Catalysts

Editorial

Jump to: Research, Review

3 pages, 207 KiB

Open AccessEditorial

Recent Developments in Rh Heterogeneous Catalysts

by János Kiss and Imre Kovács

Catalysts 2021, 11(4), 416; https://doi.org/10.3390/catal11040416 - 25 Mar 2021

Viewed by 1502

Abstract

Rh-based catalysts successfully catalyze bond making and bond breaking reactions in most cases [...] Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

Research

Jump to: Editorial, Review

25 pages, 6592 KiB

Open AccessArticle

Numerical Simulation of Methane and Propane Reforming Over a Porous Rh/Al₂O₃ Catalyst in Stagnation-Flows: Impact of Internal and External Mass Transfer Limitations on Species Profiles

by Hüseyin Karadeniz, Canan Karakaya, Steffen Tischer and Olaf Deutschmann

Catalysts 2020, 10(8), 915; https://doi.org/10.3390/catal10080915 - 10 Aug 2020

Cited by 6 | Viewed by 2711

Abstract

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al₂O₃ layer. A one-dimensional flow field is coupled with three models for [...] Read more.

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al₂O₃ layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEM^STAG. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Figure 1

14 pages, 4676 KiB

Open AccessFeature PaperArticle

The Potassium-Induced Decomposition Pathway of HCOOH on Rh(111)

by Imre Kovács, János Kiss and Zoltán Kónya

Catalysts 2020, 10(6), 675; https://doi.org/10.3390/catal10060675 - 16 Jun 2020

Cited by 8 | Viewed by 2978

Abstract

Formic acid (FA) can be considered both a CO and a H₂ carrier via selective dehydration and dehydrogenation pathways, respectively. The two processes can be influenced by the modification of the active components of the catalysts used. In the present study the [...] Read more.

Formic acid (FA) can be considered both a CO and a H₂ carrier via selective dehydration and dehydrogenation pathways, respectively. The two processes can be influenced by the modification of the active components of the catalysts used. In the present study the adsorption of FA and the decomposition of the formed formate intermediate were investigated on potassium promoted Rh(111) surfaces. The preadsorbed potassium markedly increased the uptake of FA at 300 K, and influenced the decomposition of formate depending on the potassium coverage. The work function (Δϕ) is increased by the adsorption of FA on K/Rh(111) at 300 K suggesting a large negative charge on the chemisorbed molecule, which could be probably due to the enhanced back-donation of electrons from the K-promoted Rh into an empty π orbital of HCOOH. The binding energy of the formate species is therefore increased resulting in a greater concentration of irreversibly adsorbed formate species. Decomposition of the formate species led to the formation of H₂, CO₂, H₂O, and CO, which desorbed at significantly higher temperatures from the K-promoted surface than from the K-free one as it was proven by thermal desorption studies. Transformation of surface formate to carbonate (evidenced by UPS) and its decomposition and desorption is responsible for the high temperature CO and CO₂ formation. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Graphical abstract

17 pages, 3337 KiB

Open AccessFeature PaperArticle

Hydrogenation and Hydrodeoxygenation of Oxygen-Substituted Aromatics over Rh/silica: Catechol, Resorcinol and Hydroquinone

by Kathleen Kirkwood and S. David Jackson

Catalysts 2020, 10(5), 584; https://doi.org/10.3390/catal10050584 - 22 May 2020

Cited by 8 | Viewed by 3471

Abstract

The hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) was studied in the liquid phase over a Rh/silica catalyst at 303–343 K and 3 barg hydrogen pressure. The following order of reactivity, resorcinol > catechol > hydroquinone [...] Read more.

The hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) was studied in the liquid phase over a Rh/silica catalyst at 303–343 K and 3 barg hydrogen pressure. The following order of reactivity, resorcinol > catechol > hydroquinone (meta > ortho > para) was obtained. Kinetic analysis revealed that catechol had a negative order of reaction whereas both hydroquinone and resorcinol gave positive half-order suggesting that catechol is more strongly adsorbed. Activation energies of ~30 kJ·mol⁻¹ were determined for catechol and hydroquinone, while resorcinol gave a value of 41 kJ·mol⁻¹. Resorcinol, and similarly hydroquinone, gave higher yields of the hydrogenolysis products (cyclohexanol, cyclohexanone and cyclohexane) with a cumulative yield of ~40%. In contrast catechol favoured hydrogenation, specifically to cis-1,2-dihydroxycyclohexane. It is proposed that cis-isomers are formed from hydrogenation of dihydroxycyclohexenes and high selectivity to cis-1,2-dihydroxycyclohexane can be explained by the enhanced stability of 1,2-dihydroxycyclohex-1-ene relative to other cyclohexene intermediates of catechol, resorcinol or hydroquinone. Trans-isomers are not formed by isomerisation of the equivalent cis-dihydroxycyclohexane but by direct hydrogenation of 2/3/4-hydroxycyclohexanone. The higher selectivity to HDO for resorcinol and hydroquinone may relate to the reactive surface cyclohexenes that have a C=C double bond β-γ to a hydroxyl group aiding hydrogenolysis. Using deuterium instead of hydrogen revealed that each isomer had a unique kinetic isotope effect and that HDO to cyclohexane was dramatically affected. The delay in the production of cyclohexane suggest that deuterium acted as an inhibitor and may have blocked the specific HDO site that results in cyclohexane formation. Carbon deposition was detected by temperature programmed oxidation (TPO) and revealed three surface species. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Graphical abstract

