Preferential CO Research Articles

Optimization of nano-catalysts for application in compact reformers

• Demand for H 2 necessitated the development of a compact reformer. • Core unit process of compact reformer includes SRM, WGS, and PROX. • Scale-down of reformer decrease thermal efficiency and increase processing capacity. • This review introduces the studies about each core unit and provides a guide. • It especially focuses on the selection of appropriate materials in catalysts. Climate change triggered by the excessive use of fossil fuels has resulted in an increased focus on the use of hydrogen. In addition to its clean property, hydrogen exhibits a higher efficiency for fuel cell applications compared to heat engines. Although hydrogen is one of the most common elements on Earth, it is not readily available in its elemental form in nature, indicating that it is a secondary energy source that requires the processing of hydrocarbons or water. Currently, hydrogen is predominantly produced using fossil fuels (96%), and the large-scale production of hydrogen from natural gas has already been commercialized. However, the increasing demand for on-site/distributed power generation systems has necessitated the development of a small-scale hydrogen production process. This scale-down induces a decrease in thermal efficiency and an increase in processing capacity, which compels the development of an integrated and harmonized technology. Studies are being conducted on compact reformers in this regard. The core unit processes of compact reformer include steam reforming of methane (SRM), water–gas shift (WGS), and preferential CO oxidation (PROX), with each process requiring the development of customized catalysts. This review introduces the basic thermodynamics and kinetics of these core unit processes, details their various issues, and provides a guide regarding current research trends on the development of customized catalysts for the unit processes of SRM, WGS, and PROX in compact reformers. State of the art for compact reformers are also introduced, showing that there are only several types of commercial compact reformers yet compared to their importance.

The effect of copper oxide on the CuO–NiO/CeO2 structure and its influence on the CO-PROX reaction

The effect of copper oxide species on the CuO–NiO/CeO 2 structure and the influence on the preferential CO oxidation in H 2 excess (CO-PROX) reaction at low and high temperatures were investigated. Temperature-programmed surface reaction (TPSR), In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS) results allowed determining the surface species. The maximum temperature of CO 2 formation or selectivity decreased about 40 °C for the CuO–NiO/CeO 2 catalyst compared to the NiO/CeO 2 , which suggests that the addition of Cu + species increases the active sites due to interaction with the Ni–Ce structure. Therefore, the activity of the catalyst was closely related to the oxygen in vacancies and the formation of Cu + -carbonyl species of the redox mechanism. Besides, the superior selectivity towards CO 2 below 150 °C depends on the carbonyl stabilization at the surface, inhibiting the adsorption and subsequent oxidation of H 2 . Using TPSR and spectroscopic analyzes by DRIFTS and XPS allowed us to propose the reaction mechanisms for low and high temperatures. • Synergistic interaction between dispersed CuO species and Ni–Ce–O solid solution. • The presence of Ce 3+ in CeO 2 indicate formation of oxygen vacancies on the surface. • The activity was closely related to oxygen vacancies and Cu + -carbonyl species. • The temperature of CuNiCeO catalyst decreased about 40 °C compared to the NiCeO. • The oxygen vacancies and Cu + -carbonyl species suggest a multiple-step process.

The cobalt oxidation state in preferential CO oxidation on CoOx/Pt(111) investigated by operando X-ray photoemission spectroscopy

The combination of a reducible transition metal oxide and a noble metal such as Pt often leads to active low-temperature catalysts for the preferential oxidation of CO in excess H2 gas (PROX reaction). While CO oxidation has been investigated for such systems in model studies, the added influence of hydrogen gas, representative of PROX, remains less explored. Herein, we use ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy on a CoOx/Pt(111) planar model catalyst to analyze the active phase and the adsorbed species at the CoOx/Pt(111) interface under atmospheres of CO and O2 with a varying partial pressure of H2 gas. By following the evolution of the Co oxidation state as the catalyst is brought to a reaction temperature of above 150 °C, we determine that the active state is characterized by the transformation from planar CoO with Co in the 2+ state to a mixed Co2+/Co3+ phase at the temperature where CO2 production is first observed. Furthermore, our spectroscopy observations of the surface species suggest a reaction pathway for CO oxidation, proceeding from CO exclusively adsorbed on Co2+ sites reacting with the lattice O from the oxide. Under steady state CO oxidation conditions (CO/O2), the mixed oxide phase is replenished from oxygen incorporating into cobalt oxide nanoislands. In CO/O2/H2, however, the onset of the active Co2+/Co3+ phase formation is surprisingly sensitive to the H2 pressure, which we explain by the formation of several possible hydroxylated intermediate phases that expose both Co2+ and Co3+. This variation, however, has no influence on the temperature where CO oxidation is observed. Our study points to the general importance of a dynamic reducibility window of cobalt oxide, which is influenced by hydroxylation, and the bonding strength of CO to the reduced oxide phase as important parameters for the activity of the system.

CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes.

The development of efficient catalysts is one of the main challenges in CO2 conversion to valuable chemicals and fuels. Herein, inspired by the knowledge of the thermocatalytic (TC) processes, Cu/ZnO and bare Cu catalysts enriched with Cu+1 were studied to convert CO2 via the electrocatalytic (EC) pathway. Integrating Cu with ZnO (a CO-generation catalyst) is a strategy explored in the EC CO2 reduction to reduce the kinetic barrier and enhance C–C coupling to obtain C2+ chemicals and energy carriers. Herein, ethanol was produced with the Cu/ZnO catalyst, reaching a productivity of about 5.27 mmol·gcat–1·h–1 in a liquid-phase configuration at ambient conditions. In contrast, bare copper preferentially produced C1 products like formate and methanol. During CO2 hydrogenation, a methanol selectivity close to 100% was achieved with the Cu/ZnO catalysts at 200 °C, a value that decreased at higher temperatures (i.e., 23% at 300 °C) because of thermodynamic limitations. The methanol productivity increased to approximately 1.4 mmol·gcat–1·h–1 at 300 °C. Ex situ characterizations after testing confirmed the potential of adding ZnO in Cu-based materials to stabilize the Cu1+/Cu0 interface at the electrocatalyst surface because of Zn and O enrichment by an amorphous zinc oxide matrix; while in the TC process, Cu0 and crystalline ZnO prevailed under CO2 hydrogenation conditions. It is envisioned that the lower *CO binding energy at the Cu0 catalyst surface in the TC process than in the Cu1+ present in the EC one leads to preferential CO and methanol production in the TC system. Instead, our EC results revealed that an optimum local CO production at the ZnO surface in tandem with a high amount of superficial Cu1+ + Cu0 species induces ethanol formation by ensuring an appropriate local amount of *CO intermediates and their further dimerization to generate C2+ products. Optimizing the ZnO loading on Cu is proposed to tune the catalyst surface properties and the formation of more reduced CO2 conversion products.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

Metal-organic framework derived hollow CuO/CeO2 nano-sphere: To expose more highly dispersed Cu-O-Ce interface for enhancing preferential CO oxidation

Derivation of CuO/CeO2 catalyst from metal-organic framework (MOF) precursor is able to facilitate the dispersion of active sites, resulting in an enhanced catalytic performance for preferential CO oxidation in H2-rich stream (CO-PROX). However, parts of the Cu-O-Ce active interfaces are usually sintered inside the catalyst, which will reduce the catalytic performance. To solve such problem and fully expose the catalytic active sites, herein, we design and synthesize a hollow spherical CeO2 supporting Cu3(BTC)2 precursor shell and then pyrolyze it to generate CuO/CeO2-S catalyst. The results demonstrated by the various characterizations such as PXRD, TEM, H2-TPR, O2-TPD, Raman and XPS are as follows: The quantitative Cu active species are achieved by controlling the loading layer of MOFs; The hollow structure can avoid the active Cu-O-Ce interface being coated inside; The CeO2-S provides the lowest loading energy of Cu and more oxygen vacancy. Further in-situ analysis of Raman and DRIFTs as well as kinetic detects exhibit the highest interfacial active Cu species ratio and Ce3+ concentration of the CuO/CeO2-S, comparing to catalysts with diverse morphologies and traditional impregnation method from MOF precursors. The as-synthesized CuO/CeO2-S-10-700 displays nearly 100% CO conversion at ca. 80 °C and outstanding stability for CO-PROX.

