Catalysts For Preferential Oxidation Of CO Research Articles

CuO-CeO2 nanocomposite catalysts produced by mechanochemical synthesis

Mechanochemical synthesis based on ball-milling of individual oxides was applied as a one-step preparation technique for CuO-CeO2 catalyst for preferential CO oxidation in H2 excess. The mechanical energy dose transferred to the original powder mixture determines both the catalyst composition and activity. It is found that after 90 min of milling (corresponding to a dose of 372 kJ mol–1), a mixture of 10 wt.% CuO-CeO2 powder exhibits a CO conversion of 97% at 423 K. Four active oxygen states, which are not observed in case of pure CeO2, were detected in the nanocomposite lattice and attributed to the presence of Cu in surface sites as well as in subsurface bulk sites of CeO2, in nearest neighbor and next nearest neighbor positions. Correspondingly, oxidation of CO to CO2 was found to occur in a two-stage process with Tmax = 395/460 K, and oxidation of H2 to H2O likewise in a four-stage process with Tmax = 426/448/468/516 K. The milled powder consists of CeO2 crystallites sized 8–10 nm agglomerated to somewhat larger aggregates, with CuO dispersed on the surface of the CeO2 crystallites, and to a lesser extent present as Cu2O.

Alumina-supported cobalt phosphide as a new catalyst for preferential CO oxidation at high temperatures

Novel alumina-supported cobalt phosphide catalysts (designated as CoP-3, CoP-10, CoP-20 and CoP-40) prepared from the precursors with Co loadings of 3, 10, 20 and 40 wt% by H2-temperature-programmed reaction were investigated as potential catalysts for preferential CO oxidation (PROX) in excess H2 at high temperatures. It was found that the catalytic activities of these Co2P/γ-Al2O3 catalysts were related to their Co loadings. The CoP-10 catalyst showed the best PROX performance in temperature range of 220–240 °C, which was attributed to its optimal microstructures (high surface area, small particle size and big amount of active site).

Catalytic performance of transition metals (Co, Ni, Zn, Mo) doped CuO-Ce0.8Zr0.2O2 based catalysts for CO preferential oxidation in H2-rich streams

CuO-Ce0.8Zr0.2O2 composites (CuCZ) doped by transition metals (Co, Ni, Zn, Mo) as well as Co-doped CuCZ with different molar ratios of Co/Cu were prepared by a facile urea grind combustion (UGC) method and applied for CO preferential oxidation (CO-PROX) reaction in the hydrogen-rich gas mixtures. The characterizations of XRD, N2 adsorption-desorption, SEM, TEM, XPS and H2-TPR were used to investigate the difference in the catalytic behavior. Activity results found that the modification of Co had a positive influence on the catalytic activity of CuCZ and the optimal ratio of Co/Cu was 1/10, while the addition of Ni, Zn and Mo resulted in the decrease in the catalytic activities. XRD, SEM, H2-TPR and XPS results proved that the catalytic behavior of CuCZ based catalysts largely depended on the properties and types of transition metals. The addition of Co into CuCZ led to the isomorphous substitution of Cu2+ and Ce3+ species via Co2+ and Co3+, and the redispersion of Cu species over CeO2 support promoted the generation of the highly dispersed Cu oxides. As a result, large amount of active Cu+ sites and the surface active oxygen were available. The negative effects caused by Ni, Zn and Mo species were originated from the decreased amount of highly dispersed Cu oxides, possibly due to the differences in the oxidation-reduction property, the ion radius, the metal-support interaction as well as the nucleation rate and the size growth of particles during the preparation process. Consequently, the structure-property relationship was established, and the catalytic activity was suggested to be correlated with the amount of highly dispersed Cu oxides. Furthermore, 1/10Co-CuCZ showed excellent stability in terms of both CO conversion and CO2 selectivity during 120 h time-on-stream test.

Comparison of Au–Ce and Au–Cu interaction over Au/CeO2–CuO catalysts for preferential CO oxidation

Au decelerates reduction of copper species, while it improves ceria reduction.

