Dispersed Rhodium Research Articles

One-Step Synthesis of a High Entropy Oxide-Supported Rhodium Catalyst for Highly Selective CO Production in CO2 Hydrogenation.

High entropy oxide (HEO) has shown to be a new type of catalyst support with tunable composition-function properties for many chemical reactions. However, the preparation of a metal nanoparticle catalyst supported on a metal oxide support is time-consuming and takes multiple complicated steps. Herein, we used a one-step glycine-nitrate-based combustion method to synthesize highly dispersed rhodium nanoparticles on a high surface area HEO. This catalyst showed high selectivity to produce CO in CO2 hydrogenation with 80% higher activity compared to rhodium nanoparticle-based catalysts. We also studied the effect of different metal elements in HEO and demonstrated that high CO selectivity was achieved if one of the metals in the metal oxide support favored CO production. We identified that copper and zinc were responsible for the observed high CO selectivity due to their low *CO binding strength. During hydrogenation, a strong metal-support interaction was created through charge transfer and formed an encapsulated structure between rhodium nanoparticles and the HEO support to lower the *CO binding strength, which enabled high CO selectivity in the reaction. By combining different metal oxides into HEO as a catalyst support, high activity and high selectivity can be achieved at the same time in the CO2 hydrogenation reaction.

Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts.

Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. Here, we report an approach that can address this issue. It utilizes a metal-organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·gcat-1·h-1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions.

Active Site Entropy of Atomically Dispersed Rh/Al2O3 Catalysts Dictates Activity for Ethylene Hydroformylation

The interpretation of catalytic kinetics on supported metal catalysts typically assumes that catalytic cycles occur on static active site structures apart from local rearrangement in the coordination environment. It has been reported that certain atomically dispersed metal active sites (e.g., Cu/Chabazite zeolites and Rh/γ-Al2O3) may be mobile under relevant reaction conditions, suggesting that the active sites themselves have entropy that could be relevant to apparent reaction kinetics. Here, we systematically modify the mobility (degrees of freedom or entropy) of atomically dispersed Rhodium gem-dicarbonyls, Rh(CO)2, supported on γ-Al2O3 through functionalization of the support with straight-chain alkyl-phosphonic acids of different tail lengths ranging from 1 (methyl) to 16 carbons (hexadecyl). The restricted mobility of Rh(CO)2 results in up to a 120 °C decrease in the required temperature for CO desorption from Rh(CO)2 and 1000× increase in turn over frequency for propanal formation via ethylene hydroformylation [where Rh(CO)2 is the most abundant surface intermediate] as compared to unfunctionalized Rh/γ-Al2O3. Eyring analysis suggests that the promoted rates of CO desorption and hydroformylation are due primarily to changes in apparent activation entropy [ΔΔS‡ of up to 60 J/(mol·K)], where restricted mobility of Rh(CO)2 promotes the attempt frequency of CO desorption, which is a kinetically relevant step in hydroformylation. Further, the dependence of Rh(CO)2 reactivity on alkyl phosphonic acid tail length suggests that interactions between phosphonic acid tails far from the active site modify the rigidity of the self-assembled monolayers, such that longer tails better restricted the mobility of Rh(CO)2. This work suggests that active site entropy can influence reaction kinetics on heterogeneous catalysts when changes in active site mobility are coupled to reaction coordinates and further that controlling active site entropy can be an effective design approach to increase catalytic performance.

Integrated in situ spectroscopic studies on syngas production from partial oxidation of methane catalyzed by atomically dispersed rhodium cations on ceria.

Catalytic reforming of methane to produce syngas is an important strategy for producing value-added chemicals. The conventional reforming catalyst relies on supported nickel nanoparticles. In this work, we investigated singly dispersed Rh cations anchored on a CeO2 catalyst (Rh1/CeO2) for high activity and selectivity towards the production of syngas via partial oxidation of methane (POM) in the temperature range of 600-700 °C. The yields of H2 and CO at 700 °C are 83% and 91%, respectively. The anchored Rh1 atoms on CeO2 of Rh1/CeO2 are in the cationic state, and on an average each Rh1 atom coordinates with 4-5 surface lattice oxygen atoms of CeO2. Compared to inert CeO2 for POM, via the incorporation of single-atom sites, Rh1 modifies the electronic state of oxygen atoms proximal to the Rh1 atoms and thus triggers the catalytic activity of CeO2. The high activity of single-atom catalyst Rh1/CeO2 suggests that the incorporation of single atoms of transition metals to the surface of a reducible oxide can modulate the electronic state of proximal anions of the oxide support toward forming an electronic state favorable for the selective formation of ideal products.