11 pages, 1168 KiB

Open AccessFeature PaperArticle

Unraveling Toluene Conversion during the Liquid Phase Hydrogenation of Cyclohexene (in Toluene) with Rh Hybrid Catalysts

by Mónica Rufete-Beneite and M. Carmen Román-Martínez

Catalysts 2019, 9(12), 973; https://doi.org/10.3390/catal9120973 - 20 Nov 2019

Cited by 2 | Viewed by 2971

Abstract

Monitoring hydrogen consumption has allowed studying the progress of the liquid phase hydrogenation of cyclohexene in toluene with Rh SILP (supported ionic liquid phase) catalysts prepared by the immobilization of the [{RhCl(cod)}₂] complex on different carbon materials. An excess of hydrogen [...] Read more.

Monitoring hydrogen consumption has allowed studying the progress of the liquid phase hydrogenation of cyclohexene in toluene with Rh SILP (supported ionic liquid phase) catalysts prepared by the immobilization of the [{RhCl(cod)}₂] complex on different carbon materials. An excess of hydrogen consumption with respect to the required amount for cyclohexene hydrogenation was registered and related with the solvent (toluene) hydrogenation. The study carried out led to unraveling the extent of toluene hydrogenation and to determining if the rate of this reaction is affected by the properties of the carbon material used as support. The results revealed that the Rh SILP catalysts we prepared showed acceptable toluene conversion, with 100% selectivity to the total hydrogenated product, and that the effect of the carbon support is the same as for cyclohexene hydrogenation. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Figure 1

17 pages, 3719 KiB

Open AccessArticle

Investigation of Thermal Stability and Reactivity of Rh Nanoclusters on an Ultrathin Alumina Film

by Zhen-He Liao, Po-Wei Hsu, Ting-Chieh Hung, Guan-Jr Liao, Zhao-Ying Chern, Yu-Ling Lai, Li-Chung Yu, Yao-Jane Hsu, Jeng-Han Wang, Peilong Chen and Meng-Fan Luo

Catalysts 2019, 9(11), 971; https://doi.org/10.3390/catal9110971 - 18 Nov 2019

Cited by 4 | Viewed by 2473

Abstract

We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the [...] Read more.

We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the alumina film at 300 K, had an fcc phase and grew in the (100) orientation; the annealing altered the cluster structure little—the lattice parameter decreased by a factor <2%—but the cluster morphology significantly. With elevated temperature, small clusters (diameter ≤1.5 nm) decreased little in size; in contrast, large clusters (diameter ≥2.0 nm) varied in a complex manner—their mean diameter decreased to about 1.5 nm on annealing to 450 K, despite their similar height, while it increased to above 2.0 nm at temperature ≥570 K. This atypical decrease in size was governed predominantly by energetics. Such a reduced size enhanced the total surface area as well as the reactivity of the clusters toward methanol decomposition, so increased the production of D₂ (H₂) and CO from decomposed methanol-d₄ (or methanol). The result implies a higher temperature tolerance for Rh clusters on the alumina film and a practical approach to prepare small Rh clusters with high reactivity. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Figure 1

13 pages, 3095 KiB

Open AccessFeature PaperArticle

Low Temperature Infrared Study of Carbon Monoxide Adsorption on Rh/CeO₂

by Hicham Idriss and Jordi Llorca

Catalysts 2019, 9(7), 598; https://doi.org/10.3390/catal9070598 - 11 Jul 2019

Cited by 8 | Viewed by 3991

Abstract

Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO₂ is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas [...] Read more.

Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO₂ is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas shift reaction (WGSR), and ethanol steam reforming. In this work, the adsorption of CO is studied by infrared (IR) spectroscopy, over CeO₂ and 0.6 at. % Rh/CeO₂ at a temperature range of 90 to 300 K. CeO₂ is in the form of nanoparticles with sizes between 5 and 10 nm and exposing predominantly {111} surface termination in addition to non-negligible fraction of the {100} termination, determined from high resolution transmission electron microscopy (HRTEM). The as prepared Rh/CeO₂ contained metallic Rh as well Rh cations in higher oxidation states. At 90 K two IR bands were observed at 2183–2186 and 2161–2163 cm⁻¹, with the former saturating first. The 2163 cm⁻¹ peak was more sensitive to CO pressure than the 2186 cm⁻¹. Heating resulted in the depopulation of the 2163 cm⁻¹ before the 2186 cm⁻¹ peak. The desorption energy computed, assuming a first-order desorption kinetic, was found to be 0.35 eV for the 2186 cm⁻¹ and 0.30 for the 2163 cm⁻¹ IR peak (+/−0.05 eV). The equilibrium constant at 90 K was computed equal to 1.83 and 1.33 Torr⁻¹ for the 2183 and 2161 cm⁻¹, respectively. CO adsorption at 90 K on Rh/CeO₂ resulted (in addition to the bands on CeO₂) in the appearance of a broad band in the 2110–2130 cm^-1 region that contained two components at 2116 and 2126 cm⁻¹. The high frequency of this species is most likely due to adsorption on Rh clusters with very small sizes. The desorption energy of this species was found to be equal to 0.55 eV (+/−0.05 eV). Heating the CO covered Rh/CeO₂ surface accelerated the disappearance of CO species over CeO₂ and resulted in the appearance of CO₂ bands (at about 150 K) followed by carbonate species. At 300 K, the surface was mainly composed of carbonates. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Graphical abstract