Tailoring catalytic performance of carbon nanotubes confined CuO[sbnd]CeO2 catalysts for CO preferential oxidation

Multi-wall carbon nanotubes confined CuOCeO2 binary oxide catalysts (CuxCe1-xO/CNTs, x = 0, 0.2, 0.4, 0.6 and 1.0) have been prepared by a special ultrasound-aided impregnation. The effects of CuCe molar ratios on the structure and catalytic performance for CO preferential oxidation have been systematically investigated. Judging from XRD patterns and Raman analysis, Cu2+ ions incorporate into CeO2 lattice to form ceria-base solid solution, accompanied by oxygen vacancies and an interaction produces between the catalyst particles and inner surface of carbon nanotubes. TEM microstructural analysis shows that CuO and CeO2 particles are uniformly dispersed in carbon nanotubes for the CuxCe1-xO/CNTs (x = 0.4) catalyst. Meanwhile, the reducibility of active copper species is enhanced markedly. In addition, the Cu0.4Ce0.6O/CNTs sample possesses more surface lattice oxygen and oxygen vacancies according to surface species characterization, which can provide more active oxygen species for CO oxidation and promote the mobility of lattice oxygen. The CuxCe1-xO/CNTs (x = 0.4) catalyst exhibits excellent activity, wide temperature span of full CO conversion (120–160 °C) and high selectivity especially above 110 °C (maintaining high selectivity until 140 °C).

Promotion of Au3+ reduction on catalytic performance over the Au/CuO[sbnd]CeO2 catalysts for preferential CO oxidation

A series of Au/CuOCeO2 catalysts were prepared by the different methods in order to investigate the promoting effect of Au on preferential oxidation of carbon monoxide. The composition, structure, elementary valence and reduction behavior of the catalysts were systematically characterized by an array of techniques. It is found that the catalyst prepared by liquid-phase reduction deposition (AuCeCu-lprd) method has the highest low-temperature activity and the widest temperature window of complete CO conversion in the as-prepared catalysts, which are attributed to high loading amount of Au, the small size of Au nanoparticles, good reducibility of the support and the presence of Au+ from Au3+ reduction on the surface. In addition, the AuCeCu-lprd catalyst displays satisfactory stability at 115 °C for 80 h and good resistence to H2O.

A Novel Post‐Synthesis Modification of CuO‐CeO2 Catalysts: Effect on Their Activity for Selective CO Oxidation

AbstractCopper oxide nanoparticles were dispersed onto the surface of ceria nanorods, while a post‐synthesis modification was applied to the reference sample using ammonia solutions. The modified catalysts were characterized with a variety of analytical techniques, providing useful information on the effect of the Cu:NH3 ratio, employed in the modification step, on the physicochemical properties. Re‐dispersion of the active copper species under extremely basic conditions promoted the reducibility and the oxygen mobility of the catalyst, thus resulting in a more active and selective catalyst for preferential CO oxidation.

Change of Cu+ species and synergistic effect of copper and cerium during reduction-oxidation treatment for preferential CO oxidation

The CuO-CeO2@SiO2 catalyst with flower-sphere morphology was prepared by the impregnation method and then experienced the reduction-oxidation treatment at different temperatures. The multi-technique characterization shows that the reduction-oxidation treatment can remodel CuO, improve textural and surface properties and change Cu+ content and synergistic effect of copper and cerium. The importance of this work lies in the fact that the decrease of Cu+ content and synergistic effect of copper and cerium that occurs in the reduction-oxidation process results in the decrease of catalytic activity over the CuO-CeO2@SiO2 catalyst for preferential CO oxidation. The process of reaction in rich-hydrogen streams is equivalent to a reduction procedure which decreases Cu+ content and synergistic effect of copper and cerium.

Preferential Oxidation of CO in H2-Rich Stream Over Au/CeO2–NiO Catalysts: Effect of the Preparation Method

Two methods, viz., the hydrothermal (HT) and co-precipitation (CP) methods, were used to prepare CeO2–NiO composite oxides; with them as the supports, Au/CeO2–NiO catalysts were prepared by the colloidal deposition method and used in the preferential oxidation (PROX) of CO in H2-rich stream. Various characterization measures such as N2 sorption, XRD, TEM, H2-TPR, Raman spectroscopy and XPS were used to clarify the influence of preparation method on the structure of CeO2–NiO support and the performance of Au/CeO2–NiO catalyst. The XPS and TEM results reveal that the CeO2–NiO(HT) support prepared by hydrothermal method displays a uniform rod-like shape and exposes preferentially the (110) and (100) planes of CeO2, whereas the CeO2–NiO(CP) support prepared by co-precipitation method is composed of nanorods and irregular nanoparticles dominated by (111) facets of CeO2. After deposition of gold, both the Au/CeO2–NiO(HT) and Au/CeO2–NiO(CP) catalysts are alike in the state and size distribution of deposited Au nanoparticles. The H2-TPR results indicate that the presence of Au strongly promotes the reduction of CeO2 in the Au/CeO2–NiO catalyst. Raman spectra illustrate that the incorporation of Ni ions into CeO2 remarkably increases the amount of oxygen vacancies in the CeO2–NiO supports, especially in CeO2–NiO(HT) prepared by hydrothermal method, which is beneficial to the dispersion and stabilization of gold species. The structure of CeO2–NiO support and catalytic activity of Au/CeO2–NiO in CO PROX is strongly related to the preparation method; Au/CeO2–NiO(HT) exhibits much higher activity than Au/CeO2–NiO(CP). The larger fraction of (110) and (100) CeO2 facets in CeO2–NiO(HT) can promote the dispersion of gold species, formation of oxygen vacancies and migration of oxygen species, which are effective to enhance the redox capacity and activity of the obtained Au/CeO2–NiO(HT) catalyst for CO PROX in H2-rich stream. Au supported on CeO2–NiO nanorods prepared by hydrothermal method exhibits much higher catalytic activity for CO PROX in H2-rich stream.