The role of the spatial arrangement of single rhodium sites on ZSM-5 in the oxidative methane carbonylation to acetic acid

Single atom rhodium catalysts based on microporous ZSM-5 zeolite (SiO2/Al2O3 = 30) were studied in the reaction of oxidative methane carbonylation to acetic acid. For highly dispersed rhodium distribution on the zeolite surface, nitrogen-containing polymers, namely chitosan hydrochloride, polyethyleneimine, and polyvinylpyrrolidone, were used. Using XAS spectroscopy, all rhodium was found to be as single atom metal sites in the zeolite structure, which contributes to the acetic acid formation. The catalytic properties of rhodium zeolite catalysts in the oxidative methane carbonylation to acetic acid were revealed to depend not only on the single atom rhodium distribution and the catalyst acidity, but also on the spatial rhodium atom arrangement in the zeolite, which was established by the XPS. According to DFT calculations in combination with EXAFS modeling, rhodium anchored at the zeolite channel intersections was shown to be coordinated with four zeolite oxygen atoms and one hydroxyl group oxygen atom and be the dominant catalytic species in the acetic acid formation.

Chromium Oxides as Structural Modulators of Rhodium Dispersion on Ceria to Generate Active Sites for NO Reduction

Chromium Oxides as Structural Modulators of Rhodium Dispersion on Ceria to Generate Active Sites for NO Reduction

Highly dispersed Rh prepared by the in-situ etching-growth strategy for energy-saving hydrogen evolution

BackgroundThe development of high-efficiency electrocatalysts that surpass Pt-based catalyst to achieve more efficient hydrogen evolution reactions (HER) is significant. MethodsHerein, we constructed a series of highly dispersed rhodium (Rh) catalysts anchored on the Ni-based layered double hydrotalcite, which were formed by the in-situ etching-growth of nickel-metal organic frameworks (Ni-MOFs). These electrocatalysts not only exhibit outstanding performance in HER, but also in urea oxidation reaction (UOR). Significant findingsThe nickel-aluminum layered double hydroxide anchored Rh (Rh-NiAl LDH/NF) has the best catalytic activity for HER. It only needs an overpotential of 14 mV to reach a current density of 10 mA cm−2 in 1.0 M KOH, which is in the top class of the currently reported HER catalysts. In addition, the catalyst Rh-NiFe LDH/NF has good catalytic performance for UOR, and it only needs an ultra-low potential of 1.30 V to reach 10 mA cm−2. Hence, we constructed an electrolytic cell with the two electrodes for the urea-assisted hydrogen production, which is an energy-saving protocol for HER. The current density of 50 mA cm−2 can be reached when the cell voltage is only 1.40 V, which is 150 mV less than the traditional water splitting to produce H2.

Boron Nanosheet-Supported Rh Catalysts for Hydrogen Evolution: A New Territory for the Strong Metal-Support Interaction Effect

HighlightsUltra-small and highly dispersed rhodium nanoparticles anchored in 2D ultra-thin boron nanosheets (BNS) were synthesized by a rapid NaBH4 reduction and facile freeze-dry approach.Due to the strong metal-support interaction effect, the optimized electrocatalyst exhibits excellent hydrogen evolution reaction (HER) catalytic activity and stability in the wide pH range electrolytes.Based on the stable surface, the oxidized boron surface facilitates the coupling of BNS and metal, giving catalyst outstanding HER performance.

Stabilization of atomically dispersed rhodium sites on ceria-based supports under reaction conditions probed by in situ infrared spectroscopy

In this work, atomically dispersed rhodium over co-precipitated ceria-based supports were prepared by wet impregnation. The structural and surface properties of materials were investigated by X-ray diffraction, N2 physisorption isotherms, and X-ray photoelectron spectroscopy. The fraction of isolated sites was quantified by infrared spectroscopy using CO as a probe molecule. The formation of a defective surface by incorporation of 15% (mol/mol) of zirconium on ceria structure increased ~5-fold the fraction of isolated sites on catalyst compared to unmodified ceria. The doped-support efficiently trapped rhodium atoms avoiding agglomeration and increasing stability during CO oxidation reaction in a wide range of temperatures (30–200 °C) when compared to single-sites deposited on silica.

