Increasing H2 Partial Pressure Research Articles

In Situ Fourier Transform Infrared Investigation on the Low-Level Carbon Dioxide Conversion over a Nickel/Titanium Dioxide Catalyst.

Efficiently converting atmospheric carbon dioxide (CO2) is crucial for sustainable human development. In this study, we conducted systematic in situ Fourier transform infrared tests to examine how hydrogen (H2) partial pressure affects the conversion of low-level CO2 (around 400 ppm) using nickel/titanium dioxide (Ni/TiO2). Results show that increasing H2 partial pressure significantly increases surface monodentate formate species, leading to enhanced methane (CH4) production at both 250 and 400 °C. Conversely, on Ni's surface, the key species are formyls and bidentate formate at 250 °C, but these decrease significantly at 400 °C. These findings indicate that low-level CO2 is more easily converted to CH4 over Ni/TiO2 than Ni, regardless of temperature. Additionally, the strong Ni-TiO2 interaction gives Ni/TiO2 an advantage in converting low CO2 concentrations, with excellent durability even at 400 °C. This study enhances our understanding of direct CO2 conversion and aids in the development of advanced CO2 emission reduction technologies.

Effects of steam on toluene hydrogenation over a Ni catalyst

The catalytic toluene hydrogenation over Ni/SiO2 was carried out using H2 or a H2/H2O mixture. The toluene conversion and MCH selectivity were evaluated under partial steam pressures 0−10 kPa, at H2/toluene molar ratios 1–5, and at temperatures 393−453 K. In the absence of steam, toluene conversion increased with increasing H2 partial pressure, and the MCH selectivity decreased with increasing temperature. In the presence of steam, the toluene conversion showed near-constant activities over the entire range of steam partial pressures, and the MCH selectivity increased with increasing steam partial pressure. This result indicated that the addition of steam inhibited byproduct formation. To clarify the effects of steam on the side-reaction, the byproducts composition was analyzed using a gas chromatograph. The results showed that the addition of steam inhibited demethylation, the ring-opening reaction of MCH, the disproportionation of toluene, and isomerization to convert six-membered ring compounds into five-membered ring compounds.

Rhenium-promoted selective CO2 methanation on Ni-based catalyst

Re-doped Ni(111) (Re@Ni(111)) surface was used as a model to investigate the effect of Re on the CO bond scission and on the selectivity of CO2 methanation on a Ni-based catalyst. Three pathways, including CO2 dissociation into CO* followed by CO* hydrogenation, CO2 reduction through the HCOO* and COOH* intermediates, were analyzed based on the results from the density functional theory calculations. The results indicate that the presence of Re significantly lowers the activation barrier of CO bond cleavage due to the strong affinity of Re to O but has no significant effect on the hydrogenation steps. Microkinetic analysis showed that the presence of Re greatly increases the selectivity toward CH4. Analysis of surface coverage of the adsorbed species showed that CO* and H* were the most abundant species on the Ni(111) surface whereas appreciable amount of O adatoms were present on Re@Ni(111) in addition to CO* and H*, with the O adatoms on the Re sites. On both surfaces, increasing H2 partial pressure resulted in an increase in H* coverage but decreased CO* coverage. The strong affinity of Re toward O makes Re@Ni(111) more effective for CO bond scission and thereby enhances methane selectivity.

Catalytic deoxygenation of octanoic acid over silica- and carbon-supported palladium: Support effects and reaction pathways