Negative Effects of Dopants on Copper–Ceria Catalysts for CO Preferential Oxidation Under the Presence of CO2 and H2O

The effects of non-reducible dopants on copper–ceria catalysts for the preferential oxidation of carbon monoxide (CO PROX) were experimentally investigated by adding a non-reducible rare-earth element into the ceria support. Gadolinium-doped cerium oxides were synthesized by a combustion method as a support material for the copper–ceria catalytic system. Various compositions of the catalysts, i.e., CuO/Ce1−xGdxO2−δ (x = 0, 0.05, 0.1, 0.13, 0.18, 0.25 and 0.35), were prepared. The physical, structural, redox and surface chemical properties of the prepared catalysts were characterized by inductively coupled plasma mass spectrometry (ICP-MS), N2 isotherms, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) analyses. To investigate the influence of the dopants on the CO PROX, an activity test was conducted for each sample under a reactant stream containing CO, H2, CO2 and H2O. Negative effects of the dopants on the CO PROX activity were experimentally observed. Several characteristics on the catalysts induced by the insertion of dopants caused the effects. The presence of the non-reducible dopants on the surface hindered the efficiency of the redox equilibrium between copper and cerium, which is the essential process for CO PROX. Surface oxygen vacancies, generated by the introduction of foreign dopants, were not beneficial to the CO PROX activity. Copper species on the catalysts might be penetrated through these vacancies and protected as non-reactive reduced form in these vacancies.

Promoted the reduction of Cu2+ to enhance CuO[sbnd]CeO2 catalysts for CO preferential oxidation in H2-rich streams: Effects of preparation methods and copper precursors

A series of CuOCeO2 catalyst samples synthesized by using various methods (CuCe-SF-N, CuCe-UGC-N, CuCe-SG-N and CuCe-ST-N) and copper precursors (CuCe-SF-N, CuCe-SF-C, CuCe-SF-A and CuCe-SF-S) were estimated for CO preferential oxidation in H2-rich streams. It was found that both synthesis routes and copper precursors have an important effect on catalytic behaviors of CuOCeO2 catalyst. Compared to CuCe-UGC-N, CuCe-SG-N and CuCe-ST-N, CuCe-SF-N exhibits the lowest temperature and the widest temperature window for 100% CO conversion (about 50 °C), which should be attributed to synergistic effects of smaller crystallite size, the formation of more Cu+ species together with the high ratio of Ce3+/(Ce3++Ce4+). Among the four catalysts prepared with different Cu precursors (CuCe-SF-N, CuCe-SF-C, CuCe-SF-A and CuCe-SF-S), the corresponding CO conversions of them are in the order of CuCe-SF-N > CuCe-SF-A > CuCe-SF-C » CuCe-SF-S. The lowest catalytic activity of CuCe-SF-S should be due to the presence of SO42− species covered on the surface of the catalyst, which not only results in the formation of the less Cu active species but inhibits the interaction between Cu species and CeO2. In addition, the optimal CuCe-SF-N catalyst displays relative stability during the 200 h time-on-stream test even in the presence of H2O and CO2.

Coexistence of Cu+ and Cu2 + in star-shaped CeO2/CuxO catalyst for preferential CO oxidation

Star-shaped CeO2/CuxO catalysts very effective towards the CO-PROX reaction were synthesized by the impregnation method and characterized via FESEM, XRD, N2 physisorption, H2-TPR, XPS and in-situ DRIFTS. The importance of this work lies in the fact that the Cu oxidation state in the CeO2/CuxO catalysts was correlated with their activity and the former was influenced by the calcination temperature applied. In particular, CeO2/CuxO calcined at 300 and 350°C consist of Cu+ and Cu2+ and their catalytic activity is higher than that of CeO2/CuxO calcined at 400°C for which copper is in the Cu2+ state, as well as that of CeO2/CuxO calcined at 250°C for which copper is in the Cu+ state. The star-shaped CeO2/CuxO catalysts calcined at 300 and 350°C due to the presence of sufficient surface Cu+ and Cu2+ concentrations result in an enhanced CO-PROX performance. The slight deactivation observed after 30h on reaction stream at T=115°C can result from the small progressive reduction of Cu+ and formation of carbonate and bicarbonate species on the catalyst's surface.