Isostructural Atomically Dispersed Rhodium Catalysts Supported on SAPO-37 and on HY Zeolite.

Zeolites are widely applied supports for metal catalysts, but molecular sieves with comparable structures-silicoaluminophosphates (SAPOs)-have drawn much less attention and been overlooked as supports for atomically dispersed metals. Now, we report SAPO-37 as a support for atomically dispersed rhodium in rhodium diethylene complexes, made by the reaction of Rh(η2-C2H4)2(acetylacetonate) with the support and anchored by two Rh-O bonds at framework tetrahedral sites, as shown by infrared and extended X-ray absorption fine structure spectra. The ethylene ligands were readily replaced with CO, giving sharp νCO bands indicating highly uniform supported species. A comparison of the spectra with those of comparable rhodium complexes on zeolite HY shows that the SAPO- and zeolite-supported complexes are isostructural, providing an unmatched opportunity for determining support effects in catalysis. The two catalysts had similar initial room-temperature activities per Rh atom for ethylene conversion in the presence of H2, but the SAPO-supported catalyst was selective for ethylene hydrogenation and the zeolite-supported catalyst selective for ethylene dimerization; correspondingly, the catalyst on the SAPO was more stable than that on the zeolite during operation in a flow reactor.

Atomic rhodium catalysts for hydrogen evolution and oxygen reduction reactions

High specific activity of single-atom catalysts shows great application potentials in water splitting and fuel cell systems. Herein, we report a facile pyrolysis method to fabricate an atomically dispersed rhodium catalyst (Rh@NG). The Rh@NG exhibits outstanding electrochemical activities and kinetics for oxygen reduction reaction in alkaline medium, which can compare to Pt/C catalysts, along with excellent durability and good tolerance to methanol and carbon monoxide. The half-wave potential on Rh@NG in 0.1 M KOH and 0.1 M HClO4 is 0.848 and 0.74 V vs RHE, respectively. Additionally, the Rh@NG displays superior performance for hydrogen evolution reaction, offering a low overpotential of only 29, 33, and 45 mV at 10 mA cm−2 in 0.5 M H2SO4, 1.0 M KOH, and 0.1 M KOH, respectively. Theoretical calculations reveal that the efficient active sites for HER and ORR might be RhN3 and RhN4 moiety embedded into the N-doped graphene, respectively.

Atomically Dispersed Rh Active Sites on Oxide Supports with Controlled Acidity for Gas-Phase Halide-Free Methanol Carbonylation to Acetic Acid

Acetic acid (AA) is an important bulk commodity chemical produced primarily via methanol carbonylation using homogeneous organometallic catalysts in liquid phase reactors with halides that participate in each catalytic turnover. Here we report a heterogeneous, halide-free, gas-phase process for methanol carbonylation to AA using a catalyst consisting of atomically dispersed rhodium (Rh) active sites on an acidic support. It is demonstrated that active site pairs consisting of atomically dispersed Rh and support acid sites enable highly selective AA production, whereas Rh clusters drive methanol decomposition to CO and CO2. Through a comparison of methanol carbonylation over atomically dispersed Rh species on γ-Al2O3, ZrO2, and Na-modified ZrO2, it was identified that by decreasing the concentration of acidic sites, the production of byproduct dimethyl ether could be minimized. After the acidity of the support was tuned by depositing 5 wt % Na on ZrO2 that contained atomically dispersed Rh, AA selectivity ...

An In Situ Infrared Study of CO2 Hydrogenation to Formic Acid by Using Rhodium Supported on Titanate Nanotubes as Catalysts