Octanoic acid (OA) deoxygenation was investigated over silica- and carbon-supported palladium catalysts (each containing 5wt.% Pd) at 235–300°C and 1atm in a continuous flow reactor. A commercial Pd/SiO2 (A) catalyst was active for OA decarbonylation (DCN) and hydrodeoxygenation (HDO) at 260°C under 10% H2; subsequent hydrogenation (HY) and DCN of the primary products, 1-heptene and octanal, respectively, produced n-heptane. Under equivalent conditions, a Pd/SiO2 (B) catalyst prepared using Pd(NO3)2 and Aerosil 300 produced n-heptane with very high selectivity (>99%) via DCN/HY. In contrast, a commercial Pd/C (A) catalyst was highly active and selective to n-heptane (>98%) and CO2 (65%) under these conditions. Moreover, CO2 selectivity and n-heptane yield increased with reaction temperature consistent with direct decarboxylation (DCX). Increasing H2 partial pressure resulted in markedly lower activity and CO2 selectivity; however, Pd/C (A) had negligible activity under He. Pd/C (A) exhibited greater water–gas shift (WGS) activity than Pd/SiO2 (A); however, differences in WGS activity alone cannot explain the observed support effect. A more highly dispersed Pd/C (B) catalyst was more active at 260°C under H2 than Pd/C (A); however, under 10% H2, it had lower activity, CO2 selectivity (55%), and stability. Pd/C (A) and Pd/C (B) have very similar textural properties, but Pd/C (A) has a much higher Na content. By comparison, Pd supported on high-purity acetylene carbon black exhibited only DCN activity. These results indicate that carbon surface properties (e.g., polar functional groups, alkali metal content) influence the fatty acid deoxygenation performance of Pd/C catalysts.

Nitrite removal from water by catalytic hydrogenation in a Pd-CNTs/Al2O3hollow fiber membrane reactor

BACKGROUND Nitrite contaminants in groundwater endanger public health since they may cause many diseases such as blue baby syndrome, cancer and hypertension. In this work, novel Pd-CNTs/Al2O3 catalytic hollow fiber membranes for aqueous nitrite hydrogenation reduction were fabricated by the deposition of carbon nanotubes (CNTs) and Pd nanoparticles inside porous Al2O3 hollow fibers. A hollow fiber membrane reactor was assembled for nitrite removal from water by catalytic hydrogenation reduction. RESULTS Experimental results indicated that the nitrite hydrogenation rate was highly dependent on hydrogen dissolution in water and adsorption on the catalyst. The nitrite removal increased with increasing H2 partial pressure to balance the nitrite solution, but the influence became very marginal as it approached 1.0 atm. The oxygen in the gas feed lowered the nitrite removal due to the reaction with dissolved hydrogen over the Pd-catalyst. Compared with the conventional diffusion operation of the membrane reactor, the ‘flow-through’ operation resulted in higher nitrite removals due to the intensified mass transfer rate. CONCLUSION The nitrites in water can be completely removed using the Pd-CNTs/Al2O3 membrane reactor with flow-through operation. The denitrification should be operated at around 25°C for large reaction kinetics, which is determined by both the temperature and the hydrogen solubility in water. © 2015 Society of Chemical Industry

Methanation of carbon monoxide over promoted flame-synthesized cobalt clusters stabilized in zirconia matrix

Methanation catalysts based on 20% Co–ZrO2 were synthesized in a one-step flame spray pyrolysis that at the same time allows the doping of noble metal promoters, hereby demonstrated with 0.4wt.% Pd, Rh, Pt, Ru, and Ag. Unlike conventional catalysts, the cobalt content is internally dispersed and stabilized within the zirconia matrix, allowing the formation of very fine surface Co0 clusters (1–2nm) upon reduction as unraveled from its dispersion. In situ X-ray absorption spectroscopy (XAS) showed that doping of noble metal promoters significantly improved the reducibility of cobalt in the order of undoped<Ag<Ru<Pt<Rh<Pd, as assisted by the H-spillover. The number of active Co0 sites on the surface was enhanced by almost 2-fold upon addition of promoters, and so are the corresponding rates of CO conversion. Detailed characterization by XAS and X-ray photoelectron spectroscopy (XPS) unraveled that homogeneously dispersed promoters (Rh, Pt, and Ru) merely enhanced the reducibility of cobalt and did not affect the turnover frequencies (TOFs, based on CO conversion) relative to that of the bare Co–ZrO2 catalyst. On the contrary, surface-enriched promoters (Pd and Ag) lowered the apparent TOFs due to the mixed influence of the noble metals and Co0. We further found in the representative case of Rh-promoted Co–ZrO2 that the promoter ameliorated hydrogenation and hence resulted in higher CH4 selectivity with increasing H2 partial pressure (i.e., higher reaction order), compared to the unpromoted catalyst.