Enhanced catalytic performance for CO preferential oxidation over CuO catalysts supported on highly defective CeO2 nanocrystals

Herein, we synthesized CeO2 nanocrystals (Ce250) with a large number of surface lattice defects by calcining the superfine Ce(OH)3 nanowires at 250°C. They were then utilized as the support of the CuO catalysts for CO preferential oxidation (CO PROX). The strong interaction between CuO and CeO2 is the key factor to improve the catalytic activity, and our results show that the surface lattice defects on the CeO2 support can strengthen the interaction between CuO and CeO2 during the catalyst preparation procedure. With the increase of the calcination temperature, the surface areas of the CuO/CeO2 catalysts gradually drops, but the interaction between CuO and CeO2 is strengthened. Their catalytic activities present a “volcano” type, and the CuCe500 calcined at 500°C shows the highest catalytic activity.

Rapid synthesis of Fe-doped CuO-Ce0.8Zr0.2O2 catalysts for CO preferential oxidation in H2-rich streams: Effect of iron source and the ratio of Fe/Cu

Abstract A facile route (urea grind combustion method) is described for the rapid synthesis of Fe-doped Cu-Ce-Zr catalysts within 30 min through simple grinding and combustion. The effects of iron source and Fe/Cu mass ratio on the performances of the catalysts for CO preferential oxidation (CO-PROX) are evaluated. The influences of H 2 O, CO 2 , and their mixture on the activity as well as stability of the catalysts are also investigated. The samples are characterized by XRD, N 2 adsorption-desorption, H 2 -TPR, TEM, Raman and XPS. Fe(NO 3 ) 3 is found to be superior to FeCl 3 and Fe 2 (SO 4 ) 3 as the iron source for Fe-CuCZ catalyst. Among the different synthesized catalysts, 1/10Fe (N) -CuCZ is found to be the most active catalyst, indicating that the optimal Fe/Cu mass ratio is 1/10. The influences of H 2 O, CO 2 , and H 2 O + CO 2 on the catalytic performance of 1/10Fe (N) -CuCZ are in the order of CO 2 2 + H 2 O 2 O. 1/10Fe (N) -CuCZ exhibits excellent stability during a 228 h time-on-stream test. 1/10Fe (N) -CuCZ shows the highest catalytic activity and excellent stability even in the presence of H 2 O and CO 2. The excellent catalytic performance can be attributed to the synergy between the highly dispersed copper species and ceria, as well as the formation of more oxygen vacancies and reduced copper species.

Improved Low-Temperature Activity of CuO–CeO2–ZrO2 Catalysts for Preferential Oxidation of CO in H2-Rich Streams

In this work, CuO–CeO2–ZrO2 catalysts for CO preferential oxidation have been prepared by ultrasound assisted reverse coprecipitation and slow coprecipitation, and characterized via BET, XRD, XPS and H2-TPR techniques, respectively. It is found that the catalyst prepared with ultrasound assisted reverse coprecipitation preserves large specific surface area, fine crystalline grain and high dispersion of active copper species. Meanwhile, the reducibility is improved significantly and more active copper species and oxygen vacancies are formed in the copper–ceria boundaries. The ultrasound assisted reverse coprecipitation prepared CuO–CeO2–ZrO2 catalyst exhibits superior low-temperature activity and CO2 selectivity, over which the temperature responding to 50 % CO conversion is as low as 67 °C, and the temperature window of full CO conversion is significantly widen from 100 to 160 °C. The ultrasound assisted reverse coprecipitation prepared CuO–CeO2–ZrO2 catalyst exhibits superior low-temperature activity for CO preferential oxidation and wide temperature window of full CO conversion comparing with the catalyst prepared with slow coprecipitation.