AbstractTitanate nanotubes (TiNT) were synthesized by hydrothermal method and used as support of rhodium nanoparticles. Results of X‐Ray diffraction (XRD) and Raman spectroscopy of TiNT revealed its structure of Na2Ti3O7, while the results of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and physisorption of N2 confirmed the multilayer nanotubular morphology with external diameter of ∼12 nm, length > 100 nm and a BET surface area of 195 m2 g−1. The TEM analysis of the rhodium supported sample (Rh/TiNT) showed evidence of small (∼1 nm) and highly dispersed rhodium particles. Results of X‐Ray Photoelectron Spectroscopy (XPS) revealed a strong electronic interaction between TiNT and Rh sites. The catalytic activity of Rh/TiNT for the hydrogenation of CO2 to formic acid at moderate temperature (∼40 °C) and atmospheric pressure was demonstrated as evidenced by results of Mass Spectrometry (MS) and in‐situ Diffuse Reflectance Infrared with Fourier Transform Spectroscopy (DRIFTS). The in‐situ studies showed active surface species bonded to support sites and to rhodium sites. It is proposed that under H2 atmosphere, Na+ cations near to Rh particles promote the conversion of CO2 via dissociated H, allowing the formation of formate species at low temperature. The formate species and the hydride rhodium complexes are considered reaction intermediates.

Single-Step Hydrogenolysis of Furfural to 1,2-Pentanediol Using a Bifunctional Rh/OMS-2 Catalyst.

Hydrogenolysis of biomass-derived furfural (FFA) to 1,2-pentanediol (1,2-PeD) was investigated using a bifunctional catalyst with basic and metallic sites, which was synthesized by the hydrothermal method. The synthesized catalyst consisting of rhodium (Rh) supported on an octahedral molecular sieve (OMS-2) of different loadings, such as 0.5, 1, and 1.5% w/w, was studied, and 1% (w/w) loading gave the best results. This 1% w/w Rh/OMS-2 catalyst showed excellent catalytic activity and selectivity for the hydrogenolysis reaction because of better dispersion of rhodium, later revealed by characterization. Furthermore, 1% Rh/OMS-2 catalyst was well characterized in virgin and reused states using various techniques such as Fourier-transform infrared spectroscopy, NH3-temperature-programmed desorption (TPD), CO2-TPD, temperature-programmed reduction, H2 pulse chemisorption, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller surface area, X-ray photoelectron spectroscopy, Raman spectroscopy, and differential scanning calorimetry–thermogravimetry analysis. The catalyst showed a higher surface area of 72 m2/g and the average size of the highly dispersed Rh metal of ∼2 nm. The studies were performed in a batch reactor; the catalyst offered almost 100% conversion of FFA with 87% selectivity to 1,2-PeD at 160 °C and 30 atm hydrogen pressure in 8 h. The reaction mechanism and kinetic model have been developed using a dual-site Langmuir–Hinshelwood–Hougen–Watson mechanism. The activation energies were 12.3 and 27.6 kcal/mol, correspondingly. The catalyst was found to be active, selective, and reusable.

Trace mono-atomically dispersed rhodium on zeolite-supported cobalt catalyst for the efficient methane oxidation

The partial oxidation of methane is a promising method for the efficient production of syngas. To implement this process using common stainless steel reactors, an inexpensive catalyst that functions at 650 °C or below is necessary. However, base metal catalysts typically require much higher temperatures, and they are deactivated by re-oxidation and coke formation. Here we report that modification of a zeolite-supported 3 wt% cobalt catalyst with a trace amount of mono-atomically dispersed rhodium (0.005 wt%) dramatically improves catalytic performance and durability. Cobalt/mordenite is nearly inactive due to the oxidation of cobalt, but the catalyst modified with rhodium continuously gives 85–86% methane conversion and 90–91% CO selectivity with an H2/CO ratio of 2.0 without serious coking at 650 °C. During the reaction, mono-atomically dispersed rhodium converts cobalt oxide to Co0 active species via hydrogen spillover. Use of the zeolite support is key to the high catalytic performance.

Enhanced dispersion and the reactivity of atomically thin Rh layers supported by molybdenum oxide films