DFT and kinetic evidences of the preferential CO oxidation pattern of manganese dioxide catalysts in hydrogen stream (PROX)

The oxidation functionality of Mn(IV) sites has been assessed by density functional theory (DFT) analysis of adsorption and activation energies of CO, H2 and O2 on a model Mn4O8 cluster. DFT calculations indicate that Mn(IV) atoms prompt an easy CO conversion to CO2 via a reaction path involving both catalyst and gas-phase oxygen species, while much greater energy barriers hinder H2 oxidation. Accordingly, a MnCeOx catalyst (Mnat/Ceat, 5) with large exposure of Mn(IV) sites shows a remarkable CO oxidation performance at T ≥ 293 K and no H2 oxidation activity below 393 K. Empiric kinetics disclose that the catalyst-oxygen abstraction step determines both CO and H2 oxidation rate, although different activation energies favor the preferential oxidation (PROX) pattern of the studied catalyst (353–423 K). Conversion-selectivity of 100%, high stability during 72 h reaction time and moderate inhibiting effects of water and CO2 feeding reveal the potential of MnO2 materials as efficient, low-cost and robust PROX catalysts.

Mathematical Modeling of Preferential CO Oxidation Reactions under Advection–Diffusion Conditions in a 3D-Printed Reactive Monolith

Mathematical Modeling of Preferential CO Oxidation Reactions under Advection–Diffusion Conditions in a 3D-Printed Reactive Monolith

Unraveling the Change in Multiple Cu Species Present in CuO/CeO2 over the Preferential CO Oxidation Reaction

CuO/CeO2 with strong metal–support interaction was considered to be a highly efficient catalyst in the PROX (preferential CO oxidation) reaction. Multiple Cu species from this interaction were believed to play a key role in catalytic activity. However, due to the complex chemical properties of copper, there was still controversy about the nature of active sites in the PROX reaction. In this work, the types and proportion of Cu species (Cu–[O]x–Ce, amorphous CuOx, and aggregated CuOx) could be well-controlled since CuO was loaded on surface-modified CeO2. Their properties were studied in detail through a series of characterizations. The Cu–[O]x–Ce structure was found to be the most favorable for preferential CO oxidation. While the amorphous CuOx cluster could facilitate CO oxidation, it would greatly reduce the selectivity. The aggregated CuOx caused by excessive CuO only had activity at high temperature, resulting in an upper limit of the CuO loading amount. The proportion of these Cu species varied with the CuO loading amount, and the catalytic performance was the synergetic result of all Cu species.

Effects of hydrothermal oxidation time of Al on the catalytic performance of Ru/Al@Al2O3 for selective oxidation of CO in H2

The thermal-conducting support is highly desirable for endothermic and exothermic reactions as long as highly dispersed active sites can be maintained. The core-shell Al@Al2O3 supports with different aluminum (Al) contents were prepared from Al particle by controlling the reaction time during hydrothermal surface oxidation and applied as a support to the supported Ru catalysts for preferential CO oxidation in H2 (PROX). The prepared catalysts were characterized by N2-physisorption, X-ray diffraction, CO chemisorption, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), in situ diffuse reflectance infrared Fourier transform spectroscopy after CO adsorption (CO-DRIFTS), and temperature-programmed desorption of ammonia (NH3-TPD) and ethanol (ethanol-TPD). The catalytic activity was dependent on the Al content in the Al@Al2O3 support. The most active Ru/Al@γ-Al2O3 catalyst oxidized CO selectively in H2 over a wide reaction temperature. The surface property of the outermost exterior alumina layer in the Al@γ-Al2O3 support, determined with ethanol TPD, was beneficial for formation of Ru nanoparticles with weak adsorption of CO, probed with CO-DRIFTS, results in the high catalytic performance for PROX.

Dynamic modeling of preferential CO oxidation in monolithic catalytic reactor using Pt–Fe/γ-Al2O3 catalyst

This study presents the dynamic modeling of preferential CO oxidation (CO-PROX) over a Pt–Fe/γ-Al2O3 catalyst in a monolithic reactor under both adiabatic and isothermal conditions. The modeling allowed us to investigate the effects of disturbance of the inlet conditions on transient performance. This work includes a mathematical description of the heat- and mass-transfer of species with a quasi-2D approach and a global kinetic mechanism. The model was validated by simulating the steady-state operation of the CO-PROX isothermal reactor and comparison of the predictions with experimental data obtained between 363 and 553 K at atmospheric pressure. The CO-PROX reactor model proved to be robust and accurate in the high-temperature regime (>410 K) and can be used for real-time simulation. The high response sensitivity of the adiabatic reactor predicted a significant improvement in CO conversion at temperatures from 435 K with the addition of a small amount of energy while maintaining a response time of 15 s. The isothermal reactor exhibited a reasonably fast response time when exposed to a realistic gas hourly space velocity scheme based on transient fuel demand in a proton exchange membrane fuel cell system.