Review

Jump to: Editorial, Research

29 pages, 6824 KiB

Open AccessFeature PaperReview

Rh-induced Support Transformation and Rh Incorporation in Titanate Structures and Their Influence on Catalytic Activity

by János Kiss, András Sápi, Mariann Tóth, Ákos Kukovecz and Zoltán Kónya

Catalysts 2020, 10(2), 212; https://doi.org/10.3390/catal10020212 - 10 Feb 2020

Cited by 9 | Viewed by 3035

Abstract

Rh is one of the most effective metals in several technologically important heterogeneous catalytic reactions, like the hydrogenation of CO₂, and CO, the CO+H₂O reaction, and methane and ethanol transformations. Titania and titanates are among the most frequently studied [...] Read more.

Rh is one of the most effective metals in several technologically important heterogeneous catalytic reactions, like the hydrogenation of CO₂, and CO, the CO+H₂O reaction, and methane and ethanol transformations. Titania and titanates are among the most frequently studied supports for Rh nanoparticles. The present study demonstrates that the nature of the support has a marked influence on the specific activity. For comparison, the catalytic activity of TiO₂ P25 is also presented. It is pointed out that a certain amount of Rh can be stabilized as cation (Rh⁺) in ion-exchange positions (i.e., in atomic scale distribution) of the titanate framework. This ionic form does not exists on TiO₂. We pay distinguished attention not only to the electronic interaction between Rh metal and the titania/titanate support, but also to the Rh-induced phase transitions of one-dimensional titanate nanowires (TiONW) and nanotubes (TiONT). Support transformation phenomena can be observed in Rh-loaded titanates. Rh decorated nanowires transform into the TiO₂(B) phase, whereas their pristine counterparts recrystallize into anatase. The formation of anatase is dominant during the thermal annealing process in both acid-treated and Rh-decorated nanotubes; Rh catalysis this transformation. We demonstrate that the phase transformations and the formation of Rh nanoclusters and incorporated Rh ions affect the conversion and the selectivity of the reactions. The following initial activity order was found in the CO₂ + H₂, CO + H₂O and C₂H₅OH decomposition reactions: Rh/TiO₂ (Degussa P25) ≥ Rh/TiONW > Rh/TiONT. On the other hand it is remarkable that the hydrogen selectivity in ethanol decomposition was two times higher on Rh/TiONW and Rh/TiO(NT) catalysts than on Rh/TiO₂ due to the presence of Rh⁺ cations incorporated into the framework of the titanate structures. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

► Show Figures

Figure 1

24 pages, 1108 KiB

Open AccessReview

Hydrogenation of Carbon Dioxide on Supported Rh Catalysts

by András Erdőhelyi

Catalysts 2020, 10(2), 155; https://doi.org/10.3390/catal10020155 - 29 Jan 2020

Cited by 35 | Viewed by 5828

Abstract

The constant increase in the CO₂ concentration in the atmosphere requires us to look for opportunities to convert CO₂ into more valuable compounds. In this review, the activity and selectivity of different supported metal catalysts were compared in the hydrogenation of [...] Read more.

The constant increase in the CO₂ concentration in the atmosphere requires us to look for opportunities to convert CO₂ into more valuable compounds. In this review, the activity and selectivity of different supported metal catalysts were compared in the hydrogenation of carbon dioxide, and found that Rh is one of the best samples. The possibility of the CO₂ dissociation on clean metal and on supported Rh was discussed separately. The hydrogenation of CO₂ produces mainly CH₄ and CO, but the selectivity of the reaction is affected by the support, in some cases the reduction of the support, the particle size of Rh, and the different additives. At higher pressure methanol, ethanol, and acetic acid could be also formed. The activity of the various supported Rh catalysts was compared and the results obtained for TiO₂-, SiO₂-, and Al₂O₃-supported catalysts were discussed in a separate chapter. The compounds formed on the surface of the catalysts during the reaction are shown in detail; mostly, different CO species, adsorbed formate groups, and different carbonates were detected. In a separate chapter the mechanism of the reaction was also discussed. Full article

(This article belongs to the Special Issue Recent Developments in Rh Catalysts)

Journal Menu

Journal Browser

Recent Developments in Rh Catalysts

Share This Special Issue

Special Issue Editors

Special Issue Information

Keywords

Related Special Issue

Published Papers (9 papers)

Editorial

Research

Review

Further Information

Guidelines

MDPI Initiatives

Follow MDPI