Hydrogenation of Toluene on Zirconium-Modified Hexagonal Molecular Sieve Supported Platinum and Palladium Catalysts

Hexagonal molecular sieve (HMS) and Zr-HMS materials were synthesized and employed as supports for preparation of hydrogenation catalyst. Supports and catalysts were characterized by using inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), temperature-programmed desorption, Brunauer–Emmett–Teller surface area, and CO chemisorptions techniques. Results from SEM and EDX confirm the hexagonally ordered mesoporous structure and incorporation of Zr into the HMS support, respectively. The detailed kinetic study of hydrogenation of toluene over Pt(0.15wt%)–Pd(0.15wt%)/HMS and Pt(0.15wt%)–Pd(0.15wt%)/Zr-HMS was performed in a continuous-upflow stainless steel catalytic fixed bed reactor at varied weight hourly space velocity (WHSV), hydrogen partial pressure, and temperature. It was observed that toluene conversion was increased on increasing H2 partial pressure and decreased with increasing WHSV. The conversion is dependent on temperature and shows a well-defined maximum. It was found that incorporation of Zr into the HMS structure increased the toluene conversion and reducibility of the catalysts. It was found that Pt(0.15wt%)–Pd(0.15wt%)/Zr-HMS catalysts showed better performance in the hydrogenation of toluene than Pt(0.15wt%)–Pd(0.15wt%)/HMS catalyst.

Modeling syngas permeation through a poly dimethyl siloxane membrane by Flory–Rehner theory

Poly dimethyl siloxane (PDMS) membranes show potential for the separation of carbon dioxide from hydrogen in syngas applications due to their strong affinity for CO2. The Flory–Rehner theory of mixing enables the sorption of gases within such a rubbery membrane to be modeled, as long as the Flory–Huggins interaction parameters are known. In this work, mixed gas permeation experiments are used to determine interaction parameters for a range of relevant gas pairs. For the N2–H2 system, the mixed gas performance is comparable to pure gas measurements and indicates that H2 does not interfere with the permeation of N2 through the membrane. In contrast, for the CO2–H2 system, the CO2 permeability falls with increasing H2 partial pressure, attributed to competitive sorption. The effect of a typical hydrocarbon impurity, toluene, upon CO2 and N2 permeability through PDMS was also measured. The addition of this aromatic hydrocarbon caused significant polymer swelling, which required solubility modeling with the Flory–Rehner model. A concentration dependent diffusivity was also required to fit the permeation data. The parameters determined from the laboratory experiments were used to model pilot plant results for a PDMS membrane separating CO2 from syngas, as part of the CO2CRC Mulgrave Project. It was found that the model could effectively predict experimental CO2, N2 and H2 permeabilities.

High H2O2 yield in the direct oxidation of H2 with O2 on mono dispersed Pd–Au nano colloid under pressurized conditions

Pd–Au (75 : 25) nano colloid was prepared by using various reductants and it was found that the mono dispersed Pd–Au nano colloid with 4.7 nm in diameter was prepared by reduction with oxalic acid and this Pd–Au nano colloid is highly active for the synthesis of H2O2 by direct oxidation of H2 with O2. The yield of H2O2 increased with increasing reaction pressure and at 1 MPa, H2O2 yield achieved a value of 30%. Under these conditions, H2 conversion is as high as 80%. H2O2 yield was further improved by increasing H2 partial pressure, and the optimum H2 partial pressure was 10% at 1 MPa because of safety (out of the explosion limit concentration in reactor) and the formation rate of H2O2. Because H2O2 decomposition is prevented, H2O2 yield also increased with decreasing reaction temperature and at 268 K, H2O2 yield and production rate are achieved as high as 46% and 25 mmol h−1, respectively, in aqueous solution. H2O2 concentration reaches a value of 5.8% in aqueous solution after 20 h at 268 K.