Design of Nanoalloyed Catalysts for Hydrogen Production Processes

The use of supported nanoparticles of metal alloys as the active component makes it possible to modify the activity, selectivity, and stability of conventional monometallic supported heterogeneous catalysts. An approach to directed synthesis of bimetallic powders and supported catalysts based on the decomposition of double complex salts in the pores of the support is described. The efficiency of the proposed strategy was shown earlier by the example of catalysts for hydrogen production and purification processes: Pt0.5Co0.5/SiO2 and Au0.4Cu0.6/CeO2 catalysts for preferential oxidation of CO. Nanopowders of Pt0.5M0.5 (M = Fe, Co, Cu) and Pt0.33Ag0.67 and supported Pt0.5Cu0.5/Ce0.75Zr0.25O2 and Pt–CuOx/Ce0.75Zr0.25O2 catalysts are obtained in this work. It is shown that the nanopowders possess catalytic activity in the reaction of preferential CO oxidation in excess H2, while the Pt–Cu supported catalysts, in the low-temperature water gas shift reaction.

Carbon monoxide powered fuel cell towards H2-onboard purification

Proton exchange membrane fuel cells (PEMFCs) suffer extreme CO poisoning even at PPM level (<10 ppm), owning to the preferential CO adsorption and the consequential blockage of the catalyst surface. Herein, however, we report that CO itself can become an easily convertible fuel in PEMFC using atomically dispersed Rh catalysts (Rh-N-C). With CO to CO2 conversion initiates at 0 V, pure CO powered fuel cell attains unprecedented power density at 236 mW cm−2, with maximum CO turnover frequency (64.65 s−1, 363 K) far exceeding any chemical or electrochemical catalysts reported. Moreover, this feature enables efficient CO selective removal from H2 gas stream through the PEMFC technique, with CO concentration reduced by one order of magnitude through running only one single cell, while simultaneously harvesting electricity. We attribute such catalytic behavior to the weak CO adsorption and the co-activation of H2O due to the interplay between two adjacent Rh sites.

Identification of the preferential CO and SO2 adsorption sites within NOTT-401.

NOTT-401 was found to be a highly stable adsorbent for SO2 and CO with excellent cyclability and a straightforward regeneration at room temperature. Moreover, the preferential CO binding sites within the MOF material have been identified by experimental in situ DRIFT spectroscopy coupled with DFT and QTAIM calculations. Such preferential CO adsorption sites were correlated to identify the most significant SO2 interactions within NOTT-401. This study sheds light on the role of the thiophene and hydroxo functionality, for a MOF material, in the binding of SO2 or CO.

A PdCu nanoalloy catalyst for preferential CO oxidation in the presence of hydrogen

Alloy PdCu nanoparticles for CO oxidation in the presence and absence of hydrogen.

Preferential Oxidation of CO over CoFe2O4 and M/CoFe2O4 (M = Ce, Co, Cu or Zr) Catalysts

CoFe2O4 prepared by sol-gel has been examined with respect to its catalytic performance for preferential CO oxidation in a H2-rich stream. In turn, the promoting effects of incorporation of Ce, Co, Cu, and Zr by impregnation on the surface of CoFe2O4 on the process are examined as well. The catalysts have been characterized by N2 adsorption, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), temperature programmed reduction (TPR), and X-ray photoelectron spectra (XPS), as well as diffuse reflectance infrared DRIFTS under reaction conditions with the aim of establishing structure/activity relationships for the mentioned catalyst/process. It is shown that while the presence of the various metals on CoFe2O4 hinders a low temperature CO oxidation process, it appreciably enhances the activity above 125 °C. This is basically attributed to the surface modifications, i.e. cobalt oxidation, induced in CoFe2O4 upon introduction of the metals. In turn, no methanation activity is observed in any case except for the copper-containing catalyst, in which achievement of reduced states of cobalt appears most favored.

