations. Formation of side products such as methane, methanol, formaldehyde, coke etc. was not observed under the WGS reaction conditions indicating the high selectivity of the catalyst for H2. Temperature programmed reduction of the catalyst in hydrogen (H2-TPR) shows reversible reduction of Ce4+ to Ce3+, Sn4+ to Sn2+ and Pt4+ to Pt0 oxidation state with oxygen storage capacity (OSC) of 3500μmolg−1 at 80°C. Such high value of OSC indicates the presence of highly activated lattice oxygen. CO oxidation in presence of stoichiometric O2 shows 100% conversion to CO2 at room temperature. The catalyst also exhibits 100% selectivity for CO2 at room temperature towards preferential oxidation (PROX) of residual CO in presence of excess hydrogen in the feed.