Effect of carbon dioxide and water on the performances of an iron-promoted copper/ceria catalyst for CO preferential oxidation in H2-rich streams

The catalytic behavior of an iron-promoted copper/ceria catalyst towards CO-PROX reaction under feed mixtures containing variable concentrations of CO2 (0–15 vol.%) and H2O (0–10 vol.%) was compared to that of an unpromoted sample with the same copper loading. At low temperature both species inhibit CO conversion on Fe-promoted catalyst more than on the reference Cu/CeO2 catalyst, although the performance is restored more rapidly to that corresponding to CO2-free conditions by increasing the temperature. CO2 temperature programmed desorption tests were carried out and experimental curves modeled to evaluate quantitatively the active sites for CO and H2 oxidation. Iron addition significantly reduced the amount of H2 oxidation sites, likely promoting copper dispersion, leading to the observed better selectivity under CO2-free conditions. The inhibiting effect of CO2 was ascribed to the formation of quite stable CO2 adsorbed species blocking copper active sites. The different dependence on the temperature of CO2 desorption results in a higher inhibition at low temperature for the Fe-promoted catalyst, but also in an easier CO2 desorption at higher temperature restoring the performance under CO2-free conditions for this sample.

Multishell hollow CeO2/CuO microbox catalysts for preferential CO oxidation in H2-rich stream

Multishell hollow CuO microbox was synthesized by a template method and the Ce(IV) nitrate solution was impregnated into the hollow cavity of the CuO support. The as-prepared catalysts were characterized by FESEM, TEM, XRD, N2 physisorption and H2-TPR. It is found that the alternate CuO and CeO2 multishell structure reduces the agglomeration of CuO species and improves the dispersion of CeO2. The special structure ensures even distribution of the two species in the material matrix and the optimum ratio of CeO2 to CuO provides two-component active sites with high concentration. Thus, the wider full CO conversion window and higher CO2 selectivity are achieved over the 30CeO2/CuO microbox catalyst.

Electrospun Au/CeO2 nanofibers: A highly accessible low-pressure drop catalyst for preferential CO oxidation

Au/CeO2 catalysts shaped as nanofibers were obtained by supporting Au nanoparticles (ca. 3nm) on CeO2 nanofibers of around 200nm diameter. The CeO2 support was prepared by calcining electrospun polymer nanocomposite fibers with a high Ce content; then gold nanoparticles were either synthesized in situ or deposited from a suspension. The prepared catalysts were used in the preferential oxidation of CO in a hydrogen-rich stream. The catalysts prepared by deposition of preformed gold nanoparticles were less stable and underwent sintering due to a weaker nanoparticle–support interaction. In contrast, the catalysts with Au nanoparticles synthesized in situ were active (90% conversion and 46% selectivity) and stable. The fiber-shaped catalyst was able to give maximum reactant access at a much lower pressure drop than catalyst in powder form.

High performance of Cu/CeO2-Nb2O5 catalysts for preferential CO oxidation and total combustion of toluene

Copper-based catalysts supported on niobium-doped ceria have been prepared and tested in the preferential oxidation of CO in excess of H2 (PROX) and in total oxidation of toluene. Supports and catalysts have been characterized by several techniques: N2 adsorption, ICP-OES, XRF, XRD, Raman Spectroscopy, SEM, TEM, H2-TPR and XPS, and their catalytic performance has been measured in PROX, with an ideal gas mixture (CO, O2 and H2) with or without CO2 and H2O, and in total oxidation of toluene. The effects of the copper loading and the amount of niobium in the supports have been evaluated. Remarkably, the addition of niobia to the catalysts may improve the catalytic performance in total oxidation of toluene. It allows us to prepare cheaper catalysts (niobia it is far cheaper than ceria) with improved catalytic performance.

Facile synthesis of carbon supported Pt-nanoparticles with Fe-rich surface: A highly active catalyst for preferential CO oxidation

Rational design and controlled preparation of highly effective catalysts towards the preferential oxidation of CO have significant importance for the utilization of hydrogen energy. In this work, PtFe/C bimetallic catalysts have been successfully prepared by a one-pot surfactant-free polyol process, where Pt metal nanoparticles serve as catalysts to reduce Fe ions to metallic form on the surface. The resulting PtFe/C catalysts are investigated for a PROX CO reaction and characterized by ICP-AES, XPS, XRD, TEM, HS-LEIS. It is found that PtFe/C with an optimal Fe loading shows extremely high activity and stability. These characterization results reveal that PtFe/C adopts a structure of Fe rich surface and Pt dominated cores. The excellent catalytic performances over PtFe/C catalysts are attributed to the efficient activation of O2 by Fe species located on the catalyst surface, indicated by the results of CO and O2 pulse experiments. Additionally, other PtM/C (M = Cu, Ni) catalysts possessing the similar structure with that of PtFe/C can also be synthesized using the established one-pot surfactant-free polyol process.