Catalytic Conversion of Guaiacol Catalyzed by Platinum Supported on Alumina: Reaction Network Including Hydrodeoxygenation Reactions

The conversion of guaiacol catalyzed by Pt/γ-Al2O3 in the presence of H2 was investigated with a flow reactor at 573 K and 140 kPa. Dozens of reaction products were identified, with the most abundant being phenol, catechol, and 3-methylcatechol. The kinetically significant reaction classes were found to be hydrogenolysis [including hydrodeoxygenation (HDO)], hydrogenation, and transalkylation. Selectivity–conversion data were used to determine an approximate quantitative reaction network accounting for the primary products, and a more detailed qualitative network was also inferred. Catalytic HDO was evidenced by the production of anisole and phenol. The HDO selectivity increased with an increasing H2 partial pressure and a decreasing temperature. Products formed by transalkylation reactions match those produced in the conversion catalyzed by HY zeolite, in which no deoxygenated products were observed.

Impact of gallium supersaturation on the growth of N‐polar GaN

AbstractExperimental results on growth morphology in N‐polar GaN grown by low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD) were analyzed using a thermodynamic supersaturation model for gallium. Smooth N‐polar GaN films with 1 nm RMS roughness were always obtained under extremely low Ga supersaturation in the vapor, although the growth conditions were seemingly different. It was found that increasing H2 partial pressure during the GaN growth played a role in significantly lowering the Ga supersaturation, since H2 is a product in the formation of GaN. The degree of Ga supersaturation was also controlled by adjusting the partial pressure of Ga species in the vapor. The surface‐diffusion theory dealing with step dynamics described by Burton, Cabrera, and Frank (BCF) was also used to relate Ga supersaturation to the observed smooth N‐polar GaN growth. These findings showed that a supersaturation model encompassing all major MOCVD growth parameters can be used to predict the smooth N‐polar GaN growth conditions. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Rhodium Supported Hβ Zeolite for the Hydrogenation of Toluene

Noble metal Rh supported on a large pore high acidic zeolite Hβ has been explored as a hydrodearomatization catalyst. The detail kinetic study of hydrodearomatization of toluene over a 1 wt % Rh/Hβ was done in a continuous-downflow stainless steel catalytic fixed bed reactor at varied space time, toluene feed rate, hydrogen partial pressure, hydrogen to toluene mole ratio, temperature, and in the presence of dibenzothiophene. The time on stream data and reaction order with respect to toluene were measured and was found to be of first order. Fourier transform infrared (FTIR) spectra and chemical analysis of fresh and spent catalyst suggested the presence of surface carbon species and weight percent carbon was found to be 4.43%. It was observed that toluene conversion was increased on increasing H2 partial pressure and H2/feed mole ratio. The conversion is dependent on temperature and shows a well-defined maximum. The decrease of the catalyst activity in the presence of dibenzothiophene is mainly attributed to the adsorption and decomposition of dibenzothiophene (DBT) on the metal sites, which results in a loss of metal surface available for the reaction to take place and a higher coke formation reducing the fraction of acid sites available for toluene hydrodearomatization. A nonlinear semiempirical kinetic model was also developed to have the best fit with 12% error.

A kinetic Monte Carlo Model of Silicon CVD Growth from a Mixed H2/siH4 Gas Source

AbstractWe report the development of an atomistic scale Kinetic Monte Carlo model of silicon CVD growth. By employing a variable time step algorithm, simulations have been performed over a range of time scales, enabling direct comparison with experimental data. The validity of using the kinetic theory of gases for evaluating steady state incoming particle fluxes within the model is demonstrated by comparison with computational fluid dynamics simulations. The model is applied to study hydrogen desorption rates from Si(001) and the dependence of silicon growth rate on substrate temperature, with results found to be in good agreement with experimental data. An experimentally observed decrease of growth rate with increasing H2 partial pressure is also reproduced by the model and shown to be caused by a decrease in silane adsorption on a hydrogen-rich surface.