Modulation of the Effective Metal‐Support Interactions for the Selectivity of Ceria Supported Noble Metal Nanoclusters in Atmospheric CO2 Hydrogenation

AbstractEver‐increasing attention is paid to the sustainable utilization of CO2 in the atmosphere out of environmental and economic considerations. The deep‐going clarification of the effective and ineffective metal‐support interactions (MSI) in the supported catalysts for the catalytic performance of CO2 hydrogenation is particularly needed to design highly active nanocatalysts. How to tune the MSI to be effective for the selectivity of goal hydrogenation products is a significant but rather challenging subject. In this work, we demonstrate that the selectivity in CO2 hydrogenation can be regulated from preferential CH4 production to preferential CO production by varying the nature of the electronic MSI for ceria supported noble metal nanoclusters, and the effective MSI play a pivotal role for the catalytic systems with opposite hydrogenation selectivity. The modulation of the effective MSI for the selectivity in CO2 hydrogenation sheds new light on the comprehensive understanding of the MSI and interfacial effects in terms of tailoring and controlling the performance of supported heterogeneous catalysts.

The Influence of Yttrium Stabilized Zirconia as Support of Copper-Ceria Systems on their Catalytic Properties in the Prox Process

It is shown that with the same quantitative composition of the copper-ceria-zirconia catalyst, varying only the structural and dimensional characteristics of the yttrium stabilized zirconia (YSZr), it is possible to improve the activity and selectivity of the catalyst and expand the «temperature window» of its effective operation. It has been established that the low surface area of YSZr support, due to the high degree of agglomeration of its nanoparticles, contributes to the formation of a larger number of interaction zones of copper and cerium oxides that are active sites of the process of selective oxidation of CO in hydrogen (PROX – Preferential CO oxidation).

Cyclic molecular designed dispersion (CMDD) of Fe2O3 on CeO2 promoted by Au for preferential CO oxidation in hydrogen

A novel cyclic molecular designed dispersion (CMDD) method was employed to uniformly deposit 0.0–6.5 wt% Fe on CeO2. The gold catalysts (0.0–3.8 wt%) supported on Fe2O3-CeO2 were tested for CO preferential oxidation (PROX). As the CMDD method involved grafting of Fe (acac)3 onto the surface –OH groups, 2.7 surface –OH/nm2 was determined for the CeO2 by using the Grignard reagent. The performance of CMDD catalysts was compared with the corresponding catalysts prepared by impregnation. The catalysts were characterized by XPS, XRD, TPR, HRTEM-EDX, BET and ICP. The results suggested that Au/Fe2O3-CeO2 was significantly more active and selective than Au/CeO2. The CMDD-prepared catalysts with 1.4–2.5 wt% iron showed the highest activity and selectivity, especially at temperatures as low as 323 K. The formation of Ce-Fe solid solution for CMDD catalysts promoted the dispersion of both iron and gold. Highly dispersed gold nanoparticles mainly as small as 2.2 nm were observed in HRTEM.

Ultra-Fine CeO2 Particles Triggered Strong Interaction with LaFeO3 Framework for Total and Preferential CO Oxidation.

Interactions between the active components with the support are one of the fundamentally factors in determining the catalytic performance of a catalyst. In contrast to the comprehensive understanding on the strong metal-support interactions (SMSI) in metal-based catalysts, it remains unclear for the interactions among different oxides in mixed oxide catalysts due to its complexity. In this study, we investigated the interaction between CeO2 and LaFeO3, the two important oxygen storage materials in catalysis area, by tuning the sizes of CeO2 particles and highlight a two-fold effect of the strong oxide-oxide interaction in determining the catalytic activity and selectivity for preferential CO oxidation in hydrogen feeds. It is found that the anchoring of ultra-fine CeO2 particles (<2 nm) at the framework of three-dimensional-ordered macroporous LaFeO3 surface results in a strong interaction between the two oxides that induces the formation of abundant uncoordinated cations and oxygen vacancy at the interface, contributing to the improved oxygen mobility and catalytic activity for CO oxidation. Hydrogen spillover, which is an important evidence of the strong metal-support interactions in precious metal catalysts supported by reducible oxides, is also observed in the H2 reduction process of CeO2/LaFeO3 catalyst due to the presence of ultra-fine CeO2 particles (<2 nm). However, the strong interaction also results in the formation of surface hydroxyl groups, which when combined with the hydrogen spillover reduces the selectivity for preferential CO oxidation. This discovery demonstrates that in hybrid oxide-based catalysts, tuning the interaction among different components is essential for balancing the catalytic activity and selectivity.