The behavior of rhodium layers deposited on oxidized, 0.15–20.0 ML thick Mo films formed on a nearly stoichiometric TiO2(110) single crystal was characterized by AES, TPD and work function (WF) measurements. The oxidation of 0.15–2.7 ML thick Mo deposits was performed via the redox reaction with the titania support at 1000K. Molybdenum oxide supports of MoO3 and MoO2 surface composition were formed by the oxidation of 20 ML thick Mo multilayers by O2 at 650K and 1000K, respectively. Rh grows in a layer-by-layer fashion on a mixed titanium-molybdenum oxide produced in the reaction between titania and 0.15 ML Mo, corresponding to a considerably enhanced dispersion of rhodium as compared with that on the clean TiO2(110). The surface reactivity of Rh layers supported by molybdenum oxides as a function of pre-annealing temperature was followed by carbon monoxide adsorption–desorption cycles. The CO uptake of a 0.4 ML thick Rh film formed on the MoO3 support was strongly suppressed at 300K, indicating the encapsulation of rhodium with MoOX species of low surface free energy. The CO adsorption capability of rhodium particles supported by both MoO3 and MoO2 layers was eliminated due to pre-annealing at 600K, related to the extended decoration of metal particles by MoOX moiety. The encapsulation of the rhodium films proceeded above 600K on both supports, and annealing to 1000K resulted in nearly equal WF values, indicating the formation of MoOX overlayers of similar surface composition close to MoO2. AES depth profiles revealed that the 0.4 ML thick Rh deposits covered by MoOX at 1000K preserved their island structure.

Catalytic activity of rhodium grafted on ordered mesoporous silica materials modified with aluminum in N2O decomposition

Three different ordered mesoporous silica materials, such as MCM-41, Al-containing MCM-41 (with 1.6wt% of Al incorporated into the silica framework) and SBA-15, were prepared. Furthermore, aluminum (1.8wt%) was grafted on the silica surface by the molecular designed dispersion method (MDD). In a next step, rhodium (1.4–2.9wt%), as an active metal for the catalytic reaction of N2O decomposition, was introduced by the MDD technique. The following order of the catalytic activity was found under an inert atmosphere: Al(incorporated)-MCM-41+Rh>MCM-41+Al(grafted)+Rh>SBA-15+Rh≈MCM-41+Rh. The influence of textural properties, total acidity and rhodium dispersion on the activity of catalysts was studied.

The promotion effect of hydrogen spillover on CH4 reforming with CO2 over Rh/MCF catalysts

The mesocellular silica foams (MCF) supported rhodium catalysts, prepared by wetness impregnation method, were tested in CH4 reforming with CO2. Hydrogen temperature-programmed desorption (H2-TPD) analysis showed that hydrogen spillover occurs on MCF loaded with highly dispersed rhodium. The turnover frequency of CH4 (TOFCH4(s−1)) increased with the strength rise of spilt-over hydrogen. The promotion mechanism was investigated by controlled atmosphere 13C and 1H MAS NMR. The spilt-over hydrogen atoms were found to adsorb on the bridged oxygen sites (SiOSi) of MCF and to form surface terminal silanol groups. Meanwhile, in the reforming reaction steps, CH4 dissociation into CH3* and CH3O* decomposition into CH2O* occurred. The results confirmed that hydrogen atoms can spill over from highly dispersed rhodium species to the neighboring bridged oxygen atoms of MCF, which promotes the reaction by facilitating rate determining steps of CH4 dissociation and CHxO*(x=2, 3) intermediates decomposition on metal surface.

Atomically dispersed rhodium on a support: the influence of a metal precursor and a support.

The influence of the support type and the metal precursor on the dispersion of rhodium after calcination and reduction was determined. The combination of electron microscopy and X-ray absorption analysis allowed the quantification of the amount of atomically dispersed rhodium in the samples. Higher amounts of atomically dispersed rhodium atoms are obtained when metal impregnation is performed with a rhodium acetate precursor in comparison to a rhodium chloride precursor over supports of the same composition. The stability of rhodium is improved with the addition of promoters; the co-presence of samaria and ceria in the support and metal impregnation with a rhodium acetate precursor leads to the highest amount of atomically dispersed rhodium remaining after reductive treatment at 773 K.

Structure, Activity, and Stability of Atomically Dispersed Rh in Methane Steam Reforming

Atomically dispersed rhodium catalysts supported on xSm2O3–yCeO2–Al2O3 supports were synthesized and applied in the methane steam reforming (MSR) reaction. Scanning transmission electron microscopy and MSR catalytic tests show that the dispersion of the metal, and thus the catalytic performance, depends on the support composition. The 12CeO2–Al2O3 support confers partial stability to rhodium, leading to the coexistence of atomically dispersed and nanosized rhodium particles during MSR at 773 K. A reaction mechanism with CH4 being activated on the atomically dispersed rhodium and CO being formed on the nanoparticles is proposed. A lower rate caused by the formation of carbon species is seen in the presence of atomically dispersed rhodium and in the absence of nanoparticles as CO formation is hindered.