The role of hydrogen in nitrogen-containing diamondlike films studied by photoelectron spectroscopy

The influence of H on the local structure of N-containing amorphous diamondlike films (a-CNx:H) is reported. The samples were prepared by rf sputtering of graphite in a N2, Ar, and H2 atmosphere. The chemical bonding of C and N atoms was inferred by analyzing the C 1s and N 1s electronic core-level by x-ray photoelectron spectroscopy. Hydrogen free films present N 1s peaks with a “doublet”, located at 398.2–400.5 eV. When H is introduced in the preparation chamber, the doublet evolves to a single wider band located at 399.1 eV. This new band becomes dominant when increasing H2 partial pressure, completely hiding the original structure. High H2 partial pressure interrupts the growing network formed by N and C due to the attachment of H to N and/or C. Furthermore, the experimental results suggest that the possibility of formation of the C3N4 phaselike is inhibited by the presence of hydrogen.

H 2 Partial Pressure Dependences of CH 3 Radical Density and Effects of H 2 Dilution on Carbon Thin-Film Formation in RF Discharge CH 4 Plasma

Both the CH3 radical density and carbon thin-film formation were investigated in an RF-discharge CH4/H2 plasma. In this plasma, although CH3 radical density was almost constant, the deposition rate decreased markedly with increasing H2 partial pressure. These results suggested that the surface loss of the radicals was decreased due to the change in surface composition of the film, and also the film etching was enhanced with increasing H2 partial pressure. Therefore, the deposition rate of the carbon thin film decreased.

Highly Oriented Polycrystalline Si Film on Quartz Grown from Si3H8 by Thermal and Photo-CVD

Si films deposited on quartz plates grown from Si3H8 by thermal and photo-CVD have shown a strong crystalline orientation and are preferentially grown as (110) planes at substrate temperatures below 560°C, but turn to (100) above 575°C. In the transition region, an enhancement of <100>-orientation occurs with increasing H2 partial pressure and Si3H8 flow rate, but <110>-orientation occurs at extremely high H2 pressure and Si3H8 flow rates. Surface morphologies have also been investigated, and pyramidal texture, dome like structure and random roughness are found in <100>- and <110>-oriented films and non oriented crystalline film, respectively. The mechanism of the preferred orientation is discussed, taking into consideration the bonding structure of the crystal plane and that of decomposed species in the vicinity of the surface.

Nb films sputtered with a (Ar, H2) mixture and application to superconductor-insulator-superconductor junctions

Effects of hydrogen trapping in Nb base electrodes on the superconductivity of Nb itself and superconductor-insulator-superconductor (SIS) junction characteristics have been investigated. The Nb films were prepared by sputter deposition in an atmosphere of (Ar, H2) mixture. The gap parameter ΔNb and the critical temperature Tc gradually decreased with increasing H2 partial pressure. The SIS junctions exhibited a sudden extinction of the Josephson current Ic when ΔNb was lowered to or less than 1.1 meV by H2 trapping. Such a junction could be used as a quasiparticle tunneling junction for microwave detection with no requirement for an external magnetic field.

Demethylation of toluene by reaction with steam under pressure

The catalytic toluene hydrogenation over Ni/SiO2 was carried out using H2 or a H2/H2O mixture. The toluene conversion and MCH selectivity were evaluated under partial steam pressures 0−10 kPa, at H2/toluene molar ratios 1–5, and at temperatures 393−453 K. In the absence of steam, toluene conversion increased with increasing H2 partial pressure, and the MCH selectivity decreased with increasing temperature. In the presence of steam, the toluene conversion showed near-constant activities over the entire range of steam partial pressures, and the MCH selectivity increased with increasing steam partial pressure. This result indicated that the addition of steam inhibited byproduct formation. To clarify the effects of steam on the side-reaction, the byproducts composition was analyzed using a gas chromatograph. The results showed that the addition of steam inhibited demethylation, the ring-opening reaction of MCH, the disproportionation of toluene, and isomerization to convert six-membered ring compounds into five-membered ring compounds.